Serotonin type-3 receptors mediate cholecystokinin-induced satiation through gastric distension

Effects of Palonosetron on Nausea and Vomiting Induced by Multiple-Day Chemotherapy: A Retrospective Study

Patients who undergo multiple-day chemotherapy sessions experience hard-to-treat nausea and vomiting. Currently, there is no effective standard treatment for this condition. This study compared the preventive effect of first-generation 5-hydroxytryptamine 3 receptor antagonists (5-HT₃ RAs) and second-generation 5-HT₃ RAs palonosetron in multiple-day chemotherapy-induced nausea and vomiting. The design of this study was a retrospective case-control study of patients who received a five-day cisplatin-based chemotherapy and were treated with aprepitant, dexamethasone, granisetron, and ramosetron or palonosetron. The patients were divided into two groups: patients given granisetron and ramosetron (the first-generation group), and those given palonosetron (palonosetron group). The percentage of patients with a complete response or total control was assessed. They were divided into three phases: 0-216 h (overall phase), 0-120 h (remedial phase), and 120-216 h (after phase). The remedial phase was further divided into 0-24 h (early phase) and 24-120 h (later phase). Moreover, the nutritional status of each patient was assessed by noting the patients' total calorie-intake per day and total parenteral nutrition. First-generation 5-HT₃ RAs and palonosetron were used for treatment in 18 and 28 patients, respectively. The complete response rate and caloric oral intake of the later phase were higher in the palonosetron group than in the first-generation group. We conclude that palonosetron treatment was more effective than first-generation 5-HT₃ RAs in controlling multiple-day chemotherapy-induced nausea and vomiting.

The Enteric Network: Interactions between the Immune and Nervous Systems of the Gut

Interactions between the nervous and immune systems enable the gut to respond to the variety of dietary products that it absorbs, the broad spectrum of pathogens that it encounters, and the diverse microbiome that it harbors. The enteric nervous system (ENS) senses and reacts to the dynamic ecosystem of the gastrointestinal (GI) tract by translating chemical cues from the environment into neuronal impulses that propagate throughout the gut and into other organs in the body, including the central nervous system (CNS). This review will describe the current understanding of the anatomy and physiology of the GI tract by focusing on the ENS and the mucosal immune system. We highlight emerging literature that the ENS is essential for important aspects of microbe-induced immune responses in the gut. Although most basic and applied research in neuroscience has focused on the brain, the proximity of the ENS to the immune system and its interface with the external environment suggest that novel paradigms for nervous system function await discovery.

Fatty acid sensing in the gut and the hypothalamus: in vivo and in vitro perspectives

The ability to properly sense both ingested and circulating nutrients is crucial for the maintenance of metabolic homeostasis. As such, both the gastrointestinal tract and the hypothalamus have demonstrated the capacity to sense and effectively respond to nutrients, such as fatty acids, to control food intake and glucose production to regulate energy and glucose homeostasis. In modern, Westernized societies, obesity and diabetes rates continue to rise unabated, due in part to an increase in highly palatable high-fat diet consumption. Thus, our understanding in the ability of the body to successfully monitor lipids is more vital than ever. This review details the current understanding of both the gut and the brain, specifically the hypothalamus, in sensing fatty acids. Highlighting both in vivo and in vitro studies, we explore some of the mechanisms upon which different fatty acids activate enteroendocrine and neural lipid-sensing signaling mechanisms to subsequently lower food intake and glucose production to ultimately regulate metabolic homeostasis. A better understanding of these lipid-sensing pathways could lay the groundwork for successful pharmacological targets for the treatment of obesity and diabetes.

Intraduodenal milk protein concentrate augments the glycemic and food intake suppressive effects of DPP-IV inhibition

