Beta-bungarotoxin lacking phospholipase activity is not toxic to developing motor neurons and skeletal muscle.

Effects of beta-bungarotoxin (beta-BTX) with enzymatically active and inactive phospholipase on the developing trochlear motor neurons and the superior oblique muscle of the duck embryo were examined. beta-BTX produced massive and widespread degeneration of neurons and skeletal muscle. When embryos were treated with beta-BTX lacking phospholipase activity the motor neurons and the skeletal muscle appeared essentially normal both at the light and the electron microscope level. These observations suggest that the toxic effects of the beta-BTX may be attributed to the phospholipase component. In the absence of phospholipase activity the toxin retained the ability to block neuromuscular transmission and when applied during the period of normal cell death it increased the number of trochlear motor neurons by sparing them from embryonic cell death.