Selective Activation of mGlu4 Metabotropic Glutamate Receptors Is Protective against Excitotoxic Neuronal Death

• Published in: The Journal of neuroscience Vol. 20; no. 17; pp. 6413 - 6420
• Main Authors: Bruno, V, Battaglia, G, Ksiazek, I, van der Putten, H, Catania, M. V, Giuffrida, R, Lukic, S, Leonhardt, T, Inderbitzin, W, Gasparini, F, Kuhn, R, Hampson, D. R, Nicoletti, F, Flor, P. J
• Format: Journal Article
• Language: English
• Published: United States Soc Neuroscience 01.09.2000; Society for Neuroscience
• Subjects: Aminobutyrates - pharmacology; Animals; Cells, Cultured; Cerebral Cortex - cytology; Cerebral Cortex - physiology; Excitatory Amino Acid Agonists - pharmacology; Glutamic Acid - metabolism; Glycine - analogs & derivatives; Glycine - pharmacology; Heterozygote; Mice; Mice, Knockout; N-Methyl-D-aspartic acid; N-Methylaspartate - toxicity; Nerve Degeneration - chemically induced; Nerve Degeneration - physiopathology; Neurons - cytology; Neurons - drug effects; Neurons - physiology; Neuroprotective Agents - pharmacology; Neurotoxins - pharmacology; Receptors, Metabotropic Glutamate - deficiency; Receptors, Metabotropic Glutamate - genetics; Receptors, Metabotropic Glutamate - physiology; Stereoisomerism
• ISSN: 0270-6474; 1529-2401
• DOI: 10.1523/JNEUROSCI.20-17-06413.2000

Activation of group III metabotropic glutamate receptors (mGluR4, mGluR6, mGluR7, 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 wild-type (+/+) mice and exposed to a toxic pulse of NMDA, the selective group III agonist (+)-4-phosphonophenylglycine [(+)-PPG] reversed excitotoxicity with an EC(50) value of 4.9 microm, whereas its enantiomer (-)-PPG was inactive. This correlated closely with the potency of (+)-PPG in activating recombinant mGluR4a. In cortical neurons from -/- mice, (+)-PPG showed no protection against the NMDA insult up to 300 microm, whereas group I/II mGluR ligands still retained their protective activity. Classical group III agonists (l-2-amino-4-phosphonobutyrate and l-serine-O-phosphate) were also substantially neuroprotective against NMDA toxicity in +/+ and heterozygous (+/-) cultures but were inactive in -/- cultures. Interestingly, -/- cultures were more vulnerable to low concentrations of NMDA and showed higher extracellular glutamate levels compared with +/+ cultures. We have also examined neurodegeneration induced by intrastriatal infusion of NMDA in wild-type or mGluR4-deficient mice. Low doses of (R,S)-PPG (10 nmol/0.5 microl) substantially reduced NMDA toxicity in +/+ mice but were ineffective in -/- mice. Higher doses 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 maintenance of the homeostasis of extracellular glutamate levels.