Multiple cholecystokinin (CCK) receptors and CCK-monoamine interactions are instrumental in the control of feeding

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.

Distributions and relationships of chemically defined enteroendocrine cells in the rat gastric mucosa

This paper provides quantitative data on the distributions of enteroendocrine cells (EEC), defined by the hormones they contain, patterns of colocalisation between hormones and EEC relations to nerve fibres in the rat gastric mucosa. The rat stomach has three mucosal types: non-glandular stratified squamous epithelium of the fundus and esophageal groove, a region of oxyntic glands in the corpus, and pyloric glands of the antrum and pylorus. Ghrelin and histamine were both contained in closed cells, not contacting the lumen, and were most numerous in the corpus. Gastrin cells were confined to the antrum, and 5-hydroxytryptamine (5-HT) and somatostatin cells were more frequent in the antrum than the corpus. Most somatostatin cells had basal processes that in the antrum commonly contacted gastrin cells. Peptide YY (PYY) cells were rare and mainly in the antrum. The only numerous colocalisations were 5-HT and histamine, PYY and gastrin and gastrin and histamine in the antrum, but each of these populations was small. Peptide-containing nerve fibres were found in the mucosa. One of the most common types was vasoactive intestinal peptide (VIP) fibres. High-resolution analysis showed that ghrelin cells were closely and selectively approached by VIP fibres. In contrast, gastrin cells were not selectively innervated by VIP or CGRP fibres. The study indicates that there are distinct populations of gastric EEC and selective innervation of ghrelin cells. It also shows that, in contrast to EEC of the small intestine, the majority of EEC within the stomach contained only a single hormone.

Heterogeneity of enterochromaffin cells within the gastrointestinal tract

Enterochromaffin cells were the first endocrine cells of the gastrointestinal tract to be chemically distinguished, almost 150 years ago. It is now known that the chromaffin reaction of these cells was due to their content of the reactive aromatic amine, 5-hydroxytryptamine (5-HT, also known as serotonin). They have commonly been thought to be a special class of gut endocrine cells (enteroendocrine cells) that are distinct from the enteroendocrine cells that contain peptide hormones. The study by Martin et al. in the current issue of this journal reveals that the patterns of expression of nutrient receptors and transporters differ considerably between chromaffin cells of the mouse duodenum and colon. However, even within regions, chromaffin cells differ; in the duodenum there are chromaffin cells that contain both secretin and 5-HT, cholecystokinin and 5-HT, and all three of secretin, cholecystokinin, and 5-HT. Moreover, the ratios of these different cell types differ substantially between species. And, in terms of function, 5-HT has many roles, including in appetite, motility, fluid secretion, release of digestive enzymes and bone metabolism. The paper thus emphasizes the need to define the many different classes of enterochromaffin cells and relate this to their roles.

The cholecystokinin-1 receptor antagonist devazepide increases cholesterol cholelithogenesis in mice

BACKGROUND

A defect in gallbladder contraction function plays a key role in the pathogenesis of gallstones. The cholecystokinin-1 receptor (CCK-1R) antagonists have been extensively investigated for their therapeutic effects on gastrointestinal and metabolic diseases in animal studies and clinical trials. However, it is still unknown whether they have a potential effect on gallstone formation.

DESIGN

To study whether the CCK-1R antagonists enhance cholelithogenesis, we investigated cholesterol crystallization, gallstone formation, hepatic lipid secretion, gallbladder emptying function and intestinal cholesterol absorption in male C57BL/6J mice treated by gavage with devazepide (4 mg/day/kg) or vehicle (as controls) twice per day and fed the lithogenic diet for 21 days.

RESULTS

During 21 days of feeding, oral administration of devazepide significantly accelerated cholesterol crystallization and crystal growth to microlithiasis, with 40% of mice forming gallstones, whereas only agglomerated cholesterol monohydrate crystals were found in mice receiving vehicle. Compared to the vehicle group, fasting and postprandial residual gallbladder volumes in response to the high-fat meal were significantly larger in the devazepide group during cholelithogenesis, showing reduced gallbladder emptying and bile stasis. Moreover, devazepide significantly increased hepatic secretion of biliary cholesterol, but not phospholipids or bile salts. The percentage of intestinal cholesterol absorption was higher in devazepide-treated mice, increasing the bioavailability of chylomicron-derived cholesterol in the liver for biliary hypersecretion into bile. These abnormalities induced supersaturated bile and rapid cholesterol crystallization.

CONCLUSIONS

The potent CCK-1R antagonist devazepide increases susceptibility to gallstone formation by impairing gallbladder emptying function, disrupting biliary cholesterol metabolism and enhancing intestinal cholesterol absorption in mice.

The chemical coding of 5-hydroxytryptamine containing enteroendocrine cells in the mouse gastrointestinal tract

The majority of 5-HT (serotonin) in the body is contained in enteroendocrine cells of the gastrointestinal mucosa. From the time of their discovery over 80 years ago, the 5-HT-containing cells have been regarded as a class of cell that is distinct from enteroendocrine cells that contain peptide hormones. However, recent studies have cast doubt on the concept of there being distinct classes of enteroendocrine cells, each containing a single hormone or occasionally more than one hormone. Instead, data are rapidly accumulating that there are complex patterns of colocalisation of hormones that identify multiple subclasses of enteroendocrine cells. In the present work, multiple labelling immunohistochemistry is used to investigate patterns of colocalisation of 5-HT with enteric peptide hormones. Over 95 % of 5-HT cells in the duodenum also contained cholecystokinin and about 40 % of them also contained secretin. In the jejunum, about 75 % of 5-HT cells contained cholecystokinin but not secretin and 25 % contained 5-HT plus both cholecystokinin and secretin. Small proportions of 5-HT cells contained gastrin or somatostatin in the stomach, PYY or GLP-1 in the small intestine and GLP-1 or somatostatin in the large intestine. Rare or very rare 5-HT cells contained ghrelin (stomach), neurotensin (small and large intestines), somatostatin (small intestine) and PYY (in the large intestine). It is concluded that 5-HT-containing enteroendocrine cells are heterogeneous in their chemical coding and by implication in their functions.

