Chronic high fat diet impairs glucagon like peptide-1 sensitivity in vagal afferents

Antismoking agents do not contribute synergistically to semaglutide's ability to reduce alcohol intake in rats

Preclinical studies have identified glucagon-like peptide-1 receptor (GLP-1R) agonists, and the antismoking agents varenicline and bupropion as tentative agents for treatment of alcohol use disorder (AUD). Combining different medications is a recent approach that has gained attention regarding heterogenous and difficult-to-treat diseases, like AUD. Successfully, this approach has been tested for the combination of varenicline and bupropion as it prevents relapse to alcohol drinking in rats. However, studies assessing the effects of the combination of semaglutide, an FDA-approved GLP-1R agonist for diabetes type II, and varenicline or bupropion to reduce alcohol intake in male and female rats remains to be conducted. Another approach to influence treatment outcome is to combine a medication with feeding interventions like high fat diet (HFD). While HFD reduces alcohol intake, the ability of the combination of HFD and semaglutide to alter alcohol drinking is unknown and thus the subject for a pilot study. Therefore, three intermittent alcohol drinking experiments were conducted to elucidate the effectiveness of these treatment combinations. We show that semaglutide, bupropion or HFD reduces alcohol intake in male as well as female rats. While various studies reveal beneficial effects of combinatorial pharmacotherapies for the treatment of AUD, we herein do not report any additive effects on alcohol intake by adding either varenicline or bupropion to semaglutide treatment. Neither does HFD exposure alter the ability of semaglutide to reduce alcohol intake. Although no additive effects by the combinatorial treatments are found, these findings collectively provide insight into possible monotherapeutical treatments for AUD.

Oligofructose improves small intestinal lipid-sensing mechanisms via alterations to the small intestinal microbiota

BACKGROUND

Upper small intestinal dietary lipids activate a gut-brain axis regulating energy homeostasis. The prebiotic, oligofructose (OFS) improves body weight and adiposity during metabolic dysregulation but the exact mechanisms remain unknown. This study examines whether alterations to the small intestinal microbiota following OFS treatment improve small intestinal lipid-sensing to regulate food intake in high fat (HF)-fed rats.

RESULTS

In rats fed a HF diet for 4 weeks, OFS supplementation decreased food intake and meal size within 2 days, and reduced body weight and adiposity after 6 weeks. Acute (3 day) OFS treatment restored small intestinal lipid-induced satiation during HF-feeding, and was associated with increased small intestinal CD36 expression, portal GLP-1 levels and hindbrain neuronal activation following a small intestinal lipid infusion. Transplant of the small intestinal microbiota from acute OFS treated donors into HF-fed rats also restored lipid-sensing mechanisms to lower food intake. 16S rRNA gene sequencing revealed that both long and short-term OFS altered the small intestinal microbiota, increasing Bifidobacterium relative abundance. Small intestinal administration of Bifidobacterium pseudolongum to HF-fed rats improved small intestinal lipid-sensing to decrease food intake.

CONCLUSION

OFS supplementation rapidly modulates the small intestinal gut microbiota, which mediates improvements in small intestinal lipid sensing mechanisms that control food intake to improve energy homeostasis. Video Abstract.

Neonatal overnutrition, but not neonatal undernutrition, disrupts CCK-induced hypophagia and neuron activation of the nucleus of the solitary tract and paraventricular nucleus of hypothalamus of male Wistar rats

