Effects of staurosporine on Ca2+-dependent and Ca2+-independent [14C]GABA release from rat brain synaptosomes

The effect of a protein kinase inhibitor, staurosporine, on Ca²⁺-dependent and Ca²⁺-independent release of ¹⁴C]GABA in isolated rat brain synaptosomes was studied. Calcium-dependent ¹⁴C]GABA release was stimulated by depolarization with a K⁺ channel blocker, 4-aminopyridine (4-AP), or high K⁺ concentration. It has been shown that the effect of 4-AP is Ca²⁺-dependent, while high K⁺ is able to evoke ¹⁴C]GABA release in both Ca²⁺-dependent and Ca²⁺-independent manners. In addition, Ca²⁺-independent ¹⁴C]GABA release was studied using α-latrotoxin (LTX) as a tool. Pretreatment of synaptosomes with staurosporine resulted in pronounced inhibition of 4-AP-stimulated Ca²⁺-dependent ¹⁴C]GABA release. The inhibitory effect of staurosporine on ¹⁴C]GABA release was not due to modulation of 4-AP-promoted⁴⁵Ca²⁺ influx into synaptosomes. If the process of ¹⁴C]GABA release occurred in the Ca²⁺-independent manner irrespectively of what, LTX or high K⁺, stimulated this process, it was not inhibited by staurosporine. Considering the above findings, it is reasonable to assume that the absence of Ca²⁺ in the extracellular medium created conditions for activation of the process of neurotransmitter release without Ca²⁺-dependent dephosphorylation of neuronal phosphoproteins; as a consequence, regulation of exocytotic process was modulated in such a manner that inhibition of protein kinases did not disturb exocytosis.