Dissociation between intracellular Ca2+ and modulation of [3H]noradrenaline release in chick sympathetic neurons

1. We studied the relation between cyclic AMP, intracellular Ca2+ concentration and release of [3H]noradrenaline ([3H]NA) in sympathetic neurons cultured from chick embryos. 2. Forskolin (10 microM) and vasoactive intestinal polypeptide (VIP, 3 microM) increased cellular levels of cyclic AMP 8- and 3-fold, respectively, either in the absence or presence of electrical stimulation. Electrical stimulation (1 Hz for 10 s) alone had no effect on cyclic AMP levels. 3. Electrically evoked (1 Hz for 10 s) release of [3H]NA was facilitated by 10 microM-forskolin, 3 microM-VIP and by the non-hydrolysable cyclic AMP analogue, 8-bromoadenosine 3': 5'-cyclic monophosphate (8-Br-cyclic AMP). The inactive analogue of forskolin, dideoxyforskolin, had no effect on [3H]NA release. 4. The stimulation-evoked release of [3H]NA was completely inhibited by the neuronal blocking drugs guanethidine (1 microM) and bretylium (3 microM). 5. Whole-cell voltage-clamp studies showed that forskolin and VIP did not facilitate and guanethidine and bretylium did not block voltage-activated Ca2+ currents in the cell bodies of sympathetic neurons. 6. Fluorescence measurements using the Ca(2+)-sensitive dye Indo-1 revealed that forskolin and guanethidine had no effect on the electrically stimulated increase in intracellular Ca2+ concentration recorded from the cell bodies and the growth cones. 7. We conclude that release of [3H]NA can be enhanced or blocked without affecting the increase in intracellular Ca2+ concentration produced by electrical stimulation. Therefore, it is possible that pharmacological agents enhance or depress the release of [3H]NA by acting on steps of exocytosis that are down-stream from Ca2+ mobilization.