Effects of vasoactive intestinal polypeptide, TRH, and dopamine on prolactin secretion in estrogen-primed postmenopausal women

Abstract

Abstract. Prolactin secretion is influenced by at least three important hypothalamic neurotransmitters: TRH, vasoactive intestinal polypeptide and dopamine. The purpose of this study was to determine whether, in estradiol-primed postmenopausal women, the PRL response to TRH, vasoactive intestinal polypeptide, and dopamine differed. Ten postmenopausal women were studied during treatment with estradiol benzoate at a dose of 0.625 mg per day during 15 days. TRH (200 μg iv), saline infusion, vasoactive intestinal polypeptide (75 μg infused iv during 15 min), coadministration of vasoactive intestinal polypeptide and TRH, and dopamine (4 μg · kg−1 · min−1 iv for 3 h) were administered for 5 consecutive days before and during the last 5 days of estradiol benzoate treatment. Before estradiol benzoate administration, the PRL response to TRH was significantly greater than that of vasoactive intestinal polypeptide. Estradiol benzoate treatment increased significantly the PRL release induced by TRH (p < 0.01), but did not modify the response to vasoactive intestinal polypeptide. At the end of estradiol benzoate treatment, the maximal increase in PRL after the combined (vasoactive intestinal polypeptide + TRH) test was greater than that obtained with TRH alone and occurred earlier (p < 0.04). Dopamine suppressed PRL secretion to a similar extent after estradiol benzoate treatment. Thus: 1) in postmenopausal women, estrogens act as stimulators of PRL release; 2) estrogens are able to increase TRH-stimulated PRL secretion but not the response to vasoactive intestinal polypeptide and to reveal the additive effect of TRH and vasoactive intestinal polypeptide; 3) estradiol does not modify the effectiveness of dopamine as an inhibitor of PRL secretion; 4) these results suggest different intracellular mechanisms of the action of TRH, vasoactive intestinal polypeptide and dopamine on PRL secretion.