title: Bremazocine is an agonist at kappa-opioid receptors and an antagonist at mu-opioid receptors in the guinea-pig myenteric plexus.
abstract: 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.
mesh_terms: D000818:Animals; D001575:Benzomorphans / Q000494:pharmacology*; D004558:Electric Stimulation; D006168:Guinea Pigs; D066298:In Vitro Techniques; D009019:Morphinans / Q000494:pharmacology*; D009197:Myenteric Plexus / Q000187:drug effects*; D009271:Naltrexone / Q000031:analogs & derivatives / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D011957:Receptors, Opioid / Q000378:metabolism*; D017473:Receptors, Opioid, kappa; D017450:Receptors, Opioid, mu
chemical_list: D001575:Benzomorphans; D009019:Morphinans; D009292:Narcotic Antagonists; D011957:Receptors, Opioid; D017473:Receptors, Opioid, kappa; D017450:Receptors, Opioid, mu; D009271:Naltrexone; C017120:chlornaltrexamine; C024524:beta-funaltrexamine; C027248:bremazocine