Early retrograde effects of blocking axoplasmic transport in the axons of developing neurons.

Depriving developing neurons of retrograde trophic support may disrupt their development and often causes them to die. We here report the effects, in chick embryos, of eliminating retrograde support in the isthmo-optic projection by blocking axoplasmic transport in the terminal parts of the axons, which is known ultimately to kill the isthmo-optic neurons. Within only 9 h, this had perturbed the process of cellular reorganisation that eventually leads to the laminated appearance of the mature isthmo-optic nucleus. Neuron survival in the isthmo-optic nucleus was affected even more quickly, but the earliest change, occurring in as little as 3 h, was not an increase in the number of dying neurons, but a decrease below control values. This novel effect was still present at 6 and 9 h after the injection, but at longer survival times the number of dying neurons increased well above control values as expected. Our interpretation of the transient decrease in neuronal death is that retrograde trophic signals include both death-promoting and life-promoting components, and that the former act faster in this system.