Chronic intra-ocular pressure induces neuroinflammation in retina and cerebral structures in an ocular hypertensive rat model

Abstract

Purpose: Glaucoma is a leading cause of irreversible vision loss in the world. Glaucoma is frequently initiated by pathological ocular hypertension leading to progressive retinal ganglion cells (RGCs) death and degeneration of their axons. This axonal degeneration spreads from the optic nerve to visual pathways in the brain. Thus, we hypothesize that neurodegenerative mechanism in the central visual system may contribute to the progression of glaucoma. The aim of the present study was to analyze neuroinflammation and neuronal cell death processes that occur from the pathological retina to the cerebral visual areas in a rat model of ocular hypertension induced by episcleral vein cauterization (EVC).

Methods: To induce stable intra-ocular pressure increase, 3 episcleral veins of right eyes were cauterized in Long-Evans male rats. Six weeks after, tissues (retina, optic nerve, optic tract, lateral geniculate nucleus and superior colliculus) were collected after microdissection from ocular hypertensive and control animals. In whole flat mounted retina, we counted RGCs stained with Brn3a antibody and microglia cells stained with IBA1 antibody. Neuroinflammation in retina and central visual pathway was also quantified by analyzing mRNA expression of cytokines (IL-6, IL-1β) and chemokines (CCL2). In addition expression of glial cell makers (Iba1, GFAP) and oxidative stress enzyme (Nox2, Nox4) were evaluated in the central visual structures.

Results: Gene expression analysis in retinas of hypertensive eyes revealed a significant increase in CCL2, IL-1β and Nox2 mRNA expression compared to control eyes. Furthermore, microglial cells and RGCs immunolabeled in whole flat mounted retina demonstrated peripheral RGC loss associated with microglial cell activation. These patterns were associated with a loss of optic nerve fibers and myelin destructuration. Moreover, we found an up-regulation of proinflammatory markers such as IL-6, IL-1β and CCL2 in cerebral structures in contralateral brains of ocular hypertensive rats compared to control rats.

Conclusions: This work clearly demonstrates that ocular hypertension induces neuroinflammation in the retina and in the main brain structures involved in vision. Altogether, our data highlight central inflammatory mechanisms that occur in glaucoma.