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Abstract
Voltage-gated calcium channels (VGCCs) are critical for Ca2+ influx into all types of excitable cells, but their exact function is still poorly understood. Recent reconstruction of homology models for all human VGCCs at atomic resolution provides the opportunity for a structure-based discussion of VGCC function and novel insights into the mechanisms underlying Ca2+ selective flux through these channels. In the present review, we use these data as a basis to examine the structure, function, and Zn2+-induced modulation of Cav2.3 VGCCs, which mediate native R-type currents and belong to the most enigmatic members of the family. Their unique sensitivity to Zn2+ and the existence of multiple mechanisms of Zn2+ action strongly argue for a role of these channels in the modulatory action of endogenous loosely bound Zn2+, pools of which have been detected in a number of neuronal, endocrine, and reproductive tissues. Following a description of the different mechanisms by which Zn2+ has been shown or is thought to alter the function of these channels, we discuss their potential (patho)physiological relevance, taking into account what is known about the magnitude and function of extracellular Zn2+ signals in different tissues. While still far from complete, the picture that emerges is one where Cav2.3 channel expression parallels the occurrence of loosely bound Zn2+ pools in different tissues and where these channels may serve to translate physiological Zn2+ signals into changes of electrical activity and/or intracellular Ca2+ levels.
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Affiliation(s)
- Felix Neumaier
- Forschungszentrum Jülich GmbH, Institute of Neuroscience and Medicine, Nuclear Chemistry (INM-5) , Jülich, Germany.,University of Cologne, Faculty of Medicine and University Hospital Cologne, Institute of Radiochemistry and Experimental Molecular Imaging , Cologne, Germany
| | - Toni Schneider
- Institute of Neurophysiology , Faculty of Medicine and University Hospital Cologne, Cologne, Germany
| | - Walid Albanna
- Department of Neurosurgery, RWTH Aachen University , Aachen, Germany
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2
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Xiao X, Zhu MX, Xu TL. 2-Guanidine-4-methylquinazoline acts as a novel competitive antagonist of A type γ-aminobutyric acid receptors. Neuropharmacology 2013; 75:126-37. [PMID: 23916476 DOI: 10.1016/j.neuropharm.2013.07.018] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/26/2013] [Revised: 06/13/2013] [Accepted: 07/16/2013] [Indexed: 11/18/2022]
Abstract
The pentameric A type γ-aminobutyric acid receptors (GABAARs) are the major inhibitory neurotransmitter receptors in the nervous system and have long been considered as important pharmaceutical targets for the treatment of multiple neurological or psychological disorders. Here, we show that 2-guanidine-4-methylquinazoline (GMQ), a recently identified acid-sensing ion channel (ASIC) modulator, strongly and preferentially inhibits GABAAR among the major neurotransmitter-gated ion channels in cultured rat hippocampal neurons. GMQ inhibited GABA (1 μM)-induced currents in a competitive manner, with an IC50 (0.39±0.05 μM) comparable to that of bicuculline. Schild analysis revealed a slope of 1.04±0.06 for GMQ on α1β2 GABAARs expressed in HEK293T cells. Single-channel analysis showed that GMQ decreased open probability of GABAARs without affecting conductance. Moreover, GMQ inhibited GABAergic neurotransmission in hippocampal neurons, while having no significant effect on the basal field excitatory postsynaptic potentials (fEPSPs) and the intrinsic excitability of neurons. Using site-directed mutagenesis, we further demonstrated that mutations at Glu155 of β2 subunit and Phe64 of α1 subunit, both located inside the GABA binding pocket, profoundly decreased the sensitivity of the receptor to both GABA and GMQ. Interestingly, these mutations did not significantly affect the inhibition by amiloride, a diuretic structurally similar to GMQ and a known GABAAR inhibitor. We conclude that GMQ represents a novel chemical structure that acts, possibly, by competing with GABA binding to GABAARs. It is anticipated that GMQ and its analogs will facilitate the development of new chemical probes for GABAARs.
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Huang X, Tian M, Hernandez CC, Hu N, Macdonald RL. The GABRG2 nonsense mutation, Q40X, associated with Dravet syndrome activated NMD and generated a truncated subunit that was partially rescued by aminoglycoside-induced stop codon read-through. Neurobiol Dis 2012; 48:115-23. [PMID: 22750526 DOI: 10.1016/j.nbd.2012.06.013] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/09/2012] [Revised: 06/04/2012] [Accepted: 06/22/2012] [Indexed: 01/21/2023] Open
Abstract
The GABRG2 nonsense mutation, Q40X, is associated with the severe epilepsy syndrome, Dravet syndrome, and is predicted to generate a premature translation-termination codon (PTC) in the GABA(A) receptor γ2 subunit mRNA in a position that codes for the first amino acid of the mutant subunit. We determined the effects of the mutation on γ2 subunit mRNA and protein synthesis and degradation, as well as on α1β2γ2 GABA(A) receptor assembly, trafficking and surface expression in HEK cells. Using bacterial artificial chromosome (BAC) constructs, we found that γ2(Q40X) subunit mRNA was degraded by nonsense mediated mRNA decay (NMD). Undegraded mutant mRNA was translated to a truncated peptide, likely the signal peptide, which was cleaved further. We also found that mutant γ2(Q40X) subunits did not assemble into functional receptors, thus decreasing GABA-evoked current amplitudes. The GABRG2(Q40X) mutation is one of several epilepsy-associated nonsense mutations that have the potential to be rescued by reading through the PTC, thus restoring full-length protein translation. As a first approach, we investigated the use of the aminoglycoside, gentamicin, to rescue translation of intact mutant subunits by inducing mRNA read-through. In the presence of gentamicin, synthesis of full length γ2 subunits was partially restored, and surface biotinylation and whole cell recording experiments suggested that rescued γ2 subunits could corporate into functional, surface GABA(A) receptors, indicating a possible direction for future therapy.
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Affiliation(s)
- Xuan Huang
- The Graduate Program of Neuroscience, Vanderbilt University Medical Center, Nashville, TN 37212, USA
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André VM, Cepeda C, Vinters HV, Huynh M, Mathern GW, Levine MS. Pyramidal cell responses to gamma-aminobutyric acid differ in type I and type II cortical dysplasia. J Neurosci Res 2009; 86:3151-62. [PMID: 18615638 DOI: 10.1002/jnr.21752] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
Abnormalities in the gamma-aminobutyric acid (GABA)-ergic system could be responsible for seizures in cortical dysplasia (CD). We examined responses of pyramidal neurons to exogenous application of GABA, as well as alterations of GABAergic interneuron number and size in pediatric epilepsy surgery patients with non-CD, type I CD, and type II CD pathologies. We used the dissociated cell preparation for electrophysiology along with immunohistochemistry to identify number and size of GABAergic cells. Pyramidal neurons from type I CD tissue showed increased EC(50) and faster kinetics compared with cells from non-CD and type II CD tissue. Cytomegalic pyramidal neurons showed increased GABA peak currents and decreased peak current densities, longer kinetics, and decreased sensitivity to zolpidem and zinc compared with normal pyramidal cells from non-CD and type I CD. There were fewer but larger glutamic acid decarboxylase (GAD)-containing cells in type II CD tissue with cytomegalic neurons compared with non-CD, type I CD, and type II CD without cytomegalic neurons. In addition, GABA transporters (VGAT and GAT-1) showed increased staining surrounding cytomegalic neurons in type II CD tissue. These results indicate that there are differences in GABA(A) receptor-mediated pyramidal cell responses in type I and type II CD. Alterations in zolpidem and zinc sensitivities also suggest that cytomegalic neurons have altered GABA(A) receptor subunit composition. These findings support the hypothesis that patients with type I and type II CD will respond differently to GABA-mediated antiepileptic drugs and that cytomegalic neurons have features similar to immature neurons with prolonged GABA(A) receptor open channel times.
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Affiliation(s)
- Véronique M André
- Mental Retardation Research Center, David Geffen School of Medicine, University of California at Los Angeles, Los Angeles, CA 90095, USA.
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5
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Abstract
Many clinically important drugs target ligand-gated ion channels; however, the mechanisms by which these drugs modulate channel function remain elusive. Benzodiazepines (BZDs), anesthetics, and barbiturates exert their CNS actions by binding to GABA(A) receptors and modulating their function. The structural mechanisms by which BZD binding is transduced to potentiation or inhibition of GABA-induced current (I(GABA)) are essentially unknown. Here, we explored the role of the gamma(2)Q182-R197 region (Loop F/9) in the modulation of I(GABA) by positive (flurazepam, zolpidem) and negative [3-carbomethoxy-4-ethyl-6,7-dimethoxy-beta-carboline (DMCM)] BZD ligands. Each residue was individually mutated to cysteine, coexpressed with wild-type alpha(1) and beta(2) subunits in Xenopus oocytes, and analyzed using two-electrode voltage clamp. Individual mutations differentially affected BZD modulation of I(GABA). Mutations affecting positive modulation span the length of this region, whereas gamma(2)W183C at the beginning of Loop F was the only mutation that adversely affected DMCM inhibition. Radioligand binding experiments demonstrate that mutations in this region do not alter BZD binding, indicating that the observed changes in modulation result from changes in BZD efficacy. Flurazepam and zolpidem significantly slowed covalent modification of gamma(2)R197C, whereas DMCM, GABA, and the allosteric modulator pentobarbital had no effects, demonstrating that gamma(2)Loop F is a specific transducer of positive BZD modulator binding. Therefore, gamma(2)Loop F plays a key role in defining BZD efficacy and is part of the allosteric pathway allowing positive BZD modulator-induced structural changes at the BZD binding site to propagate through the protein to the channel domain.
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Trudell JR, Yue ME, Bertaccini EJ, Jenkins A, Harrison NL. Molecular modeling and mutagenesis reveals a tetradentate binding site for Zn2+ in GABA(A) alphabeta receptors and provides a structural basis for the modulating effect of the gamma subunit. J Chem Inf Model 2008; 48:344-9. [PMID: 18197653 DOI: 10.1021/ci700324a] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Gamma-aminobutyric acid type A receptors (GABA(A)-R) containing alpha1beta2gamma2 subunits are weakly inhibited by Zn2+, whereas receptors containing only the alpha1beta2 subunits are strongly inhibited. We built homology models of the ion pores of alpha1beta2 and alpha1beta2gamma2 GABA(A)-R using coordinates of the nicotinic acetylcholine receptor as a template. Threading the GABA(A)-R beta2 sequence onto this template placed the 17' histidine and the 20' glutamate residues at adjacent locations in the mouth of the pore, such that a nearly ideal tetradentate site for Zn2+ was formed from two histidine and two glutamate residues between adjacent beta subunits in the alpha1beta2 GABA(A)-R. Following optimization with CHARMM, the distance between the alpha-carbons of the adjacent histidine residues was approximately 9.2 A, close to the ideal distance for a Zn2+ binding site. Loss of inhibition by Zn2+ in alpha1beta2gamma2 GABA(A)-R can be explained by the geometry of these residues in the arrangement alpha1beta2gamma2alpha1beta2, in which the nearest C-alpha-C-alpha distance between the histidine residues is 15.5 A, too far apart for an energetically optimal Zn2+ binding site. We then mutated the gamma subunit at the 17' and/or 20' positions. Zn2+ inhibition was not restored in alpha1beta2gamma2 (I282H) receptors. A novel finding is that the modeling shows the native 20' lysine in gamma2 can compete with Zn2+ for binding to the inserted 17' histidine. Sensitivity to Zn2+ was restored in the double mutant receptor, alpha1beta2gamma2 (I282H; K285E), in which the competition with lysine was removed and a more favorable Zn2+ binding site was formed.
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Affiliation(s)
- James R Trudell
- Department of Anesthesia, Stanford University School of Medicine, Stanford, California 94305-5117, USA.
