Further characterization of benzodiazepine receptor differences in long-sleep and short-sleep mice

Chromosomes 4 and 13 in beta-carboline-induced seizures in mice: benzodiazepine binding

Methyl beta-carboline-3-carboxylate (beta-CCM) is a ligand for the benzodiazepine (BZD) binding site of the GABA-A receptors with convulsive properties. We provided evidence for the involvement of a fragment of mouse chromosomes 4 and 13 in beta-CCM-induced seizures in a previous paper. Here, we analyzed, through [3H]-flumazenil binding, whether central BZD binding sites could be involved in the physiological processes underlying these differences of genetic sensitivities. In the JE/Le strain, where the effects of the chromosome 4 fragment can be analyzed, we found associations between [3H]-flumazenil binding and the convulsive action of beta-CCM. On the contrary, this no longer holds true in C3XtEso strain, where the effects of the chromosome 13 fragment were observed.

Changes in benzodiazepine binding in a subkindling situation

PURPOSE

A low dose of the benzodiazepine receptor inverse agonist methyl beta-carboline-3-carboxylate (beta-CCM) (1 mg/kg) was used to assess [3H]-flumazenil binding in a subkindling situation in Swiss mice.

METHODS

The brains were removed, and benzodiazepine receptor binding was studied every second day over 14 days of administration.

RESULTS

With each successive trial, Bmax values showed a steady and significant decrease, whereas Kd values showed a steady and significant increase. Behavioral data showed that at this low dose, actual kindling (seizuring) was not reached at the behavioral level.

CONCLUSIONS

The findings suggest that decreased gamma-aminobutyric acid (GABA) inhibition may occur even if behavioral effects of kindling are not observed.

Genetic selection of mouse lines differing in sensitivity to a benzodiazepine receptor inverse agonist

Mice were selectively bred according to their sensitivity or their resistance to the convulsive effects of a 4-mg/kg dose of methyl beta-carboline-3-carboxylate (beta-CCM), a benzodiazepine (BZ) receptor inverse agonist. The selection proved to be easy, with a clear separation of the two lines, convulsing with short latencies or resistant, already at the first generation of selection. Selection of a third line of animals convulsing with long latencies did not succeed. 3H-Ro 15-1788 binding analysis provided evidence for a strong decrease in Bmax in the resistant line.

Genetic factors regulate processes related to anxiety in mice

The propensity for anxiety-related behavior has been studied by comparing two highly inbred strains of mice, ABP/Le and C57BL/6ByJ, in two behavioral procedures, open-field and light-dark preference. Their Mendelian F2 population allowed us to evaluate the putative involvement of four easily identifiable loci in anxiogenic processes. In fact, chromosomal regions containing the brown, pink-eyed dilution and short-ear loci on the 4th, 7th and 9th chromosomes respectively are associated with anxiety-related behavior patterns. In addition, binding of [3H]flumazenil to brain GABA(A) receptors was measured as a biochemical index that may be associated with observed behavior patterns.

Convulsive effects of a benzodiazepine receptor inverse agonist: are they related to anxiogenic processes?

The linkage-testing strain of ABP/Le mice carries six mutations which express in easily identifiable phenotypes. By crossing this strain with a traditional inbred strain (C57BL/6ByJ) which is the 'wild type' for the mutated ABP/Le loci, we produced Mendelian populations, intercrosses and backcrosses so as to estimate whether the sensitivity to methyl beta-carboline-3-carboxylate (beta-CCM), a benzodiazepine receptor inverse agonist, and anxiety-related behaviour could be related to a common genetically determined substrate. We have shown that one locus on chromosome 9 is associated with beta-CCM-induced seizures and three loci on chromosomes 4, 7 and 9 are associated with anxiogenic processes. Analysis of [3H]flumazenil binding suggested a possible involvement of a Bmax decrease in both beta-CCM-induced seizures and anxiogenic processes. The putative common genetic regulation of both mechanisms is discussed.

