Multiple GABAA receptor alpha subunit mRNAs revealed by developmental and regional expression in rat, chicken and human brain

GABAA receptor expression and white matter disruption in intrauterine growth restricted piglets

Intrauterine growth restriction (IUGR) is one of the most common causes of perinatal mortality and morbidity. White matter and neuronal injury are major pathophysiological features of the IUGR neonatal brain. GABA_A (γ-aminobutyric acid type A) receptors have been shown to play a role in oligodendrocyte differentiation and proliferation in the neonatal brain and may be a key factor in white matter injury and myelination in IUGR neonates. Whether there are impairments to the GABAergic system and neuronal cytoskeleton in IUGR brain has yet to be elucidated. This study aims to examine GABA_A receptor α₁ and α₃ subunit protein expression and distribution in parietal cortex and hippocampus of the IUGR piglet at four different ages (term=115d - days gestational age), 100d, 104d, birth (postnatal day 0-P0) and P7 and to examine neuronal and myelination patterns. Significant alterations to GABA_A receptor α₁ and α₃ protein expression levels were observed in the IUGR piglet brain of P7 IUGR piglets with significantly greater α₃ expression compared to α₁ expression in the hippocampus while there was virtually no difference between the two subunits in the parietal cortex. However a significantly lower α₁/α₃ ratio was evident in P7 IUGR cortex when compared with P7 NG cortex. Neuronal somatodendrites studied using MAP2 immunohistochemistry showed reduced and disrupted somatodendrites while MBP immunolabelling showed loss of axonal fibres from gestational day 104d through to P7. These findings provide insights into the effects of IUGR on the development of the GABA system, altered developmental maturation of GABA_A receptor subunit expression in the IUGR brain may influence myelination and may partly explain the cognitive disabilities observed in IUGR. Understanding the mechanisms behind grey and white matter injury in the IUGR infant is essential to identifying targets for treatments to improve long-term outcomes for IUGR infants.

Developmental expression and distribution of GABA(A) receptor α1-, α3- and β2-subunits in pig brain

The principal function of the γ-aminobutyric acid (GABA) system in the adult brain is inhibition; however, in the neonatal brain, GABA provides much of the excitatory drive. As the brain develops, transmembrane chloride gradients change and the inhibitory role of GABA is initiated and continues throughout juvenile and adult life. Previous studies have shown that GABA(A) receptor subunit expression is developmentally regulated, and it is thought that the change in GABA function from excitation to inhibition corresponds to the changeover in expression of 'immature' to 'mature' subunit isoforms. We examined the protein expression pattern and distribution of GABA type A (GABA(A)) receptor α1-, α3- and β2-subunits in the parietal cortex and hippocampus of the developing piglet brain. Four perinatal ages were studied; 14 days preterm (P-14), 10 days preterm (P-10), day of birth (P0) and at postnatal day 7 (P7). Animals were obtained by either caesarean section or spontaneous birth. Protein expression levels and subunit localization were analysed by Western blotting and immunohistochemistry, respectively. In the cortex and hippocampus, GABA(A) receptor α1-subunit showed greatest expression at P7 when compared to all other age groups (p < 0.05). In contrast, α3 expression in the cortex was elevated in preterm brain, peaking at P0, followed by a significant reduction by P7 (p < 0.05); a similar trend was observed in the hippocampus. GABA(A) receptor β2-subunit protein expression appeared relatively constant across all time points studied in both the cortex and hippocampus. Immunolabelling of the α1-, α3- and β2-subunits was observed throughout all cortical layers at every age. GABA(A) receptor α3-subunit appeared to show specific localization to layers V and VI whilst labelling for the β2-subunit was observed in layer IV. In the hippocampus of all animals, the α1- and β2-subunits were shown to immunolabel various cells and processes in the dentate gyrus (DG), CA1 and CA3; the α3-subunit was barely observed except at the stratum moleculare of the DG. We report for the first time the ontogenesis of GABA(A) receptor subunits α1, α3 and β2 in the perinatal pig brain.

