Relationship of α-MSH and AgRP axons to the perikarya of melanocortin-4 receptor neurons

The central melanocortin system is composed of neurons that express either the proopiomelanocortin (POMC) or the agouti-related protein (AgRP). POMC is cleaved in bioactive peptides, including the alpha-melanocyte-stimulating hormone (alpha-MSH). alpha-MSH activates the melanocortin-4 receptor (MC4R) inducing satiety, whereas AgRP acts as an inverse agonist of MC4R. However, only limited information is available regarding possible area-specific differences in the interaction between alpha-MSH and AgRP terminals on MC4R-expressing cells. Therefore, the objective of the present study was to compare the distribution pattern of alpha-MSH and AgRP terminals on the perikarya of MC4R-expressing neurons. We performed a triple-label immunofluorescence reaction in brain series of MC4R-reporter mice to visualize MC4R-expressing neurons together with AgRP and a-MSH terminals. POMC and AgRP neurons project to areas that contain MC4R-expressing cells, although several brain nuclei exhibit AgRP and alpha-MSH terminals, but they do no express MC4R, while other brain areas contain MC4R-expressing cells and receive no apparent innervation of AgRP and POMC neurons. AgRP terminals make more presumptive appositions than a-MSH on the soma of MC4R-expressing neurons of the medial preoptic area and paraventricular nucleus of the hypothalamus (Pa). Additionally, a higher percentage of MC4R cells receive at least one presumptive apposition from AgRP terminals in the median preoptic nucleus and Pa, compared to alpha-MSH appositions. Thus, our study revealed area-specific differences in the interaction between alpha-MSH and AgRP terminals and the soma of MC4R-expressing neurons. These findings provide new insights about the relationship between first- and second-order neurons of the central melanocortin system.