CCK and 5-HT act synergistically to suppress food intake through simultaneous activation of CCK-1 and 5-HT3 receptors

CCK-expressing neurons in the NTS are directly activated by CCK-sensitive C-type vagal afferents

Vagal sensory afferents carrying information from the gastrointestinal tract (GI) terminate in the nucleus of the solitary tract (NTS). Different subpopulations of NTS neurons then relay this information throughout the brain. Cholecystokinin (CCK) is a satiety peptide that activates vagal afferents in the GI. However, CCK is also expressed by neurons in the NTS, and activation of these neurons decreases food intake. What is less clear is how these NTS CCK neurons are activated by vagal afferents and what type of information they integrate about meal size and content. To address this, we identified NTS-CCK neurons by crossing CCK-IRES-Cre mice with floxed-Rosa-tdtomato mice and made a horizontal brain slice containing vagal afferents in the solitary tract (ST). Voltage clamp recordings of NTS-CCK neurons show that activation of the ST evokes excitatory postsynaptic currents (EPSCs) mediated by both α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-d-aspartate (NMDA) receptors. Analysis of these EPSCs revealed that 80% of NTS-CCK neurons receive direct, monosynaptic inputs, with many also receiving indirect, or polysynaptic, inputs. NTS-CCK neurons are sensitive to the transient receptor potential vanilloid type 1 agonist capsaicin, suggesting that they are downstream of C-fibers. In addition, both CCK and a 5 hydroxytryptamine 3 receptor (5-HT3R) agonist increased spontaneous EPSC (sEPSC) frequency in NTS-CCK neurons, with 69% of NTS-CCK neurons sensitive to CCK and 42% to the 5-HT3 receptor agonist, as well as 45% sensitive to both and 10% to neither. Taken together with previous studies, this suggests that NTS-CCK neurons are driven primarily by vagal afferents that are sensitive to CCK and are only weakly driven by those sensitive to serotonin.NEW & NOTEWORTHY Nucleus of the solitary tract (NTS) cholecystokinin (CCK) expressing neurons are directly activated by glutamate released from vagal afferents. They are downstream of primarily C-type CCK-sensitive afferents, with a small proportion also downstream of serotonin-sensitive afferents. These findings suggest that NTS-CCK neurons integrate signals from the gut about ingestion of fats and proteins as well as stretch of the stomach, which they then relay to other brain regions important for the control of food intake.

Tryptophanylation of insulin receptor by WARS attenuates insulin signaling

Increased circulating amino acid levels have been linked to insulin resistance and development of type 2 diabetes (T2D), but the underlying mechanism remains largely unknown. Herein, we show that tryptophan modifies insulin receptor (IR) to attenuate insulin signaling and impair glucose uptake. Mice fed with tryptophan-rich chow developed insulin resistance. Excessive tryptophan promoted tryptophanyl-tRNA synthetase (WARS) to tryptophanylate lysine 1209 of IR (W-K1209), which induced insulin resistance by inhibiting the insulin-stimulated phosphorylation of IR, AKT, and AS160. SIRT1, but not other sirtuins, detryptophanylated IRW-K1209 to increase the insulin sensitivity. Collectively, we unveiled the mechanisms of how tryptophan impaired insulin signaling, and our data suggested that WARS might be a target to attenuate insulin resistance in T2D patients.

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

5-HT containing enteroendocrine cells characterised by morphologies, patterns of hormone co-expression, and relationships with nerve fibres in the mouse gastrointestinal tract

5-HT containing enteroendocrine cells (EEC), the most abundant type of EEC in the gut, regulate many functions including motility, secretion and inflammatory responses. We examined the morphologies of 5-HT cells from stomach to rectum, patterns of hormone co-expression in the stomach and colon, and the relationship of 5-HT cells with nerve fibres. We also reviewed some of the relevant literature. The morphologies of 5-HT cells were distinct, depending on their location in the gut. A noticeable feature of some 5-HT cells in the antrum and colon was their long basal processes, which resembled processes of neurons, whereas 5-HT cells in the small intestinal mucosa lacked basal processes. In the stomach, numerous 5-HT cells, including cells with basal processes, were identified as enterochromaffin-like cells by their expression of histidine decarboxylase. In the colon, we observed a small number of 5-HT cells that were in close contact with, but distinct from, oxyntomodulin (OXM) and PYY immunoreactive EEC. We did not find specific relationships between nerve fibres and the processes of colonic 5-HT cells. We conclude that five major features, i.e., gut region, morphology, hormone content, receptor repertoire and cell lineage, can be used to define 5-HT cells.

