N-ACETYLASPARTYLGLUTAMATE ACTIVATES CYCLIC AMP-COUPLED METABOTROPIC GLUTAMATE RECEPTORS IN CEREBELLAR ASTROCYTES

N-Acetylaspartylglutamate (NAAG) is the most prevalent peptide in the mammalian nervous system. NAAG meets the traditional criteria of a neu rotransmitter, including localization in synaptic vesicles, depolariza tion-induced release, low potency activation of some N-methyl-D-aspart ate receptors, and highly selective activation of a cAMP-coupled metab otropic glutamate receptor (mGluR) with potency approaching that of gl utamate. The peptide is present in cultured cortical glia in high conc entration and is hydrolyzed by cell surface peptidase activity. In the present work, we tested the hypothesis that NAAG selectively activate s a subclass of metabotropic receptors on cultured rat cerebellar glia , primarily astrocytes. These glial cells express mRNA for mGluR subty pes 1, 3, 4, 5, 6, 7, and 8. We were not able to detect message for mG luR2 in these cells using the reverse transcriptase-polymerase chain r eaction. Cerebellar glia responded to NAAG, glutamate, and trans-ACPD with a decrease in forskolin-stimulated cAMP formation. AP4, an agonis t of the group III receptors mGluR4, mGluR6, mGluR7, and mGluR8, had l ittle or no effect on stimulated cAMP levels. Treatment with low micro molar NAAG significantly increased uptake of radioactive thymidine by cultured astrocytes through activation of metabotropic glutamate recep tors. Antagonists of group II mGluRs prevented the decrease in cAMP an d the increase in uptake of thymidine by NAAG. Cultured cerebellar ast rocytes expressed 20 pmol NAAG per mg protein, a value that is at leas t 30-fold lower than that expressed by mixed glial cultures prepared f rom mouse cortex. We conclude that cerebellar astrocytes respond to NA AG via the mGluR3 receptor and that the peptide may selectively activa te this receptor subtype in astrocytes following release from neurons or glia. (C) 1998 Wiley-Liss, Inc.