Melanopsin photoreception in the eye regulates light-induced skin colour changes through the production of α-MSH in the pituitary gland

How skin colour adjusts to circadian LIGHT/dark cycles is poorly understood. Melanopsin (Opn4) is expressed in melanophores, where in vitro studies suggest it regulates skin pigmentation through a 'primary colour response' in which LIGHT photosensitivity is translated directly into pigment movement. However, the entrainment of the circadian rhythm is regulated by a population of melanopsin-expressing retinal ganglion cells (mRGCs) in the eye. Therefore, in vivo, melanopsin may trigger a 'secondary colour response' initiated in the eye and controlled by the neuro-endocrine system. We analysed the expression of opn4m and opn4x and melanin aggregation induced by LIGHT (background adaptation) in Xenopus laevis embryos. While opn4m and opn4x are expressed at early developmental times, light-induced pigment aggregation requires the eye to become functional. Pharmacological inhibition of melanopsin suggests a model whereby mRGC activation lightens skin pigmentation via a secondary response involving negative regulation of alpha-melanocyte-stimulating hormone (α-MSH) secretion by the pituitary.