Activity-independent specification of synaptic targets in the posterior lateral line of the larval zebrafish
Stimulus-evoked patterns of synaptic release are not required for polarity choice. A, B: In an anteroposterior neuromast of a tmie mutant larva, a labeled afferent fiber forms bouton synapses (blue arrowheads) with five of the ten hair cells. In this and the subsequent morphological illustrations, the two micrographs represent different planes of focus. C: The hair-bundle polarities of this neuromast reveal that the neuron innervates all five posteriorly polarized hair cells (blue arrowheads) and none of the opposite polarity. D-F: An afferent fiber in a pcdh15a mutant forms synapses with four of the five anteriorly polarized hair cells, but with none of the five cells of the opposite polarity. G-I: In a neuromast of a larva treated with 1 mM amiloride from 2 dpf to 5 dpf, the labeled fiber forms synapses with only the three anteriorly polarized hair cells.
The development of functional neural circuits requires that connections between neurons be established in a precise manner. The mechanisms by which complex nervous systems perform this daunting task remain largely unknown. In the posterior lateral line of larval zebrafish, each afferent neuron forms synaptic contacts with hair cells of a common hair-bundle polarity. We 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, and by blocking transduction during the development of wild-type fish, we found that afferent neurons could form specific synapses in the absence of stimulus-evoked patterns of synaptic release. Asking next whether this specificity arises through intrinsically generated patterns of synaptic release, we found that the polarity preference persisted in two mutant lines lacking essential synaptic proteins. These results indicate that lateral-line afferent neurons do not require synaptic activity to distinguish hair-cell polarities and suggest that molecular labels of hair-cell polarity guide prepatterned afferents to form the appropriate synapses.
