Direct pituitary inhibition of prolactin secretion by dopamine and noradrenaline in sheep

Abstract

The effects of dopamine, noradrenaline and 3,4-dihydroxyphenylacetic acid (DOPAC) on the release of prolactin were examined in ovariectomized ewes. Infusion of dopamine (0.5 or 1 μg/kg per min for 2 h i.v.) reduced plasma prolactin concentrations in a dose-dependent manner, whereas DOPAC (5 or 10 μg/kg per min for 2 h i.v.) had no effect. In a further series of experiments, ovariectomized hypothalamo-pituitary disconnected ewes were given dopamine or noradrenaline (each at 0.5 or 1 μg/kg per min for 2 h i.v.), and both amines reduced mean plasma concentrations of prolactin with similar potency in a dose-dependent manner. These effects were blocked by treatment with pimozide and prazosin resp..