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

摘要

Abstract 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 [(3)H]glutamate derived from N-acetylaspartyl-[(3)H]glutamate ([(3)H]NAAG) to measure GCPII activity. Electrical stimulation (100 Hz, 10 min) during 30 min incubation with [(3)H]NAAG increased tissue [(3)H]glutamate tenfold. This was prevented by 2-(phosphonomethyl)-pentanedioic acid (2-PMPA), a GCPII inhibitor, suggesting that stimulation increased the hydrolysis of [(3)H]NAAG and metabolic recycling of [(3)H]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 [(3)H]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 [(3)H]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 [(3)H]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.