Dopamine concentrations in pituitary stalk plasma ovariectomized rats and ovariectomized, estrogen-treated rats were measured following the subcutaneous administration of morphine or the intraventricular administration of ß-endorphin or [D-Ala2]-methionine-enkephalinamide, a synthetic enkephalin analog. The administration of morphine or the opioid peptides led to an 85-89% 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 naloxene, an opiate antagonist, prevented the opioid peptide-induced reduction in the stalk plasma concentration of dopamine. These results are supportive of the hypothesis that endogenous opioid peptides modulate the release of dopamine by tuberoinfundibular neurons into hypophysial portal blood.
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