Labeling hair cells and afferent neurons in the posterior lateral-line system of zebrafish.

The lateral line is a mechanosensory system that comprises a set of discrete sense organs called neuromasts, which are arranged in reproducible patterns on the surface of fish and amphibians. The posterior component of the system, the posterior lateral line, comprises the neuromasts on the body and tail. Each neuromast has a core of mechanosensory hair cells, each of which is depolarized by water motion in one direction and hyperpolarized by motion in the other direction, thereby enabling fish to extract information from the movements of water around their body. Neuromasts are innervated by a few afferent neurons (usually two, but sometimes more), which have their cell bodies clustered in cranial ganglia and project their central axons to the hindbrain, where they extend longitudinally along all rhombomeres. Hair cells are readily labeled by small cationic styryl pyridinium dyes such as DiASP. Afferent fibers are also progressively labeled with this dye, presumably by trans-synaptic uptake. Adjusting the dye concentration and incubation time can lead to the labeling of the entire afferent system, thereby providing a fast and easy method for visualizing the central projection in the hindbrain of live fish. The simplicity of the method makes it potentially useful for screens based on forward or reverse genetic approaches. Here we present protocols for labeling hair cells in live zebrafish and for labeling afferent neurons in zebrafish embryos.