Mice with genomic knockout of either melanocortin type 3 receptors (MC3R-/-), type 4 receptors (MC4R-/-) or knockout of both (double knockout, DKO) were tested for their anorectic response to the mixed MC3/4R agonist, MTII, injected into the anterior cerebral ventricle. Wild type (WT) mice showed a strong anorexia and, as expected, DKO were completely unresponsive to MTII. In contrast, both MC3R-/- and MC4R-/- showed a partial anorectic response. Induction of c-Fos immunoreactivity by MTII was examined in brain regions including paraventricular hypothalamus (PVN) and area postrema (AP). Compared with WT, MC4R-/- showed no activation in AP but showed normal activation in PVN, whereas MC3R-/- showed reduced activation in PVN but not in AP. RT-PCR analysis showed that hypothalamic mRNA for MC3R in MC4R-/- and for MC4R in MC3R-/- was unaltered from WT levels. These data suggest that both receptor subtypes are involved in the behavioral action of MTII, and that the critical receptors are in different brain regions.
Bibliographical noteFunding Information:
This work was funded in part by NIH grants RO1DK064712 and RO1DK057080 . We thank Dr. Dennis Huszar and Millennium Pharmaceuticals/Genelogic for providing the breeding stock MC4R−/− mice, Dr. Lex VanDerPloeg and Merck Research for providing the breeding stock MC3R−/− mice, and both David White at Genelogic and Merck Research for allowing us to generate and study the MC3R−/− × MC4R−/− double knockout mice.