GABAA-receptor alpha-subunit is an essential prerequisite for receptor formation in vivo

GABAA receptors in GtoPdb v.2021.3

The GABA_A receptor is a ligand-gated ion channel of the Cys-loop family that includes the nicotinic acetylcholine, 5-HT₃ and strychnine-sensitive glycine receptors. GABA_A receptor-mediated inhibition within the CNS occurs by fast synaptic transmission, sustained tonic inhibition and temporally intermediate events that have been termed 'GABA_A, slow' [45]. GABA_A receptors exist as pentamers of 4TM subunits that form an intrinsic anion selective channel. Sequences of six α, three β, three γ, one δ, three ρ, one ε, one π and one θ GABA_A receptor subunits have been reported in mammals [278, 235, 236, 283]. The π-subunit is restricted to reproductive tissue. Alternatively spliced versions of many subunits exist (e.g. α4- and α6- (both not functional) α5-, β2-, β3- and γ2), along with RNA editing of the α3 subunit [71]. The three ρ-subunits, (ρ1-3) function as either homo- or hetero-oligomeric assemblies [359, 50]. Receptors formed from ρ-subunits, because of their distinctive pharmacology that includes insensitivity to bicuculline, benzodiazepines and barbiturates, have sometimes been termed GABA_C receptors [359], but they are classified as GABA _A receptors by NC-IUPHAR on the basis of structural and functional criteria [16, 235, 236]. Many GABA_A receptor subtypes contain α-, β- and γ-subunits with the likely stoichiometry 2α.2β.1γ [168, 235]. It is thought that the majority of GABA_A receptors harbour a single type of α- and β - subunit variant. The α1β2γ2 hetero-oligomer constitutes the largest population of GABA_A receptors in the CNS, followed by the α2β3γ2 and α3β3γ2 isoforms. Receptors that incorporate the α4- α5-or α 6-subunit, or the β1-, γ1-, γ3-, δ-, ε- and θ-subunits, are less numerous, but they may nonetheless serve important functions. For example, extrasynaptically located receptors that contain α6- and δ-subunits in cerebellar granule cells, or an α4- and δ-subunit in dentate gyrus granule cells and thalamic neurones, mediate a tonic current that is important for neuronal excitability in response to ambient concentrations of GABA [209, 272, 83, 19, 288]. GABA binding occurs at the β+/α- subunit interface and the homologous γ+/α- subunits interface creates the benzodiazepine site. A second site for benzodiazepine binding has recently been postulated to occur at the α+/β- interface ([254]; reviewed by [282]). The particular α-and γ-subunit isoforms exhibit marked effects on recognition and/or efficacy at the benzodiazepine site. Thus, receptors incorporating either α4- or α6-subunits are not recognised by 'classical' benzodiazepines, such as flunitrazepam (but see [356]). The trafficking, cell surface expression, internalisation and function of GABA_A receptors and their subunits are discussed in detail in several recent reviews [52, 140, 188, 316] but one point worthy of note is that receptors incorporating the γ2 subunit (except when associated with α5) cluster at the postsynaptic membrane (but may distribute dynamically between synaptic and extrasynaptic locations), whereas as those incorporating the δ subunit appear to be exclusively extrasynaptic. NC-IUPHAR [16, 235, 3, 2] class the GABA_A receptors according to their subunit structure, pharmacology and receptor function. Currently, eleven native GABA_A receptors are classed as conclusively identified (i.e., α1β2γ2, α1βγ2, α3βγ2, α4βγ2, α4β2δ, α4β3δ, α5βγ2, α6βγ2, α6β2δ, α6β3δ and ρ) with further receptor isoforms occurring with high probability, or only tentatively [235, 236]. It is beyond the scope of this Guide to discuss the pharmacology of individual GABA_A receptor isoforms in detail; such information can be gleaned in the reviews [16, 95, 168, 173, 143, 278, 216, 235, 236] and [9, 10]. Agents that discriminate between α-subunit isoforms are noted in the table and additional agents that demonstrate selectivity between receptor isoforms, for example via β-subunit selectivity, are indicated in the text below. The distinctive agonist and antagonist pharmacology of ρ receptors is summarised in the table and additional aspects are reviewed in [359, 50, 145, 223]. Several high-resolution cryo-electron microscopy structures have been described in which the full-length human α1β3γ2L GABA_A receptor in lipid nanodiscs is bound to the channel-blocker picrotoxin, the competitive antagonist bicuculline, the agonist GABA (γ-aminobutyric acid), and the classical benzodiazepines alprazolam and diazepam [198].

Basmisanil, a highly selective GABAA-α5 negative allosteric modulator: preclinical pharmacology and demonstration of functional target engagement in man

GABA_A-α5 subunit-containing receptors have been shown to play a key modulatory role in cognition and represent a promising drug target for cognitive dysfunction, as well as other disorders. Here we report on the preclinical and early clinical profile of a novel GABA_A-α5 selective negative allosteric modulator (NAM), basmisanil, which progressed into Phase II trials for intellectual disability in Down syndrome and cognitive impairment associated with schizophrenia. Preclinical pharmacology studies showed that basmisanil is the most selective GABA_A-α5 receptor NAM described so far. Basmisanil bound to recombinant human GABA_A-α5 receptors with 5 nM affinity and more than 90-fold selectivity versus α1, α2, and α3 subunit-containing receptors. Moreover, basmisanil inhibited GABA-induced currents at GABA_A-α5 yet had little or no effect at the other receptor subtypes. An in vivo occupancy study in rats showed dose-dependent target engagement and was utilized to establish the plasma exposure to receptor occupancy relationship. At estimated receptor occupancies between 30 and 65% basmisanil attenuated diazepam-induced spatial learning impairment in rats (Morris water maze), improved executive function in non-human primates (object retrieval), without showing anxiogenic or proconvulsant effects in rats. During the Phase I open-label studies, basmisanil showed good safety and tolerability in healthy volunteers at maximum GABA_A-α5 receptor occupancy as confirmed by PET analysis with the tracer [¹¹C]-Ro 15-4513. An exploratory EEG study provided evidence for functional activity of basmisanil in human brain. Therefore, these preclinical and early clinical studies show that basmisanil has an ideal profile to investigate potential clinical benefits of GABA_A-α5 receptor negative modulation.

Bidirectional regulation of distinct memory domains by α5-subunit-containing GABAA receptors in CA1 pyramidal neurons

Reduction in the expression or function of α5-subunit-containing GABA_A receptors (α5GABA_ARs) leads to improvement in several hippocampus-dependent memory domains. However, studies thus far mostly lack anatomical specificity in terms of neuronal circuits and populations. We demonstrate that mice with a selective knockdown of α5GABA_ARs in CA1 pyramidal neurons (α5CA1KO mice) show improved spatial and trace fear-conditioning memory. Unexpectedly, α5CA1KO mice were comparable to controls in contextual fear-conditioning but showed an impairment in context discrimination, suggesting fine-tuning of activity in CA1 pyramidal cell dendrites through α5-mediated inhibition might be necessary for distinguishing highly similar contexts.

GABRG2 Deletion Linked to Genetic Epilepsy with Febrile Seizures Plus Affects the Expression of GABAA Receptor Subunits and Other Genes at Different Temperatures

Mutations in γ-aminobutyric acid A receptor (GABA_A) subunits and sodium channel genes, especially GABRG2 and SCN1A, have been reported to be associated with febrile seizures (FS) and genetic epilepsy with febrile seizures plus (GEFS+). GEFS+ is a well-known family of epileptic syndrome with autosomal dominant inheritance in children. Its most common phenotypes are febrile seizures often with accessory afebrile generalized tonic-clonic seizures, febrile seizures plus (FS+), severe epileptic encephalopathy, as well as other types of generalized or localization-related seizures. However, the pathogenesis of febrile seizures remains largely unknown. Here, we generated a GABRG2 gene knockout cell line (HT22^GABRG2KO) by applying the CRISPR/Cas9-mediated genomic deletion in HT-22 mouse hippocampal neuronal cell line to explore the function of GABRG2 in vitro. With mRNA-seq, we found significant changes in the expression profiles of several epilepsy-related genes when GABRG2 was knockout, some of them showing temperature-induced changes as well. Kyoto Encyclopedia Gene and Genomic (KEGG) analysis revealed a significant alteration in the MAPK and PI3K-Akt signaling pathways. We also observed an up-regulation of the matrix metalloproteinases (MMPs) family after GABRG2 knockout. Furthermore, the significant decrease in expression of GABRA1 and CACNA1A (but not others) with an increase in temperature is a novel finding. In summary, mutations in the GABA_A receptor can lead to a decrease in numbers of receptors, which may cause the impairment of GABAergic pathway signaling. This data has been the first time to reveal that GABRG2 mutations would affect the function of other genes, and based on this finding we hope this work would also provide a new direction for the research of GABRG2 in GEFS+. It also may provide a molecular basis for the severity of epilepsy, and guide the clinical medication for the treatment of the epilepsy focused on the function on GABA_A receptors, which, might be a new strategy for genetic diagnosis and targeted treatment of epilepsy.

