In situ hybridization and reverse transcription--polymerase chain reaction studies on the expression of the GABA(C) receptor rho1- and rho2-subunit genes in avian and rat brain

Synaptic Secretion and Beyond: Targeting Synapse and Neurotransmitters to Treat Neurodegenerative Diseases

The nervous system is important, because it regulates the physiological function of the body. Neurons are the most basic structural and functional unit of the nervous system. The synapse is an asymmetric structure that is important for neuronal function. The chemical transmission mode of the synapse is realized through neurotransmitters and electrical processes. Based on vesicle transport, the abnormal information transmission process in the synapse can lead to a series of neurorelated diseases. Numerous proteins and complexes that regulate the process of vesicle transport, such as SNARE proteins, Munc18-1, and Synaptotagmin-1, have been identified. Their regulation of synaptic vesicle secretion is complicated and delicate, and their defects can lead to a series of neurodegenerative diseases. This review will discuss the structure and functions of vesicle-based synapses and their roles in neurons. Furthermore, we will analyze neurotransmitter and synaptic functions in neurodegenerative diseases and discuss the potential of using related drugs in their treatment.

The adjustment of γ-aminobutyric acidA tonic subunits in Huntington's disease: from transcription to translation to synaptic levels into the neostriatum

γ-Aminobutyric acid (GABA), plays a key role in all stages of life, also is considered the main inhibitory neurotransmitter. GABA activates two kind of membrane receptors known as GABA_A and GABA_B, the first one is responsible to render tonic inhibition by pentameric receptors containing α4−6, β3, δ, or ρ1−3 subunits, they are located at perisynaptic and/or in extrasynaptic regions. The biophysical properties of GABA_A tonic inhibition have been related with cellular protection against excitotoxic injury and cell death in presence of excessive excitation. On this basis, GABA_A tonic inhibition has been proposed as a potential target for therapeutic intervention of Huntington's disease. Huntington's disease is a neurodegenerative disorder caused by a genetic mutation of the huntingtin protein. For experimental studies of Huntington's disease mouse models have been developed, such as R6/1, R6/2, HdhQ92, HdhQ150, as well as YAC128. In all of them, some key experimental reports are focused on neostriatum. The neostriatum is considered as the most important connection between cerebral cortex and basal ganglia structures, its cytology display two pathways called direct and indirect constituted by medium sized spiny neurons expressing dopamine D1 and D2 receptors respectively, they display strong expression of many types of GABA_A receptors, including tonic subunits. The studies about of GABA_A tonic subunits and Huntington's disease into the neostriatum are rising in recent years, suggesting interesting changes in their expression and localization which can be used as a strategy to delay the cellular damage caused by the imbalance between excitation and inhibition, a hallmark of Huntington's disease.

GABAρ selective antagonist TPMPA partially inhibits GABA-mediated currents recorded from neurones and astrocytes in mouse striatum

The neostriatum plays a central role in motor coordination where nerve cells operate neuronal inhibition through GABAergic transmission. The neostriatum expresses a wide range of GABA-A subunits, including GABAρ1 and ρ2 which are restricted to a fraction of GABAergic interneurons and astrocytes. Spontaneous postsynaptic currents (sPSCs) evoked by 4-aminopyridine (4-AP) were recorded from neurones of the dorsal neostriatum, and their frequency was reduced > 50% by the selective GABAρ antagonist (1,2,5,6-Tetrahydropyridine-4-yl) methylphosphinic acid (TPMPA). Additionally, we recorded GABA evoked currents from astrocytes in vitro and in situ. Astrocytes in vitro showed modulation by pentobarbital and desensitization upon consecutive applications of GABA. However, modulation by pentobarbital was absent and no significant desensitization was detected from astrocytes in situ. Moreover, TPMPA-sensitive GABA-currents that were insensitive to bicuculline were also recorded from astrocytes in situ, consistent with our previous study where GABAρ expression was demonstrated. Finally, we assessed the mRNA expression of GABAρ3, through different stages of postnatal development; double immunofluorescence disclosed GABAρ3 expression in calretinin-positive interneurons as well as in astrocytes (>70%). These results add new information about the participation of GABAρ subunits in neostriatal interneurons and astrocytes.

Leucine rich amelogenin peptide alters ameloblast differentiation in vivo

Highly mineralized tooth enamel develops from an extracellular matrix chiefly comprised of amelogenins formed by splicing of 7 (human) or 9 (rodent) exons secreted from specialized epithelial cells known as ameloblasts. Here we examined the role of the 59 amino acid alternatively spliced amelogenin known as leucine rich amelogenin peptide (LRAP) on enamel formation, using transgenic murine models in which LRAP overexpression is driven by an amelogenin promoter (TgLRAP). Beginning in the secretory stage of mouse amelogenesis, we found a reduced thickness of enamel matrix and a loss of Tomes' processes, followed by upregulated amelogenin mRNA expression, inhibited amelogenin secretion and loss of cell polarity. In the presecretory stage (P0) amelogenin m180 mRNA expression was increased 58 fold along with a 203 fold increase in MMP-20 expression and 3.5 and 3.2 fold increased in respectively enamelin and ameloblastin. When LRAP was overexpressed on an amelogenin knockout mouse model, the ameloblasts were not affected. Further, expression of the global chromatin organizer and transcription factor SATB1 was reduced in secretory stage TgLRAP ameloblasts. These findings identify a cellular role for LRAP in enamel formation that is not directly related to directing enamel crystal formation as is reported to be the primary function of full length amelogenins. The effect of LRAP overexpression in upregulating amelogenins, MMP-20 and SATB1, suggests a role in protein regulation critical to ameloblast secretion and matrix processing, to form a mineralized enamel matrix.

