Abstract

Snake toxins are a rich source of secreted phospholipases A₂ (sPLA₂), some of which are presynaptically neurotoxic (β-neurotoxic). Ammodytoxin (Atx) is a β-neurotoxic sPLA₂ from the venom of the most dangerous of European vipers, the long-nosed viper (Vipera ammodytes ammodytes). As the principal toxin of this snake’s crude venom, it is primarily responsible for the neurotoxic signs and symptoms seen in many victims of snake-bite. Its mode of action is expressed as a disruption of neuromuscular transmission at the motor end-plates.

Atx acts presynaptically at the neuromuscular (NM) junctions and is probably internalised into nerve terminals of motoneurons. It appears that internalisation of AtxA occurs via vesicular structures where binding to M-type sPLA₂ receptor (R180) is not likely to be involved, although R180 is present on the presynaptic membrane of motoneurons. β-neurotoxic action of Atx is most probably a consequence of its intracellular actions. Neurosecretion may be affected by the binding of Atx to calmodulin in the cytosol of the nerve terminal. The two proteins were shown to interact under cytosol-like conditions. Besides R180 there must be additional unknown receptors for Atx on the presynaptic membrane that are capable of concentrating it in the presynaptic region. These receptors are therefore of critical importance for the expression of the toxin’s β-neurotoxicity.

Atx causes a depletion of synaptic vesicles (SVs) from motor nerve terminals. However, the reduced number of SVs near active zones is not likely to be of crucial importance in achieving the complete blockade of indirectly elicited twitch responses of muscle. The absence of Ω-shaped profiles from the terminal boutons suggests that the recycling of SVs is not blocked at the stage of their retrieval from axolemma by endocytosis. We conclude that Atx induces loss of coupling between action potential induced depolarisation of the nerve terminal and evoked transmitter release. Endocytosis could be blocked at later stages as a secondary event in Atx-induced neuromuscular paralysis. Atx causes no disaggregation of axonal neurofilaments. On the other hand, nerve terminal mitochondria are swollen and damaged. Therefore, the reason for neurotoxic action of Atx could be a reduction in ATP levels, with de-energisation of the terminal. Last but not least, Atx could only have an influence on the activity-dependent pool of SVs. It does not affect the spontaneously recycling vesicles that are responsible for miniature end-plate potentials (MEPPs), since neither frequency nor amplitude of the MEPPs are changed after Atx action on the NM preparation.