Opiate receptor binding and opiate agonist inhibition of cyclic AMP accumulation were measured with intact neuroblastoma x glioma NG108-15 hybrid cells under identical conditions. [3H]Diprenorphine bound to intact hybrid cells with an apparent K(D) value of 1.96 ± 0.04 nM. The maximal number of binding sites on the NG108-15 cells was determined to be 345,000 ± 3,300 receptors/cell. Mu and delta ligands competitively inhibited [3H]diprenorphine binding. Apparently, cyclic AMP accumulation in hybrid cells was regulated by a single population of opiate receptors. This was concluded from observations that the pA2 values of naloxone to antagonize mu, kappa, and delta ligands were identical and that the sigma partial agonist, N-allylnormetazocine, competitively interacted with the receptor sites of the delta agonist, Leu5-enkephalin. When opiate inhibition of the prostaglandin E1-stimulated accumulation of cyclic AMP was determined, it was observed that the intrinsic activities of the opiate ligands were nonidentical. All opioid peptides tested have intrinsic activities ≥ 0.9 when compared with etorphine. Opiate alkaloids, with the exception of etorphine, ketocyclazocine, ethylketocyclazocine and GPA 1657, have partial agonist activities. Because of the putative delta nature of the receptor in the neuroblastoma cell line, opioid peptides, as a group, were more potent than the opiate alkaloids in the displacement of [3H]diprenorphine binding and in the inhibition of cyclic AMP level increase. When the ratio of the K(d) and K(i) values of opiate ligands was determined, opioid peptides, with the exception of β-endorphin, have a ratio greater than 1. Thus, opioid peptides did not require all receptors for maximal activity. With the exception of etorphine and the benzomorphans, opiate alkaloids required full occupancy of the receptor for their maximal activity (K(d)/K(i) ≃ 1). As the mu characteristic increased in the opioid peptides, the K(d)/K(i) ratio of the peptides approached unity. Hence, the efficiency of coupling between a homogeneous population of opiate receptor and the effector is determined by the receptor-ligand complex. Thus, it must follow that the opiate receptor in neuroblastoma x glioma NG108-15 cells undergoes conformational changes after binding of agonists.
|Original language||English (US)|
|Number of pages||10|
|State||Published - 1983|