Characterization of the GABA(A) receptor in the brain of the adult male bullfrog, Rana catesbeiana

Little is known about the properties of GABA receptors in the amphibian brain. The GABA(A) receptor is widespread in the mammalian brain, and can be specifically labeled with the receptor agonist [3H]muscimol. The binding of [3H]muscimol to membrane preparations from the brain of the bullfrog, Rana catesbeiana, was investigated in kinetic, saturation, and inhibition experiments to determine whether this species possessed a GABA(A)-like receptor. Binding of 20 nM [3H]muscimol to membranes was specific and could be displaced by 1 mM GABA. Association binding curves showed that steady state occurred rapidly, within 2 min, and dissociation occurred within 5 min. The receptor was saturable with a single, high-affinity binding site (K(D)=19.2 nM; B(max)=1.8 pmol/mg protein). Binding of [3H]muscimol was inhibited in a dose-dependent fashion by muscimol, GABA, bicuculline methiodide, and bicuculline (in order of potency). Baclofen (at doses from 10(-9) to 10(-3) M) failed to displace [3H]muscimol. The binding characteristics and ligand specificity of [3H]muscimol binding sites in the bullfrog brain support the hypothesis that this amphibian possesses a GABA(A)-like receptor protein similar to the GABA(A) receptor characterized in mammals.

Effects of gamma2S subunit incorporation on GABAA receptor macroscopic kinetics

GABA(A) receptors, the major inhibitory neurotransmitter receptors in the mammalian central nervous system, are heteropentameric proteins. We are interested in understanding the contribution of the gamma subunit to the kinetic properties of GABA(A) receptors. Studies in Xenopus oocytes have suggested that co-expression of alpha1, beta2, and gamma 2S subunits results in the formation of both alpha beta and alpha betagamma receptors (Boileau et al. 2002a; Boileau et al., 1998). Here, we have used an excess of the gamma 2S subunit in transfections of HEK293 cells to bias expression toward alpha beta gamma-containing receptors. Using rapid application and whole cell patch clamp techniques, we found that incorporation of the gamma subunit eliminated the rapid phases of desensitization and accelerated deactivation, consistent with a proposed role of desensitization in slowing deactivation. In addition, alpha betagamma receptors had an increased GABA EC(50), reduced sensitivity to block by Zn(2+), and did not display outward rectification as compared to alpha beta receptors.

In situ GABAergic modulation of synchronous gonadotropin releasing hormone-1 neuronal activity

Evidence indicates that gonadotropin releasing hormone-1 [GnRH-1, also known as luteinizing hormone releasing hormone (LHRH)] neurons can exhibit synchronized neuroendocrine secretory activity before entrance into the CNS. In this study, we used calcium imaging to evaluate patterns of activity in individual, embryonic, GnRH-1 neurons as well as population dynamics of GnRH-1 neurons in mouse nasal explants maintained for 1 versus 3 weeks. Independent of age, GnRH-1 neurons displayed significant calcium peaks that synchronized at an interval of approximately 20 min across multiple GnRH-1 cells within an explant. Acute tetrodotoxin treatment decreased the amplitude of calcium peaks in individual GnRH-1 neurons and the duration but not the frequency of synchronized activity in the population of GnRH-1 neurons. Acute GABA(B) receptor antagonism increased the frequency of synchronized neuronal activity at both ages, whereas acute GABA(A) receptor antagonism decreased calcium oscillations in individual GNRH-1 cells as well as synchronization of the calcium pulses within the GnRH-1 population at the 1 week time point to background non-GNRH-1 cell levels. These results indicate that developing GnRH-1 neurons rely heavily on GABAergic signaling to initiate synchronized bouts of activity but thereafter, possess an innate capacity for synchronized activity patterns that are modulated by, but not completely dependent on GABAergic signaling.

