ONTOGENY OF A PHOTIC RESPONSE IN THE SUPRACHIASMATIC NUCLEUS IN THE SIBERIAN HAMSTER (PHODOPUS-SUNGORUS)

The ontogeny of photic responsiveness in the suprachiasmatic nucleus o f the Siberian hamster (Phodopus sungorus) was studied using the enhan ced expression of the immediate early gene c-fos as a marker of neuron al activation. c-fos expression was assessed by immunocytochemical loc alization of its protein product. Hamsters were kept on a 16 h light:8 h dark photocycle. The adult Siberian hamster showed a marked increas e in the number of c-fos-immunoreactive (c-fos-ir) cells within the su prachiasmatic nuclei (SCN) in response to a 1 h light pulse delivered 1-3 h after lights off, in comparison to controls kept in the dark. Th is is consistent with previous studies in the Syrian hamster and rat, The development of the photic response was examined. The first study i nvestigated the effects of a light pulse on c-fos induction in pups at 5, 9, 12 and 24 postnatal days of age (PD). The suprachiasmatic regio n was identified by immunocytochemical localization of peptide-histidi ne-isoleucine in adjacent sections, a peptide expressed early in the d evelopment of the rodent SCN. The distribution of c-fos-ir cells was a lso compared with the location of retinal efferents, as determined by intraocular injection of the tract tracer cholera toxin B subunit 24 h previously. At PD 9, 12 and 24, significant increases in the number o f c-fos-ir cells occurred in the light pulsed animals in comparison to age-matched control animals which were moved within the non-illuminat ed room to provide a 'dark' pulse. Induction of c-fos mainly occurred in the ventrolateral region of the medial and caudal SCN, the region w hich receives the greatest density of retinal innervation. The second study focused on the earliest age at which light could induce c-fos ex pression in the SCN by investigating the effects of light on c-fos-ir cells in pups at PD 2-4. Animals at PD 3 and 4 showed enhanced c-fos e xpression in the ventrolateral region, whilst PD 2 animals showed no r esponse. The number of c-fos-ir cells in the SCN region in light-pulse d animals increased from PD 3 to adulthood. These results indicate tha t retinal input can activate cells in the SCN even before the eyelids open on PD 11, and the SCN can potentially be entrained by photic inpu ts as early as day 3 after birth.