GLIAL-NEURONAL INTERACTIONS AS STUDIED BY CEREBRAL METABOLISM OF [2-C-13]ACETATE AND [1-C-13]GLUCOSE - AN EX-VIVO C-13 NMR SPECTROSCOPIC STUDY

Mice were injected intravenously with [2-C-13]-acetate or [1-C-13]gluc ose and killed after 5, 15, or 30 min. Another group of animals was in jected three times subcutaneously during 30 min with [2-C-13]acetate t o achieve a steady-state-like situation. Brain extracts were analyzed by C-13 NMR spectroscopy, and the percent enrichment of various carbon positions was calculated for amino acids, lactate, and glucose. Resul ts obtained with [2-C-13]acetate, which is metabolized by glia and not by neurons, showed that glutamine originated from a glia[ tricarboxyl ic acid cycle (TCA cycle) that loses 65% of its intermediates per turn of the cycle. This TCA cycle was associated with pyruvate carboxylati on, which may replenish Virtually all of this loss, as seen from the l abeling of glutamine from [1-C-13]glucose. From the C-3/C-4 labeling r atios in glutamine and glutamate and from the corresponding C-3/C-2 la beling ratio in GABA obtained with [2-C-13]acetate, it was concluded t hat the carbon skeleton of glutamine to some extent was passed through TCA cycles before glutamate and GABA were formed. Thus, astrocyticall y derived glutamine is not only a precursor for transmitter amino acid s but is also an energy substrate for neurons in vivo. Furthermore, th e neuronal TCA cycles may be control points in the synthesis of transm itter amino acids. Injection of [2-C-13]acetate led to a higher C-13 e nrichment of the C-2 in glutamate and of the corresponding C-4 in GABA than in the C-3 of either compound. This could reflect cleavage of [2 -C-13]-citrate and formation of [3-C-13]oxaloacetate and acetyl-CoA, i .e., the first step in fatty acid synthesis. [3-C-13]-Oxaloacetate wou ld, after entry into a TCA cycle, give the observed labeling of glutam ate and GABA.