Pharmacological examination of cholecystokinin (CCK-8)-induced contractile activity in the rat isolated pylorus

Organization of vagal afferents in pylorus: mechanoreceptors arrayed for high sensitivity and fine spatial resolution?

The pylorus is innervated by vagal mechanoreceptors that project to gastrointestinal smooth muscle, but the distributions and specializations of vagal endings in the sphincter have not been fully characterized. To evaluate their organization, the neural tracer dextran biotin was injected into the nodose ganglia of rats. Following tracer transport, animals were perfused, and their pylori and antra were prepared as whole mounts. Specimens were processed to permanently label the tracer, and subsets were counterstained with Cuprolinic blue or immunostained for c-Kit. Intramuscular arrays (IMAs) in the circular muscle comprised the principal vagal afferent innervation of the sphincter. These pyloric ring IMAs were densely distributed and evidenced a variety of structural specializations. Morphometric comparisons between the arbors innervating the pylorus and a corresponding sample of IMAs in the adjacent antral circular muscle highlighted that sphincter IMAs branched profusely, forming more than twice as many branches as did antral IMAs (means of 405 vs. 165, respectively), and condensed their numerous neurites into compact receptive fields (∼48% of the area of antral IMAs) deep in the circular muscle (∼6μm above the submucosa). Separate arbors of IMAs in the sphincter interdigitated and overlapped to form a 360° band of mechanoreceptors encircling the pyloric canal. The annulus of vagal IMA arbors, putative stretch receptors tightly intercalated in the sphincter ring and situated near the lumen of the pyloric canal, creates an architecture with the potential to generate gut reflexes on the basis of pyloric sensory maps of high sensitivity and fine spatial resolution.

Involvement of cholecystokinin receptor in the inhibition of gastrointestinal motility by oxytocin in ovariectomized rats

The effects of oxytocin on gastric emptying, gastrointestinal transit, and plasma levels of cholecystokinin (CCK) were studied in ovariectomized rats. Gastrointestinal motility was assessed in rats 15 min after intragastric instillation of a test meal containing charcoal and Na2 51CrO4. Gastric emptying was determined by measuring the amount of radiolabeled chromium contained in the small intestine as a percentage of the initial amount received. Gastrointestinal transit was evaluated by calculating the geometric center of distribution of the radiolabeled marker. Blood samples were collected for CCK radioimmunoassay. After administration of oxytocin (0.2-0.8 mg/kg), gastric emptying and gastrointestinal transit were inhibited, whereas plasma concentration of CCK was increased in a dose-dependent manner. Atosiban, an oxytocin receptor antagonist, effectively attenuated the oxytocin-induced inhibition of gastric emptying and gastrointestinal transit. However, administration of atosiban alone had no effect on gastric emptying and gastrointestinal transit. The selective CCK1 receptor antagonists, devazepide and lorglumide, effectively attenuated the oxytocin-induced inhibition of gastric emptying and gastrointestinal transit. L-365, 260, a selective CCK2 receptor antagonist, did not alter the oxytocin-induced inhibition of gastric emptying and gastrointestinal transit. These results suggest that oxytocin inhibits gastric emptying and gastrointestinal transit in ovariectomized rats via a mechanism involving the stimulation of CCK release and CCK1 receptor activation.

Coordinated gastric and sphincter motility evoked by intravenous CCK-8 as monitored by ultrasonomicrometry in rats

Gastric and sphincter motility evoked by intravenous injection of CCK-8 were investigated in urethane-anesthetized rats. Digital ultrasonomicrometry was used to monitor pyloric (PYL), antral (ANT), corpus (COR), and lower esophageal sphincter (LES) movements while simultaneously measuring intragastric pressure (IGP) and, in some experiments, subdiaphragmatic intraesophageal pressure (sIEP). Intracrystal distances (ICD) were measured continuously between pairs of piezoelectric crystals affixed to the serosa of PYL, ANT, COR (circular and longitudinal), and LES. Consecutive intravenous injections of CCK-8 (0.3, 1, and 3 microg/kg) at 30-min intervals caused dose-dependent simultaneous tonic contractions of PYL and ANT, LES opening, and drops in IGP with peak changes at 3 microg/kg of -17.9 +/- 2.1, -7.7 +/- 2.5, 6.5 +/- 1.4, and -29.2 +/- 3.8%, respectively, whereas intravenous saline had no effect. Rhythmic contractile activity was inhibited by CCK-8. COR responses were not significantly different from vehicle controls for most metrics, and the direction of response for circular COR varied between preparations, although not for repeated trials in a single preparation. During the LES response to CCK-8, sIEP rose in parallel with drops in IGP, indicating formation of a common cavity. Recovery of LES ICD after intravenous CCK occurred more rapidly than recovery of PYL ICD, suggesting the importance of preventing simultaneous patency of gastroesophageal and gastroduodenal passages. The CCK(A) receptor antagonist devazepide (500 microg/kg intravenous) inhibited motion responses evoked by intravenous CCK-8. These data revealed CCK-8-induced gastric and sphincter activity consistent with retropulsion of gastric content.

