A Retinal Ganglion Cell That Can Signal Irradiance Continuously for 10 Hours

Aug 17, 2012The Journal of neuroscience : the official journal of the Society for Neuroscience

A retinal ganglion cell that can continuously signal light levels for 10 hours

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Abstract

ipRGCs can signal light continuously for at least 10 hours.

These retinal ganglion cells encode environmental light intensity and play a role in non-image-forming visual behaviors.
They generate responses driven by both intrinsic mechanisms and external input from rods and cones.
Action potentials in ipRGCs were evoked continuously under various light stimulation conditions for 10 hours.
During gradual changes in light intensity simulating sunrise and sunset, ipRGC firing rates increased and decreased accordingly.
Melanopsin-knock-out mice retained the ability to respond to light, suggesting that melanopsin is not essential for sustained responses.

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A recently discovered type of mammalian retinal ganglion cell encodes environmental light intensity and mediates non-image-forming visual behaviors, such as the pupillary reflex and circadian photoentrainment. These intrinsically photosensitive retinal ganglion cells (ipRGCs) generate endogenous, melanopsin-based photoresponses as well as extrinsic, rod/cone-driven responses. Because the ipRGCs' light responses and the behaviors they control are both remarkably tonic, these cells have been hypothesized to be capable of irradiance detection lasting throughout the day. I tested this hypothesis by obtaining multielectrode-array recordings from ipRGCs in a novel rat eyecup preparation that enhances the regeneration of rod/cone photopigments. I found that 10 h constant light could continuously evoke action potentials in these ganglion cells under conditions that stimulated (1) only melanopsin, (2) mainly the rod input, and (3) both intrinsic and extrinsic responses. In response to a 10 h stimulus with gradual intensity changes to simulate sunrise and sunset, ipRGC firing rates slowly increased during the "sunrise" phase and slowly decreased during the "sunset" phase. Furthermore, I recorded from putative ipRGCs of melanopsin-knock-out mice and found that these cells retained the ability to respond in a sustained fashion to 20 min light steps, indicating that melanopsin is not required for such tonic responses. In conclusion, ipRGCs can signal light continuously for at least 10 h and can probably track gradual irradiance changes over the course of the day. These results further suggest that the photoreceptors and ON bipolar cells presynaptic to ipRGCs may be able to respond to light continuously for 10 h.

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A Retinal Ganglion Cell That Can Signal Irradiance Continuously for 10 Hours

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