In Vivo Release from Cerebral Cortex of [14C]Glutamate Synthesized from [U-14C]Glutamine

Awake, unrestrained, and behaviourally normal animals with superfusion cannulae implanted over the sensorimotor cortex were used in a study of the capacity of infused [U-14C]glutamine for labelling glutamate and other amino acids released by depolarising stimuli. A spontaneous background release of [14C]glutamate was detected. This was increased by tityustoxin (1 microM). The specific radioactivity of glutamate increased eightfold during the evoked-release period. [14C]Aspartate was also detected and showed increased release, but not increased specific labelling, in response to depolarisation. Evoked gamma-aminobutyric acid (GABA) release occurred but only small amounts of [14C]GABA were detected. Glutamine showed increased rates of uptake to the sensorimotor cortex during stimulation periods, suggesting an accelerated breakdown via glutaminase.