Localization of two major GABA(A) receptor subunits in the dentate gyrus of the rat and cell type-specific up-regulation following entorhinal cortex lesion

GABA(A) receptor subunits show a specific regional distribution in the CNS during development and in the adult animal. In the hippocampal formation, individual subsets of GABAergic interneurons are highly immunoreactive for the alpha1-subunit, whereas granule and pyramidal cells show a strong expression of the alpha2-subunit. Using confocal microscopy and digital image analysis, we demonstrate that in the dentate gyrus the alpha1-subunit immunolabeling appears in differently sized clusters. The large clusters, which are confined to dendrites of interneurons, show no alpha2 labeling, whereas the smaller ones coincide with alpha2-subunit-positive clusters. In the molecular layer, the clusters of both alpha-subunits co-localize with the anchoring protein gephyrin. In the granule cell layer and hilus, we found alpha1- and alpha2-subunit-positive clusters which were devoid of gephyrin labeling. Lesions of the medial entorhinal cortex led to the deafferentation of dendrites in the middle molecular layer of the dentate gyrus. This resulted in a significantly increased concentration of alpha2-subunit-positive clusters. We also observed an increase of alpha1-subunit immunolabeling in the deafferented area. We found no change in the co-localization between alpha1 and alpha2, and no significant change in the number of large alpha1-positive clusters along individual dendritic segments of interneurons. In a previous study, we demonstrated that calbindin-immunoreactive dendrites of granule cells revealed a significant increase in gephyrin immunoreactivity following lesion, whereas parvalbumin-positive dendrites showed no such alterations. The predominant localization of small gephyrin clusters in dendrites of granule cells, which was also described in this study, leads to the conclusion that the increase of the alpha2-subunit-positive clusters, demonstrated in the present study, indicates that, following entorhinal cortex lesion, new GABAergic synapses may be formed and that they contact predominantly granule cell dendrites.

Developmental changes in the expression of GABA(A) receptor subunits (alpha(1), alpha(2), alpha(3)) in the cat visual cortex and the effects of dark rearing

The present study used Western blots and Northern slot blots to determine changes in the level of expression of GABA(A) receptor subunits alpha(1), alpha(2), and alpha(3), in relation to the "critical period" in cat visual cortex. Levels of the GABA(A) alpha(1) subunit were lowest at 1 week, increased four-fold to a maximum at 10 weeks, and declined slightly (35%) into adulthood. Levels of the GABA(A) alpha(2) and alpha(3) subunits were highest at 1 week of age, decreased two-fold by 10 weeks of age and were constant thereafter. Comparison between visual cortex from normal and dark-reared cats at 5 weeks and 20 weeks showed that alpha(1) and alpha(3) subunit expression was elevated in dark-reared animals by approximately 50% at both ages. alpha(2) expression was not affected. These results implicate the importance of a shift from putative immature to mature GABA(A) receptor subunits during the critical period of visual cortex and in conjunction with parallel analysis of NMDA receptor subunit maturation, further support the notion that a changing excitatory/inhibitory balance is critical for neuronal plasticity.

Brain benzodiazepine binding in aged rats

Membrane [3H]flunitrazepam binding to central and peripheral benzodiazepine binding sites was studied in four brain areas (cerebellum, cortex, striatum and midbrain) of young (age 2-4 months) and aged (> 24 months) rats. A generalized reduction in the density of central binding sites (Bmax) was observed in all brain areas examined in aged rats. This reduction is irrelevant of the brain area and, according to literature, may correspond to cell loss and/or differential expression of mRNAs coding for the subunits of the GABA/benzodiazepine receptor complex during ageing. In the case of the peripheral binding sites, there was a decrease of Bmax in all brain areas with the exception of the cerebellum. However, the percent reduction of peripheral binding sites varied significantly among the different brain areas. These data suggest a differential effect of ageing on brain benzodiazepine binding which may reflect the special role for each brain area during ageing.

