APStracts 2:0262R, 1995.

Entrainment of syrian hamster circadian activity rhythms by neonatal melatonin injections. Grosse, Julia, Aleksander Velickovic, and Fred C. Davis. Department of Biology, Northeastern University, Boston, MA 02115

APStracts 2:0262R, 1995.

The circadian rhythms of fetal and neonatal rodents are entrained by their mother. This entrainment is dependent upon the maternal suprachiasmatic nucleus (SCN), but the mechanism of entrainment is unknown. Administration of the pineal hormone melatonin to pregnant, SCN-lesioned female Syrian hamsters is able to entrain the activity rhythms of their hamster pups. The aim of this study was to determine whether melatonin injected directly to neonatal Syrian hamsters is able to entrain circadian rhythms, and if so, for how long this effect persists during development. Injection of melatonin to two groups of hamster pups at opposite phases on postnatal days 1-5 entrained the onset of activity rhythms on the day of weaning to two phases, 10.67h apart. Melatonin injection did not entrain activity rhythms to opposite phases on either postnatal days 6-10 or 21-25. Vehicle injection did not entrain animals to opposite phases at any of the ages studied. These results demonstrate that melatonin is able to act directly on the neonate to cause entrainment, and that this effect disappears after postnatal day 6.

Received 26 June 1995; accepted in final form 7 September 1995.
APS Manuscript Number R392-5.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 September 1995.