The distribution of immunoreactive serotonin (5-HT) was investigated in the brain and pituitary gland of the goldfish with the use of indirect immunofluorescence technique. Immunoreactive cerebrospinal fluid-contacting neurons were found in the nucleus recessus lateralis and in the nucleus recessus posterions of the hypothalamus. Additional immunoreactive neurons were observed ventro-lateral 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 5-HT-systems of the teleost brain appears to be similar to that described in mammals.
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