Distinctive striatal dopamine signaling after dieting and gastric bypass

Gut-brain mechanisms underlying changes in disordered eating behaviour after bariatric surgery: a review

Bariatric surgery results in long-term weight loss and an improved metabolic phenotype due to changes in the gut-brain axis regulating appetite and glycaemia. Neuroendocrine alterations associated with bariatric surgery may also influence hedonic aspects of eating by inducing changes in taste preferences and central reward reactivity towards palatable food. However, the impact of bariatric surgery on disordered eating behaviours (e.g.: binge eating, loss-of-control eating, emotional eating and 'addictive eating'), which are commonly present in people with obesity are not well understood. Increasing evidence suggests gut-derived signals, such as appetitive hormones, bile acid profiles, microbiota concentrations and associated neuromodulatory metabolites, can influence pathways in the brain implicated in food intake, including brain areas involved in sensorimotor, reward-motivational, emotional-arousal and executive control components of food intake. As disordered eating prevalence is a key mediator of weight-loss success and patient well-being after bariatric surgery, understanding how changes in the gut-brain axis contribute to disordered eating incidence and severity after bariatric surgery is crucial to better improve treatment outcomes in people with obesity.

Gut-brain crosstalk regulates craving for fatty food

Patients undergoing Roux-en-Y gastric bypass (RYGB) surgery elicit striking loss of body weight. Anatomical re-structuring of the gastrointestinal (GI) tract, leading to reduced caloric intake and changes in food preference, are thought to be the primary drivers of weight loss in bariatric surgery patients. However, the mechanisms by which RYGB surgery causes a reduced preference for fatty foods remain elusive. In a recent report, Hankir et al described how RYGB surgery modulated lipid nutrient signals in the intestine of rats to blunt their craving for fatty food. The authors reported that RYGB surgery restored an endogenous fat-satiety signaling pathway, mediated via oleoylethanolamide (OEA), that was greatly blunted in obese animals. In RYGB rats, high fat diet (HFD) led to increased production of OEA that activated the intestinal peroxisome proliferation activator receptors-α (PPARα). In RYGB rats, activation of PPARα by OEA was accompanied by enhanced dopamine neurotransmission in the dorsal striatum and reduced preference for HFD. The authors showed that OEA-mediated signals to the midbrain were transmitted via the vagus nerve. Interfering with either the production of OEA in enterocytes, or blocking of vagal and striatal D1 receptors signals eliminated the decreased craving for fat in RYGB rats. These studies demonstrated that bariatric surgery led to alterations in the reward circuitry of the brain in RYGB rats and reduced their preference for HFD.

Caloric Restriction-Induced Decreases in Dopamine Receptor Availability are Associated with Leptin Concentration

OBJECTIVE

It has been previously reported that early after Roux-en-Y-gastric bypass, dopamine (DA) type 2 and 3 receptor (D2/3R) binding potential (BP_ND ) was decreased from preoperative levels. The current study aimed to determine whether calorie restriction without weight loss modifies D2/3R BP_ND and whether such changes are explained by neuroendocrine regulation.

METHODS

Fifteen females with obesity (BMI = 39 ± 6 kg/m² ) were studied before and after ∼10 days of a very-low-calorie-diet (VLCD). Outcome measures included fasting insulin, leptin, acyl ghrelin, and glucose, and insulin sensitivity and disposition index were estimated using the oral-minimal model (OMM) method. Participants underwent positron emission tomography scanning with the displaceable radioligand [¹⁸ F]fallypride to estimate available regional D2/3R levels. Regions of interest included the caudate, putamen, ventral striatum, hypothalamus, and substantia nigra (SN).

RESULTS

With the VLCD, weight decreased slightly (-3 kg). Insulin, glucose, and leptin decreased significantly, but there was no change in acyl ghrelin or measures from OMM. SN D2/3R BP_ND decreased significantly, with trends toward decreased levels in the remaining regions. The decrease in leptin concentration strongly predicted the change in D2/3R BP_ND in all regions (all P ≤ 0.004).

