Physiological Characterization of Vestibular Efferent Brainstem Neurons Using a Transgenic Mouse Model

Abstract

The functional role of efferent innervation of the vestibular end-organs in the inner ear remains elusive. This study provides the first physiological characterization of the cholinergic vestibular efferent (VE) neurons in the brainstem by utilizing a transgenic mouse model, expressing eGFP under a choline-acetyltransferase (ChAT)-locus spanning promoter in combination with targeted patch clamp recordings. The intrinsic electrical properties of the eGFP-positive VE neurons were compared to the properties of the lateral olivocochlear (LOC) brainstem neurons, which gives rise to efferent innervation of the cochlea. Both VE and the LOC neurons were marked by their negative resting membrane potential <−75 mV and their passive responses in the hyperpolarizing range. In contrast, the response properties of VE and LOC neurons differed significantly in the depolarizing range. When injected with positive currents, VE neurons fired action potentials faithfully to the onset of depolarization followed by sparse firing with long inter-spike intervals. This response gave rise to a low response gain. The LOC neurons, conversely, responded with a characteristic delayed tonic firing upon depolarizing stimuli, giving rise to higher response gain than the VE neurons. Depolarization triggered large TEA insensitive outward currents with fast inactivation kinetics, indicating A-type potassium currents, in both the inner ear-projecting neuronal types. Immunohistochemistry confirmed expression of Kv4.3 and 4.2 ion channel subunits in both the VE and LOC neurons. The difference in spiking responses to depolarization is related to a two-fold impact of these transient outward currents on somatic integration in the LOC neurons compared to in VE neurons. It is speculated that the physiological properties of the VE neurons might be compatible with a wide-spread control over motion and gravity sensation in the inner ear, providing likewise feed-back amplification of abrupt and strong phasic signals from the semi-circular canals and of tonic signals from the gravito-sensitive macular organs.

Reviewing the Role of the Efferent Vestibular System in Motor and Vestibular Circuits

“…However, the mammalian EVS can possess multiple groups, dependent on species. Mouse and cat have only one cluster of EVN neurons, located near the facial nerve genu (Warr, 1975 ; Leijon and Magnusson, 2014 ; Mathews et al, 2015 ). In other mammalian studies, more than one cluster was observed with the major nucleus being referred to as group e (Goldberg and Fernàndez, 1980 ), located dorsal and/or ventral to the facial nerve (Shumilina et al, 1986 ; Perachio and Kevetter, 1989 ).…”

“…More recent work has investigated the physiological characteristics of EVN neurons including passive membrane properties, discharge profiles, and synaptic input profile. Cholinergic EVN neurons were identified in one study using transgenic mice engineered to express enhanced green fluorescent protein (eGFP) under the choline-acetyltransferase (ChAT) promotor (Leijon and Magnusson, 2014 ). In another study, the location of EVN neuronal recordings were confirmed using retrograde tracing from the posterior semicircular canal in ChAT:: tdTomato transgenic mice which confirmed the location of cholinergic EVN neurons, and the staining of each recorded neuron (Mathews et al, 2015 ).…”

Dynamic expression of transcription factor Brn3b during mouse cranial nerve development

“…A1, arrow, A2) and VNTB (Fig. B1,B2) neurons (Leijon and Magnusson, ). Note that the ChAT‐positive neurons of the abducens nucleus do not express Brn3b (Fig.…”

Efferent Inputs Are Required for Normal Function of Vestibular Nerve Afferents

“…It has been shown that cochlear efferents have to be stimulated at least at 10 Hz to facilitate their normally low probability of release (Ballestero et al, 2011), and it is a reasonable assumption that the same is required for vestibular efferents. Vestibular efferents in mouse brainstem slices (Leijon and Magnusson, 2014;Mathews et al, 2015Mathews et al, , 2017 have spontaneous resting discharges of up to 5 spikes/s. The main input to efferent neurons is provided by the vestibular nuclei, and the spontaneous activity of vestibular nuclei neurons is ϳ5 times higher in alert mice (57.5 Ϯ 4.2 spike/s) (Beraneck and Cullen, 2007) compared Figure 8.…”