Light induces chromatin modification in cells of the mammalian circadian clock

The mammalian circadian clock resides in neurons of the hypothalamic suprac hiasmatic nucleus (SCN). Light entrains phase resetting of the clock using the retino-hypothalamic tract, via release of glutamate. Nighttime light ex posure causes rapid, transient induction of clock and immediate-early genes implicated in phase-shifting the pacemaker. Here we show that a nighttime light pulse caused phosphorylation of Ser10 in histone H3's tail, in SCN cl ock cells. The effect of light was specific, and the kinetics of H3 phospho rylation were characteristic of the early response, paralleling c-fos and P er1 induction. Using fos-lacZ transgenic mice, we found that H3 phosphoryla tion and Fos induction occurRed in the same SCN neurons. Systemic treatment with the GABA(B) receptor agonist baclofen prevented light-induced c-fos a nd Per1 expression and H3 phosphorylation, indicating that one signaling pa thway governs both events. Our results suggest that dynamic chromatin remod eling in the SCN occurs in response to a physiological stimulus in vivo.