Expression of GABAρ receptors in the neostriatum: localization in aspiny, medium spiny neurons and GFAP-positive cells

GABAergic transmission in the neostriatum plays a central role in motor coordination, in which a plethora of GABA-A receptor subunits combine to modulate neural inhibition. GABAρ receptors were originally described in the mammalian retina. These receptors possess special electrophysiological and pharmacological properties, forming a characteristic class of ionotropic receptors. In previous studies, we suggested that GABAρ receptors are expressed in the neostriatum, and in this report we show that they are indeed present in all the calretinin-positive interneurons of the neostriatum. In addition, they are located in calbindin-positive interneurons and projection neurons that express the dopamine D(2) receptor. GABAρ receptors were also located in 30% of the glial fibrillary acidic protein-positive cells, and may therefore also contribute to gliotransmission. Quantitative reverse transcription-PCR suggested that the mRNAs of this receptor do not express as much as in the retina, and that GABAρ2 is more abundant than GABAρ1. Electrophysiological recordings in brain slices provided evidence of neurons expressing a cis-4-aminocrotonic acid-activated, 1,2,5,6-tetrahydropyridine-4-yl methylphosphinic acid-sensitive ionotropic GABA receptor, indicating the presence of functional GABAρ receptors in the neostriatum. Finally, electron-microscopy and immunogold located the receptors mainly in perisynaptic as well as in extrasynaptic sites. All these observations reinforce the importance of GABAρ receptors in the neostriatum and contribute to the diversity of inhibitory regulation in this area.

An Update on GABAρ Receptors

The present review discusses the functional and molecular diversity of GABAρ receptors. These receptors were originally described in the mammalian retina, and their functional role in the visual pathway has been recently elucidated; however new studies on their distribution in the brain and spinal cord have revealed that they are more spread than originally thought, and thus it will be important to determine their physiological contribution to the GABAergic transmission in other areas of the central nervous system. In addition, molecular modeling has revealed peculiar traits of these receptors that have impacted on the interpretations of the latest pharmacolgical and biophysical findings. Finally, sequencing of several vertebrate genomes has permitted a comparative analysis of the organization of the GABAρ genes.

GABAA receptor subunit profiles of tangentially migrating neurons derived from the medial ganglionic eminence

During rodent corticogenesis, a sizeable subpopulation of γ-aminobutyric acid (GABA)ergic interneurons arises extracortically from the medial ganglionic eminence (MGE). These neurons progressively acquire responsiveness to GABA in the course of corticopetal tangential migration, a process regulated by ambient GABA and mediated by GABA(A) receptors. Here, we combined patch clamp electrophysiology and single-cell reverse transcription-polymerase chain reaction to examine GABA(A) receptor expression in green fluorescent MGE-derived (eGFP+) cells in telencephalic slices from gestational day 14.5 BAC-Lhx6 embryos. GABA concentration-response curves revealed lower apparent affinity and efficacy in eGFP+ cells in and around the MGE than their counterparts in the cortex. Pharmacological tests revealed subunit-selective response profiles in the MGE and cortex consistent with differential expression of GABA(A) receptor isoforms. Profiling of GABA(A) receptor subunit transcripts (α1-5, β1-3, and γ1-3, δ) uncovered increased expression of the α1-, α2-, α5-, γ2-, and γ3-subunit messenger RNAs in the cortex. We propose that the dynamic expression of certain GABA(A) receptor subunits contributes to assembling receptor isoforms that confer functional attributes important in regulating the migration and maturation of primordial GABAergic cortical interneurons.

Redox modulation of homomeric rho1 GABA receptors

The activity of many receptors and ion channels in the nervous system can be regulated by redox-dependent mechanisms. Native and recombinant GABA(A) receptors are modulated by endogenous and pharmacological redox agents. However, the sensitivity of GABA(C) receptors to redox modulation has not been demonstrated. We studied the actions of different reducing and oxidizing agents on human homomeric GABArho(1) receptors expressed in Xenopus laevis oocytes. The reducing agents dithiothreitol (2 mM) and N-acetyl-L-cysteine (1 mM) potentiated GABA-evoked Cl(-) currents recorded by two-electrode voltage-clamp, while the oxidants 5-5'-dithiobis-2-nitrobenzoic acid (500 microM) and oxidized dithiothreitol (2 mM) caused inhibition. The endogenous antioxidant glutathione (5 mM) also enhanced GABArho(1) receptor-mediated currents while its oxidized form GSSG (3 mM) had inhibitory effects. All the effects were rapid and easily reversible. Redox modulation of GABArho(1) receptors was strongly dependent on the GABA concentration; dose-response curves for GABA were shifted to the left in the presence of reducing agents, whereas oxidizing agents produced the opposite effect, without changes in the maximal response to GABA and in the Hill coefficient. Our results demonstrate that, similarly to GABA(A) receptors and other members of the cys-loop receptor superfamily, GABA(C) receptors are subjected to redox modulation.

