Characterisation of the effects of depolarising toxins on nerve terminal action potentials: apparent block of presynaptic potassium currents

Abstract

Previous studies showed that toxic phospholipases A 2 (Pa-8 and Pa-10F) from the venom of Pseudechis australis, the Australian king brown snake, reduced acetylcholine release at mouse neuromuscular junctions and depressed motor nerve terminal action potentials [ Fatehi et al. (1994a), Toxicon 32, 1559–1572], and it was postulated that these toxins induced their effect on the action potential waveforms through nerve terminal depolarisation. To test this hypothesis, the effects of Pa-11 (another phospholipase A 2 from the venom of Pseudechis australis), and the known depolarising agents, myotoxin a, from the venom of the rattlesnake, Crotalus viridis viridis, and ouabain on these waveforms were compared with the changes induced in the nerve terminal action potentials by Pa-8 and Pa-10F. The experiments were performed on the isolated mouse triangularis sterni preparation, using extracellular recordings. Pa-11 (0.1 μM) decreased the component of nerve terminal action potential related to Na + and K + currents to about 80% and 40% of control, respectively, after 60 min. Myotoxin a (5 μM) and ouabain (50 μM) produced similar, time-dependent changes in the nerve terminal action potential. These effects are similar to those produced by Pa-8 and Pa-10F, and are consistent with a slow but partial loss of membrane potential at the nerve terminal. In addition, whole-cell patch-clamp recording was employed to investigate possible direct actions of Pa-8, Pa-10F and Pa-11 on Na + and K + currents in NG108 and PC12 cells in culture. None of these toxins (0.8 μM) reduced the Na + and K + currents in these cells. There was also no displacement of [ 125I] α-dendrotoxin bound to voltage-sensitive potassium channels on rat synaptosomal membranes induced by Pa-8, Pa-10F and Pa-11 (up to 100 μM). These results support the hypothesis that the alteration of nerve terminal waveforms by these toxic phospholipases A 2 is mediated by nerve terminal depolarisation.