Abstract
Purpose :
Retinal ganglion cell (RGC) neurons are a heterogenous population with distinct synaptic circuitry and intrinsic biophysical properties underlying diverse encoding of light information. Previously we demonstrated in mouse retina that alpha ON-Sustained (ON-S) and alpha OFF-Sustained (OFF-S) RGCs have distinct excitabilities related to differences in sensitivity to extracellular potassium (K+). In glaucomatous optic neuropathy (glaucoma), RGCs are stressed by disrupted K+ homeostasis and exhibit altered excitability. Here, we assess how elevated intraocular pressure (IOP) alters RGC sensitivity to elevated K+.
Methods :
We used bilateral microbead occlusion to elevate intraocular pressure (IOP) in C57Bl6/J mice (n=5 IOP, 10 saline control) for four weeks, sufficient time for axonopathy based on our published work. Retinas were dissected and perfused with Ames’ medium. Pipettes were filled with K-gluconate solution and Alexa555 dye to establish morphology and conduct whole-cell current clamp recording. We classified RGCs by light response, dendrite stratification, and soma size. RGC activity was recorded during stepwise current injection from 0-300pA, before and after adding extra 5mM K+ to extracellular medium (high K+). Retinas were labeled for ankyrin-G to define axon initial segment (AIS) dimensions.
Results :
We found that a 43% IOP elevation over controls depolarized RGC resting membrane potentials (RMPs; p=0.057) and decreased light response in ON-S (p=0.020) but not OFF-S RGCs. IOP elevation reduced current-evoked spiking (p<0.0001, ON-S; p=0.064, OFF-S) and increased action potential (AP) width for ON-S (p=0.013) but not OFF-S (p=0.687) cells. High K+ depolarized RMPs (p<0.0001), but RGCs exposed to elevated IOP were less depolarized by high K+ (p=0.0002, αON-S; p=0.006, αOFF-S). Likewise, current-evoked spiking was less blunted by high K+ (p=0.032), and there was less K+-induced AP widening (p=0.006). These changes were not due to differences in AIS distance from the soma (p=0.319) or length (p=0.601).
Conclusions :
Our results indicate extended IOP elevation reduces intrinsic RGC excitability and sensitivity to acutely elevated K+. Differences are related to AP generation but not solely reflective of AIS structural plasticity. This may represent an adaptive process, whereby prolonged stress in glaucoma alters RGC excitability to reduce energetic demand and slow degenerative processes that lead to vision loss.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.