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Interactions between antinociception induced by cholecystokinin antagonists and GABA agonists in the tail-flick test.

In this work, the influences of cholecystokinin receptor antagonists L-365,260, MK-329 and proglumide on antinociception induced by baclofen and GABA uptake inhibitor 4,5,6,7-tetrahydroisoxazolo [4,5-c]pyridin-3-ol (THPO) in the tail flick test has been studied. Baclofen and THPO induced antinociception in the tail flick test. Morphine, and the CCK receptor antagonists, MK-329, L-365,260 and proglumide also induced antinociception. The CCK receptor antagonists potentiated antinociceptive response induced by both baclofen and THPO. It may be concluded that cholecystokinin receptor mechanism(s) may interact with antinociception induced by GABA receptor mechanism(s).

Animals↗

Cholecystokinin receptor mechanism(s) and morphine tolerance in mice.

In a previous work, the effects of cholecystokinin receptor agonists on tolerance to morphine antinociception were evaluated. In the present study, the influence of cholecystokinin antagonists on the inhibition of tolerance to morphine antinociception by cholecystokinin agonists has been investigated. Maximum tolerance to morphine antinociception was obtained by morphine administration (50 mg/kg) to mice once daily for 4 days. The cholecystokinin receptor agonists caerulein (0.005 mg/kg) or cholecystokinin-8 (0.01 mg/kg) but not unsulfated cholecystokinin-8 (0.01 mg/kg) decreased the development of tolerance to morphine (9 mg/kg). The cholecystokininA receptor antagonist MK-329 (1 mg/kg) or the cholecystokininB receptor antagonist L-365,260 (0.25, 0.5 and 1 mg/kg) also diminished the tolerance to morphine antinociception. When animals were challenged with different doses of MK-329 (0.25, 0.5 and 1 mg/kg) against cholecystokinin-8 (0.01 mg/kg), caerulein (0.005 mg/kg) or unsulfated cholecystokinin-8 (0.01 mg/kg) on day 4 in tolerant mice, different response were obtained. Higher doses of MK-329 (1 mg/kg) caused a small decrease in attenuation of the morphine tolerance induced by cholecystokinin-8 and caerulein. Low doses of L-365, 260 diminished the effect of cholecystokinin-8 on morphine tolerance. Conversely high doses of the drug potentiated the response of caerulein (0.005 mg/kg). When animals were treated with MK-329 or L-365,260 before unsulfated cholecystokinin-8, reduction of the tolerance to morphine antinociception was obtained. These data indicate that both cholecystokinin receptors may modulate morphine tolerance.

Animals↗

Melatonin is involved in cholecystokinin-induced changes of ileal motility in rats.

The aim of this study was to determine, in the rat, the interaction between melatonin and cholecystokinin in the regulation of the ileal interdigestive motility. This was analyzed by the chronic electromyography technique. Ileal motility was defined by the presence of intermittent spike bursts corresponding to the contractile activity of the organ. In control rats, these spike bursts were organized in cyclic myoelectrical complexes. Each complex is characterized by two successive spiking activity phases: the irregular phase (ISA) followed by the regular phase (RSA). Pinealectomy suppressed the RSA phase so ileal motility was constituted only by the ISA phase. When melatonin (1 mg/kg i.v.) was injected into pinealectomized rats, RSA phases were immediately and definitively restored. RSA phases were also re-established when the "alimentary" type of cholecystokinin receptors (CCKA) were blocked by selective antagonists such as L364,718 or SR27897 (1 mg/kg i.v.). The latter had better brain accessibility than L364,718. Unlike the effects of melatonin, the effect of these antagonists was neither immediate (the latency is longer for L364,718 than for SR27897) nor definitive. In control rats, cholecystokinin (5 micrograms/kg i.v.) induced a characteristic long-lasting (29 +/- 2 min) excitomotor effect on the ileum. This effect was suppressed in pinealectomized rats and was restored after melatonin treatment. These results suggest that, via the central nervous system, melatonin is involved in the modulation of cholecystokinin action on ileal motility.

Animals↗

Effects of three different cholecystokinin receptor antagonists on basal and stimulated insulin and glucagon secretion in mice.

Cholecystokinin (CCK) receptor antagonists may be valuable tools for investigating the physiological role of CCK in islet function. In this study, the effects of the three different CCK receptor antagonists, proglumide, CR 1409 and L-364,718, on basal and stimulated insulin and glucagon secretion were investigated in vivo in the mouse. Each of the CCK antagonists was injected intravenously, either alone or together with one of the secretagogues CCK-8 (5.3 nmol kg-1), carbachol (0.16 mumol kg-1) or glucose (2.8 mmol kg-1). At a low dose level, proglumide (28 mumol kg-1) inhibited selectively CCK-8-induced insulin and glucagon secretion. However, at a higher dose level (280 mumol kg-1), proglumide inhibited also carbachol- and glucose-induced insulin secretion. Furthermore, proglumide elevated basal plasma levels of both glucagon and glucose. CR 1409 inhibited CCK-8-induced insulin secretion at a high (21 mumol kg-1) but not at a low (0.21 mumol kg-1) dose level. In contrast, CCK-8-induced glucagon secretion was not affected by CR 1409. L-364,718 (2.4 mumol kg-1) inhibited both CCK-8-induced insulin and glucagon secretion. In contrast, L-364,718 did not affect basal plasma levels of insulin, glucagon or glucose or those levels after stimulation with carbachol or glucose. We conclude that, of these three CCK antagonists, L-364,718 is the most specific CCK receptor antagonist for studies of both insulin and glucagon secretion in the mouse.

