Metabolic Precursors and Compartmentation of Cerebral GABA in Vigabatrin‐Treated Rats

Abstract

The metabolic precursors and cerebral compartmentation of the augmented GABA pool induced by vigabatrin, an irreversible inhibitor of GABA transaminase, have been investigated by 13C NMR. Adult rats receiving rat chow ad libitum were given either drinking water only or drinking water containing 2.5 g/L vigabatrin for 7 days. Both groups of animals were infused either with [1,2‐13C2]acetate (15 µmol/min/100 g body weight), an exclusive precursor of GABA formation through the glial glutamine pathway, or with [1,2‐13C2]glucose (15 µmol/min/100 g body weight), a substrate that can produce GABA through the glial glutamine pathway or by direct metabolism in the neurons. The brains were frozen in situ, extracted with perchloric acid, and analyzed by 13C NMR. In vigabatrin‐treated animals [13C]glutamine, a common intermediate for [13C]GABA synthesis from glucose or acetate, was accumulated to similar amounts during infusions with [1,2‐13C2]glucose or [1,2‐13C2]acetate. However, [13C]GABA accumulation was sevenfold higher during [1,2‐13C2]glucose infusions or twofold higher during [1,2‐13C2]acetate infusions. These results show that the direct pathway of GABA formation by neuronal metabolism of glucose predominates over the alternative pathway through glial glutamine. Near‐equilibrium relationships of the aminotransferases of GABA and aspartate imply that the observed [13C]GABA accumulation occurs initially in the neuronal compartment.

Regulation of glial metabolism studied by 13C‐NMR

“…Various ex vivo as well as in vitro NMR studies using different 13 C-labelled substrates provided concurrent evidence contradicting the neuronal localization of pyruvate 59,61,94,195 However, studies using 13 C-labelled acetate (with or without 13 Clabelled glucose) in animal models 59,65 and in primary astrocyte cultures 49,81 showed that up to 20% of lactate was derived from acetate after recycling of pyruvate. The total formation of brain lactate from TCA cycle intermediates was calculated as follows: 59 the percentage 13 Cenrichment in lactate C-2 and C-3 derived from [2-13 C 1 ]acetate was approximately 2%, and that of (whole brain) glutamate C-3 and C-4 was 2.6%.…”

“…Approximately 5 nmol/min/mg of the TCA cycle flux may be mediated by pyruvate recycling considering a relative flux of 30% 64 of the TCA cycle through pyruvate recycling and cerebral TCA cycle rates of approximately 15 nmol/ min per mg protein. 61,196,199 Comparison with maximal activities of ME and PEPCK in the brain (7.6 and 1.6 nmol/min/mg protein), 64 suggests that the capacity of these enzymes will be unlikely to limit the recycling rates in the brain in vivo. Furthermore, while the activities of ME and PEPCK are very low in neurons (approximately 0.3 nmol/min/mg protein and not detectable, respectively), they are much higher in astrocytes (approx.…”

[1‐13C] glucose MRS in chronic hepatic encephalopathy in man

“…Peak assignments based on the previous reports (3,(23)(24)(25) were confirmed in in vivo spectra and in spectra from model solutions of NAA, glutamate, glutamine, phosphorylcholine (PC), creatine (Cr), phosphocreatine (PCr), myoinositol (mI), aspartate, lactate, alanine, and HCO 3 -, as described by Bluml et al (4). All spectra were processed off-line on a Sun SPARCstation 2 using the SA/GE software package provided by General Electric.…”

Clinical experience with 13C MRS in vivo

“…13 C MRS, also developed 30 years ago, held the promise of circumventing many problems by offering dynamic metabolic analysis. 2,3 It is the purpose of this Review to examine the hypothesis that dynamic analysis of metabolite flux rates in vivo is more powerful in clinical investigation than static analysis. In other words, that 13 C NMR after enrichment provides more useful diagnostic information than other steady-state concentration measurements using 1 H, 31 P, 23 Na etc., and thereby compensates for its poor sensitivity.…”