Transient entrainment of a circadian pacemaker during development by dopaminergic activation in Syrian hamsters.

Maternal cues entrain a circadian pacemaker in fetal Syrian hamsters. These cues may act through dopaminergic activation of the fetal suprachiasmatic nucleus (SCN); injection of the dopamine D1 agonist SKF38393 to pregnant hamsters entrains activity rhythms of their pups and induces expression of c-fos in the fetal SCN. The aim of this study was to examine the ability of SKF38393 to entrain neonatal Syrian hamsters and to determine the age at which this effect is lost. SKF38393 injections given to two groups of pups at opposite times of day on postnatal days (PN) 1-5 entrained the pups' activity rhythms to average phases that differed by 9.25 h. SKF38393 failed to establish different average phases when given on PN 6-10. Injection of SKF38393 on PN 1, but not PN 6, induced expression of Fos. These results demonstrate that dopaminergic activation is a potent entraining stimulus in neonatal hamsters and that its entraining effects, as well as its ability to induce Fos, are lost by PN 6. The phase established by dopaminergic activation was approximately opposite to that previously shown to be established by melatonin injections. Dopaminergic activation and melatonin may mimic separate but complementary maternal entraining signals which represent day and night.