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.

Chronic feeding on an HF diet causes reduced synthesis of intestinal OEA () associated with diminished dorsal striatal dopamine levels (), which may lead to overconsumption of fat and subsequent weight gain. In support of this, supplementation of OEA to HF diet-induced obese (DIO) mice restores dorsal striatal dopamine release and suppresses fat ingestion, with repeated administration producing weight loss (). Here we investigated the potential causal role of this intestinal OEA-vagal afferent-dorsal striatal dopamine pathway in mediating the reduced fat appetite after RYGB. Specifically, we hypothesized that enhanced exposure of lower small intestine enterocytes to lipids due to their surgical rerouting would (re)activate local OEA synthesis. The resulting increase in nigrostriatal function would then lower the threshold of fat consumption required to entail dorsal striatal dopamine surfeit.

A mechanistic account for the reduction in fat appetite following RYGB remains elusive. Current evidence indicates that gastrointestinal reconfiguration and not physical restriction limits fat consumption after surgery (). Under physiological conditions, as fat is ingested, various lipid species are generated in the gut, which communicate with the brain to orchestrate appropriate feeding responses (). One such lipid signaler is oleoylethanolamide (OEA), an endogenous agonist of the peroxisome proliferator-associated receptor-α (PPAR-α) () produced in high amounts by enterocytes of the upper small intestine from direct conversion of dietary oleic acid (). It is thought that intestinal OEA works locally, first activating PPAR-α receptors, which in turn engages vagal sensory afferents to curb fat intake through hypothalamic () and striatal () feeding circuits.

The obesity pandemic continues to grow, and the ready accessibility to palatable, energy-dense foods is considered to be a significant driving force (). Treatment options remain of generally limited long-term efficacy, with the exception of bariatric surgery (). Roux-en-Y gastric bypass (RYGB), the most commonly employed bariatric surgical procedure, yields greater than 50% excess weight loss in the morbidly obese that is sustained in the long-term (). This positive outcome stems mainly from reductions in caloric intake commonly attributed to reduced sensations of hunger and increased satiety (). Given the modern obesogenic environment, the influence of RYGB on hedonic, motivational, and cognitive aspects of feeding behavior has garnered significant attention. Accordingly, patients after surgery have been found to display shifts in preference from high-fat (HF) to low-fat (LF) foods (), reductions in cravings for palatable foods (), diminished motivation to obtain fatty tastants (), and decreased dietary restraint and external eating (). Similar multifaceted alterations in fat intake have been reported in rodent models of RYGB ().

(E and F) Intake (E) and preference (F) of 5% Intralipid following intra-dorsolateral striatal infusion of the selective D1R antagonist SCH-23390 (0.6 μg/0.3 μL in ACSF) or ACSF as vehicle (mean ± SEM, n = 4–11/group; two-tailed t test; ∗ p < 0.05 and ∗∗∗ p < 0.001).

(C and D) Intake (C) and preference (D) of 5% Intralipid following complete sub-diaphragmatic truncal vagotomy (mean ± SEM, n = 5–10/group; one-way ANOVA; ∗∗ p < 0.01, ∗∗∗ p < 0.001, ∗∗∗∗ p < 0.0001, # p < 0.05 versus Sham-LF, and ## p < 0.05 versus Sham-LF).

(A and B) Intake (A) and preference (B) of 5% Intralipid following intra-intestinal infusion of the PPAR-α antagonist GW-6471 (4 mg/kg in DMSO/0.9% saline), the PPAR-α agonist WY-14643 (1 mg/kg in DMSO/0.9% saline), or vehicle (DMSO/0.9% saline) (mean ± SEM, n = 5–10/group; one-way ANOVA; ∗∗∗ p < 0.001 and ∗∗∗∗ p < 0.0001).

High-concentration fat emulsion (5% Intralipid) intakes and preferences of RYGB-operated and sham-operated control rats during oral two-bottle preference tests in response to interfering with gut-brain signaling. Schematic diagrams on the left demonstrate where stimulators/blockers were administered prior to experiments.

