Mechanisms by which Volatile Anesthetics Improve the Recovery of Hippocampal Slice Ca 1 Pyramidal Neurons after Hypoxia.

Abstract

Isoflurane and sevoflurane are two volatile anesthetics that are commonly used in neurologic and cardiac surgery. Isoflurane is a more potent anesthetic and its maximal clinical concentration is approximately 2%; the equivalent concentration for sevoflurane is 4%. We studied the effects of isoflurane and sevoflurane on cornus ammonis 1(CA1) pyramidal cells in rat hippocampal slices subjected to 5 minutes or 10 minutes of hypoxia (95% nitrogen, 5% carbon dioxide). Both isoflurane (2% and 4%) and sevoflurane (2% and 4%) attenuated the changes in the intracellular concentrations of ATP, K+ and Na+ caused by the hypoxic insult, however only an absolute concentration of 4% for each agent attenuated the rise in intracellular Ca2+. The effect of these concentrations of isoflurane and sevoflurane was not different for Na+, K+ or ATP concentrations at 10 minutes of hypoxia, the only difference at 5 minutes of hypoxia was that ATP was better maintained with 4% sevoflurane (2.2 vs. 1.3 nmol/mg). If the same absolute concentration (4%) of isoflurane and sevoflurane is compared then the cellular changes during hypoxia are similar for both agents and they both improve recovery. However if the same anesthetic potency and maximal clinical dose of sevoflurane (4%) and isoflurane (2%) are compared then sevoflurane better improves recovery and better attenuates the rise in Ca2+. The mechanisms of sevoflurane-induced protection include delaying and attenuating the depolarization, attenuating the increase of cytosolic calcium and delaying the fall in ATP during hypoxia. The attenuation of Na+and K+ changes during hypoxia was not different between 2% isoflurane and 4% sevoflurane treatment and is therefore not sufficient to explain the protection, although it may be required in addition to the other effects of these drugs. Anesthetic preconditioning occurs when a volatile anesthetic, such as sevoflurane, is administered before a hypoxic or ischemic insult; this has been shown to improve neuronal recovery after the insult. We found that sevoflurane-induced preconditioning in the rat hippocampal slice enhances the expression of PKMζ and this correlated with the altered electrophysiological effects and the improved recovery in the electrophysiological experiments from our lab. PKMζ is one of the atypical protein kinase C isoforms and it is mainly expressed in brain. We found that sevoflurane induces an increase of the new protein synthesis of PKMζ, and is necessary for preconditioning induced protection of neurons after short hypoxic insults. An inhibitor of this kinase, zeta inhibitory peptide (ZIP), which has been previously shown to block the altered electrophysiological effects and the improved recovery, also blocked the increase in the total amount of PKMζ protein and the amount of the activated form of this kinase, phospho-PKMζ (p-PKMζ). We conclude that sevoflurane increases PKMζ protein, which is constitutively phosphorylated to its active form, this pathway is likely one of the mechanisms by which sevoflurane-induced preconditioning improves recovery.