CONCLUSIONS

In obesity, reductions in regional D2/3R availability after VLCD are suggestive of increased endogenous DA competing with the radioligand. Changes in regional D2/3R availability were associated with decreases in leptin concentrations that occurred before clinically significant weight loss.

Effect of vertical sleeve gastrectomy on alcohol consumption and preferences in dietary obese rats and mice: A plausible role for altered ghrelin signaling

Vertical sleeve gastrectomy (VSG) and Roux-en-Y gastric bypass (RYGB) are the most common surgical options for the treatment of obesity and metabolic disorder. Whereas RYGB may result in greater and more durable weight loss, recent clinical and pre-clinical studies in rats have raised concerns that RYGB surgery may increase risk for alcohol use disorder (AUD). In contrast, recent clinical reports suggest a lesser risk for AUD following VSG, although no preclinical studies have been done to confirm that. Therefore, the present study sought to determine the effects of VSG on ethanol intake and preferences in rodent models using protocols similar to those previously used in animal studies for RYGB. Male Sprague Dawley rats and male C57B6 mice were made obese on a high fat diet (60%kcal from fat) and received VSG or no surgery (controls). All animals then were given access to increasing concentrations of ethanol (2%, 4%, 6%, and 8%), presented for few days each. Compared to controls, VSG rats consumed significantly less of 2, 6 and 8% ethanol and showed significantly reduced preferences to 6 and 8% ethanol over water. VSG mice also displayed reduced intake and preference for 6 and 8% ethanol solutions. After a two-week period of forced abstinence, 8% ethanol was reintroduced and the VSG rats and mice continued to exhibit reduced consumption and less preference for ethanol. Regarding the underlying mechanism, we hypothesized that the removal of the ghrelin producing part of the stomach in the VSG surgery is a possible contributor to the observed reduced ethanol preference. To test for functional changes at the ghrelin receptors, the VSG and control rats were given IP injections of acyl-ghrelin (2.5nmol and 5nmol) prior to ethanol access. Neither concentration of ghrelin resulted in a significant increase in 8% ethanol consumption of VSG or control subjects. Next, the rats were given IP injections of the ghrelin receptor antagonist, JMV (2.5mg/kg body weight). This dose induced a significant reduction in 8% ethanol consumption in the VSG group, but no effect on ethanol intake in the controls. While ghrelin injection was uninformative, increased sensitivity to subthreshold doses of the ghrelin receptor antagonist may indicate reduced ghrelin signaling following VSG. Overall, these findings suggest that bariatric patients with increased susceptibility to AUD may benefit from receiving VSG instead of RYGB surgery, and that changes in ghrelin signaling, at least in part, may play a role in the differential AUD risks between the two most commonly performed bariatric surgical procedures.

Roux-en-Y gastric bypass surgery suppresses hypothalamic PTP1B protein level and alleviates leptin resistance in obese rats

The present study aimed to explore the effect of Roux-en-Y gastric bypass (RYGB) surgery on protein tyrosine phosphatase 1B (PTP1B) expression levels and leptin activity in hypothalami of obese rats. Obese rats induced by a high-fat diet (HFD) that underwent RYGB (n=11) or sham operation (SO, n=9), as well as an obese control cohort (Obese, n=10) and an additional normal-diet group (ND, n=10) were used. Food efficiency was measured at 8 weeks post-operation. Plasma leptin levels were evaluated and hypothalamic protein tyrosine phosphatase 1B (PTP1B) levels and leptin signaling activity were examined at the genetic and protein levels. The results indicated that food efficiency was typically lower in RYGB rats compared with that in the Obese and SO rats. In the RYGB group, leptin receptor expression and proopiomelanocortin was significantly higher, while Neuropeptide Y levels were lower than those in the Obese and SO groups. Furthermore, the gene and protein expression levels of PTP1B in the RYGB group were lower, while levels of phosphorylated signal transducer and activator of transcription 3 protein were much higher compared with those in the Obese and SO groups. In conclusion, RYGB surgery significantly suppressed hypothalamic PTP1B protein expression. PTP1B regulation may partially alleviate leptin resistance.

