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This Week in The Journal

Fish use information about water pressure and movement to help orient their bodies in currents, evade predators, and track prey. They gather this information using the lateral line system, which consists of numerous mechanosensory structures called neuromasts that are distributed over the fish’s body.Eachneuromasthouses10–20haircells similar to those found in the inner ear. The stereocilia atop these hair cells are encapsulated by a flexible cupula structure that is deflected by changes in water pressure. Deflection of the cupula bends the stereocilia, causing mechanosensory transduction channels to open or close, depending on the direction of deflection. This leads to hair-cell depolarization or hyperpolarization and increases or decreases in glutamate release. To investigate how mechanical stimuli are converted into neural signals in the lateral line system, Pichler and Lagnado used the fluorescent glutamate sensor iGluSnFR to quantify glutamate release from individual hair cells in response to various degrees of cupula deflection. The transfer characteristics varied across cells within single neuromasts. As previously shown, some hair cells released more glutamate when the cupula was deflected toward the head, and others responded more strongly to deflections toward the tail; consequently, deflections in either direction elicited depolarization of some hair cells. Importantly, the sensitivity and working range (the difference in deflection angles eliciting minimal and maximal responses) varied across hair cells. The extent to which glutamate release was decreased by deflections in the non-preferred direction also varied across cells, and some cells exhibited only depolarizing responses. Hair cells also differed in how long glutamate release remained elevated during prolonged stimulation: some showed transient responses at stimulus onset while others showed sustained release. Finally, some hair cells exhibited transient increases in glutamate release at the offset of deflections in their non-preferred direction. The heterogeneity in transfer characteristics across hair cells likely benefits fish by enabling accurate encoding of both small and large changes in water pressure, as well as signaling onset, offset, and duration of these changes. Future work should examine the downstream connectivity of hair cells with different properties and determine how the cells contribute to various behaviors. In addition, the cellular and molecular bases of the heterogeneity should be examined.