Peripheral injected cholecystokinin-8S modulates the concentration of serotonin in nerve fibers of the rat brainstem

Serotonin and cholecystokinin (CCK) play a role in the short-term inhibition of food intake. It is known that peripheral injection of CCK increases c-Fos-immunoreactivity (Fos-IR) in the nucleus of the solitary tract (NTS) in rats, and injection of the serotonin antagonist ondansetron decreases the number of c-Fos-IR cells in the NTS. This supports the idea of serotonin contributing to the effects of CCK. The aim of the present study was to elucidate whether peripherally injected CCK-8S modulates the concentration of serotonin in brain feeding-regulatory nuclei. Ad libitum fed male Sprague-Dawley rats received 5.2 and 8.7 nmol/kg CCK-8S (n=3/group) or 0.15M NaCl (n=3-5/group) injected intraperitoneally (ip). The number of c-Fos-IR neurons, and the fluorescence intensity of serotonin in nerve fibers were assessed in the paraventricular nucleus (PVN), arcuate nucleus (ARC), NTS and dorsal motor nucleus of the vagus (DMV). CCK-8S increased the number of c-Fos-ir neurons in the NTS (mean±SEM: 72±4, and 112±5 neurons/section, respectively) compared to vehicle-treated rats (7±2 neurons/section, P<0.05), but did not modulate c-Fos expression in the DMV or ARC. Additionally, CCK-8S dose-dependently increased the number of c-Fos-positive neurons in the PVN (218±15 and 128±14, respectively vs. 19±5, P<0.05). In the NTS and DMV we observed a decrease of serotonin-immunoreactivity 90 min after injection of CCK-8S (46±2 and 49±8 pixel/section, respectively) compared to vehicle (81±8 pixel/section, P<0.05). No changes of serotonin-immunoreactivity were observed in the PVN and ARC. Our results suggest that serotonin is involved in the mediation of CCK-8's effects in the brainstem.

Modulatory role of serotonin on feeding behavior

The appearance, the odor, and the flavor of foods, all send messages to the encephalic area of the brain. The hypothalamus, in particular, plays a key role in the mechanisms that control the feeding behavior. These signals modulate the expression and the action of anorexigenic or orexigenic substances that influence feeding behavior. The serotonergic system of neurotransmission consists of neurons that produce and liberate serotonin as well as the serotonin-specific receptor. It has been proven that some serotonergic drugs are effective in modulating the mechanisms of control of feeding behavior. Obesity and its associated illnesses have become significant public health problems. Some drugs that manipulate the serotonergic systems have been demonstrated to be effective interventions in the treatment of obesity. The complex interplay between serotonin and its receptors, and the resultant effects on feeding behavior have become of great interest in the scientific community.

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

Cholecystokinin (CCK) and serotonin (5-HT) systems have been shown to cooperate interdependently in control of food intake. To assess mechanisms by which CCK and 5-HT systems interact in control of food intake we examined: (1) participation of CCK-1 and 5-HT3 receptors in 5-HT-induced suppression of sucrose intake; (2) the interaction between CCK and 5-HT in suppression of food intake; (3) the role of CCK-1 and 5-HT3 receptors in mediating this interaction. Intraperitoneal administration of 5-HT (0.25, 0.5 and 1.0 mg/kg) significantly reduced intake compared to control in a dose responsive fashion (r2=0.989). Suppression of food intake by 5-HT was significantly attenuated by prior treatment with the 5-HT3 receptor antagonist ondansetron at each 5-HT dose tested (P<0.05), while blockade of CCK-1 receptors by lorglumide had no effect on 5-HT-induced suppression of intake. Administration of CCK-8 (0.5 microg/kg) or 5-HT (0.5 mg/kg) alone significantly reduced sucrose intake by 22.9 and 22.2% respectively, compared to control (P<0.0001). Co-administration of CCK and 5-HT resulted in a synergistic suppression of intake leading to an overall 48.4% reduction in sucrose intake compared to saline (P<0.0001). Concomitant CCK-1 and 5-HT3 receptor blockade by lorglumide and ondansetron respectively, resulted in a complete reversal of the combined CCK and 5-HT-induced suppression of intake. Independent administration of lorglumide or ondansetron did not alter intake compared to control. These studies provide evidence that 5-HT causes suppression in food intake by acting at 5-HT3, not CCK-1 receptors. Furthermore, CCK and 5-HT interact to produce an enhanced suppression of food intake, an effect mediated through concomitant activation of CCK-1 and 5-HT3 receptors.

