Antagonists of GABA responses, studied using internally perfused frog dorsal root ganglion neurons

Synaptic Zinc Enhances Inhibition Mediated by Somatostatin, but not Parvalbumin, Cells in Mouse Auditory Cortex

Cortical inhibition is essential for brain activity and behavior. Yet, the mechanisms that modulate cortical inhibition and their impact on sensory processing remain less understood. Synaptically released zinc, a neuromodulator released by cortical glutamatergic synaptic vesicles, has emerged as a powerful modulator of sensory processing and behavior. Despite the puzzling finding that the vesicular zinc transporter (ZnT3) mRNA is expressed in cortical inhibitory interneurons, the actions of synaptic zinc in cortical inhibitory neurotransmission remain unknown. Using in vitro electrophysiology and optogenetics in mouse brain slices containing the layer 2/3 (L2/3) of auditory cortex, we discovered that synaptic zinc increases the quantal size of inhibitory GABAergic neurotransmission mediated by somatostatin (SOM)- but not parvalbumin (PV)-expressing neurons. Using two-photon imaging in awake mice, we showed that synaptic zinc is required for the effects of SOM- but not PV-mediated inhibition on frequency tuning of principal neurons. Thus, cell-specific zinc modulation of cortical inhibition regulates frequency tuning.

Assessment of Methods for the Intracellular Blockade of GABAA Receptors

Selective blockade of inhibitory synaptic transmission onto specific neurons is a useful tool for dissecting the excitatory and inhibitory synaptic components of ongoing network activity. To achieve this, intracellular recording with a patch solution capable of blocking GABA_A receptors has advantages over other manipulations, such as pharmacological application of GABAergic antagonists or optogenetic inhibition of populations of interneurones, in that the majority of inhibitory transmission is unaffected and hence the remaining network activity preserved. Here, we assess three previously described methods to block inhibition: intracellular application of the molecules picrotoxin, 4,4’-dinitro-stilbene-2,2’-disulphonic acid (DNDS) and 4,4’-diisothiocyanostilbene-2,2’-disulphonic acid (DIDS). DNDS and picrotoxin were both found to be ineffective at blocking evoked, monosynaptic inhibitory postsynaptic currents (IPSCs) onto mouse CA1 pyramidal cells. An intracellular solution containing DIDS and caesium fluoride, but lacking nucleotides ATP and GTP, was effective at decreasing the amplitude of IPSCs. However, this effect was found to be independent of DIDS, and the absence of intracellular nucleotides, and was instead due to the presence of fluoride ions in this intracellular solution, which also blocked spontaneously occurring IPSCs during hippocampal sharp waves. Critically, intracellular fluoride ions also caused a decrease in both spontaneous and evoked excitatory synaptic currents and precluded the inclusion of nucleotides in the intracellular solution. Therefore, of the methods tested, only fluoride ions were effective for intracellular blockade of IPSCs but this approach has additional cellular effects reducing its selectivity and utility.

Interaction of metal ions with neurotransmitter receptors and potential role in neurodiseases

There is increasing evidence that toxic metals play a role in diseases of unknown etiology. Their action is often mediated by membrane proteins, and in particular neurotransmitter receptors. This brief review will describe recent findings on the direct interaction of metal ions with ionotropic γ-aminobutyric acid (GABAA) and glutamate receptors, the main inhibitory and excitatory neurotransmitter receptors in the mammalian central nervous system, respectively. Both hyper and hypo function of these receptors are involved in neurological and psychotic syndromes and modulation by metal ions is an important pharmacological issue. The focus will be on three xenobiotic metals, lead (Pb), cadmium (Cd) and nickel (Ni) that have no biological function and whose presence in living organisms is only detrimental, and two trace metals, zinc (Zn) and copper (Cu), which are essential for several enzymatic functions, but can mediate toxic actions if deregulated. Despite limited access to the brain and tight control by metalloproteins, exogenous metals interfere with receptor performances by mimicking physiological ions and occupying one or more modulatory sites on the protein. These interactions will be discussed as a potential cause of neuronal dysfunction.

Differential GABAergic and glycinergic inputs of inhibitory interneurons and Purkinje cells to principal cells of the cerebellar nuclei

The principal neurons of the cerebellar nuclei (CN), the sole output of the olivo-cerebellar system, receive a massive inhibitory input from Purkinje cells (PCs) of the cerebellar cortex. Morphological evidence suggests that CN principal cells are also contacted by inhibitory interneurons, but the properties of this connection are unknown. Using transgenic, tracing, and immunohistochemical approaches in mice, we show that CN interneurons form a large heterogeneous population with GABA/glycinergic phenotypes, distinct from GABAergic olive-projecting neurons. CN interneurons are found to contact principal output neurons, via glycine receptor (GlyR)-enriched synapses, virtually devoid of the main GABA receptor (GABAR) subunits α1 and γ2. Those clusters account for 5% of the total number of inhibitory receptor clusters on principal neurons. Brief optogenetic stimulations of CN interneurons, through selective expression of channelrhodopsin 2 after viral-mediated transfection of the flexed gene in GlyT2-Cre transgenic mice, evoked fast IPSCs in principal cells. GlyR activation accounted for 15% of interneuron IPSC amplitude, while the remaining current was mediated by activation of GABAR. Surprisingly, small GlyR clusters were also found at PC synapses onto principal CN neurons in addition to α1 and γ2 GABAR subunits. However, GlyR activation was found to account for <3% of the PC inhibitory synaptic currents evoked by electrical stimulation. This work establishes CN glycinergic neurons as a significant source of inhibition to CN principal cells, forming contacts molecularly distinct from, but functionally similar to, Purkinje cell synapses. Their impact on CN output, motor learning, and motor execution deserves further investigation.

Maturation of GABAergic inhibition promotes strengthening of temporally coherent inputs among convergent pathways

Spike-timing-dependent plasticity (STDP), a form of Hebbian plasticity, is inherently stabilizing. Whether and how GABAergic inhibition influences STDP is not well understood. Using a model neuron driven by converging inputs modifiable by STDP, we determined that a sufficient level of inhibition was critical to ensure that temporal coherence (correlation among presynaptic spike times) of synaptic inputs, rather than initial strength or number of inputs within a pathway, controlled postsynaptic spike timing. Inhibition exerted this effect by preferentially reducing synaptic efficacy, the ability of inputs to evoke postsynaptic action potentials, of the less coherent inputs. In visual cortical slices, inhibition potently reduced synaptic efficacy at ages during but not before the critical period of ocular dominance (OD) plasticity. Whole-cell recordings revealed that the amplitude of unitary IPSCs from parvalbumin positive (Pv+) interneurons to pyramidal neurons increased during the critical period, while the synaptic decay time-constant decreased. In addition, intrinsic properties of Pv+ interneurons matured, resulting in an increase in instantaneous firing rate. Our results suggest that maturation of inhibition in visual cortex ensures that the temporally coherent inputs (e.g. those from the open eye during monocular deprivation) control postsynaptic spike times of binocular neurons, a prerequisite for Hebbian mechanisms to induce OD plasticity.

Evidence suggests that maturation of inhibition is required for the development of plasticity to proceed in the visual cortex. However, the mechanisms by which increased inhibition promotes plasticity are not clear. Here we characterized the maturation of synaptic and intrinsic ionic properties of parvalbumin-positive interneurons, a prominent subtype of inhibitory neuron in the cortex. We used a simple integrate-and-fire model to simulate the influence of maturation of inhibition on associative plasticity rules. We simulated two input pathways that converged onto a single postsynaptic neuron. The temporal pattern of activity was constructed differently for the two pathways: one pathway represented visually-driven activity, while the other pathway represented sensory-deprived activity. In mature circuits it is established that postsynaptic cells can select for sensory-driven inputs over deprived inputs, even in the case that deprived inputs have an initial advantage in synaptic size or number. We demonstrated that maturation of inhibition was required for postsynaptic cells to appropriately select sensory-driven patterns of activity when challenged with an opponent pathway of greater size. These results outline a mechanism by which maturation of inhibition can promote plasticity in the young, a period of development that is characterized by heightened learning.

Presynaptic GABAA receptors enhance transmission and LTP induction at hippocampal mossy fiber synapses

Presynaptic GABA(A) receptors (GABA(A)Rs) occur at hippocampal mossy fiber synapses. Whether and how they modulate orthodromic signaling to postsynaptic targets is poorly understood. We found that an endogenous neurosteroid that is selective for high-affinity delta subunit-containing GABA(A)Rs depolarized rat mossy fiber boutons, enhanced action potential-dependent Ca(2+) transients and facilitated glutamatergic transmission to pyramidal neurons. Conversely, blocking GABA(A)Rs hyperpolarized mossy fiber boutons, increased their input resistance, decreased spike width and attenuated action potential-dependent presynaptic Ca(2+) transients, indicating that a subset of presynaptic GABA receptors are tonically active. Blocking GABA(A)Rs also interfered with the induction of long-term potentiation at mossy fiber-CA3 synapses. Presynaptic GABA(A)Rs therefore facilitate information flow to the hippocampus both directly and by enhancing LTP.

