Proceedings of The Physiological Society

Sleep Sleep and Circadian Rhythms (London, UK) (2018) Proc Physiol Soc 42, C11

Poster Communications

The impact of colour on circadian photoentrainment in mice.

J. Mouland1, F. Martial1, T. Brown1, R. Lucas1

1. University of Manchester, Manchester, United Kingdom.

The large daily changes in ambient illumination associated with the earth's rotation are a major source of timing information for the mammalian circadian clock. These ‘irradiance' signals are encoded via the integration of extrinsic (rod) and intrinsic (melanopsin) photoreceptive signals in the retinal ganglion cells (ipRGCs) that innervate the suprachiasmatic nucleus (SCN). By contrast, cone photoreception makes minimal contributions to this process, despite the appearance of cone photoreceptive signals at the level of ipRGCs and the SCN. Instead cones may play at least one alternative function: to provide information about daily changes in the colour of ambient illumination. We previously showed that a subset of SCN neurons exhibit chromatic responses and that naturalistic changes in colour influence phase of entrainment. Here we more thoroughly investigate the contribution of chromatic information to the entrainment mechanism. Human-cone knockin mice (Opn1mwR) were used in conjunction with polychromatic lighting environments, to allow us to experimentally isolate cone signals. Animals were subjected to a variety of entrainment paradigms (constant light, shifted LD cycles, and brief light pulses) whilst locomotor activity was assessed using running wheels, or a passive infrared system. We observed that stimuli of identical irradiance (melanopsin, rod and net cone flux) but different colour (ratio of L cone to S cone activation) differentially influenced circadian responses in a paradigm dependent manner. For example, while responses to discrete light pulses were independent of colour, the period lengthening effect of constant light was significantly greater when L cone signals ‘yellow' were dominant (as in during natural daylight). These data therefore support the view that chromatic information supplied by cones influences entrainment and challenge the popular assumption that the circadian system is especially sensitive to ‘blue' light.

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