with an amidation at the C-terminus. 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.
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 porcinespinal cord and was also found to stimulate intestinal contraction.
The endogenousreceptor 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-dependent 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 serotonin (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.
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 neurotransmitterglutamate in primary afferents that respond to painful stimulation. Substance P 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.
The vomiting center in the medulla 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.
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, it is presumed, 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 an anti-inflammatory agent to reduce 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 K2 and K3 agonists. Based on recent studies, it was proposed that NK1, and possibly 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.
Substance P increases glutamate activity (NMDA) in central nervous system, and it is associated with the development of brain edema and functional deficits after traumatic brain injury.
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.
Substance P injected into pancreatic nerves has been shown to reverse diabetes in mice but effects to insulin secretion seem to be species dependent. In humans, substance P given intravenously seems to decrease insulin release and causes fluctuations in blood sugar levels.
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 suggests 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).
High levels of BDNF and substance P have been found associated with increased itching in eczema.
When the innervation to substance P nerve terminals is lost, post-synaptic cells compensate for the loss of adequate neurotransmitter by increasing the expression of post-synaptic receptors. This, ultimately, leads to a condition known as Denervation Supersensitivity as the post-synaptic nerves will become hypersensitive to any invasion of substance P into the synaptic cleft.
Naked mole rats lack cutaneous C fibers reactive to substance P (SP) and many small neurons that are normally SP-positive. Thus, these animals are insensitive to 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.
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