Screening for positive allosteric modulators of cholecystokinin type 1 receptor potentially useful for management of obesity

Obesity has become a prevailing health burden globally and particularly in the US. It is associated with many health problems, including cardiovascular disease, diabetes and poorer mental health. Hence, there is a high demand to find safe and effective therapeutics for sustainable weight loss. Cholecystokinin (CCK) has been implicated as one of the first gastrointestinal hormones to reduce overeating and suppress appetite by activating the type 1 cholecystokinin receptor (CCK1R). Several drug development campaigns have focused on finding CCK1R-specific agonists, which showed promising efficacy for reducing meal size and weight, but fell short on FDA approval, likely due to side effects associated with potent, long-lasting activation of CCK1Rs. Positive allosteric modulators (PAMs) without inherent agonist activity have been proposed to overcome the shortcomings of traditional, orthosteric agonists and restore CCK1R signaling in failing physiologic systems. However, drug discovery campaigns searching for such novel acting CCK1R agents remain limited. Here we report a high-throughput screening effort and the establishment of a testing funnel, which led to the identification of novel CCK1R modulators. We utilized IP-One accumulation to develop robust functional equilibrium assays tailored to either detect PAMs, agonists or non-specific activators. In addition, we established the CCK1R multiplex PAM assay as a novel method to evaluate functional selectivity capable of recording CCK1R-induced cAMP accumulation and β-arrestin recruitment in the same well. This selection and arrangement of methods enabled the discovery of three scaffolds, which we characterized and validated in an array of functional and binding assays. We found two hits incorporating a tetracyclic scaffold that significantly enhanced CCK signaling at CCK1Rs without intrinsically activating CCK1Rs in an overexpressing system. Our results demonstrate that a well-thought-out testing funnel can identify small molecules with a distinct pharmacological profile and provides an important milestone for the development of novel potential treatments of obesity.

Roles of Cholecystokinin in the Nutritional Continuum. Physiology and Potential Therapeutics

Cholecystokinin is a gastrointestinal peptide hormone with important roles in metabolic physiology and the maintenance of normal nutritional status, as well as potential roles in the prevention and management of obesity, currently one of the dominant causes of direct or indirect morbidity and mortality. In this review, we discuss the roles of this hormone and its receptors in maintaining nutritional homeostasis, with a particular focus on appetite control. Targeting this action led to the development of full agonists of the type 1 cholecystokinin receptor that have so far failed in clinical trials for obesity. The possible reasons for clinical failure are discussed, along with alternative pharmacologic strategies to target this receptor for prevention and management of obesity, including development of biased agonists and allosteric modulators. Cellular cholesterol is a natural modulator of the type 1 cholecystokinin receptor, with elevated levels disrupting normal stimulus-activity coupling. The molecular basis for this is discussed, along with strategies to overcome this challenge with a corrective positive allosteric modulator. There remains substantial scope for development of drugs to target the type 1 cholecystokinin receptor with these new pharmacologic strategies and such drugs may provide new approaches for treatment of obesity.

Activation of multiple receptors stimulates extracellular vesicle release from trophoblast cells

Reports of the stimulated release of extracellular vesicles (EVs) are few, and the mechanisms incompletely understood. To our knowledge, the possibility that the activation of any one of the multitudes of G-protein-coupled receptors (GPCRs) expressed by a single cell-type might increase EV release has not been explored. Recently, we identified the expression of cholecystokinin (CCK), gastrin, gastrin/cholecystokinin types A and/or B receptors (CCKAR and/or -BR), and the bitter taste receptor, TAS2R14 in the human and mouse placenta. specifically, trophoblast. These GPCR(s) were also expressed in four different human trophoblast cell lines. The current objective was to employ two of these cell lines-JAR choriocarcinoma cells and HTR-8/SVneo cells derived from first-trimester human villous trophoblast-to investigate whether CCK, TAS2R14 agonists, and other GPCR ligands would each augment EV release. EVs were isolated from the cell-culture medium by filtration and ultracentrifugation. The preparations were enriched in small EVs (<200 nm) as determined by syntenin western blot before and after sucrose gradient purification, phycoerythrin (PE)-ADAM10 antibody labeling, and electron microscopy. Activation of TAS2R14, CCKBR, cholinergic muscarinic 1 & 3, and angiotensin II receptors, each increased EV release by 4.91-, 2.79-, 1.87-, and 3.11-fold, respectively (all p < .05 versus vehicle controls), without significantly changing EV diameter. A progressive increase of EV concentration in conditioned medium was observed over 24 hr consistent with the release of preformed EVs and de novo biogenesis. Compared to receptor-mediated stimulation, EV release by the calcium ionophore, A23187, was less robust (1.63-fold, p = .08). Diphenhydramine, a TAS2R14 agonist, enhanced EV release in JAR cells at a concentration 10-fold below that required to increase intracellular calcium. CCK activation of HTR-8/SVneo cells, which did not raise intracellular calcium, increased EV release by 2.06-fold (p < .05). Taken together, these results suggested that other signaling pathways may underlie receptor-stimulated EV release besides, or in addition to, calcium. To our knowledge, the finding that the activation of multiple GPCRs can stimulate EV release from a single cell-type is unprecedented and engenders a novel thesis that each receptor may orchestrate intercellular communication through the release of EVs containing a subset of unique cargo, thus mobilizing a specific integrated physiological response by a network of neighboring and distant cells.

CCK-1 and CCK-2 receptor agonism do not stimulate GLP-1 and neurotensin secretion in the isolated perfused rat small intestine or GLP-1 and PYY secretion in the rat colon

Gastrin and cholecystokinin (CCK) are hormones released from endocrine cells in the antral stomach (gastrin), the duodenum, and the jejunum (CCK). Recent reports, based on secretion experiments in an enteroendocrine cell line (NCI-H716) and gastrin receptor expression in proglucagon-expressing cells from the rat colon, suggested that gastrin could be a regulator of glucagon-like peptide-1 (GLP-1) secretion. To investigate these findings, we studied the acute effects of CCK-8 (a CCK1/CCK2 (gastrin) receptor agonist) and gastrin-17 (a CCK2(gastrin) receptor agonist) in robust ex vivo models: the isolated perfused rat small intestine and the isolated perfused rat colon. Small intestines from Wistar rats (n = 6), were perfused intraarterially over 80 min. During the perfusion, CCK (1 nmol/L) and gastrin (1 nmol/L) were infused over 10-min periods separated by washout/baseline periods. Colons from Wistar rats (n = 6) were perfused intraarterially over 100 min. During the perfusion, CCK (1 nmol/L), vasoactive intestinal peptide (VIP) (10 nmol/L), and glucose-dependent insulinotropic polypeptide (GIP) (1 nmol/L) were infused over 10-min periods separated by washout/baseline periods. In the perfused rat small intestines neither CCK nor gastrin stimulated the release of GLP-1 or neurotensin. In the perfused rat colon, neither CCK or VIP stimulated GLP-1 or peptide YY (PYY) release, but GIP stimulated both GLP-1 and PYY release. In both sets of experiments, bombesin, a gastrin-releasing peptide analog, served as a positive control. Our findings do not support the suggestion that gastrin or CCK participate in the acute regulation of intestinal GLP-1 secretion, but that GIP may play a role in the regulation of hormone secretion from the colon.

Cholecystokinin, gastrin, cholecystokinin/gastrin receptors, and bitter taste receptor TAS2R14: trophoblast expression and signaling

We investigated expression of cholecystokinin (CCK) in humans and mice, and the bitter taste receptor TAS2R14 in the human placenta. Because CCK and gastrin activate the CCKBR receptor, we also explored placental gastrin expression. Finally, we investigated calcium signaling by CCK and TAS2R14. By RT-PCR, we found CCK/Cck and GAST/Gast mRNA expression in both normal human and mouse placentas, as well as in human trophoblast cell lines (TCL). Although both Cckar and -br mRNA were expressed in the mouse placenta, only CCKBR mRNA was detected in the human placenta and TCL. mRNA expression for TAS2R14 was also observed in the human placenta and TCL. Using immunohistochemistry, CCK protein was localized to the syncytiotrophoblast (ST) and extravillous trophoblast (EVT) in the human term placenta, and to trophoblast glycogen cells in mouse and human placentas. Gastrin and TAS2R14 proteins were also observed in ST and EVT of the human placenta. Both sulfated and nonsulfated CCK elicited a comparable rise in intracellular calcium in TCL, consistent with CCKBR expression. Three TAS2R14 agonists, flufenamic acid, chlorhexidine, and diphenhydramine, also evoked rises in intracellular calcium in TCL. These results establish CCK, gastrin, and their receptor(s) in both human and mouse placentas, and TAS2R14 in the human placenta. Both CCK and TAS2R14 agonists increased intracellular calcium in human TCL. Although the roles of these ligands and receptors, and their potential cross talk in normal and pathological placentas, are currently unknown, this study opens new avenues for placental research.

