Octopamine and tyramine modulate fictive motor program competition in Drosophila larvae

Dynamic interactions among competing motor programs shape behavioral output. Here, we use bath application of biogenic amines octopamine (OA) and tyramine (TA) to explore operating principles of competition among central pattern generating (CPG) networks controlling locomotion in 3rd 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 forward locomotion, suppressed backward locomotion, and induced bouts of fictive head sweeps during wash periods that were proportional to the promotion of forward waves. In contrast, TA, the conceptualized inhibitory antagonist to OA, promoted collisions and overlap of motor programs, 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 preapplication 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.NEW & NOTEWORTHY This study demonstrates that rhythmic motor activity in the isolated Drosophila larval CNS is dependent upon the octopaminergic/tyraminergic system. Furthermore, our findings reveal context- and state-dependent effects of adrenergic-like modulation, showing how bias toward one motor program can induce long-lasting promotions of opposing motor programs. Overall, we show how competition between different CPG networks can be explored using the Drosophila larval locomotor system as a model.