Activation of microglial N-Methyl-D aspartate receptors triggers inflamation and neuronal cell death in the developing and mature brain.

Objective: Activated microglia play a central role in the inflammatory and excitotoxic component of various acute and chronic neurological disorders. However, the mechanisms leading to their activation in the latter context are
poorly understood, particularly the involvement of N-methyl-D-aspartate receptors (NMDARs), which are critical for excitotoxicity in neurons. We hypothesized that microglia express functional NMDARs and that their activation would trigger neuronal cell death in the brain by modulating inflammation.

Methods and Results: We demonstrate that microglia express NMDARs in the murine and human central nervous system and that these receptors are functional in vitro. We show that NMDAR stimulation triggers microglia
activation in vitro and secretion of factors that induce cell death of cortical neurons. These damaged neurons are further shown to activate microglial NMDARs and trigger a release of neurotoxic factors from microglia in vitro,
indicating that microglia can signal back to neurons and possibly induce, aggravate, and/or maintain neurologic disease. Neuronal cell death was significantly reduced through pharmacological inhibition or genetically induced loss
of function of the microglial NMDARs. We generated Nr1 LoxPþ/þ LysM Creþ/ mice lacking the NMDAR subunit NR1 in cells of the myeloid lineage. In this model, we further demonstrate that a loss of function of the essential
NMDAR subunit NR1 protects from excitotoxic neuronal cell death in vivo and from traumatic brain injury.

Interpretation: Our findings link inflammation and excitotoxicity in a potential vicious circle and indicate that an
activation of the microglial NMDARs plays a pivotal role in neuronal cell death in the perinatal and adult brain.