The activity-regulated cytoskeleton-associated (Arc) gene is a new light-inducible early gene in the mouse suprachiasmatic nucleus.

The mammalian circadian system is entrained to the environmental light/dark cycle by shifting the phase of the master clock in the suprachiasmatic nucleus. Accompanying the light-induced phase-shift, a variety of immediate-early genes appears in suprachiasmatic nucleus clock cells, and here, we report the expression of a new immediate-early gene Arc (activity-regulated cytoskeleton-associated gene) in mice. Arc messenger RNAs were strongly induced at 30-120 min after the light exposure at subjective night (CT12-CT20) in neurons of the retinorecipient area of the suprachiasmatic nucleus, although their spontaneous expression was absent in usual light-dark cycles and in constant dark conditions. At protein level, ARC appeared not only in the nucleus but also in the perikarya and their processes of the suprachiasmatic nucleus neurons. These findings indicate that Arc is an activity-regulated cytoskeletal gene possibly involved in the light-induced phase-shift of the circadian rhythm.