Investigation of opioid and cholecystokinin central receptors after peripheral injection of selective and enzyme-resistant peptides

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.

Development of a capillary zone electrophoresis assay to examine the disposition of [D-pen2,5]enkephalin in rats

A novel capillary zone electrophoresis (CZE) assay method was developed to evaluate the systemic disposition of [D-pen2,5]enkephalin (DPDPE) in rats. DPDPE was recovered from serum samples (200 microliters) by solid-phase extraction. Complete resolution of DPDPE and the internal standard ([D-ser2]leucine-enkephalin; DSLET) from other serum components was achieved within 15 min on a 50-microns I.D. capillary column with borate buffer (25 mM, pH 8.3). The peak-height ratio (DPDPE to DSLET) was linear through 100 micrograms/ml, with a detection limit of 250 ng/ml in serum, when absorbance of the column eluent was monitored at 210 nm. Serum samples obtained from rats after a 10 mg/kg intravenous bolus dose of DPDPE were analyzed with the present CZE method. The results suggest that CZE is a useful technique for quantitating therapeutic peptides in biological matrices.

Evidence for a high-affinity uptake system for cholecystokinin octapeptide (CCK8) in rat cortical synaptosomes

Given the high resistance of the cholecystokinin octapeptide (CCK8) to in vivo peptidase degradation, the possible existence of a reuptake system for this peptide was investigated. Efficient accumulation of intact, tritiated propionyl CCK8 ([3H]pCCK8) was observed following its incubation with rat cortical synaptosomes but not with cerebellar synaptosomes, where no cholecystokinin immunoreactivity was found. This uptake process appeared to be dependent on temperature, duration of incubation, concentration of radioligand, the presence of glucose and the integrity of the synaptosomes. A Lineweaver-Burk analysis indicated that the putative uptake process is characterized by a single Km value of 10.7 nM and a Vmax of 8.5 fmol/min/mg of protein. Carbonyl cyanide-m-chlorophenyl hydrazone, an uncoupler of oxidative phosphorylation, blocked accumulation of [3H]pCCK8, whereas ouabain did not. The uptake was found to be highly specific since, among all the cholecystokinin analogues tested, only CCK8 and, to a lesser extent, CCK7, were able to inhibit [3H]pCCK8 uptake. The rate of [3H]pCCK8 uptake was not affected by CCK4, CCK5, D-Trp CCK8, BC 264, a potent and radioactivity was observed using [3H]pBC 264, a result which is not in favour of a cholecystokinin receptor-induced internalization mechanism. The potent and selective uptake mechanism characterized in this study could participate, in conjunction with extra and intracellular degradation of CCK8 by peptidases, in the interruption of cholecystokinin-conveyed messages in the brain.

Investigation of the structural parameters involved in the delta-opioid selectivity of several families of opioid peptides

Three series of highly delta-opioid selective peptides are now available, and each family is used as template to investigate the structural parameters involved in delta-receptor recognition and in the modulation of the selectivity of the parent peptide. The first series includes cyclic derivatives such as Tyr-D-Pen-Gly-Phe-D-Pen(DPDPE) and Tyr-D-Pen-Gly-Phe-Pen(DPLPE); the second are the synthetic linear constrained peptides [Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr(DSTBULET), Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr (OtBu)(BUBU) and especially Tyr-D-Cys(StBu)-Gly-Phe-Leu-Thr(OtBu) (BUBUC)] and the last one the natural peptides [Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2 (deltorphin or dermenkephalin) and Tyr-D-Ala-Phe-Asp-Val-Val-GlyNH2 ([D-Ala2] deltorphin I)]. In the present study, the possibly of transposing some of the decisive factors of delta-selectivity evidenced in the two other families, to the linear constrained peptides series was examined. With this aim in view, residues such as Phe3, pClPhe4 or Asp were introduced in the sequence of DSTBULET, BUBU or BUBUC. Direct comparison between the biochemical profiles of the [pClPhe4] analogs of the linear constrained peptides and their parent compounds shows that the addition of an electronegative atom on the Phe4 residue of enkephalin sequences is not an absolute parameter for delta-selectivity improvement. The hydrophobic delta-receptor subsite seems able to receive a range of molecular volumes and electronegativities. By contrast, this subsite cannot interact with a Phe3 aromatic ring introduced in this series of peptides. Moreover, the results obtained with linear peptides including additional negatively charged residues demonstrate that the proposed location of the delta-receptors in a cationic membrane environment is not adequate to explain the selectivity profile of a number of compounds.

Peptidomimetics as receptors agonists or peptidase inhibitors: a structural approach in the field of enkephalins, ANP and CCK

Stabilization of biologically active conformations of native peptides by cyclization or introduction of hindering residues led to peptidominetics endowed with high affinity and selectivity for one class of receptors and able to cross the blood brain barrier. This is the case of BUBU, Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr(OtBu) and BUBUC, Tyr-D-Cys-(OtBu)-Gly-Phe-Leu-Thr(OtBu) for the opioid delta receptors and of BC 254, Boc-gamma-D-Glu-Tyr(SO3H)-Nle-D-Lys-Trp-Nle-Asp-PheNH2 and of BC 264, Boc-Tyr(SO3H)gNle-mGly-Trp-MeNle-Asp-PheNH2 for central CCK-B receptors. Inhibition of metabolizing peptidases such as aminopeptidase N and endopeptidase 24.11 (NEP) for enkephalins and of NEP and ACE for atrial natriuretic peptide and angiotensin I by mixed inhibitors such as kelatorphan and RB 101 or ES14, rationally designed by taking into account the structural differences in the active site of these zinc-metallopeptidases, led to potent analgesics devoid of the major morphine side effects or to new antihypertensives.

Replacement of Tyr-SO3H by a p-carboxymethyl-phenylalanine in a CCK8-derivative preserves its high affinity for CCK-B receptor

The sulfated tyrosine present in the sequence of CCK8 Asp26-Tyr(SO3H)-Met-Gly-Trp-Met-Asp-PheNH2, seems to play a critical role in the recognition of CCK-A binding sites. In this work, we have investigated whether the presence of an anionic charge on the tyrosine moiety is strictly necessary and whether the sulfate moiety interacts with a divalent cation in the receptor subsite. For this purpose, the novel amino acids (L,D)Phe(p-CH2CO2H) and (L,D) Phe(p-CH2CONHOH), as well as their L-resolved forms were introduced into the sequence of Ac[X27, Nle28, Nle31]-CCK27-33 by solid phase method. The biological activities of these new derivatives were compared to two almost equiactive analogues of CCK8, Ac[Phe(p-CH2SO3H)27, Nle28, Nle31]-CCK27-33 and Boc[Nle28, Nle31]-CCK27-33 (BDNL) and to the nonsulfated analogue of the latter peptide (BDNL NS). All these new CCK-related analogues behave as agonists in stimulating pancreatic amylase release and display high affinity for brain binding sites (KI approximately 3-11 nM) but the only peptides which retain affinity for CCK-A receptors (KI approximately 20 nM) are those containing a p-carboxymethyl phenylalanine. Thus, introduction of this amino acid under an esterified form on the side chain, into specific and potent CCK-B agonists could allow compounds endowed with good bioavailabilities to be obtained.

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.