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Jo SM, Danscher G, Schrøder HD, Suh SW. Depletion of vesicular zinc in dorsal horn of spinal cord causes increased neuropathic pain in mice. Biometals 2007; 21:151-8. [PMID: 17570038 DOI: 10.1007/s10534-007-9103-x] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/12/2007] [Accepted: 05/23/2007] [Indexed: 10/23/2022]
Abstract
Zinc enriched (ZEN) neurons and terminals are abundant in the rodent spinal cord. Zinc ions have been suggested to modulate the excitability of primary afferent fibers believed to be important in nociceptive transmission. To test the hypothesis that vesicular zinc concentration is related to neuropathic pain we applied Chung's rodent pain model on BALB/c mice, and traced zinc transporter 3 (ZnT3) proteins and zinc ions with immunohistochemistry and autometallography (AMG), respectively. Under anesthesia the left fifth lumbar spinal nerve was ligated in male mice in order to produced neuropathic pain. The animals were then sacrificed 5 days later. The ZnT3 immunoreactivity was found to have decreased significantly in dorsal horn of fourth, fifth, and sixth lumbar segments. In parallel with the depressed ZnT3 immunoreactivity the amount of vesicular zinc decreased perceptibly in superficial gray matters of especially layer I-IV of the same segments. The transection-induced reduction of vesicular zinc in ZEN terminals of the dorsal horn was synchronic to reduced pain threshold, as measured by von Frey method. In a separate study, we observed intensive zinc selenite precipitation in somata of the smaller spinal ganglion cell, but 5 days after spinal nerve transection zinc precipitation was also found in the lager ganglion cells. The present results indicate that zinc may be involved in pain mechanism in the spinal ganglion level. These results support the hypothesis that vesicular zinc might have a modulatory role for neuropathic pain. Thus, increased pain sensitivity might be related to reduce vesicular zinc level in the dorsal spinal gray matter.
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Affiliation(s)
- Seung Mook Jo
- Department of Anatomy, Gachon Medical School, Namdong-Gu, Inchon, South Korea
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Chen L, Yung WH. Zinc modulates GABAergic neurotransmission in rat globus pallidus. Neurosci Lett 2007; 409:163-7. [PMID: 17046159 DOI: 10.1016/j.neulet.2006.09.033] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2006] [Revised: 09/12/2006] [Accepted: 09/15/2006] [Indexed: 11/28/2022]
Abstract
The globus pallidus plays a critical role in the regulation of movement, and abnormal activity of its neurons is associated with some basal ganglia motor diseases. A relatively high level of zinc has been reported in the globus pallidus, which is increased significantly after 6-OHDA treatments. To elucidate the action of zinc on GABAergic neurotransmission in the globus pallidus, whole-cell patch-clamp recordings were made from rat globus pallidus neurons. Superfusion of zinc significantly reduced both spontaneous and miniature inhibitory postsynaptic currents. The inhibition was selective to the amplitude with no change in the frequency, decay time and rise time. Furthermore, the reduction of spontaneous inhibitory postsynaptic currents (34.1+/-4.0%) was stronger than that of miniature inhibitory postsynaptic currents (19.7+/-3.2%). These results suggest that spontaneous inhibitory postsynaptic currents generated mainly by axonal collaterals and miniature inhibitory postsynaptic currents generated mainly by striatopallidal inputs may be mediated by different GABA(A) receptor combinations.
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Affiliation(s)
- Lei Chen
- Department of Physiology, The Chinese University of Hong Kong, Shatin, Hong Kong, China.
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9
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Abstract
AIMS We compared the antagonistic effects of state-dependent gamma-aminobutyric acid A (GABA(A)) receptor blockers picrotoxin, Zn(2+) and pregnenolone sulphate (PS) on GABA- and pentobarbital-activated currents in recombinant rat GABA(A) receptors in Xenopus oocytes. METHODS Experiments were performed with wild type rat alpha1 beta2 gamma2L and alpha1beta2 receptors, mutants alpha1V256S beta2 gamma2L and alpha1beta2A252Sgamma2L receptors by the two-electrode voltage-clamp technique. RESULTS In contrast to respective 3840- and 56-fold increases in Zn(2+) potencies to inhibit GABA- and pentobarbital-activated currents in the alpha1beta2 receptor, the corresponding potencies of PS remained unchanged in comparison with the alpha1 beta2 gamma2L receptor. A homologous mutation of the residue at 2' position closest to the cytoplasmic end of the M(2) helix to serine on both alpha1 and beta2 subunit, alpha1V256S and beta2A252S, abolished the inhibition of GABA(A) receptor by PS. In comparison with the wild type alpha1beta2gamma2L receptor, mutants alpha1V256S beta2 gamma2L and alpha1beta2 A252S gamma2L receptors did not affect the Zn(2+) inhibition. Furthermore, a significant increase in GABA potency was observed in the mutant alpha1V256S beta2 gamma2L receptor (P < 0.05), but not the mutant alpha1beta2 A252S gamma2L receptor compared with the wild type receptor. CONCLUSIONS Pregnenolone sulphate was a gamma2-subunit independent inhibitor in the GABA(A) receptor, whereas the Zn(2+) antagonism was profoundly influenced by the gamma2-subunit. It is likely that the 2' residue closest to the N-terminus of the protein at M(2) helix on both alpha1 and beta2 subunit are critical to the inhibitory actions of PS and the function of Cl(-) channels. These results are consistent with the hypothesis that PS behaves as a Cl(-) channel blocker that does not share with Zn(2+), the coincident channel property in the GABA(A) receptors.
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Affiliation(s)
- M-D Wang
- Department of Clinical Science, Obstetrics and Gynecology, Umeå Neurosteroid Research Center, Umeå University, Umeå, Sweden.
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Abstract
The gamma-aminobutyric acid type A receptor beta(3) homopentamer is spontaneously open and highly sensitive to many noncompetitive antagonists(NCAs) and Zn(2+). Our earlier study of the M2 cytoplasmic half (-1' to 10') established a model in which NCAs bind at pore-lining residues Ala(2)', Thr(6)', and Leu(9)'. To further define transmembrane 2 (M2) structure relative to NCA action, we extended the Cys scanning to the extra cellular half of the beta(3) homopentamer (11' to 20'). Spontaneous disulfides formed with T13'C, L18'C, and E20'C from M2/M2 cross-linking and with I14'C (weak), H17'C, and R19'Con bridging M2/M3 intersubunits, based on single (M2 Cys only) and dual (M2 Cys plus M3 C289S) mutations. Induced disulfides also formed with T16'C, but there were few or none with M11'C, T12'C, and N15'C. These findings show conformational flexibility/mobility in the M2 extracellular half 17' to 20' region interpreted as a deformed beta-like conformation in the open channel. The NCA radioligands used were [(3)H]1-(4-ethynylphenyl)-4-n-propyl-2,6,7-trioxabicyclo[2.2.2]octane ([(3)H]EBOB) and [(3)H]3,3-bis-trifluoromethylbicyclo[2.2.1]heptane-2,2-dicarbonitrile with essentially the same results. NCA binding was disrupted by individual Cys substitutions at 13',14',16',17', and 19'. The inactivity of T13'C/T13'S may have been due to disturbance of the channel gate; I14'S and T16'S showed much better binding activity than their Cys counterparts, and the low activities of H17'C and R19'C were reversed by dithiothreitol. Zn(2+) potency for inhibition of [(3)H]EBOB binding was lowered 346-fold by the mutation H17'A. We propose that NCAs enter their binding site both directly, through the channel pore, and indirectly, through the water cavity of adjacent subunits.
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Affiliation(s)
- Ligong Chen
- Environmental Chemistry and Toxicology Laboratory, Department of Environmental Science, Policy, and Management, College of Chemistry, University of California-Berkeley, CA 94720, USA
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11
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Abstract
Extrasynaptic GABA(A) receptors that are tonically activated by ambient GABA are important for controlling neuronal excitability. In hippocampal pyramidal neurons, the subunit composition of these extrasynaptic receptors may include alpha5betagamma and/or alpha4betadelta subunits. Our present studies reveal that a component of the tonic current in the hippocampus is highly sensitive to inhibition by Zn(2+). This component is probably not mediated by either alpha5betagamma or alpha4betadelta receptors, but might be explained by the presence of alphabeta isoforms. Using patch-clamp recording from pyramidal neurons, a small tonic current measured in the absence of exogenous GABA exhibited both high and low sensitivity to Zn(2+) inhibition (IC(50) values, 1.89 and 223 microm, respectively). Using low nanomolar and micromolar GABA concentrations to replicate tonic currents, we identified two components that are mediated by benzodiazepine-sensitive and -insensitive receptors. The latter indicated that extrasynaptic GABA(A) receptors exist that are devoid of gamma2 subunits. To distinguish whether the benzodiazepine-insensitive receptors were alphabeta or alphabetadelta isoforms, we used single-channel recording. Expressing recombinant alpha1beta3gamma2, alpha5beta3gamma2, alpha4beta3delta and alpha1beta3 receptors in human embryonic kidney (HEK) or mouse fibroblast (Ltk) cells, revealed similar openings with high main conductances (approximately 25-28 pS) for gamma2 or delta subunit-containing receptors whereas alphabeta receptors were characterized by a lower main conductance state (approximately 11 pS). Recording from pyramidal cell somata revealed a similar range of channel conductances, indicative of a mixture of GABA(A) receptors in the extrasynaptic membrane. The lowest conductance state (approximately 11 pS) was the most sensitive to Zn(2+) inhibition in accord with the presence of alphabeta receptors. This receptor type is estimated to account for up to 10% of all extrasynaptic GABA(A) receptors on hippocampal pyramidal neurons.
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Affiliation(s)
- Martin Mortensen
- Department of Pharmacology, University College London, Gower Street, London WC1E 6BT, UK
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Gallagher MJ, Song L, Arain F, Macdonald RL. The juvenile myoclonic epilepsy GABA(A) receptor alpha1 subunit mutation A322D produces asymmetrical, subunit position-dependent reduction of heterozygous receptor currents and alpha1 subunit protein expression. J Neurosci 2004; 24:5570-8. [PMID: 15201329 PMCID: PMC6729321 DOI: 10.1523/jneurosci.1301-04.2004] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Individuals with autosomal dominant juvenile myoclonic epilepsy are heterozygous for a GABA(A) receptor alpha1 subunit mutation (alpha1A322D). GABA(A) receptor alphabetagamma subunits are arranged around the pore in a beta-alpha-beta-alpha-gamma sequence (counterclockwise from the synaptic cleft). Therefore, each alpha1 subunit has different adjacent subunits, and heterozygous expression of alpha1(A322D), beta, and gamma subunits could produce receptors with four different subunit arrangements: beta-alpha1-beta-alpha1-gamma (wild type); beta-alpha1(A322D)-beta-alpha1-gamma (Het(betaalphabeta)); beta-alpha1-beta-alpha1(A322D)-gamma (Het(betaalphagamma));beta-alpha1(A322D)-beta-alpha1(A322D)-gamma (homozygous). Expression of a 1:1 mixture of wild-type andalpha1(A322D) subunits with beta2S and gamma2S subunits (heterozygous transfection) produced smaller currents than wild type and much larger currents than homozygous mutant transfections. Western blot and biotinylation assays demonstrated that the amount of total and surface alpha1 subunit from heterozygous transfections was also intermediate between those of wild-type and homozygous mutant transfections. alpha1(A322D) mutations were then made in covalently tethered triplet (gamma2S-beta2S-alpha1) and tandem (beta2S-alpha1) concatamers to target selectively alpha1(A322D) to each of the asymmetric alpha1 subunits. Coexpression of mutant and wild-type concatamers resulted in expression of either Het(betaalphabeta) or Het(betaalphagamma) receptors. Het(betaalphabeta) currents were smaller than wild type and much larger than Het(betaalphagamma) and homozygous currents. Furthermore, Het(betaalphabeta) transfections contained less beta-alpha concatamer than wild type but more than both Het(betaalphagamma) and homozygous mutant transfections. Thus, whole-cell currents and protein expression of heterozygous alpha1(A322D)beta2Sgamma2S receptors depended on the position of the mutant alpha1 subunit, and GABA(A) receptor currents in heterozygous individuals likely result primarily from wild-type and Het(betaalphabeta) receptors with little contribution from Het(betaalphagamma) and homozygous receptors.
