Balance and gaze rely on the faithful and rapid signalling of head movements by vestibular hair cells (VHCs) to primary sensory neurons. There are two types of VHCs in mammals, type-I and type-II. While type-I VHCs are contacted by a giant afferent nerve terminal, called a calyx, that encloses their basolateral membrane almost completely, type-II cells are innervated by multiple bouton afferent terminals. In both VHC types, glutamate exocytosis is triggered by Ca2+ influx through voltage-gated CaV1.3 Ca2+ channels. While signal transmission in type-I and type-II VHCs involves the Ca2+-dependent quantal exocytosis of glutamate at specialised ribbon synapses, type-I cells are also believed to exhibit a non-quantal mechanism that increases the reliability and the speed of signal transmission. However, the reliance of mature type-I hair cells on non-quantal transmission remains unknown.
In this study we investigated synaptic vesicle exocytosis in mature mammalian utricular hair cells using whole cell patch-clamp recording of Ca2+ currents and changes in membrane capacitance (ΔCm). Signal transfer from type-I cells to the calyceal afferent terminal was measured by cell attached recording of action potentials in the calyx in response to VHC depolarisation with an endolymphatic low Ca2+ solution.
We found that mature type-II hair cells responded to depolarisation with Ca2+-dependent exocytosis that showed a high-order dependence on Ca2+. By contrast, the Ca2+ current in type-I cells was approximately four times smaller and exocytosis was around ten times smaller than that observed in type-II cells. While type-II VHCs showed kinetically distinct pools of synaptic vesicles in response to increased stimulus duration, the responses of type-I cells remained comparatively small with a single pool of vesicles.
In VHCs of CaV1.3knockout mice (CaV1.3-/-) both the Ca2+ current and exocytosis were largely absent in both type-I and type-II cells. In otoferlin knockout mice (Otof-/-) the Ca2+ currents were similar to control cells but synaptic vesicle exocytosis was largely absent. Even though Ca2+-dependent exocytosis was small in control type-I hair cells, or absent in CaV1.3-/- and Otof-/- mice, these cells were able to drive action potential activity in the postsynaptic calyces, as evident from the increase in calyceal action potential activity in response to VHC depolarisation.
These findings show that mature type-I VHCs show a much smaller Ca2+-dependent exocytosis than type-II cells. The much smaller single vesicle pool in type-I cells supports a functional role for non-quantal synaptic transmission in these cells. The large vesicle pools in type-II cells would facilitate sustained transmission of tonic or low-frequency signals, whereas the restricted vesicle pool size, together with a rapid non-quantal mechanism, in type-I cells could specialise these large calyceal synapses for high-frequency phasic signal transmission.