A profile of auditory-responsive neurons in the larval zebrafish brain.

Many features of auditory processing are conserved among vertebrates, but the degree to which these pathways are established at early stages is not well explored. In this study, we have observed single cell activity throughout the brains of larval zebrafish with the goal of identifying the cellular responses, brain regions, and brain-wide pathways through which these larvae perceive and process auditory stimuli. Using GCaMP and selective plane illumination microscopy, we find strong responses to auditory tones ranging from 100 Hz to 400 Hz. We also identify different categories of auditory neuron with distinct frequency response profiles. Auditory responses occur in the medial octavolateral nucleus, the torus semicircularis, the medial hindbrain, and the thalamus, and the flow of information among these regions resembles the pathways described in adult fish and mammals. The details of these patterns, however, indicate that auditory processing is still rudimentary in larvae. The range of frequencies detected is small, and while different neurons have distinct response profiles, most are sensitive to multiple frequencies, and distinct categories show substantial overlap in their responses. Likewise, while there are signs of nascent spatial representations of frequency in the larval brain, this only faintly resembles the clear tonotopy seen in adult fish and mammals. Overall, our results show that many fundamental properties of the auditory system are established early in development, and suggest that zebrafish will provide a good model in which to study the development and refinement of these pathways.