Central 5-HTR2C in the Control of Metabolic Homeostasis

The 5-hydroxytryptamine 2C receptor (5-HTR2C) is a class G protein-coupled receptor (GPCR) enriched in the hypothalamus and the brain stem, where it has been shown to regulate energy homeostasis, including feeding and glucose metabolism. Accordingly, 5-HTR2C has been the target of several anti-obesity drugs, though the associated side effects greatly curbed their clinical applications. Dissecting the specific neural circuits of 5-HTR2C-expressing neurons and the detailed molecular pathways of 5-HTR2C signaling in metabolic regulation will help to develop better therapeutic strategies towards metabolic disorders. In this review, we introduced the regulatory role of 5-HTR2C in feeding behavior and glucose metabolism, with particular focus on the molecular pathways, neural network, and its interaction with other metabolic hormones, such as leptin, ghrelin, insulin, and estrogens. Moreover, the latest progress in the clinical research on 5-HTR2C agonists was also discussed.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

TPH2 in the Dorsal Raphe Nuclei Regulates Energy Balance in a Sex-Dependent Manner

AbstractCentral 5-hydroxytryptamine (5-HT), which is primarily synthesized by tryptophan hydroxylase 2 (TPH2) in the dorsal Raphe nuclei (DRN), plays a pivotal role in the regulation of food intake and body weight. However, the physiological functions of TPH2 on energy balance have not been consistently demonstrated. Here we systematically investigated the effects of TPH2 on energy homeostasis in adult male and female mice. We found that the DRN harbors a similar amount of TPH2+ cells in control male and female mice. Adult-onset TPH2 deletion in the DRN promotes hyperphagia and body weight gain only in male mice, but not in female mice. Ablation of TPH2 reduces hypothalamic pro-opiomelanocortin (POMC) neuronal activity robustly in males, but only to a modest degree in females. Deprivation of estrogen by ovariectomy (OVX) causes comparable food intake and weight gain in female control and DRN-specific TPH2 knockout mice. Nevertheless, disruption of TPH2 blunts the anorexigenic effects of exogenous estradiol (E2) and abolishes E2-induced activation of POMC neurons in OVX female mice, indicating that TPH2 is indispensable for E2 to activate POMC neurons and to suppress appetite. Together, our study revealed that TPH2 in the DRN contributes to energy balance regulation in a sexually dimorphic manner.

The Combination of Cholecystokinin and Stress Amplifies an Inhibition of Appetite, Gastric Emptying, and an Increase in c-Fos Expression in Neurons of the Hypothalamus and the Medulla Oblongata

Cholecystokinin (CCK) had been the first gastrointestinal hormone known to exert anorexic effects. CCK had been inferred to contribute to the onset of functional dyspepsia (FD) symptoms. To understand the pathophysiology of FD, the roles of stress have to be clarified. In this study, we aimed to clarify the influence of stress on the action of cholecystokinin (CCK) on appetite and gastric emptying. Using rats, stress was simulated by giving restraint stress or intraperitoneal injection of the stress-related peptide hormone urocortin 1 (UCN1). The effects of CCK and restraint stress, alone or in combination, on food intake and gastric motility were examined, and c-Fos expression in the neurons of appetite control network in the central nervous system was assessed by immunohistochemical staining. CCK inhibited food intake and gastric emptying in a dose-dependent manner. Food intake for 1 h was significantly lower with UCN1 (2 nmol/kg) than with the saline control. Restraint stress amplified the suppressive effects of CCK on food intake for 1 h and on gastric emptying. With regard to brain function, the CCK induced c-Fos expression in the neurons of the nucleus tractus solitarius and paraventricular nucleus of the hypothalamus was markedly and significantly amplified by the addition of restraint stress with CCK. The results suggested that stress might amplify the anorexic effects of CCK through activation of the nuclei that comprise the brain neuronal network for satiation; this might play a role in the pathogenesis of the postprandial distress syndromes of functional dyspepsia.

Daily memantine treatment blunts hedonic response to sucrose in rats

RATIONALE

Preclinical and clinical studies suggest the potential use of memantine in the treatment of binge eating disorder. The aim of this study was to further investigate the mechanisms by which memantine influences the motivational aspects of ingestion through the analysis of licking microstructure. To interpret treatment effects in relation to drug action at specific functionally relevant times, we compared the effect of two different administration schedules.

METHODS

Memantine was administered daily for a week, either 1 h before or immediately after a 30-min daily session. The effects on the microstructure of licking for a 10% sucrose solution in rats were examined in the course of treatment and for 15 days after treatment discontinuation.

RESULTS

Treatment before testing reduced ingestion due to reduced burst size and increased latency in the first session. However, a progressive increase in burst number across sessions led to a full recovery of ingestion levels by the end of treatment. Daily post-session administration induced a dramatic decrease of activation of licking behaviour, indicated by reduced burst number, accompanied to reduced burst size. A slow recovery of ingestion took place after treatment discontinuation.

CONCLUSION

These results suggest a reduced hedonic/reward evaluation response, an effect likely due to NMDA receptor blockade occurring during the testing time and support the hypothesis that memantine interferes with the hedonic/non-homeostatic mechanisms regulating food intake and food-seeking. The effect of post-session administration might be explained by the development of conditioned taste aversion.

Hypophagia induced by hindbrain serotonin is mediated through central GLP-1 signaling and involves 5-HT2C and 5-HT3 receptor activation

The overlap in neurobiological circuitry mediating the physiological and behavioral response to satiation and noxious/stressful stimuli are not well understood. The interaction between serotonin (5-HT) and glucagon-like peptide-1 (GLP-1) could play a role as upstream effectors involved in mediating associations between anorectic and noxious/stressful stimuli. We hypothesize that 5-HT acts as an endogenous modulator of the central GLP-1 system to mediate satiation and malaise in rats. Here, we investigate whether interactions between central 5-HT and GLP-1 signaling are behaviorally and physiologically relevant for the control of food intake and pica (i.e., behavioral measure of malaise). Results show that the anorexia and body weight changes induced by administration of exogenous hindbrain 5-HT are dependent on central GLP-1 receptor signaling. Furthermore, anatomical evidence shows mRNA expression of 5-HT2C and 5-HT3 receptors on GLP-1-producing preproglucagon (PPG) neurons in the medial nucleus tractus solitarius by fluorescent in situ hybridization, suggesting that PPG neurons are likely to express both of these receptors. Behaviorally, the hypophagia induced by the pharmacological activation of both of these receptors is also dependent on GLP-1 signaling. Finally, 5-HT3, but not 5-HT2C receptors, are required for the anorectic effects of the interoceptive stressor LiCl, suggesting the hypophagia induced by these 5-HT receptors may be driven by different mechanisms. Our findings highlight 5-HT as a novel endogenous modulator of the central GLP-1 system and suggest that the central interaction between 5-HT and GLP-1 is involved in the control of food intake in rats.

