title: Excitation by Axon Terminal GABA Spillover in a Sound Localization Circuit.
abstract: Synapses from neurons of the medial nucleus of the trapezoid body (MNTB) onto neurons of the lateral superior olive (LSO) in the auditory brainstem are glycinergic in maturity, but also GABAergic and glutamatergic in development. The role for this neurotransmitter cotransmission is poorly understood. Here we use electrophysiological recordings in brainstem slices from P3-P21 mice to demonstrate that GABA release evoked from MNTB axons can spill over to neighboring MNTB axons and cause excitation by activating GABAAR. This spillover excitation generates patterns of staggered neurotransmitter release from different MNTB axons resulting in characteristic "doublet" postsynaptic currents in LSO neurons. Postembedding immunogold labeling and electron microscopy provide evidence that GABAARs are localized at MNTB axon terminals. Photolytic uncaging of p-hydroxyphenacyl (pHP) GABA demonstrates backpropagation of GABAAR-mediated depolarizations from MNTB axon terminals to the soma, some hundreds of microns away. These somatic depolarizations enhanced somatic excitability by increasing the probability of action potential generation. GABA spillover excitation between MNTB axon terminals may entrain neighboring MNTB neurons, which may play a role in the developmental refinement of the MNTB-LSO pathway. Axonal spillover excitation persisted beyond the second postnatal week, suggesting that this mechanism may play a role in sound localization, by providing new avenues of communication between MNTB neurons via their distal axonal projections. Significance statement: In this study, a new mechanism of neuronal communication between auditory synapses in the mammalian sound localization pathway is described. Evidence is provided that the inhibitory neurotransmitter GABA can spill over between axon terminals to cause excitation of nearby synapses to further stimulate neurotransmitter release. Excitatory GABA spillover between inhibitory axon terminals may have important implications for the development and refinement of this auditory circuit and may play a role in the ability to precisely localize sound sources.
authors.affiliation: Department of Otolaryngology and.
authors.forename: Catherine J C
authors.identifier: http://orcid.org/0000-0002-2595-835X
authors.initials: CJ
authors.lastname: Weisz
authors.affiliation: Department of Otolaryngology and Department of Neurobiology, University of Pittsburgh School of Medicine, Eye and Ear Institute, Pittsburgh, Pennsylvania 15213, Center for the Neural Basis of Cognition, University of Pittsburgh, Pittsburgh, Pennsylvania 15213, and.
authors.forename: Maria E
authors.initials: ME
authors.lastname: Rubio
authors.affiliation: Department of Chemistry, University of Kansas, Lawrence, Kansas 66045.
authors.forename: Richard S
authors.identifier: http://orcid.org/0000-0001-6500-6671
authors.initials: RS
authors.lastname: Givens
authors.affiliation: Department of Otolaryngology and Department of Neurobiology, University of Pittsburgh School of Medicine, Eye and Ear Institute, Pittsburgh, Pennsylvania 15213, Center for the Neural Basis of Cognition, University of Pittsburgh, Pittsburgh, Pennsylvania 15213, and kkarl@pitt.edu.
authors.forename: Karl
authors.identifier: http://orcid.org/0000-0002-6053-6400
authors.initials: K
authors.lastname: Kandler
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000818:Animals; D001306:Auditory Pathways / Q000378:metabolism*; D001369:Axons / Q000378:metabolism*; D005260:Female; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D009415:Nerve Net / Q000378:metabolism*; D009847:Olivary Nucleus / Q000378:metabolism; D009924:Organ Culture Techniques; D017729:Presynaptic Terminals / Q000378:metabolism*; D013017:Sound Localization / Q000502:physiology*; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
chemical_list: D005680:gamma-Aminobutyric Acid
keywords: GABA; LSO; MNTB; caged neurotransmitter; sound localization; spillover