Effects of evodiamine on gastrointestinal motility in male rats

The effects of evodiamine on gastric emptying, gastrointestinal transit, and plasma levels of cholecystokinin (CCK) were studied in male rats. Evodiamine, isolated from the dry unripened fruit of Evodia rutaecarpa Bentham (a Chinese medicine named Wu-chu-yu), has been recommended for abdominal pain, acid regurgitation, nausea, diarrhea, and dysmenorrhea. Gastrointestinal motility was assessed in rats 15 min after intragastric instillation of a test meal containing charcoal and Na(2)51CrO(4). Gastric emptying was determined by measuring the amount of radiolabeled chromium contained in the small intestine as a percentage of the initial amount received. Gastrointestinal transit was evaluated by calculating the geometric center of distribution of the radiolabeled marker. Blood samples were collected for CCK radioimmunoassay (RIA). After administration of evodiamine (0.67-6.00 mg/kg), both gastric emptying and gastrointestinal transit were inhibited, whereas the plasma concentration of CCK was increased in a dose-dependent manner. The selective CCK(1) receptor antagonists, devazepide and lorglumide, effectively attenuated the evodiamine-induced inhibition of gastric emptying and gastrointestinal transit. L-365,260 (3R-(+)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepine-3-yl)-N'-(3-methylphenyl)-urea), a selective CCK(2) receptor antagonist, did not alter the evodiamine-induced inhibition of gastric emptying and gastrointestinal transit. These results suggest that evodiamine inhibits both gastric emptying and gastrointestinal transit in male rats via a mechanism involving CCK release and CCK(1) receptor activation.

Inhibition of gastric emptying and intestinal transit by amphetamine through a mechanism involving an increased secretion of CCK in male rats

1. The effect of amphetamine on gastrointestinal (GI) transit and the plasma levels of cholecystokinin (CCK) were studied in male rats. 2. Gastric emptying was inhibited both acutely and chronically by the administration of amphetamine. GI transit was decreased by the acute administration of amphetamine but not affected by the chronic administration of amphetamine. 3. Plasma CCK levels were increased dose-dependently by amphetamine. 4. Proglumide, a CCK receptor antagonist, prevented amphetamine-induced inhibition of gastric emptying and the decrease in GI transit in male rats. 5. The selective CCK(A) receptor antagonist, lorglumide, dose-dependently attenuated the amphetamine-induced inhibition of gastric emptying in male rats. In contrast, the selective CCK(B) receptor antagonist, PD 135,158, did not reverse the effect of amphetamine on gastric emptying. 6. Both lorglumide and PD 135,158 reversed the inhibitory effect of amphetamine on GI transit in male rats. 7. These results suggest that amphetamine-induced inhibition of gastric emptying and intestinal transit is due in part to a mechanism associated with the hypersecretion of endogenous CCK.

Whole body autoradiography of CCK-8 in rats

Rats were given i.v., intranasal or intraperitoneal doses of CCK-8 (sulfated) labelled with 125I-labeled Bolton and Hunter reagent. Radioactivity was found mainly in the liver, kidney, and the intestinal contents. No radioactivity was detected in the brain. In animals dosed i.v., specific localization occurred in the tissue of the pyloric region of the stomach, and in the pancreas. Label persisted within the pyloric region of the stomach for longer than 30 min, in spite of the reported half-life of CCK-8 in plasma of approximately 1 min. Intranasal and intraperitoneal doses had limited bioavailability. The binding to the sites in the pyloric region of the stomach, which required systemic delivery, may have identified receptors associated with appetite control.

