Introduction: The Melanin-Concentrating Hormone (MCH) is an important neuropeptide that plays major roles in the maintenance of homeostatic balance in mammals. The action of MCH is mediated by two G-protein coupled membrane receptors called MCHR1 and MCHR2. While primates synthesize both subtypes of the MCH receptor, rats, mice and hamsters produce only the MCHR1, since the gene for MCHR2 has been lost on those animals. This major difference between humans and mice creates a challenge for researchers to fully comprehend the actions of MCH in humans. The development of a transgenic mice lineage with the targeted insertion of the human Mchr2 gene (MCHR2-KI mice) may help us understand the action mechanisms of MCH. Objective: To anatomically characterize the distribution of subtypes 1 and 2 of the melanin-concentrating hormone receptor (MCHR1 and MCHR2) in the central nervous system of rats and mice, employing wild type, MCHR1-KO and MCHR2-KI mice. We also expect to identify putative species-, sex- and estrous cycle-linked differences in MCHR1 distribution. Methods: The animals will be perfused and their brains, spinal chords and hypophyses removed and then sliced. Immunohistochemical and in situ hybridization studies will then be performed to identify the regions of synthesis of MCHR1 and MCHR2 and the loci of their mRNA expression. Differences between experimental groups will be analysed by stereological means and by the integrated optic density method. Preliminary Results: In female MCHR2-KI mice we found immunoreactivity to MCHR2 in several regions: cerebral cortex, septal area, globus pallidus, hippocampus, thalamus and periventricular region. Conclusion: More studies are necessary to evaluate the specificity of the primary antibody used and to confirm the immunoreactivity pattern found for MCHR2. We should also characterize the distribution of MCHR1 on those animals to identify putative differences between rats and mice, males and females and during the different phases of the estrous cycle.
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