Neuroprotective Roles of Metformin in a Streptozotocin-Induced Dementia Model in Rats.

Alzheimer's disease (AD) is the leading cause of dementia in humans, with high social and economic costs. AD is predominantly a sporadic disorder, and its risk increases with age and in individuals with type 2 diabetes mellitus (T2DM). Metformin is considered the first line drug for treatment of T2DM and has a plethora of effects in the peripheral and nervous system. However, the neuroprotective mechanism of action of this drug is still under debate. In order to assess the effects of metformin in dementia, we investigated the optimal time to start metformin treatment in animals that were submitted to intracerebroventricular (ICV) administration of streptozotocin (STZ) (3 mg/kg) to induce a sporadic AD-like rodent model of dementia. We used two protocols of metformin administration: early metformin (50 mg/Kg/daily) treatment (2 days after STZ model induction, lasting 28 days) and late metformin (50 mg/Kg/daily) treatment (20 weeks after STZ model induction, lasting 28 days). Both time points improved cognitive behavior in STZ rats, as evaluated by the novel object recognition and Morris's water maze tasks. Moreover, both treatments reduced neuroinflammatory parameters, such as TLR4, RAGE, TNF-α and NF-κB protein expression, induced in STZ animals. Metformin downregulated the methylglyoxal/RAGE/NOX‑2 signaling pathway by restoring glyoxalase 1 activity and GSH levels, which are impaired in the STZ-induced dementia model. Our data contribute to understanding the neuroprotective role of metformin, particularly in conditions involving insulin resistance, such as diabetic encephalopathy and AD.