Diazepam sensitivity of the binding of an imidazobenzodiazepine, [3H]Ro 15-4513, in cerebellar membranes from two rat lines developed for high and low alcohol sensitivity

Natural mutation of GABAA receptor alpha 6 subunit alters benzodiazepine affinity but not allosteric GABA effects

The binding of the imidazobenzodiazepine, [3H]Ro 15-4513, to cerebellar granule cell-specific GABAA/benzodiazepine receptors is typically insensitive to benzodiazepine receptor agonists such as diazepam. A mutation in the alpha 6 subunit, causing replacement of the arginine at the 100 position by glutamine (Q100), has recently been found in an alcohol- and benzodiazepine-sensitive rat line. The mutant alpha 6(Q100)beta 2 gamma 2 recombinant receptors are sensitive to diazepam. The binding of [3H]Ro 15-4513 to cerebellar diazepam-insensitive receptors is enhanced by GABA, whereas binding to diazepam-sensitive receptors is inhibited. Recombinant receptors consisting of beta 2 and gamma 2 subunits together with the wildtype alpha 6 or mutant alpha 6(Q100) subunit showed positive modulation of [3H]Ro 15-4513 binding by GABA, whereas alpha 1 beta 2 gamma 2 receptors showed negative modulation. The picrotoxin-sensitive binding of a convulsant, t-butylbicyclophosphoro[35S]thionate ([35S]TBPS), was inhibited in the alpha 6 beta 2 gamma 2 and alpha 6(Q100) beta 2 gamma 2 receptors by GABA at concentrations less than one-tenth of those required in the alpha 1 beta 2 gamma 2 receptors. GABA effects on [35S]TBPS binding were only slightly affected by diazepam in the alpha 6(Q100) beta 2 gamma 2 receptors, while profound effects were seen in the alpha 1 beta 2 gamma 2 receptors in the presence of diazepam. The results with the mutant receptor suggest that the alpha 1 and alpha 6 subunits are responsible for differential allosteric actions by GABA on other binding sites, independently of the structures defining the benzodiazepine binding pharmacology.

Postnatal development of diazepam-insensitive [3H]Ro 15-4513 binding sites

The postnatal development of the binding sites for an imidazobenzodiazepine, [3H]Ro 15-4513, which labels all presently known GABAA receptor-associated benzodiazepine binding sites, was studied in the cerebellar, cerebrocortical and hippocampal tissues of Wistar rats. The binding sites in the hippocampal membranes were fairly similar at all ages studied (1-2, 7, 14, and 90 days), suggesting early development of the GABAA receptors. The density of the binding sites increased significantly with age in the cerebellar and cerebrocortical membranes, without any changes in the affinity for this ligand. As judged by displacement by a benzodiazepine agonist, diazepam, [3H]Ro 15-4513 seemed to bind to a homogeneous pool of receptors in the cerebral cortex and hippocampus in all age groups, whereas in the cerebellum two pharmacologically distinct sites were observed. The benzodiazepine agonist-insensitive ("diazepam-insensitive") binding first appeared at the time when the cerebellar granule cells started to mature, on the seventh postnatal day. As determined by autoradiography, such binding occurred selectively in the granule cell layer and was absent from the cerebellum of one- to two-day-old rats. The latter finding was confirmed by photoaffinity labelling experiments, in which diazepam inhibited all photolabelling in membrane homogenates from newborn rats. Diazepam-sensitive cerebellar binding, which increased steadily during development, was also localized in the molecular layer and deep nuclei.(ABSTRACT TRUNCATED AT 250 WORDS)

Benzodiazepine-induced motor impairment linked to point mutation in cerebellar GABAA receptor

