MODULATION OF MIDBRAIN VISUOMOTOR PROCESSING BY A DOPAMINERGIC CIRCUIT IN LARVAL ZEBRAFISH

Dopamine is an evolutionarily conserved neuromodulator that shapes sensorimotor behavior, yet how it dynamically controls midbrain visuomotor transformations remains poorly understood. Here we investigate dopaminergic modulation of the optic tectum in larval zebrafish, a key hub for transforming visual input into locomotor output, homologous to the mammalian superior colliculus. Using an intersectional fluorescence labelling strategy, we mapped axonal projections of pretectal dopaminergic (PrDA) neurons and found that their axons densely innervate deep layers of the tectal neuropil from where axon bundles of tectal neurons project to premotor regions in the hindbrain. To define the postsynaptic molecular substrate for dopaminergic modulation, we performed in situ hybridization chain reaction (HCR) for dopamine receptor transcripts in the optic tectum, revealing comparable overall expression of drd1 and drd2, but preferential expression of the b paralogs, indicating heterogeneous tectal dopaminergic sensitivity. Combining in-vivo Ca²⁺ (GCaMP6s) and dopamine (GRAB_DA1m) imaging with fictive swim recordings, we show that spontaneous swim bouts are associated with temporally aligned phasic Ca²⁺ signals in PrDA neuron somata and their axonal projections and dopamine release in the tectal neuropil. Within the tectal neuropil, we observe differences in the distribution of dopaminergic signals associated with spontaneous swims and those during periods of rest. Finally, targeted pharmacological elevation of dopamine in the tectal neuropil biased visually driven motor responses, producing a lateralized increase in visuomotor gain. Together, our findings reveal a layered and stimulus-dependent dopaminergic control of tectal circuits, linking anatomical innervation, receptor expression, neuromodulator dynamics, and behavior.