Glutamate transport, glutamine synthetase and phosphate‐activated glutaminase in rat CNS white matter. A quantitative study

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Abstract
Glutamatergic signal transduction occurs in CNS white matter, but quantitative data on glutamate uptake and metabolism are lacking. We report that the level of the astrocytic glutamate transporter GLT in rat fimbria and corpus callosum was ∼ 35% of that in parietal cortex; uptake of [3H]glutamate was 24 and 43%, respectively, of the cortical value. In fimbria and corpus callosum levels of synaptic proteins, synapsin I and synaptophysin were 15–20% of those in cortex; the activities of glutamine synthetase and phosphate‐activated glutaminase, enzymes involved in metabolism of transmitter glutamate, were 11–25% of cortical values, and activities of aspartate and alanine aminotransferases were 50–70% of cortical values. The glutamate level in fimbria and corpus callosum was 5–6 nmol/mg tissue, half the cortical value. These data suggest a certain capacity for glutamatergic neurotransmission. In optic and trigeminal nerves, [3H]glutamate uptake was < 10% of the cortical uptake. Formation of [14C]glutamate from [U‐14C]glucose in fimbria and corpus callosum of awake rats was 30% of cortical values, in optic nerve it was 13%, illustrating extensive glutamate metabolism in white matter in vivo. Glutamate transporters in brain white matter may be important both physiologically and during energy failure when reversal of glutamate uptake may contribute to excitotoxicity.