Glucagon-like peptide-1 (GLP-1) is an incretin hormone released from intestinal L-cells in response to food entering into the gastrointestinal tract. GLP-1-based pharmaceuticals improve blood glucose regulation and may hold promise for obesity treatment, as GLP-1 drugs reduce food intake and body weight in humans and animals. In an effort to improve GLP-1 pharmacotherapies, we focused our attention on macronutrients that, when present in the gastrointestinal tract, may enhance GLP-1 secretion and improve glycemic regulation and food intake suppression when combined with systemic administration of sitagliptin, a pharmacological inhibitor of DPP-IV (enzyme responsible for GLP-1 degradation). In particular, previous data suggest that specific macronutrient constituents found in dairy foods may act as potent secretagogues for GLP-1 and therefore may potentially serve as an adjunct dietary therapy in combination with sitagliptin. To directly test this hypothesis, rats received intraperitoneal injections of sitagliptin (6 mg/kg) or saline vehicle followed by intraduodenal infusions of either milk protein concentrate (MPC; 80/20% casein/whey; 4 kcal), soy protein (nondairy control infusate; 4 kcal), or 0.9% NaCl. Food intake was assessed 30 min postinfusion. In separate studies, regulation of blood glucose was examined via a 2-h oral glucose tolerance test (2 g/kg) following identical sitagliptin treatment and intraduodenal nutrient infusions. Collectively, results show that intraduodenal MPC, but not soy protein, significantly enhances both the food intake suppression and improved control of blood glucose produced by sitagliptin. These data support the hypothesis that dietary intake of dairy protein may be beneficial as an adjunct behavioral therapy to enhance the glycemic and food intake suppressive effects of GLP-1-based pharmacotherapies.

Spatiotemporal expression pattern of DsRedT3/CCK gene construct during postnatal development of myenteric plexus in transgenic mice

Cholecystokinin (CCK) is an early marker of both neuronal and endocrine cell lineages in the developing gastrointestinal tract. To determine the quantitative properties and the spatial distribution of the CCK-expressing myenteric neurones in early postnatal life, a transgenic mouse strain with a CCK promoter-driven red fluorescent protein (DsRedT3/CCK) was established. The cell-specific expression of DsRedT3/CCK was validated by in situ hybridization with a CCK antisense riboprobe and by in situ hybridization coupled with immunohistochemistry involving a monoclonal antibody to CCK. A gradual increase in the DsRedT3/CCK-expressing enteric neurones with clear regional differences was documented from birth until the suckling to weaning transition, in parallel with the period of rapid intestinal growth and functional maturation. To evaluate the proportion of myenteric neurones in which DsRedT3/CCK transgene expression was colocalized with the enteric neuronal marker peripherin, immunofluorescence techniques were applied. All DsRedT3/CCK neurones were peripherin-immunoreactive and the proportion of DsRedT3/CCK-expressing myenteric neurones in the duodenum was the highest after the third week of life, when the number of peripherin-immunoreactive myenteric neurones in this region had decreased. Nearly all of the DsRedT3/CCK-expressing neurones also expressed 5-hydroxytryptophan (5-HT). Thus, by utilizing a new transgenic mouse strain, we have demonstrated a small number of CCK-expressing myenteric neurones with a developmentally regulated spatiotemporal distribution. The coexistence of CCK and 5-HT in the majority of these neurones suggests their possible regulatory role in feeding at the suckling to weaning transition.

Hindbrain neurons as an essential hub in the neuroanatomically distributed control of energy balance

This Review highlights the processing and integration performed by hindbrain nuclei, focusing on the inputs received by nucleus tractus solitarius (NTS) neurons. These inputs include vagally mediated gastrointestinal satiation signals, blood-borne energy-related hormonal and nutrient signals, and descending neural signals from the forebrain. We propose that NTS (and hindbrain neurons, more broadly) integrate these multiple energy status signals and issue-output commands controlling the behavioral, autonomic, and endocrine responses that collectively govern energy balance. These hindbrain-mediated controls are neuroanatomically distributed; they involve endemic hindbrain neurons and circuits, hindbrain projections to peripheral circuits, and projections to and from midbrain and forebrain nuclei.

Endogenous leptin receptor signaling in the medial nucleus tractus solitarius affects meal size and potentiates intestinal satiation signals

Leptin receptor (LepRb) signaling in the hindbrain is required for energy balance control. Yet the specific hindbrain neurons and the behavioral processes mediating energy balance control by hindbrain leptin signaling are unknown. Studies here employ genetic [adeno-associated virally mediated RNA interference (AAV-RNAi)] and pharmacological methodologies to specify the neurons and the mechanisms through which hindbrain LepRb signaling contributes to the control of food intake. Results show that AAV-RNAi-mediated LepRb knockdown targeting a region encompassing the mNTS and area postrema (AP) (mNTS/AP LepRbKD) increases overall cumulative food intake by increasing the size of spontaneous meals. Other results show that pharmacological hindbrain leptin delivery and RNAi-mediated mNTS/AP LepRb knockdown increased and decreased the intake-suppressive effects of intraduodenal nutrient infusion, respectively. These meal size and intestinally derived signal amplification effects are likely mediated by LepRb signaling in the mNTS and not the AP, since 4th icv and mNTS parenchymal leptin (0.5 μg) administration reduced food intake, whereas this dose did not influence food intake when injected into the AP. Overall, these findings deepen the understanding of the distributed neuronal systems and behavioral mechanisms that mediate the effects of leptin receptor signaling on the control of food intake.

