Synapse loss and axon retraction in response to local muscle degeneration.

During metamorphosis in the moth, Manduca sexta, the abdominal body-wall muscle DEO1 is remodeled to form the adult muscle DE5. As the larval muscle degenerates, its motoneuron loses its end plates and retracts axon branches from the degenerating muscle. Muscle degeneration is under the control of the insect hormones, the ecdysteroids. Topical application of an ecdysteroid mimic resulted in animals that produced a localized patch of pupal cuticle. Muscle fibers underlying the patch showed a gradient of degeneration. The motoneuron showed end-plate loss and axon retraction from degenerating regions of a given fiber but maintained its fine terminal branches and end plates on intact regions. The results suggest that local steroid treatments that result in local muscle degeneration bring about a loss of synaptic contacts from regions of muscle degeneration.