MEK inhibition exacerbates ischemic calcium imbalance and neuronal cell death in rat cortical cultures

Interruption in the brain's blood supply leads to an ischemic condition, which is characterised by a depletion of energy phosphates and related failure of ionic pumps, increased extracellular potassium, neuronal depolarisation and release of excitatory amino acids, e.g. glutamate. The subsequent activation of N-methyl-d-aspartate glutamate receptors triggers a wide range of intracellular signals, including the mitogen-activated protein kinase (MAPK) pathway. Activation and inhibition of the MAPK/extracellular regulated kinases (ERK) pathway are both reported to be neuroprotective in conditions associated with excitotoxic injury. The present study was designed to explore the involvement of this signalling pathway in cultured rat cortical neurons subjected to chemically-induced ischemia obtained by coupling the mitochondrial toxin 3-nitropropionic acid with glucose deprivation. Loss of neuronal viability, reduced neuronal energy state (ATP level and mitochondrial membrane potential) and increased cytoplasmic mitochondrial calcium were all observed. The NMDA antagonist MK-801 counteracted these effects, suggesting a glutamate-dependent ischemic cell death. Addition of U0126, a selective inhibitor of MAPK kinase, exacerbated this neuronal cell death. However, non-significant changes in activated cAMP response element-binding protein were seen. The rise in cytoplasmic calcium under ischemic conditions was associated with neuronal cell swelling. Both swelling and increase in cytoplasmic calcium were exacerbated and prevented by U0126 and MK-801, respectively. These data suggest that in this ischemic model the MAPK/ERK pathway might exert a regulatory effect on calcium entry independent from gene expression.