Piperine regulates glycogen synthase kinase-3β-related signaling and attenuates cognitive decline in D-galactose-induced aging mouse model

Aging-related cholinergic dysfunction, extensive neuroinflammation and oxidative stress in brain are predominant pathogenic factors for dementia. In the present study, we aimed to evaluate the protective effects of piperine, an alkaloid nutrient component of Piper nigrum, against cognitive impairment in a senescent mouse model induced by D-galactose (D-Gal) and to explore the underlying mechanisms. Senescent mouse model was established by repeated subcutaneous injection of D-Gal (150 mg/kg, once daily for 42 days). Fourteen days after the first D-Gal exposure, piperine (2.5, 5, 10 mg/kg) or vehicle was intraperitoneally administered once daily for 28 days. The cognitive function of mice was evaluated by Morris water maze test (MWM). Twenty-four hours after behavioral test, the cholinergic function and oxidative stress level in mouse hippocampus were measured by spectrophotometric assays. In addition, the hippocampal levels of proinflammatory cytokines, including tumor necrosis factor-α, interleukin-1β and interleukin-6, were quantified using enzyme-linked immunosorbent assay. Expressions of glycogen synthase kinase-3β (GSK-3β) and its upstream or downstream molecules including phosphatidylinositol 3-kinase (PI3K)，protein kinase B (AKT), protein kinase C (PKC), NF-E2-related factor 2, nuclear factor-κB and microtubule-associated protein tau in hippocampus were determined by western blotting, immunohistochemical or immunofluorescent staining. Our data revealed that chronic D-Gal exposure in mice led to cognitive impairment in MWM, along with cholinergic malfunction, extensive oxidative stress and neuroinflammation, as well as hyperphosphorylation of tau protein in hippocampus. All these neurochemical, neuroinflammatory and cognitive alterations could be ameliorated by 4-week repeated piperine administration. Moreover, piperine also reversed D-Gal-induced GSK-3β activation through modulating PKC and PI3K/AKT pathways in senescent mouse hippocampus, suggesting GSK-3β-related signaling might be involved in the benefits of piperine against D-Gal-induced cognitive decline in mice.