Disrupted Phosphorylated Phosphoinositide 3-kinase in the NTS of Type 2 Diabetic Rats
Authors: Estrada Juan , Kim Han-Kyul , Fukazawa Ayumi , Hori Amane , Ishizawa Rie , Hotta Norio , Iwamoto Gary 
Published: May 2024

The hallmark of type 2 diabetes mellitus (T2DM) is systemic insulin resistance, where insulin signaling is impaired, blood glucose is chronically elevated, and various pathophysiological processes ensue. Much is known about the cardiovascular and metabolic consequences of T2DM. However, the impact of T2DM on autonomic nervous system function remains to be fully elucidated. Increasing evidence suggests that in both human and animal models of T2DM, the exercise pressor reflex (EPR) is abnormally augmented, characterized by heightened cardiovascular and sympathetic responses to physical activity. The underlying mechanisms are still not completely understood. Our recent studies suggest that T2DM induces alterations in peripheral somatosensory nerve activity that could explain the augmented EPR function. However, it is possible that neurons within the cardiovascular control areas of the brainstem, such as the nucleus tractus solitarius (NTS), could also be impaired by this disease. In the central nervous system, insulin has been shown to hyperpolarize neurons through activation of the phosphoinositide 3-kinase (PI3K) pathway, resulting in reduced neuronal excitability. Therefore, we hypothesized that decreased insulin transport into the brain in T2DM rats, results in increased cellular excitability as evidenced by decreased activity of the PI3K pathway in the NTS. Purpose: The aim of this study was to examine alterations in brain insulin levels and the PI3K pathway in T2DM rats as compared to control rats. Methods: Control rats were fed a low fat control diet (13% fat, 67.8% carbohydrate, and 19.2% protein) for 12-16 wks, while T2DM were generated by two low dose bolus injections of i.p. streptozotocin (35mg/kg, week 1; 25mg/kg week 2) and then fed a high fat diet (42% fat, 42.8% carbohydrate, and 15.2% protein) for 12-16wks. Cerebrospinal fluid (CSF) were collected from control and T2DM rats after an overnight fast. NTS micro-punches were performed, and then total and phosphorylated PI3K protein expressions were examined by the western blot technique. Results: CSF insulin was significantly decreased in T2DM rats as compared to control rats (0.41 ± 0.19 vs 0.11 ± 0.05 (ng/mL), Control (n=14) vs T2DM (n=4), p<0.01). Total PI3K expression in the NTS did not differ between control and T2DM (1.0 ± 0.2 vs 1.1 ± 0.2 (au), Control (n=9) vs T2DM (n=9), P=0.172). However, phosphorylated PI3K expression in the NTS of T2DM was significantly decreased as compared with control animals (1.0 ± 0.2 vs 0.8 ± 0.1 (au), Control (n=9) vs T2DM (n=9), p<0.05). Conclusion: These results suggest that 1) in consistent with earlier studies, insulin transport into the brain is impaired in high fat diet/streptozotocin-induced T2DM animals, and 2) decreased phospho-PI3K signaling in the NTS of diabetic animals is associated with lower brain insulin. Taken together, these alterations may increase neuronal excitability in the NTS, thereby contributing, at least in part, to the exaggerated EPR response to exercise in this disease. Supported by NIH HL-151632 and NIH HL-151632-01S1. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.