In mammals, day length (photoperiod) is read and encoded in the main circadian clock, the suprachiasmatic nuclei (SCN). In turn, the SCN control the seasonal rhythmicity of various physiological processes, in particular the secretion pattern of the pineal hormone melatonin. This hormone then operates as an essential mediator for the control of seasonal physiological functions on some tissues, especially the pars tuberalis (PT). In the European hamster, both hormonal (melatonin) and behavioral (locomotor activity) rhythms are strongly affected by season, making this species an interesting model to investigate the impact of the seasonal variations of the environment. The direct (on SCN) and indirect (via melatonin on PT) effect of natural short and long photoperiod was investigated on the daily expression of clock genes, these being expressed in both tissues. In the SCN, photoperiod altered the expression of all clock genes studied. In short photoperiod, whereas Clock mRNA levels were reduced, Bmal1 expression became arrhythmic, probably resulting in the observed dramatic reduction in the rhythm of Avp expression. In the PT, Per1 and Rev-erba expressions were anchored to dawn in both photoperiods. The daily proﬁles of Cry1 mRNA were not concordant with the daily variations in plasma melatonin although we conﬁrmed that Cry1 expression is regulated by an acute melatonin injection in the hamster PT. The putative role of such seasonal-dependent changes in clock gene expression on the control of seasonal functions is discussed.
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