Peripherally induced resiniferatoxin analgesia.

Selective blockade of nociceptive pathways represents a mechanism-based approach that has attracted a large variety of pharmacological and molecular investigations. A potential site for selective intervention is the primary afferent nociceptive nerve terminal. Binding of resiniferatoxin (RTX) to the vanilloid-1 receptor (VR1) stimulates and then inactivates heat and vanilloid-responsive nerve endings involved in heat and inflammatory pain signaling which can progress to localized degeneration of the peripheral ending followed by regeneration. Application of RTX directly to peripheral nerve endings produces a long term, reversible attenuation of nociceptive transmission. Heat hyperalgesia and mechanical allodynia were assessed prior to injection of RTX into the hindpaw (baseline) and at acute (minutes-hours) and more chronic (days-weeks) times after injection. Acutely, an inverse dose-to-pain response (guarding, licking) for RTX (0.0625-2.0 microg) occurs, followed by selective attenuation of peripheral pain transmission. Thermal nociception was decreased in a concentration-dependent fashion and lasted up to 21 days, without impairing motor function. Administration of RTX blocked both inflammation-induced hyperalgesia and spinal c-Fos induction. The results demonstrate the efficacy and therapeutic potential of reversible, peripheral C-fiber 'inactivation' for intermediate duration pain control.