title: Glutamatergic signaling at the vestibular hair cell calyx synapse.
abstract: In the vestibular periphery a unique postsynaptic terminal, the calyx, completely covers the basolateral walls of type I hair cells and receives input from multiple ribbon synapses. To date, the functional role of this specialized synapse remains elusive. There is limited data supporting glutamatergic transmission, K(+) or H(+) accumulation in the synaptic cleft as mechanisms of transmission. Here the role of glutamatergic transmission at the calyx synapse is investigated. Whole-cell patch-clamp recordings from calyx endings were performed in an in vitro whole-tissue preparation of the rat vestibular crista, the sensory organ of the semicircular canals that sense head rotation. AMPA-mediated EPSCs showed an unusually wide range of decay time constants, from <5 to >500 ms. Decay time constants of EPSCs increased (or decreased) in the presence of a glutamate transporter blocker (or a competitive glutamate receptor blocker), suggesting a role for glutamate accumulation and spillover in synaptic transmission. Glutamate accumulation caused slow depolarizations of the postsynaptic membrane potentials, and thereby substantially increased calyx firing rates. Finally, antibody labelings showed that a high percentage of presynaptic ribbon release sites and postsynaptic glutamate receptors were not juxtaposed, favoring a role for spillover. These findings suggest a prominent role for glutamate spillover in integration of inputs and synaptic transmission in the vestibular periphery. We propose that similar to other brain areas, such as the cerebellum and hippocampus, glutamate spillover may play a role in gain control of calyx afferents and contribute to their high-pass properties.
authors.affiliation: Departments of Otolaryngology, Head and Neck Surgery, and.
authors.forename: Soroush G
authors.initials: SG
authors.lastname: Sadeghi
authors.affiliation: Department of Otorhinolaryngology, Head and Neck Surgery, University Medical Center Groningen, University of Groningen, Groningen 9700 RB, The Netherlands.
authors.forename: Sonja J
authors.initials: SJ
authors.lastname: Pyott
authors.affiliation: Departments of Otolaryngology, Head and Neck Surgery, and Neuroscience, The Center for Hearing and Balance and The Center for Sensory Biology, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, and.
authors.forename: Zhou
authors.initials: Z
authors.lastname: Yu
authors.affiliation: Departments of Otolaryngology, Head and Neck Surgery, and Neuroscience, The Center for Hearing and Balance and The Center for Sensory Biology, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, and eglowat2@jhmi.edu.
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Glowatzki
mesh_terms: D000818:Animals; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects / Q000502:physiology; D005260:Female; D018698:Glutamic Acid / Q000378:metabolism*; D018069:Hair Cells, Vestibular / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D018408:Patch-Clamp Techniques; D011810:Quinoxalines / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D018091:Receptors, AMPA / Q000037:antagonists & inhibitors / Q000378:metabolism; D013569:Synapses / Q000187:drug effects / Q000378:metabolism*; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology*
chemical_list: D018691:Excitatory Amino Acid Antagonists; D011810:Quinoxalines; D018091:Receptors, AMPA; C062865:2,3-dioxo-6-nitro-7-sulfamoylbenzo(f)quinoxaline; D018698:Glutamic Acid
keywords: calyx; glutamate receptor; hair cell; spillover; synaptic transmission; vestibular