Brain on Fire: Incentive Salience, Hedonic Hot Spots, Dopamine, Obesity, and Other Hunger Games

This review examines human feeding behavior in light of psychological motivational theory and highlights the importance of midbrain dopamine (DA). Prospective evidence of both reward surfeit and reward deficit pathways to increased body weight are evaluated, and we argue that it is more complex than an either/or scenario when examining DA's role in reward sensitivity, eating, and obesity. The Taq1A genotype is a common thread that ties the contrasting models of DA reward and obesity; this genotype related to striatal DA is not associated with obesity class per se but may nevertheless confer an increased risk of weight gain. We also critically examine the concept of so-called food addiction, and despite growing evidence, we argue that there is currently insufficient human data to warrant this diagnostic label. The surgical and pharmacological treatments of obesity are discussed, and evidence is presented for the selective use of DA-class drugs in obesity treatment.

Gastric Bypass Surgery Recruits a Gut PPAR-α-Striatal D1R Pathway to Reduce Fat Appetite in Obese Rats

Bariatric surgery remains the single most effective long-term treatment modality for morbid obesity, achieved mainly by lowering caloric intake through as yet ill-defined mechanisms. Here we show in rats that Roux-en-Y gastric bypass (RYGB)-like rerouting of ingested fat mobilizes lower small intestine production of the fat-satiety molecule oleoylethanolamide (OEA). This was associated with vagus nerve-driven increases in dorsal striatal dopamine release. We also demonstrate that RYGB upregulates striatal dopamine 1 receptor (D1R) expression specifically under high-fat diet feeding conditions. Mechanistically, interfering with local OEA, vagal, and dorsal striatal D1R signaling negated the beneficial effects of RYGB on fat intake and preferences. These findings delineate a molecular/systems pathway through which bariatric surgery improves feeding behavior and may aid in the development of novel weight loss strategies that similarly modify brain reward circuits compromised in obesity.

The Obese Brain--Effects of Bariatric Surgery on Energy Balance Neurocircuitry

Obesity is a highly prevalent disease in the world and with a major impact on global health. While genetic components are also involved in its pathogenesis, in recent years, it has shown a critical role of the innate and adaptive immune cell response in many tissues triggered by excess of nutrients such as lipids and glucose. Free fatty acids and other nutrient-related signals induce damage such as insulin resistance in the peripheral tissues but also in the brain. Specifically in the hypothalamus, these metabolic signals can trigger significant changes in the control of energy balance. Recent studies have shown that saturated fat disrupts melanocortin signaling of hypothalamic neuronal subgroups pivotal to energy control. Bariatric surgery is a treatment option for obesity when other tools have failed, because it is more effective than pharmacotherapy concerning of weight loss itself and in improvement of obesity-related comorbidities. Here, we review the mechanisms by which Roux-en Y gastric bypass (RYGB) can change peripheral signals that modulate melanocortin circuits involved in the regulation of energy balance.

Brain-based etiology of weight regulation

Caloric intake and energy balance are highly regulated to maintain metabolic homeostasis and weight. However, hedonic-motivated food intake, in particular consumption of highly rewarding foods, may act to override hemostatic signaling and contribute to overconsumption, weight gain, and obesity. Here, we review human neuroimaging literature that has delivered valuable insight into the neural correlates of hedonic-motivated ingestive behavior, weight gain, weight loss, and metabolic status. Our primary focus is the brain regions that are thought to encode aspects of food hedonics, gustatory and somatosensory processing, and executive functioning. Further, we discuss the variability of regional brain response as a function of obesity, weight gain, behavioral and surgical weight loss, as well as in type 2 diabetes.