Ameloblast differentiation in the human developing tooth: effects of extracellular matrices

Tooth enamel is formed by epithelially-derived cells called ameloblasts, while the pulp dentin complex is formed by the dental mesenchyme. These tissues differentiate with reciprocal signaling interactions to form a mature tooth. In this study we have characterized ameloblast differentiation in human developing incisors, and have further investigated the role of extracellular matrix proteins on ameloblast differentiation. Histological and immunohistochemical analyses showed that in the human tooth, the basement membrane separating the early developing dental epithelium and mesenchyme was lost shortly before dentin deposition was initiated, prior to enamel matrix secretion. Presecretary ameloblasts elongated as they came into contact with the dentin matrix, and then shortened to become secretory ameloblasts. In situ hybridization showed that the presecretory stage of odontoblasts started to express type I collagen mRNA, and also briefly expressed amelogenin mRNA. This was followed by upregulation of amelogenin mRNA expression in secretory ameloblasts. In vitro, amelogenin expression was upregulated in ameloblast lineage cells cultured in Matrigel, and was further up-regulated when these cells/Matrigel were co-cultured with dental pulp cells. Co-culture also up-regulated type I collagen expression by the dental pulp cells. Type I collagen coated culture dishes promoted a more elongated ameloblast lineage cell morphology and enhanced cell adhesion via integrin alpha2beta1. Taken together, these results suggest that the basement membrane proteins and signals from underlying mesenchymal cells coordinate to initiate differentiation of preameloblasts and regulate type I collagen expression by odontoblasts. Type I collagen in the dentin matrix then anchors the presecretary ameloblasts as they further differentiate to secretory cells. These studies show the critical roles of the extracellular matrix proteins in ameloblast differentiation.

Catweasel mice: a novel role for Six1 in sensory patch development and a model for branchio-oto-renal syndrome

Large-scale mouse mutagenesis initiatives have provided new mouse mutants that are useful models of human deafness and vestibular dysfunction. Catweasel is a novel N-ethyl-N-nitrosourea (ENU)-induced mutation. Heterozygous catweasel mutant mice exhibit mild headtossing associated with a posterior crista defect. We mapped the catweasel mutation to a critical region of 13 Mb on chromosome 12 containing the Six1, -4 and -6 genes. We identified a basepair substitution in exon 1 of the Six1 gene that changes a conserved glutamic acid (E) at position 121 to a glycine (G) in the Six1 homeodomain. Cwe/Cwe animals lack Preyer and righting reflexes, display severe headshaking and have severely truncated cochlea and semicircular canals. Cwe/Cwe animals had very few hair cells in the utricle, but their ampullae and cochlea were devoid of any hair cells. Bmp4, Jag1 and Sox2 expression were largely absent at early stages of sensory development and NeuroD expression was reduced in the developing vestibulo-acoustic ganglion. Lastly we show that Six1 genetically interacts with Jag1. We propose that the catweasel phenotype is due to a hypomorphic mutation in Six1 and that catweasel mice are a suitable model for branchio-oto-renal syndrome. In addition Six1 has a pivotal role in early sensory patch development and may act in the same genetic pathway as Jag1.

A single amino acid in the second transmembrane domain of GABA rho receptors regulates channel conductance

GABAC receptors, expressed predominately in vertebrate retina, are thought to be formed mainly by GABA rho subunits, each of which exhibits distinct physiological and pharmacological properties. In this study, the receptors formed by perch GABA rho subunits were expressed in HEK cells, and their single channel conductances were determined using noise analysis techniques. The receptors formed by the perch rho1A subunit gate a channel with a conductance of 0.2 pS, whereas the receptors formed by GABA rho2 subunits exhibit much higher channel conductances, i.e., 3.2 and 3.5 pS for perch rho2A and rho2B receptors, respectively. A comparison of the amino acid sequences of the channel-forming TMII regions of the various subunits suggested that a single amino acid at position 2' was a potential site for the large differential in conductance. We found that switching the serine residue at that site in the GABA rho2 subunit to the proline residue present in the rho1 subunit reduced the channel conductance to a level similar to that of the wild type rho1 receptor. Conversely, mutating proline to serine in the amino acid sequence of the rho1 receptor significantly increased its unitary conductance. These results indicate that a single amino acid in the TMII region plays an important role in determining the single channel conductance of the GABAC receptors.

Interactions between ρ and γ2 subunits of the GABA receptor

The gamma-aminobutyric acid type C (GABA(C)) receptor is a ligand-gated chloride channel with distinct physiological and pharmacological properties. Although the exact subunit composition of native GABA(C) receptors has yet to be firmly established, there is general agreement that GABA rho subunits participate in their formation. Recent studies on white perch suggest that certain GABA rho subunits can co-assemble with the GABA(A) receptor gamma2 subunit to form a heteromeric receptor with electrophysiological properties that correspond more closely to the native GABA(C) receptor on retinal neurons than any of the homomeric rho receptors. In the present study we examined the interactions among various perch GABA rho and gamma2 subunits. When co-expressed in Xenopus oocytes, the gamma2 subunit co-immunoprecipitated with Flag-tagged perch rho1A, rho1B, and rho2B subunits, but not with the Flag-tagged perch rho2A subunit. Immunocytochemical studies indicated that the membrane surface expression of the gamma2 subunit was detected only when it was co-expressed with perch rho1A, rho1B, or rho2B subunit, but not with the perch rho2A subunit or when expressed alone. In addition, co-immunoprecipitation of perch rho1B and gamma2 subunits was also detected in protein samples of the teleost retina. Taken together, these findings suggest that a heteromeric rho(gamma2) receptor could represent one form of GABA(C) receptor on retinal neurons.

