The gamma 3-subunit of the GABAA-receptor confers sensitivity to benzodiazepine receptor ligands

Identification of distinct GABAA-receptor subtypes in cholinergic and parvalbumin-positive neurons of the rat and marmoset medial septum-diagonal band complex

GABAA-receptor heterogeneity is based on a multiplicity of subunits (alpha 1-6, beta 1-4, gamma 1-4, delta, rho 1-2) encoded by distinct genes. Flexibility in GABAergic signal transduction and allosteric modulation is expected to arise from the differential assembly of subunits into receptor subtypes. The aim of the present study was to investigate the potential diversity of receptor subtypes expressed by defined neuron populations, as identified by their neurotransmitter phenotype. To this end we have determined immunohistochemically the subunit repertoire of cholinergic and GABAergic neurons in the basal forebrain of rat and marmoset monkey, focusing on the medial septum-diagonal band complex. Co-localization of the GABAA-receptor subunits alpha 1, alpha 3, beta 2, beta 3, and gamma 2 with markers of cholinergic and GABAergic neurons (choline acetyltransferase and parvalbumin, respectively) was assessed by double- and triple immunofluorescence staining. The results reveal that cholinergic neurons in the rat basal forebrain are typically characterized by the subunit combination alpha 3/beta 3/gamma 2, whereas most of the parvalbumin-positive GABAergic neurons express either the subunit combination alpha 1/beta 2/gamma 2 or the combination alpha 1/alpha 3/beta 2/gamma 2. A similar pattern was observed in marmoset monkey, with GABAA-receptors containing the alpha 1-subunit being associated with parvalbumin-positive cells, but never with cholinergic neurons. Thus, the expression of distinct subunit repertoires by cholinergic and GABAergic neurons points to a functional specialization which is conserved across species. These subunit combinations are likely to correspond to different receptor subtypes, and may reflect the engagement of cholinergic and GABAergic neurons in distinct neuronal circuits in the basal forebrain.

Biological function of GABAA/benzodiazepine receptor heterogeneity

gamma-Aminobutyric acid (GABA) is the most prominent of the inhibiting neurotransmitters in the brain. It exerts its main action through GABAA receptors. The receptors respond to the presence of GABA by the opening of an intrinsic anion channel. Hence, they belong to the molecular superfamily of ligand-gated ion channels. There exist in the brain multiple GABAA receptors that show differential distribution and developmental patterns. The receptors presumably form by the assembly of five proteins from at least three different subunits (alpha 1-6, beta 1-3 and gamma 1-3). The regulation of functional properties by benzodiazepine (BZ) receptor ligands, neurosteroids, GABA and its analogs differs dramatically with the alpha variant present in the complex. Additional variation of the GABAA receptors comes with the exchange of the gamma subunits. No clear picture exists for the role of the beta subunits, though they may play an important part in the sensitivity of the channel-receptor complex. The effects of BZ receptor ligands on animal behavior range from agonist effects, e.g. anxiolysis, sedation, and hypnosis, to inverse agonist effects, e.g. anxiety, alertness, and convulsions. The diversity of effects reflects the ubiquity of the GABAA/BZ receptors in the brain. Recent data provide some insight into the mechanism of action of BZ ligands, but no clear delineation can be drawn from a single ligand to a single behavioral effect. This may be due to the fact that intrinsic efficacies of the ligands differ between receptor subtypes, so that the diversity of native receptors is further complicated by the diversity of the mode the ligands act on GABAA receptor subtypes. The behavioral actions of alcohol (ethanol) are similar to those produced by GABAA receptor agonists. In agreement, alcohol-induced potentiation of GABAergic responses has often been observed at behavioral, electrophysiological and biochemical levels. Thus, there is clearly a GABAA-dependent component in the actions of alcohol. However, the site and mode of action of ethanol on GABAA/BZ receptors remain controversial.

