IMPLICATIONS: The intraarticular administration of a vanilloid agonist resiniferatoxin inhibits pain behavior in knee joint arthritis. This effect is dose-dependent.

Vanilloids bind to the transient receptor potential type V1 (TRPV1) channels, nonselective cation ionophores that play an important role in integration of afferent noxious signals generated by inflammatory mediators (1). Vanilloid agonists (capsaicin and its analogs) can produce selective and long-lasting blockade of afferent C-fibers and some Aδ-fibers (2). The cloning of vanilloid receptors (3, 4) heralded the rapid advances in vanilloid pharmacology and has begun to play an important role in the development of new compounds with analgesic properties. This new development put a new emphasis on the results of previous studies with capsaicin. Resiniferatoxin (RTX) is an ultrapotent capsaicin analog with a unique spectrum of activities (2). Its initial excitatory effect relative to its inactivating effect is far less pronounced compared with capsaicin (2). Several studies (5–7) convincingly demonstrated that capsaicin selectively suppresses the conduction of impulses in the C-fibers and some Aδ-fibers.