TRPV1 Supports Axogenic Enhanced Excitability in Response to Neurodegenerative Stress

Abstract

Early progression in neurodegenerative disease involves challenges to homeostatic processes, including those controlling axonal excitability and dendritic organization. In glaucoma, the leading cause of irreversible blindness, stress from intraocular pressure (IOP) causes degeneration of retinal ganglion cells (RGC) and their axons which comprise the optic nerve. Previously, we discovered that early progression induces axogenic, voltage-gated enhanced excitability of RGCs, even as dendritic complexity in the retina reduces. Here, we investigate a possible contribution of the transient receptor potential vanilloid type 1 (TRPV1) channel to enhanced excitability, given its role in modulating excitation in other neural systems. We find that genetic deletion of Trpv1 (Trpv1−/−) influences excitability differently for RGCs firing continuously to light onset (αON-Sustained) vs. light offset (αOFF-Sustained). Deletion drives excitability in opposing directions so that Trpv1−/− RGC responses with elevated IOP equalize to that of wild-type (WT) RGCs without elevated IOP. Depolarizing current injections in the absence of light-driven presynaptic excitation to directly modulate voltage-gated channels mirrored these changes, while inhibiting voltage-gated sodium channels and isolating retinal excitatory postsynaptic currents abolished both the differences in light-driven activity between WT and Trpv1−/− RGCs and changes in response due to IOP elevation. Together, these results support a voltage-dependent, axogenic influence of Trpv1−/− with elevated IOP. Finally, Trpv1−/− slowed the loss of dendritic complexity with elevated IOP, opposite its effect on axon degeneration, supporting the idea that axonal and dendritic degeneration follows distinctive programs even at the level of membrane excitability.

Neuroprotection by WldS depends on retinal ganglion cell type and age in glaucoma

“…Next, we examined the influence of Wld S+/+ on the morphology and physiology of αOFF-S RGCs with IOP elevation. Following previous work, we identified αOFF-S RGCs in WT and Wld S+/+ retinas based on dendritic projections within the IPL, immunoreactivity to SMI-32, and their responses to light [ 29 , 32 – 34 ]. In WT and Wld S+/+ retinas, αOFF-S RGCs modestly express SMI-32 and have large somas with large, complex dendritic fields that project within the distal OFF sublamina of the IPL near ChAT-positive amacrine cell processes (Fig.…”

“…“Enhancing axon excitability by high-frequency stimulation or blunting excitability by blocking voltage-gated sodium channels with tetrodotoxin both accelerate axon degeneration following axotomy of neuromuscular junctions in Wld S mice” [ 54 ]. Considering these findings, αOFF-S RGCs may be more vulnerable to stress due to their intrinsically high spontaneous activity, which is further enhanced during early progression of glaucoma [ 29 , 34 ].…”

“…For RGC intracellular filling and patch-clamp recordings, we used fire-polished borosilicate glass pipettes containing (in mM) 125 K-gluconate,10 KCl, 10 HEPES, 10 EGTA, 4 Mg-ATP, 1 Na-GTP, and 0.1 ALEXA 555 (Invitrogen, Carlsbad CA; Osm 285, pH 7.35). RGC light responses were evoked using full-field light flashes generated by a light-emitting diode (365 nm, 300 μW/cm2, 3-s duration; Roithner Lasertechnik) delivered through a shutter in the microscope condenser [ 29 , 34 ].…”

“…The number of dendritic intersections was determined by Sholl analysis (ImageJ, 1.53c). Dendritic branch points were analyzed manually by counting the number of dendritic bifurcations [ 34 ].…”

“…We used the Shapiro-Wilk test for normality. If data best fit a lognormal distribution, datasets were transformed by computing the logarithm (base 10) [ 34 , 46 ]. For normal or transformed data, we performed parametric statistics.…”