Metabolic Actions of the Type 1 Cholecystokinin Receptor: Its Potential as a Therapeutic Target

Cholecystokinin (CCK) regulates appetite and reduces food intake by activating the type 1 CCK receptor (CCK1R). Attempts to develop CCK1R agonists for obesity have yielded active agents that have not reached clinical practice. Here we discuss why, along with new strategies to target CCK1R more effectively. We examine signaling events and the possibility of developing agents that exhibit ligand-directed bias, to dissociate satiety activity from undesirable side effects. Potential allosteric sites of modulation are also discussed, along with desired properties of a positive allosteric modulator (PAM) without intrinsic agonist action as another strategy to treat obesity. These new types of CCK1R-active drugs could be useful as standalone agents or as part of a rational drug combination for management of obesity.

Calcium handling in porcine coronary endothelial cells by gastrin-17

In porcine coronary artery endothelial cells (PCAEC), gastrin-17 has recently been found to increase nitric oxide (NO) production by the endothelial NO synthase (eNOS) isoform through cholecystokinin 1/2 (CCK1/2) receptors and the involvement of protein kinase A (PKA), PKC and the β2-adrenoreceptor-related pathway. As eNOS is the Ca(2)(+)-dependent isoform of the enzyme, we aimed to examine the effects of gastrin-17 on Ca(2)(+) movements. Thus, experiments were performed in Fura-2-acetoxymethyl-ester-loaded PCAEC, where changes of cytosolic Ca(2)(+) ([Ca(2)(+)]c) caused by gastrin-17 were analysed and compared with those of CCK receptors and β2-adrenoreceptors agonists/antagonists. In addition, some experiments were performed by stimulating cells with gastrin-17 in the presence or absence of cAMP/PKA activator/inhibitor and of phospholipase C (PLC) and Ca(2)(+)-calmodulin dependent protein kinase II (CaMKII) blockers. The results have shown that gastrin-17 can promote a transient increase in [Ca(2)(+)]c mainly originating from an intracellular pool sensitive to thapsigargin and from the extracellular space. In addition, the response of cells to gastrin-17 was increased by the adenylyl cyclase activator and the β2-adrenoreceptor agonists and affected mainly by the CCK2 receptor agonists/antagonists. Moreover, the effects of gastrin-17 were prevented by β2-adrenoreceptors and CaMKII blockers and the adenylyl cyclase/PKA and PLC inhibitors. Finally, in PCAEC cultured in Na(+)-free medium or loaded with the plasma membrane Ca(2)(+) pump inhibitor, the gastrin-17-evoked Ca(2)(+) transient was long lasting. In conclusion, this study shows that gastrin-17 affected intracellular Ca(2)(+) homeostasis in PCAEC by both promoting a discharge of an intracellular pool and by interfering with the operation of store-dependent channels through mainly CCK2 receptors and PKA/PLC- and CaMKII-related signalling downstream of β2-adrenoreceptor stimulation.

Functional compensation between cholecystokinin-1 and -2 receptors in murine paraventricular nucleus neurons

Cholecystokinin (CCK) and its receptor subtypes CCK-1 and -2 have diverse homeostatic functions. CCK-1 and -2 receptors share a common phosphatidylinositol signaling pathway, yet little is known regarding their possible functional coupling. We focused on CCK-mediated Ca(2+) signaling in parvocellular paraventricular nucleus (PVN) cells, which control satiety and other autonomic functions. Analysis of mouse hypothalamic slices demonstrated that the general CCK receptor agonist CCK-8s (10 nM) triggered Ca(2+) transients most significantly in the posterior subregion of the PVN (PaPo). This 10 nM CCK-8s-induced response was absent in CCK-1 receptor knock-out (CCK1R(-/-)) slices, showing that the response is mediated by CCK-1 receptors. CCK-8s concentrations higher than 30 nM triggered a Ca(2+) rise similarly in wild-type and CCK1R(-/-) slices. The large CCK-8s (100 nM)-induced Ca(2+) responses in CCK1R(-/-) slices were blocked by a CCK-2 receptor antagonist (CI-988), whereas those in wild-type slices required a mixture of CI-988 and lorglumide (a CCK-1 receptor antagonist) for complete antagonism. Therefore, CCK-1 and -2 receptors may function synergistically in single PaPo neurons and deletion of CCK-1 receptors may facilitate CCK-2 receptor signaling. This hypothesis was supported by results of real-time RT-PCR, immunofluorescence double labeling and Western blotting assays, which indicated CCK-2 receptor overexpression in PaPo neurons of CCK1R(-/-) mice. Furthermore, behavioral studies showed that intraperitoneal injections of lorglumide up-regulated food accesses in wild-type but not in CCK1R(-/-) mice, whereas CI-988 injections up-regulated food accesses in CCK1R(-/-) but not in wild-type mice. Compensatory CCK signaling via CCK-2 receptors in CCK1R(-/-) mice shed light on currently controversial satiety-controlling mechanisms.

Relationship between vulnerability to reinforcing effects of morphine and activity of the endogenous cholecystokinin system in Lewis and Fischer rats

A great number of studies have shown the presence of physiological interactions between brain neurotransmitter systems in behavioural responses. This is the case for opioid, cholecystokinin (CCK) and dopamine systems. However, so far the role that the CCK system may play in vulnerability to consumption of drugs of abuse is not clear. This was investigated in this study using Lewis rats that are more sensitive to the reinforcing properties of drugs of abuse than Fischer rats. The extraneuronal CCK(8) levels and brain CCK(2) receptors were found higher in Fischer than in Lewis rats in the nucleus accumbens, one of the most important structures involved in drug consumption. Moreover, pharmacological modulation of the CCK system by administration of a selective CCK(2) agonist blocked, in the conditioned place preference, the reinforcing effects of morphine in Lewis rats, whereas a selective CCK(2) antagonist revealed reinforcing effects of the alkaloid in Fischer rats. These results obtained following systemic administrations of the CCK ligands were confirmed following microinjection into the nucleus accumbens. Thus, a low level of CCK efflux in the nucleus accumbens could be one of the many factors involved in drug reinforcing effects, whereas a high level of CCK efflux could attenuate it.

Time-resolved quantitative analysis of CCK1 receptor-induced intracellular calcium increase

Cholecystokinin (CCK) is a gastrointestinal hormone, which regulates many physiological functions such as satiety by binding to the CCK receptor (CCKR). Molecules, which recognize this receptor can mimic or block CCK signaling and thereby influence CCKR-mediated processes. We have set up a quantitative heterologous assay with CHO cells over-expressing the rat CCK1 receptor to screen for such candidate molecules. Receptor activation, induced by agonist binding, is followed by an intracellular calcium increase, which was monitored using a fluorescent sensor dye. For quantification of the calcium increase, a population average technique using a fluorescence plate reader was optimized and subsequently compared with a single-cell approach using confocal microscopy. With both strategies, dose-response curves were generated for the natural agonist CCK-8S, the partial agonist JMV-180 as well as the antagonist lorglumide. Significant differences were found between the ligands and a strong correspondence was observed between both methods in terms of maximum response and median effect concentrations. Both highly sensitive methods proved complementary: whereas the plate reader assay allowed faster, high throughput screening, the confocal microscopy identified single-cell variations and revealed factors that reduce specificity and sensitivity.

CCK stimulation of GLP-1 neurons involves α1-adrenoceptor-mediated increase in glutamatergic synaptic inputs

OBJECTIVE

Glucagon-like peptide 1 (GLP-1) is involved in the central regulation of food intake. It is produced within the brain by preproglucagon (PPG) neurons, which are located primarily within the brain stem. These neurons project widely throughout the brain, including to the appetite centers in the hypothalamus, and are believed to convey signals related to satiety. Previous work demonstrated that they are directly activated by leptin and electrical activity of the afferent vagus. Another satiety hormone, cholecystokinin (CCK), has also been linked to activation of brain stem neurons, suggesting that it might act partially via centrally projecting neurons from the nucleus tractus solitarius (NTS). The aim of this study was to investigate the neuronal circuitry linking CCK to the population of NTS-PPG neurons.

RESEARCH DESIGN AND METHODS

Transgenic mice expressing yellow fluorescent protein (Venus) under the control of the PPG promoter were used to identify PPG neurons in vitro and to record their electrical and pharmacological profile.

