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.