UCN3 suppresses food intake in coordination with CCK and the CCK2R in Siberian sturgeon (Acipenser baerii)

Urocortin-3 (UCN3) as a brain-gut peptide inhibits food intake of animal, but the underlying mechanism is not clear. To explore the appetite mechanism about the action of UCN3 in fish, intraperitoneal injection of UCN3 with CCK8, Lorglumide (CCK1R antagonist) or LY225910 (CCK2R antagonist) were conducted. Siberian sturgeon administrated with UCN3 and CCK8 showed a drastic reduction in food intake. The anorectic effect of UCN3 was significantly blocked by LY225910, but not affected by Lorglumide. Furthermore, LY225910 could effectively reverse appetite factor mRNA expressions, including cck, pyy, cart, npy, ucn3, apelin and nucb2 in the whole brain, stomach and intestinum valvula, but Lorglumide could only partially reverse these effects, suggesting the anorectic effect of UCN3 may be primarily mediated CCK2R in Siberian sturgeon. This study indicates for the first time in fish that UCN3 may inhibit food intake in coordination with CCK and CCK2R.

Distribution and characterisation of CCK containing enteroendocrine cells of the mouse small and large intestine

There is general consensus that enteroendocrine cells, EEC, containing the enteric hormone cholecystokinin (CCK) are confined to the small intestine and predominate in the duodenum and jejunum. Contrary to this, EEC that express the gene for CCK have been isolated from the large intestine of the mouse and there is evidence for EEC that contain CCK-like immunoreactivity in the mouse colon. However, the human and rat colons do not contain CCK cells. In the current study, we use immunohistochemistry to investigate CCK peptide presence in endocrine cells, PCR to identify cck transcripts and chromatography to identify CCK peptide forms in the mouse small and large intestine. The colocalisation of CCK and 5-HT, hormones that have been hypothesised to derive from cells of different lineages, was also investigated. CCK immunoreactivity was found in EEC throughout the mouse small and large intestine but positive cells were rare in the rectum. Immunoreactive EEC were as common in the caecum and proximal colon as they were in the duodenum and jejunum. CCK gene transcripts were found in the mucosa throughout the intestine but mRNA for gastrin, a hormone that can bind some anti-CCK antibodies, was only found in the stomach and duodenum. Characterisation of CCK peptides of the colon by extraction, chromatographic separation and radioimmunoassay revealed bioactive amidated and sulphated forms, including CCK-8 and CCK-33. Moreover, CCK-containing EEC in the large intestine bound antibodies that target the biologically active sulfated form. Colocalisation of CCK and 5-HT occurred in a proportion of EEC throughout the small intestine and in the caecum but these hormones were not colocalised in the colon, where there was CCK and PYY colocalisation. It is concluded that authentic, biologically active, CCK occurs in EEC of the mouse large intestine.

Signalling from the gut lumen

The lining of the gastrointestinal tract needs to be easily accessible to nutrients and, at the same time, defend against pathogens and chemical challenges. This lining is the largest and most vulnerable surface that faces the outside world. To manage the dual problems of effective nutrient conversion and defence, the gut lining has a sophisticated system for detection of individual chemical entities, pathogenic organisms and their products, and physico-chemical properties of its contents. Detection is through specific receptors that signal to the gut endocrine system, the nervous system, the immune system and local tissue defence systems. These effectors, in turn, modify digestive functions and contribute to tissue defence. Receptors for nutrients include taste receptors for sweet, bitter and savoury, free fatty acid receptors, peptide and phytochemical receptors, that are primarily located on enteroendocrine cells. Hormones released by enteroendocrine cells act locally, through the circulation and via the nervous system, to optimise digestion and mucosal health. Pathogen detection is both through antigen presentation to T-cells and through pattern-recognition receptors (PRRs). Activation of PRRs triggers local tissue defence, for example, by causing release of antimicrobials from Paneth cells. Toxic chemicals, including plant toxins, are sensed and then avoided, expelled or metabolised. It continues to be a major challenge to develop a comprehensive understanding of the integrated responses of the gastrointestinal tract to its luminal contents.

Association Study of Serotonin 3 Receptor Subunit Gene Variants in Antipsychotic-Induced Weight Gain

BACKGROUND

Schizophrenia (SCZ) is a chronic severe neuropsychiatric disorder, where pharmacological treatment has been hindered by adverse effects, including antipsychotic-induced weight gain (AIWG) and related complications. Genetic studies have been exploring the appetite regulation and energy homeostasis pathways in AIWG with some promising leads. The serotonin system has been shown to participate in these pathways.

