Selective activation of mGlu4 metabotropic glutamate receptors is protective against excitotoxic neuronal death

Activation of group III metabotropic glutamate receptors (mGluR4, mGluR6, m GluR7, and mGluR8) has been established to be neuroprotective in vitro and in vivo. To disclose the identity of the receptor subtype(s) that exert(s) the protective effect, we have used group III agonists in combination with mGluR4 subtype-deficient mice (-/-). In cortical cultures prepared from wil d-type (+/+) mice and exposed to a toxic pulse of NMDA, the selective group III agonist (+)-4-phosphonophenylglycine [(+)-PPG] reversed excitotoxicity with an EC50 value of 4.9 mu M, whereas its enantiomer (-)-PPG was inactiv e. This correlated closely with the potency of (+)-PPG in activating recomb inant mGluR4a. In cortical neurons from -/- mice, (+)-PPG showed no protect ion against the NMDA insult up to 300 mu M, whereas group I/II mGluR ligand s still retained their protective activity. Classical group III agonists (L -2-amino-4-phosphonobutyrate and L-serine-O-phosphate) were also substantia lly neuroprotective against NMDA toxicity in +/+ and heterozygous (+/-) cul tures but were inactive in -/- cultures. Interestingly, -/- cultures were m ore vulnerable to low concentrations of NMDA and showed higher extracellula r glutamate levels compared with +/+ cultures. We have also examined neurod egeneration induced by intrastriatal infusion of NMDA in wild-type or mGluR 4-deficient mice. Low doses of (R,S)-PPG (10 nmol/0.5 mu l) substantially r educed NMDA toxicity in +/+ mice but were ineffective in -/- mice. Higher d oses of (R,S)-PPG were neuroprotective in both strains of animals. Finally, microdialysis studies showed that intrastriatal infusion of NMDA increased extracellular glutamate levels to a greater extent in -/- than in +/+ mice , supporting the hypothesis that the mGluR4 subtype is necessary for the ma intenance of the homeostasis of extracellular glutamate levels.