Doctor Larry Young describes how neuropeptides can activate circuits that can change emotions and behaviors.
So the brain has multiple different kinds of chemical communicators or transmitters; one of those is neuropeptides. A neuropeptides is a chain of amino acids that is released either into the blood where it activates receptors in our body, or directly into the brain where it can activate receptors in our brain. When a neuropeptide is released it binds to the receptor and that activates a series of events inside the cell, inside the neuron, and then that causes that neuron to become active. If that neuron is placed within a particular circuit, it can activate that circuit. So now you can begin to see how you can have a single molecule released from one neuron activating another neuron, and then activating a circuit and then a circuit can change an emotion and a behavior.