Sodium dependent 3H-noradrenaline release from rat neocortical slices in the absence of extracellular calcium: presynaptic modulation by mu-opioid receptor and adenylate cyclase activation.

In Ca2+-free EGTA (1 mmol/l)-containing medium veratrine (3 mumol/l) and ouabain (100 mumol/l) strongly enhanced the efflux of 3H-noradrenaline from superfused rat brain neocortical slices prelabelled with the radioactive amine. In both cases 3H-noradrenaline release was prevented by tetrodotoxin (1 mumol/l). These effects of veratrine and ouabain were virtually additive and independent of whether the noradrenaline uptake carrier was blocked with 1 mumol/l desipramine or not. The adenylate cyclase activator forskolin (10 nmol/l - 10 mumol/l) strongly enhanced veratrine- and ouabain-induced 3H-noradrenaline release, without affecting spontaneous tritium efflux. The release induced by both stimuli was profoundly inhibited by the selective mu-opioid receptor agonist [D-Ala, MePhe4, Gly-ol5]enkaphalin (DAGO, 3 nmol/l - 1 mumol/l) in a concentration-dependent manner. The inhibitory effects of 1 mumol/l DAGO were abolished by 1 mumol/l naloxone. On the other hand, preincubation of the slices for 1 h with the delta-opioid receptor-selective irreversible ligand fentanyl isothiocyanate (1 mumol/l) did not change the inhibitory effects of DAGO. These data show that veratrine- and ouabain-induced 3H-noradrenaline release from central noradrenergic nerve terminals is facilitated by increasing intracellular cyclic AMP levels and reduced by activation of presynaptic mu-opioid receptors, indicating the involvement of exocytotic neurotransmitter release. The results provide further evidence for the hypothesis that under these conditions neurotransmitter release from central noradrenergic neurons is triggerred by a Na+-induced efflux of Ca2+ ions from intracellular stores.