The pharmacology of the benzodiazepine site of the GABA-A receptor is dependent on the type of gamma-subunit present

The pharmacology of native and recombinant GABA-A receptors containing either gamma1, gamma2 or gamma3 subunits has been investigated. The pharmacology of native receptors has been investigated by immunoprecipitating receptors from solubilised preparations of rat brain with antisera specific for individual gamma-subunits and analysing their radioligand binding characteristics. Receptors containing a gamma1-subunit do not bind benzodiazepine radioligands with high affinity. Those containing either a gamma2 or gamma3 subunit bind [3H]flumazenil with high affinity. Some compounds compete for these binding sites with multiple affinities, reflecting the presence of populations of receptors containing several different types of alpha-subunit. Photoaffinity-labelling of GABA-A receptors from a cell line stably expressing GABA-A receptors of composition alpha1beta3gamma2 followed by immunoprecipitation of individual subunits revealed that the alpha and gamma but not the beta-subunit could be irreversibly labelled by [3H]flunitrazepam. The properties of recombinant receptors have been investigated in oocytes expressing gamma1, gamma2, or gamma3 subunits in combination with an alpha and a beta-subunit. Some compounds such as zolpidem, DMCM and flunitrazepam show selectivity for receptors containing different gamma-subunits. Others such as CL 218,872 show no selectivity between receptors containing different gamma-subunits but exhibit selectivity for receptors containing different alpha-subunits. These data taken together suggest that the benzodiazepine site of the GABA-A receptor is formed with contributions from both the alpha and gamma-subunits.

Astroglia-released factor shows similar effects as benzodiazepine inverse agonists

Media conditioned by cultured neonatal cerebral cortex microexplants (CCM) or astrocytes (ACM) contain low molecular weight (< 1,000 Da) substance(s) which inhibits the gamma aminobutyric acid (GABA)-induced inward current recorded in cerebellar granule cells and hippocampal neurons in culture using the whole-cell patch-clamp technique. This effect is specific for CCM and ACM, as medium conditioned by PC12 cells (PC12CM) does not affect the GABA response of these cells. It is also specific for GABA-induced currents because glutamate-induced currents do not change either in amplitude or in shape in the presence of CCM or ACM. The inhibitory effect on the GABA response in cerebellar granule cells of both ACM and CCM could be suppressed by flumazenil, a specific benzodiazepine (BZD) antagonist and could be mimicked by two BZD inverse agonists. These data thus demonstrate the presence of a BZD inverse agonist-like activity in CCM and ACM. This effect of ACM on different neuronal cell types was heterogenous since no detectable effect could be observed on the GABA-induced current in GABA-responsive dorsal root ganglion (DRG) neurons, presumably reflecting a functional heterogeneity of the GABAA receptors present in these different neuronal subsets. By the release of such an endogenous BZD inverse agonist-like activity, glia cells could possibly modulate GABAA receptor-mediated responses.

Recent developments in the behavioral pharmacology of benzodiazepine (omega) receptors: evidence for the functional significance of receptor subtypes

Recent research in molecular biology has demonstrated the complexity of GABAA receptors and shown that benzodiazepine (BZ-omega) receptor subtypes have a structural reality. It is therefore appropriate to ask whether the different pharmacological effects produced by benzodiazepines (anticonvulsant activity, anxiety reduction, motor incoordination, learning deficits, characteristic discriminative stimulus effects, tolerance and dependence) are associated with activity at different receptor subtypes. The present paper reviews the literature dealing with the behavioral effects of novel BZ (omega) receptor ligands relevant to the question of the functional significance of the BZ1 (omega 1) and BZ2 (omega 2) receptor subtypes. The only drugs currently available with a considerable degree of selectivity are alpidem and zolpidem. These compounds have relatively high affinity for GABAA receptors containing the alpha 1 subunit (corresponding to the BZ1 (omega 1) subtype) and very low affinity for receptors with the alpha 5 subunit (corresponding to one type of BZ2 (omega 2) receptor). Pharmacological effects observed with these, and other, less selective compounds allow several tentative conclusions to be drawn: (a) Little is known of the role of subtype selectivity in anxiolytic or amnestic effects but compounds with low intrinsic activity may reduce anxiety without giving rise to sedation or motor incoordination and BZ1 (omega 1) selective drugs appear to disrupt memory only at sedative doses; (b) Selectivity for BZ1 (omega 1) receptors may be associated with sleep-inducing activity but not with motor incoordination, suggesting that BZ2 (omega 2) receptors may be of particular importance in mechanisms of muscle relaxation; (c) The discriminative stimulus effects of different BZ (omega) receptor ligands are not identical and differences may be related to receptor selectivity; (d) Compounds with BZ1 (omega 1) selectivity and compounds with low intrinsic activity produce little or no tolerance and dependence. A wider range of selective compounds will be necessary to investigate these factors in detail and many different pharmacological profiles can be expected from drugs with selectivity and different levels of intrinsic activity.