GABA receptor rho subunit expression in the developing rat brain

Ionotropic GABA(C) receptors are composed of rho1, rho2 and rho3 subunits. Although the distribution of rho subunit mRNAs in the adult brain has been studied, information on the developmental regulation of different rho subunits in the brain is scattered and incomplete. Here, GABA(C) receptor rho subunit expression was studied in the developing rat brain. In situ hybridization on postnatal brain slices showed rho2 mRNA expression from newborn in superficial gray layer (SGL) of superior colliculus (SuC), and from the first postnatal week in the hippocampal CA1 region and pretectal nucleus of the optic tract. rho2 mRNA was also expressed in the adult dorsal lateral geniculate nucleus. Quantitative RT-PCR revealed expression of all three rho subunits in the hippocampus and superior colliculus from the first postnatal day. In the hippocampus, rho2 mRNA expression clearly dominated over rho1 and rho3, whereas in the superior colliculus, rho1 mRNA expression levels were similar to rho2. In both areas, a clear up-modulation of rho2 and rho3 mRNA during the first postnatal week was detected. GABA(C) receptor protein expression was confirmed in adult hippocampus, superior colliculus and dorsal lateral geniculate nucleus by immunohistochemistry. Our results demonstrate for the first time the expression of all three rho subunit mRNAs in several regions of the developing and adult rat brain. Our quantitative data allows assessment of putative subunit combinations in the superior colliculus and hippocampus. From the selective distribution of rho subunits, it may be hypothesized that GABA(C) receptors are specifically involved in aspects of visual image motion processing in the rat brain.

Opposing roles for GABAA and GABAC receptors in short-term memory formation in young chicks

The inhibitory neurotransmitter GABA has both inhibitory and enhancing effects on short-term memory for a bead discrimination task in the young chick. Low doses of GABA (1-3 pmol/hemisphere) injected into the multimodal association area of the chick forebrain, inhibit strongly reinforced memory, whereas higher doses (30-100 pmol/hemisphere) enhance weakly reinforced memory. The effect of both high and low doses of GABA is clearly on short-term memory in terms of both the time of injection and in the time that the memory loss occurs. We argue on the basis of relative sensitivities to GABA and to selective GABA receptor antagonists that low doses of GABA act at GABAC receptors (EC50 approximately 1 microM) and the higher doses of GABA act via GABAA receptors (EC50 approximately 10 microM). The selective GABAA receptor antagonist bicuculline inhibited strongly reinforced memory in a dose and time dependent manner, whereas the selective GABAC receptor antagonists TPMPA and P4MPA enhanced weakly reinforced in a dose and time dependent manner. Confirmation that different levels of GABA affect different receptor subtypes was demonstrated by the shift in the GABA dose-response curves to the selective antagonists. It is clear that GABA is involved in the control of short-term memory formation and its action, enhancing or inhibiting, depends on the level of GABA released at the time of learning.

Pharmacology of GABAC receptors: responses to agonists and antagonists distinguish A- and B-subtypes of homomeric rho receptors expressed in Xenopus oocytes

GABA(C) receptors, expressed predominantly in vertebrate retina, are thought to be formed mainly by GABA rho subunits. Five GABA rho subunits have been cloned from white perch retina, four of which form functional homooligomeric receptors when expressed in Xenopus oocytes. These rho subtypes, classified as rho1A, rho1B, rho2A and rho2B receptors based on amino acid sequence alignment, exhibit distinct temporal and pharmacological properties. To examine further the pharmacological properties associated with the various rho receptor subtypes, we investigated the effects of a selective GABA(C) receptor antagonist, TPMPA, on the GABA-mediated activity of receptors formed in Xenopus oocytes by the four GABA rho subunits. In addition, we recorded the activation profiles of beta-alanine, taurine, and glycine, three amino acids that modulate neuronal activity in various parts of the CNS and are purported to be rho receptor agonists. TPMPA effectively inhibited GABA-elicited responses on A-type receptors, whereas B-type receptors exhibited a relatively low sensitivity to the drug. A-type and B-type receptors also displayed distinctly different reactions to agonists. Both taurine and glycine-activated the B-type receptors, whereas these agents had no detectable effect on A-type receptors. Similarly, beta-alanine evoked large responses from B-type receptors, but was far less effective on A-type receptors. These results indicate that, in addition to the characteristic response properties identified previously, there is a pattern of pharmacological reactions that further distinguishes the A- and B-subtypes of GABA rho receptor.

