Synaptic Circuitry of Auditory Neurons

National Institute On Deafness And Other Communication Disorders

Investigators

Linked publications & trials

Abstract

The focus of the Section on Neuronal Circuitry this year has been generating data for scientific presentations and publications. Projects fall into two major categories: 1) Intrinsic electrical properties and synaptic inputs of medical olivocochlear (MOC) neurons in the brainstem, and 2) Influence of synaptic outputs of MOC neurons in the cochlea. Synaptic inputs of olivocochlear neurons in the brainstem and synaptic outputs in the cochlea Medial olivocochlear (MOC) neurons have cell bodies in the brainstem, where they receive excitatory synaptic inputs conveying sound information from the cochlea via the cochlear nucleus. Very little is known about the neuronal circuitry driving activity of MOC neurons because their cell bodies are difficult to locate in un-stained brain preparations. We completed the initial goal of the lab to develop and characterize a reliable genetic technique to perform single cell patch-clamp recordings from the MOC neurons in brain slices from mice. Using this technique, we aim to understand the diversity of synaptic inputs of MOC neurons to fully understand how the neurons are activated and modulated during auditory perception. We also aim to determine how the MOC neuron baseline electrical properties may change to contribute to neuronal hyperactivity that has been shown to occur in pathological situations such as in tinnitus or hyperacusis. Experimental accomplishments include characterization of the ChAT-IRES-Cre x tdTomato mouse line as a reliable tool for identifying MOC neurons in brainstem slices for single cell patch-clamp electrophysiology experiments. Using this mouse line, we discovered a pathway of sound-driven inhibition of MOC neurons by neurons of the medial nucleus of the trapezoid body (MNTB), and are characterizing the effect of this inhibition on MOC neuron activity. The initial report was published in January 2020 (Torres Cadenas et al 2020, JNeurosci), with an additional detailed characterization of synaptic plasticity of MNTB-MOC synapses published in 2022 (Torres Cadenas et al 2022, JPhysiol). In addition, the manuscript for a project investigating the integration of excitatory and inhibitory synaptic inputs to MOC neurons using a novel brain slice preparation termed a wedge slice is in preparation. A methods manuscript describing the technique was published in 2021 (Fischl and Weisz, JoVE). To complement patch-clamp electrophysiology projects using the wedge slice, we have developed a computational model of MOC neurons. The model includes addition of accurate post-synaptic potential waveforms and intrinsic electrical conductances from experimental data collected in our own laboratory, which is being used to investigate the integration of excitatory and inhibitory synaptic inputs to MOC neurons to determine how sound-driven inhibition alters MOC function and the MOC effects on the cochlea. These experiments will elucidate the mechanisms of synaptic activation and inhibition of MOC neurons, which in turn drives their inhibition of mechanical activity in the cochlea, thus shaping the cochlear response to sound. A manuscript utilizing the model is in preparation. Recent work uses patch-clamp electrophysiology experiments from identified MOC neurons in mouse brainstem slices to determine the effect of serotonin on the auditory efferent system. Serotonin application excites MOC neurons and lowers their activation threshold. This work indicates that the serotonergic system can modulate auditory perception via activation of the MOC system. This project is ongoing, and an abstract and poster were presented at ARO 2023. Synaptic outputs of olivocochlear neurons in the cochlea MOC neurons project to the cochlea, where they decrease the mechanical movement of the basilar membrane by inhibiting cochlear OHCs. OHC activity enhances signaling to cochlear IHCs and shapes cochlear tuning curves and gain. MOC synapses onto OHCs are implicated in inhibiting OHC via coupling of cholinergic channel coupling to an SK potassium conductance. This effect is implicated in improved hearing in background noise, and protection of the cochlea against sound trauma. MOC neurons are also thought to release other neurotransmitters in the cochlea with poorly described effects on cochlear function. Recent work has detailed GABAergic responses in type II spiral ganglion neurons, while immunohistochemistry suggests a GABAergic synapse between MOC efferent and type II afferent neurons. This work was presented at the ARO midwinter meeting in 2020, 2022, and 2023. In addition, a related review on neurotransmitters released in the cochlea was recently published in Hearing Research (Kitcher, Pederson and Weisz 2021). Genetic and functional properties of LOC neurons In a collaborative project with Dr. Lisa Goodrich (Harvard), we performed patch-clamp recordings from identified LOC neurons in mouse brainstem slices to correlate functional measures with amounts of CGRP protein by measuring CGRP-GFP fluorescence. This was combined with transcriptomic and morphological analyses from the Goodrich lab. This work was published in 2023 (Frank et al, eLife).