PRL increases during pregnancy in primates with rising levels of placental estradiol (E) and progesterone (P). However, while E will increase PRL secretion in monkey pituitary cell cultures, P has no effect. We recently localized progestin receptors (PR) to gonadotropes, but not lactotropes, with an immunocytochemical technique to double stain monkey pituitary cell cultures. The following studies were performed to confirm the immunocytochemical localization of PR in intact pituitary tissue and to determine the effect of E and P on the levels of estrogen receptors (ER) and PR in the pituitary. ER and PR levels were determined in the endometrium of the same animals for an internal comparison. Thirteen adult cycling female cynomolgus monkeys were ovariectomized and treated for 28 days with 1) an empty Silastic capsule (Spay), 2) a 2-cm Efilled capsule (E), or 3) a 2-cm E-filled capsule for 14 days plus a 6-cm P-filled capsule implanted for an additional 14 days (E+P). Blood samples were drawn daily for assay of serum E, P, and PRL levels. Serum PRL was not significantly affected by E, but the sequential addition of P significantly increased serum PRL levels over those observed in Spay animals. The anterior pituitary and endometrium were removed for measurement of ER and PR levels by a sucrose gradient shift assay incorporating monoclonal antibodies against ER and PR. Pituitary ER levels did not vary significantly with steroid treatment (158.2 ± 33.6, 135.5 ± 24.9, 104.3 ± 13.4 fmol/mg DNA in Spay, E, and E+P animals, respectively). Pituitary PR levels were undetectable in Spay animals, were induced by E (393.3 ± 53.4 fmol/mg DNA), and were suppressed to undetectable levels by the addition of P. A portion of the pituitary was frozen for immunocytochemical single staining for ER, PR, PRL, and LH and double staining for PRL+PR and LH+PR. ER staining was observed in many parenchymal cells, but there was no apparent change with steroid treatment. PR staining was absent in the Spay animals; many PR-positive cells were observed in E-treated females, and only a small number of faintly staining cells were detected in the E+P animals. Double staining for PRL+PR and LH+PR revealed PR in gonadotropes, but not lactotropes. In conclusion, PR, but not ER, are regulated by E and P in the monkey pituitary. Importantly, PR is regulated within gonadotropes, but not lactotropes. Therefore, P probably increases PRL secretion through a hypothalamic action.
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