Synaptic Physiology of the Vestibular Periphery

Project Summary. Vestibular afferent responses deviate from the coherent mechanical stimulation imparted by
their overlying accessory structures. This implicates further processing by hair cells (HCs) and primary afferent
conductances, and by afferent and efferent synapses. Processing is complicated by the parallel modes of
transmission between HCs and afferents, and the convergence of multiple HCs onto single afferents. Type I HCs
are enveloped by an afferent calyx, creating a restricted volume in the cleft between them. Type II HCs synapse
onto the external face of a calyx and/or onto bouton endings via relatively small contact areas. This results in
three types of HC-to-afferent convergence. In the simplest form, HCs converge onto an afferent solely at bouton
endings. Increased complexity is found at calyces, including both simple calyces enveloping one HC and
complex calyces encompassing two or more HCs. The highest complexity occurs at dimorphic endings that
receive input from both HC types through a combination of bouton and both inner- and outer-face calyceal
synapses. Prior studies have shown that for calyceal endings, rapid excitatory quantal transmission via
glutamatergic AMPA receptors may be modulated by K+, H+, and Ca2+ accumulation. Dynamic changes in cleft
ion concentrations occur in response to HC or afferent depolarization. These in turn impact responses in both
the type I HCs and their afferents due to changes in the conductances and equilibrium potentials facing the cleft.
As a result, properties of inner-face calyceal contacts differ significantly from those of HC and afferent
conductances bathed in the bulk perilymph. For that reason, prior single-electrode biophysical experiments on
HCs or their afferents in situ, or on isolated cells, have been unable to distinguish the contributions of HCs and
afferents resulting from reciprocal interactions created by the unique volume of the synaptic cleft coupling the
two. In this project, biophysical and morphological experiments will be performed on HC and afferent synaptic
pairs in the posterior semicircular canal crista ampullaris of the red-eared turtle, T. scripta elegans, taking
advantage of our unique ability to record simultaneously from both a HC and the afferent it contacts. This
approach will be used to characterize the ionic environment of the synaptic cleft, the biophysical properties of
HC and afferent conductances under conditions where the membrane potentials of a HC and its associated
afferent are controlled simultaneously, and the structural elements responsible for these properties. The project
has two major aims: (1) to contrast the biophysics and morphology of synaptic inputs from type I HCs onto the
internal face of the calyx with those from type II HCs onto the calyceal external face and/or bouton endings of
nearby branches, and examine their modulation by accumulation of ions and potential transmitters; and (2) to
identify the mechanisms, modulators, sites of action and regional differences in nicotinic and muscarinic
cholinergic efferent input to type I and II HCs and calyx afferents across the crista. Overall, this project will provide
an integrated structural, functional biophysical characterization of peripheral vestibular signal processing.

Grant Number: 5R01DC019953-04
NIH Institute/Center: NIH
Principal Investigator: JONATHAN ART