Daily exposure to cold phase-shifts the circadian clock of neonatal rats in vivo.

Maternal rhythms entrain the prenatal and neonatal circadian clock in the suprachiasmatic nucleus (SCN) before light entrainment is established. However, the responsible time cues for maternal entrainment are not identified. To examine the role of cyclic changes of ambient temperature in maternal entrainment, blind neonatal rats carrying a clock gene (Per2) bioluminescence reporter were exposed to either of three ambient temperatures (10, 20 or 30 °C) during 6-h maternal separation in the early light phase. Cold exposure was performed from postnatal day 1 (P1) to P5. On P6, the SCN was harvested and cultured for photometric monitoring of the circadian rhythm in Per2 expression. Here we demonstrate that the daily cold exposure phase-delayed the circadian Per2 expression rhythms at P6 in a temperature-dependent manner. Exposure to 10 °C produced the largest phase-shift of 12.7 h, and exposure to 20 and 30 °C yielded moderate shifts of 4.1 and 4.5 h, respectively. There was no significant difference in the phase-shifts between the latter two temperatures, indicating that ambient temperature is not the sole factor for the phase-shift. Behavioral rhythms that developed after weaning reflected the phase-shift of clock gene expression rhythm in the SCN. These findings indicate that a daily exposure to an ambient temperature of 10 °C during the neonatal period is capable of resetting the circadian clock in the SCN, but other factors yet unidentified are also involved in maternal entrainment.