Constant light (LL) or constant dark (DD) environmental lighting conditions cause a free-running period and activity reduction in the rodent behavioral circadian rhythm. In order to understand the molecular process underlying behavioral rhythms in LL or DD housing conditions, we examined the circadian profile of mPer2 mRNA and mPER2 in the suprachiasmatic nucleus (SCN), a main oscillator, of free-running mice. The circadian expression rhythm of mPer2 in the SCN was dampened under 7-day LL conditions, whereas that of mPER2 protein was moderately attenuated and its expression peak delayed. The circadian expression of mPer2 and its product was slightly attenuated and advanced by 7-day DD conditions. With arrhythmic behavioral activity caused by long-term LL housing, mPER2 protein lost its rhythmicity in the SCN. On the other hand, LL or DD housing did not affect the mPer2 gene and its product in the cerebral cortex. The present results suggest that mPER2 circadian expression in the SCN corresponds well with behavioral circadian oscillation under LL or DD conditions. Thus, the behavioral circadian rhythm seems to correlate with molecular clock works in the SCN.
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