In mammals, the pineal hormone melatonin is thought to be essential to process environmental photoperiodic information. In this study, we demonstrate in a circannual species, the European hamster Cricetus cricetus, the existence of a melatonin-independent second pathway. In 4 physiological parameters (reproduction, body weight, activity pattern, body temperature), a large majority of pinealectomized European hamsters were entrained to an accelerated photoperiodic regime. It compressed the natural variations in the photoperiod to a 6-month cycle, which allowed us to record up to 6 complete physiological cycles during the life span of the individuals. We show further that whether a pinealectomized animal is able to entrain to changes in the photoperiod is influenced by the season of pinealectomy. The results do not disprove that melatonin is capable of entraining a circannual rhythm, but they show clearly that melatonin is not necessary, demonstrating another melatonin-independent pathway for circannual entrainment by the photoperiod. In view of these new insights, a revision of the original literature revealed that probably the melatonin-independent pathway plays an important role in most circannual mammals but only a minor role in photoperiodic species. Thus, the present work provides also the first evidence for different synchronization mechanisms in photoperiodic and circannual species.
Keywords: activity; body temperature; body weight; circannual rhythm; melatonin; photoperiodic entrainment; pinealectomy; reproduction.