The selectively outbred alcohol-non-tolerant (ANT) rat line is highly susceptible to impairment of postural reflexes by benzodiazepine agonists such as diazepam. ANT cerebella are generally devoid of diazepam-insensitive high-affinity binding of the benzodiazepine [3H]Ro15-4513, whereas in non-selected strains such binding marks a granule-cell-specific GABAA (gamma-aminobutyric acid) receptor containing the alpha 6 subunit. A critical determinant for diazepam insensitivity of this 'wild-type' cerebellar GABAA receptor is an arginine residue in alpha 6 position 100, where other alpha subunits carry a histidine. Here we report that the alpha 6 gene of ANT rats is expressed at wild-type levels but carries a point mutation generating an arginine-to-glutamine substitution at position 100. In consequence, alpha 6(Q100)beta 2 gamma 2 receptors show diazepam-mediated potentiation of GABA-activated currents and diazepam-sensitive binding of [3H]Ro15-4513. Our results suggest that cerebellar motor control may be a distinct behavioural correlate of the alpha 6-subunit-containing GABAA receptor subtype.

Early postnatal treatment with muscimol transiently alters brain GABAA receptors and open-field behavior in rat

The long-term effects of treatment with muscimol, a GABAA agonist, and with ethanol, also shown to have a GABAergic profile of action, on subsequent behavior and on the binding parameters of GABAA receptors were studied in Wistar rats. Rats were given two daily injections of muscimol (0.1 mg/kg M1 group of 0.2 mg/kg M2 group) or ethanol (1.5 g/kg) between the 1st and 21st postnatal days, with saline-treated animals serving as controls. At the age of one month, the activity of the M2 rats was decreased in the open-field arena. At the age of four months, ethanol-treated rats consumed less 10% ethanol solution in a free choice situation. No changes were seen in the elevated plus-maze test. Maximal GABA stimulation of [3H]flunitrazepam binding was decreased in the cerebellum and hippocampus in M2 rats at the age of one month but not four months. A significant positive correlation was found in all rats between maximal GABA-stimulated [3H]flunitrazepam binding in the cerebellum and ambulation in the open-field arena at the age of one month. Ethanol intake correlated negatively with maximal GABA stimulation of [3H]flunitrazepam binding in the cerebral cortex. None of the treatments affected the cerebellar binding of an imidazobenzodiazepine, [3H]Ro 15-4513. There was an age-dependent decline in the efficacy and potency (EC50) of the GABA enhancement of [3H]flunitrazepam binding in the cerebellum in all treatment groups. Basal binding of [3H]flunitrazepam to hippocampal and cerebellar membranes as well as binding of [3H]Ro 15-4513 to cerebellar membranes was also decreased as a function of age.(ABSTRACT TRUNCATED AT 250 WORDS)

Modification of benzodiazepine receptors supports the distinctive role of histidine residues

The effect of selective protein modifying reagents was examined on benzodiazepine (BZ) receptors in synaptosomal membrane preparations of rat whole brain and cerebellum. The potency of diethyl pyrocarbonate, a histidine modifying reagent, to inactivate BZ receptor binding, correlated with the rank order of agonist-inverse agonist efficacies of BZ ligands, the binding of the partial inverse agonist [3H]Ro 15-4513 was inactivated least. Diethyl pyrocarbonate slightly enhanced the displacing potency of Ro 15-4513 and enhanced its binding in low concentrations (1-2 mM). Diazepam-sensitive and -insensitive components of [3H]Ro 15-4513 binding were separated in cerebellum. Diethyl pyrocarbonate inactivated the diazepam-sensitive component with a potency (IC50 = 1.8 mM) similar to that on the binding of other benzodiazepines, while the diazepam-insensitive component was resistant to diethyl pyrocarbonate. Tetranitromethane and 2,3-butanedione (diacetyl), reagents specific for tyrosine and arginine residues respectively, exerted concentration-dependent partial inactivation of [3H]Ro 15-4513 binding. The diazepam-insensitive component of cerebellar Ro 15-4513 binding was more sensitive to inactivation by diacetyl but less sensitive to inactivation by tetranitromethane. These findings are consistent with a distinctive role of histidine-101 in alpha 1, alpha 2, alpha 3 and alpha 5 subunits of the gamma-aminobutyric acidA receptor complex and the His is replaced by an arginine residue in the alpha 6 subunit of the diazepam-insensitive cerebellar benzodiazepine receptors. The only other point of the protein sequence where histidine residues conserved in alpha 1, alpha 2, alpha 3 and alpha 5 subunits are replaced in alpha 6 is tyrosine-214 but this residue does not appear to contribute to benzodiazepine binding.

