Substance P (SP) is the most researched and potent member of the tachykinin family. It is an undecapeptide with the amino acid sequence Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2.[6] SP binds to all three of the tachykinin receptors, but it binds most strongly to the NK1 receptor.[7]
The tachykinin NK1 receptor consists of 407 amino acid residues, and it has a molecular weight of 58,000.[6][9] NK1 receptor, as well as the other tachykinin receptors, is made of seven hydrophobic transmembrane (TM) domains with three extracellular and three intracellular loops, an amino-terminus and a cytoplasmiccarboxy-terminus. The loops have functional sites, including two cysteines for a disulfide bridge, Asp-Arg-Tyr, responsible for association with arrestin, and Lys/Arg-Lys/Arg-X-X-Lys/Arg, which interacts with G-proteins.[10][9] The binding site for substance P and other agonists and antagonists is found between the second and third transmembrane domains. The NK-1 receptor is found on human chromosome 2 and is located on the cell's surface as a cytoplasmic receptor.[11]
Function
The binding of SP to the NK1 receptor has been associated with the transmission of stress signals and pain, the contraction of smooth muscles, and inflammation.[12] NK1 receptor antagonists have also been studied in migraine, emesis, and psychiatric disorders. In fact, aprepitant has been proved effective in a number of pathophysiological models of anxiety and depression.[13] Other diseases in which the NK1 receptor system is involved include asthma, rheumatoid arthritis, and gastrointestinal disorders.[14]
Tissue distribution
The NK1 receptor can be found in both the central and peripheral nervous system. It is present in neurons, brainstem, vascular endothelial cells, muscle, gastrointestinal tracts, genitourinary tract, pulmonary tissue, thyroid gland, and different types of immune cells.[10][15][8][9]
Mechanisms of action
SP is synthesized by neurons and transported to synaptic vesicles; the release of SP is accomplished through the depolarizing action of calcium-dependent mechanisms.[6] When NK1 receptors are stimulated, they can generate various second messengers, which can trigger a wide range of effector mechanisms that regulate cellular excitability and function.
cAMP accumulation via stimulation of adenylate cyclase.[16]
It has also been reported that SP elicits interleukin-1 (IL-1) production in macrophages, sensitizes neutrophils, and enhances dopamine release in the substantia nigra region in cat brain. From spinal neurons, SP is known to evoke release of neurotransmitters like acetylcholine, histamine, and GABA. It also secretes catecholamines and plays a role in the regulation of blood pressure and hypertension. Likewise, SP is known to bind to N-methyl-D-aspartate (NMDA) receptors, eliciting excitation with calcium ion influx, which further releases nitric oxide. Studies in frogs have shown that SP elicits the release of prostaglandin E2 and prostacyclin by the arachidonic acid pathway, which leads to an increase in corticosteroid output.[8]
Clinical significance
In combination therapy, NK1 receptor antagonists appear to offer better control of delayed emesis and post-operative emesis than drug therapy without NK1 receptor antagonists. NK1 receptor antagonists block responses to a broader range of emetic stimuli than the established 5-HT3 antagonist treatments.[14] It has been reported that centrally-acting NK1 receptor antagonists, such as CP-99994, inhibit emesis induced by apomorphine and loperimidine, which are two compounds that act through central mechanisms.[15]
Neurokinin receptor 1 (NK-1R) also plays a significant role in cancer progression. NK-1R is overexpressed in various cancer types and is activated by substance P (SP).[24][25] This activation promotes tumor cell proliferation, migration, and invasion while inhibiting apoptosis.[25][26] The SP/NK-1R system is involved in angiogenesis, chronic inflammation, and the Warburg effect, all of which contribute to tumor growth.[24][25] NK-1R antagonists, such as aprepitant, have shown promise as potential anticancer treatments by inhibiting tumor growth, inducing apoptosis, and blocking metastasis.[25][27] The overexpression of NK-1R in tumors may also serve as a prognostic biomarker.[25]
Ligands
Many selective ligands for NK1 are now available, several of which have gone into clinical use as antiemetics.
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External links
"Tachykinin Receptors: NK1". IUPHAR Database of Receptors and Ion Channels. International Union of Basic and Clinical Pharmacology. Archived from the original on 2011-05-16. Retrieved 2007-10-25.
