Opioids acting through delta receptors elicit a transient increase in the intracellular free calcium concentration in dorsal root ganglion-neuroblastoma hybrid ND8-47 cells.

The neuronal cell line ND8-47 (neuroblastoma x dorsal root ganglion neuron hybrid) expressed opioid delta-type receptors. We report opioid-induced changes in cytosolic intracellular free calcium ([Ca++]i) in differentiated ND8-47 cells. Delta-opioid receptor agonists induced a transient (< 2 min) increase in [Ca++]i in a concentration-dependent fashion with the potency order: [D-Ser2,Leu5]enkephalin-Thr (DSLET) > or = deltorphin II > [D-Pen2,5] enkephalin. Their effects were blocked by naloxone (IC50 = 20 nM) and naltrindole (IC50 = 2.5 nM). Selective mu and kappa receptor agonists had no effect on [Ca++]i. The subtype specific delta receptor antagonists, 7-benzylidene naltrexone (delta-1) and naltriben (delta-2), were used to characterize further the subtype of delta receptors mediated by this response. Naltriben was more potent than 7-benzylidene naltrexone in antagonizing the DSLET-induced increase in [Ca++]i. The increase in [Ca++]i induced by DSLET was blocked by nifedipine (1 microM) or verapamil (1 microM), and was not observed in the absence of external calcium. Changes in [Ca++]i also were measured in single ND8-47 cells. The percentage of cells responding to DSLET (1 microM), deltorphin-II (1 microM) and [D-Pen2,5]enkephalin (1 microM) were 86, 84 and 37%, respectively. The results suggest that an increase in [Ca++]i induced by opioids is mediated through opioid delta receptors which can activate dihydropyridine-sensitive Ca++ channels.