Characterization of receptors for cholecystokinin and related peptides in mouse cerebral cortex

The potential role of cholecystokinin in schizophrenia: review and update

The role of the neuropeptide cholecystokinin in schizophrenia has been widely explored because of its modulating action on midbrain dopamine neurons. The recent discovery of more specific receptor subtype cholecystokinin antagonists should be considered as potential treatment for schizophrenia with fewer side effects. This paper reviews cholecystokinin/dopamine interactions in animal and human studies. Clinical trials with cholecystokinin agonists and antagonists in schizophrenia are updated.

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.

Induced association of mu opioid (MOP) and type 2 cholecystokinin (CCK2) receptors by novel bivalent ligands

Both mu-opioid (MOP) and type 2 cholecystokinin (CCK2) receptors are present in areas of the central nervous system that are involved in modulation of pain processing. We conducted bioluminescence resonance energy transfer (BRET) studies on COS cells coexpressing MOP and CCK2 receptors to determine whether receptor heterodimerization is involved in such modulation. These studies revealed the absence of constitutive or monovalent ligand-induced heterodimerization. Heterodimerization of MOP and CCK2 receptors therefore is unlikely to be responsible for the opposing effects between morphine and CCK in the CNS. However, association was induced, as indicated by a positive BRET signal, on exposure of the cells to bivalent ligands containing mu-opioid agonist and CCK2 receptor antagonist pharmacophores linked through spacers containing 16-22 atoms but not with a shorter (9-atom) spacer. These studies demonstrate for the first time that an appropriately designed bivalent ligand is capable of inducing association of G-protein-coupled receptors. The finding that opioid tolerance studies with these ligands in mice showed no correlation with the BRET data is consistent with the absence of association of MOP and CCK2 receptors in vivo.

Effect of intravenous infusion of proglumide on ruminal motility in conscious sheep (Ovis aries)

The effects of intravenous infusion of proglumide on regular ruminal contractions were examined in conscious sheep using doses that inhibit pancreatic exocrine secretion. After a control period of 20 min, proglumide was infused intravenously for 40 min at a dose of 15, 30 or 60 micromol/kg per min and venous blood was collected. The intravenous infusion of proglumide significantly increased the frequency of ruminal contractions at 15 micromol/kg per min without altering the amplitude, while it significantly decreased the frequency and amplitude of ruminal contractions at 30 and 60 micromol/kg per min in a dose-dependent manner. Proglumide did not increase contractile activity of the omasum, abomasum and duodenum or the plasma concentration of immunoreactive cholecystokinin (CCK). Application of proglumide at 1-30 mmol/L inhibited bethanechol-induced contraction in both longitudinal and circular muscle strips of the dorsal sac of the rumen. These results suggest that proglumide at a low dose acts indirectly on the rumen as a CCK receptor antagonist to increase the frequency of contractions, whereas at higher doses it inhibits cholinergic-induced contraction of the ruminal muscles or acts as an agonist to inhibit contractions in sheep. Hence, proglumide at high doses seems unsuitable for research or therapeutic use as a CCK receptor blockade in sheep.

Immunolocalization of CCK1R in rat brain using a new anti-peptide antibody

An antibody directed at the carboxy tail of the cholecystokinin-1 receptor (CCK1R) was characterized by ELISA and Western blotting. Immunohistochemistry established that CCK1R-like immunoreactivity (CCK1R-LI) was widely and topographically distributed through the neuroaxis, appearing relatively higher in rhi- and diencephalon, and intense in both neuronal somata (cytoplasmic) and processes. CCK1R-LI was found in new loci, but also in areas previously identified by receptor autoradiography, electrophysiology and in situ hybridization of CCK1R mRNA. The widespread distribution of CCK1R has implications for the functional roles of these receptors in brain. The high titre and low background seen with this new antiserum makes it of great value for cell and tissue research.

Differential bile-pancreatic secretory effects of CCK-58 and CCK-8

In this work, we 1) synthesized rat CCK-58, 2) determined the amounts and forms of rat CCK in whole blood after stimulation of its release by casein, 3) determined the potency of CCK-8 and CCK-58 peptides to displace labeled CCK-8 from CCK(A) and CCK(B) receptors transfected into Chinese hamster ovary (CHO) cells, and 4) examined the biological actions of CCK-8 and rat CCK-58 in an anesthetized rat model. CCK-58 was the only detected endocrine form of CCK in rat blood. Synthetic rat CCK-58 was less potent than CCK-8 for displacing the label from CCK(A) and CCK(B) receptors in transfected CHO cells. However, rat CCK-58 was more potent than CCK-8 for stimulation of pancreatic protein secretion in the anesthetized rat. In addition, CCK-58 but not CCK-8 stimulated fluid secretion in this anesthetized rat model. These data suggest that regions outside the COOH terminus of rat CCK-58 influence the expression of CCK biological activity. The presence of only CCK-58 in the circulation and the fact that its biological activity differs from CCK-8 suggests that CCK-58 deserves scrutiny in other physiological models of CCK activity.

Differences in receptor binding and stability to enzymatic digestion between CCK-8 and CCK-58

INTRODUCTION AND AIMS

It has been proposed that distinct tertiary structures of the C-terminus of CCK-8 and CCK-58 result in differences in stimulation of pancreatic amylase secretion. Binding of CCK-8 and CCK-58 to CCK-A and CCK-B receptors and stability to enzymatic digestion were used as independent probes for tertiary structure of the C-terminus.

METHODOLOGY

Canine CCK-58 was purified from intestinal extracts and CCK-8 was purchased. Their amounts were determined by amino acid analysis. The effect of tertiary structure on receptor binding at CCK-A receptors and CCK-B receptors was evaluated using membrane preparations from mouse pancreas and brain. The influence of C-terminal tertiary structure on stability to enzymatic digestion was evaluated by reacting CCK-8 and CCK-58 with endopeptidase 24:11.

RESULTS

CCK-58 was three times more potent than CCK-8 for binding mouse pancreatic membrane CCK-A receptors and equipotent to CCK-8 for binding mouse brain CCK-B receptors. CCK-8 was readily digested by endopeptidase 24:11, whereas CCK-58 was not.

CONCLUSIONS

The results strongly support the hypothesis that differences in tertiary structure of the carboxyl terminus of CCK-8 and CCK-58 influence receptor binding and stability to enzymatic digestion.

