Type I and type II gamma-aminobutyric acid/benzodiazepine receptors: purification and analysis of novel receptor complex from neonatal cortex

Neonatal Clonazepam Administration Induced Long-Lasting Changes in GABAA and GABAB Receptors

Benzodiazepines (BZDs) are widely used in patients of all ages. Unlike adults, neonatal animals treated with BZDs exhibit a variety of behavioral deficits later in life; however, the mechanisms underlying these deficits are poorly understood. This study aims to examine whether administration of clonazepam (CZP; 1 mg/kg/day) in 7-11-day-old rats affects Gama aminobutyric acid (GABA)ergic receptors in both the short and long terms. Using RT-PCR and quantitative autoradiography, we examined the expression of the selected GABA_A receptor subunits (α1, α2, α4, γ2, and δ) and the GABA_B B2 subunit, and GABA_A, benzodiazepine, and GABA_B receptor binding 48 h, 1 week, and 2 months after treatment discontinuation. Within one week after CZP cessation, the expression of the α2 subunit was upregulated, whereas that of the δ subunit was downregulated in both the hippocampus and cortex. In the hippocampus, the α4 subunit was downregulated after the 2-month interval. Changes in receptor binding were highly dependent on the receptor type, the interval after treatment cessation, and the brain structure. GABA_A receptor binding was increased in almost all of the brain structures after the 48-h interval. BZD-binding was decreased in many brain structures involved in the neuronal networks associated with emotional behavior, anxiety, and cognitive functions after the 2-month interval. Binding of the GABA_B receptors changed depending on the interval and brain structure. Overall, the described changes may affect both synaptic development and functioning and may potentially cause behavioral impairment.

Stress and GABA receptors

GABA(A) receptors are sensitive to subtle changes in the environment in both early-life and adulthood. These neurochemical responses to stress in adulthood are sex-dependent. Acute stress induces rapid changes in GABA(A) receptors in experimental animals, with the direction of the changes varying according to the sex of the animals and the stress-paradigm studied. These rapid alterations are of particular interest as they provide an example of fast neurotransmitter system plasticity that may be mediated by stress-induced increases in neurosteroids, perhaps via effects on phosphorylation and/or receptor trafficking. Interestingly, some studies have also provided evidence for long-lasting changes in GABA(A) receptors as a result of exposure to stressors in early-life. The short- and long-term stress sensitivity of the GABAergic system implicates GABA(A) receptors in the non-genetic etiology of psychiatric illnesses such as depression and schizophrenia in which stress may be an important factor.

Kinetics of Rat Brain and Liver Solubilized Membrane-Bound Catechol-O-Methyltransferase

Catechol-O-methyltransferase (COMT), an enzyme involved in the metabolism of catecholamines, is present in mammals as soluble (S-COMT) and membrane-bound (MB-COMT) forms. The kinetic properties of rat liver and brain solubilized MB-COMT were evaluated and compared with the ones of the respective native enzymes. Treatment with Triton X-100 did not affect the affinity of S-COMT for the substrate (adrenaline) or the activity of the enzyme. Conversely, solubilized MB-COMT presented a lower affinity for the substrate than the native protein, as evidenced by a significant increase in the Km values: 9.3 (6.2, 12) vs 2.5 (0.8, 4.3) microM for the liver enzyme and 12 (11, 13) vs 1.4 (1.0, 1.9) microM for the brain enzyme. A 1.6- and 1.5-fold increase in Vmax was also observed for the liver and brain solubilized enzymes, respectively. The actual enzyme concentrations (molar equivalence, Meq) and their efficiency in the O-methylation reaction (catalytic number, Kcat) were determined from Ackermann-Potter plots. Both liver and brain solubilized MB-COMT were more efficient in methylating adrenaline than the respective native enzymes as revealed by higher Kcat values (P < 0.05): 16.4+/-0.9 vs 10.9+/-0.8 min(-1) (brain) and 5.9+/-0.3 vs 3.3+/-0.2 min(-1) (liver). Subjecting liver solubilized MB-COMT to further purification increased the Km of the enzyme to the levels of liver S-COMT, 252 (127; 377) vs 257 (103; 411) microM. The solubilization process significantly alters MB-COMT kinetic properties but only after partial purification does the enzyme present an affinity for the subtrate identical to S-COMT.

