METABOTROPIC GLUTAMATE RECEPTORS NEGATIVELY COUPLED TO ADENYLATE-CYCLASE INHIBIT N-METHYL-D-ASPARTATE RECEPTOR ACTIVITY AND PREVENT NEUROTOXICITY IN MESENCEPHALIC NEURONS IN-VITRO

The functional effects of G protein-linked glutamate receptor activati on have been studied in mouse mesencephalic neurons in vitro. We have been able to identify two receptor classes, one linked to phosphoinosi tide hydrolysis and another that inhibits adenylate cyclase. The agoni st (1S,3R)-aminocyclo-pentane-1,3-dicarboxylate (ACPD) affected the tw o responses with similar potency (EC(50) = 2 and 7 mu M, respectively) . In contrast, (2S,3S,4S)-alpha-(carboxycyclopropyl)glycine selectivel y decreased adenylate cyclase activity (EC(50) = 150 nM), without inte rfering with the phosphoinositide pathway. Activation of ion channel-l inked glutamate receptors in mesencephalic neurons leads to cGMP forma tion. In this study, we demonstrate that cell pretreatment with ACPD o r (2S,3S,4S)-alpha-(carboxycyclopropyl)glycine prevented, in a dose-de pendent fashion, N-methyl-D-aspartate (NMDA)-induced cGMP formation bu t not the kainate-stimulated response. The pharmacological profile sug gests that receptors that are negatively coupled to adenylate cyclase are responsible for this effect. Coexposure of neurons to ACPD and Ba2 +, a K+ channel blocker, counteracted the ACPD-induced blockade of NMD A receptors, suggesting that activation of K+ conductances could be in volved in the post-transduction events triggered by metabotropic recep tors in the mesencephalon. Neuronal treatment with NMDA for 10 min cau sed a reduction in mitochondrial activity. Direct inhibition of nitric oxide synthase with the inhibitor N-G-nitro-L-arginine or removal of extracellular nitric oxide with reduced hemoglobin did not prevent thi s metabolic impairment, thus excluding a role for nitric oxide in this test for excitotoxicity. On the contrary, the mitochondrial function was maintained when neurons exposed to NMDA were preincubated with met abotropic receptor agonists. To summarize, our results suggest that me tabotropic receptors that are negatively coupled to adenylate cyclase exert modulatory control specifically on NMDA receptor activity. This event could also contribute to the reduction of neurotoxic effects due to NMDA receptor hyperactivity.