CCKA receptor antagonism inhibits mechanisms underlying CCK-8-stimulated insulin release in isolated rat islets

The Cholecystokinin Type 2 Receptor, a Pharmacological Target for Pain Management

Over the past decades, accumulating evidence has demonstrated a pivotal role of cholecystokinin type 2 receptor (CCK2R) in pain modulation. The established role of CCK2R activation in directly facilitating nociception has led to the development of several CCK2R antagonists, which have been shown to successfully alleviate pain in several rodent models of pain. However, the outcomes of clinical trials are more modest since they have not demonstrated the expected biological effect obtained in animals. Such discordances of results between preclinical and clinical studies suggest reconsidering our knowledge about the molecular basis of the pharmacology and functioning of CCK2R. This review focuses on the cellular localization of CCK2R specifically in the sensory nervous system and discusses in further detail the molecular mechanisms and signal transduction pathways involved in controlling pain perception. We then provide a comprehensive overview of the most successful compounds targeting CCK2R and report recent advances in pharmacological strategies used to achieve CCK2R modulation. We purposely distinguish between CCK2R benefits obtained in preclinical models and outcomes in clinical trials with different pain etiologies. Lastly, we emphasize the biological and clinical relevance of CCK2R as a promising target for the development of new treatments for pain management.

The Gq/G11-mediated signaling pathway is critical for autocrine potentiation of insulin secretion in mice

A variety of neurotransmitters, gastrointestinal hormones, and metabolic signals are known to potentiate insulin secretion through GPCRs. We show here that beta cell-specific inactivation of the genes encoding the G protein alpha-subunits Galphaq and Galpha11 resulted in impaired glucose tolerance and insulin secretion in mice. Interestingly, the defects observed in Galphaq/Galpha11-deficient beta cells were not restricted to loss of muscarinic or metabolic potentiation of insulin release; the response to glucose per se was also diminished. Electrophysiological recordings revealed that glucose-induced depolarization of isolated beta cells was impaired in the absence of Galphaq/Galpha11, and closure of KATP channels was inhibited. We provide evidence that this reduced excitability was due to a loss of beta cell-autonomous potentiation of insulin secretion through factors cosecreted with insulin. We identified as autocrine mediators involved in this process extracellular nucleotides such as uridine diphosphate acting through the Gq/G11-coupled P2Y6 receptor and extracellular calcium acting through the calcium-sensing receptor. Thus, the Gq/G11-mediated signaling pathway potentiates insulin secretion in response to glucose by integrating systemic as well as autocrine/paracrine mediators.

The production and role of gastrin-17 and gastrin-17-gly in gastrointestinal cancers

The gastrointestinal peptide hormone gastrin is responsible for initiating the release of gastric acid in the stomach in response to the presence of food and/or humoral factors such as gastrin releasing peptide. However, it has a role in the growth and maintenance of the gastric epithelium, and has been implicated in the formation and growth of gastric cancers. Hypergastrinemia resulting from atrophic gastritis and pernicious anemia leads to hyperplasia and carcinoid formation in rats, and contributes to tumor formation in humans. Additionally, gastrin has been suspected to play a role in the formation and growth of cancers of the colon, but recent studies have instead implicated gastrin processing intermediates, such as gastrin-17-Gly, acting upon a putative, non-cholecystokinin receptor. This review summarizes the production and chemical structures of gastrin and of the processing intermediate gastrin-17-Gly, as well as their activities in the gastrointestinal tract, particularly the promotion of colon cancers.

Therapeutic potential for novel drugs targeting the type 1 cholecystokinin receptor

Cholecystokinin (CCK) is a physiologically important gastrointestinal and neuronal peptide hormone, with roles in stimulating gallbladder contraction, pancreatic secretion, gastrointestinal motility and satiety. CCK exerts its effects via interactions with two structurally related class I guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs), the CCK(1) receptor and the CCK(2) receptor. Here, we focus on the CCK(1) receptor, with particular relevance to the broad spectrum of signalling initiated by activation with the natural full agonist peptide ligand, CCK. Distinct ligand-binding pockets have been defined for the natural peptide ligand and for some non-peptidyl small molecule ligands. While many CCK(1) receptor ligands have been developed and have had their pharmacology well described, their clinical potential has not yet been fully explored. The case is built for the potential importance of developing more selective partial agonists and allosteric modulators of this receptor that could have important roles in the treatment of common clinical syndromes.

