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 nu clei, in terms of Fos protein expression, depends on the photoperiod. In Syrian hamsters, a 4 h lengthening of the photosensitive phase occu rs within 3-4 weeks after a transfer from a long to a short photoperio d. The absence of endogenous melatonin following pinealectomy does not prevent the lengthening of the photosensitive phase. Thus, even if th e pineal production is able to convey photoperiodic information, it do es not feed back on the circadian clock to allow its integration. (C) 1997 Elsevier Science Ireland Ltd.