Formononetin attenuates LPS-induced neuroinflammation and depressive-like behaviors by regulating the Keap1/Nrf2/HO-1 pathway in the hippocampus.

Neuroinflammation and oxidative stress are major pathogenic drivers of depression, with the Keap1/Nrf2/HO-1 pathway serving as a key regulator of both processes. Formononetin (FMN) is a naturally occurring isoflavone with anti-inflammatory and antioxidant activities; however, its potential to exert antidepressant effects through activation of this pathway has not been investigated. In this study, lipopolysaccharide (LPS, 1 mg/kg, i.p.) was used to induce neuroinflammation and depressive-like behaviors in mice, and FMN (20 or 40 mg/kg) was administered for seven days prior to LPS challenge. FMN dose-dependently alleviated LPS-induced behavioral deficits, including reduced sucrose preference, decreased locomotor activity, and prolonged immobility in the tail suspension test. Hematoxylin-eosin (HE) and Nissl staining revealed that FMN markedly attenuated LPS-induced hippocampal neuronal damage. FMN also inhibited microglial activation, as evidenced by reduced Iba-1 expression, decreased NF-κB phosphorylation, and downregulation of both mRNA and protein levels of pro-inflammatory cytokines, including IL-1β, TNF-α, and IL-6. Furthermore, LPS-induced lipid peroxidation was significantly attenuated, as indicated by lower levels of 4-hydroxynonenal (4-HNE). Molecular docking suggested a potential direct interaction between FMN and Keap1, while Western blot analysis confirmed that FMN downregulated Keap1 and upregulated Nrf2 and HO-1 protein expression. Collectively, these findings demonstrate that FMN alleviates LPS-induced neuroinflammation and depressive-like behaviors, likely through activating the Keap1/Nrf2/HO-1 pathway, supporting its potential as a preventive candidate for inflammation-associated depression.