RESULTS

PPG neurons in the NTS were excited by CCK and epinephrine, but not by the melanocortin receptor agonist melanotan II. Both CCK and epinephrine acted to increase glutamatergic transmission to the PPG neurons, and this involved activation of α(1)-adrenergic receptors. Inhibition of adrenergic signaling abolished the excitatory action of CCK.

CONCLUSIONS

CCK activates NTS-PPG cells by a circuit involving adrenergic and glutamatergic neurons. NTS-PPG neurons integrate a variety of peripheral signals that indicate both long-term energy balance and short-term nutritional and digestional status to produce an output signal to feeding and autonomic circuits.

The CCK(-like) receptor in the animal kingdom: functions, evolution and structures

In this review, the cholecystokinin (CCK)(-like) receptors throughout the animal kingdom are compared on the level of physiological functions, evolutionary basis and molecular structure. In vertebrates, the CCK receptor is an important member of the G-protein coupled receptors as it is involved in the regulation of many physiological functions like satiety, gastrointestinal motility, gastric acid secretion, gall bladder contraction, pancreatic secretion, panic, anxiety and memory and learning processes. A homolog for this receptor is also found in nematodes and arthropods, called CK receptor and sulfakinin (SK) receptor, respectively. These receptors seem to have evolved from a common ancestor which is probably still closely related to the nematode CK receptor. The SK receptor is more closely related to the CCK receptor and seems to have similar functions. A molecular 3D-model for the CCK receptor type 1 has been built together with the docking of the natural ligands for the CCK and SK receptors in the CCK receptor type 1. These molecular models can help to study ligand-receptor interactions, that can in turn be useful in the development of new CCK(-like) receptor agonists and antagonists with beneficial health effects in humans or potential for pest control.

Structure-activity relationships of bifunctional cyclic disulfide peptides based on overlapping pharmacophores at opioid and cholecystokinin receptors

Prolonged opioid exposure increases the expression of cholecystokinin (CCK) and its receptors in the central nervous system (CNS), where CCK may attenuate the antinociceptive effects of opioids. The complex interactions between opioid and CCK may play a role in the development of opioid tolerance. We designed and synthesized cyclic disulfide peptides and determined their agonist properties at opioid receptors and antagonist properties at CCK receptors. Compound 1 (Tyr-c[d-Cys-Gly-Trp-Cys]-Asp-Phe-NH(2)) showed potent binding and agonist activities at delta and mu opioid receptors but weak binding to CCK receptors. The NMR structure of the lead compound displayed similar conformational features of opioid and CCK ligands.

Discovery of imidazole carboxamides as potent and selective CCK1R agonists

High-throughput screening revealed diaryl pyrazole 3 as a selective albeit modest cholecystokinin 1 receptor (CCK1R) agonist. SAR studies led to the discovery and optimization of a novel class of 1,2-diaryl imidazole carboxamides. Compound 44, which was profiled extensively, showed good in vivo mouse gallbladder emptying (mGBE) and lean mouse overnight food intake (ONFI) reduction activities.

Pharmacology of CCKRs and SAR studies of peptidic analog ligands

Cholecystokinin (CCK) is a peptide originally discovered in the gastrointestinal tract but also found in high density in the mammalian brain. The C-terminal sulphated octapeptide fragment of cholecystokinin (CCK(8)) constitutes one of the major neuropeptides in the brain. CCK8, interacting with nanomolar affinities with two different receptors designated CCK1 and CCK2, has been shown to be involved in numerous physiological functions and is involved in the modulation/control of multiple central functions. In particular, CCK is involved in the neurobiology of anxiety, depression, psychosis, cognition, nociception and feeding behavior. The functional role of CCK has been facilitated thanks to the development of potent and selective CCK receptor antagonists and agonists. In this review, the strategies followed to design peptidic analog ligands will be reported, as the pharmacology of CCK receptors.

Strategies for design of non peptide CCK1R agonist/antagonist ligands

This review mainly covers last five year literature on CCK1R agonists and antagonists. These CCK1R ligands have been found following the two usual and complementary strategies for drug discovery: rational design based on structure activity relationships on the CCK-7 and CCK-4 peptide sequences of the endogenous ligands and random screening of diverse compounds, followed by hit optimization. The first group includes: chimeric bifunctional opioid/CCK peptides, designed as opioid agonists with balanced CCK1R/CCK2R antagonist activity for the treatment of neuropathic pain, antagonist and agonist dipeptoids, and 1,3-dioxoperhydropyrido[1,2-c]pyrimidine- and anthranilic acid-based antagonists. Among the ligands derived from random screening, a few new 1,4-benzodiazepine-, 1,5-benzodiazepine-, and five member ring heterocycle-based CCK1R ligands have been reported. Finally, taking into account the importance of receptor mapping studies for ligand optimization and future precise de novo receptor structure-based design of new selective and more effective ligands, the most significant conclusions of these studies have also been reviewed.

Biochemical and Cell Biological Mechanisms of Cholecystokinin Receptor Regulation

Regulation of the cholecystokinin receptor is accomplished by biochemical and cell biological mechanisms. The major mechanism for biochemical regulation involves phosphorylation of serine and threonine residues within the receptor's intracellular third loop and carboxyl-terminal tail. This form of rapid desensitization is achieved by protein kinase C, a kinase activated in the normal signaling cascade of this Gq-coupled receptor, and/or a member of the G protein-coupled receptor kinase family that recognizes the active conformation of the receptor. Conversely, a type 2A serine protein phosphatase has been shown to selectively act on this receptor in an agonist-regulated manner, resulting in receptor dephosphorylation and resensitization. Cell biological mechanisms for desensitization involve the net removal of receptors from exposure to circulating hormone via insulation within a specialized plasma membrane domain or internalization into the cell within endocytic compartments. Internalization has been shown to occur by both clathrin-dependent and clathrin-independent mechanisms, the latter believed to represent potocytosis in caveolae, that lead to recycling of receptors to the plasma membrane for resensitization or shuttling of receptors to lysosomes for degratory down-regulation. Interestingly, receptor phosphorylation has been shown to affect intracellular domain accessibility and receptor trafficking, although the specific proteins regulating this have not yet been identified. The cholecystokinin receptor can exist in the plasma membrane as oligomeric superstructures, namely as homomers with themselves or as heteromers with closely related class I G protein-coupled receptors. Agonist occupation results in oligomer dissociation, but the functional significance of this observation has yet to be defined.

Role of lysine187 within the second extracellular loop of the type A cholecystokinin receptor in agonist-induced activation. Use of complementary charge-reversal mutagenesis to define a functionally important interdomain interaction

Activation of guanine nucleotide-binding protein (G protein)-coupled receptors is believed to involve conformational change that exposes a domain for G protein coupling at the cytosolic surface of the helical confluence, although the mechanisms for achieving this are not well understood. This conformational change can be achieved by docking a diverse variety of agonist ligands, known to occur by interacting with different regions of these receptors. In this study, we focus on the importance of a specific basic residue (Lys187) within the second extracellular loop of the receptor for the peptide hormone, cholecystokinin. Alanine-replacement and charge-reversal mutagenesis of this residue showed that it had no effect on the binding of natural peptide and nonpeptidyl ligands of this receptor but markedly interfered with agonist-stimulated signaling. It was demonstrated that this negative effect on biological activity could be eliminated with the truncation of the first 30 residues of the amino-terminal tail of this receptor. Complementary charge-reversal mutagenesis of each of the five conserved acidic residues within this region of the receptor in the presence of the charge-reversed Lys187 revealed that only the Asp5 mutant fully reversed the negative functional impact of the Lys187 charge reversal. Thus, we have demonstrated that a basic residue within the second extracellular loop of the cholecystokinin receptor interacts with a specific acidic residue within the amino terminus of this receptor. This residue-residue interaction is nicely accommodated within a new molecular model of the agonist-occupied cholecystokinin receptor.

Role of CCK/gastrin receptors in gastrointestinal/metabolic diseases and results of human studies using gastrin/CCK receptor agonists/antagonists in these diseases

In this paper, the established and possible roles of CCK1 and CCK2 receptors in gastrointestinal (GI) and metabolic diseases are reviewed and available results from human agonist/antagonist studies are discussed. While there is evidence for the involvement of CCK1R in numerous diseases including pancreatic disorders, motility disorders, tumor growth, regulation of satiety and a number of CCK-deficient states, the role of CCK1R in these conditions is not clearly defined. There are encouraging data from several clinical studies of CCK1R antagonists in some of these conditions, but their role as therapeutic agents remains unclear. The role of CCK2R in physiological (atrophic gastritis, pernicious anemia) and pathological (Zollinger-Ellison syndrome) hypergastrinemic states, its effects on the gastric mucosa (ECL cell hyperplasia, carcinoids, parietal cell mass) and its role in acid-peptic disorders are clearly defined. Furthermore, recent studies point to a possible role for CCK2R in a number of GI malignancies. Current data from human studies of CCK2R antagonists are presented and their potential role in the treatment of these conditions reviewed. Furthermore, the role of CCK2 receptors as targets for medical imaging is discussed.

