Prolactin release inhibiting and stimulating factors in the hypothalamus.

Catecholamines, especially norepinephrine and dopamine, as well as GABA extracted from porcine hypothalamic tissue, were found to possess PIF activity in vitro. However, more work is needed to determine whether or not the effects of GABA or catecholamines on prolactin release are physiological or pharmacological. Until such studies are completed and until the chemical nature and biological roles of other substances with PIF activity found in the hypothalamus are clarified, no conclusion as to the nature of the physiological inhibitor of prolactin release should be made.