MODULATION OF ACETYLCHOLINE-RELEASE AT MOUSE NEUROMUSCULAR-JUNCTIONS BY INTERACTION OF 3 HOMOLOGOUS SCORPION TOXINS WITH K+ CHANNELS

1 The effects of three scorpion toxins, charybdotoxin (CTX), iberiotox in (IbTX), and noxiustoxin (NTX) have been studied on acetylcholine re lease and on K+ channels by means of twitch tension and electrophysiol ogical recording techniques using isolated skeletal muscle preparation s and by a radioligand binding assay using I-125-labelled dendrotoxin I (DpI) and rat brain synaptosomal membranes. 2 On chick biventer cerv icis preparations, CTX and IbTX (125 nM) augmented the twitch response s 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 b eing 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) increase d 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 sin gle shock nerve stimulation after 40-50 min. 4 Extracellular recording of nerve terminal current waveforms in triangularis sterni preparatio ns revealed that CTX and IbTX (3-100 nM), but not NTX (100 nM), blocke d the Ca2+-activated K+ current, I-K-Ca. However, there was no major c hange in the portion of the nerve terminal waveform associated with vo ltage-dependent K+ currents, I-Kv. 5 In the radioligand binding assay, NTX potently displaced labelled [I-125]-DpI, whereas CTX produced onl y partial displacement. However, IbTX did not displace [I-125]-DpI fro m its binding sites on rat brain synaptosomal membranes. 6 We conclude that these three structurally homologous scorpion toxins act on diffe rent K+ channels and that this leads to different patterns of facilita tion of acetylcholine release. IbTX acts selectively on high conductan ce 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 IbT X and NTX; CTX acts on both Ca2+- and voltage-dependent K+ channels.