Effects of morphine sulfate on dorsal-horn neuronal responses to graded noxious thermal stimulation in the decerebrate cat.

Effects of morphine sulfate upon 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 utilizing an extracellular microelectrode recording technique. All single units studied (n = 30) responded to noxious thermal as well as to noxious mechanical stimulation. Their spontaneous discharge frequency was 9.7 +/- 1.5 (mean +/- 1 SE) impulses/sec (IPS), the threshold skin temperature was 44.8 +/- 0.2 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, iv, suppressed spontaneous activity by 53 +/- 6 and 84 +/- 6 per cent, respectively; increased threshold skin temperature to 46.5 +/- 0.3 and 47.9 +/- 0.5 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, iv, reversed all of these changes produced by morphine. The results of the present study are, to the authors' knowledge, the first 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.