Substance P

Substance P is an 11-amino acid neuropeptide belonging to the tachykinin family, which also includes neurokinin A, neurokinin B, and neuropeptide K. Its discovery in 1931 by von Euler and Gaddum as an uncharacterised 'substance' in intestinal and brain extracts that lowered blood pressure and stimulated smooth muscle contraction gave rise to its unusual name - "P" for "powder" in the original preparation. Despite over 90 years of research, Substance P continues to be one of the most intensively studied neuropeptides in pain biology.

Substance P is co-stored and co-released with glutamate from the central terminals of C-fibre nociceptors in the dorsal horn of the spinal cord. Following peripheral tissue injury or inflammation, C-fibres are activated and release both glutamate (mediating fast, direct AMPA/NMDA receptor activation) and Substance P (mediating slower, sustained neurokinin-1 receptor activation on second-order projection neurons). This dual mechanism underlies the temporal profile of pain: the immediate sharp sensation (glutamate-mediated) followed by the prolonged aching pain (Substance P-mediated NK1R activation and wind-up).

The concept of "central sensitisation" - the amplification of pain signalling at the spinal cord level that underlies chronic pain syndromes - is intimately tied to Substance P biology. During periods of sustained nociceptor activation, Substance P causes progressive NMDA receptor unmasking in dorsal horn neurons, leading to a state of hyperexcitability where normally innocuous stimuli are perceived as painful (allodynia) and normal painful stimuli produce exaggerated responses (hyperalgesia). Understanding and blocking this Substance P-driven sensitisation process has been a primary target of chronic pain pharmacology research.

Peripherally, Substance P mediates "neurogenic inflammation" - the sterile inflammatory response triggered by the release of neuropeptides from sensory nerve terminals. This process involves Substance P binding to NK1Rs on mast cells, endothelial cells, and immune cells, causing mast cell degranulation (histamine, tryptase release), arteriolar vasodilation, post-capillary venule permeability increase (plasma extravasation), and recruitment of immune cells. Neurogenic inflammation is fundamental to conditions including migraines (dural neurogenic inflammation), arthritis, and inflammatory bowel disease.

The development of NK1 receptor antagonists as analgesics was a major pharmaceutical research effort in the 1990s-2000s, producing compounds like aprepitant, netupitant, and rolapitant. While these compounds were ineffective as analgesics (challenging the hypothesis that Substance P blockade would reduce pain), they proved highly effective as antiemetics and are now the standard of care for chemotherapy-induced nausea and vomiting. Aprepitant (Emend) was approved by the FDA in 2003 and remains widely used. The antiemetic success demonstrates that Substance P research, even when it fails for its original purpose, generates important therapeutic tools.

The gut represents another major organ system where Substance P plays essential regulatory roles. Substance P is one of the primary excitatory neurotransmitters of the enteric nervous system (the gut's own intrinsic neural network), regulating peristaltic contractions, mucus secretion, and fluid transport. In inflammatory bowel disease, Substance P levels are markedly elevated in the inflamed gut, and NK1R blockade reduces experimental colitis in animal models - making Substance P a target of ongoing IBD research.