Cholecystokinin inhibits gastric emptying and contracts the pyloric sphincter in rats by interacting with low affinity CCK receptor sites

The aim of these experiments was to characterize the receptor affinity state through which CCK produces pyloric contraction and inhibits gastric emptying in the rat using the novel CCK heptapeptide analog CCK-JMV-180. CCK-JMV-180 has been demonstrated to act as a functional agonist at high affinity pancreatic CCKA receptors but as a functional antagonist at CCKA low affinity receptors. CCK-8 (1, 3.2 and 10 nM) induced dose dependent tension increases in isolated pyloric segments. CCK-JMV-180 (3.2 microM) or vehicle failed to mimic this action when administered alone but blocked the ability of CCK-8 (3.2 nM) to induce tension increases. CCK-8 (2 micrograms/kg) also inhibited the gastric emptying of physiological saline. CCK-JMV-180 (320 and 1000 micrograms/kg) failed to inhibit emptying when administered alone but dose dependently antagonized CCK induced inhibition of gastric emptying. Thus, in both preparations CCK-JMV-180 acted as a functional CCK antagonist. This profile is consistent with the interpretation that the actions of CCK in pyloric contraction and the inhibition of gastric emptying are mediated through CCK's interactions with receptors functionally similar to pancreatic low affinity sites.

CCK elicits and modulates vagal afferent activity arising from gastric and duodenal sites

We have begun to identify and characterize the locations and response profiles of vagal afferent fibers sensitive to CCK in the rat upper gastrointestinal tract. We found gastric and duodenal vagal afferent fibers that respond to CCK and to intraluminal loads. CCK both sensitizes and amplifies the response to loads in these fibers but may do so through separate transduction mechanisms. Thus, meal-related signals arising from the presence of gastroduodenal loads and the release of endogenous CCK can be integrated at the level of the peripheral afferent vagus nerve. These findings are consistent with behavioral results, demonstrating that combinations of gastric loads and exogenous CCK are more effective in suppressing food intake than is either stimulus presented alone. Our findings that both gastric and duodenal vagal afferent fibers are sensitive to CCK suggest that meal-related CCK may act at a range of peripheral neural sites linking the upper gastrointestinal tract to the central nervous system substrates underlying the control of food intake. The mode of activation of gastric vagal afferent by endogenously released CCK may be an endocrine action of intestinally derived CCK. Alternatively, the novel finding of duodenal load-sensitive vagal afferents close to a site of CCK release provides a potential for local paracrine actions of endogenous CCK in the mediation of satiety.

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.

Gut hormones in gastric function

This chapter has focused on many of the gut hormones that regulate gastric function. Gastrin remains the principal, and only, gastric hormone controlling gastric acid secretion during the cephalic, gastric and intestinal phases of secretion. Several other hormones, including cholecystokinin, peptide YY and secretin, released from intestinal endocrine cells in response to food substrates, have significant inhibitory effects on gastric acid secretion. Many of these hormones, including enteroglucagon and glucagon-like peptide, may act through paracrine release of somatostatin, which in turn acts as the final mediator of acid inhibition. In addition, several peptides contained in nerves, including gastrin releasing peptide and vasoactive intestinal peptide, have been shown to regulate gastric acid secretion and motor function. With the creation of specific monoclonal antibodies for use in in vivo immunoneutralization studies, and the development of selective chemical antagonists for use in receptor blockade experiments, the specific contributions of the different gut hormones in the regulation of gastric function, can be assessed.

Procolipase mRNA: tissue localization and effects of diet and adrenalectomy

Northern blot analysis has identified procolipase mRNA in rat pancreas, stomach and duodenum. Pancreatic colipase mRNA was increased by high-fat diets. Adrenalectomy increased pancreatic procolipase mRNA, an effect enhanced by high-fat diets. The results suggest that colipase is not unique to the pancreas and that diet and glucocorticoids interact in regulating the transcription of its gene.