Peripheral neural targets in obesity

Interest in pharmacological treatments for obesity that act in the brain to reduce appetite has increased exponentially over recent years, but failures of clinical trials and withdrawals due to adverse effects have so far precluded any success. Treatments that do not act within the brain are, in contrast, a neglected area of research and development. This is despite the fact that a vast wealth of molecular mechanisms exists within the gut epithelium and vagal afferent system that could be manipulated to increase satiety. Here we discuss mechano- and chemosensory pathways from the gut involved in appetite suppression, and distinguish between gastric and intestinal vagal afferent pathways in terms of their basic physiology and activation by enteroendocrine factors. Gastric bypass surgery makes use of this system by exposing areas of the intestine to greater nutrient loads resulting in greater satiety hormone release and reduced food intake. A non-surgical approach to this system is preferable for many reasons. This review details where the opportunities may lie for such approaches by describing nutrient-sensing mechanisms throughout the gastrointestinal tract.

The trophic effect of cholecystokinin on the pancreas declines in rats on total parenteral nutrition

Total parenteral nutrition (TPN) results in atrophy of the pancreas, while cholecystokinin (CCK) can significantly stimulate the exocrine pancreas in rodents. This study was designed to examine whether CCK may improve the atrophy of the pancreas in rats on TPN treatment. Forty-eight Sprague-Dawley rats were divided into orally fed and TPN groups and were infused with CCK at a dose of 5 μg/kg/h or the CCK-receptor antagonist devazepide at a dose of 200 μg/kg/h for 10 days. Infusion of CCK caused hypercholecystokininemia (hyperCCKemia) and decreased the atrophy of the pancreas resulting from TPN. The hyperplastic response to CCK in orally fed rats was decreased in the rats given TPN. Devazepide did not influence the pancreatic variables. This study further confirmed that CCK stimulates the exocrine pancreas and decreases the atrophy of the exocrine pancreas resulting from TPN. Our present findings suggest that the trophic effect of CCK on the exocrine pancreas declines in TPN.

Discovery and development of 5-HT2C receptor agonists for obesity: is there light at the end of the tunnel?

Ever since the observation of late-onset obesity during the phenotypic characterization of the 5-HT2C knock-out mouse, the serotonin 5-HT2C receptor has been a drug target for obesity. Small-molecule agonists have repeatedly been shown to reduce food intake and body weight in rodent models of obesity. To date, however, only one compound, lorcaserin, has completed Phase III trials and currently awaits an US FDA decision following a negative advisory committee meeting. Agonist selectivity versus the highly homologous 5-HT2A and 5-HT2B receptors remains a significant hurdle. Ideally, a specific 5-HT2C agonist (completely devoid of 5-HT2A and 5-HT2B activity) would be preferred. The requirement of a basic amine coupled with larger, often aromatic, hydrophobic domains, to gain selectivity, often leads to additional challenges associated with cationic amphiphilic molecules such as hERG-channel inhibition and phospholipidosis. The success of future 5-HT2C agonists will depend on further improvements in selectivity (or attainment of complete specificity) and pharmaceutical properties to permit greater and sustained receptor stimulation, while avoiding side effects associated with the activation of other 5-HT receptors.