Serotonin-type 3 receptors mediate intestinal Polycose- and glucose-induced suppression of intake

Ondansetron, a selective serotonin-type 3 (5-HT(3)) receptor antagonist, was used to test the hypothesis that duodenal infusion of isosmotic solutions of Polycose or its hydrolytic product glucose suppressed intake through 5-HT(3) receptors. Polycose suppressed sucrose intake across both concentrations infused (132 mM, 7.6 +/- 0.6 ml; 263 mM, 2.3 +/- 0.5 ml), compared with intake under control conditions (12.6 +/- 0.3 ml, P <0.001). Pretreatment with 1.0 mg/kg ondansetron attenuated reduction of sucrose intake induced only by the highest concentration of Polycose (4.6 +/- 0.8 ml, P = 0.004). Dose-response testing revealed that suppression of food intake by 263 mM Polycose was equally attenuated by ondansetron administered at 1.0, 2.0, and 5.0 mg/kg but not when given at 0.125, 0.25, and 0.5 mg/kg. Acarbose, an alpha-glucosidase inhibitor, attenuated Polycose-induced suppression of food intake, and pretreatment with 1.0 mg/kg ondansetron had no further effect. Suppression of intake after 990 mM glucose but not mannitol infusion was attenuated by pretreatment with 1.0 mg/kg ondansetron. The competitive SGLT(1) inhibitor, phloridzin, had no effect on 60-min 990 mM glucose-induced suppression of intake or the ability of ondansetron to attenuate this suppression of intake. Conversely, glucose-induced suppression of intake was attenuated by phloridzin at earlier time points and further attenuated when rats were pretreated with 1.0 mg/kg ondansetron. Ondansetron administration alone had no effect on intake at any dose tested. We conclude that 5-HT(3) receptors participate in the inhibition of food intake by intraduodenal infusion of carbohydrate solutions through a posthydrolytic, preabsorptive mechanism.

Cholecystokinin-induced satiety is mediated through interdependent cooperation of CCK-A and 5-HT3 receptors

Recent evidence supports a role for the serotonin-3 (5-HT3) receptors in the modulation of cholecystokinin (CCK)-induced satiation. Likewise, 5-HT's anorectic response has been linked to recruitment of peripheral CCK-A receptors. Evidence to date, however, does not elucidate whether there is a concomitant interaction between CCK-A and 5-HT3 receptors or whether each receptor functions independently in the negative feedback control of food intake elicited by CCK. In the present study, we used selective receptor antagonists to investigate the roles of CCK-A and 5-HT3 receptors in CCK-induced satiation. Intraperitoneal administration of CCK-8 reduced 30-min 15% sucrose intake in a dose-responsive manner. Prior treatment with ondansetron (1.0 mg/kg ip), a highly selective 5-HT3 receptor antagonist, attenuated CCK-induced suppression of food intake in a dose-responsive manner. Pretreatment with lorglumide (1.0 mg/kg ip), a selective CCK-A receptor antagonist, reversed CCK-induced inhibition of sucrose intake. Finally, simultaneous blockade of CCK-A and 5-HT3 receptors by lorglumide and ondansetron, as well as concomitant administration of the two antagonists with CCK, produced a significant synergistic increase in sucrose intake compared with intakes after administration of saline, CCK, or either antagonist alone. These findings support evidence that CCK-A and 5-HT3 receptors cooperate interdependently in control of short-term food intake. Most likely, this interconnection exists through a feed-forward parallel model arising from CCK-A and 5-HT3 receptors, where activation of one system engages the other to intensify the overall satiety signal.

5-HT3 receptors participate in CCK-induced suppression of food intake by delaying gastric emptying

Serotonin type 3 (5-HT(3)) receptors have been shown to participate in the negative-feedback control of food intake. We previously reported that cholecystokinin (CCK)-induced suppression of food intake is partly mediated through 5-HT(3) receptors when rats were tested on a preferred liquid diet, but whether such an effect occurs when they are tested on a solid maintenance diet is unknown. In the present study, we examined the effects of ondansetron, a selective 5-HT(3) antagonist, on CCK-induced suppression of solid chow intake. Intraperitoneal administration of ondansetron significantly attenuated 30- and 60-min CCK-induced reduction of food intake, with suppression being completely reversed by 120 min. It is not known whether 5-HT(3) receptors directly mediate CCK-induced satiation or whether their participation depends on CCK acting as part of a feedback cascade to inhibit ongoing intake. Because CCK-induced inhibition of sham feeding does not depend on additive gastric/postgastric-feedback signals, we examined the ability of ondansetron to reverse CCK-induced satiation in sham-feeding rats. Ondansetron did not attenuate reduction of sham feeding by CCK, suggesting that ondansetron does not directly antagonize CCK-satiation signals. CCK suppresses real feeding through a delay in gastric emptying. Ondansetron could attenuate CCK-induced reduction of food intake by reversing CCK-induced inhibition of gastric emptying. We found that blockade of 5-HT(3) receptors attenuates CCK-induced inhibition of gastric emptying of a solid meal, as well as saline and glucose loads. We conclude that 5-HT(3) receptors mediate CCK-induced satiation through indirect mechanisms as part of a feedback cascade involving inhibition of gastric emptying.

Excitotoxic lesions of the lateral parabrachial nucleus do not prevent cholecystokinin-induced suppression of milk intake in rats

We examined the effect of lateral (visceral) and medial (gustatory) parabrachial nucleus (PBN) lesions on the suppression of milk intake induced with cholecystokinin (CCK) in 24 h food-deprived rats. Irrespective of CCK dose, each type of PBN lesion significantly elevated the consumption of milk. Neither medial nor lateral PBN lesions had any discernible influence on the anorectic action of CCK, however. In combination with the results from a previous study using the same rats (Brain Res. 894 (2001) 288), the present results support the view that the role of the lateral PBN in CCK-induced anorexia depends on the nature of the food used at test.

Action of CCK and 5-HT on lateral hypothalamic neurons depends on early postnatal nutrition

Wistar rats grown up during the early postnatal life (3-21 days after birth) in artificially built normal, small or large lifters developed a significantly different body weight. This difference persisted also during adulthood when they had free access to food and water. The influence of iontophoretically administered cholecystokinin (CCK8S), serotonin (5-HT) or co-ejection of both on firing of lateral hypothalamic neurons was investigated in adult, urethane anesthetized rats of the three groups. The responsiveness to CCK8S was significantly higher in large- and small-litter rats than in the normal control group. The differences were greater in males than in females. They resulted in the male large-litter group from an increase of excitatory responses, whereas in the male small-litter group the proportion of inhibitory responses was augmented. Co-administration of 5-HT generally reduced the neuronal responsiveness. Especially in the large-litter group excitatory responses were significantly reduced. It may be speculated that the availability of food in the early postnatal life influences the development of the hypothalamic regulatory network in such a way that it stabilizes the high or low food ingestion all the life. At least in males, a changed responsiveness and type of response to cholecystokinin of lateral hypothalamic neurons might be involved in this altered regulation.

