FIGURE 1.

Zebrafish hair cells and ribbons synapses. (A) Schematic depicts a larval zebrafish. Pink patches outline the location of hair cells in the inner ear required for hearing and balance, as well as hair cells in the lateral-line system. Green patches represent the location of the anterior and posterior lateral-line ganglia. The cell bodies of neurons in these ganglia project to and innervate hair cells in the lateral line. (B) An overview of the anatomy of a single patch of hair cells in the lateral line, referred to as a neuromast. Hair cells (pink) are surrounded by supporting cells (internal, blue and peripheral, orange) and innervated by both afferent (green) and efferent neurons (yellow). Mechanosensory hair bundles (purple) at the apex of hair cells project out into the water to detect local water flow. (C) Diagram of a single hair cell. Hair cells are activated when hair bundles are deflected, for example by local water flow. This apical deflection opens mechanosensitive channels allowing in cations including potassium and calcium. This apical activity depolarizes the hair cell, resulting in presynaptic calcium influx and release of glutamate onto the afferent neuron. Inset: magnified view of a hair-cell ribbon synapse. Shown are key evolutionarily conserved synaptic proteins discussed in this review. In hair cells, a presynaptic density called a ribbon (red) helps to recruit synaptic vesicles (white circles) to the synapse near clusters of calcium channels (Ca_V1.3). The ribbon is made up primarily of the protein Ribeye. Slc17A8 (Vglut3) and DMXL2 (Rbc3a) colocalize in or near synaptic vesicles. Synaptojanin and Otoferlin are also critical for ribbon-synapse function although their precise localization has not been definitively shown.