Relative contributions of afferent vagal fibers to resistance to diet-induced obesity

Abstract

Background: We previously demonstrated vagal neural pathways, specifically subdiaphragmatic afferent fibers, regulate expression of the intestinal sodium-glucose cotransporter SGLT1, the intestinal transporter responsible for absorption of dietary glucose. We hypothesized targeting this pathway could be a novel therapy for obesity. We therefore tested the impact of disrupting vagal signaling by total vagotomy or selective vagal de-afferentation on weight gain and fat content in diet-induced obese rats.

Methods: Male Sprague-Dawley rats (n = 5-8) underwent truncal vagotomy, selective vagal de-afferentation with capsaicin, or sham procedure. Animals were maintained for 11 months on a high-caloric Western diet. Abdominal visceral fat content was assessed by magnetic resonance imaging together with weight of fat pads at harvest. Glucose homeostasis was assessed by fasting blood glucose and HbA1C. Jejunal SGLT1 gene expression was assessed by qPCR and immunoblotting and function by glucose uptake in everted jejunal sleeves.

Results: At 11-months, vagotomized rats weighed 19% less (P = 0.003) and de-afferented rats 7% less (P = 0.19) than shams. Vagotomized and de-afferented animals had 52% (P < 0.0001) and 18% reduction (P = 0.039) in visceral abdominal fat, respectively. There were no changes in blood glucose or glycemic indexes. SGLT1 mRNA, protein and function were unchanged across all cohorts at 11-months postoperatively.

Conclusions: Truncal vagotomy led to significant reductions in both diet-induced weight gain and visceral abdominal fat deposition. Vagal de-afferentation led to a more modest, but clinically and statistically significant, reduction in visceral abdominal fat. As increased visceral abdominal fat is associated with excess morbidity and mortality, vagal de-afferentation may be a useful adjunct in bariatric surgery.

Fig. 1

Animals were weighed on a weekly basis though to harvest (a). Weight is expressed as percentage change compared to the initial weight of the animal before acclimatisation. The dashed grey line shows the weight on the day of surgery. Vagotomized animals weighed significantly less than both shams and de-afferented animals after day 4 postoperatively (P <0.001 at harvest). Chow intake, as measured over a week-long period at the end of the study, was reduced in the vagotomy and deafferented animals; however, when corrected for body weight (b), food intake was unchanged in both vagotomized and de-afferented animals compared to shams (P > 0.15)

Fig. 2

Representative magnetic resonance images for each cohort of animals are shown. The section is taken at the level of the upper pole of the left kidney (which, unlike humans is more caudal than the right). In T1-weighted spin imaging, fat density is bright white, while muscle appears dark grey and bone black. In (a), the output image from 3D Slicer is shown. This is falsely coloured in (b) to highlight retroperitoneal fat, with retroperitoneal fat coloured bright yellow. Retroperitoneal fat deposition was reduced in vagotomized (Vag) animals compared to shams (Sham). To a lesser extent, the same was true for de-afferented (Cap) animals versus shams. Images were selected by picking animals with median retroperitoneal fat volumes for their cohort

Fig. 3

Representative abdominal visceral fat pads as dissected out at harvest. Images were selected by harvesting simultaneously animals with median body weights for their cohort. Cap de-afferented animals, Vag vagotomized animals, Sham shams. Scale bar = 5 cm. a Retroperitoneal fat pads were carefully dissected free for illustrative purposes, although these were not weighed as formal assessment was performed using magnetic resonance imaging. b Mesenteric fat pads, including both the mesentery and omentum. c Epididymal fat pads from each cohort, with clearly reduced fat deposition in both de-afferented and vagotomized animals compared to shams

Fig. 4

Mean visceral abdominal fat deposition by abdominal fat compartment. Vagotomized animals had significantly less body fat content (as a proportion of body weight) compared to sham animals. This was seen in each intra-abdominal compartment, including mesenteric fat (a), epididymal fat pads (b), retroperitoneal fat pads (c) and total liver fat (d). This translated to 52% reduction in total abdominal visceral fat compared to shams (e). In contrast, de-afferentation only significantly reduced epididymal and total abdominal fat mass (b, e). Compared to shams: * P <0.05; ** P < 0.01; *** P <0.001

Fig. 5

SGLT1 mRNA, as determined by qPCR relative to Actin, was unchanged between the three cohorts (a). This was reflected by unchanged levels of SGLT1 protein on Western blotting (b) and very similar levels of functional glucose transport capacity (c) as determined by radiolabelled glucose uptake by everted sleeves. Cap de-afferented animals, Vag vagotomized animals, Sham shams