Differential expressions of mPer1 and mPer2 mRNAs under a skeleton photoperiod and a complete light–dark cycle

To investigate the molecular mechanisms in photo-entrainment of circadian rhythms, expression patterns of mouse period1 ( mPer1) and mPer2 mRNAs were examined in the suprachiasmatic nucleus (SCN) and skin under a complete light–dark (LD) cycle and a corresponding skeleton photoperiod (SPP). Under the SPP, the robust daily expressions of mPer1 and mPer2 were maintained in the SCN and in the skin. A slower rise time for mPer1 mRNA expression was observed in the SPP compared with that in the LD condition both in the SCN and in the skin, although the expression pattern of mPer2 was not significantly different between the two lighting conditions. The results suggest that the expressions of mPer1 and mPer2 mRNAs are differentially regulated not only in the SCN but also in peripheral tissues. Furthermore, we propose that the persistent expression of mPer2 mRNA is required for the entrainment to the SPP.