Various proteins from rat brain, specifically and irreversibly labeled by [3H]flunitrazepam, are distinct alpha-subunits of the GABA-benzodiazepine receptor complex

Modeling of αk/γ2 (k=1, 2, 3 and 5) interface of GABAA receptor and docking studies with zolpidem: Implications for selectivity

The three-dimensional models of the alphak/gamma2 (k=1, 2, 3 and 5) interface of GABA(A) receptors, which included the agonist-binding site, were constructed and validated by molecular modeling technology. To investigate the mechanism of alpha subunit selectivity of zolpidem, docking calculations were used to illustrate the potential binding modes of zolpidem with different alpha subtypes. The results revealed that there were three reasons resulting in the distinct binding affinity of zolpidem to different alpha subtype. Firstly, the number of hydrogen bonds of agonist-receptor complex would determine the magnitude of binding affinity. Secondly, the His residue in loop A of alpha subunit was indicated as a key role of benzodiazepine binding. Thirdly, the side chain of Glu in loop C reduced the affinity of zolpidem to those receptors containing alpha2, alpha3 or alpha5 subunits.

Pharmacological and anatomical evidence for an interaction between mGluR5- and GABA(A) alpha1-containing receptors in the discriminative stimulus effects of ethanol

The discriminative stimulus properties of ethanol are mediated in part by positive modulation of GABA(A) receptors. Recent evidence indicates that metabotropic glutamate receptor subtype 5 (mGluR5) activity can influence GABA(A) receptor function. Therefore, the purpose of this work was to examine the potential involvement of mGluR5 in the discriminative stimulus effects of ethanol. In rats trained to discriminate ethanol (1 g/kg, intragastric gavage (i.g.)) from water, 2-methyl-6-(phenylethyl)-pyridine (MPEP) (1-50 mg/kg, i.p.) a selective noncompetitive antagonist of the mGlu5 receptor did not produce ethanol-like stimulus properties. However, pretreatment with MPEP (30 mg/kg) reduced the stimulus properties of ethanol as indicated by significant reductions in ethanol-appropriate responding, specifically at 0.5 and 1 g/kg ethanol, and a failure of ethanol test doses (1 and 2 g/kg) to fully substitute for the ethanol training dose. To test whether mGluR5 antagonism altered the GABA(A) receptor component of the ethanol stimulus, the ability of MPEP to modulate pentobarbital and diazepam substitution for ethanol was assessed. Pentobarbital substitution (1-10 mg/kg, i.p.) for ethanol was not altered by MPEP pretreatment. However, MPEP pretreatment inhibited the ethanol-like stimulus properties of diazepam (5 mg/kg, i.p.). To examine a potential anatomical basis for these pharmacological findings, expression patterns of mGluR5- and benzodiazepine-sensitive GABA(A) alpha1-containing receptors were examined by dual-label fluorescent immunohistochemistry with visualization by confocal microscopy. Results indicated that mGluR5- and GABA(A) alpha1-containing receptors were both coexpressed in limbic brain regions and colocalized on the same cells in specific brain regions including the amygdala, hippocampus, globus pallidus, and ventral pallidum. Together, these findings suggest an interaction between mGluR5- and benzodiazepine-sensitive GABA(A) receptors in mediating ethanol discrimination.

Changes of [3H]Muscimol, [3H]Flunitrazepam and [3H]MK-801 Binding in Rat Brain by Prolonged Ventricular Infusion of 7-Nitroindazole

In the present study, we have investigated the effects of prolonged inhibition of nitric oxide synthase (NOS) by infusion of neuronal NOS (nNOS) inhibitor, 7-nitroindazole (7-NI), to examine modulation of NMDA and GABAA receptor binding in rat brain. The duration of sleeping time was significantly increased by the pre-treatment with 7-NI (100 mg/kg) 30 min before pentobarbital (40 mg/kg) treatment in rats. However, the duration of pentobarbital-induced sleep was shortened by the prolonged infusion of 7-NI into cerebroventricle for 7 days. We have investigated the effect of NOS inhibitor on NMDA and GABAA receptor binding characteristics in discrete areas of brain regions by using autoradiographic techniques. The GABAA receptors were analyzed by quantitative autoradiography using [3H]muscimol and [3H]flunitrazepam binding, and NMDA receptor binding was analyzed by using [3H]MK-801 binding in rat brain slices. Rats were infused with 7-NI (500 pmol/10 microl/h, i.c.v.) for 7 days, through pre-implanted cannula by osmotic minipumps. The levels of [3H]muscimol were markedly elevated in cortex, caudate putamen, and thalamus while the levels of [3H]flunitrazepam binding were only elevated in cerebellum by NOS inhibitor. However, there was no change in the level of [3H]MK-801 binding except decreasing in the thalamus. These results show that the prolonged inhibition of NOS by 7-NI-infusion highly elevates [3H]muscimol binding in a region-specific manner and decreases the pentobarbital-induced sleep.

Identification of the bovine gamma-aminobutyric acid type A receptor alpha subunit residues photolabeled by the imidazobenzodiazepine [3H]Ro15-4513

Ligands binding to the benzodiazepine-binding site in gamma-aminobutyric acid type A (GABA(A)) receptors may allosterically modulate function. Depending upon the ligand, the coupling can either be positive (flunitrazepam), negative (Ro15-4513), or neutral (flumazenil). Specific amino acid determinants of benzodiazepine binding affinity and/or allosteric coupling have been identified within GABA(A) receptor alpha and gamma subunits that localize the binding site at the subunit interface. Previous photolabeling studies with [(3)H]flunitrazepam identified a primary site of incorporation at alpha(1)His-102, whereas studies with [(3)H]Ro15-4513 suggested incorporation into the alpha(1) subunit at unidentified amino acids C-terminal to alpha(1)His-102. To determine the site(s) of photoincorporation by Ro15-4513, we affinity-purified ( approximately 200-fold) GABA(A) receptor from detergent extracts of bovine cortex, photolabeled it with [(3)H]Ro15-4513, and identified (3)H-labeled amino acids by N-terminal sequence analysis of subunit fragments generated by sequential digestions with a panel of proteases. The patterns of (3)H release seen after each digestion of the labeled fragments determined the number of amino acids between the cleavage site and labeled residue, and the use of sequential proteolytic fragmentation identified patterns of cleavage sites unique to the different alpha subunits. Based upon this radiochemical sequence analysis, [(3)H]Ro15-4513 was found to selectively label the homologous tyrosines alpha(1)Tyr-210, alpha(2)Tyr-209, and alpha(3)Tyr-234, in GABA(A) receptors containing those subunits. These results are discussed in terms of a homology model of the benzodiazepine-binding site based on the molluscan acetylcholine-binding protein structure.

