In rats, the median eminence, pituitary stalk and anterior pituitary were exposed parapharyngeally and catecholamines were injected into the third ventricle or infused into a stalk portal vessel, peduncular artery, or a tuberalis ramus of an infundibular artery. Ten min after 1.25 μg dopamine hydrochloride was injected into the third ventricle, the plasma prolactin concentration was 70% of the preinjection value; 47% at 20 min; 42% at 30 min; 57% at 60 min; 69% at 90 min; and 93% at 120 min. Similar responses occurred following the administration of 2.5 μg dopamine. Quantities of dopamine greater than 2.5 jug caused less inhibition of release, however. Intraventricular injection of 2.5 or 5 μg of epinephrine or norepinephrine bitartrate did not affect prolactin release although 100 μg did. Dopamine, epinephrine, or norepinephrine, perfused into the anterior pituitary for 30 min via a hypophysial portal vein, had no effect on prolactin release. Dopamine, infused into the stalk-median eminence complex via the peduncular artery or a tuberalis ramus of an infundibular artery, had no effect on plasma prolactin levels. These findings indicate that neither dopamine, epinephrine, nor norepinephrine affected prolactin release by a direct action on the anterior pituitary but indirectly through the hypothalamic-hypophysial complex.
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