Steroid regulation of the GABAA receptor: ligand binding, chloride transport and behaviour

Certain endogenous steroids are modulators of GABAA receptors. Tetrahydroprogesterone (THP, 5 alpha-pregnan-3 alpha-ol-20-one) and tetrahydrodeoxy-corticosterone (THDOC, 5 alpha-pregnane-3 alpha, 21-diol-20-one) behave as allosteric agonists of GABAA receptors whereas pregnenolone sulphate acts as an antagonist. THP and THDOC modulate ligand binding to GABAA receptors like barbiturates; they potentiate binding of the GABAA receptor agonist muscimol and the benzodiazepine flunitrazepam and they allosterically inhibit binding of the convulsant t-butylbicyclophosphorothionate. THP and THDOC also stimulate chloride uptake and currents in synaptoneurosomes and neurons. Pregnenolone sulphate acts principally as an allosteric GABAA receptor antagonist; it competitively inhibits binding of [35S] TBPS and blocks GABA agonist-activated Cl- uptake and currents in synaptoneurosomes and neurons. In behavioural experiments the GABA-agonistic steroid THDOC shows anxiolytic actions whereas the GABA-antagonistic steroid pregnenolone sulphate antagonizes barbiturate-induced hypnosis. Changes in physiological levels of GABAergic steroids may alter GABAA receptor function, influencing neuronal excitability and CNS arousal. For example, pregnancy and the puerperium are associated with alterations in GABAA receptor binding which might be attributable to steroid actions.

Mapping convulsants' binding to the GABA-A receptor chloride ionophore: a proposed model for channel binding sites

Gamma-aminobutyric acid (GABA) type A receptors play a key role in brain inhibitory neurotransmission, and are ligand-activated chloride channels blocked by numerous convulsant ligands. Here we summarize data on binding of picrotoxin, tetrazoles, beta-lactams, bicyclophosphates, butyrolactones and neurotoxic pesticides to GABA-A ionophore, and discuss functional and structural overlapping of their binding sites. The paper reviews data on convulsants' binding sensitivity to different point mutations in ionophore-lining second trans-membrane domains of GABA-A subunits, and maps possible location of convulsants' sites within the chloride ionophore. We also discuss data on inhibition of glycine, glutamate, serotonin (5-HT3) and N-acetylcholine receptors by GABA-A channel blockers, and examine the applicability of this model to other homologous ionotropic receptors. Positioning various convulsant-binding sites within ionophore of GABA-A receptors, this model enables a better understanding of complex architectonics of ionotropic receptors, and may be used for developing new channel-modulating drugs.

GABA-induced response in spiral ganglion cells acutely isolated from guinea pig cochlea

The physiological and pharmacological properties of gamma-aminobutyric acid (GABA)-induced responses were investigated in acutely isolated spiral ganglion cells (SGCs) of guinea pig by using either a nystatin-perforated patch recording configuration or a conventional whole-cell patch recording mode combined with rapid drug application. GABA and GABA(A) subtype receptor agonist, muscimol, induced inward currents in a concentration-dependent manner in 74% of all cells. The current-voltage relationship for the GABA response indicated the GABA-induced current in SGCs is carried by Cl-. Bicuculline (BIC), strychnine (STR), and picrotoxin (PTX) suppressed the GABA response in a concentration-dependent manner. BIC and STR, and PTX blocked the GABA response in a competitive manner and in a non-competitive manner, respectively. For inorganic antagonists, Cd2+ and Ni2+ also inhibited the GABA response. On the other hand, Zn2+ failed to suppress the GABA response in SGCs. An antibiotic, benzylpenicillin, suppressed the GABA response. The GABA response was augmented by both a barbiturate derivative, pentobarbital (PB), and a benzodiazepine derivative, diazepam. The results suggest clearly that the physiological and pharmacological characteristics of GABA(A) receptor on acutely isolated guinea pig SGCs are quite similar to the common GABA(A) receptor found in other sensory ganglion cells.

Modulation of GABA(A) receptor-mediated currents by benzophenone derivatives in isolated rat Purkinje neurones

We investigated modulation of GABA(A) receptor-mediated whole-cell currents in cerebellar Purkinje neurones by several derivatives of benzophenone. A metabolite of phenazepam, 5-bromo-2'-chloro-2-aminobenzophenone (I), caused dual modification of peak amplitudes of GABA-gated currents that depended upon the concentration of applied GABA and incubation time. Following short 10 s pre-incubations, 1-30 microM I facilitated activation and delayed deactivation of currents evoked by 500 ms pulses of 20 microM GABA. In addition, 10 microM I prominently enhanced desensitisation of currents during applications of 500 microM GABA mainly by decreasing the value of the fast time constant of the desensitisation. Continuous 6 min incubation with 10 microM I during GABA stimulation or its administration between but not during 1 s pulses of 500 microM GABA led to a gradual, partly reversible attenuation of GABA-activated currents. This inhibition was not observed when I was applied only during pulses of GABA, indicating that the blockade was not use-dependent. One of the possible mechanisms of this down-modulation could be an intracellular effect of I, because when applied intracellularly it caused slow inhibition of responses to consecutive GABA pulses. When 3-30 microM I was applied on the background of small 'plateau'-like current 5-7 s after application of 500 microM GABA, it was able to block open channels with on and off rates similar to those observed with 30 microM picrotoxin but much slower than in the case of 500 microM benzylpenicillin. At a concentration of 10 microM, 5-substituted benzophenones, but not 2-aminobenzophenone or benzophenone itself, exhibited modulatory properties similar to I and distinct from those of picrotoxin and benzylpenicillin. Therefore, we conclude that derivatives of benzophenone are a novel class of GABA(A) receptor modulators with a unique pharmacological profile.

Evidence for shoulder girdle dystonia in selected patients with cervical disc prolapse

Some patients with cervical disc herniation suffer from persistent nuchal pain and muscle spasms after decompressive surgery despite the lack of clinical and radiological signs for actual spinal root compression. Sonographic examination of the brain in some of these patients showed increased echogenicity of the lentiform nuclei as described in patients with idiopathic dystonia. This has been linked to an altered Menkes protein level and copper metabolism. We suggest a relationship between persistent nuchal pain after adequate cervical disc surgery and dystonic movement disorders. Thirteen patients with persistent nonradicular nuchal pain after at least one cervical disc surgery and without evidence of continuing spinal root compression and 13 age-matched controls were included. All patients had a complete neurological examination, ultrasound, and MRI scan of the brain. In addition, Menkes protein mRNA levels of leucocytes were analyzed in patients and controls. All patients with persistent nuchal pain exhibited a constant tonic unilateral shoulder elevation associated with an ipsilateral hypertrophy of the trapezius muscle. Ultrasound examination showed an increased echogenicity of the lentiform nucleus in one patient unilaterally and in 10 patients bilaterally but in none of the controls. On MRI the T2-values of the lentiform nuclei were found to be higher in patients exhibiting a hyperechogenicity of the lentiform nuclei compared to controls (P = 0.01). In addition, Menkes protein mRNA levels were decreased in patients with cervical disc herniation (P = 0.03). Clinical, neuroimaging, and biochemical findings of this selected patient sample with chronic nuchal pain and muscle spasms after cervical disc surgery resemble alterations in patients with idiopathic cervical dystonia. This suggests a link between both disorders. A peripheral trauma to the nerve roots may precipitate dystonic movements in susceptible patients and chronic dystonic muscle contraction would account for the persistent nuchal pain.

Interaction between copper and zinc at GABA(A) receptors in acutely isolated cerebellar Purkinje cells of the rat

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.

Modulation by bicuculline and penicillin of the block by t-butyl-bicyclo-phosphorothionate (TBPS) of GABA(A)-receptor mediated Cl(-)-current responses in rat striatal neurones

1. T-butyl-bicyclo-phosphorothionate (TBPS) is a prototypical representative of the cage-convulsants which act through a use-dependent block of the GABA(A)-receptor-ionophore complex. Using current recordings from cultured neurones of rat striatum the manner was investigated in which two antagonists, bicuculline and penicillin, presumably acting at the agonist binding site and in the ionic channel, respectively, modify the rate of block by TBPS. 2. Penicillin (5 or 10 mM) did not slow the rate of block by TBPS, but produced a significant enhancement of block rate, which, however, was inversely related to the degree of antagonism by penicillin of the GABA-induced current. 3. Bicuculline (10 microM) reduced the rate of block by TBPS. However, this effect was 3 fold weaker than its GABA-antagonistic action. The slowing of block rate and the current antagonism exhibited a biphasic, positive-negative relationship. Co-application of bicuculline (100 microM) in a concentration that produced nearly complete antagonism and TBPS (10 microM) resulted in a marked ( approximately 40%) reduction of subsequent GABA response amplitudes compatible with a direct, bicuculline-induced conformational change in the receptor required for the binding of and block by TBPS. 4. The lack of protection afforded by the channel blocker penicillin as well as the lack of correlation between bicuculline antagonism of the Cl(-)-current and its efficiency in protecting against TBPS block is evidence against an open channel blocking mechanism for TBPS. TBPS does, therefore, not appear to gain access to its binding site via the open pore but through alternative routes regulated from the agonist binding site.