Changes of GABA(A) receptor binding and subunit mRNA level in rat brain by infusion of NOS inhibitor

In the present study, we have investigated the effects of prolonged inhibition of nitric oxide synthase (NOS) by infusion of NOS inhibitor, L-nitroarginine, to examine the pentobarbital-induced sleep, modulation of GABA(A) receptor binding, and GABA(A) receptor subunit mRNA level in rat brain. Pre-treatment with L-nitroarginine 30 min before pentobarbital treatment (60 mg/kg, i.p.) significantly increased the duration of sleep in rats. However, the duration of pentobarbital-induced sleep was shortened by the prolonged infusion of L-nitroarginine into ventricle. We have investigated the effect of NOS inhibitor on GABA(A) receptor binding characteristics in discrete areas of brain regions by using autoradiographic and in situ hybridization techniques. Rats were infused with L-nitroarginine (10, 100 pmol/10 microl/h, i.c.v.) for 7 days, through pre-implanted cannula by osmotic minipumps. The levels of [(3)H]muscimol and [(3)H]flunitrazepam binding were markedly elevated in almost all of brain regions including cortex, caudate putamen, thalamus, hippocampus, and cerebellum. However, there was no change in the level of [(35)S]TBPS binding. The levels of beta2-subunit were elevated in the cortex, brainstem, and cerebellar granule layers. By contrast, the levels of beta3-subunit were significantly decreased in the cortex, hippocampus, and cerebellar granule layers in L-nitroarginine-infused rats. Following L-nitroarginine treatment, the levels of alpha6- and delta-subunits which were strictly localized to the cerebellum, were not changed in the cerebellar granule layer. These results show that the prolonged inhibition of NOS by L-nitroarginine-infusion markedly elevates [(3)H]muscimol and [(3)H]flunitrazepam binding throughout the brain, and alters GABA(A) receptor subunit mRNA levels in different directions. Chronic inhibition of NO generation has differential effects on the various expressions of GABA(A) receptor subunits. These suggest the involvement of different regulatory mechanisms for the NO-induced expression of GABA(A) receptor.

Synaptic and Nonsynaptic Localization of Benzodiazepine/GABAA Receptor/Cl- Channel Complex Using Monoclonal Antibodies in the Dorsal Lateral Geniculate Nucleus of the Cat

The two monoclonal antibodies, bd-17 and bd-24, are specific for beta- and alpha-subunits of the GABAA/benzodiazepine receptor/chloride channel complex respectively. An abundance of both subunits has been revealed in the visual thalamus of the cat by light microscopic immunocytochemistry using these antibodies. The alpha-subunit specific antibody and electron microscopy were used to determine the subcellular distribution of immunoreactivity with respect to specific cell classes in the dorsal lateral geniculate nucleus. Immunoreactivity was always associated with membranes and the degree of immunoreactivity varied greatly between different types of cell as defined by: (i) immunoreactivity for GABA; (ii) soma area; (iii) presence or absence of cytoplasmic laminated bodies (CLB). GABA negative neurons with the smallest soma area showed the strongest immunoreactivity, mainly in the endoplasmic reticulum and also on the somatic plasma membrane. Cytoplasmic laminated bodies could be found in the majority of these neurons. Large GABA negative cells without CLBs were strongly immunoreactive on the plasma membrane of the soma and dendrites, but showed scant if any intracellular immunoreactivity. GABA-positive cells showed weak intracellular immunoreactivity but negligible if any immunoreactivity at the somatic and proximal dendritic plasma membrane. A similar reaction pattern was found in GABA negative cells which contained no CLBs and which constituted a medium sized cell population. It is suggested that the degree of intracellular receptor immunoreactivity is positively correlated with receptor turnover. The dendrites of projection cells, particularly outside the glomeruli, showed strong immunoreactivity on the plasma membrane. The synaptic junctions formed by many boutons (F terminals) establishing symmetrical synapses with dendrites of relay cells were immunopositive, but no immunoreactivity could be detected at the synapses established by the presynaptic dendrites of the local interneurons. Many axo-somatic F1 junctions were also immunoreactive. However, immunoreactivity for the receptor/channel complex was also widely distribution on nonsynaptic plasma membranes of somata and dendrites. Thus GABA may act at both synaptic and non-synaptic sites. Furthermore, the correlation of immunoreactivity for the GABAA receptor complex with previously published properties of physiologically identified cells suggests that the strongly immunoreactive, small, GABA negative cells with CLBs might correspond to the 'lagged' X-type cells, and the large GABA negative receptor outlined cells without CLBs might correspond to some of the Y-type neurons.

Changes of [3H]MK-801, [3H]muscimol and [3H]flunitrazepam binding in rat brain by the prolonged ventricular infusion of ginsenoside Rc and Rg1

In the present study, we have investigated the effects of centrally administered ginsenoside Rc and Rg1 on the modulation of the NMDA receptor and GABA(A)receptor binding in rat brain. The NMDA receptor binding was analysed by quantitative autoradiography using [(3)H]MK-801 binding, and the GABA(A)receptor bindings were analysed by using [(3)H]muscimol and [(3)H]flunitrazepam binding in rat brain slices. Rats were infused with ginsenoside Rc or Rg1 (10 microg/10 microl h(-1), i.c.v.) for 7 days, through pre-implanted cannula using osmotic minipumps (Alzet, model 2ML). The levels of [(3)H]MK-801 binding were highly decreased in part of the parietal layers of the cortex and cingulated by ginsenoside Rc and Rg1. The levels of [(3)H]muscimol binding were strongly elevated in almost all regions of the frontal cortex by the treatment of ginsenoside Rc but decreased by ginsenoside Rg1. However, the [(3)H]flunitrazepam binding was not modulated by ginsenoside Rc or Rg1 infusion. These results suggest that prolonged infusion of ginsenosides could differentially modulate [(3)H]MK-801 and [(3)H]muscimol binding in a region-specific manner.

Changes of GABA(A) receptor binding and subunit mRNA level in rat brain by infusion of subtoxic dose of MK-801

In the present study, we have investigated the effects of prolonged inhibition of NMDA receptor by infusion of subtoxic dose of MK-801 to examine the modulation of GABA(A) receptor binding and GABA(A) receptor subunit mRNA level in rat brain. It has been reported that NMDA-selective glutamate receptor stimulation alters GABA(A) receptor pharmacology in cerebellar granule neurons in vitro by altering the levels of selective subunit. However, we have investigated the effect of NMDA antagonist, MK-801, on GABA(A) receptor binding characteristics in discrete brain regions by using autoradiographic and in situ hybridization techniques. The GABA(A) receptor bindings were analyzed by quantitative autoradiography using [3H]muscimol, [3H]flunitrazepam, and [35S]TBPS in rat brain slices. Rats were infused with MK-801 (1 pmol/10 microl per h, i.c.v.) for 7 days, through pre-implanted cannula by osmotic minipumps (Alzet, model 2 ML). The levels of [3H]muscimol binding were highly elevated in almost all of brain regions including cortex, caudate putamen, thalamus, hippocampus, and cerebellum. However, the [3H]flunitrazepam binding and [35S]TBPS binding were increased only in specific regions; the former level was increased in parts of the cortex, thalamus, and hippocampus, while the latter binding sites were only slightly elevated in parts of thalamus. The levels of beta2-subunit were elevated in the frontal cortex, thalamus, hippocampus, brainstem, and cerebellar granule layers while the levels of beta3-subunit were significantly decreased in the cortex, hippocampus, and cerebellar granule layers in MK-801-infused rats. The levels of alpha6- and delta-subunits, which are highly localized in the cerebellum, were increased in the cerebellar granule layer after MK-801 treatment. These results show that the prolonged suppression of NMDA receptor function by MK-801-infusion strongly elevates [3H]muscimol binding throughout the brain, increases regional [3H]flunitrazepam and [35S]TBPS binding, and alters GABA(A) receptor subunit mRNA levels in different directions. The chronic MK-801 treatment has differential effect on various GABA(A) receptor subunits, which suggests involvement of differential regulatory mechanisms in interaction of NMDA receptor with the GABA receptors.

