Characterization and distribution of [ 3 H]ohmefentanyl binding sites in the human brain

Binding properties and localization of [ 3 H]ohmefentanyl, a new ligand for μ opioid receptors, were investigated on normal human brain sections. Binding assays performed at the level of the basal ganglia revealed: (1) a steady‐state binding reached after 60 min incubation at room temperature, (2) the presence, in saturation experiments, of an apparent single class of binding sites with a K d = 1.68 ± 0.45 nM and a B max = 162 ± 9 fmol/mg protein, (3) an order of potency to inhibit [ 3 H]ohmefentanyl binding as follows: ohmefentanyl > [D‐Ala 2 , MePhe 4 , Gly‐ol 5 ]enkephalin (DAGO) > ethylketocyclazocine (EKC) ≫ Tyr‐D‐Ser(OtBu)‐Gly‐Phe‐Leu‐Thr(OtBu) (BUBU) and U‐50, 488H. Quantitative autoradiography showed an heterogeneous distribution of [ 3 H]ohmefentanyl binding sites with the highest densities in amygdala, medial geniculate body, thalamus, and caudate nucleus. Binding characteristics and anatomical distribution also show that [ 3 H]ohmefentanyl may bind to a small proportion of additional sites called “DAGO‐inaccessible [ 3 H]ohmefentanyl specific binding sites.” [ 3 H]Ohmefentanyl binding to these sites can be partly inhibited by s̀ ligands such as 1, 3‐di‐o‐tolyguanidine (DTG) and haloperidol. However, unlabeled DAGO inhibited more than 80% of [ 3 H]ohmefentanyl specific binding in most of the human brain regions studied, suggesting that the major population of sites labeled by [ 3 H]ohmefentanyl represented μ opioid receptors.