substance P

substance P. It is a neuropeptide belonging to the tachykinin family, encoded in humans by the TAC1 gene. This undecapeptide acts as a neurotransmitter and neuromodulator, primarily binding to the neurokinin 1 receptor to exert its diverse physiological effects. Its functions are wide-ranging, encompassing the transmission of pain signals, the regulation of neurogenic inflammation, and roles in emotional behavior and gastrointestinal motility.

Structure and synthesis

The molecular structure is an 11-amino acid peptide with the sequence Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2, characterized by a amidated C-terminus and a hydrophobic C-terminal region critical for receptor binding. It is synthesized as a larger precursor protein from the TAC1 gene, which is located on chromosome 7 in humans and can be alternatively spliced to produce different tachykinin peptides. Post-translational processing by enzymes like prohormone convertases and carboxypeptidase E, followed by amidation via peptidylglycine alpha-amidating monooxygenase, yields the mature, active peptide. This synthesis occurs primarily in the cell bodies of specific neurons, including dorsal root ganglion cells and various central nervous system nuclei, before being packaged into vesicles and transported to nerve terminals.

Function and mechanism of action

Its primary mechanism involves binding with high affinity to the G protein-coupled receptor known as the neurokinin 1 receptor, which is coupled to Gq protein and phospholipase C signaling pathways. Activation leads to the release of intracellular calcium and the production of inositol trisphosphate, ultimately influencing neuronal excitability and gene expression. In the peripheral nervous system, it is a key mediator of pain transmission from sensory neurons to the spinal cord, particularly in response to intense or noxious stimuli. Within the central nervous system, it modulates a variety of processes, including anxiety, stress response, vomiting reflex via the area postrema, and NK1 receptor-mediated signaling in brain regions like the amygdala and hypothalamus. It also potently induces neurogenic inflammation by causing vasodilation and plasma extravasation upon release from sensory nerve endings.

Role in disease and clinical significance

Dysregulation is implicated in numerous pathological conditions, with elevated levels observed in the cerebrospinal fluid of patients with fibromyalgia, rheumatoid arthritis, and migraine. In the gastrointestinal tract, it is associated with the pathophysiology of inflammatory bowel disease, irritable bowel syndrome, and nausea related to chemotherapy. Within psychiatry, alterations in its signaling pathways have been studied in relation to major depressive disorder, with some antidepressant treatments shown to downregulate neurokinin 1 receptor expression. Furthermore, it plays a contributory role in chronic pain states, asthma through bronchoconstriction, and certain itch disorders, highlighting its broad clinical relevance across multiple organ systems.

History and discovery

The discovery process was initiated in 1931 by Ulf von Euler and John H. Gaddum, who isolated a crude preparation from equine brain and intestine that caused intestinal contraction and vasodilation; they named it "preparation P" or "powder P," with the "P" standing for preparation. The active principle was later purified and its sequence determined in 1971 by a team led by Susan E. Leeman at Brandeis University, following work on a sialagogue peptide from bovine hypothalamus. This identification firmly established it as the first mammalian tachykinin to be characterized, predating the discovery of related peptides like neurokinin A. The subsequent cloning of its receptor, the neurokinin 1 receptor, in the 1990s by teams including those of Stephen J. Peroutka and N. Michael Thayer, opened a major avenue for neuropharmacology research.

Research and therapeutic potential

Therapeutic strategies have largely focused on developing antagonists for the neurokinin 1 receptor, with aprepitant and its prodrug fosaprepitant achieving clinical success for the prevention of chemotherapy-induced nausea and vomiting, often in combination with agents like ondansetron and dexamethasone. Research into central nervous system applications, such as for major depressive disorder and anxiety disorders, has yielded mixed results in clinical trials, exemplified by compounds like lanepitant and casopitant. Current investigations explore its role in neuroinflammation associated with conditions like multiple sclerosis and Alzheimer's disease, as well as its potential as a biomarker in cerebrospinal fluid analysis. Other experimental approaches include the use of substance P-saporin conjugates to selectively ablate neurons expressing the neurokinin 1 receptor in pain research.

Category:Neuropeptides Category:Tachykinins Category:Neurotransmitters