Characterization of two cerebellar binding sites of [3H]Ro 15-4513

Ro 15-4513 (ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H- imidazo[1,5-a][1,4]benzodiazepine-3-carboxylate), a partial inverse agonist of central benzodiazepine receptors, binds to two distinct sites in the cerebellum. The binding to diazepam-sensitive (DZ-S) sites is displaced by different benzodiazepine receptor ligands, whereas the other site is insensitive to benzodiazepine agonists [diazepam-insensitive (DZ-IS)]. The binding of [3H]Ro 15-4513 was studied in pig cerebellar membranes and in receptors solubilized and purified from these. Micromolar concentrations of gamma-aminobutyric acid (GABA) decreased DZ-S binding at both 0 and 37 degrees C, whereas it had no effect on DZ-IS binding at 0 degrees C and was stimulatory at 37 degrees C. The pH profiles of [3H]Ro 15-4513 binding were quite similar in both binding sites in the pH range of 5.5-10.5 but differed at acidic pH values from those reported for flunitrazepam and Ro 15-1788 (flumazenil; ethyl-8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H- imidazol[1,5-a][1,4]benzodiazepine-3-carboxylate) binding in DZ-S sites, suggesting that [3H]Ro 15-4513 does not interact with a histidine residue apparently present in the binding site. Zn2+, Cu2+, Co2+, and Ni2+ enhanced the binding to DZ-S sites, and the first three mentioned also enhanced the binding to DZ-IS sites. [3H]Ro 15-4513 binding activity was solubilized by various detergents. All detergents tested were more efficient in solubilizing DZ-S binding activity. High ionic strength improved especially the solubility of DZ-IS binding activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of diethyl pyrocarbonate modification of benzodiazepine receptors on [3H]Ro 15-4513 binding

The effects of treatment of brain membranes with diethyl pyrocarbonate (DEP), a histidine-modifying reagent, on the binding of 3H-labeled Ro 15-4513 (ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a]- [1,4]benzodiazepine-3-carboxylate) and [3H]diazepam were compared. DEP pretreatment produced a dose-dependent decrease in [3H]diazepam binding, whereas low DEP concentrations enhanced the binding of [3H]Ro 15-4513. These effects were reversed by incubation with hydroxylamine after the treatment. The enhancement of [3H]Ro 15-4513 binding was due to an increase in the affinity of the binding sites (KD), without any effect on binding capacity (Bmax). The enhancement was perceived in cerebral cortical, cerebellar, and hippocampal membranes. DEP treatment decreased the displacement of [3H]Ro 15-4513 binding by diazepam and FG 7142 (N-methyl-beta-carboline-3-carboxamide) but not by Ro 15-4513 and Ro 19-4603 (tert-butyl-5,6-dihydro-5-methyl-6-oxo-4H-imidazol[1,5- a]thieno[2,3-f][1,4]diazepine-3-carboxylate). Although the stimulating effect of gamma-aminobutyric acid (GABA) on [3H]-diazepam binding was not affected by DEP treatment, such treatment reduced the inhibitory effect of GABA on [3H]Ro 15-4513 binding. The enhancement of [3H]Ro 15-4513 binding was observed in membranes pretreated with DEP in the presence of flunitrazepam, whereas such pretreatment reduced significantly the inhibitory effect of DEP on [3H]-diazepam binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Ro 15-4513 binding to GABAA receptors: subunit composition determines ligand efficacy