Signal transduction pathways mediating CCK-8S-induced gastric antral smooth muscle contraction

AIM

To investigate the functional and molecular mechanisms by which sulfated cholecystokinin octapeptide (CCK-8S) regulates calcium mobilization in gastric antral smooth muscle cells (SMCs) of rats.

METHODS

Isotonic contraction of antral strips was recorded using a polyphysiograph. Immunoprecipitation was used to determine the regulatory effect of protein kinase C (PKC) on regulating the phosphorylation of the type III inositol 1,4,5-triphosphate receptor (InsP(3)R3) in gastric SMCs. Alterations in the intracellular calcium ([Ca(2+)](i)) concentration were assayed using fura-2/AM-loaded microspectrofluorometry, and the L-type calcium current (I(Ca-L)) was recorded by patch-clamp techniques.

RESULTS

CCK-8S (5 x 10(-8) mol/l) significantly increased the mean contractile amplitude of circular muscle by 61.85 +/- 12.67% and the frequency of longitudinal muscle by 57.91 +/- 15.70% in gastric antral strips, which were suppressed by dexloxiglumide or thapsigargin (TG) and BAPTA-AM (BA). Treatment with chelerythrine (5 x 10(-8) mmol/l) significantly inhibited the CCK-8S-increased phosphorylation of InsP(3)R3 in SMCs. The amplitudes of the CCK-8S-triggered [Ca(2+)](i) concentration oscillations were reduced in a dose-dependent manner when the SMCs were pretreated with increasing concentrations of PMA (from 10(-8) to 10(-5) mol/l). On removal of extracellular calcium or blocking I(Ca-L) by nifedipine, a smaller but significant rise in the [Ca(2+)](i) concentration was still elicited by CCK-8S. When [Ca(2+)](i) was depleted by the administration of 10(-5) mol/l TG and 10(-5) mol/l BA or blocked by the calcium-dependent chloride current (I(Cl-Ca)) by giving 5 x 10(-6) mol/l niflumic acid, the CCK-8S-intensified I(Ca-L) (from -56.42 +/- 6.57 to -88.54 +/- 5.71 pA) was apparently inhibited by 90.34 +/- 4.71% and 82.59 +/- 4.24%.

CONCLUSIONS

These results demonstrate that the CCK-8S-evoked [Ca(2+)](i) concentration increase in gastric antral SMCs depends on the release of [Ca(2+)](i) stores which are negatively regulated by PKC-mediated phosphorylation of InsP(3)R3. Released calcium in turn activates I(Ca-L) through the activation of I(Cl-Ca), ultimately resulting in the contraction of the gastric smooth muscle.

Unexpected relationship between plasma protein binding and the pharmacodynamics of 2-NAP, a CCK1-receptor antagonist

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT?: * Two chemically diverse CCK1 receptor antagonists have been shown clinically to inhibit CCK-evoked contraction of human gallbladder [2, 3]. These studies have not examined the relationship between plasma concentration and effect, the latter usually considered to be predictive from the free drug concentration [8]. * We wanted to examine our novel CCK1 receptor antagonist in this validated model and also to explore its PK-PD relationship.

WHAT THIS STUDY ADDS

* 2-NAP inhibited CCK-evoked human gallbladder contraction in vivo but at a plasma free concentration that was, in theory, too low to have achieved adequate CCK1 receptor occupancy. * The study serves as a caveat to the assumption that free plasma concentration can be used to predict pharmacological effect.

AIMS

To study the pharmacokinetics and pharmacodynamics of 2-NAP (2-naphthalenesulfonyl-L-aspartyl-(2-phenethyl)amide), a selective CCK1 receptor antagonist in healthy volunteers.

METHODS

2-NAP was given to 12 healthy male volunteers in an ascending dose, safety and PK phase 1a study by 1 h i.v. infusion (0.6-9.6 mg kg(-1) h(-1)). A further 12 healthy male volunteers received i.v. CCK-8S (6.25 pmol kg(-1) h(-1)) to produce gallbladder contraction, measured by ultrasound recordings of gallbladder volume, and the effect of concurrent i.v. 2-NAP administration was studied. Plasma protein binding in vitro and ex vivo was measured by ultrafiltration and by equilibrium dialysis.

RESULTS

2-NAP was generally well tolerated, displayed linear pharmacokinetics and a very high degree of plasma protein binding (99.9%). A 105 min i.v. CCK-8S infusion induced a reduction in gallbladder volume of 14.9 (+/-7.0) ml during placebo co-infusion and this was reduced to 2.4 (+/-5.9) ml when 2-NAP was co-infused with CCK-8S (P = 0.00024, paired t-test, mean change 12.5 ml; 95% CI For mean 7.4, 18.3 ml). This extent of inhibition was consistent with a 2-NAP total plasma concentration of 36 microm, but when protein binding corrections were made, the 'free concentration' of 2-NAP was only 0.04 microm, a value much less than the average equilibrium dissociation constant of 2-NAP for human CCK1 receptors ( approximately 0.7 microm).

CONCLUSIONS

The pharmacological effect of a drug is usually considered to be determined by its free concentration. However, the complete inhibition of CCK-8S-evoked gallbladder contraction by a free plasma concentration of 0.04 microm 2-NAP was much greater than would have been predicted from simple drug-receptor occupancy theory and cautions against the general use of free concentration of drug for predicting pharmacological effect.

Fluorescence Characteristics of Hydrophobic Partial Agonist Probes of the Cholecystokinin Receptor

Fluorescence spectroscopic studies are powerful tools for the evaluation of receptor structure and the dynamic changes associated with receptor activation. Here, we have developed two chemically distinct fluorescent probes of the cholecystokinin (CCK) receptor by attaching acrylodan or a nitrobenzoxadiazole moiety to the amino terminus of a partial agonist CCK analogue. These two probes were able to bind to the CCK receptor specifically and with high affinity, and were able to elicit only submaximal intracellular calcium responses typical of partial agonists. The fluorescence characteristics of these probes were compared with those previously reported for structurally-related full agonist and antagonist probes. Like the previous probes, the partial agonist probes exhibited longer fluorescence lifetimes and increased anisotropy when bound to the receptor than when free in solution. The receptor-bound probes were not easily quenched by potassium iodide, suggesting that the fluorophores were protected from the extracellular aqueous milieu. The fluorescence characteristics of the partial agonist probes were quite similar to those of the analogous full agonist probes and quite distinct from the analogous antagonist probes. These data suggest that the partially activated conformational state of this receptor is more closely related to its fully active state than to its inactive state.

Estrogen and CCK1 receptor modification of mu-opioid receptor binding in the cortex of female rats

Cholecystokinin (CCK) in the nervous system has effects opposite to those of opioids. However, the mechanism by which CCK opposes the effect of opioids at the receptor or cellular level is still unknown. In the brain, distributions of CCK receptors and opioid receptors have been demonstrated to overlap. The present study was undertaken to determine the mechanism of CCK-opioid interactions in the cortex of ovariectomized rats. Furthermore, because estrogen is a powerful regulator of CCK and opioid activity, we examined whether estrogen state also modulates the interactions of these neuropeptides. mu-Opioid (MOP) receptor binding was examined in cortical membranes that were preincubated with CCK-8S and CCK receptor agonist and antagonist followed with 3H-DAMGO. Pharmacological results revealed that CCK-8S suppressed 3H-DAMGO binding in cortical membranes of ovariectomized rats. The same result was obtained using a CCK1 receptor agonist (JMV-180), whereas a CCK2 receptor agonist (CCK-4) failed to suppress 3H-DAMGO binding. Antagonism of the CCK1 receptor by JMV-179 blocked both CCK-8S and JMV-180 suppression of 3H-DAMGO binding. Furthermore, estrogen treatment to female rats resulted in a suppression of 3H-DAMGO binding in cortical membranes. These results demonstrate an estrogen regulation of the MOP receptor and a protein-protein interaction between CCK1 receptor and MOP receptor.

