Selective effects of neuronal-synaptobrevin mutations on transmitter release evoked by sustained versus transient Ca2+ increases and by cAMP

Synaptobrevin is a key constituent of the synaptic vesicle membrane. The ne uronal-synaptobrevin (n-syb) gene in Drosophila is essential for nerve-evok ed synaptic currents, but miniature excitatory synaptic currents (mESCs) re main even in the complete absence of this gene. To further characterize the defect in these mutants, we have examined conditions that stimulate secret ion. Despite the inability of an action potential to trigger fusion, high K + saline could increase the frequency of mESCs 4- to 17-fold in a Ca2+-depe ndent manner, and the rate of fusion approached 25% of that seen in wild-ty pe synapses under the same conditions. Similarly, the mESC frequency in n-s yb null mutants could be increased by a Ca2+ ionophore, A23187, and by blac k widow spider venom. Thus, the ability of the vesicles to fuse in response to sustained increases in cytosolic Ca2+ persisted in the absence of this protein, Tetanic stimulation could also increase the frequency of mESCs, pa rticularly toward the end of a train and after the train of stimuli. In con trast, these mutants did not respond to an elevation of cAMP induced by an activator of adenylyl cyclase, forskolin, or a membrane-permeable analog of cAMP, dibutyryl cAMP, which in wild-type synapses causes a marked increase in the mESC frequency even in the absence of external Ca2+, These results are discussed in the context of models that invoke a special role for n-syb in coupling fusion to the transient, local changes in Ca2+ and an as yet u nidentified target of cAMP.