Addition of Capsaicin to Local Anesthetics for Spinal Anesthesia in Rats Shortens Motor Deficits and Prolongs Anti-Nociception

Abstract

Background and Objectives: Sensory-selective anesthesia, greater or longer-lasting anti-nociception than motor or autonomic deficits, is often clinically desirable but traditional local anesthetics rarely have such selective actions. Addition of capsaicin to tertiary amine local anesthetics has recently been reported to affect a preferential prolongation of nociceptive over motor block in rat sciatic nerve. We hypothesized that this combination when used intrathecally will also prolong nociceptive block. Methods: Under sevoflurane inhalation anesthesia, rats were injected intrathecally either with local anesthetics (bupivacaine, lidocaine, and articaine) alone or simultaneously with capsaicin. Motor block was evaluated by the contractile function of foot muscles, from proximal to distal. Anti-nociception was assessed by reductions in nocifensive withdrawal and vocalization induced by pinching the skin fold over the lateral metatarsus. Durations and degrees of deficits were assessed, along with complete recovery times and compared between local anesthetics alone and in combination with capsaicin. Results: Addition of capsaicin to any of the local anesthetics shortened motor deficits. Bupivacaine, lidocaine and articaine motor blocks were reduced upon combination with capsaicin to 0.32, 0.32 ans 0.43 of the duration from the respective local anesthetic alone. Duration of anti-nociceptive action was increased by capsaicin only for articaine. The ratios of block nociceptive to sensory block durations were 3.5, 5.1 and 3.3 for the respective local anesthetics. Conclusions: Intrathecal injection of capsaicin combined with local anesthetics produced a preferentially longer anti-nociceptive deficit. These combinations have potential clinical applications, including peri-operative spinal anesthesia and pain management.