Exenatide and feeding: possible peripheral neuronal pathways

Endogenous glucagon-like peptide (GLP)-1 as alternative for GLP-1 receptor agonists: Could this work and how?

In recent years, we have witnessed the many beneficial effects of glucagon-like peptide (GLP)-1 receptor agonists, including the reduction in cardiovascular risk in patients with type 2 diabetes, and the reduction of body weight in those with obesity. Increasing evidence suggests that these agents differ considerably from endogenous GLP-1 when it comes to their routes of action, although their clinical effects appear to be the same. Given the limitations of the GLP-1 receptor agonists, could it be useful to develop agents which stimulate GLP-1 release? Here we will discuss the differences and similarities between GLP-1 receptor agonists and endogenous GLP-1, and will detail how endogenous GLP-1-when stimulated appropriately-could have clinically relevant effects.

Effect of Vagotomy and Sympathectomy on the Feeding Responses Evoked by Intra-Aortic Cholecystokinin-8 in Adult Male Sprague Dawley Rats

The vagus nerve and the celiaco-mesenteric ganglia (CMG) are required for reduction of meal size (MS) and prolongation of the intermeal interval (IMI) by intraperitoneal (ip) sulfated cholecystokinin-8 (CCK-8). However, recently we have shown that the gut regulates these responses. Therefore, reevaluating the role of the vagus and the CMG in the feeding responses evoked by CCK is necessary because the gut contains the highest concentration of enteric, vagal and splanchnic afferents and CCK-A receptors, which are required for reduction of food intake by this peptide, compared to other abdominal organs. To address this necessity, we injected sulfated CCK-8 (0, 0.1, 0.5, 1 and 3 nmol/kg) in the aorta, near the gastrointestinal sites of action of the peptide, in three groups of free-feeding rats (n = 10 rats per group), subdiaphragmatic vagotomy (VGX), celiaco-mesenteric ganglionectomy (CMGX) and sham-operated, and recorded seven feeding responses. In the sham group, CCK-8 reduced MS (normal chow), prolonged the intermeal interval (IMI, time between first and second meals), increased satiety ratio (SR, IMI/MS), shortened duration of first meal, reduced total (24 hrs) food intake and reduced number of meals relative to saline vehicle. Vagotomy attenuated all of the previous responses except IMI length and SR, and CMGX attenuated all of those responses. In conclusion, the feeding responses evoked by sulfated CCK-8 require, independently, the vagus nerve and the CMG.

Central sensory-motor crosstalk in the neural gut-brain axis

The neural gut-brain axis consists of viscerosensory and autonomic motor neurons innervating the gastrointestinal (GI) tract. Sensory neurons transmit nutrient-related and non-nutrient-related information to the brain, while motor neurons regulate GI motility and secretion. Previous research provides an incomplete picture of the brain nuclei that are directly connected with the neural gut-brain axis, and no studies have thoroughly assessed sensory-motor overlap in those nuclei. Our goal in this study was to comprehensively characterize the central sensory and motor circuitry associated with the neural gut-brain axis linked to a segment of the small intestine. We injected a retrograde (pseudorabies; PRV) and anterograde (herpes simplex virus 1; HSV) transsynaptic viral tracer into the duodenal wall of adult male rats. Immunohistochemical processing revealed single- and double-labeled cells that were quantified per nucleus. We found that across nearly all brain regions assessed, PRV + HSV immunoreactive neurons comprised the greatest percentage of labeled cells compared with single-labeled PRV or HSV neurons. These results indicate that even though sensory and motor information can be processed by separate neuronal populations, there is neuroanatomical evidence of direct sensory-motor feedback in the neural gut-brain axis throughout the entire caudal-rostral extent of the brain. This is the first study to exhaustively investigate the sensory-motor organization of the neural gut-brain axis, and is a step toward phenotyping the many central neuronal populations involved in GI control.

