Effects of photoperiod and melatonin on reproduction in the Syrian hamster

Abstract

The Syrian hamster is a spontaneous ovulator and, in the wild, a seasonal breeder. Daylength (i.e. photoperiod) appears to be an important factor in the regulation of seasonal periodicity in this species. Female hamsters became acyclic after 6-8 wk exposure to 'short days' (i.e. a 10 hr photoperiod). During the period of acyclicity these animals displayed a pronounced diurnal rhythmicity in LH secretion with peak serum LH concentrations during the mid afternoon. Serum FSH and progesterone concentrations showed a similar rhythmicity to that observed for LH, but the fluctuations were of lesser magnitude. Ovariectomy reduced serum progesterone levels and abolished the afternoon rise in serum progesterone. Serum LH was unaffected by ovariectomy while serum FSH levels were somewhat increased. Following removal of the ovaries, both gonadotropins continued to display peak concentrations in the serum during the afternoon. Daily injections of melatonin, a pineal product, inhibited ovulatory cyclicity in hamsters maintained on 'long days' (i.e. a 14 h photoperiod) when the compound was administered 4 hours prior to lights off (15.00 h). This effect was not present when the melatonin was given at 08.00 h nor was it present in pinealectomized hamsters. Similar inhibitory effects of melatonin on the male hamster reproductive system have been observed, and these effects are also dependent upon the time of day at which the compound is administered.