Melanopsin is a photopigment found in the ganglion cells of the retina that is involved in the regulation of Circadian rhythms. Structurally it is an opsin G-protein coupled receptor, it activates the G-protein transducin.
Light stimulates the melanopsin-containing ganglion cells within the retina, which causes the production of glutamate, which stimulates the suprachiasmatic nucleus of the hypothalamus. It is produced in the pineal gland.
Experiments have shown that light entrainment , the setting of behaviorally active and inactive periods based on the presence or absence of light, is not as effective in melanopsin knockout mice.
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