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

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-colour calcium imaging and optogenetic manipulation to explore how tdc²⁺ neurons track, permit, and bias fictive motor activity in 3rd instar Drosophila larvae. We find that tdc²⁺ activity in the larval ventral nerve cord is strongly coupled to motor neuron activity across multiple fictive behaviours, indicating that the system is recruited broadly across the motor repertoire. Optogenetic depolarisation of tdc²⁺neurons increases motor root bursting and induces a robust fictive forward bias, whereas optogenetic hyperpolarisation suppresses or abolishes fictive rhythms and generates a short-lasting, post-inhibitory rebound in fictive activity. Spatially-restricted stimulation reveals that posterior abdominal activation is especially effective at promoting fictive forward activity. Separating 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 post-stimulation changes in motor programme 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 signalling to motor programme competition.