Substance P is found in the gut and in the brain. It is responsible for a number of excitatory effects on both central and peripheral neurons. Substance P contracts smooth muscle, constricts bronchioles, and increases capillary permeability. When released from afferent nerves, Substance P causes a neurogenic inflammatory response, including mast cell degranulation. It's an NK-1 agonist; a potent vasodilator and hypotensive agent; it induces salivation; it's a putative neurotransmitter in sensory (pain) afferents.