Systemic lidocaine induces expansion of the receptive field of spinal dorsal horn neurons in rats.

The effect of intravenous administration of the local anaesthetic lidocaine (1, 3 or 5 mg/kg) on the responsiveness and size of the cutaneous receptive fields of 18 lumbar dorsal horn neurons was examined in intact urethane-anaesthetized rats. Lidocaine induced expansion of the receptive field in the majority of neurons examined, particularly after the two higher doses. The expansion occurred usually within 10 min after lidocaine injection and the effect was reversible. Lidocaine also altered the responsiveness of dorsal horn neurons to peripheral mechanical stimulation. The responses of wide-dynamic-range neurons to noxious pinch were usually inhibited by lidocaine. However, some low-threshold neurons started to react to noxious mechanical stimulation and some high-threshold neurons started to respond to innocuous brushing after lidocaine injection. The present results show that moderate doses of systemic lidocaine induce complex changes in the excitability of dorsal horn neurons, including an increase in the size of the receptive field and altered response characteristics to mechanical stimulation.