GABA receptor mechanisms in the central nervous system

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.

Interactions of etifoxine with the chloride channel coupled to the GABA(A) receptor complex

This study examined the nature of the interactions of etifoxine, an anxiolytic and anticonvulsant compound, with the GABA(A) receptor/chloride channel complex. In membrane preparations of Sprague-Dawley rat cerebral cortex, etifoxine competitively inhibited the binding of [35S]t-butylbicyclophosphoro-thionate (TBPS), a specific ligand of the GABA(A) receptor chloride channel site. In vivo studies demonstrated an anticonvulsant effect of etifoxine (50 and 75 mg/kg, i.p.) against the clonic convulsions induced by TBPS in CD1 mice. Flumazenil (10 and 40 mg/kg, i.p.), an antagonist of benzodiazepine sites at GABA(A) receptors, had no effect on the action of etifoxine. These findings suggest that etifoxine exerts its effect by interacting with the Cl- channel of GABA(A) receptors and probably by facilitating GABAergic inhibition.

Prevention of muscimol-induced long-term depression by brain-derived neurotrophic factor

1. The authors have recently reported a new protocol for inducing long-term depression through activation of GABAA receptors in the hippocampal slices. This long-term depression is reversed by bicuculline and potentiated by neurosteroids such as alphaxalone. It was also shown that glutamate receptor activity is not involved in the induction of this novel type of long-term depression. Brain derived neurotrophic factor is a member of the neurotrophins family widely expressed in the central nervous system. There is increasing evidence that indicate an important role for brain-derived neurotrophic factor in synaptic plasticity. It has been reported that brain-derived neurotrophic factor level is downregulated by GABA system. The present study investigated a possible relation between muscimol-induced long-term depression and brain-derived neurotrophic factor level. 2. Extracellular recordings were made in the CA1 pyramidal cell layer of rat hippocampal slices following orthodromic stimulation of Schaffer collateral fibers in stratum radiatum. 3. It was observed that brain-derived neurotrophic factor at concentration that did not have any effect itself on the population spike, prevents the induction of long-term depression by muscimol. In addition to this, K-252a an inhibitor of Trk type kinase blocked the prevention of muscimol-induced LTD by brain-derived neurotrophic factor. 4. The results suggest that there is an interaction between muscimol-induced long-term depression and brain-derived neurotrophic factor and may explain the post receptor mechanism of muscimol-induced long-term depression through a bilateral relation between GABAA activity and brain-derived neurotrophic factor.

GABAA-Dependent chloride influx modulates GABAB-mediated IPSPs in hippocampal pyramidal cells

The relationship between postsynaptic inhibitory responses [the fast GABA(A)-mediated inhibitory postsynaptic potential (IPSP) and the slow GABA(B)-mediated IPSP] were investigated in hippocampal CA3 pyramidal cells. Mossy fiber-evoked GABA(B)-mediated IPSPs were, paradoxically, of greater amplitude in cells with resting membrane potential of -62 mV (13.6 +/- 0.5 mV; mean +/- SE) as compared with cells with resting membrane potential of -54 mV (7.0 +/- 0.8 mV). In addition, when a cell's membrane potential was artificially manipulated, GABA(B)-mediated IPSPs were reduced at relatively depolarized levels (-55 mV) and enhanced at relatively hyperpolarized potentials (at least -60 mV). In contrast, the preceding GABA(A)-mediated IPSPs were larger at the more positive membrane potentials and smaller as the cell was hyperpolarized. Similar voltage dependency was obtained when monosynaptic GABA(A)- and GABA(B)-mediated IPSPs were isolated in the presence of glutamatergic receptor antagonists. However, monosynaptic GABA(B)-mediated IPSPs isolated in the presence of glutamatergic and GABA(A) receptor antagonists were not reduced at the more positive membrane potentials, and were significantly larger in amplitude than GABA(B)-mediated IPSPs preceded by a monosynaptic GABA(A)-mediated IPSP. The amplitude of the isolated monosynaptic GABA(B)-mediated IPSPs recorded with potassium chloride-containing microelectrodes was significantly smaller than the comparable potential recorded with potassium acetate microelectrodes without chloride. We conclude that voltage-dependent chloride influx, via GABA(A) receptor-gated channels, modulates postsynaptic GABA(B)-mediated inhibition in hippocampal CA3 pyramidal cells.

Three GABA receptor-mediated postsynaptic potentials in interneurons in the rat lateral geniculate nucleus

Inhibition is crucial for the thalamus to relay sensory information from the periphery to the cortex and to participate in thalamocortical oscillations. However, the properties of inhibitory synaptic events in interneurons are poorly defined because in part of the technical difficulty of obtaining stable recording from these small cells. With the whole-cell recording technique, we obtained stable recordings from local interneurons in the lateral geniculate nucleus and studied their inhibitory synaptic properties. We found that interneurons expressed three different types of GABA receptors: bicuculline-sensitive GABA(A) receptors, bicuculline-insensitive GABA(A) receptors, and GABA(B) receptors. The reversal potentials of GABA responses were estimated by polarizing the membrane potential. The GABA(A) receptor-mediated responses had a reversal potential of approximately -82 mV, consistent with mediation via Cl(-) channels. The reversal potential for the GABA(B) response was -97 mV, consistent with it being a K(+) conductance. The roles of these GABA receptors in postsynaptic responses were also examined in interneurons. Optic tract stimulation evoked a disynaptic IPSP that was mediated by all three types of GABA receptors and depended on activation of geniculate interneurons. Stimulation of the thalamic reticular nucleus evoked an IPSP, which appeared to be mediated exclusively by bicuculline-sensitive GABA(A) receptors and depended on the activation of reticular cells. The results indicate that geniculate interneurons form a complex neuronal circuitry with thalamocortical and reticular cells via feed-forward and feedback circuits, suggesting that they play a more important role in thalamic function than thought previously.

Late vagal inhibition in neurons of the ventrolateral medulla oblongata in the rat

Stimulation of cervical vagal afferents elicits long-lasting inhibitory effects in a variety of neuronal populations, although little is known concerning the cellular mechanisms that are involved in these effects. In the present study, the electrophysiological characteristics of responses elicited by cumulative activation of vagal afferents were examined in neurons of the rostral ventrolateral medulla oblongata, which play an important role in the coordination of cardiovascular and other visceral activities. The study has focused on the late-onset, slow inhibitory component of vagal responses, which is likely to affect the temporal modulation of postsynaptic effects. Vagal stimulation elicited four distinct response patterns in intracellularly penetrated neurons (n = 78): excitation, inhibition, excitation-inhibition and inhibition-inhibition. The late inhibitory component was encountered in 43 (55%) of the cells, including five putative medullospinal neurons. It was due to a postsynaptic hyperpolarization which reversed at potentials more negative than -83 mV. The voltage dependency, as well as the average onset latency (93+/-3.0 ms), duration (270+/-16.5 ms) and amplitude (1.3+/-0.2 mV as measured at resting membrane potentials), of late inhibition were clearly different from those of the short-latency inhibitory response. The differences in the voltage dependency and time-course of the short-latency responses and the late inhibition indicate that they are mediated by different central relays. In the majority of neurons, late inhibition could be elicited by stimulating only myelinated vagal afferents. The magnitude of the response was, however, significantly enhanced in 63% of the examined cells when the intensity of stimulation was raised to recruit further myelinated and non-myelinated fibres. This indicates that late vagal inhibition is often elicited by a cumulative activation of convergent afferent inputs. The intracellularly labelled vagally responsive neurons were present at all rostrocaudal levels of the rostral ventrolateral medulla, with an accumulation in the region of the lateral paragigantocellular nucleus. Neurons that exhibited late vagal inhibition were dominant in the juxtafacial region of this nucleus. Due to its slow time-course, late vagal inhibition may contribute to a tonic modulation of the activity of neurons in the rostral ventrolateral medulla oblongata. It is proposed that late vagal inhibition plays an important role in the temporal integration of sensory inputs in neurons of the rostral ventrolateral medulla oblongata. The time-course and strength of this modulatory effect are related to the level of activity in those visceral sensory inputs that converge onto the inhibitory interneurons that mediate late inhibition to rostral ventrolateral medulla oblongata neurons.

