A review of synaptic mechanisms of vestibular efferent signaling in turtles: Extrapolation to efferent actions in mammals
Issue title: ARO 2012 Symposium on Vestibular Neuropharmacology, New Insights on Vestibular Neuropharmacology: From Bench to Bedside - San Diego, CA, USA, February 28, 2012
Affiliations: [a] Department of Otolaryngology, University of Rochester, Rochester, NY, USA | [b] Department of Neurobiology and Anatomy, University of Rochester, Rochester, NY, USA | [c] Department of Pharmacology and Physiology, University of Rochester, Rochester, NY, USA | INSERM U1051 Montpellier and UMR 7260 CNRS Aix-Marseille University, Marseille, France
Correspondence:
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Corresponding author: J. Chris Holt, Department of Otolaryngology, University of Rochester, 601 Elmwood Avenue, Box 603, Rochester, NY 14642, USA. Tel.: +1 585 273 3064; Fax: +1 585 271 8552; E-mail: joseph_holt@urmc.rochester.edu
Abstract: The vestibular labyrinth of nearly every vertebrate class receives a prominent efferent innervation that originates in the brainstem and ends as bouton terminals on vestibular hair cells and afferents in each end organ. Although the functional significance of this centrifugal pathway is not well understood, it is clear that efferent neurons, when electrically stimulated under experimental conditions, profoundly impact vestibular afferent discharge. Effects range from chiefly excitation in fish and mammalian vestibular afferents to a more heterogeneous mixture of inhibition and/or excitation in amphibians, reptiles, and birds. What accounts for these diverse response properties? Recent cellular and pharmacological characterization of efferent synaptic mechanisms in turtle offers some insight. In the turtle posterior crista, vestibular efferent neurons are predominantly cholinergic and the effects of efferent stimulation on vestibular afferent discharge can be ascribed to three distinct signaling pathways: (1) Hyperpolarization of type II hair cells mediated by α9/α10-nAChRs and SK-potassium channels; (2) Depolarization of bouton and calyx afferents via α4β2*-containing nAChRs; and (3) A slow excitation of calyx afferents attributed to muscarinic AChRs. In this review, we discuss the evidence for these pathways in turtle and speculate on their role in mammalian vestibular efferent actions where synaptic mechanisms are largely unknown.
