Abstract
Purpose :
Optic neuropathies are the leading cause of irreversible blindness, characterized by degeneration of retinal ganglion cells (RGCs) and their axons comprising the optic projection. Not all RGC types appear equally susceptible to disease or injury. Cells with sustained firing to light decrement (OFF-S) appear to be more susceptible than those firing to light increment (ON-S), which may be due to intrinsic physiologic properties. Here, we evaluate if action potential (AP) shape reflects differences in firing dynamics and susceptibility to stress.
Methods :
C57Bl6/J mice were sacrificed, and retinas dissected in the dark. Retinas were perfused with glucose-supplemented Ames’ medium. Recording pipettes were filled with K-gluconate based solution and Alexa-fluor dye. Whole-cell current clamp signals were amplified and digitized. RGCs were classified by light response and dendritic stratification. 41 ON-S and 34 OFF-S RGCs were recorded during stepwise depolarizing current injections from 0-300pA. An additional 7 ON-S and 6 OFF-S were recorded before and after adding 5mM KCl to the extracellular medium.
Results :
OFF-S RGCs had more depolarized resting potentials (p < 0.001), higher spontaneous spike rates (p < 0.001), and faster spiking to current injections (p < 0.001). A plateau of the current-spike rate relationship for OFF-S began at 200pA of current injection, indicating depolarization block, but did not occur at baseline for ON-S. APs widened with increased depolarization for both cells, but more appreciably in OFF-S. However, plots of spike rate vs. measures of AP shape (area, depolarization and repolarization rates) had similar slopes for both RGC types (p range 0.17-0.38). Adding 5mM KCl to the extracellular medium induced depolarization block in ON-S RGCs at 200pA. AP shape analysis showed that K+ significantly altered the repolarization rate of ON-S (p < 0.001), and that depolarization block in ON-S + K occurred at similar repolarization rate values as baseline OFF-S RGCs.
Conclusions :
Increasing firing rate causes a widening of RGC APs. OFF-S RGCs undergo depolarization block at lower current injections at baseline, but it can be experimentally induced in ON-S. Analyses of AP shape vs. spike rate demonstrate that differences correlate with rate and occur at similar thresholds for both cell types, suggesting that mechanisms underlying depolarization block are cell type-agnostic.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.