The stems of Syringa pinnatifolia alleviate insomnia based on the brain-gut-microbiota axis mediated by 5-HT in mice and rats.

The peeled stems of Syringa pinnatifolia Hemsl. (SP) have long been used for the treatment of insomnia in Inner Mongolia, China, but their hypnotic constituents and underlying pharmacological basis remain poorly defined. In this study, we investigated the hypnotic activity of SP and identified its active fraction and major constituents using an equal mass to equivalent effect transmission (EMEET)-guided fractionation strategy. SP extracts and fractions were evaluated in pentobarbital-induced sleep tests and a PCPA-induced insomnia model in mice and rats. The phenolic fraction (Ps) significantly shortened sleep latency, prolonged sleep duration, and reduced hyperactivity in insomnia animals. Further fractionation and pharmacological evaluation identified alashinol F (F), alashinol G (G), and (-)-secoisolariciresinol (K) as major constituents contributing to the hypnotic activity of Ps. Mechanistically, Ps decreased serotonin transporter (SERT) expression and increased tryptophan hydroxylase (TPH) expression in the hypothalamus and colon. Ps treatment was also associated with decreased hypothalamic 5-HT2C and PLCβ1 expression, restoration of Gαq/11 expression, and alterations in gut microbial composition, including changes in Bacteroidetes, Prevotella_9, and Blautia. Collectively, these findings identify the major hypnotic fraction and active lignans of SP and suggest that its sleep-promoting effects are associated with coordinated regulation of serotonergic signaling and the gut microbiota.