Does our fat tell our brain what to do? A sympathetic' appraisal

Adipocytes initiate an adipose-cerebral-peripheral sympathetic reflex to induce insulin resistance during high-fat feeding

The underlying mechanism by which amassing of white adipose tissue in obesity regulates sympathetic nerve system (SNS) drive to the tissues responsible for glucose disposal, and causes insulin resistance (IR), remains unknown. We tested the hypothesis that high-fat (HF) feeding increases afferent impulses from white adipose tissue that reflexively elevate efferent nerve activity to skeletal muscle (SM) and adipose tissue to impair their local glucose uptake. We also investigated how salt-intake can enhance IR. HF-fed rats received a normal salt (0.4%) or high salt (4%) diet for 3 weeks. High-salt intake in HF fed rats decreased insulin-stimulated 2-deoxyglucose uptake by over 30% in white adipose tissue and SM, exacerbated inflammation, and impaired their insulin signaling and glucose transporter 4 (Glut4) trafficking. Dietary salt in HF fed rats also increased the activity of the adipose-cerebral-muscle renin-angiotensin system (RAS) axes, SNS, and reactive oxygen species (ROS). Insulin sensitivity was reduced by 32% in HF rats during high-salt intake, but was improved by over 62% by interruption of central RAS and SNS drive, and by over 45% by denervation or deafferentation of epididymal fat (all P<0.05). Our study suggest that a HF diet engages a sympathetic reflex from the white adipose tissue that activates adipose-cerebral-muscle RAS/ROS axes and coordinates a reduction in peripheral glucose uptake. These are all enhanced by salt-loading. These findings provide new insight into the role of a reflex initiated in adipose tissue in the regulation of glucose homeostasis during HF feeding that could lead to new therapeutic approaches to IR.

Blunted sympathoinhibitory responses in obesity-related hypertension are due to aberrant central but not peripheral signalling mechanisms

The gut hormone cholecystokinin (CCK) acts at subdiaphragmatic vagal afferents to induce renal and splanchnic sympathoinhibition and vasodilatation, via reflex inhibition of a subclass of cardiovascular-controlling neurons in the rostroventrolateral medulla (RVLM). These sympathoinhibitory and vasodilator responses are blunted in obese, hypertensive rats and our aim in the present study was to determine whether this is attributable to (i) altered sensitivity of presympathetic vasomotor RVLM neurons, and (ii) aberrant peripheral or central signalling mechanisms. Using a diet-induced obesity model, male Sprague-Dawley rats exhibited either an obesity-prone (OP) or obesity-resistant (OR) phenotype when placed on a medium high fat diet for 13-15 weeks; control animals were placed on a low fat diet. OP animals had elevated resting arterial pressure compared to OR/control animals (P < 0.05). Barosensitivity of RVLM neurons was significantly attenuated in OP animals (P < 0.05), suggesting altered baroreflex gain. CCK induced inhibitory responses in RVLM neurons of OR/control animals but not OP animals. Subdiaphragmatic vagal nerve responsiveness to CCK and CCK1 receptor mRNA expression in nodose ganglia did not differ between the groups, but CCK induced significantly less Fos-like immunoreactivity in both the nucleus of the solitary tract and the caudal ventrolateral medulla of OP animals compared to controls (P < 0.05). These results suggest that blunted sympathoinhibitory and vasodilator responses in obesity-related hypertension are due to alterations in RVLM neuronal responses, resulting from aberrant central but not peripheral signalling mechanisms. In obesity, blunted sympathoinhibitory mechanisms may lead to increased regional vascular resistance and contribute to the development of hypertension.