Development of new potent agonists able to interact with two postulated subsites of the cholecystokinin CCK-B receptor

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.

Peripheral stimulation of CCK-B receptors by BC264 induces a hyperexploration, dependent on the delta opioid system in the nucleus accumbens of rat

This study analyses the influence of the CCKergic system on the enkephalinergic system in the exploratory behavior of rats, using both behavioral and biochemical approaches. The results show that the increase of the spontaneous alternation behavior induced by the selective CCKB agonist, BC264 (3 microg/kg) was not suppressed by the opioid antagonists, naloxone (100 microg/kg), or naltrindole (300 microg/kg). In contrast, BC264 injected at the same dose induced a hyperlocomotor activity measured in the open-field test, which was antagonized by the selective delta opioid antagonist, naltrindole. BC264 (3 microg/kg) significantly increased the extracellular levels of Met-LI in the anterior part of the nucleus accumbens. Furthermore, local injection of naltrindole (0.25 microg/0.5 microl) in the anterior nucleus accumbens completely suppressed the hyperlocomotion induced by BC264. The behavioral effects induced by BC264 cannot be explained by its interaction with gastrinic receptors mediating gastric acid release, since BC264 produced a long-lasting increase of gastric acid output from conscious gastric fistula rats only at doses 100 times higher than those inducing behavioral modifications. The hyperlocomotion obtained after stimulation by BC264 of probably peripheral CCKB receptors, indicates that this receptor type could participate in the transmission of information between the peripheral system and some regions of the CNS involved in motivations and emotions.

The cholecystokininB receptor is coupled to two effector pathways through pertussis toxin-sensitive and -insensitive G proteins

Previous binding studies have suggested the existence of two affinity states for type B cholecystokinin receptors (CCK(B)R), which could correspond to different coupling states of the receptor to G proteins. To test this hypothesis, we have further investigated signal transduction pathways coupled to rat CCK(B)R stably transfected in Chinese hamster ovary cells. We show that CCK(B)R are coupled to two distinct transduction pathways involving two different G proteins, a pertussis toxin-insensitive/phospholipase C pathway leading to the production of inositol phosphate and arachidonic acid, and a pertussis toxin-sensitive/phospholipase A2 pathway leading to the release of arachidonic acid. We further demonstrate that the relative degree of activation of each effector pathway by different specific CCK(B)R agonists is the same, and that a specific CCK(B)R antagonist, RB213, can differentially antagonize the two signal transduction pathways elicited by these agonists. Taken all together, these data could be explained by the recently proposed theory assuming that the receptor can exist in a three-state model in which two active conformations corresponding to the complex formed by the receptor with two different G proteins coexist. According to this model, agonists or antagonists could recognize preferentially either conformation of the activated receptor, leading to variable behavior in a system containing a single receptor type.