The α5-Containing GABAA Receptors-a Brief Summary

GABA_A receptors are the major inhibitory neurotransmitter receptor in the human brain. The receptors are assembled from combination of protein subunits in pentameric complex which may consist of α1-6, β1-3, γ1-3, ρ1-3, δ, ε, θ, or π subunits. There are a theoretical > 150,000 possible assemblies and arrangements of GABA_A subunits, although only a few combinations have been found in human with the most dominant consists of 2α1, 2β2, and 1γ2 in a counterclockwise arrangement as seen from the synaptic cleft. The receptors also possess binding sites for various unrelated substances including benzodiazepines, barbiturates, and anesthetics. The α5-containing GABA_ARs only make up ≤ 5% of the entire receptor population, but up to 25% of the receptor subtype is located in the crucial learning and memory-associated area of the brain-the hippocampus, which has ignited myriads of hypotheses and theories in regard to its role. As well as exhibiting synaptic phasic inhibition, the α5-containing receptors are also extrasynaptic and mediate tonic inhibition with continuously occurring smaller amplitude. Studies on negative-allosteric modulators for reducing this tonic inhibition have been shown to enhance learning and memory in neurological disorders such as schizophrenia, Down syndrome, and autism with a possible alternative benzodiazepine binding site. Therefore, a few α5 subunit-specific compounds have been developed to address these pharmacological needs. With its small population, the α5-containing receptors could be the key and also the answer for many untreated cognitive dysfunctions and disorders.

Partial inactivation of GABAA receptors containing the α5 subunit affects the development of adult-born dentate gyrus granule cells

Alterations of neuronal activity due to changes in GABAA receptors (GABAA R) mediating tonic inhibition influence different hippocampal functions. Gabra5-null mice and α5 subunit((H105R)) knock-in mice exhibit signs of hippocampal dysfunction, but are capable of improved performance in several learning and memory tasks. Accordingly, alleviating abnormal GABAergic tonic inhibition in the hippocampal formation by selective α5-GABAA R modulators represents a possible therapeutic approach for several intellectual deficit disorders. Adult neurogenesis in the dentate gyrus is an important facet of hippocampal plasticity; it is regulated by tonic GABAergic transmission, as shown by deficits in proliferation, migration and dendritic development of adult-born neurons in Gabra4-null mice. Here, we investigated the contribution of α5-GABAA Rs to granule cell development, using retroviral vectors expressing eGFP for labeling precursor cells in the subgranular zone. Global α5-GABAA R knockout (α5-KO) mice showed no alterations in migration and morphological development of eGFP-positive granule cells. However, upregulation of α1 subunit-immunoreactivity was observed in the hippocampal formation and cerebral cortex. In contrast, partial gene inactivation in α5-heterozygous (α5-het) mice, as well as single-cell deletion of Gabra5 in newborn granule cells from α5-floxed mice, caused severe alterations of migration and dendrite development. In α5-het mice, retrovirally mediated overexpression of Cdk5 resulted in normal migration and dendritic branching, suggesting that Cdk5 cooperates with α5-GABAA Rs to regulate neuronal development. These results show that minor imbalance of α5-GABAA R-mediated transmission may have major consequences for neuronal plasticity; and call for caution upon chronic therapeutic use of negative allosteric modulators acting at these receptors.

Regulation of adult neurogenesis by GABAergic transmission: signaling beyond GABAA-receptors

In the adult mammalian brain, neurogenesis occurs in the olfactory bulb (OB) and in the dentate gyrus (DG) of the hippocampus. Several studies have shown that multiple stages of neurogenesis are regulated by GABAergic transmission with precise spatio-temporal selectivity, and involving mechanisms common to both systems or specific only to one. In the subgranular zone (SGZ) of the DG, GABA neurotransmitter, released by a specific population of interneurons, regulates stem cell quiescence and neuronal cell fate decisions. Similarly, in the subventricular zone (SVZ), OB neuroblast production is modulated by ambient GABA. Ambient GABA, acting on extrasynaptic GABAA receptors (GABAAR), is also crucial for proper adult-born granule cell (GC) maturation and synaptic integration in the OB as well as in the DG. Throughout adult-born neuron development, various GABA receptors and receptor subunits play specific roles. Previous work has demonstrated that adult-born GCs in both the OB and the DG show a time window of increased plasticity in which adult-born cells are more prone to modification by external stimuli. One mechanism that controls this "critical period" is GABAergic modulation. Indeed, depleting the main phasic GABAergic inputs in adult-born neurons results in dramatic effects, such as reduction of spine density and dendritic branching in adult-born OB GCs. In this review, we systematically compare the role of GABAergic transmission in the regulation of adult neurogenesis between the OB and the hippocampus, focusing on the role of GABA in modulating plasticity and critical periods of adult-born neuron development. Finally, we discuss signaling pathways that might mediate some of the deficits observed upon targeted deletion of postsynaptic GABAARs in adult-born neurons.

Selective distribution of GABA(A) receptor subtypes in mouse spinal dorsal horn neurons and primary afferents

In the spinal cord dorsal horn, presynaptic GABA(A) receptors (GABA(A)Rs) in the terminals of nociceptors as well as postsynaptic receptors in spinal neurons regulate the transmission of nociceptive and somatosensory signals from the periphery. GABA(A)Rs are heterogeneous and distinguished functionally and pharmacologically by the type of α subunit variant they contain. This heterogeneity raises the possibility that GABA(A)R subtypes differentially regulate specific pain modalities. Here, we characterized the subcellular distribution of GABA(A)R subtypes in nociceptive circuits by using immunohistochemistry with subunit-specific antibodies combined with markers of primary afferents and dorsal horn neurons. Confocal laser scanning microscopy analysis revealed a distinct, partially overlapping laminar distribution of α1-3 and α5 subunit immunoreactivity in laminae I-V. Likewise, a layer-specific pattern was evident for their distribution among glutamatergic, γ-aminobutyric acid (GABA)ergic, and glycinergic neurons (detected in transgenic mice expressing vesicular glutamate transporter 2-enhanced green fluorescent protein [vGluT2-eGFP], glutamic acid decarboxylase [GAD]67-eGFP, and glycine transporter 2 (GlyT2)-eGFP, respectively). Finally, all four subunits could be detected within primary afferent terminals. C-fibers predominantly contained either α2 or α3 subunit immunoreactivity; terminals from myelinated (Aβ/Aδ) fibers were colabeled in roughly equal proportion with each subunit. The presence of axoaxonic GABAergic synapses was determined by costaining with gephyrin and vesicular inhibitory amino acid transporter to label GABAergic postsynaptic densities and terminals, respectively. Colocalization of the α2 or α3 subunit with these markers was observed in a subset of C-fiber synapses. Furthermore, gephyrin mRNA and protein expression was detected in dorsal root ganglia. Collectively, these results show that differential GABA(A)R distribution in primary afferent terminals and dorsal horn neurons allows for multiple, circuit-specific modes of regulation of nociceptive circuits.

The cooperation of sustained and phasic inhibitions increases the contrast of ITD-tuning in low-frequency neurons of the chick nucleus laminaris

Neurons in the nucleus laminaris (NL) of birds detect the coincidence of binaural excitatory inputs from the nucleus magnocellularis (NM) on both sides and process the interaural time differences (ITDs) for sound localization. Sustained inhibition from the superior olivary nucleus is known to control the gain of coincidence detection, which allows the sensitivity of NL neurons to ITD tolerate strong-intensity sound. Here, we found a phasic inhibition in chicken brain slices that follows the ipsilateral NM inputs after a short time delay, sharpens coincidence detection, and may enhance ITD sensitivity in low-frequency NL neurons. GABA-positive small neurons are distributed in and near the NL. These neurons generate IPSCs in NL neurons when photoactivated by a caged glutamate compound, suggesting that these GABAergic neurons are interneurons that mediate phasic inhibition. These IPSCs have fast decay kinetics that is attributable to the α1-subunit of the GABAA receptor, the expression of which dominates in the low-frequency region of the NL. Model simulations demonstrate that phasic IPSCs narrow the time window of coincidence detection and increase the contrast of ITD-tuning during low-level, low-frequency excitatory input. Furthermore, cooperation of the phasic and sustained inhibitions effectively increases the contrast of ITD-tuning over a wide range of excitatory input levels. We propose that the complementary interaction between phasic and sustained inhibitions is the neural mechanism that regulates ITD sensitivity for low-frequency sound in the NL.

