Facilitating microglia M2 polarization alleviates p-Synephrine-induced depressive-like behaviours in CSDS mice via the 5-HT6R-FYN-ERK1/2 pathway

In recent years, modulation of microglial phenotype transformation has emerged as a promising strategy for treating central nervous system disorders. Aurantii Fructus Immaturus (Zhishi), a traditional Chinese medicine with versatile applications, contains p-Synephrine (p-SYN) as its principal bioactive compound, recognized for its anti-inflammatory efficacy. However, the molecular mechanisms underlying these effects remain unclear. This study aimed to elucidate the mechanisms through which p-SYN modulates the microglial phenotype and alleviates neuroinflammation using both a chronic social defeat stress (CSDS) model and a lipopolysaccharide-induced human microglial cell (HMC-3) system. The antidepressant effects of p-SYN were assessed using various behavioural tests including social interaction, three-chambered social interaction, sucrose preference, tail suspension, forced swimming, open field, and novelty-suppressed feeding tests. Additionally, brain penetration of p-SYN was determined using LC-MS. The results indicated that p-SYN mitigated CSDS-induced social deficits and depressive-like behaviours, and lowered inflammatory responses, as evidenced by decreased levels of TNF-α and IL-6 and increased IL-10 levels. Moreover, p-SYN reduced the expression of M1 markers CD86 and iNOS, while increasing that of M2 markers Arg-1 and CD206. Cellular thermal shift assay, drug affinity reaction target stabilization technology, and co-immunoprecipitation demonstrated that p-SYN directly binds to the 5-hydroxytryptamine 6 receptor (5-HT6R), leading to weakening 5-HT6R and tyrosine kinase FYN interaction and inhibiting the 5-HT6R-FYN-ERK1/2 pathway. This enhances the production of anti-inflammatory factors, subsequently shifts microglia to the M2 phenotype, and eventually mitigates neuroinflammation, thereby exhibiting antidepressant properties.