Photoperiod and melatonin affect testicular growth in the marsh rice rat (Oryzomys palustris)

Reproduction in rice rats is subject to photoperiodic control and the pineal gland mediates this effect. We examined the effects of the pineal gland hormone melatonin on testicular weight when administered via implants, injections, and infusions. Testicular weight was modified by photoperiod and the size of the melatonin implant. Twenty-millimeter implants suppressed testicular weight in rice rats housed on 12- and 16-hr photoperiods, while those housed on a 14-hr photoperiod were more sensitive to melatonin; in these animals 10- and 20-mm implants inhibited testicular weight. Melatonin implants also prevented rice rats from responding to a change in photoperiod with the appropriate alteration of testicular growth. Melatonin injections inhibited testicular growth when administered before lights out on LD 14:10, but not on LD 16:8. Morning injections had no effect on either photoperiod. Finally, 12-hr duration melatonin infusions inhibited testicular growth in pinealectomized rice rats on LD 16:8, while 6-hr duration infusions were without effect. These data show that the pineal, through the secretion of melatonin, is a phototransducing organ intimately involved in testicular growth in rice rats.