Postnatal development of GABAA receptor beta1, beta2/3, and gamma2 immunoreactivity in the rat retina

GABA (gamma-aminobutyric acid) is the major inhibitory neurotransmitter in the mammalian central nervous system and plays an important role in neuronal physiology during ontogenesis. The distribution of the beta1-, beta2/3-, and gamma2-subunit of the GABAA receptor in the rat retina was studied during postnatal development using immunohistochemical methods. All subunits were found at birth. However, each subunit showed a unique staining pattern with a different local distribution. The immunoreactivity pattern changed during the time course of postnatal development for each of the proteins investigated. A clustered distribution at presumptive synaptic sites as indicated by a punctate staining pattern of the inner plexiform layer was detected as early as the second day of postnatal development. However, diffuse staining of presumptive extrasynaptic sites was found throughout development. The typical adult layering of immunoreactivity into distinctive bands appeared later in development, characteristically in the second postnatal week. The results of the present study suggest that GABAA receptor expression precedes the formation of functional synapses and changes along with cellular differentiation of the rat retina. Developmentally regulated changes in GABAA receptor composition and distribution indicate possible functions for this receptor during retinal ontogeny.

A furosemide-sensitive K+-Cl- cotransporter counteracts intracellular Cl- accumulation and depletion in cultured rat midbrain neurons

Efficacy of postsynaptic inhibition through GABAA receptors in the mammalian brain depends on the maintenance of a Cl- gradient for hyperpolarizing Cl- currents. We have taken advantage of the reduced complexity under which Cl- regulation can be investigated in cultured neurons as opposed to neurons in other in vitro preparations of the mammalian brain. Tightseal whole-cell recording of spontaneous GABAA receptor-mediated postsynaptic currents suggested that an outward Cl- transport reduced dendritic [Cl-]i if the somata of cells were loaded with Cl- via the patch pipette. We determined dendritic and somatic reversal potentials of Cl- currents induced by focally applied GABA to calculate [Cl-]i during variation of [K+]o and [Cl-] in the patch pipette. [Cl-]i and [K+]o were tightly coupled by a furosemide-sensitive K+-Cl- cotransport. Thermodynamic considerations excluded the significant contribution of a Na+-K+-Cl- cotransporter to the net Cl- transport. We conclude that under conditions of normal [K+]o the K+-Cl- cotransporter helps to maintain [Cl-]i at low levels, whereas under pathological conditions, under which [K+]o remains elevated because of neuronal hyperactivity, the cotransporter accumulates Cl- in neurons, thereby further enhancing neuronal excitability.

A molecular model for the synergic interaction between gamma-aminobutyric acid and general anaesthetics

Within the context of the discussion about rational polytherapy, we determined the effects of four anaesthetics on the binding of [3H]t-butylbicycloorthobenzoate ([3H]TBOB) to the GABA(A) receptor complex in the presence of several concentrations of GABA (gamma-aminobutyric acid), in order to build a molecular model that can describe and quantify the interactions between the compounds. The empirical isobole method revealed that GABA and the anaesthetics acted synergically in displacing [3H]TBOB. This synergy could be described by a simple molecular model in which both GABA and the anaesthetics displaced [3H]TBOB allosterically and in which GABA allosterically enhanced the binding of the anaesthetics. To get information about the interaction between GABA and anaesthetics, we used [3H]TBOB as a tracer ligand. The model indicated that GABA enhanced the affinity of thiopental 3.0-fold, propofol 5.0-fold, the neuroactive steroids Org 20599 3.5-fold and Org 20549 13-fold. Insight into the molecular mechanism and strength of these interactions can help clinicians to choose therapeutically optimal drug and dose combinations: a step towards rational polytherapy.

An update on GABAA receptors

Recent advances in molecular biology and complementary information derived from neuropharmacology, biochemistry and behavior have dramatically increased our understanding of various aspects of GABAA receptors. These studies have revealed that the GABAA receptor is derived from various subunits such as alpha1-alpha6, beta1-beta3, gamma1-gamma3, delta, epsilon, pi, and rho1-3. Furthermore, two additional subunits (beta4, gamma4) of GABAA receptors in chick brain, and five isoforms of the rho-subunit in the retina of white perch (Roccus americana) have been identified. Various techniques such as mutation, gene knockout and inhibition of GABAA receptor subunits by antisense oligodeoxynucleotides have been used to establish the physiological/pharmacological significance of the GABAA receptor subunits and their native receptor assemblies in vivo. Radioligand binding to the immunoprecipitated receptors, co-localization studies using immunoaffinity chromatography and immunocytochemistry techniques have been utilized to establish the composition and pharmacology of native GABAA receptor assemblies. Partial agonists of GABAA receptors are being developed as anxiolytics which have fewer and less severe side effects as compared to conventional benzodiazepines because of their lower efficacy and better selectivity for the GABAA receptor subtypes. The subunit requirement of various drugs such as anxiolytics, anticonvulsants, general anesthetics, barbiturates, ethanol and neurosteroids, which are known to elicit at least some of their pharmacological effects via the GABAA receptors, have been investigated during the last few years so as to understand their exact mechanism of action. Furthermore, the molecular determinants of clinically important drug-targets have been investigated. These aspects of GABAA receptors have been discussed in detail in this review article.

Bicuculline-resistant, Cl- dependent GABA response in the rat spinal dorsal horn

Receptors for gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the mammalian central nervous system (CNS), have been divided into three subtypes. GABA(A) receptor is a ligand-gated chloride channel that is competitively antagonized by bicuculline, whereas GABA(B) receptor regulate Ca2+ or K+ channels through G proteins. Recently, GABA(C) receptor has been identified in mammalian and fish retina. Unlike GABA(A) receptors, the GABA(C) receptor is a bicuculline-resistant chloride channel that is selectively activated by cis-4-aminocrotonic acid (CACA), and antagonized by imidazole-4-acetic acid (I4AA) and to some extent by picrotoxin. We report here that bicuculline-resistant GABA responses mediated by chloride channels are also expressed in substantia gelatinosa (SG) neurons in the dorsal horn, which receive predominantly nociceptive inputs from periphery. The GABA responses are, however, not mimicked by CACA nor affected by I4AA, but abolished by picrotoxin. Moreover, these responses are modulated by benzodiazepines (flunitrazepam) and barbiturates (thiopental), although GABA(C) responses are not affected. Thus, the pharmacological characteristics of the GABA responses observed in SG neurons are distinct from those responses mediated by the known GABA receptors. These differences may reflect the presence of receptor subunits unique to SG neurons.

Dopamine receptor subtypes modulate olfactory bulb gamma-aminobutyric acid type A receptors

The gamma-aminobutyric acid type A (GABAA) receptor is the predominant Cl- channel protein mediating inhibition in the olfactory bulb and elsewhere in the mammalian brain. The olfactory bulb is rich in neurons containing both GABA and dopamine. Dopamine D1 and D2 receptors are also highly expressed in this brain region with a distinct and complementary distribution pattern. This distribution suggests that dopamine may control the GABAergic inhibitory processing of odor signals, possibly via different signal-transduction mechanisms. We have observed that GABAA receptors in the rat olfactory bulb are differentially modulated by dopamine in a cell-specific manner. Dopamine reduced the currents through GABA-gated Cl- channels in the interneurons, presumably granule cells. This action was mediated via D1 receptors and involved phosphorylation of GABAA receptors by protein kinase A. Enhancement of GABA responses via activation of D2 dopamine receptors and phosphorylation of GABAA receptors by protein kinase C was observed in mitral/tufted cells. Decreasing or increasing the binding affinity for GABA appears to underlie the modulatory effects of dopamine via distinct receptor subtypes. This dual action of dopamine on inhibitory GABAA receptor function in the rat olfactory bulb could be instrumental in odor detection and discrimination, olfactory learning, and ultimately odotopic memory formation.

Antagonism by (1,2,5,6-tetrahydropyridine-4-yl) methylphosphinic acid of synaptic transmission in the neonatal rat spinal cord in vitro: an electrophysiological study

The effect of the novel GABAc receptor antagonist (1,2,5,6-tetrahydropyridine-4-yl)methyl-phosphinic acid (TPMPA) on synaptic transmission and GABA-mediated responses was investigated with electrophysiological recordings from the in vitro spinal cord preparation of the neonatal rat. Bath-applied TPMPA (10 microM) had no effect on spinal reflexes evoked by dorsal root stimulation, on ventral root polarization level or amplitude of ventral root depolarizations induced by exogenously applied GABA (0.5 mM). TPMPA significantly attenuated the depressant action of GABA on spinal reflexes without changing responses induced by the GABA(A) receptor agonist isoguvacine (50 microM) or the GABA(B) receptor agonist baclofen (0.5-2 microM). Following block of GABA(A) receptors by bicuculline (20 microM) and of glycine receptors by strychnine (1 microM), regular bursting activity recorded from ventral roots developed spontaneously and persisted unchanged for many hours. This bursting pattern, which is generated at the level of the interneuronal network, was significantly slowed down by TPMPA, which also increased the duration of individual bursts and the number of intraburst oscillations. These results suggest that in the neonatal rat spinal cord some functional GABAc receptors exist: their role was clearly unmasked following pharmacological block of GABA(A) (and glycine) receptors. Under these conditions GABAc receptors appeared to contribute to the excitation of spinal interneurons supporting rhythmic bursting activity.

