Abstract
Purpose :
Increasing the electrical activity of retinal ganglion cells (RGCs) following an optic nerve injury can be protective and promote axon growth. However, we do not fully understand why an injury would reduce RGC activity, including spike firing, in the first place. Here, we tested the hypothesis that both synaptic and intrinsic mechanisms contribute to reduced RGC firing in a model of optic nerve injury.
Methods :
Unilateral optic nerve crush (ONC) was performed on 6-10 week old mice. Results were compared to uninjured controls. We focused on an RGC type that is resilient to injury, the ON Alpha RGC, also known as the M4 intrinsically-photosensitive RGC (ipRGC). Cells were targeted based on their large soma size and ‘ON’ response to light increments. Extracellular and whole cell patch-clamp recordings were performed using standard methods, in vitro. Cells were assessed for light responses (high-contrast spots), for both firing and synaptic currents, and for firing responses to positive current injection.
Results :
Extracellular recordings showed reduced background firing at 7 days post-crush (dpc) (22 ± 27 spikes/s; mean ± SD, n=90) versus controls (49 ± 18, n=46, p<0.0001), which was independent of sex (p=0.60). Whole cell voltage-clamp recordings showed no significant difference between groups in peak excitatory postsynaptic currents (control: 561 ± 135 pA, n=5; 7dpc: 396 ± 231, n=7, p=0.18) or inhibitory currents (control: 223 ± 154 pA, n=7; 7dpc: 167 ± 77, n=7, p = 0.40). There was also no significant difference in resting membrane potential (control: -63.2 ± 6.2 mV, n=4; 7dpc: -63.9 ± 3.5, n=7, p=0.76) or input resistance (control: 52 ± 28 MΩ, n=15; 7dpc: 77 ± 47, n=38, p=0.062). However, injured RGCs showed reduced firing, relative to controls, to positive current injection (0-300 pA, n = 4 control, 12 7dpc cells; adjusted p<0.05 for all levels).
Conclusions :
ON Alpha RGCs showed reduced spike firing seven days following an optic nerve injury. Reduced firing was not associated with altered synaptic inputs, membrane potential, or membrane resistance. Instead, reduced firing seems to depend on an intrinsic mechanism that reduces excitability. Further experiments will aim to identify and target specific ion channels associated with reduced RGC activity.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.