Stage-specific activity patterns affect motoneuron axonal retraction and outgrowth during the metamorphosis of Manduca sexta.

During the metamorphosis of holometabolous insects, most larval muscles and sensory neurons are replaced by new adult elements, whereas most motoneurons persist and are remodelled to serve new adult functions. In Manduca sexta, the formation of the anlagen of the adult dorsal longitudinal flight muscle (DLM) is characterized by retraction of axonal terminals and dendrites of persisting larval motoneurons, partial target muscle degeneration and myoblast accumulation during late larval life. Most of these structural changes have been attributed to hormonal control, not only because ecdysteroids govern metamorphosis, but also because motoneurons express ecdysteroid receptors and experimental manipulations of ecdysteroid titres perturb normal development. To test whether activity-dependent mechanisms also came into play, chronic extracellular recordings were conducted in vivo from the five future DLM motoneurons throughout the last 3 days of larval life. Motoneuron activity is regulated developmentally. The types of motoneurons recruited, the number of motor spikes and the duration of bursts change in a stereotypical fashion during different stages, indicating an internal control of motor activity. A characteristic cessation in the activity of the five future DLM motoneurons coincides in time with the retraction of their dendrites and their terminal arborizations, whereas their activation during ecdysis coincides with the onset of new outgrowth. Inducing advanced activity by stimulating the motoneurons selectively with ecdysis-like patterns results in significant outgrowth of their terminal arborizations. Therefore, steroids might act in concert with activity-dependent mechanisms during the postembryonic modifications of neuromuscular systems.