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

Abstract

Vestibular nerve afferents are divided into regular and irregular groups based on the variability of interspike intervals in their resting discharge. Most afferents receive inputs from bouton terminals that contact type II hair cells as well as from calyx terminals that cover the basolateral walls of type I hair cells. Calyces have an abundance of different subtypes of KCNQ (Kv7) potassium channels and muscarinic acetylcholine receptors (mAChRs) and receive cholinergic efferent inputs from neurons in the brainstem. We investigated whether mAChRs affected membrane properties and firing patterns of calyx terminals through modulation of KCNQ channel activity. Patch clamp recordings were performed from calyx terminals in central regions of the cristae of the horizontal and anterior canals in 13 -18 day old Sprague-Dawley rats. KCNQ mediated currents were observed as voltage sensitive currents with slow kinetics (activation and deactivation), resulting in spike frequency adaptation so that calyces at best fired a single action potential at the beginning of a depolarizing step. Activation of mAChRs by application of oxotremorine methiodide or inhibition of KCNQ channels by linopirdine dihydrochloride decreased voltage activated currents by ~30%, decreased first spike latencies by ~40%, decreased spike thresholds by ~50%, and resulted in continuous firing during depolarizing steps. Interestingly, some of the calyces showed spontaneous discharge in the presence of these drugs. Together, these findings suggest that cholinergic efferents can modulate the response properties and encoding of head movements by afferents.

Dopaminergic Inhibition of Na+ Currents in Vestibular Inner Ear Afferents

“…A similar mechanism exists in the cochlea where the efferent-mediated release of acetylcholine inhibits outer hair cell function through α9 receptors ( Fuchs and Lauer, 2019 ). Conversely, acetylcholine can result in excitation through muscarinic receptor activation which inhibits M-like K + currents in calyx afferents of turtle and rat crista ( Holt et al, 2017 ; Ramakrishna et al, 2021 ). Other transmitters may also contribute to efferent transmission ( Lee and Jones, 2017 ).…”

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

“…The turtle crista preparation is well-suited for pharmacological characterization of vestibular efferent synaptic mechanisms as these cholinergic drugs could be directly applied to the neuroepithelium without access issues. Ease of drug access has also been advantageous in characterizing efferent cholinergic mechanisms in vestibular endorgan preparations from mice ( Poppi et al, 2018 , 2020 ; Ramakrishna et al, 2020 ; Yu et al, 2020 ). However, in characterizing efferent-mediated afferent responses in the intact ear of anesthetized mice, there had to be considerations about whether some of the aforementioned drugs would make it to the perilymphatic space during systemic or middle ear administration.…”

“…The general consensus has been that scopolamine’s effectiveness is attributed to mAChR blockade in central vestibular circuitry ( Soto and Vega, 2010 ; Idoux et al, 2018 ). But evidence regarding efferent activation of mAChRs on vestibular afferents and its sensitivity to mAChR antagonists, now including glycopyrrolate and methscopolamine, suggest we include mAChRs in the vestibular periphery as potential contributors ( Weerts et al, 2015 ; Holt et al, 2017 ; Ramakrishna et al, 2020 ; Schneider et al, 2021 ). While differences in the effectiveness of peripherally and centrally-active mAChR antagonists in alleviating motion sickness are mixed ( Kirsten and Schoener, 1975 ; Uijdehaage et al, 1993 ; Hasler et al, 1995 ; Lang et al, 1999 ; Spinks and Wasiak, 2011 ; Qi et al, 2019 ), glycopyrrolate has been utilized as a vestibular suppressant in Meniere’s patients and for treating vertigo after cochleostomy during cochlear implants ( Storper et al, 1998 ; Chakrabarty et al, 2011 ).…”

“…Recent pharmacological evidence in mice has identified at least three distinct cholinergic mechanisms. 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 ).…”

“…Many pharmacological studies characterizing the underlying cholinergic efferent receptor mechanisms in the mammalian inner ear have been carried out in anesthetized or reduced preparations ( Sridhar et al, 1995 ; Maison et al, 2007 ; Poppi et al, 2018 ; Ramakrishna et al, 2020 ; Schneider et al, 2021 ), while insights into vestibular and auditory efferent function in behaving animals have been primarily performed in transgenic animals missing key efferent synaptic mechanisms ( Lauer and May, 2011 ; May et al, 2011 ; Luebke et al, 2014 ; Hübner et al, 2015 , 2017 ; Terreros et al, 2016 ; Clause et al, 2017 ; Morley et al, 2017 ; Tu et al, 2017 ; Jones et al, 2018 ; Wang et al, 2021 ). The further incorporation of pharmacological tools in probing efferent function in behaving animal models could be used to corroborate those observations in transgenic animals, provided that the drugs used are selective and their application can be restricted to the inner ear while limiting CNS entry.…”