5-Hydroxytryptamine-mediated behaviour in male and female rats

The responses of male and female rats to drugs causing the behavioural syndrome induced by 5-hydroxytryptamine (5-HT) were compared. Preliminary experiments showed that females had largely similar responses to the releaser of 5-HT, p-chloroamphetamine (PCA) and the 5-HT receptor agonist 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) at different stages of the oestrus cycle. The behavioural responses to 5-MeODMT (with and without the monoamine oxidase inhibitor pargyline) or to p-chloroamphetamine were not significantly different to those of males except for tremor after p-chloroamphetamine which was more marked in the females. However, concentrations of p-chloroamphetamine in brain in these animals, when killed immediately after behavioural recording were greater in the females. When rats, pretreated with the monoamine oxidase inhibitor, pargyline, were given the precursor of 5-HT, tryptophan, the females showed substantially greater hypothermia and larger scores for components of the 5-HT syndrome than the males. This sex difference may have been due to the moderately but significantly higher levels of 5-HT (and possibly tryptamine) in brain attained by the female rats, than by similarly-treated males. The results as a whole therefore suggest that the greater behavioural response of female rats to pargyline and tryptophan reflects a greater effect of this treatment on the synthesis of indoleamines than that occurring in males.