GABAergic synapses between auditory efferent neurons and type II spiral ganglion afferent neurons in the mouse cochlea

Abstract

Cochlear outer hair cells (OHCs) are electromotile and are implicated in mechanisms ofamplification of responses to sound that enhance sound sensitivity and frequency tuning. Theysend information to the brain through glutamatergic synapses onto a small subpopulation ofneurons of the ascending auditory nerve, the type II spiral ganglion neurons (SGNs). The OHCsynapses onto type II SGNs are sparse and weak, suggesting that type II SGNs respond primarilyto loud and possibly damaging levels of sound. OHCs also receive innervation from the brainthrough the medial olivocochlear (MOC) efferent neurons. MOC neurons are cholinergic yetexert an inhibitory effect on auditory function as they are coupled to alpha9/alpha10 nicotinicacetylcholine receptors (nAChRs) on OHCs, which leads to calcium influx that gates SKpotassium channels. The net hyperpolarization exerted by this efferent synapse reduces OHCactivity-evoked electromotility and is implicated in cochlear gain control, protection againstacoustic trauma, and attention. MOC neurons also label for markers of gamma-aminobutyric acid(GABA) and GABA synthesis. GABAB autoreceptor (GABABR) activation by GABA releasedfrom MOC terminals has been demonstrated to reduce ACh release, confirming importantnegative feedback roles for GABA. However, the full complement of GABAergic activity in thecochlea is not currently understood, including the mechanisms that regulate GABA release fromMOC axon terminals, whether GABA diffuses from MOC axon terminals to other postsynapticcells, and the location and function of GABAA receptors (GABAARs). Previous electronmicroscopy studies suggest that MOC neurons form contacts onto several other cell types in thecochlea, but whether these contacts form functional synapses, and what neurotransmitters areemployed, are unknown. Here we use immunohistochemistry, optical neurotransmitter imagingand patch-clamp electrophysiology from hair cells, afferent dendrites, and efferent axons todemonstrate that in addition to presynaptic GABABR autoreceptor activation, MOC efferentaxon terminals release GABA onto type II SGN afferent dendrites with postsynaptic activitymediated by GABAARs. This synapse may have multiple roles including developmentalregulation of cochlear innervation, fine tuning of OHC activity, or providing feedback to thebrain about MOC and OHC activity.