Abstract
Previous research has shown that the toxicities of single S (-) enantiomers levobupivacaine and ropivacaine to the cardiovascular and central nervous systems are weaker than a racemic mixture of S (-) and R (+) enantiomers such as bupivacaine. In this study, we investigated effects of levobupivacaine, ropivacaine, bupivacaine and R (+) bupivacaine on excitatory synaptic inputs to spinal dorsal horn neurons evoked by dorsal root stimulation, and on action potentials (APs) in dorsal root ganglion (DRG) neurons generated by the dorsal root stimulation. In the spinal dorsal horn, levobupivacaine reversibly suppressed the amplitude of monosynaptic Aδ and C fiber-evoked EPSCs. However, Aβ fiber-evoked EPSCs were slightly inhibited in amplitude at the same concentration. On the other hand, bupivacaine equally suppressed those of the three fiber-evoked EPSCs. These local anesthetics did not change the frequency and amplitude of miniature EPSCs. In DRG neurons, APs were reversibly inhibited in amplitude by the local anesthetics. Half-maximum inhibitory concentrations (IC50) of bupivacaine and R (+) bupivacaine were almost equal on Aβ , Aδ and C neurons. On the other hand, IC50 of levobupivacaine and ropivacaine on Aδ and C neurons were lower than that on Aβ neurons. The present results suggest that pure S (-) enantiomers especially levobupivacaine effectively inhibits noxious transmission to the spinal dorsal horn by the blockade of AP conduction through C and Aδ fibers. [J Physiol Sci. 2006;56 Suppl:S180]
