The goal of this project is to characterize the resonant frequencies and ionic currents of toadfish saccular hair cells in an attempt to reveal either a regional mapping of these properties or a grouping of particular electrophysiological properties. Such grouping may reveal a coding in the hair cells of the saccule that is neither regional nor solely for frequency, but perhaps for other properties of the sensory stimulus. It is anticipated that this work will lead to an understanding of the function of the saccule and to future experiments that will define the role of these ionic currents in sensory coding in hair cells in general. The ionic currents in the hair cells will be characterized and then related to the hair cell morphology. The saccular macula has macro- and microscopic morphological division (by light and scanning electron microscopy); cells will be sampled from these regions for the experiments to be performed. The magnitude and kinetics of activation and deactivation of the four potassium currents found in the hair cell will be determined. I(k) will be isolated by blocking IK(Ca) with TEA or charybdotoxin at voltages where the A current is inactivated. Voltage paradigms designed to reveal the A current and inward rectifier will provide a map of these currents by cell type and location. Also, the resonant frequency of each hair cell will be determined. Hair cell resonant frequencies will be correlated with the currents measured below and mapped on the saccule. While in currrent clamp, action potential production by the hair cell will be tested using positive current steps. The hypothesis underlying this research is that the diverse complements of ionic currents found in toadfish saccular hair cells are not configured to code only frequency, but also may enable the cell to respond optimally to other attributes of the auditory stimulus. There is considerable unexplained diversity in hair cell ionic currents. It does not appear that the diversity is due to a regional distribution of cell types attributable to segregation by auditory (fast kinetics, high frequency) or vestibular (slow kinetics, low frequency) function. Additionally, the range of resonant frequencies in the saccule (107 - 175 Hz) does not include either the toadfish calling frequency (up to 250 Hz) or hearing freqency (up to 700 Hz). These and other observations cast some doubt on the concept that current kinetics are coding only characteristic frequency. Perhaps one set of cells codes frequency and another transients, e.g. The experiments are therefore designed to look for sub-sets of cells with specific ionic current composition that may respond differently to simulated transducer currents. These sub-sets will be classed with their morphology, particularly hair cell bundle morphology, site of origin and response to three putative efferent transmitters. Due to the technical difficulties of using either sound or bimorph stimulation at characteristic frequency ranges in isolated cells, simulated transducer currents will be used. Simulated currents also have an advantage in that features of the stimulus can be manipulated to test their interaction with ionic currents.
