Brain inflammation and neuronal activity in diabetes and metabolic syndrome
Creators: name: Al Zahrani, Omar Awad
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 320101
subject: Cardiology (incl. cardiovascular diseases)
classificationCode: 320208
subject: Endocrinology
classificationCode: 320901
subject: Autonomic nervous system

description: It has been previously suggested that inflammation in the cardiovascular control centres of the brain can lead to hypertension and cardiac dysfunction in the streptozotocin-induced (STZ) type one diabetes rat model. Brain inflammation in these animals was found 6-8 weeks after administration of STZ. However, when given access to 1% saline to drink as sole source of fluids, hypertension and brain inflammation were increased at early time points (2 weeks) but inhibited at later time points (8 weeks). This suggests that brain inflammation contributes to cardiovascular complications in type one diabetes and that saline intake may have beneficial effects in the long term. However, increased brain inflammation observed at early time points shows that the effects of saline intake are complex and time dependent in this diabetic rat model. Thus, the utility and safety of saline intake as a therapeutic strategy in human diabetes requires more study before it can be considered. A better understanding of the mechanisms of the increased inflammation and the switch from a pro- to an anti-inflammatory action of saline is therefore required, particularly whether the pro-inflammatory effects of saline at early time points can be reduced or prevented by altering the administered concentrations of saline. Furthermore, since changes in electrolyte concentrations, particularly of magnesium and potassium, also occur in diabetes, an investigation into whether electrolytes other than sodium influence inflammation in the brain in diabetes needs to be undertaken..
 
STZ-treated rats are an established model of type one diabetes and have been previously reported to exhibit activation of microglia and neurons within the cardiovascular brain regions of the PVN and the NTS. Furthermore, preliminary results from our laboratory found that giving STZ-treated diabetic rats saline as sole fluid source for 8 weeks reduced microglial activation. Thus initially the study addressed whether different electrolytes including NaCl, KCl, and MgCl2, would exert similar anti-inflammatory and neuronal activation effects. In an attempt to minimise the activation previously seen at 2 weeks by the administration of 1% saline, in these experiments the saline dose was reduced to 0.9%. The model was validated by measurement of blood glucose, increased body weight and fluid intake. The main finding was that STZ-treated diabetic rats given access to 0.9% saline had reduced neuronal activation assessed by C-Fos expression and microglial activation assessed by CD11b expression in the both the paraventricular nucleus of the hypothalamus (PVN) and the nucleus of the solitary tract (NTS), in comparison to STZ-treated diabetic rats given access to tap water (vehicle treatment). Similar results were found when STZ-treated diabetic rats were given KCl and MgCl2. Measurement of blood pressure and heart rate indicated that administration of 0.9% saline was not accompanied by deleterious effects at 2 weeks. These novel findings indicated that in STZ-treated diabetic rats drinking 0.9% saline, and other electrolytes, reduced neuronal activation and inflammation in regions of the brain.

To examine the effect of treating rats with different electrolytes on parameters related to cardiovascular complications of diabetes, STZ diabetic rats were given access to 0.9% saline (NaCl) and systolic blood pressure, heart rate and kidney weight were compared to STZ-treated diabetic rats given access to tap water alone (vehicle treatment). The results of this study suggested a potential protective role of 0.9% NaCl treatment on blood pressure in diabetes. However, no differences in cardiac function were observed between groups.
Given the positive effects of saline administration on diabetes-related parameters in STZ-treated rats, it was of interest to ask whether STZ-treated diabetic rats given a choice of tap water or 0.9% saline would choose to self-regulate their fluid intake to achieve equivalent beneficial effects of enforced saline intake. The main finding of this study was that STZ-treated diabetic rats preferred to drink 0.9% NaCl even when given the choice of tap water and that they had reduced microglial activation and inflammation, even compared to STZ-treated diabetic rats given only 0.9% NaCl to drink. This suggests that the anti-inflammatory mechanisms of electrolyte supplementation is exacerbated in STZ-treated diabetic rats when given a choice of both the electrolyte solution and tap water to drink. A major limitation of this study however was that in these experiments the beneficial effects of 0.9% saline treatment observed in preliminary work from our laboratory and in the earlier studies reported in this thesis, were not observed in this group of rats. Constraints of time and resources did not permit repetition of these experiments and further work needs to be done to establish these findings.

To extend the observations on the effects of saline treatment on brain inflammation and neuronal activation a mouse model of type one diabetes was used in which C57BL/6J mice were injected with STZ, in collaboration with Baker IDI Institute. Additionally, a metabolic syndrome mouse model was used in collaboration with Professor Jiming Ye which represents early stages of type two diabetes induced by a high fat diet. In type one diabetes, microglia were found to be activated in both the PVN and the NTS prior to the activation of neurons in STZ-treated diabetic mice. This suggests that the same mechanisms that occur in the brains of diabetic rats may also occur in diabetic mice. In metabolic syndrome mice, brain inflammation and neuronal activity was investigated. The main finding was that that mice exposed to high fat diet showed increases in microglia and neuronal activation within the PVN and the NTS. The results suggest that neuroinflammation and neuronal activation in central cardiovascular control regions occur in mouse models of type one diabetes and metabolic syndrome. 

In conclusion, the results of the present thesis highlight that inflammation and increased neuronal activation occurs in a rat and a mouse model of type one diabetes and in a murine model of metabolic syndrome in key integrative brain regions. There were time differences observed between the PVN and the NTS and the intake of electrolytes can influence this. The physiological correlates of such changes in the PVN and the NTS require further investigation