D1-dopamine receptor binding and tyrosine hydroxylase-immunoreactivity in the fetal and neonatal hamster suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) of the anterior hypothalamus is the site of an endogenous biological clock that regulates mammalian circadian rhythms. Circadian rhythms, although endogenously driven, are synchronized or entrained to daily environmental cues. Developmentally, the SCN begins to oscillate before birth and is entrained to the maternal circadian rhythm by a mechanism that is still unclear. Recent evidence in rats and hamsters suggests that a fetal dopaminergic system and D1-dopamine receptors may be involved in the process of entraining the fetal clock. The present study using [ 3H]SCH 23390 autoradiography and tyrosine hydroxylase (TH) immunocytochemistry determined the developmental time courses of the appearance of D1 receptors in, and catecholaminergic input to, the hamster SCN. [ 3H]SCH 23390 binding to D1-dopamine receptors was first detected in the fetal SCN on embryonic day (E) 15, the day before birth in this species, and persisted through adulthood. The TH immunoreactive fibers were first observed on day E15 coursing just ventral to the fetal SCN but TH-immunoreactive cells and fibers were not seen within the SCN until postnatal day (P) 5. The presence of D1-dopamine receptor binding in the fetal hamster SCN is consistent with the role of these receptors in entrainment of the fetal circadian pacemaker to maternal cues. However, a receptor–transmitter mismatch exists between D1-dopamine receptors and TH-immunoreactive fibers in the fetal SCN suggesting that the role of dopamine in maternal–fetal entrainment may be as a paracrine or humoral signal.