Molecular effects of intranasal insulin in the rodent brain: Examination of insulin receptor signaling and the glutamatergic system.

Abstract

As brain insulin resistance has been identified as a pathological feature of age-related cognitive decline (ARCD), intranasal insulin (INI) is being explored as a potential treatment for patients with ARCD. Previous studies have demonstrated that INI enhances memory, but the underlying mechanisms remain unclear. To investigate the mechanistic basis for the pro-cognitive effects of INI, insulin receptor (IR) signaling and the expression of glutamate receptors and transporters, due to the role of the glutamatergic system in hippocampal synaptic transmission, was examined. Adult male Sprague-Dawley rats received bilateral hippocampal injections of a control lentiviral vector (LV-Con) or a lentivirus containing a selective insulin receptor antisense sequence (LV-IRAS) to induce hippocampal-specific insulin resistance. Seven months later, these animals were administered INI 30 minutes before euthanasia. In a separate cohort of Fischer 344 x Brown Norway F1 hybrid male rats (no virus) we investigated the effects of acute and chronic (10 days) INI dosing paradigms in young (3 months old) and aged (26 months old) rats, with a final INI administration 30 minutes before euthanasia. In both experiments, the hippocampus was processed for immunoblot analysis to assess changes in central IR signaling and phosphorylation/expression of glutamate receptor subunits and transporters. We previously observed that LV-IRAS injection selectively downregulated hippocampal IR expression and insulin-stimulated IR phosphorylation without affecting peripheral insulin sensitivity. Additionally, LV-IRAS animals showed reduced hippocampal basal glutamate levels and decreased phosphorylation/expression of glutamate receptor subunits. In this study, vesicular glutamate transporter 2 (vGluT2) expression, but not vGluT1, was significantly decreased in the hippocampus of LV-IRAS animals after INI administration. In the other cohort, age- and dosing paradigm-dependent effects were observed vis-à-vis IR signaling and glutamate receptor phosphorylation/expression. More studies are needed to fully understand the mechanistic changes following INI and to determine the most effective treatment strategies, but these data indicate that INI may improve cognition through the enhancement of IR signaling in the brain and/or through exerting synaptic effects on glutamate neurotransmission. It is important to understand the mechanism of action, as INI could eventually be used in the broader clinical setting to treat ARCD.