Preconditioning induces tolerance by suppressing glutamate release in neuron culture ischemia models

J. Neurochem. (2012) 122, 470–481.

Abstract

This study determined how preconditioned neurons responded to oxygen-glucose deprivation (OGD) to result in neuroprotection instead of neurotoxicity. Neurons preconditioned using chronically elevated synaptic activity displayed suppressed elevations in extracellular glutamate ([glutamate_ex]) and intracellular Ca²⁺ (Ca²⁺_in) during OGD. The glutamate uptake inhibitor TBOA induced neurotoxicity, but at a longer OGD duration for preconditioned cultures, suggestive of delayed up-regulation of transporter activity relative to non-preconditioned cultures. This delay was attributed to a critically attenuated release of glutamate, based on tolerance observed against insults mimicking key neurotoxic signaling during OGD (OGD-mimetics). Specifically, in the presence of TBOA, preconditioned neurons displayed potent protection to the OGD-mimetics: ouabain (a Na⁺/K⁺ ATPase inhibitor), high 55 mM KCl extracellular buffer (plasma membrane depolarization), veratridine (a Na⁺ ionophore), and paraquat (intracellular superoxide producer), which correlated with suppressed [glutamate_ex] elevations in the former two insults. Tolerance by preconditioning was reversed by manipulations that increased [glutamate_ex], such as by exposure to TBOA or GABA_A receptor agonists during OGD, or by exposure to exogenous NMDA or glutamate. Pre-synaptic suppression of neuronal glutamate release by preconditioning, possibly via suppressed exocytic release, represents a key convergence point in neuroprotection during exposure to OGD and OGD-mimetics.