Beyond classical benzodiazepines: novel therapeutic potential of GABAA receptor subtypes

GABA(A) (γ-aminobutyric acid, type A) receptors are a family of ligand-gated ion channels that are essential for the regulation of central nervous system function. Benzodiazepines - which non-selectively target GABA(A) receptors containing the α1, α2, α3 or α5 subunits - have been in clinical use for decades and are still among the most widely prescribed drugs for the treatment of insomnia and anxiety disorders. However, their use is limited by side effects and the risk of drug dependence. In the past decade, the identification of separable key functions of GABA(A) receptor subtypes suggests that receptor subtype-selective compounds could overcome the limitations of classical benzodiazepines; furthermore, they might be valuable for novel indications such as chronic pain, depression, schizophrenia, cognitive enhancement and stroke.

Decreased GABAA receptors and benzodiazepine binding sites in the anterior cingulate cortex in autism

The anterior cingulate cortex (ACC; BA 24) via its extensive limbic and high order association cortical connectivity to prefrontal cortex is a key part of an important circuitry participating in executive function, affect, and socio-emotional behavior. Multiple lines of evidence, including genetic and imaging studies, suggest that the ACC and gamma-amino-butyric acid (GABA) system may be affected in autism. The benzodiazepine binding site on the GABA(A) receptor complex is an important target for pharmacotherapy and has important clinical implications. The present multiple-concentration ligand-binding study utilized (3)H-muscimol and (3)H-flunitrazepam to determine the number (B(max)), binding affinity (K(d)), and distribution of GABA(A) receptors and benzodiazepine binding sites, respectively, in the ACC in adult autistic and control cases. Compared to controls, the autistic group had significant decreases in the mean density of GABA(A) receptors in the supragranular (46.8%) and infragranular (20.2%) layers of the ACC and in the density of benzodiazepine binding sites in the supragranular (28.9%) and infragranular (16.4%) lamina [corrected]. These findings suggest that in the autistic group this downregulation of both benzodiazepine sites and GABA(A) receptors in the ACC may be the result of increased GABA innervation and/or release disturbing the delicate excitation/inhibition balance of principal neurons as well as their output to key limbic cortical targets. Such disturbances likely underlie the core alterations in socio-emotional behaviors in autism.

Gamma-aminobutyric acid A receptor functional decrease in the hypothalamus during pancreatic regeneration in rats

OBJECTIVE

In the present study, we investigated the alteration of gamma-aminobutyric acid A (GABAA) receptors in the hypothalamus of rats during pancreatic regeneration.

METHODS

Three groups of rats were used for the study: sham operated, 72 hours partially pancreatectomized, and 7 days partially pancreatectomized. The GABA receptor assay was performed by using the [H]GABA as ligand to the Triton X-100-treated membranes, and displacement with unlabeled GABA and [H]bicuculline binding to the GABAA receptors was assayed in Triton X-100-treated synaptic membranes and displacement with unlabeled bicuculline.

RESULTS

The GABA content in the brain regions and pancreas of the sham and experimental rat groups was quantified by displacement method. In the hypothalamus, the high-affinity GABAA receptor binding showed a significant decrease in maximal binding (P < 0.01) and equilibrium dissociation constant (P < 0.05) in 72 hours and 7 days partially pancreatectomized rats. The content of GABA has significantly decreased in the hypothalamus during the regeneration of pancreas.

CONCLUSIONS

This effect of GABAA receptors in hypothalamus suggests a regulatory role during active regeneration of pancreas that will have significance in insulin secretion.

Effect of the alpha subunit subtype on the macroscopic kinetic properties of recombinant GABA(A) receptors

The GABA(A) receptors (GABARs) are chloride-permeable ligand-gated ion channels responsible for fast inhibitory neurotransmission. These receptors are structurally heterogeneous, and in mammals can be formed from a combination of sixteen different subunit subtypes. Much of this variety comes from the six different alpha subunit subtypes. All neuronal GABARs contain an alpha subunit, and the identity of the alpha subtype affects the pharmacological properties of the receptors. The expression of each of the different alpha subtypes is regulated developmentally and regionally and changes with both normal physiological processes such development and synaptic plasticity, and pathological conditions such as epilepsy. In order to understand the functional significance of this structural heterogeneity, we examined the effect of the alpha subtype on the receptor's response to GABA. Each of the six alpha subtypes was transiently co-expressed with the beta3 and gamma2L subunits in mammalian cells. The sensitivity to GABA was measured with whole-cell recordings. We also determined the activation, deactivation, desensitization, and recovery kinetics for the six isoforms using rapid application recordings from excised macropatches. We found unique characteristics associated with each alpha subunit subtype. These properties would be expected to influence the post-synaptic response to GABA, creating functional diversity among neurons expressing different alpha subunits.

Reorganization of GABAergic circuits maintains GABAA receptor-mediated transmission onto CA1 interneurons in alpha1-subunit-null mice

The majority of hippocampal interneurons strongly express GABA(A) receptors containing the alpha1 subunit, suggesting that inhibitory control of interneurons is important for proper function of hippocampal circuits. Here, we investigated with immunohistochemical and electrophysiological techniques how these GABA(A) receptors are replaced in mice carrying a targeted deletion of the alpha1-subunit gene (alpha1(0/0) mice). Using markers of five major populations of CA1 interneurons (parvalbumin, calretinin, calbindin, neuropeptide Y and somatostatin), we show that these interneurons remain unaffected in alpha1(0/0) mice. In triple immunofluorescence staining experiments combining these markers with the GABA(A) receptor alpha1, alpha2 or alpha3 subunit and gephyrin, we demonstrate a strong increase in alpha3- and alpha2-GABA(A) receptors clustered at postsynaptic sites along with gephyrin in most CA1 interneurons in alpha1(0/0) mice. The changes were cell type-specific and resulted in an increased number of GABAergic synapses on interneurons. These adjustments were mirrored functionally by retention of spontaneous IPSCs with prolonged decay kinetics, as shown by whole-cell patch-clamp recordings of CA1 interneurons. However, a significant decrease in frequency and amplitude of miniature IPSCs was evident, suggesting reduced affinity of postsynaptic receptors and/or impaired vesicular GABA release. Finally, to assess whether these compensatory changes are sufficient to protect against a pathological challenge, we tested the susceptibility of alpha1(0/0) mice against kainic acid-induced excitotoxicity. No genotype difference was observed in the effects of kainic acid, indicating that the absence of a major GABA(A) receptor subtype is functionally compensated for in hippocampal interneurons by a reorganization of inhibitory circuits.

GABAergic phenotype of periglomerular cells in the rodent olfactory bulb

Periglomerular (PG) cells in the rodent olfactory bulb are heterogeneous anatomically and neurochemically. Here we investigated whether major classes of PG cells use gamma-aminobutyric acid (GABA) as a neurotransmitter. In addition to three known subtypes of PG cells expressing tyrosine hydroxylase (TH), calbindin D-28k (CB), and calretinin (CR), we identified a novel PG cell population containing the GABAA receptor alpha5 subunit. Consistent with previous studies in the rat, we found that TH-positive cells were also labeled with antibodies against GABA, whereas PG cells expressing CB or the alpha5 subunit were GABA-negative. Using GAD67-GFP knockin mice, we found that all PG cell subtypes expressed GAD67-GFP. Calretinin labeled the major fraction (44%) of green fluorescent protein (GFP)-positive cells, followed by TH (16%), CB (14%), and the alpha5 subunit (13%). There was no overlap between these neuronal populations, which accounted for approximately 85% of GAD67-GFP-positive cells. We then demonstrated that PG cells labeled for TH, CB, or CR established dendrodendritic synapses expressing glutamic acid decarboxylase (GAD) or the vesicular inhibitory amino acid transporter, VGAT, irrespective of their immunoreactivity for GABA. In addition, CB-, CR-, and TH-positive dendrites were apposed to GABAA receptor clusters containing the alpha1 or alpha3 subunits, which are found in mitral and tufted cells, and the alpha2 subunit, which is expressed by PG cells. Together, these findings indicate that all major subtypes of PG cells are GABAergic. In addition, they show that PG cells provide GABAergic input to the dendrites of principal neurons and are interconnected with other GABAergic interneurons, which most likely are other PG cells.