Meal-induced changes in extracellular 5-HT in medial hypothalamus of lean (Fa/Fa) and obese (fa/fa) Zucker rats

Hypothalamic serotonin (5-HT) is involved in appetite regulation and sympathetic stimulation of thermogenesis. This study tested the hypothesis that the enhanced energetic efficiency of obese Zucker rats involves blunted serotonergic release within the medial hypothalamus (MH). We used microdialysis and HPLC-EC to measure dynamic changes in extracellular 5-HT levels in the MH of 10-13-week-old male lean (Fa/Fa) and obese (fa/fa) Zucker rats before and after a meal. No differences were noted in basal levels of 5-HT between lean and obese rats. Consistent with the suggestion that hypothalamic 5-HT plays a physiological role in feeding, extracellular 5-HT levels increased significantly in both lean and obese rats given a meal. This increase was observed in the 20 min interval in which they ate the 8.1 kcal meal and remained for an additional 60 min. The net release of 5-HT during the meal interval was comparable in the lean (1.46+/-0.38 fmol/microl) and obese (1.21+/-0.82 fmol/microl) rats. However, the 5-HT levels of the leans (1.80+/-0.29 fmol/microl) plateaued in the next 20 min interval, whereas they continued rising (2.74+/-0.53 fmol/microl) in obese rats and were significantly higher than those in the leans during the 40 and 60 min intervals after the meal was presented. This resulted in a total net release during the meal plus the next three 20 min intervals that was significantly higher in obese (9.83+/-1.16 fmol/microl) than in lean (5.59+/-0.85 fmol/microl) rats. Thus, the enhanced energetic efficiency of the obese Zucker rats may not be associated with attenuated serotonin release in response to a meal. Rather their enhanced release of 5-HT in the MH may reflect compensatory mechanisms for the elevated orexigen NPY, the reduction in meal-induced CCK release, and/or a functional resistance to 5-HT.

Interaction of serotonin and cholecystokinin in the lateral parabrachial nucleus to control sodium intake

Serotonin [5-hydroxytryptamine (5-HT)] and CCK injected into the lateral parabrachial nucleus (LPBN) inhibit NaCl and water intake. In this study, we investigated interactions between 5-HT and CCK into the LPBN to control water and NaCl intake. Male Holtzman rats with cannulas implanted bilaterally in the LPBN were treated with furosemide + captopril to induce water and NaCl intake. Bilateral LPBN injections of high doses of the 5-HT antagonist methysergide (4 microg) or the CCK antagonist proglumide (50 microg), alone or combined, produced similar increases in water and 1.8% NaCl intake. Low doses of methysergide (0.5 microg) + proglumide (20 microg) produced greater increases in NaCl intake than when they were injected alone. The 5-HT(2a/2c) agonist 2,5-dimetoxy-4-iodoamphetamine hydrobromide (DOI; 5 microg) into the LPBN reduced water and NaCl intake. After proglumide (50 microg) + DOI treatment, the intake was not different from vehicle treatment. CCK-8 (1 microg) alone produced no effect. CCK-8 combined with methysergide (4 microg) reduced the effect of methysergide on NaCl intake. The data suggest that functional interactions between 5-HT and CCK in the LPBN may be important for exerting inhibitory control of NaCl intake.

Does increased endogenous CCK interact with serotonin to reduce food intake in rats?

The present study was aimed to test the hypothesis that increased endogenous CCK may interact with the anorectic serotonergic agent dl-fenfluramine to reduce food intake in rats. Previous studies, using selective CCK receptor antagonists, could demonstrate CCK-dependent 5-HT-induced anorexia. In the present approach, we used protease inhibitors to increase levels of endogenous CCK instead of blocking CCK receptors by antagonists. The protease inhibitors we used were soybean trypsin inhibitor (STI) and camostate. We hypothesized that combining the anorectic serotonergic drug dl-fenfluramine with either STI or camostate should result in an enhanced hypophagic effect when compared to single drug treatment. All feeding experiments were performed in non-deprived rats during night time feeding. Given alone, STI (500 mg/kg, po), camostate (200 mg/kg po) and also fenfluramine (1-9 mg/kg ip) reduced significantly food intake, with a more pronounced effect following fenfluramine. However, the experiments do not provide evidence for any additive or synergistic action between camostate or STI and the anorectic serotonergic drug dl-fenfluramine on food intake.

Cholecystokinin receptors do not mediate the suppression of food-motivated behavior by lipopolysaccharide and interleukin-1 beta in mice

During the course of an infection, profound metabolic and behavioral changes are observed. The resulting decrease in food intake can be reproduced by administration of lipopolysaccharide (LPS) or the proinflammatory cytokines (e.g., interleukin-1 [IL-1] and tumor necrosis factor it induces. To test the possibility that cholecystokinin (CCK) mediates anorexia induced by IL-1 beta and LPS, mice trained to poke their noses in a hole to obtain a food reward according to a fixed ratio (1 reward per 20 actions) were pretreated with the CCK-A receptor antagonist L364,718 (at 1 mg/kg) or with the CCK-B receptor antagonist L365,260 (50 microg/kg) before being injected with LPS (100 microg/kg) or IL-1 beta (20 microg/kg). All injections were given via the intraperitoneal (i.p.) route. In spite of its ability to block the effects of exogenous CCK-8 on food-motivated behavior in mice, the CCK-A receptor antagonist did not block the depressive actions of LPS and IL-1 beta on food-motivated behavior. The CCK-B receptor antagonist was not more effective at blocking. These results do not support a role for CCK in the anorexic effect of LPS and IL-1 beta.

