Abstract
Purpose :
Mammals sense light primarily with rods, cones, and intrinsically photosensitive retinal ganglion cells (ipRGCs). IpRGCs support a variety of visual processes, one of the most important being synchronization of the circadian clock with the solar day. These neurons provide practically all retinal input to the master clock, the suprachiasmatic nucleus (SCN; located in the hypothalamus). The clock responds to irradiance, the overall intensity of illumination, rather than image detail. We hypothesized that ipRGCs send an effective representation of irradiance to the master clock.
Methods :
To preserve natural ipRGC responses and obtain a high level of experimental control, we used an ex vivo, acute brain slice that contained most of the SCN and retained intact connectivity with both retinas (Wong, Graham, and Berson 2007 J Biol Rhythms). We made these explants from adult mice in which ipRGC axons and presynaptic terminals expressed an optical reporter of activity. We imaged this reporter with multiphoton microscopy while delivering visual stimuli to the retinas.
Results :
The signals conveyed from ipRGCs to the SCN are complex and dynamic. IpRGC terminals are differentially tuned to light intensity, with distinct subsets activated across the environmental range. The most common terminal is silent when the intensity is below or above its preferred range. Terminals also vary in their response kinetics, with some remaining activated long after the period of illumination. Terminals with different tunings and response kinetics are intermingled within the volume of the SCN. Finally, the retina-SCN explant is robust, with light responses remaining for at least 24 hours after preparation.
Conclusions :
IpRGC signals within the SCN appear to support an effective representation of irradiance. By preferring different irradiance ranges, each ipRGC encodes its range at high resolution and the population of ipRGCs collectively encodes a broad range. By silencing outside of their preferred ranges, ipRGCs potentially save energy. By producing persistent responses, ipRGCS smooth temporal contrast to favor the representation of ambient illumination rather than image detail. Moreover, the full spectrum of ipRGC signals appears to be repeated throughout the SCN and is thus broadly available to postsynaptic neurons.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.