Two neuropharmacological types of rabbit ON-alpha ganglion cells express GABAC receptors

The major inhibitory neurotransmitters GABA and glycine provide the bulk of input to large-field ganglion cells in the retina. Whole-cell patch-clamp recordings were used to characterize the glycine- and GABA-activated currents for morphologically identified ON-alpha ganglion cells in the rabbit retina. Cells identified as ON-alpha cells by light evoked currents were intracellularly stained and examined by light microscopy which revealed dendritic stratification in the vitreal half of the inner plexiform layer and confirmed their physiological identity. All Ca(2+)-mediated synaptic influences were abolished with Co(2+), revealing two types of ON-alpha cell characterized by their different inhibitory current profiles. One group exhibited larger glycine- than GABA-activated currents, while the other group had larger GABA- than glycine-activated currents. Both cell types demonstrated strychnine-sensitive glycine-activated currents and bicuculline-sensitive GABAA-activated currents. Surprisingly, both cell types expressed functional GABAC receptors demonstrated by their sensitivity to TPMPA. In addition, the cells with larger glycine-activated currents also possessed GABAB receptors, whereas those with larger GABA-activated currents did not. Immunocytochemical experiments confirmed the presence of glycine, GABAA, and GABAC receptor subunits on all physiologically identified ON-alpha ganglion cells in this study. In addition, the GABAB receptor immunolabeled puncta were present on the cells with larger glycine-activated currents, but not on the cells with the larger GABA-activated currents. In conclusion, the presence of different functional GABA and glycine receptors determined physiologically correlated well with the specific GABA and glycine receptor immunolabeling for two neuropharmacological types of rabbit ON-alpha ganglion cells.

Gamma-aminobutyric acid-induced calcium signalling in rat superior collicular neurones

Ionotropic gamma-aminobutyric acid (GABA) receptors are known to mediate excitation in neonatal neurones as a crucial developmental factor. In the present study we employed calcium imaging techniques with the calcium indicator Fura-2-AM to study the pharmacology of GABA-induced calcium responses in cultures prepared from neonatal rat superficial superior colliculus (SC), after immunocytochemical labelling confirmed the presence of GABA(C) rho(1) subunits in 35% of neurones. Rises in neuronal intracellular calcium were obtained in response to GABA and also to the subtype-specific GABA(A) agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol. However, the GABA(C) agonist cis-4-aminocrotonic acid induced calcium response only at unspecifically high concentrations (500 microM). Co-application of GABA antagonists revealed that both GABA(A&C) agonists' actions could be blocked by the GABA(A) antagonist bicuculline but not the GABA(C) antagonists 1,2,5,6-tetrahydro-(pyridin-4-yl) methylphosphinic acid. This suggests that activation of GABA(A) but not GABA(C) receptors contributes to excitatory GABA responses and related calcium signals in neonatal SC neurones.

Combinatorial expression of GATA4, Nkx2-5, and serum response factor directs early cardiac gene activity

Herein, the restricted expression of serum response factors (SRF) closely overlapped with Nkx2-5 and GATA4 transcripts in early chick embryos coinciding with the earliest appearance of cardiac alpha-actin (alphaCA) transcripts and nascent myocardial cells. The combinatorial expression of SRF, a MADS box factor Nkx2-5 (a NK4 homeodomain), and/or GATA4, a dual C4 zinc finger protein, in heterologous CV1 fibroblasts and Schneider 2 insect cells demonstrated synergistic induction of alphaCA promoter activity. These three factors induced endogenous alphaCA mRNA over a 100-fold in murine embryonic stem cells. In addition, the DNA-binding defective mutant Nkx2-5pm efficiently coactivated the alphaCA promoter in the presence of SRF and GATA4 in the presence of all four SREs and was substantially weakened when individual SREs were mutated and or serially deleted. In contrast, the introduction of SRFpm, a SRF DNA-binding mutant, blocked the activation with all of the alphaCA promoter constructions. These assays indicated a dependence upon cooperative SRF binding for facilitating the recruitment of Nkx2-5 and GATA4 to the alphaCA promoter. Furthermore, the recruitment of Nkx2-5 and GATA4 by SRF was observed to strongly enhance SRF DNA binding affinity. This mechanism allowed for the formation of higher ordered alphaCA promoter DNA binding complexes, led to a model of SRF physical association with Nkx2-5 and GATA4.

Co-assembly of GABA rho subunits with the GABA(A) receptor gamma(2) subunit cloned from white perch retina

Although it is well established that GABA(C) receptors are comprised in part of GABA rho subunits, the exact subunit composition of neuronal GABA(C) receptors is yet to be determined. A detailed comparison of GABA(C)-mediated neuronal responses elicited from retinal neurons with those obtained from receptors formed by GABA rho subunits revealed a number of significant differences both in the kinetics and the pharmacology of the responses. Our previous studies indicated that the human GABA(A) receptor gamma(2) subunit could co-assemble with one (rho(1B)) of the white perch GABA rho subunits to form a heterooligomeric receptor with properties that resembled those of the GABA(C) receptors on white perch bipolar cells. In this study, we cloned the white perch gamma(2) subunit, and investigated its co-assembly with four white perch GABA rho subunits. When expressed in Xenopus oocytes, perch gamma(2) and rho(1B) subunits form heterooligomeric receptors with distinct properties: the GABA-elicited responses have fast kinetics and are sensitive to pentobarbital modulation. The enhancement of GABA-elicited responses by pentobarbital on the heterooligomeric receptors could be eliminated by a single mutation in the third transmembrane domain of the gamma(2) subunit, indicating that pentobarbital sensitivity is mediated by the incorporated gamma(2) subunit. On the other hand, co-expression of the perch gamma(2) subunit with the other perch GABA rho subunits produced no detectable changes in the kinetics of GABA-elicited response or the sensitivity to pentobarbital modulation. These results suggest that the gamma(2) subunit can co-assemble only with one (rho(1B)), but not with other white perch GABA rho subunits.

