Molecular cloning of the human brain and gastric cholecystokinin receptor: structure, functional expression and chromosomal localization

YF476 is a new potent and selective gastrin/cholecystokinin-B receptor antagonist in vitro and in vivo

BACKGROUND

We newly synthesized YF476 ((R)-1-[2,3-dihydro-2-oxo-1-pivaloylmethyl-5-(2'-pyridyl)-1H-1, 4benzodiazepin-3-yl]-3-(3-methylamino-phenyl)urea) as a gastrin/cholecystokinin-B (CCK-B) receptor antagonist. We investigated the pharmacological profile of YF476 in vitro and in vivo.

METHODS

We examined the binding properties of YF476 to the rat brain, cloned canine and cloned human gastrin/CCK-B receptors, and the effect of YF476 on secretagogue-induced gastric acid secretion in rats and Heidenhain pouch dogs.

RESULTS

YF476 replaced the specific binding of [125I]CCK-8 to the rat brain, cloned canine and cloned human gastrin/CCK-B receptors, with Ki values of 0.068, 0.62 and 0.19 nM, respectively. The affinity of YF476 for rat brain gastrin/CCK-B receptor was 4100-fold higher than that for rat pancreatic CCK-A receptor. In anaesthetized rats, intravenous YF476 inhibited pentagastrin-induced acid secretion with an ED50 value of 0.0086 micromol/kg, but did not affect histamine- and bethanechol-induced acid secretion at a dose of 10 micromol/kg. In Heidenhain pouch dogs, intravenous and oral YF476 inhibited pentagastrin-stimulated gastric acid secretion in a dose-dependent manner with ED50 values of 0.018 and 0.020 micromol/kg, respectively, but did not affect histamine-induced acid secretion.

CONCLUSION

These results suggest that YF476 is an extremely potent and highly selective gastrin/CCK-B receptor antagonist, and that the gastrin/CCK-B receptor is not involved in histamine- or bethanechol-induced gastric acid secretion in dogs or rats.

Pharmacological and biochemical evidence for the simultaneous expression of CCKB/gastrin and CCKA receptors in the pig pancreas

1. In the pig, the secretory response of the pancreas is not inhibited by the antagonist MK329 suggesting that cholecystokininA (CCKA) receptors are not involved. 2. Membranes were isolated from the pancreas of 6 Large White pigs to characterize their CCK receptors. 3. The binding of [125I]-BH-[Thr, Nle]CCK-9 was dependent on pH, maximal after a 90 min incubation period, saturable and reversible. Saturation analysis of the binding demonstrated a single class of high affinity sites (Kd = 0.22 +/- 0.02 nM) and a binding capacity, Bmax = 110.64 +/- 12.50 fmol mg-1 protein. 4. Competition binding by agonists and antagonists of CCKA and CCKB/gastrin receptors demonstrated the presence of two distinct binding components, sites presenting a high affinity for [Thr, Nle]CCK-9, gastrin, PD 135158, L-365, 260 and a low affinity for MK329, SR 27897, and sites presenting a high affinity for [Thr, Nle]CCK-9, MK329, SR 27897 and a low affinity for gastrin, PD 135158, L-365,260. 5. These pharmacological data demonstrate the presence of both CCKA and CCKB/gastrin receptors in the pig pancreas, the latter being predominant. 6. Two distinct membrane proteins (50 and 85-100 kDa, respectively) display pharmacological features of CCKB/gastrin and CCKA receptors. 7. In pigs, as in calves and humans, CCKB/gastrin receptors are predominant in the pancreas.

Cholecystokinin cells

Cholecystokinin (CCK) is an important hormonal regulator of the digestive process. CCK cells are concentrated in the proximal small intestine, and hormone is secreted into the blood upon the ingestion of food. The physiological actions of CCK include stimulation of pancreatic secretion and gallbladder contraction, regulation of gastric emptying, and induction of satiety. Therefore, in a highly coordinated manner, CCK regulates the ingestion, digestion, and absorption of nutrients. CCK is produced by two separate cell types: endocrine cells of the small intestine and various neurons in the gastrointestinal tract and central nervous system. Accordingly, CCK can function as either a hormone or a neuropeptide. This review focuses on the physiology of the CCK cell in the intestine and, in particular, on how the CCK cell is regulated to secrete its hormone product. The effects of ingested nutrients on the CCK cell and the intracellular messenger systems involved in controlling secretion are reviewed. A summary is provided of recent studies examining the electrophysiological properties of CCK cells and newly discovered proteins that act as releasing factors for CCK, which mediate feedback pathways critical for regulated secretion in the intact organism.

mRNAs encoding CCKB but not CCKA receptors are expressed in human T lymphocytes and Jurkat lymphoblastoid cells

We previously reported the existence of pharmacologically related gastrin/CCKB type receptors (CCKB-R) in a variant of Jurkat T lymphoblastoid cells (JK(CD3- CD4+)). We studied here the expression of mRNAs encoding CCKA and CCKB receptors in various human white cells by means of Reverse Transcription-Polymerase Chain Reaction (RT-PCR). Using CCKB-R specific primers, we detected a significant expression of CCKB-R mRNA in JK(CD3- CD4+) cells. These transcripts were also expressed, at a lower level, in two other Jurkat clones (JK(CD3+ CD4-) and JK(CD3+ CD4+)), in peripheral blood lymphocytes (PBL) and in purified CD4+ and CD8+ lymphocytes. Activation of Jurkat cells and PBL by T cells mitogenic lectins (jacalin, phytohemaglutinin) did not modify CCKB-R mRNA expression. In all these cells, using CCKA-R specific primers, we could not amplify any specific cDNA fragment corresponding to this receptor. Neither CCKB-R nor CCKA-R mRNAs could be detected in monocytic cells. Our data show for the first time a constitutive expression of CCKB-R transcripts in lymphoid cells. Moreover, the modulation of immunocyte functions by cholecystokinin-related peptides could occur through CCKB-R rather than CCKA-R and affect lymphocytes rather than monocytes.

Pharmacological analysis of the CCKB/gastrin receptors mediating pentagastrin-stimulated gastric acid secretion in the isolated stomach of the immature rat

1. The CCKB/gastrin receptors mediating pentagastrin stimulation of gastric acid secretion by histamine release and by direct stimulation of oxyntic cells have been characterized in the immature rat isolated stomach assay. This was achieved by estimating antagonist affinity values for competitive antagonists from three distinct chemical classes (L-365,260, PD134,308 and JB93190) in the absence and presence of a high concentration of the histamine H2-receptor antagonist, famotidine (30 microM). 2. Pentagastrin produced concentration-dependent stimulation of gastric acid secretion in the absence and presence of famotidine. Famotidine depressed the maximum secretory response to pentagastrin although the degree of depression varied between experimental replicates (25-60%). This variation was attributed to the histamine-release mediated component of acid secretion, as judged by the consistency of the maximum responses obtained in the presence, but not absence, of famotidine. 3. All three CCKB/gastrin receptor antagonists behaved as surmountable antagonists in the absence and presence of famotidine. JB93190 (pKB approximately 9.1, approximately 8.9, in the absence and presence of famotidine, respectively) was approximately 30 fold more potent than either L-365,260 (pKB approximately 7.4, approximately 7.1) or PD134,308 (pKB approximately 7.6, approximately 7.4). 4. It was assumed that the famotidine treatment converted pentagastrin-stimulated acid secretion from a combination of an indirect action due to the release of histamine and a direct action on the oxyntic cell to solely a direct action on the oxyntic cell. A simple mathematical model of this two-receptor system was developed. The direct and indirect components were assumed to sum to produce the total response to pentagastrin obtained in the absence of famotidine. It was found that this model could account quantitatively for the behaviour of the three antagonists without invoking a difference in antagonist affinity for the CCKB/gastrin receptors mediating the direct and indirect actions of pentagastrin. However, a conclusion of receptor homogeneity has to be qualified because the model was also used to generate simulations which indicated that the analysis could only detect antagonist affinity differences of greater than one log-unit between enterochromaffin-like (ECL) and oxyntic cell CCKB/gastrin receptor populations.

