Presence and possible site of action of secretin in the rat pituitary and hypothalamus

Secretin-like immunoreactivity was detected in extracts of several rat brain structures by radioimmunoassay, most notably in the pituitary, hypothalamus, pineal and septum. Its localization to these structures suggested that it might play a role in neuroendocrine events similar to its structural homolog vasoactive intestinal peptide. Dose-related stimulations (MED, 10(-7) M) of prolactin (PRL) release were observed after incubation of synthetic secretin with dispersed, cultured pituitary cells from male and ovariectomized (OVX) female rats. In OVX females, i.v. infusion of a high dose of secretin (10 micrograms) resulted in a significant elevation of PRL levels. Doses of secretin as low as 0.1 micrograms when administered into the third cerebroventricle were capable of significantly inhibiting PRL release in both males and OVX females, suggesting an ultrashort-loop, negative feedback of secretin. Secretin can now be added to the growing list of putative PRL-releasing agents.