Prior vestibular research has shown that afferent responses from semicircular canals and otolith organs deviate from the coherent mechanical stimulation imparted by the overlying accessory structures. This suggests further signal processing by hair cells (HCs) and primary afferent conductances, and by the HC?afferent synapse. Processing is complicated by the parallel modes of synaptic 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 cup-shaped cleft between the two elements. By contrast, type II HCs synapse onto bouton endings and/or the external face of a calyx, with relatively small areas of cellular apposition. Further complexity is conferred by three classes of HC-to-afferent convergence. In the simplest configuration, HCs converge onto an afferent solely via bouton endings. Increased complexity is found at calyceal endings, either as simple calyces enveloping a single HC or as complex calyces where the afferent encompasses two or more HCs. The highest complexity occurs at dimorphic endings that contact both type I and II HCs via a combination of bouton and inner? and outer?face calyceal synapses. Prior experiments in turtles have shown that for calyceal endings, rapid excitatory synaptic transmission, via glutamatergic AMPA receptors, is modulated by K+, H+, and Ca2+ accumulation. In response to HC depolarization, there are dynamic changes in ion concentration in the cleft. These in turn impact responses in both the type I HCs and their afferents due to changes in the equilibrium potentials and driving forces for conductances facing the cleft. As a result, properties of these calyceal contacts are significantly different from those for HC and afferent conductances bathed in the bulk perilymph. Consequently, prior single-electrode biophysical experiments on either HCs or their afferents in situ, or using isolated cells, have been unable to dissect the contributions of HCs and afferents resulting from reciprocal interactions created by the unique volume of the synaptic cleft coupling the two. I now have preliminary biophysical results on isolated anterior semicircular canal epithelia in the mouse, Mus musculus. These experiments demonstrate that I will be able to extend and refine the development of a mammalian preparation in which I can characterize the ionic environment of the synaptic cleft and the biophysical characteristics of synaptic transmission between HCs and afferents under conditions where the membrane potentials of the HC and its associated afferent are controlled simultaneously. I will focus on two areas: (1) the gating of type I HC conductances exposed to the dynamic environment of the synaptic cleft; and (2) the integration of synaptic inputs from type I HCs on the internal face of the calyx and type II HCs synapsing either on the external face of the calyx, or on bouton endings of dimorphic afferents.
The proposed research utilizes a new and technically innovative approach to identify the biophysical bases for variations in the modes of synaptic transmission between vestibular sensory hair cell receptors and the afferent endings they innervate. This research will advance our understanding of the biological bases for dizziness, vertigo, imbalance, and vestibulo-autonomic disorders due to peripheral vestibular dysfunction, and will provide a clear direction for future efforts addressing treatments for these disorders.