Adeno-associated virus (AAV) vector antisense gene transfer in vivo decreases GABA(A) alpha1 containing receptors and increases inferior collicular seizure sensitivity

In the inferior colliculus, adeno-associated virus (AAV) vectors are capable of gene transfer and stable, long-term expression, but it remained to be shown if this in vivo gene transfer could alter focal seizure sensitivity in the inferior colliculus. Because GABA receptors directly modulate inferior collicular seizures, AAV vectors were constructed with a cytomegalovirus (CMV) promoter and a truncated, human GABA(A) alpha1 cDNA in both the sense and antisense orientations. Seven days after collicular microinjection of the sense vectors (1 microl; 3 x 10(9) particles/microl), neurons exhibited GABA(A) alpha-like immunoreactivity in amounts far exceeding endogenous concentrations. Unilateral or bilateral sense vector infusion had no effect on inferior collicular seizure parameters or on [3H]zolpidem binding. In contrast, bilateral infusion of the antisense AAV-GABA(A) alpha1 vector (1 microl; 3 x 10(8) particles/microl) caused a 137% increase in the seizure duration. Moreover, unilateral antisense vector infusion produced a localized, 48% decrease in [3H]zolpidem binding. Thus, in the inferior colliculus, antisense AAV-CMV vectors can reduce a specific receptor subunit protein and change receptor function that directly influences in vivo seizure sensitivity.

Structural and pharmacological aspects of the GABAA receptor: involvement in behavioral pathogenesis

The gamma-aminobutyric acidA (GABAA) receptor is a complex hetero-oligomeric protein. It is composed of several subunits which assemble to form a functional chloride channel. The precise molecular organization of the receptor is as yet unknown. In the first part, we review recent literature dealing with the molecular and pharmacological aspects of the GABAA receptor, the second part will review some of the pathologies probably associated with gene defects and/or quantitative differential expression of transcripts encoding GABAA receptor subunits.

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

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

Differential expression of the short and long forms of the gamma 2 subunit of the GABAA/benzodiazepine receptors

The distribution of the mRNAs encoding the gamma 2S and gamma 2L subunits of the GABAA receptor in the rat brain has been revealed by in situ hybridization, northern blot and dot blot analysis using specific antisense oligonucleotides. In addition, the quantitative distribution of the gamma 2S and gamma 2L subunit peptides participating in the fully assembled GABAA receptors/benzodiazepine receptors has been mapped by immunoprecipitation with specific anti-gamma 2S and anti-gamma 2L antibodies. Several neuronal types and brain regions are enriched in gamma 2L such as neurons of the layer II of striate cortex and cerebellar Purkinje cells as well as the inferior colliculus, superior colliculus, deep cerebellar nuclei, medulla and pons. Other neuronal types and regions are enriched in gamma 2S such as the mitral cells of the olfactory bulb, pyramidal neurons of the pyriform cortex, layer VI of the neocortex, granule cells of the dentate gyrus and pyramidal cells of the hippocampus. Other cortical areas and cerebellar granule cells express both gamma 2S and gamma 2L in comparable amounts. There is a good correlation between the relative expression of gamma 2S and gamma 2L mRNAs and the relative presence of these protein subunits in fully assembled and mature receptors in the studied brain regions. The differential distribution of gamma 2S and gamma 2L might result in differential ethanol sensitivity of the neurons expressing these GABAA receptor subunits.

Immunochemical characterization of the beta 2 subunit of the GABAA receptor

To date three beta subunits of the GABAA receptor have been identified in rat brain as a result of cDNA library screening. The beta 2 subunit has been reported to have a wide distribution in rat brain based on in situ hybridization studies quantifying beta 2 mRNA. To study the beta 2 subunit more directly, we have raised a polyclonal antibody to a synthetic peptide representing residues 315-334 of the intracellular loop of the beta 2 subunit. The antibody, which had been affinity-purified, recognized the beta 2 peptide but did not immunolabel homologous beta 1 and beta 3 subunit peptides, indicating that this antibody is specific for the beta 2 subunit of the receptor. In western blots of the purified receptor, the antibody recognized a major diffuse band of 54-58 kDa and exhibited minor labeling of lower-molecular-mass polypeptides. In western blots of cortex homogenate, the antibody exhibited nervous system-specific labeling of a 55-kDa band that comigrated with the 55-kDa band of the purified receptor. Quantitative immunolabeling of this 55-kDa polypeptide permitted direct determination of the relative amounts of the beta 2 subunit in different brain regions. The brainstem contained the highest relative specific activity of the beta 2 subunit, followed by the inferior colliculus, olfactory lobe, and cerebellum. Lower levels of immunolabeling were seen in hypothalamus, hippocampus, thalamus, and cortex.