Epileptiform propagation patterns mediated by NMDA and non-NMDA receptors in rat neocortex

PURPOSE

The neocortex can generate various forms of epileptiform activity, including one that depends on N-methyl-D-aspartate (NMDA)-type glutamate receptors (NMDARs), and another dependent on non-NMDA-type (AMPA) glutamate receptors (AMPARs). Previous work in vitro suggests that both forms of activity are initiated by neurons of layer 5, but the spatial patterns of horizontal propagation have been studied only for the AMPAR form. We have tested the hypothesis that both types of epileptiform activity spread via common pathways in one cortical layer, suggesting that lamina-specific intervention might selectively interrupt both.

METHODS

Slices of rat somatosensory cortex were maintained in vitro and treated with the gamma-aminobutyric acid type A (GABA(A))-receptor antagonist picrotoxin. Single all-or-none epileptiform discharges were evoked with an electrical stimulus, and extracellular microelectrodes were used to track the vertical and lateral spread of the discharges.

RESULTS

In both high and low concentrations of picrotoxin, the non-NMDAR antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) completely blocked propagation, whereas the NMDAR antagonist D-2-amino-5-phosphonovaleric acid (DAPV) only shortened the duration of discharges. When extracellular [Mg2+] was reduced in the presence of picrotoxin and CNQX, NMDAR-dependent epileptiform discharges could be initiated. NMDAR-dependent discharges spread at about one fifth the conduction velocity of AMPAR-dependent events. Analysis of spatiotemporal field-potential patterns suggested that both NMDAR- and AMPAR-mediated propagation involved early activity in layers 5 and 6, followed by larger-amplitude activity in upper cortical layers along the path of propagation.

CONCLUSIONS

Our results imply that a common pathway mediates the propagation of these two forms of epileptiform activity, and suggests that lamina-specific surgical intervention might maximize anticonvulsant effect while minimally disrupting cortical function.

Layer-specific pathways for the horizontal propagation of epileptiform discharges in neocortex

PURPOSE

Epileptiform discharges that resemble interictal spikes can be generated by slices of neocortex treated with antagonists of gamma-aminobutyric acid A (GABA(A)) receptors. These discharges can propagate horizontally for long distances. We tested the hypothesis that propagation occurs through preferred horizontal pathways that lie in a particular cortical layer.

METHODS

Slices were prepared from the primary somatosensory cortex of rats, maintained in vitro, and bathed with the GABA(A) receptor antagonist picrotoxin. Electrical stimuli were used to evoke single all-or-none paroxysmal field potentials (PFP) that were recorded with pairs or arrays of field potential electrodes.

RESULTS

To test which laminae are necessary for propagation, vertical cuts were made to force the PFP to spread horizontally through particular layers. If slices were bathed in a high dose of picrotoxin (35 microM), a bridge of cortex 350 microm thick placed at any lamina was sufficient to support PFP propagation. However, in low picrotoxin doses (2.5 microM), similarly sized bridges had to include tissue from layers 4/5 or 5/6 to support propagation. When slices were cut horizontally (i.e., parallel to the pia) in strips. either upper-, middle-, or lower-layer strips were sufficient to support PFP propagation if the picrotoxin concentration was high; however, in low picrotoxin doses, only horizontal strips that included layer 5 could support propagation. Finally, in intact picrotoxin-treated slices, focal applications of GABA were systematically applied to different laminae as the PFP propagated past; GABA was most effective at blocking or delaying propagation when it was applied to layer 5b.

CONCLUSIONS

We conclude that epileptiform propagation can occur through a variety of horizontal pathways when cortical inhibition is strongly impaired. However, when inhibition is reduced only moderately, axonal pathways in layer 5 are critical for seizure spread.

Interaction of H+ and Zn2+ on recombinant and native rat neuronal GABAA receptors

1. The interaction of Zn2+ and H+ ions with GABAA receptors was examined using Xenopus laevis oocytes expressing recombinant GABAA receptors composed of subunits selected from alpha1, beta1, gamma2S and delta types, and by using cultured rat cerebellar granule neurones. 2. The potency of Zn2+ as a non-competitive antagonist of GABA-activated responses on alpha1beta1 receptors was reduced by lowering the external pH from 7.4 to 5.4, increasing the Zn2+ IC50 value from 1.2 to 58.3 microM. Zinc-induced inhibition was largely unaffected by alkaline pH up to pH 9.4. 3. For alpha1beta1delta subunits, concentration-response curves for GABA were displaced laterally by Zn2+ in accordance with a novel mixed/competitive-type inhibition. The Zn2+ IC50 at pH 7.4 was 16.3 microM. Acidification of Ringer solution resulted in a reduced antagonism by Zn2+ (IC50, 49.0 microM) without affecting the type of inhibition. At pH 9.4, Zn2+ inhibition remained unaffected. 4. The addition of the gamma2S subunit to the alpha1beta1delta construct caused a marked reduction in the potency of Zn2+ (IC50, 615 microM), comparable to that observed with alpha1beta1gamma2S receptors (IC50 639 microM). GABA concentration-response curves were depressed in a mixed/non-competitive fashion. 5. In cultured cerebellar granule neurones, Zn2+ inhibited responses to GABA in a concentration-dependent manner. Lowering external pH from 7.4 to 6.4 increased the IC50 from 139 to 253 microM. 6. The type of inhibition exhibited by Zn2+ on cerebellar granule neurones, previously grown in high K+-containing culture media, was complex, with the GABA concentration-response curves shifting laterally with reduced slopes and similar maxima. The Zn2+-induced shift in the GABA EC50 values was reduced by lowering the external pH from 7.4 to 6.4. 7. The interaction of H+ and Zn2+ ions on GABAA receptors suggests that they share either a common regulatory pathway or coincident binding sites on the receptor protein. The apparent competitive mode of block induced by Zn2+ on alpha1beta1delta receptors is shared by GABAA receptors on cerebellar granule neurones, which are known to express delta-subunit-containing receptors. This novel mechanism is masked when a gamma2 subunit is incorporated into the receptor complex, revealing further diversity in the response of native GABAA receptors to endogenous cations.

High-affinity copper block of GABA(A) receptor-mediated currents in acutely isolated cerebellar Purkinje cells of the rat

The actions of Cu2+ ions on GABAA receptor-mediated currents in acutely isolated Purkinje cells from rat cerebellum were studied using the whole-cell patch-clamp technique and a rapid perfusion system. Bath application of Cu2+ reduced currents induced by 2 microM gamma-aminobutyric acid (GABA) in a concentration-dependent manner with an IC50 of 35 nM. The Cu2+-induced block of GABA responses was not voltage-dependent. Increasing the GABA concentration (from 2 to 50 microM) decreased the blocking effect of Cu2+. Dose-response analysis for activation of GABAA receptors revealed a twofold decrease in apparent affinity for GABA in the presence of 0.1 microM Cu2+. Recovery from the block required several minutes after removal of Cu2+ from the medium. The block was removed by histidine, which preferentially forms complexes with Cu2+, or by other chelating substances. Application of 10 microM histidine immediately before application of 2 microM GABA completely relieved the block of GABA responses produced by 0.1 microM Cu2+. The effect of histidine was concentration-dependent with an EC50 of 0.75 microM. The results demonstrate that Cu2+ is a potent inhibitor of GABA-evoked responses in rat Purkinje cells. Copper may be an endogenous synaptic modulating factor. Cu2+ toxicity, notably in Wilson's disease, could result to some extent from chronic GABAA receptor blockade.

