BRAIN REGION AND DOSE-SPECIFIC EFFECTS OF ACUTE INTRANASAL INSULIN ON INSULIN SIGNALLING IN RATS

Brain insulin resistance disrupts central signaling and contributes to neurodegeneration, while intranasal insulin may help restore brain insulin signaling. The aim of this study was to investigate the dose- and time-dependent effects of acute intranasal insulin on insulin receptor activation and downstream signaling in different brain regions. Adult male Wistar rats received a single bilateral intranasal dose of human fast-acting insulin (0.5, 2, or 6 IU), while untreated animals served as intact controls. Rats were sacrificed at 3, 7.5, 15 or 30 minutes after administration. Insulin signaling was assessed by Western blot analysis in the temporal cortex, hippocampus, and hypothalamus by quantifying phosphorylated insulin receptor (Tyr1150/1151) and phosphorylated insulin receptor substrate-1 (Ser307 and Tyr608) relative to total protein levels. Acute intranasal insulin induced distinct region- and dose-dependent signaling responses. In the temporal cortex, insulin receptor phosphorylation showed minimal changes, whereas inhibitory IRS1 phosphorylation progressively increased, particularly at the 0.5 and 6 IU doses. In the hippocampus, insulin receptor phosphorylation remained stable, but IRS1 signaling was highly dynamic, with early activation at 0.5 and 6 IU and mixed activating and inhibitory phosphorylation at the 2 IU dose. In the hypothalamus, insulin receptor phosphorylation was strongly dose-dependent, with increased activation at 0.5 and 6 IU and reduced activation at 2 IU, while IRS1 activation was most pronounced at the highest dose. These findings demonstrate that acute intranasal insulin exerts complex, dose-dependent, and region-specific effects on brain insulin signaling, underscoring the importance of dose selection and regional sensitivity when targeting central insulin pathways.