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Affiliation(s)
- Martin J Gallagher
- Department of Neurology, Vanderbilt University, Nashville, Tennessee 37212, USA
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Das P, Bell-Horner CL, Huang RQ, Raut A, Gonzales EB, Chen ZL, Covey DF, Dillon GH. Inhibition of type A GABA receptors by L-type calcium channel blockers. Neuroscience 2004; 124:195-206. [PMID: 14960351 DOI: 10.1016/j.neuroscience.2003.12.005] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 12/04/2003] [Indexed: 11/26/2022]
Abstract
Modulation of type A GABA receptors (GABAA) by L-type Ca++ channel blockers was investigated. The dihydropyridines nifedipine and nitrendipine, and the phenylalkylamine verapamil inhibited recombinant rat alpha1beta2gamma2 receptors recorded from human embryonic kidney (HEK) 293 cells; nifedipine at low concentrations also elicited modest stimulatory effects on GABA-gated current. The IC50 for GABA current inhibition was lowest for nitrendipine (17.3 +/- 1.3 microM), so subsequent studies were focused on further exploring its mechanism and possible site of action. When co-applied with GABA, nitrendipine had minimal effects on initial current amplitude, but significantly enhanced current decay rate. Nitrendipine-mediated inhibition was subunit-selective, as its IC50 was 10-fold lower in alpha1beta2 receptors. Nitrendipine's effect in recombinant human alpha1beta2gamma2 receptors was similar (IC50=23.0 +/- 1.3 microM) to that observed in rat receptors of the same configuration, indicating the site of action is conserved in the two species. The inhibitory effects were dependent on channel gating, were independent of transmembrane voltage, and were also observed in GABAA receptors recorded from hypothalamic brain slices. The pharmacologic mechanism of inhibition by nitrendipine was non-competitive, indicating it does not act at the GABA binding site. Nitrendipine block was retained in the presence of the benzodiazepine antagonist flumazenil, indicating it does not interact at the benzodiazepine site. The actions of nitrendipine were not affected by a mutation (beta2T246F) that confers resistance to the channel blocker picrotoxin, and they were not altered in the presence of the picrotoxin site antagonist alpha-isopropyl-alpha-methyl-gamma-butyrolactone, demonstrating nitrendipine does not act at the picrotoxin site of the GABAA receptor. Possible interaction of nitrendipine with the Zn++ site was also eliminated, as mutation of beta2 H267 to A, which confers resistance to Zn++, had no effect on nitrendipine-mediated inhibition. Our data suggest some of the central effects of dihydropyridines may be due to actions at GABAA receptors. Moreover, the effects may be mediated through interaction with a novel modulatory site on the GABAA receptor.
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Affiliation(s)
- P Das
- Department of Pharmacology and Neuroscience, University of North Texas Health Science Center at Fort Worth, 3500 Camp Bowie Boulevard, Fort Worth, TX 76107, USA
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Akabas MH. GABAA Receptor Structure–Function Studies: A Reexamination in Light of New Acetylcholine Receptor Structures. International Review of Neurobiology 2004; 62:1-43. [PMID: 15530567 DOI: 10.1016/s0074-7742(04)62001-0] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Affiliation(s)
- Myles H Akabas
- Department of Physiology and Biophysics, Albert Einstein College of Medicine, Yeshiva University, Bronx, New York 10461, USA
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Abstract
In the suprachiasmatic nucleus (SCN), electrical activity, secretion, and other cellular functions undergo profound rhythm during day-night cycle due to oscillatory expression of clock gene constituents. Although SCN is enriched with gamma-aminobutyric acid (GABA)-ergic neurons, it is unknown whether there are circadian changes in the GABAA receptor expression and/or function. Here we investigated the possible daily variations in zinc sensitivity of GABAA channels in rat SCN neurons maintained in brain slices. Extracellular zinc inhibited GABA-induced currents in all ventrolateral (VL) and dorsomedial (DM) SCN neurons studied, as well as in neurons of non-SCN regions. In SCN neurons, the currents evoked by 30 microM GABA were inhibited by Zn2+ with an IC50 of 50.3+/-3.2 microM, whereas currents evoked by 100 microM GABA were inhibited with an IC50 of 181.6+/-32.0 microM. The antagonist action of zinc saturated at 97.4+/-0.7% for 30 microM GABA and 91.6+/-2.7% for 100 microM GABA. These observations indicate that Zn2+ inhibits SCN GABAA receptor competitively and in part non-competitively. In SCN neurons, but not in other neurons, the zinc sensitivity varied with daily time. During the day, the calculated IC50 for zinc was significantly lower than during the night (43.9+/-4.7 microM vs. 58.6+/-3.8, respectively). These results indicate that native GABAA receptors in SCN neurons display pharmacological properties of receptors having and not having gamma subunit and that the proportionality of these receptors could change during the day and night.
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Affiliation(s)
- Karla Kretschmannova
- Institute of Physiology, Academy of Sciences of the Czech Republic, Vídenská 1083, 142 20 Prague 4, Czech Republic
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16
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Lopantsev V, Wenzel HJ, Cole TB, Palmiter RD, Schwartzkroin PA. Lack of vesicular zinc in mossy fibers does not affect synaptic excitability of CA3 pyramidal cells in zinc transporter 3 knockout mice. Neuroscience 2003; 116:237-48. [PMID: 12535956 DOI: 10.1016/s0306-4522(02)00570-5] [Citation(s) in RCA: 39] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Zinc is found throughout the CNS in synaptic vesicles of glutamatergic neurons and has been suggested to have a modulatory role in the brain because of its interaction with voltage- and ligand-gated ion channels. We took advantage of zinc transporter 3 knockout mice, which lack vesicular zinc, to study the possible physiological role of this heavy metal in hippocampal mossy fiber neurotransmission. We examined postsynaptic responses evoked by mossy fiber activation, recorded in CA3 pyramidal cells in hippocampal slices prepared from zinc transporter 3 knockout and wild-type mice. Field-potential response threshold and amplitude, input-output curves, and paired-pulse evoked responses were the same in slices from zinc transporter 3 knockout and wild-type mice. Furthermore, neither amplitude nor duration of pharmacologically isolated N-methyl-D-aspartate, non-N-methyl-D-aspartate, GABA(A), and GABA(B) receptor-mediated postsynaptic potentials differed between zinc transporter 3 knockout and wild-type mice. There was no difference in the magnitude of epileptiform discharges evoked by repetitive stimulation or kainic acid application. However, in slices from zinc transporter 3 knockout mice, there was greater attenuation of GABA(A)-mediated inhibitory postsynaptic potentials during tetanic stimulation compared with slices from wild-type animals. We conclude that lack of vesicular zinc in mossy fibers does not significantly affect the mossy fiber-associated synaptic excitability of CA3 pyramidal cells; however, zinc may modulate GABAergic synaptic transmission under conditions of intensive activation.
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Affiliation(s)
- V Lopantsev
- Department of Pharmacology, University of Washington, Box 357280, Seattle, WA 98195-7280, USA.
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17
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Abstract
GABA(A) receptors, the major inhibitory neurotransmitter receptors in the mammalian central nervous system, are heteropentameric proteins. We are interested in understanding the contribution of the gamma subunit to the kinetic properties of GABA(A) receptors. Studies in Xenopus oocytes have suggested that co-expression of alpha1, beta2, and gamma 2S subunits results in the formation of both alpha beta and alpha betagamma receptors (Boileau et al. 2002a; Boileau et al., 1998). Here, we have used an excess of the gamma 2S subunit in transfections of HEK293 cells to bias expression toward alpha beta gamma-containing receptors. Using rapid application and whole cell patch clamp techniques, we found that incorporation of the gamma subunit eliminated the rapid phases of desensitization and accelerated deactivation, consistent with a proposed role of desensitization in slowing deactivation. In addition, alpha betagamma receptors had an increased GABA EC(50), reduced sensitivity to block by Zn(2+), and did not display outward rectification as compared to alpha beta receptors.
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Affiliation(s)
- A J Boileau
- Department of Physiology, University of Wisconsin, Madison, WI 53706, USA
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18
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Hosie AM, Dunne EL, Harvey RJ, Smart TG. Zinc-mediated inhibition of GABA(A) receptors: discrete binding sites underlie subtype specificity. Nat Neurosci 2003; 6:362-9. [PMID: 12640458 DOI: 10.1038/nn1030] [Citation(s) in RCA: 205] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/23/2002] [Accepted: 02/05/2003] [Indexed: 11/09/2022]
Abstract
Zinc ions are concentrated in the central nervous system and regulate GABA(A) receptors, which are pivotal mediators of inhibitory synaptic neurotransmission. Zinc ions inhibit GABA(A) receptor function by an allosteric mechanism that is critically dependent on the receptor subunit composition: alphabeta subunit combinations show the highest sensitivity, and alphabetagamma isoforms are the least sensitive. Here we propose a mechanistic and structural basis for this inhibition and its dependence on the receptor subunit composition. We used molecular modeling to identify three discrete sites that mediate Zn2+ inhibition. One is located within the ion channel, and the other two are on the external amino (N)-terminal face of the receptor at the interfaces between alpha and beta subunits. We found that the characteristically low Zn2+ sensitivity of GABA(A) receptors containing the gamma2 subunit results from disruption to two of the three sites after receptor subunit co-assembly.
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Affiliation(s)
- Alastair M Hosie
- Department of Pharmacology, University College London, Gower Street, London WC1E 6BT, UK
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Kim TY, Hwang JJ, Yun SH, Jung MW, Koh JY. Augmentation by zinc of NMDA receptor-mediated synaptic responses in CA1 of rat hippocampal slices: mediation by Src family tyrosine kinases. Synapse 2002; 46:49-56. [PMID: 12211081 DOI: 10.1002/syn.10118] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Normal neuronal activity results in the release of zinc from the synaptic vesicles of glutamatergic terminals and subsequent entry into postsynaptic neurons. Although the exact physiological role of zinc translocation is currently unknown, it is very likely that intracellular zinc exerts long-term modulatory effects upon synaptic transmission since zinc affects various molecules involved in signaling pathways. In this study we used rat hippocampal slices to examine the effect of zinc on glutamatergic synaptic transmission in the Schaffer collateral-CA1 synapses. Following a 10-min exposure to 0.3-1 mM zinc, the magnitude of NMDA receptor-mediated field excitatory postsynaptic potentials (fEPSP) gradually increased over the subsequent 30-40 min. In contrast, the magnitude of AMPA/kainate receptor-mediated fEPSPs remained unchanged. The selective potentiation of NMDA receptor-mediated fEPSPs by zinc was unlikely to be a presynaptic event, since the degree of paired-pulse facilitation was unaltered. Interestingly, the specific Src family tyrosine kinase inhibitor PP2 completely blocked zinc-induced potentiation of NMDA receptor-mediated fEPSP while the inactive analog PP3 had no effect, thereby suggesting the involvement of Src family tyrosine kinases. Furthermore, zinc exposure increased levels of total and tyrosine-phosphorylated forms of NR2A and NR2B in a PP2-dependent manner in both hippocampal slices and cell cultures. In addition, zinc treatment of hippocampal cultures increased the levels of tyrosine phosphorylation at the two positive regulatory sites of Src family tyrosine kinases. Our results demonstrate that zinc increases NMDA receptor function via Src family tyrosine kinase-mediated increases of NR2A and 2B tyrosine phosphorylation. We speculate that intense release of endogenous synaptic zinc may potentiate NMDA receptor-mediated transmission in zinc-containing glutamatergic pathways by a similar mechanism.