Cloning and functional expression of the first Drosophila melanogaster sulfakinin receptor DSK-R1

Described in this report is a successful cloning and characterization of a functionally active Drosophila sulfakinin receptor designated DSK-R1. When expressed in mammalian cells, DSK-R1 was activated by a sulfated, Met(7-->Leu(7)-substituted analog of drosulfakinin-1, FDDY(SO(3)H)GHLRF-NH(2) ([Leu(7)]-DSK-1S). The interaction of [Leu(7)]-DSK-1S with DSK-R1 led to a dose-dependent intracellular calcium increase with an EC(50) in the low nanomolar range. The observed Ca(2+) signal predominantly resulted from activation of pertussis toxin (PTX)-insensitive signaling pathways pointing most likely to G(q/11) involvement in coupling to the activated receptor. The unsulfated [Leu(7)]-DSK-1 was ca. 3000-fold less potent than its sulfated counterpart which stresses the importance of the sulfate moiety for the biological activity of drosulfakinin. The DSK-R1 was specific for the insect sulfakinin since two related vertebrate sulfated peptides, human CCK-8 and gastrin-II, were found inactive when tested at concentrations up to 10(-5) M. To our knowledge, the cloned DSK-R1 receptor is the first functionally active Drosophila sulfakinin receptor reported to date.

Peptide/benzodiazepine hybrids as ligands of CCK(A) and CCK(B) receptors

The (neuro)hormones gastrin and cholecystokinin (CCK) share a common C-terminal tetrapeptide amide sequence that has been recognized as the message portion while the N-terminal extensions are responsible for the CCK(A) and CCK(B) receptor subtype selectivity and avidity. 1,4-Benzodiazepine derivatives are potent and selective antagonists of these receptors, and according to comparative molecular field analysis, the structures of these nonpeptidic compounds could well mimic the message sequence of the peptide agonists at least in terms of spatial array of the aromatic residues. Docking of a larger series of low molecular weight nonpeptide antagonists to a homology modeling derived CCK(B) receptor structure revealed a consensus binding mode that is further validated by data from site-directed mutagenesis studies of the receptors. Whether this putative binding pocket of the nonpeptide antagonists is identical to that of the message portion of the peptide agonists, or whether it is distinct and spatially separated, or overlapping, but with distinct interaction sites, is still object of debate. Using a 1,4-benzodiazepine core amino-functionalized at the C3 position, related tryptophanyl derivatives were synthesized as mimics of the tetrapeptide and subsequently extended N-terminally with gastrin and CCK address sequences. All hybrid constructs were recognized as antagonists by the CCK(A) and CCK(B) receptors, but their address portions were incapable of enhancing in significant manner selectivity and avidity. Consequently, the binding of the peptide/benzodiazepine hybrids has to be dictated mainly by the benzodiazepine moiety, which apparently prevents optimal interactions of the address peptides with extracellular receptor subdomains. These findings would strongly support the view of distinct binding sites for the message portion of the peptide agonists and the benzodiazepine-based nonpeptide antagonists.

Hepatic uptake of cholecystokinin octapeptide by organic anion-transporting polypeptides OATP4 and OATP8 of rat and human liver

BACKGROUND & AIMS

Cholecystokinin (CCK) is a major gastrointestinal peptide hormone that is released postprandially from the small intestine and exerts marked effects on gallbladder and gastrointestinal motility. The smaller isoforms CCK-8 and CCK-4 are rapidly taken up into hepatocytes, metabolized, and excreted into bile. Our aim was to identify and characterize the hepatocellular CCK-8 uptake system.

METHODS

CCK-8 uptake was measured in Xenopus laevis oocytes expressing the organic anion-transporting polypeptides of rat liver (Oatp1, Oatp2, Oatp3, or Oatp4) and of human liver (OATP-A, OATP-B, OATP-C, or OATP8) and in primary cultured rat hepatocytes.

RESULTS

Rat Oatp4 and human OATP8 efficiently mediated CCK-8 uptake in oocytes, with Michaelis constant (Km) values of 14.9 +/- 2.9 micromol/L and 11.1 +/- 2.9 micromol/L, respectively. CCK-8 uptake by hepatocytes was also saturable, with a Km of 6.7 +/- 2.1 micromol/L. The Km value in rat hepatocytes is consistent with Oatp4-mediated transport.

CONCLUSIONS

CCK-8 is selectively transported by rat Oatp4 and human OATP8, both of which are exclusively expressed at the basolateral membrane of hepatocytes. These 2 transporters are the first and probably the predominant hepatic uptake systems for CCK-8 and may be critical for the rapid clearance of this hormone from the circulation.

New Ca2+-releasing messengers: are they important in the nervous system?

In the nervous system, Ca2+ signalling is determined primarily by voltage-gated Ca2+-selective channels in the plasma membrane, but there is increasing evidence for involvement of intracellular Ca2+ stores in such signalling. It is generally assumed that neurotransmitter-elicited release of Ca2+ from internal stores is primarily mediated by Ins(1,4,5)P3, as originally discovered in pancreatic acinar cells. The more-recently discovered Ca2+-releasing messenger, cyclic ADP-ribose (cADPR), which activates ryanodine receptors, has so far only been implicated in a few cases, and the possible importance of another Ca2+-releasing molecule, nicotinic acid adenine dinucleotide phosphate (NAADP), has been ignored. Recent investigations of the action of the brain-gut peptide cholecystokinin on pancreatic acinar cells have indicated that NAADP and cADPR receptors are essential for Ca2+ release. Tools are available for testing the possible involvement of NAADP and cADPR in neurotransmitter-elicited intracellular Ca2+ release, and such studies could reveal complex mechanisms that control this release in the nervous system.

A potent nonpeptide neuropeptide Y Y1 receptor antagonist, a benzodiazepine derivative

We describe here a nonpeptide neuropeptide Y Y1 receptor antagonist, 2,4-dioxo-1,5-bis(2-oxo-2-orthotolyl-ethyl)-3-[3-[3-([3-[3-(3-p iperidin-1-ylmethyl-phenoxy)-propylcarbamoyl]-propyl]-car bamoyloxymethyl)-phenyl]-ureido]-2,3,4,5-tetrahydro-1H-benzo[b][1,4]diaz epine (Compound 1), which was previously synthesized as a linked type of dual cholecystokinin (CCK)-B and histamine H2 receptor antagonist. Compound 1 competitively inhibited [125I]peptide YY (PYY) binding to Y1 receptors in human neuroblastoma SK-N-MC cells with Ki of 6.4 +/- 1.0 nM, while it had no effect on [125I]PYY binding to Y2 or Y5 receptors even at 1 microM. Functionally, Compound 1 inhibited the Y1 receptor-mediated increase in cytosolic free Ca2+ concentration and Y1 receptor-mediated attenuation of cAMP accumulation in a dose-dependent manner with IC50 values of 95 +/- 5 and 320 +/- 10 nM in SK-N-MC cells, respectively. Neither its CCK-B receptor antagonistic moiety of Compound 1 (Compound 2) nor its histamine H2 receptor antagonistic moiety of Compound 1 (Compound 3) had any effect on [125I]PYY binding, suggesting that the entire structure of Compound 1 is essential for Y1 receptor blocking activity. It showed no significant activity (IC50 > 1 microM) in 30 receptor binding assays and 5 enzyme assays, with the exception of CCK-B and histamine H2 receptors. We conclude that Compound 1 is a useful molecule not only for studying the physiological role of neuropeptide Y but also for exploring more specific Y1 receptor antagonists.

