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NEUROSURGICAL ANESTHESIA

-Aminobutyric Acid-A Receptors Contribute to Isoflurane Neuroprotection in Organotypic Hippocampal Cultures

Philip E. Bickler, MD PhD^*, David S. Warner, MD, Greg Stratmann, MD^*, and Jennifer A. Schuyler, BS^*

*Department of Anesthesia and Perioperative Care, University of California San Francisco; and Department of Anesthesiology, Duke University Medical Center, Durham, North Carolina

Address correspondence and reprint requests to Dr. Bickler, Sciences 255, Box 0542, University of California Medical Center, 513 Parnassus Ave., San Francisco, CA 94143-0542. Address e-mail to bicklerp{at}anesthesia.ucsf.edu

The mechanisms by which anesthetics such as isoflurane reduce cell death in rodent models of cerebral ischemia remain incompletely defined. Reduction in glutamate excitotoxicity explains some but not all of isoflurane’s neuroprotection. Because isoflurane potentiates -aminobutyric acid (GABA) receptor-mediated ion fluxes and GABA_A receptor agonists have neuroprotective effects, we hypothesized that GABA_A receptors contribute to isoflurane neuroprotection. As a model of cerebral ischemia and recovery, we used rat hippocampal slice cultures. Survival of CA1, CA3, and dentate neurons was examined 2 and 3 days after 1-h combined oxygen-glucose deprivation (OGD) at 37°C. To define the role of GABA_A receptors in mediating protection, the effect of 1% isoflurane on cell survival was examined in the presence of the GABA_A antagonist bicuculline during OGD. Cell death was measured with propidium iodide fluorescence. Isoflurane and the selective GABA_A agonist muscimol (25 µM) reduced cell death after OGD to values similar to slices not exposed to OGD, with the exception that muscimol did not reduce cell death in CA3 neurons 2 days after OGD. The GABA_A antagonist bicuculline reduced the neuroprotective effects of isoflurane on hippocampal neurons 2 and 3 days after OGD. We conclude that GABA_A receptors contribute to neuroprotection against OGD produced by isoflurane in the hippocampal slice model. Based on this and other studies, it is likely that neuroprotection produced by isoflurane is multifactorial and includes actions at both GABA_A and glutamate receptors and possibly other mechanisms.

IMPLICATIONS: Isoflurane is neuroprotective in rodent brain ischemia models, but the mechanisms for this effect remain incompletely defined. In organotypic cultures of rat hippocampus, we show that protection of CA1, CA3, and dentate neurons by 1% isoflurane from death caused by oxygen and glucose deprivation involves GABA_A receptors.

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