Differential solubilization of lipids along with membrane proteins by different classes of detergents

Membrane proteins are typically extracted by detergent concentrations of 0.5-2.0%, using detergent/protein ratios of 1:1 to 3:1. We have compared the ability of 14 different detergents from seven different structural and ionic classes, at a concentration of 2.0% and a detergent/protein ratio of 2:1, to extract an integral membrane protein (the serotonin 5-HT1A receptor) in active form and have observed profound differences in both lipids and proteins. All extracts were freed from detergents and dialyzed to form vesicles containing 95-100% of the extracted lipids, prior to [3H]8-hydroxy-2-(N,N-di-n-propylamino)tetralin ([3H]8-OH-DPAT) binding. The most efficient detergents in extracting active 5-HT1A receptor protein were the zwitterionic 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) and 3-[(cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate (CHAPSO), followed by the neutral n-dodecyl-beta-D-maltoside. Zwitterionic detergents also produced the highest solubilized lipid/protein ratio (3.0 and 2.5, respectively) and in general the relative amounts of extracted lipids and proteins followed inverse profiles. Thus, hydrophobic detergents such as Tritons (with critical micelle concentrations similar to CHAPS) and Thesit (structurally similar to Lubrol) extracted the most protein, but relatively little lipid (ratios of less than 0.2) and very little active 5-HT receptor. Dramatic differences were also observed in the ratios of individual lipids extracted by the same concentrations of different detergents and resolved by high-performance thin-layer chromatography. For example, galactosylceramide (GalCer) content ranged from 2.7% (CHAPSO) to 13.4% (sodium cholate) of the total lipid extract and cholesterol ranged from 0% (digitonin) to 17.9% (Triton X-100). The detergent-extractability profile for phosphatidylethanolamine (PE) (range 15-40% of total lipid) paralleled that of phosphatidylinositol (PI) (range 4-10%), but was inverse to that for GalCer and cholesterol. Detergent-extractability profiles for phosphatidylcholine (PC) and phosphatidylserine (PS) also followed inverse profiles, with zwitterionic detergents giving high PS/PC and high PE/PC ratios (approximately 2:1), whereas the Tritons and digitonin gave ratios of 1:2. We believe that differential solubilization of lipids, as well as proteins, by detergents is important for the biological activity of the extracted proteins, and lipid extractability should be taken into account when purifying membrane proteins.

Allosteric modulation of [35S]TBPS-binding in the cerebral cortex of the rat during postnatal development

The ontogenesis of the GABA-gated Cl- channel was investigated in the cerebral cortex of the rat by monitoring the binding parameters of [35S]t-butylbicyclophosphorothionate ([35S]TBPS) at intervals after birth (1-90 days). To investigate the influence of the developmental changes in the content of GABA on [35S]TBPS-binding, the assays were carried out in unwashed membranes, in which the concentration of GABA was dependent on its content in vivo, and in repeatedly washed membranes in the presence of defined concentrations of exogenous GABA. At birth, the density (Bmax) of [35S]TBPS-binding sites in unwashed membranes was similar to that found in well-washed membranes. However, in unwashed membranes, the number of [35S]TBPS-binding sites increased by two-fold within 10 days after birth whereas in washed membranes it increased by four-fold during the same period. The higher density of [35S]TBPS-binding sites in washed membranes as compared with the unwashed counterparts persisted throughout development. In unwashed membranes, the apparent Kd for [35S]TBPS-binding increased with age whereas in washed membranes the affinity of [35S]TBPS for its binding sites remained constant throughout development. The binding of [35S]TBPS to the GABA-gated Cl- channel is allosterically modulated by drugs acting on different sites of the GABAA receptor complex. Thus, GABA and diazepam decrease [35S]TBPS-binding whereas the GABAA receptor antagonist, bicuculline, and the inverse agonist for benzodiazepine receptors, 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylic acid methyl ester, increase it.(ABSTRACT TRUNCATED AT 250 WORDS)

Coincidental appearance of the alpha 1 subunit of the GABA-A receptor and the type I benzodiazepine receptor near birth in macaque monkey visual cortex

