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Research by Lawrence Morin, PhD and Keith Studholme Advances Knowledge of Nonvisual Effects of Light on Animals

Lawrence Morin, PhD

September 22, 2014 - Professor Lawrence Morin, PhD and Keith Studholme continued their quest for a fuller understanding of the nonvisual effects of light on animals by demonstrating that suppression of activity and circadian rhythm phase shifts are regulated differentially when mice are exposed to light of various durations. The results of their study were published in the Journal of Biological Rhythms.

Previous research has shown that shining light on mice during their normal nocturnal active phase causes them to stop moving and fall asleep, and produces a shift in circadian rhythm phase. In a previous paper, Dr. Morin argued that these effects should be studied not in isolation but as part of a integrated biological system involving a common light-processing pathway terminating in the suprachiasmatic nucleus (SCN). In this paper, the researchers add nuance to their thesis by offering evidence to suggest that exposure to light regulates suppression of activity differently from the way it regulates circadian rhythm phase shifts.

By exposing mice to light of various durations (30 seconds, 5 minutes, 20 minutes, 50 minutes) and measuring subsequent changes in activity levels during the ensuing 10 days, the researchers demonstrated that shorter exposures generally yielded smaller responses and longer exposures yielded larger responses. Variations in the duration of exposure to light, however, affected suppression of activity and phase shift in different ways.

A 30-second exposure to light produced the same effect on suppression of activity as an exposure of 5 minutes, although a longer exposure to light (20 minutes or greater) further increased suppression of activity; while a 5-minute exposure produced a significantly larger effect in the magnitude of phase shift than a 30-second exposure, but a exposure of more than 5 minutes had no additional effect.

These findings imply differences in how light energy is processed to produce the two effects. The authors suggest that although there is one input pathway through which light suppresses nocturnal activity, induces sleep and causes circadian rhythm phase shifts, the differences in response reported in this paper “imply the existence of mechanisms within the input pathway that differentially govern any of the several functions modulated by photic stimulation.” This is not unexpected since changes in phase shift are intrinsic to the SCN while the other responses involve neural circuitry regulated by the SCN but external to it.

The article, titled “Light Pulse Duration Differentially Regulates Mouse Locomotor Suppression and Phase Shifts” was published in the September 17, 2014 issue of the Journal of Biological Rhythms. Dr. Morin is Director of the Chronobiology Laboratory in the Department of Psychiatry at Stony Brook University.