CHARACTERIZATION OF NEUROPROTECTION FROM EXCITOTOXICITY BY MODERATE AND PROFOUND HYPOTHERMIA IN CULTURED CORTICAL-NEURONS UNMASKS A TEMPERATURE-INSENSITIVE COMPONENT OF GLUTAMATE NEUROTOXICITY

Although profound hypothermia has been used for decades to protect the human brain from hypoxic or ischemic insults, little is known about t he underlying mechanism. We therefore report the first characterizatio n of the effects of moderate (30 degrees C) and profound hypothermia ( 12 degrees to 20 degrees C) on excitotoxicity in cultured cortical neu rons exposed to excitatory amino acids (EAA; glutamate, N-methyl-D-asp artate [NMDA], AMPA, or kainate) at different temperatures (12 degrees to 37 degrees C). Cooling neurons to 30 degrees C and 20 degrees C wa s neuroprotective, but cooling to 12 degrees C was toxic. The extent o f protection depended on the temperature, the EAA receptor agonist emp loyed, and the duration of the EAA challenge. Neurons challenged brief ly (5 minutes) with all EAA were protected, as were neurons challenged for 60 minutes with NMDA, AMPA, or kainate. The protective effects of hypothermia (20 degrees and 30 degrees C) persisted after rewarming t o 37 degrees C, but rewarming from 12 degrees C was deleterious. Surpr isingly, however, prolonged (60 minutes) exposures to glutamate unmask ed a temperature-insensitive component of glutamate neurotoxicity that was not seen with the other, synthetic EAA; this component was still mediated via NMDA receptors, not by ionotropic or metabotropic non-NMD A receptors. The temperature-insensitivity of glutamate toxicity was n ot explained by effects of hypothermia on EAA-evoked [Ca2+](i) increas es measured using high- and low-affinity Ca2+ indicators, nor by effec ts on mitochondrial production of reactive oxygen species. This first characterization of excitotoxicity at profoundly hypothermic temperatu res reveals a previously unnoticed feature of glutamate neurotoxicity unseen with the other EAA, and also suggests that hypothermia protects the brain at the level of neurons by blocking, rather than slowing, e xcitotoxicity.