GABA(A) receptor modulates the activity of inner hair cell afferents in guinea pig cochlea

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) is mediated by two main categories of receptors: the GABA(A) and GABA(B) receptor. Recent immunocytochemical and electron microscopical studies revealed the existence of GABA at the efferent olivocochlear innervation of the guinea pig cochlea. In this microiontophoretic study we examined the effect of GABA on spontaneous and glutamate or acetylcholine induced activity of afferent fibres in the dendritic region of inner hair cells. Furthermore, the receptor subtypes being responsible for this GABA action were analysed using specific agonists and antagonists on alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartic acid (NMDA) induced activity. The spike activities of the subsynaptic area were recorded in the third or fourth turn of the cochlea of anaesthetised guinea pigs. Application of GABA had little effect on spontaneous activity whereas the glutamate or acetylcholine induced firing rate could be depressed by GABA. AMPA and NMDA induced activity was reduced by the GABA(A) agonist muscimol but not by the GABA(B) agonist baclofen. The GABA(A) antagonist blocked the inhibition of both GABA and the GABA(A) agonist. In contrast, the GABA(B) antagonist saclofen was without effect. These results reveal that GABA reduces the activated firing rate of inner hair cell afferents mediated by the GABA(A) receptor subtype.

Neurotransmitter activation of inwardly rectifying potassium current in dissociated hippocampal CA3 neurons: interactions among multiple receptors

We characterized potassium current activated by G-protein-coupled receptors in acutely dissociated hippocampal CA3 neurons. Agonists for serotonin, adenosine, and somatostatin receptors reliably activated a potassium-selective conductance that was inwardly rectifying and that was blocked by 1 mM external Ba2+. The conductance had identical properties to that activated by GABAB receptors in the same cells. In one-half of the CA3 neurons that were tested, the metabotropic glutamate agonist 1S,3R-ACPD also activated inwardly rectifying Ba2+-sensitive potassium current. Activation of the current by serotonin and adenosine agonists occurred with a time constant of 200-700 msec after a lag of 50-100 msec; on removal of agonist the current deactivated with a time constant of 1-2 sec after a lag of 200-400 msec. These kinetics are similar to GABAB-activated current and consistent with a direct action of G-protein on the channels. For somatostatin, both activation and deactivation were approximately fourfold slower, probably limited by agonist binding and unbinding. The half-maximally effective agonist concentrations were approximately 75 nM for somatostatin, approximately 100 nM for serotonin, and approximately 400 nM for 2-chloroadenosine. Dose-response relationships had Hill coefficients of 1.2-1.9, suggesting cooperativity in the receptor-to-channel coupling mechanism. At saturating concentrations of agonists, the combined application of baclofen and either somatostatin, serotonin, or 2-chloroadenosine produced effects that were subadditive and often completely occlusive. However, at subsaturating concentrations the effects of baclofen and 2-chloroadenosine were supra-additive. Thus, low levels of different transmitters can act synergistically in activating inwardly rectifying potassium current.

The acetylcholine, GABA, glutamate triangle in the rat forebrain

The present overview demonstrates that stress, fear, novelty, and learning processes are associated with arousal and increases of extracellular levels of cortical and hippocampal ACh, independently of increases of motor activity. Forebrain cholinergic systems appears to be regulated by GABAergic and glutamatergic inputs. However, several other neurotransmitter systems play a role.

Modulation by substance P of synaptic transmission in the mouse hippocampal slice

The modulatory action of substance P on synaptic transmission of CA1 neurons was studied using intra- or extracellular recording from the mouse hippocampal slice preparation. Bath-applied substance P (2-4 microM) or the selective NK1 receptor agonist substance P methylester (SPME, 10 nM-5 microM) depressed field potentials (recorded from stratum pyramidale) evoked by focal stimulation of Schaffer collaterals. This effect was apparently mediated via NK1 receptors since it was completely blocked by the selective NK1 antagonist SR 140333. The field potential depression by SPME was significantly reduced in the presence of bicuculline. Intracellular recording from CA1 pyramidal neurons showed that evoked excitatory postsynaptic potentials (EPSPs) and evoked inhibitory postsynaptic potentials (IPSPs) were similarly depressed by SPME, which at the same time increased the frequency of spontaneous GABAergic events and reduced that of spontaneous glutamatergic events. The effects of SPME on spontaneous and evoked IPSPs were prevented by the ionotropic glutamate receptor blocker kynurenic acid. In tetrodotoxin (TTX) solution, no change in either the frequency of spontaneous GABAergic and glutamatergic events or in the amplitude of responses of pyramidal neurons to 4 microM alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or 10 microM N-methyl-D-aspartate (NMDA) was observed. On the same cells, SPME produced minimal changes in passive membrane properties unable to account for the main effects on synaptic transmission. The present data indicate that SPME exerted its action on CA1 pyramidal neurons via a complex network mechanism, which is hypothesized to involve facilitation of a subset of GABAergic neurons with widely distributed connections to excitatory and inhibitory cells in the CA1 area.

The diversity of GABAA receptors. Pharmacological and electrophysiological properties of GABAA channel subtypes

The amino acid gamma-aminobutyric-acid (GABA) prevails in the CNS as an inhibitory neurotransmitter that mediates most of its effects through fast GABA-gated Cl(-)-channels (GABAAR). Molecular biology uncovered the complex subunit architecture of this receptor channel, in which a pentameric assembly derived from five of at least 17 mammalian subunits, grouped in the six classes alpha, beta, gamma, delta, sigma and epsilon, permits a vast number of putative receptor isoforms. The subunit composition of a particular receptor determines the specific effects of allosterical modulators of the GABAARs like benzodiazepines (BZs), barbiturates, steroids, some convulsants, polyvalent cations, and ethanol. To understand the physiology and diversity of GABAARs, the native isoforms have to be identified by their localization in the brain and by their pharmacology. In heterologous expression systems, channels require the presence of alpha, beta, and gamma subunits in order to mimic the full repertoire of native receptor responses to drugs, with the BZ pharmacology being determined by the particular alpha and gamma subunit variants. Little is known about the functional properties of the beta, delta, and epsilon subunit classes and only a few receptor subtype-specific substances like loreclezole and furosemide are known that enable the identification of defined receptor subtypes. We will summarize the pharmacology of putative receptor isoforms and emphasize the characteristics of functional channels. Knowledge of the complex pharmacology of GABAARs might eventually enable site-directed drug design to further our understanding of GABA-related disorders and of the complex interaction of excitatory and inhibitory mechanisms in neuronal processing.

Distinct GABAB actions via synaptic and extrasynaptic receptors in rat hippocampus in vitro

Intracellular recordings were obtained from pyramidal cells to examine gamma-aminobutyric acid-B (GABAB)-mediated synaptic mechanisms in the CA1 region of rat hippocampal slices. To investigate if heterogeneous ionic mechanisms linked to GABAB receptors originate from distinct sets of inhibitory fibers, GABAB-mediated monosynaptic late inhibitory postsynaptic potentials (IPSPs) were elicited in the presence of antagonists of ionotropic glutamate and GABAA receptors and of an inhibitor of GABA uptake and were compared after direct stimulation of inhibitory fibers in three different CA1 layers: stratum oriens, radiatum, and lacunosum-moleculare. No significant differences were found in mean amplitude, rise time, or time to decay to half-amplitude of IPSPs evoked from the three layers. Mean equilibrium potential (Erev) of late IPSPs was similar for all groups and close to the equilibrium potential of K+. Bath application of the GABAB antagonist CGP55845A blocked all monosynaptic late IPSPs. During recordings with micropipettes containing guanosine-5'-O-(3-thiotriphosphate) (GTPgammaS), the mean amplitude of all GABAB IPSPs gradually was reduced. Bath application of Ba2+ completely eliminated monosynaptic late IPSPs evoked from any of the stimulation sites. Late IPSPs were blocked completely during Ba2+ applications that reduced the GABAB-mediated hyperpolarizations elicited by local application of exogenous GABA only by approximately 50%. These results indicate that heterogenous K+ conductances activated by GABAB receptors do not originate from separate sets of inhibitory fibers in these layers. To examine if synchronous release of GABA from a larger number of inhibitory fibers could activate heterogeneous GABAB mechanisms, giant GABAB IPSPs were induced by 4-aminopyridine (4-AP) in the presence of antagonists of ionotropic glutamate and GABAA receptors. The amplitude and time course 4-AP-induced late IPSPs were approximately double that of evoked monosynaptic late IPSPs, but their voltage sensitivity, Erev, and antagonism by the GABAB antagonist CGP55845A and intracellular GTPgammaS were similar. Ba2+ completely abolished 4-AP-induced late IPSPs, whereas responses elicited by exogenous GABA were only reduced by approximately 50% in the same cells. These results indicate that synchronous activation of large numbers of inhibitory fibers, as induced by 4-AP, may not activate heterogenous GABAB-mediated conductances. Similarly, Ba2+ almost completely blocked late inhibitory postsynaptic currents evoked by stimulus trains. Overall, our results show that exogenous GABA can activate heterogenous K+ conductances via GABAB receptors, but that GABA released synaptically, either by electrical stimulation or 4-AP application, can only activate K+ conductances homogeneously sensitive to Ba2+. Thus GABAB receptors located at synaptic and extrasynaptic sites on hippocampal pyramidal cells may be linked to distinct K+ conductances.