Molecular genetic analysis of the role of GABAergic systems in the behavioral and cellular actions of alcohol

Recent studies implicate the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in many neurochemical actions of ethanol and a variety of behavioral responses to acute and chronic ethanol treatment. However, the molecular mechanisms responsible for genetic differences in initial neurochemical or behavioral sensitivity to ethanol, and adaptation following chronic or repeated ethanol administration, remain to be elucidated. Pharmacogenetic research will increasingly move toward mapping, cloning, identification, and functional analysis of the genes underlying the actions of ethanol. The approaches discussed here permit molecular analysis of both known and previously unknown genes regulating behavioral sensitivity to ethanol. The synthesis of molecular methods and behavioral genetics offers immediate hope for delineating the role of the GABA(A) receptor complex, and other determinants of GABAergic neurotransmission, in determining genetic variation in behavioral responses to ethanol.

Regional brain metabolic response to lorazepam in subjects at risk for alcoholism

The mechanisms underlying the blunted response to alcohol administration observed in subjects at risk for alcoholism are poorly understood and may involve GABA-benzodiazepine receptors. The purpose of this study was to investigate if subjects at risk for alcoholism had abnormalities in brain GABA-benzodiazepine receptor function. This study measured the effects of 30 micrograms/kg (i.v.) of lorazepam, on regional brain glucose metabolism using positron emission tomography and 2-deoxy-2[18F]fluoro-D-glucose in subjects with a positive family history for alcoholism (FP) (n = 12) and compared their response with that of subjects with a negative family history for alcoholism (FN) (n = 21). At baseline, FP subjects showed lower cerebellar metabolism than FN. Lorazepam decreased whole-brain glucose metabolism, and FP subjects showed a similar response to FN in cortical and subcortical regions, but FP showed a blunted response in cerebellum. Lorazepam-induced changes in cerebellar metabolism correlated with its motor effects. The decreased cerebellar baseline metabolism in FP as well as the blunted cerebellar response to lorazepam challenge may reflect disrupted activity of benzodiazepine-GABA receptors in cerebellum. These changes could account for the decreased sensitivity to the motor effects of alcohol and benzodiazepines in FP subjects.

Genetic animal models of alcohol and drug abuse

Behavioral and pharmacological responses of selectively bred and inbred rodent lines have been analyzed to elucidate many features of drug sensitivity and the adverse effects of drugs, the underlying mechanisms of drug tolerance and dependence, and the motivational states underlying drug reward and aversion. Genetic mapping of quantitative trait loci (QTLs) has been used to identify provisional chromosomal locations of genes influencing such pharmacological responses. Recent advances in transgenic technology, representational difference analysis, and other molecular methods now make feasible the positional cloning of QTLs that influence sensitivity to drugs of abuse. This marks a new period of synthesis in pharmacogenetic research, in which networks of drug-related behaviors, their underlying pharmacological, physiological, and biochemical mechanisms, and particular genomic regions of interest are being identified.

Solubilization of benzodiazepine receptors from long- and short-sleep mice

Previous studies have shown that long-sleep (LS) and short-sleep (SS) mice, which were selectively bred for differential responses to the sedative-hypnotic actions of ethanol, also differ in response to several other agents that act at the GABAergic receptor. Binding at cortical benzodiazepine receptors is enhanced differentially by GABA and ethanol in membranes prepared from the two lines of mice with SS receptors enhanced to a greater extent. Heat denaturation studies and other biochemical characterizations of these receptors suggest that the GABAergic receptor complex from the two lines of mice differs. The present study examined whether perturbation of receptor-membrane interactions by treatment with detergent altered either GABA enhancement of [3H]flunitrazepam binding or ethanol enhancement of this binding. Octylglucopyranoside (OCTG), 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate (CHAPS), or deoxycholate solubilization of cortical membranes resulted in a loss of the LS/SS difference in GABA enhancement. Ethanol's effects on binding were altered differently from those of GABA by treatment with OCTG as an increase, not a decrease, in enhancement was observed in both lines of mice. These results suggest that protein-membrane interactions play an important role in mediating LS/SS differences in the allosteric interactions within the GABAergic receptor complex.

