Limits to the dependence of developing neurons on protein synthesis in their axonal target territory

Our basic question was whether the survival of developing neurons is critically dependent on the level of protein synthesis in the axonal target region. The experiments were carried out on the projection from the isthmo-optic nucleus (ION) to the contralateral retina in chick embryos. The ION is known to undergo almost 60% neuronal death between embryonic days (E) 12 and E17 and to be critically dependent on the retina for trophic support throughout this period and shortly afterwards. Various concentrations of the protein synthesis inhibitor cycloheximide were infused into one eye from E15 to E19. Moderate inhibition (up to about 40%) of retinal protein synthesis, which did not lead to retinal degeneration, had no detectable effects on the number of neurons, nor on the general morphology, in the ION. Only when the inhibition was as high as 50%, leading to widespread degeneration in the retina, did massive degeneration occur also in the ION. It was also shown that a single intraocular injection of cycloheximide at E15 that inhibited retinal protein synthesis by as much as 70-90% during the subsequent 24 h had little effect on the ION in embryos fixed at E19. These results indicate that although the ION neurons are critically dependent on the retina, they can resist major reductions in the level of retinal protein synthesis, which argues against the widespread belief that neuronal survival during development is regulated by the limited production of trophic molecules in the axonal target area. The data are, however, compatible with alternative hypotheses. Most plausibly, survival may be regulated by limited access to a nonlimiting supply of trophic molecules.