Role of cholecystokinin in appetite control and body weight regulation

Summary Cholecystokinin (CCK), a peptide that is distributed widely throughout the gastrointestinal tract and the central nervous system, has a number of physiological effects including the stimulation of gallbladder contraction and pancreatic and gastric acid secretion, slowing of gastric emptying and suppression of energy intake. This review focuses on current knowledge relating to (i) the effects of CCK on energy intake; (ii) the role for CCK in the pathophysiology of obesity; and (iii) the therapeutic potential for strategies which modulate the action or secretion of CCK in the management of obesity. While CCK plays a role in the acute regulation of appetite and energy intake, there is little evidence to suggest that specific CCK receptor agonists, or modulation of the actions of endogenous CCK by dietary manipulation, have sustainable inhibitory effects on energy intake. Hence, it appears unlikely that manipulating the pathways by which CCK modulates energy intake will prove to be an effective strategy in the long term management of obesity.

A family of octopamine [corrected] receptors that specifically induce cyclic AMP production or Ca2+ release in Drosophila melanogaster

In invertebrates, the biogenic-amine octopamine is an important physiological regulator. It controls and modulates neuronal development, circadian rhythm, locomotion, 'fight or flight' responses, as well as learning and memory. Octopamine mediates its effects by activation of different GTP-binding protein (G protein)-coupled receptor types, which induce either cAMP production or Ca(2+) release. Here we describe the functional characterization of two genes from Drosophila melanogaster that encode three octopamine receptors. The first gene (Dmoa1) codes for two polypeptides that are generated by alternative splicing. When heterologously expressed, both receptors cause oscillatory increases of the intracellular Ca(2+) concentration in response to applying nanomolar concentrations of octopamine. The second gene (Dmoa2) codes for a receptor that specifically activates adenylate cyclase and causes a rise of intracellular cAMP with an EC(50) of approximately 3 x 10(-8) m octopamine. Tyramine, the precursor of octopamine biosynthesis, activates all three receptors at > or = 100-fold higher concentrations, whereas dopamine and serotonin are non-effective. Developmental expression of Dmoa genes was assessed by RT-PCR. Overlapping but not identical expression patterns were observed for the individual transcripts. The genes characterized in this report encode unique receptors that display signature properties of native octopamine receptors.

The rat pancreatic islets: a reliable tool to study islet responses to cholecystokinin receptor occupation

This study was undertaken to show that rat purified islets can be used as a reliable tool to study some aspects of human islet's physiology related to CCKR occupation. Therefore, isolated foetal, adult human and rat islets were compared for (1) CCKR subtypes mRNA and protein expression and somatostatin (SS) mRNA and (2) co-localization of these receptors with insulin, glucagon and SS. Finally, rat islets were tested for their responsiveness to stimulation. Purified human and rat islets were used for CCKR subtypes and SS mRNA estimation by RT-PCR and protein by Western blots. Receptors and hormones co-localizations were evaluated by confocal microscopy. Hormones secretion served to determine rat islets responsiveness. Islets of both species express CCKA and CCKBR mRNA and proteins and SS mRNA. The CCKAR co-localizes with insulin and glucagon and the CCKBR with SS. Insulin release was increased 5-fold in response to 16 mM glucose and SS secretion reached 1.3- and 1.7-fold increments above basal in response to forskolin and IBMX. In conclusions, human and rat islets have comparable CCKR subtypes localized on the same cells; they also express SS mRNA. The rat islets are functional as they secrete but their response to hormonal stimulation remains to be clarified. These rat islets can thus serve as tools to study islets physiology.

Fluorescence resonance energy transfer analysis of the antagonist- and partial agonist-occupied states of the cholecystokinin receptor

Changes in receptor conformation are believed to be key for ligand-induced regulation of cellular signaling cascades. However, little information exists about specific conformations of a receptor. We recently applied fluorescence resonance energy transfer to determine distances from distinct points distributed over the surface and within the helical bundle of the cholecystokinin receptor to the amino terminus of a full agonist CCK analogue (Harikumar, K. G., Pinon, D. I., Wessels, W. S., Dawson, E. S., Lybrand, T. P., Prendergast, F. G., and Miller, L. J. (2004) Mol. Pharmacol. 65, 28-35). Here, we apply the same experimental strategy to determine distances from the same receptor positions to an analogous point at the amino terminus of structurally related partial agonist (Alexa488-Gly-[(Nle(28,31))CCK-26-32]phenethyl ester) and antagonist (Alexa488-Gly-[(D-Trp31, Nle(28,31))CCK-26-32]phenethyl ester) ligands. A high degree of spectral overlap and fluorescence transfer was observed for ligand-occupied fluorescent-tagged receptors with no transfer observed for the ligand-occupied pseudo-wild type null cysteine-reactive mutant receptor (C94S). For the partial agonist, calculated distances to receptor positions 94, 102, 204, and 341, representing sites within the helical confluence, and the first, second, and third loops, were 21 +/- 0.4, 18 +/- 0.4, 25 +/- 1, and 17 +/- 1 angstroms, not different from those measured previously for the analogous full agonist. For the antagonist, the analogous distances were 21 +/- 2, 28 +/- 2, 15 +/- 1 and 21 +/- 1 angstroms. Distances to the first and third loops were longer and the distance to the second loop was shorter for the antagonist relative to both the full and partial agonist probes, whereas all three probes demonstrated similar distances to the intrahelical reference point. This supports the possibilities of changes in the conformation of the probe and/or the receptor induced by structurally similar ligands having distinct intrinsic biological activities.

Cholecystokinin tetrapeptide improves water maze performance of neonatally 6-hydroxydopamine-lesioned young rats

This study addressed the proposed memory-modulating effect of the cholecystokinin (CCK) 2 agonist Boc-CCK-4 in rats using a Morris water maze. In the brain, CCK is colocalized and interacts with dopamine, respectively. To impair dopaminergic neurotransmission, and consequently, dopamine-mediated learning and memory, rat pups received the neurotoxin 6-hydroxydopamine (6-OHDA) into the left [Day 5 postnatal (p.n.)] and right (Day 8 p.n.) ventricles (50 microg/5 microl each). After 6-OHDA treatment, dopamine brain levels were reduced by 60% on Day 50 p.n. Lesioned rats had a lower body weight but normal swimming abilities. In the acquisition phase of the water maze (Day 50 p.n.), sham-lesioned rats learned quickly, compared to lesioned rats. Treatment with Boc-CCK-4 (40 microg/kg ip) did not affect performance in sham-lesioned rats but restored the learning curve in lesioned rats without increasing swimming speed indicating a better spatial learning in the dopamine-depleted rats. In summary, these findings demonstrate that stimulation of CCK2 receptors may counteract cognitive deficits of dopamine-depleted rats.

Distinct molecular mechanisms for agonist peptide binding to types A and B cholecystokinin receptors demonstrated using fluorescence spectroscopy

Fluorescence spectroscopy provides a direct method for evaluating the environment of a fluorescent ligand bound to its receptor. We utilized this methodology to determine the environment of Alexa within a cholecystokinin (CCK)-like probe (Alexa488-Gly-[(Nle(28,31))CCK-26-33]; CCK-8 probe) bound to the type A CCK receptor (Harikumar, K. G., Pinon, D. L., Wessels, W. S., Prendergast, F. G., and Miller, L. J. (2002) J. Biol. Chem. 277, 18552-18560). Here, we study this probe at the type B CCK receptor and develop another probe with its fluorophore closer to the carboxyl-terminal pharmacophore of type B receptor ligands (Alexa488-Trp-Nle-Asp-Phe-NH2; CCK-4 probe). Both probes bound to type B CCK receptors in a saturable and specific manner and represented full agonists. Similar to the type A receptor, at the type B receptor these probes exhibited shorter lifetimes and lower anisotropy when the receptor was in the active conformation than when it was shifted to its inactive, G protein-uncoupled state using guanosine 5'-[beta,gamma-imido]-triphosphate trisodium salt. Absolute values for lifetime and anisotropy were lower for the CCK-8 probe bound to the type B receptor than for this probe bound to the type A receptor, and Alexa fluorescence was more easily quenched by iodide at the type B receptor. This represents the first direct evidence that, despite having identical affinities for binding and potencies for activating type A and B receptors, CCK is docked via distinct mechanisms, with the amino terminus more exposed to the aqueous milieu when bound to the type B CCK receptor than to the type A CCK receptor. Of interest, despite this difference in binding, activation of both receptors results in analogous direction of movement of the fluorescent indicator probes.

“Dipeptoids”: From the Chemical Structure of the Endogenous Peptide to the Design of Peptidomimetics

The present review details the rational multi-step process followed for the discovery of a family of non-peptide CCK receptor ligands ("dipeptoids"), starting from the structure of the endogenous peptide, CCK(8). Emphasis will be made on the N- and C-terminal modifications, on the singular effects of the stereochemical changes and the incorporation of conformational constraints into the structure of "dipeptoids", and on the modifications directed to improve the pharmacological profile of these compounds to afford valuable clinical candidates.

