Intranasal metformin treatment ameliorates cognitive functions via insulin signaling pathway in ICV-STZ-induced mice model of Alzheimer's disease

The relationship between type 2 diabetes and Alzheimer's disease (AD) provides evidence that insulin and insulin sensitizers may be beneficial for the treatment of AD. The present study investigated the effect and mechanism of action of intranasal metformin treatment on impaired cognitive functions in an experimental mice model of AD. Intracerebroventricularly (ICV) streptozotocin (STZ)-injected mice were treated with intranasal or oral metformin for 4 weeks. Learning and memory functions were evaluated using Morris water maze. Metformin and Aβ42 concentrations were determined by liquid chromatography tandem mass spectrometry and ELISA respectively. The expressions of insulin receptor, Akt and their phosphorylated forms were determined in the hippocampi and cerebral cortices of mice. ICV-STZ-induced AD mice displayed impaired learning and memory functions which were improved by metformin treatment. ICV-STZ injection or intranasal/oral metformin treatments had no effect on blood glucose concentrations. Intranasal treatment yielded higher concentration of metformin in the hippocampus and lower in the plasma compared to oral treatment. ICV-STZ injection and metformin treatments did not change amyloid β-42 concentration in the hippocampus of mice. In hippocampal and cortical tissues of ICV-STZ-induced AD mice, insulin receptor (IR) and Akt expressions were unchanged, while phosphorylated insulin receptor (pIR) and pAkt expressions decreased compared to control. Metformin treatments did not change IR and Akt expressions but increased pIR and pAkt expressions. The present study showed for the first time that intranasal metformin treatment improved the impaired cognitive functions through increasing insulin sensitivity in ICV-STZ-induced mice model of AD.