Vertebrate ancient (VA) opsin and extraretinal photoreception in the Atlantic salmon (Salmo salar)

A member of a new photopigment family first isolated from teleost fish, ver tebrate ancient (VA) opsin, has recently been shown to form a functional ph otopigment and to be expressed within a subset of horizontal and amacrine c ells of the inner retina. These sites of expression (and structural feature s) of VA opsin suggest that this photopigment might mediate non-image-formi ng light-detection tasks. We attempted to gain support for this hypothesis by examining the expression of VA opsin within the central nervous system ( CNS) (pineal and deep brain) of the Atlantic salmon Salmo salar. In additio n, we examined the sites of rod-opsin, cone-opsin and alpha-transducin expr ession within the salmon CNS to provide a more complete description of the extraretinal photoreceptors of a teleost vertebrate, We show that multiple populations of cells within the salmon CNS appear to contain photoreceptors : VA opsin was strongly expressed in the pineal organ and in bilateral colu mns of subependymal cells in the epithalamus; anti-cone-opsin antibodies la belled cells within the pineal and numerous cells in the anterior hypothala mus (suprachiasmatic nucleus, nucleus preopticus magnocellularis, nucleus p reopticus parvocellularis); anti-rod-opsin antibodies labelled cells within the pineal but no other areas within the central brain; and anti-alpha-tra nsducin antibodies labelled cells within the pineal and the ventral telence phalon. Collectively, our results suggest that VA opsin is a photopigment s pecialised for irradiance detection tasks within the eye, pineal and centra l brain, and that the salmon has multiple and varied populations of photore ceptors within the CNS. We review the significance of these findings within the broad context of vertebrate extraretinal photoreception.