Deduction of amino acid residues in the GABA(A) receptor alpha subunits photoaffinity labeled with the benzodiazepine flunitrazepam

Peptide mapping and microsequencing were used to infer the site of photoaffinity labeling by the gamma-aminobutyric acidA receptor modulator [3H]flunitrazepam. Peptide mapping with and without N-deglycosylation was used to restrict the domain for photoaffinity labeling to residues 74-123 of the bovine alpha1 subunit, in agreement with a previously predicted labeling domain between residues 59-148 based on cyanogen bromide fragmentation. Edman degradation of partially purified photolabeled peptides gave release of 3H counts in the ninth cycle of a tryptic peptide sequence. A second V8/chymotryptic peptide produced an impure sequence with release of 3H counts in the seventh through ninth cycle of sequence. The combined data support those previously reported, i.e., that the primary site for photoaffinity labeling by [3H]flunitrazepam is His102 of the bovine alpha1 subunit. In addition we also detected possible secondary labeling of Pro97.

Changes of [3H]muscimol binding and GABA(A) receptor beta2-subunit mRNA level by tolerance to and withdrawal from pentobarbital in rats

Effects of continuous pentobarbital administration on binding characteristics of [3H]muscimol were examined by autoradiography, and levels of GABA(A) receptor beta2-subunit mRNA were investigated by in situ hybridization histochemistry in the rat brain. In order to eliminate the induction of hepatic metabolism by systemic administration of pentobarbital, an i.c.v. infusion model of tolerance to and withdrawal from pentobarbital was used. An experimental model of barbiturate tolerance and withdrawal was developed using i.c.v. infusion of pentobarbital (300 microg/10 microl/hr for 7 days) by osmotic minipumps and abrupt withdrawal from pentobarbital. The levels of [3H]muscimol binding were elevated in cingulate of frontal cortex (46%) and granule layer of cerebellum (32%) of rats 24-hr after withdrawal from pentobarbital, while it was only elevated in cingulate (58%) of tolerant rats. The GABA(A) receptor beta2-subunit mRNA was increased in the withdrawal rats only: in the cortex (9-14%), hippocampus (15-21%), inferior colliculus (21%), and granule layer of cerebellum (24%). These results show the involvement of GABA(A) receptor and its beta2-subunit up-regulations in pentobarbital withdrawal rats, and suggest that the levels of [3H]muscimol binding and GABA(A) receptor beta2-subunit mRNA are altered in a region-specific manner during pentobarbital withdrawal.

Two tyrosine residues on the alpha subunit are crucial for benzodiazepine binding and allosteric modulation of gamma-aminobutyric acidA receptors

Benzodiazepines (BZs) exert their therapeutic effects in the mammalian central nervous system at least in part by modulating the activation of gamma-aminobutyric acid (GABA)-activated chloride channels. To gain further insight into the mechanism of action of BZs on GABA receptors, we have been investigating structural determinants required for the actions of the BZ diazepam (dzp) on recombinant alpha1 beta2 gamma2 GABA(A) receptors. Site-directed mutagenesis was used to introduce point mutations into the alpha1 and gamma2 GABA(A) receptor subunits. Wild-type and mutant GABA(A) receptors were then expressed in Xenopus laevis oocytes or human embryonic kidney 293 (HEK 293) cells and studied using two-electrode voltage-clamp and ligand-binding techniques. With this approach, we identified two tyrosine residues on the alpha1 subunit (Tyr159 and Tyr209) that when mutated to serine, dramatically impaired modulation by dzp. The Y209S substitution resulted in a >7-fold increase in the EC50 for dzp, and the Y159S substitution nearly abolished dzp-mediated potentiation. Both of these mutations abolished binding of the high affinity BZ receptor antagonist [3H]Ro 15-1788 to GABA(A) receptors expressed in HEK 293 cells. These tyrosine residues correspond to two tyrosines of the beta2 subunit (Tyr157 and Tyr205) previously postulated to form part of the GABA-binding site. Mutation of the corresponding tyrosine residues on the gamma2 subunit produced only a slight increase in the EC50 for dzp (approximately 2-fold) with no significant effect on the binding affinity of [3H]Ro 15-1788. These data suggest that Tyr159 and Tyr209 of the alpha1 subunit may be components of the BZ-binding site on alpha1 beta2 gamma2 GABA(A) receptors.

Diazepam physical dependence and withdrawal in rats is associated with alteration in GABAA receptor function

Alteration in the function of the GABAA receptor complex and its relation to changes in withdrawal signs in diazepam (DZP)-dependent rats were studied. Physical dependence on DZP was induced in male F344 rats by using the drug-admixed food method. After cessation of treatment, withdrawal signs such as spontaneous convulsions were observed and withdrawal scores were maximal at 39 approximately 45 hr after the DZP withdrawal. Furthermore, these withdrawal signs almost disappeared by 159 approximately 168 hr after the DZP withdrawal. GABA-stimulated 36Cl- influx into cerebral cortical membrane vesicles was significantly decreased in rats 0 hr after DZP withdrawal and significantly increased in rats 42 hr after DZP withdrawal compared with control rats Flunitrazepam (FZ)-induced potentiation and an antagonistic effect of Ro 15-1788 on GABA-stimulated 36Cl- influx were observed in control rats. No FZ-potentiated GABA-stimulated 36Cl- influx was observed in rats 0 hr after DZP withdrawal: however, such an effect of FZ was recognized in rats 42 hr and 162 hr after DZP withdrawal. No antagonistic effect of Ro15-1788 on the FZ-induced stimulation was recognized in rats 0 hr and 42 hr after DZP withdrawal but was recognized at 162 hr after DZP treatment, although it was not significant. In a [3H]FZ assay of binding to benzodiazepine (BZ) receptors. Bmax values were significantly decreased in rats 0 hr after DZP withdrawal, but increased at 42 hr after DZP withdrawal, compared with control rats Bmax had almost returned to the control level at 162 hr after DZP treatment rats. In conclusion, these results indicate that functional changes in the GABAA/BZ receptor/CI- channel complex, i.e. increased sensitivity in GABAA receptors and impairment in the functional coupling between BZ receptors and GABAA receptors, may possibly be involved in the biochemical mechanism of the severe withdrawal symptoms appearing after chronic treatment with DZP.

Pharmacology of barbiturate tolerance/dependence: GABAA receptors and molecular aspects

Barbiturates are central nervous system depressants that are used as sedatives, hypnotics, anesthetics and anticonvulsants. However, prolonged use of the drugs produces physical dependence, and the drugs have a high abuse liability. The gamma-aminobutyric acidA (GABAA) receptor is one of barbiturates' main sites of action, and therefore it is thought to play a pivotal role in the development of tolerance to and dependence on barbiturates. Recent advances in the study of the GABAA receptor/chloride channel complex allow us to examine possible mechanisms that underlie barbiturate tolerance/dependence in a new light. In this minireview, we mainly focus on molecular and cellular aspects of the action of barbiturates and the possible mechanisms that contribute to development of tolerance to and dependence on barbiturates.

The major site of photoaffinity labeling of the gamma-aminobutyric acid type A receptor by [3H]flunitrazepam is histidine 102 of the alpha subunit

The alpha subunit of the gamma-aminobutyric acid type A (GABA(A)) receptor is known to be photoaffinity labeled by the classical benzodiazepine agonist, [3H]flunitrazepam. To identify the specific site for [3H]flunitrazepam photoincorporation in the receptor subunit, we have subjected photoaffinity labeled GABA(A) receptors from bovine cerebral cortex to specific cleavage with cyanogen bromide and purified the resulting photolabeled peptides by immunoprecipitation with an anti-flunitrazepam polyclonal serum. A major photolabeled peptide component from reversed-phase high performance liquid chromatography of the immunopurified peptides was resolved by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The radioactivity profile indicated that the [3H]flunitrazepam photoaffinity label is covalently associated with a 5.4-kDa peptide. This peptide is glycosylated because treatment with the enzyme, peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase, reduced the molecular mass of the peptide to 3.2 kDa. Direct sequencing of the photolabeled peptide by automated Edman degradation showed that the radioactivity is released in the twelfth cycle. Based on the molecular mass of the peptides that can be generated by cyanogen bromide cleavage of the GABA(A) receptor alpha subunit and the potential sites for asparagine-linked glycosylation, the pattern of release of radioactivity during Edman degradation of the photolabeled peptide was mapped to the known amino acid sequence of the receptor subunit. The major site of photoincorporation by [3H]flunitrazepam on the GABA(A) receptor is shown to be alpha subunit residue His102 (numbering based on bovine alpha 1 sequence).

