In vitro inhibition of pituitary prolactin synthesis and release by hypothalamic extract

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Abstract
When rat anterior pituitary (AP) was incubated at 37.5 C in a Dubnoff metabolic shaker for 2 hr, 169% more prolactin was found in the combined medium and AP than in nonincubated AP. When AP was incubated together with homogenate or acid extract of rat hypothalamus, prolactin levels in the medium and AP were markedly decreased (36–75%), indicating inhibition of synthesis and release. Acid extract of rat cerebral cortex had no effect on prolactin synthesis or release. Incubation of ovine or rat prolactin, with or without hypothalamus, did not decrease prolactin activity, demonstrating that hypothalamic inhibition of AP prolactin production was not due to hormone inactivation. Acetylcholine, epinephrine, norepinephrine, serotonin, histamine, substance P, oxytocin, and arginine or lysine vasopressin had no effect on AP prolactin release. These results indicate that the hypothalamus contains a factor(s) which inhibits synthesis and release of prolactin by the rat AP in vitro, and this factor(s) is not any of the recognized neurohumors in the hypothalamus.