The Vagus Nerve and the Celiaco-mesenteric Ganglia Participate in the Feeding Responses Evoked by Non-sulfated Cholecystokinin-8 in Male Sprague Dawley Rats

We have shown that non-sulfated cholecystokinin-8 (NS CCK-8) reduces food intake in adult male Sprague Dawley rats by activating cholecystokinin-B receptor (CCK-BR). Here, we tested the hypothesis that the vagus nerve and the celiaco-mesenteric ganglia may play a role in this reduction. The hypothesis stems from the following facts. The vagus and the celiaco-mesenteric ganglia contain NS CCK-8, they express and have binding sites for CCK-BR, NS CCK-8 activates CCK-BR on afferent vagal and sympathetic fibers and the two structures link the gastrointestinal tract to central feeding nuclei in the brain, which also contain the peptide and CCK-BR. To test this hypothesis, three groups of free-feeding rats, vagotomy (VGX), celiaco-mesenteric ganglionectomy (CMGX) and sham-operated, received NS CCK-8 (0, 0.5 and 1 nmol/kg) intraperitoneally prior to the onset of the dark cycle and various feeding behaviors were recorded. We found that in sham-operated rats both doses of NS CCK-8 reduced meal size (MS), prolonged the intermeal interval (IMI, time between first and second meal), increased satiety ratio (SR = IMI/MS), reduced 24-h food intake and reduced the number of meals relative to saline control. In the VGX and the CMGX groups, all of the previous responses were attenuated. Consistent with our hypothesis, the findings of the current work suggest a role for the vagus nerve and the celiaco-mesenteric ganglia in the feeding responses evoked by NS CCK-8.

Long-term effect of parasympathetic or sympathetic denervation on intestinal epithelial cell proliferation and apoptosis

Intestinal epithelial tissue is constantly regenerated as a means to maintain proper tissue function. Previous studies have demonstrated that denervation of the parasympathetic or sympathetic nervous system to the intestine alters this process. However, results are inconsistent between studies, showing both increases and decreases in proliferation after denervation of the parasympathetic or sympathetic. The effect appears to correlate with (1) the timing post-denervation, (2) denervation-induced changes in food intake, (3) the denervation technique used, and (4) which intestinal segment is investigated. Thus, we proposed that parasympathetic or sympathetic denervation does not have an effect on intestinal epithelial regeneration when you (1) evaluate denervation after long-term denervation, (2) control for post-surgical changes in food intake, (3) use minimally invasive surgical techniques and (4) include a segmental analysis. To test this, adult male Sprague Dawley rats underwent parasympathetic denervation via subdiaphragmatic vagotomy, sympathetic denervation via celiacomesenteric ganglionectomy, a parasympathetic denervation sham surgery, or a sympathetic denervation sham surgery. Sham surgery ad libitum-fed groups and sham surgery pair-fed groups were used to control for surgically induced changes in food intake. Three weeks post-surgery, animals were sacrificed and tissue from the duodenum, jejunum, and ileum was excised and immunohistochemically processed to visualize indicators of proliferation (bromodeoxyuridine-positive cells) and apoptosis (caspase-3-positive cells). Results showed no differences between groups in proliferation, apoptosis, or total cell number in any intestinal segment. These results suggest that parasympathetic or sympathetic denervation does not have a significant long-term effect on intestinal epithelial turnover. Thus, intestinal epithelial regeneration is able to recover after autonomic nervous system injury. Impact statement This study investigates the long-term effect of autonomic denervation on intestinal epithelial cell turnover, as measured by proliferation, apoptosis, and total cell number. Although previous research has established that autonomic denervation can alter intestinal epithelial turnover under short-term conditions, here we establish for the first time that these changes do not persist long-term when you control for surgical-induced changes in food intake and use targeted denervation procedures. These findings add to the base of knowledge on autonomic control of tissue turnover, highlight the ability of the intestinal epithelium to recover after autonomic injury and reveal possible implications of the use of ANS denervation for disease treatment in humans.

Coinfusion of low-dose GLP-1 and glucagon in man results in a reduction in food intake

Obesity is a growing epidemic, and current medical therapies have proven inadequate. Endogenous satiety hormones provide an attractive target for the development of drugs that aim to cause effective weight loss with minimal side effects. Both glucagon and GLP-1 reduce appetite and cause weight loss. Additionally, glucagon increases energy expenditure. We hypothesized that the combination of both peptides, administered at doses that are individually subanorectic, would reduce appetite, while GLP-1 would protect against the hyperglycemic effect of glucagon. In this double-blind crossover study, subanorectic doses of each peptide alone, both peptides in combination, or placebo was infused into 13 human volunteers for 120 min. An ad libitum meal was provided after 90 min, and calorie intake determined. Resting energy expenditure was measured by indirect calorimetry at baseline and during infusion. Glucagon or GLP-1, given individually at subanorectic doses, did not significantly reduce food intake. Coinfusion at the same doses led to a significant reduction in food intake of 13%. Furthermore, the addition of GLP-1 protected against glucagon-induced hyperglycemia, and an increase in energy expenditure of 53 kcal/day was seen on coinfusion. These observations support the concept of GLP-1 and glucagon dual agonism as a possible treatment for obesity and diabetes.

