Brief low [Mg2+]o-induced Ca2+ spikes inhibit subsequent prolonged exposure-induced excitotoxicity in cultured rat hippocampal neurons

Reducing Mg²⁺]_o to 0.1 mM can evoke repetitive Ca²⁺]_i spikes and seizure activity, which induces neuronal cell death in a process called excitotoxicity. We examined the issue of whether cultured rat hippocampal neurons preconditioned by a brief exposure to 0.1 mM Mg²⁺]_o are rendered resistant to excitotoxicity induced by a subsequent prolonged exposure and whether Ca²⁺ spikes are involved in this process. Preconditioning by an exposure to 0.1 mM Mg²⁺]_o for 5 min inhibited significantly subsequent 24 h exposure-induced cell death 24 h later (tolerance). Such tolerance was prevented by both the NMDA receptor antagonist D-AP5 and the L-type Ca²⁺ channel antagonist nimodipine, which blocked 0.1 mM Mg²⁺]_o-induced Ca²⁺]_i spikes. The AMPA receptor antagonist NBQX significantly inhibited both the tolerance and the Ca²⁺]_i spikes. The intracellular Ca²⁺ chelator BAPTA-AM significantly prevented the tolerance. The nonspecific PKC inhibitor staurosporin inhibited the tolerance without affecting the Ca²⁺]_i spikes. While Gö6976, a specific inhibitor of PKCα had no effect on the tolerance, both the PKCε translocation inhibitor and the PKCζ pseudosubstrate inhibitor significantly inhibited the tolerance without affecting the Ca²⁺]_i spikes. Furthermore, JAK-2 inhibitor AG490, MAPK kinase inhibitor PD98059, and CaMKII inhibitor KN-62 inhibited the tolerance, but PI-3 kinase inhibitor LY294,002 did not. The protein synthesis inhibitor cycloheximide significantly inhibited the tolerance. Collectively, these results suggest that low Mg²⁺]_o preconditioning induced excitotoxic tolerance was directly or indirectly mediated through the Ca²⁺]_i spike-induced activation of PKCε and PKCξ, JAK-2, MAPK kinase, CaMKII and the de novo synthesis of proteins.