Key Differences in Molecular Complexes of the Cholecystokinin Receptor with Structurally Related Peptide Agonist, Partial Agonist, and Antagonist

The molecular basis of docking of receptor ligands having differences in biological activity and their subsequent effects on receptor conformation represent areas of great interest. In this work, we focus on the sulfated tyrosyl residue in position 27 of cholecystokinin (CCK) and its spatial approximation with the type A CCK receptor residue Arg(197) that has been predicted from mutagenesis experiments. We have examined the requirement for sulfation of this residue in a series of structurally related peptide agonists, partial agonists, and antagonists using assays of receptor binding and biological activity. Whereas sulfation of CCK position 27 was critical for affinity and potency of a full agonist, it had progressively less effect as the biological activity of the ligand was reduced. It had an intermediate effect on the partial agonist and no effect on the antagonist. In addition, photoaffinity labeling was used to determine the spatial approximations between the receptor and residue 27 of the agonist and antagonist in this series. Direct photoaffinity labeling with a full agonist probe confirmed the spatial approximation of ligand residue 27 and receptor residue Arg(197) in the active complex. Of note, the analogous antagonist probe labeled a distinct region within the receptor amino terminus, confirming a key structural difference in active and inactive complexes.

Analysis of strain difference in behavior to Cholecystokinin (CCK) receptor mediated drugs in PVG hooded and Sprague–Dawley rats using elevated plus-maze test apparatus

This study investigated the behavioral mechanisms underlying the anxiogenic, or anxiolytic mediated effects of CCK(2) receptor mediated agonist (CCK-4) and antagonist drugs (LY225910, LY288513, CR2945) in PVG hooded and Sprague-Dawley (SD) rats using the elevated plus maze test apparatus. In addition, the effects of a CCK(1) antagonist (CR1409) were investigated for its possible mediation in anxiety behavior between PVG hooded and SD rats. PVG hooded rats treated with CCK-4, decreased the time spent in the open arm and increased the time spent in the closed arm and correspondingly showed increase in the number of entries in the open arms while the number of entries in closed arm was insignificant, whereas SD rats decreased the time spent in the closed arm, while other parameters remained insignificant. PVG hooded rats administered with various CCK(2) antagonists (LY225910, LY288513, and CR2945) significantly increased the time spent in the open arm and correspondingly decreased the time spent in the closed arm, while the number of entries in the open or closed arm was insignificant, in contrast, SD rats failed to show any reliable significance. PVG hooded rats administered with the CCK(1) antagonist (CR1409), failed to show any reliable significance, in contrast, SD rats significantly increased the time spent in the open arm. The strain differences observed in this study suggests that CCK plays mainly as a neuromodulator, in which the various CCK(2) antagonists may not affect baseline anxiety state, but instead they modulate heightened states of anxiety through differential effects of CCK(1)/CCK(2) receptors.

Physiological phenotyping of cholecystokinin-responsive rat taste receptor cells

The recent discovery that subsets of rat taste receptor cells (TRCs) express the peptide cholecystokinin (CCK) and that subsets of TRCs respond to CCK with altered potassium currents or elevated intracellular calcium via CCK-A receptor has lead to the hypothesis that CCK may play a novel signaling role within the taste bud, perhaps modifying tastant responses by co-transmission with a classic transmitter. To better understand this phenomenon, CCK-responsive TRCs were characterized for sensitivity to two bitter stimuli, quinine or caffeine, or to the neurotransmitter ACh using a ratiometric procedure with the calcium sensitive dye fura-2. In characterizing TRC responses to quinine, it was observed that quinine-induced elevations of intracellular calcium were not due to endogenous fluorescence of the quinine molecule. Most (60-70%) CCK-responsive cells were also sensitive to either bitter stimuli or to cholinergic stimulation. These data suggest that TRCs expressing CCK-receptors also express receptors to bitter stimuli and/or muscarinic receptors. They further support the notion of a putative modulatory role of CCK with convergence of multiple inputs occurring at the level of intracellular calcium.

Transient agonist-induced regulation of the cholecystokinin-A and cholecystokinin-B receptor mRNA levels in rat pancreatic acinar AR42J cells

BACKGROUND

CCK-8 and gastrin exert multiple effects in the gastrointestinal tract and the nervous system. Their actions are mediated via the G-protein coupled CCK-A and CCK-B receptors.

METHODS

Rat pancreatic acinar tumor AR42J cells express both CCK receptor subtypes. This cell line was used to characterize the agonist-dependent regulation of CCK-A and CCK-B receptor gene expression.

RESULTS

CCK-8 (10 nM) or gastrin (10 nM) reduced CCK-A receptor mRNA expression to 56% and 53%, respectively 2 h after hormonal exposure. In contrast, the level of CCK-B receptor gene expression was upregulated to 157% and 153%, respectively. These effects are most probably linked to the CCK-B receptor in AR42J cells. The phorbolester PMA (100 nM), a protein kinase C activator, downregulated CCK-A receptor expression but did not affect CCK-B receptor gene transcription. Activation of protein kinase A by forskolin (10 microM) or Bt(2)cAMP (100 microM) is not involved in the transient regulation of CCK receptor mRNA expression. Both elevated CCK-B and decreased CCK-A receptor mRNA expression returned to basal levels 6 h after continuous stimulation.

CONCLUSION

These results demonstrate that CCK-A and CCK-B receptor mRNA levels are differentially regulated by their agonists via distinct signal transduction mechanisms in AR42J cells.

Intermolecular interactions between peptidic and nonpeptidic agonists and the third extracellular loop of the cholecystokinin 1 receptor

Intermolecular interactions were determined between a synthetic peptide corresponding to the third extracellular loop and several residues from the adjoining sixth and seventh transmembrane domains of the human cholecystokinin-1 receptor, CCK(1)-R(329-357), and the synthetic agonists Ace-Trp-Lys[NH(epsilon)CONH-o-(MePh)]-Asp-MePhe-NH(2) (GI5269) and the C1 N-isopropyl-N-(4-methoxyphenyl)acetamide derivative of 3-(1H-Indazol-3ylmethyl)-3-methyl-5-pyridin-3-yl-1,5-benzodiazepine (GI0122), using high-resolution nuclear magnetic resonance spectroscopy and computer simulations. Addition of the ligands to CCK(1)-R(329-357) in an aqueous solution of DPC micelles produced a number of intermolecular nuclear Overhauser enhancements (NOEs) to residues in TMs 6 and 7 of the receptor fragment. NOE-restrained molecular models of the GI5269 and GI0122/CCK(1)-R complexes provide evidence for overlapping ligand-binding sites for peptidic and nonpeptidic agonists. The proposed binding modes of GI5269 and GI0122 are supported by the structure-activity relationship of analogues and mutagenesis data for the CCK(1)-R selective antagonist L-364,718.

Effect of peripheral cholecystokinin receptor agonists on c-Fos expression in brain sites mediating food consumption in rats

Peripheral cholecystokinin (CCK) elicits satiety by acting on hypothalamic nuclei. The anoretic effect of CCK is mediated by the vagus nerve and involves brainstem areas receiving vagal inputs, such as the nucleus tractus solitarius (NTS) and the area postrema (AP). This work aims to analyze, by measuring c-Fos expression, the effect of selective CCK receptor agonists on brain areas involved in food-intake/satiety process. We observed that SR-146,131, a CCK(1)R agonist, increased c-Fos expression in NTS and AP as well as in some hypothalamic nuclei. CCK-4, a CCK(2)R agonist which does not cross the blood-brain barrier (BBB), only was effective in the hypothalamus. Our data show that the activation of the brainstem is not a requisite to obtain a hypothalamic effect of peripheral CCK and suggest that CCK-4 may indirectly stimulate hypothalamic areas endowed with BBB, without previous activation of neither NTS nor AP.

Identification of a series of CCK-2 receptor nonpeptide agonists: sensitivity to stereochemistry and a receptor point mutation

The search for small-molecule drugs that act at peptide hormone receptors has resulted in the identification of a wide variety of antagonists. In contrast, the discovery of nonpeptide agonists has been far more elusive. We have used a constitutively active mutant of the cholecystokinin 2 receptor (CCK-2R) as a sensitive screen to detect ligand activity. Functional assessment of structural analogs of the prototype CCK-2R antagonist, L-365,260 [3R-N- (2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepin-3-yl)-N'-(3-methylphenyl)urea], resulted in the identification of a series of agonists. Each of the active molecules is an S enantiomer, whereas the corresponding R stereoisomers have little or no activity. Further in vitro and in vivo assessment at the wild-type receptor indicated that efficacy of the two most active ligands approached that of the endogenous hormone. The function of selected R and S enantiomers was differentially sensitive to a point mutation, N353L, within the putative CCK-2R ligand pocket. The results of this study highlight the potential of constitutively active receptors as drug screening tools and the interdependence of ligand stereochemistry and receptor conformation in defining drug efficacy.