Repeated swim-stress reduces GABAA receptor alpha subunit mRNAs in the mouse hippocampus

The effects of brief repeated swim stress on the expression of GABAA receptor alpha 1 subunit mRNAs was investigated in the mouse. Adult male mice were exposed to repeated brief (10 min) swim-stress once daily for 7 or 14 days and the levels of GABAA receptor alpha subunit mRNAs were quantified in the hippocampus 24 h after the last session by Northern analysis. Repeated swim stress for 14 days resulted in a 47.3% +/- 6.5 and 39.8% +/- 7.6 decrease in the levels of the 4.8 kb and 4.4 kb GABAA receptor alpha 1 subunit mRNAs, respectively. While there was a trend toward a reduction in the level of GABAA receptor alpha 1 subunit mRNAs following 7 days of repeated swim stress, the latter did not reach statistical significance. In contrast, no significant alterations in the levels of glutamic acid decarboxylase or beta-actin mRNAs were observed at either time point. The reduction in GABAA receptor alpha 1 subunit mRNAs following repeated swim stress may underlie similar alteration(s) in hippocampal GABAA receptor density previously observed following repeated swim stress.

The GABAA receptor channel mediated chloride ion translocation through the plasma membrane: new insights from 36Cl- ion flux measurements

GABAA receptors in plasma membranes of neurons are integral oligomers which form chloride channels. The binding of GABA molecules at recognition sites for channel opening triggers a transient increase in transmembrane chloride ion flux. The multiplicity and drug specificity of GABAA receptor, kinetics of channel opening, and desensitization of GABAA receptor and its short- and long-term regulation have been investigated by the use of tracer amounts of the radioactive chloride isotope, 36Cl- ion. Results and new insights from 36Cl- ion flux measurements have been reviewed.

GABAA receptor function and regional analysis of subunit mRNAs in long-sleep and short-sleep mouse brain

The greater sensitivity of long-sleep (LS), as compared with short-sleep (SS), mice to ethanol is due in part to differences in GABAA receptor function in specific brain regions. To determine if differences in subunit composition of GABAA receptors contribute to this differential sensitivity, we measured alpha 1 and gamma 2 subunit mRNAs with Northern analysis and in situ hybridization and gamma 2S, gamma 2L and alpha 6 subunit mRNAs with polymerase chain reaction (PCR) amplification. No differences in mRNAs in whole brain were apparent by Northern analysis. In situ hybridization revealed that alpha 1 and gamma 2 subunit mRNAs were co-localized in many brain regions but that they still had distinct patterns of hybridization. However, the few differences observed between LS and SS mice in the levels of hybridization for these subunits did not show a regional distribution consistent with ethanol sensitivity differences. Similar ratios of gamma 2L, and gamma 2S subunit mRNAs were found in LS and SS mouse cerebral cortex and hippocampus, and both mouse lines expressed essentially only gamma 2L subunit mRNA in cerebellum. mRNA for the alpha 6 subunit was detected only in cerebellum and also was qualitatively similar between LS and SS mice. Studies of muscimol-stimulated 36Cl- uptake by cortical membrane vesicles confirmed earlier findings that ethanol does not enhance function of GABAA receptors in SS mice when assayed at 30 degrees C. However, at 34 degrees C ethanol did increase this function in SS mice although the enhancement remained greater in LS mice. These functional results, together with the results showing similar levels of alpha 1, gamma 2S, gamma 2L and alpha 6 subunits in LS and SS mice, suggest that the ethanol-insensitivity of SS mouse GABAA receptors cannot be due solely to lack of subunits required for ethanol action and further suggest that differences in catalytic mechanisms affecting post-translational processing may account for some genetic differences in ethanol sensitivity of GABAA receptors.

