Circadian Modulation of Melanopsin-Driven Light Response in Rat Ganglion-Cell Photoreceptors

Sep 17, 2009Journal of biological rhythms

Daily rhythm affects light response in rat eye cells that detect brightness

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Abstract

Bright light triggers significantly more spiking in rat intrinsically photosensitive retinal ganglion cells (ipRGCs) early in the subjective night.

The sensitivity of intrinsic photoresponses of rat ipRGCs was measured at 4 circadian times.
There was little circadian modulation in the threshold of ipRGC photoresponses.
Increased spiking was observed in response to bright light during the early night (circadian time 12-13).
The gain of the melanopsin-driven response appears to be slightly elevated in the early night.
The modest gain change is unlikely to significantly influence the phase response curve for behavioral photoentrainment.

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Intrinsically photosensitive retinal ganglion cells (ipRGCs) project to the suprachiasmatic nucleus (SCN) and are essential for normal photic entrainment of global circadian rhythms in physiology and behavior. The effect of light on the central clock is dependent on circadian phase, and the retina itself contains intrinsic circadian oscillators that can alter its sensitivity to light. This raises the possibility that the ipRGCs, and hence the photoentraining signals in the retinohypothalamic tract, are subject to circadian modulation. Although the ipRGC photopigment melanopsin reportedly exhibits circadian variations in expression, there has been no direct test of the hypothesis that ipRGC sensitivity is under circadian control. Here, the authors provide such a test by measuring the sensitivity of intrinsic photoresponses of rat ipRGCs at 4 circadian times (CTs) using multielectrode array recording. There was little if any circadian modulation in the threshold of intrinsic ipRGC photoresponses. However, very bright light evoked significantly more spiking early in the subjective night (CT12-13) than at other circadian phases. Thus, the gain of the melanopsin-driven response is slightly increased in the early night, at roughly the circadian phase when melanopsin synthesis is thought to be elevated. However, this gain change is probably too modest to contribute much to shape the phase response curve (PRC) for behavioral photoentrainment.

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Circadian Modulation of Melanopsin-Driven Light Response in Rat Ganglion-Cell Photoreceptors

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