The expression of subtypes of the GABA-A/benzodiazepine receptor complex has been studied during pre- and postnatal development of Macaca monkey visual cortex using complementary radioligand and immunocytochemical labeling. Type I benzodiazepine receptors were labeled directly by [3H]zolpidem. Type II receptors were determined by the amount of binding for [3H]flunitrazepam (FZ) persisting in the presence of the type I-specific ligand CL218872. Monoclonal antibody bd24 was used to label alpha 1 subunits and bd17 to label beta 2 and beta 3 subunits of the GABA-A receptor. Radioligand binding data and bd17 immunoreactivity indicated that type II benzodiazepine receptors were present by fetal day (Fd) 74 (44% of gestation). Immunoreactivity for the beta 2/beta 3 subunits increased until 3-6 weeks after birth, and then declined somewhat into adulthood. Neither radioligand labeling for type I receptors nor immunocytochemical staining for the alpha 1 subunit were apparent until mid-gestation. Both markers appeared shortly before birth in layer 4C, and then in other cortical layers after birth. Immunoreactivity for the alpha 1 subunit increased steadily after birth until it became more intense than that for beta 2/3 subunits in the adult. Quantitative densitometry of CL218872 competition for [3H]FZ binding showed that type I/II distribution was 22%/78% at Fd103; 42%/58% at Fd131; 67%/33% at 9 months; and 61%/39% in adult visual cortex. This "switch" between benzodiazepine receptor subtypes overlaps the postnatal critical period for geniculostriate development, suggesting that the change from type II to type I receptors and the appearance of alpha 1 subunits may play a decisive role in the maturation of geniculocortical axon terminations and cortical response properties. It remains to be shown whether this "switch" is dependent on functional visual input.

Differential effects of GABAergic ligands in mouse and rat hippocampal neurons

Previous electrophysiological studies suggested that GABAA receptors in rat hippocampal neurons might be less sensitive to ethanol than mouse neurons. Therefore, we examined the effects of ethanol (0.5-850 mM) in cultured mouse (C57BL/6) and rat (Sprague-Dawley) neurons. In 35% of the mouse neurons, the Cl- current was potentiated by ethanol starting at 0.5 mM. In all of the rat neurons examined, on the other hand, the current was potentiated by concentrations starting at 200 mM. We also studied the effects of GABA and other GABAergic ligands. GABAA receptors in rat and mouse neurons displayed EC50s for GABA of 9 +/- 0.3 and 17 +/- 0.8 microM, respectively and ethanol did not significantly change these values. The EC50 for diazepam was 92 +/- 3 and 120 +/- 8 nM in rat and mouse, respectively. Pentobarbital enhanced the current with EC50s of 84 +/- 3 and 106 +/- 6 microM in rat and mouse, respectively. The sensitivity for Cl-218,872, which binds preferentially to the Type I benzodiazepine receptor, was similar in all the neurons. RO 15-4513, an inverse partial agonist to the benzodiazepine receptor, was not effective in reversing the potentiation of the Cl- current in rat neurons and only slightly reduced the potentiation in mouse neurons. The receptors in rat neurons were more sensitive to external Zn2+; the current was inhibited by 50% with a concentration of 93 +/- 3 and 244 +/- 9 microM in rat and mouse, respectively. Analysis of mRNA encoding for the gamma 2L receptor subunit showed similar levels in rat and mouse neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory neurotransmission in rat spinal cord: co-localization of glycine- and GABAA-receptors at GABAergic synaptic contacts demonstrated by triple immunofluorescence staining

