• Eser Yilmaz-Rastoder, Michael S Gold, Karen A Hough, G F Gebhart, and Brian A Williams.
• Department of Anesthesiology and Center for Pain Research, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261, USA.
• Reg Anesth Pain Med. 2012 Jul 1; 37 (4): 403-9.

Background And ObjectivesThere is increasing clinical use of adjuvant drugs to prolong the duration of local anesthetic-induced block of peripheral nerves. However, the mechanistic understanding regarding drug interactions between these compounds in the periphery is quite limited. Accordingly, we undertook this study to determine whether selected adjuvant drugs are efficacious in blocking action potential propagation in peripheral nerves at concentrations used clinically and whether these drugs influence peripheral nerve block produced by local anesthetics.MethodsIsolated rat sciatic nerves were used to assess (1) the efficacy of buprenorphine, clonidine, dexamethasone, or midazolam, alone and in combination, on action potential propagation; and (2) their influence on the blocking actions of local anesthetics ropivacaine and lidocaine. Compound action potentials (CAPs) from A- and C-fibers were studied before and after drug application.ResultsAt estimated clinical concentrations, neither buprenorphine nor dexamethasone affected either A- or C-waves of the CAP. Clonidine produced a small but significant attenuation of the C-wave amplitude. Midazolam attenuated both A- and C-wave amplitudes, but with greater potency on the C-wave. The combination of clonidine, buprenorphine, and dexamethasone had no influence on the potency or duration of local anesthetic- or midazolam-induced block of A- and C-waves of the CAP.ConclusionsThese results suggest that the reported clinical efficacy of clonidine, buprenorphine, and dexamethasone influences the actions of local anesthetics via indirect mechanisms. Further identification of these indirect mechanisms may enable the development of novel approaches to achieve longer-duration, modality-specific peripheral nerve block.

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