Light-induced variations in AP-1 binding activity and composition in the rat suprachiasmatic nucleus

Expression of immediate early genes, including fos-like and jun-like genes, in the suprachiasmatic nucleus is believed to be part of the mechanism for photic entrainment of circadian rhythms to the environmental light/dark cy cle. However, the effects of a light stimulus on activating protein-1 (AP-1 ) complexes in the suprachiasmatic nucleus remain unclear. The photic regul ation of AP-1 DNA-binding activity and composition in the rat suprachiasmat ic nucleus was evaluated by using an electrophoretic mobility shift assay. A light pulse given during subjective night induced an increase in AP-1 bin ding activity when either nuclear or whole-cell extracts from suprachiasmat ic nuclei were used. Under constant dark conditions, proteins that are pred ominant components of AP-1 complexes are Fra-2 and Jun-D. Under light stimu lation, c-Fos and Jun-B consistently increased, as expected, but this was a lso the case for Fra-2, Jun-D, and c-Jun, although to a lesser extent. An i mmunocytochemical study of the Fra-2 expression pattern demonstrated the pr esence of the protein in the ventrolateral as well as in the dorsomedial su bdivisions of the suprachiasmatic nucleus. Light regulation of Fra-2 immuno reactivity, however, appeared to be restricted to the ventrolateral subdivi sion. It is concluded that light may be acting both by increasing constitut ive AP-1 complexes and by inducing the expression of specific complexes.