Non conventional signal transmission at the mouse vestibular Type I hair cell - calyx synapse

Vestibular sensory epithelia of Amniotes contain two types of hair cells, Type I and Type II, which differ in electrophysiological properties and synaptic contacts. Type I hair cells alone express a low-voltage activated outward rectifying K+ conductance, named GK,L. Moreover, each Type II hair cell is contacted by several bouton afferent endings, while a single large calyceal afferent terminal encloses the basolateral membrane of Type I hair cells, where voltage-gated Ca2+ and K+ channels and the presynaptic sites for glutamate release are located. Besides vesicular transmission, a nonquantal transmission has been hypothesized to occur at the calyx synapse, whereby K+ exiting the hair cell directly depolarizes the calyx terminal. To investigate K+ involvement in signal transmission, we whole-cell recorded from in vitro mouse Type I hair cells or their associated calyx. We found that intercellular (in the synaptic cleft) K+ increased or decreased depending upon hair cell membrane potential as a consequence of GK,L negative voltage-range of activation . Moreover, we found evidence for the calyx inner membrane facing the synaptic cleft expressing voltage-gated K+ channels of the KV1 and KV7 type. Present results suggests a scenario where hair bundle deflection produces calyx depolarization or hyperpolarization by modulating K+ flux across the hair cell and through postsynaptic voltage-gated K+ channels.