METHODS

In the current study, we examined single nucleotide polymorphisms across the serotonin receptor genes HTR3A and HTR3B. Prospective weight change was assessed for a total of 149 SCZ patients of European ancestry.

RESULTS

We did not find the tested HTR3A or HTR3B gene markers to be associated with AIWG in our sample.

CONCLUSION

Our preliminary findings suggest that these receptors may not play a major role in predicting AIWG.

Altered intestinal neuroendocrine gene expression in humans with obesity

OBJECTIVE

Gastrointestinal hormones are critically involved in the regulation of food intake and body weight. Previous studies support an interplay between gastrointestinal hormones and the serotonergic system. This study explored intestinal neuroendocrine expression patterns in humans with obesity versus nonobese humans.

METHODS

Jejunum samples were collected from 164 humans with obesity (120 women; BMI (mean ± SD): 43.5 ± 6.6 kg/m(2) ) while they underwent Roux-en-Y gastric bypass surgery and from 18 nonobese humans (7 women; BMI: 23.5 ± 3.0 kg/m(2) ) undergoing distinct intestinal surgeries. mRNA expression of cholecystokinin (CCK), peptide YY3-36 (PYY), nesfatin1, ghrelin, ghrelin O-acyltransferase (GOAT), leptin, leptin receptor (leptinR), glucagon-like-peptide 1 receptor (GLP1R), serotonin transporter (SERT), tryptophan hydroxylase 1 (TPH1), and serotonin receptor 3A (5HT3A R) was determined with qRT-PCR. Ghrelin and GOAT protein expression was quantified using immunohistological stainings. Statistical analyses were performed with SPSS.

RESULTS

Jejunum samples from humans with obesity showed a higher expression of GOAT (mRNA and protein), TPH1, and SERT mRNA compared with the nonobese humans (all P < 0.05). Positive correlations were observed between TPH1, CCK, PYY, and nesfatin1 in nonobese and GOAT, ghrelin, TPH1, SERT, CCK, and PYY in humans with obesity (all P < 0.01).

CONCLUSIONS

Our top-down approach substantiates the dysregulation of jejunal neuroendocrine hormones in obesity.

Neuroendocrine control of satiation

Abstract Eating is a simple behavior with complex functions. The unconscious neuroendocrine process that stops eating and brings a meal to its end is called satiation. Energy homeostasis is mediated accomplished through the control of meal size via satiation. It involves neural integrations of phasic negative-feedback signals related to ingested food and tonic signals, such as those related to adipose tissue mass. Energy homeostasis is accomplished through adjustments in meal size brought about by changes in these satiation signals. The best understood meal-derived satiation signals arise from gastrointestinal nutrient sensing. Gastrointestinal hormones secreted during the meal, including cholecystokinin, glucagon-like peptide 1, and PYY, mediate most of these. Other physiological signals arise from activation of metabolic-sensing neurons, mainly in the hypothalamus and caudal brainstem. We review both classes of satiation signal and their integration in the brain, including their processing by melanocortin, neuropeptide Y/agouti-related peptide, serotonin, noradrenaline, and oxytocin neurons. Our review is not comprehensive; rather, we discuss only what we consider the best-understood mechanisms of satiation, with a special focus on normally operating physiological mechanisms.

Role of 5-HT3 receptor on food intake in fed and fasted mice

Background

Many studies have shown that 5-hydroxytryptamine (5-HT) receptor subtypes are involved in the regulation of feeding behavior. However, the relative contribution of 5-HT₃ receptor remains unclear. The present study was aimed to investigate the role of 5-HT₃ receptor in control of feeding behavior in fed and fasted mice.

Methodology/Principal Findings

Food intake and expression of c-Fos, tyrosine hydroxylase (TH), proopiomelanocortin (POMC) and 5-HT in the brain were examined after acute treatment with 5-HT₃ receptor agonist SR-57227 alone or in combination with 5-HT₃ receptor antagonist ondansetron. Food intake was significantly inhibited within 3 h after acute treatment with SR 57227 in fasted mice but not fed mice, and this inhibition was blocked by ondansetron. Immunohistochemical study revealed that fasting-induced c-Fos expression was further enhanced by SR 57227 in the brainstem and the hypothalamus, and this enhancement was also blocked by ondansetron. Furthermore, the fasting-induced downregulation of POMC expression in the hypothalamus and the TH expression in the brain stem was blocked by SR 57227 in the fasted mice, and this effect of SR 57227 was also antagonized by ondansetron.

Conclusion/Significance

Taken together, our findings suggest that the effect of SR 57227 on the control of feeding behavior in fasted mice may be, at least partially, related to the c-Fos expression in hypothalamus and brain stem, as well as POMC system in the hypothalamus and the TH system in the brain stem.

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.

