Binge alcohol intake triggers microglial activation and TNF-dependent aberrant synaptic pruning, causing synapse loss and increased anxiety

The effects of alcohol in the central nervous system (CNS) often lead to behavioral deficits which can include anxiety, cognitive decline, and motor dysfunction. These effects were associated with impairment of synaptic function, which is a major cellular and molecular hallmark of excessive alcohol use. Upon a well-established dysfunction that excessive alcohol intake imposes on neurons, abnormal activation of microglia immune function, overproduction of inflammatory mediators and exacerbated phagocytic activity can also be detrimental to synapse activity and negatively affect behavior. Consistent with a neuroimmune hypothesis for alcohol addiction, knocking out various genes that are exclusively expressed or enriched in microglia, decreases voluntary alcohol intake in mice. As such, in this work we explored how microglia activation by binge alcohol intake impacts on synapse pruning and behavior.

We resorted to mice models that allow improved visualization of microglia, selective ablation of microglia, or silencing tumor necrosis factor (TNF) expression, in association with in vitro experiments using a human microglia cell line and sophisticated live imaging tools, to validate pharmacological approaches that target TNF production and counteract the action of binge alcohol intake.

We show that alcohol intake over ten consecutive days resulted in substantial loss of excitatory synapse in the prefrontal cortex, a consequence of aberrant synaptic pruning, which led to increased anxiety-like behavior. Mechanistically, these effects of alcohol intake were mediated by a detrimental increase of microglia engulfment capacity via Src-dependent activation of nuclear factor-kB (NF-kB) and consequent TNF production. Accordingly, pharmacological blockade of Src activation or TNF production by microglia, genetic ablation of TNF, or diphtheria toxin-mediated conditional ablation of microglia attenuated aberrant synaptic pruning preventing excitatory synapse loss and anxiety-like behavior.

Overall, our data suggest that aberrant pruning of excitatory synapses by microglia might disrupt synaptic transmission during alcohol use.

This work was supported by FEDER - Fundo Europeu de Desenvolvimento Regional funds through the COMPETE 2020 Operational Programme for Competitiveness and Internationalisation (POCI), Portugal 2020, and by Portuguese funds through FCT - Fundação para a Ciência e a Tecnologia/Ministério da Ciência, Tecnologia e Ensino Superior in the framework of the projects PTDC/SAU-TOX/30647/2017 and PTDC/ MED-NEU/31318/2017.