Cholecystokinin in anxiety

Cholecystokinin (CCK) plays an important role in both the alimentary tract and the central nervous system (CNS). At present it seems to be the most abundant neuropeptide in the CNS. This paper reviews the CCK neuronal system and its interactions with gamma-aminobutyric acid (GABA) and serotonin (5-hydroxytryptamine; 5-HT). In addition, its putative role in anxiety will be discussed on the basis of animal data and studies in healthy volunteers and panic disorder patients. According to these investigations, the CCK4 challenge test fulfills most criteria for an ideal panicogenic agent and evidence has been found that CCKB receptor antagonists might possess anxiolytic properties in man.

Behavioral effects of CCKB receptor ligands in a validated simulation of panic anxiety in rats

Animals or human subjects receiving brain stimulation in the dorsal periaqueductal gray matter (dPAG) show sudden fear-suggestive behavioral reactions and physical signs of autonomic activation which are reminiscent of the symptom profile characterizing a panic attack. An experimental situation in rats measuring dPAG stimulation self-interruption thresholds has been validated as realistically simulating several aspects of panic anxiety with objective signs of symptomatic and predictive validity using established antipanic and panicogenic agents; it was utilized here to evaluate the effects of various cholecystokinin B receptor ligands. A dose-dependent increase in self-interruption thresholds (antipanic-like effect) was recorded following injection of L-365,260 (3.2, 10 and 32 mg/kg i.p.), a CCKB receptor antagonist with good brain penetration, whereas no significant changes in thresholds were recorded following CI-988 (3.2, 10 and 32 mg/kg i.p.), a dipeptoid CCKB receptor antagonist with poor brain penetration. Latencies for self-interruption were not modified, suggesting that motor functions remained intact. No significant changes in self-interruption thresholds were recorded following peripheral administration of the CCKB receptor agonists CCK4 (0.03 to 0.32 mg/kg i.v.; 0.01 to 3.2 mg/kg i.p.) or the metabolically stabilized analog Boc-CCK4 (0.1 to 10 mg/kg i.p.). Systemic administration of the panicogenic compounds caffeine and yohimbine enhance acute anxiety in this model. These data indicate that, in the dPAG simulation of panic anxiety, central CCKB receptor blockade by L-365,260 induces antiaversive effects analogous to those observed following benzodiazepine receptor activation by clonazepam or alprazolam. Potency and efficacy of L-365,260 were lower than those of clonazepam or alprazolam, suggesting modest, but nonetheless authentic, antiaversive properties for this CCKB receptor antagonist. Lack of effects observed following peripheral administration of the agonists CCK4, and Boc-CCK4 or of the dipeptoid antagonist CI-988 is likely to reflect restricted brain penetration of those compounds in rats; it furthermore excludes a contribution of peripheral gastrin and CCKA receptors to the antipanic-like properties of selective CCKB receptor antagonists such as L-365,260.

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.

Evaluation of cholecystokinin, gastrin, CCK-A receptor, and CCK-B/gastrin receptor gene expressions in gastric cancer

The brain-gut hormones, cholecystokinin (CCK) and gastrin, regulate the growth of gastrointestinal mucosa and tumor cells. In this study, reverse transcription-polymerase chain reaction (RT-PCR) was used to evaluate messenger RNA expression for CCK, gastrin, CCK-A receptor, and CCK-B/gastrin receptor in surgical specimens of gastric cancers and in normal antrum and body mucosa of the stomach. The CCK mRNA expression was detectable in 4/14 (29%) samples of gastric cancer and in 3/12 (25%) samples of antral mucosa. However, the gastrin mRNA expression was not detectable in any gastric cancer samples, although it was detectable in all the samples of antral mucosa. The CCK-A receptor mRNA expression was detectable in 5/14 (36%) samples of gastric cancer and in 7/12 (58%) body mucosa. Three cases out of 14 (21%) of gastric cancer expressed both CCK gene and CCK-A receptor gene. The CCK-B receptor mRNA expression was detectable in only 1/14 (7%) samples of gastric cancer, although it was detectable in 10/12 (83%) body mucosa of the stomach. These findings may suggest a greater role for CCK and CCK-A receptor than for gastrin and CCK-B receptor in gastric cancers.

Evaluation of the specificity and potency of a series of cholecystokinin-B/gastrin receptor antagonists in vivo

The potency and specificity of five proposed cholecystokinin-B receptor antagonists, YM022, RP73870, L-740,093, L-365,260 and LY288513, were studied in rats and mice. Gastrin activates rat stomach histidine decarboxylase via cholecystokinin-B/gastrin receptors. To examine cholecystokinin-B receptor-mediated effects of the five drugs, they were infused intravenously to fasted rats and the histidine decarboxylase activity in the oxyntic mucosa was determined. While YM022, RP73870, L-740,093 and L-365,260 failed to activate histidine decarboxylase, they dose-dependently antagonized the gastrin-induced histidine decarboxylase activation. LY288513 had no effect in the doses tested. The maximal inhibitory effect of L-365,260, L-740,093, RP73870 and YM022 on histidine decarboxylase, activated by the intravenous infusion of an ED50 does of gastrin (0.4 nmoles/kg/hr), was seen at doses of 3, 0.3, 0.1 and 0.1 mumoles/kg/hr, respectively; the corresponding ID50 values were 0.4, 0.02, 0.007 and 0.004 mumoles/kg/h. In a follow-up study, YM022 and RP73870 were found to produce a rightward shift of the gastrin dose-response curve, which is consistent with competitive inhibition. The effect of the five drugs on a cholecystokinin-A receptor-mediated response was examined by studying gastric emptying in mice. Cholecystokinin-8s, given by a subcutaneous bolus injection, dose-dependently inhibits gastric emptying. The specific cholecystokinin-A receptor antagonist devazepide (given intravenously as a bolus injection) antagonized the effect of cholecystokinin-8s in a dose-dependent manner, with an ID50 value of 28 nmoles/kg. None of the drugs inhibited the gastric emptying or prevented the cholecystokinin-8s-induced effect at the doses tested. The results indicate that YM022, RP73870, L-740,093 and L-365,260 act as cholecystokinin-B receptor antagonists in vivo, being without measurable agonistic activity. Furthermore, they do not interact with cholecystokinin-A receptors at te doses tested. Among the cholecystokinin-B receptor antagonists studied YM022 and RP73870 are superior, the rank order of potency being YM022 > or = RP73870 > L-740,093 > L-365,260.

