Regulation of calcium-dependent [ 3H]noradrenaline release from rat cerebrocortical synaptosomes by protein kinase C and modulation of the actin cytoskeleton

Abstract

The effects that active phorbol esters, staurosporine, and changes in actin dynamics, might have on Ca 2+-dependent exocytosis of [ 3H]-labelled noradrenaline, induced by either membrane-depolarizing agents or a Ca 2+ ionophore, have been examined in isolated nerve terminals in vitro. Depolarization-induced openings of voltage-dependent Ca 2+ channels with 30 mM KCl or 1 mM 4-aminopyridine induced limited exocytosis of [ 3H]noradrenaline, presumably from a readily releasable vesicle pool. Application of the Ca 2+ ionophore calcimycin (10 μM) induced more extensive [ 3H]noradrenaline release, presumably from intracellular reserve vesicles. Stimulation of protein kinase C with phorbol 12-myristate,13-acetate increased release evoked by all secretagogues. Staurosporine (1 μM) had no effect on depolarization-induced release, but decreased ionophore-induced release and reversed all effects of the phorbol ester. When release was induced by depolarization, internalization of the actin-destabilizing agent DNAase I into the synaptosomes gave a slight increase in [ 3H]NA release and strongly increased the potentiating effect of the phorbol ester. In contrast, when release was induced by the Ca 2+ ionophore, DNAase I had no effect, either in the absence or presence of phorbol ester. The results indicate that depolarization of noradrenergic rat synaptosomes induces Ca 2+-dependent release from a releasable pool of staurosporine-insensitive vesicles. Activation of protein kinase C increases this release by staurosporine-sensitive mechanisms, and destabilization of the actin cytoskeleton further increases this effect of protein kinase C. In contrast, ionophore-induced noradrenaline release originates from a pool of staurosporine-sensitive vesicles, and although activation of protein kinase C increases release from this pool, DNAase I has no effect and also does not change the effect of protein kinase C. The results support the existence of two functionally distinct pools of secretory vesicles in noradrenergic CNS nerve terminals, which are regulated in distinct ways by protein kinase C and the actin cytoskeleton.