Time- and region-specific effects of intranasal insulin on oxidative stress parameters in the rat brain

Understanding how intranasal insulin affects brain signaling and metabolism is essential for elucidating its therapeutic potential in neurodegenerative disorders with underlying metabolic dysfunction, such as Alzheimer’s disease (AD). Oxidative stress, which increases with aging, has been observed in both AD and type 2 diabetes, indicating a potential link between oxidative stress, brain insulin resistance and cognitive impairment. This study aimed to investigate the effect of intranasal insulin on the redox potential in a brain region- and time- dependent manner. Male Wistar rats received 2 IU of regular insulin intranasally and were sacrificed 3, 7.5, 15, 30, 60, and 120 minutes after administration. Six animals served as intact control. Redox homeostasis was assessed by measuring lipid peroxidation, total reductive capacity, concentrations of free thiol groups and low-molecular-weight thiols, and superoxide dismutase activity in plasma, nasal epithelia, and brain regions. The results were correlated with insulin signaling analyses from the same samples. Intranasal insulin induced rapid but highly heterogeneous redox responses. The most pronounced alterations occurred in nasal epithelia, where respiratory and olfactory regions exhibited distinct and opposing redox profiles. In the brain, redox responses varied markedly across regions, with the most prominent alterations involving thiol parameters in the cortices, hippocampus, hypothalamus, olfactory bulb, and cerebellum. Plasma redox parameters remained largely unchanged, supporting the predominantly central action of intranasally delivered insulin. Correlation analyses revealed associations between oxidative stress markers and insulin signaling parameters, suggesting complex interactions between metabolic signaling pathways and redox regulation. These findings demonstrate that intranasal insulin modulates redox homeostasis in a rapid, region-specific, and time-dependent manner, highlighting the importance of spatial and temporal factors in insulin-mediated regulation of brain oxidative balance.