ULTRAVIOLET-LIGHT ENTRAINS RODENT SUPRACHIASMATIC NUCLEUS PACEMAKER

It has long been assumed that, in contrast to other vertebrates, mamma ls are ultraviolet blind.(12) Recent evidence indicates, however, that the spectral sensivity of the retina in rodents extends into the ultr aviolet range(14-16) This finding, combined with reports that ultravio let light can suppress nocturnal melatonin release(2) and reverse the effect of short photoperiod on the gonads,(3) invites speculation abou t the role of ultraviolet light in photoperiodic control of physiologi cal and behavioral functions, One idea is that ultraviolet light parti cipates in retinally mediated processes underlying photic entrainment of a pacemaker located in the hypothalamic suprachasmatic nucleus that generates circadian rhythms.(29) Consistent with such a function,,ve now show that ultraviolet light is capable of inducing phase shifts in circadian rhythms in the rat and of inducing in the hypothalamic supr achiasmatic nucleus expression of the transcription factor Fos, a know n cellular correlate of Light-induced phase shifts of the hypothalamic suprachiasmatic nucleus pacemaker.(6,18,28,30)