Vol. 285, Issue 2, 595-601, May 1998

Kappa-Opioid Receptor Binding Populations in Rhesus Monkey Brain: Relationship to an Assay of Thermal Antinociception¹

Eduardo R. Butelman² , Mei-Chuan Ko, Katarzyna Sobczyk-Kojiro, Henry I. Mosberg, Barbara Van Bemmel, Gerald Zernig³ and James H. Woods

Departments of Pharmacology (E.R.B., M.C.K., B.V.B., G.Z., J.H.W.) and Psychology (J.H.W.) and Medicinal Chemistry, College of Pharmacy (H.I.M., K.S.K.), University of Michigan, Ann Arbor, Michigan

The binding characteristics of the kappa opioid ligands [³H]U69,593 and [³H]bremazocine, the mu 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 of kappa opioid receptors (kappa-1 receptors: K_d, 1.2 nM; B_max, 66 fmol/mg). [³H]Bremazocine (in the presence of mu and delta 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 defined kappa-1 sites were similar to previously reported sites in other mammalian species, particularly in terms of the higher kappa-1 selectivity observed with arylacetamide (e.g., U50,488) vs. benzomorphan kappa 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 putative kappa-1-agonist, the arylacetamide U50,488 (0.1-3.2 mg/kg s.c.), vs. those of the benzomorphan kappa 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-fold kappa-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-BNI in vivo. Overall, the present results suggest that at least two functional kappa receptor populations may be present in rhesus monkey brain.