Expression and dendritic mRNA localization of GABAC receptor rho1 and rho2 subunits in developing rat brain and spinal cord

The cellular distribution of GABAC receptor rho1 and rho2 subunits in the rat central nervous system remains controversial. We investigated how these subunits were distributed in cerebellum, hippocampus and spinal cord at postnatal day 1, 7 or in adult life. We found that in the adult cerebellum rho1 and rho2 mRNAs were expressed in Purkinje cells and basket-like cells only. In the hippocampus both subunits were expressed throughout the CA1 pyramidal layer, dentate gyrus and scattered interneurons with maximum staining intensity at P7. In the adult hippocampus in situ staining was predominantly found on interneurons. GABAC antibody labelling in P7 and adult hippocampus was largely overlapping with the in situ staining. Western blot analysis showed GABAC receptor in retina, ovary and testis. In the spinal cord the rho2 signal was consistently stronger than rho1 with overlapping expression patterns. At P1, the most intensely labelled cells were the motoneurons while on P7 and adult sections, interneurons and motoneurons were likewise labelled. On spinal neurons both rho1 and rho2 mRNAs showed somatodendritic localization, extending out for >100 microm with punctate appearance especially in adult cells. A similar spinal distribution pattern was provided with polyclonal antibody labelling, suggesting close correspondence between mRNA and protein compartmentalization. Electrophysiological experiments indicated that P1 spinal motoneurons did possess functional GABAC receptors even though GABAC receptors played little role in evoked synaptic transmission. Our results suggest a pattern of rho1 and rho2 subunit distribution more widespread than hitherto suspected with strong developmental regulation of subunit occurrence.

gamma-Aminobutyric acidA rho receptor subunits in the developing rat hippocampus

The RT-PCR approach was used to estimate the expression of gamma-aminobutyric acid (GABA)(A) rho receptor subunits in the hippocampus of neonatal and adult rats. All three rho subunits were detected at postnatal day (P) 2, the rho3 subunit being expressed at an extremely low level. The rho1 and rho2 products appeared to be developmentally regulated; they were found to be more pronounced in adulthood. In another set of experiments, to correlate gene expression with receptor function, GABA(A) rho subunit mRNAs were detected with single-cell RT-PCR in CA3 pyramidal cells (from P3-P4 hippocampal slices), previously characterized with electrophysiological experiments for their bicuculline-sensitive or -insensitive responses to GABA. In 6 of 19 cells (31%), pressure application of GABA evoked at -70 mV inward currents that persisted in the presence of 100 microM bicuculline (314 plus minus 129 pA). RT-PCR performed in two of these neurons revealed the presence of rho1 and rho2 subunits, the latter being present with the alpha2 subunit. A rho2 subunit was also found in 1 neuron (among 9) exhibiting a response to GABA, which was completely abolished by bicuculline. This might be due to the lack of putative accessory GABA(A) subunits that can coassemble with rho2 to make functional receptors. Similar experiments from 10 P15 CA3 pyramidal cells failed to reveal any rho1-3 transcripts. However, these neurons abundantly express alpha3 subunits. It is likely that in CA3 pyramidal cells of neonatal and adult hippocampus GABA(A) rho subunits are present but at very low levels of expression.

Positive allosteric modulation by ultraviolet irradiation on GABA(A), but not GABA(C), receptors expressed in Xenopus oocytes

1. Recombinant rat GABA(A) (alpha1beta2, alpha1beta2gamma2, beta2gamma2) and human GABA(C) (rho1) receptors were expressed in Xenopus oocytes to examine the effect of ultraviolet (UV) light on receptor function. 2. GABA-induced currents in individual oocytes expressing GABA receptors were tested by two-electrode voltage clamp before, and immediately after, 312 nm UV irradiation. 3. UV irradiation significantly potentiated 10 microM GABA-induced currents in alpha1beta2gamma2 GABA receptors. The modulation was irradiation dose dependent, with a maximum potentiation of more than 3-fold. 4. The potentiation was partially reversible and decayed exponentially with a time constant of 8.2 +/- 1.2 min toward a steady-state level which was still significantly elevated (2.7 +/- 0.3-fold) compared to the control level. 5. The effect of UV irradiation on GABA(A) receptors varied with receptor subunit composition. UV irradiation decreased the EC50 of the alpha1beta2, alpha1beta2gamma2 and beta2gamma2 GABA(A) receptors, but exhibited no significant effect on the rho1 GABA(C) receptor. 6. UV irradiation also significantly increased the maximum current 2-fold in alpha1beta2 GABA(A) receptors with little effect on the maximum of alpha1beta2gamma2 (1.1-fold) or beta2gamma2 (1.1-fold) GABA(A) receptors. 7. The effect of UV irradiation on GABA(A) receptors did not overlap the effect of the GABA receptor- allosteric modulator, diazepam. 8. The UV effect on GABA(A) receptors was not prevented by the treatment of the oocytes before and during UV irradiation with one of the following free-radical scavengers: 40 mM D-mannitol, 40 mM imidazole or 40 mM sodium azide. In addition, the effect was not mimicked by the free-radical generator, H2O2. 9. Potential significance and mechanism(s) of the UV effect on GABA receptors are discussed.

