Melanocortin-3 receptors (MC3R) are an important component of the central nervous melanocortin system. While this system is under active investigation with respect to energy homeostasis and obesity, surprisingly little is known about the specific functions of MC3Rs expressed in the brain. Our long term goal has been to investigate and understand the functions of MC3R in the central nervous system. In the previous funding cycle, we developed the overarching research hypothesis that MC3R(+ve) neurons are a point of convergence for systems governing the anticipatory response to nutrient intake. The overall objective of the next funding cycle is to identify and characterize MC3R(+ve) neurons involved in expression of the anticipatory response to food intake. To achieve this objective, we developed a strain of C57BL/6J mice where Mc3r transcription is inhibited by a "lox-stop-lox" (LoxTB) sequence inserted 5'of the open reading frame. We will use this model in pursuing the following specific aims to test the central hypothesis.
Aim 1 will identify MC3R signaling pathways involved in regulating the expression of ingestive behaviors anticipating nutrient intake. The anticipatory response appears to involve a circadian or clock-like mechanism. Our data suggests that MC3Rs are inputs into systems governing the synchronization of circadian rhythms with food intake.
This aim will investigate the working hypothesis that the expression of rhythms anticipating food presentation involves actions of MC3Rs in the dorsomedial and lateral hypothalamus and in mesolimbic dopaminergic neurons. These objectives will be met by assessing FAA and entrainment of circadian rhythms to food in LoxTB MC3R mice crossed with transgenic mouse strains expressing Cre in areas of the mediobasal hypothalamus and ventral tegmental area.
Aim 2 will identify MC3R signaling pathways responsible for maintaining metabolic homeostasis and synchronizing metabolic rhythms during restricted feeding. It is well established that the central nervous Melanocortin system is coupled to efferent pathways governing glucose and fatty acid metabolism. Our data suggests that MC3Rs regulate inputs into systems that govern hepatic glucose production and insulin action during periodic cycles of fasting and re-feeding.
This aim will investigate the working hypothesis that actions of MC3Rs expressed in the lateral and ventromedial hypothalami are involved in synchronizing rhythms in metabolism with food availability to minimize the impact of nutrients on the body. Melanocortin analogs are being actively investigated in the development of therapies against obesity and diabetes, so the outcomes from the proposed research may have a positive impact on the development of new therapies. Outcomes from the proposed research also have the potential to impact on strategies for treating metabolic disorders through using MC3R modulators. Moreover, investigating the regulation of circadian rhythms by MC3Rs could also have an impact on the development of treatments for behavioral disorders linked to aberrant circadian function.
Relevance of the proposed research to public health is based on clinical data showing that the Melanocortin system functions to prevent obesity in humans. This system is considered an attractive target for developing drugs against obesity and eating disorders, however little is known about the functions of melanocortin-3 receptors. The proposed research may therefore have a direct impact on the development of drugs targeting this system.
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