An Analgesic Action of Intravenously Administered Lidocaine on Dorsal-horn Neurons Responding to Noxious Thermal Stimulation

Abstract

Using extracellular single-unit recording techniques, effects of i.v. administered lidocaine on dorsal-horn nociceptive neurons were studied in cats made decerebrate whose spinal cords were transected. Neurons (37) in Rexed lamina V responding to high-threshold mechanical and noxious thermal stimuli (radiant heat, using Hardy-Wolff-Goodell dolorimeter) were studied. Lidocaine hydrochloride, 2.5, 5 and 10 mg/kg, i.v., produced dose-related suppression of both spontaneous activity and responses of these neurons to noxious thermal stimulation. Spontaneous discharge frequencies at maximum suppression, seen 3-7 min after administration of each of the 3 doses of lidocaine were 64 .+-. 14 (mean .+-. 1 SE), 32 .+-. 8 and 25 .+-. 9% of control values, respectively; responses to noxious thermal stimuli were 83 .+-. 5, 52 .+-. 8 and 39 .+-. 7% of the control values, respectively. Threshold skin temperature to noxious thermal stimulation increased from 44.7 .+-. 0.4.degree. C (control) to 46.3 .+-. 0.7.degree. C with lidocaine, 5 mg/kg (P < 0.05), to 47.8 .+-. 0.8.degree. C with lidocaine, 10 mg/kg (P < 0.01). The times necessary for recovery varied in a dose-related fashion. Plasma lidocaine concentrations 5 min after lidocaine, 5 mg/kg, averaged 3.6 .+-. 0.7 .mu.g/ml. Apparently i.v. administered lidocaine produces analgesia at plasma concentrations of 3-10 .mu.g/ml. Lidocaine may block conduction of nociceptive impulses, by suppression of spinal-cord nociceptive neurons.