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In mammals, changing daylength (photoperiod) is the main synchronizer of seasonal functions. The photoperiodic information is transmitted through the retino-hypothalamic tract to the suprachiasmatic nuclei (SCN), site of the master circadian clock. To investigate effects of day length change on the sheep SCN, we used  in-situ  hybridization to assess the daily temporal organization of expression of circadian clock genes ( Per1 ,  Per2 ,  Bmal1  and  Fbxl21 ) and neuropeptides ( Vip ,  Grp  and  Avp ) in animals acclimated to a short photoperiod (SP; 8h of light) and at 3 or 15 days following transfer to a long photoperiod (LP3, LP15, respectively; 16h of light), achieved by an acute 8-h delay of lights off. We found that waveforms of SCN gene expression conformed to those previously seen in LP acclimated animals within 3 days of transfer to LP. Mean levels of expression for  Per1-2  and  Fbxl21  were nearly 2-fold higher in the LP15 than in the SP group. The expression of  Vip  was arrhythmic and unaffected by photoperiod, while, in contrast to rodents,  Grp  expression was not detectable within the sheep SCN. Expression of the circadian output gene  Avp  cycled robustly in all photoperiod groups with no detectable change in phasing. Overall these data suggest that synchronizing effects of light on SCN circadian organisation proceed similarly in ungulates and in rodents, despite differences in neuropeptide gene expression.

Effects of Photoperiod Extension on Clock Gene and Neuropeptide RNA Expression in the SCN of the Soay Sheep