Biochemical and behavioral effects of steroids on GABAA receptor function in long- and short-sleep mice

The in vitro and in vivo effects of alphaxalone, a steroid anesthetic, and two physiological steroids, tetrahydrodeoxycorticosterone (THDOC) and pregnenolone sulfate (PS), on GABAA receptor function were evaluated in long-sleep (LS) and short-sleep (SS) mice. In vitro, both alphaxalone and THDOC enhanced GABAergic inhibition as measured by [3H]FNZ binding and GABA-stimulated 36Cl- flux. However, with the exception of alphaxalone potentiation of [3H]FNZ binding, which was greater in SS brain regions, LS and SS mice did not differ in their degree of enhancement. Pregnenolone sulfate produced mixed agonistic and antagonistic effects on GABAergic function, dependent upon brain region, with few differences between the lines of mice. In vivo effects of these steroids on sleep time indicated that, like other anesthetic agents, THDOC and alphaxalone induced longer sleep times in LS mice. Antagonism by PS of ethanol-induced sleep time was observed in LS mice only; however, this effect was dependent upon the dose of ethanol used and on the vehicle used to prepare the steroid. Pentobarbital-induced sleep time was not reduced by PS treatment in either line of mouse. These results demonstrate that few differences in sensitivity of the GABAergic receptor to these steroids exist between LS and SS mice. Thus, unlike the differences between LS and SS mice in GABAergic mediation of responses to ethanol and benzodiazepines, there is little genetic variability in subtypes of GABAA receptors capable of modulation by steroids in these lines of mice.

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

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

Adrenalectomy and stress modulate GABAA receptor function in LS and SS mice

The effects of manipulation of adrenal steroids by adrenalectomy (ADX) or stress on GABAA receptor function were characterized in long-sleep (LS) and short-sleep (SS) mice. 36Chloride flux was not altered in either line of mouse after ADX; however, exposure to a behavioral stressor resulted in a highly significant inhibition of ion channel activity measured in cortical membranes from both LS and SS mice. Adrenalectomy also had no effect on [3H]FNZ binding; whereas exposure to stress differentially altered benzodiazepine binding in LS and SS mice. In LS cortex both Bmax and Kd values increased, whereas in SS cerebellum, Bmax and Kd values were decreased after stress. In SS mice ADX did not affect GABA-enhancement of [3H]FNZ binding. In LS mice, however, ADX resulted in a potentiation of GABA-enhanced [3H]FNZ binding in cortex and an inhibition of enhancement in cerebellum. Corticosterone (CCS) replacement in ADX-LS mice returned enhancement values to those of sham-operated mice, indicating a role for basal levels of CCS in maintaining normal receptor coupling function in this line of mouse. These results suggest that GABAA receptor sensitivity is more labile under stressful conditions. Differential receptor responses to adrenal manipulation between LS and SS mice may be due to genetic variation in GABAA receptor subunit combinations in these lines of mice.

Genetic difference in sensitivity to beta-carboline: evidence for the involvement of brain benzodiazepine receptors

The convulsive effects of methyl beta-carboline-3-carboxylate (beta-CCM), a benzodiazepine receptor ligand, are different in two inbred strains of mice: BALB/cBy mice are more sensitive to beta-CCM than C57BL/6J mice. In the present article, we report the effects of [3H]flunitrazepam binding in these two strains, which suggest a possible explanation of the differences in their sensitivity to beta-CCM by the involvement of brain benzodiazepine receptors.

gamma-Aminobutyric acid-activated chloride channels in rats selectively bred for differential acute sensitivity to alcohol

Effects of various sedative hypnotic agents on GABA-mediated chloride flux were evaluated in whole brain membrane vesicles (microsacs) prepared from rats selectively bred for high (HAS) and low sensitivity (LAS) to an acute hypnotic dose of alcohol. The HAS rats were more sensitive to the effects of pentobarbital, phenobarbital, flunitrazepam, and ethanol on GABA-mediated chloride flux compared with the LAS rats. No differences between the lines in GABA-stimulated chloride flux were observed. Modulation of 1-[3H]-phenyl-4-butyl-2,6,7-trioxabicyclo(2.2.2)octane ([3H]-TBOB) and [3H]-diazepam binding also was measured. The lines did not differ in inhibition of [3H]-TBOB binding by pentobarbital, phenobarbital, muscimol or picrotoxin. Although the lines displayed almost identical KD and Bmax for [3H]-diazepam binding, the GABA agonist, muscimol, was a more potent stimulator of [3H]-diazepam binding in membranes prepared from HAS rats than from LAS rats. These findings are discussed in light of previous work using other selected lines.

