N-acetylaspartylglutamate (NAAG) is the probable mediator of axon-to-glia signaling in the crayfish medial giant nerve fiber

Glial cell hyperpolarization previously has been reported to be induced by high frequency stimulation or glutamate. We now report that it also is prod uced by the glutamate-containing dipeptide N-acetylaspartylglutamate (NAAG) , by its non-hydrolyzable analog beta -NAAG, and by NAAG in the presence of 2-(phosphonomethyl)-pentanedioic acid (2-PMPA), a potent inhibitor of the NAAG degradative enzyme glutamate carboxypeptidase II. The results indicate that NAAG mimics the effect of nerve fiber stimulation on the glia. Althou gh glutamate has a similar effect, the other presumed product of NAAG hydro lysis, N-acetylaspartate, is without effect on glial cell membrane potentia l, as is aspartylglutamate (in the presence of 2-PMPA). The hyperpolarizati on induced by stimulation, glutamate, NAAG, beta -NAAG, or NAAG plus 2-PMPA is completely blocked by the Group II metabotropic glutamate receptor anta gonist (S)-alpha -ethylglutamate but is not altered by antagonists of Group I or III metabotropic glutamate receptors. The N-methyl-D-aspartate recept or antagonist MK801 reduces but does not eliminate the hyperpolarization ge nerated by glutamate, NAAG or stimulation. These results, in combination wi th those of the preceding paper, are consistent with the premise that NAAG could be the primary axon-to-glia signaling agent. When the unstimulated nerve fiber is treated with cysteate, a glutamate reu ptake blocker, there is a small hyperpolarization of the glial cell that ca n be substantially reduced by pretreatment with 2-PMPA before addition of c ysteate. A similar effect of cysteate is seen during a 50 Hz/5 s stimulatio n. From these results we suggest that glutamate derived from NAAG hydrolysis a ppears in the periaxonal space under the conditions of these experiments an d may contribute to the glial hyperpolarization. (C) 2001 IBRO. Published b y Elsevier Science Ltd. All rights reserved.