Mild mitochondrial inhibition in vivo enhances glutamate-induced neuronal damage through calpain but not caspase activation: role of ionotropic glutamate receptors

Glutamate neurotoxicity is exacerbated when energy metabolism is impaired. In vitro studies show that neuronal death in these conditions is related to mitochondrial dysfunction, ATP depletion, and the loss of calcium homeostasis. We have recently observed that, in vivo, enhancement of glutamate toxicity elicited by previous mitochondrial inhibition does not involve severe ATP depletion, suggesting the involvement of other processes. Factors such as the activation of different proteases may determine the extent and type of cell death. Protease activation might be triggered by internal or external factors, such as mitochondrial damage or the activation of a particular glutamate receptor subtype. In the present study we aimed to investigate whether moderate inhibition of mitochondrial metabolism facilitates glutamate toxicity through caspase-3 or calpain activation, as well as the contribution of NMDA and non-NMDA glutamate ionotropic receptors to this activation. Rats were pre-treated with a subtoxic dose of 3-NP and 4 h later intrastriatally injected with glutamate. Results show that neither of these treatments alone (3-NP or Glu) or in combination (3-NP+Glu) activated caspase-3. Conversely, calpain activity is induced after glutamate injection both in intact and 3-NP pre-treated rats. Inhibition of calpain activity by MDL-28170 significantly prevented striatal damage. NMDA and non-NMDA receptors contributed equally to calpain activation and to the induction of neuronal death. Results suggest that enhancement of glutamate toxicity due to inhibition of mitochondrial metabolism in vivo, does not recruit caspase-dependent apoptosis but favors calpain activation through the stimulation of both subtypes of glutamate ionotropic receptors.