Knockdown of GLP-1 Receptors in Vagal Afferents Affects Normal Food Intake and Glycemia

Abstract

Nutrient stimulation of enteroendocrine L cells induces the release of the incretin and satiating peptide glucagonlike peptide 1 (GLP-1). The vagus nerve innervates visceral organs and may contribute to the mediation of gut-derived GLP-1's effects on food intake, energy homeostasis, and glycemic control. To test the hypothesis that vagal afferent neuron (VAN) GLP-1 receptors (GLP-1Rs) are necessary for these effects of endogenous GLP-1, we established a novel bilateral nodose ganglia injection technique to deliver a lentiviral vector and to knock down VAN GLP-1Rs in male Sprague Dawley rats. We found that a full expression of VAN GLP-1Rs is not necessary for the maintenance of longterm energy balance in normal eating conditions. VAN GLP-1R knockdown (kd) did, however, increase meal size and accelerated gastric emptying. Moreover, postmeal glycemia was elevated and insulin release was blunted in GLP-1R kd rats, suggesting that VAN GLP-1Rs are physiological contributors to the neuroincretin effect after a meal. Collectively, our results highlight a crucial role for the VANs in mediating the effects of endogenous GLP-1 on food intake and glycemia and may promote the further development of GLP-1-based therapies.Glucagon-like peptide 1 (GLP-1) is an incretin and satiating hormone that has provided new tools for the pharmacotherapy of obesity and diabetes (1,2). Yet, despite the clinical effectiveness of GLP-1-based drugs in ameliorating the symptoms of type 2 diabetes, the role of endogenous GLP-1 in the control of energy intake and glucose homeostasis is not fully understood. Vagal afferent neurons (VANs) express GLP-1 receptors (GLP-1Rs) (3,4) and terminate in the lamina propria of the intestinal mucosa as well as in the wall of the hepatic portal vein (HPV) (5). VANs may therefore relay the gut GLP-1-derived signals to the brain and, hence, mediate satiating and glucoregulatory responses. Previous studies using lesioning approaches have implicated the vagus nerve in the effects of peripherally administered GLP-1 on food intake and glycemia (reviewed in 6,7). In more recent studies, sudiaphragmatic vagal deafferentation (SDA) in rats clearly attenuated the acute eating-inhibitory effect of intraperitoneally (IP) infused GLP-1 (8) and exendin-4 (Ex-4), a GLP-1R agonist (9). Moreover, unlike Sham-operated rats, SDA rats failed to show a GLP-1R-mediated incretin response (10). Based on these findings, it is reasonable to hypothesize that endogenous gut-derived GLP-1 could activate GLP-1Rs on VANs in a paracrine-like fashion to reduce food intake, limit gastric emptying, and trigger a neural component of the incretin effect. Disruption of this endogenous GLP-1 signaling mechanism in the VANs due to genetic or environmental factors may contribute to the pathophysiology of obesity and diabetes. Hence, we examined the physiological role of in the control of food intake and regulation of glucose homeostasis by generating a specific knockdown (kd) of VAN GLP-1R expression in rats. Our approach is based on the delivery of a...

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