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.

[3H]PD 140376: a novel and highly selective antagonist radioligand for the cholecystokininB/gastrin receptor in guinea pig cerebral cortex and gastric mucosa.

The specific binding characteristics of the novel cholecystokinin (CCK)B/gastrin receptor-selective peptoid antagonist radioligand [3H]PD 140376 were investigated using membrane homogenates prepared from guinea pig cerebral cerebral cortex and gastric fundic mucosa. [3H]PD 140376 (0.01-10 nM) bound to both cerebral cortex and gastric gland homogenates with comparable high affinity (Kd, 0.1-0.2 nM) and to an apparent single population of sites with Bmax values of 119 and 296 fmol/mg of protein, respectively. The level of specific binding, defined as that displaced by unlabeled CCK sulfated octapeptide, was routinely between 60 and 70% in the cortex and between 50 and 60% in the fundic mucosa. Pharmacological characterization of the [3H]PD 140376-labeled binding sites with a series of agonist and antagonist ligands selective for each of the CCK receptor subtypes demonstrated, in both preparations, an affinity profile consistent with that of the CCKB/gastrin receptor. However, Hill slopes for the competition curves for the unlabeled agonist ligands against specific [3H]PD 140376 binding were significantly less than unity, whereas those for the antagonist ligands, including unlabeled PD 140376, were close to unity. The affinity and Hill slope for PD 140376 and the related CCKB/gastrin antagonist CI-988 were unaffected by the presence of the nonhydrolyzable GTP analogue guanylyl-5'-imidodiphosphate. In contrast, guanylyl-5'-imidodiphosphate caused a characteristic decrease in affinity and an increase in the Hill slopes towards unity for the agonist ligands CCK sulfated octapeptide and pentagastrin. The binding characteristics of unlabeled PD 140376 were also unaffected by the presence of the monovalent cation sodium. In conclusion, the present study has demonstrated that [3H]PD 140376 is the most potent and selective antagonist radioligand yet described for the characterization of CCKB/gastrin receptors in the central and peripheral nervous systems.

CI-977, a novel and selective agonist for the kappa-opioid receptor.

1. CI-977 is a new, nonpeptide kappa-opioid compound that has been synthesized and its pharmacological properties determined in a series of in vitro and in vivo rodent models. 2. In a radioligand binding studies, with guinea-pig forebrain homogenates, CI-977 bound with high affinity to [3H]-U69593-labelled kappa-sites (Ki = 0.11 nM) but with low affinity to [3H]-[D-Ala2, MePhe4, Gly-ol5] enkephalin (DAMGO) labelled mu-sites (Ki = 99 nM) and [3H]-[D-Pen2.5]enkephalin (DPDPE) labelled delta-sites (Ki = 1.04 microM). CI-977 also bound with negligible affinity to [3H]-(+)-3-(1-propyl-3-piperi-dinyl)phenol (3-PPP) labelled sigma-sites (Ki = 1.9 microM) and [3H]-1-(1-[2-thienyl]cyclohexyl)piperidine (TCP) labelled PCP sites (Ki greater than 10 microM). 3. CI-977 produced a potent inhibition of the electrically-evoked contractions of the guinea-pig ileum and rabbit vas deferens with IC50 values of 0.087 nM and 3.3 nM, respectively. The pKB values for the opioid antagonists naloxone (7.6) and norbinaltorphimine (10.5) supported the kappa nature of the CI-977-mediated effects in the smooth muscle assays. 4. CI-977 was a potent antinociceptive agent against a mechanical noxious stimulus in rats following intravenous, intramuscular, subcutaneous and oral administration. CI-977 was also effective against mechanical and chemical noxious stimuli in the mouse but ineffective against a thermal stimulus. The antinociceptive effects produced by CI-977 were completely reversed by naloxone (1 mg kg-1, s.c.). 5. At doses close to those required to produce antinociception, CI-977 also caused a naloxone-reversible diuresis and inhibition of locomotor activity.6. The in vitro and in vivo pharmacological profile of CI-977 demonstrates that it is a potent and selective agonist at the Kappa-opioid receptor.

An in vitro profile of activity for the (+) and (-) enantiomers of spiradoline and PD117302.

Kappa- and mu-opioid binding site affinities of the kappa-selective ligand U62066 and its optical isomers, (+)-U63639 and (-)-U63640 were compared with those of the structurally related ligand PD117302 and its respective isomers, (+)-PD123497 and (-)-PD123475. The relative efficacies of each compound were also established using the guinea-pig ileum, rat and rabbit vas deferens smooth muscle bioassays. The specific opioid receptor mediating the agonist behaviour was determined in the guinea-pig ileum bioassay by obtaining pKB values for naloxone and for the kappa-selective antagonist nor-binaltorphimine. Both racemic compounds and the (-)-enantiomers displayed high selectivity for the kappa-receptor with (-)-PD123475 the most selective. The (+)-enantiomer, PD123497, was approximately equipotent at mu-/kappa-sites while (+)-U63639 displayed a 140-fold mu-receptor selectivity. Bioassay studies showed each compound to be interacting at the kappa-receptor, with the exception of (+)-U63639 which displayed a profile consistent with that of a weak mu-receptor agonist.

Pharmacological profile of PD 117302, a selective kappa-opioid agonist.

1 PD 117302, a new nonpeptide opioid compound shown in in vitro studies to be a selective kappa-opioid agonist, has been evaluated in vivo for antinociceptive activity and other effects characteristic of kappa-receptor activation. 2 Dose-related long lasting antinociception was produced by PD 117302 against a mechanical noxious stimulus in rats following intravenous, subcutaneous or oral administration. 3 PD 117302 was effective in raising the nociceptive threshold to mechanical and chemical but not to thermal noxious stimuli in the mouse. This effect was attenuated in animals pretreated with the opioid antagonist naloxone. 4 In addition to producing antinociception, PD 117302 also caused naloxone-reversible locomotor impairment and diuresis, effects that are typical of kappa-agonists. 5 PD 117302 did not cause respiratory depression, inhibition of gastrointestinal motility or naloxone-precipatated withdrawal jumping in mice, effects that are associated with actions at the mu-opioid receptor. 6 The pharmacological profile of PD 117302 in vivo is consistent with in vitro data suggesting that PD 117302 is a selective agonist at the kappa-opioid receptor.