TRPV1-/- Accelerates Neurodegeneration From Microbead-Induced Ocular Hypertension

Abstract

Purpose: Transient receptor potential (TRP) channels transduce information about the extracellular environment via influx of Ca²⁺. In the central nervous system (CNS), transient receptor potential vanilloid 1 (TRPV1) mediates diverse neuronal responses to stress and is known to modulate synaptic activity. Since TRPV1 activation affects retinal ganglion cell (RGC) survival in vitro, we sought to characterize its influence in vivo using microbead-induced ocular hypertension (OHT) in TRPV1-/- mice.

Methods: OHT was induced in 4 month C57 and TRPV1-/- mice through microbead occlusion of the anterior chamber. The contralateral eye received an equivalent volume saline injection. Ocular pressure was monitored for 5 weeks using Tono-Pen XL rebound tonometry, and fluorescent cholera toxin subunit β (CTB) was injected intravitreally 2 days prior to sacrifice. CTB transport along the optic projection was quantified in coronal sections through perfusion-fixed midbrains containing superior colliculus. RGC axons were quantified in counter-stained semi-thin sections through optic nerve. C57 paraffin sections were immunolabeled against TRVP1 and RIBEYE, a pre-synaptic marker.

Results: Microbead-injected eyes exhibited a significant increase in IOP over their saline-injected controls in both C57 (19.77±4.23 mmHg vs. 14.75±3.64 mmHg) and TRPV1-/- (19.79±4.47 mmHg vs. 14.96±4.04 mmHg) mice (p<0.001, n=13). OHT resulted in diminished CTB transport to the colliculus compared to saline-injection that was worse for TRPV1-/- mice (63.6% decrease) compared to C57 (43.4%) and (p<0.001). Similarly, axon loss in the optic nerve with OHT was significantly worse in the TRPV1-/- cohort (p<0.001). We found peri-synaptic expression of TRPV1 in normal retina as indicated by its proximity to RIBEYE label. This is consistent with RGC dendritic expression.

Conclusions: Our results indicate that TRPV1-/- accelerates neurodegenerative outcomes resulting from OHT with microbead occlusion. Within the CNS, the Ca²⁺-mediated activity of TRPV1 has been linked to both protective and degenerative functions depending on the context of its signaling. TRPV1 may constitutively contribute to survival of RGCs in response to stressors like OHT, possibly through a pro-survival response modulating synaptic activity.

Keywords: 615 neuroprotection • 531 ganglion cells • 439 calcium