Neuroscience
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Interleukin-6 (IL-6) is a major cytokine that promotes anti- and pro-inflammatory outcomes by activating the membrane IL-6 receptor (IL-6R) or the soluble IL-6 receptor (sIL-6R). IL-6R and sIL-6R signaling engage the JAK1/2/3 targets and the downstream transcription of STAT1 and STAT3 family. In the brain, physiological IL-6 signaling preserves neurogenesis, neuronal differentiation, and neuroprotection against tissue injury, but IL-6 has been proposed as a biomarker for poor prognosis in several mental pathologies such as depressive disorders, schizophrenia, bipolar disorder, and autism. ⋯ Notably, definition of anti- or pro-inflammatory profiles by IL-6 signaling in the brain are sensitive to the levels, cellular source, and targets of the IL-6 itself, as well as IL-6 receptor signaling, and its activation/inhibition ratio. We propose that a mutual IL-6 crosstalk between microglia, astrocytes, neurons, and endothelial cells defines the anti- and pro-inflammatory outcomes in the brain, modulating brain function. This review will describe the cellular, molecular and context-dependent signaling pathways that define anti- or pro-inflammatory profiles setting by IL-6 during physiological or pathological outcomes in the brain.
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Toll-like receptor-4 (TLR4), a member of the TLR family, plays a key role in inflammation-related diseases of the nervous system. TLR4 knockout mice are widely used in various neurological disease studies, and there is a clear correlation between inflammation and behavior. Therefore, elucidating the effect of TLR4 on neurobehavioral function is essential, and the related mechanisms need to be explored. ⋯ TLR4 knockout significantly attenuated the fear response in 16-m-old mice. The TLR4-/- mice also had better blood-brain barrier integrity, increased expression of tight junction-associated proteins, increased cerebral cortical blood flow and reduced proinflammatory cytokine expression in the cortex and cerebrospinal fluid. Our results suggest that TLR4 deletion ameliorates significant neurobehavioral dysfunction during the aging stage, as well as multiple abnormalities in brain function and structure due to alterations in tight junction-associated proteins and inflammatory factors.