Mapping of GABAA receptor sites that are photoaffinity-labelled by [3H]flunitrazepam and [3H]Ro 15-4513

The GABAA receptor in brain membranes prepared from bovine cerebral cortex and cerebellum has been photoaffinity-labelled by the classical benzodiazepine agonist, [3H]flunitrazepam, or by the partial inverse agonist [3H]Ro 15-4513. Following solubilization and precipitation with trichloroacetic acid, the photoaffinity-labelled receptor preparations were subjected to specific chemical cleavage using hydroxylamine, a reagent which cleaves specifically at a relatively rare Asn-Gly bond. The resulting peptides were resolved by denaturing polyacrylamide gel electrophoresis and mapping of these peptides to the known amino acid sequences of the GABAA receptor subunits has localized the photoaffinity-labelling sites for these two ligands to distinct portions of the alpha subunits. It is shown that the site for [3H]flunitrazepam photoaffinity-labelling in the receptor populations of both the cerebral cortex and cerebellum occurs within residues 1-103 of the bovine alpha 1 subunit sequence (or within analogous segments of homologous alpha subunits). In contrast, the site of photoaffinity-labelling by [3H]Ro 15-4513 in the cerebral cortex and in the diazepam-sensitive GABAA receptor population of the cerebellum lies between residues 104 and the carboxy-terminus of the bovine alpha 1 or homologous alpha subunits. However, the [3H]Ro 15-4513 photoaffinity-labelling site for the diazepam-insensitive receptors of the cerebellum is shown to occur within residues 1-101 (alpha 6 subunit numbering). These results demonstrate that the photoaffinity-labelling sites for [3H]flunitrazepam and [3H]Ro 15-4513 on the GABAA receptor are localized to distinct domains of the alpha 1 subunit and that [3H]Ro 15-4513 photoaffinity labels a site on the alpha 6 subunit that is unique from its site of labelling on the alpha 1 subunit.

Identification of domains in human recombinant GABAA receptors that are photoaffinity labelled by [3H]flunitrazepam and [3H]Ro15-4513

We have used [3H]flunitrazepam and [3H]Ro15-4513 as photoaffinity labelling agents in combination with a chemical cleavage technique to localize the benzodiazepine recognition sites of specific human recombinant alpha 1 beta 1 gamma 2, alpha 1 beta 3 gamma 2 and alpha 6 beta 3 gamma 2 GABAA receptor subtypes. The chemical agent utilized was hydroxylamine, whose substrate is a relatively rare asparagine-glycine amide bond that occurs only in the alpha subunits of the receptors examined in this study. Cleavage products were resolved using sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). The results of these experiments show that, in the alpha 1 subunit-containing receptors, incorporation of [3H]flunitrazepam occurs within residues 1-103 of the alpha 1 subunit, while incorporation of [3H]Ro15-4513 occurs within the region of the alpha 1 subunit that lies between residue 104 and the C-terminus. Photolabelling of membranes prepared from the alpha 6 beta 3 gamma 2-expressing cell line with [3H]Ro15-4513 resulted in the incorporation of radiolabel into two major protein species of M(r) 56,000 and M(r) 48,000, indicating incorporation into the alpha 6 subunit and possibly also the gamma 2 subunit. Hydroxylamine cleavage of alpha 6-containing receptors labelled with [3H]Ro15-4513 produced a gel profile consistent with the incorporation of the label occurring between residue 125 and the C-terminal. Thus, we have shown that the recognition sites for the agonist [3H]flunitrazepam and the inverse agonist [3H]Ro15-4513 occur within distinct domains of the human GABAA receptor.

Dependence of the GABAA receptor gating kinetics on the alpha-subunit isoform: implications for structure-function relations and synaptic transmission

1. To examine the dependence of gamma-aminobutyric acid (GABAA) receptor gating on the alpha-subunit isoform, we studied the kinetics of GABA-gated currents (IGABA) of receptors that differed in the alpha-subunit subtype, alpha 1 beta 2 gamma 2S and alpha 3 beta 2 gamma 2S. cDNAs encoding rat brain subunits were co-expressed heterologously in HEK-293 cells and the resultant receptors studied with the whole-cell patch clamp technique and rapidly applied GABA pulses (5-10 s). 2. IGABA of both receptors showed a loosely similar dependence on GABA concentration over a wide range (1-5000 microM). Generally, IGABA manifested activation reaching an early current peak, subsequent slower spontaneous desensitization, and deactivation of open channels at pulse termination. Lowering GABA concentrations reduced peak currents and slowed activation and desensitization kinetics. 3. The presence of alpha 3 altered the peak IGABA concentration-response relationship by shifting the fitted Hill equation to tenfold greater GABA concentrations (GABA concentration at half amplitude: alpha 1, 7 microM; and alpha 3, 75 microM) without affecting Hill coefficients (alpha 1, 1.6; alpha 3, 1.5). These findings indicate a reduction in the apparent activating site affinity and are consistent with previous reports. 4. To investigate differences in gating, we normalized for apparent activating site affinities by analysing the time course of macroscopic gating at equi-activating GABA concentrations. The presence of alpha 3 slowed activation fourfold (time to current peak (means +/- S.E.M.): alpha 1, 1.2 +/- 0.06 s (2 microM); alpha 3, 4.7 +/- 0.5 s (20 microM)), desensitization nearly twofold (reciprocal of time to 80% decay: alpha 1, 2.5 +/- 0.48 s-1 (100 microM); alpha 3, 1.5 +/- 0.15 s-1 (1000 microM)) and deactivation threefold (monoexponential decay time constant: alpha 1, 0.22 +/- 0.026 s (2 microM); alpha 3, 0.68 +/- 0.1 s (20 microM)). 5. To gain an insight into the gating mechanisms underlying macroscopic desensitization, we extended a previous gating model of GABAA receptor single-channel activity to include a desensitization pathway. Such a mechanism reproduced empirical alpha 1 beta 2 gamma 2S activation, desensitization and deactivation kinetics. 6. To identify molecular transitions underlying the gating differences between alpha 1 beta 2 gamma 2S and alpha 3 beta 2 gamma 2S receptors, we explored parameter alterations of the alpha 1 beta 2 gamma 2S gating model that provided an accounting of alpha 3 beta 2 gamma 2S empirical responses. Remarkably, alteration of rates and rate constants involved in ligand binding alone allowed reproduction of alpha 3 beta 2 gamma 2S activation, desensitization and deactivation. 7. These results indicate that substitution of the alpha 3 subunit variant in an alpha 1 beta 2 gamma 2S receptor alters transition rates involved in ligand binding that underlie changes in apparent activating site affinity and macroscopic current gating. Furthermore, they argue strongly that the structural determinants of these functional features reside on the alpha-subunit.

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.

