Immunocytochemical localization and circadian variations of serotonin and N-acetylserotonin in photoreceptor cells. Light and electron microscopic study in the teleost pineal organ.

Using two immunocytochemical procedures (i.e., immunofluorescence and the unlabeled peroxidase-antiperoxidase method), the localization of a serotonin(HT)-like and of a N-acetylserotonin (aHT)-like immunoreactivity in the pineal organ of the pike was studied during winter. It was shown that immunostaining was exclusively restricted to the cells of the receptor line (CRL = typical and modified photoreceptors). The intensity of the reactions varied through the light-dark cycle, HT-like immunoreactivity being high during the photophase and low during the scotophase. In contrast, aHT-like immunoreactivity was highest at the beginning of the scotophase. HT and aHT-like immunoreactivities were detected in all cell types of the pineal epithelium after administration of a monoamine oxidase inhibitor. Up to now, only HT immunoreactivity could be localized at the ultrastructural level. In a number of typical and modified photoreceptors, a HT-positive staining seemed to be confined within the hyaloplasm of the inner segment, particularly with that of the perikaryon and basal pedicle. Our previous and present results strongly suggest that indole compounds, which are involved in the regulation of various neuroendocrine processes in fish, are synthetized within the CRL. Taking into account that the CRL of the pike are also photosensitive, it appears more and more likely that they are photoneuroendocrine cells involved in mediating the effects of the photoperiod on various physiological and behavioral processes.