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

Emotional Impairments and Neuroinflammation are Induced in Male Mice Invulnerable to Repeated Social Defeat Stress

Prolonged stress triggers neuroinflammation, which plays a significant role in the development of depression; however, stressed people do not always suffer from depression because of individual differences in stress vulnerability. Negative cognitive bias (NCB) toward pessimistic judgment often underlies depressive episodes. However, a relationship between stress vulnerability, neuroinflammation, and NCB remains elusive. In addition, an animal model with all the traits would be a powerful tool for studying the etiology of depression and its therapeutic approaches. Accordingly, this study evaluated the effect of stress vulnerability on neuroinflammation and depression-related behaviors, including NCB in males, using a modified version of repeated social defeat stress (mRSDS) paradigm, a validated animal model of psychosocial stress. Exposure to mRSDS, consisting of 5 min of social defeat by unfamiliar CD-1 aggressor mice for five consecutive days, caused NCB, which co-occurred with depressive- and anxiety-like behaviors, and neuroinflammation in male BALB/c mice. Treatment with minocycline, an antibiotic with anti-inflammatory property, blocked mRSDS-induced depressive-like behaviors and neuroinflammation, but not NCB, indicating the limited effect of an anti-inflammatory intervention. In addition, marked differences were found in neuroinflammatory profiles and hippocampal gene expression patterns between resilient and unstressed mice, as well as between susceptible and resilient mice. Therefore, mice resilient to mRSDS are indeed not intact. Our findings provide insights into the unique features of the mRSDS model in male BALB/c mice, which could be used to investigate the etiological mechanisms underlying depression as well as bridge the gap in the relationship between stress vulnerability, neuroinflammation, and NCB in males.

Differential effects of etifoxine on anxiety-like behaviour and convulsions in BALB/cByJ and C57BL/6J mice: any relation to overexpression of central GABAA receptor beta2 subunits?

Dysfunction of GABAergic transmission related to abnormal expression of GABA(A) receptor subunits in specific brain regions underlies some pathological anxiety states. Besides involvement of the benzodiazepine recognition site of GABA(A) receptor in the expression of anxiety-like behaviour, the roles of the β(2)/β(3) subunits are not well characterized. To address this issue, the experimental design of this study utilized the GABAergic compound etifoxine (with a preferential effectiveness after binding to a specific site at β(2)/β(3) subunits) tested in two inbred mouse strains: BALB/cByJ and C57BL/6J mice using three behavioural paradigms (light/dark box, elevated plus maze and restraint stress-induced small intestinal transit inhibition) and the t-butylbicyclophosphorothionate-induced convulsions model. Etifoxine plasma and brain levels and β(2)/β(3) mRNAs and protein expression levels in various brain regions were compared between the two strains. The two mouse strains differed markedly in basal anxiety level. Etifoxine exhibited more pronounced anxiolytic and anticonvulsant effects in the BALB/cByJ mice compared to the C57BL/6J mice. The etifoxine brain/plasma ratios of the two strains were not different. Beta2 subunit mRNA and protein expression levels were around 25 and 10% higher respectively in the anterodorsal nucleus of the thalamus and the CA3 field of hippocampus of BALB/cByJ mice compared to C57BL/6J mice. Beta3 subunit mRNA and protein expression levels did not differ between the two strains. Based on these results, it is suggested that overexpression of GABA(A) receptor β(2) subunit in BALB/cByJ mice relative to C57BL/6j mice contributes to the dysfunction in GABA(A) transmission in regions of brain known to regulate responses to stress. The dysregulated GABA(A) function in BALB/cByJ mice may be corrected by the administration of etifoxine.

Differential effects of lorazepam on sleep and activity in C57BL/6J and BALB/cJ strain mice

