Estradiol and progesterone influence L-5-hydroxytryptophan-induced myoclonus in male guinea pigs: sex differences in serotonin-steroid interactions.

The effects of castration in males and sex differences in the effects of estradiol and progesterone on L-5-hydroxytryptophan (L-5-HTP)-induced myoclonus in guinea pigs were examined. Castration had no effect on L-5-HTP-induced myoclonus in males. There were sex differences in sensitivity to L-5-HTP. In the absence of steroids, L-5-HTP-induced myoclonus was higher in gonadectomized males than females. A low dose of estradiol benzoate (EB; 3.5 micrograms) given 46 h before L-5-HTP (100 mg/kg) enhanced myoclonus in gonadectomized females but not males. However, at a higher dose of EB (10 micrograms) and a lower dose of L-5-HTP (80 mg/kg), myoclonic responding was enhanced in males. These findings indicate that estradiol has a similar effect on L-5-HTP-induced myoclonus in males and females, but do not rule out the possibility of sex differences in sensitivity to L-5-HTP when both sexes are given estradiol priming. When L-5-HTP was given 6 h after 0.5 mg progesterone in estradiol-primed males, myoclonus was enhanced. Progesterone treatment reverses the facilitative effect of EB on L-5-HTP-induced myoclonus in females. Therefore, progesterone has opposite effects on L-5-HTP-induced myoclonus in males and females. These findings were discussed with respect to the interaction of steroids and 5-HT transmission in the regulation of steroid-dependent reproductive behavior.