Preliminary data indicate that the melanocortin-3 receptor (MC3R) is important for the entrainment of circadian rhythms of locomotory behavior to feeding cues. Normally, the daily rhythms of ingestive behavior, physical activity, and the metabolic activity of key digestive organs are entrained to light. However, eating outside the normal time results in a phase shift in the circadian rhythms of physical activity and metabolism. In mammals, circadian rhythms are maintained at the cellular level by oscillators distributed throughout the body. Oscillators responding to feeding and light appear to be distinct. One component of the core oscillator that responds to feeding cues is the transcription factor neuronal PAS domain protein 2 (NPAS2). NPAS2 exhibits a circadian profile of expression and activity in the brain that correlates with peak periods of ingestive behavior. NPAS2 is not required for maintaining circadian rhythms in the absence of light, however it is important for entrainment to feeding cues. The mechanisms involved in the regulation of the oscillators by feeding is not clear. Two structures in the hypothalamus: the ventromedial hypothalamus and the subparaventricular zone, have been demonstrated to be important for circadian rhythms of ingestive and locomotory behavior. The MC3R is highly expressed in a region of the ventromedial nucleus of hypothalamus that projects to the subparaventricular zone. Pro-opiomelanocortin (POMC) neurons that release the endogenous ligands of the MC3R project from the arcuate nucleus to the ventromedial hypothalamus, and respond to metabolite and endocrine factor that indicate nutrient consumption and energy balance. The central hypothesis of this application is that hypothalamic POMC neurons, acting on second order neurons expressing the MC3R in the ventromedial hypothalamus, are involved in the entrainment of oscillators, including NPAS2, to feeding cues. The first and second aims of this proposal use gene knockout mice to demonstrate that MC3R are involved in the food entrainment of oscillators in the central nervous system and liver, an organ vital to the digestive process.
The third aim uses a combination of conditional knockout (Mc3r flox/flox, Sf1-Cre) and recombinant adenovirus constructs for site- and neuron- specific targeting of MC3R expression in hypothalamus. The objective of the third aim is to identify the MC3R neurons in the central nervous system that transmit inputs into oscillators responding to feeding cues.
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