Study for the physiological characteristics of the pineal ultraviolet photoreceptor cells.

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Lower vertebrates can detect UV light with the pineal complex independently of eyes. Electrophysiological studies, together with chromophore extraction analysis, have suggested that the underlying pigment in the lamprey pineal exhibits a bistable nature, that is, reversible photoreaction by UV and visible lights, which is never achieved by known UV pigments. Here we addressed the molecular identification of the pineal UV receptor. Our results showed that the long-hypothesized pigment is a lamprey homologue ofparapinopsin, which exhibits an absorption maximum at 370 nm. UV light causes cis-trans isomerization of its retinal_2 chromophore,.forming a stable photoproduct having an absorption maximum at 515 nm. The photoproduct reverts to the original pigment upon visible light absorption, showing photoregeneration of the pigment. In situ hybridization showed that parapinopsin is selectively expressed in the cells located at the dorsal region of the pineal organ. We successfully obtained the hyperpolarizing responses with a maximum sensitivity about 380 nm from the photoreceptor cells at the dorsal region, in which the outer segment was clearly stained with anti-parapinopsin antibody. These results demonstrated that parapinopsin is the pineal UV pigment having photo-interconvertible two stable states. The bistable nature of the parapinopsin can account for the photorecovery of the pineal UV-sensitivity by background green light in the lamprey. Furthermore, we isolated the parapinopsin homologues from fish and frog pineal complexes which exhibit UV-sensitivity, suggesting that parapinopsin is a common molecular basis for pineal UV reception in the vertebrate.