Central projections from the goldfish pineal organ traced by HRP-immunocytochemistry.

Pineal efferent projections have been traced in the brain of the goldfish (Carassius auratus) by administration of a concentrated solution of horseradish peroxidase onto the pineal organ. After different survival times, fish were sacrificed and the administered peroxidase was revealed by immunocytochemistry on paraffin sections using an anti-horseradish peroxidase antiserum. Immunoreactive fibres were seen in the anterior hypothalamus, habenula, dorsal thalamus, ventral thalamus, optic tectum, torus longitudinalis, area pretectalis, torus semicircularis and dorsal tegmentum. No immunoreactive cell bodies were visualized in the central nervous system, thus suggesting the absence of central pinealopetal innervation. Since all areas showing pineal labelled fibres are also known to receive retinal inputs, it can be suggested that an overlapping of information from retinal and extraretinal photoreceptors may be important to processes depending on photic stimulation such as entrainment of circadian rhythms or photoneuroendocrine responses.