BIOCHEMICAL AND PHARMACOLOGICAL CHARACTERIZATION OF MU-OPIOID, DELTA-OPIOID AND KAPPA(3)-OPIOID RECEPTORS EXPRESSED IN BE(2)-C NEUROBLASTOMA-CELLS

Total opioid binding in the human neuroblastoma cell line BE(2)-C has a density similar to that found in brain, with a B, value of 383 +/- 60 fmol/mg protein and a K-D of 0.4 +/- 0.07 nM for the nonselective opioid antagonist H-3-diprenorphine. Selective assays reveal a binding distribution of mu (38%), delta (16%) and kappa(3) (43%) opioid receptors. There is no observable kappa(1) or kappa(2) binding. The sum of the B-max values in the selective binding assays (370 +/- 39 fmol/mg protein) approximates closely that observed with H-3-diprenorphine, suggesting that mu, delta and kappa(3) sites account for most of the binding. The binding selectivities of various opiates and opioid peptides in the BE(2)C cells are similar to those in rat brain. Delta and mu binding are defined easily by traditional selective ligands. The binding profiles also distinguish clearly mu from kappa(3) binding. The selective mu ligand DAMGO competes with mu binding over 35-fold more potently than kappa(3) binding, whereas morphine shows a 10-fold selectivity. Functionally, selective mu, delta and kappa(3) agonists inhibit forskolin-stimulated cAMP accumulation through distinct receptor mechanisms that are pertussis toxin-sensitive. In addition to demonstrating that BE(2)-C cells provide a useful model system for studying mu, kappa(3) and delta receptors, these studies confirm that kappa(3) receptors represent a pharmacologically distinct receptor class in this cell line.