morphinans and chlornaltrexamine

1. Ethylketocyclazocine (EKC) and U50,488H have been employed widely as kappa-receptor agonists in the study of opioid receptor systems. However, the quantification of their agonism in terms of affinities and relative efficacies has not been investigated. 2. In this study, operational model-fitting was used to analyse the effects of irreversible receptor alkylation by beta-chlornaltrexamine (beta-CNA) on the kappa-receptor mediated effects of EKC and U50,488H in the isolated, coaxially stimulated ileum of the guinea-pig. 3. EKC produced monophasic inhibitory concentration-effect curves which were readily amenable to analysis. In contrast U50,488H produced biphasic curves characterized by a higher potency phase of agonism that was susceptible to antagonism by 16-methylcyprenorphine (RX8008M) and a lower potency phase that was apparently non-opioid in nature. 4. Analysis of the kappa-receptor-mediated effects of both agonists indicated that EKC has fifteen fold higher affinity than U50,488H and that the two agonists possess similar intrinsic efficacies.

Topics: 3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; Analgesics; Animals; Binding, Competitive; Cyclazocine; Electric Stimulation; Ethylketocyclazocine; Guinea Pigs; In Vitro Techniques; Male; Models, Biological; Morphinans; Muscle, Smooth; Naltrexone; Narcotic Antagonists; Pyrrolidines; Receptors, Opioid; Receptors, Opioid, kappa

1. The irreversible opioid receptor antagonist beta-chlornaltrexamine (beta-CNA) has been shown previously to have agonist activity in the guinea-pig ileum preparation. However, the receptor type or types mediating this effect have not been established. 2. In this study, the agonism of beta-CNA was investigated by use of the competitive antagonist 16-methylcyprenorphine (RX8008M). Non-cumulative concentration-effect curves for beta-CNA were displaced in a non-parallel fashion indicating that the agonism was mediated by both mu- and kappa-receptors. 3. In principle, expression of agonism by an irreversible receptor antagonist could compromise its use in estimating agonist dissociation constants (pKAs) due to desensitization operating in addition to receptor inactivation. For kappa-receptors, this possibility was checked by use of ethylketocyclazocine (EKC) to mimic the agonist effects of beta-CNA and test whether subsequent EKC concentration-effect curves were displaced. For mu-receptors it was necessary to perform more involved experiments in which [D-Ala2, MePhe4, Gly-ol5]enkephalin (DAGOL) was used as a standard agonist and its pKA was estimated under different conditions of beta-CNA incubation. 4. These analyses indicated that neither the mu- nor the kappa-receptor-mediated agonism of beta-CNA was associated with appreciable receptor desensitization. In turn it was concluded that the usefulness of beta-CNA as a pharmacological tool for the estimation of mu- and kappa-opioid receptor agonist dissociation constants is not compromised by the agonist effects that the compound demonstrates at these receptors.

Topics: Animals; Cyclazocine; Drug Interactions; Electric Stimulation; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalins; Ethylketocyclazocine; Guinea Pigs; Ileum; In Vitro Techniques; Male; Morphinans; Muscle, Smooth; Naltrexone; Narcotic Antagonists; Receptors, Opioid; Receptors, Opioid, kappa; Receptors, Opioid, mu

The opioid agonist and antagonist activities of diprenorphine have been tested in four in vitro bioassay preparations. Diprenorphine is an antagonist at delta-receptors in the hamster vas deferens, at mu-receptors in the rat vas deferens and at kappa-receptors in the rabbit vas deferens. In the guinea-pig ileum it is an antagonist at mu-receptors and an agonist at kappa-receptors.

Topics: Animals; Cricetinae; Diprenorphine; Guinea Pigs; Ileum; In Vitro Techniques; Male; Morphinans; Muscle Contraction; Myenteric Plexus; Naltrexone; Rats; Receptors, Opioid; Receptors, Opioid, kappa; Receptors, Opioid, mu; Vas Deferens

The agonist and antagonist activity of bremazocine at opioid receptors in the guinea-pig myenteric plexus preparation was determined in untreated tissues and in tissues in which either mu-9 or kappa-opioid receptors were blocked preferentially. After pretreatment of the tissue with beta-funaltrexamine for 90 min followed by washing out, the IC50 value of the selective mu-ligand [D-Ala2,MePhe4,Gly-ol5]enkephalin was increased 67 fold whereas the IC50 values of the selective kappa-ligand U-69,593 and of the non-selective kappa-ligand bremazocine were not significantly changed. In this experimental design bremazocine acted only on kappa-receptors. After pretreatment of the tissue with beta-chlornaltrexamine and 10 microM of the mu-ligand for 30 min followed by washout, the IC50 value of the mu-ligand was increased 2 fold whereas the IC50 value of the selective kappa-ligand was increased 32 fold and that of bremazocine 62 fold. Under these experimental conditions, it was shown that bremazocine is an antagonist against [D-Ala2,MePhe4,Gly-ol5]enkephalin at the mu-receptor (Ke = 1.6 nM). The residual agonist activity of bremazocine is at the kappa-receptor. In naive myenteric plexus preparations the mu-antagonist activity of bremazocine cannot be demonstrated because its potency at the kappa-receptor is very high. This dual action may be of importance for the responses of bremazocine in other peripheral and central tissues.

Topics: Animals; Benzomorphans; Electric Stimulation; Guinea Pigs; In Vitro Techniques; Morphinans; Myenteric Plexus; Naltrexone; Narcotic Antagonists; Receptors, Opioid; Receptors, Opioid, kappa; Receptors, Opioid, mu