In vivo and in vitro biophysical properties of hair cells from the lateral line and inner ear of developing and adult zebrafish.

Hair cells detect and process sound and movement information, and transmit this with remarkable precision and efficiency to afferent neurons via specialized ribbon synapses. The zebrafish is emerging as a powerful model for genetic analysis of hair cell development and function both in vitro and in vivo. However, the full exploitation of the zebrafish is currently limited by the difficulty in performing systematic electrophysiological recordings from hair cells under physiological recording conditions. Thus, the biophysical properties of developing and adult zebrafish hair cells are largely unknown. We investigated potassium and calcium currents, voltage responses and synaptic activity in hair cells from the lateral line and inner ear in vivo and using near-physiological in vitro recording conditions. We found that the basolateral current profile of hair cells from the lateral line becomes more segregated with age, with cells positioned in the centre of the neuromast showing more mature characteristics and those towards the edge retaining a more immature phenotype. The proportion of mature-like hair cells within a given neuromast increased with zebrafish development. A developmental change in current profile was also seen in hair cells from the inner ear between juvenile and adult zebrafish. In lateral line hair cells from juvenile zebrafish, exocytosis also became more efficient, needing less calcium for vesicle fusion. In hair cells from mature zebrafish, the biophysical characteristics of ion channels and exocytosis resembled those of hair cells from other lower vertebrates and, to some extent, those in the immature mammalian vestibular and auditory systems. We show that whilst the zebrafish provides a suitable animal model for studies on hair cell physiology, it is advisable to consider that the age at which the majority of hair cells acquire a mature-type configuration is only reached in the juvenile lateral line and in the inner ear from >2 months after hatching.