Distribution of [3H]zolpidem binding sites in relation to messenger RNA encoding the alpha 1, beta 2 and gamma 2 subunits of GABAA receptors in rat brain

Localization of the messenger RNAs that encode the alpha 1, beta 2 and gamma 2 subunits of GABAA showed a distinct topographic pattern in rat brain which corresponded with [3H]zolpidem binding in most brain regions. The close topographic correspondence between the specific receptor subunits examined and the distribution of [3H]zolpidem binding sites provides support for the hypothesis that this benzodiazepine type 1 selective ligand binds to a GABAA receptor that consists of alpha 1, beta 2 and gamma 2 subunits in the rat brain. Brain regions with relatively high densities of alpha 1, beta 2 and gamma 2 subunits of GABAA and [3H]zolpidem binding included olfactory bulb, medial septum, ventral pallidum, diagonal band, inferior colliculus, substantia nigra pars reticulata and specific layers of the cortex. Two areas with low [3H]zolpidem binding and a virtual absence of these GABAA receptor subunit messenger RNAs were the lateral septum and the striatum. In contrast to the discrete pattern observed for alpha 1 and beta 2 subunit messenger RNAs, the gamma 2 subunit messenger RNA was distributed more diffusely in brain. Only the hippocampus, layer 2 of the piriform cortex and the cerebellum showed a strong concentration of the gamma 2 subunit messenger RNA. It was determined with a polymerase chain reaction assay that both long and short variants of the gamma 2 subunit messenger RNAs were present within several of the brain sites selected for examination. Sites with high densities of [3H]zolpidem binding sites had a greater relative abundance of the gamma 2 long splice variant, compared to the gamma 2 short variant. There were some regions that expressed high levels of alpha 1, beta 2 and gamma 2S subunit messenger RNAs but low [3H]zolpidem binding, suggesting that gamma 2 splice variant expression may modulate high-affinity [3H]zolpidem binding. To determine relationships between in vitro [3H]zolpidem binding and functional sensitivity in vivo, interactions between zolpidem and GABA were assessed in brain regions that contained high and low densities of [3H]zolpidem binding sites. In the medial septum, a brain region with a high concentration of [3H]zolpidem binding sites, iontophoretic application of zolpidem enhanced the inhibitory effect of GABA responses on 70% of the neurons examined. In the lateral septum, which contains very low densities of [3H]zolpidem binding sites, neurons were not sensitive to zolpidem enhancement of GABA-induced inhibition. These electrophysiological results demonstrate a correspondence between the regional distribution of [3H]zolpidem binding in vitro and functional sensitivity to the drug in vivo.

Heterogeneity and differential expression of the gamma-aminobutyric acidA (GABAA)/benzodiazepine receptor in the avian brain during development

1. The changes in the GABAA/benzodiazepine receptor in chicken brain during development has been studied by using 3H-flunitrazepam as the probe for the benzodiazepine modulator site and the antibodies recognizing the receptor protein. In the telencephalon and optic tectum, the proteins of 48, 50, and 51 kD were markedly labeled by 3H-flunitrazepam from embryonic day 18 to postnatal days, as revealed by photoaffinity labeling and SDS-PAGE of the brain membranes; the 51-kD protein appeared to be the predominant one in labeling intensity except at embryonic day 18 and postnatal days 14 and 28, whereas the 47- and 50-kD proteins were dominant in the cerebellum. However, the 47- and 48-kD proteins were faintly seen after postnatal day 28 in the three regions examined. 2. Immunoblotting using a monoclonal antibody against the 50- and 51-kD proteins showed that the straining pattern in the developing telecephalon or optic tectum was similar to the 50 kD/51 kD pattern obtained from fluorography. The antibody also stained the 50- and 51-kD proteins in the cerebellum despite the fact that the 51-kD protein was barely seen in the fluorogram. Moreover, the 50-kD protein was recognized by an antiserum raised against a partial sequence of the alpha 1 subunit of the receptor expressed in bacteria. The staining levels for the 50-kd protein by the antiserum on immunoblots of the brain regions were low in embryonic animals but higher during postnatal stages, consistent with that seen in fluorograms. 3. Receptor binding autoradiography using 3H-flunitrazepam exhibited that varying degrees of labeling intensity occurred among various brain areas at different ages. High densities of binding were present in the olfactory bulb, paleostriatum, optic tectum, and midbrain. These results support the diversity of the GABAA/benzodiazepine receptor in the vertebrate CNS.

Radioligands for PET studies of central benzodiazepine receptors and PK (peripheral benzodiazepine) binding sites--current status

The status of the radiochemical development and biological evaluation of radioligands for PET studies of central benzodiazepine (BZ) receptors and the so-called peripheral benzodiazepine binding sites, here discriminated and referred to as PK binding sites, is reviewed against current pharmacological knowledge, indicating those agents with present value and those with future potential. Practical recommendations are given for the preparation of two useful radioligands for PET studies, [N-methyl-11C]flumazenil for central BZ receptors, and [N-methyl-11C]PK 11195 for PK binding sites. Quality assurance and plasma metabolite analysis are also reviewed for these radioligands and practical recommendations are given on methodology for their performance.

m-Sulfonate benzene diazonium chloride: a powerful affinity label for the gamma-aminobutyric acid binding site from rat brain

m-Sulfonate benzene diazonium chloride (MSBD) was used to affinity-label the gamma-aminobutyric acid (GABA) binding site from rat brain membranes. To assess the irreversibility of the labeling reaction, we used an efficient ligand dissociation procedure combined to a rapid [3H]muscimol binding assay, both steps being performed on filter-adsorbed membranes. Inactivation of specific [3H]-muscimol binding sites by MSBD and its prevention by GABA were both time- and concentration-dependent. The time course of MSBD labeling was shortened as the pH of the incubation medium was increased from 6.2 to 8. These data suggest that MSBD can efficiently label the GABA binding site through alkylation of a residue having an apparent dissociation constant around neutrality.

Multiple benzodiazepine receptors: no reason for anxiety

Since the introduction of the benzodiazepines into clinical practice in 1960, these drugs have been widely employed as anxiolytics, sedative/hypnotics and anticonvulsants. In recent years, concern has been expressed about their side-effects, and their use has declined. During this latter period many advances have been made in understanding the molecular mechanisms by which these drugs produce their effects. Adam Doble and Ian Martin review this progress and highlight the possibilities that these advances may hold for the development of more efficacious and specific medicines.

A prominent epitope on GABAA receptors is recognized by two different monoclonal antibodies

The monoclonal antibody 62-3G1 raised against the GABAA/benzodiazepine receptor complex was tested for its subunit selectivity using recombinantly expressed GABAA receptor subunits. The antibody bound selectively to beta 2 and beta 3 but not beta 1 nor any other GABAA receptor subunit. Using enzyme-linked immunosorbent assay, the epitope on beta 2 and beta 3 subunits was determined to be residues 1-3. MAb bd 17, which displays identical subunit selectivity as mAb 62-3G1, was seen to bind to the same epitope. These results resolve the subunit selectivity of mAbs 62-3G1 and bd 17 and reveal the identity and localization of a prominent immunogenic epitope on the GABAA/benzodiazepine receptor.

