The Effects of Maternal Isolation on the Ontogeny of Circadian Activity Rhythms and the Growth of Rat Pups

Abstract

<p>The circadian rhythms of young animals can be entrained by prenatal and postnatal maternal cues. The present research examined the effects of early postnatal maternal isolation on the ontogeny of the activity rhythm in rat pups. The studies also examined the effects of different light cycles, as well as other stimuli which may serve as synchronizers of the activity rhythm during the postnatal period. Activity rhythms synchronized to light-dark (LD) cycle appeared as early as five days after birth in mother-reared pups. This result contrasts with data from other reports which indicate a later onset. Pups reared without their mothers (AR pups) between 3 or 4 to 18 days postnatally on a LD cycle had rhythms which were of lower amplitude and of shorter duration than those in the mother-reared group. AR pups with a LD cycle and a feeding cycle which approximated the normal nursing rhythm showed more synchronization of their activity than pups with only a LD or feeding cycle. The mean period of the activity rhythms of pups-raised under constant light deviated the most from 24 hrs. The introduction of a temperature cycle attenuated the AR pups' activity. These results indicate that the nursing rhythm, in conjunction with LD cycles, may serve as synchronizers of rat pups' rhythms. However, nursing and LD cycles represent only a part of the complex postnatal environment which includes temperature, as well as other stimuli not investigated here, such as olfactory cues.</p> <p>There was also evidence suggesting that rhythmic factors during the early postnatal period may influence growth. AR pups with a cyclic feeding schedule had heavier spleens and lighter livers than animals with a noncyclic schedule. Heavier forebrains were associated with a predominantly diurnal feeding cycle. These growth factors may, in turn, influence the rhythmicity of locomotor activity.</p>