Leptin, which serves as a lipid-modulating hormone to control metabolic energy availability, is decreased in Alzheimer's disease (AD) patients, and serum levels are inversely correlated to severity of dementia. We have previously described the effects of leptin in reducing amyloid beta protein both ill vitro and in vivo, and tau phosphorylation in Vitro. Herein, we systematically investigated the signaling pathways activated by leptin, leading to these molecular endpoints, to better understand its mechanism of action. Inhibition of amyloid beta production and tau phosphorylation in leptin-treated human and/or rat neuronal cultures were both dependent on activation of AMP-activated protein kinase (AMPK). Direct stimulation of AMPK with the cell-permeable activator, 5-aminoimidazole-4-carboxyamide ribonucleoside (AICAR), replicated leptin's effects and conversely, Compound C, an inhibitor of AMPK, blocked leptin's action. The data implicate that AMPK is a key regulator of both AD-related pathways. (C) 2009 Elsevier Inc. All rights reserved.