Long-term alterations in body weight do not affect the expression of melanocortin receptor-3 and -4 mRNA in the ovine hypothalamus

The pro-opiomelanocortin-derived peptides and the melanocortin receptors are implicated in various functions within the CNS including the regulation of food intake. In the present study, we used in situ hybridization, with specific 35S-labelled ovine riboprobes to map the expression of melanocortin receptor-3 (MC3-R) and -4 (MC4-R) mRNA in the diencephalon and brainstem of normal female sheep. Furthermore, we examined the effect of long-term alterations in energy balance on the distribution and expression of MC3-R and MC4-R mRNA in food-restricted and ad libitum-fed ovariectomized female sheep. The distribution of melanocortin receptors generally resembled that of the rat. A high number of MC3-R-labelled cells were seen in the ventral division of the lateral septum and the medial preoptic area. In the hypothalamus, a moderate number of MC3-R-labelled cells was observed in the lateral hypothalamic area while other nuclear groups had low to intermediate numbers of MC3-R-labelled cells. The distribution of MC4-R mRNA was generally similar to that of MC3-R mRNA in the septal/preoptic and hypothalamic regions, with a high number of labelled cells present in the intermediate division of the lateral septum. Within the hypothalamus, no MC4-R mRNA expression was observed in the arcuate nucleus. There was more widespread distribution of moderate to low numbers of MC4-R mRNA-expressing cells in the brainstem compared to that of MC3-R mRNA. Unlike findings in the rat, only a low number of cells expressed melanocortin receptor mRNA in the ovine hypothalamic nuclei associated with feeding behavior. The number of melanocortin receptor-labelled cells and the level of expression (silver grains/cell) in the hypothalamic feeding centers was similar in food-restricted and ad libitum-fed animals. These findings suggest that long-term alterations in metabolic status do not change the melanocortin receptor mRNA distribution and/or expression in the sheep hypothalamus.