Effects of Morphine Sulfate on Dorsal-horn Neuronal Responses to Graded Noxious Thermal Stimulation in the Decerebrate Cat

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Abstract
Effects of morphine sulfate on activity of the neurons of dorsal-horn lamina V as evoked by graded noxious thermal stimuli applied on the receptive field were studied in spinal cord-transected, decerebrate cats using an extracellular microelectrode recording technique. All single units studied (30) responded to noxious thermal and to noxious mechanical stimulation. Their spontaneous discharge frequency was 9.7 .+-. 1.5 (mean .+-. 1 SE) impulses/s (IPS), the threshold skin temperature was 44.8 .+-. 0.2.degree. C, and a linear correlation existed between skin temperature and discharge frequency at 6.7 .+-. 0.6 IPS/.degree. C. Morphine, 1 and 2 mg/kg i.v., suppressed spontaneous activity by 53 .+-. 6% and 84 .+-. 6%, respectively; increased threshold skin temperature to 46.5 .+-. 0.3% and 47.9 .+-. 0.5.degree. C, respectively; and maintained the linear correlation between skin temperature and discharge frequency but depressed the mean slope of the regression line to 4.5 .+-. 0.7 and to 2.4 .+-. 0.4 IPS/.degree. C, respectively. Naloxone, 0.02-0.04 mg/kg i.v., reversed all the changes produced by morphine. This is the 1st demonstration of the suppressive effect of morphine on the spinal nociceptive neurons in Rexed lamina V as they respond to graded noxious thermal stimuli. These results may explain the analgesic action of morphine at the spinal level.