Signaling in the inner ear: @EatockLab

The vestibular inner ear in mammals encodes head motion into neural signals that drive and calibrate motor reflexes that compensate in real time as we move, stabilizing vision and posture. These pathways operate sub-consciously, sparing us cognitive effort but also leaving us unaware of the vital roles of the vestibular inner ear in facilitating motion at speed through the environment.

We are taking both single-cell and population approaches to analyze encoding of vestibular sensory information. Our questions range from What ion channels underlie functional differences between types of hair cells, synapses and afferents? to How do populations of hair cells and afferents dynamically represent head motions?

Our model preparation is the mouse utricle, which detects linear accelerations. The utricle lends itself to sensory analysis because (1) like all amniote vestibular organs, the utricle has 2 hair cell types (I vs. II) with afferent synaptic contacts (calyx vs. bouton) that use remarkably different transmission mechanisms, and (2) the sensory epithelium has clear zones (striola vs. lateral and medial extrastriola) that give rise to irregular and regular afferent populations with distinct encoding mechanisms (temporal vs. rate). The and hair cell types (I vs. II) with different synapses (calyx vs. bouton) that use unique non-quantal mechanisms in addition to quantal mechanisms found in all hair cells. Through single-cell recordings, we have learned that specialized voltage-gated potassium channels underlie the salient functional differences between hair cell types, synaptic types, and afferent types.

Type I and II hair cells transduce linear head accelerations and head tilt into receptor potentials, which drive transmission across highly specialized calyx and bouton afferent terminals. Postsynaptic responses in the afferent terminals arrive at hemi-nodes where they initiate spike trains that propagate along the nerve fibers through cell bodies (vestibular ganglion neurons, VGNs) and on to central targets in the brainstem and cerebellum.

To analyze activity at the single-cell level, we record whole-cell currents and voltages from hair cells, afferent terminals, or neuronal cell bodies, evoked by controlled motions of the mechanosensitive hair bundles, or voltage or currents injected directly through the recording pipette. This work has provided key insights into ion channels, non-quantal synaptic transmission mechanisms, and spike timing mechanisms of the amniote vestibular inner ear.

An unusual low-voltage-activated K conductance (gK,L) in type I hair cells (Martin et al. 2024; collaboration with Anna Lysakowski (UIC)) requires Kv1.8 (Kcna10) subunits. This conductance speeds up the receptor potential (Martin et al. 2025; collaboration with K. Cullen Lab (Hopkins)). and is implicated in fast non-quantal transmission across the type I-calyx synapse (below).

Martin HR, Lysakowski A, Eatock RA. The potassium channel subunit Kv1.8 (Kcna10) is essential for the distinctive outwardly rectifying conductances of type I and II vestibular hair cells. Elife. 2024 Dec 3;13:RP94342. doi: 10.7554/eLife.94342. PMID: 39625061; PMCID: PMC11614384.

Martin HR, Verdone BM, López-Ramírez O, Green M, Silvian D, Scott E, Cullen KE, Eatock RA. Kv1.8 (Kcna10) potassium channels enhance fast, linear signaling in vestibular hair cells and facilitate vestibulomotor reflexes and balance. bioRxiv [Preprint]. 2025 Jan 28:2025.01.28.634388. doi:10.1101/2025.01.28.634388. PMID: 39975259; PMCID: PMC11838376.

A realistic computational model based on known anatomy, ion channel expression, and ion channel properties and validated on independent datasets suggests distinct fast (ephaptic) and slow (K+ accumulation) mechanisms of non-quantal transmission, both of which depend on dense expression of K-LV channels in presynaptic (hair cell, Kv1.8) and postsynaptic (calyx Kv7) channels, and the specialized calyx geometry (Govindaraju et al. 2023; collaboration with R. Raphael, A. Govindaraju, A. Lysakowski).

Govindaraju AC, Quraishi IH, Lysakowski A, Eatock RA, Raphael RM. Nonquantal transmission at the vestibular hair cell-calyx synapse: K-LV currents modulate fast electrical and slow K+ potentials. Proc Natl Acad Sci USA. 2023 Jan 10;120(2):e2207466120. doi:10.1073/pnas.2207466120. Epub 2023 Jan 3. PMID: 36595693; PMCID: PMC9926171.

