Studies in vitamin A. 25. Visual pigments in tadpoles and adult frogs

Immunocytochemical localization of vitamin A in the retina and pineal organ of the frog, Rana esculenta

Vitamin A immunoreactive sites were studied in the retina and pineal organ of the frog, Rana esculenta, by the peroxidase antiperoxidase, avidin-biotinperoxidase and immunogold methods. In dark-adapted material, strong immunoreaction was found in the outer and inner segments of the photoreceptor cells of both retina and pineal organ, as well as in the pigment epithelium, retinal Müller cells and pineal ependymal cells. In light-adapted retina, cones and green (blue-sensitive) rods were immunopositive. At the electron microscopic level, immunogold particles were found on the membranes of the photoreceptor outer segments as well as on the membranes of the endoplasmic reticulum and mitochondria. Individual retinal photorecptor cells exhibited strong immunoreaction in the distal portion of the inner segment, the ciliary connecting piece and the electron-dense material covering the outer segment. In the pigment epithelium, the immunolabeling varied in intensity in the basal and apical cytoplasm and phagocytosed outer segments. The immunocytochemical results indicate that retinoids (retinal, retinol and possibly retinoic acid) are present not only in the photoreceptor cells of the retina but also in those of the pineal organ. The light-dependent differences in the immunoreactivity of vitamin A underlines its essential role in the visual cycle of the photopigments. Our results suggest that the pineal ependyma plays a role comparable to that of the Müller cells and pigment epithelium of the retina with regard to the transport and storage of vitamin A. The presence of a retinoid in nuclei, mitochondria and cytoplasmic membranes suggests an additional role of vitamin A in other metabolic processes.

Seasonal variation of the vitamin A2-based visual pigment in the retina of adult bullfrog, Rana catesbeiana

Visual pigment composition was determined by HPLC analysis over a one-year period in the adult bullfrog (Rana catesbeiana). In Japan, the vitamin A2-based pigment was only 5% of the total visual pigment from the middle of July to October. The vitamin A2-based pigment increased in November and reached a maximum of 32-36% between January and June. This seasonal variation may relate to the average of outdoor temperature rather than the daylight hours. The amount of vitamin A2-based pigment began to increase when the average temperature became lower than 20 degrees C and it decreased rapidly as the average temperature was higher than 20 degrees C.