Remote control of wildtype zebrafish optomotor response with a photoswitchable drug.

Introduction: For animals to interact effectively with their environment, the brain must integrate sensory information and generate appropriate motor responses. Multiple neuronal circuits contribute to this process, and identifying their roles remains a central focus in neuroscience. In this context, photopharmacology provides light-controllable compounds that enable remote and spatially and temporally precise modulation of neuronal activity without requiring genetic modification. This approach minimizes off-target activation of unwanted brain regions while allowing reversible control of neural circuits. The recently developed photoswitchable compound Carbadiazocine has been shown to reversibly control neuronal firing across multiple species in a light-dependent manner. It modulates larval zebrafish locomotion and alleviates neuropathic pain in rodents in a reversible, light-induced manner. Given the established Methods: We focused on the optomotor response in zebrafish larvae and assessed its potential as a tool for circuit perturbation and behavioral analysis, for the first time combined with photopharmacology. We performed experiments in head-fixed and free-swimming larvae to assess their capacity to detect and follow optic flow, and to characterize swimming speed patterns and tail bout properties following administration of the two Carbadiazocine photoisomers. Results: In both paradigms, treatment with the pre-illuminated compound decreased accuracy in responding to optic flow (correct turning percentage dropping from ~95% to ~80% in head-fixed experiments and from ~65% to ~20% in free-swimming experiments). Speed analysis revealed more frequent and prolonged fast movements, with fewer slow movements, even without visual stimulation. Tail bout analysis showed increased 15-30 Hz bout frequencies, corresponding to irregular tail movements. These effects were absent with the dark-relaxed compound. Discussion: Together, these findings provide a foundation to probe neuronal circuits underlying behavior in diverse animal species using a dynamic range of photostimulation patterns.