Characterization of secondary hyperalgesia produced by topical capsaicin jelly--a new experimental tool for pain research.

Peripheral administration of the nociceptive agent capsaicin is used as an experimental tool to study neurophysiological and pharmacological aspects of the generation and control of pain. When investigating secondary hyperalgesia phenomena, current topical and intradermal capsaicin delivery methods have two key limitations. Intradermal injection can evoke severe chemogenic pain and both delivery methods produce an unstable area of dynamic mechanical allodynia. We present validity studies of a new preparation for capsaicin delivery that overcomes these limitations. The novel capsaicin formulation consists of a water-based semisolid jelly preparation containing 1% capsaicin which is applied topically under adhesive-free occlusion to a small area of the skin. We demonstrate that in healthy human subjects this model evokes areas of flare, punctate hyperalgesia and mechanical allodynia which are equivalent to established models and that these areas are stable over time and reproducible on repeat experiments. The jelly formulation evokes only minimal chemogenic pain and, as the preparation remains in situ throughout the study providing constant capsaicin exposure, a stable area of dynamic mechanical allodynia is produced. These validation studies show that this novel capsaicin administration method is a practical, reliable and viable tool for investigating experimental secondary hyperalgesia.