Abstract

Organisms rely on environmental, sensory cues to survive. The inner ear houses two sensory modalities: the sense of equilibrium which allows for free mobility in one's environment and hearing which enables localization and communication. The mechanotransducers of both of these sensory systems are the hair cells. These have stereocilia with mechanically gated ion channels that open in response to stimulation, allowing cationic current flow that depolarizes the basolateral membrane and causes release of neurotransmitters in a process called exocytosis. The neurotransmitters bind postsynaptic receptors in the secondary, afferent neurons which spike in response to stimulation. In this work, I focused on understanding the mechanism of information transfer from the hair cell to the afferent neuron. Together this body of work addresses three important questions: (1) Do the tuning properties of the hair cell affect the basic synaptic physiology of neurotransmitter release? To this end, I used an assay of cell surface area as an indication of vesicular release. I studied two groups of differentially tuned hair cells in a frog auditory organ, the amphibian papilla (AP). I identified not only differences in their ability to release vesicles, but differences in the calcium dependence of release in response to sustained sinusoidal stimulation. For strong step depolarizations, I uncovered a novel kinetic mode of release. (2) Do differentially tuned hair cells use different mobile calcium buffers to sculpt synaptic physiology? I addressed this question using immunohistological techniques that examined the differential distribution of two calcium buffers in the AP. I found differences in the distribution of these mobile buffers that were not tonotopic. (3) Can hair cell type affect neurotransmitter release? Using a vestibular organ of the rat, the utricle, and the cell surface area assay, I provide the first direct demonstration that depolarizing voltage steps elicit vesicle release from utricular hair cells. Together, these studies identify release components present in all hair cells. Subtleties in underlying mechanisms of information transfer are likely due to the different demands placed upon each hair cell, which can range from transfer of differentially tuned information to the need for tonic versus phasic information transfer.