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Affiliation(s)
- Tae-Youn Kim
- National Creative Research Initiative Center for the Study of CNS Zinc and Department of Neurology, University of Ulsan College of Medicine, Seoul 138-736, Korea
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20
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Abstract
The hypothalamus influences a number of autonomic functions. The activity of hypothalamic neurons is modulated in part by release of the inhibitory neurotransmitter GABA onto these neurons. GABA(A) receptors are formed from a number of distinct subunits, designated alpha, beta, gamma, delta, epsilon, and theta, many of which have multiple isoforms. Little data exist, however, on the functional characteristics of the GABA(A) receptors present on hypothalamic neurons. To gain insight into which GABA(A) receptor subunits are functionally expressed in the hypothalamus, we used an array of pharmacologic assessments. Whole cell recordings were made from thin hypothalamic slices obtained from 1- to 14-day-old rats. GABA(A) receptor-mediated currents were detected in all neurons tested and had an average EC(50) of 20 +/- 1.6 microM. Hypothalamic GABA(A) receptors were modulated by diazepam (EC(50) = 0.060 microM), zolpidem (EC(50) = 0.19 microM), loreclezole (EC(50) = 4.4 microM), methyl-6,7-dimethoxy-4-ethyl-beta-carboline (EC(50) = 7.7 microM), and 5alpha-pregnan-3alpha-hydroxy-20-one (3alpha-OH-DHP). Conversely, these receptors were inhibited by Zn(2+) (IC(50) = 70.5 microM), dehydroepiandrosterone sulfate (IC(50) = 16.7 microM), and picrotoxin (IC(50) = 2.6 microM). The alpha4/6-selective antagonist furosemide (10-1,000 microM) was ineffective in all hypothalamic neurons tested. The results of our pharmacological analysis suggest that hypothalamic neurons express functional GABA(A) receptor subtypes that incorporate alpha1 and/or alpha2 subunits, beta2 and/or beta3 subunits, and the gamma2 subunit. Our results suggest receptors expressing alpha3-alpha6, beta1, gamma1, and delta, if present, represent a minor component of functional hypothalamic GABA(A) receptors.
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Affiliation(s)
- Ren-Qi Huang
- Department of Pharmacology and Neuroscience, University of North Texas Health Science Center, Fort Worth, Texas 76107, USA
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Barberis A, Petrini EM, Cherubini E, Mozrzymas JW. Allosteric interaction of zinc with recombinant alpha(1)beta(2)gamma(2) and alpha(1)beta(2) GABA(A) receptors. Neuropharmacology 2002; 43:607-18. [PMID: 12367606 DOI: 10.1016/s0028-3908(02)00109-0] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
In a recent study we have provided evidence that inhibition of native GABA(A) receptors by zinc depends primarily on the allosteric modulation of receptor gating. Both the kinetics and the sensitivity of the GABA(A) receptor to zinc depend on subunit composition, especially on the presence of the gamma(2) subunit. To analyze the mechanism of action of zinc its effects have been tested on recombinant alpha(1)beta(2)gamma(2) and alpha(1)beta(2) receptors expressed in HEK 293 cells. The currents produced by ultrafast application of GABA have been measured to assess the impact of zinc ions on GABA(A) receptor gating with resolution corresponding to the time scale of synaptic currents. While, as expected, zinc markedly reduced the peak amplitude of alpha(1)beta(2)-mediated currents, its effect on kinetics was significantly different from that observed for alpha(1)beta(2)gamma(2). In particular, unlike alpha(1)beta(2)gamma(2), zinc did not affect the onset of alpha(1)beta(2)-mediated responses. Moreover, zinc increased the extent of desensitisation of alpha(1)beta(2)gamma(2) receptors and reduced desensitisation of alpha(1)beta(2) ones. Quantitative analysis suggests that zinc exerts an allosteric modulation on both alpha(1)beta(2)gamma(2) and alpha(1)beta(2) receptors. Zinc effects on alpha(1)beta(2)gamma(2) were qualitatively similar to those reported for native receptors.
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Affiliation(s)
- A Barberis
- Neuroscience Programme and Istituto Nazionale Fisica della Materia (INFM) Unit, International School for Advanced Studies, Via Beirut 2-4, 34014 Trieste, Italy
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22
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Boileau AJ, Baur R, Sharkey LM, Sigel E, Czajkowski C. The relative amount of cRNA coding for gamma2 subunits affects stimulation by benzodiazepines in GABA(A) receptors expressed in Xenopus oocytes. Neuropharmacology 2002; 43:695-700. [PMID: 12367615 DOI: 10.1016/s0028-3908(02)00036-9] [Citation(s) in RCA: 100] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
Benzodiazepine (BZD) potentiation of GABA-activated Cl(-)-current (I(GABA)) in recombinant GABA(A) receptors requires the presence of the gamma subunit. When alpha1, beta2 and gamma2S cRNA are expressed in a 1:1:1 ratio in Xenopus oocytes, BZD potentiation of I(GABA) is submaximal, variable and diminishes over time. Potentiation by BZDs is increased, more reproducible and is stabilized over time by increasing the relative amount of cRNA coding for the gamma2S subunit. In addition, GABA EC(50) values for alpha1beta2gamma2 (1:1:1) receptors are intermediate to values measured for alpha1beta2 (1:1) and alpha1beta2gamma2 (1:1:10) receptors. We conclude that co-expression of equal ratios of alpha1, beta2 and gamma2 subunits in Xenopus oocytes produces a mixed population of alpha1beta2 and alpha1beta2gamma2 receptors. Therefore, for accurate measurements of BZD potentiation it is necessary to inject a higher ratio of gamma2 subunit cRNA relative to alpha1 and beta2 cRNA. This results in a purer population of alpha1beta2gamma2 receptors.
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Affiliation(s)
- A J Boileau
- Department of Physiology, University of Wisconsin, Madison 53706, USA
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23
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Wilkins ME, Hosie AM, Smart TG. Identification of a beta subunit TM2 residue mediating proton modulation of GABA type A receptors. J Neurosci 2002; 22:5328-33. [PMID: 12097484 DOI: 20026556] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
GABA type A (GABA(A)) receptors are functionally regulated by external protons in a manner dependent on the receptor subunit composition. Although H(+) can regulate the open probability of single GABA ion channels, exactly what residues and receptor subunits are responsible for proton-induced modulation remain unknown. This study resolves this issue by using recombinant alpha1betai subunit GABA(A) receptors expressed in human embryonic kidney cells. The potentiating effect of low external pH on GABA responses exhibited p(Ka) in accord with the involvement of histidine and/or cysteine residues. The exposure of GABA(A) receptors to the histidine-modifying reagent DEPC ablated regulation by H(+), implicating the involvement of histidine residues rather than cysteines in proton regulation. Site-specific substitution of all conserved external histidines to alanine on the beta subunits revealed that H267 alone, in the TM2 domain, is important for H(+) regulation. These results are interpreted as a direct protonation of H267 on alpha1betai receptors rather than an involvement in signal transduction. The opposing functional effects induced by Zn(2+) and H(+) at this single histidine residue most likely reflect differences in charge delocalization on the imidazole rings in the mouth of the GABA(A) receptor ion channel. Additional substitutions of H267 in beta subunits with other residues possessing charged side chains (glutamate and lysine) reveal that this area of the ion channel can profoundly influence the functional properties of GABA(A) receptors.
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Kanematsu T, Jang IS, Yamaguchi T, Nagahama H, Yoshimura K, Hidaka K, Matsuda M, Takeuchi H, Misumi Y, Nakayama K, Yamamoto T, Akaike N, Hirata M, Nakayama KI. Role of the PLC-related, catalytically inactive protein p130 in GABA(A) receptor function. EMBO J 2002; 21:1004-11. [PMID: 11867528 PMCID: PMC125885 DOI: 10.1093/emboj/21.5.1004] [Citation(s) in RCA: 118] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
The protein p130 was isolated from rat brain as an inositol 1,4,5-trisphosphate-binding protein with a domain organization similar to that of phospholipase C-delta1 but lacking PLC activity. We show that p130 plays an important role in signaling by the type A receptor for gamma-aminobutyric acid (GABA). Yeast twohybrid screening identified GABARAP (GABA(A) receptor-associated protein), which is proposed to contribute to the sorting, targeting or clustering of GABA(A) receptors, as a protein that interacts with p130. Furthermore, p130 competitively inhibited the binding of the gamma2 subunit of the GABA(A) receptor to GABARAP in vitro. Electrophysiological analysis revealed that the modulation of GABA-induced Cl- current by Zn2+ or diazepam, both of which act at GABA(A) receptors containing gamma subunits, is impaired in hippocampal neurons of p130 knockout mice. Moreover, behavioral analysis revealed that motor coordination was impaired and the intraperitoneal injection of diazepam induced markedly reduced sedative and antianxiety effects in the mutant mice. These results indicate that p130 is essential for the function of GABA(A) receptors, especially in response to the agents acting on a gamma2 subunit.