Distinct developmental patterns of expression of rat alpha 1, alpha 5, gamma 2S, and gamma 2L gamma-aminobutyric acidA receptor subunit mRNAs in vivo and in vitro

We have quantitated the alpha 1, alpha 5, gamma 2S, and gamma 2L gamma-aminobutyric acidA (GABAA) receptor subunit mRNAs in the maturing cerebellum in vivo and in cerebellar granule neurons differentiating in vitro. Absolute amounts of mRNA were measured by reverse transcription and competitive polymerase chain reaction (PCR) analysis with appropriate internal standards. The alpha 1 and gamma 2L mRNA content increased continuously during postnatal cerebellar maturation and their changes with time matched very closely those of the cerebellar granule cells differentiating in vitro. The gamma 2S subunit mRNA showed a relatively constant pattern of expression both in vivo and in vitro, with comparable absolute concentrations in both developmental paradigms. The alpha 5 mRNA was initially high in vivo and decreased (eight-fold) to adult levels as postnatal cerebellar development progressed. In vitro the amount of alpha 5 GABAA receptor subunit mRNA was higher than in vivo at 3 days, increased by more than twofold by 8 days, and declined to approximately the initial values at 23 and 28 days in vitro. Collectively, the results indicate that the alpha 1, alpha 5, gamma 2S, and gamma 2L GABAA receptor subunit mRNAs are regulated differentially in a temporal manner during in vivo and in vitro maturation. Moreover, a comparison of the ontogenetic profiles of the gamma 2S and gamma 2L mRNAs indicates that alternative splicing of the gamma 2 primary RNA transcript is regulated developmentally during postnatal maturation of the rat cerebellum.

Bicuculline- and baclofen-insensitive gamma-aminobutyric acid binding to rat cerebellar membranes

Up to 60% of gamma-[3H]aminobutyric acid ([3H]GABA) bound specifically to rat cerebellar membranes in the absence of Ca2+ was insensitive to the GABAA antagonist bicuculline and to the GABAB agonist baclofen. This indicates that a significant component of specifically bound [3H]GABA is associated with non-GABAA, non-GABAB binding sites. The presence of this binding component appeared seasonal, peaking in the month of September (early spring) each year over a 4-year period. The calcium independence and bicuculline and baclofen insensitivity of the binding indicate that this binding is not to the classical GABAA and GABAB binding sites. High concentrations of muscimol and isoguvacine inhibited non-GABAA, non-GABAB binding. Scatchard analysis of the non-GABAA, non-GABAB binding sites indicated two kinetic components: KD1 = 42 nM and KD2 = 9.2 microM; Bmax1 = 1.6 pmol/mg of protein and Bmax2 = 28 pmol/mg of protein.

Aging reduces the mRNA of alpha 1 GABAA receptor subunit in rat cerebral cortex

The effect of aging on the binding of ligands to the GABA, benzodiazepine and picrotoxin binding sites as well as alpha subunit mRNA level of GABAA receptor was investigated in cerebral cortex of male Fischer F-344 rats. In aged (730- to 770-day-old) rats, the binding of [35S]t-butylbicyclophosphorothionate (TBPS) was significantly reduced. Also, alpha 1 mRNA level was markedly decreased (86% suppression). In contrast, alpha 1 mRNA remained unchanged in cerebellum. These findings indicate a selective age-related structural change in GABAA receptor in rat cerebral cortex.

Diazepam enhances intrasynaptosomal free calcium concentrations

Synaptosomes prepared from brains of rats were incubated in different concentrations of diazepam under conditions designed to reduce the action of a reversed Na+/Ca2+ exchanger. In synaptosomes depolarized in the presence of added Ca2+, doses of diazepam ranging from 0.1 to 100 microM were found to significantly enhance Ca2+ levels measured with the fluorescent dye fura-2, compared to control incubations without drug. Furthermore, doses of diazepam as low as 1 microM significantly increased the concentration of Ca2+ in non-depolarized synaptosomes without added Ca2+ in the medium. The effects of depolarization and diazepam treatment were synergistic in increasing the levels of intrasynaptosomal Ca2+.