Met-195 of the cholecystokinin-A receptor interacts with the sulfated tyrosine of cholecystokinin and is crucial for receptor transition to high affinity state

Sulfation of the tyrosine at the seventh position from the C terminus of cholecystokinin (CCK) is crucial for CCK binding to the CCK-A receptor. Using three-dimensional modeling, we identified methionine 195 of the CCK-A receptor as a putative amino acid in interaction with the aromatic ring of the sulfated tyrosine of CCK. We analyzed the role played by the two partners of this interaction. The exchange of Met-195 for a leucine caused a minor decrease (2. 8-fold) on the affinity of the high affinity sites for sulfated CCK-9, a strong drop (73%) of their number, and a 30-fold decrease on the affinity of the low and very low affinity sites for sulfated CCK-9, with no change in their number. The mutation also caused a 54-fold decrease of the potency of the receptor to induce inositol phosphates production. The high affinity sites of the wild-type CCK-A receptor were highly selective (800-fold) toward sulfated versus nonsulfated CCK, whereas low and very low affinity sites were poorly selective (10- and 18-fold). In addition, the M195L mutant bound, and responded to, sulfated CCK analogues with decreased affinities and potencies, whereas it bound and responded to nonsulfated CCK identically to the wild-type receptor. Thus, Met-195 interacts with the aromatic ring of the sulfated tyrosine to correctly position the sulfated group of CCK in the binding site of the receptor. This interaction is essential for CCK-dependent transition of the CCK-A receptor to a high affinity state. Our data should represent an important step toward the identification of the residue(s) of the receptor in interaction with the sulfate moiety of CCK and the understanding of the molecular mechanisms that govern CCK-A receptor activation.

Role of fluidity of membranes on the guanyl nucleotide-dependent binding of cholecystokinin-8S to rat brain cortical membranes

The binding of [3H]cholecystokinin octapeptide (sulphated) ([3H]CCK-8S), an agonist of the cholecystokinin receptors, to rat cortical membranes was fast, specific and saturable, with pH optimum at 6.5-7.0. The divalent cations Mg2+ and Ca2+ clearly enhanced [3H]CCK-8S binding, whereas the monovalent cations Na+ and K+ were inhibitors. Inactivation of the ligand binding ability of these membranes was dependent on the incubation temperature and corresponding tau1/2 values were 11 days at 4 degrees , 12 hr at 21 degrees , 154 min at 30 degrees and 51 min at 37 degrees , which revealed the apparent activation energy of this process to be 130+/-4 kJ/mol. Scatchard analysis of the saturation curves of [3H]CCK-8S binding was best described by a one site binding model with a Kd = 0.63+/-0.18 nM and a maximum binding of 32+/-2 fmol/mg protein. The stable GTP analogue guanosin-5'-O-(3-thiotriphosphate) (GTPgammaS) decreased the affinity of [3H]CCK-8S binding only up to 2-fold without significant influence on maximal binding. Modulation of membrane properties by different detergents revealed that only in the case of digitonin (0.03-0.04%) did the GTP-dependence of [3H]CCK-8S binding considerably increase without significant influence on the ligand binding properties in the absence of GTPgammaS. Other detergents studied (sodium cholate, sodium deoxycholate, 3-(3-cholamidopropyl)dimethylammonio-1-propanesulfonate (CHAPS), sucrose monolaurate, series Triton X and Tween) either had little influence on GTP-gammaS-dependence of [3H]CCK-8S binding or inactivated the receptor. Parallel studies of fluorescent polarization of diphenylhexatriene (DPH) in rat cortical membranes indicated that digitonin was the only detergent which at low concentrations caused a rapid increase in membrane fluidity and thereafter stabilized it at a certain level. Other detergents studied had only moderate influence on membrane fluidity (CHAPS, cholate, deoxycholate) or caused fast and continuous increase of membrane fluidity (Triton X-100, Tween 80). These data together point to the essential influence of the fluidity of membranes on the regulation of the interactions between G proteins and CCK receptors in rat cortical membranes. Under standard experimental conditions (temperature lower than 30 degrees), the CCK receptor-G protein complex is active for quantitative characterization of the receptors, but the membranes are too rigid for natural communication and regulation.

Synthesis and structure-activity relationships of dual histamine H2 and gastrin receptor antagonists with modified benzodiazepine skeletons

Three different types of dual histamine H2 and gastrin receptor antagonists, e.g. those bearing a benzazepine, benzoxazepine, or benzothiazepine skeleton instead of the benzodiazepine one as a gastrin receptor antagonistic moiety were synthesized to reduce high hydrophobicity of parent compounds and evaluated for the dual activities. These skeletal modifications significantly potentiated the binding affinity of dual antagonists with histamine H2 receptor but markedly diminished their binding affinity with the gastrin receptor and the gastrin versus CCK-A receptor selectivity. We evaluated in vivo gastric acid antisecretory activities for some representative compounds by the rat pylorus ligation method for 10 mg kg(-1) dose by oral route. However, they exerted only low inhibitory activities for oral dose with % inhibition values ranging between 32 and 53%.

Synthesis and structure-activity relationships of dual histamine H2 and gastrin receptor antagonists with noncyclic gastrin receptor antagonistic moieties

In order to study structure-activity relationships of dual histamine H2 and gastrin receptor antagonists as well as to improve their low oral absorbability, their prototype benzodiazepine gastrin receptor antagonistic moieties were altered to a conformationally flexible noncyclic dipeptide equivalent. This skeletal modification significantly potentiated the binding affinity of hybrid compounds for the histamine H2 receptor, whereas their affinity for the gastrin receptor and receptor selectivity over the CCK-A receptor varied widely with the substituents on the gastrin moiety. Among them, [3-[3-(3-piperidin-1-ylmethylphenoxy)propylcarbamoyl]p ropyl carbamic acid 3-[3-([(3-methoxyphenyl)[(methylphenylcarbamyl)methyl]carbamoyl]me thyl)ureido]benzyl ester (7a) showed the highest dual histamine H2 and gastrin receptor antagonistic activities. It also displayed distinct gastric acid antisecretory activity in vivo for two assays, namely, Schild's rat method by i.d. administration and the rat pylorus ligation method by oral administration. With the latter case, dose-response relationships were observed for the first time, suggesting its substantially improved oral absorbability. However, 7a did not display distinct in vivo gastric acid antisecretory activity for the assay with Heidenhain pouch dogs.