Neuronal GLP1R mediates liraglutide's anorectic but not glucose-lowering effect

Glucose control and weight loss are cornerstones of type 2 diabetes treatment. Currently, only glucagon-like peptide-1 (GLP1) analogs are able to achieve both weight loss and glucose tolerance. Both glucose and body weight are regulated by the brain, which contains GLP1 receptors (GLP1R). Even though the brain is poised to mediate the effects of GLP1 analogs, it remains unclear whether the glucose- and body weight-lowering effects of long-acting GLP1R agonists are via direct action on CNS GLP1R or the result of downstream activation of afferent neuronal GLP1R. We generated mice with either neuronal or visceral nerve-specific deletion of Glp1r and then administered liraglutide, a long-acting GLP1R agonist. We found that neither reduction of GLP1R in the CNS nor in the visceral nerves resulted in alterations in body weight or food intake in animals fed normal chow or a high-fat diet. Liraglutide treatment provided beneficial glucose-lowering effects in both chow- and high-fat-fed mice lacking GLP1R in the CNS or visceral nerves; however, liraglutide was ineffective at altering food intake, body weight, or causing a conditioned taste aversion in mice lacking neuronal GLP1R. These data indicate that neuronal GLP1Rs mediate body weight and anorectic effects of liraglutide, but are not required for glucose-lowering effects.

Intravenous infusion of gastrin-releasing peptide-27 and bombesin in rats reveals differential effects on meal size and intermeal interval length

We have previously shown that the intraperitoneal (i.p.) administration of gastrin-releasing peptide-27 (GRP-27) or bombesin (BN) (at 0.21, 0.41 and 1.03nmol/kg) reduces meal size (MS) and prolongs the intermeal interval (IMI). Here, we hypothesized that the intravenous (i.v.) administration of the same doses of GRP-27 and BN will be as effective as the i.p. administration in evoking these feeding responses. To test this hypothesis, we administered GRP-27 and BN i.v. and measured first MS (10% sucrose), IMI, satiety ratio (SR, IMI/MS) and second MS in overnight food-deprived but not water-deprived male Sprague Dawley rats. We found that (1) only GRP-27 reduced the first MS, (2) BN prolonged the IMI, (3) GRP-27 and BN increased the SR and (4) only BN reduced the size of the second meal. Contrary to our hypothesis, the i.v. administration of GRP-27 and BN affected the MS and IMI differently than did the i.p. administration. In conclusion, this pharmacological study suggests that the MS and IMI are regulated at different sites.

Vagal afferents mediate early satiation and prevent flavour avoidance learning in response to intraperitoneally infused exendin-4

Glucagon-like peptide-1 receptor (GLP-1R) agonists such as exendin-4 (Ex-4) affect eating and metabolism and are potential candidates for treating obesity and type II diabetes. In the present study, we tested whether vagal afferents mediate the eating-inhibitory and avoidance-inducing effects of Ex-4. Subdiaphragmatic vagal deafferentation (SDA) blunted the short-term (< 1 h) but not long-term eating-inhibitory effect of i.p.-infused Ex-4 (0.1 μg/kg) in rats. A dose of 1 μg/kg Ex-4 reduced 0.5, 1, 2 and 4 h cumulative food intake in SDA and sham-operated rats to a similar extent. Paradoxically, SDA but not sham rats developed a conditioned flavour avoidance (CFA) after i.p. Ex-4 (0.1 μg/kg). SDA completely blunted the induction of c-Fos expression by Ex-4 in the hypothalamic paraventricular nucleus. Ex-4, however, increased the number of c-Fos expressing cells, independent of intact vagal afferents, in the nucleus accumbens and in the central nucleus of the amygdala, the lateral external parabrachial nucleus, the caudal ventrolateral medulla and the dorsal vagal complex. These data suggest that intact vagal afferents are only necessary for the full expression of the early satiating effect of Ex-4 but not for later eating-inhibitory actions, when circulating Ex-4 might reach the brain via the circulation. Our data also dissociate the satiating and avoidance-inducing effects of the low Ex-4 dose tested under our conditions and suggest that vagal afferent signalling may protect against the development of CFA. Taken together, these findings reveal a complex role of vagal afferents in mediating the effects of GLP-1R activation on ingestive behaviour.