Dopamine-induced hypophagia is mediated by D1 and 5HT-2c receptors in chicken

The present study was designed to examine the effects of intracerebroventricular (ICV) injection of Dopamine (10, 20 and 40 nmol), L-DOPA (dopamine precursor; 62.5, 125 and 250 nmol), 6-OHDA (dopamine inhibitor; 75, 150 and 300 nmol), SCH 23390 (D1 antagonist; 2.5, 5 and 10 nmol), AMI-193 (D2 antagonist; 2.5, 5 and 10 nmol), NGB2904 (D3 antagonist; 3.2, 6.4 and 12.8 nmol), L-741 T742 (D4 antagonist; 1.5, 3 and 6 nmol) on food intake in FD3 chickens. At following, birds were ICV injected using 8-OH-DPAT (5-HT1A agonist; 15.25 nmol) and SB242084 (5-HT2C antagonist; 1.5 μg) prior dopamine (40 nmol) injection. Cumulative food intake was determined until 3 h post-injection. According to the results, dopamine significantly decreased food intake in chickens (p<0.05). The inhibitory effect of dopamine on food intake was decreased by SCH 23390 pretreatment (P<0.05), but AMI-193, NGB2904 and L-741, 742 had no effect on food intake induced by dopamine. In addition, hypophagic effect of dopamine was attenuated by SB242084 (P<0.05), but 8-OH-DPAT had no effect. These results suggest that dopamine decrease food intake via D1 receptor and there is an interaction between dopaminergic and serotonergic systems via 5-HT2C receptor in chickens.

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.

Current and emerging concepts on the role of peripheral signals in the control of food intake and development of obesity

The gastrointestinal peptides are classically known as short-term signals, primarily inducing satiation and/or satiety. However, accumulating evidence has broadened this view, and their role in long-term energy homeostasis and the development of obesity has been increasingly recognised. In the present review, the recent research involving the role of satiation signals, especially ghrelin, cholecystokinin, glucagon-like peptide 1 and peptide YY, in the development and treatment of obesity will be discussed. Their activity, interactions and release profile vary constantly with changes in dietary and energy influences, intestinal luminal environment, body weight and metabolic status. Manipulation of gut peptides and nutrient sensors in the oral and postoral compartments through diet and/or changes in gut microflora or using multi-hormone 'cocktail' therapy are among promising approaches aimed at reducing excess food consumption and body-weight gain.

Regulation of energy balance and body weight by the brain: a distributed system prone to disruption

Maintaining adequate energy supply via regulation of food intake and energy expenditure is crucial for survival and reproduction. The neural control of energy balance is highly complex, occurs across distributed central and peripheral areas, and incorporates multiple domains of control (including homeostatic and hedonic processes). The sheer number of active compounds (such as leptin and GLP-1) involved in the regulation of food intake speaks to the redundancy and complexity of the system. The balance between energy intake and expenditure is under CNS control. Constant bidirectional communication between the brain and the GI tract, as well as between the brain and other relevant tissues (ie, adipose tissue, pancreas, and liver), ensures that the brain constantly perceives and responds accordingly to the energy status/needs of the body. This elegant biological system is subject to disruption by a toxic obesogenic environment, leading to syndromes such as leptin and insulin resistance, and ultimately further exposing obese individuals to further weight gain and T2DM. Recent imaging studies in humans are beginning to examine the influence that higher-order/hedonic brain regions have on homeostatic areas, as well as their responsiveness to homeostatic peripheral signals. With greater understanding of these mechanisms, the field moves closer to understanding and eventually treating the causalities of obesity.

Gut fat sensing in the negative feedback control of energy balance--recent advances

Infusions of lipids into the small intestine potently suppress ongoing feeding. Prior work has identified potential roles for gut extrinsic vagal and non-vagal sensory innervation in mediating the ability of gut lipid infusions to reduce food intake, but the local biochemical processes underlying gut lipid sensing at the level of the small intestine remain unclear. This manuscript will summarize recent progress in the identification and characterization of several candidate gut lipid sensing molecules important in the negative feedback control of ingestion, including the fatty acid translocase CD36, peroxisome proliferator-activated receptor alpha (PPAR-α), and the fatty acid ethanolamide oleoylethanolamide (OEA). In addition, this manuscript addresses a larger role for gut lipid sensing in the overall control of energy availability by modulating not only food intake but also hepatic glucose production.

Ongoing ingestive behavior is rapidly suppressed by a preabsorptive, intestinal "bitter taste" cue

The discovery that cells in the gastrointestinal (GI) tract express the same molecular receptors and intracellular signaling components known to be involved in taste has generated great interest in potential functions of such post-oral "taste" receptors in the control of food intake. To determine whether taste cues in the GI tract are detected and can directly influence behavior, the present study used a microbehavioral analysis of intake, in which rats drank from lickometers that were programmed to simultaneously deliver a brief yoked infusion of a taste stimulus to the intestines. Specifically, in daily 30-min sessions, thirsty rats with indwelling intraduodenal catheters were trained to drink hypotonic (0.12 M) sodium chloride (NaCl) and simultaneously self-infuse a 0.12 M NaCl solution. Once trained, in a subsequent series of intestinal taste probe trials, rats reduced licking during a 6-min infusion period, when a bitter stimulus denatonium benzoate (DB; 10 mM) was added to the NaCl vehicle for infusion, apparently conditioning a mild taste aversion. Presentation of the DB in isomolar lithium chloride (LiCl) for intestinal infusions accelerated the development of the response across trials and strengthened the temporal resolution of the early licking suppression in response to the arrival of the DB in the intestine. In an experiment to evaluate whether CCK is involved as a paracrine signal in transducing the intestinal taste of DB, the CCK-1R antagonist devazepide partially blocked the response to intestinal DB. In contrast to their ability to detect and avoid the bitter taste in the intestine, rats did not modify their licking to saccharin intraduodenal probe infusions. The intestinal taste aversion paradigm developed here provides a sensitive and effective protocol for evaluating which tastants-and concentrations of tastants-in the lumen of the gut can control ingestion.

