Single prenatal injections of melatonin or the D1-dopamine receptor agonist SKF 38393 to pregnant hamsters sets the offsprings' circadian rhythms to phases 180 degrees apart.

Pregnant Syrian hamsters with lesions of the suprachiasmatic nucleus (SCN) received single injections of melatonin or the D1-dopamine receptor agonist, SKF 38393 on day 15 of gestation (1 day before birth). Pups were weaned on postnatal day 20 and their freerunning activity rhythms recorded for 3-4 weeks. The pups' phases on the day of weaning were significantly clustered in both of the treatment groups, but the average phases differed by approximately 180 degrees. The results demonstrate that a single prenatal stimulus is sufficient to set the phases of the hamsters' rhythms and that the phase established depends on the stimulus. Both c-fos mRNA and Fos protein were expressed in the fetal SCN after SKF 38393 injection but neither were expressed after melatonin injection. Simulations showed that a single stimulus could produce the observed synchrony from a population of uniformally distributed phases if the phase shifts were three to four times the magnitude of the adult hamster light phase response curve (PRC). A light pulse PRC mimicked the effect of an SKF 38393 injection and a dark-pulse PRC mimicked the effects of a melatonin injection. Together these results suggest that dopamine and melatonin either are, or mimic, maternal entraining signals that represent day and night.