EFFECTS OF CHLORDIAZEPOXIDE ON MATERNAL AGGRESSION IN MICE DEPEND ON EXPERIENCE OF RESIDENT AND SEX OF INTRUDER

Lactating mice respond differentially to intruders of differing sex, d isplaying defensive attack against the male and offensive attack again st the female. Such a phenotypic dichotomy appears to have adaptive va lue in that unfamiliar males pose a much greater threat to the offspri ng than do females. The present study examined the effects of the benz odiazepine anxiolytic chlordiazepoxide (CDP) (2.5-10.0 mg/kg) on this differential response pattern in aggression-naive (nonscreened) (NS) a nd aggression-experienced (screened) (S) lactating female mice (Mus mu sculus domesticus) confronting intruders of either sex in a 10-min tes t. This procedure was used to evaluate the influence of both the type of opponent and previous aggressive experience on basal behavioural pr ofiles and drug action. Results showed that both intruder sex and prio r screening for attack modulated the behaviour of lactating females to ward intruders. In turn, both variables strongly influenced CDP effect s on maternal aggression. In particular, in S dams, CDP dose-dependent ly increased maternal attack against males but decreased attack agains t female intruders. Conversely, in NS dams, CDP decreased attack (and fear) against males but did not affect it against females. In both S a nd NS conditions, CDP modified the attack strategy of lactating female s against the male, switching it from a defensive to an offensive patt ern. Exploration, social investigation, eating, and immobility were di fferentially affected by the drug treatment, depending on screening an d/or intruder sex condition. These differential effects of CDP between S and NS conditions, toward either male or female intruders, cannot b e fully explained by differences in the baseline levels of these behav iours. Alternative hypotheses are discussed. These findings demonstrat e that the effects of CDP on maternal attack behaviour depend on not o nly the drug but also the object of attack, and hence the function of attack and the prior experience of the attacker.