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.

Specific Aims

1. 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 rats 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 findings in the guinea pig were inconclusive. We opted to carry out the final measurements in squirrel monkeys, instead of rats, because squirrel monkeys are foveate animals with larger eye movements. 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 under way and will continue into the coming year.

2. 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 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 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 Aim 2, 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, which is 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. This is defined morphologically as the region with calretinin positive complex calyces and is 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 findings 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.

Earth-based Applications of Research Project
Understanding the frequency tuning of the mammalian saccule will help us answer basic science questions pertaining to the specialization of the saccule and may allow us to improve existing clinical measures of saccular function (the vestibular-evoked myogenic potentials) in addition to leading to new clinical measures, such as the vestibulo-ocular reflexes in response to long-duration tones, which can be used both in flight and during re-adaptation to assess vestibular function.

Project Description
NASA Task Book Entry