DEFINITION OF THE DEVELOPMENTAL TRANSITION FROM DOPAMINERGIC TO PHOTIC REGULATION OF C-FOS GENE-EXPRESSION IN THE RAT SUPRACHIASMATIC NUCLEUS

D-1-dopamine receptor stimulation induces c-fos gene expression in the fetal suprachiasmatic nucleus (SCN), but not in the adult rat SCN. Li ght exposure at night induces c-fos gene expression in the adult SCN. The present experiments were designed to define the developmental tran sition from dopaminergic to photic regulation of c-fos gene expression in the SCN. Treatment with the D-1-dopamine receptor agonist SKF 3839 3 (10 mg/kg) increased SCN c-fos gene expression during both day and n ight on postnatal day (PD) 0, 1, and at night on PD 2, but the c-fos r esponse disappeared by PD 4. Photic induction of c-fos gene expression was apparent during both day and night at each age examined, from PD 0 through PD 6. The magnitude and distribution of c-fos expression fol lowing light during the daytime was distinguishable from the response to light at night beginning on PD 2, indicating that the circadian clo ck regulates (gates) the c-fos response by PD 2. Orbital enucleation p revents the induction of c-fos by light at night on PD 2, indicating r etinal mediation. Developmental loss of the c-fos response to SKF is n ot precipitated by the arrival of the retinohypothalamic tract; animal s enucleated on PD 0 were insensitive to SKF on PD 6, as were visually intact controls. The results demonstrate that both dopaminergic and p hotic inputs can regulate SCN c-fos gene expression early in the neona tal period, and that the developmental loss of sensitivity to SKF is n ot due to the arrival of photic input to the SCN. The developmental tr ansition from dopaminergic to photic regulation of c-fos gene expressi on roughly parallels the developmental transition from maternal to pho tic entrainment of the developing biological clock.