Synaptic and nonsynaptic localization of GABAA receptors containing the alpha5 subunit in the rat brain

The alpha5 subunit of the GABA(A) receptors (GABA(A)Rs) has a restricted expression in the brain. Maximum expression of this subunit occurs in the hippocampus, cerebral cortex, and olfactory bulb. Hippocampal pyramidal cells show high expression of alpha5 subunit-containing GABA(A)Rs (alpha5-GABA(A)Rs) both in culture and in the intact brain. A large pool of alpha5-GABA(A)Rs is extrasynaptic and it has been proposed to be involved in the tonic GABAergic inhibition of the hippocampus. Nevertheless, there are no studies on the localization of the alpha5-GABA(A)Rs at the electron microscope (EM) level. By using both immunofluorescence of cultured hippocampal pyramidal cells and EM postembedding immunogold of the intact hippocampus we show that, in addition to the extrasynaptic pool, there is a pool of alpha5-GABA(A)Rs that concentrates at the GABAergic synapses in dendrites of hippocampal pyramidal cells. The results suggest that the synaptic alpha5-GABA(A)Rs might play a role in the phasic GABAergic inhibition of pyramidal neurons in hippocampus and cerebral cortex.

Alteration of GABAergic synapses and gephyrin clusters in the thalamic reticular nucleus of GABAA receptor alpha3 subunit-null mice

Multiple GABAA-receptor subtypes are assembled from alpha, beta and gamma subunit variants. GABAA receptors containing the alpha3 subunit represent a minor population with a restricted distribution in the CNS. In addition, they predominate in monoaminergic neurons and in the nucleus reticularis thalami (nRT), suggesting a role in the regulation of cortical function and sleep. Mice with a targeted deletion of the alpha3 subunit gene (alpha3(0/0)) are viable and exhibit a subtle behavioural phenotype possibly related to dopaminergic hyperfunction. Here, we investigated immunohistochemically the consequences of the loss of alpha3 subunit for maturation of GABAA receptors and formation of GABAergic synapses in the nRT. Throughout postnatal development, the regional distribution of the alpha1, alpha2, or alpha5 subunit was unaltered in alpha3(0/0) mice and the prominent alpha3 subunit staining of nRT neurons in wildtype mice was not replaced. Subcellularly, as seen by double immunofluorescence, the alpha3 and gamma2 subunit were clustered at postsynaptic sites in the nRT of adult wildtype mice along with the scaffolding protein gephyrin. In alpha3(0/0) mice, gamma2 subunit clustering was disrupted and gephyrin formed large aggregates localized at the cell surface, but unrelated to postsynaptic sites, indicating that nRT neurons lack postsynaptic GABAA receptors in mutant mice. Furthermore, GABAergic terminals were enlarged and reduced in number, suggesting a partial deficit of GABAergic synapses. Therefore, GABAA receptors are required for gephyrin clustering and long-term synapse maintenance. The absence of GABAA-mediated transmission in the nRT may have a significant impact on the function of the thalamo-cortical loop of alpha3(0/0) mice.

Compensatory alteration of inhibitory synaptic circuits in cerebellum and thalamus of gamma-aminobutyric acid type A receptor alpha1 subunit knockout mice

Targeted deletion of the alpha1 subunit gene results in a profound loss of gamma-aminobutyric acid type A (GABA(A)) receptors in adult mouse brain but has only moderate behavioral consequences. Mutant mice exhibit several adaptations in GABA(A) receptor subunit expression, as measured by Western blotting. By using immunohistochemistry, we investigated here whether these adaptations serve to replace the missing alpha1 subunit or represent compensatory changes in neurons that normally express these subunits. We focused on cerebellum and thalamus and distinguished postsynaptic GABA(A) receptor clusters by their colocalization with gephyrin. In the molecular layer of the cerebellum, alpha1 subunit clusters colocalized with gephyrin disappeared from Purkinje cell dendrites of mutant mice, whereas alpha3 subunit/gephyrin clusters, presumably located on dendrites of Golgi interneurons, increased sevenfold, suggesting profound network reorganization in the absence of the alpha1 subunit. In thalamus, a prominent increase in alpha3 and alpha4 subunit immunoreactivity was evident, but without change in regional distribution. In the ventrobasal complex, which contains primarily postsynaptic alpha1- and extrasynaptic alpha4-GABA(A) receptors, the loss of alpha1 subunit was accompanied by disruption of gamma2 subunit and gephyrin clustering, in spite of the increased alpha4 subunit expression. However, in the reticular nucleus, which lacks alpha1-GABA(A) receptors in wild-type mice, postsynaptic alpha3/gamma2/gephyrin clusters were unaffected. These results demonstrate that adaptive responses in the brain of alpha1(0/0) mice involve reorganization of GABAergic circuits and not merely replacement of the missing alpha1 subunit by another receptor subtype. In addition, clustering of gephyrin at synaptic sites in cerebellum and thalamus appears to be dependent on expression of a GABA(A) receptor subtype localized postsynaptically.

Clustered and non-clustered GABAA receptors in cultured hippocampal neurons

In cultured hippocampal neurons, gamma2 subunit-containing GABA(A) Rs form large postsynaptic clusters at GABAergic synapses and small clusters outside GABAergic synapses. We now show that a pool of non-clustered gamma2 subunit-containing GABA(A) Rs are also present at the cell surface. We also demonstrate that myc- or EGFP-tagged gamma2, alpha2, beta3 or alpha1 subunits expressed in these neurons assemble with endogenous subunits, forming GABA(A) Rs that target large postsynaptic clusters, small clusters outside GABAergic synapses or a pool of non-clustered surface GABA(A) Rs. In contrast, myc- or EGFP-tagged delta subunits only form non-clustered GABA(A) Rs, which can be induced to form clusters by antibody capping. A myc-tagged chimeric gamma2 subunit possessing the large intracellular loop (IL) of the delta-subunit IL (myc gamma2S/delta-IL) assembled into GABA(A) Rs, but it did not form clusters, therefore behaving like the delta subunit. Thus, the large intracellular loops of gamma2 and delta play an important role in determining the synaptic clustering/non-clustering capacity of the GABA(A) Rs.

Requirement of alpha5-GABAA receptors for the development of tolerance to the sedative action of diazepam in mice

Despite its pharmacological relevance, the mechanism of the development of tolerance to the action of benzodiazepines is essentially unknown. The acute sedative action of diazepam is mediated via alpha1-GABA(A) receptors. Therefore, we tested whether chronic activation of these receptors by diazepam is sufficient to induce tolerance to its sedative action. Knock-in mice, in which thealpha1-,alpha2-,alpha3-, oralpha(5)-GABA(A) receptors had been rendered insensitive to diazepam by histidine-arginine point mutation, were chronically treated with diazepam (8 d; 15 mg x kg(-1) x d(-1)) and tested for motor activity. Wild-type, alpha2(H101R), and alpha3(H126R) mice showed a robust diminution of the motor-depressant drug action. In contrast, alpha5(H105R) mice failed to display any sedative tolerance. alpha1(H101R) mice showed no alteration of motor activity with chronic diazepam treatment. Autoradiography with [3H]flumazenil revealed no change in benzodiazepine binding sites. However, a decrease in alpha5-subunit radioligand binding was detected selectively in the dentate gyrus with specific ligands. This alteration was observed only in diazepam-tolerant animals, indicating that the manifestation of tolerance to the sedative action of diazepam is associated with a downregulation of alpha5-GABA(A) receptors in the dentate gyrus. Thus, the chronic activation of alpha(5)-GABA(A) receptors is crucial for the normal development of sedative tolerance to diazepam, which manifests itself in conjunction with alpha1-GABA(A) receptors.

Variability in the benzodiazepine response of serotonin 5-HT1A receptor null mice displaying anxiety-like phenotype: evidence for genetic modifiers in the 5-HT-mediated regulation of GABA(A) receptors

Benzodiazepines (BZs) acting as modulators of GABA(A) receptors (GABA(A)Rs) are an important group of drugs for the treatment of anxiety disorders. However, a large inter-individual variation in BZ sensitivity occurs in the human population with some anxiety disorder patients exhibiting diminished sensitivity to BZ and reduced density of GABA(A)Rs. The mechanism underlying BZ treatment resistance is not known, and it is not possible to predict whether an anxiety patient will respond to BZ. 5-hydroxytryptamine1A receptor (5-HT1AR) null mice (R-/-) on the Swiss-Webster (SW) background reproduce several features of BZ-resistant anxiety; they exhibit anxiety-related behaviors, do not respond to BZ, have reduced BZ binding, and have decreased expression of the major GABA(A)R subunits alpha1 and alpha2. Here, we show that R-/- mice on the C57Bl6 (B6) background also have anxiety phenotype, but they respond to BZ and have normal GABA(A)R subunit expression. This indicates that the 5-HT1AR-mediated regulation of GABA(A)R alpha subunit expression is subject to genetic modification. Hybrid SW/B6-R-/- mice also exhibit BZ-resistant anxiety, suggesting that SW mice carry a genetic modifier, which mediates the effect of the 5-HT1AR on the expression of GABA(A)Ralpha subunits. In addition, we show that this genetic interaction in SW mice operates early in postnatal life to influence the expression of GABA(A)R alpha subunits at the transcriptional level. These data indicate that BZ-resistant anxiety results from a developmental arrest of GABA(A)R expression in SW-R-/- mice, and a similar mechanism may be responsible for the BZ insensitivity of some anxiety patients.

