Hyperresponsiveness on Washout of Volatile Anesthetics from Isolated Spinal Cord Compared to Withdrawal from Ethanol

We performed experiments in spinal cords isolated from neonatal rats to probe the mechanisms responsible for hyperresponsiveness of the population excitatory evoked potential (pEPSP) observed on washout of the volatile anesthetics halothane and isoflurane (1 minimal alveolar anesthetic concentration equivalent, MAC) compared with that observed after an anesthetic concentration of ethanol. After 30 min exposure to each anesthetic and washout, pEPSP area increased to levels significantly more than control (P < 0.01–0.001). Exposure to a very small (0.025 MAC) concentration of isoflurane over the same period itself produced a similarly exaggerated pEPSP (P < 0.05) in the continued presence of the drug, suggesting that the phenomenon is a direct excitatory effect of the small concentrations of anesthetic on washout, unlike the true withdrawal observed with ethanol. Isoflurane, but not halothane, significantly increased the amount of potassium-stimulated release of the excitatory neurotransmitters glutamate, aspartate, and substance P, suggesting the hyperresponsiveness for that drug is the result of a presynaptically mediated increase in transmitter release. A broad spectrum specific protein kinase C inhibitor, GF109203X, blocked ethanol withdrawal hyperresponsiveness but not hyperresponsiveness after halothane. If the behavioral symptoms of emergence from anesthesia are based on excitatory actions similar to those observed in the spinal cord, the results show that they represent direct excitatory actions rather than withdrawal and are attributable to direct actions on ion channels or receptors, rather than indirect effects mediated by protein kinase C.