L-364,718 and L-365,260, two CCK antagonists, have no affinity for central benzodiazepine binding sites in chickens

It has recently been demonstrated that L-365,260, a CCK-B antagonist in mammals, causes an increase in food intake in chickens. In contrast, L-364, 718, a CCK-A antagonist in mammals, shows this effect only at very high dose levels. It has been shown that L-365,260 has very low affinity for chicken CCK receptors. Thus, the mechanism of action of L-365,260 remains unknown. As L-365,260 is a benzodiazepine derivative, one may hypothesize that it would be acting on benzodiazepine binding sites. The aims of this work were to establish the existence of benzodiazepine binding sites in the chicken brain, and to check the possibility that L-365,260 was acting on these receptors, determining the affinity of L-364,718 and L-365,260 for them. We have found specific binding for tritiated flunitrazepam (a benzodiazepine agonist) ([3H]-flunitrazepam) in chicken brain membranes. A single binding site was detected with a Kd of 3.58 +/- 0.97 nM and a Bmax of 451.6 +/- 23.3 fmol/mg protein L-365,260 and L-364,718 exhibited very low affinity for these binding sites (Ki = 1.17 x 10(-6) +/- 0.16 x 10(-6) M and Ki > 10(-5) M, respectively). Thus, these results demonstrate that the increase in food intake caused by L-365,260 in the chicken is not due to a direct action on benzodiazepine receptors. Other possible explanations for its effect are discussed.

The influence of guanyl nucleotide on agonist and antagonist affinity at guinea-pig CCK-B/gastrin receptors: binding studies using [3H]PD140376

The novel radioligand [3H]PD140376 was used to label receptors that bind cholecystokinin (CCK) and related peptides in membranes prepared from guinea-pig brain and gastric glands. Under control conditions, measurements of the apparent affinity of 11 agonist and 16 antagonist ligands in both tissues revealed a strong positive relationship between the affinity of a compound in either tissue (slope of the regression line = 0.89, r2 = 0.908). Agonists consistently showed higher affinity for sites in gastric glands compared to brain. If agonists were excluded from the analysis, the degree of correspondence between affinities measured in each tissue was almost perfect (slope = 0.93, r2 = 0.986). In the presence of the guanyl nucleotide 5'-guanylimidodiphosphate (GppNHp), agonist affinity in gastric glands, but not brain, was reduced such that there was a direct relationship between binding affinity in each tissue. These data are consistent with the notion that the receptor sites in brain and gastric glands, which recognise CCK and gastrin related compounds, are the same and of the CCK-B/gastrin subtype. The receptors in the two respective tissues, however, do appear to differ in the degree of post-receptor coupling. These findings may explain previously reported differences between gastrin and CCK-B receptors that were based upon binding studies using agonist ligands.

Analysis of variation in L-365,260 competition curves in radioligand binding assays

1. For several years, we have used the cholecystokinin (CCK)B/gastrin receptor selective antagonist, L-365,260, as a reference compound in a variety of studies in CCKB/gastrin receptor radioligand binding assays. Here, we have analysed the competition curve data sets obtained between L-365,260 and [125I]-BH-CCK8S in guinea-pig gastric gland and mouse and rat cerebral cortex preparations. 2. Competition curves obtained for L-365,260 in the mouse cortex assay were not different from rectangular hyperbolae (slope = 1.01 +/- 0.02) implying the presence of a single population of binding sites (pKI = 8.41 +/- 0.01; data from 47 experiments, slope constrained to unity). However, in the rat cortex and guinea-pig gastric gland assays, the mean slope of the competition curves was significantly less than one and the mean apparent pKI significantly lower than that obtained in the mouse cortex (slope = 0.85 +/- 0.03, 0.90 +/- 0.03; apparent pKI = 7.98 +/- 0.05, 8.07 +/- 0.05; 48 and 45 experiments, in rat and guinea-pig, respectively). The distribution of the individual pKI and slope estimates of the competition curves in these two assays was consistent with expectations for the variable expression (in terms of absolute number and proportion) of two binding sites. The two sites were characterized by pKI values for L-365,260 of 8.50 +/- 0.04 and 8.48 +/- 0.04 for the high affinity site and 7.32 +/- 0.04 and 7.22 +/- 0.06 for the low affinity site in guinea-pig and rat, respectively. 3. The affinity estimates for L-365,260, although obtained on different tissues, are consistent with data obtained from the analysis of L-365,260 antagonism of pentagastrin-stimulated responses in mouse and rat stomach (acid secretion) and guinea-pig gastric muscle (isotonic contraction) assays. To this extent, these data suggest the existence of two CCKB/gastrin receptor subtypes.

Analysis of the variation in the action of L-365,260 at CCKB/gastrin receptors in rat, guinea-pig and mouse isolated gastric tissue assays

1. Since L-365,260 was first described as a selective antagonist at cholecystokinin (CCK)B/gastrin receptors, we have used it periodically as a reference compound in isolated tissue assays of guinea-pig gastric muscle and lumen-perfused stomachs from mouse and immature rat. L-365,260 behaved as a surmountable antagonist and produced parallel rightward shifts of pentagastrin concentration-effect curves' in each of the replicate experiments. The experiments were performed by several different experimenters in the same laboratories over a five year period. 2. In the isolated, lumen-perfused, immature rat stomach assay, L-365,260 behaved as a simple competitive antagonist (Schild plot slope = 1.00 +/- 0.10, pKB = 7.54 +/- 0.03 from a global analysis of the data) acting at a homogeneous population of receptors in five separate, highly-reproducible, experiments. In contrast, the replicate data sets obtained from the interaction in the isolated, lumen-perfused mouse stomach and guinea-pig gastric muscle assays, over the same period, were not consistent with the presence of a single receptor population. The guinea-pig gastric muscle data were relatively reproducible between experiments but some individual Schild plot slopes and the slope estimated from a global analysis of all the data were significantly less than unity (slope = 0.80 +/- 0.07, pA2 = 8.56 +/- 0.05 from the global analysis). The data obtained in the mouse stomach were significantly more variable than that obtained in the same assay, during the same period, from the interaction between histamine and the H2-receptor antagonist, famotidine. The individual Schild plot slopes ranged from being very flat (0.20) to being not significantly different from unity (1.23) and the pA2 values ranged from 7.68 to 8.70. 3. Overall, the data could be accounted for by assuming the variable expression of two receptor subtypes across the assays. The rat stomach appeared to express a single receptor characterized by a low affinity constant for L-365,260 (pKB approximately 7.5). The guinea-pig gastric muscle and mouse stomach data could be explained by the presence of this receptor and a second one characterized by a high affinity constant for L-365,260 (pKB approximately 8.6). The activity of the two proposed receptor subtypes was consistent between experiments in the guinea-pig and the high affinity receptor appeared to be predominant. In contrast, the mouse stomach data could only be simulated by assuming that the proportion and absolute number of each subtype varied significantly between the replicate experiments. 4. The L-365,260 affinity estimates at the inferred receptor subtypes were indistinguishable from those obtained in a corresponding analysis of the behaviour of L-365,260 in CCKB/gastrin receptor radioligand binding experiments in guinea-pig gastric gland and mouse and rat cerebral cortex preparations.