Adrenalectomy increases bicuculline-induced seizure sensitivity in long-sleep and short-sleep mice

Susceptibility to bicuculline-induced seizure onset and tonus was increased in LS and SS mice after adrenalectomy (ADX). Replacement with 10% corticosterone (CCS) in ADX animals resulted in a return to seizure latencies equal to those of sham-operated (SHAM) mice. In SS mice, dexamethasone (DEX) and cholesterol-control replacement was as effective as 10% CCS in returning seizure thresholds to SHAM values. In LS mice, DEX was only effective at a low bicuculline dose. Within the sham-operated group SS mice were more susceptible to bicuculline-induced seizure onset than LS mice; however, after ADX latencies did not differ between the two lines. These results suggest that seizure thresholds are regulated to some extent by the hypothalamic-pituitary-adrenal (HPA) axis. The effects of ADX on GABA-related seizure activity may also be influenced by genotype, such that genetic differences in GABAA receptor function and adrenocortical responses in LS and SS mice may be responsible for the differential seizure latencies observed in sham-operated mice.

t-[35S]butylbicyclophosphorothionate binding under equilibrium and nonequilibrium conditions: differential effects of barbiturates and gamma-aminobutyric acid in the long-sleep and short-sleep selected mouse lines

Significant differences were demonstrated between the long-sleep (LS) and short-sleep (SS) selected mouse lines in the abilities of barbiturates and gamma-aminobutyric acid (GABA) to inhibit t-[35S]butylbicyclophosphorothionate [( 35S]TBPS) binding to well-washed cerebral cortical membranes. Thus, using phenobarbital to initiate the dissociation of [35S]TBPS, the extent of inhibition was significantly greater in LS mice (but not SS mice) than would be predicted using equilibrium conditions. Pentobarbital had the opposite effect, causing [35S]TBPS to dissociate to a greater extent in SS than LS membranes. [35S]TBPS binding was dissociated from LS and SS membranes by GABA to a greater and lesser extent, respectively, than would be predicted from equilibrium studies. Because no line differences in the potencies of these drugs to inhibit [35S]TBPS binding were found using equilibrium conditions, these results indicate that the association rates of barbiturates and GABA may be different between these lines. These findings are consistent with neurochemical studies indicating differences in the benzodiazepine/GABA receptor-chloride channel complex in these selected lines and may explain their differential sensitivities to certain agents acting through this supramolecular complex.

Cocaine toxicity: genetic evidence suggests different mechanisms for cocaine-induced seizures and lethality

While low doses of cocaine have been shown to decrease locomotor activity, and moderate doses produce marked increases in locomotor activity, high doses commonly produce seizures and, eventually, death. However, the biological mechanisms associated with cocaine-related death, as well as the contribution of seizure activity to death are poorly understood. This study examined the effects of a broad range of cocaine doses on acute occurrence of seizures and death in LS/Ibg (LS) and SS/Ibg (SS) mice. Large differences in sensitivity to cocaine-induced seizures were seen, with ED50 values being 41.7 and 80.9 mg/kg for the SS and LS mice, respectively. Conversely, no significant differences in cocaine LD50 values were found, being 100.7 and 107.2 mg/kg for the SS and LS mice, respectively. That these lines of mice differ substantially in convulsant responses to cocaine, but show no difference in lethal response to this drug, supports the conclusion that cocaine causes death by mechanisms distinct from those which contribute to seizures. The distinction between cocaine-induced seizures and lethality in these lines may prove useful in elucidating the underlying mechanisms of these traits. In a subsequent study, the incidence of seizures in F1 and F2 generation mice from hybrid matings of LS and SS parents was not consistent with a single gene hypothesis, suggesting that multiple genes and receptors mediate seizurgenic responses to cocaine. Further, albino mice from the F2 generation did not show a significantly different incidence of seizures than non-albino mice, indicating that of the genes contributing to seizures following cocaine, none appear to be closely associated with the albino locus on mouse chromosome seven.

