5-HT2C receptors in the glutamatergic neurons of dorsal raphe nucleus orchestrate the comorbidity of pain and anxiety in mice

BACKGROUND: Pain-anxiety comorbidity represents a prevalent clinical concern. This study aims to investigate the molecular mechanisms underlying the comorbidities via focusing on the activity of dorsal raphe nucleus (DRN) glutamatergic neurons and the functional role of their 5-hydroxytryptamine 2C (5-HT2C) receptors in relation to gut microbiota. METHODS: We established a Complete Freund's Adjuvant (CFA)-induced pain-anxiety comorbidity model in mice and systematically investigated the role of the brain-gut axis in the comorbidity using behavioral phenotyping, molecular biology, pharmacological/chemogenetic modulation, and gut microbiota profiling. RESULTS: Heightened activity in the glutamatergic neurons of the DRN was found in mice with comorbidity. Chemogenetic activation of DRN glutamatergic neurons replicated the comorbid phenotype in naïve mice, while the selective inhibition of DRN glutamatergic neurons effectively reversed the behavioral and physiological impairments induced by CFA. Notably, a significant upregulation in the protein levels of 5-HT2C receptors in the DRN was detected in the comorbid state. Bidirectional manipulation of 5-HT2C receptors in the DRN glutamatergic neurons bidirectionally regulates neuronal excitability and comorbid phenotypes: agonism or overexpression exacerbates comorbidity, while antagonism or knockdown attenuates CFA-induced deficits. CONCLUSIONS: These findings uncover the role of 5-HT2C receptors in DRN glutamatergic neurons in pain-anxiety comorbidity, thereby presenting novel targets for potential therapeutic interventions.