EFFECTS OF TAMOXIFEN ON SERUM PROLACTIN LEVELS, PITUITARY IMMUNOREACTIVE PROLACTIN CELLS AND UTERINE GROWTH IN ESTRADIOL-TREATED OVARIECTOMIZED RATS

Pituitary effects of the antiestrogen tamoxifen are not well establish ed, although estrogen is known to have a stimulatory role in prolactin secretion. Effects of tamoxifen on serum prolactin levels, pituitary wet weight and number of prolactin cells were studied, Ovariectomized female Wistar rats were injected, subcutaneously, with estradiol valer ate, 50 or 300 mu g/rat per week for 2 or 10 weeks. Tamoxifen was inje cted during the last days of estrogen treatment. Data were compared wi th two other groups, treated with estradiol valerate alone or estradio l valerate plus the dopamine agonist bromocriptine. Serum prolactin le vels were increased by estrogen treatment with all doses used. Further more, rats treated with 300 mu g of estradiol valerate, for 2 and 10 w eeks, showed a clear increase in pituitary weight and number of prolac tin cells (p < 0.05), Bromocriptine decreased prolactin levels, pituit ary weight and the number of prolactin cells (p < 0.05). Tamoxifen ass ociated to subacute period of estrogen administration resulted in a si gnificant reduction of serum prolactin levels and pituitary weight (p < 0.05). No effects on prolactin levels or number of prolatin cells we re observed with tamoxifen associated to chronic estrogen treatment. T amoxifen also presented a dose-related inhibitory effect upon estrogen -stimulated rises in uterine weight and DNA content. In conclusion, th e results of the present paper showed that tamoxifen reduced estrogen- stimulated prolactin levels in some, but not in other hormonal conditi ons and that these effects were not mediated by an inhibition of lacto troph cell growth, Further studies are needed to define the exact role of antiestrogens at molecular level in hyperprolactinemic states and their eventual connection with dopamine and its agonists.