A multispecies probiotic accelerates fear extinction and inhibits relapse in mice: Role of microglia

Microglial activation in the medial prefrontal cortex after remote fear recall participates in the regulation of auditory fear extinction

Excessive or inappropriate fear responses can lead to anxiety-related disorders, such as post-traumatic stress disorder (PTSD). Studies have shown that microglial activation occurs after fear conditioning and that microglial inhibition impacts fear memory. However, the role of microglia in fear memory recall remains unclear. In this study, we investigated the activated profiles of microglia after the recall of remote-cued fear memory and the role of activated microglia in the extinction of remote-cued fear in adult male C57BL/6 mice. The results revealed that the expression of the microglia marker Iba1 increased in the medial prefrontal cortex (mPFC) at 10 min and 1 h following remote-cued fear recall, which was accompanied by amoeboid morphology. Inhibiting microglial activation through PLX3397 treatment before remote fear recall did not affect recall, reconsolidation, or regular extinction but facilitated recall-extinction and mitigated spontaneous recovery. Moreover, our results demonstrated reduced co-expression of Iba1 and postsynaptic density protein 95 (PSD95) in the mPFC, along with decreases in the p-PI3K/PI3K ratio, p-Akt/Akt ratio, and KLF4 expression after PLX3397 treatment. Our results suggest that microglial activation after remote fear recall impedes fear extinction through the pruning of synapses in the mPFC, accompanied by alterations in the expression of the PI3K/AKT/KLF4 pathway. This finding can help elucidate the mechanism involved in remote fear extinction, contributing to the theoretical foundation for the intervention and treatment of PTSD.

Microbial reconstitution reverses prenatal stress-induced cognitive impairment and synaptic deficits in rat offspring

Stress during pregnancy is often linked with increased incidents of neurodevelopmental disorders, including cognitive impairment. Here, we report that stress during pregnancy leads to alterations in the intestinal flora, which negatively affects the cognitive function of offspring. Cognitive impairment in stressed offspring can be reproduced by transplantation of cecal contents of stressed pregnant rats (ST) to normal pregnant rats. In addition, gut microbial dysbiosis results in an increase of β-guanidinopropionic acid in the blood, which leads to an activation of the adenosine monophosphate-activated protein kinase (AMPK) and signal transducer and activator of transcription 3 (STAT3) in the fetal brain. Moreover, β-guanidinopropionic acid supplementation in pregnant rats can reproduce pregnancy stress-induced enhanced glial differentiation of the fetal brain, resulting in impaired neural development. Using probiotics to reconstruct maternal microbiota can correct the cognitive impairment in the offspring of pregnant stressed rats. These findings suggest that microbial reconstitution reverses gestational stress-induced cognitive impairment and synaptic deficits in male rat offspring.

Astrocytic phagocytosis in the medial prefrontal cortex jeopardises postoperative memory consolidation in mice

Memory impairment is one of the main characteristics of postoperative cognitive dysfunction. It remains elusive how postoperative pathological changes of the brain link to the memory impairment. The clinical setting of perioperation was mimicked via partial hepatectomy under sevoflurane anaesthesia together with preoperative restraint stress (Hep-Sev-stress) in mice. Memory changes were assessed with fear conditioning. The medial prefrontal cortex (mPFC)-dorsal hippocampus connectivity was evaluated with injecting neurotracer 28 days before surgery. Astrocytic activation was limited via injecting AAV-GFAP-hM4Di-eGFP into the mPFC. Astrocytic and microglial phagocytosis of synapses were visualised with co-labelling hippocampal neuronal axon terminals with PSD-95 and S100β or Iba1. Neuroinflammation and oxidative stress status were also detected. Hep-Sev-stress impaired the memory consolidation (mean [standard error], 49.91 [2.55]% vs. 35.40 [3.97]% in the contextual memory, p = 0.007; 40.72 [2.78]% vs. 27.77 [2.22]% in cued memory, p = 0.002) and the cued memory retrieval (39.00 [3.08]% vs. 24.11 [2.06]%, p = 0.001) in mice when compared with these in the naïve controls. Hep-Sev-stress damaged the connectivity from the dorsal hippocampus to mPFC but not from the mPFC to the dorsal hippocampus and increased the astrocytic but not microglial phagocytosis of hippocampal neuronal axon terminals in the mPFC. The intervention also induced neuroinflammation and oxidative stress in the dorsal hippocampus and the mPFC in a regional-dependent manner. Limiting astrocyte activation in the mPFC alleviated memory consolidation impairment induced by Hep-Sev-stress. Postoperative memory consolidation was impaired due to astrocytic phagocytosis-induced connectivity injury from the dorsal hippocampus to the medial prefrontal cortex.

