Neurobiology of substance P and the NK₁ receptor

Substance P belongs to a group of neurokinins (NKs), small peptides that are broadly distributed in the central nervous system (CNS) and peripheral nervous system (PNS). The biological effects of substance P in the CNS, namely regulation of affective behavior and emesis in the brain and nociception in the spinal cord, are mediated by its binding to the NK₁ receptor. The substance P-NK₁ (SP-NK₁) receptor system is the most extensively studied NK pathway, and in contrast to receptors for other neurotransmitters, such as glutamate, which have high expression throughout the CNS, only a minority of neurons (5% to 7%) in certain CNS areas express the NK₁ receptor. The NK₁ receptor is distributed in the plasma membrane of cell bodies and dendrites of unstimulated neurons, but upon substance P binding, the NK₁ receptor undergoes rapid internalization, followed by rapid recycling to the plasma membrane, Release of substance P is induced by stressful stimuli, and the magnitude of its release is proportional to the intensity and frequency of stimulation. More potent and more frequent stimuli allow diffusion of substance P farther from the site of release, allowing activation of an approximately 3- to 5-times greater number of NK₁ receptor-expressing neurons. Recent studies employing pharmacologic or genetic inactivation of NK₁ receptors demonstrate the important role of the SP-NK₁ receptor system in the regulation of affective behavior and suggest that inhibition of this pathway may be a useful approach to treatment of depression and associated anxiety.