Regulation of glutamate carboxypeptidase II hydrolysis of N-acetylaspartylglutamate (NAAG) in crayfish nervous tissue is mediated by glial glutamate and acetylcholine receptors.

Glutamate carboxypeptidase II (GCPII), a glial ectoenzyme, is responsible for N-acetylaspartylglutamate (NAAG) hydrolysis. Its regulation in crayfish nervous tissue was investigated by examining uptake of [H-3]glutamate derived from N-acetylaspartyl-[H-3]glutamate ([H-3]NAAG) to measure GCPII activity. Electrical stimulation (100 Hz, 10 min) during 30 min incubation with [H-3]NAAG increased tissue [H-3]glutamate tenfold. This was prevented by 2-(phosphonomethyl)-pentanedioic acid (2-PMPA), a GCPII inhibitor, suggesting that stimulation increased the hydrolysis of [H-3]NAAG and metabolic recycling of [H-3]glutamate. Antagonists of glial group II metabotropic glutamate receptors (mGLUR(II)), NMDA receptors and acetylcholine (ACh) receptors that mediate axon-glia signaling in crayfish nerve fibers decreased the effect of stimulation by 58-83%, suggesting that glial receptor activation leads to stimulation of GCPII activity. In combination, they reduced [H-3]NAAG hydrolysis during stimulation to unstimulated control levels. Agonist stimulation of mGLUR(II) mimicked the effect of electrical stimulation, and was prevented by antagonists of GCPII or mGLUR(II). Raising extracellular K+ to three times the normal level stimulated [H-3]NAAG release and GCPII activity. These effects were also blocked by antagonists of GCPII and mGLUR(II). No receptor antagonist or agonist tested or 2-PMPA affected uptake of [H-3]glutamate. We conclude that NAAG released from stimulated nerve fibers activates its own hydrolysis via stimulation of GCPII activity mediated through glial mGLUR(II,) NMDA and ACh receptors.