Neuroprotection against NMDA excitotoxicity by group I metabotropic glutamate receptors is associated with reduction of NMDA stimulated currents.

The neurotransmitter glutamate can have both excitotoxic and protective effects on neurons. The excitotoxic effects have been intensively studied, whereas the protective effects, including the involvement of metabotropic glutamate receptors (mGluRs), remain unclear. In the present study, we tested the protective effects of the group-I-mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) on organotypic hippocampal slice cultures exposed to excitotoxic concentrations of N-methyl-D-aspartate (NMDA). Effects of DHPG on electrophysiological responses induced by NMDA receptor activation were also recorded. Experiments were performed on organotypic hippocampal slice cultures derived from 7-day-old rats, with cellular uptake of propidium iodide as a marker for neuronal cell death. Slice cultures pretreated with DHPG (10 or 100 microM) for 2 h prior to exposure to 50 microM NMDA for 30 min displayed reduced propidium iodide uptake, compared to cultures exposed to NMDA only. The neuroprotective effect was confirmed by Hoechst 33342 staining, where the appearance of pycnotic nuclei after NMDA treatment was prevented by the DHPG pretreatment. Using caspase-3 activity to monitor the presence of apoptosis, failed to demonstrate this type of cell death in CA1 after NMDA application. The protective effect of DHPG was abolished by the mGluR1 selective antagonist (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid (LY367385; 5 or 10 microM), whereas the mGluR5-selective antagonist 2-methyl-6-phenylethynylpyridine (MPEP; 1 microM) had no effect. Voltage-clamping of CA1 pyramidal cells in cultures treated with 10 microM DHPG for 2 h showed a significant depression of NMDA-induced inward currents compared to untreated controls. We conclude that neuroprotection induced by activation of group-I-mGluRs involve mGluR1 and is associated with decreased NMDA-stimulated currents.