Our recent research work is focused on the development and application of the state of the art nuclear magnetic resonance (NMR) spectroscopy and stable ¹³C isotope techniques to study the neurotransmission energetics in rodent brain. We used these in vivo NMR techniques to investigate the relationship between the rates of the neurotransmitter substrate cycles involving glutamate and gamma-aminobutyrate (GABA) and neuronal activity.In the past I have used in-vivo NMR Spectroscopy to study neurotransmitters metabolism in rats brain. Our earlier studies have shown that glutamine, a neutral amino acid produced in the astroglia, is a major precursor of GABA. Further we discovered that GAD67, one of the two isoforms of glutamate decaboxylase (GAD), the enzyme which catalyzes the GABA synthesis mediates majority of GABA synthesis at resting. In continuation of this project, very recently we have shown that GAD65 supports major fraction of the increased GABA synthesis during intense neuronal activation. Using stable ¹³C labeled acetate, and glucose we developed a method to separate the contribution of GABA to total neurotransmitter cycling.




We discovered that GABA contributes to significant fraction (~20%) of total neurotransmitter cycling and neuronal glucose oxidation, and GABAergic flux actually increased but not decreased with increased cortical activity. This finding contradicts the conventional view that increased excitability corresponds to decreased inhibition and not increased as we found. Very recently we have shown that the contribution of neuronal reuptake to synaptically released GABA is minimal. In collaboration with others we found that both glutamatergic and GABAergic neurotransmission increased and tightly coupled to energetics during postnatal development in rat cortex.The fate of glutamate/glutamine cycle was investigated by using in vivo 1H-{¹³C}-NMR spectroscopy during [1,6-¹³C₂]glucose infusion. We discovered that glutamate/glutamine cycling and neuronal glucose oxidation flux increased proportionately during intense neuronal activation. In contrast to what had previously been suggested, non-oxidative glycolytic processes during intense neuronal activation do not appear to supply the energy required during neuronal activity. We have also shown that the intense neuronal activation did not increase the de novo synthesis of oxaloacetate in astroglia through pyruvate carboxylase, despite more than two fold increase in glutamate/glutamine cycle under similar condition. Recently, we consolidated these in vivo and in vitro findings into a comprehensive model for coupling of brain glucose metabolism and neuronal activity. The model predicts neurons produce at least 88% of total oxidative ATP, and take up ~26% of the total glucose oxidized. In a very recent study we found that nerve terminals glutamate are turning over at similar rate as of whole tissue and neurotransmitter cycling flux measured using in vivo ¹³C NMR spectroscopy is true measure of the flux. Succinate semialdehyde dehydrogenase (SSADH) deficiency is a rare autosomal recessive inherited defect of GABA catabolism. In collaboration with others we discovered that both astroglila and glutamatergic metabolism was impaired with no change in GABA metabolism despite elevated GABA level in SSADH knockout mice.