Zn2+ inhibition of recombinant GABAA receptors: an allosteric, state-dependent mechanism determined by the gamma-subunit

1. The gamma-subunit in recombinant gamma-aminobutyric acid (GABAA) receptors reduces the sensitivity of GABA-triggered Cl- currents to inhibition by Zn2+ and transforms the apparent mechanism of antagonism from non-competitive to competitive. To investigate underlying receptor function we studied Zn2- effects on macroscopic and single-channel currents of recombinant alpha 1 beta 2 and alpha 1 beta 2 gamma 2 receptors expressed heterologously in HEK-293 cells using the patch-clamp technique and rapid solution changes. 2. Zn2+ present for > 60 s (constant) inhibited peak, GABA (5 microM)-triggered currents of alpha 1 beta 2 receptors in a concentration-dependent manner (inhibition equation parameters: concentration at half-amplitude (IC50) = 0.94 microM; slope related to Hill coefficient, S = 0.7) that was unaffected by GABA concentration. The gamma 2 subunit (alpha 1 beta 2 gamma 2 receptor) reduced Zn2+ sensitivity more than fiftyfold (IC50 = 51 microM, S = 0.86); increased GABA concentration (100 microM) antagonized inhibition by reducing apparent affinity (IC50 = 322 microM, S = 0.79). Zn2+ slowed macroscopic gating of alpha 1 beta 2 receptors by inducing a novel slow exponential component in the activation time course and suppressing a fast component of control desensitization. For alpha 1 beta 2 gamma 2 receptors, Zn2+ accelerated a fast component of apparent desensitization. 3. Zn2+ preincubations lasting up to 10 s markedly increased current depression and activation slowing of alpha 1 beta 2 receptors, but had little effect on currents from alpha 1 beta 2 gamma 2 receptors. 4. Steady-state fluctuation analysis of macroscopic alpha 1 beta 2 gamma 2 currents (n = 5) resulted in control (2 microM GABA) power density spectra that were fitted by a sum of two Lorentzian functions (relaxation times: 37 +/- 5.6 and 1.41 +/- 0.15 ms, means +/- S.E.M.). Zn2+ (200 microM) reduced the total power almost sixfold and accelerated the slow (23 +/- 2.8 ms, P < 0.05) without altering the fast (1.40 +/- 0.16 ms) relaxation time. The ratio (fast/slow) of Lorentzian areas was increased by Zn2+ (control, 3.39 +/- 0.55; Zn2+, 4.9 +/- 0.37, P < 0.05). 5. Zn2+ (500 microM) depression of previously activated current amplitudes (% control) for alpha 1 beta 2 gamma 2 receptors was independent of GABA concentration (5 microM, 13.2 +/- 0.72%; 100 microM, 12.2 +/- 2.9%, P < 0.8, n = 5). Both onset and offset inhibition time courses were biexponential. Onset rates were enhanced by Zn2+ concentration. Inhibition onset was also biexponential for preactivated alpha 1 beta 2 receptors with current depression more than fourfold less sensitive (5 microM GABA, IC50 = 3.8 microM, S = 0.84) relative to that in constant Zn2+. 6. The results lead us to propose a general model of Zn2+ inhibition of GABAA receptors in which Zn2+ binds to a single extracellular site, induces allosteric receptor inhibition involving two non-conducting states, site affinity is state-dependent, and the features of state dependence are determined by the gamma-subunit.

Inhibition of ATP-activated current by zinc in dorsal root ganglion neurones of bullfrog

1. The effect of Zn2+ on ATP-activated current was studied in bullfrog dorsal root ganglion (DRG) neurones using the whole-cell patch-clamp technique. 2. Zn2+ (2-800 microM) inhibited current activated by submaximal concentrations of ATP. The Zn2+ concentration that produced 50% inhibition (IC50) of current activated by 2.5 microM ATP was 61 +/- 9.8 microM. When ATP concentrations were adjusted to account for chelation of Zn2+, the IC50 of Zn2+ was 86 +/- 18 microM. 3. The inhibitory action of Zn2+ on ATP-gated channels did not appear to be due to a decrease in the concentration of one or more species of ATP. 4. Zn2+ inhibition of ATP-activated current was independent of membrane potential between -80 and +40 mV, and did not involve a shift in the reversal potential of the current. 5. Zn2+ (100 microM) shifted the ATP concentration-response curve to the right in a parallel manner, increasing the EC50 for ATP from 2.5 +/- 0.5 microM to 5.5 +/- 0.4 microM. 6. Zn2+ decreased the time constant of deactivation of ATP-gated ion channels without affecting the time constant of activation or desensitization. 7. Dithiothreitol (DTT) reversed Zn2+ inhibition of ATP-activated current. 8. 2-Methylthio ATP, alpha,beta-methylene ATP and ADP activated current with EC50 values of 2.4 +/- 0.3. 50.1 +/- 5.8 and 303.1 +/- 53.9 microM, respectively. Adenosine, AMP or beta,gamma-methylene ATP did not evoke detectable current. 9. Reactive Blue 2 and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid inhibited ATP-activated current. 10. The results suggest that Zn2+ can inhibit P2X purinoceptor function by decreasing the affinity of the binding site for ATP. These observations provide the first evidence for this action of Zn2+ on a neurotransmitter-gated ion channel. Furthermore, the receptor-channel in these neurones appears to be a novel member of the P2X purinoceptor class.

Action of furosemide on GABA- and glycine currents in rat septal cholinergic neurons in culture

The action of furosemide on GABA- and glycine-induced currents in rat septal cholinergic neurons in culture was examined by use of the whole-cell voltage-clamp technique. Furosemide inhibited the peak amplitude of either GABA or glycine current in a dose-dependent manner; its half-maximal concentration for them was some 2 and 1 mM, respectively. Furosemide at a concentration of 1 mM depressed the maximal peak amplitude of the dose-response curve for either GABA or glycine to about 70% of control. These results indicate that furosemide inhibits in a noncompetitive fashion GABA and glycine receptor-channels in septal neurons.

The pharmacology of amino-acid responses in septal neurons

Septal cholinergic neurons are known to play an important role in cognitive processes including learning and memory through afferent innervation of the hippocampal formation and cerebral cortex. The septum contains not only cholinergic neurons but also various types of neurons including GABA (gamma-aminobutyric acid)-ergic neurons. Although synaptic transmission in the septum is mediated primarily by the activation of excitatory and inhibitory amino-acid receptors, it is possible that a distinct phenotype of neuron is endowed with a different type for each of the amino-acid receptors and thus they play different roles from each other, since it has been demonstrated within the septum that there is a regional distribution of various types of amino-acid receptor subunits, their expression as different combinations within a specific cell may produce receptor channels with disparate functional properties. As a first step towards knowing the various functions of septal cholinergic neurons, we characterized the functional properties of glutamate, GABA (type A; GABAA) and glycine receptor channels on cultured rat septal neurons which were histologically identified to be cholinergic. These were similar to those of receptor channels on other types of neurons, except for the actions of some neuromodulators. The septal N-methyl-D-aspartate receptor channel was distinct in being less sensitive to Mg2+ and in a voltage-dependent action of Zn2+. The septal GABAA receptor channel exhibited a lanthanide site whose activation resulted in a positive allosteric interaction with a binding site of pentobarbital. The septal glycine receptor channel was only positively modulated by Zn2+; this action of Zn2+ was not accompanied by an inhibitory effect. Our data suggest that the amino-acid receptors on septal cholinergic neurons may play a distinct role compared to other types of neurons; this difference depends on the actions of neuromodulators and metal cations. It would be interesting to compare these effects recorded in tissue culture to those observed with septal cholinergic neurons in slice preparations.

Pharmacological evidence for two types of postsynaptic glycinergic receptors on the Mauthner cell of 52-h-old zebrafish larvae

The presence of homooligomeric and heterooligomeric glycine receptors (GlyRs) on the Mauthner (M) cell in the isolated medulla of 52-h-old zebrafish larvae was investigated by analysis of the effects of picrotoxin on glycine-gated channels and on glycinergic miniature inhibitory postsynaptic currents (mIPSCs). Two functionally different GlyRs have been previously described on the M cell. The effects of picrotoxin on these two GlyRs were first analyzed by measuring the relative change in their total open probability (NP(o)) with picrotoxin concentration. Picrotoxin had no significant effect on the glycine channel with a single conductance level of 40-46 pS. In contrast, picrotoxin application decreased the NP(o) of the GlyR with multiple subconductance levels. On this GlyR, picrotoxin decreased in a concentration-dependent manner the occurrence of the 80- to 86-pS substate (median inhibiting concentration = 0.89 microM) and had no apparent effect on the 40- to 46-pS opening probability. Opening frequency and the mean open times of the 80- to 88-pS main conductance state were reduced in the presence of 10 microM picrotoxin, but their relative weight remained unchanged. These effects of picrotoxin were not voltage dependent. Picrotoxin also modified 40- to 46-pS kinetics. At 100 microM, picrotoxin evoked voltage-independent flickering during channel openings. Short and long mean open times were significantly decreased, whereas the relative proportion of long mean open times was increased. The medium closed time was decreased, whereas medium burst duration was increased. The burst frequency remained unchanged. Spontaneous glycinergic mIPSCs were recorded in the presence of 1 microM tetrodotoxin + 25 microM bicuculline (holding potential = -50 mV). Application of 10 microM picrotoxin did not change the frequency of the synaptic activity, whereas it decreased the amplitude of large mIPSCs. No effect was observed on the time to peak (0.8 ms) or the mean decay time constant (tau(d) = 7.7 ms). Increasing picrotoxin concentration to 100 microM resulted in a decrease of mIPSC frequency (35.6%), amplitude (39.8%), and tau(d) (from 7.7 to 5 ms). These results suggest that these two functionally different GlyRs correspond to alpha1 homooligomeric-like and alpha1/beta-heterooligomeric-like GlyRs, and that both are synaptically activated. Variation in the proportions of GlyR subtypes from one synapse to another could partly account for the broad amplitude distribution of mIPSCs recorded from the zebrafish M cell.

