Up-regulation of opiate receptors by opiate antagonists in neuroblastoma-glioma cell culture: The possibility of interaction with guanosine triphosphate-binding proteins

Neuroblastoma-glioma NG108-15 cells that were cultured for 48 h with the opiate antagonist, naloxone, respond to the guanosine 5′-triphosphate (GTP) analogue guanosine 5′-[β,γ-imido]-triphosphate (GMP-PNP) in the binding assay as the control, non-treated, cells. This was observed when the guanyl nucleotide was tested in the presence or absence of sodium chloride and also after subcellular fractionation of the membranes on a sucrose gradient which separated between two receptor-containing fractions. The findings suggest that the increase in δ type enkephalin receptors in naloxone-treated NG108-15 cells does not reflect an alteration in the interaction between the receptor and the adenylate cyclase-GTP-binding protein system.