Intraperitoneal injection of the Gram-negative bacterial endotoxin lipopolysaccharide (LPS) elicits a rapid innate immune response. While this systemic inflammatory response can be destructive, tolerable low doses of LPS render the brain transiently resistant to subsequent injuries. However, the mechanism by which microglia respond to LPS stimulation and participate in subsequent neuroprotection has not been documented. In this study, we first established a novel LPS treatment paradigm where mice were injected intraperitoneally with 1.0 mg/kg LPS for four consecutive days to globally activate CNS microglia. By using a reciprocal bone marrow transplantation procedure between wild-type and Toll-like receptor 4 (TLR4) mutant mice, we demonstrated that the presence of LPS receptor (TLR4) is not required on hematogenous immune cells but is required on cells that are not replaced by bone marrow transplantation, such as vascular endothelia and microglia, to transduce microglial activation and neuroprotection. Furthermore, we showed that activated microglia physically ensheathe cortical projection neurons, which have reduced axosomatic inhibitory synapses from the neuronal perikarya. In line with previous reports that inhibitory synapse reduction protects neurons from degeneration and injury, we show here that neuronal cell death and lesion volumes are significantly reduced in LPS-treated animals following experimental brain injury. Together, our results suggest that activated microglia participate in neuroprotection and that this neuroprotection is likely achieved through reduction of inhibitory axosomatic synapses. The therapeutic significance of these findings rests not only in identifying neuroprotective functions of microglia, but also in establishing the CNS location of TLR4 activation.

Microglia actively survey the brain microenvironment and play essential roles in sculpting synaptic connections during brain development. While microglial functions in the adult brain are less clear, activated microglia can closely appose neuronal cell bodies and displace axosomatic presynaptic terminals. Microglia-mediated stripping of presynaptic terminals is considered neuroprotective, but the cellular and molecular mechanisms are poorly defined. Using 3D electron microscopy, we demonstrate that activated microglia displace inhibitory presynaptic terminals from cortical neurons in adult mice. Electrophysiological recordings further establish that the reduction in inhibitory GABAergic synapses increased synchronized firing of cortical neurons in γ-frequency band. Increased neuronal activity results in the calcium-mediated activation of CaM kinase IV, phosphorylation of CREB, increased expression of antiapoptotic and neurotrophic molecules and reduced apoptosis of cortical neurons following injury. These results indicate that activated microglia can protect the adult brain by migrating to inhibitory synapses and displacing them from cortical neurons.

Recent studies have described significant demyelination and microglial activation in the cerebral cortex of brains from multiple sclerosis patients. To date, however, experimental models of cortical demyelination or cortical inflammation have not been extensively studied. In this report we describe focal cortical inflammation induced by stereotaxic injection of killed bacteria (BCG), followed 1 month later by subcutaneous injection of the same antigen, a protocol that overcomes the immune privilege of the cortex. Intracerebral BCG injection produced focal microglial activation at the injection site (termed acute lesion). Ten days after peripheral challenge (termed immune-mediated lesion), larger areas and higher densities of activated microglia were found near the injection site. In both paradigms, activated microglia and/or their processes closely apposed neuronal perikarya and apical dendrites. In the immune-mediated lesions, 45% of the axosomatic synapses was displaced by activated microglia. Upon activation, therefore, cortical microglial migrate to and strip synapses from neuronal perikarya. Since neuronal pathology was not a feature of either the acute or immune-mediated lesion, synaptic stripping by activated microglia may have neuroprotective consequences. V V C 2006 Wiley-Liss, Inc.

Preconditioning is a phenomenon in which the brain protects itself against future injury by adapting to low doses of noxious insults. Preconditioning stimuli include ischemia, low doses of endotoxin, hypoxia, hypothermia and hyperthermia, cortical spreading depression, anesthetics, and 3-nitropropionic acid, among others. Understanding of the mechanisms underlying preconditioning has been elusive, but NMDA receptor activation, nitric oxide, inflammatory cytokines, and suppression of the innate immune system appear to have a role. Elucidation of the endogenous cell survival pathways involved in preconditioning has significant clinical implications for preventing neuronal damage in susceptible patients. All authors reported that they have no financial relationships that pose a potential conflict of interest with this article.

The medial nucleus of the trapezoid body (MNTB) in the superior olivary complex (SOC) is an inhibitory hub considered critical for binaural sound localization. We show that genetic ablation of MNTB neurons in mice only subtly affects this ability by prolonging the minimum time required to detect shifts in sound location. Furthermore, glycinergic innervation of the SOC is maintained without an MNTB, consistent with the existence of parallel inhibitory inputs. These findings redefine the role of MNTB in sound localization and suggest that the inhibitory network is more complex than previously thought.