Endogenous hindbrain glucagon-like peptide-1 receptor activation contributes to the control of food intake by mediating gastric satiation signaling

Exogenous activation of central nervous system glucagon-like peptide-1 (GLP-1) receptors (GLP-1Rs) reduces food intake. Experiments addressed whether endogenous central GLP-1R activity is involved in the control of normal feeding and examined which gastrointestinal satiation signals contribute to this control. Given that nucleus tractus solitarius (NTS) neurons are the source of central GLP-1, that caudal brainstem circuits mediate the intake suppression triggered by exogenous hindbrain GLP-1R activation, and that these neurons process gastrointestinal vagal signals, the role of endogenous hindbrain GLP-1R activation to intake control was the focus of the analysis. Food intake increased with GLP-1R antagonist [Exendin-(9-39) (Ex-9)] [10 microg, fourth intracerebroventricular (icv)] delivery to overnight food-deprived rats after ingestion of 9 ml Ensure diet. Direct medial NTS injection of a ventricle subthreshold dose (1.0 microg) of Ex-9 increased food intake and established the contribution of this GLP-1R population to the effect observed with ventricular administration. To determine whether satiation signals of gastric vs. intestinal origin drive the GLP-1R-mediated NTS effect on food intake, two experiments were performed in overnight-fasted rats. In one, Ensure was infused intraduodenally (0.4 ml/min for 20 min); in another, the stomach was distended (9 ml SILASTIC brand balloon) for 15 min before fourth icv Ex-9. The intake suppression by duodenal nutrient infusion was not affected by GLP-1R blockade, but the feeding suppression after gastric distension was significantly attenuated by fourth icv Ex-9. We conclude that endogenous NTS GLP-1R activation driven by gastric satiation signals contributes to the control of normal feeding.

Progress in developing cholecystokinin (CCK)/gastrin receptor ligands that have therapeutic potential

Gastrin and cholecystokinin (CCK) are two of the oldest hormones and within the past 15 years there has been an exponential increase in knowledge of their pharmacology, cell biology, receptors (CCK1R and CCK2R), and roles in physiology and pathological conditions. Despite these advances there is no approved disease indication for CCK receptor antagonists and only a minor use of agonists. In this review, the important factors determining this slow therapeutic development are reviewed. To assess this it is necessary to briefly review what is known about the roles of CCK receptors (CCK1R and CCK2R) in normal human physiology, their role in pathologic conditions, the selectivity of available potent CCKR agonists/antagonists as well as to review their use in human conditions to date and the results. Despite extensive studies in animals and in humans, recent studies suggest that monotherapy with CCK1R agonists will not be effective in obesity, nor CCK2R antagonists in panic disorders or CCK2R antagonists to inhibit growth of pancreatic cancer. Areas that require more study include the use of CCK2R agonists for imaging tumors and radiotherapy, CCK2R antagonists in hypergastrinemic states especially with long-term PPI use and for potentiation of analgesia as well as use of CCK1R antagonists for a number of gastrointestinal disorders [motility disorders (irritable bowel syndrome, dyspepsia, and constipation) and pancreatitis (acute and chronic)].

Cholecystokinin

PURPOSE OF REVIEW

The hormone cholecystokinin was discovered in 1928 because of its ability to induce gallbladder contraction. Since then, cholecystokinin has been shown to possess multiple functions in the gastrointestinal tract and brain. This review discusses several significant developments in cholecystokinin biology that show how it plays a role in gastrointestinal diseases, including control of appetite.

RECENT FINDINGS

Cholecystokinin was shown to induce satiety by interacting through CCK-1 receptors located in specialized regions of the hindbrain. Cholecystokinin also inhibits expression of orexigenic peptides in the hypothalamus and prevents stimulation of specialized neurons by ghrelin. In the pancreas, cholecystokinin increased the proliferation of insulin-producing beta cells and reduced insulin-induced hyperphagia. Elevated cholecystokinin levels decreased appetite and reduced intestinal inflammation caused by parasites and bacterial toxins.

SUMMARY

Understanding the mechanisms by which cholecystokinin regulates orexigenic pathways in the body may lead to strategies for controlling appetite-related disorders such as obesity and bulimia. The reduction of intestinal inflammation by dietary fats (by elevating cholecystokinin) suggests that the hormone plays an integrated role in regulating the ingestion and digestion of food that may be relevant to inflammatory diseases of the gastrointestinal tract.