Binge ethanol exposure delays development of GABAergic miniature postsynaptic currents in septal neurons

Whole cell GABA(A)R currents of septal neurons isolated from rat pups increase rapidly during the first weeks of life when inhibitory synapses are forming. Early postnatal binge ethanol intubation on days 4-9 delays this maturational up-regulation in septal neurons isolated several days later suggesting inhibitory synapse formation could be disrupted [S.-H. Hsiao, J.L. Acevedo, D.W. DuBois, K.R. Smith, J.R. West, G.D. Frye, Early postnatal ethanol intubation blunts GABA(A) receptor upregulation and modifies 3alpha-hydroxy-5alpha-pregnan-20-one sensitivity in rat MS/DB neurons, Brain Res. Dev. Brain Res. 130 (2001) 25-40]. Surprisingly, whole cell GABA(A)R function does not increase rapidly when septal neurons are grown for the same period in vitro and is not blunted by comparable ethanol exposure of the cultures [S.-H. Hsiao, D.W. DuBois, R.C. Miranda, G.D. Frye, Critically timed ethanol exposure reduces GABA(A)R function on septal neurons developing in vivo but not in vitro, Brain Res Dev. Brain Res. 1008 (2004) 69-80]. Because GABAergic miniature postsynaptic currents (mPSCs) show parallel patterns of maturation whether cortical neurons are growing in vivo or in vitro [D.D. Dunning, C.L. Hoover, I. Soltesz, M.A. Smith, D.K. ODowd, GABA(A) receptor-mediated miniature postsynaptic currents and alpha-subunit expression in developing cortical neurons, J. Neurophysiol. 82 (1999) 3286-3297], we examined the impact of binge ethanol exposure on synaptic receptors activated by these currents in septal cultures. Binge ethanol treatment of embryonic septal neurons over 6-11 days in vitro (DIV) slightly reduced GABA(A)R-mediated mPSC amplitude and frequency, but also substantially slowed decay kinetics when mPSCs were recorded later on DIV 13-18. Decreased frequency and slowed mPSC decay kinetics after ethanol were consistent with parameters measured in immature neurons. Untreated septal neurons exhibited decreased mPSC amplitude and frequency with acute 30-100 mM ethanol, without changing decay kinetics suggesting a direct inhibition of postsynaptic receptors. Sustained inhibition of GABA(A)Rs with 100 microM picrotoxin on DIV 6-11 decreased mPSC amplitude and frequency and slowed decay kinetics similar to binge ethanol exposure. These results suggest that binge ethanol exposure delays mPSC maturation by interfering with trophic postsynaptic GABA(A)R signaling during the early development of septal neurons.

GABAA receptor abnormalities in Prader–Willi syndrome assessed with positron emission tomography and [11C]flumazenil

Prader-Willi syndrome (PWS) is a multi-system disorder characterized clinically by abnormal mental and physical development. PWS patients have a deletion in an imprinted region on paternal chromosome 15 (15q11-13), maternal disomy for this segment, or rarely, a chromosomal imprinting center deletion that gives rise to suppression of the equivalent paternal genes. Within the affected segment of chromosome 15 are genes encoding the alpha(5), beta(3) and gamma(3) subunits of the gamma-aminobutyric acid type-A (GABA(A)) receptor. Therefore, altered neurobehavioral function could arise in PWS due directly to altered GABA(A) receptor composition and expression, or alternatively, from brain developmental and maturational effects of these or other genes in the imprinted region. The aim of the present study was to assess cerebral GABA(A) receptors in PWS with the use of positron emission tomography of the benzodiazepine binding site employing [11C]flumazenil ([11C]FMZ). A reduction in [11C]FMZ binding was found predominantly in the cingulate, frontal and temporal neocortices and insula in six adult PWS patients compared to nine normal subjects. A possible role for the deleted beta(3) subunit gene in PWS is supported in part by the wide cortical distribution of its mRNA expression and the effects of experimental knockouts on benzodiazepine binding described in prior studies. Altered GABA(A) receptor composition or number in these cortical regions may account for neurobehavioral abnormalities in PWS including mild mental retardation, poor impulse control, and impaired responses to somatic pain.

ANALYSIS OFGABAARECEPTORFUNCTION ANDDISSECTION OF THEPHARMACOLOGY OFBENZODIAZEPINES ANDGENERALANESTHETICSTHROUGHMOUSEGENETICS

GABAA receptors are molecular substrates for the regulation of vigilance, anxiety, muscle tension, epileptogenic activity, and memory functions, and the enhancement of GABAA receptor-mediated fast synaptic inhibition is the basis for the pharmacotherapy of various neurological and psychiatric disorders. Two kinds of GABAA receptor-targeted mutant mice have been generated: (a) knockout mice that lack individual GABAA receptor subunits (alpha1, alpha5, alpha6, beta2, beta3, gamma2, delta, and rho1) and (b) knockin mice that carry point mutations affecting the action of modulatory drugs [alpha1(H101R), alpha2(H101R), alpha3(H126R), alpha5(H105R), and beta3(N265M)]. Whereas the knockout mice have provided information primarily with respect to the regulation of subunit gene transcription, receptor assembly, and some physiological functions of individual receptor subtypes, the point-mutated knockin mice in which specific GABAA receptor subtypes are insensitive to diazepam or some general anesthetics have revealed the specific contribution of individual receptor subtypes to the pharmacological spectrum of diazepam and general anesthetics.

GABA and schizophrenia: a review of basic science and clinical studies

BACKGROUND

A converging body of evidence implicates the gamma-aminobutyric acid (GABA) neurotransmitter system in the pathogenesis of schizophrenia.

METHODS

The authors review neuroscience literature and clinical studies investigating the role of the GABA system in the pathophysiology of schizophrenia. First, a background on the GABA system is provided, including GABA pharmacology and neuroanatomy of GABAergic neurons. Results from basic science schizophrenia animal models and human studies are reviewed. The role of GABA in cognitive dysfunction in schizophrenia is then presented, followed by a discussion of GABAergic compounds used in monotherapy or adjunctively in clinical schizophrenia studies.

RESULTS

In basic studies, reductions in GABAergic neuronal density and abnormalities in receptors and reuptake sites have been identified in several cortical and subcortical GABA systems. A model has been developed suggesting GABA's role (including GABA-dopamine interactions) in schizophrenia. In several clinical studies, the use of adjunctive GABA agonists was associated with greater improvement in core schizophrenia symptoms.

CONCLUSIONS

Alterations in the GABA neurotransmitter system are found in clinical and basic neuroscience schizophrenia studies as well as animal models and may be involved in the pathophysiology of schizophrenia. The interaction of GABA with other well-characterized neurotransmitter abnormalities remains to be understood. Future studies should elucidate the potential therapeutic role for GABA ligands in schizophrenia treatment.

Sex differences in GABA(A)ergic system in rat substantia nigra pars reticulata

The substantia nigra pars reticulata (SNR) is involved in the control of movement disorders including seizures through its GABAergic neurons. Microinfusions of muscimol (a GABA(A) receptor agonist) produce specific effects on seizures depending on sex, infusion site (SNR(anterior) or SNR(posterior)) and age. To assess whether these effects are due to sex differences in GABAergic indices within the SNR we analyzed the expression of alpha(1) subunit mRNA of the GABA(A) receptor and the levels of GABA immunoreactivity (IR) of male and female rats at postnatal day 15 (PN15) and PN30. In each age, within the same SNR region, expression of alpha(1) subunit mRNA and intensity of GABA IR per neuron was higher in females compared to males. At PN15, in both sexes, there were no regional differences in expression of alpha(1) subunit mRNA and intensity of GABA IR. However, at PN30 in both sexes, expression of alpha(1) subunit mRNA and intensity of GABA IR per cell was higher in SNR(anterior) than in SNR(posterior). These results demonstrate that expression of alpha(1) subunit mRNA for GABA(A) receptor and levels of GABA IR in the SNR are sex- and site-specific, which may contribute to sex-, regional- and age-related differences in the expression of movement disorders and seizures.

