Sites and mechanisms of action of lidocaine upon the isolated spinal cord of the frog

The sites of action of lidocaine on the responses evoked by stimulation of lateral column (LC) and dorsal root (DR) were studied in the isolated, intra-arterially perfused spinal cord of the bullfrog. When the ventral root volley produced by stimulation was abolished by lidocaine, the presynaptic focal potential was almost unchanged. Intracellular recordings from motoneurons clearly demonstrated a marked reduction in amplitude of the EPSPs before the block of conduction of presynaptic fibers and the block of invasion of the neuron soma by antidromic spike potential. At low concentrations of lidocaine, the EPSPs elicited by LC stimulation produced shortening in time to peak, slowing in the decay time, decrease in amplitude and smaller changes in the later EPSPs of a train than the earlier ones. From the observations, it was concluded that the low concentrations of lidocaine affected primarily synaptic transmission in the spinal cord. The possible mechanisms of action of lidocaine were discussed.