The bidirectional modulation of ligand binding to benzodiazepine receptors (BzR) by GABA (the "GABA shift") has been widely used to predict ligand efficacy. The present study examined the effects of GABA and muscimol on [3H]Ro 15-4513 binding to "diazepam-insensitive" (DI) and "diazepam-sensitive" (DS) BzR. Neither GABA nor muscimol significantly altered [3H]Ro 15-4513 binding to DI in cerebellum, while both compounds inhibit [3H]Ro 15-4513 binding to cerebellar DS in a concentration-dependent fashion. The maximum reductions in [3H]Ro 15-4513 binding to cerebral cortical and hippocampal membranes elicited by GABA were comparable to those obtained in cerebellar DS, but significantly less than obtained with the full inverse agonist [3H]3-carbomethoxy-beta-carboline. The qualitatively different effect of GABAmimetics on [3H]Ro 15-4513 binding to DS and DI is not species specific since identical effects were obtained in rat and mouse brain. Based on previously established criteria, Ro 15-4513 can be classified as a "GABA-neutral" (antagonist) ligand at DI and "GABA negative" (inverse agonist) at other BzR. These findings suggest that GABAA receptor subunit composition determines not only ligand affinity but also ligand efficacy.

Functional properties of GABAA receptors in two rat lines selected for high and low alcohol sensitivity

The effects of lorazepam and sodium barbital on GABAA receptor function were evaluated in rat lines selected for differential sensitivity to the motor-impairing effects of ethanol [alcohol-insensitive (AT) and alcohol-sensitive (ANT) lines]. The effect of GABA on [3H]flunitrazepam and [3H]Ro 15-4513 binding and the effects of lorazepam and sodium barbital on [3H]muscimol binding were measured in cerebellar, cerebrocortical, and hippocampal membrane preparations. The effects of lorazepam and sodium barbital on muscimol-stimulated 36Cl- influx were measured using membrane vesicle suspensions from the same brain areas. No differences were found between the rat lines in the GABA-induced stimulation of [3H]flunitrazepam binding or in the lorazepam and sodium barbital-induced enhancement of either [3]muscimol binding or muscimol-stimulated 36Cl- flux. Neither was desensitization of the 36Cl- flux affected differently by ethanol, lorazepam, and barbital in vitro between the lines. The affinity of cerebellar diazepam-insensitive (DZ-IS) [3H]Ro 15-4513-binding sites for benzodiazepine agonists has been shown to be much greater in the ANT than the AT rats. In the present study, at 0 degrees C, GABA decreased [3H]Ro 15-4513 binding in the presence of diazepam only in ANT rats. Similarly, GABA decreased this binding at 37 degrees C in ANT rats having a high affinity for diazepam, whereas it enhanced the binding in all AT samples in those ANT samples where diazepam had a poor AT-like affinity. The decrease in binding in ANT samples is apparently caused by the enhancing effect of GABA on diazepam binding to DZ-IS [3H]Ro 15-4513-binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebellar GABAA receptors in two rat lines selected for high and low sensitivity to moderate alcohol doses: pharmacological and genetic studies

Alcohol-sensitive (ANT) rat line produced by selective outbreeding for high acute sensitivity to the motor-impairing effects of ethanol, displays unusual cerebellar GABAA receptor pharmacology. The ANT rats have enhanced benzodiazepine agonist affinity at their binding sites for an imidazobenzodiazepine, [3H]Ro 15-4513, normally not affected by agonists at all, and reduced GABAA agonist, [3H]muscimol, binding, when compared to the alcohol-insensitive (AT) rat line. In the present study, the benzodiazepine receptor difference was localized to the cerebellar granule cell layer. This receptor difference was not found in ex vivo binding studies after lorazepam administration, although brain lorazepam concentrations in both rat lines similarly exceeded 1 microM. An indication for differential binding in vivo between the lines was, however, observed, as pretreatment with lorazepam accentuated the relative accumulation of radioactivity only in the cerebellum of the AT rat line after an intravenous injection of a trace amount of [3H]Ro 15-4513, thus revealing benzodiazepine insensitivity for a portion of the cerebellar [3H]Ro 15-4513 binding in the AT but not in the ANT rats. In the second generation of AT/ANT cross-breeding, there was no clear association of alcohol sensitivity and cerebellar receptor binding. There was, however, a significant positive correlation between the [3H]muscimol binding and the diazepam-insensitive [3H]Ro 15-4513 binding in the cerebellum. In conclusion, the receptor defect in the cerebellar granular cell layer of the alcohol-sensitive ANT rats was also detectable in vivo, but it may not explain the enhanced alcohol sensitivity of these rats.

