Reidentification of larval interneurons in the pupal stage of the tobacco hornworm, Manduca sexta

The abdominal prolegs are the primary locomotory appendages of Manduca sexta larvae. After the prolegs are lost at pupation, some of the proleg motoneurons die while the survivors are respecified to carry out different functions in the adult moth. As a first step toward investigating the process of functional respecification at the synaptic level, we searched for larval interneurons that affected the activity of proleg motoneurons, and followed these interneurons into the pupal stage. Interneurons were judged to be individually identifiable based on their effects on proleg motoneuron activity and their anatomical features. Seven larval interneurons were identified and placed in five physiological classes based on their effects on proleg motoneurons: ipsilateral excitors, contralateral excitors, ipsilateral inhibitors, contralateral inhibitors, and bilateral inhibitor-excitors. Four of the larval interneurons produced apparently monosynaptic postsynaptic potentials in proleg motoneuron. Of the five larval interneurons that were reidentified in the early pupal stage, two showed minor but consistent structural modifications from the larval stage. Interneurons that produced unitary postsynaptic potentials in larval motoneurons continued to do so in pupal motoneurons. These studies demonstrate that individually identified interneurons can be followed through the larval-pupal transformation, during the initial stages of motoneuron respecification.