GABAA receptor subtypes immunopurified from rat brain with alpha subunit-specific antibodies have unique pharmacological properties

The unique cytoplasmic loop regions of the alpha 1, alpha 2, alpha 3, and alpha 5 subunits of the GABAA receptor were expressed in bacterial and used to produce subunit-specific polyclonal antisera. Antibodies immobilized on protein A-Sepharose were used to isolate naturally occurring alpha-specific populations of GABAA receptors from rat brain that retained the ability to bind [3H]muscimol, [3H]flunitrazepam, [3H]Ro15-1788, and [125I]iodo-clonazepam with high affinity. Pharmacological characterization of these subtypes revealed marked differences between the isolated receptor populations and was generally in agreement with the reported pharmacological profiles of GABAA receptors in cells transiently transfected with alpha 1 beta 1 gamma 2, alpha 2 beta 1 gamma 2, alpha 3 beta 1 gamma 2, and alpha 5 beta 1 gamma 2 combinations of subunits. Additional subtypes were also identified that bind [3H]muscimol but do not bind benzodiazepines with high affinity. The majority of GABAA receptor oligomers contains only a single type of alpha subunit, and we conclude that alpha 1, alpha 2, alpha 3, and alpha 5 subunits exist in vivo in combination with the beta subunit and gamma 2 subunit.

Ontogeny of the benzodiazepine receptor in human brain: fluorographic, immunochemical, and reversible binding studies

The prenatal and postnatal human ontogeny of the central benzodiazepine receptor was investigated in six different brain regions between week 24 postconception and age 14 years. Binding studies, which were performed with [3H]flunitrazepam [( 3H]FNZ), revealed a steep increase in receptor density postnatally in frontal cortex and cerebellum. Bmax values were higher in medulla oblongata, pons, and thalamus than in cortex and cerebellum up to week 26. After that, receptor densities declined significantly in medulla and olive. The same tendency was apparent in pons, whereas receptor density remained unchanged in thalamus. The early ontogeny of the benzodiazepine receptor was also evaluated in fluorographs [( 3H]FNZ) and immunoblots using the alpha 1-subunit-specific monoclonal antibody (mAb) bd-24. Specific radiolabeled proteins with molecular weights of 53K and 59K were visible in cortical membranes from gestational week 8, the earliest time investigated. During further development, the intensity of the 53K band increased without changes in the 59K band. As in other species, postmortem proteolysis in human brain led to a specifically labeled peptide of 47K. The mAb bd-24 immunolabeled only the 53K protein and the 47K peptide.

N-deglycosylation and immunological identification indicates the existence of beta-subunit isoforms of the rat GABAA receptor

beta 2- and beta 3-subunits of GABAA receptors purified from the brains of 5-10-day-old rats were investigated in the intact or completely N-deglycosylated state using the beta-subunit-specific monoclonal antibody bd-17 and polyclonal antibodies directed against synthetic amino acid sequences specific for the GABAA receptor beta 2- or beta 3-subunits. The present results seem to indicate the existence of two different isoforms of the beta 3-subunit and several different isoforms of the beta 2-subunit of the GABAA receptor which probably are produced by alternative splicing.

Kindling does not induce persistent changes in fluorographic labeling patterns of benzodiazepine binding proteins in various rat brain regions

The GABAA receptor has been implicated in the mechanisms underlying the phenomenon of kindling. Photoaffinity labeling with 3H-flunitrazepam followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis allows the fluorographic visualization of GABAA receptor proteins with benzodiazepine binding sites which presumably correspond to different alpha-subtypes. This method offers an opportunity to investigate whether up- or down-regulation of single benzodiazepine binding proteins occurs. In the present study, labeling patterns of benzodiazepine binding proteins were determined in 12 brains regions of amygdala-kindled rats (2 weeks after the last fully kindled seizure) and sham-operated controls. For most brain regions, labeling patterns were separately determined for the ipsi- and contralateral side. A comparison of the labeling patterns thus obtained revealed no persistent changes between kindled animals and controls in any of the brain regions, including amygdala, substantia nigra and hippocampus. Thus, we conclude that kindling does not induce fluorographically detectable changes in the expression patterns of the benzodiazepine binding proteins. The results confirm the existence of regional heterogeneity of benzodiazepine binding proteins and extend the findings to brain regions which had previously not been investigated.

Peptide subunits of gamma-aminobutyric acidA/benzodiazepine receptors from bovine cerebral cortex

The gamma-aminobutyric acidA/benzodiazepine receptor complexes from bovine cerebral cortex were purified by immunoaffinity chromatography, and the main component peptide subunits were characterized. The peptide band originally thought to be a single beta subunit [57,000 Mr band in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE)] is composed of at least four different peptides of 54,000-57,000 Mr. Two peptides of 55,000 and 57,000 Mr were recognized by the beta subunit-specific monoclonal antibody 62-3G1. Peptides in the range of 54,000-57,000 Mr were photoaffinity-labeled with [3H]muscimol. A different 57,000 Mr peptide was photoaffinity-labeled by [3H]flunitrazepam, but neither was recognized by the monoclonal antibody 62-3G1 nor photoaffinity-labeled with [3H]muscimol. Some peptides could be identified by their differential mobility shift in SDS-PAGE after treatment with endoglycosidase H. Two additional subunit peptides of 51,000 and 53,000 Mr were also photoaffinity-labeled by [3H]flunitrazepam and reacted with antiserum A. However, the 57,000 Mr peptide that also was photoaffinity-labeled by [3H]flunitrazepam did not react with antiserum A.

Purification of the gamma-aminobutyric acidA/benzodiazepine receptor complex by immunoaffinity chromatography

The bovine gamma-aminobutyric acidA/benzodiazepine receptor complex has been purified by a novel immunoaffinity chromatography method on immobilized monoclonal antibody 62-3G1. Immunopurification of the complex was achieved in a single step with an improved yield over affinity chromatography on the benzodiazepine Ro 7-1986/1. High-resolution sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of the immunoaffinity-purified receptor revealed three major peptide bands of 51,000, 55,000, and 57,000 Mr which were also present in the Ro 7-1986/1 affinity-purified receptor. Peptide mapping, immunoblotting with subunit specific antibodies, and photoaffinity labeling with [3H]flunitrazepam and [3H]muscimol have been used for the identification of receptor subunits, including several which comigrated in a single band in SDS-PAGE.

Single channels activated by high concentrations of GABA in superior cervical ganglion neurones of the rat

1. Single-channel currents evoked by high concentrations of GABA (10-2000 microM) have been analysed to investigate the characteristics of GABAA receptor channels in outside-out patches from rat sympathetic neurones. When high concentrations of GABA were applied to a patch, channel openings occurred in prolonged clusters (3.8 +/- 3.7 s (mean +/- S.D.) at 50 microM-GABA) consisting, on average, of 350 apparent openings per cluster. Individual clusters were separated by long silent intervals. 2. Channel openings were to many (often ill-defined) conductance states (range 7-36 pS), but the most frequently observed conductance level was approximately 30 pS, (29.6 +/- 0.34 pS). Only these clusters during which the channel was open to this main state conductance for at least 95% of the cluster open time were used in the analysis of probability of being open. Other less frequently observed conductance levels were 15-18 and 22-23 pS, while levels of 33-36 and 7-9 pS were occasionally, but reliably, observed. 3. Bursts within clusters were defined as a series of openings separated by closed intervals shorter than some critical value, tc. At 50 microM-GABA the mean burst length was 439 +/- 434 ms (+/- S.D., tc = 50 ms). 4. The probability of being open, po, during bursts within clusters has been analysed as a function of GABA concentration. As expected, increasing the concentration of GABA resulted in an overall increase in po. However, for a given agonist concentration there was a wide spread in po, far greater than that predicted for a population of identical and independent receptor channels (demonstrated by comparison with stimulated channel activity). 5. The wide range of po values at a particular concentration of GABA was not due to inappropriate selection of tc. At 50 microM-GABA the range of po values was similar for tc of 20-1000 ms, although the overall mean po became lower (0.64 rather than 0.81). 6. On the basis of simulated channel activity, it appears that most clusters which do not contain multiple openings, arise from the activity of one individual channel. Furthermore, there was no detectable tendency for gaps between bursts to be shorter in the middle of a cluster than at its ends. Therefore it is unlikely that variability in po arose from overlapping activity of two or more channels. 7. The values for po, mean open time, and mean shut time for bursts within the same cluster, and between different clusters, were compared by a randomization test.(ABSTRACT TRUNCATED AT 400 WORDS)

