Pre‐incubation of guinea‐pig myenteric plexus with β‐funaltrexamine: discrepancy between binding assays and bioassays

Abstract

1 The acute effects of P-funaltrexamine and the effects of pre-incubation with this compound were examined in five in vitro assay tissues and in selective binding assays in homogenates of guinea-pig brain and myenteric plexus. 4 These findings indicate that the effects of pre-incubation with P-funaltrexamine on agonist potency of the IA-receptor ligand are due to an interference with the coupling mechanism between the n-binding site and the effector system.

Alkylation with β‐funaltrexamine suggests differences between μ‐opioid receptor systems in guinea‐pig brain and myenteric‐plexus

“…Assuming affinities of [3H]-DAMGO C-1 Macmillan Press Ltd, 1991 p-OPIOID RECEPTOR SYSTEMS AND /i-FUNALTREXAMINE 719 for the p-site in myenteric plexus of 2.27 nM (Corbett et al, 1985) and for the b-and K-sites of 407 nM (Cotton et al, 1985) and 4960 (Kosterlitz et al, 1981) respectively and a p: 6 : K binding site ratio of 25%: 26%: 49% (Corbett et al, 1985) (McPherson, 1985).…”

“…Homogenates of tissues, either strips of myenteric plexus longitudinal muscle from the ileum or whole brain (without cerebellum), were prepared in Tris buffer (pH 7.4, 50mM), or Krebs-HEPES, essentially as described by Kosterlitz and colleagues (Gillan et al, 1980;Corbett et al, 1985 (Handa et al, 1981 (Corbett et al, 1985). Assuming affinities of [3H]-DAMGO C-1 Macmillan Press Ltd, 1991 p-OPIOID RECEPTOR SYSTEMS AND /i-FUNALTREXAMINE 719 for the p-site in myenteric plexus of 2.27 nM (Corbett et al, 1985) and for the b-and K-sites of 407 nM (Cotton et al, 1985) and 4960 (Kosterlitz et al, 1981) respectively and a p: 6 : K binding site ratio of 25%: 26%: 49% (Corbett et al, 1985) (McPherson, 1985).…”

“…The loss of potency of p-ligands in intact myenteric-plexus longitudinal muscle preparations following /-FNA treatment has therefore been attributed to an interference with the receptor-effector coupling system in this tissue, rather than a direct blockade of binding sites (Corbett et al, 1985).…”

“…It is effective in preventing p-receptor activation by opioids in both in vitro bioassay systems (Ward et al, 1982a;Hayes et al, 1985;Corbett et al, 1985) and in vivo (Ward et al, 1982b;Hayes et al, 1986;Takemori & Portoghese, 1987). Ligand binding studies in brain homogenates suggest that this action is due to an irreversible reduction in the number of opioid binding sites (Rothman et al, 1983;Ward et al, 1985;Tam & Liu-Chen, 1986), possibly due to Michael addition with a thiol function close to the ligand-recognition site (Tam & Liu-Chen, 1986).…”

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

“…The potencies of agonists were obtained from dose-response curves with the exception that, after Pchlornaltrexamine pretreatment, the agonist IC50 values of bremazocine were determined by the 'singledose' method (Kosterlitz & Watt, 1968 (Corbett et al, 1985b). Agonist potencies were determined before and after P-chlornaltrexamine or P-funaltrexamine treatment.…”