μ and δ opioid agonists at low concentrations decrease voltage-dependent K + currents in F11 neuroblastoma × DRG neuron hybrid cells via cholera toxin-sensitive receptors

Abstract

In a previous study, we showed that μM concentrations of mu or delta opioid agonists increase voltage-dependent outward K + currents in neuroblastoma × DRG neuron hybrid F11 cells via pertussis toxin-sensitive receptors. The present study demostrates that much lower concentrations (fM to nM) of these opioids (DAGO and DPDPE) decreased voltage-dependent outward K + currents during step depolarization. The opioid antagonist, naloxone (3 nM) prevented these decreasesin K + current as did the cholera toxin subunits A or B (ca. 1 nM). Furthermore, the specific μ opioid receptor antagonist, β-funaltrexamine (5 nM) blocked the decrease by DAGO and the specific δ antagonist, naltrindole (1 nM) blocked that by DPDPE. Acute GM1 ganglioside (1 μM) treatment markedly enhanced the efficacy of opioid-induced decrease in K + current. After treating the cells with pertussis toxin (1 μg/ml) for 2 days or more, these opioids decreased the K + current even when tested at concentrations as high as 1 μM. These results indicate that the decrease in K + current elicited in F11 cells by low concentrations of μ and δ opioid agonists resembles the opioid-induced prolongation of the action potential duration and decrease in voltage-dependent K + conductance that occur in DRG neurons in primary cultures. The F11 cell line provides therefore a valuable model system for correlative pharmacologic, electrophysiologic and biochemical analyses of G s-coupled, GM1 ganglioside-regulated excitatory opioid receptor functions, in addition to G i/G o-coupled inhibitory receptor functions, in sensory neurons.