In vivo analysis of cortical amino acid neurotransmitters collected in the rat by a new double lumen push-pull catheter system.

The release of both endogenous and newly synthesized amino acid neurotransmitters was examined simultaneously in different areas of the cerebral cortex in the freely moving rat. An array of push-pull guide tubes was implanted permanently to rest above the frontal, parietal, temporal and occipital areas of the cortex of each rat. Then a new double-lumen catheter system, specially adapted for localized push-pull perfusion of the conscious animal, was used to perfuse an artificial cerebrospinal fluid at each cortical site. For the new synthesis experiments, 0.5 microCi of [14C]glucose in a volume of 2.0 microliter was first microinjected into the perfusion site as a precursor to label amino acids. After the site was perfused at a rate of 12.0 microliter/min, each of the samples was assayed by two-dimensional thin-layer chromatography. In a second analysis, the content of six endogenous amino acids present in unlabeled samples of push-pull perfusate was quantified by high-performance liquid chromatography analysis with electrochemical detection. The results showed a notable homogeneity among each of the four cortical areas in the content of four of the six amino acids examined. Endogenous glutamine exhibited the highest proportional content in the cortical perfusates, whereas glutamic acid was proportionally higher in terms of new synthesis. An anatomical analysis revealed that the level of endogenous glutamic acid in the frontal area was significantly lower than that found in the occipital or temporal regions of the rat's cortex. An opposite result was obtained when the proportional synthesis of glutamic acid from [14C] glucose was compared in different cortical regions in that a statistically higher release occurred in the frontal than in the occipital cortex.