Compared to C57BL/6 mice, BALB/c mice exhibit greater 'anxiousness' on behavioural tests of anxiety, and can show significantly longer sleep disruptions after exposure to anxiogenic situations. Relative to C57BL/6 mice, BALB/c mice also have reduced benzodiazepine (BZ) receptor densities in the brain and fivefold less BZ receptor density in the amygdala, a region important in anxiety and in the control of arousal. Lorazepam is a BZ receptor full agonist and has been used to treat both anxiety and insomnia. Differences between C57BL/6 and BALB/c mice raise the question of whether BZ agonists would impact sleep and activity differentially in the two strains. We examined the effects of two doses of lorazepam (0.5 and 1.5 mg kg(-1)) or saline alone (0.2 mL) on sleep and activity in C57BL/6 (n = 8) and BALB/c (n = 7) mice. Compared to saline, both doses of lorazepam significantly increased non-rapid eye movement (NREM) and reduced activity in both strains. In C57BL/6 mice, rapid eye movement (REM) was increased at both doses. In BALB/c mice, the 0.5 mg kg(-1) dose had no significant influence on REM, whereas REM was reduced significantly after the 1.5 mg kg(-1) dose. The results demonstrate significant differences between C57BL/6 and BALB/c mice in the effects of lorazepam on REM, whereas the effects on NREM and activity were similar. Strain differences in the number of BZ receptors in the amygdala, but not other brain regions, suggests possible site specificity in the effects of lorazepam on REM. These differences in BZ-binding sites in the amygdala could be a significant factor in differences in the sleep response between C57 and BALB/c mice.

Selective effects of benzodiazepines on the acquisition of conditioned taste aversion compared to attenuation of neophobia in C57BL/6 mice

INTRODUCTION

The effects of pre-conditioning administration of anxiolytic benzodiazepines on the acquisition of a conditioned taste aversion (CTA) and on the acquisition of attenuation of neophobia (AN) were investigated in C57BL/6 mice.

MATERIALS AND METHODS

A CTA was induced by injecting lithium chloride (LiCl; 6 mEq x kg(-1)) 30 min after the animal had imbibed a novel 0.5% saccharin solution. In other animals, neophobia was attenuated by a single access to the novel 0.5% saccharin solution, followed only by injection of saline.

RESULTS AND DISCUSSION

Pre-conditioning administration of chlordiazepoxide (CDZ; 6-24 mg x kg(-1), i.p.) and alprazolam (0.3-1 mg x kg(-1), p.o.) resulted in a CTA that did not differ initially from that observed in vehicle-treated controls, but which showed faster extinction. The acquisition of AN was impaired only after the higher doses of CDZ (12-24 mg x kg(-1), i.p.) or alprazolam (1 mg x kg(-1), i.p.). The results show that in this test, altered acquisition of an aversive CTA memory by anxiolytic benzodiazepines is reflected in more rapid extinction. Moreover, at low doses, these drugs showed selectivity for weakening CTA learning compared to AN learning. Evidence is discussed that selective weakening of aversive memory formation is a clinically relevant effect of anxiolytic benzodiazepines.

Relations between open-field, elevated plus-maze, and emergence tests in C57BL/6J and BALB/c mice injected with GABA- and 5HT-anxiolytic agents

Two 5HT(1A) receptor agonists and chlordiazepoxide were examined in open-field, elevated plus maze, and emergence tests. At doses with no effect in the open-field, chlordiazepoxide increased open and open/total arm visits as well as open arm duration in the elevated plus maze, whereas 5HT(1A) receptor agonists showed an anxiolytic response on a single measure. The anxiolytic action of chlordiazepoxide was limited to the less active BALB/c strain. Unlike the 5HT(1A) receptor agonists, chlordiazepoxide was also anxiolytic in the emergence test, once again only in BALB/c and not C57BL/6J mice. Significant correlations were found between emergence latencies and specific indicators of anxiety in the elevated plus-maze in chlordiazepoxide-treated but not in mice treated with buspirone and 8-OH-DPAT. These results indicate that elevated plus-maze and emergence tests depend on benzodiazepine receptors. In contrast, 5HT(1A) receptor agonists were ineffective in the emergence test and no correlation was found between emergence latencies and specific indicators of anxiety in the elevated plus-maze. Though superficially similar, the emergence test seems to tap into a partially separate facet of anxiety.

Post-traumatic stress behavioural responses in inbred mouse strains: can genetic predisposition explain phenotypic vulnerability?

