Octopamine and Tyramine Modulate Motor Program Competition in Drosophila larvae

Dynamic interactions amongst competing motor programs shape behavioural output. Here, we use bath application of biogenic amines octopamine (OA) and tyramine (TA) to explore operating principles of competition amongst central pattern generating (CPG) networks controlling locomotion in 3^rd instar Drosophila larvae. OA and TA share the same synthesis pathway and are contained in overlapping sets of neurons. Bath application of OA promoted fictive forwards locomotion, suppressed backwards locomotion, and induced bouts of fictive head sweeps during wash periods that were proportional to the promotion of forward waves. In contrast, TA, the conceptualised inhibitory antagonist to OA, promoted collisions and overlap of motor programmes, activity-dependent inhibition, and bouts of silence during wash periods. Co-application of OA and TA revealed bimodal state-dependent modulation with forward bias promotion exclusively if the pre-application network was not already forward dominant. Overall, our work provides insights into how adrenergic-like systems can modulate dynamics of motor competition among interacting CPGs within locomotor networks.