Steroid regulation of neuronal death in the moth nervous system

The emergence of the adult Manduca sexta moth is followed by the programmed degeneration of about 50% of the insect's abdominal interneurons and motoneurons. Neurons in implanted ganglia undergo degeneration in concert with neurons of the host, thereby indicating that a hormonal cue is important. The neuronal death follows the normal decline in the levels of the steroid hormones, ecdysteroids, that occurs at the end of metamorphosis. Manipulations that cause a precocious ecdysteroid decline result in an early death of the neurons. Also, prolongation of the ecdysteroid titer extend the life of the neurons. By properly timed steroid application it was possible to dissociate the neuronal degeneration from the behavioral events and muscle degeneration that also occur at emergence. Also the stereotyped sequence of death among identified motoneurons could be stopped at various points in the sequence by the carefully timed application of steroid. It was concluded that the signal for neuronal death was the withdrawal of ecdysteroids at the end of metamorphosis and that the death was probably a direct response of the individual neurons to the hormone withdrawal.