A major locus for autosomal recessive retinitis pigmentosa on 6q, determined by homozygosity mapping of chromosomal regions that contain gamma-aminobutyric acid-receptor clusters

Retinitis pigmentosa (RP) is the most common inherited retinal dystrophy, with extensive allelic and nonallelic genetic heterogeneity. Autosomal recessive RP (arRP) is the most common form of RP worldwide, with at least nine loci known and accountable for approximately 10%-15% of all cases. Gamma-aminobutyric acid (GABA) is the major inhibitory transmitter in the CNS. Different GABA receptors are expressed in all retinal layers, and inhibition mediated by GABA receptors in the human retina could be related to RP. We have selected chromosomal regions containing genes that encode the different subunits of the GABA receptors, for homozygosity mapping in inbred families affected by arRP. We identify a new locus for arRP, on chromosome 6, between markers D6S257 and D6S1644. Our data suggest that 10%-20% of Spanish families affected by typical arRP could have linkage to this new locus. This region contains subunits GABRR1 and GABRR2 of the GABA-C receptor, which is the effector of lateral inhibition at the retina.

Molecular composition of GABAC receptors

In the central nervous system inhibitory neurotransmission is primarily achieved through activation of receptors for gamma-aminobutyric acid (GABA). Three types of GABA receptors have been identified on the basis of their pharmacology and electrophysiology. The predominant type, termed GABAA and a recently identified type, GABAC, have integral chloride channels, whereas GABAB receptors couple to separate K+ or Ca2+ channels via G-proteins. By analogy to nicotinic acetylcholine receptors, native GABAA receptors are believed to be heterooligomers of five subunits, drawn from five classes (alpha, beta, gamma, delta, epsilon/chi). An additional class, called rho, is often categorized with GABAA receptor subunits due to a high degree of sequence similarity. However, rho subunits are capable of forming functional homooligomeric and heterooligomeric receptors, whereas GABAA receptors only express efficiently as heterooligomers. Intriguingly, the pharmacological properties of receptors formed from rho subunits are very similar to those exhibited by GABAC receptors and rho subunits and GABAC responses have been colocalized to the same retina cells, indicating that rho subunits are the sole components of GABAC receptors. In contrast, the propensity of GABAA receptor and rho subunits to form multimeric structures and their coexistence in retinal cells suggests that GABAC receptors might be heterooligomers of rho and GABAA receptor subunits. This review will summarize our current understanding of the molecular composition of GABAC receptors based upon studies of rho subunit assembly.

GABA-Induced intrinsic light-scattering changes associated with voltage-sensitive dye signals in embryonic brain stem slices: coupling of depolarization and cell shrinkage

We have found new evidence for gamma-aminobutyric acid (GABA)-induced intrinsic optical changes associated with a voltage-sensitive dye signal in the early embryonic chick brain stem slice. The slices were prepared from 8-day-old embryos, and they were stained with a voltage-sensitive dye (NK2761). Pressure ejection of GABA to one site within the preparation elicited optical changes. With 580-nm incident light, two components were identified in the GABA-induced optical change. The first component was wavelength dependent, whereas the second, slower change was independent of wavelength. Comparison with the known action spectrum of the dye indicates that the first component reflects a depolarization of the membrane and that the second, slow component is a light-scattering change resulting from cell shrinkage coupled with the depolarization. Similar optical changes also were induced by glycine, although the amplitude of both the first and second signals was much smaller than for GABA. The optical changes induced by GABA persisted in the presence of picrotoxin and 2-hydroxysaclofen, suggesting that these optical responses include a novel GABA response, which has been termed GABAD in our previous reports.

Release of endogenous glutamate, aspartate, GABA, and taurine from hippocampal slices from adult and developing mice under cell-damaging conditions

The releases of endogenous glutamate, aspartate, GABA and taurine from hippocampal slices from 7-day-, 3-, 12-, and 18-month-old mice were investigated under cell-damaging conditions using a superfusion system. The slices were superfused under hypoxic conditions in the presence and absence of glucose and exposed to hydrogen peroxide. In the adult hippocampus under normal conditions the basal release of taurine was highest, with a response only about 2-fold to potassium stimulation (50 mM). The low basal releases of glutamate, aspartate, and GABA were markedly potentiated by K+ ions. In general, the release of the four amino acids was enhanced under all above cell-damaging conditions. In hypoxia and ischemia (i.e., hypoxia in the absence of glucose) the release of glutamate, aspartate and GABA increased relatively more than that of taurine, and membrane depolarization by K+ markedly potentiated the release processes. Taurine release was doubled in hypoxia and tripled in ischemia but K+ stimulation was abolished. In both the mature and immature hippocampus the release of glutamate and aspartate was greatly enhanced in the presence of H2O2, that of aspartate particularly in developing mice. In the immature hippocampus the increase in taurine release was 10-fold in hypoxia and 30-fold in ischemia, and potassium stimulation was partly preserved. The release processes of the four amino acids in ischemia were all partially Ca2+-dependent. High concentrations of excitatory amino acids released under cell-damaging conditions are neurotoxic and contribute to neuronal death during ischemia. The substantial amounts of the inhibitory amino acids GABA and taurine released simultaneously may constitute an important protective mechanism against excitatory amino acids in excess, counteracting their harmful effects. In the immature hippocampus in particular, the massive release of taurine under cell-damaging conditions may have a significant function in protecting neural cells and aiding in preserving their viability.

Ontogenesis of presynaptic GABAB receptor-mediated inhibition in the CA3 region of the rat hippocampus

gamma-Aminobutyric acid-B(GABAB) receptor-dependent and -independent components of paired-pulse depression (PPD) were investigated in the rat CA3 hippocampal region. Intracellular and whole cell recordings of CA3 pyramidal neurons were performed on hippocampal slices obtained from neonatal (5-7 day old) and adult (27-34 day old) rats. Electrical stimulation in the hilus evoked monosynaptic GABAA postsynaptic currents (eIPSCs) isolated in the presence of the ionotropic glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) and D(-)2-amino-5-phosphovaleric acid (-AP5, 50 microM) with 2(triethylamino)-N-(2,6-dimethylphenyl) acetamine (QX314) filled electrodes. In adult CA3 pyramidal neurons, when a pair of identical stimuli was applied at interstimulus intervals (ISIs) ranging from 50 to 1,500 ms the amplitude of the second eIPSC was depressed when compared with the first eIPSC. This paired-pulse depression (PPD) was partially blocked by P-3-aminoprophyl -P-diethoxymethylphosphoric acid (CGP35348, 0.5 mM), a selective GABAB receptor antagonist. In neonates, PPD was restricted to ISIs shorter than 200 ms and was not affected by CGP35348. The GABAB receptor agonist baclofen reduced the amplitude of eIPSCs in a dose-dependent manner with the same efficiency in both adults and neonates. Increasing the probability of transmitter release with high Ca2+ (4 mM)/low Mg2+ (0.3 mM) external solution revealed PPD in neonatal CA3 pyramidal neurons that was 1) partially prevented by CGP35348, 2) independent of the membrane holding potential of the recorded cell, and 3) not resulting from a change in the reversal potential of GABAA eIPSCs. In adults the GABA uptake blocker tiagabine (20 microM) increased the duration of eIPSCs and the magnitude of GABAB receptor-dependent PPD. In neonates, tiagabine also increased duration of eIPSCs but to a lesser extent than in adult and did not reveal a GABAB receptor-dependent PPD. These results demonstrate that although GABAB receptor-dependent and -independent mechanisms of presynaptic inhibition are present onGABAergic terminals and functional, they do not operate at the level of monosynaptic GABAergic synaptic transmission at early stages of development. Absence of presynaptic autoinhibition of GABA release seems to be due to the small amount of transmitter that can access presynaptic regulatory sites.

