Impact of adolescent ethanol binge on serotonin signaling and pain sensitivity post-withdrawal

Adolescence is a critical neurodevelopmental period characterized by heightened neuroplasticity. While the acute effects of binge ethanol (EtOH) consumption are documented, its long-term impact on both pain sensitivity and microglial activation during adolescence remains unclear. Given serotonin's (5-HT) known involvement in pain processing and sensitivity to EtOH, this study examined the effects of adolescent EtOH binge on microglia-induced neuroinflammation in serotonergic nuclei, downstream 5-HT signaling, and pain sensitivity at different time points after EtOH withdrawal. Adolescent male C57BL/6J mice received triweekly oral gavage of 20 % EtOH or water for 4 weeks and were assessed after 24 h and 3 weeks post-withdrawal. We used immunohistochemistry to assess neuroinflammation in the dorsal raphe, median raphe, and raphe magnus by labeling 5-HT, CD68, and P2Y12. Further analyses examined downstream signaling via 5-HT and serotonin transporter (SERT) expression in the nucleus accumbens, anterior cingulate cortex, thalamus, amygdala, hypothalamus, and raphe magnus. Pain sensitivity was then assessed using the Hargreaves test. EtOH exposure led to widespread serotonergic and neuroinflammatory changes. Significant increases in microglia-induced neuroinflammation were observed in the dorsal raphe nucleus, median raphe nucleus, and raphe magnus nucleus after both 24 h and 3 weeks post-withdrawal, along with significant deficits in 5-HT. Similar 5-HT deficits were observed in downstream regions-notably in the anterior cingulate cortex, thalamus, amygdala, and hypothalamus-at varying time points post-withdrawal. EtOH-exposed mice also showed lasting hyperalgesia at both 24 h and 3 weeks post-withdrawal that persisted for up to 9 weeks. These results suggest that persistent hyperalgesia following adolescent EtOH binge may be driven by changes in serotonergic function and microglial activation.