LIGHT-INDUCED RESETTING OF A MAMMALIAN CIRCADIAN CLOCK IS ASSOCIATED WITH RAPID INDUCTION OF THE MPER1 TRANSCRIPT

To understand how light might entrain a mammalian circadian clock, we examined the effects of light on mPer1, a sequence homolog of Drosophi la per, that exhibits robust rhythmic expression in the SCN.mPer1 is r apidly induced by short duration exposure to light at levels sufficien t to reset the clock, and dose-response curves reveal that mPer1 induc tion shows both reciprocity and a strong correlation with phase shifti ng of the overt rhythm. Thus, in both the phasing of dark expression a nd the response to light mPer1 is most similar to the Neurospora clock gene fro. Within the SCN there appears to be localization of the indu ction phenomenon, consistent with the localization of both light-sensi tive and light-insensitive oscillators in this circadian center.