Changes in intracellular calcium concentration affect desensitization of GABAA receptors in acutely dissociated P2-P6 rat hippocampal neurons

The whole cell configuration of the patch-clamp technique was used to study the effects of different cytosolic calcium concentrations [Ca2+]i on desensitization kinetics of gamma-aminobutyric acid (GABA)-activated receptors in acutely dissociated rat hippocampal neurons. Two different intrapipette concentrations of the calcium chelator 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA; 11 and 0.9 mM, respectively) were used to yield a low (1.2 x 10(-8) M) or a high (2.2 x 10(-6) M) [Ca2+]i. In low [Ca2+]i, peak values of GABA-evoked currents (20 microM) evoked at -30 mV, were significantly larger than those recorded in high calcium [2,970 +/- 280 (SE) pA vs. 1,870 +/- 150 pA]. The extent of desensitization, assessed from steady-state to peak ratio was significantly higher in high calcium conditions (0.14 +/- 0.007 vs. 0.11 +/- 0.008). Similar effects of -Ca2+-i on desensitization were observed with GABA (100 microM). Recovery from desensitization, measured at 30 s interval with double pulse protocol was significantly slower in high [Ca2+]i than in low [Ca2+]i (54 +/- 3% vs. 68 +/- 2%). The current-voltage relationship of GABA-evoked currents was linear in the potential range between -50 and 50 mV. The kinetics of desensitization process including the rate of onset, extent of desensitization, and recovery were voltage independent. The run down of GABA-evoked currents was faster with the higher intracellular calcium concentration. The run down process was accompanied by changes in desensitization kinetics: in both high and low [Ca2+]i desensitization rate was progressively increasing with time as the slow component of the desensitization onset was converted into the fast one. In excised patches, the desensitization kinetics was much faster and more profound than in the whole cell configuration, indicating the involvement of intracellular factors in regulation of this process. In conclusion, [Ca2+]i affects the desensitization of GABAA receptors possibly by activating calcium-dependent enzymes that regulate their phosphorylation state. This may lead to modifications in cell excitability because of changes in GABA-mediated synaptic currents.

GABA, GAD, and GABA(A) receptor alpha4, beta1, and gamma1 subunits are expressed in the late embryonic and early postnatal neocortical germinal matrix and coincide with gliogenesis

Increasing evidence indicates that the classical, fast-acting neurotransmitter gamma-amino butyric acid (GABA) may initially act as morphogen in cell proliferation and differentiation via specific receptors. In view of the potential roles for GABA in central nervous system development, we examined the expression of GABA, GABA(A) receptor beta1 and gamma1 subunits by immunocytochemistry and the expression of transcripts for two GABA-synthesizing enzymes, glutamate decarboxylase (GAD65, GAD67 mRNAs), and for alpha4, beta1, and gamma1 subunits of GABA(A) receptor by in situ hybridization in the developing neocortex. Tissue sections were taken from embryonic days (E) 17 and E20 embryos and newborn rats (P0). The embryos' mothers and newborn rats had been injected with 5-bromo-2'-deoxyuridine (BrdU) and had survived for 2 hours. At E17, BrdU-positive cells were largely restricted in the synthetic zone at the ventricular margin when cortical neurogenesis was still active. GAD mRNAs and GABA immunoreactivity were detected in the subventricular zone, while alpha4, beta1, and gamma1 subunits were abundant in the ventricular zone. At E20 and P0, when neurogenesis had largely ceased and gliogenesis had commenced, BrdU-positive cells were found throughout the ventricular zone with GABA, GAD mRNAs, and alpha4, beta1, and gamma1 subunits. GABA, GAD mRNAs and alpha4, beta1, and gamma1 subunit signals intensified in the ventricular zone from E17 to P0 as gliogenesis proceeded. Thus, specific components of a putative GABAergic circuit are expressed in cells of the ventricular zone during the late embryonic/early postnatal period coincident with gliogenesis, suggesting a role for GABA in glial cell proliferation.

The effects of strychnine, bicuculline, and ketamine on 'immersion-inhibited' dorsal horn convergent neurons in intact and spinalized rats

In both intact and spinalized rats, this study examined the effects of strychnine (a glycine antagonist), bicuculline (a GABAA antagonist), and ketamine (a non-competitive NMDA receptor antagonist) on one particular class of lumbar dorsal horn convergent neurons. This group of convergent neurons are inhibited when a rat's entire ipsilateral hindpaw is immersed in 50 degrees C water and has a strong afterdischarge as soon as the paw is removed from the water. Strychnine (2 mg/kg, iv) increased ongoing activity and blocked the 'inhibition phase' in both intact and spinalized rats demonstrating that a spinal-related glycine mechanism was involved in the inhibition. However, only in intact rats did the firing rate of the 'afterdischarge phase' increase significantly from pre-drug levels, suggesting that supraspinal sites may be involved in modulating this phase. Ketamine (15 mg/kg, iv) depressed ongoing activity and the firing rate in the afterdischarge phase of these neurons. Additionally, ketamine reversed the strychnine-induced increase in ongoing activity. Bicuculline (2 mg/kg, iv) had no effect on the activity of this cell class. As shown previously, and replicated here, these 'immersion-inhibited' neurons invariably have both inhibitory and excitatory mechano-receptive fields on the ipsilateral hindpaw. Thus, the response of this class of convergent neurons to noxious stimulation may be a function of relative inputs of glycine and EAA's, each possibly triggered by the stimulation of different receptive fields/regions on the same paw. Furthermore, when both fields are co-stimulated during noxious immersion of the entire paw, glycine has a stronger influence on activity than does the EAA's.

Activation of muscarinic receptors during blockade of GABA(A)-mediated inhibition induces synchronous epileptiform activity in immature rat hippocampus

We investigated the effects of the cholinergic agonist carbachol (25 microM) on the synaptic potentials recorded extracellularly and intracellularly from the CA3 area of immature hippocampal slices of the rat (postnatal days 10-20). In control conditions, carbachol reduced the amplitude of evoked synaptic responses (n=8) and did not induce any spontaneous synchronous activity (n=12); the depressant effect of carbachol was mimicked by acetylcholine (100 microM, in eserine 10 microM, n=5) and was reversed by the muscarinic antagonist atropine (1 microM, n=2). The GABA(A)-receptor antagonist bicuculline (10 microM) enhanced the amplitude and duration of the evoked synaptic responses and induced infrequent (0.016-0.045 Hz) spontaneous synchronous discharges in 23/37 of the slices. Application of carbachol in the presence of bicuculline reduced the amplitude of the evoked synaptic responses (n=21) and in addition induced synchronous discharges with rates of occurrence 0.075-0.225 Hz, in 64/68 slices. Both effects were mimicked by acetylcholine and eserine, and antagonized by atropine. The specific muscarinic antagonists pirenzepine (M1-type), tripitramine (M2-type), 4-diphenylacetoxy-N-methylpiperidine methiodide (M3-type) and tropicamide (M4-type) (all tested at 0.1-1 microM) reversibly reduced the frequency of synchronous carbachol-induced discharges. In addition, these discharges were reversibly blocked by high Ca2+ perfusion medium (7 mM CaCl2, n=4) and by the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM, n=7). Synchronous epileptiform discharges were recorded from both CA1 and CA3 areas in intact slices (n=3), but only from CA3 following disruption of the CA1-CA3 synaptic connections (n=3). These experiments suggest that activation of muscarinic receptors during blockade of GABA(A)-mediated potentials, may enhance synchronous epileptiform activity in immature (postnatal days 10-20) hippocampus, through activation of local excitatory circuits and that endogenous acetylcholine may be sufficient to play this role.

