Background: This review presents basic knowledge on glutamate and glutamine homeostasis in the central nervous system and relates this knowledge to some aspects of cerebral ischaemia.
Results and interpretation: The amino acid glutamate is the main excitatory neurotransmitter in the central nervous system. Following stimulation of the postsynaptic glutamate receptors, glutamate must rapidly be removed from the synaptic cleft. Whereas several other neurotransmitters are taken up directly into the presynaptic nerve terminal, glutamate is mainly transported into the surrounding glial cells. Glial glutamate can be amidated to glutamine, and since glutamine is a precursor of glutamate without being a neurotransmitter itself, it can be returned to the presynaptic neuron without eliciting new synaptic signals. In the nerve terminal, glutamine can be converted back to glutamate, thereby completing the postulated glutamate-glutamine cycle. During ischaemia, this cycle is impaired as it depends on energy-consuming membrane transport proteins and enzymes.