The GABAA receptor family in the mammalian brain

The GABAA-benzodiazepine receptor protein from bovine brain was purified by affinity chromatography and the subunit composition examined by gel electrophoresis in sodium dodecyl sulfate. Protein staining revealed a doublet at 51-53 kDa, a band at 55 kDa, and a broad band at 57-59 kDa. The 51 and 53 kDa bands co-migrated with the alpha 1 and alpha 2 gene products identified by Western blotting with subtype-specific antibodies. These two bands were also photoaffinity labeled by [3H]flunitrazepam, as was a breakdown product at 44 kDa. Partial sequencing of proteolytic fragments of these polypeptides yielded sequences found in all alpha clones, and identified the benzodiazepine binding site within residues 8-297 and probably between 106-297 of alpha 1; the 44 kDa and 31 kDa bands yielded fragments containing alpha 3 sequence. The native alpha 3 polypeptide was identified with subtype-specific antibody at 57 kDa overlapping with the two major bands photolabeled with [3H]muscimol at 55 and 58 kDa. Antisera to a beta-selective peptide recognized four bands at 60, 58, 57 and 55 kDa. Thus, one can identify 6-8 distinct polypeptides with the possibility of another 4-6 in purified GABAA receptor proteins, depending on brain region, consistent with the family of gene products suggested by molecular cloning.

Region-specific expression of the mRNAs encoding beta subunits (beta 1, beta 2, and beta 3) of GABAA receptor in the rat brain

The expression of mRNAs encoding different beta subunits (beta 1, beta 2, and beta 3) of the gamma-aminobutyric acid A (GABAA) receptor in the rat brain was investigated by in situ hybridization histochemistry. Transcripts of each subunit showed region-specific localization. Some areas contained all three beta subunit mRNAs, while other areas showed marked differences in the location and level of expression of each subunit mRNA. The regions with strong to very strong labeling by the beta 3 probe were as follows: the olfactory bulb, the cortex, the caudate-putamen, the accumbens nucleus, the hypothalamus, the amygdala, the hippocampal formation, and some areas of the brainstem and spinal cord. Weak to moderate labeling was detected in the thalamus, superior and inferior colliculus, and many areas of the brainstem. The regions with strong to very strong labeling by the beta 2 probe were as follows: the olfactory bulb, the ventral pallidum, the globus pallidus, the nucleus of the diagonal band, the preoptic magnocellular nucleus, the thalamus, the subthalamic nucleus, the substantia nigra, and the cerebellum. Weak to moderate labeling was detected in the regions that showed strong expression of beta 3 subunit mRNA. Expression of beta 1 subunit mRNA was only weak to moderate compared to that of the beta 2 and beta 3 subunit mRNAs. The regions with moderate labeling by the beta 1 probe were the following: the cortex, the claustrum, the bed nucleus of the stria terminale, some nuclei of thalamus and amygdala, the hypothalamus, the hippocampal formation, the subiculum, the cerebellum, and several areas of the brainstem and spinal cord. Thus, our findings showed that the beta subunit mRNAs of the GABAA receptor were differentially expressed in the rat brain, perhaps indicating the existence of different subtypes of the GABAA receptor in different brain regions.

Region-specific expression of messenger RNAs encoding GABAA receptor subunits in the developing rat brain