Separate systems for serotonin and leptin in appetite control

Appetite control involves an integration of the drive signals arising form energy stores in the body with the satiety signals generated by periodic episodes of food consumption. Serotonin (5-hydroxytryptamine, 5-HT) has been implicated in the processes of within-meal satiation and postmeal satiety (5-HT1B and 5-HT2C postsynaptic receptors) which are concerned with the signals arising form the pattern of food intake. Central nervous system (CNS) 5-HT is sensitive to circulating levels of the precursor tryptophan, certain macronutrients and peripheral satiety factors such as cholecystokinin (CCK) and enterostatin. Hypothalamic 5-HT receptor systems inhibit neuropeptide Y (NPY), a potent stimulator of hunger and food intake. In contrast to the linking of 5-HT with the consequences of food ingestion, the hormone leptin (OB protein) is regarded as a signal linking adipose tissue status with a number of key CNS circuits. Leptin itself stimulates CNS leptin receptors (OB-r receptor) which link with pro-opiomelanocortin (POMC)/ MC-4 receptors. The effects of leptin may also be modulated by factors such as the corticotrophin-releasing factor (CRF), cocaine and amphetamine-regulated transcript (CART), orexins and galanin. Very little evidence exists to support any direct link between the actions of 5-HT and leptin, suggesting that they are separate systems. 5-HT is a part of an integrated network for short-acting satiety signals (episodic in nature), and leptin is a hormonal indicator of long-term (tonic) energy reserves. At a conceptual level, these may represent the distinction between 'satiety' and 'drive'. Interestingly, both 5-HT and leptin modulate the action of NPY, which may form a part of a common output pathway for the expression of appetite.

Action of cholecystokinin and serotonin on lateral hypothalamic neurons of rats

Discharges of spontaneously active lateral hypothalamic neurons were extracellularly recorded during iontophoretic administration of cholecystokinin (CCK-8S) or/and serotonin (5-HT) in anesthetized rats. The main results are the following. (1) The proportion of neurons responsive to CCK-8S was 62% (61/99) and that responsive to 5-HT 42% (33/78). (2) Out of the neuronal sample, 36% were influenced by both transmitters, allowing an interaction between the two systems. (3) Co-ejection of CCK and 5-HT elicited a response in 40% of the tested neurons, which was a significantly smaller responsiveness than with separate ejection of CCK-8S. The effect resulted from a reduced number of excited neurons whereas the number of inhibitions did not change. The results show that effects of 5-HT and CCK can converge on the same neuron within the lateral hypothalamus. This might be of relevance in the regulation of feeding behavior.

Cholecystokinin and serotonin receptors in the regulation of fat-induced satiety in rats

The present study investigated the relationship between endogenous CCK and serotonin (5-HT) in fat-induced satiety. Male Wistar rats with duodenal cannulas were adapted to eating 6 h/day along with receiving an infusion of saline or one of two isocaloric solutions (10 ml, 1 kcal/ml, 0.45 ml/min) varying in fat and carbohydrate content (20 or 80% energy from fat). Rats were infused 10 min after food presentation. The satiation/satiety response was determined from measures of meal size (MS), intermeal interval (IMI), and total food intake (TFI). Infusion with either fat solution reduced MS compared with saline; however, the 80% fat infusate reduced TFI and lengthened the IMI compared with saline and the 20% fat infusate. CCK and 5-HT involvement in fat-induced satiety was investigated by preceding the 80% fat infusate with CCK and/or 5-HT3 receptor antagonists Devazepide (Dev) and Tropisetron (Trop). A CCK releaser, trypsin inhibitor (TI), was added to the 20% fat infusate to enhance satiety. Pretreatment with Dev or Trop alone attenuated the inhibitory effects of the 80% solution on IMI, whereas reversal of the inhibitory effects on MS and TFI were sensitive only to Dev at the doses provided. Both antagonists together completely blocked the satiating effects of the 80% fat infusate on all feeding variables measured. Addition of TI to the 20% fat infusate lengthened the IMI but did not affect MS or TFI. These results provide evidence for the participation of both endogenous CCK and 5-HT in the satiety response to fat in the intestine.

Cholecystokinin actions in the parabrachial nucleus: effects on thirst and salt appetite

The present study investigated the effects of bilateral injections of the nonselective CCK receptor antagonist proglumide or CCK-8 into the lateral parabrachial nuclei (LPBN) on the ingestion of 0.3 M NaCl and water induced by intracerebroventricular injection of ANG II or by a combined treatment with subcutaneous furosemide (Furo) + captopril (Cap). Compared with the injection of saline (vehicle), bilateral LPBN injections of proglumide (50 micrograms . 200 nl-1 . site-1) increased the intake of 0.3 M NaCl induced by intracerebroventricular ANG II (50 ng/1 microliter). Bilateral injections of proglumide into the LPBN also increased ANG II-induced water intake when NaCl was simultaneously available, but not when only water was present. Similarly, the ingestion of 0.3 M NaCl and water induced by the treatment with Furo (10 mg/kg) + Cap (5 mg/kg) was increased by bilateral LPBN proglumide pretreatment. Bilateral CCK-8 (0.5 microgram . 200 nl-1 . site-1) injections into the LPBN did not change Furo + Cap-induced 0.3 M NaCl intake but reduced water consumption. When only water was available after intracerebroventricular ANG II, bilateral LPBN injections of proglumide or CCK-8 had no effect or significantly reduced water intake compared with LPBN vehicle-treated rats. Taken together, these results suggest that CCK actions in the LPBN play a modulatory role on the control of NaCl and water intake induced by experimental treatments that induce hypovolemia and/or hypotension or that mimic those states.

