The suprachiasmatic nucleus (SCN), locus of the central circadian clock, consists of two neuronal populations (i.e., a light-recipient ventral SCN subpopulation directly entrained by light and a dorsal SCN subpopulation with an autonomous oscillatory function possessing an indirect or weak light response). However, the mechanism underlying the transmission of photic signals from the ventral to dorsal SCN remains unclear. Because gastrin-releasing peptide (GRP), expressed mainly in the ventral SCN, exerts phase-shifting actions, loss of the GRP receptor intuitively implies a reduction of photic information from the ventral to dorsal SCN. Therefore, using GRP receptor-deficient mice, we examined the involvement of GRP and the GRP receptor in light- and GRP-induced entrainment by the assessment of behavioral rhythm and induction of mouse-Period (mPer) gene in the SCN, which is believed to be a critical for photic entrainment. Administration of GRP during nighttime dose dependently produced a phase delay of behavior in wildtype but not GRP receptor-deficient mice. This phase-shift by GRP was closely associated with induction of mPer1 and mPer2 mRNA as well as c-Fos protein in the dorsal portion of the SCN, where the GRP receptor was also expressed abundantly. Both the light-induced phase shift in behavior and the induction of mPer mRNA and c-Fos protein in the dorsal SCN were attenuated in GRP receptor-deficient mice. Our present studies suggest that GRP neurons in the retinorecipient ventral area of the SCN convey the photic entrainable signals from the ventral SCN to the dorsal SCN via induction of the mPer gene.
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