Ammonium and Glutamate Released by Neurons Are Signals Regulating the Nutritive Function of a Glial Cell

Abstract

Glial cells transform glucose to a fuel substrate taken up and used by neurons. In the honeybee retina, photoreceptor neurons consume both alanine supplied by glial cells and exogenous proline. Ammonium (NH4+) and glutamate, produced and released in a stimulus-dependent manner by photoreceptor neurons, contribute to the biosynthesis of alanine in glia. Here we report that NH4+ and glutamate are transported into glia and that a transient rise in the intraglial concentration of NH4+ or of glutamate causes a net increase in the level of reduced nicotinamide adenine dinucleotides [NAD(P)H]. Biochemical measurements indicate that this is attributable to activation of glycolysis in glial cells by the direct action of NH4+ and glutamate on at least two enzymatic reactions: those catalyzed by phosphofructokinase (PFK; ATP:D-fructose-6-phosphotransferase, EC2.7.1.11) and glutamate dehydrogenase (GDH; L-glutamate:NAD oxidoreductase, deaminating; EC1.4.1.3). This activation leads to an increase in the production and release of alanine by glia. This signaling, which depends on the rate of conversion of NH4+ and glutamate to alanine and alpha-ketoglutarate, respectively, in the glial cells, raises the novel possibility of a tight regulation of the nutritive function of glia.

The Ammonium-Induced Increase in Rat Brain Lactate Concentration is Rapid and Reversible and is Compatible with Trafficking and Signaling Roles for Ammonium

“…Several laboratories have suggested that glycolysis in astrocytes is coupled to the activity of glutamatergic synapses (see Pellerin and Magistretti, 2005). This proposal originally stemmed from the observation that lactate release from astrocytes was increased by glutamate (see Pellerin and Magistretti, 2005), but it has subsequently been found that glycolysis is also increased by application of ammonium to astrocytes (Kala and Hertz, 2005) and to glial cells of bee retina (Tsacopoulos et al, 1997), and by ammonium plus glutamate to retinal Mü ller glial cells (Poitry et al, 2000). In contrast, application of ammonium to neurons does not increase net lactate production (Kala and Hertz, 2005).…”

Short-Term Hyperhomocysteinemia-Induced Oxidative Stress Activates Retinal Glial Cells and Increases Vascular Endothelial Growth Factor Expression in Rat Retina

“…Tel: +82-2-3277-3468; Fax: +82-2-3277-2862; E-mail: nschang@ewha.ac.kr Abbreviations: BRB, blood-retinal barrier; GCL, ganglion cell layer; GFAP, glial fibrillary acidic protein; HPLC, high-performance liquid chromatography; INL, inner nuclear layer; LOX-1, lectin-like oxidized low-density lipoprotein; NFL, nerve fiber layer; NIH, National Institutes of Health; ONL, outer nuclear layer; PBS, phosphate-buffered saline; SOD, superoxide dismutase; TBARS, thiobarbituric acid reactive substances; VEGF, vascular endothelial growth factor modulating the microenvironment of individual neurons by maintaining tissue integrity and ion homeostasis, and these cells also regulate the transport, uptake, and metabolism of neurotransmitters. 14) Even though there is evidence that suggests that glial cells might play a crucial role in development, injury repair, and regeneration in the nervous system and retina, [15][16][17] the effects of hyperhomocysteinemia and homocystine diet-induced oxidative stress on retinal glial cells and retinal injury have not been studied to date.…”

The nutritive function of glia is regulated by signals released by neurons

“…For fluorescence imaging experiments, aliquots of freshly isolated glial cells Tsacopoulos et al, 1997) were pipetted into solutionfilled wells made from Petri dishes fitted with glass coverslips coated with poly-L-lysine to immobilize cells during pressure injections, and then left 3-12 h at 4-8°C in 2 ml of Ringer's solution (in mM: NaCl 200; KCl 10; CaCl 2 2.0; MgCl 2 4.5; MOPS 10; sucrose 200; and trehalose 40; pH 6.9) prior to fluorescence measurements. NAD(P)H fluorescence was measured on an inverted Axiovert 135 TV epi-fluorescence microscope equipped with Zeiss Pan-neofluar 63ϫ oil immersion lens (1.25 NA) and fitted with a xenon arc lamp, neutral density filters, 380 nm excitation filter, a 425 nm dichroic mirror, and a 470 nm emission filter.…”

“…For maintaining primary cultures of metabolically active glial cells up to 24 h, only the presence of 40 mM trehalose was necessary. Quantitative analysis of amino acids in isolated cells was performed using reverse phase chromatography with precolumn derivatization with ortho-phthalaldehyde and fluorescence detection (HP 1090 HPLC system, Hewlett-Packard, Palo Alto, CA) Tsacopoulos et al, 1997).…”