Light pulses suppress responsiveness within the mouse photic entrainment pathway

We have characterized a decrease in photic responsiveness of the mammalian circadian entrainment pathway caused by light stimulation. Phase delays of the running-wheel activity rhythm were used to quantify the photic responsi veness of the circadian system in mice (C57BL/6J). In an initial experiment , the authors measured the responsiveness to single "saturating" light puls es ("white" fluorescent light; approximate to 1876 mu W; 15 min). In two ad ditional experiments, the authors measured responses to this stimulus at se veral time points following a saturating pulse at CT 14 or CT 16. Data from these experiments were analyzed in two manners. Experiment 2 was analyzed assuming that the phase of the circadian pacemaker was unchanged by an init ial pulse, and Experiment 3 was analyzed assuming that the initial pulse in duced an instantaneous phase delay. Results reveal a significant reduction in responsivity to light that persists for at least 2 h and possibly up to 4 h after the initial stimulus. Immediately after the stimulus, the respons iveness of the photic entrainment pathway was reduced to levels less than o r equal to 12% of normal. After 2 h, the responsiveness was less than or eq ual to 42% of normal, and by 4 h, responsiveness had recovered to levels th at were less than or equal to 60% of normal (levels not statistically diffe rent from controls). By the following circadian cycle, responsiveness was m ore completely recovered, although the magnitude of some phase delays remai ned less than or equal to 85% of normal. These major reductions in the magn itude of phase delays land phase response curve amplitude) caused by satura ting light pulses confound interpretations of two-pulse experiments designe d to measure the rate of circadian phase delays. In addition, the time cour se for this reduced responsiveness may reflect the time course of cellular and molecular events that underlie light-induced resetting of the mammalian circadian pacemaker.