Synthesis and structure-activity relationships of dual histamine H2 and gastrin receptor antagonists with decreased hydrophobicity

In order to study structure-activity relationships of the previously reported dual histamine H2 and gastrin receptor antagonists and also to improve their low oral absorbability, we tried two chemical modifications. One tried to decrease the high hydrophobicity of the parent hybrid compounds to an appropriate level by incorporating a hydrophilic group into the molecule and the other by replacing the more hydrophobic groups with less hydrophobic ones. The former compounds (type I) involved hybrid compounds with a hydroxyl group at a position of a spacer, a piperidine moiety of H2A, or a phenyl ring at the C5 of the benzodiazepine skeleton as well as those with a free carboxyl group in the piperidine moiety of H2A. The latter (type II) involved hybrid compounds with the C5-phenyl group replaced with either a methyl group or hydrogen atom. Among them, only a type I compound, ([2-[3-(3-piperidin-1-ylmethylphenoxy)propylcarbamoyl] ethylcarbamoyl]methyl)carbamic acid 3-[3-[5-(3-hydroxyphenyl)-1-methyl-2-oxo-2,3-dihydro-1H-1,4-benzodiaz epin-3-yl]ureido]benzyl ester (18), showed potent dual histamine H2 and gastrin receptor antagonistic activity, whereas others resulted in a significant decrease of histamine H2 receptor antagonistic activity. The in vivo gastric acid antisecretory activity of 18 evaluated by Schild's rat method, however, did not suggest any notable improvement in oral absorbability.

Activation of amygdala cholecystokininB receptors potentiates the acoustic startle response in the rat

The acoustic startle reflex is a sensitive index of "anxiety" and "fear." Potentiation of startle by conditioned and unconditioned fear stimuli appears to be mediated by the amygdala. CholecystokininB (CCKB) agonists increase "anxiety" in laboratory animals and induce "panic" in humans. Here, we investigate the role CCKB receptor-mediated mechanisms in the amygdala in the potentiation of startle. First, intra-amygdala infusions of the CCKB receptor agonist pentagastrin (0, 0.01, 0.1, 1, and 10 nM) produced a dose-related potentiation of acoustic startle responses. At the highest dose, startle amplitudes were increased up to 90% above preinfusion baseline levels. Second, similar infusions of pentagastrin had no effect on locomotor activity over the same time course, showing that increases in startle responsivity after infusions of pentagastrin are not attributable to nonspecific changes in motor activity. Third, infusions of similar doses of pentagastrin into the striatum or nucleus accumbens did not potentiate startle responses. Fourth, pretreatment with the CCKB receptor antagonist L-365,260 (0.1 mg/kg, i.p.) attenuated the potentiation of startle produced by intra-amygdala infusions of pentagastrin. Finally, intra-amygdala infusion of the CCKB receptor-selective antagonist PD-135158 (10 micro;g) blocked the potentiation of startle produced by i.c.v. infusions of pentagastrin, suggesting that i.c.v. infusions of pentagastrin potentiate startle responses via activation of amygdala CCKB receptors. These results show that amygdala CCKB receptor-mediated mechanisms are involved in the potentiation of acoustic startle responses.

Direct contractile effect of cholecystokinin octapeptide on caecal circular smooth muscle cells of guinea pig via both CCK-A and CCK-B/gastrin receptors

This study was designed to investigate the participation of cholecystokinin(CCK)-A and/or CCK-B/gastrin receptors in CCK-8-induced contraction of guinea pig caecal circular smooth muscle cells, using a novel selective CCK-A receptor antagonist, (S)-N-[1-(2-fluorophenyl)-3,4,6,7-tetrahydro-4-oxo-pyrrolo-[3,2,1-jk] [1,4]benzodiazepine-3-yl]-1H-indole-2-carboxamide (FK480), and a novel selective CCK-B/gastrin receptor antagonist, (R)-1-[2,3-dihydro-1-(2'-methylphenacyl)-2-oxo-5-phenyl-1H-1, 4-benzodiazepine-3-yl]-3-(3-methylphenyl)urea (YM022). Concentration-response curves for the contractile effect of CCK-8 alone and in the presence of 0.1nM FK480, 0.1 nM YM022, or a combination of 0.1 nM FK480 and 0.1 nM YM022 on isolated smooth muscle cells were determined. In addition, the inhibitory effects of various concentrations of FK480 or YM022 on 1 nM CCK-8-induced contraction were examined. At a concentration of 0.1 nM, both FK480 and YM022 shifted the concentration-response curve for CCK-8 to the right (about 100 times) with the same potency. In addition, a concentration-response curve for a combination of 0.1 nM FK480 and 0.1 nM YM022 was shifted to the right (about 100 times) of the curves for 0.1 nM FK480 alone or 0.1 nM YM022 alone. Both antagonists inhibited 1 nM CCK-8-induced contraction of caecal circular smooth muscle cells in a concentration-dependent manner, with the similar inhibitory potency. A significant inhibition was obtained at a concentration as low as 0.1 nM FK480 and 0.1 nM YM022. This study strongly suggested the presence of both CCK-A and CCK-B/gastrin receptors in caecal circular smooth muscle cells of guinea pig, and that the contractile effect of CCK-8 on these cells was mediated via both of these receptors.

G protein-coupled cholecystokinin-B/gastrin receptors are responsible for physiological cell growth of the stomach mucosa in vivo

Many peptide hormone and neurotransmitter receptors belonging to the seven membrane-spanning G protein-coupled receptor family have been shown to transmit ligand-dependent mitogenic signals in vitro. However, the physiological roles of the mitogenic activity through G protein-coupled receptors in vivo remain to be elucidated. Here we have generated G protein-coupled cholecystokinin (CCK)-B/gastrin receptor deficient-mice by gene targeting. The homozygous mice showed a remarkable atrophy of the gastric mucosa macroscopically, even in the presence of severe hypergastrinemia. The atrophy was due to a decrease in parietal cells and chromogranin A-positive enterochromaffin-like cells expressing the H+,K(+)-ATPase and histidine decarboxylase genes, respectively. Oral administration of a proton pump inhibitor, omeprazole, which induced hypertrophy of the gastric mucosa with hypergastrinemia in wild-type littermates, did not eliminate the gastric atrophy of the homozygotes. These results clearly demonstrated that the G protein-coupled CCK-B/gastrin receptor is essential for the physiological as well as pathological proliferation of gastric mucosal cells in vivo.

A segment of five amino acids in the second extracellular loop of the cholecystokinin-B receptor is essential for selectivity of the peptide agonist gastrin

The two known receptors mediating the actions of cholecystokinin (CCK) and gastrin, CCK type A (CCKAR) and CCK type B (CCKBR) receptors, are G protein-coupled receptors having approximately 50% amino acid homology. Both the CCKAR and CCKBR have high affinity for sulfated CCK peptides, while only the CCKBR has high affinity for gastrin peptides. To determine the structural basis for the selectivity of the CCKBR for gastrin, we first constructed a series of CCKB/AR chimeras in which restriction endonuclease-defined segments of the CCKBR were replaced with the corresponding segments of the CCKAR. Chimeras transiently expressed in COS-1 cells were screened for the selective loss of gastrin affinity according to the displacement of 125I-labeled Bolton-Hunter-CCK-8 binding by gastrin-17-I and CCK-8. The sequence spanning from transmembrane domain III (TM III) to TM V was the only segment that resulted in the selective loss of gastrin affinity. This segment could account for 100 of the expected 300-fold lower affinity of gastrin-17-I observed for the control CCKAR compared to the control CCKBR. Using site-directed mutagenesis in this segment of the CCKBR, we identified a sequence of 5 amino acids in the second extracellular loop responsible for this 100-fold selective loss in gastrin affinity. 125I-labeled Bolton-Hunter-CCK-8 binding displacement by L365,260 (a CCKBR selective antagonist) was unaffected by the changes in these 5 amino acids. These results present for the first time the identification of the amino acid sequence of the CCKBR conferring the majority of the selectivity for gastrin.