Anticonvulsant steroids and the GABA/benzodiazepine receptor-chloride ionophore complex

The ability of steroids to influence brain excitability is well documented. Certain 3 alpha-hydroxylated pregnanes are known to possess anticonvulsant and sedative-hypnotic/anesthetic properties. It has been observed that the seizure susceptibility in menstruating women with catamenial epilepsy appears to be correlated with changes in ovarian steroid levels. However, the underlying mechanism of these steroid influences on brain activity has only been recently revealed by pharmacological studies. These studies have provided compelling evidence for the presence of a novel steroid recognition site on the GABAA-benzodiazepine receptor complex (GBRC). Steroids may interact with this site with high affinity and stereospecificity to enhance chloride channel conductance in a manner similar to that produced by benzodiazepines (BZs) or barbiturates. The existence of such a steroid site on the GBRC is further supported by recent experiments involving the transfection of GABAA receptor cDNAs into a human embryonic kidney cell line. Based on the knowledge of the structure-activity requirements for the interaction of steroids with this novel recognition site, it is conceivable that the development of new anticonvulsant steroids with high therapeutic indices can be achieved.

GABAA-receptor expressed from rat brain alpha- and beta-subunit cDNAs displays potentiation by benzodiazepine receptor ligands

In mammalian brain, the activation of GABAA-receptors is associated with the opening of chloride channels, whose function can be allosterically modulated by drugs, in particular by ligands of the benzodiazepine receptor. Agonistic ligands potentiate while inverse agonists reduce the efficiency of GABA. We have cloned cDNAs encoding alpha 1- and beta 1-subunits of the GABAA-receptor from rat brain. When the corresponding RNAs were co-expressed in Xenopus oocytes. GABA-induced currents were recorded which were inhibited by bicuculline and potentiated by pentobarbital. GABA activated the channel in a weakly cooperative manner. Furthermore, the GABA-response was modulated by benzodiazepine receptor ligands. However, not only various agonists but also the antagonist flumazenil and the inverse agonist DMCM potentiated the GABA-response. Thus, alpha 1- and beta 1-subunits are sufficient to form GABAA-receptors which contain benzodiazepine binding sites, although in a functionally restricted form.

Chronic benzodiazepine antagonist treatment and its withdrawal upregulates components of GABA-benzodiazepine receptor ionophore complex in cerebral cortex of rat

Effect of chronic administration of benzodiazepine (BZ) receptor antagonist Ro 15-1788 (flumazenil) (4 mg/kg once daily for 14 days) treatment and its withdrawal on locomotor activity, body temperature, and the binding pattern of receptor ligands that bind to GABA-BZ receptor ionophore complex in different regions of the brain of the rat was studied. Ro 15-1788 (x 14 d) increased the specific binding of [3H]ethyl-8-fluoro-5-6-dihydro-5-methyl-6-oxo-4H- imidazo[1,5 alpha][1,4]benzodiazepine-3-carboxylate [( 3H]Ro 15-1788), [3H]ethyl-8-azido-5-6-dihydro-5-methyl-6-oxo-4H- imidazo[1,5 alpha][1,4]benzodiazepine-3-carboxylate [( 3H]Ro 15-4513), [3H]flunitrazepam, and [35S]t-butylbicyclophosphorothionate [( 35S]TBPS) in cerebral cortex, and this increase in binding remained upregulated during the drug withdrawal at 24 h. The binding of [3H]Ro 15-1788 was also found significantly increased in the hippocampus, but not in cerebellum and striatum. The chronic Ro 15-1788 treatment did not alter the specific binding of [3H]GABA. Rosenthal analysis of the saturation isotherms indicated that the observed upregulation in the binding pattern of [3H]Ro 15-1788 and [3H]Ro 15-4513 in the cerebral cortex was due to an increase in the binding capacity (Bmax). The receptor affinity (Kd) was not changed. The withdrawal of Ro 15-1788 following its chronic administration also enhanced locomotor activity. However, no apparent change in body temperature was observed either due to chronic treatment or withdrawal. These data indicate that chronic Ro 15-1788 treatment and its withdrawal may produce an upregulation of subunits which bind the positive (benzodiazepines), negative (inverse agonist), and neutral (antagonist) ligands of benzodiazepine receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Subunit selectivity and epitope characterization of mAbs directed against the GABAA/benzodiazepine receptor

mAbs bd 17, bd 24, and bd 28 raised against bovine cerebral gamma-aminobutyric acid (GABAA)/benzodiazepine receptors were analyzed for their ability to detect each of 12 GABAA receptor subunits expressed in cultured mammalian cells. Results showed that mAb bd 17 recognizes epitopes on both beta 2 and beta 3 subunits while mAb bd 24 is selective for the alpha 1 subunit of human and bovine, but not of rat origin. The latter antibody reacts with the rat alpha 1 subunit carrying an engineered Leu at position four, documenting the first epitope mapping of a GABAA receptor subunit-specific mAb. In contrast to mAbs bd 17 and bd 24, mAb bd 28 reacts with all GABAA receptor subunits tested but not with a glycine receptor subunit, suggesting the presence of shared epitopes on subunits of GABA-gated chloride channels.

GABAA-receptors: structural requirements and sites of gene expression in mammalian brain

GABAA-receptors, the major synaptic targets for the neurotransmitter GABA, are gated chloride channels. By their allosteric drug-induced modulation they serve as molecular control elements through which the levels of anxiety, vigilance, muscle tension and epileptiform activity can be regulated. Despite their functional prominence, the structural requirements of fully functional GABAA-receptors are still elusive. Expression of cDNAs coding for the alpha 1- beta 1-subunits of rat brain yielded GABA-gated chloride channels which were modulated by barbiturates but displayed only agonistic responses to ligands of the benzodiazepine receptor. GABAA-receptors with fully functional benzodiazepine receptor sites were formed when the alpha 1- and beta 1-subunits were co-expressed with the gamma 2-subunit of rat brain. These receptors, however, failed to show cooperativity of GABA in gating the channel. In order to determine the subunit repertoire available for receptor assembly in different neuronal populations in vivo, the sites of subunit gene expression were (alpha 1, alpha 2, alpha 3, alpha 5, alpha 6, beta 1, beta 2, beta 3, gamma 2) mapped by in situ hybridization histochemistry in brain sections. The mRNAs of the alpha 1-, beta 1- and gamma 2-subunits were co-localized e.g. in mitral cells of olfactory bulb, pyramidal cells of hippocampus as well as granule cells of dentate gyrus and cerebellum. The lack of colocalization in various other brain areas points to an extensive receptor heterogeneity. The presence of multiple GABAA-receptors in brain may contribute to synaptic plasticity, differential responsiveness of neurons to GABA and to variations in drug profiles.

