MORPHINE-INDUCED AND OPIOID PEPTIDE-INDUCED INHIBITION OF THE RELEASE OF DOPAMINE FROM TUBEROINFUNDIBULAR NEURONS

Dopamine concentrations in pituitary stalk plasma of ovariectomized rats and ovariectomized, estrogen-treated rats were measured after morphine or intraventricular .beta.-endorphin or [D-Ala2[2-alanine]]-methionine-enkephalinamide, a synthetic enkephalin analog. The administration of morphine or the opioid peptides led to an 85-95% reduction in the concentration of dopamine in pituitary stalk plasma when compared to the mean concentration of dopamine in stalk plasma of vehicle-treated animals. Pretreatment of rats with naloxone, an opiate antagonist, prevented the opioid peptide-induced reduction in the stalk plasma concentration of dopamine. Apparently, endogenous opioid peptides modulate the release of dopamine by tuberoinfundibular neurons into hypophysial portal blood.