Enkephalin is a competitive antagonist of cholecystokinin in the gastrointestinal tract, as predicted from prior conformational analysis

Prior calculations based on ECEPP (Empirical Conformational Energies for Peptides Program) of the low energy minima for cholecystokinin (CCK) and Met-enkephalin have demonstrated that significant structural features of these two peptides are identical. This result suggested the possibility that Met-enkephalin, as well as other enkephalin analogues of similar structure, could associate with receptors for CCK. To test this theoretical result, we examined the ability of Met-enkephalin and its analogues to bind to peripheral CCK receptors in the rat gastrointestinal tract; in particular, we measured the ability of the opiate peptide to inhibit the effects of CCK in a physiological assay system which we have previously characterized: CCK-induced contraction of the isolated rat pyloric sphincter. We find that Met-enkephalin is an antagonist of the CCK-8-induced contraction, with a IC50 of 110 nM. Furthermore, antibodies against CCK were found to cross-react with Met-enkephalin and its analogues in a manner which suggests a distinct structure-activity relationship. These experimental results strongly support the theoretical results of conformational analysis showing structural similarity between enkephalin and CCK. They further suggest that enkephalins could modulate the response of CCK systems under physiological conditions.

Peripheral factors in the mediation of cholecystokinin-induced satiety as assessed by comparative potencies of cholecystokinin antagonists

Cholecystokinin COOH-terminal octapeptide (CCK-8) produces a satiating effect in the rat and other animals upon peripheral administration. Although it has been demonstrated that the receptors which mediate this action are located in the periphery and are of the CCK-A subtype, their anatomical location has not been firmly established. A dense population of CCK receptors in the pyloric sphincter has been suggested as a candidate. We here quantify the potency of several CCK antagonists to inhibit the contractile effect of CCK-8 on the rat pyloric sphincter in vitro. The potent and selective antagonist MK-329 has a Schild pK of 8.85; the less potent but selective antagonist lorglumide (CR-1409) a pK of 6.37; the related antagonist phenoxyacetylproglumide (phi oAc proglumide) a pK of 5.1; and the weak parent compound proglumide a pK of about 3.3. These data can be compared with the potencies of these compounds to inhibit the actions of CCK-8 to produce satiety in the rat; this comparison supports the contention that CCK receptors of the rat pyloric sphincter could in part mediate the satiety effect produced by exogenous CCK-8.

Vagal afferent innervation and regulation of gastric function

In this article, we have presented evidence that vagal capsaicin-sensitive afferent fibers are involved in the regulation of gastric mucosal and motor function. Gastric acid secretion stimulated by gastric distension, histamine and central injection of TRH analog are all partly dependent on vagal capsaicin-sensitive afferent mechanisms. It is possible that as vagal efferent activity releases histamine, the common final pathway is the reduction in the response to histamine. At present, it is unclear as to the mechanism by which capsaicin-sensitive afferents are involved in the secretory response to histamine. With regard to the gastric acid and mucosal blood flow responses to TRH, it is not clear whether the sensory neurons represent a component of the efferent pathway that is activated by TRH or whether their role is to set the sensitivity of, or exert feedback control on this efferent pathway. As perineural capsaicin application decreases peptide content in the peripheral terminal fields of sensory neurons and these peptides may produce local effector functions within the tissue, it is possible that alterations in the gastric responses to TRH result from a decrease in the local effector functions of vagal neurons. From the experiments on electrical stimulation of the vagus nerve, it is evident that antidromic stimulation of vagal afferents can stimulate gastric mucosal blood flow, although under these experimental conditions there was no evidence for a capsaicin-sensitive stimulation of gastric acid secretion. The physiological relevance of this stimulation of gastric mucosal blood flow is at present unclear, but it is possible that physiological stimuli, such as distension or nutrients, may stimulate afferents and signal for an increase in gastric mucosal blood flow. In addition, pathophysiological or noxious stimulation of vagal afferents may also signal for an increase in gastric mucosal blood flow and may play a role in the response of the mucosa to injury.

Autoradiographic and functional development of gastric cholecystokinin receptors in the rat

To determine the functional role for and the pharmacological specificity of developing gastrointestinal CCK receptors, in vitro pyloric contractility and autoradiographic CCK receptor binding were examined in pups aged 1-20 days. CCK contracted the gastroduodenal junction at all ages, while nonsulfated CCK-8 (d-CCK) was less potent. Autoradiographic studies revealed CCK binding localized to the gastroduodenal junction throughout development. MK-329, a specific type A CCK receptor antagonist, completely displaced 125I CCK-8 binding at all ages, while d-CCK displaced binding at ages at which d-CCK elicited gastroduodenal contractility. The results demonstrate a physiological role for and pharmacological specificity of neonatal gastroduodenal CCK receptors.