Dual mechanisms of GABAA response inhibition by beta-lactam antibiotics in the pyramidal neurones of the rat cerebral cortex

1. The effects of beta-lactam antibiotics on the gamma-aminobutyric acid (GABA)-induced Cl- current were investigated in pyramidal neurones freshly dissociated from the rat frontal cortex by the use of a nystatin-perforated patch recording mode under voltage-clamp conditions. 2. The GABA-induced inward current increased in a concentration-dependent manner with an EC50 of 6.7 x 10(-6) M at a holding potential of -40 mV. The GABA response was accompanied by an increase in the membrane conductance and reversed at near the Cl- equilibrium potential. 3. All beta-lactams (penicillin, imipenem, aztreonam and cefotiam) inhibited the 10(-5) M GABA-induced response in a concentration-dependent manner with an IC50 and Hill coefficient of 1.3 x 10(-3) M and 0.64 for penicillin, 9.6 x 10(-4) M and 0.83 for imipenem, 2.5 x 10(-3) M and 9.99 for aztreonam, and 2.9 x 10(-4) M and 1.03 for cefotiam. 4. Imipenem inhibited the GABA-response competitively while penicillin inhibited the same response in a noncompetitive fashion. 5. The inhibitory action of imipenem showed no voltage-dependency, whereas the effect of penicillin was voltage-dependent. 6. It is thus proposed that some classes of beta-lactams, including imipenem may have a mechanism that is different from penicillin and competitively affects the GABAA receptor.

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.

Mutations affecting the glycine receptor agonist transduction mechanism convert the competitive antagonist, picrotoxin, into an allosteric potentiator

Contrary to its effects on the gamma-aminobutyric acid type A receptor, picrotoxin antagonism of the alpha 1 subunit of the human glycine receptor is shown to be competitive, not use-dependent, and nonselective between the picrotoxin components, picrotin, and picrotoxinin. Competitive antagonism and non-use dependence are consistent with picrotoxin binding to a site in the extracellular domain. The mutations Arg-->Leu or Arg-->Gln at residue 271 of the glycine receptor alpha 1 subunit, which are both associated with human startle disease, have previously been demonstrated to disrupt the transduction process between agonist binding and channel activation. We show here that these mutations also transform picrotoxin from an allosterically acting competitive antagonist to an allosteric potentiator at low (0.01-3 microM) concentrations and to a noncompetitive antagonist at higher (> or = 3 microM) concentrations. This demonstrates that arginine 271 is involved in the transduction process between picrotoxin binding and its mechanism of action. Thus, the allosteric transduction pathways of both agonists and antagonists converge at a common residue prior to the activation gate of the channel, suggesting that this residue may act as an integration point for information from various extracellular ligand binding sites.

Enhancement by GABA of the association rate of picrotoxin and tert-butylbicyclophosphorothionate to the rat cloned alpha 1 beta 2 gamma 2 GABAA receptor subtype

1. We examined how gamma-aminobutyric acid (GABA) influences interaction of picrotoxin and tert-butylbicyclophosphorothionate (TBPS) with recombinant rat alpha 1 beta 2 gamma 2 GABAA receptors stably expressed in human embryonic kidney cells (HEK293), as monitored with changes in Cl- currents measured by the whole-cell patch clamp technique. 2. During application of GABA (5 microM) for 15 s, picrotoxin and TBPS dose-dependently accelerated the decay of inward GABA-induced currents (a holding potential of -60 mV under a symmetrical Cl- gradient). The drugs, upon preincubation with the receptors, also reduced the initial current amplitude in a preincubation time and concentration-dependent manner. This indicates their interaction with both GABA-bound and resting receptors. 3. The half maximal inhibitory concentration for picrotoxin and TBPS at the beginning of a 15 s GABA (5 microM) pulse was several times greater than that obtained at the end of the pulse. GABA thus appears to enhance picrotoxin and TBPS potency, but only at concentrations leading to occupancy of both high and low affinity GABA sites, i.e., 5 microM. Preincubation of the receptors with the drugs in the presence of GABA at 200 nM, which leads to occupancy of only high affinity GABA sites in the alpha 1 beta 2 gamma 2 subtype, produced no appreciable change in potency of picrotoxin or TBPS. This indicates that they preferentially interact with multiliganded, but not monoliganded receptors, unlike U-93631, a novel ligand to the picrotoxin site, which has higher affinity to both mono- and multiliganded receptors than resting receptors. 4. The time-dependent decay and preincubation time-dependent reduction of initial amplitude of GABA-induced Cl- currents followed monoexponential time courses, and time constants thus obtained displayed a linear relationship with drug concentration. Analysis of the data using a kinetic model with a single drug site showed that GABA (5 microM) enhanced the association rate for picrotoxin and TBPS nearly 100 fold, but their dissociation rate only 10 fold. The dissociation rate obtained from current recovery from picrotoxin or TBPS block yielded nearly identical values to the above analysis.5. We conclude that picrotoxin and TBPS interact with both resting and GABA-bound receptors, but their affinity for the latter is about 10 times greater than that for the former, largely due to a markedly increased association rate to the multiliganded receptors (but not monoliganded ones). This and our earlier study with U-93631 improves our understanding of functional coupling between GABA and picrotoxin sites, which appears to be useful in characterizing the mode of interaction for various picrotoxin site ligands.

Comparison of the actions of glycine and related amino acids on isolated third order neurons from the tiger salamander retina

Whole cell voltage and current clamp recordings were obtained from third order neurons isolated from the salamander retina. Using cross desensitization, the structure-function relationship of short chain amino acids on the glycine receptor were examined. L-Serine, L-alanine, beta-alanine and taurine all cross desensitized with glycine, but did not show significant cross desensitization with GABA. This indicates that these amino acids act at the glycine receptor. The order of potency was glycine >> beta-alanine > taurine >> L-alanine > L-serine. TAG, a reputed selective taurine antagonist, was equally effective in blocking taurine and glycine currents. There is no evidence for distinct receptors for taurine. Amino acids with larger moieties at the alpha carbon, such as threonine and valine, produced inactive ligands. Placing a methyl group on the amine of glycine or esterification of the carboxyl group also greatly reduced activity. Based on these modifications of the glycine molecule, it appears that selectivity at the glycine receptor results in part from steric restrictions at all three sites in the glycine chain. The steric interference is most critical at the carboxyl and amino ends, and less limiting at the alpha carbon. Doses of L-serine that had only slight effects in voltage clamp experiments, nevertheless produced large effects in current clamp experiments. This indicates that several endogenous amino acids can have significant effects on membrane voltage, even when their shunting activity may be small. High concentrations of agonists produced desensitization in the voltage clamp records, but there was little evidence of desensitization in the current clamp experiments. These results indicate that several endogenous amino acids can activate the glycine receptor, but there is no evidence for a discrete receptor for taurine, beta-alanine, L-alanine or L-serine. Since all these endogenous amino acids have similar amino and acid terminals, reduction in potency results from steric interference around the alpha carbon. This graded potency may have functional significance in mediating inhibition.

Zinc inhibition of GABA-stimulated Cl- influx in rat brain regions is unaffected by acute or chronic benzodiazepine

Zinc modulation of GABAA receptor function was studied using GABA-stimulated 36Cl- influx into microsacs prepared from rat cerebral cortex, cerebellum and hippocampus. Zinc (10-100 microM) did not affect the basal influx, but significantly inhibited GABA-stimulated 36Cl- influx. The inhibition appeared to be noncompetitive. Zinc produced differing degrees of inhibition of GABA-stimulated 36Cl- influx in different brain regions. The order of sensitivity to zinc inhibition of GABA-stimulated 36Cl- influx was hippocampus > cerebral cortex > cerebellum. These regional differences may reflect the structural heterogeneity of GABAA receptors among brain areas. Zinc inhibition was not affected by the short-term addition of three benzodiazepines, diazepam, bretazenil and triazolam. The effect of diazepam and bretazenil to potentiate GABA-stimulated 36Cl- influx was not affected by zinc, but the effect of triazolam was decreased by zinc. In brain tissue prepared from flurazepam-treated rats, there was no difference compared with controls in zinc inhibition of GABA-stimulated 36Cl- influx. The results indicate that the effects of zinc on the GABAA receptor are largely independent of drugs acting on the benzodiazepine binding site.

Orientation selectivity of cortical neurons during intracellular blockade of inhibition

Neurons in the primary visual cortex of the cat are selectively activated by stimuli with particular orientations. This selectivity can be disrupted by the application of antagonists of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) to a local region of the cortex. In order to determine whether inhibitory inputs are necessary for a single cortical neuron to show orientation selectivity, GABA receptors were blocked intracellularly during whole cell recording. Although the membrane potential, spontaneous activity, subfield antagonism, and directional selectivity of neurons were altered after they were perfused internally with the blocking solution, 18 out of 18 neurons remained selective for stimulus orientation. These results indicate that excitatory inputs are sufficient to generate orientation selectivity.