Pig pancreatic acinar cells possess predominantly the CCK-B receptor subtype

Studies performed on pig indicated that its pancreas was insensitive to the gastrointestinal hormone cholecystokinin (CCK) and suggested that its secretions were rather under the control of the neurotransmitter acetylcholine. This study was performed to determine reasons for this insensitivity by comparing secretory responses to different secretagogues and establishing the dominant CCK receptor type. Pancreatic acini prepared from weaned piglets were evaluated for their sensitivity to carbamylcholine (Cch), caerulein, JMV-180, and secretin. RNA were extracted for CCK-A and CCK-B receptor expression using specific cRNA probes. Results indicate that pig pancreatic acini are sensitive to Cch and relatively insensitive to caerulein with no response to JMV-180, a CCKA agonist, or secretin; MK-329, a CCK-A receptor antagonist, significantly inhibited caerulein-induced enzyme secretion from 10(-8) M. The pig pancreas expresses few CCK-A mRNA receptors but a majority of CCK-B. These data demonstrate that the pig pancreas expresses a majority of CCK-B receptors. In conclusion, the pig pancreas possesses a large majority of CCK-B receptors responsible for their low sensitivity to CCK.

Missense mutation of the cholecystokinin B receptor gene: lack of association with panic disorder

Cholecystokinin tetrapeptide (CCK4) is known to induce panic attacks in patients with panic disorder at a lower dose than in normal controls. Therefore, the cholecystokinin B (CCKB) receptor gene is a candidate gene for panic disorder. We searched for mutations in the CCKB gene in 22 probands of panic disorder pedigrees, using single-strand conformation polymorphism (SSCP) analysis. Two polymorphisms were detected. A polymorphism in an intron (2491 C-->A) between exons 4 and 5 was observed in 10 of 22 probands. A missense mutation in the extracellular loop of exon 2 (1550 G-->A, Val125-->Ile) was found in only one proband. This mutation was also examined in additional 34 unrelated patients with panic disorder and 112 controls. The prevalence rate of this mutation was 8.8% in patients with panic disorder (3/34) and 4.4% in controls (5/112). The mutation did not segregate with panic disorder in two families where this could be tested. These results suggest no pathophysiological significance of this mutation in panic disorder.

3-[2-(N-phenylacetamide)]-1,5-benzodiazepines: orally active, binding selective CCK-A agonists

A series of modifications were made to the C-3 substituent of the 1,5-benzodiazepine CCK-A agonist 1. Replacement of the inner urea NH and addition of a methyl group to generate a C-3 quaternary carbon resulted in acetamide 6, which showed CCK-A receptor binding selectivity and sub-micromolar agonist activity in vitro. Benzodiazepine 6 was active in an in vivo mouse gallbladder emptying assay and represents a novel orally active, binding selective CCK-A agonist.

Effect of CCK receptor antagonists on the antinociceptive, reinforcing and gut motility properties of morphine

1. The ability of a selective CCKA receptor antagonist PD 140548 and a selective CCKB receptor antagonist CI-988 (formerly PD 134308) to modulate the various in vivo properties of morphine was investigated in the rat. 2. PD 140548 dose-dependently (0.001-1.0 mg kg-1, i.p.) antagonised the development of conditioned place preference to morphine (2.0 mg kg-1, s.c.). In contrast, CI-988 (0.01-1.0 mg kg-1, i.p.) did not affect this morphine-induced behaviour. Neither of the CCK receptor antagonists blocked or generalised to the morphine (3.0 mg kg-1, i.p.) discriminative stimulus. 3. CI-988 (0.001-10.0 mg kg-1, s.c.) at doses of 0.05 and 0.1 mg kg-1 (s.c.), potentiated the antinociceptive action of a threshold dose of morphine (5.0 mg kg-1, i.p.) in a radiant heat model of acute nociception, the rat tail flick test. Furthermore, at 0.01 mg kg-1 it potentiated the antinociceptive action of morphine (3.0 mg kg-1) during the acute phase of the rat paw formalin test. And at doses of 0.01 and 0.1 mg kg-1 it also potentiated the antinociceptive action of morphine (1.0 mg kg-1) during the tonic phase of the formalin test. However, in both models, higher doses of CI-988 were ineffective. In contrast, PD 140548 (0.001-10 mg kg-1, s.c.) was only active at a dose of 1.0 mg kg-1 (s.c.) and only in the tonic phase of the formalin test. Neither CI-988 nor PD 140548 possessed any intrinsic antinociceptive action in either of the tests. Chronic treatment with CI-988 (0.01 mg kg-1, s.c.) prevented the development of tolerance to morphine antinociception (4 mg kg-1, s.c.) following a 6 day period of twice daily injections of morphine escalating from 1 to 16 mg kg-1 (i.p.). 4. Morphine dose-dependently (1-10 mg kg-1, s.c.) reduced the distance travelled by a charcoal meal in the rat intestine. Neither PD 140548 (0.01-1.0 mg kg-1, i.p.) nor CI-988 (0.01-1.0 mg kg-1, i.p.) potentiated or suppressed this inhibitory action of morphine. 5. In conclusion, the results of the present study indicate that CCKA and CCKB receptors modulate different properties of morphine. Thus, whilst a selective CCKA receptor antagonist blocked the rewarding properties of morphine, a selective CCKB receptor antagonist potentiated the antinociceptive action. However, neither compound displayed a potential for modulating the influence of morphine on gastro-intestinal motility. It is suggested that these findings may have important implications for development of CCK receptor antagonists as analgesic adjuncts to the therapeutic use of morphine.

Antiproliferative effect of a novel cholecystokinin-B/gastrin receptor antagonist, YM022

Cholecystokinin (CCK)-B and gastrin receptors are expressed on a variety of human tumor cells. Recently, we have demonstrated that the human brain CCK-B receptors are identical to the gastrin receptors derived from the stomach mucosa, and that the brain-gut peptides, CCK-8 and gastrin I are mitogenic for mouse NIH 3T3 fibroblasts expressing human CCK-B/gastrin receptors (N-hCCKBR). In this report, we evaluated the antiproliferative potency of CCK-B/gastrin receptor antagonists by using N-hCCKBR cells. Among several antagonists, a benzodiazepine derivative, YM022 had the most potent activities in competing with [125I]CCK-8 or [125I]gastrin I binding, inhibition of CCK-8- or gastrin I-induced phosphoinositide hydrolysis and increasing cytoplasmic free calcium. Interestingly, a potent antagonist for rat CCK-B/gastrin receptors did not have such activities in N-hCCKBR cells. YM022 inhibited the CCK-8- or gastrin I-induced [methyl-3H]thymidine incorporation of N-hCCKBR cells in a dose-dependent manner. In the absence of exogenous peptide ligands, YM022 also inhibited the proliferation of several human cancer cell lines expressing the genes for both gastrin and its receptor. These results suggest that YM022 could intervene in the autocrine stimulation of human tumor cell lines through CCK-B/gastrin receptors. N-hCCKBR cells are an excellent tool to screen for novel human CCK-B/gastrin receptor antagonists possessing antiproliferative activity for human cancer cells.

