The Circannual Clock in the European Hamster: How Is It Synchronized by Photoperiodic Changes?

Abstract

In a seasonal environment, mammals time their reproductive phase so that the offspring are born in spring and summer. Two strategies have evolved to ensure accurate seasonal timing of reproduction, but both share a common Zeitgeber, the seasonal changes in photoperiod. The reproductive axis might be directly controlled, as in photoperiodic species, which require photoperiodic input to show seasonal changes in reproductive competence. In contrast, in circannual species photoperiodic changes act indirectly, namely, on an endogenous circannual clock that then times reproduction. This circannual clock generates self-sustained rhythms with a period length of about 1 year, and photoperiodic information is only needed to synchronize these rhythms. Concerning the mechanism that imparts the photoperiodic message internally, so far no differences between photoperiodic and circannual mammals have been reported. Recent results however, strongly suggest that the circannual European hamster (Cricetus cricetus) uses a fundamentally different mechanism than photoperiodic species. In the latter, photoperiod induces a change in the reproductive state via a melatonin-dependent pathway, whereas the circannual clock of the European hamsters can be synchronized via a melatonin-independent pathway. Instead, a circadian mechanism based on a specific organizational state is involved. Juvenile European hamsters use probably both pathways: the melatonin-dependent photoperiodic pathway for the short-term timing in the year of birth and the melatonin-independent circannual pathway for long-term timing of the seasonal events in the next year.