Intercellular K⁺ accumulation depolarizes Type I vestibular hair cells and their associated afferent nerve calyx.

Mammalian vestibular organs contain two types of sensory receptors, named Type I and Type II hair cells. While Type II hair cells are contacted by several small afferent nerve terminals, the basolateral surface of Type I hair cells is almost entirely enveloped by a single large afferent nerve terminal, called calyx. Moreover Type I, but not Type II hair cells, express a low-voltage-activated outward K(+) current, I(K,L), which is responsible for their much lower input resistance (Rm) at rest as compared to Type II hair cells. The functional meaning of I(K,L) and associated calyx is still enigmatic. By combining the patch-clamp whole-cell technique with the mouse whole crista preparation, we have recorded the current- and voltage responses of in situ hair cells. Outward K(+) current activation resulted in K(+) accumulation around Type I hair cells, since it induced a rightward shift of the K(+) reversal potential the magnitude of which depended on the amplitude and duration of K(+) current flow. Since this phenomenon was never observed for Type II hair cells, we ascribed it to the presence of a residual calyx limiting K(+) efflux from the synaptic cleft. Intercellular K(+) accumulation added a slow (τ>100ms) depolarizing component to the cell voltage response. In a few cases we were able to record from the calyx and found evidence for intercellular K(+) accumulation as well. The resulting depolarization could trigger a discharge of action potentials in the afferent nerve fiber. Present results support a model where pre- and postsynaptic depolarization produced by intercellular K(+) accumulation cooperates with neurotransmitter exocytosis in sustaining afferent transmission arising from Type I hair cells. While vesicular transmission together with the low Rm of Type I hair cells appears best suited for signaling fast head movements, depolarization produced by intercellular K(+) accumulation could enhance signal transmission during slow head movements.

Elementary properties of Kir2.1, a strong inwardly rectifying K(+) channel expressed by pigeon vestibular type II hair cells.

By using the patch-clamp technique in the cell-attached configuration, we have investigated the single-channel properties of an inward rectifier potassium channel (Kir) expressed by pigeon vestibular type II hair cells in situ. In high-K(+) external solution with 2 mM Mg(2+), Kir inward current showed openings to at least four amplitude levels. The two most frequent open states (L2 and L3) had a mean slope conductance of 13 and 28 pS, respectively. L1 (7 pS) and L4 (36 pS) together accounted for less than 6% of the conductive state. Closed time distributions were fitted well using four exponential functions, of which the slowest time constant (tau(C4)) was clearly voltage-dependent. Open time distributions were fitted well with two or three exponential functions depending on voltage. The mean open probability (P(O)) decreased with hyperpolarization (0.13 at -50 mV and 0.03 at -120 mV). During pulse-voltage protocols, the Kir current-decay process (inactivation) accelerated and increased in extent with hyperpolarization. This phenomenon was associated with a progressive increase of the relative importance of tau(C4). Kir inactivation almost disappeared when Mg(2+) was omitted from the pipette solution. At the same time, P(O) increased at all membrane voltages and the relative importance of L4 increased to a mean value of 47%. The relative importance of tau(C4) decreased for all open states, while L4 only showed a significantly longer open time constant. The present work provides the first detailed quantitative description of the elementary properties of the Kir inward rectifier in pigeon vestibular type II hair cells and specifically describes the Kir gating properties and the molecule's sensitivity to extracellular Mg(2+) for all subconductance levels. The present results are consistent with the Kir2.1 protein sustaining a strong inwardly rectifying K(+) current in native hair cells, characterized by rapid activation time course and slow partial inactivation. The longest closed state (tau(C4)) appears as the main parameter involved in time- and Mg(2+)-dependent decay. Finally, in contrast to Kir2.1 results described so far for mammalian cells, external Mg(2+) had no effect on channel conductance.