Pharmacological profile of T-0632, a novel potent and selective CCKA receptor antagonist, in vitro

The pharmacological profile of a new CCKA receptor antagonist, T-0632 [sodium (S)-3-[1-(2-fluorophenyl)-2,3-dihydro-3-[(3-isoquinolinyl)-carbonyl] amino-6-methoxy-2-oxo-1-H-indole]propanoate], was examined in in vitro studies and compared with those of L-364,718 [3S(-)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1, 4-benzodiazepine-3-yl)-1H-indole-2-carboxamide] and loxiglumide [D,L-4-(3,4-dichlorobenzoylamino)-5-(N-3-methoxypropyl-pentylam ino)-5- oxopentanoic acid]. T-0632 inhibited the specific binding of [125I]CCK-8 to rat pancreatic CCKA receptor in a concentration-dependent and competitive manner. The Ki value of T-0632 for the CCKA receptor was estimated to be 0.24 nM, which was 23 000-fold less than the Ki value (5600 nM) for guinea pig CCKB receptor. L-364,718 and loxiglumide were 1500- and 64-fold selective for CCKA over CCKB receptor, respectively. T-0632, L-364,718 and loxiglumide inhibited CCK-8 (100 pM)-stimulated amylase release from rat pancreatic acini in a concentration-dependent manner with IC50 values of 5.0 nM, 5.0 nM and 3.0 microM, respectively. In the isolated rabbit gallbladder smooth muscle, T-0632 and loxiglumide competitively inhibited CCK-8-induced contraction with pA2 values of 8.5 and 7.0, respectively. However, L-364,718 showed an apparent non-competitive antagonism. The IC50 values of T-0632, L-364,718 and loxiglumide for CCK-8 (30 nM)-induced contraction were 31 nM, 4.9 nM and 1300 nM, respectively. The inhibitory effects of T-0632 and loxiglumide in gallbladder smooth muscle were readily reversible, but L-364,718 showed a long-lasting inhibition. These results suggest that T-0632 is a potent, reversible and more selective CCKA receptor antagonist compared with L-364,718 and loxiglumide.

Role of N- and C-terminal substituents on the CCK-B agonist-antagonist pharmacological profile of Boc-Trp-Phg-Asp-Nal-NH2 derivatives

Among the CCK derivatives, the tetrapeptide Boc-Trp-Phg-Asp-Nal-NH2 (1) behaves as a short potent CCK-B agonist which led to the development of an efficient peptidase-resistant CCK-B antagonist by bismethylation of its terminal CONH2 group. Further modifications of the N- and C-terminal moieties of 1 have been performed and are described in this paper, together with the pharmacological profile of the novel synthetized compounds. Introduction of more bulky substituents than NalNH2 on the C-terminal part decreased the CCK-B receptor binding affinity. In the series of N-protected tetrapeptides X30-Phg31-Asp32-Nal33-N(CH3)2, the Boc-substituent was shown to be optimal among the N-protecting groups Boc, 2Adoc, propionyl or acetyl when X = Trp. On the other hand, when X = alpha MeTrp, its optimal N-protecting group was 2Adoc and its configuration was preferentially D. In the newly synthesized compounds, 13: 2Adoc-D-alpha MeTrp-Phg-Asp-NalN(CH3)2 and 16: 2Adoc-D-alpha MeTrp-Phg-Asp-NalNH2 had the best CCK-B receptor affinities (KI = 3.5 and 3.4 nM, respectively) and were selected for further biological evaluation. Interestingly, when tested for their capacity to influence inositol phosphate formation, induced by CCK8 in CHO cells transfected with the rat CCK-B receptor, compound 13 behaved as a full CCK-B antagonist with an IC50 value of 18 +/- 1 nM, being as potent as the antagonist L-365,260 and PD-134,308 (IC50 values respectively, 39 +/- 17 and 30 +/- 2 nM), whereas compound 16 was found to behave as a partial CCK-B agonist. Indeed 16 behaved as an antagonist on the firing rate of rat CA1 hippocampal neurons and acted as an agonist in the pentagastrin stimulated gastric acid secretion (EC50 = 12 nmol/kg) in anesthetized rats. Compound 13 in contrast, was found to inhibit the pentagastrin action at a dose (ID50 = 0.56 mumol/kg) similar to the potent antagonist PD-134,308 (ID50 = 0.4 mumol/kg). The antagonist/agonist properties of compounds 13 and 16 show that both N- and C-terminal substituents modulate the pharmacological properties in the Boc-CCK4 derivatives presented here.

Cholecystokinin-A and cholecystokinin-B/gastrin receptor mRNA expression in the gastrointestinal tract and pancreas of the rat and man. A polymerase chain reaction study

BACKGROUND

Gastrin and cholecystokinin (CCK) are thought to exert trophic effects on the gastrointestinal tract and pancreas. Two types of receptors have been cloned, CCK-A and CCK-B/ gastrin. We have examined the occurrence of CCK-A and CCK-B receptor mRNA in the brain, digestive tract, pancreas, and kidney of the rat and man by Northern blot and reverse transcribed polymerase chain reaction (RT-PCR).

METHODS

Total RNA was isolated from rat tissues and reverse transcribed into cDNA. cDNA from brain, kidney, and pancreas of the rat and man and from human whole stomach were commercially available. Northern blot and a PCR technique based on Taq polymerase-antibody interaction and using CCK-A and CCK-B receptor-specific primers, followed by Southern blot analysis, were the methods used.

RESULTS

By means of Northern blots, CCK-A receptor mRNA was detected in rat fundus mucosa and pancreas but not in the remaining GI tract or brain. By means of RT-PCR, CCK-A receptor mRNA was demonstrated in the brain and the mucosa of the fundus, antrum, duodenum, and colon, kidney, pancreas and pancreatic islets. CCK-B receptor mRNA was detected by Northern blot analysis in the brain and the fundus mucosa but not in the rest of the digestive tract and not in the pancreas, pancreatic islets, or kidney. By RT-PCR, expression of CCK-B receptor mRNA could also be detected in antrum mucosa. In man, CCK-A receptor mRNA was detected in the brain, stomach, pancreas, and kidney, whereas CCK-B receptor mRNA was found in the brain, stomach, and pancreas but not in the kidney. Cloning and DNA-sequence analysis of the PCR-amplified rat and human CCK-A and CCK-B receptor DNA fragments, which cover the protein-encoding regions of the intracellular loop C3, showed complete sequence homology as compared with published rat and human sequences.

CONCLUSIONS

It appears unlikely that CCK will have effects in the ileum, at least not effects mediated by CCK-A receptors. It also appears unlikely that physiologic concentrations of gastrin in the circulation will promote growth (or exert other effects) in the pancreas, duodenum, ileum, and colon, since CCK-B receptor mRNA is not expressed or is poorly expressed in these tissues.