Clinical studies of twin pairs and families of post-traumatic stress disorder (PTSD) patients raise questions as to possible genetic predisposition to PTSD. Studies using isogenic animal populations exposed to a stress paradigm could elucidate the relative contributions of genotype and environment to endophenotypic expression. The prevalence of individuals displaying severely compromised behavioural responses to predator scent stress (PSS) was assessed in six inbred strains of mice in an animal model of PTSD that classifies individuals into groups according to the degree of their behavioural response. The choice of strains was based on the frequent use of these mice in transgenic research. The prevalence of extreme behavioural response in the elevated plus maze and the acoustic startle response paradigms, performed in sequence, was assessed at baseline and 7 d after PSS exposure between and within strains, and compared to differences in circulating corticosterone levels. Narrow-sense trait heritability was determined by comparing the between-strain variance to the total variance. Although strain-specific differences in anxiety-like behaviours were demonstrated, the results revealed a significant degree of individual variability in response patterns within each of the inbred strains, yielding a baseline heritability factor for anxiety-like behaviours of 30%, but only 10% for response to stress exposure. Baseline anxiety-like behaviours were found not to be predictive of post-exposure behavioural responses. The response of the individual to stress is multifactorial and environmental factors play a predominant role in characterizing the individual response to stress exposure, although there are significant genetic underpinnings.

Relations between open-field, elevated plus-maze, and emergence tests as displayed by C57/BL6J and BALB/c mice

The relations between open-field, elevated plus-maze, and emergence tests were examined in two strains of mice. In the open-field, C57BL/6J mice had more ambulatory movements and rears but not stereotyped movements relative to BALB/c. In addition, C57BL/6J mice entered more often than BALB/c into enclosed and open arms of the elevated plus-maze. When placed inside a large enclosure, C57BL/6J mice emerged more quickly than BALB/c from a small toy object. In the entire series of mice, ambulation and rears in the open-field were linearly correlated with open and enclosed arm visits in the elevated plus-maze. Ambulatory movements and rears were also correlated with emergence latencies. In contrast, stereotyped movements were correlated with emergence latencies, but not with any elevated plus-maze value. These results specify the extent and limits of association between the three tests.

Strain- and model-dependent effects of chlordiazepoxide, L-838,417 and zolpidem on anxiety-like behaviours in laboratory mice

The promise of subtype-selective GABA(A) receptor drugs with anxiolytic properties but with a much reduced side-effect burden (compared to benzodiazepines) is an attainable goal. However, its achievement necessitates the availability of in vivo preclinical assays capable of demonstrating differences as well as similarities between subtype-selective agents and non-selective benzodiazepines. In this study, we have compared three mouse strains (NMRI, C57BL/6J and DBA/2) in four models of anxiety-like behaviour (plus-maze, zero-maze, light-dark, and Vogel conflict). Furthermore, in each model, we have contrasted in detail the behavioural responses of each strain to the non-selective benzodiazepine chlordiazepoxide (CDP; 5-20 mg/kg), and the subtype-selective agents L-838,417 (GABA(A)-alpha(2/3/5); 3-30 mg/kg) and zolpidem (GABA(A)-alpha1; 0.3-3.0 mg/kg). The data show a complex mouse strainxmodelxpharmacological agent interaction. Most importantly, not all mouse strainxmodel test systems showed a positive response to CDP or predicted the response to L-838,417. This dissociation between CDP and L-838,417 opens up opportunities for preclinical test systems that differentiate subtype-selective and non-selective GABA(A) receptor agents, an attribute that might well be important in providing the necessary confidence for further drug development. Present findings suggest the need for a much greater focus on defining test systems appropriate for screening novel chemical entities, rather than self-selection of models or genotypes based on responses to known pharmacological agents. For example, if current data with L-838,417 are confirmed with compounds showing similar selectivity profiles, such agents may in future be best identified and characterised using test systems comprising NMRI mice in the zero-maze and/or C57 mice in the Vogel conflict and/or light-dark tests.

Use of a platform in an automated open-field to enhance assessment of anxiety-like behaviors in mice

The present report describes a setup for simultaneously measuring anxiety-like behaviors and locomotor activity in mice. Animals are placed in a brightly lit, standard automated open-field (OF) in which a rectangular ceramic platform 8 cm high covers one quadrant of the floor. Mice preferred to stay under the platform, avoiding the area with bright illumination. Activities under and outside the platform were measured for 5 min. Chlordiazepoxide and buspirone dose-dependently increased time spent outside the platform (L-Time) and the light distance to total OF distance ratio (L:T-TD) in both genders without changing total OF distance. By contrast, amphetamine decreased L-Time and L:T-TD in males, thus displaying an anxiogenic effect. Imipramine was without selective effect on L-Time or L:T-TD, but decreased total OF distance at the highest dose indicative of a sedative effect. Drug effects were also evaluated in the OF without platform using conventional anxiety measures. Introduction of the platform into the OF apparatus strongly enhanced the sensitivity to anxiolytics. Comparison of strains differing in activity or anxiety levels showed that L-Time and L:T-TD can be used as measures of anxiety-like behavior independent of locomotor activity. Changes in motor activity are reflected in the total distance traveled under and outside the platform. Therefore, the platform test is fully automated, sensitive to both anxiolytic and anxiogenic effects of drugs and genetic phenotypes with little evidence of gender-specific responses, and can be easily utilized by most laboratories measuring behavior.

