Harmine exerts anxiolytic effects by regulating neuroinflammation and neuronal plasticity in the basolateral amygdala

Increasing evidence indicates that an altered immune system is closely linked to the pathophysiology of anxiety disorders, and inhibition of neuroinflammation may represent an effective therapeutic strategy to treat anxiety disorders. Harmine, a beta-carboline alkaloid in various medicinal plants, has been widely reported to display anti-inflammatory and potentially anxiolytic effects. However, the exact underlying mechanisms are not fully understood. Our recent study has demonstrated that dysregulation of neuroplasticity in the basolateral amygdala (BLA) contributes to the pathological processes of inflammation-related anxiety. In this study, using a mouse model of anxiety challenged with Escherichia coli lipopolysaccharide (LPS), we found that harmine alleviated LPS-induced anxiety-like behaviors in mice. Mechanistically, harmine significantly prevented LPS-induced neuroinflammation by suppressing the expression of pro-inflammatory cytokines including IL-1 beta and TNF-alpha. Meanwhile, ex vivo whole-cell slice electrophysiology combined with optogenetics showed that LPS-induced increase of medial prefrontal cortex (mPFC)-driven excitatory but not inhibitory synaptic transmission onto BLA projection neurons, thereby alleviating LPS-induced shift of excitatory/inhibitory balance towards excitation. In addition, harmine attenuated the increased intrinsic neuronal excitability of BLA PNs by reducing the medium after-hyperpolarization. In conclusion, our findings provide new evidence that harmine may exert its anxiolytic effect by downregulating LPS-induced neuroinflammation and restoring the changes in neuronal plasticity in BLA PNs.