Photoreceptors and CSF-contacting neurons in the pineal organ of a teleost fish have direct axonal connections with the brain: an HRP-electron-microscopic study.

Neural signals transmitted from the pineal organ to the brain in cold-blooded vertebrates provide information about ambient illumination, information of importance for the synchronization of activity rhythms with the light-dark cycle. The ultrastructure of intrapineal projection neurons (pineal "ganglion cells") was studied after retrograde filling with HRP through their cut axons. The dominating neuronal type is a small bipolar cell. It is present in largest numbers in the pineal stalk. This cell type displays several morphological features characteristic of cerebrospinal fluid (CSF)-contacting neurons. An apical dendritelike process extends toward the central lumen of the pineal organ. This dendritic process contains numerous mitochondria, it may have several fine branches, and it may possess a ciliumlike structure. An axon emerges from the basal pole of the neuron and joins the pineal tract. This CSF-contacting neuron is postsynaptic to photoreceptor basal pedicles with ribbon-type synapses. Such synapses may occur on the neuronal soma but are mostly observed on small, basally located processes in the vicinity of the axon. There is a significant similarity between this cell type and the bipolar cells bearing a Landolt's club in the retina. In the rostral part of the pineal end-vesicle, several large photoreceptors were labeled. These photoreceptors may, consequently, have axons more than 1 mm long. An intriguing possibility is that this previously unknown vertebrate photoreceptor type conveys graded potentials over long distances.