Progesterone withdrawal effects in the open field test can be predicted by elevated plus maze performance

Allopregnanolone (3alpha-hydroxy-5alpha-pregnane-20-one) is a ring-A-reduced metabolite of progesterone, which is naturally produced during the luteal phase of the menstrual cycle, during pregnancy and by stressful events. The steroid hormone inhibits neural functions through increased chloride ion flux through the GABA(A) receptor. The effects and subsequent withdrawal symptoms are similar to those caused by alcohol, benzodiazepines and barbiturates. This study examined the withdrawal effects of progesterone with regards to the influence of individual baseline exploration and risk taking. Rats were tested on the elevated plus maze (EPM) before hormonal treatment, in order to evaluate differences in risk taking and exploration of open and elevated areas. Treatment consisted of ten consecutive once a day progesterone or vehicle s.c. injections. On the last day of treatment, estradiol was injected in addition to progesterone, followed by a 24-h withdrawal before testing in the open field test (OF). Progesterone-treated rats showed a withdrawal effect of open area avoidance in the OF. The vehicle-treated control rats showed strong correlations between the EPM and OF parameters. This relationship was not found for the progesterone group at withdrawal. Rats with greater numbers of open arm entrance in the EPM pretest showed an increased sensitivity to progesterone withdrawal (PWD) compared to rats with low exploration and risk taking. The results indicate that the effects of PWD relate to individual exploration and risk taking. Furthermore, the possible analogy of PWD and PMS/PMDD in relation to individual traits is discussed.

Maternal behavior regulates benzodiazepine/GABAA receptor subunit expression in brain regions associated with fear in BALB/c and C57BL/6 mice

Inbred strains of mice, such as BALB/cByJ and C57BL/6ByJ, have been used repeatedly to study genotype-phenotype relations. These strains differ on behavioral measures of fear. In novel environments, for example, BALB/c mice are substantially more neophobic than C57BL/6 animals. The benzodiazepine (BZ)/GABAA receptor system has been proposed as a regulator of behavioral responses to stress, and BALB/c and C57BL/6 mice differ in BZ/GABAA receptor binding. In the present study, we found increased BZ receptor levels in C57BL/6 mice in the central and basolateral nuclei of the amygdala as well as the locus coeruleus using either flunitrazepam (nonselective) or zolpidem (alpha1 subtype selective) as radioligands. Differences in receptor binding were most pronounced in the amygdala and locus coeruleus using [3H]zolpidem. C57BL/6 mice showed increased alpha1 mRNA levels in the locus coeuruleus compared to BALB/c mice. In addition, gamma2 mRNA expression in BALB/c mice was decreased in the central nucleus of the amygdala to levels that were 2-2.5-fold lower than those of C57BL/6 mice. The results of an adoption study revealed that the biological offspring of C57BL/6 mothers fostered after birth to BALB/c dams showed decreased levels of gamma2 mRNA expression in the central nucleus of the amygdala in comparison to peers fostered to other C57BL/6 mothers (the reverse was found for the biological offspring of BALB/c mothers). In a step-down exploration paradigm, BALB/cByJ mice crossfostered onto a C57BL/6ByJ dam expressed reduced anxiety responses. However, among C57BL/6ByJ mice, the relatively low levels of anxiety ordinarily evident were not increased when mice of this strain were reared by a BALB/cByJ dam. These preliminary findings suggest that the strain differences in the BZ/GABAA receptor system occur, at least in part, as a function of parental care. Such findings may reflect a mammalian example of an indirect genetic effect mediated by maternal care.

