Putative targets of CNS melanocortin receptor activity.

Chronic antagonism of hypothalamic melanocortin receptors, primarily melanocortin-4 receptor (MC4R), is the molecular basis for "agouti obesity syndrome," whereas suppression of MC4R gene activity due to genetic mutations induces obesity in both rodents and humans. However, little is known about the neurocircuitry of MC4R-mediated control of energy balance, the regulation of MC4R gene expression, or how suppression of MC4R activity leads to differential expression of potential downstream central nervous system (CNS) targets or effectors of melanocortin signaling. This paper focuses on strategies for mapping CNS melanocortin circuits using transgenic mouse models for conditional expression of MC4R and MC3R as well as progress in characterizing the murine MC4R promoter. Additionally, preliminary studies that focus on putative targets of melanocortinergic signaling will include a discussion of CD81, a gene identified using the polymerase chain reaction-based method of suppression subtractive hybridization. CD81, first described as TAPA-1 (target of antiproliferative antibody), is a member of the tetraspanin family of cell surface proteins believed to function in cell-cell adhesion, signal transduction, and possibly neuronal plasticity. Elevated expression of CD81 mRNA in hypothalamic regions of obese yellow mice suggests that loss of MC4R activity may lead to altered neuronal function via modulation of the cell surface protein CD81.