Membrane excitability and secretion from peptidergic nerve terminals

1. Thin slices of the posterior pituitary can be used as a preparation for the study of biophysical mechanisms underlying neuropeptide secretion. Patch-clamp techniques in this preparation have revealed the properties of ion channels that control the excitability of the nerve terminal membrane and have clarified the relation between Ca2+ and exocytosis. 2. Repetitive electrical activity at high frequencies broadens action potentials to allow more Ca2+ entry and thus enhance exocytosis. Action potential broadening results from the inactivation of a voltage-dependent K+ channel. 3. When repetitive electrical activity is sustained, secretion is depressed. This depression can be attributed in part to action potential failure caused by the opening of a Ca(2+)-activated K+ channel. This channel can be modulated by protein kinases, phosphatases, and G-proteins. 4. The inhibitory neurotransmitter GABA activates a GABAA receptor in the nerve terminal membrane. The gating of the associated Cl- channel depolarizes the membrane slightly to inactivate voltage-gated Na+ channels and block action potential propagation. 5. The response of the nerve terminal GABAA receptor is enhanced by neuroactive steroids and this can potentiate the inhibition of neurosecretion by GABA. The action of neurosteroids at this site could play a role in changes in neuropeptide secretion associated with reproductive transitions. 6. Ca2+ channels in the nerve terminal membrane are inactivated by sustained depolarization and by trains of brief pulses. Ca2+ entry promotes Ca2+ channel inactivation during trains by inhibiting the recovery of Ca2+ channels from inactivation. The inactivation of Ca2+ channels can play a role in defining the optimal frequency and train duration for evoking neuropeptide secretion. 7. Measurements of membrane capacitance in peptidergic nerve terminals have revealed rapid exocytosis and endocytosis evoked by Ca2+ entry through voltage-gated Ca2+ channels. Exocytosis is too rapid to account for the delays in neuropeptide secretion evoked by trains of action potentials. Endocytosis sets in rapidly after exocytosis with a time course comparable to that of the rapid endocytosis observed in nerve terminals at rapid synapses. Our results support the finding in rapid synaptic nerve terminals that endocytosis is inhibited by intracellular Ca2+. Multiple pools of vesicles were revealed, and these pools may reflect different stages in the mobilization and release of neuropeptide.

Inhibitory control of plateau properties in dorsal horn neurones in the turtle spinal cord in vitro

1. The role of inhibition in control of plateau-generating neurones in the dorsal horn was studied in an in vitro preparation of the spinal cord of the turtle. Ionotropic and metabotropic inhibition was found to condition the expression of plateau potentials. 2. Blockade of gamma-aminobutyric acid (GABAA) and glycine receptors by their selective antagonists bicuculline (10-50 microM) and strychnine (5-20 microM) enhanced the excitatory response to stimulation of the dorsal root and facilitated the expression of plateau potentials. 3. Bicuculline and strychnine also facilitated the generation of plateau potentials in response to depolarizing current pulses, suggesting the presence of tonic ionotropic inhibitory mechanisms in turtle spinal cord slices. 4. Activation of GABAB receptors also inhibited plateau-generating neurones. The selective agonist baclofen (5-50 microM) inhibited wind-up of the response to repeated depolarizations induced synaptically or by intracellular current pulses. 5. Baclofen reduced afferent synaptic input. This effect was not affected by bicuculline or strychnine and was blocked by the selective GABAB receptor antagonist 2-hydroxysaclofen (2-OH-saclofen, 100-400 microM). 6. Postsynaptically, baclofen inhibited plateau properties. Activation of GABAB receptors produced a hyperpolarization (7.0 +/- 0.5 mV, mean +/- S.E.M., n = 29) with an associated decrease in input resistance (22.7 +/- 3.1%, n = 24). These effects were blocked by extracellular Ba2+ (1-2 mM). 7. When the baclofen-induced hyperpolarization and shunt were compensated for by adjusting the bias current and the strength of the stimulus, baclofen still inhibited generation of plateau potentials. Wind-up and after-discharges were also inhibited by baclofen. These effects remained in the presence of tetrodotoxin (1 microM) and were antagonized by 2-OH-saclofen. 8. The inhibition of plateau properties was observed even when the baclofen-induced hyperpolarization and shunt were blocked by Ba2+ and when potassium channels were blocked by Ba2+ (3 mM), tetraethylammonium (TEA, 15 mM) and apamin (0.25-0.5 microM). The baclofen-sensitive component of the plateau potential was reduced by nifedipine (10 microM), suggesting a modulation of postsynaptic L-type Ca2+ channels. 9. We suggest that inhibitory regulation of plateau properties plays a role in somatosensory processing in the dorsal horn. The inhibitory control of wind-up and after-discharges may be particularly significant in physiological and therapeutic control of central sensitization to pain.

Changes in [K+]o evoked by baclofen in guinea pig hippocampus

K+-sensitive microelectrodes were used to record changes evoked by baclofen in extracellular potassium concentration ([K+]o) and field potentials in the stratum pyramidale (SP) and stratum radiatum (SR) in the CA1b region of guinea pig hippocampal slices in vitro. Bath applications of ( ±)-baclofen (1 µM - 3 mM for approx 5 min) evoked changes in [K+]o, which were in most cases sustained throughout agonist application and reversed during washout. The maximal (Rmax) values for curves fitted to the concentration-response data were for SP and SR, respectively, 0.59 ± 0.03 and 0.65 ± 0.03 mM, and EC50 values were 39.7 and 39.4 µM, respectively. The evoked K+ and field potential changes were significantly correlated and could be blocked by 2-OH-saclofen (50 µM) and CGP 35348 (50 µM). In <= 10% of experiments baclofen (10-50 µM) induced either a decrease or a transient increase ( <= 1 min duration) in [K+]o; in some slices with concentrations >=20 µM an initial decrease preceded a progressive increase. Pressure ejection of baclofen (100 µM for 100-900 ms) evoked increases in [K+]o and field potentials, which were larger in SR than in SP. In <= 10% of slices brief and (or) sustained application of baclofen (by either bath perfusion or pressure ejection) also evoked synchronous, repetitive interictal and ictal discharges at frequencies approx 1/s and 1/12 s, respectively, an observation that affirms a proconvulsant capacity. It is concluded that (i) although increases in [K+]o evoked by baclofen in SR compared with SP are slightly larger, they are not significantly different, (ii) GABAB receptor subtype(s) in SR and SP appear similar, as they have identical affinities, and (iii) [K+]o accumulations evoked by GABA likely include a contribution from a GABAB receptor activated K+ conductance, especially in dendritic regions.Key words: brain slices, stratum pyramidale, stratum radiatum, GABAB receptors, ion-selective microelectrodes, epileptiform activity.

Cloning and characterization of mouse GABA(C) receptor subunits

Retinal gamma-aminobutyric acid type C (GABA[C]) receptors are believed to consist of rho subunits. The pharmacological properties of GABA(C) receptors appear to vary among different species. Since the mouse is an important model for gene targeting and behavioral studies, this study was aimed at cloning rho subunits in the mouse. Here we report the isolation of two full length cDNAs corresponding to mouse rho1 and rho2 subunits. In contrast to the rat, both subunits in mouse are able to form homomeric receptors, but only rho2 receptors are insensitive to picrotoxinin. Mouse rho1 and rho2 also co-assemble to form heteromeric receptors. These findings form the basis for further study on GABA(C) receptors using the mouse model.

Functional GABA(A) receptors on human glioma cells

Glioma cells in acute slices and in primary culture, and glioma-derived human cell lines were screened for the presence of functional GABA(A) receptors. Currents were measured in whole-cell voltage clamp in response to gamma-aminobutyric acid (GABA). While cells from the most malignant glioma, the glioblastoma multiforme, did not respond to GABA, an inward current (under our experimental conditions with high Cl- concentration in the pipette) was induced in gliomas of lower grades, namely in 71% of oligodendroglioma cells and in 62% of the astrocytoma cells. Glioma cell lines did not express functional GABA(A) receptors, irrespective of the malignancy of the tumour they originate from. The currents elicited by application of GABA were due to activation of GABA(A) receptors; the specific agonist muscimol mimicked the response, the antagonists bicuculline and picrotoxin blocked the GABA-activated current and the benzodiazepine receptor agonist flunitrazepam augmented the GABA-induced current and the benzodiazepine inverse agonist DMCM decreased the GABA current. Cells were heterogeneous with respect to the direction of the current flow as tested in gramicidin perforated patches: in some cells GABA hyperpolarized the membrane, while in the majority it triggered a depolarization. Moreover, GABA triggered an increase in [Ca2+]i in the majority of the tumour cells due to the activation of Ca2+ channels. Our results suggest a link between the expression of GABA receptors and the growth of glioma cells as the disappearance of functional GABA(A) receptors parallels unlimited growth typical for malignant tumours and immortal cell lines.

GABA-gated Cl- channels in the rat retina

gamma-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the mammalian retina, and its physiological action is well established. GABA receptors have been localized immunocytochemically in the retina of different mammalian species, and all major retinal cell types have been found to express GABAA receptor subunits. Recently, a new type of GABA receptor with pharmacological and electrophysiological properties different from the known GABAA and GABAB receptors, has been described. These GABAC receptors are found predominantly in the vertebrate retina. This review concentrates on the electrophysiological characterization of GABA receptors expressed by amacrine and bipolar cells of the rat retina. We recorded GABA-induced currents from cultured neonatal amacrine and bipolar cells as well as from isolated bipolar cells of adult animals. While amacrine cells contain a homogeneous population of GABAA receptors, bipolar cells exhibit both GABAA and GABAC responses. Although both receptors gate chloride-selective ion channels, their biophysical and pharmacological properties differ markedly. These functional differences and the cellular distribution of GABAA and GABAC receptors suggest that they have different inhibitory functions in the rat retina.

