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POSTDOCTORAL FELLOWSHIP.
During spaceflight and after re-entry, astronauts experience a number of symptoms associated with vestibular dysfunction including the disruption of balance, locomotion, eye-head coordination and space motion sickness. These symptoms may result from changes in the otolith organs. Our long-term goal is to develop and refine sound-evoked diagnostic tests associated with otolith function, which will lead to better ways to evaluate vestibular function in the clinic, in-flight and during re-adaptation. More specifically, the goal of this project is to link mammalian saccular tuning to the frequency tuning of sound-evoked vestibular reflexes. This research originally had two aims.
The first aim is to characterize the frequency tuning of saccular reflexes in response to short duration tones using measurements of both vestibular evoked myogenic potentials (VEMPs) and vestibulo-ocular reflexes (VORs) in rat as well as the responses to long duration tones. In the first year we have worked to establish our animal model of sound-evoked VOR responses. We successfully adapted the surgical techniques previously used in the lab for instrumenting guinea pigs to rats and did preliminary tests of sound-evoked VORs in guinea pigs. Preliminary finding in the guinea pig were inconclusive and we opted to carry out the final measurements in squirrel monkeys (because squirrel monkeys are foveate animals with larger eye movements) instead of rats. These measurements have required the design of a new sound source with specialized fittings to reproducibly introduce sound (with minimal artifact) to the monkey sitting within the eyecoil frame. These tests are currently underway and will continue into the coming year.
The second aim is to use whole cell patch clamp recordings to compare hair cell receptor potentials in the cental (striolar) and peripheral (extrastriolar) zones of the saccular epithelium. In the first year I have learned how to do basic whole-cell voltage clamp measurements and have begun to record receptor potentials and transduction currents from both type I and type II hair cells in the saccular striolar and extrastriolar zones. Preliminary results suggest that the tuning in the different cells vary: some cells exhibit transduction currents that appear to be highpass up to frequencies in the hundreds of Hertz and others that appear to be tuned to more specific frequencies (at tens of Hertz). Despite these promising initial findings, more work needs to be done to improve the methods of hair cell stimulation and to collect sufficient data to see if these differences vary between cell types and zones. Over the course of the upcoming year, we will continue to improve the stability of the measurement apparatus and will collect sufficient data to characterize differences between cell types and zones.
Supplementary to the second aim, I am evaluating the developmental morphology of the saccular epithelium to better assess the location of the striolar and extrastriolar zones. To determine the location of the zones we are evaluating the line of polarity reversal (easily observable both in the fixed tissue used here as well as the live tissue used for the physiological studies) and its location relative to the striola, defined morphological as the region with calretinin positive complex calyces, and observable using confocal microscopy. Over the course of the last year I have been trained in the use of the confocal microscope and have learned about immunohistochemisty. Preliminary results suggest that the striola of the rat saccule is located medial to the line of polarity reversal, which is consistent with previous finding in the rat utricle. In the coming year efforts will continue in characterizing the epithelium at the developmental ages for which I am collecting physiological data.
This work is the first step in linking saccular tuning to the frequency tuning of sound-evoked vestibular reflexes. Not only will this help us answer basic science questions pertaining to specializations of the saccule, it may allow us to improve existing clinical measures of saccular function and explore new measures of saccular function. |
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