Central and peripheral cholecystokinin receptors in chickens differ from those in mammals

Specific binding for the radioligand [3H]CCK-8s has been identified in chicken brain, hypothalamus, pancreas, gallbladder and caecum membranes. This binding was found to be of high affinity, low capacity and saturable, suggesting the presence of specific CCK receptors in these tissues. Scatchard analysis indicated the existence of a single binding site for each tissue. Dissociation constant (kd) values were 0.63 +/- 0.18, 0.73 +/- 0.13, 0.85 +/- 0.12, 1.47 +/- 0.21 and 0.96 nM for brain, hypothalamus, pancreas, caecum and gallbladder, respectively. Binding densities (Bmax) were higher for brain, pancreas and caecum (32.60 +/- 10.70, 30.33 +/- 2.40 and 35.83 +/- 5.10 fmol/mg protein, respectively) than for the other two tissues (9.75 +/- 1.90 and 6.31 fmol/mg protein for hypothalamus and gallbladder, respectively). As in mammals, CCK-4 shows high affinity for CCK receptors located in chicken brain and hypothalamus, and very low affinity for those located in peripheral structures. L-364,718 (a CCK-A antagonist) showed a relative selectivity and a high affinity for those receptors located in central tissues, whereas L-365,260 (a CCK-B antagonist) is almost inactive in all studied tissues. These results give support for the existence of at least two distinct CCK receptors in birds and that these receptors are relatively different from those described in mammals.

Pharmacological properties of 403U76, a new chemical class of 5-hydroxytryptamine- and noradrenaline-reuptake inhibitor

403U76 (5-chloro-[[2-[(dimethylamino)methyl]phenyl]thio]benzene- methanol hydrochloride) is a potent, competitive, inhibitor of 5-hydroxytryptamine (5-HT) and noradenaline reuptake into rat brain synaptosomes. Inhibition of 5-HT uptake in-vivo by 403U76 was demonstrated by potentiation of the behavioural effects of 5-hydroxytryptophan in rats and mice and blockade of p-induced depletion of 5-HT in rats. The firing of 5-HT-ergic dorsal raphe neurons in rats was decreased after intravenous administration of low doses of 403U76 as would be predicted for a 5-HT uptake inhibitor. 403U76 antagonized tetrabenazine-induced sedation, an effect associated with inhibitors of noradrenaline uptake, but not with inhibitors of 5-HT uptake. Thus 403U76 affects noradrenergic as well as 5-HT-ergic neurotransmission in-vivo. Potential anxiolytic activity was indicated by reductions in isolation-induced vocalizations in neonates after 403U76 treatment. Low intravenous doses of 403U76 were well tolerated and had no sustained cardiovascular effects. There were no deleterious behavioural side-effects at active doses. Effects observed on isolated tissues or transmitter receptors occurred only at very high concentrations and were pharmacologically unimportant. Thus 403U76 can be considered a potential antidepressant/anxiolytic agent that is a potent, selective inhibitor of 5-HT and noradrenaline reuptake.

Cholecystokinin and gastrin are not equally sensitive to GTP gamma S at CCKB receptors: importance of the sulphated tyrosine

We have shown that gastrin and cholecystokinin octapeptide (CCK-8) are differently coupled to G protein (GTP-binding protein) through type B cholecystokinin receptors in guinea-pig brain membranes and Jurkat cells. Indeed, the gastrin-13 binding affinity is strongly reduced by stable guanyl nucleotides, whereas CCK-8 binding is only slightly affected. In order to determine the structural requirements regulating such coupling, we have synthesized several gastrin and cholecystokinin fragments (sulphated or unsulphated) elongated at the N-terminus of the common C-terminal tetrapeptide. We investigated their interaction with CCKB receptors in guinea pig brain membranes and Jurkat cells and their involvement in the G protein coupling. Their apparent binding affinities to CCKB receptors were measured by inhibition of [125I]Bolton Hunter-CCK-8 (3-[125I]iodo-4-hydroxyphenyl)propionyl-CCK-8) binding in the presence or absence of GTP gamma S (guanosine 5'-O-(3-thio)triphosphate) or aluminum tetrafluoride (AlF4-). Activation of the G proteins by GTP gamma S or AlF4- led to a decrease in binding affinity for the gastrin related peptides, the common CCK-gastrin C-terminal forms, the cholecystokinin hexapeptide and the unsulphated cholecystokinin heptapeptide. Sulphated CCK-7, CCK-8, and cionin apparent binding affinities were not affected. These finding indicated that the sulphated tyrosine in position 7 in CCK (as counted from the C-terminus), provides the cholecystokinin selectivity for the CCKB receptor compared to gastrin. The results are discussed with the aim to better clarify the physiological relevance of gastrin and cholecystokinin toward CCKB receptors and their related intracellular events.

A placebo-controlled trial of L-365,260, a CCKB antagonist, in panic disorder

The functional role of cholecystokinin in the central nervous system is unknown. The tetra peptide CCK-4 was previously observed to induce panic attacks in a majority of normal volunteers and patients with panic disorder. Furthermore, it had been demonstrated that pretreatment with 10-50 mg of L-365,260, a selective CCKB antagonist, blocked CCK-4 induced panic in patients with panic disorder. Therefore, the present multicenter, placebo-controlled, double-blind trial was designed to investigate the efficacy of L-365,260, a CCKB antagonist, in patients with panic disorder with or without agoraphobia. Following a 1-week, single-blind placebo period, 88 patients were randomized to double-blind treatment in which they received either L-365,260, 30 mg qid, or placebo for 6 weeks. At the dose tested, there were no clinically significant differences between L-365,260 and placebo in global improvement ratings, Hamilton anxiety rating scale scores, panic attack frequency, panic attack intensity, or disability measures. The possible reasons for lack of effect with L-365,260 are discussed.

