Pastras, C. J. | Search bioRxiv

bioRxiv2023

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. SignificanceThe mammalian vestibular system drives some of the fastest reflex pathways in the nervous system, ensuring stable gaze and postural control for locomotion on land. To achieve this, terrestrial amniotes evolved a large, unique calyx afferent terminal which completely envelopes one or more pre-synaptic vestibular hair cells, which transmits mechanosensory signals mediated by quantal and nonquantal (NQ) synaptic transmission. We present several lines of data in the guinea pig that reveal the pre-synaptic transmission of the most sensitive vestibular afferents are faster than their auditory nerve counterparts. Here, we present neurophysiological and pharmacological evidence that this vestibular speed advantage arises from ultrafast NQ electrical synaptic transmission from Type I hair cells to their calyx partners.

neuroscience↗

bioRxiv2022

Vestibular Compound Action Potentials and Macular Velocity Evoked by Sound and Vibration in the Guinea Pig.

To examine mechanisms responsible for vestibular afferent sensitivity to transient air conducted sounds (ACS) and inter-aural bone conducted vibration (BCV), we performed simultaneous measurements of stimulus-evoked vestibular compound action potentials (vCAPs), utricular macula or stapes velocity, and Vestibular Microphonics (VMs) in the anaesthetized guinea pig. For short duration punctate stimuli (<1ms), the vCAP increases magnitude in close proportion to macular velocity and temporal bone (ear-bar) acceleration, rather than other kinematic variables. For longer duration stimuli, the vCAP magnitude switches from acceleration sensitive to linear jerk sensitive. vCAP input-output (IO) functions suggest primary afferent response generation has the same origins for both BCV and ACS, with similar macular velocity thresholds and IO functions for both stimuli. Frequency tuning curves evoked by tone-burst stimuli also show the vCAP increases magnitude in proportion to macular velocity, while in contrast, the VM increases magnitude in proportion to macular displacement across the entire frequency bandwidth tested. The subset of vestibular afferent neurons responsible for synchronized firing and vCAPs make calyceal synaptic contacts with type I hair cells in the striolar region of the epithelium and have irregularly spaced inter-spike intervals at rest. Present results provide new insight into mechanical and neural mechanisms underlying synchronized action potentials in these sensitive afferents, with clinical relevance for understanding the activation and tuning of neurons responsible for driving rapid compensatory reflex responses. Significant statementCalyx-bearing afferents in the utricle have the remarkable ability to fire an action potential at a precise time following the onset of a transient stimulus and provide temporal information required for compensatory vestibular reflex circuits, but specifically how transient high-frequency stimuli lead to mechanical activation of hair cells and neural responses is poorly understood. Here, we dissect the relative contributions of mechanics, hair cell transduction, and action potential generation on short-latency responses to transient stimuli. Results provide a framework for the interpretation of synchronized vestibular afferent responses, with relevance to understanding origins of myogenic reflex responses commonly used in the clinic to assay vestibular function, and vestibular short latency potentials commonly used for vestibular phenotyping in rodents.

neuroscience↗