We have previously demonstrated that anoxic preconditioning (APC) protects against a subsequent otherwise 'lethal' anoxic insult in hippocampal slices. Tested here are two hypotheses: (a) APC requires calcium to improve electrical recovery in hippocampa slices; and (b) mild excitation promotes preconditioning neuroprotection. Control hippocampal slices were given a single 'test' anoxic insult followed by reoxygenation. Experimental slices were preconditioned by three short anoxic insults of 1 min separated by 10 min of reoxygenation. At 30 min after the third 'conditioning' insult, slices underwent a 'test' anoxic insult [1 min of anoxic depolarization (AD)], and then slices were reoxygenated. Evoked potentials (EPs) were recorded throughout the experiment. In other slices, APC was emulated by inducing spreading depression (as determined by a negative DC shift) with KCL or by inducing increased neuronal excitability with the excitatory agent 8- cyclopentyl-1,3-dipropylxanthine (DPCPX) (an adenosine A1 receptor blocker). 'Test' anoxic insults lasted 2 min of AD in these groups. To determine the role of calcium during APC, extracellular CaCl2 was decreased to 0.5 mM but only during the APC episodes ('test' anoxia, 1 min of AD). EP amplitudes recovered significantly better after anoxia in preconditioned slices, and in KCl- and DPCPX-treated slices (147.2 ± 33.3, n = 8, ** p < 0.01, 71.7 ± 13.5, n = 7, ** p < 0.01, and 117.8 ± 37.3, n = 5, *** p < 0.001, respectively) compared to controls. Decreases in extracellular CaCl2 during APC blocked the recovery of EPs after 'test' anoxia (80.6 ± 23.0, n = 8). These data confirm that increases in excitability can emulate APC. These data also demonstrate that calcium influx during preconditioning is required for the induction of tolerance during APC.
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