Saikosaponin- d -mediated downregulation of neurogenesis results in cognitive dysfunction by inhibiting Akt/Foxg-1 pathway in mice

Saikosaponin-d (SSd), one of the main constituents of the total saikosaponins extracted from Bupleurum falcatum L, possesses anti-inflammatory and anti-apoptosis effect. Recently, SSd was proved to improve depressive symptoms although exhibit hepatotoxicity in animals, but the central nervous system (CNS) toxicity of SSd remains unclear. The present study investigated the SSd-induced impairment in hippocampal cognitive function and explored the possible mechanisms involved. After intragastric administration of SSd (4 mg/kg, 8 mg/kg) for 7 days, the learning and memory abilities of mice were evaluated by behavioral experiments. In the step-down passive avoidance test, we found that the mice treated with SSd showed a significant decrease of step-down latency and increase of the frequency of errors. In the Morris water maze task, both the escape latency and swimming distance of the mice treated with SSd were increased, correspondingly, both the time of mice staying in the target zone and the frequency of crossing platform were decreased. These neurobehavioral changes were accompanied by the reduction of the expression of 5-bromo-2′-deoxyuridine (BrdU), nestin, doublecortin (Dcx) and microtubule associated protein 2 (MAP2). Moreover, SSd significantly inhibited the expression of p-Akt, Foxg-1 and fibroblast growth factor 2 (FGF2) in the hippocampus of mice. These results indicated that SSd had a toxic effect on cognitive function in mice, which was associated with inhibiting the hippocampal neurogenesis via Akt/Foxg1 pathway.