[3H]pBC 264, first highly potent and very selective radioligand for CCK-B receptors

Stimulation of δ-Opioid Receptors Reduces the In Vivo Binding of the Cholecystokinin (CCK)-B-Selective Agonist [3H]pBC 264: Evidence for a Physiological Regulation of CCKergic Systems by Endogenous Enkephalins

Cholecystokinin (CCK) and enkephalins appear to be colocalized in several brain structures, and a physiological interaction between these peptides has been suggested by a large number of pharmacological studies. In this work we have shown, by in vivo binding experiments, that the endogenous enkephalins, protected from degrading enzymes by mixed inhibitors such as kelatorphan and N-[(R,S)-2-benzyl-3-[(S)-2-amino-4-methylthiobutyldithio]-1-oxo pro pyl]- L-phenylalanine benzyl ester (RB 101), a systemically active prodrug, modulate CCK release in mouse brain, leading to an overall increase in the extracellular levels of CCK. This was quantified by measuring the effects of both inhibitors on the in vivo binding of [3H]propionyl-Tyr(SO3H)-gNle-mGly-Trp-(N-Me)Nle-Asp-Phe-NH2 ([3H]pBC 264), a selective and highly potent CCK-B agonist. Thus, intracerebroventricular injection of kelatorphan produced a dose-dependent inhibition of the in vivo binding of [3H]pBC 264 with a maximal effect (40%) at 50 nmol. A similar response was observed after intravenous injection of RB 101 (40 mg/kg). The specific binding of [3H]pBC 264 was also inhibited (25%) by intravenous injection of the selective delta-opioid agonist H-Tyr-D-Cys(StBu)-Gly-Phe-Leu-Thr(OtBu)-OH (BUBUC; 2 mg/kg) but not by the mu-agonist H-Tyr-D-Ala-Gly-(N-Me)Phe-Gly-ol (5 mg/kg), suggesting a preferential involvement of delta-opioid receptors in the modulation of CCK release. This was confirmed by using the selective delta-opioid antagonist naltrindole, which prevented the inhibitory effects of BUBUC and of enkephalin-degrading enzyme inhibitors on [3H]pBC 264 binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Autoradiographic Localization of Cholecystokinin A and B Receptors in Rat Brain Using [125I]d-Tyr25 (Nle28,31)-CCK 25 - 33S

The distribution of receptors for the sulphated octapeptide cholecystokinin 26 - 33 (CCK - 8S) in rat brain was investigated by radioligand binding in conjunction with autoradiography using the novel iodinable, non-oxidizable, amino- and thiolendopeptidase-resistant CCK analogue, d-Tyr25(Nle28,31)-CCK 25 - 33S. Labelling of the peptide was achieved by synthesis utilizing Na125I and Chloramine-T. [125I]d-Tyr25(Nle28,31)-CCK 25 - 33S (100 pM) bound rapidly and reversibly to a single population of sites on slide-mounted coronal sections of rat forebrain with a dissociation constant of 34 pM. Specific binding was fully inhibited by CCK-8S, CCK-8, CCK-4, L-365,260 and L-364,718, with inhibition constants 2.7, 9.8, 35, 7.0 and 130 nM, respectively. These inhibition data may indicate that the [125I] ligand binds preferentially to a CCKB subtype of receptor, but may also reflect the relative paucity of CCKA receptors in the rat forebrain. Optimum conditions for autoradiography combined the preincubation of brain sections in unlabelled 10 pM d-Tyr25(Nle28,31)-CCK 25 - 33S with a 60-min wash after incubation with the [125I] ligand. Analyses of the autoradiograms obtained from the use of coronal and horizontal brain sections were aided by the high levels of specific binding (80 - 90%), and revealed that CCK receptors were topographically distributed through the neuroaxis. High densities of receptor-associated silver grains were found in the olfactory bulb (internal plexiform layer), neocortex (layer III), nucleus accumbens, parasubiculum, subbrachial nucleus, parabigeminal nucleus, dorsal vagal complex, area postrema and the A2 region. Moderate labelling was observed in many telencephalic and diencephalic nuclei. The majority of these receptors were of the CCKB subtype, as shown by the use of subtype-selective antagonists, although CCKA receptors were present in moderate to high densities in the A2 area, area postrema and nucleus tractus solitarii, and at low density in the interpeduncular nucleus and central amygdala. These findings provide further evidence for the widespread, topographic distribution of CCK receptors and indicate that [125I]d-Tyr25(Nle28,31)-CCK 25 - 33S is very suitable for autoradiographic investigations because of its low non-specific binding.

CCK-B receptor: chemistry, molecular biology, biochemistry and pharmacology

Cholecystokinin (CCK) is a peptide originally discovered in the gastrointestinal tract but also found in high density in the mammalian brain. The C-terminal sulphated octapeptide fragment of cholecystokinin (CCK8) constitutes one of the major neuropeptides in the brain; CCK8 has been shown to be involved in numerous physiological functions such as feeding behavior, central respiratory control and cardiovascular tonus, vigilance states, memory processes, nociception, emotional and motivational responses. CCK8 interacts with nanomolar affinities with two different receptors designated CCK-A and CCK-B. The functional role of CCK and its binding sites in the brain and periphery has been investigated thanks to the development of potent and selective CCK receptor antagonists and agonists. In this review, the strategies followed to design these probes, and their use to study the anatomy of CCK pathways, the neurochemical and pharmacological properties of this peptide and the clinical perspectives offered by manipulation of the CCK system will be reported. The physiological and pathological implication of CCK-B receptor will be confirmed in CCK-B receptor deficient mice obtained by gene targeting (Nagata el al., 1996. Proc. Natl. Acad. Sci. USA 93, 11825-11830). Moreover, CCK receptor gene structure, deletion and mutagenesis experiments, and signal transduction mechanisms will be discussed.

