Abstract
Purpose :
Intrinsically photosensitive retinal ganglion cells (ipRGCs) of the M1 type are principal regulators of the circadian clock and other non-image visual functions. These cells diversify their intrinsic light responses to encode a broad range of environmental irradiances as a population. Here, we test the hypothesis that this intrinsic, functional diversity combines with circuit influences to produce effective irradiance encoding.
Methods :
We recorded electrophysiologically from M1 ipRGCs in the dark-adapted, isolated retinas of adult, transgenic mice. These cells were labeled genetically or retrogradely from their projections in the suprachiasmatic nucleus, the master circadian clock. We used optical stimulation and pharmacological manipulation to characterize the contributions of intrinsic and synaptic drives to cellular activity.
Results :
Our experiments indicate that the synaptic input to M1 ipRGCs is highly diverse in both sensitivity and dynamics. The most sensitive response in all M1 ipRGCs examined was synaptic in origin. This synaptic input was required for spike firing of some cells but dispensable for that of many others. This spike firing exhibits features that are advantageous for irradiance encoding. In particular, it encodes the total photon count with fidelity over an orders-of-magnitude variation in time scale. This integrative ability is evident for indivual cells that have varying balances of synaptic and intrinsic drive, and is more precise at the population level.
Conclusions :
Synaptic and intrinsic drives vary across M1 ipRGCs to produce population activity that is suitable for representing irradiance. Temporal integration, a hallmark of circadian photoregulation, is already present in the set of cells that carry light information to the clock.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.