The distribution and levels of messenger RNAs encoding the alpha 1, beta 1, beta 2, beta 3, and gamma 2 subunits of the GABAA receptor in the developing and adult rat brain were investigated using quantitative in situ hybridization histochemistry and subunit-specific probes. Regional localization of the subunit messenger RNAs was determined with film autoradiography and expression in identified neuronal cell populations was examined using higher resolution techniques. Each of the GABAA receptor subunit messenger RNAs exhibits a distinct pattern of localization in the developing and adult brain. Of the subunits examined, the alpha 1, beta 2, and gamma 2 are the most abundant and are found in many brain regions, including the olfactory bulb, cortex, hippocampus, thalamic nuclei, and inferior colliculus. In addition, these subunit messenger RNAs are prominent in the cerebellum where virtually all cells of the deep cerebellar nuclei and Purkinje cell layer are labeled. The levels of most of the subunit messenger RNAs, with the exception of that encoding the beta 1 subunit, increase during postnatal development. While the alpha 1, beta 2, and gamma 2 subunit messenger RNAs rise in parallel in many regions and identified cell populations, different subsets of receptor subunit messenger RNAs are co-ordinately expressed at other sites. The greatest increases in subunit messenger RNA levels occur in the cerebellar cortex during the second postnatal week, a period coincident with cerebellar maturation. The co-distribution of different GABAA receptor subunit messenger RNAs in various regions of the developing and adult nervous systems supports the hypothesis that multiple receptor compositions exist. Moreover, that different subunit messenger RNAs exhibit coordinate changes in expression in different regions and cell populations suggests that receptor gene expression is modulated by cell type-specific signals. The temporal changes in subunit messenger RNA levels in the cerebellum raise the possibility that synaptogenesis may play a role in receptor gene regulation in this brain region.

Developmental expression of the GABAA receptor alpha 1 subunit mRNA in the rat brain

Recent studies have suggested that the GABAA, receptor complex, the site of action of the inhibitory neurotransmitter gamma amino-butyric acid (GABAA) and the anxiolytic benzodiazepines, is heterogeneous. Moreover, its composition may change during development. To better understand the molecular basis of receptor heterogeneity, the levels and distribution of the mRNA encoding the alpha 1 receptor subunit were examined in the developing and adult rat brain with quantitative in situ hybridization histochemistry. Our studies demonstrate that alpha 1 subunit mRNA expression changes during ontogeny. At late embryonic stages and in the first postnatal week, low levels of the mRNA were detected in the cortex, inferior colliculus, and hippocampus. The mRNA levels in these regions increased during the second and third postnatal weeks. Furthermore, a dramatic change in the distribution of the alpha 1 subunit mRNA was seen in the second postnatal week when the message first became detectable in the cerebellar cortex. During subsequent development and in the mature brain, the alpha 1 subunit mRNA was most abundant in the cerebellum, olfactory bulb, and inferior colliculus, although the absolute levels of mRNA varied by as much as sixfold in selected brain regions. The mature distribution of alpha 1 subunit mRNA, along with its temporal appearance in the cerebellum, suggests that this subunit is a constituent of the Type 1 benzodiazepine site of the GABAA receptor complex. Furthermore, the onset of alpha 1 subunit mRNA expression in the cerebellar cortex coincides with a period of extensive synapse formation, raising the possibility that synaptic interactions modulate the appearance of this GABAA receptor subunit in the cerebellum.

Chronic ethanol and pentobarbital administration in the rat: effects on GABAA receptor function and expression in brain

Chronic exposure of rats to ethanol significantly decrease GABAA receptor-mediated 36Cl- uptake in cerebral cortical synaptoneurosomes. Muscimol and pentobarbital stimulation as well as ethanol enhancement of muscimol-stimulated 36Cl- flux are significantly decreased following chronic ethanol inhalation. Repeated pentobarbital administration has a similar effect on muscimol and pentobarbital-stimulated 36Cl- uptake in cerebral cortical synaptoneurosomes. We have postulated that these adaptive response may be associated with an alteration of GABAA receptor gene expression. Chronic ethanol exposure resulted in a significant reduction in the levels of GABAA receptor alpha-subunit mRNA's. The most abundant mRNA species in the rat cerebral cortex were reduced 40-50% (4.4 Kb mRNA, 43%, 4.8 Kb mRNA, 47%). beta-Actin mRNA and poly(A)+ RNA levels were not significantly reduced following chronic ethanol exposure. Repeated pentobarbital administration had no effect on the level of the 4.4 and 4.8 Kb transcripts of alpha-subunit mRNAs in rat cerebral cortex. These data suggest that chronic ethanol exposure alters the level of mRNA's coding for the alpha-subunit of the GABAA receptor. This decrease may reflect an alteration of mRNA processing in the cell or an alteration in GABAA receptor gene expression.