Ontogenesis of circadian rhythm of melatonin synthesis in pineal gland of rat.

Serotonin N-acetyltransferase (NAT) activity regulates the circadian rhythm of melatonin synthesis in pineal gland. NAT rhythm persisted in continuous darkness or in blinded rats. Development of the circadian rhythm of NAT activity was followed in the infant rats raised under various lighting schedules. The pups born and raised in continuous darkness or under constant illumination showed a rhythmic change indicating that the biological clock for NAT is generated inborn independent of environmental light-dark schedules. In the absence of light, the rhythms of pups synchronized with mother's rhythm. When rats were maintained under a ultradian light-dark schedule (LD 6:6), NAT activity showed a circadian change increasing once a day. When pups were born and raised under the ultradian lighting condition, NAT activity rhythm coincided with mother's rhythm. When pups were raised by a foster mother who had an inverted rhythm from that of an original mother, the rhythm of the pups synchronized with that of the foster mother. Whe pups were separated from their mother daily for 12 hours, their NAT rhythms reappeared when they were separated from their mother. These observations indicated a novel mechanism that in the absence of light-dark schedule, mothers taught the circadian rhythm to the pups as they raised them.