SYNTHESIS AND RELEASE OF N-ACETYLASPARTYLGLUTAMATE (NAAG) BY CRAYFISH NERVE FIBERS: IMPLICATIONS FOR AXON^GLIA SIGNALING

AbstractöEarly physiological and pharmacological studies of cray¢sh and squid giant nerve ¢bers suggested that glu-tamate released from the axon during action potential generation initiates metabolic and electrical responses of periaxonal glia. However, more recent investigations in our laboratories suggest that N-acetylaspartylglutamate (NAAG) may be the released agent active at the glial cell membrane. The investigation described in this paper focused on NAAG metabolism and release, and its contribution to the appearance of glutamate extracellularly. Axoplasm and periaxonal glial cell cytoplasm collected from medial giant nerve ¢bers (MGNFs) incubated with radiolabeled L-glutamate contained radiolabeled glutamate, glutamine, NAAG, aspartate, and GABA. Total radiolabel release was not altered by electrical stimulation of nerve cord loaded with [14C]glutamate by bath application or loaded with [14C]glutamate, [3H]-D-aspartate or [3H]NAAG by axonal injection. However, when radiolabeled glutamate was used for bath loading, radiolabel distribution among glutamate and its metabolic products in the superfusate was changed by stimulation. NAAG was the largest fraction, accounting for approximately 50 % of the total recovered radiolabel in control conditions. The stimulated increase in radioactive NAAG in the superfusate coincided with its virtual clearance from the medial giant axon (MGA). A small, stimulation-induced increase in radiolabeled glutamate in the superfusate was detected only when a glutamate uptake inhibitor was present. The increase in [3H]glutamate in the superfusion solution of nerve incubated with [3H]NAAG was reduced when L-NAAG, a competitive glutamate carboxypeptidase I