Serotonin and cholecystokinin activate different populations of rat mesenteric vagal afferents

This electrophysiological study was performed to elucidate the interactions of serotonin (5-hydroxytryptamine, 5-HT) and cholecystokinin (CCK) on mesenteric afferents supplying the rat jejunum. 5-HT and CCK produced characteristic responses in multi-unit recordings of mesenteric afferents. Waveform analysis to extract single units from the whole nerve recording identified populations of single afferents that were sensitive to either 5-HT or CCK, but not both. Furthermore, devazepide (0.5 mg/kg) completely abolished the response to CCK without altering the response to 5-HT while granisetron (0.5 mg/kg) abolished the response to 5-HT with no effect on the response to CCK. These results suggest that there are discrete, noninteractive populations of jejunal afferents that possess either 5-HT3 or CCK-A receptors but not both.

Food for thought: a critique on the hypothesis that endogenous cholecystokinin acts as a physiological satiety factor

This review evaluates the various lines of evidence supporting the hypothesis that cholecystokinin (CCK) released from the small intestine during feeding plays a physiological satiety. Issues considered include, the effects of systemic injection of CCK on consummatory and operant feeding, the role of the vagus nerve, the effects of CCKB receptor antagonists, and the neuroendocrine responses to exogenous CCK. A critical appraisal of this research indicates that while it is clearly demonstratable that exogenous peripheral CCK can alter food intake by acting on CCKA receptors, the mechanism involved may be more closely related to the induction if aversion and nausea, rather than satiety. With regard to peripheral endogenous CCK, the available evidence also does not seem to support a role for the hormone in satiety. In particular, it is doubtful whether plasma concentrations of CCK following a meal are sufficiently high to inhibit feeding. Moreover, CCKA receptor antagonist which do not cross the blood brain barrier fail to increase meal size, as would be expected if peripheral CCK was an effective satiety factor. In addition, the recent literature concerned with the possibility that CCK may have a direct action within the brain in the control of food intake has been reviewed. These studies show that CCK administered intracerebroventicularly, or by micoinjection into discrete brain regions, also inhibits feeding via a CCKA receptor mechanism. However, the physiological relevance of these findings have yet to be determined.

Lack of interaction between devazepide and 8-OH-DPAT-induced hyperphagia in the rat

Recently, a number of studies have provided evidence suggesting that CCK and 5-HT interact in the control of food intake. However, the majority of these studies have relied on the administration of exogenous CCK to investigate potential interactions. The aim of the present study was to focus on the potential role of endogenous CCK in 5-HT-CCK interactions. Our prediction was that the CCKA antagonist, devazepide, alone would potentiate the hyperphagic effect of the 5-HT1A agonist, 8-OH-DPAT, in free-feeding rats. The results showed that devazepide, at a dose that had no intrinsic effect (1.0 mg/kg), did not enhance the hyperphagic effect of 8-OH-DPAT (100 and 300 micrograms/kg). This suggests that when serotonergic inhibitory activity is reduced by 5-HT1A-receptor stimulation, there is no compensatory increase of endogenous CCK activity to excite 5-HT neurons and thereby inhibit food intake.

The physiology and brain mechanisms of feeding

This article is designed as an introduction to the major theoretical models in the field of regulation of eating behavior, and a selective review of relevant neurobiological data. We first critically consider the paradigm of homeostasis as it relates to body energy content, and argue that additional theoretical constructs will be needed to account for the complexity of eating behavior in both nonhumans and humans. We then summarize some of the methods available to the neuroscientist in this area, and address some of their limitations. We review treatments and potential mechanisms that increase food intake, including deprivation, antimetabolites, norepinephrine, and several peptides including neuropeptide Y. We next review treatments that decrease food intake, including a variety of humoral, gastrointestinal, and pancreatic factors, as well as examine central pathways of satiety. This includes a discussion of leptin and other potential anorectic agents. We conclude with a discussion of human obesity and anorexias, and prospects for pharmacotherapy of eating disorders. We emphasize throughout that most regions of the human brain probably make some contribution to feeding behavior, and so a focus on any one area of transmitter/hormone is an unrealistic approach both in basic and applied areas.

Baclofen pretreatment attenuates the suppressant effect of intraperitoneal administration of cholecystokinin (CCK) on food intake in rats

The aim of this study was to investigate whether pretreatment with the GABAB receptor agonist baclofen could prevent the inhibitory effect of systemically administered cholecystokinin (CCK) on food intake in rats. Baclofen (2 mg/kg, SC) administered 60 min prior to IP injection of CCK (5 micrograms/kg) significantly attenuated the suppressant effect of the peptide on feeding in nondeprived rats (Experiment 1) and rats that had been deprived of food for 22 h (Experiment 2). Baclofen had no significant effects on food intake when administered alone. The results suggest that the inhibitory effect of exogenous peripheral CCK on food intake may be dependent on an interaction with a GABAB-receptor mediated mechanism.

