Melanopsin (Opn4) positive cells in the cat retina are randomly distributed across the ganglion cell layer

A rare type of rodent retinal ganglion cell expresses melanopsin (Opn4), the majority of which project to the suprachiasmatic nuclei. Many of these cells are directly light sensitive and appear to regulate the circadian system in the absence of rod and cone photoreceptors. However, the rodent retina contains no overt regions of specialization, and the different ganglion cell types are hard to distinguish. Consequently, attempts to distinguish the distribution of melanopsin ganglion cells in relation to regions of retinal specialization or subtype have proved problematic. Retinal cells with a common function tend to be regularly distributed. In this study, we isolate cat melanopsin and label melanopsin expressing cells using in situ hybridization. The labelled cells were all confined to the ganglion cell layer, their density was low, and their distribution was random. Melanopsin containing cells showed no clear center-to-periphery gradient in their distribution and were comprised of a relatively uniform cellular population.