Human experimental pain models 3: heat/capsaicin sensitization and intradermal capsaicin models.

The heat/capsaicin sensitization and intradermal capsaicin injection models are safe and noninvasive paradigms to generate stable, long-lasting, and reproducible injury capable of producing an area of both primary and secondary hyperalgesia. Risk of skin injury is substantially reduced since lower levels of thermal and chemical irritation produce long-lasting cutaneous hyperalgesia. Rekindling sustains central sensitization by providing peripheral nociceptive input. The intradermal capsaicin model has been widely used to test analgesic efficacy for a wide range of analgesics. Unlike the heat/capsaicin sensitization model, intradermal capsaicin results in a brief painful stimulus followed by a long lasting area of secondary hyperalgesia. The intradermal injection of capsaicin results in a transient, intense stinging sensation at the site of injection (e.g. heat allodynia) followed by a persistent area of secondary tactile allodynia.