Modulation by photoperiod of gonadotrophin secretion in intact and castrated Djungarian hamsters.

Reproductive activity in the Djungarian hamster, Phodopus sungorus, is suppressed by short daylengths and induced and maintained by long daylengths. To determine the time course of changes in plasma and pituitary gonadotrophin concentrations during the photoperiodic response, sexually immature male hamsters were moved from short (9L:15D) to long days (14D:10D). This induced an increase in testicular weight in 5 days and full sexual maturity in about 1 month. Plasma and pituitary FSH concentrations were significantly elevated after 3 and 5 long days respectively, reaching peak levels after 7--14 days and then declining. In contrast, pituitary and plasma LH concentrations did not increase until 10 and 21 days of photostimulation respectively. Castration of hamsters kept in short days led to a marked increase in plasma and pituitary FSH titres. Transfer to long days further increased plasma FSH although pituitary content did not change. Castration of hamsters kept in long days led to an elevation of plasma and pituitary FSH concentration to these same levels. Transfer to short days reduced plasma FSH to the levels seen in hamsters castrated and kept in short days. Pituitary content did not change. The results suggest that while there is substantial steroid negative feedback in intact animals on both long and short days, the photoperiodic modulation of gonadotrophin secretion does not necessarily involve changes in feedback sensitivity.