Influence of the 5-HT3 receptor antagonist ondansetron on gastric sensorimotor function and nutrient tolerance in healthy volunteers

BACKGROUND

Serotonin is believed to be involved in the regulation of the gastric accommodation reflex in man however which receptor subtype(s) are involved remains to be elucidated.

METHODS

Eleven healthy subjects (nine men, age 19-30) underwent a gastric barostat and a drinking test after treatment with either placebo or ondansetron (8 mg intravenously). During the barostat protocol an intragastric flaccid bag was stepwise distended (2 mmHg increments 2 min) to determine gastric compliance and sensitivity to distention. Subsequently, the pressure level was set at intra-abdominal pressure +2 mmHg while volume was followed before and after administration of a liquid meal (200 mL; 300 kcal). During the drink test volunteers drank at a rate of 15 mL min(-1) until maximal satiation. Results (mean ± SEM) were compared using t-tests and mixed model analysis.

KEY RESULTS

Gastric compliance was not significantly altered by ondansetron (51.5 ± 5.6 vs 49.2 ± 5.2 mL mmHg(-1)), neither were the pressure thresholds for first perception or discomfort. Ondansetron treatment did not affect basal gastric tone (173 ± 14 vs 156 ± 12 mL), neither did it affect the amplitude of the meal-induced relaxation (160 ± 52 vs 131 ± 43 mL) or the maximum volume increase after the meal (264 ± 54 mL vs 234 ± 51 mL). During the drinking test the amount of liquid meal ingested at maximum satiation was significantly increased by ondansetron (784 ± 74 vs 907 ± 64 mL, P < 0.05).

CONCLUSIONS & INFERENCES

These data suggest that 5-HT acting at 5-HT(3) receptors is not involved in the control of gastric sensorimotor function, but contributes to the regulation of hunger and satiation in man.

Impaired intestinal afferent nerve satiety signalling and vagal afferent excitability in diet induced obesity in the mouse

Gastrointestinal vagal afferents transmit satiety signals to the brain via both chemical and mechanical mechanisms. There is indirect evidence that these signals may be attenuated in obesity. We hypothesized that responses to satiety mediators and distension of the gut would be attenuated after induction of diet induced obesity. Obesity was induced by feeding a high fat diet (60% kcal from fat). Low fat fed mice (10% kcal from fat) served as a control. High fat fed mice were obese, with increased visceral fat, but were not hyperglycaemic. Recordings from jejunal afferents demonstrated attenuated responses to the satiety mediators cholecystokinin (CCK, 100 nm) and 5-hydroxytryptamine (5-HT, 10 μm), as was the response to low intensity jejunal distension, while responses to higher distension pressures were preserved. We performed whole cell patch clamp recordings on nodose ganglion neurons, both unlabelled, and those labelled by fast blue injection into the wall of the jejunum. The cell membrane of both labelled and unlabelled nodose ganglion neurons was less excitable in HFF mice, with an elevated rheobase and decreased number of action potentials at twice rheobase. Input resistance of HFF neurons was also significantly decreased. Calcium imaging experiments revealed reduced proportion of nodose ganglion neurons responding to CCK and 5-HT in obese mice. These results demonstrate a marked reduction in afferent sensitivity to satiety related stimuli after a chronic high fat diet. A major mechanism underlying this change is reduced excitability of the neuronal cell membrane. This may explain the development of hyperphagia when a high fat diet is consumed. Improving sensitivity of gastrointestinal afferent nerves may prove useful to limit food intake in obesity.

Naturally occurring variants in the HTR3B gene significantly alter properties of human heteromeric 5-hydroxytryptamine-3A/B receptors

OBJECTIVES

The 5-hydroxytryptamine-3 (5-HT3) receptor, a ligand-gated ion channel, is known to be involved in gut motility and peristalsis, the mediation of pain and psychiatric diseases. 5-HT3 receptor antagonists are effectively used to treat chemotherapy-induced emesis and irritable bowel syndrome. We have characterized the impact of four naturally occurring variants in the HTR3B gene leading to amino acid exchanges within the respective subunit of heteromeric 5-HT3A/B receptors on a functional and expressional level.

METHODS AND RESULTS

For functional characterization, a Ca influx assay based on aequorin bioluminescence was used. Radioligand-binding studies with the 5-HT3 receptor antagonist [H]GR65630 were carried out to determine expression levels of heteromeric 5-HT3A/B receptors. Transiently transfected human embryonic kidney 293 cells using 5-HT3A and 5-HT3B complementary DNA constructs were shown to coexpress homopentameric 5-HT3A next to heteromeric 5-HT3A/B receptors. The variant p.V183I decreased surface expression, whereas p.Y129S and p.S156R led to pronounced increases of 5-HT maximum responses, despite nearly unaltered surface expression levels of heteromeric 5-HT3A/B receptors.

CONCLUSION

These results may help to explain earlier reported association findings of the frequent p.Y129S and p.V183I variants with psychiatric diseases. Replication studies with larger sample pools, especially regarding the rare p.S156R variant would be useful, to obtain an idea about the predisposing role of these single nucleotide polymorphisms as susceptibility variants.

How cytokines can influence the brain: a role for chemokines?

