Efferent Vestibular Neurons Show Homogenous Discharge Output But Heterogeneous Synaptic Input Profile In Vitro

Abstract

Despite the importance of our sense of balance we still know remarkably little about the central control of the peripheral balance system. While previous work has shown that activation of the efferent vestibular system results in modulation of afferent vestibular neuron discharge, the intrinsic and synaptic properties of efferent neurons themselves are largely unknown. Here we substantiate the location of the efferent vestibular nucleus (EVN) in the mouse, before characterizing the input and output properties of EVN neurons in vitro. We made transverse serial sections through the brainstem of 4-week-old mice, and performed immunohistochemistry for calcitonin gene-related peptide (CGRP) and choline acetyltransferase (ChAT), both expressed in the EVN of other species. We also injected fluorogold into the posterior canal and retrogradely labelled neurons in the EVN of ChAT:: tdTomato mice expressing tdTomato in all cholinergic neurons. As expected the EVN lies dorsolateral to the genu of the facial nerve (CNVII). We then made whole-cell current-, and voltage-clamp recordings from visually identified EVN neurons. In current-clamp, EVN neurons display a homogeneous discharge pattern. This is characterized by a high frequency burst of action potentials at the onset of a depolarizing stimulus and the offset of a hyperpolarizing stimulus that is mediated by T-type calcium channels. In voltage-clamp, EVN neurons receive either exclusively excitatory or inhibitory inputs, or a combination of both. Despite this heterogeneous mixture of inputs, we show that synaptic inputs onto EVN neurons are predominantly excitatory. Together these findings suggest that the inputs onto EVN neurons, and more specifically the origin of these inputs may underlie EVN neuron function.

Heading in the right direction: the importance of direction selectivity for cerebellar motor learning

“…Recent electrophysiological characterization of EVN neurons revealed heterogeneous synaptic input (Mathews et al . ) consistent with polysynaptic tracing work demonstrating connections between the EVN and many other brain structures including the flocculus (Metts et al . ).…”

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

“…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 ). Both studies were conducted in vitro .…”

“…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 ).…”

“…Both studies were conducted in vitro . EVN neurons displayed a negative membrane potential and characteristic firing pattern (Leijon and Magnusson, 2014 ; Mathews et al, 2015 ), that appear to be mediated by fast transient outward K + currents ( I A ) (Leijon and Magnusson, 2014 ). EVN neurons demonstrated two discharge profiles at rest—spontaneous and non-spontaneously active (Highstein and Baker, 1985 ; Mathews et al, 2015 ), perhaps indicating a neuronal heterogeneity within the EVN.…”

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.…”