Pharmacologically Distinct Nicotinic Acetylcholine Receptors Drive Efferent-Mediated Excitation in Calyx-Bearing Vestibular Afferents

Abstract

Electrical stimulation of vestibular efferent neurons rapidly excites the resting discharge of calyx/dimorphic (CD) afferents. In turtle, this excitation arises when acetylcholine (ACh), released from efferent terminals, directly depolarizes calyceal endings by activating nicotinic ACh receptors (nAChRs). Although molecular biological data from the peripheral vestibular system implicate most of the known nAChR subunits, specific information about those contributing to efferent-mediated excitation of CD afferents is lacking. We sought to identify the nAChR subunits that underlie the rapid excitation of CD afferents and whether they differ from ␣9␣10 nAChRs on type II hair cells that drive efferent-mediated inhibition in adjacent bouton afferents. We recorded from CD and bouton afferents innervating the turtle posterior crista during electrical stimulation of vestibular efferents while applying several subtype-selective nAChR agonists and antagonists. The ␣9␣10 nAChR antagonists, ␣-bungarotoxin and ␣-conotoxin RgIA, blocked efferent-mediated inhibition in bouton afferents while leaving efferent-mediated excitation in CD units largely intact. Conversely, 5-iodo-A-85380, sazetidine-A, varenicline, ␣-conotoxin MII, and bPiDDB (N,N-dodecane-1,12-diyl-bis-3-picolinium dibromide) blocked efferent-mediated excitation in CD afferents without affecting efferent-mediated inhibition in bouton afferents. This pharmacological profile suggested that calyceal nAChRs contain ␣6 and ␤2, but not ␣9, nAChR subunits. Selective blockade of efferent-mediated excitation in CD afferents distinguished dimorphic from calyx afferents by revealing type II hair cell input. Dimorphic afferents differed in having higher mean discharge rates and a mean efferentmediated excitation that was smaller in amplitude yet longer in duration. Molecular biological data demonstrated the expression of ␣9 in turtle hair cells and ␣4 and ␤2 in associated vestibular ganglia.

Nicotinic acetylcholine receptors regulate vestibular afferent gain and activation timing

“…With regard to α9/α10 mechanisms of inhibition, the published work on turtle and frog extrapolated receptor subunit composition assumed the receptor was α9/10 primarily on the basis of pharmacology. Calcium permeability of the α9* receptor in nonmammals is low and is similar to that of the α4β2 receptor (Lipovsek et al, ), although the pharmacology of the α9/10 in both species is similar (see, e.g., Holt et al, ). The pharmacological profiles of the mammalian α9 homomer and the α9/10 heteromer are also similar even though the biophysical properties are different (Elgoyhen et al, ).…”

The mammalian efferent vestibular system plays a crucial role in the high-frequency response and short-term adaptation of the vestibuloocular reflex

“…both type I and type II hair cells (Baird et al 1988). As shown in turtles (Holt et al 2006), and more recently in mammals (C57BL/6 and cba129 mice: Poppi et al 2014), EVS activation, specifically the cholinergic component, is thought to have a dual effect. One effect of EVS activation is the inhibition of type II hair cells (i.e., strictly a reduction of resting discharge rate and attenuation of sensitivity/gain) via ␣9 nAChRs coupled to SK channels (Holt et al 2006;Poppi et al 2014).…”

“…As shown in turtles (Holt et al 2006), and more recently in mammals (C57BL/6 and cba129 mice: Poppi et al 2014), EVS activation, specifically the cholinergic component, is thought to have a dual effect. One effect of EVS activation is the inhibition of type II hair cells (i.e., strictly a reduction of resting discharge rate and attenuation of sensitivity/gain) via ␣9 nAChRs coupled to SK channels (Holt et al 2006;Poppi et al 2014). The other effect is the excitation of afferents (Boyle and Highstein 1990;Goldberg and Fernandez 1980), through nAChRs that contain ␣4, ␣6, and ␤2 subunits (Holt et al 2015).…”

“…For a review about efferent synaptic mechanisms see Jordan et al (2013). The cholinergic component of EVS activation likely functions by inhibition of type II hair cells (i.e., strictly a reduction of resting discharge rate and attenuation of sensitivity/gain) via ␣9 nAChRs coupled to calcium-activated potassium (SK) channels (Holt et al 2006;Poppi et al 2014) and excitation of afferents (Boyle and Highstein 1990;Goldberg and Fernandez 1980), through nAChRs that contain ␣4, ␣6, and ␤2 subunits (Holt et al 2015). In ␣9-knockout mice this inhibition/excitation dual effect would be partially compromised.…”

Cellular signaling protective against noise-induced hearing loss – A role for novel intrinsic cochlear signaling involving corticotropin-releasing factor?

“…While this topic will not be considered further, it should be recognized that the neural mechanisms of afferent nerve activation and modulation appear to be very similar (although perhaps not identical, e.g. [27]) to the processes used in the cochlea at the level of neurotransmitters and receptors, and most genes associated with cochlear hair cells are also expressed by vestibular hair cells. Yet, a description of various protective mechanisms associated with the cochlea discussed below have never been assessed as protective for the vestibular end organs.…”