Arousal and appetite through MC4R

MC4R sits in the hypothalamus and the spinal cord, and it is a central node for both feeding and sexual function. Driving it produces erection through brain and spinal pathways rather than through blood vessels in the penis, which is why the original work saw erections appear with no sexual stimulation at all, and why it works differently from the vessel-acting medicines. The same receptor is the body's own satiety switch, so driving it also damps appetite. Yawning, stretching and nausea come from the same central receptors, which is why they arrive together.