The left habenular nucleus contains a discrete serotonin-immunoreactive subnucleus in the coho salmon (Oncorhynchus kisutch).

By use of antibodies against serotonin, a discrete subnucleus of putatively serotoninergic neurons was observed in the dorsal subdivision of the left habenular nucleus in the brain of the coho salmon. The subnucleus was observed in salmon of different life-stages: in fingerlings, during smolt transformation, after smolt transformation (in seawater), and after spawning. This finding further emphasizes the close relationship between the pineal organ and the habenular nuclei-not only in terms of topographical proximity but also in terms of cytological similarities: cells of the habenular nucleus and the pineal complex have previously been shown to be immunoreactive also with antibodies directed against retinal phototransduction proteins. It also underlines the asymmetric organization of the epithalamic region.