Background: 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.
Methods: 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.
Results: 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.
Conclusion: 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.