α-Latrotoxin affects mitochondrial potential and synaptic vesicle proton gradient of nerve terminals

Ca2+-independent [3H]GABA release induced by α-latrotoxin was found to consist of two sequential processes: a fast initial release realized via exocytosis and more delayed outflow through the plasma membrane GABA transporters [Linetska, M.V., Storchak, L.G., Tarasenko, A.S., Himmelreich, N.H., 2004. Involvement of membrane GABA transporters in α-latrotoxin-stimulated [3H]GABA release. Neurochem. Int. 44, 303–312]. To characterize the toxin-stimulated events attributable to the transporter-mediated [3H]GABA release from rat brain synaptosomes we studied the effect of α-latrotoxin on membrane potentials and generation of the synaptic vesicles proton gradient, using fluorescent dyes: potential-sensitive rhodamine 6G and pH-sensitive acridine orange. We revealed that α-latrotoxin induced a progressive dose-dependent depolarization of mitochondrial membrane potential and an irreversible run-down of the synaptic vesicle proton gradient. Both processes were insensitive to the presence of cadmium, a potent blocker of toxin-formed transmembrane pores, indicating that α-latrotoxin-induced disturbance of the plasma membrane permeability was not responsible to these effects. A gradual dissipation of the synaptic vesicle proton gradient closely coupled with lowering the vesicular GABA transporter activity results in a leakage of the neurotransmitter from synaptic vesicles to cytoplasm. As a consequence, there is an essential increase in GABA concentration in a soluble cytosolic pool that appears to be critical parameter for altering the mode of the plasma membrane GABA transporter operation from inward to outward. Thus, our data allow clarifying what cell processes underlain a recruitment of the plasma membrane transporter-mediated pathway in α-LTX-stimulated secretion.