Somatostatin receptor-GTP binding regulatory protein-adenylyl cyclase system in hippocampal membranes of strychnine-treated rats

Wistar rats were injected with either a non-convulsive dose (37.5 micrograms/100 g body weight (b.wt.), intravenously (i.v.)) or a convulsive dose (50 or 80 micrograms/100 g b.w.t, i.v.) of strychnine. Binding of 125I-Tyr11-somatostatin (125I-Tyr11-SS) to its specific receptors was measured in hippocampal membranes 15 min after strychnine injection at these three doses. The non-convulsive dose of strychnine did not affect binding of SS in the hippocampus whereas both convulsive doses decreased the number of specific SS receptors without influencing their apparent affinity. Somatostatin-like immunoreactivity (SSLI), SS-modulated adenylyl cyclase (AC) activity and the inhibitory guanine-nucleotide binding regulatory protein were also measured in rats treated with 80 micrograms/100 g b.wt. of strychnine. SSLI content remained stable. No significant differences were seen for the basal and forskolin (FK)-stimulated AC enzyme activities in the hippocampus of strychnine-treated rats when compared to the control group. The capacity of SS to inhibit basal and FK-stimulated AC activity in the hippocampus was significantly lower in the strychnine group than in the control group. The ability of the stable GTP analogue 5'-guanylylimidodiphosphate [Gpp(NH)p] to inhibit FK-stimulated AC activity was also decreased in hippocampal membranes from strychnine-treated rats. These results suggest that the attenuated inhibition of AC by SS in hippocampal membranes from strychnine-treated rats may be caused by decreases in both Gi activity and in the number of SS receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of lindane (gamma-BHC) and related convulsants on GABAA receptor-operated chloride channels in frog dorsal root ganglion neurons

Effects of lindane (gamma-benzenehexachloride; gamma-BHC) on GABA-evoked Cl- current (IGABA) in freshly dissociated frog sensory (dorsal root ganglion) neurons were studied and compared with those of tert-butylbicycloortho benzoate (TBOB) and picrotoxin by the use of the suction-pipette method [13]. Drugs were applied with a rapid drug-application method, "Concentration-clamp" technique. At concentration of GABA of > 3 x 10(-6) M, at least two components of the IGABA were recognized distinct degree of desensitization. Those were defined as the peak and plateau components in the text. At low concentration (3 x 10(-7) M) of gamma-BHC, only the plateau component of IGABA at 10(-5) M were depressed without changing the peak amplitude. While gamma-BHC at high concentration (3 x 10(-5) M) depressed both the peak and plateau current components. The gamma-BHC-induced depression of IGABA seemed to be IGABA-component-dependent. A detailed analysis of the gamma-BHC action in the concentration-response relationship for GABA revealed that the IGABA with strong desensitization was preferentially blocked by gamma-BHC (3 x 10(-5) M). The rate of recovery of the IGABA from gamma-BHC-induced block depended on the concentration of GABA. The lower the concentration of GABA, the slower the recovery. The GABAA receptor Cl- channels were proposed to be classified into two types of the gamma-BHC-sensitive and -resistant ones.(ABSTRACT TRUNCATED AT 250 WORDS)

Penicillin-induced triphasic modulation of GABAA receptor-operated chloride current in frog sensory neuron

Effects of penicillin-G (PCN) on GABA-evoked Cl- current (IGABA) were investigated in freshly dissociated frog sensory neurons by the use of the concentration-clamp technique combined with the suction-pipette method. Under conditions where the internal and external solutions allowed only Cl- permeability, PCN elicited triphasic modulation on IGABA, consisting of two modes of blockade on IGABA and a following rebound (rebound-like transient IGABA). Simultaneously applied PCN and GABA depressed IGABA immediately (phasic blockade), with the depressed IGABA slightly recovering in amplitude to achieve a stable level of blockade (tonic blockade). When a solution containing a mixture or PCN and GABA was quickly replaced by one containing GABA alone, a rebound-like transient Cl- current (IR) was evoked. Each component of the PCN actions on IGABA was PCN- and GABA-concentration-dependent. The reversal potential for each component of the PCN actions on IGABA was close to the chloride equilibrium potential (ECl) calculated using the Nernst equation. The current-voltage (I-V) relations for both the phasic and tonic blockade revealed inward rectification, while I-V curves for the control IGABA and the IR were outwardly rectified. The degree of IGABA-desensitization and the amplitude of the IR correlated well. The data suggest that partial removal of the GABAA receptor-desensitization may result in generation of the IR.

Characterization of two cerebellar binding sites of [3H]Ro 15-4513

Ro 15-4513 (ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H- imidazo[1,5-a][1,4]benzodiazepine-3-carboxylate), a partial inverse agonist of central benzodiazepine receptors, binds to two distinct sites in the cerebellum. The binding to diazepam-sensitive (DZ-S) sites is displaced by different benzodiazepine receptor ligands, whereas the other site is insensitive to benzodiazepine agonists [diazepam-insensitive (DZ-IS)]. The binding of [3H]Ro 15-4513 was studied in pig cerebellar membranes and in receptors solubilized and purified from these. Micromolar concentrations of gamma-aminobutyric acid (GABA) decreased DZ-S binding at both 0 and 37 degrees C, whereas it had no effect on DZ-IS binding at 0 degrees C and was stimulatory at 37 degrees C. The pH profiles of [3H]Ro 15-4513 binding were quite similar in both binding sites in the pH range of 5.5-10.5 but differed at acidic pH values from those reported for flunitrazepam and Ro 15-1788 (flumazenil; ethyl-8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H- imidazol[1,5-a][1,4]benzodiazepine-3-carboxylate) binding in DZ-S sites, suggesting that [3H]Ro 15-4513 does not interact with a histidine residue apparently present in the binding site. Zn2+, Cu2+, Co2+, and Ni2+ enhanced the binding to DZ-S sites, and the first three mentioned also enhanced the binding to DZ-IS sites. [3H]Ro 15-4513 binding activity was solubilized by various detergents. All detergents tested were more efficient in solubilizing DZ-S binding activity. High ionic strength improved especially the solubility of DZ-IS binding activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Penicillin-induced potentiation of glycine receptor-operated chloride current in rat ventro-medial hypothalamic neurones

1. Effects of penicillin G (PCN) on glycine (Gly)-evoked Cl- current (IGly) were investigated in acutely dissociated rat ventro-medial hypothalamic (VMH) neurones by the whole cell mode of patch clamp technique. 2. When PCN was applied simultaneously with Gly, PCN depressed IGly like a Cl- channel blocker. 3. The PCN-induced blocking action was clearly observed at a low PCN concentration (30 u), while the maximal blockade was achieved by 600 u (units per 10 ml) PCN. 4. When tested solution containing both PCN and Gly was quickly substituted with one containing Gly only, a new rebound-like transient current (I(T)) which also passed through Cl- channel, was elicited. 5. The peak amplitude of I(T) induced by PCN at concentrations higher than 100 u was greater than that induced by glycine alone. We termed this phenomenon PCN-induced potentiation of IGly. In all cells tested, PCN potentiated IGly. 6. At a lower PCN concentration below 30 u, I(T) generation was not clear in the presence of 10(-5) M gamma-aminobutyric acid. With PCN a higher concentration than 300 u, I(T) amplitude was greater than that of the original peak IGly. This was observed in 18 neurones out of 21. The maximal amplitude of the I(T) was achieved with 600 u PCN.

