Regional synthesis of 14C-amino acids in cerebral cortex of the unanesthetized rat: simultaneous analysis by "push-pull" perfusion.

Rats were surgically prepared with two sets of guide cannulae positioned bilaterally to rest below the superficial layer of the prefrontal, parietal and occipital regions of the cerebral cortex. Following post-operative recovery, two cortical sites were labeled simultaneously by microinjection of a 1.0 microliter volume of 5.0 microCi of D-[U-14C]-glucose. After a 20 min interval for incorporation into amino acids, two sites were concurrently perfused over an interval of 20 min by means of a push-pull cannula system with an artificial CSF solution at a rate of 25 microliters/min. A thin-layer chromatographic assay for separation of amino acid neurotransmitters was used to estimate the content in each sample of perfusate of labeled gamma-amino-butyric acid (GABA), glutamate, glutamine, aspartate and glycine. The results show that recovery of three amino acids, GABA, aspartate and glycine, was uniform among the three areas of the rat's cortex. However, the synthesis of labeled glutamate and glutamine was significantly lower within the parietal cortex in comparison to that of the prefrontal and occipital regions. Overall, the recovery of GABA was proportionally higher whereas glycine was lower than that of the other amino acids. The difference in the results from previous observations can be attributed to the distribution of amino acids in neurons and their terminals in deeper layers of cortex, which may give a different profile of release when epidural measurements are obtained.