Abstract

The effects of bupivacaine (1-2 mM), etidocaine (1-2 mM), and lidocaine (1-20 mM) on sensory and motor pathways were studied in isolated sciatic nerve-spinal cord preparations obtained from winter bullfrogs. Application of these three anesthetics to short segments (approximately 5-7 mm) of the various nerve structures depressed mostly slow conducting fibers (<50 m/sec); however, when these anesthetics were applied to longer segments (up to 48 mm) of the sciatic nerve, the depression included the fast conducting fibers (>50 m/sec) and was greater at high rate of stimulation (frequency-dependent effect) with all three anesthetics. Bupivacaine (2 mM) and lidocaine (10 mM) preferentially depressed the more complex and wider compound action potential of sensory fibers over the simpler potential of motor fibers when applied to the nerve roots. Etidocaine, unlike the other two anesthetics, had similar effect on both roots during the first 10 minutes of exposure; however at the end of 30 minutes all three anesthetics equally depressed all fibers. All three anesthetics preferentially depressed the sensory pathway when applied over the dorsal root ganglion.

In conclusion, the dorsal root ganglion is the structure in the afferent pathway most vulnerable to the actions of bupivacaine, etidocaine and lidocaine, especially during transmission of rapid nerve signals. Bupivacaine and lidocaine, when applied to the roots have a faster depressant effect on slow conducting sensory than motor fibers and show more frequency-dependent effects than etidocaine. This difference among the three anesthetics reflects experimental conditions such as time and length of nerve exposed to these drugs.