Circadian and photic regulation of immediate-early gene expression in the hamster suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus is the site of an endogenous circa dian clock synchronized by daily light-dark cycles. At some daily phases, l ight exposure both shifts the clock and alters the expression of several im mediate-early genes in cells of the suprachiasmatic nucleus. We have studie d both spontaneous circadian and light-induced expression of several immedi ate-early gene messenger RNAs and proteins in hamsters in constant darkness or in response to brief light exposure. There was no detectable spontaneou s expression of NGFI-A messenger RNA in suprachiasmatic nucleus cells at an y circadian phase, but light pulses induced its expression selectively duri ng the subjective night, with highest levers of expression 6 h into the nig ht. We also found that there are two independent rhythms of expression of j unB messenger RNA and JunB protein, as well as c-fos messenger RNA and c-Fo s protein, in the suprachiasmatic nucleus of hamsters: a rhythm of photic s ensitivity expressed throughout the night and a spontaneous rhythm of expre ssion triggered around dawn. Induction of NGFI-A messenger RNA and c-Sos me ssenger RNA. and c-Fos protein in response to a light pulse were found thro ughout the suprachiasmatic nucleus, with the highest levels of expression i n the ventrolateral subdivision; however, the spontaneous expression of Jun B and c-Fos proteins was confined mainly to the dorsomedial suprachiasmatic nucleus. The temporal and anatomical differences in the expression of these immediat e-early genes in the mammalian suprachiasmatic nucleus suggest that their p rotein products may be involved in different signaling mechanisms mediating either photic entrainment or endogenous oscillations within distinct subpo pulations of suprachiasmatic nucleus cells. (C) 1999 IBRO. Published by Els evier Science Ltd.