Clonidine is an alpha-2 adrenergic receptor agonist that crosses the blood-brain barrier and stimulates receptors in the nucleus tractus solitarius and locus coeruleus. This reduces sympathetic outflow, decreases norepinephrine release, and lowers peripheral vascular resistance, heart rate, and blood pressure. In ADHD, modulation of prefrontal cortical noradrenergic signaling improves executive function and impulse control.