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Carl Weitlauf, Nicholas Ward, Brian Carlson, Wendi Lambert, David Calkins; Upregulation of TRPV1 Signaling Boosts RGC Excitation Early in a Mouse Model of Glaucoma. Invest. Ophthalmol. Vis. Sci. 2013;54(15):802.
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© ARVO (1962-2015); The Authors (2016-present)
Sensitivity to elevated intraocular pressure (IOP) is a major risk factor for degeneration of retinal ganglion cells (RGCs) in glaucoma. Rodent RGCs express the transient receptor potential vanilloid-1 (TRPV1) cation channel, which gates a rapid influx of Ca2+ with exposure to stress. Here, we used an inducible mouse model of glaucoma to investigate physiologically whether TRPV1 in RGCs could intrinsically counter loss of excitatory activity in RGCs challenged by elevated IOP.
IOP of month-old C57 mice was elevated 30-40% for 2-15 days via microbead occlusion of the anterior chamber (15 µm; 1.5 µL). Retinas from both microbead- and saline-injected (control) eyes were paraffin-embedded for immunohistochemistry or acutely prepared as whole mounts for electrophysiological patch-clamp recording and morphological analysis. Under whole-cell current clamp, spontaneous and evoked spike-firing were measured in RGCs before and after bath application of the specific TRPV1 agonists capsaicin (2 µM) and N-oleoyldopamine (N-OLDA; 10 µM). In a subset of experiments, the TRPV1 competitive antagonist iodoresiniferatoxin (I-RTX; 100 nM) was pre-applied before the agonist. 1% Lucifer Yellow was included in the recording solution to label dendritic arbors.
TRPV1 protein expression was increased in the inner plexiform layer following 1 week of microbead injection. Correspondingly, capsaicin significantly enhanced firing rate in RGCs from microbead-injected eyes compared to saline-injected eyes (p<0.001). Pre-application of I-RTX had no effect on baseline firing rate (p=0.469) but did block capsaicin-mediated enhancement (p=0.062). N-OLDA elicited the same enhancement of firing rate (p=0.012) that was blocked by I-RTX (p=0.512). RGCs from microbead-injected eyes exhibited no overt changes in overall dendritic complexity in the period studied.
Our results reveal a putative neuroadaptative response to short-term elevations in IOP through TRPV1-mediated enhancement of neural firing in RGCs. These data further implicate TRPV1 Ca2+ channels in the intrinsic stress response to elevated pressure in the eye. Our results suggest these changes precede dendritic pruning. A better understanding of how this cascade could promote RGC activity in the face of glaucomatous stressors could reveal a novel therapeutic avenue to counter degeneration.
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