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Affiliation(s)
| | - Il-Sung Jang
- Laboratory of Molecular and Cellular Biochemistry, Faculty of Dental Science, and Station for Collaborative Research, Kyushu University, Fukuoka 812-8582,
Laboratory of Cellular and System Physiology, Faculty of Medical Science, Department of Pharmacology, Faculty of Pharmaceutical Science, and Department of Molecular and Cellular Biology, Medical Institute of Bioregulation, Kyushu University, Fukuoka 812-8582, CREST, Japan Science and Technology Corporation (JST), Kawaguchi, Saitama 332-0012 and Departmentof Biochemistry, Fukuoka University School of Medicine,Fukuoka 814-0180, Japan Corresponding author e-mail:
| | - Taku Yamaguchi
- Laboratory of Molecular and Cellular Biochemistry, Faculty of Dental Science, and Station for Collaborative Research, Kyushu University, Fukuoka 812-8582,
Laboratory of Cellular and System Physiology, Faculty of Medical Science, Department of Pharmacology, Faculty of Pharmaceutical Science, and Department of Molecular and Cellular Biology, Medical Institute of Bioregulation, Kyushu University, Fukuoka 812-8582, CREST, Japan Science and Technology Corporation (JST), Kawaguchi, Saitama 332-0012 and Departmentof Biochemistry, Fukuoka University School of Medicine,Fukuoka 814-0180, Japan Corresponding author e-mail:
| | - Hiroyasu Nagahama
- Laboratory of Molecular and Cellular Biochemistry, Faculty of Dental Science, and Station for Collaborative Research, Kyushu University, Fukuoka 812-8582,
Laboratory of Cellular and System Physiology, Faculty of Medical Science, Department of Pharmacology, Faculty of Pharmaceutical Science, and Department of Molecular and Cellular Biology, Medical Institute of Bioregulation, Kyushu University, Fukuoka 812-8582, CREST, Japan Science and Technology Corporation (JST), Kawaguchi, Saitama 332-0012 and Departmentof Biochemistry, Fukuoka University School of Medicine,Fukuoka 814-0180, Japan Corresponding author e-mail:
| | | | | | | | | | - Yoshio Misumi
- Laboratory of Molecular and Cellular Biochemistry, Faculty of Dental Science, and Station for Collaborative Research, Kyushu University, Fukuoka 812-8582,
Laboratory of Cellular and System Physiology, Faculty of Medical Science, Department of Pharmacology, Faculty of Pharmaceutical Science, and Department of Molecular and Cellular Biology, Medical Institute of Bioregulation, Kyushu University, Fukuoka 812-8582, CREST, Japan Science and Technology Corporation (JST), Kawaguchi, Saitama 332-0012 and Departmentof Biochemistry, Fukuoka University School of Medicine,Fukuoka 814-0180, Japan Corresponding author e-mail:
| | - Keiko Nakayama
- Laboratory of Molecular and Cellular Biochemistry, Faculty of Dental Science, and Station for Collaborative Research, Kyushu University, Fukuoka 812-8582,
Laboratory of Cellular and System Physiology, Faculty of Medical Science, Department of Pharmacology, Faculty of Pharmaceutical Science, and Department of Molecular and Cellular Biology, Medical Institute of Bioregulation, Kyushu University, Fukuoka 812-8582, CREST, Japan Science and Technology Corporation (JST), Kawaguchi, Saitama 332-0012 and Departmentof Biochemistry, Fukuoka University School of Medicine,Fukuoka 814-0180, Japan Corresponding author e-mail:
| | - Tsuneyuki Yamamoto
- Laboratory of Molecular and Cellular Biochemistry, Faculty of Dental Science, and Station for Collaborative Research, Kyushu University, Fukuoka 812-8582,
Laboratory of Cellular and System Physiology, Faculty of Medical Science, Department of Pharmacology, Faculty of Pharmaceutical Science, and Department of Molecular and Cellular Biology, Medical Institute of Bioregulation, Kyushu University, Fukuoka 812-8582, CREST, Japan Science and Technology Corporation (JST), Kawaguchi, Saitama 332-0012 and Departmentof Biochemistry, Fukuoka University School of Medicine,Fukuoka 814-0180, Japan Corresponding author e-mail:
| | - Norio Akaike
- Laboratory of Molecular and Cellular Biochemistry, Faculty of Dental Science, and Station for Collaborative Research, Kyushu University, Fukuoka 812-8582,
Laboratory of Cellular and System Physiology, Faculty of Medical Science, Department of Pharmacology, Faculty of Pharmaceutical Science, and Department of Molecular and Cellular Biology, Medical Institute of Bioregulation, Kyushu University, Fukuoka 812-8582, CREST, Japan Science and Technology Corporation (JST), Kawaguchi, Saitama 332-0012 and Departmentof Biochemistry, Fukuoka University School of Medicine,Fukuoka 814-0180, Japan Corresponding author e-mail:
| | - Masato Hirata
- Laboratory of Molecular and Cellular Biochemistry, Faculty of Dental Science, and Station for Collaborative Research, Kyushu University, Fukuoka 812-8582,
Laboratory of Cellular and System Physiology, Faculty of Medical Science, Department of Pharmacology, Faculty of Pharmaceutical Science, and Department of Molecular and Cellular Biology, Medical Institute of Bioregulation, Kyushu University, Fukuoka 812-8582, CREST, Japan Science and Technology Corporation (JST), Kawaguchi, Saitama 332-0012 and Departmentof Biochemistry, Fukuoka University School of Medicine,Fukuoka 814-0180, Japan Corresponding author e-mail:
| | - Kei-Ichi Nakayama
- Laboratory of Molecular and Cellular Biochemistry, Faculty of Dental Science, and Station for Collaborative Research, Kyushu University, Fukuoka 812-8582,
Laboratory of Cellular and System Physiology, Faculty of Medical Science, Department of Pharmacology, Faculty of Pharmaceutical Science, and Department of Molecular and Cellular Biology, Medical Institute of Bioregulation, Kyushu University, Fukuoka 812-8582, CREST, Japan Science and Technology Corporation (JST), Kawaguchi, Saitama 332-0012 and Departmentof Biochemistry, Fukuoka University School of Medicine,Fukuoka 814-0180, Japan Corresponding author e-mail:
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Klausberger T, Sarto I, Ehya N, Fuchs K, Furtmuller R, Mayer B, Huck S, Sieghart W. Alternate use of distinct intersubunit contacts controls GABAA receptor assembly and stoichiometry. J Neurosci 2001; 21:9124-33. [PMID: 11717345 [PMID: 11717345 DOI: 10.1523/jneurosci.21-23-09124.2001] [Citation(s) in RCA: 58] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023] Open
Abstract
GABA(A) receptors are the major inhibitory transmitter receptors in the CNS. Recombinant GABA(A) receptors composed of alpha(1)beta(3)gamma(2) subunits have been demonstrated to assemble as pentamers consisting of two alpha(1), two beta(3), and one gamma(2) subunit. Using truncated and chimeric alpha(1) subunits, we identified the alpha(1)(80-100) sequence as a major binding site for gamma(2) subunits. In addition, we demonstrated its direct interaction with gamma(2)(91-104), a sequence that previously has been identified to form the contact to alpha(1) subunits. The observation that the amino acid residues alpha(1)P96 and alpha(1)H101, which can be photolabeled by [(3)H]flunitrazepam, are located within or adjacent to the alpha(1)(80-100) sequence, indicates that the benzodiazepine binding site of GABA(A) receptors is located close to this intersubunit contact. The observation that alpha(1)(80-100) interacts with gamma(2) but not with beta(3) subunits indicates the existence of an additional beta(3) binding site on alpha(1) subunits. The preferred alternate use of the gamma(2) and beta(3) binding sites in two different alpha(1) subunits of the same receptor ensures the incorporation of only a single gamma(2) subunit and thus, determines subunit stoichiometry of alpha(1)beta(3)gamma(2) receptors. Distinct binding sites and their alternate use can therefore explain how subunits of hetero-oligomeric transmembrane proteins assemble into a defined protein complex.
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Abstract
To gain insight into specific GABA(A) receptor configurations functionally expressed in the nucleus tractus solitarius (NTS), we conducted several physiological and pharmacological assessments. NTS neurons were characterized in thin brain slices from 1-14 day old rats using whole-cell patch clamp recordings. GABA(A-) receptor-mediated currents were detected in all neurons tested, with an average EC(50) of 22.2 microM. GABA currents were consistently stimulated by diazepam (EC(50)=63 nM), zolpidem (EC(50)=85 nM), loreclezole (EC(50)=10.1 microM) and the neurosteroid 5alpha-pregnan-3alpha-hydroxy-20-one (3alpha-OH-DHP). In contrast, GABA-gated currents of the NTS were inhibited by the divalent cation Zn(2+) (IC(50)=33.6 microM) picrotoxin (IC(50)=2.4 microM) and blockade of endogenous protein tyrosine kinase. GABA-activated currents were insensitive to furosemide (10-1000 microM) in all NTS neurons tested. Collectively, the data suggest that in neonatal rats, the predominant alpha subunit isoform present in GABA(A) receptors of the NTS appears to be the alpha1 and/or alpha2 subunit. beta2 and/or beta3 subunits are the major beta isoform, while the predominant gamma subunit is likely gamma2. Our data suggest the contribution to NTS GABA currents by alpha3-alpha6, beta1, gamma1 and delta subunits, if present, is minor by comparison.
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Affiliation(s)
- R Q Huang
- Department of Pharmacology and Neuroscience, University of North Texas Health Science Center, 3500 Camp Bowie Blvd., Fort Worth, TX 76107, USA
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27
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Klausberger T, Sarto I, Ehya N, Fuchs K, Furtmuller R, Mayer B, Huck S, Sieghart W. Alternate use of distinct intersubunit contacts controls GABAA receptor assembly and stoichiometry. J Neurosci 2001; 21:9124-33. [PMID: 11717345 PMCID: PMC6763904] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2001] [Revised: 08/22/2001] [Accepted: 09/12/2001] [Indexed: 02/22/2023] Open
Abstract
GABA(A) receptors are the major inhibitory transmitter receptors in the CNS. Recombinant GABA(A) receptors composed of alpha(1)beta(3)gamma(2) subunits have been demonstrated to assemble as pentamers consisting of two alpha(1), two beta(3), and one gamma(2) subunit. Using truncated and chimeric alpha(1) subunits, we identified the alpha(1)(80-100) sequence as a major binding site for gamma(2) subunits. In addition, we demonstrated its direct interaction with gamma(2)(91-104), a sequence that previously has been identified to form the contact to alpha(1) subunits. The observation that the amino acid residues alpha(1)P96 and alpha(1)H101, which can be photolabeled by [(3)H]flunitrazepam, are located within or adjacent to the alpha(1)(80-100) sequence, indicates that the benzodiazepine binding site of GABA(A) receptors is located close to this intersubunit contact. The observation that alpha(1)(80-100) interacts with gamma(2) but not with beta(3) subunits indicates the existence of an additional beta(3) binding site on alpha(1) subunits. The preferred alternate use of the gamma(2) and beta(3) binding sites in two different alpha(1) subunits of the same receptor ensures the incorporation of only a single gamma(2) subunit and thus, determines subunit stoichiometry of alpha(1)beta(3)gamma(2) receptors. Distinct binding sites and their alternate use can therefore explain how subunits of hetero-oligomeric transmembrane proteins assemble into a defined protein complex.
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Affiliation(s)
- T Klausberger
- Section of Biochemical Psychiatry, University Clinic for Psychiatry, Institute for Theoretical Chemistry Divisions of Biochemistry, Brain Research Institute, University of Vienna, A-1090 Vienna, Austria
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28
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Abstract
Tetanic stimuli of peripheral C fibers produces long-term potentiation (LTP) in the spinal cord, which may contribute to sensitization of spinal pain-sensitive neurons. Zn2+ is widely distributed in the central nervous system and has blocked (LTP) in the hippocampus. The present study examined the effects of Zn2+ on the induction and maintenance of C fiber-evoked LTP in the deep dorsal horn of spinalized rats in vivo. The sciatic nerve was stimulated by tetanic stimuli for inducing LTP. (1) Topical administration of Zinc chloride (15 microM) to the spinal cord 15 min before tetanic stimulation completely blocked the induction of LTP, but not the baseline C responses. When Zn2+ was given 2 h after induction of LTP, no significant effect occurred. (2) Chelation of Zn2+ by disodium calcium ethylene diaminetelraacetate (CaEDTA) (500 microM) resulted in no effect on LTP. (3) Coadministration of Zn2+ (15 microM) and N-methyl-D-aspartic acid (NMDA) (5 microM) significantly attenuated C fiber-evoked potentials, which was prevented by the NMDA receptor antagonist AP-5 (100 microM). The present results showed that Zn2+ may contribute to the modulation of the formation, but not the maintenance, of spinal LTP. NMDA receptors may be involved in Zn2+-induced modulation.
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Affiliation(s)
- J Y Ma
- Institute of Shanghai Physiology, Chinese Academy of Sciences, Shanghai, China
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29
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Teissére JA, Czajkowski C. A (beta)-strand in the (gamma)2 subunit lines the benzodiazepine binding site of the GABA A receptor: structural rearrangements detected during channel gating. J Neurosci 2001; 21:4977-86. [PMID: 11438573 [PMID: 11438573 DOI: 10.1523/jneurosci.21-14-04977.2001] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Benzodiazepines (BZDs) exert their effects in the CNS by binding to a modulatory site on GABA(A) receptors. Individual amino acids have been implicated in BZD recognition and modulation of the GABA(A) receptor, but the secondary structure of the amino acids contributing to the BZD binding site has not been elucidated. In this report we used the substituted cysteine accessibility method to understand the structural dynamics of a region of the GABA(A) receptor implicated in BZD binding, gamma(2)Y72-gamma(2)Y83. Each residue within this region was mutated to cysteine and expressed with wild-type alpha(1) and beta(2) subunits in Xenopus oocytes. Methanethiosulfonate (MTS) reagents were used to modify covalently the engineered cysteines, and the subsequent effects on BZD modulation of the receptor were monitored functionally by two-electrode voltage clamp. We identified an alternating pattern of accessibility to sulfhydryl modification, indicating that the region gamma(2)T73-gamma(2)T81 adopts a beta-strand conformation. By monitoring the ability of BZD ligands to impede the covalent modification of accessible cysteines, we also identified two residues within this region, gamma(2)A79 and gamma(2)T81, that line the BZD binding site. Sulfhydryl modification of gamma(2)A79C or gamma(2)T81C allosterically shifts the GABA EC(50) of the receptor, suggesting that certain MTS compounds may act as tethered agonists at the BZD binding site. Last, we present structural evidence that a portion of the BZD binding site undergoes a conformational change in response to GABA binding and channel gating (opening and desensitization). These data represent an important step in understanding allosteric communication in ligand-gated ion channels.
