Effects of Halothane on Spinal Neuronal Responses to Graded Noxious Heat Stimulation in the Cat

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Abstract
The dose-response effects of clinical concentrations of halothane were examined on the activity of wide-dynamic-range (WDR) neurons in the dorsal horn of the spinal cord of the decerebrate, spinal cord-transected cat. All cells (40) responded maximally to high-intensity (G 45.degree. C) noxious heat stimulation. Following administration of halothane, 0.5, 1.0 and 1.5%, the mean spontaneous discharge frequency was significantly decreased (P < 0.01) by 44, 74 and 87%, respectively. The mean evoked discharge frequencies were also significantly decreased at all temperatures (46, 48.5, 51.degree. C) by all concentrations of halothane. The slope of the regression line relating heat intensity and evoking neuronal discharge frequency was significantly decreased (P < 0.01) with 1.0 and 1.5% halothane by 46 and 75%, respectively. Since the spinal cord was transected, these effects were the result of a direct action at the level of the spinal cord. The neuronal activity that was suppressed was evoked by stimuli that were exclusively noxious. This substantiates the ability of halothane to modify the transmission of noxious information at the spinal cord level and thus explains a mechanism by which halothane may induce analgesia.