Having established that RYGB potentiates dorsal striatal dopamine release in response to an HF meal in particular, we next determined postsynaptic alterations in dopamine receptor expression to discern striatal direct or indirect pathway recruitment after RYGB. Since previous trials with [C] raclopride have failed to demonstrate a convincing change in dopamine D2/D3R availability following RYGB (), it therefore appears unlikely that D2R-expressing striatopallidal/indirect pathway neurons govern the newly adopted eating behaviors observed after surgery. Whether RYGB influences striatal dopamine 1 receptor (D1R) expression/direct pathway neurons, however, has yet to be addressed. To this end, we performed in vivo small-animal positron emission tomography (PET) imaging studies with the D1R selective radiotracer [C] SCH-23390. Importantly, and in contrast to [C] raclopride, the high binding affinity of the D1R antagonist [C] SCH-23390 to the D1R renders the radiotracer non-displaceable by endogenous dopamine, thus providing an accurate measure of D1R density in the intact striatum (). In order to control for the effect of diet in addition to body weight and treatment, PET scans were performed on two separate series of animals, each comprising Sham, RYGB, and Sham-BWM groups. The first series of animals was postoperatively maintained on standard LF chow and the second series of animals on a two-choice diet of standard LF chow and an HF diet. In keeping with the findings on extracellular neurotransmitter release, no significant differences in striatal D1R expression levels were identifiable between experimental groups postoperatively maintained on standard LF chow ( Figure S4 ). Remarkably, when animals were postoperatively maintained on a choice diet, RYGB-LF/HF rats displayed markedly higher striatal D1R density than Sham-LF/HF and Sham-BWM-LF/HF rats ( Figure 4 ). This was despite the ratio of LF/HF intake in RYGB-LF/HF rats being the same as Sham-BWM-LF/HF controls. Together with the results obtained from the cerebral microdialysis experiments, these results indicate that the body weight-independent effect of RYGB surgery on dorsal striatal dopamine signaling emerges in response to consumption of an HF diet. They further suggest that RYGB chronically engages D1R containing striatonigral direct pathway neurons.

Left panel: representative brain PET images of RYGB-operated and sham-operated control rats that received the dopamine 1 receptor (D1R) selective radioligand [ 11 C] SCH-23390. Animals were previously maintained on a choice diet of LF and HF food for 14 weeks postoperatively up until the time of scanning. Right panel: the mean standardized uptake value (SUV) of [ 11 C] SCH-23390 in striatum normalized to that of the mean SUV in cerebellum and plotted against time (data are presented as linear regression curves with 95% confidence intervals denoted by the dotted lines; n = 5/group; p values for slopes of linear regression curves were obtained from one-way ANOVA).

Intra-intestinal OEA supplementation has previously been shown to potentiate dorsal striatal dopamine release in response to intra-gastric lipid infusion in mice (). Based on this and the findings above of increased intestinal OEA synthesis after RYGB, we determined if there is an accompanying postoperative potentiation in dorsal striatal dopamine release by performing oral feeding studies with HF and LF meals concomitantly to in vivo brain dopamine measurements. Microdialysate samples were collected at 10 min intervals from the dorsolateral aspect of the striatum. RYGB-treated rats exhibited significantly higher dopamine effluxes in response to an HF meal than obese and body weight-matched control groups ( Figures 3 A and 3B ), despite lower calorie intake ( Figure 3 C). These effects were observed in the absence of any differences in basal striatal dopamine concentrations between experimental groups ( Figure S1 , available online). In line with the requirement of vagal afferents for the central effects of OEA, vagotomy greatly attenuated the potentiated dopamine release caused by RYGB ( Figures 3 A and 3B). Notably, calorie restriction-induced weight loss failed to increase dopamine release in response to an HF meal ( Figures 3 A and 3B). Consumption of an LF meal expectedly elicited a lower-magnitude dopamine response compared to consumption of an HF meal in all study groups ( Figures S2 A and S2B). Nevertheless, RYGB-treated rats still displayed higher dorsal striatal dopamine release compared to rats from Sham-LF, Sham-HF, and RYGB-VAG groups ( Figure S2 B), despite consuming a similar amount of calories from the LF meal ( Figure S2 C). Of note is that Sham-BWM rats consumed significantly more LF meal than RYGB-treated rats ( Figure S2 C), but the corresponding dopamine release still tended to be lower ( Figure S2 B). Importantly, and in contrast to previous data suggesting an oxytocin-mediated satiety effect of intestinal OEA (), hypothalamic mRNA expression levels of oxytocin in RYGB-treated rats were unaltered ( Figure S3 ). Together, these findings imply that RYGB targets hedonic striatal pathways that are dysfunctional in DIO. This modification required intact vagal signaling and occurred independently of body weight loss.

