Abstract
[Introduction] Circadian rhythms in clock gene expression have been reported to show photoperiodic responses in the suprachiasmatic nucleus(SCN) of nocturnal rodents. On the other hand, different parts of the SCN were demonstrated to response differentially to photoperiods. In the present study, we examined photoperiodic responses of circadian rhythms in Per1 and Per2 expression in the rat SCN. [Material and Methods] Male Wister rats were exposed to either to a long (LD18:6) or to a short (LD6:18) photoperiod for more than 3 weeks. The brains were obtained at 3 hours intervals under darkness. Coronal sections of 30μm in thickness were made from the rostral to the caudal portions of the SCN and in situ hybridization was performed to measure Per1 and Per2 expressions in individual cells with digoxigenin-labeled probes. The number of positive cells were counted in regular squares of 75μm length, and the circadian rhythms in gene expressions were obtained in discrete areas of the SCN. [Results and Conclusions] Under a long photoperiod, the circadian rhythm in Per1 expression cells showed a bimodal pattern in the rostral part, whereas it showed one peak in the caudal. On the other hand, the circadian rhythms in Per2 expression showed essentially a single peak thoughout the SCN. These findings indicate that there are more than two oscillating cell populations in the rat SCN, which respond differentially to different photoperiod. [J Physiol Sci. 2007;57 Suppl:S138]
