INDIVIDUAL-DIFFERENCES IN RESPONSE TO PSYCHOLOGICAL STRESS AND CHLORDIAZEPOXIDE IN ADULT MICE - RELATIONS WITH CHANGES IN EARLY SOCIAL MILIEU

Individual differences in the adult behavior of mice have been associa ted with variations in early social milieu, such as age of weaning and the sex of littermates. To better understand the roles that these two variables play in the organization of a behavioral profile, the long- term effects of their manipulation in CD-1 mice were assessed. On Post natal Day (PND) 15, mouse litters were split in half. Each half litter contained 4 males or 4 females, or 2 of each sex At the same time, on e half litter was weaned (precocious weaning); the other half was left with the dam (regular weaning). At adulthood (PND 70), the animals we re challenged with the benzodiazepine agonist chlordiazepoxide (CDP; a t a 0-, 2.5, or 5.0 mg/kg dose) and assessed in sequential order in an animal model for anxiety (black/white exploration test) and for pain reactivity (hot-plate test, set at 55 degrees C +/- 1 degrees C) after a 2-min exposure to a threatening stimulus represented by cat urine. Balanced-sex reared males exhibited a shorter latency to leave the whi te area than did males raised unisexually, being apparently more promp t to react in a novel situation. No such difference appeared in the fe male group. In the hot-plate test, precocious weaning significantly re duced the latency to lick a forepaw, and prior exposure to cat's urine induced a clear-cut analgesia The latter response was dose-dependentl y reduced by CDP administration in mice raised in a sex-balanced condi tion only. Baseline activity levels also differed as a consequence of gender and of rearing condition, and were slightly increased by CDP tr eatment. Overall, these results suggest that subtle variations of soci al environment early in development can exert long-term effects on bot h behavioral reactivity to environmental stimuli and on BDZ-induced be havioral changes.