Synaptic inhibition in rat spinal cord is mediated by the amino acids gamma-aminobutyric acid (GABA) and glycine. Most spinal cord neurons respond to both neurotransmitters, suggesting co-expression of GABAA- and strychnine-sensitive glycine-receptors in individual cells. While the distribution of glycine-receptors has been extensively characterized, much less is known about the cellular localization of GABAA-receptors in spinal cord neurons. In the present study, the distribution of GABAA-receptors was analyzed immunohistochemically with a subunit-specific antiserum recognizing the alpha 1-subunit. Their co-localization with glycine-receptors and their apposition to GABAergic axon terminals were assessed by confocal laser microscopy in sections processed for double- and triple-immunofluorescence staining, using a monoclonal antibody against the 93 kDa glycine-receptor-associated protein, gephyrin, and an antiserum to glutamic acid decarboxylase. Staining for the GABAA-receptor alpha 1-subunit decorated the soma and dendrites of numerous neurons in laminae III-VIII and X of the spinal cord, revealing their morphology in clear detail. By contrast, laminae II and IX contained little immunoreactivity for these GABAA-receptors. Double-immunofluorescence staining showed that most GABAA-receptor-positive cells in layers III-VIII and X also exhibited a prominent glycine-receptor immunoreactivity. Both types of receptors had very similar distribution patterns in the cell membrane and were frequently co-localized in sites apposed to GABAergic axon terminals. These results indicate that GABAA- and glycine-receptors may co-exist within single postsynaptic densities, suggesting a possible synergism in the action of GABA and glycine in spinal cord neurons.

Differential expression of GABAA receptor alpha-subunits in rat brain during development

Unique cytoplasmic loop regions of the alpha 1, alpha 2, alpha 3, and alpha 5 subunits of the GABAA receptor have been expressed in E. coli and used to generate polyclonal antisera specific for these subunits. The antibodies identify proteins by SDS-polyacrylamide gel electrophoresis and western blotting of molecular size 51 kDa, 53 kDa, 59 kDa and 55 kDa, respectively, which show differential patterns of expression during development. Whereas the alpha 2 and alpha 3 subunits are present at early stages, the expression of alpha 1 and alpha 3 subunits is low at birth and increases with age. This differential expression could be correlated with previous studies examining the developmental expression of BZ1 and BZ2 benzodiazepine binding sites.

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.

Developmental study of [3H]TCP and [3H]glycine binding sites in the rat hippocampus

The postnatal development of [3H]thienylphencyclidine ([3H]TCP) sites in rat hippocampus has been studied autoradiographically and with membrane preparations. [3H]TCP binding increased progressively from birth to adulthood; this is due to a change in the maximal number of sites (Bmax) but not in the affinity (Kd). A different developmental pattern was found for strychnine-insensitive [3H]glycine binding which also increased after birth, but reached adult levels earlier than [3H]TCP binding. The ontogenesis of TCP or glycine sites also differed from that previously described for N-methyl-D-aspartate (NMDA) sites in the hippocampus. In neonatal, as in adult hippocampus, [3H]TCP binding was enhanced by NMDA or glycine and reduced by Mg2+. We suggest that TCP sites are functionally coupled to the NMDA receptor-ion channel complex in developing as in mature hippocampus, but that there are developmental changes in the receptor channel complex.

Differential effects of chronic lorazepam and alprazolam on benzodiazepine binding and GABAA-receptor function

1. Chronic benzodiazepine administration has been associated with tolerance and with downregulation of gamma-aminobutyric acidA (GABAA)-receptor binding and function. However, effects of individual benzodiazepines on brain regions have varied. 2. To compare the effects of chronic lorazepam and alprazolam, we have administered these drugs to mice for 1 and 7 days (2 mg kg-1 day-1) and determined benzodiazepine receptor binding in vivo with and without administration of CL 218,872, 25 mg kg-1 i.p., and GABA-dependent chloride uptake in 3 brain regions at these time points. 3. Benzodiazepine binding was decreased in the cortex and hippocampus at day 7 compared to day 1 of lorazepam, with an increase in CL 218,872-resistant (Type 2) sites in both regions. Maximal GABA-dependent chloride uptake was also decreased in the cortex and hippocampus at day 7. 4. Binding was decreased only in the cortex after 7 days of alprazolam, with no significant change in Type 2 binding. Maximal GABA-dependent chloride uptake was also decreased only in the cortex. 5. These data suggest that the effects of chronic benzodiazepine administration on the GABAA-receptor may be both region-specific and receptor subtype-specific.

