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Bremazocine is an agonist at K‐opioid receptors and an antagonist at μ‐opioid receptors in the guinea‐pig myenteric plexus

1 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 μ‐ or k‐opioid receptors were blocked preferentially. 2 After pretreatment of the tissue with β‐funaltrexamine for 90 min followed by washing out, the IC50 value of the selective μ‐ligand [D‐Ala2,MePhe4,Gly‐ol5]enkephalin was increased 67 fold whereas the IC50 values of the selective k‐ligand U‐69,593 and of the non‐selective k‐ligand bremazocine were not significantly changed. In this experimental design bremazocine acted only on k‐receptors. 3 After pretreatment of the tissue with β‐chlornaltrexamine and 10 μM of the μ‐ligand for 30 min followed by washout, the IC50 value of the μ‐ligand was increased 2 fold whereas the IC50 value of the selective k‐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 μ‐receptor (Ke. = 1.6 nM). The residual agonist activity of bremazocine is at the k‐receptor. 4 In naive myenteric plexus preparations the μ‐antagonist activity of bremazocine cannot be demonstrated because its potency at the k‐receptor is very high. This dual action may be of importance for the responses of bremazocine in other peripheral and central tissues.