What are the clinical and investigational roles of capsaicin?
Capsaicin, the algogenic substance in hot peppers, selectively stimulates primary afferent C fibers. These C fibers express TRV1, capsaicin receptors that nonselectively gate cations, including sodium and calcium, which depolarize axons. Selective antagonists to capsaicin have been developed. These may reduce the contribution of this channel in conditions in which the environment of injury (e.g., low pH) results in prolonged opening of the channel.
Capsaicin itself may help as an analgesic. When administered to neonatal animals, capsaicin destroys C fibers; when administered to adults, it produces a long-term desensitization of the C fibers, possibly by depletion of their peptide neurotransmitters, such as substance P, or by transient ablation of the nociceptor terminals. The desensitization is associated with a decreased response to noxious stimulation, which provides a rational basis for the therapeutic use of capsaicin in patients. To date, topical application of capsaicin has shown some promise in the treatment of postherpetic neuralgia pain and postmastectomy intercostal neuralgia.