METABOLISM OF LACTATE IN CULTURED GABAERGIC NEURONS STUDIED BY C-13 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY

Primary cultures of mouse cerebral cortical neurons (GABAergic) were i ncubated for 4 hours in media without glucose containing 1.0 mmol/L [U -C-13]lactate in the absence or presence of 0.5 mmol/L glutamine. Redi ssolved, lyophilized cell extracts were analyzed by C-13 nuclear magne tic resonance spectroscopy to investigate neuronal metabolism of lacta te and by HPLC for determination of the total amounts of glutamate (Gl u), gamma-aminobutyric acid (GABA), and aspartate (Asp). The C-13 nucl ear magnetic resonance spectra of cell extracts exhibited multiplets f or Glu, GABA, and Asp, indicating pronounced recycling of labeled tric arboxylic acid cycle constituents. There was extensive incorporation o f C-13 label into amino acids in neurons incubated without glutamine, with the percent enrichments being approximately 60% for Glu and Asp, and 27% for GABA. When 0.5 mmol/L glutamine was added to the incubatio n medium, the enrichments for Asp, Glu, and GABA were 25%, 35%, and 25 %, respectively. This strongly suggests that glutamine is readily conv erted to Glu and Asp but that conversion to GABA may be complex. The o bservation that enrichment in GABA was identical in the absence and pr esence of glutamine whereas cycling was decreased in the presence of g lutamine indicates that only C-2 units derived from glutamine are used for GABA synthesis, that is, that metabolism through the tricarboxyli c acid cycle is a prerequisite for GABA synthesis from glutamine. The current study gives further support to the hypothesis that cellular me tabolism is compartmentalized and that lactate is an important fuel fo r neurons in terms of energy metabolism and extensively labels amino a cids synthesized from tricarboxylic acid cycle intermediates (Asp and Glu) as well as the neurotransmitter in these neurons (GABA).