3.1 NAAG is Found in Both Axons and Glia of the Crayfish Medial Giant Nerve Fiber
Lieberman et al7 were the first to report that NAAG appeared to be a product of glutamate metabolism in crayfish nerve cord. This finding has since been confirmed9-11 and illustrated in Fig. 1. Following a 2 hour incubation of the medial giant nerve fiber with bath-applied radiolabeled glutamate, axons (right panel) and glia (left panel) contained significant amounts of radiolabeled NAAG, glutamate and glutamine with lesser amounts of GABA and aspartate (not shown). Stimulation of the nerve fiber caused a 30% reduction of NAAG in glia and a total depletion of NAAG in the axon. The radiolabeled NAAG that appeared in the superfusate following stimulation was increased by 300% over non-stimulated controls and represented 49% of the total recovered radiolabel efflux. Glutamate had the next largest efflux at approximately 20% and was unchanged by stimulation. The remaining radiolabel was distributed between aspartate, glutamine and GABA. Total radiolabel recovered in these four glutamate metabolites was greater than 95%.
NAAG content of axons and glia, normalized to 1 ^ of cytoplasm, suggested that glia synthesize and accumulate NAAG an order of magnitude greater than axons. When the volume of the axon, relative to glia is considered (axoplasm represents 95% of the nerve fiber cytoplasm) the increase in radiolabel in axonal NAAG content gives approximately the same rate of synthesis as that seen in glia.
Radiolabeled glutamate directly injected into the axoplasm of medial giant axons appeared in axonal NAAG at 3x the quantity in glia. Stimulation of the nerve fiber
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