Effects of short-term treatment with testosterone on the secretion of FSH and LH in castrated golden hamsters exposed to short days.

Castrated hamsters which were transferred from long (14L:10D) to short (9L:15D) days and received testosterone-filled capsules for 1 week after transfer failed to show a significant suppression in the plasma levels of FSH and LH after capsule removal. In contrast, gonadotrophin concentrations were suppressed in hamsters in which the long-day castration response had been blocked with exogenous testosterone. After castration on long days and exposure to 10 weeks of short days pituitary gland weight and gonadotrophin content, as well as plasma FSH titres, were higher in control animals than in those that had received testosterone implants for 7 weeks of short days. The results suggest that failure of castrated hamsters to respond to the suppressive effects of short days reflects castration-induced changes in hypothalamo-pituitary physiology rather than a neuroendocrine mechanism by which photoperiod modulates gonadotrophin secretion.