Modulation of acetylcholine release at mouse neuromuscular junctions by interaction of three homologous scorpion toxins with K+ channels

Abstract

1 The effects of three scorpion toxins, charybdotoxin (CTX), iberiotoxin (IbTX), and noxiustoxin (NTX) have been studied on acetylcholine release and on K+ channels by means of twitch tension and electrophysiological recording techniques using isolated skeletal muscle preparations and by a radioligand binding assay using 125I‐labelled dendrotoxin I (DpI) and rat brain synaptosomal membranes. 2 On chick biventer cervicis preparations, CTX and IbTX (125 nM) augmented the twitch responses to indirect muscle stimulation. Further, the increase (about 70–80% of control twitch height) was fast in onset, reaching a maximum within 25–30 min. NTX at 125 nM produced a slower augmentation of the twitch responses to indirect muscle stimulation, with the maximum response being seen after 40–50 min. 3 On mouse triangularis sterni preparations, CTX (300 nM after 35–40 min) and IbTX (100 nM after 15 min) increased quantal content of the evoked endplate potentials (e.p.p.) by about two fold. However, NTX (300 nM) caused only a small increase in e.p.p. amplitude, which was followed by repetitive e.p.ps in response to single shock nerve stimulation after 40–50 min. 4 Extracellular recording of nerve terminal current waveforms in triangularis sterni preparations revealed that CTX and IbTX (3‐100nM), but not NTX (100 nM), blocked the Ca2+‐activated K+ current, 7K.ca‐ However, there was no major change in the portion of the nerve terminal waveform associated with voltage‐dependent K+ currents, IKv. 5 In the radioligand binding assay, NTX potently displaced labelled [125I]‐DpI, whereas CTX produced only partial displacement. However, IbTX did not displace [125I]‐DpI from its binding sites on rat brain synaptosomal membranes. 6 We conclude that these three structurally homologous scorpion toxins act on different K+ channels and that this leads to different patterns of facilitation of acetylcholine release. IbTX acts selectively on high conductance Ca2+‐activated K+ channels, leading to an increase in the amplitude of e.p.ps without any other changes. NTX acts on voltage‐dependent K+ channels that are sensitive to dendrotoxin and causes repetitive e.p.ps. CTX shares amino acid residues that exist in the structures of IbTX and NTX; CTX acts on both Ca2+‐ and voltage‐dependent K+ channels. 1995 British Pharmacological Society