In keeping with the observation that the hypothalamus exerts tonic inhibitory effects on prolactin secretion, is the finding that crude hypothalamic extracts inhibit prolactin release. This bioactivity has been termed PIF by Meites (1966). PIF has been identified in portal vessel blood (1971), thus satisfying one of the critical requirements for proof of physiological significance of a hypophysiotrophic hormone. Although a number of substances that can inhibit prolactin release have been isolated from the hypothalamus, most workers believe that dopamine is the principal factor.