Following inflammation or infection, cytokines are released in the blood. Besides their effect on the immune system, cytokines can also act in the brain to modulate our behaviors, inducing for example anorexia when produced in large amount. This review focuses on our current knowledge on how cytokines can influence the brain and the behaviors through several possible pathways: modulating peripheral neurons which project to the brain through the vagus nerve, modulating the levels of hormones such as leptin which can act to the brain through the humoral pathway and possibly acting directly in the brain, through the local production of cytokines and chemokines such as SDF-1alpha/CXCL12.

Urocortins and cholecystokinin-8 act synergistically to increase satiation in lean but not obese mice: involvement of corticotropin-releasing factor receptor-2 pathway

Interactions between gastrointestinal signals are a part of integrated systems regulating food intake (FI). We investigated whether cholecystokinin (CCK)-8 and urocortin systems potentiate each other to inhibit FI and gastric emptying (GE) in fasted mice. Urocortin 1 and urocortin 2 (1 microg/kg) were injected ip alone or with CCK (3 microg/kg) in lean, diet-induced obese (DIO) or corticotropin-releasing factor receptor-2 (CRF(2))-deficient mice. Gastric vagal afferent activity was recorded from a rat stomach-vagus in vitro preparation. When injected separately, urocortin 1, urocortin 2, or CCK did not modify the 4-h cumulative FI in lean mice. However, CCK plus urocortin 1 or CCK plus urocortin 2 decreased significantly the 4-h FI by 39 and 27%, respectively, compared with the vehicle + vehicle group in lean mice but not in DIO mice. Likewise, CCK-urocortin-1 delayed GE in lean but not DIO mice, whereas either peptide injected alone at the same dose had no effect. CCK-urocortin 2 suppression of FI was observed in wild-type but not CRF(2)-deficient mice. Gastric vagal afferent activity was increased by intragastric artery injection of urocortin 2 after CCK at a subthreshold dose, and the response was reversed by devazepide. These data establish a peripheral synergistic interaction between CCK and urocortin 1 or urocortin 2 to suppress FI and GE through CRF(2) receptor in lean mice that may involve CCK modulation of gastric vagal afferent responsiveness to urocortin 2. Such synergy is lost in DIO mice, suggesting a resistance to the satiety signaling that may contribute to maintain obesity.

Intestinal nutrients elicit satiation through concomitant activation of CCK1 and 5-HT3 receptors

Previous studies demonstrate that cholecystokinin type-1 (CCK(1)) and serotonin type-3 (5-HT(3)) dependent pathways are independently involved in intestinal nutrient-induced meal termination. In the current study, we employed selective antagonists to investigate the relative contribution of CCK(1) and 5-HT(3) receptors in mediating the anorexia produced by duodenal infusion of Polycose or Intralipid in rats. Combined administration of 1 mg/kg ondansetron (Ond) and 1 mg/kg devazepide (Dev) reversed 132 mM Polycose-induced suppression to the level of control intake and significantly attenuated 263 mM Polycose-induced suppression greater than either antagonist alone. Similar results were observed when subthreshold doses of Ond (500 microg/kg) and Dev (5 microg/kg) were co-administered prior to 263 mM Polycose infusion. Suppression of intake resulting from 130 mM Intralipid was reversed to the level of control when Ond and Dev were co-administered at both independent effective doses (1 mg/kg each) and subthreshold doses (500 microg/kg and 5 microg/kg, respectively). Finally, combined administration of the antagonists increased sucrose intakes beyond intakes following control or treatment with either antagonist alone when rats were infused with saline. These data demonstrate that intestinal carbohydrates and lipids inhibit food intake through simultaneous CCK(1) and 5-HT(3) receptor activation and that these receptors appear to completely mediate the Intralipid-induced suppression of intake.

Naturally occurring variations in the human 5-HT3A gene profoundly impact 5-HT3 receptor function and expression

BACKGROUND

The serotonin [5-hydroxytryptamine (5-HT)]-gated ion channel 5-HT3 is involved in the mediation of postoperative and radiotherapy/chemotherapy-induced nausea/emesis and in irritable bowel syndrome. It has also been suggested to play a role in various psychiatric diseases. Five naturally occurring single nucleotide polymorphisms leading to amino acid changes have been identified in the human 5-HT3A gene.

METHODS AND RESULTS

We investigated the functional effects of these polymorphisms on the 5-HT3A receptor using fluorescence-based cellular assays. Notably, variants A33T, S253N, and M257I displayed 5-HT-induced maximal responses of 3-64% of the wild-type response, whereas R344H and P391R exhibited wild-type-like function. All variants displayed wild-type-like potencies of 5-HT and three 5-HT3 antagonists. Furthermore, all variants displayed Kd values similar to that of the wild-type receptor in a [H]GR65630-binding assay. The surface expression of A33T, M257I, and R344H was reduced 2-4-fold compared with the wild-type, despite similar total expression levels. Finally, coexpression of wild-type 5-HT3A or 5-HT3B subunits with 5-HT3A variants A33T, S253N, or M257I resulted in mixed or heteromeric receptors, characterized by significantly reduced maximal responses to 5-HT compared with the wild-type receptors.

CONCLUSIONS

Three polymorphisms of the 5-HT3A gene gave rise to functionally impaired receptors whose function could not be rescued by either wild-type 5-HT3A or 5-HT3B. Three of the variant receptors were surface-expressed at reduced levels in spite of total expression levels similar to wild-type, indicating that these variants affect receptor biogenesis and/or trafficking. These severe single nucleotide polymorphism effects hold promise for identification of new 5-HT3A gene-disease causalities.