(B and C) Associated area under the curve (B) and corresponding HF intake (C) after 30 min refeeding (mean ± SEM, n = 4–8/group; one-way ANOVA; ∗ p < 0.05, ∗∗ p < 0.01, ∗∗∗ p < 0.001, and ∗∗∗∗ p < 0.0001).

(A) Dorsolateral striatal dopamine levels determined by HPLC of RYGB-operated and sham-operated control rats that were food restricted overnight and refed with an HF meal for 30 min (indicated by the gray area). Samples were collected by in vivo cerebral microdialysis at 10 min intervals at baseline, during the HF meal, and for 90 min afterward. Values for each group are expressed relative to baseline (mean ± SEM, n = 4–8/group).

As diet-derived fatty acids contacting the apical surface of enterocytes serve as the initial substrate of OEA production following a meal (), we investigated whether rerouting of intestinal fat passage by RYGB reactivates intestinal OEA synthesis after its downregulation by HF DIO. By using liquid chromatography coupled with mass spectrometry (LC-MS), OEA levels were analyzed in the biliopancreatic limb (previously the duodenum), the proximal Roux limb (previously the proximal jejunum), and the proximal common channel (previously the distal jejunal/proximal ileal segments) in the fasted and re-fed state with an HF meal. In line with previous evidence (), Sham-LF groups displayed the highest concentrations of OEA in the duodenum ( Figure 2 A), followed by a gradual decline in progressively distal gut segments ( Figures 2 B and 2C), while prolonged exposure to an HF diet () decreased post-prandial intestinal OEA synthesis ( Figures 2 A and 2C). A strikingly divergent pattern was found after RYGB surgery. As would be expected, the biliopancreatic limb exhibited negligible OEA synthesis due to the diversion of ingested nutrients away from this gut region ( Figure 2 A). In the proximal Roux limb, however, post-prandial OEA synthesis substantially increased ( Figure 2 B), with peak amounts found in the proximal common channel ( Figure 2 C), where biliopancreatic secretions now first enter the reconfigured small intestine. Remarkably, post-prandial OEA concentrations in the common channel were markedly higher compared to those in physiologically corresponding segments of the control groups ( Figures 2 D and 2E). This occurred despite significantly lower dietary fat intake in RYGB-treated animals ( Figure 2 F). There was no stimulatory effect found from elevated intestinal OEA synthesis on circulating markers of systemic fatty acid oxidation or ketogenesis (data not shown). Importantly, vagotomy had no influence on OEA synthesis ( Figures 2 A–2E). These findings indicate that RYGB-mediated rerouting of fat passage uniquely increases OEA synthesis in more distal gut regions than would occur under normal physiological conditions.