Partial hippocampal kindling decreases efficacy of presynaptic GABAB autoreceptors in CA1

The effect of partial hippocampal kindling, a model of temporal lobe seizure, on monosynaptic inhibition mediated by GABA was studied. Kindled rats were given 15 nonconvulsive hippocampal afterdischarges, and control rats were given low frequency or no stimulations. At 1-2 d after kindling, paired-pulse depression (PPD) of the IPSCs recorded in CA1 neurons in vitro was significantly smaller in kindled as compared with control rats. The difference in PPD persisted for at least 21 d after kindling. The decrease in PPD of the IPSCs after partial hippocampal kindling was likely caused by a reduced GABA autoinhibition after downregulation of presynaptic GABAB receptors. The GABAB antagonist CGP35348 (1 mM) suppressed PPD of the IPSCs more strongly in control than in kindled rats. Direct activation of the presynaptic GABAB receptors by baclofen suppressed the monosynaptic IPSCs significantly more in control than in kindled rats. The decay rate of a single-pulse IPSC was faster in kindled than in control rats on day 1 or day 21 after partial kindling. The difference in IPSC decay between kindled and control rats was found with or without a GABAB receptor antagonist. The low efficacy of the presynaptic GABAB receptors in kindled rats may provide compensatory stabilization of the postsynaptic membrane against further seizures or plasticity.

GABA receptors of bipolar cells from the skate retina: actions of zinc on GABA-mediated membrane currents

GABA receptors of bipolar cells from the skate retina: actions of zinc on GABA-mediated membrane currents. J. Neurophysiol. 78: 2402-2412, 1997. gamma-Aminobutyric acid (GABA)-induced currents were recorded from isolated bipolar cells of the skate retina using perforated patch-clamp methodology. Pharmacological analysis of the responses, using selective agonists and antagonists of the major classes of GABA receptor, revealed the presence of both GABAA and GABAC receptors at both the dendrites and axon terminals of the bipolar cells. The two receptor types showed very different reactions to zinc, a divalent metallic cation that was detected in the synaptic terminal region of skate photoreceptors. Currents mediated by the activation of GABAC receptors were down-regulated by zinc, a feature that is typical of the action of zinc on GABAC receptors. On the other hand, the effects of zinc on GABAA receptor-mediated activity was highly dependent on zinc concentration. Unlike the GABAA receptors on other neurons, responses mediated by activation of the GABAA receptor of skate bipolar cells were significantly enhanced by zinc concentrations in the range of 0. 1-100 mu M; at higher concentrations of zinc (>100 mu M), response amplitudes were suppressed below control levels. The enhancement of GABAA receptor activity on skate bipolar cells showed little voltage dependence, suggesting that zinc is acting on the extracellular domain of the GABAA receptor. In the presence of 10 mu M zinc, the dose-response curve for 4,5,6, 7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP; a GABAA agonist that suppresses GABAC-activated currents) was shifted to the left of the curve obtained in the absence of zinc, but without a significant change in the response maximum. This finding indicates that the enhancing effect of zinc is due primarily to its ability to increase the sensitivity of the GABAA receptor. The novel enhancement of neuronal GABAA receptor activity by zinc, observed previously in the GABAA-mediated responses of skate Müller (glial) cells, may reflect the presence of a unique subtype of GABAA receptor on the bipolar and Müller cells of the skate retina.

The attenuation of kainate-induced neurotoxicity by chlormethiazole and its enhancement by dizocilpine, muscimol, and adenosine receptor agonists

Systemically administered kainate (10 mg.kg-1) caused neuronal loss in both the hippocampus and the entorhinal regions of the rat brain. This resulted in a loss of 68.3 +/- 13.8 and 53.3 +/- 12.8% of pyramidal neurones in the hippocampal CA1 and CA3a regions, respectively. Chlormethiazole attenuated the loss of neurones in the hippocampal cell layers CA1 (cell loss 10 +/- 3.2%) and CA3a (cell loss 10 +/- 7.7%). The neuroprotective activity of chlormethiazole was apparent in the presence or absence of a low dose of clonazepam (200 micrograms.kg-1 i.p.). The kainate-induced damage could also be measured by the increase in binding of the peripheral benzodiazepine ligand ([3H]PK11195) in the hippocampus. In kainate-treated rats there was a 350-500% increase in binding indicative of reactive gliosis. Chlormethiazole prevented this elevation in a dose- and time-dependent manner, with an ED50 of 10.64 mg.kg-1 and an effective therapeutic window from 1 to 4 h posttreatment. Dizocilpine also attenuated damage significantly. The GABAA agonist muscimol was also able to attenuate the increase in [3H]PK11195 binding in a dose-dependent manner, with an ED50 of approximately 0.1 mg.kg-1. If muscimol, dizocilpine, or the adenosine A1 receptor agonist R-N6-phenylisopropyl-adenosine were administered together with chlormethiazole at their respective ED25 doses, a potentiation was apparent in the degree of neuroprotection. It is concluded that the combination of neuroprotective agents with different mechanisms of action can lead to a synergistic protection against excitotoxicity.

The role of the GABA(A) receptor alpha1 subunit N-terminal extracellular domain in propofol potentiation of chloride current

Propofol (2,6-diisopropylphenol), an intravenous general anesthetic in active clinical use today, potentiates the action of gamma-aminobutyric acid (GABA) at the type-A receptor and also directly induces current in the absence of GABA. We expressed different combinations of murine GABA(A) receptor alpha1, beta3 and gamma2 subunits in Xenopus oocytes to investigate the subunit dependence of propofol potentiation of pentobarbital-induced current. Pentobarbital induces current in all beta3-subunit-containing receptors, whereas current gating by GABA requires the presence of both alpha1 and beta3 subunits. Therefore, pentobarbital rather than GABA was used to induce current in order to separate the subunit dependence of current gating from the subunit dependence of potentiating action of propofol. alpha1beta3gamma2, alpha1beta3, beta3gamma2, or beta3 subunit combinations all responded to pentobarbital in a dose-dependent manner. True potentiation was defined as the current magnitude to simultaneous application of pentobarbital and propofol exceeding the additive responses to individual drug applications. A dose-dependent propofol potentiation of pentobarbital-induced current was observed in oocytes injected with alpha1beta3 or alpha1beta3gamma2 but not in beta3gamma2 or beta3 subunits, suggesting that the alpha1 subunit was necessary for this modulatory action of propofol. Further examination of the propofol potentiation in chimeras between the alpha1 and beta3 subunits showed that the extracellular amino-terminal half of the alpha1 subunit was sufficient to support propofol potentiation. The different requirements of the receptor structure for the agonistic (gating) and the potentiating actions suggest that these two actions of propofol are distinct processes mediated through its action at distinct sites.

Action of zolpidem on responses to GABA in relation to mRNAs for GABA(A) receptor alpha subunits within single cells: evidence for multiple functional GABA(A) isoreceptors on individual neurons

The relationship between zolpidem sensitivity and GABA(A) receptor alpha subunits was studied in individual dissociated neurons from rat brain. Using whole-cell recording, similar EC50 values were demonstrated for the effect of gamma-aminobutyric acid (GABA) on gated-chloride currents from substantia nigra reticulata (SNR) and lateral septal neurons. Subsequently, many neurons from both the SNR or lateral septum were found to exhibit enhanced GABA-gated chloride currents across concentrations of zolpidem ranging from 10 to 300 nM. Some neurons exhibited a greater than 20% increase in responsiveness to GABA at 30 nM of zolpidem without further increase at higher concentrations of zolpidem. Conversely, zolpidem enhancement of GABA from another group of neurons was not observed at 30 nM zolpidem, but between 100 and 300 nM the response to GABA increased greater than 20%. Finally, a third group of neurons reached both of these criteria for zolpidem enhancement of GABA. This latter spectrum of responses to GABA after varying concentrations of zolpidem was consistent with the presence of either two GABA(A) receptors or a single receptor with differing affinities for zolpidem on an individual neuron. Following determination of the sensitivity of neurons from SNR or lateral septum to zolpidem, cytoplasm was extracted from some individual cells to allow identification of cellular mRNAs for the alpha1, alpha2 and alpha3 GABA(A) receptor subunits with RT-PCR. Those neurons that responded to the 30 nM zolpidem concentration invariably expressed the alpha1-GABA(A) receptor subunit. This result is consistent with the GABA(A) alpha1-receptor subunit being an integral part of a functional high-affinity zolpidem type 1-BZD receptor complex on neurons in brain. Those neurons which showed enhancement of GABA from 100 to 300 nM zolpidem contained mRNAs for the alpha2 and/or the alpha3 receptor subunits, a finding consistent with these alpha subunits forming type 2-BZD receptors. Some individual dissociated SNR neurons were sensitive to both low and high concentrations of zolpidem and contained mRNAs for all three alpha-receptor subunits. These latter individual neurons are proposed to have at least two functional GABA(A) receptor subtypes. Thus, the present investigation emphasizes the importance of characterizing the relationship between endogenous GABA(A) receptor function and the presence of specific structural components forming GABA(A) receptor subtypes on neurons.

