After focal ischaemia, the brain parenchyma suffers from a profound loss of oxygen and glucose, the
brain’s major sources for energy generated by oxidative phosphorylation. The subsequent local energy
deficit induces depolarization of neurons and glia, resulting in activation of voltage gated calcium
channels and the release of excitotoxic amino acids into the extracellular matrix. Particularly the
release of glutamate, which under physiological conditions undergoes an active presynaptic or
astrocytic reuptake, and subsequent binding to and activation of ionotropic N-methyl-D-aspartate
(NMDA) and a-amino-3-hydroxy-5-methyl-4-isoazole propionic acid (AMPA) receptors, promotes
excessive direct and indirect calcium influx.12 Increased intracellular calcium acts as a universal second
and third messenger to trigger an array of downstream phospholipases and proteases that degrade
membranes and proteins essential for cellular integrity.13–15 This intracellular calcium overload also
triggers mitochondrial outer membrane depolarization (MOMP) and subsequent cell death.16
Furthermore, sodium and chloride enter neurons via channels for monovalent ions (for example the
AMPA receptor) and are passively followed by water, causing intracellular (“cytotoxic”) edema. This
may negatively impact perfusion in the peri-infarct region. Imbalances of other ions are also significant:
for example large amounts of zinc that are stored in vesicles of excitatory neurons are released
upon depolarization and contribute to excitotoxic cell death.17–19 Finally, excitotoxicity may be an
initiator of molecular events that lead to apoptosis and inflammation (see below) in regions where
rapid necrosis does not occur.10,12