Further evidence that the CCK2 receptor is coupled to two transduction pathways using site-directed mutagenesis

A heterogeneity of CCK2 receptors has been reported which could correspond to different states of coupling to G proteins and/or association with different second messenger systems. To investigate these hypotheses, the wild-type CCK2 receptor and three mutants F347A, D100N and K333M/K334T/R335L, expected to modify the coupling of the G protein with the third intracellular loop of the receptor, were transfected into Cos-7 cells and their binding and signalling properties were evaluated using the natural ligand CCK8. Activation of wild-type as well as F347A, D100N or K333M/K334T/R335L CCK2 receptors by this ligand led to a similar arachidonic acid release which was blocked by pertussis toxin and the phospholipase A2 inhibitor, mepacrine. Nevertheless, in contrast to the wild-type CCK2 receptor, addition of CCK8 to cells transfected with the F347A or K333M/K334T/R335L mutants did not result in the production of inositol phosphates while the maximum increase in this second messenger formation was reduced by 30% with the D100N mutant. Taken together, these results suggest that the CCK2 receptor is coupled to two G proteins and that Phe347 and the cluster of basic residues K333/K334/R335 probably play a key role in Gq protein stimulation leading to inositol phosphate production but not in activation of the G protein coupled to phospholipase A2. These data bring additional support at the molecular level to the existence of different affinity states of CCK2 receptors suggested from the results of binding assays and behavioural studies.

Hypothalamic-pituitary adrenal response to cholecystokinin-B receptor agonism is resistant to cortisol feedback inhibition

Intravenous injection of the cholecystokinin (CCK)-B receptor agonist, pentagastrin, produces robust, dose-dependent release of adrenocorticotropin (ACTH) and cortisol, supporting the hypothesis that CCK-B agonists pharmacologically activate the hypothalamic-pituitary-adrenal (HPA) axis. The mechanism of activation and its physiological relevance remain uncertain. Preliminary data suggest that the ACTH response to pentagastrin may be differentiated from the response to exogenous corticotropin releasing hormone (CRH) by its relative resistance to cortisol feedback inhibition. To more directly test the relationship between cortisol levels and ACTH response to pentagastrin, this study examined responses to pentagastrin (a) during a peak (8 a.m.) and a nadir (4 p.m.) period of endogenous cortisol secretion and (b) when cortisol levels were artificially reduced to low levels by administration of metyrapone. ACTH responses to pentagastrin were identical in the morning and afternoon, despite substantial differences in basal cortisol levels. Suppression of cortisol with metyrapone had little impact on ACTH response to pentagastrin. These data support the hypothesis that CCK-B receptor mediated activation of the HPA axis is relatively resistant to cortisol feedback inhibition. This differentiates it from CRH-mediated activation and raises the possibility that CCK could contribute to acute activation of the HPA axis even in the face of elevated basal cortisol levels, such as those seen in chronic stress or some psychiatric disorders.

Cholecystokinin and learning and memory processes

Cholecystokinin (CCK) is a classical brain-gut peptide that exerts a variety of physiological actions in the gastrointestinal tract and central nervous system. CCK occurs in several molecular forms of varying aminoacid length, the sulphated octapeptide (CCK-8) being the predominant form in the brain. CCK mediates its effects through interaction with specific receptors subdivided in two subtypes--CCK-A (present in the periphery and in few selected brain nuclei) and CCK-B (the predominant receptor subtype in the brain). CCK is implicated in variety of behavioral functions as satiety, anxiety, exploratory and locomotor activity and learning and memory. After a brief description of the distribution, molecular forms, release, inactivation, etc. of CCK in the brain, the present review summarizes the recent data on the role of CCK in learning and memory. The memory-enhancing effects of CCK have been demonstrated in various types of memory. Data showing that CCK-A receptors mediate mnemonic while CCK-B receptors mediate amnestic effects are also presented.

Applications of fluorescence in the characterization of the ligand-binding domain and activation of the cholecystokinin receptor

Fluorescence techniques can be applied to guanine nucleotide-binding protein-coupled receptors to gain insights into the environment and molecular motion of a fluorophore that is either incorporated into a ligand or more directly into a specific site within the receptor. Fluorescence studies can provide insight into the environment of that indicator. By situating the same indicator into an analogous position in both agonist and antagonist ligands, comparisons can provide insights into differences between active and inactive states of the receptor. These types of studies have been performed for the cholecystokinin receptor and show that receptor activation by agonist occupation is associated with a movement of the CCK-like ligand out of a protected environment and into a more accessible hydrophilic milieu.

Structure of cholecystokinin receptor binding sites and mechanism of activation/inactivation by agonists/antagonists

Delineation of CCK receptor binding sites is a prerequisite for the understanding of the molecular basis for ligand recognition, partial agonism, ligand-induced traffiking of receptor signalling. In the current paper, we illustrate how, in the past 5 years, studies from our laboratory and others have provided new data on the molecular basis of the pharmacology and functioning of CCK1 and CCK2 receptors. Available data on CCK1 and CCK2R binding sites indicate that 1) homologous regions of the two receptors are involved in the binding site of CCK, however, positioning of CCK slightly differs; 2) binding sites of non-peptide agonists/antagonist are buried in the pocket formed by transmembrane helices and overlap that of CCK and 3) residues of the binding sites as well as of conserved motifs such as E/DRY, NPXXY are crucial for receptor activation.

Anxiogenic effects of the CCK(B) agonist pentagastrin in humans and dose-dependent increase in plasma C-peptide levels

RATIONALE

Cholecystokinin type B (CCK(B)) receptor agonists such as pentagastrin or CCK-4 have panic-like anxiogenic effects in humans. It has also been shown that CCK-4 can stimulate insulin release and thus C-peptide release from pancreatic islet cells. Combined, these mechanisms may provide a basis for a bioassay.

OBJECTIVES

Our aim was to study if a pentagastrin bolus injection evokes insulin release (as measured by C-peptide) and if the levels of C-peptide correlate to the anxiogenic effect of pentagastrin.

METHODS

Pentagastrin was given in bolus IV injections to healthy volunteers at increasing doses (0.003, 0.012, 0.05 and 0.2 microg/kg).

RESULTS

A significant increase in the plasma level of C-peptide was observed 2-4 min after the highest dose of pentagastrin. This increase was accompanied by a transient panic-like anxiety within 2 min following pentagastrin, measured using a state anxiety scale. Also, 0.05 microg/kg pentagastrin gave a minor but significant subjective discomfort at the same time interval. The basal plasma level of C-peptide preceding the pentagastrin injection showed a positive correlation to the intensity of the subsequent pentagastrin-induced panic-like anxiety as rated on the state anxiety scale. In addition, basal plasma levels of cortisol were positively correlated to the subsequent pentagastrin-induced increase in plasma C-peptide levels.

CONCLUSIONS

Our results imply a possible relationship between insulin/C-peptide release and sensitivity to psychotropic activation by CCK(B) receptor stimulation. Furthermore, we postulate that both basal and pentagastrin-induced plasma levels of C-peptide may possess characteristic phenotype properties for anxiety related traits.

Cholecystokinin and panic disorder: past and future clinical research strategies

The involvement of cholecystokinin (CCK) in human anxiety is well documented. Exogenous administration of CCK-2 receptor agonists, such as cholecystokinin-tetrapeptide and pentagastrin, provoke panic attacks in man. Patients with panic disorder (PD) are hypersensitive to CCK-2 receptor stimulation compared to healthy volunteers and patients with other anxiety disorders, and they differ from healthy subjects in CCK metabolism and genetic characteristics of the CCK-2 receptor system. This article reviews the corpus of work supporting the role of CCK in anxiety and suggests three research approaches which can further enhance our understanding of the CCK-2 system in PD. These approaches include: i) searching for a specific anomaly of the CCK-2 receptor system, ii) establishing a relationship between CCK-2 receptor polymorphism and vulnerability to pharmacologically-induced or spontaneous panic attacks, and iii) evaluating the therapeutic efficacy of CCK-2 receptor antagonists which possess adequate pharmacokinetic properties.