Effects of stress and etifoxine on pentobarbital-induced loss of righting reflex in Balb/cByJ and C57BL/6J mice

We hypothesized that functional changes in the GABAergic system induced by stress would differ between two inbred mouse strains BALB/cByJ and C57BL/6J. We compared the effects of restraint stress and of the anxiolytic drug etifoxine (EFX) on the duration of pentobarbital-induced loss of righting reflex (hypnotic effect) in the two strains. Naive BALB/cByJ mice were less sensitive than naive C57BL/6J mice to the hypnotic effect of pentobarbital. C57BL/6J mice exhibited a shortening in the duration of pentobarbital-induced hypnosis following stress whereas stress had no effect in BALB/cByJ mice. EFX reversed the shortening of pentobarbital-induced hypnosis elicited by stress in C57BL/6J and shortened the duration of pentobarbital-induced hypnosis after stress in BALB/cByJ mice. Alterations in the GABAergic function in BALB/cByJ mice could be corrected by EFX, an enhancer of GABAergic transmission.

Difference in anxiety and sensitization of the acoustic startle response between the two inbred mouse strains BALB/cAN and DBA/2N

The inbred mouse strain BALB has been proposed to be an animal model for pathological anxiety. BALB exhibits a stronger acoustic startle response (ASR) than the 'less emotional' inbred strain DBA. Four experiments were conducted to determine whether this strong ASR is due to a higher anxiety level and/or to greater sensitization in BALB than in DBA, with the following results: (1) The ASR to the very first startle stimulus was found to be much stronger in BALB than in DBA, and freezing behavior evoked by startle stimuli was more pronounced in BALB than in DBA. These findings indicate a higher level of anxiety in this strain. (2) ASR amplitudes of BALB initially rose much higher during consecutive startle stimuli and remained at a high level much longer than in DBA. Thereafter, ASR amplitude dropped more slowly and to a lesser degree than in DBA. Startle amplitudes decreased similarly in both strains (strong exponential decrease) only when a low sound pressure level (SPL) was used which elicited approximately the same low ASR in both strains. These results can only be explained by increased sensitization in BALB. (3) The slope of the i/o-function, which represents the relation between sensory input and motor output, was steeper in BALB than in DBA. As it has been shown recently, sensitization increases the slope of the startle i/o-function indicating increased sensitization in BALB. It is discussed, however, whether anxiety also contributes to this effect. (4) Footshocks increased the ASR much less in BALB than in DBA, again showing increased sensitization in BALB. Both a higher level of anxiety and greater sensitization therefore determined the greater strength of the ASR in BALB than in DBA.

Measuring normal and pathological anxiety-like behaviour in mice: a review

Measuring anxiety-like behaviour in mice has been mostly undertaken using a few classical animal models of anxiety such as the elevated plus-maze, the light/dark choice or the open-field tests. All these procedures are based upon the exposure of subjects to unfamiliar aversive places. Anxiety can also be elicited by a range of threats such as predator exposure. Furthermore, the concepts of "state" and "trait" anxiety have been proposed to differentiate anxiety that the subject experiences at a particular moment of time and that is increased by the presence of an anxiogenic stimulus, and anxiety that does not vary from moment to moment and is considered to be an "enduring feature of an individual". Thus, when assessing the behaviour of mice, it is necessary to increase the range of behavioural paradigms used, including animal models of "state" and "trait" anxiety. In the last few years, many mice with targeted mutations have been generated. Among them some have been proposed as animal models of pathological anxiety, since they display high level of anxiety-related behaviours in classical tests. However, it is important to emphasise that such mice are animal models of a single gene dysfunction, rather than models of anxiety, per se. Inbred strains of mice, such as the BALB/c line, which exhibits spontaneously elevated anxiety appear to be a more suitable model of pathological anxiety.

Differential sensitivity to inverse agonists of GABA(A)/benzodiazepine receptors in rats with genetic absence-epilepsy

A strain of Wistar rats, genetic absence epilepsy rats from Strasbourg (GAERS), was selected and inbred over 40 generations for occurrence of spontaneous spike-wave discharges characteristic of absence seizures, simultaneously with a strain of non-epileptic rats (NER). GAERS demonstrate an excessive sensitivity to antagonists of the GABA(A) receptor. The sensitivity to convulsions induced by various inverse agonists of the GABA(A)/benzodiazepine receptor was compared in GAERS and NERs. The beta-carbolines FG 7142 and DMCM, and the imidazobenzodiazepines RO 19-4603 and the alpha 5-selective RY 024 were several times more convulsant in GAERS than in NERs. The largest differences were found with the non-selective RO 19-4603- and FG 7142. The proconvulsant imidazobenzodiazepine RO 15-4513, binding also to diazepam-insensitive receptors, had low efficacy. The high affinity binding of GABA(A)/BZD receptors with (3H) RO 15-1788 in the brain of naive rats and after administration of FG 7142 did not differ in GAERS and NERs. The data indicate that the hypersensitivity of GAERS to various inverse agonists of the GABA(A)/benzodiazepine receptor involves cortical GABA(A) receptors and is not related to differential activity of a subunit-selective receptor.