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Teissére JA, Czajkowski C. A (beta)-strand in the (gamma)2 subunit lines the benzodiazepine binding site of the GABA A receptor: structural rearrangements detected during channel gating. J Neurosci 2001; 21:4977-86. [PMID: 11438573 PMCID: PMC6762856] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/20/2023] Open
Abstract
Benzodiazepines (BZDs) exert their effects in the CNS by binding to a modulatory site on GABA(A) receptors. Individual amino acids have been implicated in BZD recognition and modulation of the GABA(A) receptor, but the secondary structure of the amino acids contributing to the BZD binding site has not been elucidated. In this report we used the substituted cysteine accessibility method to understand the structural dynamics of a region of the GABA(A) receptor implicated in BZD binding, gamma(2)Y72-gamma(2)Y83. Each residue within this region was mutated to cysteine and expressed with wild-type alpha(1) and beta(2) subunits in Xenopus oocytes. Methanethiosulfonate (MTS) reagents were used to modify covalently the engineered cysteines, and the subsequent effects on BZD modulation of the receptor were monitored functionally by two-electrode voltage clamp. We identified an alternating pattern of accessibility to sulfhydryl modification, indicating that the region gamma(2)T73-gamma(2)T81 adopts a beta-strand conformation. By monitoring the ability of BZD ligands to impede the covalent modification of accessible cysteines, we also identified two residues within this region, gamma(2)A79 and gamma(2)T81, that line the BZD binding site. Sulfhydryl modification of gamma(2)A79C or gamma(2)T81C allosterically shifts the GABA EC(50) of the receptor, suggesting that certain MTS compounds may act as tethered agonists at the BZD binding site. Last, we present structural evidence that a portion of the BZD binding site undergoes a conformational change in response to GABA binding and channel gating (opening and desensitization). These data represent an important step in understanding allosteric communication in ligand-gated ion channels.
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Affiliation(s)
- J A Teissére
- Neuroscience Training Program, Department of Physiology, University of Wisconsin, Madison, Wisconsin 53706, USA
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Lim MS, Birnir B. Heterogeneity of functional GABA(A) receptors in rat dentate gyrus neurons revealed by a change in response to drugs during the whole-cell current time-course. Neuropharmacology 2001; 40:1034-43. [PMID: 11406195 DOI: 10.1016/s0028-3908(01)00032-6] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
We examined if the drug sensitivity of GABA(A) receptors in dentate gyrus granule neurons changed during the whole-cell current time-course. Effects of drugs on currents evoked immediately (the peak current) upon drug application and currents remaining about two seconds later (semi-plateau current) were compared. The apparent affinity for GABA (EC(50)) of the peak and the semi-plateau current were 14 and 4 microM, respectively. Bicuculline inhibited 50% of the peak and the semi-plateau current (IC(50)) at 7 and 36 microM, respectively, while 100 microM was required for full inhibition of the 100 microM GABA-evoked current. Zinc inhibited about 50% of the peak current with an IC(50) value of 94 microM whereas biphasic, but complete inhibition of the semi-plateau current was recorded with IC(50) values of 3 and 558 microM. The decay phase of the 100 microM GABA-evoked current was fitted by a fast (tau(1), 100-300 ms) and a slow (tau(2), 1-2 s) time-constants in all cells. The relative current amplitude associated with the fast (A1) and the slow (A2) component varied. The A1 current amplitude appeared more sensitive to bicuculline than the A2 current while the opposite was true for zinc. The results are consistent with heterogenous population of functional GABA(A) receptors in the dentate gyrus granule neurons.
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Affiliation(s)
- M S Lim
- Membrane Biology Program, John Curtin School of Medical Research, Australian National University, Canberra ACT, Australia
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Abstract
The sensitivity of GABAA receptors (GABARs) to Zn2+ inhibition depends on subunit composition. The predominant neuronal forms of mammalian GABARs, alpha(beta)gamma and, alpha(beta)delta are differentially sensitive to Zn2+ inhibition; alpha(beta)gamma receptors are substantially less sensitive than alpha(beta)delta receptors. Recently, functional domains involved in Zn2+ sensitivity have been identified in and subunits. Our aim in the present study was to localize functional domains of low Zn2+ sensitivity within gamma2L subunits. Chimeric subunits were constructed by progressively replacing the rat gamma2L subunit sequence with that of the rat delta subunit sequence. Whole-cell currents were recorded from mouse L929 fibroblasts coexpressing wild-type rat alpha1 and beta3 subunits with a chimeric delta-gamma2L subunit. Unlike alpha and beta subunits, the gamma2L subunit was found to contain a determinant of low Zn2+ sensitivity in the N-terminal extracellular region. In addition, we identified determinants in the M2 segment and the M2-M3 loop of the gamma2L subunit that are homologous to those found in beta and alpha subunits. We postulate that the interface between the latter two domains, which may form the outer vestibule of the channel, represents a single functional domain modulating Zn2+ sensitivity. Thus, the Zn2+ sensitivity of ternary GABARs appears to be determined by two functional domains, one in the N-terminal extracellular region and one near the outer mouth of the channel.
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Affiliation(s)
- N Nagaya
- Department of Neurology, University of Michigan Health System, University of Michigan, Ann Arbor, MI 48104-1687, USA
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Abstract
Zinc is abundantly present in the CNS, and after nerve stimulation is thought to be released in sufficient quantity to modulate the synaptic transmission. Although it is known that this divalent cation inhibits the GABAergic synaptic currents, the underlying mechanisms were not fully elucidated. Here we report that zinc reduced the amplitude, slowed the rise time, and accelerated the decay of mIPSCs in cultured hippocampal neurons. The analysis of current responses to rapid GABA applications and model simulations indicated that these effects on mIPSCs are caused by zinc modulation of GABA(A) receptor gating. In particular, zinc slowed the onset of GABA-evoked currents by decreasing both the binding (k(on)) and the transition rate from closed to open state (beta(2)). Moreover, slower onset and recovery from desensitization as well as an increased unbinding rate (k(off)) were shown to underlie the accelerated deactivation kinetics in the presence of zinc. The nonequilibrium conditions of GABA(A) receptor activation were found to strongly affect zinc modulation of this receptor. In particular, an extremely fast clearance of synaptic GABA is implicated to be responsible for a stronger zinc effect on mIPSCs than on current responses to exogenous GABA. Finally, the analysis of currents evoked by GABA coapplied with zinc indicated that the interaction between zinc and GABA(A) receptors was too slow to explain zinc effects in terms of competitive antagonism. In conclusion, our results provide evidence that inhibition of mIPSCs by zinc is attributable to the allosteric modulation of GABA(A) receptor gating.
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Barberis A, Cherubini E, Mozrzymas JW. Zinc inhibits miniature GABAergic currents by allosteric modulation of GABAA receptor gating. J Neurosci 2000; 20:8618-27. [PMID: 11102466 PMCID: PMC6773059] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/18/2023] Open
Abstract
Zinc is abundantly present in the CNS, and after nerve stimulation is thought to be released in sufficient quantity to modulate the synaptic transmission. Although it is known that this divalent cation inhibits the GABAergic synaptic currents, the underlying mechanisms were not fully elucidated. Here we report that zinc reduced the amplitude, slowed the rise time, and accelerated the decay of mIPSCs in cultured hippocampal neurons. The analysis of current responses to rapid GABA applications and model simulations indicated that these effects on mIPSCs are caused by zinc modulation of GABA(A) receptor gating. In particular, zinc slowed the onset of GABA-evoked currents by decreasing both the binding (k(on)) and the transition rate from closed to open state (beta(2)). Moreover, slower onset and recovery from desensitization as well as an increased unbinding rate (k(off)) were shown to underlie the accelerated deactivation kinetics in the presence of zinc. The nonequilibrium conditions of GABA(A) receptor activation were found to strongly affect zinc modulation of this receptor. In particular, an extremely fast clearance of synaptic GABA is implicated to be responsible for a stronger zinc effect on mIPSCs than on current responses to exogenous GABA. Finally, the analysis of currents evoked by GABA coapplied with zinc indicated that the interaction between zinc and GABA(A) receptors was too slow to explain zinc effects in terms of competitive antagonism. In conclusion, our results provide evidence that inhibition of mIPSCs by zinc is attributable to the allosteric modulation of GABA(A) receptor gating.
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Affiliation(s)
- A Barberis
- Neuroscience Program and Istituto Nazionale Fisica della Materia Unit, International School for Advanced Studies (SISSA), 34014 Trieste, Italy
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Mellor JR, Wisden W, Randall AD. Somato-synaptic variation of GABA(A) receptors in cultured murine cerebellar granule cells: investigation of the role of the alpha6 subunit. Neuropharmacology 2000; 39:1495-513. [PMID: 10854895 DOI: 10.1016/s0028-3908(00)00007-1] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
Electrophysiological investigation of cultured cerebellar murine granule cells revealed differences between the GABA(A) receptors at inhibitory synapses and those on the cell body. Specifically, mIPSCs decayed more rapidly than cell body receptors deactivated, the mean single channel conductance at the synapse (32 pS) was greater than that at cell body (21 pS) and only cell body receptors were sensitive to Zn(2+) (150 microM), which depressed response amplitude by 82+/-5% and almost doubled the rate of channel deactivation. The GABA(A) receptor alpha6 subunit is selectively expressed in cerebellar granule cells. Although concentrated at synapses, it is also found on extrasynaptic membranes. Using a mouse line (Deltaalpha6lacZ) lacking this subunit, we investigated its role in the somato-synaptic differences in GABA(A) receptor function. All differences between cell body and synaptic GABA(A) receptors observed in wild-type (WT) granule cells persisted in Deltaalpha6lacZ cells, thus demonstrating that they are not specifically due to the cellular distribution of the alpha6 subunit. However, mIPSCs from WT and Deltaalpha6lacZ cells differed in both their kinetics (faster decay in WT cells) and underlying single channel conductance (32 pS WT, 25 pS Deltaalpha6lacZ). This provides good evidence for a functional contribution of the alpha6 subunit to postsynaptic GABA(A) receptors in these cells. Despite this, deactivation kinetics of mIPSCs in WT and Deltaalpha6lacZ granule cells exhibited similar benzodiazepene (BDZ) sensitivity. This suggests that the enhanced BDZ-induced ataxia seen in Deltaalpha6lacZ mice may reflect physiological activity at extrasynaptic receptors which, unlike those at synapses, display differential BDZ-sensitivity in WT and Deltaalpha6lacZ granule cells (Jones, A.M., Korpi, E.R., McKernan, R.M., Nusser, Z., Pelz, R., Makela, R., Mellor, J.R., Pollard, S., Bahn, S., Stephenson, F.A., Randall, A.D., Sieghart, W., Somogyi, P., Smith, A.J.H., Wisden, W., 1997. Ligand-gated ion channel partnerships: GABA(A) receptor alpha(6) subunit inactivation inhibits delta subunit expression. Journal of Neuroscience 17, 1350-1362).
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Affiliation(s)
- J R Mellor
- Division of Neurobiology, MRC Laboratory of Molecular Biology, Hills Road, CB2 2QH, Cambridge, UK
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Sharonova IN, Vorobjev VS, Haas HL. Interaction between copper and zinc at GABA(A) receptors in acutely isolated cerebellar Purkinje cells of the rat. Br J Pharmacol 2000; 130:851-6. [PMID: 10864892 PMCID: PMC1572144 DOI: 10.1038/sj.bjp.0703392] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022] Open
Abstract
Nanomolar concentrations of Cu(2+) induce a slowly reversible block of GABA(A) receptor-mediated currents which can be removed by chelating substances. The possible interaction of Cu(2+) with the Zn(2+) binding site on the GABA(A) receptor complex was studied in acutely isolated Purkinje cells using whole-cell recording and a fast drug application system. When Zn(2+) was applied together with 2 microM GABA, the Zn(2+)-induced block of GABA-mediated currents was not additive to the Cu(2+)-induced block. In the presence of 0.1 microM Cu(2+) in the bath solution the degree of inhibition of GABA-mediated responses by Zn(2+) was strongly attenuated. Preapplication of 100 microM Zn(2+) during 10 s, terminated 1 s before exposure to 2 microM GABA did not affect the GABA current in Cu(2+)-free solution, but relieved its block by 0.1 microM Cu(2+). This effect of Zn(2+) was concentration-dependent with an EC(50) of 72 microM. When the Cu(2+)-induced block was removed by histidine, preapplication of Zn(2+) did not increase the GABA current, indicating that the relief of Cu(2+) block by Zn(2+) is the result of its ability to actively remove Cu(2+) from the GABA receptor complex. It is proposed that the inhibitory effects of Zn(2+) and Cu(2+) on GABA-induced currents result from an action of these metal ions at distinct, but conformationally linked sites on the GABA(A) receptor protein. Under physiological conditions Zn(2+) would liberate Cu(2+) from the GABA(A) receptor, thus facilitating Cu(2+) turnover and its binding by other endogenous chelating molecules.
