Opsin-immunoreactive outer segments in the pineal and parapineal organs of the lamprey (Lampetra fluviatilis), the eel (Anguilla anguilla), and the rainbow trout (Salmo gairdneri).

The pineal complex of Lampetra fluviatilis, Anguilla anguilla and Salmo gairdneri was studied by means of the indirect immunohistochemical antiopsin reaction. Opsin-immunoreactive material was demonstrated in the outer segments of the photoreceptor cells in the pineal organ of all three species investigated. In the lamprey, the opsin-positive outer segments were located in the lumen of the pineal vesicle and atrium. In the two teleost species, the immunoreactive outer segments were observed in abundance in the pineal end-vesicle and stalk. These structures were found to accumulate in the prominent initial portion of the pineal stalk of the eel. In the rainbow trout, immunoreactive outer segments occurred in the wide orifice of the pineal recess at the roof of the third ventricle. In addition, outer segments of photoreceptor cells of the parapineal organ ("parapinealocytes") displayed opsin immunoreactivity. In the lamprey, opsin immunoreactivity was restricted to the central portion of the ventral parapineal retina, while the parapinealocytes in the lateral portions did not bind the antibody. In the two teleosts, immunoreactive outer segments displayed a scattered pattern. These immunocytochemical results provide direct evidence that the photosensitivity of the pineal demonstrated electrophysiologically in lampreys and teleosts (cf. Dodt 1973) is based on an opsin-containing photopigment. The presence of opsin in cells of the parapineal organ strengthens the view that also this organ may be capable of direct light perception. In the lamprey, the exclusive opsin immunoreactivity of a circumscribed group of parapineal cells suggests the existence of two types of parapinealocytes. The significance of opsin-containing photoreceptor outer segments occurring in the most proximal portion of the teleost pineal stalk is discussed, especially with regard to the interpretation of results obtained from pinealectomy experiments.