During nervous system development, axon terminals synapse on precise domains of target (post-synaptic) dendrites, but what controls this specificity is not well understood. Now, Samantha Galindo and colleagues in the lab of Wesley Grueber use a combination of cell ablations and synaptic markers in Drosophila to show that axon-axon interactions contribute to synapse positioning. The authors focus on cIII touch-sensing and cIV nociceptive (pain-sensing) somatosensory neurons, which both innervate down-and-back (DnB) neuron dendrites on adjacent, non-overlapping areas. They show that the ablation of nociceptor axons causes touch neurons to expand their axon terminals into the nociceptive domain of DnB dendrites. However, the loss of touch neuron axons does not impact nociceptive positioning. Using live imaging of a sensory neuron (pannier) reporter line, the researchers reveal that sensory axons target sequentially in a pattern consistent with nociceptor axons arriving before touch neuron axons. Together, these data show that touch axons are competent to synapse more broadly with DnB dendrites, but are restricted from doing so by the presence of nociceptor axons, revealing that axon-axon interactions and the temporal specificity of axon targeting are important for specific synapse positioning on dendrites.
