Previous studies have shown that the hypothalamic content of β-endorphin (βEP) and other POMC-derived peptides increases 4 weeks after orchiectomy and that this increase can be prevented by testosterone replacement. To determine if these changes in hypothalamic βEP content are secondary to changes in the biosynthesis of βEP we have measured POMC mRNA levels in intact and castrated adult male rats with and without testosterone replacement. After either 2 or 4 weeks of treatment the medial basal hypothalamus (MBH) was collected and homogenized. An aliquot was removed for βEP RIA, RNA was isolated, and the amount of POMC mRNA was measured using a solution hybridization S1 nuclease protection assay. In agreement with previous results, the content of βEP in the MBH from 4-week castrated animals (7930 ± 568 pg/mg protein) was significantly increased compared to that in either castrated and testosterone-replaced rats (5792 ± 568 pg/mg; P < 0.02) or intact controls (6027 ± 349 pg/mg; P < 0.02). POMC mRNA in the MBH from the 4-week castrated group was significantly higher compared to that in the castrated and testosterone- replaced group (2.61 ± 0.33 vs. 1.7 ± 0.22 pg/fig RNA; P < 0.05), which is parallel to the changes we found in βEP peptide levels. Two weeks after castration no significant change was detected in the βEP content in the MBH from castrated rats (5401 ± 318 pg/mg protein) compared to that in the castrated and testosterone-replaced group (4848 ± 304 pg/mg protein). However, we were able to detect a significant difference in the amount of POMC mRNA in the 2-week castrated group (1.47 ± 0.267 pg/μg RNA) compared to that in the 2-week castrated and testosterone-replaced group (0.815 ± 0.061 pg/μg RNA; P < 0.05). the finding that testosterone replacement for either 2 or 4 weeks to castrated rats significantly reduced POMC mRNA levels suggests that sex steroids have an inhibitory effect on the biosynthesis of POMC in the MBH.
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