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-selective, opioid peptide-insensitive binding site in N18TG2 mouse neuroblastoma cells and in late passage hybrid F-11 cells, derived from N18TG2 neuroblastoma cells and rat dorsal root ganglion cells. Those cells lacked classical opioid peptide-sensitive receptor subtypes, but contained [H-3]morphine and [H-3]diprenorphine binding sites with affinity 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 dynorphin 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 presence of a single site with K-d = 49 nM and B-max = 1510 fmol/mg protein. This novel binding site was not present in F-11 hybrid cells at early passage. Instead the hybrid cells contained conventional opioid receptors (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 occurred in mouse chromosome number, the peptide-insensitive binding appeared and the opioid peptide-binding (delta and mu) receptors were lost reciprocally. Thus, expression of the peptide-insensitive binding normally may be repressed when conventional opioid receptors are expressed. The peptide-insensitive opiate binding site described here appears to correspond to the mu(3) receptor subtype, recently identified pharmacologically and functionally in several cell types of the immune system. It is proposed that this opiate alkaloid-sensitive mu(3) receptor of macrophages and certain other immunocytes is also present in certain neuronal cell lines and thus may possibly exist in certain neurons of the intact organism.