We are using Drosophila larval locomotion as a model to elucidate the working principle of motor circuit. Motoneurons and sensory neurons are well characterized in this system. However, little is known about the identity of the interneurons involved and how the interneurons regulate larval locomotion.
Here, we searched for cholinergic interneurons that regulate larval locomotion. Although previous studies showed that pre-motor excitatory interneurons are cholinergic, their identity is unknown. We generated Gal4 lines which express Gal4 under the control of various regulatory regions of the Cha (Choline acetyltransferase) gene. Using calcium imaging with G-CaMP, we found that one of these lines, Cha12-Gal4, expresses Gal4 in a small subset of interneurons including two types of interneurons whose activity patterns were correlated with forward locomotion. We tentatively named these neurons O (oscillatory) neurons and W (wave-like) neurons, because they showed oscillatory and wave-like activity related to peristaltic locomotion, respectively. O neurons were located in a posterior-most neuromere and extended a long axon anteriorly towards a thoracic neuromere. They were activated prior to the initiation of motor wave during forward peristalsis. W neurons were present in each abdominal segment and extended neurites towards a dorsal neuropile region covering 2 neuromeres. They were activated sequentially from the posterior to anterior segments in a manner correlated with the activity propagation of motoneurons. We screened a publically available Gal4 collection (from Janelia Farm) for those specific to O neurons or W neurons, and identified a single line specific to W neurons. Optogenetic activation of W neurons using this Gal4 line resulted in contraction of the abdominal body muscles, suggesting that W neurons activate motor neurons. We are now performing more detailed functional and anatomical analyses to further study the role of W neurons in larval locomotion.