A comparison of the local anesthetic effects of meperidine, fentanyl, and sufentanil on dorsal root axons

The local anesthetic effects of opioids have been demonstrated in both clinical and laboratory studies. Clinically, both meperidine and sufentanil can produce seg mental sensory anesthesia. However, previous studies of the effects of opioids on nerve conduction have all made use of peripheral nerve preparations and yielded conflicting results. In the present study we describe the local anesthetic effects of phenylpiperidine opioids on individual dorsal root axons, the probable target for intrathecal local anesthetics. Dorsal roots were removed from anesthetized adult male rats and maintained in vitro. Standard single fiber recording techniques were used to isolate activity in the individual axons. Drug exposure was accomplished by perfusing the isolated dorsal root with an artificial cerebrospinal fluid containing the study drug at a clinically relevant concentration. Steady-state drug effects were measured after 15-30 min of exposure and compared to control measurements in the same preparation. Meperidine (705 mu M) blocked conduction in 61.5% of 39 myelinated and unmyelinated axons, and significantly reduced conduction velocity in the remaining unblocked axons. These effects were not naloxone reversible. Fentanyl (0.6 mu M and 3 mu M) and sufentanil (1.04 mu M) failed to affect the nerve conduction in any dorsal root axon. The discrepancy between laboratory and clinical observations is discussed. We suggest that the site of conduction block may occur at the proximal end of the dorsal root as it passes through the dorsal root entry zone, an anatomically unique segment of the primary sensory pathway with decreased conduction safety for action potential propagation.