Postnatal alterations of GABA receptor profiles in the rat superior colliculus

Midbrain sections taken from Sprague-Dawley rats of varying ages within the first four postnatal weeks were used to determine, immunocytochemically, putative changes of GABA(A) receptor beta2/3 subunits, GABA(B) receptor (R1a and R1b splice variants), and GABA(C) receptor rho1 subunit expression and distribution in the superficial, visual layers of the superior colliculus. Immunoreactivity for the GABA(A) receptor beta2/3 subunits was found in the superficial grey layer from birth. The labelling changed with age, with an overall continuous reduction in the number of cells labelled and a significant increase in the labelling intensity distribution (neuropil vs soma). Further analysis revealed an initial increase in the labelling intensity between postnatal days 0 and 7 in parallel with an overall reduction of labelled neurones. This was followed by a significant decrease in labelling intensity distribution between postnatal days 7 and 16, and a subsequent increase in intensity between postnatal days 16 and 28. The labelling profiles for GABA(B) receptors (R1a and R1b splice variants) and GABA(C) receptors (rho1 subunit) showed similar patterns. Both receptors could be found in the superficial layers of the superior colliculus from birth, and the intensity and distribution of labelling remained constant during the first postnatal month. However, the cell body count showed a significant decrease between postnatal days 7 and 16. These changes may be related to the time-point of eye opening, which occurred approximately two weeks after birth. For all three receptor types, the cell body count remained constant after postnatal day 16. By four weeks of age, there was no significant difference between the cell numbers obtained for the different receptors. Both GABA itself and neurofilament labelling were also obtained in the superficial superior colliculus at birth. Neurofilament, although found at birth, showed very little ordered arrangement until 16days after birth. When slices were double labelled for GABA(C) receptors and neurofilament, some overlap was observed. Double labelling for the presynaptic protein synaptophysin and GABA(C) receptors showed proximity in some places, indicative of a partly synaptic location of GABA(C) receptors. When GABA(C) and GABA(A) receptors were labelled simultaneously, some but not all neurones showed immunoreactivity for both receptor types. In conclusion, all three GABA receptor types were found to be present in the superior colliculus from birth, and all show some form of postnatal modification, with GABA(A) receptors demonstrating the most dramatic changes. However, GABA(B) and GABA(C) receptors are modified significantly around the onset of input-specific activity. Together, this points towards a contribution of the GABAergic system to processes of postnatal maturation in the superficial superior colliculus.

Response kinetics and pharmacological properties of heteromeric receptors formed by coassembly of GABA rho- and gamma 2-subunits

Two of the gamma-aminobutyric acid (GABA) receptors, GABAA and GABAC, are ligand-gated chloride channels expressed by neurons in the retina and throughout the central nervous system. The different subunit composition of these two classes of GABA receptor result in very different physiological and pharmacological properties. Although little is known at the molecular level as to the subunit composition of any native GABA receptor, it is thought that GABAC receptors are homomeric assemblies of rho-subunits. However, we found that the kinetic and pharmacological properties of homomeric receptors formed by each of the rho-subunits cloned from perch retina did not resemble those of the GABAC receptors on perch bipolar cells. Because both GABAA and GABAC receptors are present on retinal bipolar cells, we attempted to determine whether subunits of these two receptor classes are capable of interacting with each other. We report here that, when coexpressed in Xenopus oocytes, heteromeric (rho 1B gamma 2) receptors formed by coassembly of the rho 1B-subunit with the gamma 2-subunit of the GABAA receptor displayed response properties very similar to those obtained with current recordings from bipolar cells. In addition to being unresponsive to bicuculline and diazepam, the time-constant of deactivation, and the sensitivities to GABA, picrotoxin and zinc closely approximated the values obtained from the native GABAC receptors on bipolar cells. These results provide the first direct evidence of interaction between GABA rho and GABAA receptor subunits. It seems highly likely that coassembly of GABAA and rho-subunits contributes to the molecular organization of GABAC receptors in the retina and perhaps throughout the nervous system.

Permeability and single channel conductance of human homomeric rho1 GABAC receptors

1. Homomeric human rho1 GABAC receptors were expressed in Xenopus oocytes and in human embryonic kidney cells (HEK293) in order to examine their conductance and permeability. 2. Reversal potentials of currents elicited by gamma-aminobutyric acid (GABA) were measured in extracellular solutions of various ionic composition to determine relative permeability of homomeric rho1 receptors. The rank order of anionic permeability was: SCN- > I- > NO3- > Br- > Cl- > formate (For-) > HCO3- > acetate (Ac-) approximately proprionate (Prop-) approximately isethionate (Ise-) approximately F- approximately PO4-. 3. In the oocyte expression system, relative permeabilities to SCN-, I-, NO3-, Br- and HCO3- were higher for rho1 GABAC receptors than alpha1beta2gamma2L GABAA receptors. 4. Expression of rho1 GABAC receptors in Xenopus oocytes and in HEK293 cells gave similar relative permeabilities for selected anions, suggesting that the expression system does not significantly alter permeation properties. 5. The pore diameter of the homomeric rho1 GABAC receptor expressed in oocytes was estimated to be 0.61 nm, which is somewhat larger than the 0.56 nm pore diameter estimated for alpha1beta2gamma2L GABAA receptors. 6. Homomeric rho1 GABA receptors expressed in oocytes had a single channel chord conductance of 0.65 +/- 0.04 pS (mean +/- s.e.m.) when the internal chloride concentration ([Cl-]i) was 20 mM. With a [Cl-]i of 100 mM, the single channel chord conductance was 1.59 +/- 0.24 pS. 7. The mean open time directly measured from 43 GABA-induced channel openings in six patches was 3. 2 +/- 0.8 s. The mean open time in the presence of 100 microM picrotoxin was 0.07 +/- 0.01 s (77 openings from 3 patches). 8. The differences observed in ionic permeabilities, pore size, single channel conductance and mean open time suggest that the rho1 homomeric receptor may not be the native retinal GABAC receptor reported previously.