Effects of benzodiazepine receptor agonists in neurones acutely dissociated from the rat neostriatum

To understand the properties of benzodiazepine receptor in the neostriatum, we examined the potentiating effects of diazepam, triazolam and brotizolam on the GABA(A) receptor-mediated Cl- current in dissociated rat neostriatal neurones using the nystatin-perforated patch recording configuration. Neurones were classified into large and small neurones, on their somatic size. In the large neurones, which are putative cholinergic interneurones, all the benzodiazepine receptor agonists recognized a single effective site. However, in the small neurones, which are mostly considered to be projecting neurones, the effect of brotizolam was best described when two effective sites were assumed. Therefore, the properties of benzodiazepine receptor differed among large and small neurones while at least two kinds of functional binding sites were also found to exist in small neurones.

GABA-ergic transmission in deep cerebellar nuclei

gamma-Aminobutyric acid (GABA) is the inhibitory transmitter released at Purkinje cell axon terminals in deep cerebellar nuclei (DCN). Neurons in DCN also receive excitatory glutamatergic inputs from the inferior olive. The output of DCN neurons, which depends on the balance between excitation and inhibition on these cells, is involved in cerebellar control of motor coordination. Plasticity of synaptic transmission observed in other areas of the mammalian central nervous system (CNS) has received wide attention. If GABA-ergic and/or glutamatergic synapses in DCN also undergo plasticity, it would have major implications for cerebellar function. In this review, literature evidence for GABA-ergic synaptic transmission in DCN as well as its plasticity are discussed. Studies indicate that fast inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) in neurons of DCN are mediated by GABAA receptors. While GABAB receptors are present in DCN, they do not appear to be activated by Purkinje cell axons. The IPSPs undergo paired-pulse, as well as frequency-dependent, depressions. In addition, tetanic stimulation of inputs can induce a long-term depression (LTD) of the IPSPs and IPSCs. Excitatory synapses do not appear to undergo long-term potentiation or LTD. The LTD of the IPSP is not input-specific, as it can be induced heterosynaptically and is associated with a reduced response of DCN neurons to a GABAA receptor agonist. Postsynaptic Ca2+ and protein phosphatases appear to contribute to the LTD. The N-methyl-D-aspartate receptor-gated, as well as the voltage-gated Ca2+ channels are proposed to be sources of the Ca2+. It is suggested that LTD of GABA-ergic transmission, by regulating DCN output, can modulate cerebellar function.

Optical characterization of a novel GABA response in early embryonic chick brainstem

To examine the functional expression of embryonic GABA receptors, the inhibitory effects were studied of GABA (GABA responses) on the excitatory postsynaptic potentials evoked by vagal stimulus in seven- to 10-day-old embryonic chick brainstem slice preparations. A multiple-site optical recording technique was used, with a multiple element photodiode array system and a fast voltage-sensitive merocyanine-rhodanine dye (NK2761). First, in the GABA response, three components were pharmacologically identified: component 1, related to GABA(A) receptors; component 2, related to GABA(B) receptors; and component 3 which is insensitive to GABA(A) and GABA(B) antagonists, but is stimulated by both GABA(A) and GABA(B) agonists. Subsequently. the embryogenesis and early development of the three components were investigated, and early developmental maps of regional distribution patterns of the three components were constructed. Components 1 and 3 have already emerged in the seven-day-old embryonic brainstem preparation; component 2 appeared in the eight-day-old preparations. No component related to GABA(C) receptors was observed in the seven- to 10-day-old embryonic stages. From the pharmacological properties of component 3, we suggest that it is related to a new subtype, the GABA(D) receptor.

Inhibition of voluntary activity by thalamic stimulation in humans: relevance for the control of tremor

The motor effects of stimuli delivered through four-channel, quadripolar macroelectrodes chronically implanted in the ventrolateral thalamus were studied in 20 awake cooperating human subjects. Single stimuli could inhibit voluntary contraction of the contralateral first dorsal interosseous muscle (FDI) for up to 200 ms. The inhibition was often followed by a rebound facilitation or by oscillatory activity. This inhibition appeared to arise from the ventrolateral thalamus and could not be obtained in other patients by stimulation of the periventricular grey matter (PVG), the globus pallidus internus (GPI), or the subthalamic nucleus (STN). The neural elements activated by the stimulus had a short chronaxie and a short refractory period, implying that they were large-diameter axons. Similar effects were obtained from each of the four electrodes in the row, suggesting that this fiber system lay parallel rather than perpendicular to the implanted macroelectrode. The inhibition resulting from a single stimulus was diminished by a prior stimulus or train of stimuli. A continuous train of stimuli produced inhibition for only the first 200 ms. We propose that the thalamic stimulus activates a neural network which includes thalamic relay cells and neurons of the thalamic reticular nucleus and that the inhibition of thalamic relay cells habituates with repeated stimuli. It has been suggested that parkinsonian rest tremor results from synchronization of the oscillatory activity of this network. If this is the case, continuous thalamic stimulation might disrupt this oscillation by diminishing the inhibitory phase.

Modulation of GABAA receptor function by tyrosine phosphorylation of beta subunits

Protein tyrosine phosphorylation is a key event in diverse intracellular signaling pathways and has been implicated in modification of neuronal functioning. We investigated the role of tyrosine phosphorylation in regulating type A GABA (GABAA) receptors in cultured CNS neurons. Extracellular application of genistein (50 microM), a membrane-permeable inhibitor of protein tyrosine kinases (PTKs), produced a reversible reduction in the amplitude of GABAA receptor-mediated whole-cell currents, and this effect was not reproduced by daidzein (50 microM), an inactive analog of genistein. In contrast, intracellular application of the PTK pp60(c-src) (30 U/ml) resulted in a progressive increase in current amplitude, and this potentiation was prevented by pretreatment of the neurons with genistein. Immunoprecipitation and immunoblotting of cultured neuronal homogenates indicated that the beta2/beta3 subunit(s) of the GABAA receptor are tyrosine phosphorylated in situ. Moreover, genistein (50 microM) was found to be capable of decreasing GABAA currents in human embryonic kidney 293 cells transiently expressing functional GABAA receptors containing the beta2 subunit. Thus, the present work provides the first evidence that native GABAA receptors are phosphorylated and modulated in situ by endogenous PTKs in cultured CNS neurons and that phosphorylation of the beta subunits may be sufficient to support such a modulation. Given the prominent role of GABAA receptors in mediating many brain functions and dysfunctions, modulation of these receptors by PTKs may be important in a wide range of physiological and pathological processes in the CNS.

GABA(A) receptor activation triggers a Cl- conductance increase and a K+ channel blockade in cerebellar granule cells

GABA(A) receptor activation in cerebellar granule cells induced a complex physiological response, namely the activation of a Cl- conductance in concert with a blockade of the resting K+ outward conductance (by 71% as compared to controls). Both responses were mediated by the activation of GABA(A) receptors, since they were both mimicked by the GABA(A) receptor agonist muscimol and antagonized by picrotoxin and bicuculline. A substantial decrease of the mean open time of single, outwardly rectifying K+ channels was triggered by GABA as revealed from cell-attached recordings; this finding implies that an intracellular pathway links GABA(A) receptors and K+ channels. Furthermore, this action of GABA is mediated through the cytoplasm, as experiments with the cell-attached patch-clamp technique show. GABA induced a prominent membrane depolarization ranging from 10 to 25 mV as revealed by current-clamp recordings of gramicidin (or nystatin) permeabilized patches, thus selecting conditions not to perturb the physiological Cl- gradient across the cell. Our findings imply that the GABA-activated Cl- current depolarized the membrane as described for immature neurons. The blockade of the resting K+ channel conductance acts in concert and both mechanisms lead to this substantial depolarizing event.