Selective and interactive down-regulation of μ- and δ-opioid receptors in human neuroblastoma SK-N-SH cells

Human neuroblastoma SK-N-SH cells, which contain both μ- and δ-opioid receptors, were grown under conditions that provided a μ:δ ratio of 1.5:1. Both receptors were down-regulated after 72 hr of exposure to 100 nM etorphine. Selective down-regulation was demonstrated using selective opioid agonists; the μ agonist Tyr-D-Ala²-Gly-(Me)Phe⁴-Gly-ol down-regulated μ- but not δ-opioid receptors, whereas prolonged exposure to the selective δ agonist D-Pen²,D-Pen⁵-enkephalin resulted in δ- but not μ-opioid receptor down-regulation. Morphine, which binds μ- as well as δ-opioid receptors, down-regulated both receptor subtypes. NG108-15 cells, which contain δ receptors exclusively, were also tested. NG108-15 cells did not exhibit δ-opioid receptor down-regulation when exposed to morphine. The discrepancy between the effect of chronic morphine treatment on δ receptors in SK-N-SH cells and in NG108-15 cells raised the question of whether the coexistence of μ receptors in the former allowed morphine to down-regulate δ receptors. The role of μ-opioid receptors in morphine-induced δ receptor down-regulation was studied by using the irreversible μ antagonist β-funaltrexamine. Pretreatment of SK-N-SH cells with β-funaltrexamine prevented down-regulation of δ receptors in response to chronic exposure to morphine but did not affect down-regulation of δ receptors in response to D-Pen²,D-Pen⁵-enkephalin. The experimental data indicate that morphine-induced δ-opioid receptor down-regulation is dependent on the presence of functional μ receptors in the same cell.