Proton-induced sodium current in freshly dissociated hypothalamic neurones of the rat

1. The proton-gated current was investigated in freshly dissociated ventromedial hypothalamic (VMH) neurones from 4-week-old Wistar rats, under whole-cell configuration by the use of the 'concentration-clamp' technique which combines intracellular perfusion with the rapid exchange of external solution within 1-2 ms under a single-electrode voltage-clamp condition. 2. The proton-gated current increased in a sigmoidal fashion as extracellular pH (pHo) decreased. In external solution containing 2 mM-Ca2+, the threshold of current activation was at pHo 6.5, and the maximum response appeared at pHo 4.1-3.9. The dissociation constant (Kd) and Hill coefficient were 10(-4.9) M (pHo = 4.9) and 1.5 respectively. 3. Decreasing extracellular Na+ concentration reduced the proton-gated current. The current reversed direction at the Na+ equilibrium potential (ENa), indicating that it was carried by Na+. 4. The activation phase kinetics of proton-induced current was single exponential. The time constant of activation (tau a) did not have a potential dependence but decreased slightly by decreasing pHo. The inactivation phase kinetics was two-exponential. The time constant of inactivation (tau i) consisted of fast and slow components (tau if and tau is, respectively). Like tau a, both tau if and tau is did not have any potential dependence, but they slightly increased with decreasing pHo. 5. The steady-state inactivation curve, constructed by decreasing pHo from various conditioning pHos to 4.1, revealed that the proton-induced current had a half-maximum inactivation at pHo 6.2. 6. The proton-induced current was suppressed as the extracellular Ca2+ concentration ([Ca2+]o) increased from almost free (0.01 mM) to 80 mM. Increasing [Ca2+]o increased tau a, but slightly decreased both tau if and tau is. 7. Recovery of proton-induced current from complete inactivation of proton-induced current depended on the degree of pHo change. A bigger change in pHo induced faster recovery than a smaller change. 8. External divalent cations inhibited the proton-induced current, and the inhibitory potency was in the order of Mn2+ greater than Co2+ greater than Ca2+ greater than Sr2+ greater than Ba2+ greater than Mg2+. 9. Tetrodotoxin (TTX) at relatively low concentration (less than 10(-7) M) did not inhibit the peak amplitude of the proton-induced current, but at a higher concentration (10(-6) M) it slightly inhibited the peak amplitude of the current and accelerated the inactivation process. Scorpion toxin markedly increased the peak amplitude of the proton-induced current and prolonged the inactivation phase. The tau is was also increased by scorpion toxin in a concentration-dependent manner. Veratridine had no effect on the proton-induced current. 10. The membrane properties of the proton-operated channel were similar to those of the voltage-gated Na+ channel rather than the Ca2+ channel.

On the mechanism of action of picrotoxin on GABA receptor channels in dissociated sympathetic neurones of the rat

1. The mechanism of action of picrotoxin on GABA receptor channels in rat sympathetic neurones has been investigated with whole-cell clamp. In addition, the action of picrotoxin on single GABA channels has been examined in outside-out membrane patches from these cells. 2. Picrotoxin, at concentrations which dramatically reduced the amplitude of whole-cell GABA currents, did not alter the spectral time constants or single-channel conductance estimated by analysis of GABA-activated current noise. This was observed at potentials both negative and positive to the GABA reversal potential (i.e. for both inward and outward GABA currents). In control conditions, the slow and fast time constants from GABA noise were 40 +/- 14 ms and 2 +/- 0.4 ms, while the estimated single-channel conductance was 14 +/- 2 pS. In the presence of picrotoxin, the time constants and estimated single-channel conductance were 41 +/- 5 ms, 2.7 +/- 0.6 ms and 15 +/- 2.3 pS. 3. Picrotoxin did not alter the shape of the whole-cell GABA current-voltage relationship, indicating that the steady-state block was not voltage dependent. The lack of effect of picrotoxin on the GABA noise spectra and the lack of outward rectification makes it unlikely that picrotoxin acts by a simple voltage-dependent (or voltage-independent) channel blocking mechanism. In the presence of picrotoxin the reversal potential for GABA remained at approximately 0 mV in symmetrical chloride. 4. Distributions of total burst durations, obtained from single-channel records with low concentrations of GABA, were fitted with three or four exponential components. Picrotoxin had no consistent effect on the time constants of the total burst length distributions. It also did not alter the amplitude of the main conductance state. However, picrotoxin did reduce the frequency of channel openings. 5. The application of brief ionophoretic pulses of GABA, to cells under whole-cell voltage clamp, revealed that the rate of onset of block by picrotoxin was accelerated in the presence of GABA. In the absence of agonist, picrotoxin produced a more slowly equilibrating block. 6. Our data are consistent with a mechanism whereby picrotoxin binds preferentially to an agonist bound form of the receptor and stabilizes an agonist-bound shut state. This could, for example, mean that picrotoxin enhances the occurrence of a desensitized state or an allosterically blocked state.

Non-competitive inhibition of GABAA responses by a new class of quinolones and non-steroidal anti-inflammatories in dissociated frog sensory neurones

1. The interaction of a new class of quinolone antimicrobials (new quinolones) and non-steroidal anti-inflammatory agents (NSAIDs) with the GABAA receptor-Cl- channel complex was investigated in frog sensory neurones by use of the internal perfusion and 'concentration clamp' techniques. 2. The new quinolones and the NSAIDs (both 10(-6)-10(-5) M) had little effect on the GABA-induced chloride current (ICI) when applied separately. At a concentration of 10(-4) M the new quinolones, and to a lesser degree the NSAIDs, produced some suppression of the GABA response. 3. The co-administration of new quinolones and some NSAIDs (10(-6)-10(-14) M) resulted in a marked suppression of the GABA response. The size of this inhibition was dependent on the concentration of either the new quinolone or the NSAID tested. The inhibitory potency of new quinolones in combination with 4-biphenylacetic acid (BPAA) was in rank order norfloxacin (NFLX) much greater than enoxacin (ENX) greater than ciprofloxancin (CPFX) much greater than ofloxacin (OFLX), and that of NSAIDs in combination with ENX was BPAA much greater than indomethacin = ketoprofen greater than naproxen greater than ibuprofen greater than pranoprofen. Diclofenac, piroxicam and acetaminophen did not affect GABA responses in the presence of ENX. 4. In the presence of ENX or BPAA, there was a small shift to the right of the concentration-response curve for GABA without any effect on the maximum response. However, the co-administration of these drugs suppressed the maximum of the GABA concentration-response curve, indicating a non-competitive inhibition, for which no voltage-dependency was observed.5. Simultaneous administration of ENX and BPAA also suppressed pentobarbitone (PB)-gated Icl. On the other hand, both PB and phenobarbitone reversed the inhibition of GABA-induced Ic, by coadministration of ENX and BPAA.6. The effect on GABAA responses of co-administration of new quinolones and NSAIDs was not via an interaction with benzodiazepine receptors coupled to the GABAA receptor, since this effect was not reversed by Rol5-1788 or diazepam.7. It is concluded that the co-administration of new quinolones and some of the NSAIDs inhibit GABAergic transmission, and could result in convulsions.

Differential blockade of bicuculline and strychnine on GABA- and glycine-induced responses in dissociated rat hippocampal pyramidal cells

The inhibitory effects of bicuculline (BIC) and strychnine (STR) on GABA- and glycine-induced responses were studied in the rat dissociated hippocampal CA1 pyramidal neurons in whole-cell mode by using the conventional patch-clamp technique. Both GABA and glycine elicited inward Cl- currents in a dose-dependent manner and had almost the same maximal responses. The half-maximum dose (Ka) and Hill coefficient were 6.4 microM and 1.1 for the GABA response, and 74 microM and 1.5 for the glycine response. BIC and STR antagonized both GABA and glycine responses in a competitive manner. The blocking potency of BIC and STR on the GABA response was comparable. The half inhibition dose (IC50) was 2.7 microM for BIC and 6.7 microM for STR. STR blocked the glycine response about 3,000 x more effectively than BIC. The IC50 was 28 nM for STR and 100 microM for BIC. The BIC and STR did not have voltage-dependent blocking effects on either GABA or glycine responses. Neither GABA nor glycine showed outward rectification in their current-voltage relationships. The functional role of glycine in the rat hippocampal CA1 region is discussed.

gamma-Aminobutyric acid-induced response in acutely isolated nucleus solitarii neurons of the rat

The gamma-aminobutyric acid (GABA)-induced macroscopic Cl- current (ICl) was investigated in acutely isolated nucleus tractus solitarii (NTS) neurons by a conventional patch-clamp technique combined with a rapid drug application method. The GABA- and muscimol-induced ICl increased in a concentration-dependent manner. The reversal potentials were close to the Cl- equilibrium potential. Pentobarbital sodium (PB) itself elicited a current. Bicuculline (BIC), strychnine (STR), picrotoxin, benzylpenicillin (PCG), Cd2+, and Zn2+ suppressed the GABA response in a concentration-dependent manner. Both BIC and STR shifted the concentration-response curve for GABA response to the right, whereas PCG suppressed the maximum response without affecting the threshold, indicating that BIC and STR antagonized competitively and PCG noncompetitively. The inhibitory action of PCG on GABA response was in a highly voltage-dependent manner. PB shifted the concentration-response curve for GABA response to the left. The augmentatory effect of PB was voltage dependent.

