PRESENCE IN NEUROBLASTOMA-CELLS OF A MU(3) RECEPTOR WITH SELECTIVITY FOR OPIATE ALKALOIDS BUT WITHOUT AFFINITY FOR OPIOID-PEPTIDES

Evidence is presented for the occurrence of a unique opiate alkaloid-s elective, opioid peptide-insensitive binding site in N18TG2 mouse neur oblastoma cells and in late passage hybrid F-11 cells, derived from N1 8TG2 neuroblastoma cells and rat dorsal root ganglion cells. Those cel ls lacked classical opioid peptide-sensitive receptor subtypes, but co ntained [H-3]morphine and [H-3]diprenorphine binding sites with affini ty for certain opiate alkaloids but not for any endogenously occurring opioid peptide or peptide analog tested, including D-ala(2)-D-leu(5)- enkephalin (DADLE), D-Ala(2),N-Me-Phe(4),Gly(5)-ol (DAGO) and dynorphi n A(1-17). The binding site differed from hitherto described mu, delta and kappa neuronal opioid receptors not only on the basis of peptide insensitivity, but also on the basis of selectivity and affinities of alkaloids. Saturation experiments with [H-3]morphine indicated the pre sence of a single site with K-d = 49 nM and B-max = 1510 fmol/mg prote in. This novel binding site was not present in F-11 hybrid cells at ea rly passage. Instead the hybrid cells contained conventional opioid re ceptors (predominantly delta and also mu) capable of binding DADLE and other peptides as well as opiate alkaloids. With additional passage ( cell divisions) of the hybrid cells, during which a limited change occ urred in mouse chromosome number, the peptide-insensitive binding appe ared and the opioid peptide-binding (delta and mu) receptors were lost reciprocally. Thus, expression of the peptide-insensitive binding nor mally may be repressed when conventional opioid receptors are expresse d. The peptide-insensitive opiate binding site described here appears to correspond to the mu(3) receptor subtype, recently identified pharm acologically and functionally in several cell types of the immune syst em. It is proposed that this opiate alkaloid-sensitive mu(3) receptor of macrophages and certain other immunocytes is also present in certai n neuronal cell lines and thus may possibly exist in certain neurons o f the intact organism.