Hyper-insulinemia increases the glutamate-excitotoxicity in cortical neurons: A mechanistic study.

Insulin resistance in type-2 diabetic condition increases the risk of stroke and cognitive deficits in which involvement of glutamate has been postulated. It has been hypothesized that hyper-insulinemia in cortical neurons increases the vulnerability towards glutamate-induced excitotoxicity. To mimic insulin resistance, cortical neurons were incubated with high insulin (1 µM) and high glucose (50 mM final concentration) in in-vitro condition for 24 h. Pre-treatment of cortical neurons with high insulin blocked acute insulin-induced activation of Akt and GSK-3β but not in the case of high glucose. Our results demonstrate that chronic high insulin exposure increases glutamate-induced excitotoxity, which was blocked by insulin receptor antagonist (S961) and GSK-3β inhibitor (SB216763). These inhibitors also ameliorated pAkt (Ser473) and pGSK-3β(Ser9) levels after chronic insulin exposure. Increase in glutamate-excitotoxicity in insulin-resistant cortical neurons was found to be associated with increased expression of PICK1. However, GluR2 did not get altered in hyper-insulinemia condition. This study demonstrates that hyper-insulinemia increases glutamate excitotoxicity which could be attributed to activation of GSK-3β and increased expression of PICK1.