Dopamine can also be classified as a hypophysiotrophic pituitary hormone since it is present in hypophysical-portal vessel blood in sufficient concentrations to duplicate all of its known inhibitory effects on the secretion of prolactin (Fig. 1.10). Several hormones structure thought to act as prolactin-releasing factors (PRFs) include TRH and vasoactive intestinal polypeptide (VIP), the latter also being found in the gut.
