UC San Diego

Light entering the mammalian eye activates a neural pathway independent of the classical visual system, serving to regulate circadian rhythms via input to the central pacemaker in the suprachiasmatic nuclei (SCN) of the hypothalamus. Various factors affect light regulation of the circadian system, including the physical properties of the light stimulus and the position of the light stimulus relative to the eyes. Photic response is also markedly altered by prior light exposure and thus, the non -image forming visual system represents an attractive model of neural plasticity. Seasonal changes in the duration of daylight (i.e. photoperiod) provide a rich example of how photic history can influence circadian regulation by light. Specifically, previous entrainment to a short winter-like photoperiod results in an approximately two-fold greater maximal phase shift as compared to relatively longer summer days (Goldman and Elliott, 1988; Evans et al., 2004). The work presented here is the first to characterize sensitivity to light as a function of photoperiod history. Indeed, there is a 40- fold increase in photic sensitivity for phase advancing of activity rhythms in Syrian hamsters previously entrained to shorter days, refining our understanding of how seasonal changes in photoperiod modulate circadian response to light. Additional experiments test the effects of photoperiod on various measures associated with non- visual, light-mediated responses, including: circadian phase delays, light sampling behavior, light-induced melatonin suppression and photic-induction of proteins in the SCN. Collectively, these projects further identify the level of regulation by which photoperiod modulates photic sensitivity of the circadian system