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Pineal N-acetyltransferase (NAT) activity in 10-day-old rat pups was used to examine several aspects of the developing circadian timing system. When born and reared under constant darkness, 10-day-old animals manifested a clear daily rhythm of NAT activity whose phase was in time with the estimated circadian time of the mother (set by the lighting cycle during pregnancy). When pups were reared in constant darkness from birth by a foster mother whose circadian time was 180 degrees (12 h) out of phase with that of the natural mother, the resulting population profiles of NAT activity were arrhythmic. Analysis of the individual litter profiles from this experiment showed a variable postnatal influence of the maternal circadian system on the timing of the developing circadian system. Neither cross-fostering per se nor social interactions among litter mates contributed significantly to the apparent maternal influence. The magnitude of the postnatal maternal influence was not the same throughout development, but was most apparent during the first 5 days of life. Neonatal blinding (within 24 h of birth) did not appreciably alter the maternal influence. Extraretinal photoreception does not contribute to entrainment of the circadian clock during postnatal development, and retina-mediated photic entrainment is present by 10 days of age. The results indicate that pineal NAT activity monitored in 10-day-old rats provides a useful paradigm for studying the developing circadian system.

Cerebrospinal fluid (CSF) was serially withdrawn in individual, unanesthetized, unrestrained rats and assayed for vasopressin using a sensitive and specific radioimmunoassay. A prominent daily rhythm in the CSF concentrations of this peptide was found under diurnal lighting conditions. Low levels during the dark period alternated with high values during the light period; the rhythm appeared to anticipate the artificial 'dawn' and 'dusk' by a few hours. An 8-h phase shift in diurnal lighting caused a corresponding phase shift in the CSF rhythm. In addition, the rhythm persisted for at least 10 days in the absence of periodic environmental lighting cues in animals blinded by bilateral orbital enucleation; the rhythm was disrupted after 10 days of constant light. Blood vasopressin concentrations did not show a daily rhythm. Our results indicate that the daily vasopressin rhythm in rodent CSF is endogenously generated and that its phase is synchronized to the environmental light-dark cycle.