Evidence that ultrafast non-quantal transmission underlies short-latency vestibular evoked potentials

Amniotes evolved a unique calyceal postsynaptic terminal in the vestibular organs of the inner ear that underpins quantal and non-quantal transmission at the synapse of sensory hair cells and vestibular afferent neurons. The non-quantal component is of particular interest as it includes an ultrafast synaptic current thought to underlie the exquisite synchronization of action potentials in vestibular afferent fibres to dynamic stimuli such as sound and vibration. Here we demonstrate evidence that non-quantal transmission is responsible for short latency vestibular evoked potentials (vCAPs) in the guinea pig utricle. We first show that, unlike auditory nerve responses which are completely abolished, vCAPs are insensitive to local administration of the AMPA receptor agonist CNQX. Moreover, latency comparisons between presynaptic hair cell and postsynaptic neural responses reveal that the vCAP occurs without measurable synaptic delay. Finally, using a paired-pulse stimulus designed to deplete the readily releasable pool of synaptic vesicles in hair cells, we reveal that forward masking is lacking in vestibular responses, compared to the equivalent cochlear responses. Our data support the hypothesis that the fast component of non-quantal transmission at calyceal synapses is indefatigable and responsible for ultrafast responses of vestibular organs evoked by transient stimulation.