The genetic basis of the pharmacological effects of anxiolytics: a review based on rodent models

Anxiolytic drugs exert their pharmacological actions by binding to molecular targets, such as benzodiazepine receptors or 5-hydroxytryptamine (5-HT) receptors. Specific genes encode these receptors, or the subunits of which they are formed. Therefore, genetic factors may influence strongly the ability of anti-anxiety agents to produce their behavioural effects. The literature on this subject is reviewed here, with emphasis on data derived from studies with rodents. We present in a critical way the animal models used in the studies aimed at investigating the genetic basis of the action of anxiolytic compounds, including inbred mice, selected lines, linkage strains or mice generated by targeted mutation. Data show that increased anxiety-like behaviour is not a predictive factor for increased sensitivity to anxiolytic treatment, and it is possible that gene deletion might not be isomorphic to pharmacological antagonism. It is suggested that the strain differences in anxiety-like behaviour may be used as a tool in assaying anxiolytic activity of new drugs.

Divergent levels of anxiety in mice selected for differences in sensitivity to a convulsant agent

Spontaneous behavior patterns were assessed in eight different behavioral situations in two lines of mice, BR and BS, previously selected for their sensitivity to an anxiogenic benzodiazepine (BZ) receptor inverse agonist, Methyl beta-carboline-3-carboxylate (beta-CCM). BR is highly resistant, and BS, highly sensitive to beta-CCM-induced seizures. Tests used included an assessment of general locomotor activity, several situations classically used for measuring fear-motivated behaviors (open field, thigmotaxis, elevated plus-maze, light-dark discrimination, staircase), a test for measuring exploration (holeboard), and a test for measuring nociception (hot-plate). In the absence of beta-CCM, the results provide evidence of reduced motor activity and higher levels of anxiety in the BR line as compared to the BS line.

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.

Hypoexpression of benzodiazepine receptors in the amygdala of neophobic BALB/c mice compared to C57BL/6 mice

The distribution of benzodiazepine receptors in the brain of neophobic BALB/c mice was studied by autoradiographic analysis using [3H]-diazepam and compared to that of the same receptors of the "nonemotional" C57BL/6 mice. This technique revealed no significant interstrain difference except for a lower density of diazepam binding sites in the amygdala of BALB/c mice. Therefore, the expression of benzodiazepine receptors in the amygdala of the two strains of mice were quantified by binding studies on brain membranes. The amygdala of BALB/c mice exhibited a fivefold decrease in the density of benzodiazepine receptors compared to C57BL/6 mice. These results suggest that the trait anxiety (neophobia) that characterizes BALB/c mice could be due, at least in part, to a genetic modulation of benzodiazepine receptor expression in the amygdala, a structure known to be strongly involved in fear behavior.

Mouse strains differ in their sensitivity to alprazolam effect in the staircase test

The behavioral responses of five mouse strains (inbred: C57 and BALB/c; outbred: Swiss, ICR and HS/Ibg) to alprazolam was examined in the staircase test, an animal model sensitive to benzodiazepines (BZs). Alprazolam administration resulted in a dose-dependent suppression of rearing behavior, but to a different extent among the strains. By contrast, the number of stairs ascended was not suppressed by alprazolam at doses of 0.25 and 0.5 mg/kg, except in the C57 mice. The addition of flumazenil antagonized the alprazolam effect on rearing and climbing in all strains. There was a consistency within strains in sensitivity to alprazolam, with some strains being highly sensitive (C57 and HS) or less sensitive (Swiss, ICR and BALB/c) with regard to both rearing and climbing behaviors. Serum alprazolam levels did not differ significantly among the strains. This strain-dependent pattern of response to alprazolam seems to indicate a genetic component, rather than pharmacokinetic, in the behavior sensitivity to the BZ, with a spectrum of degree of responsivity among strains.

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.

