Development of Per2, Bmal1 and Arg-vasopressin mRNA circadian rhythms in the suprachiasmatic nucleus of rat pups under a light-dark cycle and constant dim light

A clock in the suprachiasmatic nucleus (SCN) is regulated by the negative feedback control of transcription. The developmental mechanism of the clock controlled genes remains unclear. To clarify the development and entraining mechanisms of the clock of pups, we measured the circadian rhythms of Per2, Bmal1 and arg-vasopressin (AVP) mRNAs in the SCN of sighted rat-pups nursed by their native dam under a light-dark (LD) cycle and constant dim light (DD). All pups were born under the LD cycle. The circadian rhythm of Per2 was significant from postnatal day 6 (P6) until P20. The peak time of the Per2 rhythm shifted even under the LD cycle. Development of the Bmal1 rhythm was delayed by one week from the Per2 rhythm. The circadian rhythm of the AVP peptide, contained in the output neuronal path from the SCN, had already started to oscillate before postnatal day 5. On P13, the AVP peptide rhythm was out of phase with that of mRNA, however, on P20, the AVP mRNA rhythm coincided well with the peptide rhythm. Bmal1 transcriptional activation by PER2 might function after the maturation of clock mechanisms. The results indicate that the nursing dam partially entrains the pup's circadian rhythms.