Cerebellar and frontal cortical benzodiazepine receptors in human alcoholics and chronically alcohol-drinking rats

Postmortem cerebellar and frontal cortical membrane homogenates from human alcoholics, control subjects without neurological or psychiatric illnesses, and rats that chronically drank alcohol were studied to determine the binding characteristics of an imidazobenzodiazepine, [3H]Ro 15-4513. This ligand binds to classical gamma-aminobutyric acidA (GABAA)/benzodiazepine receptors, as well as to a "diazepam-insensitive" site associated with the GABAA receptor complex in the cerebellar granule cell layer. There were no differences in the density of the binding sites between alcoholics and their controls, between alcohol-drinking AA rats that had a choice between 10% alcohol or water for about 10 weeks and their controls, or between Wistar rats that had been given 20% alcohol as their only fluid for 4 months and their controls, which were pair-fed isocalorically with sucrose. The affinity for the cerebellar binding of [3H]Ro 15-4513 was higher in the alcoholics than the controls. No differences were observed in the frontocortical binding. No affinity differences were observed in the rat models. There were no differences between the groups in the characteristics of [3H]Ro 15-4513 binding to human cerebellum in the presence of micromolar diazepam, thus revealing the diazepam-insensitive binding. When this component was subtracted from the total cerebellar binding, to reveal the diazepam sensitive binding, both the KD and Bmax were lower in the alcoholic than the control group. The binding of [3H]muscimol, a GABAA agonist, tended to be higher in the frontal cortices of alcoholics; a similar trend for greater effects was observed in the alcoholics for the GABA inhibition of [3H]Ro 15-4513 binding. These results suggest that no drastic changes occur through chronic alcohol abuse in the numbers of cerebellar and frontocortical benzodiazepine receptors in humans and rodent models; however, the data indicate that the alcoholics have either acquired or innate differences in classical benzodiazepine recognition sites of the cerebellum and in the coupling of these sites to GABAA sites in the frontal cortex, without any differences in cerebellar granule cell-specific diazepam-insensitive [3H]Ro 15-4513 binding sites.

Substrate specificity of diazepam-insensitive cerebellar [3H]Ro 15-4513 binding sites

[3H]Ro 15-4513, a negative modulator at the benzodiazepine receptor, binds with high affinity to all known benzodiazepine binding sites associated with the GABAA receptors. The present experiments surveyed a number of benzodiazepine receptor ligands with different chemical structures and different intrinsic pharmacological profiles for their ability to displace [3H]Ro 15-4513 binding in rat cerebellar membranes. The latter have been shown to possess two types of binding sites, one sensitive to positive modulators such as diazepam and the other insensitive to them (diazepam-insensitive). Whereas the full positive modulators from the benzodiazepine, beta-carboline, and quinoline classes did not displace binding at the diazepam-insensitive sites, the partial positive modulators' affinities for these sites varied much more, some being very potent and others having no affinity at all. All the neutral antagonists tested displaced binding at both types of binding sites, while some of the negative modulators were apparently not potent at the diazepam-insensitive sites. In an alcohol-sensitive rat line (ANT, alcohol non-tolerant), with an enhanced affinity for positive modulators at the diazepam-insensitive sites, these sites also exhibited enhanced affinity for some of the partial positive and negative modulators. The results suggest that the cerebellar diazepam-insensitive [3H]Ro 15-4513 binding sites have unique substrate specificity. It remains to be established which behaviours are affected by activation or inhibition of these receptors that are characterized by their insensitivity to benzodiazepine-positive modulators.

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.

