Isolation and characterization of melanopsin photoreceptors of Atlantic salmon (Salmo salar).

Melanopsins constitute a recently described group of vertebrate opsin photoreceptors that are involved in nonvisual photoreception. Here we describe the identification of six melanopsin genes of Atlantic salmon (Salmo salar), a valuable teleost model for studying nonvisual photoreception and the basis of photoperiodism. The results show that genes belonging to two different groups, the mammalian-like (Opn4m) and the Xenopus-like (Opn4x) melanopsins have been duplicated in teleosts. In addition, two pairs of salmon duplicates were identified, presumably originating from the salmon lineage whole genome duplication event. The expression pattern of melanopsins was studied by in situ hybridization. The results show that Opn4m and Opn4x melanopsins are differentially expressed in the brain and retina, indicating a functional divergence. In the retina, Opn4m and Opn4x melanopsin are differentially expressed in ganglion, amacrine, and horizontal cells. In the brain, Opn4m is expressed in the dorsal thalamus and in the nucleus lateralis tuberis of the hypothalamus, which is closely connected to and involved in the regulation of pituitary function. Opn4x melanopsins are expressed in the dopaminergic, hypophysiotrophic cell population of the suporaoptic/chiasmatic nucleus and in the serotonergic cell population of the left habenula. The results suggest that melanopsin photoreceptors can be involved in signaling of photoperiodic information through multiple pathways, involving both the retina and possibly as deep-brain photoreceptors directly transmitting photoperiodic information to the hypothalamus-pituitary axis.