Group I metabotropic glutamate receptors reduce excitotoxic injury and may facilitate neurogenesis

Abstract

Group I metabotropic glutamate receptor (mGluR) agonist DHPG reduced nerve cell death caused by their exposure to NMDA ("neuroprotective effect") and attenuated NMDA receptor-mediated currents [Blaabjerg, M., Baskys, A., Zimmer, J., Vawter, M. P., 2003b. Changes in hippocampal gene expression after neuroprotective activation of group I metabotropic glutamate receptors. Brain Research, Molecular Brain Research 117, 196-205.]. In the present study, we used organotypic hippocampal culture preparation to examine specific phospholipase C (PLC) inhibitor U73122 effects on DHPG-induced neuroprotection, changes in excitatory synaptic transmission associated with the neuroprotective DHPG treatment and a role of group I mGluR ligands in neurogenesis. Results show that short (10 min) DHPG treatment did not result in neuroprotection but significantly depressed field synaptic potentials (fEPSP) in the Schaffer collateral-CA1 pathway. The fEPSP depression was not affected by the PLC inhibitor U73122. In contrast, prolonged (2-h) treatment of cultures with DHPG induced a significant protective effect that was blocked by a PLC inhibitor U73122 but not by its inactive analog U73343. Voltage-clamp measurements of spontaneous miniature excitatory post-synaptic currents (EPSCs) recorded in CA1 neurons from cultures treated with DHPG (10 microM, 2 h) showed a significant reduction of the EPSC amplitude in DHPG-treated but not control (untreated) cultures. This reduction was completely abolished by U73122, suggesting a PLC involvement. Since activation of PLC is thought to be associated with cell proliferation, we investigated whether group I mGluR agonist DHPG or subtype antagonists LY367385 and MPEP have an effect on dentate granule cells expressing immature neuronal marker TOAD-64. DHPG (100 microM, 72 h) slightly but not significantly increased the number of TOAD-64 positive cells. The mGluR1 antagonists LY367385 (10 microM, 72 h) markedly decreased the number of TOAD-64 positive cells and mGluR5 antagonist MPEP (1 microM, 72 h) had no effect. These data suggest that (1) prolonged activation of group I mGluRs reduces nerve cell susceptibility to excitotoxic injury in a PLC-dependent manner; (2) this reduction is associated with a PLC-dependent depression of excitatory synaptic transmission; and (3) mGluR1 activation may facilitate neurogenesis.