Effects of inhalation anesthetics on parasympathetic reflex vasodilatation

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The present experiments were designed to examine the effects of inhalation anesthetics (isoflurane, halothane, and sevoflurane) on the parasympathetic reflex vasodilation in the lower lip and palate elicited by electrical stimulation of the central cut end of the lingual nerve in vago-sympathectomized cats. Isoflurane (1.5%), halothane (1.0 %), sevoflurane (2.5%), each at a concentration of 1.0 MAC (minimum alveolar concentra-tion), markedly suppressed the evoked blood flow increases in the lower lip, whereas nitrous oxide (70 % in 30% oxygen) and morphine (2 mg/kg, iv) did not. Prior administra-tion of picrotoxin, a GABA antagonist, (2 mg/kg, iv) reversed the inhibitory effect of isoflurane on the parasympathetic reflex response. Decerebration had no significant effect on the isoflurane-induced inhibition. These findings suggest that there is a GABA-mediated suppressive mechanism acting on this parasympathetic reflex response ; the sites at which inhalation anesthetics exert such an inhibitory action could be in the mid-brain, pons, or medulla, but not in the hypothalamus or higher structures.