Localization of muscarinic acetylcholine receptor dependent rhythm generating modules in the Drosophila larval locomotor network

Mechanisms of rhythm generation have been extensively studied in motor systems that control locomotion over terrain in limbed animals; however, much less is known about rhythm generation in soft-bodied terrestrial animals. Here we explored how muscarinic acetylcholine receptor (mAChR) dependent rhythm generating networks are distributed in the central nervous system (CNS) of soft-bodied Drosophila larvae. We measured fictive motor patterns in isolated CNS preparations using a combination of Ca2+ imaging and electrophysiology while manipulating mAChR signalling pharmacologically. Bath application of the mAChR agonist oxotremorine potentiated rhythm generation in distal regions of the isolated CNS, whereas application of the mAChR antagonist scopolamine suppressed rhythm generation in these regions. Oxotremorine raised baseline Ca2+ levels and potentiated rhythmic activity in isolated posterior abdominal CNS segments as well as isolated anterior brain and thoracic regions, but did not induce rhythmic activity in isolated anterior abdominal segments. Bath application of scopolamine to reduced preparations lowered baseline Ca2+ levels and abolished rhythmic activity. These results suggest the presence of a bimodal gradient of rhythmogenicity in the larval CNS, with mAChR dependent rhythm generating networks in distal regions separated by medial segments with severely reduced rhythmogenic abilities. This work furthers our understanding of motor control in soft-bodied locomotion and provides a foundation for study of rhythm generating networks in an emerging genetically tractable locomotor system.