Calcium and increase excitability promote tolerance against anoxia in hippocampal slices

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 elect rical recovery in hippocampal slices; and (b) mild excitation promotes prec onditioning neuroprotection. Control hippocampal slices were given a single 'test' anoxic insult followed by reoxygenation. Experimental slices were p reconditioned by three short anoxic insults of 1 min separated by 10 min of reoxygenation. At 30 min after the third 'conditioning' insult, slices und erwent a 'test' anoxic insult [1 min of anoxic depolarization (AD)], and th en slices were reoxygenated. Evoked potentials (EPs) were recorded througho ut the experiment. In other slices, APC was emulated by inducing spreading depression las determined by a negative DC shift) with KCL or by inducing i ncreased neuronal excitability with the excitatory agent 8-cyclopentyl-1,3- dipropylxanthine (DPCPX) (an adenosine Al receptor blocker). 'Test' anoxic insults lasted 2 min of AD in these groups. To determine the role of calciu m during APC, extracellular CaCl2 was decreased to 0.5 mM but only during t he APC episodes ('test' anoxia, 1 min of AD). EP amplitudes recovered signi ficantly better after anoxia in preconditioned slices, and in KCl- and DPCP X-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 recov ery of EPs after 'test' anoxia (80.6 +/- 23.0, n = 8). These data confirm t hat increases in excitability can emulate APC. These data also demonstrate that calcium influx during preconditioning is required for the induction of tolerance during APC. (C) 1999 Elsevier Science B.V. All rights reserved.