Substance P: Tachykinin Neuropeptide in Pain and Neurogenic Inflammation
A detailed examination of substance P, an undecapeptide of the tachykinin family that mediates pain transmission and neurogenic inflammation via NK1 receptors.
Chemical Identity
Substance P (SP) is an 11-amino-acid neuropeptide of the tachykinin family, named for its ability to induce rapid smooth muscle contraction. It is widely distributed in the central and peripheral nervous systems, where it serves as a key transmitter in nociceptive pathways.
| Property | Value |
|---|---|
| Chemical Formula | C₆₃H₉₈N₁₈O₁₃S |
| Molecular Weight | 1347.63 g/mol |
| CAS Number | 33507-62-3 |
| Isoelectric Point (pI) | 11.5 |
| Amino Acid Count | 11 |
| Disulfide Bridge | None (linear) |
Primary Structure
Substance P belongs to the tachykinin family, characterized by a conserved C-terminal pentapeptide sequence. The full sequence is:
Arg–Pro–Lys–Pro–Gln–Gln–Phe–Phe–Gly–Leu–Met–NH₂
Key structural features:
- C-terminal amidation (Met–NH₂) essential for full receptor efficacy
- Conserved tachykinin motif: –Phe–X–Gly–Leu–Met–NH₂ shared by neurokinin A and neurokinin B
- Proline residues at positions 2 and 4 introduce conformational rigidity
- Basic N-terminal region (Arg, Lys) contributing to membrane interaction
Receptor Interactions
Substance P acts primarily through the NK1 receptor (also known as the substance P receptor), a seven-transmembrane G-protein coupled receptor:
| Receptor | Primary Ligand | Coupling | Distribution |
|---|---|---|---|
| NK1 | Substance P | Gq/11 → PLC → IP₃/Ca²⁺ | CNS, dorsal horn, immune cells |
| NK2 | Neurokinin A | Gq/11 | Smooth muscle, peripheral tissues |
| NK3 | Neurokinin B | Gq/11 | CNS, hypothalamus |
- NK1 binding: Sub-nanomolar affinity; expressed on dorsal horn neurons, microglia, and immune cells
- Receptor internalization: NK1 undergoes rapid β-arrestin-mediated internalization upon SP binding
- Desensitization: Prolonged SP exposure leads to receptor phosphorylation and functional desensitization
Biological Functions
Substance P is a critical mediator in nociception and neurogenic inflammation:
- Pain Transmission: SP is released from unmyelinated C-fibers in the dorsal horn of the spinal cord, where it activates NK1-expressing second-order neurons, facilitating nociceptive signal propagation.
- Neurogenic Inflammation: Peripheral SP release from sensory nerve terminals induces vasodilation (via NO), plasma extravasation (via endothelial gap junctions), and immune cell recruitment.
- Immune Modulation: Stimulates cytokine release (IL-1, IL-6, TNF-α) from monocytes and macrophages, enhancing inflammatory responses.
- Behavioral Effects: Central SP administration produces anxiety-like behavior, while NK1 antagonists show anxiolytic properties in preclinical models.
Clinical Relevance
NK1 receptor antagonists, such as aprepitant (Emend), are used clinically as antiemetics in chemotherapy-induced nausea. Substance P is also implicated in chronic pain syndromes, inflammatory bowel disease, asthma, and depression, making NK1 antagonism an area of active pharmacological investigation.
References
- O’Connor, T. M., et al. (2024). Substance P and neurogenic inflammation. Neuroscience, 530, 11–25.
- Severini, C., et al. (2002). Pharmacology of tachykinins. Pharmacological Reviews, 54, 837–872.
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