Poster Abstract #3: mGluR5 Activation Reduces Microglial-Mediated Neurotoxicity in Vitro and Improves Outcome After Experimental Traumatic Brain Injury

Abstract

In traumatic brain injury, neuronal cell death continues long after the initial insult; persistent activation of microglial cells may contribute to this process. Metabotropic glutamate receptors (mGluRs) are G-protein coupled receptors. Group I mGluRs modulate neuronal cell death: mGluR1 activation enhances necrotic cell death and mGluR5 activation inhibits apoptosis. We have found that microglia strongly express mGluR5 but not mGluR1, and speculated that they may serve to modulate microglial-mediated inflammation and neuronal toxicity. We explored this question using both primary rat microglial cultures and a mouse microglial cell line (BV2). Stimulation of BV2 microglia by lipopolysaccharide (LPS) increased inducible nitric oxide synthase expression and nitric oxide production, a key inflammatory signaling molecule. Pretreatment with the mGluR5 agonist CHPG significantly attenuated these responses, which were reversed by the mGluR5 antagonist MTEP. LPS stimulation also increased the expression of p22PHOX, a component of the NADPH oxidase enzyme complex that generates reactive oxygen species; both p22PHOX and reactive oxygen species were attenuated by CHPG. Conditioned media from LPS-stimulated BV2 microglia significantly increased neuronal cell death, which was blocked by CHPG pretreatment. Similar observations were made using primary microglial cultures. After controlled cortical impact injury, mice received intracerebroventricular injection of either CHPG or vehicle at 30 minutes after injury. CHPG significantly reduced lesion volume (by T2-weighted magnetic resonance imaging) at 3 weeks and improved motor function and spatial learning performance when compared to vehicle-treated mice; each of the effects was blocked by MTEP. Moreover, at 21 days post injury, the CHPG-treated mice showed a marked reduction in ED1 expression, a marker of activated microglia. These data support a potential therapeutic role of mGluR5 agonists for inhibiting microglial activation and related neurotoxicity. Given the important proposed role of neuroinflammation in acute and chronic neurodegeneration, these observations may have broader implications for many neurological disorders.