Abstract
Purpose :
Persistent activity in the nervous system supports tasks that range from working memory to the encoding of eye position. Within the retina, it is a prominent feature of intrinsically photosensitive retinal ganglion cells (ipRGCs), which are vital for processes that include the regulation of circadian rhythms and sleep. IpRGCs respond directly to light using a receptor called melanopsin. Melanopsin’s active state is stable and drives phototransduction for minutes even after illumination ceases. Thus, successive periods of illumination can increment the number of activated melanopsin molecules and the rate of persistent spike firing. Because small currents have large impacts on ipRGCs, the existence of graded persistent firing suggests a mechanism of precise control. We tested the hypothesis that persistent activity is controlled by negative feedback in melanopsin phototransduction.
Methods :
We focused on ipRGCs of the M1 type, which are principal regulators of the master circadian clock and exhibit large, melanopsin-driven responses that are amenable to quantitative analysis. We identified these neurons in the ex vivo mouse retina using a fluorescent reporter and isolated their intrinsic responses with antagonists of synaptic transmission. We made patch-clamp electrophysiological recordings in voltage clamp and current clamp to examine the melanopsin-driven photocurrent and voltage change, respectively. We delivered optical stimuli to probe persistent responses and negative feedback. We altered negative feedback by manipulating Ca2+, one of its mediators.
Results :
We found that persistent photocurrent drove negative feedback, which was evident in response desensitization. When we suppressed the persistent photocurrent, by delivering light that depopulates melanopsin’s active state, sensitivity increased for a subset of M1 ipRGCs. Acute removal of extracellular Ca2+, which reduces negative feedback, increased the persistent photocurrent in a reversible manner.
Conclusions :
Collectively, our experiments indicate that persistent activity in M1 ipRGC phototransduction recruits and is subject to negative feedback. The latter is mediated in part by Ca2+ influx. The balance of persistent activation and negative feedback varies across cells. In some cells, delivering light to suppress persistent activation produces a net increase in sensitivity, despite negative feedback produced by the light itself.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.