Glutamine and Glucose as Precursors of Transmitter Amino Acids: Ex Vivo Studies

Abstract

Radiolabelled glutamine and glucose were infused into lateral ventricles of rats in order to label transmitter amino acid pools in vivo. Brain regions close to the lateral ventricle (hippocampus, corpus striatum, hypothalamus) were labelled more effectively than more distant structures such as cerebral cortex or cerebellum. All regions were labelled to much the same extent over 30-150 min by [U-14C]glucose, [U-14C]glutamine, or [3H]glutamine administered alone or together in double-label experiments when allowance was made for any differences in precursor specific radioactivities. Slices of cerebral cortex or hippocampus from brains labelled in vivo were incubated and stimulated in vitro with veratrine (75 microM); tetrodotoxin (1 microM) was present in the control medium. Single-label experiments showed that [U-14C]glutamine was more effective than [U-14C]glucose for labelling releasable glutamate and GABA. Double-label experiments showed that [3H]glutamine and [U-14C]glucose given together in vivo labelled glutamate and GABA releasable in vitro to a similar extent. Both types of experiment emphasise the large contribution made by glutamine in vivo to pools of transmitter glutamate and GABA.

Glutamate immunoreactivity in rat cerebral cortex is reversibly abolished by 6-diazo-5-oxo-L-norleucine (DON), an inhibitor of phosphate-activated glutaminase.

“…As reported in the Introduction, a large body of experimental data obtained by a variety of methods supports the notion that PAG is a crucial although not exclusive enzyme for the synthesis of transmitter Glu (Ji et al, 1991;Sherman and Mott, 1986;Thanki et al, 1983;Ward et al, 1983;Reubi, 1980;Hamberger et al, 1979a,b;Hertz, 1979;Bradford et al, 1978Bradford et al, ,1983. These data have been recently confirmed by showing an elevation of KCI-and 4-aminopyridine-induced Glu release from synaptosomal preparations pre-incubated with glutamine (Sherman, 1991).…”

“…Whether the fraction of PAG-dependent Glu immunoreactivity that is critical for immunocytochemical visualization is produced via "perikaryal" or "synaptosomal" PAG (Aoki et al, 1991) is at present completely unknown, as is the possible role of eventual perikaryal high-affinity uptake sites. Whatever the mechanism(s) responsible for cell body staining, given the crucial role of PAG in the formation of "releasable" Glu Ui et al, 1991;Sherman and Mott, 1986;Thanki et al, 1983;Ward et al, 1983;Reubi, 1980;Hamberger et al, 1979a,b;Hertz, 1979;Bradford et al, 1978Bradford et al, ,1983, the results reported here give some strength to the hypothesis that, under our experimental conditions, Glu immunoreactivity in perikarya of cortical neurons might reflect the neurotransmitter pool of Glu (i.e., that Glu immunoreactivity in cell bodies might indeed be a marker of cortical neurons releasing Glu at their nerve terminals). In the cerebral cortex, this possibility is supported by the following observations: (a) the selectivity of staining, both at light and at electron microscopic levels.…”

Metabolic Challenge to Glia Activates an Adenosine-Mediated Safety Mechanism that Promotes Neuronal Survival by Delaying the Onset of Spreading Depression Waves

“…However, this interpretation is weakened by the fact that glial-derived glutamine is not the only source of the neurotransmitter glutamate, as it can also be synthesized from glucose in neurons (Ward et al, 1983). Furthermore, the highly variable rate and degree of synaptic depression observed in different studies (from 30 mins to 5 h and from total suppression to only 85%; BergJohnsen et al, 1993;Keyser and Pellmar, 1994;Largo et al, 1996;Martin et al, 2007) raise some doubt whether the lack of transmitter is the main cause of synaptic failure during glial poisoning.…”

EAT-4, a Homolog of a Mammalian Sodium-Dependent Inorganic Phosphate Cotransporter, Is Necessary for Glutamatergic Neurotransmission inCaenorhabditis elegans

“…Results from metabolic labeling experiments showed that glutamine is used preferentially over glucose as the precursor for depolarization-releasable glutamate in brain slice and synaptosomal preparations (Bradford et al, 1978;Hamberger et al, 1979a,b;Ward et al, 1983). A phosphateactivated glutaminase (PAG; EC 3.5.1.2) hydrolyzes glutamine to glutamate and ammonia in the mitochondria of glutamatergic neurons (Kvamme, 1983;Erecinska and Silver, 1990;Fonnum, 1993).…”