Genetic analysis of active avoidance performance in mice

Positive allosteric modulation of GABAB receptors ameliorates sensorimotor gating in rodent models

BACKGROUND

Converging evidence points to the involvement of γ-amino-butyric acid B receptors (GABABRs) in the regulation of information processing. We previously showed that GABABR agonists exhibit antipsychotic-like properties in rodent models of sensorimotor gating deficits, as measured by the prepulse inhibition (PPI) of the acoustic startle reflex. The therapeutic potential of these agents, however, is limited by their neuromuscular side effects; thus, in this study, we analyzed whether rac-BHFF, a potent GABABR-positive allosteric modulator (PAM), could counter spontaneous and pharmacologically induced PPI deficits across various rodent models.

METHODS

We tested the antipsychotic effects of rac-BHFF on the PPI deficits caused by the N-methyl-D-aspartate glutamate receptor antagonist dizocilpine, in Sprague-Dawley rats and C57BL/6 mice. Furthermore, we verified whether rac-BHFF ameliorated the spontaneous PPI impairments in DBA/2J mice.

RESULTS

rac-BHFF dose-dependently countered the PPI deficits across all three models, in a fashion akin to the GABABR agonist baclofen and the atypical antipsychotic clozapine; in contrast with these compounds, however, rac-BHFF did not affect startle magnitude.

CONCLUSIONS

The present data further support the implication of GABABRs in the modulation of sensorimotor gating and point to their PAMs as a novel promising tool for antipsychotic treatment, with fewer side effects than GABABR agonists.

Are DBA/2 mice associated with schizophrenia-like endophenotypes? A behavioural contrast with C57BL/6 mice

RATIONALE

Due to its intrinsic deficiency in prepulse inhibition (PPI), the inbred DBA/2 mouse strain has been considered as an animal model for evaluating antipsychotic drugs. However, the PPI impairment observed in DBA/2 mice relative to the common C57BL/6 strain is confounded by a concomitant reduction in baseline startle reactivity. In this study, we examined the robustness of the PPI deficit when this confound is fully taken into account.

MATERIALS AND METHODS

Male DBA/2 and C57BL/6 mice were compared in a PPI experiment using multiple pulse stimulus intensities, allowing the possible matching of startle reactivity prior to examination of PPI. The known PPI-enhancing effect of the antipsychotic, clozapine, was then evaluated in half of the animals, whilst the other half was subjected to two additional schizophrenia-relevant behavioural tests: latent inhibition (LI) and locomotor reaction to the psychostimulants-amphetamine and phencyclidine.

RESULTS

PPI deficiency in DBA/2 relative to C57BL/6 mice was essentially independent of the strain difference in baseline startle reactivity. Yet, there was no evidence that DBA/2 mice were superior in detecting the PPI-facilitating effect of clozapine when startle difference was balanced. Compared with C57BL/6 mice, DBA/2 mice also showed impaired LI and a different temporal profile in their responses to amphetamine and phencyclidine.

CONCLUSION

Relative to the C57BL/6 strain, DBA/2 mice displayed multiple behavioural traits relevant to schizophrenia psycho- and physiopathology, indicative of both dopaminergic and glutamatergic/N-methyl-D: -aspartic acid receptor dysfunctions. Further examination of their underlying neurobiological differences is therefore warranted in order to enhance the power of this specific inter-strain comparison as a model of schizophrenia.

Activation of GABA(B) receptors reverses spontaneous gating deficits in juvenile DBA/2J mice

RATIONALE

Gamma-amino-butyric acid (GABA)(B) receptors play a key role in the pathophysiology of psychotic disorders. We previously reported that baclofen, the prototypical GABA(B) agonist, elicits antipsychotic-like effects in the rat paradigm of prepulse inhibition (PPI) of the startle, a highly validated animal model of schizophrenia.

OBJECTIVES

We studied the role of GABA(B) receptors in the spontaneous PPI deficits displayed by DBA/2J mice.

MATERIALS AND METHODS

We tested the effects of baclofen (1.25-5 mg/kg, intraperitoneal [i.p.]) in DBA/2J and C57BL/6J mice, in comparison to the antipsychotic drugs haloperidol (1 mg/kg, i.p.) and clozapine (5 mg/kg, i.p.). Furthermore, we investigated the expression of GABA(B) receptors in the brain of DBA/2J and C57BL/6J mice by quantitative autoradiography.

RESULTS

Baclofen dose-dependently restored PPI deficit in DBA/2J mice, in a fashion similar to the antipsychotic clozapine (5 mg/kg, i.p.). This effect was reversed by pretreatment with the GABA(B) antagonist SCH50211 (50 mg/kg, i.p.). In contrast, baclofen did not affect PPI in C57BL/6J mice. Finally, quantitative autoradiographic analyses assessed a lower GABA(B) receptor expression in DBA/2J mice in comparison to C57BL/6J controls in the prefrontal cortex and hippocampus but not in other brain regions.

CONCLUSIONS

Our data highlight GABA(B) receptors as an important substrate for sensorimotor gating control in DBA/2J mice and encourage further investigations on the role of GABA(B) receptors in sensorimotor gating, as well as in the pathophysiology of psychotic disturbances.

