A subset of kappa opioid ligands bind to the membrane glucocorticoid receptor in an amphibian brain.

Previous studies demonstrated that a membrane receptor for glucocorticoids (mGR) exists in neuronal membranes from the roughskin newt (Taricha granulosa) and that this receptor appears to be a G protein-coupled receptor (GPCR). The present study investigated the question of whether this mGR recognizes nonsteroid ligands that bind to cognate receptors in the GPCR superfamily. To address this question, ligand-binding competition studies evaluated the potencies of various ligands to displace [ 3 H]corticosterone (CORT) binding to neuronal membranes. Initial screening studies tested 21 different competitors and found that [ 3 H]CORT binding was displaced only by dynorphin 1‐13 amide (an endogenous k-selective opioid peptide), U50,488 (a synthetic k-specific agonist) and naloxone (a nonselective opioid antagonist). Follow-up studies revealed that the k agonists bremazocine (BRE) and ethylketocyclazocine (EKC) also displaced [ 3 H]CORT binding to neuronal membranes, but that U69,593 (a k specific agonist) and nor-BNI (a k specific antagonist) were ineffective. The Ki values measured for the opioid competitors were in the subnanomolar to low micromolar range and had the following rankorder: dynorphin . U50,488 . naloxone . BRE . EKC. Because these ligands displaced, at most, only 70% of [ 3 H]CORT specific binding, it appears that some [ 3 H]CORT binding sites are opioid insensitive. Kinetic analysis of [ 3 H]CORT off-rates in the presence of U50,488 and/or CORT revealed no differences in dissociation rate constants, suggesting that there is a direct, rather than allosteric, interaction with the [ 3 H]CORT binding site. In summary, these results are consistent with the hypothesis that the high-affinity membrane binding site for [ 3 H]CORT is located on a k opioid-like receptor.