Environment and mobility of a series of fluorescent reporters at the amino terminus of structurally related peptide agonists and antagonists bound to the cholecystokinin receptor

Fluorescence is a powerful biophysical tool for the analysis of the structure and dynamics of proteins. Here, we have developed two series of new fluorescent probes of the cholecystokinin (CCK) receptor, representing structurally related peptide agonists and antagonists. Each ligand had one of three distinct fluorophores (Alexa(488), nitrobenzoxadiazolyl, or acrylodan) incorporated in analogous positions at the amino terminus just outside the hormone's pharmacophore. All of the probes bound to the CCK receptor specifically and with high affinity, and intracellular calcium signaling studies showed the chemically modified peptides to be fully biologically active. Quenching by iodide and measurement of fluorescence spectra, anisotropy, and lifetimes were used to characterize the response of the fluorescence of the probe in the peptide-receptor complex for agonists and antagonists. All three fluorescence indicators provided the same insights into differences in the environment of the same indicator in the analogous position for agonist and antagonist peptides bound to the CCK receptor. Each agonist had its fluorescence quenched more easily and showed lower anisotropy (higher mobility of the probe) and shorter lifetime than the analogous antagonist. Treatment of agonist-occupied receptors with a non-hydrolyzable GTP analogue shifted the receptor into its inactive low affinity state and increased probe fluorescence lifetimes toward values observed with antagonist probes. These data are consistent with a molecular conformational change associated with receptor activation that causes the amino terminus of the ligand (situated above transmembrane segment six) to move away from its somewhat protected environment and toward the aqueous milieu.

Modulation of cholestasis-induced antinociception by CCK receptor agonists and antagonists

Acute cholestasis is associated with increased activity of the endogenous opioid system. Agonists and antagonists of cholecystokinin (CCK) receptors are known to modulate opioid-induced antinociception. In the present study, the effect of the CCK receptor agonist caerulein and the antagonist proglumide on antinociception induced during acute cholestasis was investigated in rats using the tail-flick test. A significant increase in nociception threshold was observed in bile duct ligated (BDL) rats compared to sham-operated controls that was maximum on day 7 after the operation and decreased thereafter. Proglumide (40 mg/kg, i.p.) did not affect nociception in unoperated and sham-operated animals, but exerted a significant potentiation of antinociception in cholestatic rats in a way similar to its potentiation effect on unoperated morphine-treated (2 mg/kg, s.c.) animals. Caerulein (0.005, 0.001, 0.01 and 0.02 mg/kg, s.c.), which did not change nociception per se or in sham-operated animals, also significantly potentiated the antinociception in BDL rats as well as in morphine-treated unoperated controls. Caerulein-induced potentiation of antinociception in BDL animals was completely reversed by proglumide pretreatment. Our findings show that, in cholestatic animals, modulation of nociception by the CCK system is different from normal subjects and resembles the state observable in morphine-administered subjects.

Differential effects of the CCKA receptor ligands PD-140,548 and A-71623 on latent inhibition in the rat

Latent inhibition (LI) is a behavioural paradigm in which repeated exposure to a stimulus without consequence inhibits the formation of any new associations with that stimulus. To the extent that LI reflects a process of leaming to ignore irrelevant stimuli, disrupted LI has been suggested as an animal model for the attentional deficits observed in schizophrenia. The antipsychotic potential of cholecystokinin (CCK) stems from its colocalization with dopamine (DA) in the mesolimbic pathway, where it demonstrates both excitatory and inhibitory effects on dopaminergic activity. This may be explained by mediation through different receptor subtypes. A variety of hypotheses has emerged regarding the potential clinical application of subtype-selective CCK-based drugs. The present experiments examined the effects on LI of two selective CCK(A) ligands: PD-140,548 (a CCK(A) antagonist, Experiment 1: 0.001, 0.01, and 0.1 mg/kg) and A-71623 (a CCK(A) agonist, Experiment 2: 0.02, 0.05, and 0.1 mg/kg). In both experiments, the effects of haloperidol (0.1 mg/kg) were also investigated. Animals receiving 0.1 mg/kg of haloperidol or 0.001 or 0.1 mg/kg (but not 0.01 mg/kg) of PD-140,548 treated the preexposed stimulus as irrelevant after a low number of preexposures. In contrast, no facilitatory effect on LI was detectable at any of the A-71623 doses. The finding that A-71623 failed to enhance LI indicates that it is unlikely that this compound would have any antipsychotic effect within the clinical setting. Considering the facilitatory effect exerted by PD-140,548 on LI, it is probable that the inhibition of CCK activity might prove a more promising strategy for the pharmacological treatment of schizophrenia.

The biologically crucial C terminus of cholecystokinin and the non-peptide agonist SR-146,131 share a common binding site in the human CCK1 receptor. Evidence for a crucial role of Met-121 in the activation process

The cholecystokinin (CCK) receptor-1 (CCK1R) is a G protein-coupled receptor, which mediates important central and peripheral cholecystokinin actions. Our aim was to progress in mapping of the CCK1R binding site by identifying residues that interact with the methionine and phenylalanine residues of the C-terminal moiety of CCK because these are crucial for its binding and biological activity, and to determine whether CCK and the selective non-peptide agonist, SR-146,131, share a common binding site. Identification of putative amino acids of the CCK1R binding site was achieved by dynamics-based docking of the ligand CCK in a refined three-dimensional model of the CCK1R using, as constraints, previous results that identified contact points between residues of CCK and CCK1R (Kennedy, K., Gigoux, V., Escrieut, C., Maigret, B., Martinez, J., Moroder, L., Frehel, D., Gully, D., Vaysse, N., and Fourmy, D. (1997) J. Biol. Chem. 272, 2920-2926 and Gigoux, V., Escrieut, C., Fehrentz, J. A., Poirot, S., Maigret, B., Moroder, L., Gully, D., Martinez, J., Vaysse, N., and Fourmy, D. (1999) J. Biol. Chem. 274, 20457-20464). By this approach, a series of residues forming connected hydrophobic clusters were identified. Pharmacological and functional analysis of mutated receptors indicated that a network of hydrophobic residues including Cys-94, Met-121, Val-125, Phe-218, Ile-329, Phe-330, Trp-326, Ile-352, Leu-356, and Tyr-360, is involved in the binding site for CCK and in the activation process of the CCK1R. Within this hydrophobic network, the physico-chemical nature of residue 121 seems to be essential for CCK1R functioning. Finally, the biological properties of mutants together with dynamic docking of SR-146,131 in the CCK1R binding site demonstrated that SR-146,131 occupies a region of CCK1R binding site which interacts with the C-terminal amidated tripeptide of CCK, i.e. Met-Asp-Phe-NH(2). These new and important insights will serve to better understand the activation process of CCK1R and to design or optimize ligands.

Effects of the incorporation of IBTM beta-turn mimetics into the dipeptoid CCK(1) receptor agonist PD 170292

Replacement of the 2-Adoc-D-alphaMeTrp residue in the non-selective CCK(1) receptor agonist PD 170292 by the Z-(2R,5R,11bS)-IBTM skeleton, able to fix a type II beta-turn-like conformation, led to a conformationally restricted dipeptoid analogue, namely 3a, which exhibited a notable increase in the CCK(1) selectivity and antagonist properties.

Agonist-dependent dissociation of oligomeric complexes of G protein-coupled cholecystokinin receptors demonstrated in living cells using bioluminescence resonance energy transfer

Dimerization of some G protein-coupled receptors has recently been demonstrated, but how widespread this phenomenon might be and its functional implications are not yet clear. We have utilized biophysical and biochemical techniques to evaluate whether the type A cholecystokinin (CCK) receptor can form oligomeric complexes in the plasma membrane and the impact of ligand binding and signaling on such complexes. We investigated the possibility of bioluminescence resonance energy transfer (BRET) between receptor constructs that included carboxyl-terminal tags of Renilla luciferase or yellow fluorescent protein. Indeed, co-expression of these constructs in COS cells resulted in the constitutive presence of a significant BRET signal above that in a series of controls, with this signal reduced by co-expression of competing non-tagged CCK receptors. The presence of an oligomeric complex of CCK receptor molecules was confirmed in co-immunoprecipitation experiments. Occupation of CCK receptors with agonist ligands (CCK or gastrin-4) resulted in the rapid reduction in BRET signal in contrast to the enhancement of such a signal reported after agonist occupation of the beta(2)-adrenergic receptor. These effects on CCK receptor oligomerization were concentration-dependent, correlating with the potencies of the agonists. A smaller effect was observed for a partial agonist, and no effect was observed for antagonist occupation of this receptor. Agonist-induced reduction in BRET signal was also observed for pairs of CCK receptors with a donor-acceptor pair situated in other positions within the receptor. Manipulation of the phosphorylation state of CCK receptor using protein kinase C activation with phorbol ester or inhibition with staurosporine had no effect on the basal level or agonist effect on CCK receptor oligomerization. This provides the first evidence for CCK receptor oligomerization in living cells, with insights that the active conformation of this receptor dissociates these complexes in a phosphorylation-independent manner.