Ultraviolet light entrains rodent suprachiasmatic nucleus pacemaker.

It has long been assumed that, in contrast to other vertebrates, mammals are ultraviolet blind. Recent evidence indicates, however, that the spectral sensitivity of the retina in rodents extends into the ultraviolet range. This finding, combined with reports that ultraviolet light can suppress nocturnal melatonin release and reverse the effect of short photoperiod on the gonads, invites speculation about the role of ultraviolet light in photoperiodic control of physiological and behavioral functions. One idea is that ultraviolet light participates in retinally mediated processes underlying photic entrainment of a pacemaker located in the hypothalamic suprachasmatic nucleus that generates circadian rhythms. Consistent with such a function, we now show that ultraviolet light is capable of inducing phase shifts in circadian rhythms in the rat and of inducing in the hypothalamic suprachiasmatic nucleus expression of the transcription factor Fos, a known cellular correlate of light-induced phase shifts of the hypothalamic suprachiasmatic nucleus pacemaker.