Substance P (SP), is an 11-amino acid neuropeptide. SP is a member of the tachykinin family of peptides because it induces rapid smooth muscle contraction in guinea pig ileum and rat duodenum. Other members of the tachykinin family, sharing common carboxyl terminal Phe-X-Gly-Leu-met-NH2 sequences in mammals, include neurokinin A and neurokinin B. In mammals, tachykinins are produced by two genes, preprotachykinin-A (PPT-A) and preprotachykinin-B (PPT-B), and SP is a product of the PPT-A gene. SP is localized in the central nervous system as well as in several peripheral tissues, including the entire length of the gastrointestinal tract as well as the colon. The main sources of SP in the gut include the myenteric and submucosal plexus, intrinsic sensory neurons, as well as sensory neurons originating from the dorsal root ganglia. A newly identified gene, preprotachykinin C gene, encodes for the sequence of a new preprotachykinin protein designated hemokinin (HK) and produced primarily by hematopoietic cells. HK binds with high selectivity to NK-1R and has similar in vivo potency to SP. Like SP, HK is an 11-amino acid peptide having ~55% amino acid similarity to SP. The effects of SP are mediated by three different G-protein-coupled receptors (GPCRs), namely neurokinin (NK)-1, 2, and 3. SP binds with high affinity to NK-1 receptor (NK-1R), and with low affinity to NK-2 and 3 receptors. NK-1 receptors are present in both small intestine and colon of animals and humans and are localized in a variety of cells, including nerves, smooth muscle, immune cells, glands, endothelial cells, as well as epithelial cells. Although NK-1 receptors have been associated with several intestinal pathophysiologic conditions (see below), NK-2 receptors have been linked mostly with circular muscle contraction, and are localized in circular muscle and muscularis mucosae. Although NK-2 receptors are present predominantly on smooth muscle and, like NK-1, can affect gut motility, NK-3 receptors are expressed predominantly in neurons and can stimulate or diminish muscle contraction indirectly following SP binding to neuronal cells in the submucosal and myenteric nerve plexuses of the gastrointestinal tract. NK-3 receptors also provide slow excitatory synaptic input to neurons in ganglia of the sphincter of Oddi. Thus, both NK-2 and NK-3 receptors affect motility responses in the GI, but there is very little evidence that they are involved in neuroimmune interactions. (PMID: 17192554).