Visualization of alpha5 subunit of GABAA/benzodiazepine receptor by 11C Ro15-4513 using positron emission tomography

Although [(11)C]Ro15-4513 and [(11)C]flumazenil both bind to the central benzodiazepine (BZ) receptors, the distributions of the two ligands are not identical in vivo. Moreover, the in vivo pharmacological properties of [(11)C]Ro15-4513 have not been thoroughly examined. In the present study, we examined the pharmacological profile of [(11)C]Ro15-4513 binding in the monkey brain using positron emission tomography (PET). [(11)C]Ro15-4513 showed relatively high accumulation in the anterior cingulate cortex, hippocampus, and insular cortex, with the lowest uptake being observed in the pons. Accumulation in the cerebral cortex was significantly diminished by the BZ antagonist flumazenil (0.1 mg/kg, i.v.), but not that in the pons. Using the pons as a reference region, the specific binding of [(11)C]Ro15-4513 in most of the cerebral cortex including the limbic regions clearly revealed two different affinity sites. On the other hand, specific binding in the occipital cortex and cerebellum showed only a low affinity site. Zolpidem with affinity for alpha1, alpha2, and alpha3 subunits of GABA(A)/BZ receptor fully inhibited [(11)C]Ro15-4513 binding in the occipital cortex and cerebellum, while only about 23% of the binding was blocked in the anterior cingulate cortex. Diazepam with affinity for alpha1, alpha2, alpha3, and alpha5 subunits inhibited the binding in all brain regions. Since Ro15-4513 has relatively high affinity for the alpha5 subunit in vitro, these in vivo bindings of [(11)C]Ro15-4513 can be interpreted as the relatively high accumulation in the fronto-temporal limbic regions representing binding to the GABA(A)/BZ receptor alpha5 subunit.

alpha5 Subunit-containing GABA(A) receptors form clusters at GABAergic synapses in hippocampal cultures

We have used triple-label fluorescence immunocytochemistry to demonstrate that alpha5 subunit-containing GABA(A) receptors (GABA(A)Rs) form large clusters at GABAergic synapses in dendrites and axon initial segment of cultured hippocampal neurons. The large synaptic clusters of alpha5 subunit-containing GABA(A)Rs also contained alpha1, beta2/3, gamma2 GABA(A)R subunits and gephyrin. The alpha5 subunit-containing GABA(A)Rs also formed small clusters. The small clusters were not associated with GABAergic synapses and often did not co-localize with gephyrin.

Neuroactive steroid mechanisms and GABA type A receptor subunit assembly in hypothalamic and extrahypothalamic regions

Gonadal- and neuronal-derived steroids are capable of altering brain functions through two basic mechanisms: slow (genomic) and rapid (novel nongenomic membrane) types of activities. The genomic activities that are circumscribed to the numerous neuronal and glial expressed receptor actions involve transcriptional processes regulated largely by classical steroids. On the other hand, rapid nongenomic activities are linked to the stereoselective interactions of potent neuroactive steroids. It appears that both of these steroid mechanisms can be successfully evoked at the ligand-gated heteroligomeric GABA type A receptor. However, the precise structural prerequisites and type of molecular steroid interactions implicated in this neuronal target have not been fully investigated. This article reviews the most common subunits (alpha, beta, and gamma) of the native GABA type A receptor involved in signaling pathways of slow and rapid steroidal mechanisms. Different beta-containing compositions (alpha1beta1-3gamma2) are necessary for the slow type of mechanism, whereas different alpha-containing constructs (alpha2-6beta 1/2 gamma2/2L) are linked to the rapid type. Because of the major role played by neuroactive steroids in GABA-dependent neuroendocrine and sociosexual events, distinction of the specific subunit combination is essential not only for elucidating neuronal communicative expressions during such events but also for elucidating their potential neuroprotective role in neurodegenerative disorders.

GABA(A) receptor alpha-1 subunit deletion alters receptor subtype assembly, pharmacological and behavioral responses to benzodiazepines and zolpidem

Potentiation of GABA(A) receptor activation through allosteric benzodiazepine (BZ) sites produces the anxiolytic, anticonvulsant and sedative/hypnotic effects of BZs. Using a mouse line lacking alpha1 subunit expression, we investigated the contribution of the alpha1 subunit to GABA(A) receptor pharmacology, function and related behaviors in response to BZ site agonists. Competitive [(3)H]flunitrazepam binding experiments using the Type I BZ site agonist, zolpidem, and the Type I and II BZ site non-specific agonist, diazepam, demonstrated the complete loss of Type I BZ binding sites in alpha1(-/-) mice and a compensatory increase in Type II BZ binding sites (41+/-6%, P<0.002). Chloride uptake analysis in alpha1(-/-) mice revealed an increase (108+/-10%, P<0.001) in the efficacy (E(max)) of flunitrazepam while the EC(50) of zolpidem was increased 495+/-26% (alpha1(+/+): 184+/-56 nM; alpha1(-/-): 1096+/-279 nM, P<0.01). An anxiolytic effect of diazepam was detected in both alpha1(+/+) and alpha1(-/-) mice as measured on the elevated plus maze; however, alpha1(-/-) mice exhibited a greater percentage of open arm entries and percentage of open arm time following 0.6 mg/kg diazepam. Furthermore, alpha1(-/-) mice were more sensitive to the motor impairing/sedative effects of diazepam (1-10 mg/kg) as measured by locomotor activity in the open field. Knockout mice were insensitive to the anticonvulsant effect of diazepam (1-15 mg/kg, P<0.001). The hypnotic effect of zolpidem (60 mg/kg) was reduced by 66% (P<0.001) in alpha1(-/-) mice as measured by loss of righting reflex while the effect of diazepam (33 mg/kg) was increased 57% in alpha1(-/-) mice (P<0.05). These studies demonstrate that compensatory adaptations in GABA(A) receptor subunit expression result in subunit substitution and assembly of functional receptors. Such adaptations reveal important relationships between subunit expression, receptor function and behavioral responses.

Molecular and pharmacological characterization of GABA(A) receptor alpha1 subunit knockout mice

GABA(A) receptors mediate fast inhibitory neurotransmission in the central nervous system (CNS), and approximately half of these receptors contain alpha1 subunits. GABA(A) receptor alpha1 subunits are important for receptor assembly and specific pharmacological responses to benzodiazepines. Plasticity in GABA(A) receptor alpha1 subunit expression is associated with changes in CNS excitability observed during normal brain development, in animal models of epilepsy, and upon withdrawal from alcohol and benzodiazepines. To examine the role of alpha1 subunit-containing GABA(A) receptors in vivo, we characterized receptor subunit expression and pharmacological properties in cerebral cortex of knockout mice with a targeted deletion of the alpha1 subunit. The mice are viable but exhibit an intention tremor. Western blot analysis confirms the complete loss of alpha1 subunit peptide expression. Stable adaptations in the expression of several GABA(A) receptor subunits are observed in the fifth to seventh generations, including decreased expression of beta2/3 and gamma2 subunits and increased expression of alpha2 and alpha3 subunits. There was no change in alpha4, alpha5, or delta subunit peptide levels in cerebral cortex. Knockout mice exhibit loss of over half of GABA(A) receptors measured by [(3)H]muscimol, [(3)H]2-(3-carboxyl)-3-amino-6-(4-methoxyphenyl)-pyridazinium bromide ([(3)H]SR-95531), and t-butylbicyclophosphoro[(35)S]thionate ([(35)S]TBPS) binding. [(3)H]Ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylate ([(3)H]Ro15-4513) binding is reduced by variable amounts in different regions across brain. GABA(A) receptor alpha1(-/-) mice lose all high-affinity [(3)H]zolpidem binding and about half of [(3)H]flunitrazepam binding in the cerebral cortex. The potency and maximal efficacy of muscimol-stimulated (36)Cl(-) uptake in cerebral cortical synaptoneurosomes are reduced in alpha1(-/-) mice. Furthermore, knockout mice exhibit increased bicuculline-induced seizure susceptibility compared with wild-type mice. These data emphasize the significance of alpha1 subunit expression and its involvement in the regulation of CNS excitability.

