Dopamine levels in hypophysial stalk blood in the rat are sufficient to inhibit prolactin secretion in vivo

Much evidence indicates that dopamine can inhibit PRL secretion. However, it is still unclear whether dopamine acts on hypothalamic neurons to stimulate PRL-inhibiting factor (PIF) release or whether dopamine is itself PIF, acting directly on the pituitary. This study was designed to determine if PRL secretion is inhibited by dopamine infusions producing plasma dopamine levels which mimic those found in hypophysial stalk plasma. Stalk plasma dopamine concentration in urethane-anesthetized diestrus-1 rats was 6.0 ± 1.1 ng/ml (mean ± SE; n = 10). During a dopamine infusion, stalk plasma dopamine levels were 70% of arterial levels. Therefore, dopamine was infused at a rate which achieved arterial levels of 9–10 ng/ml and the effect on PRL secretion in urethane-anesthetized female rats was measured. In diestrus rats, dopamine did not inhibit PRL secretion. In rats pretreated with α-methyl-p-tyrosine to elevate serum PRL levels, dopamine suppressed PRL secretion 70%. In rats with PRL levels elevated by median eminence lesions, dopamine suppressed PRL secretion 42%. A negligible amount of the infused dopamine was converted to epinephrine or norepinephrine. Thus, PRL secretion was inhibited by physiological levels of dopamine in rats in which hypothalamic dopamine secretion was blocked (α-methyl-p-tyrosine-treated) and in rats in which all hypothalamic input to the pituitary was eliminated (median eminence lesions). These results support the hypothesis that dopamine is a physiological PIF acting directly on the pituitary.