Insect systems for the study of programmed neuronal death.

In the moth Manduca sexta and the fly Drosophila melanogaster, the emergence of the adult is followed by a period of neuronal death. The steroid hormones, the ecdysteroids, are involved in two aspects of this death. Ecdysteroid action early in metamorphosis is somehow essential for fixing the fates of the neurons that will die. The actual triggering of their death, however, occurs a few weeks later and requires the withdrawal of the steroid and, at least in some cases, a trans-synaptic signal. These "execution" events involve both RNA and protein synthesis and result in the onset of degeneration about 10 h later. The early "decision" of a neuron to opt for the degeneration fate is associated with an altered pattern of ecdysteroid receptor expression. A model is presented to relate how the levels of steroid receptors may be related to the activation of death-related genes.