Octopamine and Tyramine Modulate Motor Program Competition in Drosophila larvae

Dynamic Interactions amongst competing motor programs shape behavioural output. Here, we use a combination of opto- and electro-physiology to explore how the tdc2+adrenergic-like system modulates competition amongst central pattern generating (CPG) networks controlling locomotion in 3rd instar Drosophila larvae. Bath application of octopamine (OA) promoted fictive forwards locomotion, suppressed backwards locomotion, and induced bouts of fictive head sweeps during wash period that were proportional to the promotion of forward waves. In contrast, tyramine (TA), the conceptualised inhibitory antagonist to OA, promoted collisions and overlap of motor programmes, activity-dependent inhibition, and bouts of silence during wash periods. Dual-colour calcium imaging of the tdc2+system with motor neurons revealed that the tdc2+ neurons are recruited phasically prior to a majority of fictive locomotor behaviours. Optogenetic manipulation of the tdc2+system recapitulated a subset of effects and further revealed that activity in tdc2+ system neurons necessary and sufficient for locomotor CPG activity in the system. Overall, our work generates testable predictions for future behavioural and connectome-constrained computational studies for Drosophila motor competition and provides insights into how adrenergic-like systems can modulate dynamics of motor competition among interacting CPGs within locomotor networks.