Light Enhances Learned Fear: From Photoreceptor, to Brian, to Behavior

Light modulates behavior and physiology in numerous ways. Among the many effects of light are modulation of defensive behavior, emotion, and cognition. In mammals, the sensation of light occurs exclusively via the three photoreceptor classes of the retina: the rods, the cones, and the melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs). The actual behavioral and physiological outputs of light are mediated downstream of the retina in central brain regions. The amygdala, a central region critical for learning, memory, defensive behavior, and emotion, is downstream of the retina by both monosynaptic and polysynaptic pathways. Here we have used tone-cued fear conditioning to investigate the effects of light on learning, memory, and defensive behavior in mice. Specifically, we have studied the effect of light on the acquisition, recall, and expression of learned fear in C57BL/6J mice. Our studies have shown that light enhances learned fear in these mice, and that this effect is specific to a learned cue. This represents a novel effect of light on mammalian learning and behavior. Furthermore, using several photoreceptor mutants, we have investigated the photoreceptors responsible for the light-dependent enhancement of learned fear. Our experiments have revealed that the rods and/or cones are the primary drivers of this effect, and that the role of melanopsin and the ipRGCs is minimal. Finally, using immunohistochemistry in mouse brain slices we have investigated the central pathways mediating the effect of light on learned fear. Our results show an enhancement of activity in the anterior basolateral amygdala of mice fear conditioned in light, pointing to the involvement of this region in the effects of light on learned fear. Our work has extended 3 the ever-growing list of biological photoeffects, and expanded our understanding of the mechanisms by which light modulates learning, memory, and defensive behavior.

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