The suprachiasmatic nucleus of the hypothalamus receives visual cues for timing the circadian rhythm from specialized “melanopsin cells” – retinal ganglion cells that respond to light using melanopsin as the photopigment. These melanopsin cells appear to have no role in the analysis of visual images, and likely are completely dedicated to non-image forming visual activities, including circadian rhythm regulation. It is known, however, that stimulation of rods and cones will influence the circadian rhythm, so perhaps there is more than one route by which the suprachiasmatic nucleus can be stimulated. The present investigators developed an immunotoxin that selectively ablated melanopsin cells, and found that the circadian rhythm response to light was disrupted. This suggests that any influence provided by rods and cones likely passes through the melanopsin cells to the suprachiasmatic nucleus, and further confirms the central role of these cells in the regulation of the circadian rhythm.
