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), and investigation of integration of excitatory and inhibitory synaptic inputs to MOC neurons using a novel brain slice preparation termed a ‘wedge slice’ (Fischl and Weisz 2021, Fischl et al 2024). Recent work to investigate synaptic inputs to MOC neurons uses patch-clamp electrophysiology experiments to determine the effect of serotonergic synapses on the auditory efferent system. Serotonin application excites MOC neurons and lowers their activation threshold, with a metabotropic mechanism that acts to close voltage-gated potassium channels and excite the neurons. This work indicates that the serotonergic system can modulate auditory perception via activation of the MOC system. A manuscript for the project is in preparation. 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. Model parameters were analyzed from experimental data using machine learning models. The first version of the model was published along with its supporting experimental data (Fischl et al 2024), and the model is currently undergoing expansion through the work of a Masters Degree student that is part of the Center for Alzheimer’s and Related Dementias (CARD) program. These experiments continue to 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 second manuscript utilizing the model is expected in 2026-2027, to be published with experimental data on voltage-gated potassium channels. 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. We published a review on neurotransmitters released in the cochlea in Hearing Research (Kitcher, Pederson and Weisz 2021). Recent experimental 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. Optical experiments utilizing fluorescent neurotransmitter indicators indicates that MOC neurons release GABA directly onto type II SGN afferents. A manuscript was recently published in PNAS (Bachman et al 2025). Follow up work in progress in investigating the role of GABA activity in the cochlea both in development and in the mature system, while a collaborative study investigating release of GABA from MOC neurons in development was published in BioRXiv (Castagnola et al 2024) and the manuscript is currently in revision in JNeurosci (Castagnola et al 2025). Genetic and functional properties of LOC and MOC neurons In a collaborative project with Dr. Lisa Goodrich (Harvard), we first performed PatchSeq experiments from identified MOC and LOC neurons to extract both functional and transcriptomic data, and also 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). A dataset containing the PatchSeq data will be published in September 2026.