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Affiliation(s)
- Irina N Sharonova
- Brain Research Institute, Russian Academy of Medical Sciences, Moscow, Russia
| | - Vladimir S Vorobjev
- Brain Research Institute, Russian Academy of Medical Sciences, Moscow, Russia
| | - Helmut L Haas
- Department of Neurophysiology, Heinrich-Heine-Universität, POB 101007, D-40001 Düsseldorf, Germany
- Author for correspondence:
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Abstract
The gamma-aminobutyric acid (GABA) response profiles of IMR-32 human neuroblastoma cells were examined using whole-cell patch clamp and RT-PCR techniques. GABA activated a concentration-dependent and bicuculline-sensitive current, and RT-PCR revealed the expression of multiple GABA(A) receptor subunit mRNAs (alpha(1), alpha(3), alpha(4), beta(1), beta(3), gamma(2), and delta). A pharmacological profile of the GABA-induced current was derived using several subunit-selective agents. Diazepam, which requires the presence of a gamma subunit in order to modulate GABA(A) receptor-mediated responses, potentiated GABA-induced currents in a subset of IMR-32 cells. Two populations of GABA-activated currents were also evident based on sensitivity to modulation by zinc. Comparison of zinc- and diazepam-induced modulation of GABA-induced current responses in the same cells revealed an inverse correlation between these two modulators. No differences, however, were observed with the GABA(A) receptor modulators loreclezole, allopregnanolone, and pentobarbital. Thus, IMR-32 cells maintained in culture are heterogeneous in terms of expression of GABA(A) receptor isoforms.
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Affiliation(s)
- D W Sapp
- Department of Pharmacology, University of Connecticut Health Center, Farmington 06030, USA
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Gibbs JW, Zhang YF, Shumate MD, Coulter DA. Regionally selective blockade of GABAergic inhibition by zinc in the thalamocortical system: functional significance. J Neurophysiol 2000; 83:1510-21. [PMID: 10712476 DOI: 10.1152/jn.2000.83.3.1510] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The thalamocortical (TC) system is a tightly coupled synaptic circuit in which GABAergic inhibition originating from the nucleus reticularis thalami (NRT) serves to synchronize oscillatory TC rhythmic behavior. Zinc is colocalized within nerve terminals throughout the TC system with dense staining for zinc observed in NRT, neocortex, and thalamus. Whole cell voltage-clamp recordings of GABA-evoked responses were conducted in neurons isolated from ventrobasal thalamus, NRT, and somatosensory cortex to investigate modulation of the GABA-mediated chloride conductance by zinc. Zinc blocked GABA responses in a regionally specific, noncompetitive manner within the TC system. The regional levels of GABA blockade efficacy by zinc were: thalamus > NRT > cortex. The relationship between clonazepam and zinc sensitivity of GABA(A)-mediated responses was examined to investigate possible presence or absence of specific GABA(A) receptor (GABAR) subunits. These properties of GABARs have been hypothesized previously to be dependent on presence or absence of the gamma2 subunit and seem to display an inverse relationship. In cross-correlation plots, thalamic and NRT neurons did not show a statistically significant relationship between clonazepam and zinc sensitivity; however, a statistically significant correlation was observed in cortical neurons. Spontaneous epileptic TC oscillations can be induced in vitro by perfusion of TC slices with an extracellular medium containing no added Mg(2+). Multiple varieties of oscillations are generated, including simple TC burst complexes (sTBCs), which resemble spike-wave discharge activity. A second variant was termed a complex TC burst complex (cTBC), which resembled generalized tonic clonic seizure activity. sTBCs were exacerbated by zinc, whereas cTBCs were blocked completely by zinc. This supported the concept that zinc release may modulate TC rhythms in vivo. Zinc interacts with a variety of ionic conductances, including GABAR currents, N-methyl-D-aspartate (NMDA) receptor currents, and transient potassium (A) currents. D-2-amino-5-phosphonovaleric acid and 4-aminopyridine blocked both s- and cTBCs in TC slices. Therefore NMDA and A current-blocking effects of zinc are insufficient to explain differential zinc sensitivity of these rhythms. This supports a significant role of zinc-induced GABAR modulation in differential TC rhythm effects. Zinc is localized in high levels within the TC system and appears to be released during TC activity. Furthermore application of exogenous zinc modulates TC rhythms and differentially blocks GABARs within the TC system. These data are consistent with the hypothesis that endogenously released zinc may have important neuromodulatory actions impacting generation of TC rhythms, mediated at least in part by effects on GABARs.
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Affiliation(s)
- J W Gibbs
- Department of Anatomy, Medical College of Virginia of Virginia Commonwealth University, Richmond, Virginia 23298-0599, USA
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Louiset E, McKernan R, Sieghart W, Vaudry H. Subunit composition and pharmacological characterization of gamma-aminobutyric acid type A receptors in frog pituitary melanotrophs. Endocrinology 2000; 141:1083-92. [PMID: 10698184 DOI: 10.1210/endo.141.3.7397] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
The frog pars intermedia is composed of a single population of endocrine cells directly innervated by gamma-aminobutyric acid (GABA)ergic nerve terminals. We have previously shown that GABA, acting through GABA(A) receptors, modulates both the electrical and secretory activities of frog pituitary melanotrophs. The aim of the present study was to take advantage of the frog melanotroph model to determine the relationship between the subunit composition and the pharmacological properties of native GABA(A) receptors. Immunohistochemical labeling revealed that in situ and in cell culture, frog melanotrophs were intensely stained with alpha2-, alpha3-, gamma2-, and gamma3-subunit antisera and weakly stained with a gamma1-subunit antiserum. Melanotrophs were also immunolabeled with a monoclonal antibody to the beta2/beta3-subunit. In contrast, frog melanotrophs were not immunoreactive for the alpha1-, alpha5-, and alpha6-isoforms. The effects of allosteric modulators of the GABA(A) receptor on GABA-activated chloride current were tested using the patch-clamp technique. Among the ligands acting at the benzodiazepine-binding site, clonazepam (EC50, 5 x 10(-9) M), diazepam (EC50, 10(-8) M), zolpidem (EC50, 3 x 10(-8) M), and beta-carboline-3-carboxylic acid methyl ester (EC50, 10(-6) M) were found to potentiate the whole cell GABA-evoked current in a dose-dependent manner. Methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (IC50, 3 x 10(-5) M) inhibited the current, whereas Ro15-4513 had no effect. Among the ligands acting at other modulatory sites, etomidate (EC50, 2 x 10(-6) M) enhanced the GABA-evoked current, whereas 4'-chlorodiazepam (IC50, 4 x 10(-7) M), ZnCl2 (IC50, >5 x 10(-5) M), and furosemide (IC50, >3 x 10(-4) M) depressed the response to GABA. PK 11195 did not affect the GABA-evoked current or its inhibition by 4'-chlorodiazepam. The results indicate that the native GABA(A) receptors in frog melanotrophs are formed by combinations of alpha2-, alpha3-, beta2/3-, gamma1-, gamma2-, and gamma3-subunits. The data also demonstrate that clonazepam is the most potent, and zolpidem is the most efficient positive modulator of the native receptors. Among the inhibitors, 4'-chlorodiazepam is the most potent, whereas ZnCl2 is the most efficient negative modulator of the GABA(A) receptors. The present study provides the first correlation between subunit composition and the functional properties of native GABA(A) receptors in nontumoral endocrine cells.
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Affiliation(s)
- E Louiset
- European Institute for Peptide Research (IFRMP 23), Laboratory of Cellular and Molecular Neuroendocrinology, INSERM U-413, Centre National de la Recherche Scientifique, University of Rouen, Mont-Saint-Aignan, France
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Abstract
GABA(A)-mediated IPSCs typically decay more rapidly than receptors in excised patches in response to brief pulses of applied GABA. We have investigated the source of this discrepancy in CA1 pyramidal neurons. IPSCs in these cells decayed rapidly, with a weighted time constant tau(Decay) of approximately 18 msec (24 degrees C), whereas excised and nucleated patch responses to brief pulses of GABA (2 msec, 1 mM) decayed more than three times as slowly (tau(Decay), approximately 63 msec). This discrepancy was not caused by differences between synaptic and exogenous transmitter transients because (1) there was no dependence of tau(Decay) on pulse duration for pulses of 0.6-4 msec, (2) responses to GABA at concentrations as low as 10 microM were still slower to decay (tau(Decay), approximately 41 msec) than IPSCs, and (3) responses of excised patches to synaptically released GABA had decay times similar to brief pulse responses. These data indicate that the receptors mediating synaptic versus brief pulse responses have different intrinsic properties. However, synaptic receptors were not altered by the patch excision process, because fast, spontaneous IPSCs could still be recorded in nucleated patches. Elevated calcium selectively modulated patch responses to GABA pulses, with no effect on IPSCs recorded in nucleated patches, demonstrating the presence of two receptor populations that are differentially regulated by intracellular second messengers. We conclude that two receptor populations with distinct kinetics coexist in CA1 pyramidal cells: slow extrasynaptic receptors that dominate the responses of excised patches to exogenous GABA applications and fast synaptic receptors that generate rapid IPSCs.
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Banks MI, Pearce RA. Kinetic differences between synaptic and extrasynaptic GABA(A) receptors in CA1 pyramidal cells. J Neurosci 2000; 20:937-48. [PMID: 10648698 PMCID: PMC6774173] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/15/2023] Open
Abstract
GABA(A)-mediated IPSCs typically decay more rapidly than receptors in excised patches in response to brief pulses of applied GABA. We have investigated the source of this discrepancy in CA1 pyramidal neurons. IPSCs in these cells decayed rapidly, with a weighted time constant tau(Decay) of approximately 18 msec (24 degrees C), whereas excised and nucleated patch responses to brief pulses of GABA (2 msec, 1 mM) decayed more than three times as slowly (tau(Decay), approximately 63 msec). This discrepancy was not caused by differences between synaptic and exogenous transmitter transients because (1) there was no dependence of tau(Decay) on pulse duration for pulses of 0.6-4 msec, (2) responses to GABA at concentrations as low as 10 microM were still slower to decay (tau(Decay), approximately 41 msec) than IPSCs, and (3) responses of excised patches to synaptically released GABA had decay times similar to brief pulse responses. These data indicate that the receptors mediating synaptic versus brief pulse responses have different intrinsic properties. However, synaptic receptors were not altered by the patch excision process, because fast, spontaneous IPSCs could still be recorded in nucleated patches. Elevated calcium selectively modulated patch responses to GABA pulses, with no effect on IPSCs recorded in nucleated patches, demonstrating the presence of two receptor populations that are differentially regulated by intracellular second messengers. We conclude that two receptor populations with distinct kinetics coexist in CA1 pyramidal cells: slow extrasynaptic receptors that dominate the responses of excised patches to exogenous GABA applications and fast synaptic receptors that generate rapid IPSCs.
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Affiliation(s)
- M I Banks
- Department of Anesthesiology, University of Wisconsin, Madison, Wisconsin 53706, USA.