Sequence and novel distribution of the chicken homologue of the mammalian gamma-aminobutyric acidA receptor gamma 1 subunit

cDNAs have been cloned that encode the chicken (Gallus domesticus) gamma-aminobutyric acidA receptor gamma 1 subunit, the mature sequence of which shares 90, 79, and 69% identity with those of the rat gamma 1, gamma 2, and gamma 3 subunits, respectively. In situ hybridization reveals that there are pronounced differences in the regional and cellular localizations of the corresponding gamma-aminobutyric acidA receptor gamma-subunit mRNA compared with that of the gamma 2-subunit mRNA in 1-day-old chick brain. The absence of the gamma 1-subunit transcript in certain chick brain nuclei of visual and auditory pathways, in which gamma 2-subunit mRNA is present, points to differences in the functional roles of receptors containing one or other of these polypeptides. Certain cells in other brain regions appear to contain both gamma 1- and gamma 2-subunit mRNAs, suggesting that they either have two gamma-aminobutyric acidA receptor subtypes or possess receptors incorporating two different gamma subunits. We have also found contrasts in the distribution patterns, in homologous brain regions, of the chicken gamma 1-subunit mRNA and the rat gamma 1-subunit mRNA. These data may reflect different functional roles of the chicken and rat gamma 1 subunits.

Differential properties of type I and type II benzodiazepine receptors in mammalian CNS neurones

1. The effects of benzodiazepine receptor (BZR) partial agonists, Y-23684 and CL218,872, were compared with its full agonist, diazepam, on gamma-aminobutyric acid (GABA)-induced Cl- current (ICl) in acutely dissociated rat cerebral cortex (CTX), cerebellar Purkinje (CPJ) and spinal ventral horn (SVH) neurones, by the whole-cell mode patch-clamp technique. 2. The GABA-induced responses were essentially the same in both SVH and CPJ neurones, but the KD value of the GABA response in CTX neurone was lower than those in the other two brain regions. 3. Enhancement of the GABA response by the two partial agonists was about one-third of that by diazepam in the SVH neurones (where type II subtype of BZR, BZ2, is predominant), whereas these partial agonists potentiated the GABA response as much as diazepam in CPJ neurones (where the type I subtype of BZR, BZ1, is predominant). In CTX neurones where both type I and II variants are expressed, the augmentation ratio of the GABA response by diazepam was between the values in CPJ and SVH neurones. 4. In concentration-response relationships of BZR partial agonists, the threshold concentrations, KD values and maximal augmentation ratio of the GABA response were similar in all CTX, CPJ and SVH neurones. Also, in all preparations, the threshold concentration and KD values of diazepam action were 10 fold less than those induced by partial agonists. 5. All BZR agonists shifted the concentration-response relationship for GABA to the left without changing the maximum current amplitude, indicating that activation of both BZ1 and BZ2 increase the affinity of the GABAA receptor for GABA. 6. The results are important in clarifying the mechanism of anxiety and might explain the anxioselectivity of BZR partial agonists.