Cholecystokinin and gastrin receptors

Cholecystokinin and gastrin receptors (CCK1R and CCK2R) are G protein-coupled receptors that have been the subject of intensive research in the last 10 years with corresponding advances in the understanding of their functioning and physiology. In this review, we first describe general properties of the receptors, such as the different signaling pathways used to exert short- and long-term effects and the structural data that explain their binding properties, activation, and regulation. We then focus on peripheral cholecystokinin receptors by describing their tissue distribution and physiological actions. Finally, pathophysiological peripheral actions of cholecystokinin receptors and their relevance in clinical disorders are reviewed.

Effect of exogenous cholecystokinin on islet blood flow in anesthetized rats

Although a number of studies have investigated the effect of cholecystokinin (CCK) on pancreatic blood flow and exocrine function, few have addressed the effect of CCK on islet blood flow. Here, we studied the effect of exogenous CCK on islet blood flow in anesthetized rats. Islet blood flow was measured by the color microsphere method. Bolus intravenous administration of CCK (10 microg/kg) significantly increased pancreatic and islet blood flow in control Long-Evans Tokushima Otsuka (LETO) rats, but not in Otsuka Long-Evans Tokushima Fatty (OLETF) rats lacking CCK-A receptors. Since fractional islet blood flow expressed as a percentage of whole pancreatic blood flow was decreased after CCK administration in LETO rats, the vasodilating effect of CCK appeared to be stronger in exocrine than endocrine tissue. Although vagotomy failed to alter the CCK-induced increase in pancreatic and islet blood flow, pretreatment with nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine completely prevented the increase in pancreatic and islet blood flow. Our results demonstrated that exogenous CCK is a potent vasodilator of exocrine as well as islet vasculature via CCK-A receptors, and that such action is mediated by a NO-dependent mechanism rather than by vagal mechanisms.

CCK receptor subtype in insulin-producing cells: a combined functional and in situ hybridization study in rat islets and a rat insulinoma cell line

Cholecystokinin (CCK) stimulates insulin secretion. It is, however, not established whether CCK receptors are expressed in insulin-producing cells. We therefore investigated, by in situ hybridization, whether CCK-A or CCK-B receptor mRNA could be detected in normal rat pancreatic islets and in the rat insulinoma cell line, RINm5F. The CCK-A, but not the CCK-B, receptor transcript was detected in both islets and RINm5F cells. Islet CCK-A receptors were mostly confined to the center of the islets corresponding to the distribution of the B cells. In RINm5F cells, insulin release was not significantly affected by cholecystokinin (CCK)-8-S (10(-13) to 10(-7) M), which is in contrast to the insulinotropic effect of CCK-8-S in normal rat islets. Similarly, in FURA-2AM-loaded cells, CCK-8-S (10(-11) to 10(-7) M) was without effect on the intracellular Ca2+ concentration ([Ca2+]ic) in RINm5F cells, whereas CCK-8-S (10(-7) M) markedly increased [Ca2+]ic (by 366+/-2 nM (P < 0.001) in normal rat islet cells. We conclude that the CCK-A, but not the CCK-B, receptor subtype is expressed in both normal rat islets and in the rat insulinoma-derived cell line RINmS5F. There is, however, a functional difference between normal islets and the RINm5F cells with respect to effects of CCK-8-S on insulin release and [Ca2+]ic.

