Photoperiod history, melatonin, and reproductive responses of male Syrian hamsters.

The action of melatonin (MEL) in mediating photoperiodic history (PPH) effects among male Syrian hamsters was investigated. In Exp. 1, pineal intact males in LD 14:10 received daily injections of MEL (15 micrograms) or ethanol:saline vehicle (SAL) 1 h before lights off for 8 wk to generate two groups experiencing identical photoperiods but distinctly different MEL histories. Following the cessation of injections, males were transferred to either LD 12:12 or LD 8:16 for 8 wk to evaluate whether their reproductive response to the new photoperiod would be more influenced by prior PPH or prior MEL history; MEL history was the significant variable. LD 12:12 caused gradual recrudescence in hamsters that were gonadally regressed following MEL injections. In contrast, LD 12:12 caused gonadal regression in hamsters that had large testes following SAL injections. Exp. 2 evaluated whether PPH influences might be mediated by aftereffects on the period (tau) of the circadian pacemaker regulating many behavioral and physiological rhythms. Pineal intact hamsters were exposed to long or short T cycles consisting of an 8 h photoperiod, repeated every 24.67 h (long T) or 23.33 h (short T) to mimic the aftereffects generated by short or long photoperiods. After 5 wk in these T-cycle conditions, all males were transferred to LD 12:12 for 11 wk. The reproductive response to LD 12:12 was modestly influenced by T-cycle history, even though each T-cycle generated different patterns of entrainment to LD 12:12. These findings support the hypothesis that the response of the reproductive system of male hamsters to an intermediate-duration photoperiod depends upon the duration of nocturnal melatonin secretion associated with hamsters' previous PPH.