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Selective astrocytic gap junctional trafficking of molecules involved in the glycolytic pathway: impact on cellular brain imaging

To assess the specificity of metabolite trafficking among gap junction‐coupled astrocytes, we developed novel, real‐time, single‐cell enzymatic fluorescence assays to assay cell‐to‐cell transfer of unlabeled glycolytic intermediates and report (i) highly restricted transfer of glucose‐6‐phosphate (P) and two analogs, deoxyglucose (DG)‐6‐P, and 2‐[N‐(7‐nitrobenz‐2‐oxa‐1,3‐diazol‐4‐yl)amino]‐2‐DG‐6‐P, compared with DG and 2‐ and 6‐[N‐(7‐nitrobenz‐2‐oxa‐1,3‐diazol‐4‐yl)amino]‐2‐DG, (ii) extensive junctional diffusion of glyceraldehyde‐3‐P, NADH, and NADPH plus three anionic fluorescent dyes used as internal standards for transfer assays, and (iii) stimulation of gap junctional communication by increased intracellular Na+ that also evokes metabolic responses in nearby coupled astrocytes. Thus, dye transfer does not predict gap junctional permeability of endogenous metabolites. Intracellular retention of flux‐regulating compounds (e.g. glucose‐6‐P) may be necessary for local metabolic control, whereas ‘syncytial sharing’ may dissipate the work load on peri‐synaptic astrocytes. Imaging of brain functional activity depends on local accumulation of exogenous or endogenous signals, and DG‐6‐P is trapped in the cell where it is phosphorylated, whereas rapid dispersal of cytoplasmic NAD(P)H and labeled glucose metabolites throughout the astrocytic syncytium can interfere with cellular assessment of neuron–astrocyte relationships in autoradiographic, fluorescence microscopic, and magnetic resonance spectroscopic studies.