Localization of the murine cholecystokinin A and B receptor genes

We have determined the chromosomal locations of the two cholecystokinin (CCK) receptor genes in the mouse. Genetic localization utilized an interspecific backcross panel formed from the cross (C57BL/6J x Mus spretus) F1 x Mus spretus. Genomic DNAs from 94 individuals in the backcross were analyzed by Southern hybridization with rat CCKA and CCKB receptor cDNA probes. Unique map positions were determined by haplotype analysis with 650 previously mapped loci in the mouse backcross. The CCKA receptor gene (Cckar) mapped to mouse Chromosome (Chr) 5, in tight linkage with the DNA marker D5Bir8. The CCKB receptor gene (Cckbr) mapped to mouse Chr 7, tightly linked to the beta-hemoglobin locus (Hbb). This localization places Cckbr in the same region as the mouse obesity mutation tubby (tub), which also maps near Hbb (2.4 +/- 1.4 cM). Since CCK can function as a satiety factor when administered to rodents, localization of Cckbr near the tub mutation identifies this receptor as a possible candidate gene for this obesity mutation.

Effects of vagotomy on cholecystokinin- and dexfenfluramine-induced Fos-like immunoreactivity in the rat brain

This study compared the effects of bilateral subdiaphragmatic vagotomy on the Fos-like immunoreactivity (FLI), a marker of neuronal activation, in rat brain induced by two anorectic agents, cholecystokinin (CCK) and the serotonin agonist, dexfenfluramine (DFEN). In the nonvagotomized rats, both CCK (5 micrograms/kg, IP) and DFEN (2 mg/kg, IP) induced FLI in the nucleus of the solitary tract (NST), the external subdivision of the lateral parabrachial nuclei (LPBE), the lateral subdivision of the central amygdaloid nucleus (CeL), and the bed nucleus of the stria terminalis (BST). However, subregional distribution of the FLI induced by the two agents was different in most of these regions. Additionally, the area postrema and the medial subdivision of the hypothalamic paraventricular nucleus were preferentially activated by CCK but not DFEN, while the caudate-putamen was activated by DFEN but not CCK. Bilateral subdiaphragmatic vagotomy completely abolished CCK-induced FLI in all the brain regions but did not attenuate DFEN-induced FLI in any of these regions, including the NST. The results of the present study suggest that DFEN-activation of the NST-LPBE-CeL/BST neuraxis is not mediated by the vagus nerve. On the other hand, and consistent with a variety of other data, activation of various parts of the brain by peripherally administered CCK depends on a vagal pathway. These data are discussed in relation to a previously proposed interaction between CCK and serotonin in mediating satiety.

Role of neuropeptides in the regulation of feeding behavior: a review of cholecystokinin, bombesin, neuropeptide Y, and galanin

The purpose of this report is to provide a review of four peptides (cholecystokinin, bombesin, neuropeptide Y, galanin) and their role in feeding behavior. Cholecystokinin (CCK) and bombesin (BBS) are considered satiety peptides, and neuropeptide Y (NPY) and galanin (GAL) have been proposed as appetite peptides. For the purposes of this review, satiety refers to the physiological cessation of feeding, and appetite refers to the drive to eat and exists in gradations.

Cholecystokinin- and dexfenfluramine-induced anorexia compared using devazepide and c-fos expression in the rat brain

It has been proposed that there might be a link between the anorectic actions of cholecystokinin (CCK) and serotonin (5HT). The present study compared the patterns of c-fos protein-like immunoreactivity (FLI) induced in rat brain by CCK and the indirect 5HT agonist dexfenfluramine (DFEN), as well as the ability for devazepide, a CCK-A receptor antagonist, to antagonize both anorexia and FLI induced by these agents. Devazepide reversed the anorectic effect of CCK but not that of DFEN in food deprived rats. The FLI induced by CCK and DFEN occurred in similar brain regions, but in different subdivisions. Such regions included the bed nucleus of the stria terminalis (BST), the lateral central nucleus of the amygdala (CeL), and the lateral parabrachial nucleus (LPB). Devazepide abolished the FLI induced by CCK in most of these brain regions, but had no effect on FLI induced by DFEN. These results suggest that the LPB-CeL/BST pathway might be responsible for the anorectic effects of both CCK and DFEN, but different parts or neuronal populations in these structures might be differentially engaged by CCK and DFEN. The putative interaction between CCK and 5HT might happen along this pathway, rather than in the periphery.

Effects of BOC-CCK-4 and L 365.260 on cortical 5-HT release in guinea-pigs on exposure to the elevated plus maze

The elevated plus maze is a well-established model of anxiety, with previous results showing that guinea-pigs handled daily from birth exhibit behaviour in this test similar to rats. In the present microdialysis study exposure of the guinea-pig to the elevated plus maze increased extracellular 5-HT in the lateral prefrontal cortex. The CCK-B receptor agonist BOC-CCK-4 (10 micrograms/kg) produced 'anxious' behaviour and potentiated the rise in 5-HT observed on exposure to the X-maze. The basal release of cortical extracellular 5-HT was not affected by BOC-CCK-4. Pretreatment with the selective CCK-B antagonist L 365.260 (100 micrograms/kg) antagonized both the 'anxious' behaviour and the neurochemical changes induced by BOC-CCK-4 while L 365.260 alone produced 'anxiolytic' behaviour, decreased basal extracellular 5-HT and prevented the increase in extracellular 5-HT seen when the guinea-pigs were exposed to the X-maze. Our results show that CCK-B receptor stimulation and blockade induce changes in central extracellular 5-HT levels associated with 'anxious' and 'anxiolytic' behaviour, respectively.

Biological actions of cholecystokinin

Cholecystokinin (CCK) has emerged as an important mammalian neuropeptide, localized in peripheral organs and in the central nervous system. This review presents an overview of the molecular aspects of CCK peptides and CCK receptors, the anatomical distribution of CCK, the neurophysiological actions of CCK, release of CCK and effects of CCK on release of other neurotransmitters, and the actions of CCK on digestion, feeding, cardiovascular function, respiratory function, neurotoxicity and seizures, cancer cell proliferation, analgesia, sleep, sexual and reproductive behaviors, memory, anxiety, and dopamine-mediated exploratory and rewarded behaviors. Human clinical studies of CCK in feeding disorders and panic disorders are described. New findings are presented on potent, nonpeptide CCK antagonists, selective for the two CCK receptor subtypes, which demonstrate that endogenous CCK has biologically important effects on physiology and behavior.

