Abstract

The binding of a radiolabeled opiate agonist ([3H]etorphine) and antagonist ([3H]naloxone) was studied using quantitative film autoradiography of rat-brain sections labeled by in vitro dipping methods. The binding activities of both [3H]naloxone and [3H] etorphine were saturable in three brain regions: noncluster striatum, nucleus accumbens and cingulate cortex. Eadie-Hofstee analysis of these regions yielded the following binding affinities and capacities: noncluster striatum binding affinity (KD) +/- S.E. = 1.59 +/- 0.23 nM, maximal binding capacity (Bmax) +/- S.E. = 28.3 +/- 1.9 fmol/mg, S.D. error of the raw data (Erad) = 6.4%; nucleus accumbens, KD +/- S.E. = 1.74 +/- 0.28 nM, Bmax +/- S.E. = 73.3 +/- 5.2 fmol/mg, S.D. (Erad) = 6.2%; cingulate cortex, KD +/- S.E. = 1.44 +/- 0.15 nM, Bmax +/- S.E. = 37.6 +/- 1.4 fmol/mg, S.D. (Erad) = 2.5%. A KD +/- S.E. = 1.72 +/- 0.29 nM, Bmax +/- S.E. = 74.1 +/- 5.3 fmol/mg, S.D. (Erad) = 5.0% was found for [3H]etorphine binding in the noncluster striatum. Hill plots of both [3H]naloxone and [3H]etorphine binding in noncluster striatum demonstrated an absence of cooperativity with slopes of 1.01 and 1.07, respectively. Stereospecificity of binding was confirmed by competition for 2.0 nM [3H]naloxone in the noncluster striatum with a levorphanol IC50 = 5.5 nM and a dextrorphan IC50 greater than 1000 nM. Rank order potency for competition for 2.0 nM [3H]naloxone binding in noncluster striatum was etorphine greater than naloxone greater than levorphanol greater than morphine greater than dextrorphan. The regional order of binding activities (femtomoles per milligram +/- S.D.) for 2.0 nM [3H]naloxone was as follows: striatal clusters (111.1 +/- 24.5) greater than interpeduncular nucleus (77.8 +/- 10.1) greater than central nucleus of amygdala (64.5 +/- 9.7) greater than nucleus accumbens (34.4 +/- 6.9) greater than median raphe (24.4 +/- 6.1) greater than striatal noncluster (23.3 +/- 3.5) greater than superior colliculus striatum grieseum (22.2 +/- 4.0). Thus, quantitative film autoradiography of brain sections labeled in vitro may be used to characterize the pharmacological binding properties of ligands in many small brain regions not amendable to study in membrane preparations.