Melanopsin - Function

Function

Evidence supporting prior theories that melanopsin is the photopigment responsible for the entrainment of the central "body clock", the suprachiasmatic nuclei (SCN), in mammals. Fluorescent immunocytochemistry was used to visualize melanopsin distribution throughout the rat retina and showed that melanopsin was found in approximately 2.5% of the total rat retinal ganglion cells (RGCs) and that these cells were indeed ipRGCs. Using β-galactosidase as a marker for the melanopsin gene, X-gal labeling of these ipRGCs showed that their axons directly target the SCN, providing further evidence that melanopsin is important in entrainment through the retinohypothalamic tract (RHT).

Melanopsin-containing ganglion cells exhibit both light and dark adaptation, that is, that they adjust their sensitivity according to the recent history of light exposure. In this respect, they are similar to rods and cones. Whereas rods and cones are responsible for the analysis of images, patterns, motion and color, a number of studies have shown that melanopsin-containing ganglion cells contribute to various reflexive responses of the brain and body to the presence of (day)light.

A mouse paraneuronal cell line (Neuro-2a), which normally is not photosensitive, is rendered photoreceptive by the addition of human melanopsin. Under such conditions, melanopsin acts as a sensory photopigment, performing physiological light detection. The melanopsin photoresponse is selectively sensitive to short-wavelength light (peak absorption ~480 nm), while it also has an intrinsic photoisomerase regeneration function that is chromatically shifted to longer wavelengths.

Melanopsin photoreceptors are sensitive to a range of light wavelengths. Melanopsin photoreceptors reach peak light absorption at blue light wavelengths around 488 nanometers. Other wavelengths of light activate the melanopsin signaling system with decreasing efficiency as they get shorter or longer than 488 nm. For example, shorter wavelengths around 445 nm (closer to violet in the visible spectrum) are half as efficient at melanopsin photoreceptor stimulation as light at 488 nm. Longer wavelengths around 529 nm (in the bluish-green part of the visible spectrum) are also half as efficient as light at 488 nm.

Dopamine (DA) is a factor in the regulation of melanopsin mRNA in ipRGCs. Because DA synthesis and release in the rat retina are under the control of rods and cones, it appears that rods and cones, in conjunction with or possibly with the exclusion of direct circadian or photic input, control transcription of melanopsin.