Nicotinic ACh receptor blockade and spontaneous nerve cell death in various brain regions.

Transmission blockade has been performed with alpha- and beta-bungarotoxin and with d-tubocarine in 10-to 18-day-old chick embryos. The naturally occurring nerve cell death in the trochlear nucleus and some other motor cranial nerve nuclei has been prevented after postsynaptic blockade with alpha-BTX or dTC, but an increased cell death has been found after presynaptic blockade with beta-BTX. The ultrastructural appearance and some cytological parameters of the motoneurons are comparable with these neurons in controls, but the Golgi preparations show a significant decrease in dendritic branching after postsynaptic blockade. As demonstrated by HRP labelling, the motoneurons have maintained contacts with their peripheral target even after 8 days of postsynaptic blockade. Distinct alterations have been observed in the retina, the tectum and in some other visual brain regions by quantitative histological, ultrastructural and autoradiographic methods. A hypothesis is proposed explaining the observed changes as a common mechanism effective in the ontogenesis and the regeneration of neuromuscular systems.