Rather recently a new photoreceptor had been discovered i the human retina. It is responsible for the non-visual detection of light, so that our circadian and neuroendocrine rhythms can be synchronized by the day-night cycle. The photoreceptor is an intrinsically photosensitive retinal ganglion cell located in the inner retina, unlike the rods and cones that are located in the outer retina. Melanopsin is the functional photopigment used by the new photoreceptor, a photopigment that is much more similar to the opsins of invertebrate rhabdomeric photoreceptors than the opsins of vertebrate ciliary photoreceptors. It seems like the Melanopsin phototransduction cascade is similar to that of the invertebrates, involving G-protein activation and opening of a light-gated channel, leading to depolarization of the cell membrane. This in turn leads to an action potential that follows the retinohypothalamic tract to the suprachiasmatic nucleus in hypothalamus, the location of our biological clock that is responsible for the circadian rhythms. A projection from the suprachiasmatic nucleus to the pineal gland is the main regulatory factor for melatonin production.
Clinically, the new photoreceptor and melatonin are involved in many physiological aspects, involving the phase-shifting the circadian rhythms, affecting the quality of sleep, emotions and psychiatric disorders – most importantly depression. Some studies have even suggested that cognitive functions like learning and memory are involved; others have indicated that somatic diseases like cancer, heart disease and Alzheimer may be affected by imbalances in the daily melatonin rhythms. Much research is necessary to fully understand the cellular physiology and clinical possibilities of light induced regulation of melatonin.