Blockade of spontaneous posttraining performance improvement in mice by NMDA antagonists

We investigated the effects of immediate post-training systemic administration of gamma-L-glutamyl-L-aspartate (gamma-LGLA) and 3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonate (CPP), antagonists at the N-methyl-D-aspartate receptor, in a lever-press task in two inbred strains of mice. When retention performance was tested in control animals 24 h after partial acquisition of the task. BALB/c mice exhibited a spontaneous performance improvement whereas C57BL/6J mice did not gamma-LGLA at doses of 2.5 and 25 mumol/kg and CPP at doses ranging between 0.025 and 2.5 mumol/kg blocked the spontaneous performance improvement found in BALB/c mice but had no apparent effects on the retention performance of C57BL/6J mice. These data suggest that retention impairment induced by CPP and gamma-LGLA in BALB/c mice results from an interference with posttraining memory processes.

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.

Down-regulation of benzodiazepine receptors by ethyl beta-carboline-3-carboxylate in cerebrocortical neurons

Effect of exposure of primary cultured cerebral cortical neurons to ethyl beta-carboline-3-carboxylate (beta-CCE) on the function of benzodiazepine receptors was studied. Exposure of neurons to beta-CCE (0.1-10 microM) decreased the binding of [3H]flunitrazepam to extensively washed membrane fractions in a dose- and time-dependent manner, whereas the binding of [3H]flunitrazepam to the cytosolic fractions increased (180%) under the same conditions as described above. Ethyl-8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a] [1,4] benzodiazepine-3-carboxylate (Ro15-1788), an antagonist of the central type of benzodiazepine receptors, completely abolished the beta-CCE-induced decrease in [3H]flunitrazepam binding and the IC50 value for [3H]flunitrazepam binding to the extensively washed membrane fractions prepared from beta-CCE-treated neurons was similar to that from non-treated neurons. Scatchard analysis revealed that only the Bmax value for [3H]flunitrazepam binding decreased after the exposure to beta-CCE (1 microM) for 12 h, although the Kd value was not altered. These results indicate that beta-CCE induces the down-regulation of benzodiazepine receptors by an increase in benzodiazepine receptor internalization.

Genetic analysis of anxiety-related behaviors and responses to benzodiazepine-related drugs in AXB and BXA recombinant inbred mouse strains

Recombinant inbred (RI) strains derived from the C57BL/6J and A/J mouse strains were used for behavioral studies designed to estimate the number and location of chromosomal loci responsible for anxiety-related behaviors and differential sensitivity to agonists and inverse agonists of the gamma-aminobutyric acidA (GABAA)/benzodiazepine receptor complex. The phenotypes of the parental inbred strains and of 28 RI strains were characterized for the number of transitions in the light<-->dark exploratory model, anxiolytic response to diazepam, vertical and ambulatory activities in an open field, and sensitivity to the convulsant properties of methyl-beta-carboline-3-carboxylate (beta-CCM). The strain distribution patterns and estimates of the minimal number of loci obtained for each trait suggest that multiple chromosomal loci contribute to differences in anxiety-related behavioral phenotypes and the behavioral responses to diazepam and beta-CCM between C57BL/6J and A/J mice. The best probabilities of linkage were found between the variables characterizing response to diazepam and loci on chromosomes 1 (Xmv-41) and 10 (D10Mit2) and between the sensitivity to the convulsant actions of beta-CCM and locus D15Mit5 on chromosome 15.

Characterization of benzodiazepine-sensitive behaviors in the A/J and C57BL/6J inbred strains of mice

Exploratory behaviors as well as pharmacological actions of gamma-aminobutyric acidA (GABAA)/benzodiazepine receptor agonists and inverse agonists were characterized in C57BL/6J and A/J strains of mice. C57BL/6J mice displayed higher levels of exploratory behavior than A/J mice in the light in equilibrium with dark exploration model of anxiety and in an open-field test, suggesting that C57BL/6J mice are less "emotional" and more active than A/J mice, respectively. However, C57BL/6J mice were more sensitive than A/J mice to the anxiolytic effects of diazepam in the light in equilibrium with dark exploration model. In contrast, A/J mice were more sensitive than C57BL/6J mice to the convulsant effects of methyl-beta-carboline-3-carboxylate. C57BL/6J mice showed no evidence of acquisition of a passive avoidance task, while A/J readily acquired this memory task at low levels of footshock. C57BL/6J and A/J mice should be useful parental strains in recombinant inbred lines for investigating the genetic determinants of benzodiazepine-sensitive behaviors and sensitivity to drugs acting on the GABAA/benzodiazepine receptor complex.