Advances in the study of the effects of gut microflora on microglia in Alzheimer's disease

Alzheimer's disease (AD) is a central nervous system (CNS) degenerative disorder, is caused by various factors including β-amyloid toxicity, hyperphosphorylation of tau protein, oxidative stress, and others. The dysfunction of microglia has been associated with the onset and advancement of different neurodevelopmental and neurodegenerative disorders, such as AD. The gut of mammals harbors a vast and complex population of microorganisms, commonly referred to as the microbiota. There's a growing recognition that these gut microbes are intrinsically intertwined with mammalian physiology. Through the circulation of metabolites, they establish metabolic symbiosis, enhance immune function, and establish communication with different remote cells, including those in the brain. The gut microbiome plays a crucial part in influencing the development and performance of microglia, as indicated by recent preclinical studies. Dysbiosis of the intestinal flora leads to alterations in the microglia transcriptome that regulate the interconversion of microglia subtypes. This conversation explores recent research that clarifies how gut bacteria, their byproducts, and harmful elements affect the activation and characteristics of microglia. This understanding opens doors to innovative microbial-based therapeutic strategies for early identification and treatment goals in AD.

Fluoxetine Decreases Phagocytic Function via REV-ERBα in Microglia

Although fluoxetine (FLX) is a commonly used drug in psychiatric disorders, such as major depressive disorder, anxiety disorder, panic disorder, and obsessive-compulsive disorder, the mechanism by which FLX exerts its therapeutic effect is not completely understood. In this study, we aimed to determine the possible mechanism by which FLX focuses on microglial phagocytosis. FLX reduced phagocytic function in BV2 cells and increased REV-ERBα without affecting other microglia-related genes, such as inflammation and phagocytosis. Although FLX did not change BMAL1 protein levels, it restricted the nucleocytoplasmic transport (NCT) of BMAL1, leading to its cytosolic accumulation. REV-ERBα antagonist SR8278 rescued the decreased phagocytic activity and restricted NCT of BMAL1. We also found that REV-ERBα mediates the effect of FLX via the inhibition of phospho-ERK (pERK). The ERK inhibitor FR180204 was sufficient to reduce phagocytic function in BV2 cells and restrict the NCT of BMAL1. These results were recapitulated in the primary microglia. In conclusion, we propose that FLX decreases phagocytic function and restricts BMAL1 NCT via REV-ERBα. In addition, ERK inhibition mimics the effects of FLX on microglia.

The microbiota-gut- hippocampus axis

Introduction

It is well known that the intestinal bacteria substantially affect physiological processes in many body organs. Especially, through a bidirectional communication called as gut-microbiota-brain axis, the gut microbiota deeply influences development and function of the nervous system. Hippocampus, as a part of medial temporal lobe, is known to be involved in cognition, emotion, and anxiety. Growing evidence indicates that the hippocampus is a target of the gut microbiota. We used a broad search linking the hippocampus with the gut microbiota and probiotics.

Methods

All experimental studies and clinical trials published until end of 2021 were reviewed. Influence of the gut microbiota on the behavioral, electrophysiological, biochemical and histological aspects of the hippocampus were evaluated in this review.

Results

The effect of disrupted gut microbiota and probiotic supplements on the microbiota-hippocampus link is also considered. Studies show that a healthy gut microbiota is necessary for normal hippocampus dependent learning and memory and synaptic plasticity. The known current mechanisms are production and modulation of neurotrophins, neurotransmitters and receptors, regulation of intracellular molecular processes, normalizing the inflammatory/anti-inflammatory and oxidative/antioxidant factors, and histological stability of the hippocampus. Activity of the hippocampal neuronal circuits as well as behavioral functions of the hippocampus positively respond to different mixtures of probiotic bacteria.

Discussion

Growing evidence from animal researches indicate a close association between the hippocampus with the gut microbiota and probiotic bacteria as well. However, human studies and clinical trials verifying such a link are scant. Since the most of papers on this topic have been published over the past 3 years, intensive future research awaits.

Human threat learning is associated with gut microbiota composition

The ability to learn about threat and safety is critical for survival. Studies in rodent models have shown that the gut microbiota can modulate such behaviors. In humans, evidence showing an association with threat or extinction learning is lacking. Here, we tested whether individual variability in threat and extinction learning was related to gut microbiota composition in healthy adults. We found that threat, but not extinction learning, varies with individuals' microbiome composition. Our results provide evidence that the gut microbiota is associated with excitatory threat learning across species.

Restorative therapy using microglial depletion and repopulation for central nervous system injuries and diseases

Microglia are important resident immune cells in the central nervous system (CNS) and play an important role in its development, homeostasis, and disease treatments. Activated microglia perform diverse functions in mouse models of CNS neurodegenerative diseases or deficits. In humans, microglia have been linked to various neurodegenerative diseases. Following brain or spinal cord injury, microglia express pro- and anti-inflammatory phenotypes at different stages of recovery. With the development of pharmacological and genetic tools for microglial depletion, studies have demonstrated that microglial depletion exerts both positive and negative effects in the treatment of CNS diseases. Notably, microglial depletion provides an empty niche that stimulates production of new microglia. Microglial depletion and repopulation can not only treat diseases by eliminating dysfunctional microglia but can also provide an indication of the molecular mechanisms of diseases. Although this approach has shown impressive results, its use is still in its infancy. In this review, we summarize the current pharmacological and genetic tools for microglial depletion and highlight recent advances in microglial repopulation therapy for the treatment and functional recovery of neurological diseases and deficits. Finally, we briefly discuss the therapeutic challenges and prospective uses of microglial repopulation therapy.