GABAA/benzodiazepine receptor gamma 2 subunit gene expression in developing normal and mutant mouse cerebellum

Recent studies have identified several subunits (alpha, beta, gamma and delta) of the gamma-aminobutyric acidA/benzodiazepine receptor; each consists of several variants. The gamma 2 subunit appears to mediate the interaction of the alpha and beta subunits making the receptor capable of modulation by benzodiazepines. In the present studies, the expression of mRNA encoding the gamma 2 subunit was examined in the cerebellum during development and in adult Purkinje cell degeneration, lurcher and reeler mutant mice. In the normal adult cerebellum, in situ hybridization with [35S]cRNA probes revealed a strong signal over the Purkinje cell layer and deep cerebellar nuclei, and a weaker signal over basket, stellate and granule cells. Labeling over Purkinje cells was detectable at birth, gradually becoming stronger and more punctate during postnatal weeks 1 and 2, as Purkinje cells formed a monolayer between the molecular and granule cell layers. Adult levels of grain density were reached by P20. The external germinal layer, which contained proliferating granule cells, was unlabeled throughout development; however, weak labeling was detected over the internal granular layer at the end of postnatal week 1, as granule cells began their migration across the molecular layer. During the second postnatal week, punctate labeling became visible over the molecular layer in a distribution indicative of basket and stellate cells. In adult Purkinje cell degeneration and lurcher mutants, in which Purkinje cells have degenerated, no punctate labeling characteristic of mature Purkinje cells was detected. In adult and developing reeler mutants, where all classes of cells are malpositioned throughout the cerebellum, the punctate hybridization signal was present and clearly associated with Purkinje cells in all cortical regions. Our results suggest that developing Purkinje cells express the gamma 2 gene at a time prior to receiving GABAergic inhibitory input, and that the continued expression in the adult is not affected by the absence of afferents.

Neuron-specific expression of GABAA-receptor subtypes: differential association of the alpha 1- and alpha 3-subunits with serotonergic and GABAergic neurons

GABAA-receptors in the brain display a striking structural heterogeneity, which is based on a multiplicity of diverse subunits. The allocation of GABAA-receptor subtypes to identified neurons is essential for an analysis of the functional significance of receptor heterogeneity. Among GABA-receptive neurons, well-characterized examples include the serotonergic and GABAergic neurons in the raphe nuclei. The GABAA-receptor subtypes expressed in these two types of neurons were analysed using antisera which recognize selectively the alpha 1- and alpha 3-subunits, and their co-localization with serotonin and glutamate decarboxylase was assessed by confocal laser microscopy in double and triple immunofluorescence staining in the rat. The vast majority of serotonergic neurons express strong alpha 3-subunit-immunoreactivity, but are devoid of alpha 1-subunit staining. In contrast, both the alpha 1- and alpha 3-subunit-immunoreactivities are present in glutamate decarboxylase-positive neurons. Thus, serotonergic and GABAergic neurons selectively express distinct patterns of alpha subunits, suggesting that they possess distinct subtypes of GABAA-receptors. The occurrence of neuron-specific GABAA-receptor subtypes may open new possibilities for the targeting of drugs with selective therapeutic actions.

Co-existent expression of GABAA receptor beta 2, beta 3 and gamma 2 subunit messenger RNAs during embryogenesis and early postnatal development of the rat central nervous system

