Photoperiod does not act on the suprachiasmatic nucleus photosensitive phase through the endogenous melatonin, in the Syrian hamster.

The duration of the sensitive phase to light of the suprachiasmatic nuclei, in terms of Fos protein expression, depends on the photoperiod. In Syrian hamsters, a 4 h lengthening of the photosensitive phase occurs within 3-4 weeks after a transfer from a long to a short photoperiod. The absence of endogenous melatonin following pinealectomy does not prevent the lengthening of the photosensitive phase. Thus, even if the pineal production is able to convey photoperiodic information, it does not feed back on the circadian clock to allow its integration.