Previous work from our lab identified two subtypes of the opioid kappa receptor. Whereas the kappa1 receptor can be labeled by [3H]U69,593 (5α, 7α, 8β-(-)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro (4,5)dec-8-yl]-phenyl-benzeneacetamide), the kappa2 receptor can be labeled by [125I]IOXY (6β-125iodo-3,14-dihydroxy-17-cyclopropylmethyl-4,5α-epoxymorphinan). Other data demonstrate that [125I]IOXY, like [3H]bremazocine, labels two populations of kappa2 receptors in guinea pig brain: kappa2a and kappa2b binding sites. In the present study, we tested the hypothesis that certain dihydrocodeinone and oxicodone derivatives, which have been shown to irreversibly block low affinity [3H]naloxone binding sites, would also bind irreversibly to opioid kappa receptor subtypes. We also tested the novel irreversible mu receptor antagonist, clocinnamox (14β-(p-chlorocinnamoylamino)-7,8-dihydro-N-cyclopropylmethylnormorphinone mesylate). Wash-resistant inhibition (WRI) assays were conducted to detect apparent irreversible inhibition. The proportion of WRI attribuable to inhibition of receptor binding, termed receptor inhibition (RI), was calculated by the equation: RI=WRI (wash-resistant inhibition)-SI (supernatant inhibition or inhibition attributable to residual drug.) Dihydrocodeinone-hydrazone, dihydrocodeinone-oxime and naloxone-3-OMe-oxime failed to produce any wash-resistant inhibition of kappa receptor binding. In contrast, preincubating guinea pig membranes with 1 μM clocinnamox produced a substantial degree of wash-resistant inhibition (greater than 90%) at kappa1 and kappa2 binding sites. However, as indicated by supernatant inhibition values of 70% to 90%, there was a large amount of residual clocinnamox which remained despite the use of an extensive washing procedure. Thus, it is apparent that clocinnamox has essentially no irreversible effect on kappa binding sites. Moreover, these results clearly demonstrate the requirement to determine supernatant inhibition when testing putative irreversible ligands. The apparent inactivity of dihydrocodeinone-hydrazone, dihydrocodeinone-oxime or naloxone-3-OMe-oxime as irreversible inhibitors of kappa receptors suggests that the low affinity [3H]naloxone binding site eliminated by these agents may not be a kappa binding site.
- Opioid kappa receptor
ASJC Scopus subject areas
- Cellular and Molecular Neuroscience