The expression of beta 1, beta 2, beta 3, gamma 2 and delta subunit messenger RNAs of the GABAA receptor was followed by in situ hybridization histochemistry using radiolabeled oligodeoxynucleotide probes in sections of embryonic (E12-21) and early postnatal (P1-5) rat. beta 2, beta 3 and gamma 2 subunit messenger RNAs were first detectable at E15 in the spinal cord (ventral > dorsal) and lower central nervous system regions (e.g. pons, medulla and thalamus). beta 3 subunit messenger RNA was abundantly expressed in olfactory bulb neurons at E15. At E17, the expression pattern of these subunit messenger RNAs continued in the lower central nervous system. In the upper central nervous system, beta 2, beta 3, and gamma 2 subunit messenger RNAs were first detectable in the outer layer of the hippocampal and entire cortical neuroepithelium. The expression for both beta 3 and gamma 2 subunit messenger RNAs increased significantly over that observed at E15, whereas beta 2 subunit messenger RNA increased to a lesser extent and was more discretely expressed in inferior colliculus, cerebellar neuroepithelium and spinal cord (ventral = dorsal). By E19, messenger RNAs for beta 2, beta 3 and gamma 2 subunits a widespread and abundant co-existent distribution throughout the central nervous system. Exceptions to this co-expression were the absence of beta 2 messenger RNA in the dentate gyrus and beta 3 messenger RNA in entorhinal cortex, areas in which they are present in adult. There was also a differential distribution of subunit messenger RNAs in developing olfactory bulb at E19-20: the glomerular cells preferentially expressed beta 3 and gamma 2 subunit messenger RNAs; the mitral cells preferentially expressed beta 2 subunit messenger RNA; inner granule cells expressed moderate levels of beta 2, beta 3 and gamma 2 subunit messenger RNAs. Expression of beta 2, beta 3 and gamma 2 messenger RNAs was also anatomically co-existent at P5. In addition, significant expression of beta 1 and delta subunit messenger RNAs was apparent in hippocampus and entorhinal cortex. The identity of the gamma 2 expressed between E15 and E21 was shown to be mostly the short isoform of gamma 2 subunit messenger RNA. Expression of both forms was evident beginning around P3-5. These results indicate that during the late embryonic and early postnatal period of development, beta 2, beta 3 and gamma 2 subunit messenger RNAs are abundantly expressed and co-localized to most central nervous system regions.(ABSTRACT TRUNCATED AT 400 WORDS)

Antibodies to the human gamma 2 subunit of the gamma-aminobutyric acidA/benzodiazepine receptor

A gamma-aminobutyric acidA (GABAA) receptor (GABAAR) gamma 2 subunit (short form) was cloned from an adult human cerebral cortex cDNA library in bacteriophage lambda gt11. The 261-bp intracellular loop (IL) located between M3 and M4 was amplified using the polymerase chain reaction and inserted into the expression vectors lambda gt11 and pGEX-3X. Both beta-galactosidase (LacZ) and glutathione-S-transferase (GST) fusion proteins containing the gamma 2IL were purified, and a rabbit antibody to the LacZ-gamma 2IL was made. The antibody reacted with the gamma 2IL of both LacZ and GST fusion proteins and immunoprecipitated the GABAAR/benzodiazepine receptor (GABAAR/BZDR) from bovine and rat brain. The antibody reacted in affinity-purified GABAAR/BZDR immunoblots with a wide peptide band of 44,000-49,000 M(r). Immunoprecipitation studies with the anti-gamma 2IL antibody suggest that in the cerebral cortex, 87% of the GABAARs with high affinity for benzodiazepines and 70% of the GABAARs with high affinity for muscimol contain at least a gamma subunit, probably a gamma 2. These results indicate that there are [3H]muscimol binding GABAARs that do not bind [3H]flunitrazepam with high affinity. Immunoprecipitations with this and other anti-GABAAR/BZDR antibodies indicate that the most abundant combination of GABAAR subunits in the cerebral cortex involves alpha 1, gamma 2 (or other gamma), and beta 2 and/or beta 3 subunits. These subunits coexist in > 60% of the GABAAR/BZDRs in the cerebral cortex. The results also show that a considerable proportion (20-25%) of the cerebellar GABAAR/BZDRs is clonazepam insensitive. At least 74% of these cerebellar receptors, which likely contain alpha 6, also contain gamma 2 (or other gamma) subunit(s). The alpha 1 and beta 2 or beta 3 subunits are also frequently associated with gamma 2 (or other gamma) and alpha 6 in these cerebellar receptors.

Comparative molecular neuroanatomy of cloned GABAA receptor subunits in the rat CNS