Metabolic programming may be induced by reduction or enhancement of litter size, which lead to neonatal over or undernutrition, respectively. Changes in neonatal nutrition can challenge some regulatory processes in adulthood, such as the hypophagic effect of cholecystokinin (CCK). In order to investigate the effects of nutritional programming on the anorexigenic function of CCK in adulthood, pups were raised in small (SL, 3 pups per dam), normal (NL, 10 pups per dam), or large litters (LL, 16 pups per dam), and on postnatal day 60, male rats were treated with vehicle or CCK (10 µg/Kg) for the evaluation of food intake and c-Fos expression in the area postrema (AP), nucleus of solitary tract (NTS), and paraventricular (PVN), arcuate (ARC), ventromedial (VMH), and dorsomedial (DMH) nuclei of the hypothalamus. Overnourished rats showed increased body weight gain that was inversely correlated with neuronal activation of PaPo, VMH, and DMH neurons, whereas undernourished rats had lower body weight gain, inversely correlated with increased neuronal activation of PaPo only. SL rats showed no anorexigenic response and lower neuron activation in the NTS and PVN induced by CCK. LL exhibited preserved hypophagia and neuron activation in the AP, NTS, and PVN in response to CCK. CCK showed no effect in c-Fos immunoreactivity in the ARC, VMH, and DMH in any litter. These results indicate that anorexigenic actions, associated with neuron activation in the NTS and PVN, induced by CCK were impaired by neonatal overnutrition. However, these responses were not disrupted by neonatal undernutrition. Thus, data suggest that an excess or poor supply of nutrients during lactation display divergent effects on programming CCK satiation signaling in male adult rats.

Activation of glucagon-like peptide-1 receptors reduces the acquisition of aggression-like behaviors in male mice

Aggression is a complex social behavior, which is provoked in the defense of limited resources including food and mates. Recent advances show that the gut-brain hormone ghrelin modulates aggressive behaviors. As the gut-brain hormone glucagon-like peptide-1 (GLP-1) reduces food intake and sexual behaviors its potential role in aggressive behaviors is likely. Therefore, we investigated a tentative link between GLP-1 and aggressive behaviors by combining preclinical and human genetic-association studies. The influence of acute or repeated injections of a GLP-1 receptor (GLP-1R) agonist, exendin-4 (Ex4), on aggressive behaviors was assessed in male mice exposed to the resident-intruder paradigm. Besides, possible mechanisms participating in the ability of Ex4 to reduce aggressive behaviors were evaluated. Associations of polymorphisms in GLP-1R genes and overt aggression in males of the CATSS cohort were assessed. In male mice, repeated, but not acute, Ex4 treatment dose-dependently reduced aggressive behaviors. Neurochemical and western blot studies further revealed that putative serotonergic and noradrenergic signaling in nucleus accumbens, specifically the shell compartment, may participate in the interaction between Ex4 and aggression. As high-fat diet (HFD) impairs the responsiveness to GLP-1 on various behaviors the possibility that HFD blunts the ability of Ex4 to reduce aggressive behaviors was explored. Indeed, the levels of aggression was similar in vehicle and Ex4 treated mice consuming HFD. In humans, there were no associations between polymorphisms of the GLP-1R genes and overt aggression. Overall, GLP-1 signaling suppresses acquisition of aggressive behaviors via central neurotransmission and additional studies exploring this link are warranted.

GLP-1 attenuates intestinal fat absorption and chylomicron production via vagal afferent nerves originating in the portal vein

Background/Objective

GLP-1R agonists have been shown to reduce fasting and postprandial plasma lipids, both of which are independent risk factors for the development of cardiovascular disease. However, how endogenous GLP-1 – which is rapidly degraded – modulates intestinal and hepatic lipid metabolism is less clear. A vagal gut-brain-axis originating in the portal vein has been proposed as a possible mechanism for GLP-1’s anti-lipemic effects. Here we sought to examine the relationship between vagal GLP-1 signalling and intestinal lipid absorption and lipoprotein production.

Methods

Syrian golden hamsters or C57BL/6 mice received portal vein injections of GLP-1^(7-36), and postprandial and fasting plasma TG, TRL TG, or VLDL TG were examined. These experiments were repeated during sympathetic blockade, and under a variety of pharmacological or surgical deafferentation techniques. In addition, hamsters received nodose ganglia injections of a GLP-1R agonist or antagonist to further probe the vagal pathway. Peripheral studies were repeated in a novel GLP-1R KO hamster model and in our diet-induced hamster models of insulin resistance.

