Calcium‐Dependent and ‐Independent Acetylcholine Release from Electric Organ Synaptosomes by Pardaxin: Evidence of a Biphasic Action of an Excitatory Neurotoxin

Abstract: The effect of pardaxin, a new excitatory neurotoxin, on neurotransmitter release was tested using purely cholinergic synaptosomes of Torpedo marmorata electric organ. Pardaxin elicited the release of acetylcholine with a biphasic dose dependency. At low concentrations (up to 3 × 10⁻⁷M), the release was calcium‐dependent and synaptosomal structure was well preserved as revealed by electron microscopy and measurements of occluded lactate dehydrogenase activity. At concentrations from 3 × 10⁻⁷M to 10⁻⁵M, the pardaxin‐induced release of acetylcholine was independent of extracellular calcium, and occluded synaptosomal lactate dehydrogenase activity was lowered, indicating a synaptosomal membrane perturbation. Electron microscopy of 10⁻⁶M pardaxin‐treated synaptosomes revealed nerve terminals depleted of synaptic vesicles and containing cisternae. At higher toxin concentrations ( 10⁻⁵M), there were striking effects on synaptosomal morphology and occluded lactate dehydrogenase activity, suggesting a membrane lytic effect. We conclude that, at low concentrations, this neurotoxin is a promising tool to investigate calcium‐dependent mechanisms of neurotransmitter release in the nervous system.