Figure 2.

Proposed pathway of glutamate/glutamine neurotransmitter cycling between neurons and glia (22, 23), whose flux has been quantitated recently by ¹³C MRS experiments (9). Action potentials reaching the presynaptic neuron cause release of vesicular glutamate into the synaptic cleft, where it is recognized by glutamate receptors post-synaptically and is cleared by Na⁺ -coupled transport into glia. There it is converted enzymatically to glutamine, which passively diffuses back to the neuron and, after reconversion to glutamate, is repackaged into vesicles. The rate of the glutamate-to-glutamine step in this cycle (V_cycle), has been derived from recent ¹³C experiments (8–10).