Cocaine produces locomotor stimulation in SS but not LS mice: relationship to dopaminergic function

Cocaine produces several behavioral effects, most notably locomotor stimulation. While low doses of cocaine have been shown to decrease locomotor activity, moderate to high doses in the range of 5-50 mg/kg usually produce a marked increase in locomotor activity in rodents. This study examined the effects of a range of cocaine doses, 1-75 mg/kg, on locomotor activity in LS/Ibg (LS) and SS/Ibg (SS) mice. At the lowest doses, activity was depressed in both lines, but to a greater extent in LS mice. As the dose of cocaine was increased, activity returned to baseline, and at the highest doses, increases in locomotor activity were found, but only in SS mice. In LS mice, cocaine was ineffective in increasing locomotor activity at any of the doses tested. Since striatal dopaminergic neurons influence locomotor activity, we also assessed ligand affinity and receptor density of dopamine transporters and dopaminergic D1 and D2 receptors in striatal tissue obtained from these two selected lines. No differences in these receptor binding parameters were found. However, because of their anomalous locomotor response to cocaine, LS mice may prove to be a valuable tool in increasing our understanding of those sites which mediate specific effects of cocaine.

[35S]TBPS binding sites are decreased in the colliculi of mice with a genetic predisposition to bicuculline-induced seizures

Several differences have been found in GABAergic function between the long sleep (LS) and short sleep (SS) mice which were genetically selected for different ethanol-induced sleeptimes, and it has been suggested that these differences may explain their differential ethanol sensitivity. However, these lines also differ in seizure susceptibility, a behavior which may also be mediated by GABAergic pathways. Thus, it is difficult to associate differences in GABA neurochemistry with either of these behaviors, particularly when only two selected lines are used. We measured differences in the density and affinity of the [35S]TBPS binding site on the GABAA receptor/Cl- ionophore complex in discrete brain areas; and in order to determine the relationship between receptor binding and behavioral differences, we included mice from 5 of the LS and SS recombinant inbred strains (LS x SS RI) in addition to mice from the LS and SS lines. [35S]TBPS binding in sagittal brain sections was analyzed by quantitative autoradiography, and the amount of binding differed depending on whether bicuculline was added to inhibit endogenous GABA binding. In the presence of bicuculline, the number of [35S]TBPS sites in SS mice was highest in the colliculi (4.5 +/- 0.5 pmol/mg protein), cerebellum (4.8 +/- 0.6 pmol/mg), hippocampus (3.2 +/- 0.7 pmol/mg) and cortex (2.9 +/- 0.3 pmol/mg). The Bmax was two-fold lower in both superior and inferior colliculi (IC) of LS mice. There were no differences between lines in Bmax in any other area and in Kd values in any area (58 +/- 4.0 nM).(ABSTRACT TRUNCATED AT 250 WORDS)

Selected mouse lines, alcohol and behavior

The technique of selective breeding has been employed to develop a number of mouse lines differing in genetic sensitivity to specific effects of ethanol. Genetic animal models for sensitivity to the hypnotic, thermoregulatory, excitatory, and dependence-producing effects of alcohol have been developed. These genetic animal models have been utilized in numerous studies to assess the bases for those genetic differences, and to determine the specific neurochemical and neurophysiological bases for ethanol's actions. Work with these lines has challenged some long-held beliefs about ethanol's mechanisms of action. For example, lines genetically sensitive to one effect of ethanol are not necessarily sensitive to others, which demonstrates that no single set of genes modulates all ethanol effects. LS mice, selected for sensitivity to ethanol anesthesia, are not similarly sensitive to all anesthetic drugs, which demonstrates that all such drugs cannot have a common mechanism of action. On the other hand, WSP mice, genetically susceptible to the development of severe ethanol withdrawal, show a similar predisposition to diazepam and phenobarbital withdrawal, which suggests that there may be a common set of genes underlying drug dependencies. Studies with these models have also revealed important new directions for future mechanism-oriented research. Several studies implicate brain gamma-aminobutyric acid and dopamine systems as potentially important mediators of susceptibility to alcohol intoxication. The stability of the genetic animal models across laboratories and generations will continue to increase their power as analytic tools.

