serotonin in the brain of the goldfish carassius auratus an immunocytochemical study the distribution of immunoreactive serotonin 5ht was investigated in the brain and pituitary gland of the goldfish with the use of indirect immunofluorescence technique immunoreactive cerebrospinal fluidcontacting neurons were found in the nucleus recessus lateralis and in the nucleus recessus posterioris of the hypothalamus additional immunoreactive neurons were observed ventrolateral to the posterior commissure in the nucleus dorsolateralis thalami this group of cell bodies extends posteriorly as far as the rostral midbrain tegmentum at the level of the isthmus numerous immunoreactive cell bodies were located medially between the fiber bundles of the fasciculus longitudinalis medialis finally a few isolated immunoreactive cells were observed in the medulla oblongata in the pituitary gland immunoreactive fibers and cells were found primarily in the pars distalis the origin of the numerous fibers of the telencephalon is not clear however they may arise from the perikarya located in the raphe region the general organization of the ascending and descending 5htsystems of the teleost brain appears to be similar to that described in mammals