[3H]ohmefentanyl preferentially binds to mu-opioid receptors but also labels sigma-sites in rat brain sections

Ohmefentanyl has been shown to be 6300 times more potent than morphine for analgesia. The receptor binding characteristics and distribution of [3H]ohmefentanyl in rat brain sections are presented. [3H]Ohmefentanyl bound with high affinity to opioid receptors in a saturable manner (Kd = 0.95 +/- 0.08 nM, Bmax = 337 +/- 14 fmol/mg protein). We used various currently available specific mu, delta and kappa ligands to show that [3H]ohmefentanyl has a high selectivity for the mu opioid receptor. However, [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO) was unable to completely inhibit [3H]ohmefentanyl specific binding, while complete inhibition was observed with fentanyl derivatives and the benzomorphan derivative, ethylketocyclazocine. This remaining 20% DAGO-inaccessible [3H]ohmefentanyl specific binding did not correspond to either mu1, delta or kappa sites. Haloperidol and 1,3-di-o-tolylguanidine were able to inhibit DAGO-inaccessible [3H]ohmefentanyl specific binding, suggesting that [3H]ohmefentanyl might also bind to haloperidol-sensitive sigma sites. The topographical distribution of [3H]ohmefentanyl found by autoradiography was generally similar to that of [3H]DAGO. However, in agreement with the biochemical results, quantitative analysis revealed additional sites in several rat brain regions, the greatest discrepancies with [3H]DAGO distribution being observed in cerebellum, central grey, hippocampal formation and locus coeruleus. Finally, our results suggest that this capacity of binding to both mu and sigma sites is shared by various fentanyl derivatives.