Molecular cloning, developmental expression and pharmacological characterization of the CCKB/gastrin receptor in the calf pancreas

We have cloned the calf predominant pancreatic cholecystokinin B (CCKB)/gastrin receptor cDNA. It encodes a 454 amino acid protein with 90% identity with the CCKB/gastrin receptor cloned in other species and tissues. However, the calf pancreatic CCKB/gastrin receptor contains a pentapeptide cassette within the third intracellular loop which is absent in the cloned human brain and stomach receptor. Quantification of the CCKB/gastrin receptor mRNA levels by reverse transcription polymerase chain reaction demonstrated the same level of transcripts at birth, +7 and +28 days. On the other hand, binding study with pancreatic membranes showing a dramatic increase (600-fold) in the number of CCKB/gastrin receptor sites between at birth and +28 days indicates that the development of the calf pancreatic CCKB/gastrin receptor occurs during the first 4 weeks of post-natal life. COS monkey cells (COS-7 cells) transiently transfected by the cloned cDNA exhibit binding of 125I-Bolton-Hunter-[Thr28,Ahx31]CCK-(25-33) and 125I-Bolton-Hunter-[Leu15]human gastrin-(2-17) to two affinity classes of sites. Kd values of the high affinity binding components indicate a 4-fold higher affinity of the receptor for sulfated gastrin than for CCK. Finally, the recombinant receptor is coupled to G proteins and [Ca2+]i mobilization, and is expressed as a glycoprotein of 82 kDa.

His381 of the rat CCKB receptor is essential for CCKB versus CCKA receptor antagonist selectivity

A great interest is devoted to antagonists of the cholecystokinin type B (CCKB) receptor such as L-365,260, which reduces panic attacks in humans and to antagonists of the cholecystokinin type A (CCKA) receptor, such as L-364,718 which might be efficient in mental diseases. The A/B specificity of these antagonists was proposed to be mainly dependent on the amino acid sequence of the seventh transmembrane domain (Mantamadiotis and Baldwin (1994) Biochem. Biophys. Res. Commun. 201,1382). In our study, one of these residues, His381 was replaced in the rat CCKB receptor by leucine (the corresponding residue in the CCKA receptor), phenylalanine or arginine using site-directed mutagenesis. Changing histidine for leucine or phenylalanine did not modify significantly the affinity of the CCKB receptor antagonists, L-365,260 and PD-134,308 although both compounds belong to different chemical classes, but strongly improved the affinity of the CCKA receptor antagonists tested. Interestingly, the A selectivity of these CCKA receptor antagonists was recovered by substituting His381 by arginine. Moreover, these results are discussed on the basis of a three dimentional model of the CCKB receptor. The mutated receptors possessed unchanged binding properties for agonists, suggesting that determinants confering specificity for agonists and antagonists are different.

CCKB/gastrin receptor antagonists: recent advances and potential uses in gastric secretory disorders

Cholecystokinin (CCK) and the structurally related peptide, gastrin, have numerous effects on tissues in the central nervous system and gastrointestinal tract. Recent studies show these effect are mediated by a CCKA and CCKB receptor. Knowledge of the physiological role and role of CCKB receptors in pathologic processes has been particularly limited by the availability of selective, potent receptor antagonists. Recently, new members of five different classes of non-peptide CCKB receptor antagonists are reported and are reviewed briefly. these include compounds isolated from Streptomyces (tetronothiodin, virginiamycin analogues), ureido-acetamide analogues (RP 69758, RP 72540, RP 73870), newer benzodiazepine analogues (L-368,935, L-740,093, YM022), pyrazolidimine analogues (LY 262,691) and glutamic acid analogues (CR2194). Many of these compounds have greater than 1000-fold selectivity for the CCKB over the CCKA receptor and some have greater than 10,000-fold selectivity. The pharmacology and effects of CCKB receptor antagonists on gastric acid secretion is briefly reviewed. Furthermore, the possible clinical usefulness of CCKB receptor antagonists in treating disorders of gastric acid secretion, in inhibiting the trophic effects of gastrin and in other clinical conditions is briefly discussed.

Are C-terminal octapeptide of cholecystokinin and [Leu11]gastrin-(5-17) different in stimulating acid secretion in isolated rabbit gastric glands?

In the present study we compared various CCK(B) receptor antagonists and tried to detect a difference in biological activity between the C-terminal octapeptides of cholecystokinin (CCK-8) and [Leu11]gastrin-(5-17) in isolated rabbit gastric glands. Binding experiments showed that different CCK(B)/gastrin receptor agonists bound with high affinity and that antagonists inhibited this binding in accordance with a CCK(B)/gastrin pharmacological profile. [Leu11]gastrin-(5-17), CCK-8 and cionin were found to induce [14C]aminopyrine accumulation to 25% above the basal level. Under the same experimental conditions, histamine induced a response twice as great as the response obtained with [Leu11]gastrin-(5-17) or CCK-8. [Leu11]gastrin-(5-17) (10(-7) M), CCK-8 (10(-8) M) and cionin (10(-8) M) appeared to be full agonists. CCK(B)/gastrin receptor antagonists including L-365,260 (3R-(+)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepin++ +-3-yl)-N-(3-methylphenyl) urea), L-364,718 (3S-(-)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepin++ +-3-yl)-1H-indole-2-carboximide) (a selective CCK(A) receptor antagonist), PD-135,158 (4([2-[[3-(1H-indol-3-yl)-2-methyl-1-oxo-2-[[[1.7.7-trimethyl-bicyclo[2. 2.1]hept-2-yl)oxy]carbonyl]amino]propyl]amino]-1-phenylethyl] amino-4-oxo-[1S-1alpha.2beta[S*(S*)]4alpha]]-butano nate N-methyl-D-glucamine) (bicyclo system 1S-endo), YM-022 ((R)-1-[2,3-dihydro-1-(2'-methylphenacyl)-2-oxo-5-phenyl-1H-1,4-++ +benzodiazepin-3-yl]-3-(3-methylphenyl)urea) and JMV-180 (Boc-Tyr(SO3H)-Nle-Gly-Trp-Nle-Asp-O-CH2-CH2-C6H5) exhibited the same profile for inhibition of [Leu11]gastrin-(5-17) or CCK-8-induced [14C]aminopyrine accumulation in rabbit gastric glands. These results suggested that [Leu11]gastrin-(5-17) and CCK-8 induced [14C]aminopyrine accumulation by the same mechanism. [Leu11]gastrin-(5-17)- or CCK-8-induced [14C]aminopyrine accumulation was inhibited by about 40% by the histamine H2 receptor blocker cimetidine. These results are consistent with there being cooperativity between [Leu11]gastrin-(5-17) (or CCK-8) and histamine in the acid secretory pathway. Similarly, the CCK(B)/gastrin receptor antagonists were tested against histamine-induced [14C]aminopyrine accumulation and surprisingly, only compound L-365,260 appeared active and even more potent than cimetidine.

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.