Animals↗

Cholecystokinin receptor family. Molecular cloning, structure, and functional expression in rat, guinea pig, and human.

A review of the literature encompassing numerous pharmacological, physiological, and biochemical studies indicates the presence of at least four CCK receptor types, CCKA, CCKB, gastrin, and CG-4 receptors. Multiple subtypes of the CCKAR have been postulated to account for the differences in pharmacology or affinity cross-linking of CCKARs between pancreas and gallbladder and the presence of high and low affinity CCKARs on pancreatic acini. Multiple subtypes of the CCKBR have been postulated to explain the differences in pharmacology and physiology between gastric and gallbladder smooth muscle CCKBRs. We recently cloned and functionally expressed both the CCKAR and the CCKBR from rat, guinea pig, and human. The CCKAR and CCKBR are 48% homologous and constitute a family of receptors within the guanine nucleotide-binding regulatory protein-coupled superfamily of receptors. Each receptor is highly conserved between species. A single cDNA encoding a single protein is present in both pancreas and gallbladder and can account for both high and low affinity CCKARs found on pancreatic acini when transfected into COS-7 cells. A single cDNA encoding a single CCKBR protein is present in both the central nervous system and the periphery including the gastrointestinal system. Therefore, the gastrin receptor is actually a CCKBR present on parietal cells. Genomic and cDNA library hybridization as well as Northern and Southern hybridization studies among rat, guinea pig, and human species identifies only two members of the CCK receptor family, CCKAR and CCKBR. Although these studies do not identify other closely related members of the CCK receptor family, they do not rule out the existence of other less closely related members. Furthermore, differences in tissue and species-specific posttranslational processing, receptor coupling, and associated membrane protein and lipid heterogeneity may be among some of the other factors that may account for the phenotypic expression of more receptor subtypes than molecular studies would predict.

Amino Acid Sequence↗

Integration of postprandial function in the proximal gastrointestinal tract. Role of CCK and sensory pathways.

Cholecystokinin (CCK) stimulates vagal afferent fiber discharge, both gastric and intestinal, which seems to result in reflex decrease in gastric motility, gastric acid secretion, and stimulation of pancreatic protein secretion. Endogenous release of CCK by fat or soybean trypsin inhibitor also alters function by way of a capsaicin-sensitive pathway. We suggest that CCK is released locally from the intestine and acts locally or systemically to stimulate vagal afferent fiber discharge to alter proximal gastrointestinal function (Fig. 14). In this way, in addition to its effect on food intake, CCK and the neural pathway integrate function in the proximal gastrointestinal tract, regulating the entry of food into the duodenum to ensure effective digestion and absorption.

Afferent Pathways↗

Effect of cholecystokinin on gastric motility in humans.

Gastric emptying after food ingestion is regulated by neural and hormonal factors. However, the relative contributions of each pathway is not yet clearly defined. The classic gut hormone CCK seems to be involved in the regulation of gastric emptying in humans. Experimental evidence is best for gastric emptying of liquid meals that release CCK from the duodenum: (1) CCK infused at postprandial plasma concentrations inhibits gastric emptying of a liquid and a semisolid meal. (2) Administration of the CCK antagonist loxiglumide significantly accelerated gastric emptying of a liquid mixed meal and a glucose meal. Discrepant results with the antagonist MK329 are difficult to explain considering the marked acceleration of gastric emptying rates by the specific and potent antagonist MK329 shown in several animal studies. Taken together, current information favors the conclusion, however, that CCK mainly controls gastric emptying of the liquid but not the solid components. Thus, CCK is involved in the physiologic regulation of gastric emptying and gastric motility in man. Blocking CCK-A receptors accelerates gastric emptying of liquid meals and abolishes the gastrocolonic reflex. Therefore, CCK may play a role as a common regulator of postprandial gallbladder contraction and pancreatic enzyme secretion as well as of gastric emptying rates under certain conditions. Such common control would optimize the nutrient-to-digestive juices concentration ratio. The importance of endogenous CCK on gastric emptying of solid meals, however, is poorly understood and remains to be defined. Only very limited information is available on gastric motility. Much more work has to be done before a clear concept can be developed.

Benzodiazepinones↗