Previous studies have shown that the hypothalamic concentrations of β-endorphin (β-EP) and other proopiomelanocortin (POMC)-derived peptides change in the female rat following castration and gonadal steroid replacement. In this study we have measured POMC mRNA by solution hybridization assay in the medial basal hypothalamus (MBH) of ovariectomized rats treated with a regimen of estradiol (E2) that we have previously shown alters brain β-EP peptide content. In addition the effect of progesterone (P) was also studied. In the first experiment the concentration of β-EP and α-melanocyte-stimulating hormone (α-MSH) in the MBH of castrated rats decreased significantly after 3 weeks of E2 treatment compared to castrated unreplaced rats: β-EP decreased from 6.00 ± 0.46 to 4.32 ± 0.38 ng/mg protein and α-MSH decreased from 3.00 ± 0.23 to 2.35 ± 0.15 ng/mg protein (p < 0.05). A similar decrease in peptide content was noted in the anterior hypothalamus/ preoptic area. A parallel reduction in the concentration of POMC mRNA was measured in the MBH of the E2-replaced animals: 1.17 ± 0.14 vs. 0.72 ± 0.08 pg/µg RNA (p < 0.02). In a second study castrated rats were studied after 2 weeks of E2 or E2 plus P treatment. After 2 weeks, POMC peptide levels did not change significantly in the MBH of either the E2- or E2 plus P-treated rats. POMC mRNA, however, was significantly reduced from 1.10 ± 0.10 pg/µg RNA in the unreplaced rats to 0.58 ± 0.05 and 0.61 ± 0.06 pg/µg RNA after E2 or E2 plus P, respectively (p < 0.001). No significant change in the concentration of β-EP or POMC mRNA was noted after 48 h of E2. We conclude that E2 has significant effects on POMC gene expression and peptide levels in the MBH of female rats. The parallel decrease in POMC mRNA and peptide levels and the fact that the changes in gene expression precede the changes in peptide content are consistent with an inhibitory effect of E2 on the biosynthesis of POMC in the MBH under these conditions.

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