A biophysical approach to sensory transduction by vertebrate hair cells

Citation

Corey, David Paul (1980) A biophysical approach to sensory transduction by vertebrate hair cells. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/42z7-s748. https://resolver.caltech.edu/CaltechTHESIS:01252017-090732325

Abstract

An in vitro preparation of the bullfrog sacculus was developed, in which the macular epithelium was arranged as a septum between two chambers. Hair cells are stimulated by moving the overlying otolithic membrane with a glass stimulus probe; the response is recorded as the transepithelial current with the epithelium voltage-clamped. The preparation complements the intracellular studies of individual hair cells under way in this laboratory, and has the advantages of lower electrical noise, higher temporal resolution, greater stability over time, and separate control of solutions bathing apical and basal surfaces of the epithelium. Its major disadvantage is that stimulation and recording methods are somewhat less direct. Four projects were carried out with this preparation:

The generation of the transepithelial "microphonic" current was analyzed in terms of the passive electrical properties of the epithelium and time- and voltage-dependent conductances of the hair cell membrane. The summed receptor currents are modified by the change in membrane potential of hair cells, by a voltage-dependent potassium conductance, and by an adaptive shift of the responsive range of hair bundles. A mathematical model that includes these effects predicts the observed waveform of the microphonic current.

The ionic selectivity of the transduction channel was studied with this preparation and with intracellular voltage-clamping of individual hair cells. The transduction channel is permeable to all alkali cations, to at least some of the divalent alkaline earth metals, and to many small organic cations. The permeability to anions is not clear. The channel constitutes a large, water-filled pore, of at least 0.65 nm diameter, which appears to contain a binding site for permeant organic cations. While a good fit to the current-voltage relation in Na⁺ saline can be obtained with a simple model that includes two energy barriers to permeation near the middle of the membrane, the distribution of barriers is not known with any confidence.

The hair cell response saturates if the mechanically sensitive hair bundle is displaced by more than about 0.4 μm. An adaptation, which follows such saturation and had been seen intracellularly, was shown to be an adaptive shift of the responsive range such as to restore the sensitivity. The shape of the response curve is unchanged by adaptation. The adaptation constitutes a relaxation of the link between hair bundle displacement and bias on the transduction element. The position and shape of the response curve is changed by intracellular Ca⁺⁺ and/or pH, and the rate of the shift is increased by increasing the Ca⁺⁺ concentration.

Following a step displacement of the otolithic membrane, the microphonic current approaches a new equilibrium value over several tens of microseconds. The latency of the response is less than 40 μs at 22°C. The kinetics of the approach to equilibrium are slower at lower temperatures, and depend on the position of the hair bundle. A three-state model for transduction channel gating is presented that supposes that changing the position of the hair bundle directly and continuously changes the free energies of states of the channel. The model can quantitatively predict the observed kinetics of the current.

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