(A–E) Intestinal OEA concentrations determined by LC-MS of RYGB-operated and sham-operated control rats either food restricted overnight or refed with an HF meal for 60 min. Top panel shows a schematic of the gastrointestinal anatomy of “sham groups” (Sham-LF, Sham-VAG, Sham-HF, and Sham-BWM) and “RYGB groups” (RYGB and RYGB-VAG). (A) Duodenum/biliopancreatic limb, (B) proximal jejunum/Roux limb, and (C) proximal ileum/common channel. Concentrations of OEA in the common channel of RYGB and RYGB-VAG rats were compared to (D) the duodenum and (E) the proximal jejunum of sham-operated control animals (mean ± SEM, n = 3–7/group; repeated-measures two-way ANOVA; ∗∗ p < 0.01, ∗∗∗ p < 0.001, and ∗∗∗∗ p < 0.0001 for effects of refeeding; a = p < 0.05 and aa = p < 0.001 versus Sham-HF, b = p < 0.001 versus RYGB and RYGB-VAG, c = p < 0.05 versus RYGB and RYGB-VAG, and d = p < 0.0001 versus RYGB and RYGB-VAG for group effects in the refed state).

To test our hypothesis, we utilized an established rat model of RYGB that closely mimics the clinical features of the surgical procedure in humans (). Intervention specificity for feeding outcome was tested by controlling for (1) surgical stress (sham surgery, postoperatively maintained on free access to standard LF laboratory chow [Sham-LF]), (2) body weight (sham surgery, postoperatively calorie-restricted and body weight-matched [BWM] to RYGB rats [Sham-BWM]), (3) vagus nerve signaling (RYGB and sham surgeries with complete sub-diaphragmatic truncal vagotomy [RYGB-VAG and Sham-VAG]), and (4) dietary effects (sham surgery, postoperatively maintained on an HF-diet [Sham-HF]). Weekly body weight and feeding measurements revealed substantial and sustained weight reduction following RYGB and RYGB-VAG interventions from postoperative week 2 ( Figure 1 A), accompanied by persistent hypophagia ( Figure 1 B). A separate group of rats was postoperatively offered a two-choice diet comprising HF and LF chow to establish the exact time point at which the switch in fat preference after RYGB occurred. Preoperatively, preference for HF over LF chow was similarly high (over 90%) for RYGB-treated and Sham-treated animals ( Figure 1 C). Postoperatively, however, HF preference gradually declined to below 40% from week 7 in RYGB-treated rats, but remained high (over 70%) in Sham-treated controls ( Figure 1 C).

(C) Weekly preference for HF diet over regular chow for a separate cohort of RYGB-operated and sham-operated control rats given free access to both (mean ± SEM, n = 6/group; repeated-measures two-way ANOVA; ∗ p < 0.05, ∗∗ p < 0.01, and ∗∗∗ p < 0.001).

(A) Weekly postoperative body weight measurements of high-fat (HF) DIO rats that had undergone Roux-en-Y gastric bypass surgeries with (RYGB-VAG) or without (RYGB) total sub-diaphragmatic truncal vagotomy compared to sham-operated control groups. Postoperatively, animals were maintained on a regular chow diet with the exception of Sham-HF rats, which were constantly maintained on an HF diet (mean ± SEM, n = 7–16/group; repeated-measures two-way ANOVA; # p < 0.05 for Sham-HF versus Sham-LF and Sham-VAG; ## p < 0.01, ### p < 0.001, and #### p < 0.0001 for RYGB, RYGB-VAG, and Sham-BWM versus Sham-LF, Sham-VAG, and Sham-HF).

Discussion

We have provided evidence for a causal role of modified gut-brain communication in mediating the decreased calorie intake that occurs following gastric bypass surgery. We found that synthesis of the diet-derived, lipid signaling molecule OEA is uniquely increased in the lower small intestine of RYGB-treated rats. This was associated with a robust vagus nerve-mediated and nutritionally sensitive enhancement in feeding-induced dorsal striatal dopamine release. Small-animal PET imaging further revealed a pronounced upregulation in striatal postsynaptic D1R density in vivo after RYGB. As with dopamine release, this could be dissociated from weight loss and was brought out under conditions of HF diet ingestion. Finally, intestinal PPAR-α, vagal, and dorsal striatal D1R signaling were all found to be essential components of the postoperative reduction in HF appetite caused by RYGB.