Effects of a mixture of peptidase inhibitors (amastatin, captopril and phosphoramidon) on Met-enkephalin-, beta-endorphin-, dynorphin-(1-13)- and electroacupuncture-induced antinociception in rats

The effects of a mixture of three peptidase inhibitors (PIs), amastatin, captopril and phosphoramidon, on methionine-enkephalin (Met-enk)-, beta-endorphin (beta-end)-, dynorphin-(1-13) (Dyn)- and electroacupuncture (EA)-induced antinociception were compared in rats. EA was performed by passing electric pulses (3 Hz, 0.1-msec duration, for 45 min) through acupuncture needles inserted into the Hoku-point. The antinociceptive effect was estimated by the hind paw pressure test. The antinociceptive effects of Met-enk and beta-end injected i.c.v. or i.t. and of Dyn injected i.t. were clearly potentiated by the PIs pretreated by the same administration routes as used for the injection of opioid peptides. The antinociceptive effects of Met-enk, beta-end and Dyn injected i.c.v. were also potentiated significantly by i.t.-PIs. PIs injected into the periaqueductal gray (PAG) potentiated EA antinociception. However, the EA effect was not affected by i.t.-PIs and was rather attenuated by i.c.v.-PIs. These results suggest that: i) Met-enk hydrolyzing enzymes are involved in the degradation of not only Met-enk but also beta-end and Dyn in the rat central nervous system; ii) Met-enk and beta-end act on both supraspinal and spinal sites, while Dyn acts only on the spinal site; iii) EA antinociception is mediated by supraspinal Met-enk and/or beta-end; and iv) an anti-opiate peptide system may be activated by EA stimulation, being susceptible to Met-enk hydrolyzing enzymes.

Systemic administration of CCK-8S, but not CCK-4, enhances dopamine turnover in the posterior nucleus accumbens: a microdialysis study in freely moving rats

The present study was carried out to examine the effects of peripheral administration of sulfatedcholecystokinin octapeptide (CCK-8S) on dopamine (DA) turnover in the posterior nucleus accumbens (PNAc) and the caudate-putamen (CP) in awake rats. Microdialysis was used to quantify the extracellular concentrations of DA and its two metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). Intraperitoneal injections of CCK-8S (0.3 mg/kg b.wt.) caused a significant increase in DOPAC and HVA concentrations in the PNAc, but did not affect the DA level. Such increases in the metabolite contents were not found in the CP. Similar injections of vehicle (1% NaHCO3 solution, 1 ml/kg b.wt.) did not have an effect in either brain region. In an attempt to determine the type of receptor involved in the CCK-8S-induced changes, CCK tetrapeptide (CCK-4, 0.3 mg/kg b.wt.) known to have high affinity for CCKB subtype or vehicle (10% DMSO-saline, 1 ml/kg b.wt.) was administered intraperitoneally. Neither CCK-4 nor vehicle caused significant changes in any of extracellular DA, DOPAC and HVA contents in the PNAc. These results suggest that peripherally administered CCK-8S has stimulatory effects on the dopaminergic system in the PNAc, and raise the possibility that the effect appears to be mediated via CCKA receptors.

Pharmacological properties of ureido-acetamides, new potent and selective non-peptide CCKB/gastrin receptor antagonists

We present here the pharmacological properties of 3 ureido-acetamide members of a novel family of non-peptide cholecystokinin-B (CCKB) receptor antagonists. RP 69758 (3-(3-[N-(N-methyl N-phenyl-carbamoylmethyl) N-phenyl-carbamoylmethyl] ureido)phenylacetic acid), RP 71483 ((E)-2-[3-(3-hydroxyiminomethyl phenyl) ureido] N-(8-quinolyl) N-[(1,2,3,4-tetrahydro 1-quinolyl)carbonylmethyl]acetamide) and RP 72540 ((RS)-2-[3-(3-[N-(3-methoxy phenyl) N-(N-methyl N-phenyl-carbamoylmethyl) carbamoylmethyl] ureido) phenyl] propionic acid) displayed nanomolar affinity for guinea-pig, rat and mouse CCKB receptors labelled with [3H]pCCK-8 or with the selective CCKB receptor ligand [3H]pBC264. RP 69758 and RP 72540 showed selectivity factors in express of 200 for CCKB versus CCKA receptors. All three compounds had also high affinity for gastrin binding sites in the stomach. The ureido-acetamides behaved as potent antagonists of CCK-8-induced neuronal firing in rat hippocampal slices in vitro, a functional model of brain CCKB receptor mediated responses. RP 69758 is also a potent gastrin receptor antagonist in vivo that dose dependently inhibits gastric acid secretion induced by i.v. injection of pentagastrin in the rat. None of the three ureido-acetamides, at concentrations up to 1 microM, significantly blocked CCK-8-evoked contractions of the guinea-pig ileum in vitro, a CCKA receptor bioassay. In ex vivo binding studies, i.p. administration of RP 69758 and RP 72540 resulted in a dose-dependent inhibition of [3H]pCCK-8 binding in mouse brain homogenate. However, the relative penetration of these ureido-acetamides into the forebrain after peripheral administration was below 0.01%. RP 71483 did not appear to cross the blood-brain barrier in quantities sufficient to prevent [3H]pCCK-8 binding at low doses, a property that makes it suitable for the exploration of the peripheral versus central origin of the behavioural effects observed following systemic administration of CCK. RP 69758, RP 71483 and RP 72540 are highly potent and selective non-peptide CCKB receptor antagonists which are useful tools to explore the physiological functions of CCKB receptors.

Nucleus accumbens dopamine-CCK interactions in psychostimulant reward and related behaviors

The present paper provides an overview of dopamine (DA) and cholecystokinin (CCK) mechanisms that contribute to psychostimulant reward-related behaviors. Three different behavioral paradigms are focused on: intravenous psychostimulant reward, psychostimulant-induced locomotor activity, and psychostimulant-induced feeding. Based on evidence derived from these different behavioral paradigms, this paper reviews data indicating that nucleus accumbens CCK and DA play an important role in mediating reward. Neurobiological mechanisms are proposed to explain the functional relationship between CCK and DA in the nucleus accumbens.

Role of CCK in gallbladder function

Cholecystokinin may play a role in regulation of interdigestive motility, but this still remains to be investigated. CCK constitutes the major hormonal stimulus for postprandial gallbladder emptying. CCK exerts its contractile effects mainly through interaction directly with receptors on the gallbladder smooth muscle cells in the muscle layer, but also through interaction with cholinergic nerves extrinsic and/or intrinsic in nature. Furthermore, CCK can enhance ongoing nicotinic ganglionic transmission occurring in the serosal layer by release of acetylcholine. CCK interaction with the gallbladder smooth muscle CCKA receptor was studied in further detail. CCK contracts strips of gallbladder muscle in a concentration-dependent way with a potency in the nanomolar range in all tested species. The potency is 1,000-fold better than that of gastrin; thus, the receptor is of type CCKA. CCK binding to this receptor is specific and of high affinity, 1,000-fold better than that of gastrin with no differences between the tested species including bovine, porcine, and human. Also, CCK binding affinity was independent of age, gender, or weight of the person and pathology of the human gallbladder. The biochemistry of the CCKA receptor varies between the tested species (bovine and human). Both CCKA receptors are heavily glycosylated, but of different size and carbohydrate content. The bovine CCKA receptor is of apparent size M(r) = 70-85 kD with N-linked complex carbohydrates and sialic acids. The human CCKA receptor is of M(r) = 85-95 kD, with N-linked complex carbohydrates, but no sialic acids. They both have a protein core of apparent size M(r) = 43 kD, with almost identically sized fragments after enzymatic cleavage. Probably the protein cores contain the receptor binding region, which seems well preserved between species. CCK and the CCKA gallbladder muscularis receptor are main regulators of postprandial gallbladder emptying. The biochemistry of the CCKA gallbladder smooth muscle receptor is in accord with newly generated data of purification and cloning of the rat pancreatic CCKA receptor.

