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Neurovestibular adaptation in the utricular otolith following extended periods of 3G exposure and re-adaptation to 1G.  R. Boyle¹, Y. Popova¹, J. Varelas¹, A. Mofrad¹, A. Kondrachuk^{2      1}BioVIS Center, NASA Ames Research Center, Moffett Field, CA 94035, and ²Institute of Physics, National Acad. Science, Kiev, Ukraine.

   The utricular organ senses the sum of inertial force due to head translation and head tilt relative to the gravitational vertical. A change in force has a profound effect on how an organism maintains equilibrium, and the neural response might involve the peripheral otolith receptors, the brain or both. If the influence of µG leads to adaptation and subsequent re-adaptation in otolith function upon return to 1G, then: does the transfer from 1G to 3G impart the opposite effects on changes of structure and function response seen following µG exposure? Do the effects accompanying transfer from 3G to 1G conditions resemble in part (as an analog) the transfer from 1G to µG? Here we record the impulse response of utricular afferents to servo-controlled linear accelerations and tilt profiles in the anesthetized oyster toadfish, Opsanus tau, after 1-, 2-, 3-, 4-, 5-, 8- and 16-day exposure to 3G centrifugation, and following 1-8 days recovery to study re-adaptation to 1G.  Typically >60 afferents are well characterized in each animal. Afferents were also examined during electric shocks applied to the efferent vestibular pathway at rest and during motion tests. In each fish 45-60 horizontal canal afferents are also studied to yaw. Synaptic body counts in striolar and extrastriolar hair cells are performed on serial sections of the macula using transmission electron microscopy. Results to date show a biphasic pattern: an initial sensitivity up-regulation (3- and 4-day) followed by a significant decrease after 16-day exposures. Return to control values following 16-day exposure is on the order of 4 days. A significant decrease by 50% in hair cell synaptic bodies is observed between control and 16-day exposure fish. The otolith sensitivity to acceleration is clearly regulated by altered gravity exposure, and indicates that this paradigm might be used to study the neural and behavioral responses, including the response to countermeasure intervention, to altered gravity in a ground-based model.