A subset of interneurons required for Drosophila larval locomotion.

Efforts to define the neural circuits generating locomotor behavior have produced an initial understanding of some of the components within the spinal cord, as well as a basic understanding of several invertebrate motor pattern generators. However, how these circuits are assembled during development is poorly understood. We are defining the neural circuit that generates larval locomotion in the genetically tractable fruit fly Drosophila melanogaster to study locomotor circuit development. Forward larval locomotion involves a stereotyped posterior-to-anterior segmental translocation of body wall muscle contraction and is generated by a relatively small number of identified muscles, motor and sensory neurons, plus an unknown number of the ~270 bilaterally-paired interneurons per segment of the 1st instar larva. To begin identifying the relevant interneurons, we have conditionally inactivated synaptic transmission of interneuron subsets and assayed for the effects on locomotion. From this screen we have identified a subset of 25 interneurons per hemisegment, called the lateral locomotor neurons (LLNs), that are required for locomotion. Both inactivation and constitutive activation of the LLNs disrupt locomotion, indicating that patterned output of the LLNs is required. By expressing a calcium indicator in the LLNs, we found that they display a posterior-to-anterior wave of activity within the CNS corresponding to the segmental translocation of the muscle contraction wave. Identification of the LLNs represents the first step toward elucidating the circuit generating larval locomotion.