Effects of halothane on spinal neuronal responses to graded noxious heat stimulation in the cat

This study was undertaken to examine the dose-response effects of clinical concentrations of halothane on activity of wide-dynamic-range (WDR) neurons in the dorsal horn of the spinal cord of the decerebrate, spinal cord-transected cat. All cells (n = 40) responded maximally to high-intensity (greater than 45 C) noxious heat stimulation. Following administration of halothane, 0.5, 1.0, and 1.5 per cent, the mean spontaneous discharge frequency was significantly decreased (P < 0.01) by 44, 74, and 87 per cent, respectively. The mean evoked discharge frequencies were also significantly decreased at all temperatures (46, 48.5 51 C) by all concentrations of halothane. The slope of the regression line relating heat intensity and evoked neuronal discharge frequency was significantly decreased (P < 0.01) with both 1.0 and 1.5 per cent halothane by 46 and 75 per cent, respectively. Since the spinal cord was transected, these results indicate that 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.