Photoperiod can entrain circannual rhythms in pinealectomized European hamsters

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.