The CCK-B agonist, BC264, increases dopamine in the nucleus accumbens and facilitates motivation and attention after intraperitoneal injection in rats

Although it is known that panic attacks are triggered by the cholecystokinin fragment CCK4, the specific involvement of peripheral or central cholecystokinin CCK receptors in various adaptive processes such as emotion, memory and anxiety has yet to be demonstrated. With this aim, we have investigated the biochemical and pharmacological effects resulting from the administration of BC264, a highly potent and selective CCK-B agonist able to cross the blood-brain barrier. Very low doses of BC264 (microg/kg i.p.), increased the exploration of animals submitted to an unknown territory but were devoid of anxiogenic properties in the elevated plus maze. BC264 increased locomotion and rearings of rats newly placed in an open field and improved their spontaneous alternation in a Y-maze. The use of vagotomized animals showed that the increased alternation induced by BC264 did not require an intact vagus nerve, unlike the locomotor activation. These behavioural effects, prevented by the prior i.p. administration of the CCK-B antagonist L-365,260 but not by the CCK-A antagonist L-364,718, were shown to depend on dopaminergic systems, since they were blocked by D1 (SCH23390, 25 microg/kg i.p.) or D2 (sulpiride, 50 or 100 mg/kg i.p.) antagonists. In addition, bilateral perfusion in freely moving rats of BC264 at pharmacologically active doses, using a newly designed microdialysis system, was found to increase the extracellular levels of DA, DOPAC and HVA in the anterior part of the nucleus accumbens. These results show that activation of CCK-B receptors by BC264 does not produce anxiogenic-like effects but appears to improve motivation and attention, whereas other CCK-B agonists such as BocCCK4 induce anxiogenic responses. Several explanations, including the existence of different sub-sites of the CCK-B receptor, could account for these differential effects.

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.

CCK-A and CCK-B selective receptor agonists and antagonists modulate olfactory recognition in male rats

Modulation of learning and memory is one of the physiological roles that the neuropeptide cholecystokinin (CCK-8) may play. We have used a behavioural model of olfactory recognition among rats to test this hypothesis and to explore the relationship between CCK-A and CCK-B receptors and memory retention. Adult male rats form a transient memory of a juvenile congenere as indicated by a reduction in the duration of investigatory behaviour upon re-exposure 30 min after an initial exposure, but not when re-exposure is delayed until 120 min afterwards. In the present study, rats were treated after the first contact with various compounds; inhibition and facilitation of olfactory recognition were evaluated as the persistence in investigation 30 min and the decrease in investigation 120 min after pharmacological manipulations, respectively. Systemic injection of CCK-8, of a selective CCK-A agonist, or of non-peptide CCK-B antagonists (CI-988 and LY-262691) enhanced olfactory recognition. In contrast, the CCK-B selective agonist BC 264 and the tetrapeptide CCK-4 both disrupted it. Taken together with previous evidence of the detrimental effect of the nonpeptide. CCK-A antagonist devazepide on olfactory recognition, these results confirm and extend the hypothesis that there is a balance between CCK-A-mediated facilitative effects and CCK-B-mediated inhibitory effects on memory retention.

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.

In vivo binding affinities of cholecystokinin agonists and antagonists determined using the selective CCKB agonist [3H]pBC 264

The respective role of central vs. peripheral CCK-B receptors in the recently reported anxiolytic effects of CCK-B antagonists remains to be firmly established. We therefore investigated the in vivo binding properties of cerebral CCK receptors after i.c.v. injection into mice of [3H]pBC 264 ([3H]propionyl-Tyr(SO3H)-gNle-mGly- Trp-(NMe)Nle-Asp-Phe-NH2), a highly potent, peptidase-resistant and selective CCK-B agonist. The specific binding of [3H]pBC 264 was reversible and saturable. The dose producing 50% receptor occupancy was 25 pmol and the Bmax was 0.9 pmol/brain 15 min after injection. I.c.v. administered CCK8 (ID50 8500 pmol) was 200-fold less potent than pBC 264 (ID50 43 pmol) in inhibiting specific [3H]pBC 264 binding; CCK8NS, CCK5 and CCK4 being slightly less potent than CCK8. Aminopeptidases play a major role in degrading CCK8 since the protected analog pCCK8 or CCK8 in the presence of an aminopeptidase inhibitor exhibited higher affinities than CCK8. I.v. administration of pBC 264 (20 mg/kg) inhibited [3H]pBC 264 specific binding by about 72%, confirming its ability to enter the brain. In contrast, CCK4 was unable to modify [3H]pBC 264 binding. As expected, the CCK-A antagonist (L364,718) did not inhibit [3H]pBC 264 binding, while at the highest dose used (40 mg/kg i.p.) the CCK-B antagonist (L365,260) inhibited binding by 20%. Several hypotheses are discussed to account for the very low i.v. doses of CCK4 and L365,260 needed to produce anxiogenic and anxiolytic responses, respectively.

Effect of a highly selective central CCK-B receptor agonist: BC-264 on rat sleep

The possible involvement of the CCK-B receptor type in the atypical somnolence and EEG changes induced by low doses of CCK-B was investigated by intraperitoneal administration of three different doses (8, 16 and 32 micrograms/kg) of the new highly potent and selective CCK-B agonist, BC-264, on sleep parameters in the fasted rat. At the dose of 8 micrograms/kg BC-264 induced a significant increase in waking in the second 120 min of recording without effect on slow wave sleep (SWS). BC-264 did not modify the others sleep parameters. Taken all together these results suggest that CCK-B type receptors are probably not critically involved in satiety and sleep.