Expression of gastrin, gastrin/CCK-B and gastrin/CCK-C receptors in human colorectal carcinomas

To investigate further the presence of an autocrine proliferative loop involving gastrin in colorectal carcinomas and to clarify the receptor responsible, 102 human colorectal carcinomas and 10 hepatic metastases were investigated for the expression of the genes encoding gastrin, the gastrin/CCK-B receptor and the gastrin/CCK-C receptor. Levels of RNA expression were assayed by RNase protection assay. In addition, gastrin/CCK receptors on crude membranes of tumour tissue were assayed by radioligand binding. High-affinity gastrin/CCK-B receptors were not detected in any of the carcinomas investigated, whereas in 36% low-affinity binding was observed, consistent with the expression of the gastrin/CCK-C receptor. RNase protection assay detected the RNA for the gastrin/CCK-B receptor in 11% of the carcinomas investigated, whereas the RNA for the gastrin/CCK-C receptor was demonstrated in 75% and the RNA for gastrin in 86% of the carcinomas investigated. These results confirm the recent demonstration of progastrin fragments in colorectal carcinomas. One possible explanation for progastrin expression is that such progastrin fragments may participate in an autocrine proliferative loop. The receptor involved in this loop is more likely to be the low-affinity gastrin/CCK-C receptor rather than the gastrin/CCK-B receptor, which is rarely expressed in colorectal carcinomas.

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.

Effects of guanyl nucleotides on CCKB receptor binding in brain tissue and continuous cell lines: a comparative study

The effects of non-hydrolyzable guanyl nucleotide analogue GTP-gamma S on CCKB receptor binding in human and guinea-pig cortex, Jurkat T-cells, rat pituitary GH3 cells, rat glioma C6 cells and human small cell lung cancer NCI-H69 cells were investigated by using [3H]CCK-8S saturation and competition binding studies. GTP-gamma S caused inhibition of specific [3H]CCK-8S binding in a concentration dependent manner with a plateau at 10-25 microM. 25 microM GTP-gamma S resulted in a small but significant increase in Kd and IC50 values with amount very similar in all CCKB receptor models tested. However, the maximal number of specific [3H]CCK-8S binding sites (Bmax) was unaffected. Results suggest that CCKB receptors are G-protein coupled in a similar way to human and guinea-pig cortex, Jurkat cells, GH3 cells, C6 cells and NCI-H69 cells.

Localization of the murine cholecystokinin A and B receptor genes

We have determined the chromosomal locations of the two cholecystokinin (CCK) receptor genes in the mouse. Genetic localization utilized an interspecific backcross panel formed from the cross (C57BL/6J x Mus spretus) F1 x Mus spretus. Genomic DNAs from 94 individuals in the backcross were analyzed by Southern hybridization with rat CCKA and CCKB receptor cDNA probes. Unique map positions were determined by haplotype analysis with 650 previously mapped loci in the mouse backcross. The CCKA receptor gene (Cckar) mapped to mouse Chromosome (Chr) 5, in tight linkage with the DNA marker D5Bir8. The CCKB receptor gene (Cckbr) mapped to mouse Chr 7, tightly linked to the beta-hemoglobin locus (Hbb). This localization places Cckbr in the same region as the mouse obesity mutation tubby (tub), which also maps near Hbb (2.4 +/- 1.4 cM). Since CCK can function as a satiety factor when administered to rodents, localization of Cckbr near the tub mutation identifies this receptor as a possible candidate gene for this obesity mutation.

Characterization of a serine peptidase responsible for the inactivation of endogenous cholecystokinin in human brain

Depolarization of slices of human cerebral cortex releases cholecystokinin-8 immunoreactivity, only a fraction of which is recovered in intact immunoreactive form in the medium. This suggests that extensive hydrolysis takes place during short incubations. In the presence of diisopropylfluorophosphate, a serine reagent, the recovery is nearly doubled, however, consistent with the involvement of a serine peptidase activity. The latter was characterized by assessing the protective effects of a series of serine protease inhibitors belonging to the families of peptide chloromethylketones or boronic acids. The relative potency of these inhibitors was similar to corresponding values previously found with rat brain slices indicating that a similar serine peptidase activity is responsible for endogenous cholecystokinin inactivation in the two species.

The role of gastrin and cholecystokinin in normal and neoplastic gastrointestinal growth

Gastrin and cholecystokinin (CCK) act as growth factors for the gastric mucosa and the pancreas, respectively. CCK is also responsible, via the CCK-A receptor, for the pancreatic hyperplasia observed following the feeding of protease inhibitors or pancreaticobiliary diversion. Hypergastrinaemia does not increase the incidence of spontaneous gastrointestinal carcinoma, but does stimulate the proliferation of gastric enterochromaffin-like cells via the gastrin/CCK-B receptor, with a consequent increase in the incidence of gastric carcinoids. Whether gastrin influences mutagen-induced gastrointestinal carcinogenesis is still controversial, but CCK clearly enhances the induction by carcinogens of acinar tumours in the pancreas. While gastrin increases xenograft growth of 50% of gastrointestinal tumours tested, effects on the proliferation of gastrointestinal tumour cell lines in vitro have been more difficult to demonstrate, perhaps because many cell lines are already maximally stimulated by autocrine gastrin. Gastrin mRNA and progastrin, but not mature amidated gastrin, have been detected in all gastrointestinal cell lines tested. Although cell proliferation is inhibited by gastrin/CCK receptor antagonists, the spectrum of antagonist affinities is not consistent with binding to either CCK-A or gastrin/CCK-B receptors. Definition of the molecular structure of the receptor involved in the autocrine loop may lead to novel therapies for gastrointestinal cancer.

Chromosomal localization of the gastric and brain receptors for cholecystokinin (CCKAR and CCKBR) in human and mouse

Receptors for cholcystokinin (CCK) can be pharmacologically classified into at least two distinct subtypes, CCKAR and CCKBR. In an effort to determine whether the CCKA and CCKB receptors may be associated with certain CNS or gastrointestinal diseases, we have localized and compared the human and mouse chromosomal loci encoded by the CCKAR and CCKBR genes. The gene encoding the CCKA receptor maps to a syntenic region of human chromosome 4 and mouse chromosome 5. The CCKB receptor gene, on the other hand, resides on a syntenic region of human chromosome 11 and distal mouse chromosome 7. Localization of the CCK receptors with two dopamine receptors, DRD5 (4p15.1-p15.3) and DRD4 (11p15), provides the interesting possibility of coinvolvement in neuropsychiatric or CNS illnesses.

[N-methylnorleucine-(28,31)]cholecystokinin-(26-33) (SNF 8702) activity at a cloned rat CCKB receptor

[N-methyl-Nle 28,31)]cholecystokinin-(26-33) (SNF 8702) is a highly selective ligand for the CCKB type of receptor present in the vertebrate central nervous system. Radioligand binding data showing that SNF 8702 binding affinity is reduced by the GTP analog guanylyl-imidodiphosphate suggest that SNF 8702 is an agonist but the ability of SNF 8702 to activate CCKB receptors has not been demonstrated. The present study shows that SNF 8702 is a highly potent agonist at rat CCKB receptors expressed on COS-7 cells and that these receptors are coupled to the mobilization of intracellular calcium. The A50 measured for SNF 8702-induced calcium mobilization (66 pM) is over 6-fold less than that of cholecytstokinin octapeptide (420 pM). Data are also presented showing that SNF 8702 has high binding affinity for these receptors with a Kd value (760 pM) consistent with previous measurements using guinea pig brain tissue preparations.

