Optogenetic control of activity in descending Tdc²⁺ neurons modulates motor program bias in the Drosophila larval...

Motor systems must flexibly select between competing outputs while preserving stability of rhythmic outputs. In Drosophila larvae, the isolated central nervous system is capable of maintaining rhythmicity by generating multiple different fictive motor programs. The biogenic amines octopamine and tyramine are known to regulate larval locomotion, however, how the tdc²⁺ octopaminergic/tyraminergic system regulates motor program competition is not well understood. Here, we combine dual-color calcium imaging and optogenetic manipulation to explore how tdc²⁺ neurons track, permit, and bias fictive motor activity in third instar Drosophila larvae. We find that tdc²⁺ activity in the larval ventral nerve cord is strongly coupled to motor neuron activity across multiple fictive behaviors, indicating that the system is recruited broadly across the motor repertoire. Optogenetic depolarization of tdc²⁺neurons increases motor root bursting and induces a robust fictive forward bias, whereas optogenetic hyperpolarization suppresses or abolishes fictive rhythms and generates a short-lasting, postinhibitory rebound in fictive activity. Spatially restricted stimulation reveals that posterior abdominal activation is especially effective at promoting fictive forward activity. Separating ventral nerve cord (VNC)-residing from brain-residing tdc²⁺ populations further shows that activation of descending brain-residing tdc²⁺ projections is sufficient to recapitulate this forward bias. Finally, tdc²⁺ activation induces short-lived poststimulation changes in motor program probability, including transient elevation of competing fictive backward instantaneous frequency. Together, these findings suggest that tdc²⁺ neurons act as a permissive and biasing modulatory layer within larval motor circuits, linking adrenergic-like signaling to motor program competition.NEW & NOTEWORTHY Rhythmically active motor systems must continually cycle between competing motor programs. How biogenic amines regulate competition among competing central pattern generating networks is not well understood. Here, we use calcium imaging and optogenetics to explore how a population of adrenergic-like neurons establishes and controls biases in motor programs. What is new and novel is that we uncover a modulatory layer that appears to control biases as well as form a core component of rhythm generation.