Pinealectomy and constant release of melatonin or 5-methoxytryptamine induce testicular atrophy in the European hamster (Cricetus cricetus, L.).

The role of the pineal and of photoperiod in the regulation of gonadal activity has been studied in the European hamster, a wild hamster commonly found in the vicinity of Strasbourg, France. Besides the presence of a probable endogenous circannual rhythm in reproductive capacities, it appears that, as in all other hamster species studied to date, the sexual activity of this animal is directly regulated by changes in the photoperiodic environment. Pinealectomy as well as a constant release of melatonin or of 5-methoxytryptamine has no effect on the gonadal atrophy induced by short photoperiod, but induces testicular regression in animals kept under long photoperiod. The pineal in this species appears thus to be involved in the maintenance of sexual activity in long photoperiod, while it is apparently not required for the gonadal inhibitory effect of short photoperiod. These results are similar to those obtained in the Turkish hamster. 5-Methoxytryptophol is not implicated in the phenomenon of synchronization of the sexual activity with the photoperiodic changes.