Ra. Cruciani et al., PRESENCE IN NEUROBLASTOMA-CELLS OF A MU(3) RECEPTOR WITH SELECTIVITY FOR OPIATE ALKALOIDS BUT WITHOUT AFFINITY FOR OPIOID-PEPTIDES, Brain research, 667(2), 1994, pp. 229-237
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.