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The Influence of Gonadal Steroids on Pre‐Pro Melanin‐Concentrating Hormone mRNA in Female Rats

Melanin‐concentrating hormone (MCH) may have a regulatory role in the control of luteinizing hormone (LH) release. We have investigated if gonadal steroids induce changes in the expression of pre‐pro MCH (ppMCH) that are associated with changes in the pattern of LH release. Using quantitative in‐situ hybridization histochemistry we have determined the effect of administration of either oestradiol benzoate (5 μg/rat) or oestradiol benzoate followed 44 or 48 h later by progesterone (0.5 mg/rat) to ovariectomized rats on the expression of ppMCH in the medial and lateral zona incerta and the lateral hypothalamus. The prevalence of ppMCH transcripts in the intact female rat at 12.00 and 19.00 h on proestrus and the first day of dioestrus was also examined. Oestrogen reduced the intensity of hybridization signal for ppMCH mRNA and this was associated with both a decrease in the number of cells in which the message was detected in the medial zona incerta and a negative feedback effect on LH release in ovariectomized rats. Progesterone administration to oestradiol benzoate‐primed rats did not alter the reduced expression in the medial zona incerta in spite of its positive feedback effect on LH release. We suggest that progesterone may act only on post‐translational events. Expression in the MCH cell bodies of the lateral zona incerta were not affected but there was a transient decrease 4 h after progesterone treatment in the oestradiol benzoate‐primed rats in expression in the lateral hypothalamus. No changes in ppMCH mRNA were seen in intact animals on proestrus or the first day of dioestrus indicating that gonadal steroids are not important in the modulation of ppMCH gene expression during the oestrous cycle. In other steroid‐dependent physiological situations, however, oestrogen may influence the expression of ppMCH in a subpopulation of cell bodies in the medial zona incerta.