Abstract

The binding characteristics of the kappa opioid ligands [³H]U69,593 and [³H]bremazocine, themu opioid ligand [³H][d-ala²,N-Me-Phe⁴,glycol⁵]enkephalin and the delta opioid ligand [³H]p-Cl-[d-pen^2,5]enkephalin were studied in rhesus monkey brain membranes in saturation binding experiments and were followed by competition binding experiments with a variety of peptidic and nonpeptidic opioid ligands. The [³H]U69,593 sites appeared to be a subset ofkappa opioid receptors (kappa-1 receptors: K_d, 1.2 nM;B_max, 66 fmol/mg). [³H]Bremazocine (in the presence of mu anddelta receptor-masking agents), bound to a larger population of kappa receptors (kappa-all:K_d, 0.39 nM;B_max, 227 fmol/mg), which presumably included the aforementioned kappa-1 sites. Competition binding experiments revealed that the presently definedkappa-1 sites were similar to previously reported sites in other mammalian species, particularly in terms of the higherkappa-1 selectivity observed with arylacetamide (e.g., U50,488) vs. benzomorphankappa agonists (e.g., ethylketocyclazocine). The kappa-selective antagonist norbinaltorphimine (nor-BNI) displayed a very small (2.3-fold) selectivity for kappa-1 vs.kappa-all sites. This led to the prediction that in rhesus monkeys (n = 3), systemically administered nor-BNI (10 mg/kg s.c.) should have a very moderate degree of antagonist selectivity for the antinociceptive effects of a putativekappa-1-agonist, the arylacetamide U50,488 (0.1–3.2 mg/kg s.c.), vs. those of the benzomorphankappa agonist ethylketocyclazocine (0.01–056 mg/kg s.c.). This prediction was confirmed in vivo because nor-BNI (10 mg/kg) caused a robust and long lasting (up to 21 days) antagonism of the antinociceptive effects of U50,488 and a small but significant antagonism of ethylketocyclazocine. The arylacetamide congener CI-977 (enadoline), which displayed an 11-foldkappa-1 vs.kappa-all binding selectivity, was not sensitive to nor-BNI pretreatment. This indicates that the kappa subtype-binding profile of an agonist is not necessarily predictive of its sensitivity to nor-BNIin vivo. Overall, the present results suggest that at least two functional kappa receptor populations may be present in rhesus monkey brain.