N-ACETYLASPARTYLGLUTAMATE SELECTIVELY ACTIVATES MGLUR3 RECEPTORS IN TRANSFECTED CELLS

In previous studies, we demonstrated that the neuropeptide, N-acetylas partylglutamate (NAAG), meets the traditional criteria for a neurotran smitter and selectively activates metabotropic glutamate receptor mGlu R2 or mGluR3 in cultured cerebellar granule cells and glia. Sequence h omology and pharmacological data suggest that these two receptors are highly related structurally and functionally. To define more rigorousl y the receptor specificity of NAAG, cloned rat cDNAs for mGluR1-6 were transiently or stably transfected into Chinese hamster ovary cells an d human embryonic kidney cells and assayed for their second messenger responses to the two endogenous neurotransmitters, glutamate and NAAG, as well as to metabotropic receptor agonists, trans-1-aminocyclopenta ne-1 ,3-dicarboxylate (trans-ACPD) and L-2-amino-4-phosphonobutyrate ( L-AP4). Despite the high degree of relatedness of mGluR2 and mGluR3, N AAG selectively activated the mGluR3 receptor. NAAG activated neither mGluR2 nor mGluR1, mGluR4, mGluR5, or mGluR6. The mGluR agonist, trans -ACPD, activated each of the transfected receptors, whereas L-AP4 acti vated mGluR4 and mGluR6, consistent with the published selectivity of these agonists. Hybrid cDNA constructs of the extracellular domains of mGluR2 and mGluR3 were independently fused with the transmembrane and cytoplasmic domain of mGluR1a. This latter receptor domain is coupled to phosphoinositol turnover, and its activation increases intracellul ar calcium. The cells transfected with these chimeric receptors respon ded to activation by glutamate and trans-ACPD with increases in intrac ellular calcium. NAAG activated the chimeric receptor that contained t he extracellular domain of mGluR3 and did not activate the mGluR2 chim era.