• Mi Geum Lee, Dong Kyu Lee, Billy K Huh, Sang Sik Choi, Hee Zoo Kim, Byung Gun Lim, Hong Soon Kim, Yun Suk Choi, Won Seok Hur, and Mi Kyoung Lee.
• Department of Anesthesiology and Pain Medicine, Gachon University Gil Hospital, Incheon 405-760, Korea.
• Acta Med Okayama. 2015 Jan 1; 69 (2): 95-103.

AbstractResiniferatoxin (RTX) is an ultrapotent synthetic TRPV1 (transient receptor potential vanilloid subtype 1) agonist with significant initial transient hyperalgesia followed by a prolonged analgesic effect in response to thermal stimulus. Using a rat model of neuropathic pain, we evaluated the effect of pretreatment with clonidine-which has been shown to relieve intradermal capsaicin-induced hyperalgesia-on the initial hyperalgesic response and the thermal analgesic property of RTX. Thirty-six male rats were divided into 6 treatment groups (n=6 each):RTX 500 ng, RTX 1 μg, clonidine 20 μg (Cl), Cl＋RTX 500 ng, Cl＋RTX 1 μg, or normal saline 20 μL (control). We evaluated the short-term (180 min) and long-term (20 days) analgesic effects of RTX after thermal stimulation and mechanical stimulation. RTX had significant initial transient hyperalgesia followed by a prolonged analgesic effect in response to the thermal stimulus, but the RTX 500 ng and RTX 1 μg groups showed no initial short-term thermal hyperalgesic responses when pretreated with clonidine. The Cl＋RTX 1 μg rats' behavior scores indicated that they were more calm and comfortable compared to the RTX 1 μg rats. Even though we cannot precisely confirm that pretreatment with clonidine potentiates or adds to the analgesic effect of RTX, clonidine pretreatment with epidural RTX eliminated the initial RTX-associated hyperalgesic response and systemic toxicity in this neuropathic pain rat model.

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