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 established, 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 valerate, 50 or 300 micrograms/rat per week for 2 or 10 weeks. Tamoxifen was injected during the last days of estrogen treatment. Data were compared with two other groups, treated with estradiol valerate alone or estradiol valerate plus the dopamine agonist bromocriptine. Serum prolactin levels were increased by estrogen treatment with all doses used. Furthermore, rats treated with 300 micrograms of estradiol valerate, for 2 and 10 weeks, showed a clear increase in pituitary weight and number of prolactin cells (p < 0.05). Bromocriptine decreased prolactin levels, pituitary weight and the number of prolactin cells (p < 0.05). Tamoxifen associated to subacute period of estrogen administration resulted in a significant reduction of serum prolactin levels and pituitary weight (p < 0.05). No effects on prolactin levels or number of prolatin cells were observed with tamoxifen associated to chronic estrogen treatment. Tamoxifen also presented a dose-related inhibitory effect upon estrogen-stimulated rises in uterine weight and DNA content. In conclusion, the results of the present paper showed that tamoxifen reduced estrogen-stimulated prolactin levels in some, but not in other hormonal conditions and that these effects were not mediated by an inhibition of lactotroph 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.

Effects of norethisterone acetate and tamoxifen on serum prolactin levels, uterine growth and on the presence of uterine immunoreactive prolactin in estradiol-treated ovariectomized rats

The aim of the present work was to study the effects of the antiestrogen tamoxifen (TAM), of progestin norethisterone acetate (NA) and of their combination on serum prolactin levels, uterine growth and the presence of uterine immunoreactive prolactin in estradiol-treated rats. Ovariectomized female Wistar rats were injected sc with estradiol valerate (VE, 50 micrograms/rat per week) or oil vehicle. During the second week, estradiol-treated rats also received NA (0.12 or 1.0 mg/rat, sc, daily) or TAM (0.06 mg/rat) alone or in combination with NA (0.12 mg). Serum prolactin levels were suppressed to the same extent in the TAM- and 1.0 mg NA-treated groups compared with rats given estrogen alone (2.3 +/- 0.3 and 5.6 +/- 1.5 ng/ml for TAM and NA groups vs 39.7 +/- 3.6 ng/ml for VE groups, P < 0.05). Except for the lowest dose of NA, uterine wet weight and DNA content were significantly reduced in all groups compared to estradiol alone (236.8 +/- 18.0 and 295.6 +/- 27.8 mg vs 309.4 +/- 32.2 mg for uterine weight in TAM and NA groups vs VE, respectively, P < 0.05; and 1.14 +/- 0.05 and 0.93 +/- 0.04 mg/uterus vs 1.33 +/- 0.06 mg/uterus for uterine DNA in TAM and NA groups vs VE groups). The combination of NA and TAM resulted in a higher degree of suppression of uterine growth than when each drug was used alone, indicating an additive antiproliferative effect of NA and TAM. Although no prolactin immunostaining was detected in the uterus of rats treated with estradiol, uterine immunoreactive prolactin was identified in those treated with NA, TAM or both.(ABSTRACT TRUNCATED AT 250 WORDS)