Characterization with antibodies of the gamma-aminobutyric acidA/benzodiazepine receptor complex during development of the rat brain

The postnatal development of the gamma-aminobutyric acidA/benzodiazepine receptor (GABAR/BZDR) complex of the rat brain has been investigated using the monoclonal antibody 62-3G1 and the polyclonal rabbit antiserum A, specific for the 57,000 and 51,000 Mr receptor subunits, respectively. Both GABAR and BZDR binding activities co-precipitated during all postnatal ages. Adult rats showed a main 51,000 Mr[3H]flunitrazepam photoaffinity-labeled peptide, whereas newborn rats showed several photolabeled peptides of higher Mr. All the photolabeled peptides could be immunoprecipitated with each antibody regardless of the age of the rats. These results suggest that the physical coupling between the GABAR and the BZDR is already present in newborn animals and it is maintained afterwards during development. Glycosidase and peptidase treatments of the immunoprecipitated GABAR/BZDR complex indicated that all the [3H]flunitrazepam-photolabeled subunits are different peptides, although they seem to conserve a high degree of homology. In addition to the age-dependent heterogeneity, the results also suggest that for each age, there is heterogeneity in the subunit composition of the GABAR/BZDR complex.

Sequence and expression of a novel GABAA receptor alpha subunit

Cloned cDNA encoding the bovine alpha 4 subunit of the GABAA receptor has been isolated. The predicted 51 amino acid long mature protein contains an exceptionally long intracellular domain and shares 53-56% sequence similarity to the previously characterized alpha 1, alpha 2 and alpha 3 subunits. Co-expression of alpha 4 and beta 1 in Xenopus oocytes resulted in the formation of GABA-gated chloride channels with expected pharmacology, although no benzodiazepine potentiation was observed. Northern analysis indicates that a 4 kb alpha 4 mRNA is expressed in the calf cerebellum, cortex and hippocampus but is barely detectable in the rat brain.

Multiplicity of GABAA--benzodiazepine receptors

Binding studies suggest the presence of at least two pharmacologically distinct 'central' benzodiazepine receptors in the brain. Since central benzodiazepine receptors are allosteric modulatory sites on GABAA receptors, this evidence indirectly points to the existence of at least two GABAA receptors. Werner Sieghart describes biochemical studies that have identified several different alpha- and beta-subunits of these receptors, and molecular biological studies in which the genes encoding a variety of different alpha-, beta- and gamma-subunits have been isolated, sequenced and expressed in Xenopus oocytes. These studies all point to the existence of multiple GABAA receptors in the brain.

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.

Phosphorylation of gamma-aminobutyrate (GABA)/benzodiazepine receptors by cyclic AMP-dependent protein kinase

Preparations of gamma-aminobutyrate (GABA)/benzodiazepine receptor from pig cerebral cortex are composed of three major bands of polypeptides (51, 55 and 57 kDa) which are purified in a ratio of approx. 2:1:1 respectively. Treatment of purified receptor preparations with cyclic AMP-dependent protein kinase resulted in major incorporation of 32P into the 55 kDa band only. The maximum incorporation achieved was 0.6 mol of 32P/mol of 55 kDa polypeptide. The phosphorylated receptor subunit (beta-subunit) displays the same apparent Mr as a band labelled irreversibly with the GABA receptor agonist [3H]muscimol. The two nonphosphorylated subunit polypeptides (51 and 57 kDa) are each labelled irreversibly with [3H]flunitrazepam and are recognized by anti-peptide antibodies specific for alpha-subunits.

Importance of a novel GABAA receptor subunit for benzodiazepine pharmacology

Neurotransmission effected by GABA (gamma-aminobutyric acid) is predominantly mediated by a gated chloride channel intrinsic to the GABAA receptor. This heterooligomeric receptor exists in most inhibitory synapses in the vertebrate central nervous system (CNS) and can be regulated by clinically important compounds such as benzodiazepines and barbiturates. The primary structures of GABAA receptor alpha- and beta-subunits have been deduced from cloned complementary DNAs. Co-expression of these subunits in heterologous systems generates receptors which display much of the pharmacology of their neural counterparts, including potentiation by barbiturates. Conspicuously, however, they lack binding sites for, and consistent electrophysiological responses to, benzodiazepines. We now report the isolation of a cloned cDNA encoding a new GABAA receptor subunit, termed gamma 2, which shares approximately 40% sequence identity with alpha- and beta-subunits and whose messenger RNA is prominently localized in neuronal subpopulations throughout the CNS. Importantly, coexpression of the gamma 2 subunit with alpha 1 and beta 1 subunits produces GABAA receptors displaying high-affinity binding for central benzodiazepine receptor ligands.

Functional reconstitution of the bovine brain GABAA receptor from solubilized components

The GABAA/benzodiazepine receptor has been solubilized from membrane preparations of bovine cerebral cortex and has been reconstituted, in a functionally active form, into phospholipid vesicles. In preliminary experiments, the receptor was labeled with the photoactive benzodiazepine [3H]flunitrazepam prior to solubilization. A peptide of apparent molecular weight 53,500 was specifically labeled by this method, and this was used as a marker for the receptor during the reconstitution procedures. The labeled protein was solubilized with approximately 40% efficiency by 1% beta-octyl glucoside. Reconstitution was achieved by mixing the solubilized proteins with a 4:1 mixture of soybean asolectin and bovine brain phospholipids, followed by chromatography on Sephadex G-50-80 to remove detergent. The incorporation of the GABAA receptor into membrane vesicles has been verified by sucrose gradient centrifugation in which the [3H]-flunitrazepam-labeled peptide comigrated with [14C]phosphatidylcholine used as a lipid marker. Vesicles prepared without labeled markers retained the ability to bind both [3H]flunitrazepam and the GABA analogue [3H]muscimol. Furthermore, the binding parameters were very similar to those measured using native membrane preparations. A novel fluorescence technique has been used to measure chloride transport mediated by the GABAA receptor in reconstituted vesicles. Chloride influx was rapidly stimulated in the presence of micromolar concentrations of muscimol and was blocked by preincubation of the membranes with muscimol (desensitization). Flux was also blocked by pretreatment with the competitive GABAA receptor blocker bicuculline or with the noncompetitive GABAA receptor antagonist picrotoxin.

Evidence for the existence of several different alpha- and beta-subunits of the GABA/benzodiazepine receptor complex from rat brain

gamma-Aminobutyric acid (GABA)/benzodiazepine receptors purified from the brains of young and adult rats were photolabeled by [3H]flunitrazepam or [3H]muscimol. Gel electrophoresis revealed 3 different proteins with apparent molecular weight (Mr) 51,000, 53,000 and 59,000 which were specifically and irreversibly labeled by [3H]flunitrazepam and recognized by the alpha-subunit specific antibody bd-28. Similarly, 3 different proteins with Mrs 51,000, 53,000 and 56,000 were irreversibly labeled by [3H]muscimol and recognized by the beta-subunit-specific antibody bd-17. Comparison of the photolabeling and immunological staining pattern from young and adult rats seems to indicate that proteins labeled by [3H]flunitrazepam and bd-28 are different from those labeled by [3H]muscimol and bd-17.

Photolabeled tryptic degradation products of benzodiazepine-binding proteins are glycopeptides. Implications for localization of cleavage sites

Crude synaptic membranes of avian and mammalian brain tissue were photolabeled with the benzodiazepine-receptor ligand [3H]flunitrazepam and subsequently treated extensively with trypsin followed by incubation with endoglycosidase F. SDS-polyacrylamide gel electrophoresis and fluorography revealed that the final tryptic degradation product of 25 kDa in both pigeon and calf brain is deglycosylated in two steps. These results were confirmed by immunoblots of similarly pretreated membranes of pig brain using the alpha-subunit-specific monoclonal antibody bd-24. Benzodiazepine-receptor binding and its enhancement by GABA are largely retained after trypsinization. Based on the proposed transmembrane topology for the alpha-subunits of the GABA/benzodiazepine receptor, we suggest that the large N-terminal domain of benzodiazepine-binding proteins is protected against tryptic cleavage.