Intranasal insulin improves cerebral blood flow, Nrf-2 expression and BDNF in STZ (ICV)-induced memory impaired rats☆

Abstract Aims Insulin/insulin receptor signaling is involved in cognitive functions. Clinical studies have shown that intranasal insulin administration improves memory functions. However, the molecular mechanisms associated with improvement in memory functions are largely unexplored. Therefore, we investigated the protective effect of intranasal insulin in intracerebroventricular (ICV) streptozotocin (STZ) induced memory impairment in rats. Main methods Rats were injected with STZ (3 mg/kg, ICV) bilaterally twice, on days 1 and 3 and intranasal insulin (2 IU/rat/day) was given for 14 days. Memory was assessed by Morris water maze test. Cerebral blood flow (CBF) was measured by laser-Doppler flowmetry. The biochemical and molecular studies were done in cortex and hippocampus of rat brain. Key findings STZ (ICV) administration caused memory impairment along with the reduction of CBF, ATP level, and Nrf-2 expression. Treatment with intranasal insulin significantly improved memory functions as well as restored CBF, ATP content and Nrf-2 expression in STZ injected rats. STZ administration stimulated oxidative-nitrosative stress as evidenced by a significant increase in ROS, malondialdehyde, NO level and inducible nitric oxide synthase expression and the decrease in glutathione level; which was normalized by intranasal insulin delivery. STZ-induced cholinergic dysfunction (AChE activity and α7-nAChR expression), and mitochondrial hypofunction was largely prevented by treatment with intranasal insulin. Intranasal insulin delivery successfully restored BDNF level and pCREB expression in STZ injected rats. Significance The study shows the beneficial effects of intranasal insulin against STZ-induced memory impairment, which attributed to improved CBF, cholinergic function, brain energy metabolism, BDNF, Nrf-2 expression and antioxidative action.