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et al. PPARγ in vagal neurons regulates high-fat diet induced thermogenesis. Due to the intestinal reconfiguration caused by RYGB, ingested fat (1) is diverted away from the proximal small intestine (biliopancreatic limb/duodenum) toward the distal small intestine (Roux limb/proximal jejunum) (2). As ingested fat traverses through the gut, it eventually meets with bile in the common channel (distal jejunum/proximal ileum) (3). Here, fatty acids (FAs), liberated from the catalytic action of pancreatic lipases, cross the apical membrane of enterocytes (4) via CD36-mediated transport (). Inside the enterocyte, FAs are converted into OEA through a series of enzymatic reactions. From this stage, the precise signaling pathway has not been defined, but two distinct possibilities exist: (5) within the enterocyte the lipid messenger OEA and/or FAs activate the nuclear receptor PPAR-α, resulting in rapid non-genomic membrane depolarization () and the release of an unidentified excitatory paracrine (across the basolateral membrane) onto vagal afferents, and/or (6) OEA is released by the enterocyte and enters vagal afferents, where it activates PPAR-α (), also resulting in membrane depolarization. This signal is propagated via the vagus nerve to hindbrain neurons in the nucleus tractus solitarius (NTS) (7), which send indirect projections to midbrain dopaminergic substantia nigra pars compacta neurons (SNpc) (8). The increased function of nigrostriatal dopaminergic neurons culminates in the release of dopamine, which binds to and activates dopamine 1 receptors (D1Rs) predominantly expressed in direct pathway medium spiny neurons (9), thereby reducing fat appetite (10). As a critical mediator of the rewarding properties of food, regulated striatal dopamine signaling is required for appropriate feeding behavior (). As such, dysregulated dorsal striatal function has been widely considered to lead to compensatory overeating in obesity (). Our findings suggest that re-sensitized gut lipid sensing caused by the increased access of fat to the lower small intestine after RYGB potentiates dorsal striatal dopamine signaling. Consequently, the amount of dietary fat required to signal reward receipt is reduced, possibly contributing to overall decreased calorie intake after surgery ( Figure 6 ). It should also be noted, however, that interfering with dorsal striatal D1R signaling did not affect feeding in any of the control groups used in the present study. Therefore, other brain circuits are likely modified after RYGB, which, in concert with enhanced dorsal striatal D1R signaling, leads to changes in fat appetite.

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Fenske W.K. Distinctive striatal dopamine signaling after dieting and gastric bypass. Gut reconfiguration after RYGB may also affect ventral striatal dopamine signaling to impact the motivational and reinforcing aspects of feeding (). For instance, acutely blocking gut hormone release in patients who had undergone RYGB causes an increase in their breakpoint to obtain a food reward in a progressive ratio task associated with increases in ventral striatal neuronal activity (). From this, it can be concluded that RYGB reduces the motivation to obtain a food reward by gut hormone-mediated suppression of ventral striatal function (). In combination with the findings from the present study, it can be speculated that RYGB may have differential effects on ventral versus dorsal striatal dopamine signaling to regulate the hedonic aspects of feeding ().

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Geliebter A. How does the brain implement adaptive decision making to eat?. From an alternative perspective, dorsal striatal dopamine has well-established roles in goal-directed behavior and habit formation. For instance, in mice, elevations in dorsal striatal dopamine sufficiently underlie amphetamine-induced hypophagia, thought to be due to impaired action selection (). It can thus be reasoned that the deliberation of RYGB-treated rats in response to calorie-dense food cues in the present study ceased following pharmacological attenuation of elevated dorsal striatal dopamine signaling. In terms of habitual behaviors, animal () and human () studies have shown that obesity has a detrimental effect. In the situation after RYGB, it may be that enhanced (dorsolateral) striatal D1R signaling generates healthier feeding habits that promote a negative energy balance. This prediction derives mainly from our finding that following surgery, reduced intake of calories from fat is required to elicit dorsolateral striatal dopamine signaling through the D1R, which drives new habit formation (that of less fat intake) (). In this context, persistent hypophagia in anorexia nervosa has been attributed to progressively altered dorsal striatal structure/function underpinning feeding habits that override homeostatic pressures and perpetuate sustained weight loss ().