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

The ontogeny of photic responsiveness in the suprachiasmatic nucleus of the Siberian hamster (Phodopus sungorus) was studied using the enhanced expression of the immediate early gene c-fos as a marker of neuronal activation. c-fos expression was assessed by immunocytochemical localization of its protein product. Hamsters were kept on a 16 h light:8 h dark photocycle. The adult Siberian hamster showed a marked increase in the number of c-fos-immunoreactive (c-fos-ir) cells within the suprachiasmatic 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. This is consistent with previous studies in the Syrian hamster and rat, The development of the photic response was examined. The first study investigated 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 region was identified by immunocytochemical localization of peptide-histidine-isoleucine in adjacent sections, a peptide expressed early in the development of the rodent SCN. The distribution of c-fos-ir cells was also 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 of c-fos-ir cells occurred in the light pulsed animals in comparison to age-matched control animals which were moved within the non-illuminated room to provide a 'dark' pulse. Induction of c-fos mainly occurred in the ventrolateral region of the medial and caudal SCN, the region which receives the greatest density of retinal innervation. The second study focused on the earliest age at which light could induce c-fos expression 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 expression in the ventrolateral region, whilst PD 2 animals showed no response. The number of c-fos-ir cells in the SCN region in light-pulsed animals increased from PD 3 to adulthood. These results indicate that 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 inputs as early as day 3 after birth.