The influence of gonadal steroids on pre-pro melanin-concentrating hormonemRNA in female rats

Melanin-concentrating hormone (MCH) may have a regulatory role in the contr ol of luteinizing hormone (LH) release. We have investigated if gonadal ste roids induce changes in the expression of pre-pro MCH (ppMCH) that are asso ciated with changes in the pattern of LH release. Using quantitative in-sit u hybridization histochemistry we have determined the effect of administrat ion of either oestradiol benzoate (5 mu g/rat) or oestradiol benzoate follo wed 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 lat eral 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 a lso examined. Oestrogen reduced the intensity of hybridization signal for p pMCH mRNA and this was associated with both a decrease in the number of cel ls in which the message was detected in the medial zona incerta and a negat ive feedback effect on LH release in ovariectomized rats. Progesterone admi nistration to oestradiol benzoate-primed rats did not alter the reduced exp ression in the medial zona incerta in spite of its positive feedback effect on LH release. We suggest that progesterone may act only on post-translati onal 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 late ral hypothalamus. No changes in ppMCH mRNA were seen in intact animals on p roestrus or the first day of dioestrus indicating that gonadal steroids are not important in the modulation of ppMCH gene expression during the oestro us cycle. In other steroid-dependent physiological situations, however, oes trogen may influence the expression of ppMCH in a subpopulation of cell bod ies in the medial zona incerta.