Natural and synthetic CCK-58. Novel reagents for studying cholecystokinin physiology

CCK-58 is a unique reagent for testing how segments of a peptide far removed from its active site can influence the expression of its biological activity. Indications of tertiary structure have come from studies with natural peptide purified from canine small intestine. These studies gave clear indications that tertiary structure affects CCK-58 bioactivity, but the small quantities of CCK-58 available made it impossible to characterize completely how tertiary structure influenced bioactivity. Canine CCK-58 was synthesized manually using a solid support and was purified by reverse phase high pressure liquid chromatography (HPLC). Synthetic CCK-58 was characterized by isocratic reverse phase and gradient HPLC, amino acid analysis, mass spectral analysis, sequence analysis, and three bioassays. Synthetic and natural canine CCK-58 had the same elution profiles, amino acid composition, sequence, and mass. The two peptides were equipotent for the stimulation of pancreatic secretion. Natural canine CCK-58 was equipotent to CCK-8 for CCK "B" receptor binding, a further indication of the purity of the natural peptide. However, natural CCK-58 was more potent than CCK-8 for CCK "A" receptor binding and less potent than CCK-8 for stimulation of pancreatic secretion. These data support the concept that CCK-58 has a stable tertiary structure. This structure does not affect its binding to CCK "B" receptors, enhances its binding to low affinity CCK "A" receptors, and decreases its activity expressed through binding to high affinity CCK "A" receptors. The concept of a stable tertiary structure is also supported by the fact that many antibodies directed towards the carboxyl terminus of cholecystokinin react better with CCK-8 than CCK-58. The availability of synthetic CCK-58 will allow analysis of its tertiary structure by physical and chemical methods as well as studies on how peptide tertiary structure can affect receptor binding, receptor activation, metabolism in blood, degradation in interstitial fluid, and inactivation at the receptor. Evaluating all of these systems will help investigators understand the regulation of cholecystokinin activity by its major endocrine form, CCK-58.

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

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

Biological actions of cholecystokinin

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

Peripheral biological activity of SR 27897: a new potent non-peptide antagonist of CCKA receptors

SR 27897 is a new non-peptide antagonist of CCKA receptors: 1-[[2-(4-(2-chlorophenyl)thiazol-2-yl)aminocarbonyl] indolyl] acetic acid. This compound is a potent ligand for CCKA binding sites (rat pancreatic membranes, Ki = 0.2 nM) and is highly selective (CCKB and gastrin/CCKA IC50 ratios of 800 and 5000 respectively). In vitro, it is a competitive antagonist of cholecystokinin (CCK)-stimulated amylase release in isolated rat pancreatic acini (pA2 = 7.50) and of CCK-induced guinea pig gall bladder contractions (pA2 = 9.57). In in vivo gastrointestinal models, SR 27897 confirmed the potency obtained in vitro: at 1 mg/kg (i.v.) it completely reversed the CCK-induced amylase secretion, at 3 micrograms/kg (p.o.) it antagonized by 50% the CCK-induced inhibition of gastric emptying of a charcoal meal in mice, and 72 micrograms/kg (p.o.) was the median effective dose for inhibiting CCK-induced gall bladder emptying in mice. SR 27897 was also very active (ED50 = 27 micrograms/kg p.o.) in the gall bladder emptying protocol with egg yolk as an inducer of endogenous CCK release. SR 27897 had a long-lasting action in all the experiments, with no differences between oral and intravenous routes of administration. SR 27897 was more or less effective than L-364,718, depending on the model and the species. Both compounds increased the gall bladder volume of fasting mice, but the effect of SR 27897 was 10 times lower than that of L-364,718. In summary, SR 27897 is a selective antagonist of CCKA receptors, is highly potent in animal models whatever the route of administration and has a long duration of action.(ABSTRACT TRUNCATED AT 250 WORDS)

CCKB receptor activation reduces glutamate-induced depolarization in slices of rat cerebral cortex

Recent studies on cell cultures have indicated that the neuropeptide cholecystokinin (CCK) can prevent glutamate-induced cytotoxicity. In a preparation of rat cortical tissue placed into a two-compartment bath, the cortical tissue could be depolarized, relative to the corpus callosum, by superfusions of KCl or glutamate (1.25-10 mM). Caerulein (1-100 nM), a CCK receptor agonist, caused a rightward shift of the glutamate dose-response curve. The effect of caerulein was abolished by adding L365,260 (1 microM), a selective CCKB receptor antagonist. These findings suggest that CCK may be a physiological antagonist of glutamate-mediated neurotransmission in the rat brain.

Biological effects of human gastrin I and II chemically modified at the C-terminal tetrapeptide amide

Binding to gastrin receptors and gastric acid secretion experiments were performed with gastrin derivatives modified at the C-terminal tetrapeptide amide from HG-13 sequence. 1. When the ultimate phenylalanine amide was replaced by a phenethylester or a phenetylamide moiety, the resulting compound bound to gastrin receptors (Kd approximately 10 nM) and exhibited antagonist activity on gastrin-induced acid secretion in the anesthetized rat. 2. Changing the peptide bond between Trp and Leu residues to a -omega(CH2-NH)- bond resulted in a compound which also bound to gastrin receptors (Kd approximately 10 nM) but presented agonist activity on acid secretion in the rat. In contrast, when the peptide bond between Leu and Asp residues was replaced by a -omega(CH2-NH)- bond, the resulting compound was devoid of any affinity for gastrin receptor (Kd greater than 10(-6) M) and of any biological activity. 3. The HG-13 derivatives were synthesized in sulfated and unsulfated forms: O-sulfation of the HG-13 tyrosine residue did not change its intrinsic in vivo activity but enhanced its affinity for gastrin receptors (Kd approximately 0.3 nM). On the contrary, O-sulfation of the various chemically modified HG-13 had no significant effect in either in vitro or in vivo experiments. 4. Finally, no significant difference between binding on parietal (F3) and nonparietal (F1) cells was observed, in agreement with the presence of a gastrin-type receptor in these two cell populations.

