Effect of systemic lidocaine on dorsal horn neuron hyperactivity following chronic peripheral nerve injury in rats

Systemic lidocaine (3-4 mg/kg) was tested for its effect on identified spinal cord wide-dynamic-range (WDR) neurons in rats with a unilateral chronic neuropathy induced by two different peripheral nerve injuries (section or compression by ligatures). In both cases, neurons on the side ipsilateral to the injuries showed a spontaneous firing frequency higher than that of the opposite intact side (23.5 +/- 3.4 vs 4.2 +/- 1.5 spikes/sec). The hyperactivity was not affected by a sensory block of the peripheral receptive field. It was consistently found that lidocaine suppressed or reduced the spontaneous neuronal hyperactivity on the ipsilateral side, whereas it had no effect on the normal spontaneous activity of the neurons on the intact side. In all recordings, the hyperactivity returned to the prelidocaine injection rates within 20 min. These results indicate a preferential action of subanesthetic doses of lidocaine on the hyperactive WDR neurons. Such preferential action is related to a susceptibility acquired by WDR neurons of the peripherally injured side and is not simply attributable to the increased frequency of firing.