GABA-immunoreactive neurons in the photosensory pineal organ of the rainbow trout: two distinct neuronal populations.

The distribution of putative GABA-ergic neurons in the photosensory pineal organ of the rainbow trout was investigated by use of a specific antiserum against gamma-aminobutyric acid (GABA). GABA-immunoreactive (GABA-IR) neurons were located in the rostral portion of the pineal end-vesicle, presumably constituting a population of interneurons. GABA-IR neurons were also found in the pineal stalk. The axons of these neurons were traced along the pineal stalk toward the brain. The terminal areas of these axons could not be established. GABA-IR glial cells were observed in the pineal end-vesicle, but not in the pineal stalk.