Ontogeny of light-induced Fos-like immunoreactivity in the hamster suprachiasmatic nucleus

Light induction of Fos within the Syrian hamster suprachiasmatic nucleus (SCN) occurred first at postnatal day 4. The number of cells with light-induced Fos-like immunoreactivity (Fos-LI) per unit volume of SCN increased with age. Blinding experiments were used to demonstrate that the eye, though possessing an immature retina, appears to be necessary for light induction of Fos. In neonatal hamsters, environmental cycles (e.g., light and darkness) may be able to reinforce the effect of maternal melatonin in synchronizing the pup's clock.