Pharmacological characterization of RP 62203, a novel 5-hydroxytryptamine 5-HT2 receptor antagonist

1. RP 62203 (2-[3-(4-(4-fluorophenyl)-piperazinyl)propyl]naphto[1,8- ca]isothiazole-1,1-dioxide) is a novel naphtosultam derivative which shows very high affinity for 5-HT2 receptors in the rat cerebral cortex (Ki = 50.0 pM). 2. RP 62203 is relatively selective for this sub-type of 5-hydroxytryptamine (5-HT) receptor, having lower affinity for the 5-HT1A receptor and very low affinity for the 5-HT, receptor. RP 62203 displayed low to moderate affinity for alpha 1-adrenoceptors, dopamine D2 receptors and histamine H1 receptors. 3. In vivo binding experiments demonstrated that oral administration of low doses of RP 62203 led to a long-lasting (greater than 6 h) occupation of cortical 5-HT2 receptors (ID50 = 0.39 mgkg-1). 4. In cortical slices from the neonatal rat, RP 62203 potently inhibited inositol phosphate formation evoked by 5-HT, with an IC50 of 7.76 nM. 5. The activity of neurones in the raphé and their responses to microiontophoretically applied 5-HT were studied with extracellular recording electrodes in the anaesthetized rat. RP 62203 potently and dose-dependently blocked excitations evoked by 5-HT when administered at doses of 0.5-4.0 mg kg-1, i.p. In contrast, neither 5-HT-evoked depressions nor glutamate-evoked excitations of raphé neuronal firing were blocked by RP 62203 at doses as high as 8.0 mg kg-1, i.p. 6. Head twitches induced by 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) could be abolished by low doses of RP 62203 in mice (ED50 = 0.44 mg kg-1, p.o.) and in rats (ED50 = 1.54 p.o.). Similar results were obtained with mescaline and 5-hydroxytryptophan (5-HTP). 7. The potency of RP 62203 was compared with that of three other 5-HT2 receptor antagonists, ritanserin, ICI 169,369 and ICI 170,809. In all models, RP 62203 showed similar activity to ritanserin, whilst either ICI 169,369 or ICI 170,809 was several fold less active. 8. It is concluded that RP 62203 is a potent and selective antagonist at 5-HT2 receptors in the rodent central nervous system.

Somatostatin 28 interacts with CCK receptor in brain and pancreas

The ability of somatostatin analogs to interact with the binding of cholecystokinin has been studied in pancreatic and brain cortical membranes. Only the 28 amino-acid forms of somatostatin (S28), [Nle8]S28 and [Des Lys14,DTrp22]S28 were found to inhibit the binding of cholecystokinin to rat pancreatic plasma membranes and to increase the amylase release from pancreatic acini. This effect was independent of somatostatin receptor and resulted from an interaction between S28 and CCK receptor. This interaction was not observed with [Leu8, DTrp22, Tyr25]S28, indicating that this analog does not possess the biological activity of the native peptide and that the iodinated peptide could not label specific S28 receptors. S28 interacted also with CCK receptors in cortical brain membranes. Our results support the concept that S28, but not S14, may function as a regulatory molecule at CCK receptors and emphasize that S28 and S14 may be distinct neuromodulators.

Cholecystokinin modulates the release of dopamine from the anterior and posterior nucleus accumbens by two different mechanisms

The effects of various cholecystokinin (CCK)-related peptides were investigated on 35 mM K(+)-stimulated endogenous dopamine release from slices of either anterior or posterior nucleus accumbens of the rat. CCK sulphated octapeptide (1-10 microM), but not pentagastrin or CCK unsulphated octapeptide, was found to cause a dose-dependent increase in the release from the posterior nucleus accumbens. This effect was blocked by low doses of the CCKA receptor antagonist L364,718 (10 nM) but not the CCKB receptor antagonist L365,260. In the anterior nucleus accumbens CCK sulphated octapeptide (1 microM) and CCK unsulphated octapeptide (0.1-1 microM) inhibited the dopamine release, and this effect was blocked by L365,260 (10-100 nM) but not by L364,718. These results suggest that CCK has a different effect on dopamine release from the anterior and posterior nucleus accumbens and that these effects are mediated by two different types of CCK receptor.

Characterization of cholecystokinin receptors on guinea pig gastric chief cell membranes

The binding of cholecystokinin (CCK) to its receptors on guinea pig gastric chief cell membranes were characterized by the use of 125I-CCK-octapeptide (CCK8). At 30 degrees C optimal binding was obtained at acidic pH in the presence of Mg2+, while Na+ reduced the binding. In contrast to reports on pancreatic and brain CCK receptors, scatchard analysis of CCK binding to chief cell membranes revealed two classes of binding sites. Whereas, in the presence of a non-hydrolyzable GTP analog, GTP gamma S, only a low affinity site of CCK binding was observed. Chief cell receptors recognized CCK analogs, with an order of potency of: CCK8 greater than gastrin-I greater than CCK4. Although all CCK receptor antagonists tested (dibutyryl cyclic GMP, L-364718 and CR1409) inhibited labeled CCK binding to chief cell membranes, the relative potencies of these antagonists in terms of inhibiting labeled CCK binding were different from those observed in either pancreatic membranes or brain membranes. The results indicate, therefore, that on gastric chief cell membranes there exist specific CCK receptors, which are coupled to G protein. Furthermore, chief cell CCK receptors may be distinct from pancreatic or brain type CCK receptors.

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

Almost two decades ago, exogenous cholecystokinin (CCK) was shown to suppress food consumption in rats. Since then, CCK has been detected not only in peripheral tissue but extensively throughout the central nervous system. Furthermore, specific CCK receptors have been described, and a distinction drawn between CCK-A and CCK-B receptors. Recently, potent, orally active CCK antagonists, which show a high degree of selectivity for either CCK-A or CCK-B receptors, have been introduced. The present report reviews recent evidence obtained in studies using devazepide (a selective CCK-A receptor antagonist) and L-365,260 (a selective CCK-B/gastrin receptor antagonist). Both compounds increased food consumption and postponed the onset of satiety in well-satiated rats. L-365,260 was more potent, suggesting that central CCK-B type receptors may mediate the satiety effects of endogenously released CCK. Only devazepide was effective in blocking the feeding-suppressant effect of exogenous CCK, indicating that CCK-A type receptors mediate this effect. In a second series of studies, devazepide but not L-365,260 antagonized the anorectic effect of either d-fenfluramine or systemically administered 5-HT. Hence, CCK-A type receptors appear to be involved in the anorectic effects of these serotonergic drugs. We propose that CCK and 5-HT mechanisms involved in mediating satiety are mutually interdependent. Possible interactions between CCK and catecholaminergic mechanisms are also briefly considered.

Cholecystokinin analogues with high affinity and selectivity for brain membrane receptors

A series of CCK analogues in which positions 28 and 31 have been replaced by N-methylnorleucine residues have been synthesized. It has been found that most of these N-methylnorleucine containing analogues of CCK are highly potent and some are extraordinarily selective for the central vs. peripheral receptor in two animal models (guinea pig and rat). [N-MeNle28,31]CCK26-33 nonsulfated exhibited both high potency (IC50 = 0.13 nM) and selectivity for central vs. peripheral receptors. The pancrease to brain cortex binding affinity ratio for this analogue is 5100 in the rat model. NMR studies reveal that there is cis/trans isomerism about the N-methylnorleucine residue that may be related to high selectivity.