Endothelin modulates the circadian expression of non-visual opsins

Abstract

The non-visual opsin, melanopsin, expressed in the mammalian retina, is considered a circadian photo- 24 pigment because it is responsible to entrain the endogenous biological clock. This photopigment is also 25 present in the melanophores of Xenopus laevis, where it was first described, but its role in these cells is not 26 fully understood. X. laevis melanophores respond to light with melanin granule dispersion, the maximal 27 response being achieved at the wavelength of melanopsin maximal excitation. Pigment dispersion can 28 also be triggered by endothelin-3 (ET-3). Here we show that melanin translocation is greater when a blue 29 light pulse was applied in the presence of ET-3. In addition, we demonstrated that mRNA levels of the 30 melanopsins Opn4x and Opn4m exhibit temporal variation in melanophores under light/dark (LD) cycles 31 or constant darkness, suggesting that this variation is clock-driven. Moreover, under LD cycles the oscil- 32 lations of both melanopsins show a circadian profile suggesting a role for these opsins in the photoen- 33 trainment mechanism. Blue-light pulse decreased Opn4x expression, but had no effect on Opn4m. ET-3 34 abolishes the circadian rhythm of expression of both opsins; in addition the hormone increases Opn4x 35 expression in a dose-, circadian time- and light-dependent way. ET-3 also increases the expression of 36 its own receptor, in a dose-dependent manner. The variation of melanopsin levels may represent an 37 adaptive mechanism to ensure greater melanophore sensitivity in response to environmental light con- 38 ditions with ideal magnitude in terms of melanin granule dispersion, and consequently color change.