Efferent Synaptic Transmission at the Vestibular Type II Hair Cell Synapse

Abstract

In the vestibular peripheral organs, type I and type II hair cells (HCs) transmit incoming signals via glutamatergic quantal transmission onto afferent nerve fibers. Additionally, type I HCs transmit via 'non-quantal' transmission to calyx afferent fibers, by accumulation of glutamate and potassium in the synaptic cleft. Vestibular efferent inputs originating in the brainstem contact type II HCs and vestibular afferents. Here, we aimed at characterizing the synaptic efferent inputs to type II HCs using electrical and optogenetic stimulation of efferent fibers combined with in vitro whole-cell patch clamp recording from type II HCs in the rodent vestibular crista. Properties of efferent synaptic currents in type II HCs were similar to those found in cochlear hair cells and mediated by activation of 9/nicotinic acetylcholine receptors (AChRs) and SK potassium channels. While efferents showed a low probability of release at low frequencies of stimulation, repetitive stimulation resulted in facilitation and increased probability of release. Notably, the membrane potential of type II HCs measured during optogenetic stimulation of efferents showed a strong hyperpolarization even in response to single pulses and was further enhanced by repetitive stimulation. Such efferent-mediated inhibition of type II HCs can provide a mechanism to adjust the contribution of signals from type I and type II HCs to vestibular nerve fibers. As a result, the relative input of type I hair cells to vestibular afferents will be strengthened, emphasizing the phasic properties of the incoming signal that are transmitted via fast non-quantal transmission. New and NoteworthyType II vestibular hair cells (HCs) receive inputs from efferent fibers originating in the brainstem. We used in vitro optogenetic and electrical stimulation of efferent fibers to study their synaptic inputs to type II HCs. Efferent inputs inhibited type II HCs, similar to cochlear efferent effects. We propose that efferent inputs adjust the contribution of signals from type I and type II HCs that report different components of the incoming signal to vestibular nerve fibers.

Cholinergic Modulation of Membrane Properties of Calyx Terminals in the Vestibular Periphery

“…It is important to note that while efferent terminals are present in this preparation, efferent fibers are cut and separated from their somata located in the brainstem [52,53]. These efferent fibers and terminals can be stimulated by electrical or optogenetic stimulation [55]. Note that while the drugs applied in the current study can potentially affect these efferent terminals, this will not affect our conclusions.…”

The mammalian efferent vestibular system utilizes cholinergic mechanisms to excite primary vestibular afferents

“…Similar responses in frog and turtle are mediated through α9nAChRs-SK activation that hyperpolarizes type II hair cells to reduce transmitter release and inhibit afferent discharge 16 , 17 , 57 – 59 . Recently, patch clamp data has demonstrated the α9nAChR-SK mechanism is widely present in mouse type II hair cells 26 , 27 , which begs the question why efferent-mediated afferent inhibition is not commonly observed? In fish and turtle 17 , 60 , one explanation is inhibitory influences on afferent discharge during EVS stimulation are often masked by competing efferent-mediated excitatory mechanisms.…”

“…In situ hybridization and RT-PCR data exists for numerous mAChRs and nAChRs in the mammalian vestibular periphery 19 – 25 . Furthermore, recent pharmacological data have confirmed that locally-released ACh activates an α9nAChR/SK mechanism in mouse type II hair cells 5 , 26 , 27 . While exogenous application of cholinergic and GABAergic agonists excites mouse vestibular afferent neurons 5 , 28 – 30 , it is unclear how these effects are related to the afferent excitation seen with direct efferent stimulation.…”

Characterizing the Access of Cholinergic Antagonists to Efferent Synapses in the Inner Ear

“…Our data with glycopyrrolate and methscopolamine indicated that the systemic administration of some charged compounds can make it to the inner ear. Although α9α10nAChRs are expressed in mammalian vestibular endorgans and their activation gives rise to hyperpolarization of type II hair cells ( Poppi et al, 2018 , 2020 ; Yu et al, 2020 ), direct observations of efferent-mediated inhibition of mouse vestibular afferents are infrequent ( Goldberg and Fernández, 1980 ; Schneider et al, 2021 ; also see Figure 1A ). One likely explanation is that the inhibitory component of efferent-mediated afferent responses is obscured by efferent-mediated fast excitation ( Holt et al, 2015 ).…”

“…Efferent-mediated slow excitation requires activation of afferent muscarinic ACh receptors (mAChRs) while efferent-mediated fast excitation depends on activation of afferent α4β2*-containing nicotinic AChRs (nAChRs) ( Ramakrishna et al, 2020 ; Schneider et al, 2021 ). Efferent-mediated inhibition of vestibular afferents is thought to proceed through the sequential activation of α9α10nAChRs and SK2 potassium channels in type II vestibular hair cells ( Poppi et al, 2018 , 2020 ; Yu et al, 2020 ). While patch clamp recordings demonstrate that α9α10nAChRs and SK2 are widely expressed in type II hair cells, direct observations of efferent-mediated inhibition of mammalian vestibular afferents are infrequent ( Goldberg and Fernández, 1980 ; Marlinski et al, 2004 ; Schneider et al, 2021 ).…”