Pyloroplasty does not disrupt liquid phase gastric emptying or CCK-induced satiety

To assess the hypothesis that cholecystokinin (CCK) induces satiety by constricting the pylorus and inhibiting the rate of gastric emptying, we impaired pyloric function in 8 experimental animals using a Heincke-Mikulicz pyloroplasty procedure. Liquid phase gastric emptying was measured with a double-sampling procedure. We found that pyloroplasty did not disrupt the pattern of gastric emptying of saline or nutrient under control conditions. CCK also showed emptying in both pyloroplasty and control animals. Finally, pyloroplasty did not affect the ability of CCK to induce satiety. We provide functional and morphological evidence that the pyloroplasty procedure successfully impaired constriction of the pyloric sphincter. These findings suggest that the pylorus is not critical to the control of liquid phase emptying and, together with the absence of a pyloroplasty effect on CCK-induced satiety, seriously question the adequacy of the pyloric mediation hypothesis.

In vitro response of rat gastrointestinal segments to cholecystokinin and bombesin

In previous studies of the rat gastrointestinal (GI) tract, we have demonstrated specific binding of cholecystokinin (CCK) to the pylorus and of bombesin (BN) to the gastric fundus, gastric antrum, duodenum, and ileum. We now present the results of an investigation of the in vitro response of the same regions of the rat GI tract to CCK-8 (the active octapeptide of CCK) and BN. Sections of rat fundus, antrum, pylorus, duodenum, and ileum were suspended in a Tyrode buffer and attached to an isometric pressure transducer in a longitudinal orientation. Dose-response curves to CCK-8 and BN were generated for each tissue. CCK-8 consistently induced a change only in pylorus, while BN induced a response from fundus, antrum and duodenum. With the exception of the lack of ileal response to BN, the regions of the rat GI tract which biologically respond (i.e., contract or relax) to CCK-8 or BN were the same regions in which we have located BN and CCK-8 binding sites. This correlation supports the hypothesis that GI function is modified by specified hormone-receptor interactions with these peptides.

Effect of gastrointestinal peptides on ingestion in old and young mice

Early satiety may play a role in the anorexia of aging. The effects of the peripheral satiety agents cholecystokinin (CCK), bombesin, glucagon, and calcitonin were studied in 8 and 25 month old mice. During normal feeding behavior, the older mice consumed more than their younger counterparts, however, when food deprived, the younger consumed more. All peptides inhibited food intake over the first hour after administration in young and old mice. CCK, bombesin, and calcitonin suppressed feeding in 25 month old mice to a greater extent than in 8 month old mice. However, CCK demonstrated the greatest age-related suppression of food intake. CCK has a potential role to play in the pathogenesis of the anorexia of aging.

Two proglumide analogues are equipotent antagonists of the inhibition of food intake by CCK-8

The reduction in food intake produced by exogenous CCK-8 (8 micrograms.kg-1, IP) in 18 hr food-deprived rats was significantly reversed by either of two proglumide analogues at doses of 0.44 and 4.4 microM.kg-1. The two glutamic acid derivatives tested were CR-1409 [N-(3,4-dichlorobenzoyl)-L-glutamic acid-1-di-n-pentylamide], effective at doses of 0.2 and 2.0 mg.kg-1, IP, and PGDPA [N-(phenoxyacetyl)-L-glutamic acid-1-di-n-propylamide], effective at the equimolar doses of 0.16 and 1.6 mg.kg-1, IP, as well as at 16 mg.kg-1 (44 microM.kg-1). By comparison, proglumide reversed the inhibition of food intake by CCK-8 at 160 mg.kg-1 (470 microM.kg-1), but not at 16 mg.kg-1 (47 microM.kg-1). At the 0.44 microM.kg-1 dose which antagonized CCK-8-induced satiety, neither PGDPA nor CR-1409 reduced the inhibition of food intake induced by bombesin, supporting the behavioral specificity of these CCK antagonists. Previous in vitro studies have shown that CR-1409 was approximately 4000-fold more potent than proglumide and PGDPA was 100-fold more potent than proglumide as antagonists of CCK-8-induced amylase secretion and binding in pancreatic acinar cells. Here, we found no potency difference between PGDPA and CR-1409; each was more than 1000-fold more potent than proglumide as an antagonist of the inhibition of food intake produced by CCK-8. This nonparallelism between the potencies of these antagonists at CCK receptors located upon pancreatic acinar cells and at CCK receptors involved in CCK-8-induced satiety suggests that the two receptor populations differ pharmacologically.