Loss of melanopsin-containing retinal ganglion cells in a rat glaucoma model

Glaucoma can cause progressive damage to retinal ganglion cells. These cells can be classified as cells projecting to the superior colliculus and melanopsin-containing retinal ganglion cells, which project to the suprachiasmatic nucleus. This study was to investigate the effects of chronic intraocular pressure elevation on melanopsin-containing retinal ganglion cells in rats.

Chronic intraocular pressure elevation was induced in one eye of adult Wistar rats by cauterization of three episcleral veins. Intraocular pressure was measured at different intervals with a rebound tonometer. Superior collicular retinal ganglion cells were retrogradely labeled from the superior colliculus with Fluorogold. Melanopsin-containing retinal ganglion cells were visualized by free-floating immunohistochemistry on whole-mount retinas. The number of labeled superior collicular and melanopsin-containing retinal ganglion cells were counted in the sample areas on flat-mounted retinas.

Compared with contralateral control eyes, the numbers of both superior collicular and melanopsin-containing retinal ganglion cells were significantly reduced after 12 weeks of experimental intraocular pressure elevation ((2317.41 +/- 29.96)/mm(2) vs (1815.82 +/- 24.25)/mm(2); (26.20 +/- 2.10)/mm(2) vs (20.62 +/- 1.52)/mm(2), respectively). The extent of cell loss of the two types of retinal ganglion cells was similar. However, no morphologic changes were found in melanopsin-containing retinal ganglion cells.

Both melanopsin-containing and superior collicular retinal ganglion cells were damaged by chronic ocular hypertension, indicating that glaucomatous neural degeneration involves the non-image-forming visual pathway.