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Functional changes in central vestibular relay circuits following 2G centrifugation. S.M. Jones¹, L. Warren², R. Shukla¹, A. Browning¹, C.A. Fuller² and T.A. Jones¹. ¹School of Medicine, University of Missouri, Columbia; ²SNPB, University of California, Davis, CA 95616. 

Linear vestibular evoked potentials (VsEPs) were used to characterize peripheral and central vestibular function in birds following embryogenesis at 2G centrifugation or at elevated levels of vibration (+20dBre: back-ground levels). Additionally, peripheral and central vestibular responses were characterized in normal hatchlings exposed to 2G centrifugation for 7 days. Linear VsEP response peak latencies, amplitudes, thresholds and input/output functions were quantified. Birds vibrated throughout embryogenesis and up to one-week post-hatch revealed no changes in linear VsEP compared to control siblings. Birds centrifuged at 2G throughout embryogenesis also evidenced no changes in the linear VsEP measured at hatch (P0). Significant changes were seen, however, for linear VsEPs of post-hatch birds placed at 2G for 7 days beginning on post-hatch day 5. Linear VsEPs for these animals displayed significant reductions in response amplitudes for central neural relays of gravity receptors (P2, N2 and P3). In contrast, peripheral nerve responses (i.e., P1, N1) were not significantly altered with the 7-day exposure to 2G. Thus, there was no evidence of generalized changes in peripheral gravity receptor excitability or in the rate of maturation in developing animals under increased levels of gravity or vibration. If gravity level plays a critical role in shaping peripheral vestibular ontogeny at magnitudes between 1 and 2G, then it must operate on more subtle physiological features than can be resolved by the VsEP. In contrast, exposure to elevated gravity during post-hatch periods does alter central vestibular function thus providing direct evidence for central vestibular adaptation to the gravitational environment. The fact that central functional change was observed in hatchlings and not embryos raises the possibility that the first 2-weeks post-hatch may be a critical period of "heightened developmental sensitivity" to hypergravity. 

(Supported by NASA: NAGW 1275, 3910 and NAG 2-1032.)