High affinity ligands for 'diazepam-insensitive' benzodiazepine receptors

Structurally diverse compounds have been shown to possess high affinities for benzodiazepine receptors in their 'diazepam-sensitive' (DS) conformations. In contrast, only the imidazobenzodiazepinone Ro 15-4513 has been shown to exhibit a high affinity for the 'diazepam-insensitive' (DI) conformation of benzodiazepine receptors. We examined a series of 1,4-diazepines containing one or more annelated ring systems for their affinities at DI and DS benzodiazepine receptors, several 1,4-diazepinone carboxylates including Ro 19-4603, Ro 16-6028 and Ro 15-3505 were found to possess high affinities (Ki approximately 2.6-20 nM) for DI. Nonetheless, among the ligands examined, Ro 15-4513 was the only substance with a DI/DS potency ratio approximately 1; other substances had ratios ranging from 13 to greater than 1000. Ligands with high to moderate affinities at DI were previously classified as partial agonists, antagonists, or partial inverse agonists at DS benzodiazepine receptors, but behaved as 'GABA neutral' (antagonist) substances at DI. The identification of several additional high affinity ligands at DI benzodiazepine receptors may be helpful in elucidating the pharmacological and physiological importance of these sites.

Neonatal administration of a GABA-T inhibitor alters central GABAA receptor mechanisms and alcohol drinking in adult rats

Long-term effects of chronic treatment with a GABA-T (GABA-transaminase) inhibitor, ethanolamine O-sulphate (EOS) (200 mg/kg/day for the postnatal days 3-21) on the binding parameters of GABAA receptors, hypothalamic monoamines and subsequent behavior were studied in Wistar rats. At the age of 1 month, EOS-treated rats showed reduced activity in the open-field and, at the age of 4 months, their voluntary alcohol consumption was increased. No changes were seen in Porsolt's swim test or in the plus-maze test. Weight gain was significantly retarded in EOS-treated rats. Maximal stimulation of [3H] flunitrazepam binding by GABA was decreased in the cerebral cortex and the EC50-value for the GABA stimulation increased in the hippocampus in the EOS rats at the age of 4 months. EOS treatment did not alter the cerebellar diazepam sensitive and insensitive binding components of the imidazobenzodiazepine [3H]Ro 15-4513. No changes were observed in the hypothalamic monoamine concentrations. The results are in agreement with the idea that GABA-T inhibitor treatment permanently alters GABAA mechanisms. Moreover, altering the CNS GABA level during development increases adult alcohol intake in rat.

Diversity in receptor signalling: cellular individuality and the search for selective drugs

Recent developments in the molecular pharmacology of receptor signalling are reviewed. Receptors and other signalling molecules belong to superfamilies characterized by variations on given themes. A remarkable degree of diversity is generated by unique combinations of molecular entities, and there are also interactions between parallel signal transduction pathways. Molecular biology techniques have revealed new targets for drug development, as well as new methods of drug testing. However, it is not yet clear whether molecular and biochemical specificity always correspond to clinical selectivity.

Specific alterations in the cerebellar GABA(A) receptors of an alcohol-sensitive ANT rat line

The AT (alcohol-tolerant) and ANT (alcohol-nontolerant) rat lines, selected for differential sensitivity to the acute motor-impairing effects of ethanol, have been shown to differ in the ligand binding characteristics of their cerebellar GABAA receptors. In the present study, we characterized these binding differences further and determined whether similar differences are present in other rodent line pairs produced by selective breeding for differences in ethanol sensitivity. The alcohol-insensitive AT rats had more high-affinity [3H]muscimol binding sites in the cerebellum than the alcohol-sensitive ANT rats. The cerebellar "diazepam-insensitive" [3H]Ro 15-4513 binding sites were displaced by several benzodiazepine agonists (diazepam, lorazepam, clonazepam, and midazolam) at micromolar concentrations with greater efficacy in the ANT than the AT rats. Analyses of the displacement curves indicated that the "diazepam-insensitive" [3H]Ro 15-4513 binding sites have 30 to 300 times higher affinity to benzodiazepine agonists in the ANT than AT rats. There was no difference between the rat lines in the displacing potency of Ro 15-1788, a weak partial agonist; Ro 15-4513, a partial inverse agonist; or Ro 5-4864, a peripheral-type benzodiazepine receptor ligand. Thus, the affinity difference seen in the cerebellar [3H]Ro 15-4513 binding sites seems to be specific for benzodiazepine agonists. This difference in affinity may explain the behavioral difference in sensitivity to lorazepam between the rat lines. No differences in [3H]muscimol binding or in the sensitivity of [3H]Ro 15-4513 binding to micromolar diazepam concentrations were found between other rodent line pairs tested (LS/SS, HAS/LAS, HOT/COLD, FAST/SLOW, AA/ANA).(ABSTRACT TRUNCATED AT 250 WORDS)

