The effect of halothane on thermosensitive neurons in the preoptic region of the anterior hypothalamus in acutely instrumented cats.

Normal thermoregulatory processes are significantly impaired by halothane anesthesia. However, the direct effects of halothane on thermosensitive neurons in the preoptic region of the anterior hypothalamus, a major thermoregulatory site, have not been previously investigated. Thirty-eight cats were anesthetized with alpha-chloralose (60 mg/kg) and urethane (600 mg/kg) and placed in stereotactic restraint. Stainless steel thermodes for highly selective local heating and cooling were stereotactically placed into the preoptic region with thermocouples used to monitor regional temperature. Using tungsten microelectrodes, 148 single neurons in the preoptic region were identified and subjected to local heating (to 42 degrees C) and cooling (to 30 degrees C). Eighteen percent (n = 27) in 15 different cats were classified as thermosensitive by accepted criteria (change in firing rate per degree centigrade of greater than 0.8 spikes.s-1.degrees C-1 or less than -0.6 spikes.s-1.degrees C-1). Thermosensitve units were then subjected to graded concentrations of halothane (0.25-1.0% end-tidal), and local heating and cooling were repeated. The spontaneous firing rate (spikes per second) at 37 degrees C of 21 warm-sensitive neurons was significantly (P less than 0.05) reduced, to 65.5 +/- 8.3, 42.6 +/- 10.7, 28.0 +/- 9.5, and 18.1 +/- 6.0% of control at 0.25, 0.50, 0.75, and 1% halothane, respectively. Spontaneous firing rate returned to 99.5 +/- 19.8% of control within 30 min after discontinuation of halothane. Thermosensitivity (change, per degree centigrade, in spikes per second) was also significantly reduced, to 33.3 +/- 5.6, 28.5 +/- 14.6, and 13.9 +/- 6.6% of control at 0.50, 0.75 and 1.0% halothane (all P less than 0.05 compared to control).(ABSTRACT TRUNCATED AT 250 WORDS)