Synaptic Specificity in the Zebrafish Lateral Line

The proper wiring of the vertebrate brain represents an extraordinary developmental challenge, requiring billions of neurons to select their appropriate synaptic targets. In view of this complexity, simple vertebrate systems provide necessary models for understanding how synaptic specificity arises. The posterior lateral-line organ of larval zebrafish consists of polarized hair cells organized in discrete clusters known as neuromasts. Here I show that each afferent neuron of the posterior lateral line establishes specific contacts with hair cells of the same hair-bundle polarity. I quantify this specificity by modeling the neuron as a biased selector of hair-cell polarity and find evidence for bias from as early as 2.5 days post-fertilization. More than half of the neurons form contacts on multiple neuromasts, but the innervated organs are spatially consecutive and the polarity preference is consistent. Using a novel reagent for correlative electron microscopy, HRP-mCherry, I show that these contacts are indeed afferent synapses bearing vesicle-loaded synaptic ribbons. Moreover, afferent neurons reassume their biased innervation pattern after hair-cell ablation and regeneration. By documenting specificity in the pattern of neuronal connectivity during development and in the context of organ regeneration, these results establish the posterior lateral-line organ as a vertebrate system for the in vivo study of synaptic specificity. In order to shed light on the mechanism for this specificity, I investigated whether afferent neurons distinguish hair-cell polarities by analyzing differences in the synaptic signaling between oppositely polarized hair cells. By examining two mutant zebrafish lines with defects in mechanoelectrical transduction, I found that afferent neurons can form specific synapses in the absence of stimulus-evoked patterns of synaptic release. Asking next whether this specificity could arise through intrinsically generated patterns of synaptic release, I found that the polarity preference persisted in two mutant lines lacking essential synaptic proteins. These results indicate that lateral-line afferent neurons do not utilize synaptic activity to distinguish hair-cell polarities and suggest that molecular markers of hair-cell polarity guide pre-patterned afferents to form the appropriate synapses