Restoration of Prolactin Synthesis and Release by the Administration of Monoaminergic Blocking Agents to Pituitary Tumor-bearing Rats1

tin present in the pituitary gland and incubation medium was measured. A significant decrease in prolactin produc tion was observed in the glands from tumor-bearing rats. Incubation of these glands or normal pituitary glands with 4-(2-aminoethyl)pyrocatechol (dopamine) caused a sig nificant decrease in the amount of radioactive prolactin secreted into the incubation medium. The administration of monoaminergic blocking agents, i.e., perphenazine, haloperidol, or pimozide, significantly overcame the dopaminemediated inhibition of prolactin secretion. Injection of tumor-bearing rats with these agents completely restored to normal levels the incorporation of radioactive leucine into prolactin. These data suggest that the elevated serum prolactin concentration resulting from pituitary tumor secretion decreases the host's pituitary gland production of