The compartmentation of glutamate and its metabolites in fractions of neuron cell bodies and neuropil; studied by intra-ventricular injection of [u-14C] glutamate.

Abstract

Abstract— The in vivo metabolism of glutamate in rat neuron cell bodies and neuropil was studied after intraventricular injection of (U‐14C)glutamic acid followed by separation of the tissue into neuronal and neuropil fractions. The losses of amino acid and of radioactivity during the fractionation were equivalent. Recoveries were: glutamate, 32; glutamine, 15; aspartate, 25; GABA, 41; alanine, 30 per cent. In the washed cell fractions glutamine was 45 per cent and alanine 132 per cent higher in the neuronal fraction, glutamate was 62, GABA 77 and aspartate 95 per cent of neuropil levels. This contrasted with results obtained previously for in vitro incorporation. Calculation from these results indicated that 28 per cent of the original cell suspension was neuronal, 72 per cent neuropil. In the final cell preparations, 29 per cent of the neuron cell bodies and 26 per cent of the neuropil were recovered. Specific activity of glutamate in the neuronal fraction 15 min after injection was higher than in the original suspension, but had declined to 30 per cent of its initial value by 2 h. In the neuropil, specific activity of glutamate was below that of the cell suspension at 15 min, but at later times rose above it by up to 40 per cent. Radioactivity was detected in aspartate and glutamine 15 min after injection and GABA by 60 min after injection. In the original cell suspension the specific activity of glutamine was higher than that of glutamate at all times (the Waelsch effect) but aspartate and GABA were lower than glutamate. In the neuronal fraction the specific activity of glutamine was below that of glutamate at all times, indicating a precursor‐product relationship. In the neuropil fraction, glutamine specific activity remained above glutamate for the first hour. These results are discussed in relation to the interpretation of the Waelsch effect in terms of metabolic compartmentation.