The effects of ethanol and Ro 15-4513 on elevated plus-maze and rotarod performance in long-sleep and short-sleep mice

The effects of ethanol and diazepam were examined in long-sleep (LS) and short-sleep (SS) mice using the elevated plus-maze. Ethanol had more pronounced effects in SS mice than in LS mice. In contrast, LS mice were more sensitive to the effects of diazepam on the elevated plus-maze. The ataxic effects of ethanol were measured by rotarod performance. SS mice were more resistant to the ataxic effects of a 2.0 g/kg dose of ethanol than LS mice. Ro 15-4513 reversed ethanol's ataxic effects when administered after ethanol in both LS mice and SS mice. Pentobarbital-induced ataxia was unaffected by treatment with Ro 15-4513. Studies of competition of Ro 15-4513 on 3H-flunitrazepam binding indicated that LS and SS mice did not differ in this measure in cortex, cerebellum or hippocampus.

Interaction of ethanol and stress with the GABA/BZ receptor in LS and SS mice

The interaction of stress and ethanol with the GABA/BZ receptor system was evaluated in LS and SS mice. The effects of two separate in vivo treatments, a 2.5 g/kg injection of ethanol or a behavioral stressor, on GABA-enhanced [3H]-FNZ binding were nearly identical in both lines of mice. A 2.5 g/kg ethanol- or stress-pretreatment resulted in increased enhancement in SS cortex, but not LS. In cerebellum, treatment effects were demonstrated in both SS and LS mice. Intraperitoneal injections of increasing doses of ethanol produced biphasic stimulation of GABA-enhanced [3H]-FNZ binding in LS brain regions, but not SS. Adrenalectomies performed one week prior to ethanol administration produced a loss of ethanol enhancement in cerebellum of both lines. However, in cortex, removal of the adrenals had no effect. The in vitro addition of 30 mM ethanol to brain preparations incubated at 37 degrees C from stressed and unstressed animals resulted in greater enhancement of binding in cortex, but not cerebellum of stressed mice. Differences in the degree of enhancement between the lines of mice were lost if the animals were stressed prior to sacrifice or if membrane preparations were incubated at 4 degrees C. The results of this study suggest that the interaction between ethanol and stress is mediated by the GABAergic system, but responses vary dependent on brain region, dose of ethanol, and degree of ethanol sensitivity.

Genetic differences in coupling of benzodiazepine receptors to chloride channels

The affinity and density of [3H]flunitrazepam binding sites and the ability of flunitrazepam to augment GABA-activated chloride flux were determined using membrane vesicles prepared from cortex of long-sleep (LS, alcohol-sensitive), short-sleep (SS, alcohol-resistant) and individual heterogeneous stock (HS) mice. LS membranes were more sensitive than SS membranes to the effects of flunitrazepam (and ethanol) on chloride flux but did not differ in the binding of [3H]flunitrazepam. Likewise, individual HS mice displayed marked differences in enhancement of chloride flux by flunitrazepam, but little difference in binding of flunitrazepam. These results suggest that there is little genetic variation in the binding of benzodiazepines to receptors, but there is a marked diversity in the coupling of these receptors to chloride channels. This implies the existence of 'spare receptors' in LS and most HS mice, but not in SS mice. These findings also suggest that genetic differences in alcohol sensitivity in vivo may be related to coupling of benzodiazepine binding to chloride channel function.

Genotype-dependent effects of GABAergic agents on sedative properties of ethanol

Two lines of mice, selectively bred for differential sensitivity to the soporific effects of ethanol (ETOH), were administered GABAergic drugs in an effort to evaluate a role for GABA in ETOH sensitivity. ETOH sensitive Long-Sleep mice (LS) showed potentiated ETOH sedation when administered bicuculline, muscimol and aminooxyacetic acid (AOAA). ETOH-insensitive SS mice exhibited reduced ETOH sedation in the presence of the antagonists, bicuculline and picrotoxin, and potentiated sedation in the presence of muscimol and AOAA. These changes in narcosis duration were interpreted as central effects, since blood ethanol levels at waking from ETOH sedation varied with GABAergic drug treatment. Picrotoxin antagonized pentobarbital-induced narcosis in both lines, but to a greater extent in SS mice. These and other experiments with a genetically heterogeneous stock suggest GABA involvement in genotype-dependent ETOH sensitivity, but do not support a simple role of GABA receptor involvement.