Abstract
It is well known that light resets the circadian clock only at specific times of day. The mechanisms mediating such gating of environmental input to the CNS are not well understood. We show that calbindinD28K (CalB)-containing cells of the suprachiasmatic nucleus (SCN), which are directly retinorecipient, gate photic entrainment of cellular circadian oscillators and thereby determine the timing of locomotor rhythmicity. Specifically, we demonstrate a circadian rhythm of subcellular localization of CalB: whereas the protein is detected at all times in the cytoplasm, it is low or absent in the nucleus during the night. Under normal circumstances, light-induced behavioral phase shifts and Period (Per) gene expression in the SCN occur only during the subjective night. Surprisingly, both behavioral phase shifts and light-induced Per are blocked during the subjective night and enhanced during the subjective day after administration of CalB antisense oligodeoxynucleotides. These results suggest a cellular basis for temporal gating of photic input to the circadian clock.