Alterations in spontaneous transmitter release by divalent cations after treatment of the neuromuscular junction withβ-bungarotoxin

1. Spontaneous transmitter release was studied at the frog sartorius neuromuscular junction in the presence of a variety of cations before and after treatment with the specific presynaptic neurotoxin,β-bungarotoxin (β-BuTX). 2. Treatment withβ-BuTX produced a maintained increase in spontaneous release, as indicated by the miniature end-plate potential (m.e.p.p.) frequency. It was demonstrated that the m.e.p.p. frequency remained dependent on the extracellular calcium concentration. 3. A 30 mM increase in extracellular sodium chloride produced a reversible increase in frequency only afterβ-BuTX treatment, indicating thatβ-BuTX had increased the permeability of the presynaptic terminal. 4. Furthermore, several divalent cations other than calcium were shown to either maintain or greatly increase the m.e.p.p. frequency afterβ-BuTX treatment (before toxin treatment replacement of calcium by these divalent cations produced only small changes in frequency). The relative effectiveness of the divalent cations tested in increasing spontaneous transmitter release after toxin treatment was Co2+ ≃ Ni2+ > Mg2+ > Ca2+≃ Sr2+ > Mn2+. The effect of cobalt, which increased the m.e.p.p. frequency 6.5 times after toxin treatment, was studied in detail. 5. It is proposed thatβ-BuTX, through its phospholipase activity, increases the ionic permeability of the terminal membrane and allows access to intracellular sites of relatively impermeant cations. This allowed us to demonstrate that several divalent cations other than calcium can influence transmitter release either directly at release sites or by altering internal calcium buffering.