Immunocytochemical localization of serotonin and gonadotropin-releasing hormone in the brain and pituitary gland of the Atlantic croaker Micropogonias undulatus.

The distribution of serotonin-immunoreactive (5-HT-ir) and gonadotropin-releasing hormone-immunoreactive (GnRH-ir) elements in the brain and pituitary gland of the Atlantic croaker (Micropogonias undulatus) was studied using immunoperoxidase and immunofluorescence techniques. 5-HT- and GnRH-ir perikarya were observed in the ventrolateral region of the olfactory bulbs and in the preoptic-anterior hypothalamic area bordering the third ventricle as well as in regions distal to the ependyma of the third ventricle. The immunoreactive elements of the two systems appear to overlap in these regions. Cerebrospinal fluid-contacting 5-HT-ir neurons were found in the periventricular hypothalamus, particularly in the preoptic area and in the nucleus posterior periventricularis, nucleus ventromedialis thalami, nucleus recessus lateralis, and nucleus recessus posterioris. Immunoreactivity to 5-HT in the pituitary was strongest in the proximal pars distalis (PPD) and in a few scattered cells bordering the pars intermedia. Dendritic processes of 5-HT neurons could be seen contacting the adjoining hypophysial cells in the PPD. GnRH-ir fibers were seen entering the pituitary stalk and the immunoreaction was concentrated in the ventral PPD. Several large 5-HT-ir perikarya were located in the raphe region forming a column in the mediosagittal plane. The results suggest that the general organization of the 5-HT system in the brain of the Atlantic croaker is similar to that described in other vertebrates. The demonstration that 5-HT and GnRH systems are in close proximity, particularly in the olfactory bulbs and in the preoptic-anterior hypothalamic area of the croaker brain, suggests that the two systems may interact in these regions of the teleostean brain to control several reproductive functions including spawning behavior and gonadotropin release.