DIFFERENTIAL-EFFECTS OF CHLORDIAZEPOXIDE ON AGGRESSIVE-BEHAVIOR IN MALE-MICE - THE INFLUENCE OF SOCIAL-FACTORS

The present study examined the influence of prior social experience on the effects of chlordiazepoxide (CDP; 5.0, 10.0 and 20.0 mg/kg) on in tra-sexual aggression in male mice. Prior to drug testing, animals wer e either individually housed or screened in dyadic encounters in a neu tral cage. This novel method yielded four experimental groups comprisi ng animals with different social experiences and different aggressive/ defensive characteristics: 1) individually-housed males (I): 2) aggres sive males (A); 3) counter-attacking males (C), which actively respond ed to but did not initiate attack; and 4) defeated males (D). Twenty-f our hours after screening, animals were treated with CDP and subjected to a resident-intruder test with untreated intruders. Results indicat ed that the lowest dose of CDP (5 mg/kg) increased aggressive behaviou r but only in A males. At higher doses (10-20 mg/kg), CDP reduced atta cks towards intruders in A, C and I, but not D, males. In A and C male s, the antiaggressive action of CDP was associated with a prosocial ef fect (increased social investigation), whereas in I males, reduced agg ression was associated with an increase in fear-related behaviours. As these differential effects of CDP on intermale aggression cannot be f ully explained by differences in behavioural baselines, present data h ighlight the importance of experiential background as a powerful varia ble in determining behavioural responses to benzodiazepines. Present f indings therefore suggest that an understanding of drug effects on soc ial behaviour demands consideration of biological variability in pheno type.