The role of cholecystokinin in interleukin-1-induced anorexia

Anorexia is a common response to infection which is thought to be mediated, at least in part, by interleukin (IL-1), an immunoregulatory peptide secreted by activated monocytes. Cholecystokinin (CCK) is a neuropeptide that suppresses food intake and gastric emptying when injected into healthy animals. There is increasing evidence of bidirectional interactions between neuropeptides, immune cell function, and secretion of immunoregulatory cytokines. Therefore, the present study was undertaken to determine if administration of L364,718 (L364), a CCK receptor antagonist, might block the anorexigenic effects of recombinant human IL-1 alpha (IL-1 alpha) in rats. We report that injection of IL-1 alpha significantly increased plasma CCK 1 h after injection, and decreased food intake and emptying of gastric contents. Pretreatment with 1 mg/kg L364 partially blocked the decrease in food intake and gastric stasis induced by IL-1 alpha. We conclude CCK may mediate, at least in part, IL-1 alpha-induced anorexia.

Dexfenfluramine induces Fos-like immunoreactivity in discrete brain regions in rats

Administration of the anorectic agent, dexfenfluramine (DFEN) to rats induced dose- and time-dependent expression of Fos-like immunoreactivity in several discrete brain regions of rats. At moderate doses, the regions showing the most intense Fos immunostaining included the bed nucleus of the stria terminalis, the lateral part of the central amygdala, midline thalamic nuclei, habenular nuclei, lateral parabrachial nucleus, and nucleus of the solitary tract. It is suggested that these nuclei, many of which are known to receive gustatory or visceral input, may form part of a functional circuit via which DFEN modulates food intake. Rats that were made hungry by either food deprivation or administration of insulin also showed induction of Fos in several brain regions, but only that in the supramamillary/ventral tegmental area was suppressed by pretreatment with an anorectic dose of DFEN. The functional significance of these sites requires further investigation.

The CCK-A receptor antagonist, devazepide, blocks the anorectic action of CCK but not peripheral serotonin in rats

A role has been proposed for cholecystokinin (CCK)-A-type receptors in mediating the anorectic action produced by serotonergic stimulation in rats. We examined the effect of pretreatment with the CCK-A antagonist devazepide (DVZ) on anorexia produced by peripheral administration of serotonin [5-hydroxytryptamine (5-HT)] or CCK-8 in 3-h food-deprived rats consuming a 30-min test meal of sweetened mash. The anorectic effect of CCK-8 (4.0 nmol/kg, IP) was antagonized in a dose-dependent manner by DVZ (0.03, 0.10, and 0.30 mumol/kg, IP), with even the lowest dose producing a significant reversal. Under identical testing conditions, a supramaximal dose of DVZ (.75 mumol/kg) did not attenuate the reductions in food intake produced by either a moderate (4.0 mumol/kg) or a high dose (10.0 mumol/kg) of 5-HT. These data confirm established findings that the anorectic action of peripheral CCK depends upon CCK-A receptors. However, peripherally administered 5-HT reduces food intake independently of CCKergic function.

Systemic administration of a cholecystokinin analogue, ceruletide, protects against ischemia-induced neurodegeneration in gerbils

The neuroprotective action of a cholecystokinin octapeptide analogue, ceruletide, was evaluated in models of cerebral ischemia using Mongolian gerbils. Ceruletide significantly suppressed the hyperactivity and amnesia induced by ischemia when injected s.c. 30 min before 5-min occlusion of the bilateral common carotid arteries at room temperature or immediately after their reperfusion. Ceruletide also reduced behavioral changes in ischemic gerbils whose body temperature was maintained at 37 degrees C during the 3-min occlusion. In these groups, delayed neuronal cell death in the hippocampal CA1 area following ischemia was markedly attenuated by s.c. administration of ceruletide. On the other hand, ceruletide could not inhibit the behavioral changes or the neurodegeneration induced in the hippocampal CA1 area by 5-min occlusion at 37 degrees C. These findings indicate that peripheral injection of ceruletide produces a neuroprotective action against moderate cerebral ischemia, which is the first evidence suggesting the efficacy of ceruletide in neurodegenerative diseases.

Morphine place conditioning is differentially affected by CCKA and CCKB receptor antagonists

In the present study we have examined the interaction between the selective cholecystokinin (CCK)A and CCKB receptor antagonists, devazepide and L365-260 on morphine conditioned place preference (CPP). Using an unbiased procedure, morphine (1.5 mg/kg) produced a reliable CPP which was observed irrespective of the conditioning compartment type. Pretreatment with devazepide (0.001-0.01 mg/kg s.c.) produced a dose related attenuation of this response. At higher doses (0.1-1 mg/kg) this antagonism became variable and dependent on the training compartment with blockade only observed when conditioning was to the white/rough textured environment. This profile has also been reported for the serotonin (5-HT)3 receptor antagonist ondansetron. The CCKB antagonist L365-260 (0.000001-0.01 mg/kg) failed to antagonize the morphine CPP, if anything a mild potentiation was observed. To study this further we examined the interaction between L365-260 (0.01 mg/kg) and a subthreshold dose of morphine (0.3 mg/kg). At these doses neither drug elicited CPP, however when co-administered a significant CPP was recorded. Finally, L365-260 at 1 mg/kg induced a mild but significant CPP when administered alone. These results suggest a differential role of CCK receptor subtypes on reward-related behaviour and complement previous studies suggesting bimodal effects of CCK systems on mesolimbic dopamine function.