Tachykinin neuropeptides are involved in axonal and synaptic differentiation of the pioneer motor axon in zebrafish

In contrast to many other developing systems, in which axon pathfinding and synaptogenesis are separated in time, the pioneering axon of the individually identifiable caudal primary motor neuron in embryonic zebrafish forms en passant synapses during its stereotypical ventral growth. How simultaneous synaptic differentiation and axon pathfinding are coordinated is not fully understood. Here we ask what the role of the tac1 gene, coding for the synaptic tachykinin neuropeptides, is in this unique axon differentiation process. The gene is expressed during axon outgrowth and its disruption results in increased branch length of CaP axons and subtle morphological defects of the pre-synapse. These abnormalities are accompanied by a robust ∼1.5-fold increase in motor neuron activity and in spontaneous early contractions in tac1 -deficient embryos. Furthermore, pharmacological inhibition of the tachykinin receptor (Tacr1) leads to altered CaP axonal morphology, mimicking the axonal phenotype observed in tac1 -deficient zebrafish. These findings suggest that tachykinin neuropeptides modulate formation and activity of en passant synapses and prevent aberrant axon branching during growth of zebrafish motor axons.