Ultrastructural changes accompanying the disturbances of neuromuscular transmission caused by Pardachirus toxin

Toxic secretion from the Red Sea flatfish Pardachirus marmoratus (Soleidae) was applied to frog neuromuscular preparations while intracellular recording was done with microelectrodes. Endplates from treated preparations were examined in the electron microscope. Lyophilized Pardachirus toxin (0.01 mg/ml) caused a sudden great increase in spontaneous transmitter release lasting 20-120 min, a short-lived increase in the size of neurally evoked responses, and a short period (1-3 min) of spontaneous muscle tremor which could not be blocked completely by d-tubocurarine. Neuromuscular junctions viewed in the electron microscope revealed lamellar and occasionally concentric arrangements of Schwann cells surrounding and intruding into nerve terminals, while active zones on the presynaptic membrane remained in place opposite junctional folds. Synaptic vesicle aggregation and great density of neurofilaments were in evidence in many terminals, while muscle structure in preparations treated with higher concentrations (0.1 mg/ml) was radically disrupted.