Localization of cholecystokinin A and cholecystokinin B/gastrin receptors in the canine upper gastrointestinal tract

BACKGROUND/AIMS

Cholecystokinin (CCK) is a neuropeptide that exerts numerous effects in the gut. To determine the sites of action of CCK, the distribution and properties of CCK receptor subtypes were studied.

METHODS

CCK receptors were localized by autoradiographic analysis of 125I-CCK binding to frozen sections of the canine upper gastrointestinal tract.

RESULTS

In the cardiac and fundic stomach, CCK-B/gastrin receptors were found in the mucosa and in a subpopulation of neuronal elements in the circular muscle. The antrum expressed CCK-B/gastrin receptors in a few neurons in the circular muscle and in the entire myenteric plexus; no receptors were observed in the antral mucosa or esophagus. The duodenum showed a high concentration of CCK-B/gastrin receptors in the myenteric plexus. The cardiac and fundic basal mucosae expressed CCK-A receptors. Two nonpeptide CCK receptor antagonists were unable to differentiate between the receptor subtypes.

CONCLUSIONS

The differential expression of CCK receptor subtypes in the gastric mucosa provides a morphological basis for the separate regulatory roles of CCK and gastrin in gastric function. CCK-B/gastrin receptor expression in a subset of neurons in gastric circular muscle suggests a novel site of action for CCK and/or gastrin.

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.

Heterogeneity of CCK-B receptors involved in animal models of anxiety

The effects of the selective CCK-B agonists, BC 264 and BC 197, and the nonselective CCK agonist BDNL were investigated in the elevated plus-maze in rats. BDNL and BC 197 induced anxiogeniclike effects, in contrast to BC 264, which had no effect. The behavioral responses induced by BDNL were not significantly blocked by L-365,260, but were suppressed by CI-988, another selective CCK-B antagonist, and by high doses of L-364,718, a selective CCK-A antagonist. BC 197-induced effects were also blocked by CI-988. Competition experiments performed with [3H]pBC 264 using brain membranes of guinea pig, mouse, and rat were significantly better fitted when analyzed by a two site model than by a one site model with BC 197 but not with BC 264. Moreover, BC 264 produced anxiogeniclike effects when administered with increasing doses of L-365,260 and opposing effects with increasing doses of CI-988. Together these results give pharmacological and behavioral evidence for the existence of CCK-B receptor subtypes.

Selective non-peptide ligands for an accommodating peptide receptor. Imidazobenzodiazepines as potent cholecystokinin type B receptor antagonists

A series of imidazobenzodiazepines, non-peptide antagonists of the peptide hormone cholecystokinin (CCK), are described. Derived by chemical modification of the benzodiazepine ring system embedded within the CCK-B antagonist L-365,260, these compounds display CCK-B/CCK-A selectivity and some analogs have receptor binding affinities in the subnanomolar range. This group of novel imidazobenzodiazepines, among which N-[(2S,4R)-methyl-6-phenyl-2,4-dihydro-1H-imidazo[1,2- alpha][1,4]benzodiazepin-4-yl]-N'-[3-methylphenyl]-urea (12) is the principal compound, expands the structural diversity of the collection of non-peptide CCK-B antagonists and will be useful in further delineating the function of CCK in the central nervous system.

Antiproliferative gastrin/cholecystokinin receptor antagonists target the 78-kDa gastrin-binding protein

Inhibition of colon carcinoma cell growth by the nonselective gastrin/cholecystokinin (CCK) receptor antagonists proglumide and benzotript provided evidence that gastrin functions as an autocrine growth factor. However, the molecular properties of the receptor mediating the antagonist effects have not been identified. A 78-kDa gastrin-binding protein (GBP), the sequence of which is related to the family of enzymes possessing enoyl-CoA hydratase and 3-hydroxyacyl-CoA dehydrogenase activities, has been previously purified from porcine gastric mucosal membranes. I now report that covalent cross-linking of 125I-labeled [Nle15]gastrin2,17 to the 78-kDa GBP is inhibited by crotonyl-CoA and by acetoacetyl-CoA. Gastrin, CCK, and their analogues also inhibit cross-linking, and the spectrum of analogue affinities correlates better with the values previously reported for binding to the gastrin/CCK-C receptor than with the values reported for binding to either the CCK-A or the gastrin/CCK-B receptor. Cross-linking is also inhibited by proglumide and benzotript, but no inhibition is seen with either the CCK-A receptor-selective antagonist L364,718 or the gastrin/CCK-B receptor-selective antagonist L365,260. The affinities of antagonists for the GBP correlate well with their affinities for the gastrin/CCK-C receptor and with their potencies for inhibition of colon carcinoma cell growth. I conclude that the 78-kDa gastrin-binding protein is (i) a member of the hydratase/dehydrogenase family of fatty acid oxidation enzymes, (ii) the gastrin/CCK-C receptor, and (iii) the target for the antiproliferative action of two gastrin/CCK receptor antagonists.

Pentagastrin infusions in patients with panic disorder. I. Symptoms and cardiovascular responses

Cholecystokinin (CCK) may mediate human anxiety and animal data suggest that cholecystokinin antagonists could provide an important advance in the treatment of anxiety disorders. The study of CCK receptor systems in psychiatric patients has, however, been severely limited by the lack of available probes. We utilized intravenous infusions of pentagastrin, a selective CCK-B receptor agonist, and studied behavioral and cardiovascular responses in 10 patients with panic disorder and 10 normal controls. Pentagastrin produced substantial symptomatology, including anxiety, and increases in heart rate and blood pressure, in both patients and controls. Patients were more sensitive to the panicogenic effects of the pentagastrin. Panic attacks occurred in 70% of patients and 0% of controls. Patients' symptom responses were very similar to their "typical" panic attacks and to symptoms produced by CCK4. Pentagastrin provides a readily available alternative to CCK4 for studying the CCK receptor system and exploring its involvement in human anxiety.

Molecular and pharmacological characterization of the human CCKB receptor

The human cholecystokinin B (CCKB) receptor has been isolated from a human temporal cortex cDNA library. Transient transfection of the receptor into COS-M6 cells resulted in high specific binding of 125I-sulphated CCK-8 labelled with Bolton and Hunter Reagent (KD = 31 pM). Competition experiments yielded the expected CCKB receptor ligand binding profile for agonists and antagonists. Similar results were obtained in human small cell lung carcinoma cells, which express an endogenous CCKB receptor. Extensive functional characterization of the receptor was performed in stably transfected HeLa cells using intracellular calcium imaging and microphysiometry techniques. Molecular analysis of the human CCKB receptor using Southern blotting of genomic DNA suggests the presence of a single gene for the CCKB receptor with no closely related homologues. This was confirmed by the polymerase chain reaction cloning of identical receptor coding sequences from human small cell lung carcinoma cells and human gastric enterochromaffin-like cell-oma (ECLoma) tissue.