Insect satiety: sulfakinin localization and the effect of drosulfakinin on protein and carbohydrate ingestion in the blow fly, Phormia regina (Diptera: Calliphoridae)

Sulfakinins, which are satiety factors in invertebrates, have previously been shown to inhibit feeding in the German cockroach and desert locust. This study examines the occurrence of sulfakinin immunoreactivity and the role of sulfakinin as a feeding satiety factor in the black blow fly, Phormia regina. Specifically, this study examines the effect of sulfakinin on two of the blow fly's nutrient requirements (i.e., carbohydrates and proteins). We observed sulfakinin immunoreactive cells in the brains of both male and female flies. We found that drosulfakinin I (DrmSKI, FDDY[SO(3)H]GHMRFa) significantly inhibited carbohydrate feeding by 44% at the most effective dose (10 nmol) in female flies. Statistically, there was no significant effect on males; however, injections of 10 nmol DrmSKI reduced carbohydrate feeding by 34% compared to the sham. Drosulfakinin had no effect on protein feeding and no significant inhibition was detected in females or males. The results of this study lend further support to the idea that carbohydrate and protein feeding are regulated by separate control mechanisms, especially in Calliphoridae.

Serotonin-type 3 receptors mediate intestinal lipid-induced satiation and Fos-like immunoreactivity in the dorsal hindbrain

Several gastrointestinal stimuli, including some intestinal nutrients, have been shown to exert their satiating effect via activation of serotonin type-3 (5-HT(3)) receptors. The presence of lipids in the small intestine potently suppresses food intake; however, whether 5-HT(3) receptors play a role in this response has not been directly examined. Therefore, using the selective 5-HT(3) receptor antagonist ondansetron, we tested the hypothesis that duodenal infusion of lipid suppresses intake of both sucrose solution and chow through 5-HT(3) receptor activation. Rats duodenally infused with 72 and 130 mM Intralipid suppressed 1-h 15% sucrose intake by 33 and 67%, respectively. Suppression of sucrose intake by 72 mM Intralipid was significantly attenuated by ondansetron at all doses tested (0.5, 1.0, 2.0, and 5.0 mg/kg ip), whereas the lowest effective dose of ondansetron to attenuate suppression of intake by 130 mM Intralipid was 1.0 mg/kg. Furthermore, infusion of 130 mM Intralipid suppressed 1- and 4-h chow intake by 35 and 20%, respectively. Ondansetron administered as low as 0.5 mg/kg significantly attenuated 1-h Intralipid-induced suppression of chow intake and completely reversed the suppression by 4 h. Administration of ondansetron alone did not alter sucrose or chow intake compared with vehicle injection at any time. Finally, to test whether Intralipid-induced neuronal activation of the dorsal vagal complex is mediated by 5-HT(3) receptors, Fos-like immunoreactivity (Fos-LI) was quantified in ondansetron-pretreated rats following intestinal lipid infusion. Ondansetron (1 mg/kg) significantly attenuated duodenal intralipid-induced Fos-LI in the dorsal hindbrain. These data support the hypothesis that 5-HT(3) receptors mediate both satiation, as well as hindbrain neuronal responses evoked by intestinal lipids.

Gastric distension enhances CCK-induced Fos-like immunoreactivity in the dorsal hindbrain by activating 5-HT3 receptors

The combination of gastric distension and cholecystokinin (CCK) enhances both suppression of food intake and induction of c-Fos-like immunoreactivity (Fos-LI) in the dorsal vagal complex (DVC). Previously, we have shown that serotonin type-3 (5-HT3) receptor mediation of suppression of food intake by CCK requires gastric participation. Therefore, we hypothesized that 5-HT3 receptors mediate CCK-induced Fos-LI in the dorsal hindbrain through a mechanism that involves gastric distension. To test this hypothesis, we counted Fos-LI in the DVC of ondansetron (1 mg/kg; 5-HT3 receptor antagonist) and vehicle-treated rats following gastric balloon distension (5 ml), CCK (1 microg/kg) administration, or CCK combined with gastric distension. Ondansetron administration attenuated DVC Fos-LI by CCK administration. Likewise, ondansetron attenuated Fos-LI by gastric distension in the DVC, specifically within the nucleus of the solitary tract (NTS) and area postrema (AP) nuclei. The most pronounced attenuation of distension-induced Fos-LI by ondansetron occurred in the NTS, particularly in the medial and intermedial NTS. When combined, CCK and gastric distension enhanced Fos-LI in the DVC greater than each treatment alone. Furthermore, ondansetron administration attenuated the overall DVC enhanced Fos-LI induced by CCK + gastric distension, in particular at the NTS and AP nuclei. We found that, within the mid-to-caudal regions of the NTS and AP, 5-HT3 receptors most significantly mediate neuronal activation by CCK + distension. In conjunction with previous behavioral data, these results show that gastric distension enhances CCK-induced neuronal activation in the DVC by activating 5-HT3 receptors.