More than one alpha variant may exist in a GABAA/benzodiazepine receptor complex

GABAA/benzodiazepine receptors are formed by the assembly of presumably five polypeptides with unknown stoichiometry. Six alpha, three beta, two gamma, and one delta subunit have been characterized on the molecular level. In analogy to the nicotinic acetylcholine receptor, and supported by functional analysis of recombinantly expressed GABAA receptor subunits, a structure containing at least three different polypeptides has been proposed for the functional GABAA and benzodiazepine regulated Cl(-)-channel. Using an alpha 1 subunit specific antiserum we could show that additional alpha variants are present in alpha 1 subunit containing GABAA/Benzodiazepine receptor complexes. This suggests that the diversity of GABAA/Benzodiazepine receptors may be larger than previously thought.

Cerebellar GABAA receptor selective for a behavioural alcohol antagonist

Benzodiazepines are widely prescribed anxiolytics and anticonvulsants which bind with high affinity to sites on the GABAA receptor/Cl- channel complex and potentiate the effect of the neurotransmitter GABA (gamma-aminobutyric acid). The heterogeneity of benzodiazepine recognition sites in the central nervous system was revealed by studies showing different classes of GABAA receptor subunits (classes alpha, beta and gamma) and variant subunits in these classes, particularly in the alpha-class. Expression of recombinant subunits produces functional receptors; when certain alpha-variants are coexpressed with beta- and gamma-subunits the resulting receptors have pharmacological properties characteristic of GABAA-benzodiazepine type I or type II receptors. The alpha-variants are differentially expressed in the central nervous system and can be photoaffinity-labelled with benzodiazepines. Here we report a novel alpha-subunit (alpha 6) of cerebellar granule cells. We show that recombinant receptors composed of alpha 6, beta 2 and gamma 2 subunits bind with high affinity to the GABA agonist [3H]muscimol and the benzodiazepine [3H]Ro15-4513 but not the other benzodiazepines or beta-carboniles. The same distinctive pharmacology is observed with GABAA receptors from rat cerebellum immunoprecipitated by an antiserum specific for the alpha 6 subunit. We conclude that this alpha-subunit is part of a cerebellar receptor subtype, selective for Ro15-4513, an antagonist of alcohol-induced motor incoordination and ataxia.

Mechanisms of alcohol intoxication in a rodent model: blunted alcohol-opposing reaction in "alcohol-sensitive" rats

In this paper we describe new data and review some studies on the mechanisms of alcohol-induced motor impairment in rats. Habituation to handling did not affect the naive behavioural differences between the alcohol sensitive and alcohol insensitive rat lines. Nor was there any effect on the differential sensitivities of the lines to the motor impairing and hypnotic effects of alcohol. Peripheral mechanisms may be involved in the differential behaviours of these lines, as the plasma corticosterone response was much weaker in the alcohol sensitive animals, suggesting a limited capacity to react to stress and alcohol. A similar blunted response to acute ethanol exposure was found in the uptake of the benzodiazepine antagonist [3H]Ro 15-1788 in vivo by the cerebellum of alcohol sensitive rats. The finding that these rat lines do not have any general differences in their brain inhibitory GABAergic receptors was extended to the spinal cord inhibitory glycinergic receptors, which showed only a modest line difference in their dissociation constant. The apparent localisation of the two main receptor differences (high-affinity [3H]muscimol binding and diazepam sensitivity of [3H]Ro 15-4513 binding) to the cerebellar granule layer suggests a genetic modification in the granule cells of alcohol-sensitive rats. In conclusion, our studies on acute intoxication by moderate alcohol doses show that several central nervous and peripheral factors may be involved in this behaviour. As many of these factors mitigate the effects of alcohol, alcohol antagonistic treatments should be aimed at activating and supporting multiple adaptive phenomena.