Toll-like receptor 4 in the interface between neuroimmune response and behavioral alterations caused by stress

Abstract

Different stressors can elicit neuroinflammatory responses modulated by innate immunity receptors, such as the family of Toll-like receptors (TLRs). The TLR4, a pattern recognition receptor (PRR), is involved in many diseases, such as inflammatory and central nervous system (CNS) diseases. Stress exposure can regulate the expression of PRRs, including TLR4, in the brain of animals, especially in the hippocampus and prefrontal cortex. Moreover, TLR4 modulates behavior and neuroinflammatory responses in the brain. In addition, to TLR4, the endocannabinoid (eCB) system plays a role in stress response and immunity, acting as a regulatory, stress-buffer system. This system is involved in many TLRs-mediated immune responses, such as microglia activation. Therefore, pharmacological approaches targeting the eCB system could modulate neuroinflammatory responses to stress by interfering with the TLR4 pathway. Although the connection between TLR4, stress, and neuroinflammation is well documented, almost no pre-clinical studies investigate the possible direct relationship between TLR4, behavior, stress, and the eCB system. Studies exploring the relationship between stress, neuroinflammation, TLR4, and the eCB system were searched using Pubmed, Web of Science, and Embase databases. Based on this search, this review is focused on the involvement of TLR4 receptors and signaling in neuroinflammation and the behavioral consequences of stress exposure. Moreover, evidence of the eCB system modulating TLR4-mediated responses was brought to the attention, pointing out a possible regulatory role of these responses by eCBs in behavior changes related to mood disorders.