The effect of parachlorophenylalanine on social interaction of male rats.

1. Juvenile male rats treated with parachlorophenylalanine showed hair loss round the head and neck extending down the chest and abdomen.2. Treated isolated rats did not have this loss of hair, while untreated animals living in the same cage as treated rats lost their hair. The loss therefore seems to be caused by increased social behaviour. This consists of a greater frequency of chasing each other, rolling over and social grooming.3. Adult male rats show an increase in mounting after treatment with parachlorophenylalanine, and this change in behaviour was counteracted by treatment with 5-hydroxytryptophan.4. It is concluded that 5-hydroxytryptamine inhibits sexual behaviour in male rats. The increase in social interaction seen in juvenile rats may be the behavioural precursor of adult sexual behaviour.5. Atropine 2.5 mg/kg blocked all forms of social interaction in adult male rats, although other activity was not altered.