The zebrafish lateral line is an established model for hair cell organ damage, yet few studies link mechanistic disruptions to changes in biologically relevant behavior. We used larval zebrafish to determine how damage via ototoxic compounds impact rheotaxis. Larvae were treated with CuSO4 or neomycin to disrupt lateral line function then exposed to water flow stimuli. Their swimming behavior was recorded on video then DeepLabCut and SimBA software were used to track movements and classify rheotaxis behavior, respectively. Lateral line-disrupted fish performed rheotaxis, but they swam greater distances, for shorter durations, and with greater angular variance than controls. Furthermore, spectral decomposition analyses confirmed that lesioned fish exhibited ototoxic compound-specific behavioral profiles with distinct changes in the magnitude, frequency, and cross-correlation between fluctuations in linear and angular movements. Our observations demonstrate that lateral line input is needed for fish to hold their station in flow efficiently and reveals that commonly used lesion methods have unique effects on rheotaxis behavior.

The synthetic glucocorticoid dexamethasone is commonly used to treat inner ear disorders. Previous work in larval zebrafish has shown that dexamethasone treatment enhances hair cell regeneration, yet dexamethasone has also been shown to inhibit regeneration of peripheral nerves after lesion. We therefore used the zebrafish model to determine the impact of dexamethasone treatment on lateral-line hair cells and primary afferents. To explore dexamethasone in the context of regeneration, we used copper sulfate (CuSO4) to induce hair cell loss and retraction of nerve terminals, and then allowed animals to recover in dexamethasone for 48 h. Consistent with previous work, we observed significantly more regenerated hair cells in dexamethasone-treated larvae. Importantly, we found that the afferent processes beneath neuromasts also regenerated in the presence of dexamethasone and formed an appropriate number of synapses, indicating that innervation of hair cells was not inhibited by dexamethasone. In addition to regeneration, we also explored the effects of prolonged dexamethasone exposure on lateral-line homeostasis and function. Following dexamethasone treatment, we observed hyperpolarized mitochondrial membrane potentials (ΔΨm) in neuromast hair cells and supporting cells. Hair cells exposed to dexamethasone were also more vulnerable to neomycin-induced cell death. In response to a fluid-jet delivered saturating stimulus, calcium influx through hair cell mechanotransduction channels was significantly reduced, yet presynaptic calcium influx was unchanged. Cumulatively, these observations indicate that dexamethasone enhances hair cell regeneration in lateral-line neuromasts, yet also disrupts mitochondrial homeostasis, making hair cells more vulnerable to ototoxic insults and possibly impacting hair cell function.

The synthetic glucocorticoid dexamethasone is commonly used to treat inner ear disorders. Previous work in larval zebrafish has shown that dexamethasone treatment enhances hair cell regeneration, yet dexamethasone has also been shown to inhibit regeneration of peripheral nerves after lesion. We therefore used the zebrafish model to determine the impact of dexamethasone treatment on lateral-line hair cells and primary afferents. To explore dexamethasone in the context of regeneration, we used copper sulfate (CuSO4) to induce hair cell loss and retraction of nerve terminals, and then allowed animals to recover in dexamethasone for 48 hours. Consistent with previously work, we observed significantly more regenerated hair cells in dexamethasone-treated larvae. Importantly, we found that the afferent processes beneath neuromasts also regenerated in the presence of dexamethasone and formed an appropriate number of synapses, indicating that innervation of hair cells was not inhibited by dexamethasone. In addition to regeneration, we also explored the effects of prolonged dexamethasone exposure on lateral-line homeostasis and function. Following dexamethasone treatment, we observed more negatively charged mitochondrial membrane potentials (ΔΨm) in neuromast hair cells and supporting cells. Hair cells exposed to dexamethasone were also more vulnerable to neomycin-induced cell death. In response to a fluid-jet delivered saturating stimulus, calcium influx through hair cell mechanotransduction channels was significantly reduced, yet presynaptic calcium influx was unchanged. Cumulatively, these observations indicate that dexamethasone enhances hair cell regeneration in lateral-line neuromasts, yet also disrupts mitochondrial homeostasis, making hair cells more vulnerable to ototoxic insults and possibly impacting hair cell function.