The objective of this study is to understand how the auditory periphery maintains function under conditions of ionic and osmotic stress. The auditory subsystem comprised of the hair cell end organs, afferent synapses and VIIIn will be examined in two species of anuran amphibians - a toad (Bufo marinus) which is dehydration-tolerant and a frog (Rana catesbeiana) which is dehydration-sensitive. This study is specifically designed to determine if auditory function is ultimately compromised due principally to: 1) ionic or osmetic effects on the fluid spaces of the hair cells (otic capsule/endolymph) or 2) direct effects on hair cells and their ability to ion-regulate or volume-regulate. Amphibians make an excellent model for this research because they are uniquely adapted to endure dehydration; at the same time, they cannot use renal mechanisms to compensate for increased plasma salt load. This means that frogs and toads are largely dependent on inner ear regulatory mechanisms to maintain function. Species of anurans differ in their dehydration tolerance, thus the amphibian model will permit an understanding of the compensatory mechanisms and adaptations specific to this tolerance. The anatomy of the anuran inner ear permits access to the endolymphatic space of the hair cells, and this space can be directly manipulated and monitored relative to ion and osmolyte concentration. Finally, the presence of two independent auditory organs in each anuran ear, the amphibian and basilar papillae, allow for comparison of differential effects of ionic and osmotic stressors on non- identical hair cell end organs. The recording of evoked potentials (Brainstem Auditory Evoked Potential and Frequency Following Response) and single-unit recordings from the VIIIn will be performed while challenging R. catesbeiana and B. marinus with increased concentrations of ions (NaCl) and osmotic agents (sucrose, urea). Comparisons will be made of induced auditory dysfunction with both whole body and direct otic capsule challenges. How hair cell systems deal with ionic and osmotic stress is of critical importance in understanding pathological dysfunction of inner hair cell homeostatic mechanisms, as occurs with Meniere's disease, and mechanisms protecting sensory systems from extreme ionic and osmotic disequilibrium.