Effects of C-terminal fragments of cholecystokinin on plasma insulin and glucagon concentrations in sheep

The effects of three C-terminal fragments of cholecystokinin (CCK) (CCK-8-sulphated form [SF], CCK-8-non-sulphated form [NSF] and CCK-4) on insulin and glucagon secretion were examined in sheep in vivo. Each CCK fragment was injected intravenously at a wide range of doses (1 pmol to 3 x 10(5) pmol kg-1). CCK-8(SF) had the lowest threshold dose (10 pmol kg-1) and a maximal response dose of 10(3) pmol kg-1 for increasing plasma insulin concentration; the respective threshold doses of CCK-8(NSF) and CCK-8 for increasing plasma insulin were 30 and 100 times greater than that of CCK-8(SF). A maximal insulin response was not obtained at the highest doses of CCK-8(NSF) or CCK-4 tested (3 x 10(3) and 3 x 10(5) pmol kg-1, respectively). These results indicate that CCK-A type receptors rather than CCK-B receptors may be involved in CCK-induced insulin secretion in sheep. None of the CCK fragments affected plasma glucagon concentration. The lack of a glucagon response to exogenous CCK-fragments may be one of the characteristics of the endocrine pancreatic responses of ruminant species.

Effects of CCK-8 on the cytoplasmic free calcium concentration in isolated rat islet cells

The C-terminal octapeptide of cholecystokinin (CCK-8) is known to stimulate insulin secretion. We examined its effects on the cytoplasmic free calcium concentration ([Ca2+]IC) in isolated rat pancreatic islet cells. At 8.3 mM glucose and 1.28 mM Ca2+, CCK-8 (100 nM) rapidly increased [Ca2+]IC to a short-lived peak, whereafter the [Ca2+]IC, within 1.5 minutes, fell to values below baseline. CCK-8 also rapidly increased the [Ca2+]IC at 3.3 mM glucose and in a calcium deficient medium. However, either at low glucose or in the absence of extracellular Ca2+, the post-peak [Ca2+]IC did not fall below baseline levels. The CCKA receptor antagonist, L-364,718 (20 nM), inhibited the effects of CCK-8 on [Ca2+]IC. The results suggest that CCK-8 in islet cells liberates calcium from intracellular stores by activating CCKA receptors.

CCK-8-stimulated insulin secretion in vivo is mediated by CCKA receptors

The effects of the cholecystokinin A (CCKA) receptor antagonist, L-364,718, and the CCKB receptor antagonist, L-365,260, on CCK-8-stimulated insulin secretion were studied in vivo in the mouse. It was found that CCK-8-stimulated insulin secretion was suppressed by L-364,718 at a low dose level (0.078 mumol/kg). In contrast, L-365,260 caused a partial inhibition of CCK-8-stimulated insulin release only at the high dose level (24 mumol/kg). It is concluded that the CCK-8-stimulated insulin release in vivo is mediated by CCK receptors of the CCKA subtype.

Studies on the mechanism by which galanin inhibits insulin secretion in islets

The mechanism by which the neuropeptide galanin inhibits insulin secretion in normal islets is not yet fully elucidated. Isolated rat or mouse islets were perifused in a medium containing glucose (8.3 mM) and galanin (10(-6) M) or the sulphonamide diazoxide (400 microM). In rat islets prelabelled with 86Rb+ or 45Ca2+, galanin inhibited glucose-induced insulin secretion at the same time as increasing 86Rb+ efflux and reducing 45Ca2+ efflux. The diazoxide-induced 86Rb+ efflux was not affected by galanin, indicating that galanin activates ATP-regulated K+ channels in rat islets. In mouse islets prelabelled with 86Rb+, galanin (10(-6) M) decreased 86Rb+ efflux. These results suggest that galanin inhibits insulin release in isolated islets by increasing K+ and decreasing Ca2+ permeability. The increased K+ permeability, which is probably regulated differently in rat and mouse islets, is followed by a reduced Ca2+ influx, possibly through voltage-dependent Ca2+ channels. In addition, during a 60-min incubation with isolated islets, galanin inhibited insulin secretion induced by forskolin (1 microM), dibutyryl cyclic AMP (1 mM), or TPA (12-O-tetradecanoylphorbol-13-acetate; 0.1 microM). Galanin also reduced the content of cyclic AMP in islets stimulated by 16.7 mM glucose. We therefore conclude that the inhibitory action of galanin on insulin secretion in normal islets includes increasing K+ permeability as well as interference with the activation of adenylate cyclase and the activity of protein kinase C and cyclic AMP.