a neuropeptide that functions as a neurotransmitter in both peripheral and central nervous systems. It belongs to the neurokinin family of transmitters. High concentrations of neurons containing substance P are localized in the dorsal horn of the spinal cord, where they play a role in the modulation of pain. In peripheral tissues, substance P acts as a vasodilator. It also has a role in sexual behavior and has been implicated in the regulation of mood.