Epigenetic repression of the serotonergic neuron phenotype following adolescent binge drinking is restored through inhibition of proinflammatory signaling by exercise and glycyrrhizic acid

The serotonergic system regulates diverse social and emotional processes, including formation of social networks that continue to mature throughout adolescence. In adolescent intermittent ethanol (AIE)-exposed male and female rats and post-mortem human dorsal raphe nucleus (DRN) tissue from adolescent-onset alcohol use disorder (AUD) cases, we observed persistent reductions of serotonergic markers (TPH2+, 5HT+) within the DRN and its projections, accompanied by robust neuroimmune activation (HMGB1, TLR4, pNFκBp65). Reduced DRN prodynorphin and selective reductions of medial DRN serotonergic neurons (TPH2+, 5HT+) suggest that specific serotonergic subpopulations are suppressed by AIE. We further found increased repressive histone markers (H3K9me2) and decreased activating markers (H3K4me3Q5ser) at the <i>Tph2</i> gene promoter in the DRN, consistent with epigenetic repression of the serotonergic neuron phenotype. Post-AIE treatment with glycyrrhizic acid-an HMGB1 antagonist-or post-AIE exercise restored serotonergic neuron populations and their projections, reversed induction of neuroimmune signaling molecules, and normalized <i>Tph2</i> promoter chromatin states, indicating reversible neuroimmune-mediated suppression of serotonergic phenotype rather than neuronal loss. Parallel changes in human AUD DRN tissue support a conserved mechanism. Behaviorally, AIE induced social deficits in rats that mirrors social withdrawal in AUD, an effect that was rescued by post-AIE glycyrrhizic acid treatment. In humans, early-onset problematic alcohol use is associated with social dysfunction, which is mitigated in individuals with high physical activity or anti-inflammatory interventions. Together, these findings implicate epigenetic regulation of serotonergic neuron phenotype as a key mechanism linking adolescent alcohol exposure to social pathology and identify potential targets for therapeutic interventions in individuals with adolescent-onset AUD.<b>Significance statement</b> Adolescence is a critical window for maturation of the serotonergic system that shapes emotional and social behavior. We report that adolescent intermittent ethanol (AIE) exposure persistently suppresses dorsal raphe nucleus (DRN) serotonergic neurons through neuroimmune activation and epigenetic repression of the <i>Tph2</i> gene, effects that parallel findings in human DRN tissue from adolescent-onset alcohol use disorder (AUD) cases. Importantly, pharmacological blockade of proinflammatory HMGB1 signaling or post-AIE exercise restored serotonergic neuron integrity and social behavior, demonstrating reversibility of this neuroimmune-mediated epigenetic repression. These results identify a conserved neuroimmune-epigenetic mechanism linking adolescent alcohol exposure to lasting serotonergic and social dysfunction, suggesting new therapeutic avenues for adolescent-onset AUD and related psychopathologies.