Exacerbation of Neuronal Cell Death by Activation of Group I Metabotropic Glutamate Receptors: Role of NMDA Receptors and Arachidonic Acid Release Jason W. Allen,* , † Stefano Vicini,† , ‡ and Alan I. Faden* , † , § ,1 *Institute for Cognitive and Computational Sciences, †Interdisciplinary Program in Neuroscience, ‡Department of Physiology and Biophysics, and §Department of Neuroscience, Georgetown University, Washington, DC 20007 Received August 31, 2000; accepted February 22, 2001 Both ionotropic and metabotropic glutamate recep- tors have been implicated in the pathogenesis of neu- ronal injury. Activation of group I metabotropic glu- tamate receptors (mGluR) exacerbates neuronal cell death, whereas inhibition is neuroprotective. How- ever, the mechanisms involved remain unknown. Ac- tivation of group I mGluR modulates multiple signal transduction pathways including stimulation of phos- phoinositide hydrolysis, potentiation of NMDA recep- tor activity, and release of arachidonic acid. Here we demonstrate that whereas activation of group I mGluR by (S)-3,5-dihydroxyphenylglycine (DHPG) po- tentiates NMDA-induced currents and intracellular calcium increases in rat cortical neuronal cultures, partial effects of group I mGluR activation or inhibi- tion on neuronal injury induced by oxygen– glucose deprivation remain despite NMDA receptor blockade. DHPG stimulation also increases basal arachidonic acid release from rat neuronal– glial cultures and po- tentiates injury-induced arachidonic acid release in these cultures. Thus, activation of group I mGluR may exacerbate neuronal injury through multiple mecha- nisms, which include positive modulation of NMDA receptors and enhanced release of arachidonic acid. © 2001 Academic Press Key Words: CNS injury; ischemia; trauma; calcium; neuronal– glial culture; signal transduction. INTRODUCTION CNS injury is characterized by activation of both ionotropic and metabotropic glutamate receptors. The former are directly linked to cation channels, whereas the latter are coupled to G-proteins (48). Numerous reports have demonstrated the neuroprotective effects of ionotropic glutamate receptor antagonists following trauma or ischemia, both in vitro (35, 42) and in vivo (21, 22, 55). Modulation of metabotropic glutamate re- ceptors (mGluR) also affects neuronal survival follow- ing injury, with the result depending upon the mGluR group involved and the type of injury model used (1, 9, 23, 26, 41, 47, 51). A role for group I mGluR in neuronal injury has been established in a variety of in vitro and in vivo models. In vitro, activation of group I mGluR exacerbates, whereas inhibition of group I mGluR attenuates, neu- ronal injury induced by application of NMDA (8, 9), oxygen– glucose deprivation (OGD) (3, 9, 47), or me- chanical trauma (41, 43). Similar results have been described in vivo, including models of ischemia–reper- fusion (51) and traumatic brain injury (26, 41). Much early work relating to signal transduction pathways activated by group I mGluR focused on phos- pholipase C, activation of which results in phosphoino- sitide (PI) hydrolysis, liberation of intracellular cal- cium stores, and stimulation of protein kinase C (13, 48). Yet it has been recognized that other pathways may also be induced by group I mGluR; these include activation of adenylyl cyclase, release of arachidonic acid, stimulation of phospholipase D, potentiation of NMDA receptors, and modulation of calcium and po- tassium channels (13, 48). However, many of these earlier studies were performed before the advent of subtype selective agonists and antagonists, utilizing transfection studies in cell lines to isolate each receptor subtype. Thus, the role of such signal transduction pathways in mediating effects of group I mGluR in neurons or during neuronal injury remains largely un- explored. We have previously demonstrated that activation of group I mGluR increased phosphoinositides in neuro- nal– glial cultures and that antisense oligonucleotides directed against both mGluR1 and mGluR5 attenuated this response (41). Interestingly, antisense directed against mGluR1, but not mGluR5, selectively attenu- 1 To whom correspondence should be addressed at Department of Neuroscience, EP-12b Research Building, 3970 Reservoir Road, NW, Washington, DC 20007. Fax: (202) 687-0617. E-mail: fadena@giccs.georgetown.edu. Experimental Neurology 169, 449 – 460 (2001) doi:10.1006/exnr.2001.7672, available online at http://www.idealibrary.com on 449 0014-4886/01 $35.00 Copyright © 2001 by Academic Press All rights of reproduction in any form reserved.