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.

Lack of interaction between peripheral injection of CCK and obestatin in the regulation of gastric satiety signaling in rodents

Obestatin is a new peptide for which anorexigenic effects were recently reported in mice. We investigate whether peripheral injection of obestatin or co-injection with cholecystokinin (CCK) can modulate food intake, gastric motor function (intragastric pressure and emptying) and gastric vagal afferent activity in rodents. Obestatin (30, 100 and 300 microg/kg, i.p.) did not influence cumulative food intake for the 2h post-injection in rats or mice nor gastric emptying in rats. In rats, obestatin (300 microg/kg) did not modify CCK (1 microg/kg, i.p.)-induced significant decrease in food intake (36.6%) and gastric emptying (31.0%). Furthermore, while rats injected with CCK (0.3 microg/kg, i.v.) displayed gastric relaxation, no change in gastric intraluminal pressure was elicited by obestatin (300 microg/kg, i.v.) pre- or post-CCK administration. In in vitro rat gastric vagal afferent preparations, 20 units that had non-significant changes in basal activity after obestatin at 30 microg responded to CCK at 10 ng by a 182% increase. These data show that obestatin neither influences cumulative food intake, gastric motility or vagal afferent activity nor CCK-induced satiety signaling.

Dorsal hindbrain 5-HT3 receptors participate in control of meal size and mediate CCK-induced satiation

We have previously shown that systemic administration of ondansetron, a selective serotonin type-3 (5-HT3) receptor antagonist, attenuates cholecystokinin (CCK)-induced suppression of food intake. The exact location of 5-HT3 receptors mediating this action is not clear and may involve hindbrain 5-HT3 receptors. In this study, we first examined sucrose intake in response to direct injections of ondansetron into various sites of the dorsal hindbrain. Ondansetron (1.0 and 2.0 microg/100 nl) delivered into the medial nucleus of the solitary tract (NTS) significantly increased 15% sucrose intake (12.2 +/- 0.6 and 13.5 +/- 0.7 ml, respectively) compared to control (10.2 +/- 0.7 ml), while equivalent injections into ipsilateral adjacent sites such as the lateral NTS, dorsal medial nucleus of the vagus, and other areas of the dorsal hindbrain had no effect on sucrose intake. Second, we examined the effects of hindbrain 5-HT3 receptor blockade on suppression of intake by systemic CCK. Fourth ventricular (i.c.v.) administration of ondansetron (10.0 microg/3.0 microl) significantly attenuated suppression of intake by CCK (9.1 +/- 1.0 vs. 6.4 +/- 0.4 ml, respectively). Ondansetron alone had no effect on sucrose intake at any i.c.v. dose tested. In a separate group of rats, CCK administration suppressed 60-min intake significantly (8.9 +/- 0.8 ml) compared to control (12.4 +/- 0.4 ml). Administration of ondansetron into the medial NTS completely reversed suppression of intake by CCK (11.8 +/- 1.0 and 12.3 +/- 1.4 ml, for 0.5 microg and 1.0 microg/100 nl, respectively). These data demonstrate that 5-HT3 receptors located in the medial NTS participate in control of meal size and mediate CCK-induced suppression of food intake.

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

We have previously shown that serotonin type-3 (5-HT3) receptors mediate cholecystokinin (CCK)-induced satiation and that this effect is dependent on postoropharyngeal feedback. However, the independent contributions of gastric and intestinal feedback in 5-HT3 receptor mediation of suppression of food intake by CCK have not been determined. Using a sham-feeding preparation combined with intraduodenal sucrose infusion, we show that blockade of 5-HT3 receptors by ondansetron (1 mg/kg ip) had no effect on suppression of sham feeding by intraduodenal 15% sucrose infusion (4 ml/10 min), CCK (2 μg/kg ip) administration, or the combination of the two treatments. In separate experiments consisting of either sham-feeding rats that received gastric distension with the use of a balloon or real-feeding rats whose stomachs were distended using gastric loads of saline after the occlusion of the pylorus, we tested the hypothesis that gastric feedback signals are necessary for activation of 5-HT3 receptors. Ondansetron significantly attenuated suppression of sham sucrose intake after a 10-ml gastric balloon distension (30.5 ± 2.2 vs. 20.2 ± 2.2 ml, respectively) and gastric distension combined with CCK (21.9 ± 1.4 vs. 12.0 ± 1.7 ml, respectively). When intestinal feedback was eliminated in a real-feeding paradigm by closing the pylorus using a cuff preparation, ondansetron attenuated suppression of sucrose intake produced by a 10-ml saline gastric load (6.8 ± 0.7 vs. 4.2 ± 0.4 ml, respectively). Finally, when CCK (1 μg/kg) was administered in combination with a 5-ml saline gastric load in a real-feeding preparation, ondansetron significantly attenuated suppression of sucrose intake by CCK (9.0 ± 0.9 vs. 6.3 ± 0.5 ml, respectively), as well as the enhanced suppression of intake by CCK plus gastric load (6.9 ± 0.6 vs. 4.6 ± 0.5 ml, respectively). These findings demonstrate that CCK-induced activation of 5-HT3 receptors requires gastric, but not intestinal feedback.

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.