Effects of the calyx on the apparent properties of vestibular type I hair cells K+ currents

Vestibular Type I hair cells are almost entirely enveloped by a single large afferent nerve terminal, called calyx, whose functional meaning is still enigmatic. Another defining property of Type I cells is the expression of a low-voltage-activated outward rectifying K+ current, named IK,L. By patch-clamp whole-cell recordings from in situ mouse vestibular Type I cells, we have found that IK,L activation can result in K+ accumulation around the cell, as inferred from the positive shift of K+ currents reversal potential (VrevK+), presumably due to the presence of a residual calyx [1]. This phenomenon accompanied to a slower IK,L deactivation during hyperpolarizing voltage steps. We have investigated this aspect in more detail, and found that IK,L deactivated with a slow complex time course, not consistent with the reported exponential decay [2]. Since most previous studies were done in isolated Type I cells, we repeated the experiments in enzimatically dissociated cells and found that both the shift of VrevK+ and the alteration of IK,L deactivation were absent or much less obvious. Our hypothesis is that most of the calyx survives in situ , but not after cell dissociation, which would represent a restriction to K+ diffusion, and a resistance (Re) to current flow between the ynaptic cleft and the bath. As a consequence, large ion currents would produce a significant voltage drop (Ve) across Re. Ve would be maximal at the peak amplitude of the instantaneous IK,L, and then decrease with IK,L deactivation. Thus, IK,L deactivation would be distorted by voltage-clamp failure due to Re, which we estimated in tens of MΩ, i.e. in the same magnitude order of the calyx input resistance (~90 MΩ; [1]). In conclusion, our data demonstrate for the first time that the calyx can significantly influence, via a purely electrical mechanism which adds to the effects of K+ accumulation in the cleft, the behavior of the currents generated by the hair cell membrane.