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Ontogenesis of the circadian adreno cortical rhythm in rats role of mother rats in determining the phase of the circadian rhythm

Ontogenesis of the circadian adreno cortical rhythm in rats role of mother rats in determining the phase of the circadian rhythm

Journal of Kyoto Prefectural University of Medicine 89(6): 491-499

To determine the role of mother rats in setting the phase of the pups' endogenous circadian rhythm, the author studied the influence of foster mothers, who had a reversed rhythm to that of the original mothers, on the development of the circadian adrenocortical rhythm in blinded pups. Wister albino rats were divided into 2 groups. One group was laid under LD [light:dark] (light 0700-1900), the other under DL (light 1900-0700) condition. After acclimation to the environmental conditions, they were mated within each group. Offspring were blinded within 24 h after birth by optic enucleation. Two litters of each (LD and DL) group were reared by their original mothers under the same lighting conditons, while another 2 litters were transferred immediately after blinding to the reversed lighting condition and reared by foster mothers who had reversed their rhythm to the original pattern. Four and 8 wk after birth, serial blood sampling was performed every 4 h over 48 h periods. Three pups were selected from each litter and repeatedly used for each serial blood sampling. Blood was obtained by tail tip incision and corticosterone was determined by the competitive protein binding method of Murphy. In most of the pups, peaks of plasma corticosterone concentration appeared at the same time of the day .+-. 4 h during 2 consecutive days. At the end of the 4th wk, regardless of original or foster mother, most peaks occurred at the transition time of light to dark. This fact indicates that the pups' rhythm was entrained by their mothers during the nursing period. At the end of the 8th wk, the phase of the pups' rhythm was shifted by 12 h from that of the 4th wk. Free-running of this rhythm indicates that their rhythm was endogenous. To examine the possibility of existence of an extraretinal pathway of light in newborn rats, pups who were born under LD or DL together with their original mothers were enucleated at the day of the pups' birth and transferred to the reversed lighting conditions. At the end of the 4th wk, the pups' circadian adrenocortical rhythm phases were entrained to that of the mother and not to the lighting condition, i.e., mother played a predominant role in determining the pups' rhythm. Their rhythm free-ran. The biological clock regulating the endogenous rhythm itself is thought to be inherited, but these results suggest the importance of the mother's role after birth in setting the phase of the endogenous rhythm.

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