Trace fear conditioning involves hippocampal alpha5 GABA(A) receptors

The heterogeneity of gamma-aminobutyric acid type A (GABA(A)) receptors contributes to the diversity of neuronal inhibition in the regulation of information processing. Although most GABA(A) receptors are located synaptically, the small population of alpha5GABA(A) receptors is largely expressed extrasynaptically. To clarify the role of the alpha5GABA(A) receptors in the control of behavior, a histidine-to-arginine point mutation was introduced in position 105 of the murine alpha5 subunit gene, which rendered the alpha5GABA(A) receptors diazepam-insensitive. Apart from an incomplete muscle relaxing effect, neither the sedative, anticonvulsant, nor anxiolytic-like activity of diazepam was impaired in alpha5(H105R) mice. However, in hippocampal pyramidal cells, the point mutation resulted in a selective reduction of alpha5GABA(A) receptors, which altered the drug-independent behavior. In line with the role of the hippocampus in certain forms of associative learning, trace fear conditioning, but not delay conditioning or contextual conditioning, was facilitated in the mutant mice. Trace fear conditioning differs from delay conditioning in that the conditioned and unconditioned stimulus are separated by a time interval. Thus, the largely extrasynaptic alpha5GABA(A) receptors in hippocampal pyramidal cells are implicated as control elements of the temporal association of threat cues in trace fear conditioning.

Drug interactions at GABA(A) receptors

Neurotransmitter receptor systems have been the focus of intensive pharmacological research for more than 20 years for basic and applied scientific reasons, but only recently has there been a better understanding of their key features. One of these systems includes the type A receptor for the gamma-aminobutyric acid (GABA), which forms an integral anion channel from a pentameric subunit assembly and mediates most of the fast inhibitory neurotransmission in the adult vertebrate central nervous system. Up to now, depending on the definition, 16-19 mammalian subunits have been cloned and localized on different genes. Their assembly into proteins in a poorly defined stoichiometry forms the basis of functional and pharmacological GABA(A) receptor diversity, i.e. the receptor subtypes. The latter has been well documented in autoradiographic studies using ligands that label some of the receptors' various binding sites, corroborated by recombinant expression studies using the same tools. Significantly less heterogeneity has been found at the physiological level in native receptors, where the subunit combinations have been difficult to dissect. This review focuses on the characteristics, use and usefulness of various ligands and their binding sites to probe GABA(A) receptor properties and to gain insight into the biological function from fish to man and into evolutionary conserved GABA(A) receptor heterogeneity. We also summarize the properties of the novel mouse models created for the study of various brain functions and review the state-of-the-art imaging of brain GABA(A) receptors in various human neuropsychiatric conditions. The data indicate that the present ligands are only partly satisfactory tools and further ligands with subtype-selective properties are needed for imaging purposes and for confirming the behavioral and functional results of the studies presently carried out in gene-targeted mice with other species, including man.

Loss of the major GABA(A) receptor subtype in the brain is not lethal in mice

The alpha1beta2gamma2 is the most abundant subtype of the GABA(A) receptor and is localized in many regions of the brain. To gain more insight into the role of this receptor subtype in the modulation of inhibitory neurotransmission, we generated mice lacking either the alpha1 or beta2 subunit. In agreement with the reported abundance of this subtype, >50% of total GABA(A) receptors are lost in both alpha1-/- and beta2-/- mice. Surprisingly, homozygotes of both mouse lines are viable, fertile, and show no spontaneous seizures. Initially half of the alpha1-/- mice died prenatally or perinatally, but they exhibited a lower mortality rate in subsequent generations, suggesting some phenotypic drift and adaptive changes. Both adult alpha1-/- and beta2-/- mice demonstrate normal performances on the rotarod, but beta2-/- mice displayed increased locomotor activity. Purkinje cells of the cerebellum primarily express alpha1beta2gamma2 receptors, and in electrophysiological recordings from alpha1-/- mice GABA currents in these neurons are dramatically reduced, and residual currents have a benzodiazepine pharmacology characteristic of alpha2- or alpha3-containing receptors. In contrast, the cerebellar Purkinje neurons from beta2-/- mice have only a relatively small reduction of GABA currents. In beta2-/- mice expression levels of all six alpha subunits are reduced by approximately 50%, suggesting that the beta2 subunit can coassemble with alpha subunits other than just alpha1. Our data confirm that alpha1beta2gamma2 is the major GABA(A) receptor subtype in the murine brain and demonstrate that, surprisingly, the loss of this receptor subtype is not lethal.

Input-dependent synaptic targeting of alpha(2)-subunit-containing GABA(A) receptors in synapses of hippocampal pyramidal cells of the rat

Pyramidal cells, expressing at least 14 subunits of the heteropentameric GABA(A) receptor, receive GABAergic input on their soma and proximal dendrites from basket cells, activating GABA(A) receptors and containing either parvalbumin or cholecystokinin and vasoactive intestinal polypeptide. The properties of GABA(A) receptors are determined by the subunit composition, and synaptic receptor content governs the effect of the presynaptic neuron. Using a quantitative electron microscopic immunogold technique, we tested whether the synapses formed by the two types of basket cell show a difference in the subunit composition of GABA(A) receptors. Terminals of one of the basket cells were identified by antibodies to parvalbumin. Synapses made by parvalbumin-negative terminals showed five times more immunoreactivity for the alpha(2) subunit than synapses made by parvalbumin-positive basket cells, whose synapses were frequently immunonegative. This difference is likely to be due to specific GABA(A) receptor alpha subunit composition, because neither synaptic size nor immunoreactivity for the beta(2/3) subunits, indicating total receptor content, was different in these two synapse populations. Synapses established by axo-axonic cells on axon initial segments showed an intermediate number of immunoparticles for the alpha(2) subunit compared to those made by basket cells but, due to their smaller size, the density of the alpha(2) subunit immunoreactivity was higher in synapses on the axon. Because the two basket cell types innervate the same domain of the pyramidal cell, the results indicate that pyramidal cells have mechanisms to target GABA(A) receptors, under presynaptic influence, preferentially to distinct synapses. The two basket cell types act via partially distinct GABA(A) receptor populations.

Relative contribution by GABA or glycine to Cl(-)-mediated synaptic transmission on rat hypoglossal motoneurons in vitro

The relative contribution by GABA and glycine to synaptic transmission of motoneurons was investigated using an hypoglossus nucleus slice preparation from neonatal rats. Spontaneous, miniature, or electrically evoked postsynaptic currents (sPSCs, mPSCs, ePSCs, respectively) mediated by glycine or GABA were recorded under whole cell voltage clamp after blocking excitatory glutamatergic transmission with kynurenic acid. The overall majority of Cl(-)-mediated sPSCs was glycinergic, while only one-third was GABAergic; 70 +/- 10% of mPSCs were glycinergic while 22 +/- 8% were GABAergic. Tetrodotoxin (TTX) application dramatically reduced the frequency (and slightly the amplitude) of GABAergic events without changing frequency or amplitude of glycinergic sPSCs. These results indicate that, unlike spontaneous GABAergic transmission, glycine-mediated neurotransmission was essentially independent of network activity. There was a consistent difference in the kinetics of GABAergic and glycinergic responses as GABAergic events had significantly slower rise and decay times than glycinergic ones. Such a difference was always present whenever sPSCs, mPSCs, or ePSCs were measured. Finally, GABAergic and glycinergic mPSCs were differentially modulated by activation of glutamate metabotropic receptors (mGluRs), which are abundant in the hypoglossus nucleus. In fact, the broad-spectrum mGluR agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (50 microM), which in control solution increased the frequency of both GABAergic and glycinergic sPSCs, enhanced the frequency of glycinergic mPSCs only. These results indicate that on brain stem motoneurons, Cl(-)-mediated synaptic transmission is mainly due to glycine rather than GABA and that GABAergic and glycinergic events differ in terms of kinetics and pharmacological sensitivity to mGluR activation or TTX.

Expression of GABA(A) receptor alpha5 subunit-like immunoreactivity in human hippocampus

To investigate the distribution of GABA(A) receptor alpha5 subunit expression in brain, polyclonal antisera were raised, characterised and applied to human and rat brain sections. The resultant antibodies detected a major band of 53 kDa in sodium dodecyl sulphate-polyacrylamide gel electrophoresis immunoblots. Abundant immunostaining was demonstrated in the hippocampal formation in multiple cell types, although predominantly in pyramidal neurons. These data are supportive of GABA-ergic involvement in cognition, and suggest that this influence may be mediated through receptors containing the alpha5 subunit.

