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It is known that a high-cholesterol diet induces oxidative stress, inflammatory response, and beta-amyloid (Abeta) accumulation in mouse brain, resulting in neurodegenerative changes. Quercetin, a naturally occurring flavonoid, has been reported to possess numerous biological activities beneficial to health. Our previous studies have demonstrated that quercetin protects mouse brain against D-galactose-induced oxidative damage. Against this background, we evaluated the effect of quercetin on high-cholesterol-induced neurotoxicity in old mice and explored its potential mechanism. Our results showed that oral administration of quercetin significantly improved the behavioural performance of high-cholesterol-fed old mice in both a step-through test and the Morris water maze task. This is at least in part caused by decreasing ROS and protein carbonyl levels and restoring Cu--Zn superoxide dismutase (Cu, Zn-SOD) activity. Furthermore, quercetin also significantly activated the AMP-activated protein kinase (AMPK) via down-regulation of protein phosphatase 2C (PP2C), which reduced the integral optical density (IOD) of activated microglia cells and CD11b expression, down-regulated iNOS and cyclooxygenase-2 (COX-2) expression, and decreased IL-1beta, IL-6, and TNF-alpha expression in the brains of high-cholesterol-fed old mice through the suppression of NF-kappaB p65 nuclear translocation. Moreover, AMPK activation significantly increased 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase and acetyl-CoA carboxylase (ACC) phosphorylation and reduced fatty acid synthase (FAS) expression in the brains of high-cholesterol-fed old mice, which reduced cholesterol levels, down-regulated cholesterol 24-hydroxylase (CYP46A1) and beta-amyloid converting enzyme 1 (BACE1) expression, decreased eukaryotic translation initiation factor 2alpha (eIF2alpha) phosphorylation, and lowered Abeta deposits. However, the neuroprotective effect of quercetin was weakened by intraperitoneal injection of compound C, an AMPK inhibitor. These results suggest that AMPK activated by quercetin may be a potential target to enhance the resistance of neurons to age-related diseases.

Evidence shows that administration of D-galactose (D-gal) induces reactive oxygen species (ROS) production and inflammatory response resulting in neurodegenerative changes. Ursolic acid (UA), a triterpenoid compound, has been reported to possess antioxidant and anti-inflammatory properties. Our previous studies have demonstrated that UA could protect mouse brain against D-gal-induced oxidative damage. In the present study, we examined the protective effect of UA against D-gal-induced inflammatory response in the prefrontal cortex and explored the potential mechanism of its action. Our results showed that UA administration significantly improved behavioral performance of D-gal-treated mice in step-through test and Morris water maze task. One of the potential mechanisms of this action was decreased advanced glycation end products (AGEs), ROS, and protein carbonyl levels in the prefrontal cortex of D-gal-treated mice. Furthermore, the results also showed that UA significantly reduced the number of activated microglia cells and astrocytes, decreased the expression of CD11b and glial fibrillary acidic protein, downregulated the expression of iNOS and COX-2, and decreased interleukin (IL)-1β, IL-6, and tumor necrosis factor-α levels in the prefrontal cortex of D-gal-treated mice. Moreover, UA significantly decreased AGEs induced the expression of receptor for advanced glycation end products and inhibited NF-κB p65 nuclear translocation in the prefrontal cortex of D-gal-treated mice. The aforementioned effects of UA could attenuate brain inflammatory response.

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