Results

GLP-1^(7-36) site-specifically reduced postprandial and fasting plasma lipids in both hamsters and mice. These inhibitory effects of GLP-1 were investigated via pharmacological and surgical denervation experiments and found to be dependent on intact afferent vagal signalling cascades and efferent changes in sympathetic tone. Furthermore, GLP-1R agonism in the nodose ganglia resulted in markedly reduced postprandial plasma TG and TRL TG, and fasting VLDL TG and this nodose GLP-1R activity was essential for portal GLP-1s effect. Notably, portal and nodose ganglia GLP-1 effects were lost in GLP-1R KO hamsters and following diet-induced insulin resistance.

Conclusion

Our data demonstrates for the first time that portal GLP-1 modulates postprandial and fasting lipids via a complex vagal gut–brain–liver axis. Importantly, loss or interference with this signalling axis via surgical, pharmacological, or dietary intervention resulted in the loss of portal GLP-1s anti-lipemic effects. This supports emerging evidence that native GLP-1 works primarily through a vagal neuroendocrine mechanism.

Interactions between the microbiota and enteric nervous system during gut-brain disorders

For the last 20 years, researchers have focused their intention on the impact of gut microbiota in healthy and pathological conditions. This year (2021), more than 25,000 articles can be retrieved from PubMed with the keywords "gut microbiota and physiology", showing the constant progress and impact of gut microbes in scientific life. As a result, numerous therapeutic perspectives have been proposed to modulate the gut microbiota composition and/or bioactive factors released from microbes to restore our body functions. Currently, the gut is considered a primary site for the development of pathologies that modify brain functions such as neurodegenerative (Parkinson's, Alzheimer's, etc.) and metabolic (type 2 diabetes, obesity, etc.) disorders. Deciphering the mode of interaction between microbiota and the brain is a real original option to prevent (and maybe treat in the future) the establishment of gut-brain pathologies. The objective of this review is to describe recent scientific elements that explore the communication between gut microbiota and the brain by focusing our interest on the enteric nervous system (ENS) as an intermediate partner. The ENS, which is known as the "second brain", could be under the direct or indirect influence of the gut microbiota and its released factors (short-chain fatty acids, neurotransmitters, gaseous factors, etc.). Thus, in addition to their actions on tissue (adipose tissue, liver, brain, etc.), microbes can have an impact on local ENS activity. This potential modification of ENS function has global repercussions in the whole body via the gut-brain axis and represents a new therapeutic strategy.

Reappraising the role of the vagus nerve in GLP-1-mediated regulation of eating

Here, we provide a focused review of the evidence for the roles of the vagus nerve in mediating the regulatory effects of peripherally and centrally produced GLP-1 on eating behaviour and energy balance. We particularly focus on recent studies which have used selective genetic, viral, and transcriptomic approaches to provide important insights into the anatomical and functional organisation of GLP-1-mediated gut-brain signalling pathways. A number of these studies have challenged canonical ideas of how GLP-1 acts in the periphery and the brain to regulate eating behaviour, with important implications for the development of pharmacological treatments for obesity.

Central and peripheral GLP-1 systems independently suppress eating

The anorexigenic peptide glucagon-like peptide-1 (GLP-1) is secreted from gut enteroendocrine cells and brain preproglucagon (PPG) neurons, which, respectively, define the peripheral and central GLP-1 systems. PPG neurons in the nucleus tractus solitarii (NTS) are widely assumed to link the peripheral and central GLP-1 systems in a unified gut-brain satiation circuit. However, direct evidence for this hypothesis is lacking, and the necessary circuitry remains to be demonstrated. Here we show that PPGNTS neurons encode satiation in mice, consistent with vagal signalling of gastrointestinal distension. However, PPGNTS neurons predominantly receive vagal input from oxytocin-receptor-expressing vagal neurons, rather than those expressing GLP-1 receptors. PPGNTS neurons are not necessary for eating suppression by GLP-1 receptor agonists, and concurrent PPGNTS neuron activation suppresses eating more potently than semaglutide alone. We conclude that central and peripheral GLP-1 systems suppress eating via independent gut-brain circuits, providing a rationale for pharmacological activation of PPGNTS neurons in combination with GLP-1 receptor agonists as an obesity treatment strategy.