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 granulo sa) and that this receptor appears to be a G protein-coupled receptor (GPCR ). The present study investigated the question of whether this mGR recogniz es nonsteroid ligands that bind to cognate receptors in the GPCR superfamil y. To address this question, ligand-binding competition studies evaluated t he potencies of various ligands to displace [H-3]corticosterone (CORT) bind ing to neuronal membranes. Initial screening studies tested 21 different co mpetitors and found that [H-3]CORT binding was displaced only by dynorphin 1-13 amide (an endogenous kappa-selective opioid peptide), U50,488 (a synth etic kappa-specific agonist) and naloxone (a nonselective opioid antagonist ). Follow-up studies revealed that the kappa agonists bremazocine (BRE) and ethylketocyclazocine (EKC) also displaced [H-3]CORT binding to neuronal me mbranes, but that U69,593 (a kappa specific agonist) and nor-BNI (a kappa s pecific antagonist) were ineffective. The K-i values measured for the opioi d competitors were in the subnanomolar to low micromolar range and had the following rank-order: dynorphin > U50,488 > naloxone > BRE > EKC. Because t hese ligands displaced, at most, only 70% of [H-3]CORT specific binding, it appears that some [H-3]CORT binding sites are opioid insensitive. Kinetic analysis of [H-3]CORT off-rates in the presence of U50,488 and/or CORT reve aled no differences in dissociation rate constants, suggesting that there i s a direct, rather than allosteric, interaction with the [H-3]CORT binding site. In summary, these results are consistent with the hypothesis that the high-affinity membrane binding site for [H-3] CORT is located on a kappa o pioid-like receptor.