Protein kinase C and cAMP-dependent protein kinase phosphorylate the beta subunit of the purified gamma-aminobutyric acid A receptor

A number of recent studies have suggested that phosphorylation of the gamma-aminobutyric acid A (GABAA) receptor could modulate receptor function. Activators of protein kinase C and cAMP-dependent protein kinase have been shown to influence GABAA receptor function. In addition, Sweetnam et al. [Sweetnam, P. M., Lloyd, J., Gallombardo, P., Malison, R. T., Gallager, D. W., Tallman, J. F. & Nestler, E. J. (1988) J. Neurochem. 51, 1274-1284] have reported that a kinase associated with a partially purified preparation of the receptor could phosphorylate the alpha subunit of the receptor. Moreover, Kirkness et al. [Kirkness, E. F., Bovenkerk, C. F., Ueda, T. & Turner, A. J. (1989) Biochem. J. 259, 613-616] have recently shown that cAMP-dependent protein kinase could phosphorylate a muscimol binding polypeptide of the GABAA receptor. To explore the issue further, we have examined the ability of specific kinases to catalyze significant phosphorylation of the GABAA receptor that has been purified to near homogeneity. The GABAA receptor was purified as previously described using benzodiazepine affinity chromatography. The purified receptor possessed no detectable kinase activity. Protein kinase C and cAMP-dependent protein kinase catalyzed the phosphorylation of the beta and alpha subunits of the receptor. However, most of the phosphate incorporation was associated with the beta subunit. Two muscimol binding polypeptides designated beta 58 (Mr 58,000) and beta 56 (Mr 56,000) were present in the preparation. The higher molecular weight polypeptide, beta 58, was phosphorylated specifically by cAMP-dependent protein kinase. beta 56 was phosphorylated specifically by protein kinase C. beta 58 and beta 56 gave distinct patterns in a one-dimensional phosphopeptide analysis. The stoichiometry of phosphorylation (mol of phosphate/mol of muscimol binding) catalyzed by cAMP-dependent protein kinase was 0.52 and that catalyzed by protein kinase C was 0.38. Taken together these data confirm that there are two forms of the beta subunit of the GABAA receptor and suggest that these two forms of the beta subunit are phosphorylated by distinct kinases.

Activating the damaged basal forebrain cholinergic system: tonic stimulation versus signal amplification

The hypothesis that the cognitive decline in senile dementia is related to the loss of cortical cholinergic afferent projections predicts that pharmacological manipulations of the remaining cholinergic neurons will have therapeutic effects. However, treatment with cholinesterase inhibitors or muscarinic agonists has been, for the most part, largely unproductive. These drugs seem to disrupt the normal patterning of cholinergic transmission and thus may block proper signal processing. An alternative pharmacological strategy which focuses on the amplification of presynaptic activity without disrupting the normal patterning of cholinergic transmission appears to be more promising. Such a strategy may make use of the normal GABAergic innervation of basal forebrain cholinergic neurons in general, and in particular of the inhibitory hyperinnervation of remaining cholinergic neurons which may develop under pathological conditions. Disinhibition of the GABAergic control of cholinergic activity is assumed to intensify presynaptic cortical cholinergic activity and to enhance cognitive processing. Although the extent to which compounds such as the benzodiazepine receptor antagonist beta-carboline ZK 93,426 act via the basal forebrain GABA-cholinergic link is not yet clear, the available data suggest that the beneficial behavioral effects of this compound established in animals and humans are based on indirect cholinomimetic mechanisms. It is proposed that an activation of residual basal forebrain cholinergic neurons can be achieved most physiologically via inhibitory modulation of afferent GABAergic transmission. This modulation may have a therapeutic value in treating behavioral syndromes associated with cortical cholinergic denervation.

