Characterization of opioid binding sites in murine neuroblastoma

The binding of [³H][d-Ala²,MePhe⁴,Gly-ol⁵]enkephalin ([³H]DAGO), [³H]d-Ala²,d-Leu⁵]enkephalin [³H]DADLE) and (±)-[³H]ethylketocyclazocine ([³H]EKC) to neurotumor tissues derived from S20Y neuroblastoma cells transplanted into A/Jax mice was examined. Specific and saturable binding to [³H]DADLE and [³H]EKC was detected, and the data fit a single homogenous binding site for each ligand. Scatchard analysis for [³H]DADLE and [³H]EKC yielded K_d values of 0.65 and 0.45 nM, respectively, and B_max values of 9.2 and 116 fmol/mg protein. Binding was dependent on time, temperature, and pH, and was sensitive to Na⁺ and guanine nucleotides. Pretreatment of the tumor homogenates with trypsin markedly reduced binding to both ligands, suggesting that the binding sites were proteinaceous in character. Displacement experiments indicated that delta (δ) receptor related compounds (e.g. DPDPE, ICI 174,864) avidly displaced [³H]DADLE, whereas kappa (κ) related compounds (e.g. U50, 488, dynorphin) markedly competed with [³H]EKC. Mu (μ) receptor drugs (e.g. DAGO, β-FNA, morphine) were not potent in displacing either [³H]DADLE or [³H]EKC. These results are the first to characterize opioid binding sites in tumor tissue. The function of these sites is nuclear, but previous evidence as to the growth regulatory properties of endogenous opioid systems may suggest that either one, or both, binding sites may be involved in carcinogenic events.