Maternal behavior in female C57BL/6J and DBA/2J inbred mice

Inbred strains of mice exhibit different patterns of maternal behavior, providing material for studies of genetic influences on the expression of maternal behavior. Beginning 1 day after birth, maternal behavior was recorded daily for 14 days in the first and second litters of C57BL/6J (B6) and DBA/2J (D2) mothers. D2 mice had higher pup survival than B6 mice, and pup survival was higher in both strains in second litters than in first litters. D2 mothers spent more time engaged in maternal behavior, especially resting with, crouching over, and nursing pups than B6 mothers with first litters, but not with second litters. Not all measures of maternal behavior were correlated with pup survival; with both litters, B6 mothers retrieved pups faster than D2 mothers.

Active avoidance performance in genetically defined mice

It has been concluded by several investigators that active avoidance performance in mice is primarily influenced or even determined by a single gene. The genetically defined strains C57BL/6 and BALB/c have provided evidence that an aberrant development of pyramidal cells and mossy fiber configuration in the hippocampus of BALB/c mice also is determined by a single gene. As a test of the generality of the single gene influence on avoidance learning, and to examine the relationship of the hippocampal defect to avoidance learning, adult male mice of the inbred progenitor strains C57BL/6ByJ and BALB/cByJ and their seven recombinant inbred strains were tested in a variant of the shuttle-box paradigm used in previous studies. BALB/c were found to acquire the avoidance response at a faster rate than C57BL/6, consistent with most earlier reports, but performances of the recombinant inbred strains failed to dichotomize about the progenitor strains. The rank order of performance scores for the recombinant inbred strains was different from that reported in previous studies. Thus the present data failed to support the interpretation of a single major gene influencing active avoidance learning. It is concluded that avoidance learning and performance cannot be considered as unitary variables and that the interaction of genetic with environmental factors, including the conditions of the specific testing situation, are important considerations in any interpretation of genetic effects. No relationship between the hippocampal lamination defect and avoidance performance was demonstrated.

Conditioned lever-press avoidance response in mice: acquisition processes and effects of diazepam

Acquisition of conditioned lever-press avoidance behavior on a continuous avoidance schedule (response-shock interval = 30 s and shock-shock interval = 5 s) and on a discrete avoidance schedule (intertrial interval = 25 s and duration of the warning presentation = 5 s with an escape contingency) in dd mice was investigated. When the behavioral baseline had stabilized, the effects of diazepam on avoidance behaviors were examined. About 60% and 80% of the mice achieved criterion levels of avoidance behavior under continuous avoidance (shock rate being less than 0.5/min) and discrete avoidance (avoidance rate being higher than 75%), respectively. Diazepam (0.5--4 mg/kg SC) dose-dependently impaired avoidance behavior of mice which had a low baseline shock rate and a high baseline avoidance rate under continuous and discrete avoidances, respectively. The changes in avoidance behavior in mice after diazepam were almost identical with those previously found in rats.

Interaction of genotype and fostering in the development of behavior of DBA and C57 mice

Effects of fostering on behavior were studied in DBA/1Bg and C57BL/10Bg mice. Two-day-old pups were either infostered to mothers of their own strain, crossfostered to mothers of the other strain, or left as controls with their own mothers. Body weight, latency of emergence into an open field, open-field activity, defecation in an open field, spontaneous alternation, passive avoidance learning, and active escape learning were measured when the mice were 22 and 43 days old. Strain differences were observed for each trait except for spontaneous alternation. Fostering per se affected open-field activity and active escape learning. Similar effects of fostering per se on aggressive behavior have been reported by others. These may involve a role of ovarian cytoplasm, X-chromosomal genes, or uterine environment.

Effects of situational variables on performance of inbred mice in active- and passive-avoidance situations

A group of experiments were conducted to assess the interaction between environmental factors and specific genetic loci upon the performance of mice in avoidance learning situations. The performance of all mice was better in passive-avoidance than in active-avoidance situations. The performance of C57BL/6J mice was generally superior to that of DBA/2J mice. C57BL/6J mice usually avoided foot-shocks, while DBA/2J mice attempted to escape them. DBA/2J mice performed poorly in the presence of mild foot-shocks, while C57-BL/6J mice performed poorly in the presence of intense foot-shocks. Analysis of the data suggested that the loci involved probably exerted their influence by affecting the subject's response to its environment.

Behavioral characteristics of C57BL/6J. DBA/2J, and B6D2F1 mice which are potentially useful for gerontological research

This review summarizes the behavioral characteristics of C57BL/6J, DBA/2J, and B6D2F1, hybrid mice which are most likely to be of interest to gerontological scientists. Where possible, the characteristics of young and old mice are included. Where no information exists about the behavior of older mice of these genotypes, but there is reason to believe that information about mice might be useful for aging research, this information is included. The purpose of the review is to acquaint gerontologists with the availability of genetically defined animal models for aging research, to present what is known about these models, and to suggest areas of research which could profit from more attention.

Avoidance learning, behavior genetics, and aging: a critical review and comment on methodology

Animal models of human aging problems are potentially of enormous value to gerontological research. Models of behavioral problems have not often lived up to their promise. This review examines one such model system, avoidance learning in aging rodents. This model system has become increasingly popular among behavioral gerontologists, yet it has not provided the unambiguous answers to experimental questions which investigators expected. Our effort has been to determine why the system has failed, and to provide useful suggestions for future research. At the same time we have tried to provide a comprehensive review of the relevant literature and to assess the strengths and weaknesses of studies which have been conducted in the last two decades.