Effects of the conformationally restricted GABA analogues, cis- and trans-4-aminocrotonic acid, on GABA neurotransmission in primary neuronal cultures

The effects of the GABA analogues, cis- and trans-4-aminocrotonic acid (ACA) on GABA(A) receptor function and GABA uptake, together with the presence of p-1 subunit mRNA and putative GABAc receptors, were studied in primary cultures of neocortical neurons and cerebellar granule cells. Both isomers induced a Cl- influx, which was inhibited by bicuculline, t-butylbicyclophosphorothionate (TBPS), picrotoxinin (PTX), and gamma-hexachlorocyclohexane (gamma-HCH or lindane). [3H]-flunitrazepam binding was also increased by both isomers and this increase was inhibited by bicuculline. In neocortical neurons, the transisomer completely inhibited the [3H]GABA uptake, whereas the cis-isomer produced only a 25% inhibition at the highest concentration used. The possible presence of GABAc receptors was investigated only in neocortical cultures by using RT-PCR in order to detect the presence of the mRNA encoding the p-1 subunit which assembles to form homooligomeric Cl-channels. The results presented here show that p-1 subunits, and thus GABAc receptors, may represent a very minor population of GABA receptors in these neuronal preparations. We conclude that both GABA analogues may act as agonists at the GABA(A) receptors, although with very different potencies.

GABAC receptor rho subunits are heterogeneously expressed in the human CNS and form homo- and heterooligomers with distinct physical properties

In the central nervous system, receptors for gamma-aminobutyric acid (GABA) are responsible for inhibitory neurotransmission. Anatomical and electrophysiological studies indicate that GABAC receptors are composed of rho subunits. While the rho 1 subunit of various species forms homooligomeric receptors with GABAC-like properties, molecular cloning has identified additional rho subunits whose functional role is unclear. By RT-PCR, we demonstrated that rho 1 expression is primarily restricted to the retina, whereas the rho 2 subunit was present in all brain regions tested. Transfection of HEK-293 cells with rho 2 cDNA resulted in GABA-gated whole-cell currents that differed from those mediated by the rho 1 subunit in two respects: maximal amplitude (rho 1:rho 2 approximately 4:1) and inactivation time course (rho 1:rho 2 approximately 2:1). Cotransfection of rho 1 and rho 2 cDNA in a 1:1 ratio generated whole-cell currents with large amplitudes characteristic of rho 1 but more rapid inactivation typical for rho 2. This observation suggested formation of heterooligomeric GABAC receptors with distinct features. Therefore, we tested the assembly of rho 1 and rho 2 subunits by cotransfecting rho 2 cDNA together with a chimeric rho 1 beta 1 subunit, known to interfere with rho 1 assembly in a dominant-negative fashion. Reduction of rho 2 generated currents correlated with the ratio of chimeric to rho 2 cDNA. Secondly, we determined that the picrotoxinin sensitivity of cells transfected with various ratios of rho 1 and rho 2 cDNA differed from that expected of a pure mixture of homooligomeric receptors. The latter two observations support the idea that rho 1 and rho 2 subunits form heterooligomeric GABAC receptors in mammalian cells. Together, our results indicate that the presence of both rho subunits enables the formation of heterooligomeric receptors with physical properties distinct from homooligomers, thus increasing the diversity of GABAC receptors in the CNS.

[3H]t-butylbicycloorthobenzoate binding to recombinant alpha1beta2gamma2s GABA(A) receptor

The interaction of several selected compounds with the binding of the cage convulsant t-[3H]butylbicycloorthobenzoate ([3H]TBOB) to membranes isolated from human embryonic kidney (HEK) 293 cells stably transfected with alpha1beta2gamma2s subtype of GABA(A) receptors was studied. Scatchard analysis of binding data revealed the existence of a single type of binding site for [3H]TBOB with a Kd of 47.06+/-4.06 nM and a Bmax value of 6.72+/-0.52 pmol/mg protein. GABA, thiopental, TBOB, picrotoxin and the neurosteroid dehydroepiandrosterone sulfate displaced concentration-dependently the binding of [3H]TBOB to this recombinant receptor. Dehydroepiandrosterone sulfate reversed the 5 microM GABA-induced inhibition of specific [3H]TBOB binding. It is concluded that membranes isolated from HEK 293 cells stably transfected with alpha1beta2gamma2s subunits exhibit specific high-affinity [3H]TBOB binding. The potency of drugs to inhibit [3H]TBOB binding mainly corresponded to that observed for the inhibition of the binding of cage convulsants to the native receptors or to transiently transfected HEK 293 cells.