gamma-Aminobutyric acidA (GABAA) receptors in the mammalian central nervous system (CNS) are members of a family of ligand-gated ion channels consisting of heterooligomeric glycoprotein complexes in synaptic and extrasynaptic membranes. Although molecular cloning studies have identified 5 subunits (with approximately 40% amino acid homology) and isoforms thereof (approximately 70% homology), namely alpha 1-6, beta 1-4, gamma 1-3, delta, and rho, the subunit composition and stoichiometry of native receptors are not known. The regional distribution and cellular expression of GABAA receptor messenger RNAs (mRNAs) in the rat CNS have now been investigated by in situ hybridization histochemistry with subunit-specific 35S-labelled oligonucleotide probes on adjacent cryostat sections. Whereas alpha 1, beta 2, and gamma 2 transcripts were the most abundant and ubiquitous in the rat brain--correlating with the radioautographic distribution of GABAA receptors revealed by an ionophore ligand--others had a more restricted expression while often being abundant. For example, alpha 2 transcripts were found only in the olfactory bulb, cerebral cortex, caudate putamen, hippocampal formation, and certain lower brain stem nuclei; alpha 3 only in the olfactory bulb and cerebral cortex; alpha 5 in the hippocampal formation; and alpha 6 only in cerebellar granule cells. In addition, beta 1, beta 3, gamma 1, and delta mRNAs were also uniquely expressed in restricted brain regions. Moreover, in the spinal cord, alpha 1-3, beta 2,3, and gamma 2 mRNAs were differently expressed in Rexed layers 2-9, with alpha 2, beta 3, and gamma 2 transcripts most prominent in motoneurons of layer 9. Although differential protein trafficking could lead to the incorporation of some subunits into somatic membranes and others into dendritic membranes, some tentative conclusions as to the probable composition of native proteins in various regions of the CNS may be drawn.(ABSTRACT TRUNCATED AT 400 WORDS)

Receptor classes and the transmitter-gated ion channels

Transmitter-gated channels, which can be selective for cations or for anions, form an important class among the membrane receptors responsible for signal transduction. Thirteen principal types of these channels can now be recognized and most of these are available for analysis in recombinant form. It is instructive to contrast their characteristic structural features with those of the two other primary classes of the signal-transducing receptors of membranes.

GABAA receptor channels: from subunits to functional entities

GABAA receptor channels mediate postsynaptic inhibition. The functional diversity of these receptors rests on differences in subunit composition and on a large repertoire of subunits. Subunit expression patterns in the brain have been found to predict in vivo compositions of GABAA receptors. In addition, molecular determinants underlying the differential binding properties of allosteric ligands to receptor subtypes have been identified.

Gene expression in cells of the central nervous system

This review summarized a part of our studies over a long period of time, relating them to the literature on the same topics. We aimed our research toward an understanding of the genetic origin of brain specific proteins, identified by B. W. Moore and of the high complexity of the nucleotide sequence of brain mRNA, originally investigated by W. E. Hahn, but have not completely achieved the projected goal. According to our studies, the reason for the high complexity in the RNA of brain nuclei might be the high complexity in neuronal nuclear RNA as described in the Introduction. Although one possible explanation is that it results from the summation of RNA complexities of several neuronal types, our saturation hybridization study with RNA from the isolated nuclei of granule cells showed an equally high sequence complexity as that of brain. It is likely that this type of neuron also contains numerous rare proteins and peptides, perhaps as many as 20,000 species which were not detectable even by two-dimensional PAGE. I was possible to gain insight into the reasons for the high sequence complexity of brain RNA by cloning the cDNA and genomic DNA of the brain-specific proteins as described in the previous sections. These data provided evidence for the long 3'-noncoding regions in the cDNA of the brain-specific proteins which caused the mRNA of brain to be larger than that from other tissues. During isolation of such large mRNAs, a molecule might be split into a 3'-poly(A)+RNA and 5'-poly(A)-RNA. In the studies on genomic DNA, genes with multiple transcription initiation sites were found in brain, such as CCK, CNP and MAG, in addition to NSE which was a housekeeping gene, and this may contribute to the high sequence complexity of brain RNA. Our studies also indicated the presence of genes with alternative splicing in brain, such as those for CNP, MAG and NGF, suggesting a further basis for greater RNA nucleotide sequence complexity. It is noteworthy that alternative splicing of the genes for MBP and PLP also produced multiple mRNAs. Such a mechanism may be a general characteristic of the genes for the myelin-specific proteins produced by oligodendrocytes. In considering the high nucleotide sequence complexity, it is interesting that MAG and S-100 beta genes etc. possess two additional sites for poly(A).(ABSTRACT TRUNCATED AT 400 WORDS)