Ontogeny of the circadian oscillation: zeitgebers of maternal netrainment and its mechanism.

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Most circadian rhythms of the rat appear around the 3rd postnatal week, but the oscillator which drives the overt rhythms is functionate prenatallyy and entrained by maternal rhythms. In this experiment, the mechanism and zeitgeber(s) of the maternal entrainment of pup's circadian oscillator was investigated. No circadian rhythms can be longitudinally measured from suckling pups. Therefore, artificial cyclicities of various putative zeitgebers were imposed on pregnant and/or nursing mothers at different phases of the day, and the entainability was examined by calculating the phase and period of pup's locomotor rhythm appeared after weaning. Results 1. Maternal rhythmicities which entrained to the light-dark cycle can entrain pup's oscillator when the litter size was small. 2. The restricted daily feeding (RF; food available for 4 h per a day), which did not entrain the circadian osicllation of adult rats, entrained the pup's sircadian oscillation when imposed before the 2nd postnatal d … More ay. Maternal rhythm(s) which coupled to the food associated oscillation is the zeitgeber of mother-pups entrainment. Melatonin, which couples to the light entrainable circadian oscillator under RF, is not considered to be the zeitgeber. 3. The body temperature rhythm of mother couples to the food-associated oscillator and pups were prevented from heat loss by mother. Therefore, pups were idolated from 4 or 2 h daily, and 3 different temperature cyclicities (10, 22 or 30゜C)were imposed until weaning. Pup's oscillation was entrained by 4 h isolation. Cold exposure during the isolation significantly increased the entrainability. Two hour isolation of any temperature cycle had no effect. 4. Catecholaminergic neurons in the CNS is the responsible pathway for prefeeding corticosterone peak under RF. But catecholamine depletion by intraventricular injection of 6-hydroxydopamin on the day of birth did not affect on the mother-pups entrainmnnt, either under RF or ad libitum feeding. It is suggested that this entrainment is mediated by the pathways other than the catecholaminergic neurons. Less