Ak. Urazaev et al., Synthesis and release of N-acetylaspartylglutamate (NAAG) by crayfish nerve fibers: Implications for axon-glia signaling, NEUROSCIENC, 106(1), 2001, pp. 237-247
Early physiological and pharmacological studies of crayfish and squid giant
nerve fibers suggested that glutamate released from the axon during action
potential generation initiates metabolic and electrical responses of peria
xonal 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 g
lutamate extracellularly.
Axoplasm and periaxonal glial cell cytoplasm collected from medial giant ne
rve fibers (MGNFs) incubated with radiolabeled L-glutamate contained radiol
abeled glutamate, glutamine, NAAG, aspartate, and GABA. Total radiolabel re
lease was not altered by electrical stimulation of nerve cord loaded with [
C-14]glutamate by bath application or loaded with [14C]glutamate, [H-3]-D-a
spartate or [H-3]NAAG by axonal injection. However, when radiolabeled gluta
mate 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 re
covered radiolabel in control conditions. The stimulated increase in radioa
ctive NAAG in the superfusate coincided with its virtual clearance from the
medial giant axon (MGA). A small, stimulation-induced increase in radiolab
eled glutamate in the superfusate was detected only when a glutamate uptake
inhibitor was present. The increase in [3H]glutamate in the superfusion so
lution of nerve incubated with [H-3]NAAG was reduced when beta -NAAG, a com
petitive glutamate carboxypeptidase II (GCP II) inhibitor, was present.
Overall, these results suggest that glutamate is metabolized to NAAG in the
giant axon and its periaxonal glia and that, upon stimulation, NAAG is rel
eased from the axon and converted in part to glutamate by GCP Il. A quisqua
late-and beta -NAAG-sensitive GCP II activity was detected in nerve cord ho
mogenates. These results, together with those in the accompanying paper dem
onstrating that NAAG can activate a glial electrophysiological response com
parable to that initiated by glutamate, implicate NAAG as a probable mediat
or of interactions between the MGA and its periaxonal glia. (C) 2001 IBRO.
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