Steroid and barbiturate modulation of the GABAa receptor. Possible mechanisms

This review describes the modulation of the GABAa receptor by steroid hormones and barbiturates and proposes guidelines for further research. Having examined the complex organization of the GABAa receptor complex and the multiple allosteric interactions between its drug and transmitter/modulator binding sites, the possibility that conformational changes of the receptor molecule may explain most of its characteristics is explored. On the basis of considerable evidence, we propose that the GABAa receptor may adopt as many as five different conformations. However, the heterogeneity of central GABAa receptor binding cannot only be explained by different configurations of a single protein. It also has been shown that different GABAa receptor subtypes exist within different brain regions. These receptor subtypes may differ from each other in their subunit composition. By describing the GABAa receptor as a macromolecular complex that may adopt different conformations and whose subunit composition may vary, it becomes possible to understand the molecular mechanisms by which steroid hormones modulate the receptor. This has led to two models of hormone actions. A first model addresses the direct effects that steroids exert on the GABAa receptor and predicts that steroid hormones may cause the conformation of the receptor complex to change between active and inactive states. A second model, which addresses the observed heterogeneity of GABAa receptor binding within the brain, suggests that steroid hormones may change the expression of the different subunits of the receptor complex by acting at the genomic level. This review complements other recent reviews describing the modulation of the GABAa receptor (Olsen and Venter, 1986; Gee, 1988).

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

GABAA receptor alpha subunit transcripts were detected by Northern analysis of rat, chicken and human brain mRNA using a series of 32P-labelled antisense RNA probes derived from human alpha 1 subunit cDNAs. These alpha subunit mRNAs differ in their distribution among various brain regions in the rat and at least one species is detected primarily in fetal brain. GABAA receptor alpha 1 subunit probes encoding the putative extracellular domain detect at least five alpha subunit transcripts in rat brain, whereas probes encoding the putative intracellular domain detect only two mRNAs. These data suggest the presence in brain of multiple GABAA receptor alpha subunits having homologous extracellular domains and whose expression is regionally and developmentally regulated. These alpha subunit transcripts may encode proteins that comprise GABAA isoreceptors differing in their pharmacological and physiological properties.

Immunological identification of multiple alpha-like subunits of the gamma-aminobutyric acidA receptor complex purified from neonatal rat cortex

Antibodies were prepared against a synthetic peptide corresponding to amino acid sequences 174-203 of the bovine gamma-aminobutyric acidA (GABAA) receptor alpha 1-subunit. The antibodies recognized this synthetic alpha 1-peptide, but failed to react with the homologous peptide sequence, 170-199, of the bovine beta 1-subunit. On Western blots, anti-alpha 1-subunit antibody recognized a 50-kilodalton (kDa) protein in affinity-purified receptor preparations from adult rat cortex and cerebellum. In receptor purified from neonatal cortex, the anti-alpha 1-antibody reacted with 50-kDa, 53-54-kDa, and 59-kDa proteins. After digestion with endoglycosidase F, these three protein bands retained differing electrophoretic mobilities. The 50-kDa and 59-kDa subunits of affinity-purified neonatal receptor, which were photoaffinity-labeled with [3H]flunitrazepam, were immunoprecipitated to different extents by alpha-subunit antibody. These data suggest the existence in GABAA receptor from neonatal cortex of three proteins (50 kDa, 53 kDa, and 59 kDa) which have immunological homology to alpha 1-subunit of bovine GABAA receptor. The presence of an alpha- and a beta-like subunit with similar mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis may account for the relatively high concentration of protein in the 53-54-kDa band which has been observed in receptor purified from neonatal cortex. The presence of multiple alpha-like subunits may be related to the presence of a relatively high concentration of type II GABA receptor in this tissue.

Type I and type II GABAA-benzodiazepine receptors produced in transfected cells

GABAA (gamma-aminobutyric acid A)-benzodiazepine receptors expressed in mammalian cells and assembled from one of three different alpha subunit variants (alpha 1, alpha 2, or alpha 3) in combination with a beta 1 and a gamma 2 subunit display the pharmacological properties of either type I or type II receptor subtypes. These receptors contain high-affinity binding sites for benzodiazepines. However, CL 218 872, 2-oxoquazepam, and methyl beta-carboline-3-carboxylate (beta-CCM) show a temperature-modulated selectivity for alpha 1 subunit-containing receptors. There were no significant differences in the binding of clonazepam, diazepam, Ro 15-1788, or dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) to all three recombinant receptors. Receptors containing the alpha 3 subunit show greater GABA potentiation of benzodiazepine binding than receptors containing the alpha 1 or alpha 2 subunit, indicating that there are subtypes within the type II class. Thus, diversity in benzodiazepine pharmacology is generated by heterogeneity of the alpha subunit of the GABAA receptor.