Colocalization of multiple GABA(A) receptor subtypes with gephyrin at postsynaptic sites

Clustering of gamma aminobutyric acid (GABA)(A) receptors to postsynaptic sites requires the presence of both the gamma2 subunit and gephyrin. Here, we analyzed by double-immunofluorescence staining the colocalization of gephyrin and major GABA(A)-receptor subtypes distinguished by the subunits alpha1, alpha2, alpha3, or gamma2 in adult rat brain. By using confocal laser scanning microscopy, GABA(A)-receptor subunit staining revealed brightly stained clusters that were colocalized with gephyrin-positive clusters of similar size and distribution in several brain regions, including cerebellum, hippocampus, thalamus, and olfactory bulb. In addition, a diffuse staining was observed for GABA(A)-receptor subunits in the neuropil, presumably representing extrasynaptic receptors. Overall, only few gephyrin-positive clusters were not colocalized with GABA(A)-receptor subunit clusters. Electron microscopic analysis in cerebellar cortex confirmed the selective postsynaptic localization of gephyrin. High-resolution images (voxel size, 50 x 50 x 150 nm) were restored with an iterative image deconvolution procedure based on a measured point-spread function to analyze the colocalization between GABA(A)-receptor subunits and gephyrin in individual clusters. This analysis revealed a considerable heterogeneity in the micro-organization of these presumptive GABAergic postsynaptic sites. For instance, whereas gephyrin- and gamma2 subunit-positive clusters largely overlapped in the cerebellar molecular layer, the colocalization was only partial in glomeruli of the granule cell layer, where small gephyrin clusters typically were "embedded" in larger GABA(A)-receptor clusters. These findings show that gephyrin is associated with a majority of GABA(A)-receptor subtypes in brain, and document the usefulness of image deconvolution for analyzing the structural organization of the postsynaptic apparatus by fluorescence microscopy.

Differential sensitivity to Zolpidem of IPSPs activated by morphologically identified CA1 interneurons in slices of rat hippocampus

Hippocampal pyramidal cells express several alpha-subunits, which determine the affinity of GABAA (gamma-aminobutyric acid) receptors for benzodiazepine site ligands. This study asked whether inhibitory postsynaptic potentials (IPSPs) elicited by specific interneuronal subclasses were differentially sensitive to the alpha1-preferring agonist Zolpidem, i.e. whether different receptors mediate different inhibitory connections. Paired intracellular recordings in which the presynaptic cell was an interneuron and the postsynaptic cell a CA1 pyramid were performed in slices of adult rat hippocampus. Resultant IPSPs were challenged with Zolpidem, cells filled with biocytin and identified morphologically. IPSPs elicited by fast spiking (FS) basket cells (n = 9) were enhanced more than IPSPs elicited by regular spiking (RS) basket cells (n = 10). At FS basket cell synapses the efficacy of Zolpidem was equivalent to that of Diazepam, while RS basket cell IPSPs are enhanced 50% less by Zolpidem than by Diazepam. Thus, while alpha1 subunits may dominate at synapses supplied by FS basket cells, RS basket cell synapses also involve alpha2/3 subunits. Two bistratified cell IPSPs tested with Zolpidem did not increase in amplitude, despite powerful enhancements of bistratified cell IPSPs by Diazepam, consistent with previous indications that these synapses utilize alpha5-containing receptors. Enhancements of basket cell IPSPs by Zolpidem and Diazepam were bi- or triphasic with steep amplitude increases separated by plateaux, occurring 10-15, 25-30 and 45-55 min after adding the drug to the bath. The entire enhancement was, however, blocked by the antagonist Flumazenil (n = 7). Flumazenil, either alone (n = 3), or after Zolpidem, reduced IPSP amplitude to approximately 90% of control, suggesting that alpha4-containing receptors were not involved.

Prevalence of the GABAA receptor assemblies containing alpha1-subunit in the rat cerebellum and cerebral cortex as determined by immunoprecipitation: lack of modulation by chronic ethanol administration

The anti-alpha1 antibody elicited higher immunoprecipitation (%) values of the [3H]flunitrazepam and [3H]muscimol binding activity in the rat cerebellum vs. cerebral cortex, whereas immunoprecipitation values for [3H]Ro 15-4513 and [3H]zolpidem were comparable in these brain regions. Chronic ethanol administration neither changed the radioligand binding to the immunoprecipitated pellet nor the percentage immunoprecip-itation values, thereby indicating that chronic ethanol did not result in down-regulation of the GABAA receptor assemblies containing alpha1-subunit.

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.

Postsynaptic clustering of major GABAA receptor subtypes requires the gamma 2 subunit and gephyrin

Most fast inhibitory neurotransmission in the brain is mediated by GABAA receptors, which are mainly postsynaptic and consist of diverse alpha and beta subunits together with the gamma 2 subunit. Although the gamma 2 subunit is not necessary for receptor assembly and translocation to the cell surface, we show here that it is required for clustering of major postsynaptic GABAA receptor subtypes. Loss of GABAA receptor clusters in mice deficient in the gamma 2 subunit, and in cultured cortical neurons from these mice, is paralleled by loss of the synaptic clustering molecule gephyrin and synaptic GABAergic function. Conversely, inhibiting gephyrin expression causes loss of GABAA receptor clusters. The gamma 2 subunit and gephyrin are thus interdependent components of the same synaptic complex that is critical for postsynaptic clustering of abundant subtypes of GABAA receptors in vivo.

Rat and human hippocampal alpha5 subunit-containing gamma-aminobutyric AcidA receptors have alpha5 beta3 gamma2 pharmacological characteristics

The gamma-aminobutyric acid (GABA)A receptor is a hetero-oligomer consisting of five subunits, the combination of which confers unique pharmacological properties to the receptor. To understand the physiological role of native GABAA receptors, it is critical to determine their subunit compositions. The pharmacological characteristics of human alpha5 beta3 gamma2 and alpha5beta3gamma3 GABAA receptors stably expressed in L(tk-) cells were characterized with the alpha5-selective ligand [3H]L-655,708 and compared with the pharmacological characteristics of [3H]L-655,708 binding sites from rat and human hippocampus. Saturation analyses revealed a 9-fold selective affinity of [3H]L-655,708 for alpha5 beta3 gamma2 receptors (Kd = 1.7 +/- 0.4 nM), compared with alpha5 beta3 gamma3 receptors (Kd = 15 +/- 3 nM). Rat and human hippocampal [3H]L-655,708 binding sites had affinities of 2.2 +/- 0.6 and 1.0 +/- 0.2 nM, respectively, comparable to the affinity of alpha5 beta3 gamma2 receptors. Pharmacological analysis of [3H]L-655,708 binding sites in rat and human hippocampi revealed a strong correlation with the affinities of seven benzodiazepine site ligands for alpha5 beta3 gamma2 but not alpha5 beta3 gamma3 receptors. Immunoprecipitation of [3H]L-655,708 binding sites from rat hippocampus with a gamma2-selective antibody yielded 19 +/- 4% of total benzodiazepine binding sites measured using [3H]Ro15-1788, whereas no specific binding was measured after immunoprecipitation with an anti-gamma3 antibody. Combinatorial immunoprecipitations of [3H]muscimol binding sites with anti-alpha5 and anti-gamma2 or anti-alpha5 and anti-gamma3 antibodies established the preferential expression of alpha5 gamma2 receptors, accounting for 22 +/- 2% of total rat hippocampal GABAA receptors. These observations provide pharmacological and structural evidence for the prevalence of alpha5 beta3 gamma2 GABAA receptors in rat hippocampus, despite the clustering of alpha5 and gamma3 loci on the same chromosome.

Independent assembly and subcellular targeting of GABA(A)-receptor subtypes demonstrated in mouse hippocampal and olfactory neurons in vivo

The ability of neurons to display more than a single GABA(A)-receptor subtype per cell requires intricate targeting mechanisms. Analysis by confocal laser scanning microscopy revealed that the alpha2- and alpha5-subunits differed strikingly in their subcellular distribution in hippocampal pyramidal cells and olfactory bulb granule cells, while the distribution of the gamma2-subunit was rather uniform. In mutant mice lacking the alpha5-subunit gene due to a chromosomal deletion, the absence of the alpha5-subunit was accompanied by a corresponding decrease of the gamma2-subunit immunoreactivity. In striking contrast, the subcellular distribution of the alpha2-subunit was unchanged in these mutant mice. These findings indicate that the assembly of distinct GABA(A)-receptor subtypes in the same neuron is regulated independently. Furthermore, the alpha-subunit is a prime candidate for providing domains which direct subcellular targeting.

GABA(A)-receptor assembly in vivo: lessons from subunit mutant mice

The rules governing the assembly of GABA(A) receptors in vivo were assessed in subunit mutant mice. The transcription of individual subunit genes was regulated independently. The lack of a particular subunit did not result in a molecular rescue by an enhanced transcription of other subunits. In addition, the availability of an alpha- and beta-subunit was essential for receptor formation. Finally, highly selective recognition processes directed the subcellular targeting of receptors. The loss of a particular receptor subtype (alpha5) did not lead to a subcellular redistribution of the remaining subtype (alpha2) present in the same cell.