Substance P

In the field of neuroscience, substance P (SP) is a neuropeptide: an undecapeptide that functions as a neurotransmitter and as a neuromodulator. It belongs to the tachykinin neuropeptide family. Substance P and its closely related neuropeptide neurokinin A (NKA) are produced from a polyprotein precursor after differential splicing of the preprotachykinin A gene. The deduced amino acid sequence of substance P is as follows: Substance P is released from the terminals of specific sensory nerves, it is found in the brain and spinal cord, and is associated with inflammatory processes and pain.
 * Arg Pro Lys Pro Gln Gln Phe Phe Gly Leu Met

Discovery
Substance P was originally discovered in 1931 by Ulf von Euler and John H. Gaddum as a tissue extract that caused intestinal contraction in vitro. Its tissue distribution and biologic actions were further investigated over the following decades. In 1983, NKA (previously known as substance K or neuromedin L) was isolated from porcine spinal cord and was also found to stimulate intestinal contraction.

Receptor
The endogenous receptor for Substance P is neurokinin 1 receptor (NK1-receptor, NK1R). It belongs to the tachykinin receptor sub-family of GPCRs. Other neurokinin subtypes and neurokinin receptors that interact with SP have also been reported. Amino acid residues that are responsible for the binding of SP and its antagonists are present in the extracellular loops and transmembrane regions of NK-1. Binding of SP to NK-1 results in internalization by the clathrin-dependant mechanism to the acidified endosomes where the complex disassociates. SP is subsequently degraded and NK-1 is re-expressed on the cell surface. Substance P and the NK1 receptor are widely distributed in the brain and are specifically found in brain regions that regulate emotion (hypothalamus, amygdala, and the periaqueductal gray). They are also found in close association with 5-hydroxytryptamine (5-HT) and neurons containing norepinephrine that are targeted by the currently used antidepressant drugs. The SP receptor promoter contains regions that are sensitive to cAMP, AP-1, AP-4, CEBPB and epidermal growth factor. Because these regions are related to complexed signal transduction pathways mediated by cytokines, it has been proposed that cytokines and neurotropic factors can induce NK-1. SP can also induce the cytokines that are capable of inducing NK-1 transcription factors.

Function
Substance P is an important element in pain perception. The sensory function of substance P is thought to be related to the transmission of pain information into the central nervous system. Substance P coexists with the excitatory neurotransmitter glutamate in primary afferents that respond to painful stimulation. SP has been associated with the regulation of mood disorders, anxiety, stress, reinforcement, neurogenesis, respiratory rhythm, neurotoxicity, nausea/emesis, pain and nociception. Substance P and other sensory neuropeptides can be released from the peripheral terminals of sensory nerve fibers in the skin, muscle and joints. It is proposed that this release is involved in neurogenic inflammation which is a local inflammatory response to certain types of infection or injury. The regulatory function of SP also involves the regulation of its high-affinity receptor, NK-1. Substance P receptor antagonists may have important therapeutic applications in the treatment of a variety of stress-related illnesses, in addition to their potential as analgesics.

Vomiting
The vomiting center in the brainstem contains high concentrations of substance P and its receptor, in addition to other neurotransmitters such as choline, histamine, dopamine, serotonin, and opioids. Their activation stimulates the vomiting reflex. Different emetic pathways exist, and substance P/NK1R appears to be within the final common pathway to regulate vomiting. Substance P antagonist (SPA) aprepitant is available in the market in the treatment of chemotherapy-induced nausea/emesis.

Pain
Substance P is involved in nociception, transmitting information about tissue damage from peripheral receptors to the central nervous system to be converted to the sensation of pain. It has been theorized that it plays a part in fibromyalgia. Capsaicin has been shown to reduce the levels of Substance P probably by reducing the number of C-fibre nerves or causing these nerves to be more tolerant. Thus, Capsaicin is clinically used as an analgesic and anti-inflammatory agent to relieve pain associated with arthritis and many types of neuralgia. A role of substance P and NKA in nociception is suggested by the reduction in response thresholds to noxious stimuli by central administration of NK1 and NK2 agonists. Based on recent studies, it was proposed that NK1, and possibly the NK2 receptor antagonists could be developed as analgesic drugs. It has been studied that the mice carrying a disruption of the gene encoding SP/NKA, show severely reduced nociceptive pain responses when the stimuli are moderate to intense. Pain behaviors induced by mechanical, thermal and chemical stimulation of somatic and visceral tissues were reduced in the mutant mice lacking SP/NKA. However, it has been proposed that the importance of SP and NKA in animal's pain response apply only to a certain 'window' of pain intensities and when the intensity of the pain stimuli is further increased, the responses of the knockout mice is not severely different from the wild-type mice.

Cell growth
Substance P has been known to stimulate cell growth in culture, and it was shown that Substance P could promote wound healing of non-healing ulcers in humans. It has also been shown to reverse diabetes in mice.

Vasodilation
Substance P also has effects as a potent vasodilator. Substance P-induced vasodilatation is dependent on nitric oxide release. Substance P is involved in the axon reflex-mediated vasodilatation to local heating and wheal and flare reaction. It has been shown that vasodilatation to substance P is dependent on the NK1 receptor located on the endothelium. In contrast to other neuropeptides studied in human skin, substance P-induced vasodilatation has been found to decline during continuous infusion. This possibly suggest an internalization of neurokinin-1 (NK1). As is typical with many vasodilators, it also has bronchoconstrictive properties, administered through the non-adrenergic, non-cholinergic nervous system (branch of the vagal system).

Eczema
High levels of BDNF and Substance P have been found associated with increased itching in eczema.

Gastrointestinal infection
Entamoeba histolytica is a single-celled parasitic protozoan that infects the lower gastrointestinal tract of humans. The symptoms of infection are diarrhea, constipation, and abdominal pain. This protozoan was found to secrete serotonin as well as substance P and neurotensin.

Deficiency
Naked mole rats lack Substance P and do not feel pain when painful stimuli are administered to the skin. New studies have shown that when the function of SP is genetically disrupted in mice, the animals demonstrated reduced responses to painful stimuli. Moreover, the response to capsaicin was absent or severely reduced in knockout mice.