Anesthetic Protection of Neurons Injured by Hypothermia and Rewarming Roles of Intracellular Ca2+ and Excitotoxicity

Background: Mild hypothermia is neuroprotective after cerebral ischemia but surgery involving profound hypothermia (PH, temperature less than 18 degrees C) is associated with neurologic complications. Rewarming (RW) from PH injures hippocampal neurons by glutamate excitotoxicity, N-methyl-D-aspartate receptors, and intracellular calcium. Because neurons are protected from hypoxia-ischemia by anesthetic agents that inhibit N-methyl-D-aspartic acid receptors, we tested whether anesthetics protect neurons from damage caused by PH/RW. Methods: Organotypic cultures of rat hippocampus were used to model PH/RW injury, with hypothermia at 4 degrees C followed by RW to 37 degrees C and assessment of cell death 1 or 24 h later. Cell death and intracellular Ca2+ were assessed with fluorescent dye imaging and histology. Anesthetic agents were present in the culture media during PH and RW or only RW. Results: Injury to hippocampal CA1, CA3, and dentate neurons after PH and RW involved cell swelling, cell rupture, and adenosine triphosphate (ATP) loss; this injury was similar for 4 through 10 h of PH. Isoflurane (1% and 2%), sevoflurane (3%) and xenon (60%) reduced cell loss but propofol (3 mu M) and pentobarbital (100 mu M) did not. Isoflurane protection involved reduction in N-methyl-D-aspartate receptor-mediated Ca2+ influx during RW but did not involve gamma-amino butyric acid receptors or K-ATP channels. However, cell death increased over the next day. Conclusion: Anesthetic protection of neurons rewarmed from 4 degrees C involves suppression of N-methyl-D-aspartate receptor-mediated Ca2+ overload in neurons undergoing ATP loss and excitotoxicity. Unlike during hypoxia/ischemia, anesthetic agents acting predominantly on gamma-aminobutyric acid receptors do not protect against PH/RW. The durability of anesthetic protection against cold injury may be limited.