New type of cone cell identified

FORT LAUDERDALE, Fla. — Researchers believe they have found a new type of cone photoreceptor in the human retina, which may play a role in regulating circadian rhythms and other nonvisual but light-dependent mechanisms.

Howard Cooper, MD, and associates at the University of Lyon said they unexpectedly identified the presence of a cone photoreceptor in human retina containing the photopigment melanopsin. They presented their findings here at the annual meeting of the Association for Research in Vision and Ophthalmology.

None of the melanopsin-positive cones contain any of the other known types of opsins, such as SW or MW/LW opsins, the researchers said.

The melanopsin was detected by performing anti-melanopsin immunohistochemistry on free-floating sections of retina obtained from human cadaver eyes.

Previous studies have shown that melanopsin is expressed in a subpopulation of human retinal ganglion cells that are intrinsically photosensitive. The task remains to “define the presence of melanopsin-positive neurons in human retina in order to determine the involvement of this pigment in human circadian photoreception,” Dr. Cooper said.

Melanopsin expressing ganglion cells are considered to play a role in a broad range of nonvisual irradiance-detection processes due to their intrinsic photosensitivity and projections to structures mediating circadian rhythms, pupillary reflex and seasonal rhythms. A similar role can be hypothesized for melanopsin in the human retina, according to Dr. Cooper.