The long range goal of this project is to determine the role(s) of outer hair cells in mammalian hearing. Their contribution to cochlear transduction is analyzed by measuring changes in inner ear currents resulting from biochemical and pharmacological manipulations that are known to affect outer hair cells. The biophysics of these changes is investigated directly using isolated cell culture preparations. The major energy source for the sensory and motor functions of the outer hair cell comes from the """"""""silent current"""""""" generated by- metabolically active ion pumps in the stria vascularis. The silent current is modulated by conductance changes in outer hair cell membranes. Parallel in vivo and in vitro experiments measure the electrical and chemical gradients at the organ and cellular level respectively. Electrical potentials are recorded in vivo at fixed intervals along a microelectrode track and the current density is calculated by taking the first spatial derivative of the potential field. The resulting analysis permits a complete three dimensional characterization of the strial current across the cochlear partition in silence and during acoustic stimulation. The effect of medial olivocochlear component efferent fiber stimulation on the standing current is measured and can be attributed to a direct action on outer hair cells. Perfusion of the cochlea with putative efferent neurotransmitter substances may provide additional insight into the postulated sensory-motor feedback loop that adjusts the electro-mechanical pin of the organ of Corti. In vitro measurements are made of the effect of putative efferent neurotransmitters on the outer hair cell's unique hydraulic support system. Their effects on rapid electromotility and on whole cell conductivity are also measured using a variety of modern recording techniques. These include whole cell recording, and the analysis of images obtained using either video enhanced microscopy or a laser scanning confocal microscope. Our results are described and extended by developing models of the outer hair cell's static and dynamic mechanics. The models, in turn. suggest critical experiments to verify hypotheses. Aspirin ototoxicity and other I manipulations affect rapid electromotility by changing the cell's turgor pressure. The mechanisms of outer hair cell volume regulation are investigated by measuring the effect of aspirin. aminoglycosides and other ototoxic drugs as well as simple sugars on cell volume and turgor pressure. Clarification of their action on the isolated cell coupled with recordings made while perfusing the cochlea with these agents in vivo will lead to a better understanding of the pathology of the ototoxic drugs. Our measurements of extracellular, transmembrane and intracellular potential and chemical gradients in the cochlea will clarify the sensory and motor contributions of the outer hair cell in cochlear transduction and help to explain the basis for hearing loss associated with outer hair cell pathology.
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