Sensitivity to extracellular potassium underlies type-intrinsic differences in retinal ganglion cell excitability

Abstract

Neuronal type-specific physiologic heterogeneity can be driven by both extrinsic and intrinsic mechanisms. In retinal ganglion cells (RGCs), which carry visual information from the retina to central targets, evidence suggests intrinsic properties shaping action potential (AP) generation significantly impact the responses of RGCs to visual stimuli. Here, we explored how differences in intrinsic excitability further distinguish two RCG types with distinct presynaptic circuits, alpha ON-sustained (αON-S) cells and alpha OFF-sustained (αOFF-S) cells. We found that αOFF-S RGCs are more excitable to modest depolarizing currents than αON-S RGCs but excitability plateaued earlier as depolarization increased (i.e., depolarization block). In addition to differences in depolarization block sensitivity, the two cell types also produced distinct AP shapes with increasing stimulation. αOFF-S AP width and variability increased with depolarization magnitude, which correlated with the onset of depolarization block, while αON-S AP width and variability remained stable. We then tested if differences in depolarization block observed in αON-S and αOFF-S RGCs were due to sensitivity to extracellular potassium. We found αOFF-S RGCs more sensitive to increased extracellular potassium concentration, which shifted αON-S RGC excitability to that of αOFF-S cells under baseline potassium conditions. Finally, we investigated the influence of the axon initial segment (AIS) dimensions on RGC spiking. We found that the relationship between AIS length and evoked spike rate varied not only by cell type, but also by the strength of stimulation, suggesting AIS structure alone cannot fully explain the observed differences RGC excitability. Thus, sensitivity to extracellular potassium contributes to differences in intrinsic excitability, a key factor that shapes how RGCs encode visual information.

Retinal ganglion cells adapt to ionic stress in experimental glaucoma

“…Since potassium homeostasis may be altered in glaucoma ( Fischer et al, 2019a , b ), we next sought to investigate how 4 weeks of IOP elevation alters the sensitivity of RGCs to acutely elevated extracellular potassium. As previously described ( Boal et al, 2022 ), we performed a within-subjects experimental design with recordings before and after application of extracellular medium containing additional KCl (extra 5 mM, high K + , Figure 3A ). For both αON-S and αOFF-S high K + depolarized the RMP, regardless of experimental group ( Figure 3B , p < 0.0001 for both cell types).…”

“…We obtained 15 C57Bl6/J mice (8 males, 7 females, 12–20 weeks old) from Jackson Laboratories (Bar Harbor, ME). These numbers were determined, based upon our previous experience with this model and recording strategy ( Risner et al, 2018 , 2021 ; Boal et al, 2022 ), to provide a sufficient number of each cell type for statistical comparisons. Mice were housed at the Vanderbilt University Division of Animal Care and maintained on 12-h light/dark cycle.…”

“…After 4 weeks mice were sacrificed and retinas prepared for electrophysiologic recordings. As previously described (Risner et al, 2018(Risner et al, , 2020a(Risner et al, , 2021(Risner et al, , 2022Boal et al, 2022), we targeted αRGCs for recording by identifying large cell bodies and confirmed cell types by characterizing soma size, dendritic stratification within the inner plexiform layer (Famiglietti and Kolb, 1976;Galli-Resta et al, 2000), and light-evoked responses (Figure 2). We focused analysis on two well-characterized and readily identifiable αRGC types: αON-Sustained (αON-S) and αOFF-Sustained (αOFF-S; Krieger et al, 2017).…”

“…Importantly, such intrinsic differences may predispose certain RGC types to be particularly sensitive to IOP-related stress ( Della Santina et al, 2013 ; El-Danaf and Huberman, 2015 ; Ou et al, 2016 ; Risner et al, 2021 ). Previously, we established that different RGC subtypes exhibit varied sensitivities to elevated extracellular potassium ( Boal et al, 2022 ). Dysregulation of potassium ion (K + ) homeostasis and channel expression contribute to altered excitability in neurodegenerative diseases, including glaucoma, and represent potential targets for early diagnosis and treatment ( Hall et al, 2015 ; Frazzini et al, 2016 ; Cacace et al, 2019 ; Fischer et al, 2019a , b ; Kim et al, 2021 ).…”

“…IOP elevation due to microbead occlusion of the anterior chamber. We previously found differences in excitability between αON-S and αOFF-S were reflected in the shape of their action potentials (APs), and that high K + promoted rate-dependent AP widening (Boal et al, 2022). We measured AP half-widths in both experimental groups to determine if decreased potassium sensitivity in 4wk IOP RGCs was reflected at the level of AP generation (Figure 5).…”