Differences in afferent spike timing reflect zonal differences in expression of afferent voltage-gated K channels (Kalluri et al. 2010; Lysakowski et al. 2011; collaboration with A. Lysakowski).

Kalluri R, Xue J, Eatock RA. Ion channels set spike timing regularity of mammalian vestibular afferent neurons. J Neurophysiol. 2010 Oct;104(4):2034-51. doi: 10.1152/jn.00396.2010. Epub 2010 Jul 21. PMID: 20660422; PMCID: PMC2957450.

Lysakowski A, Gaboyard-Niay S, Calin-Jageman I, Chatlani S, Price SD, Eatock RA. Molecular microdomains in a sensory terminal, the vestibular calyx ending. J
Neurosci. 2011 Jul 6;31(27):10101-14. doi: 10.1523/JNEUROSCI.0521-11.2011. PMID:21734302; PMCID: PMC3276652.

Different sodium (Nav) channels (Liu et al. 2016) and Nav current modes through a given channel also have the potential to contribute to differences in spike timing and rate (Baeza-Loya & Eatock, 2024).

Kalluri R, Xue J, Eatock RA. Ion channels set spike timing regularity of mammalian vestibular afferent neurons. J Neurophysiol. 2010 Oct;104(4):2034-51.
doi: 10.1152/jn.00396.2010. Epub 2010 Jul 21. PMID: 20660422; PMCID: PMC2957450.

Liu XP, Wooltorton JR, Gaboyard-Niay S, Yang FC, Lysakowski A, Eatock RA. Sodium channel diversity in the vestibular ganglion: NaV1.5, NaV1.8, and
tetrodotoxin-sensitive currents. J Neurophysiol. 2016 May 1;115(5):2536-55. doi:10.1152/jn.00902.2015. Epub 2016 Mar 2. PMID: 26936982; PMCID: PMC4922472.

Baeza-Loya S, Eatock RA. Effects of transient, persistent, and resurgent sodium currents on excitability and spike regularity in vestibular ganglion neurons. Front Neurol. 2024 Nov 18;15:1471118. doi: 10.3389/fneur.2024.1471118. PMID: 39624672; PMCID: PMC11608953.

To analyze population activity, we record calcium signals from tens of individual hair cells or afferents evoked by common stimuli (fluid jets), with the goal of understanding simultaneous and moment-by-moment processing by the sensory epithelium of a common stimulus, as occurs naturally for head motions. Correlational analyses across different cell types and zones should complement the large database of single-afferent responses collected one at a time and subsequently averaged.

Other collaborations:

How different hair cell types acquire their identities; regeneration in the mature utricle (with Jenny Stone, University of Washington)

González-Garrido A, Pujol R, López-Ramírez O, Finkbeiner C, Eatock RA, Stone JS. The Differentiation Status of Hair Cells That Regenerate Naturally in the Vestibular Inner Ear of the Adult Mouse. J Neurosci. 2021 Sep 15;41(37):7779-7796. doi: 10.1523/JNEUROSCI.3127-20.2021. Epub 2021 Jul 23. PMID: 34301830; PMCID: PMC8445055.

How epithelial zones and maps of hair bundle polarity are specified, and their functional impact

Ono K, Keller J, López Ramírez O, González Garrido A, Zobeiri OA, Chang HHV, Vijayakumar S, Ayiotis A, Duester G, Della Santina CC, Jones SM, Cullen KE,
Eatock RA, Wu DK. Retinoic acid degradation shapes zonal development of vestibular organs and sensitivity to transient linear accelerations. Nat Commun.
2020 Jan 2;11(1):63. doi: 10.1038/s41467-019-13710-4. PMID: 31896743; PMCID: PMC6940366. (with Doris Wu and colleagues, NIDCD)

Ono K, Jarysta A, Hughes NC, Jukic A, Chang HHV, Deans MR, Eatock RA, Cullen KE, Kindt KS, Tarchini B. Contributions of mirror-image hair cell orientation to
mouse otolith organ and zebrafish neuromast function. Elife. 2024 Nov 12;13:RP97674. doi: 10.7554/eLife.97674. PMID: 39531034; PMCID: PMC11556791 (with Basile Tarchini, Jackson Laboratory, Katie Kindt, NIDCD, Kathy Cullen, The Johns Hopkins University, Michael Deans, Emory, and their colleagues)