In this study, we investigated the effects of NMDA receptor antagonists on calcium transients induced by a single 2-min preconditioning anoxia (PA) in rat olfactory cortical slices, and on the ability of PA to prevent pathological calcium overload induced by subsequent 10-min test anoxia (TA). Relative changes in the intracellular Ca²⁺ concentration (Ca_i) and in the level of Ca²⁺ bound to intracellular hydrophobic domains (Ca_b) were monitored using fura-2 and chlortetracycline, respectively. Our results confirmed that TA induces prominent long-lasting increases in Ca_i and Ca_b, reflecting cellular calcium overload. It was found that PA produces moderate increases in both Ca²⁺ pools and prevents Ca²⁺ overload induced by TA carried out 90 min later. Calcium transients and the protective effects of PA were significantly suppressed in slices treated with NMDA receptor antagonists during and 30 min after PA. These results indicate that moderate activation of the NMDA receptors participates in the mechanism of the PA-induced anoxic tolerance of cortical neurons.

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Ca(2+)-mediated signal transduction of group I metabotropic glutamate receptors (ImGluR) was studied in the brain of young (15 days) and old rats (90 days) exposed to severe hypobaric hypoxia on gestation days 14-16. Changes in the concentration of bound intracellular Ca(2+) (Ca(2+) response) were evaluated after repeated application of a selective ImGluR agonist 3,5-dihydroxyphenylglycine (DHPG) to cultured brain slices. Primary application of DHPG for 2 min induced a negative Ca(2+) response in slices from 15-day-old intact animals, while repeated application caused a positive response. In slices from 90-day-old control animals, both responses were negative. In slices from rats of both age groups subjected to severe prenatal hypobaric hypoxia, both responses were mainly positive, but short-term negative components were present in adult animals. Our results suggest that severe hypobaric hypoxia changes the balance between the two constitutive signal pathways triggered by ImGluR (inosine triphosphate and diacylglycerol pathways). This procedure is followed by the increased influx of extracellular Ca(2+) (as compared to Ca(2+) release from the intracellular stores). This imbalance is particularly pronounced at the early stage of ontogeny.

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The involvement of the calcium and phosphoinositide intracellular regulatory systems in the molecular-cellular mechanisms of adaptation of the brain to hypoxia induced by transient anoxia were studied in slices of rat olfactory cortex. Anoxia lasting 2 min initiated the development of moderate but stable activation of intracellular regulatory systems during the reoxygenation period, with increases in binding of Ca2+ to intracellular hydrophobic domains and increases in the level of polyphosphoinositide metabolism. During this period, cells in the slices released neuromediator factors into the perfusion fluid; transfer of these to recipient slices induced similar changes in the activities of intracellular regulatory system components in the recipient slices. After anoxia lasting 10 min, NMDA-mediated pathogenic hyperactivity of the calcium and phosphoinositide systems developed. Preliminary moderate activation of these systems by transient anoxia or neuromodulator factors released by cells in response to transient anoxia prevented disruption of intracellular regulatory system activity induced by subsequent longer-lasting anoxia.