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Pawelzik H, Bannister AP, Deuchars J, Ilia M, Thomson AM. Modulation of bistratified cell IPSPs and basket cell IPSPs by pentobarbitone sodium, diazepam and Zn2+: dual recordings in slices of adult rat hippocampus. Eur J Neurosci 1999; 11:3552-64. [PMID: 10564363 DOI: 10.1046/j.1460-9568.1999.00772.x] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Simultaneous intracellular recordings from presynaptic Stratum pyramidale interneurons and postsynaptic pyramidal cells in adult rat hippocampal slices were performed to investigate the strength of the modulation of single-axon inhibitory postsynaptic potentials (IPSPs) by the GABAA receptor modulators pentobarbitone, diazepam and zinc. The processing of biocytin-filled interneurons for light microscopy revealed that these single-axon IPSPs were generated by basket cells (n = 33), bistratified cells (n = 18) and axo-axonic cells (n = 2). The IPSPs generated by these three groups of interneurons had amplitudes and widths at half amplitude with similar ranges, but when bistratified cell IPSPs were compared with basket cell IPSPs with similar half widths their rise times were slower. Pentobarbitone sodium (250 microM) powerfully enhanced 13 tested IPSPs generated by all three cell types. Amplitudes were enhanced by 82 +/- 56%, 10-90% rise times by 150 +/- 101% and the widths at half amplitude by 71 +/- 29%. Diazepam (1-2 microM) also increased all IPSPs tested, although the changes were more moderate in basket cell IPSPs (amplitudes increased by 19 +/- 11%, n = 8) than in bistratified cell IPSPs (amplitudes increased by 66 +/- 48%, n = 5). Basket cell IPSP 10-90% rise times and widths at half amplitude were not significantly increased. Bistratified cell IPSP 10-90% rise times were increased by 44 +/- 24% and the widths at half amplitude by 32 +/- 35%. The one tested IPSP generated by an axo-axonic cell was also diazepam-sensitive. Zinc, 250 microM, decreased four out of 10 IPSPs generated by basket cells and four out of five IPSPs generated by bistratified cells. The one tested axo-axonic cell IPSP was zinc-insensitive. These data suggest that IPSPs generated in CA1 pyramidal cells by basket and bistratified cells display different pharmacologies and may be mediated by different receptors or receptor combinations.
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Affiliation(s)
- H Pawelzik
- Department of Physiology, Royal Free and University College Medical School London, UK.
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43
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Abstract
The effect of Waglerin-1, a 22-amino acid peptide purified from the venom of Wagler's pit viper on the whole cell current response (I(GABA)) to gamma-aminobutyric acid (GABA) was examined for neurons freshly isolated from the nucleus accumbens of 3- to 7-day-old rats. Waglerin-1 depressed I(GABA) induced by subsaturating concentrations of GABA; the IC(50) for I(GABA) induced by 10 microM GABA was 2.5 microM Waglerin-1. This concentration of Waglerin-1 shifted the GABA concentration-response curve to the right in a parallel manner, increasing the GABA EC(50) from 12+/-3 to 27+/-5 microM. The depressant effect of Waglerin-1 was greater at negative holding potentials. Zn(2+) also inhibited I(GABA) with an IC(50) of 0.3 microM. Phosphorylation state appeared to modulate GABA(A) receptor sensitivity to the inhibitory effect of Waglerin-1 since dialysis of neurons with N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide HCl (H-89), an inhibitor of protein kinase A, prevented inhibition. The data are discussed in terms of developmental influences on the subunit composition of GABA(A) receptors in neurons of the nucleus accumbens.
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Affiliation(s)
- J H Ye
- Departments of Anesthesiology, and Pharmacology and Physiology, New Jersey Medical School (UMDNJ), 185 South Orange Avenue, Newark, NJ 07103-2714, USA
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Guyon A, Laurent S, Paupardin-Tritsch D, Rossier J, Eugène D. Incremental conductance levels of GABAA receptors in dopaminergic neurones of the rat substantia nigra pars compacta. J Physiol 1999; 516 ( Pt 3):719-37. [PMID: 10200421 PMCID: PMC2269308 DOI: 10.1111/j.1469-7793.1999.0719u.x] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022] Open
Abstract
1. Molecular and biophysical properties of GABAA receptors of dopaminergic (DA) neurones of the pars compacta of the rat substantia nigra were studied in slices and after acute dissociation. 2. Single-cell reverse transcriptase-multiplex polymerase chain reaction confirmed that DA neurones contained mRNAs encoding for the alpha3 subunit of the GABAA receptor, but further showed the presence of alpha4 subunit mRNAs. alpha2, beta1 and gamma1 subunit mRNAs were never detected. Overall, DA neurones present a pattern of expression of GABAA receptor subunit mRNAs containing mainly alpha3/4beta2/3gamma3. 3. Outside-out patches were excised from DA neurones and GABAA single-channel patch-clamp currents were recorded under low doses (1-5 microM) of GABA or isoguvacine, a selective GABAA agonist. Recordings presented several conductance levels which appeared to be integer multiples of an elementary conductance of 4-5 pS. This property was shared by GABAA receptors of cerebellar Purkinje neurones recorded in slices (however, with an elementary conductance of 3 pS). Only the 5-6 lowest levels were analysed. 4. A progressive change in the distribution of occupancy of these levels was observed when increasing the isoguvacine concentration (up to 10 microM) as well as when adding zolpidem (20-200 nM), a drug acting at the benzodiazepine binding site: both treatments enlarged the occupancy of the highest conductance levels, while decreasing that of the smallest ones. Conversely, Zn2+ (10 microM), a negative allosteric modulator of GABAA receptor channels, decreased the occupancy of the highest levels in favour of the lowest ones. 5. These properties of alpha3/4beta2/3gamma3-containing GABAA receptors would support the hypothesis of either single GABAA receptor channels with multiple open states or that of a synchronous recruitment of GABAA receptor channels that could involve their clustering in the membranes of DA neurones.
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Affiliation(s)
- A Guyon
- Neurobiologie Cellulaire, Institut des Neurosciences, CNRS-Universite Pierre et Marie Curie, 9 quai Saint-Bernard, F-75005 Paris, France.
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45
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Abstract
1. Regional distinctions in GABA type A (GABAA) miniature IPSC responses are thought to be determined by postsynaptic receptor composition. The kinetics of receptor activation and deactivation were studied using rapid exchange (100 micros) of GABA at excised patches containing recombinant (alpha1beta1gamma2 or alpha2beta1gamma2) and native (cortical) GABAA receptors. 2. Receptors activated by brief (< 1 ms) pulses of GABA demonstrated a characteristic current response, hereby referred to as the 'receptor system response'. System response properties included agonist concentration-dependent peak amplitudes and concentration-independent maximal rates of activation and deactivation. Receptor subtypes were characterized functionally and phenotyped using the system response characteristics. 3. System responses obtained for alpha1beta1gamma2 receptors exhibited a single phenotype while alpha2beta1gamma2 receptors exhibited either a predominant slow deactivation (type I) or a relatively infrequent faster (type II) phenotype. Receptor system responses of alpha2beta1gamma2 receptors reached peak currents twice as fast as those of alpha1beta1gamma2 receptors (0.5 versus 1.0 ms) but decayed 2 or 6 times more slowly (taulong of approximately 190 and 62 ms for type I and II alpha2beta1gamma2, and approximately 34 ms for alpha1beta1gamma2 receptors). 4. Receptor system responses from cultured fetal mouse cortical neurons could be statistically separated and classified into five major types with little intragroup variability, primarily based on variations in the current deactivation phases. 5. Receptors subjected to pharmacological modulation exhibited alterations in system response properties consistent with known mechanisms of action, such that distinctions between binding and gating modulations were possible. 6. Brief agonist exposure places limits on receptor activation and deactivation response kinetics. Consequently, receptor system responses may be used to characterize and functionally phenotype an excised patch receptor population. Furthermore, since synaptic exposure to transmitter is postulated to be similarly brief, IPSC kinetics may reflect a functional fingerprint of synaptic receptors.
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Affiliation(s)
- A M McClellan
- Programs in Neuroscience and in Human Molecular Biology and Genetics, Departments of Neurology and Pharmacology, University of Utah, Salt Lake City, UT 84112, USA.
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Doi A, Kishimoto K, Ishibashi H. Modulation of glycine-induced currents by zinc and other metal cations in neurons acutely dissociated from the dorsal motor nucleus of the vagus of the rat. Brain Res 1999; 816:424-30. [PMID: 9878863 DOI: 10.1016/s0006-8993(98)01172-x] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Effects of Zn2+ and other polyvalent cations on glycine-induced currents in the freshly dissociated rat dorsal motor nucleus of the vagus neurons were investigated under voltage-clamp conditions by the use of the nystatin-perforated patch recording configuration. Glycine evoked a Cl- current in a concentration-dependent manner with a half-maximum effective concentration (EC50) of 3.3x10-5 M. Strychnine inhibited the 3x10-5 M glycine-induced current in a concentration-dependent manner with a half-maximal inhibitory concentration (IC50) of 6.8x10-7 M. At low concentrations (3x10-8 M-3x10-6 M), Zn2+ potentiated the current elicited by 3x10-5 M glycine. On the other hand, at concentrations higher than 10-5 M, Zn2+ inhibited the glycine response. The biphasic action of Zn2+ was mimicked by Ni2+, but La3+ and Co2+ had only potentiating effect. Zn2+ shifted the concentration-response curve for the glycine-induced current without changing the maximum response, and the EC50 values for the glycine response in the absence and presence of 10-6 M and 10-4 M Zn2+ were 3.3x10-5 M, 1.3x10-5 M and 1.3x10-4 M, respectively. These results suggest that the biphasic modulation of glycine response by Zn2+ results from changes in apparent glycine affinity.
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Affiliation(s)
- A Doi
- Department of Physiology, Faculty of Medicine, Kyushu University, Fukuoka 812-8582, Japan
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47
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Abstract
The suprachiasmatic nucleus (SCN) of the hypothalamus is responsible for generating circadian rhythms in mammals, and GABA is the predominant neurotransmitter in the SCN. Properties of gamma-aminobutyric acid-A (GABAA) responses in SCN neurons were examined in acutely prepared hypothalamic slices from 3- to 8-wk-old rats with the use of whole cell voltage-clamp techniques. Zn2+ reduced the amplitude of GABAA-mediated spontaneous inhibitory postsynaptic currents (sIPSCs) in a concentration-dependent manner ranging from a reduction of control amplitude to 88% at 10 microM to 27% at 1,000 microM. Zn2+ reduced IPSC amplitude to a similar degree in the presence of tetrodotoxin and also significantly reduced the amplitude of currents evoked by application of exogenous GABA (100 microM, pressure applied). Zn2+ increased the frequency of IPSCs at lower concentrations and decreased it at higher ones. Flunitrazepam (100 nM) usually failed to potentiate the amplitude of sIPSCs, but prolonged sIPSC kinetics. Two exponential components were normally resolved in the sIPSC decay constants, and flunitrazepam significantly increased those two components. Thus flunitrazepam increased the duration of sIPSCs and potentiated the amplitude of currents evoked by pressure application of GABA. Zn2+ and benzodiazepine each modulated the effect of GABA in nearly all cells, suggesting that most SCN neurons have a similar GABAA receptor subunit composition in this respect. Zn2+ also affected sIPSC frequency, which suggests that Zn2+ increased neuronal firing rate at lower concentrations. These results begin to define the cellular roles that these GABAA receptor modulators might play in circadian regulation.
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Affiliation(s)
- G J Strecker
- Department of Anatomy and Neurobiology, Colorado State University, Fort Collins, Colorado 80523-1670, USA
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48
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Abstract
Pharmacological modulation of gamma-aminobutyric acid-A (GABAA) receptors can provide important information on the types of subunits composing these receptors. In recombinant studies, zinc more potently inhibits alphabeta subunits compared with the alphabetagamma combination, whereas modulation by nanomolar concentrations of the benzodiazepine type 1-selective agonist zolpidem is conferred by the alpha1betagamma2 subunit combination. We examined four properties of miniature inhibitory postsynaptic currents (mIPSCs) from identified necortical pyramidal cells in rat brain slices: decay time constant, peak amplitude, rate of rise, and interevent interval. Exposure to 50 microM zinc reduced the decay time constant, peak amplitude, and rate of rise with no effect on interevent interval. Zolpidem enhanced mIPSCs in a concentration-dependent manner. Both 20 and 100 nM zolpidem increased the decay time constants of mIPSCs. In some cells, both peak amplitude and rate of rise were also enhanced. All cells treated with zinc were also responsive to zolpidem. These results show that neocortical pyramidal cells have a population of GABAA receptors sensitive to both zinc and zolpidem.
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Affiliation(s)
- T Defazio
- Department of Neurobiology, University of Alabama at Birmingham, Birmingham, Alabama 35294, USA
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