Somatostatin-like immunoreactivity in the pituitary and brain of three teleost fish species: somatostatin as a potential regulator of prolactin cell function

Somatostatin-like immunofluorescence occurs in the hypothalamus and neurohypophysis of three euryhaline teleosts: tilapia, killifish, and mudsucker. This immunofluorescence was eliminated by incubating the primary antibody with excess somatostatin or somatostatin-28 but not with urotensin II, a partial analogue of somatostatin. In all three fishes, the strongest reaction was seen in the proximal pars distalis and parts of the pars intermedia. Strongly fluorescing processes from cells of the preoptic nucleus extend toward the pituitary. Distinct fluorescence was also associated with the neurohypophysis penetrating into the rostral pars distalis in the tilapia but not in the killifish or mudsucker. In the tilapia, an extensive network of immunofluorescent fibers and small cells were present in the anterior dorsolateral telencephalon, in addition to a moderately fluorescing group of cells anterolateral to the preoptic nucleus. A small area of diffuse fluorescence was also seen in the anterior dorsolateral midbrain tegmentum. Previous physiological studies have implicated somatostatin as a regulator of prolactin cell activity in tilapia. The present study demonstrates the route by which somatostatin may be delivered to the rostral pars distalis to inhibit prolactin secretion.