A role for calcium/calmodulin kinase(s) in the regulation of GABA exocytosis.

A possible role for protein kinases in the regulation of GABA exocytosis in nerve endings was investigated. The effect on the release of the radioactive neurotransmitter ([3H]GABA) from mouse brain synaptosomes of several protein kinase inhibitors was estimated after treatment with 37 mM K+ in the absence of external Na+, a condition under which [3H]GABA release is completely Ca2+ dependent. Among the inhibitors one group inhibit the kinases by the catalytic site (i.e. staurosporine and H7) and others (TFP, sphingosine and W7) act on the regulatory site of protein kinases. The compounds of the second group, which are reported to inhibit calmodulin dependent events and the increase in cytosolic Ca2+ (Cai) induced by high K+ depolarization, were the most efficient inhibitors of [3H]GABA release. The selective inhibitor of CaMPK II, KN-62, also markedly diminished [3]GABA release as well as the increase in Cai induced by high K+. The kinase inhibitors from the first group that are unable to diminish the increase in Cai induced by high K+ were also less efficient inhibitors of [3H]GABA release even at high concentrations. The present results indicate that at the doses tested all the drugs inhibit to some extent the release of the Ca2+ dependent fraction of [3H]GABA perhaps by inhibiting a CaMPK II mediated phosphorylation step triggered by depolarization and facilitated by the elevation of Cai. In addition, the second group of antagonists and KN-62 inhibit the elevation of Cai to high K+ thus exhibiting a higher efficiency on [3H]GABA release than the first group of antagonists.