Strychnine-induced potassium current in isolated dorsal root ganglion cells of the rat

1. Effects of strychnine (Str) on the dissociated dorsal root ganglion (DRG) cells of the rat have been investigated in whole-cells configuration by a conventional patch-clamp technique. 2. gamma-Aminobutyric acid (GABA)-induced Cl- current (ICl) increased sigmoidally with increasing concentration. The half-maximal response (Ka) was 3 x 10(-5) M and the Hill coefficient was 1.5. Both Str and bicuculline inhibited the GABA-induced ICl in a concentration-dependent manner. 3. Str itself could elicit the current at concentrations over 10(-5) M, at which concentrations the GABA response was completely suppressed. The concentration-response curve for the Str-induced current was bell-shaped, and a nearly maximum response occurred at 3 x 10(-4) M. A transient 'hump' current appeared immediately after the wash-out of external solution containing high concentrations of Str over 3 x 10(-4) M. 4. The Str-induced outward current and a transient 'hump' current were augmented by the removal of extracellular K+ and were suppressed by the substitution of intracellular K+ for Cs+. But the current was not sensitive to extracellular Na+, Ca2+ and Cl-. 5. The reversal potential of Str-induced current (EStr) was -75 mV, which was close to the K+ equilibrium potential (EK = -76.3 mV). The change of EStr for a ten fold change in extracellular K+ concentration was 58 mV, indicating that the membrane behaves like a K+ electrode in the presence of Str. The reversal potential of the 'hump' current was also close to EK. 6. The Str-induced outward current was antagonized by K+ channel blockers such as Ba2+, tetraethylammonium (TEA)-chloride, and 4-aminopyridine (4-AP) in a concentration-dependent manner. 7. The Str-induced K+ current was not affected by internal perfusion of bis(gamma-aminophenoxy)ethane-N, N,N',N-tetraacetic acid (BAPTA), indicating that the Str response does not result in the activation of K+ conductance by the intracellular Ca2+.

Differential effect of zinc on the vertebrate GABAA-receptor complex

1. gamma-Aminobutyric acid (GABA) responses were recorded from rat superior cervical ganglia (SCG) in culture using the whole cell recording technique. 2. Zinc (50-300 microM) reversibly antagonized the GABA response in embryonic and young post-natal neurones, while neurones cultured from adult animals were far less sensitive and occasionally resistant to zinc blockade. Cadmium (100-300 microM) also antagonised the GABA response, while barium (100 microM-2 mM) was ineffective. 3. The differential blocking effect of zinc on cultured neurones of different ages also occurred in intact SCG tissue. 4. The GABA log dose-response curve constructed with foetal or adult cultured neurones was reduced in a non-competitive manner by zinc. This inhibition was minimally affected by the membrane potential. 5. The GABA response recorded intracellularly from guinea-pig pyriform cortical slices was enhanced by zinc (300-500 microM), which occurred concurrently with a decrease in the input conductance of the cell. The enhancement was unaffected by prior blockade of the GABA uptake carrier by 1 mM nipecotic acid. This phenomenon could be reproduced by barium (300 microM) and cadmium (300 microM). 6. We conclude that the vertebrate neuronal GABAA-receptor becomes less sensitive to zinc with neural (GABAA-receptor?) development, and the enhanced GABA response recorded in the CNS is a consequence of the reduction in the input conductance and not due to a direct effect on the receptor complex.

Influences of external Ca2+ on the GABA-induced chloride current and the efficacy of diazepam in internally perfused frog sensory neurons

The effects of extracellular Ca2+ on the gamma-aminobutyric acid (GABA)-induced Cl- current and the efficacy of diazepam in the facilitation of GABA response were studied in frog isolated sensory neurons, using a 'concentration clamp' technique which combines a suction pipette (internal perfusion and voltage clamp) and a rapid drug application system. When nominal Ca2+-free external solution was changed to the solution containing 2 mM Ca2+, the response elicited by 1 x 10(-5) M GABA was reduced by about 40% of the control obtained in nominal Ca2+-free solution. The dose-response curve for GABA was shifted to the right without affecting the maximum response. It can be suggested that the application of external Ca2+ modulates the affinity of the GABA receptor to its agonist, GABA. Diazepam at the concentration of 3 x 10(-6) M shifted the dose-response curve for GABA to the left without changing the maximum response with or without external Ca2+. However, the augmentatory action of diazepam on the GABA response was reduced in the presence of external Ca2+. Possible mechanisms for inhibitory action of external Ca2+ on the GABA-gated response and the reduced efficacy of diazepam are discussed.

In vitro electrophysiological analysis of mature rat hippocampal transplants in oculo

We have investigated the maturation of isolated rat hippocampus grafted into the anterior chamber of the eye. Electrophysiological responses from transplants were compared to those recorded from the in vitro hippocampal slice preparation. Intracellular recording demonstrated that the passive membrane characteristics of intraocular hippocampal neurons were similar to those of the CA1 pyramidal cells in the in vitro slice preparation. However, the slow after-hyperpolarization which normally follows depolarization-induced action potentials was reduced or completely absent in the intraocular transplants, and the excitatory postsynaptic potential (EPSP) evoked by local stimulation was prolonged. The duration of the EPSP was reduced by perfusion with D-aminophosphonovaleric acid (2.5-50 microM), an N-methyl-D-aspartate receptor antagonist. Normal levels of glutamate decarboxylase (a marker for gamma-aminobutyric acidergic neurons) were found in the transplants, and responses to adenosine, bicuculline, and norepinephrine were similar in the in oculo transplants and in vitro slices. The data suggest that although many properties of hippocampal neurons are intrinsically determined, other aspects of the physiology of mature hippocampus either fail to develop, or develop abnormally in the absence of external inputs in oculo.

Nerve endings from rat brain tissue release copper upon depolarization. A possible role in regulating neuronal excitability

Membrane vesicles from rat cerebral cortex were prepared and the functional response of the GABAA receptor was followed by monitoring GABA-activated influx of the radiotracer 36Cl- ion. CuCl2 decreased GABA-activated 36Cl- influx into synaptosomal membrane vesicles. The effect of Cu2+ was concentration dependent (5-500 microM CuCl2) and occurred with saturating (1 mM) as well as low (30 microM) GABA concentrations. A similar inhibition of the responses to muscimol (30 microM) was also observed with 50 microM CuCl2. In addition, release of copper from cortical synaptosomes and median eminence was followed by atomic absorption technique. An increased release of copper into the extracellular space was observed upon depolarization with 50 mM K+. A minimal concentration of copper was estimated to be 100 microM in the synaptic cleft. These findings suggest that copper may play a role in regulating neuronal excitability.

Desensitization kinetics of a chloride acetylcholine response in Aplysia

The kinetics of desensitization of acetylcholine-evoked Cl conductance increased response of Aplysia RC neurons of the abdominal ganglion were studied under voltage-clamp conditions for comparison with results of similar studies on acetylcholine Na and K responses. The response evoked by acetylcholine on RC neurons was an outward current at resting potential (about -45 mV) that reversed at about -65 mV and was blocked by D-tubocurarine and strychnine but not hexamethonium and was not activated by arecoline. From the current-voltage relation this response can be ascribed to a pure conductance increase to Cl. The apparent KD was 40.6 microM. Upon prolonged exposure to acetylcholine the response peaked within 200-400 ms, and then decayed to a plateau current in the continued presence of the agonist. The peak and plateau currents reversed at the same potential, indicating that there had not been significant redistribution of Cl. The current decay in every cell was best fit by a double exponential function plus a constant, and the average time constants were tau fast = 1.8 +/- 0.2 s and tau slow = 16.2 +/- 1.0 s. Both components were slowed by cooling. While tau fast did not change with dose, tau slow increased with dose. Both components accelerated with hyperpolarization and upon application of trifluoperazine (2 microM). These results are consistent with the interpretation that desensitization of the acetylcholine Cl response is composed of two independent processes. This conclusion is the same as that derived from studies of the acetylcholine Na and K responses, and is in general consistent with desensitization being a property of a common acetylcholine receptor, and independent of the ionic selectivity of the associated channel. There are, however, significant differences in voltage, temperature and trifluoperazine dependence of the two components of the three ionic responses which may reflect influence of the different ion channels and/or transduction mechanisms.

Epileptiform discharges evoked in hippocampal brain slices from epileptic patients

We have recorded from diseased hippocampal tissue which was surgically removed from epileptic patients for therapeutic purposes. When the perforant path was stimulated at a low frequency (1 Hz), the number of population spikes evoked in the dentate gyrus increased by a factor of as great as 8 during a 15 s train. This effect was transient. A similar epileptiform discharge could be generated in normal rat hippocampal brain slices by the same stimulus paradigm, but only in the presence of a low concentration (0.2 microM) of bicuculline. These results suggest that this frequency-dependent epileptiform discharge, evoked in the dentate gyrus of epileptic patients, may be due to a small reduction in GABAA-mediated inhibition and may involve factors that lead to the initiation of seizures.

Pregnenolone sulfate antagonizes GABAA receptor-mediated currents via a reduction of channel opening frequency

Our previous study showed antagonism of GABAA receptor-mediated whole-cell currents by pregnenolone sulfate (PS). Here, the effects of PS, picrotoxin (PTX) and pentobarbital (PB) were tested on GABA-activated single Cl- channels recorded from membrane patches of rat cortical neurons in primary cultures. PS and PTX selectively decreased the opening frequency of the channels, while PB increased mean open time and burst duration without affecting opening frequency. It is suggested that PS and PTX may antagonize GABAA receptor function through the same mechanism and/or the same binding site.