MU AND DELTA-OPIOID AGONISTS AT LOW CONCENTRATIONS DECREASE VOLTAGE-DEPENDENT K+ CURRENTS IN F11 NEUROBLASTOMA X DRG NEURON HYBRID-CELLS VIA CHOLERA TOXIN-SENSITIVE RECEPTORS

In a previous study, we showed that muM concentrations of u or delta opioid agonists increase voltage-dependent outward K+ currents in neuroblastoma x DRG neuron hybrid F11 cells via pertussis toxin-sensitive receptors. The present study demonstrates 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 decreases in K+ current as did the cholera toxin subunits A or B (ca. 1 nM). Furthermore, the specific mu opioid receptor antagonist, beta-funaltrexamine (5 nM) blocked the decrease by DAGO and the specific delta antagonist, naltrindole (1 nM) blocked that by DPDPE. Acute GM1 ganglioside (1 muM) treatment markedly enhanced the efficacy of opioid-induced decrease in K+ current. After treating the cells with pertussis toxin (1 mug/ml) for 2 days or more, these opioids decreased the K+ current even when tested at concentrations as high as 1 muM. These results indicate that the decrease in K+ current elicited in F11 cells by low concentrations of mu and delta 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.