mGluR7-like metabotropic glutamate receptors inhibit NMDA-mediated excitotoxicity in cultured mouse cerebellar granule neurons

Glutamate-induced glutamate release may be involved in the delayed neuronal death induced by N-methyl-D-aspartate (NMDA). In order to examine a possib le modulatory effect of the presynaptic group III mGluRs on glutamate excit otoxicity, the effect of L-2-amino-4-phosphonobutyrate (L-AP4) was examined on NMDA-induced delayed death of mouse cerebellar granule neurons in cultu re. We found that L-AF4, at high concentration tin the millimolar range), i nhibited in a non-competitive manner the NMDA-induced toxicity. This effect was mimicked by high concentration of L-serine-o-phosphate (L-SOP), and wa s inhibited by pertussis toxin (PTX) indicating the involvement of a Gi/o p rotein. This suggests the involvement of mGluR7 in the L-AP4 effect, and th is was consistent: with the detection of both mGluR7 protein and mRNA in th ese cultured neurons. To examine the mechanism of the L-AP4-induced protect ion from excitotoxic damage, the effect of L-AP4 on glutamate release was e xamined. L-AP4 (greater than or equal to 1 mM) noncompetitively inhibited b y more than 60% the glutamate release induced by NMDA during the insult. We also observed that the 10-min NMDA receptor stimulation resulted in a dram atic increase in the extracellular glutamate concentration reaching 6000% o f the control value 24 h after the insult. This large increase was also inh ibited when NMDA was applied in the presence of greater than or equal to 1 mM L-AP4. Part of the L-AP4-induced protection from excitotoxic damage of g ranule neurons may therefore result from the inhibition of the vicious cycl e: dying cells release glutamate, glutamate induced cell death. The present results add to the hypothesis that presynaptic mGluRs, probably mGluR7, ma y be the targets of drugs decreasing glutamate release and then neuronal de ath observed in some pathological situations.