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THE EFFECTS OF HYPERGRAVITY ON VESTIBULAR NEURAL FUNCTION IN POSTNATAL RATS.  S. Irons-Brown¹, S. M. Jones², W. Avniel², K. Paggett², G. Subramanian², Lisa Baer³, A. E. Ronca³ and T. A. Jones^1&2. ¹Dept of Physiology, ²Dept of Surgery-ENT, Univ of Missouri-Columbia, and ³Life Sciences Division, NASA Ames, Moffett Field, CA.

     The gravitational environment can have profound influences on physiological systems.  In this study, we evaluated whether vestibular functional adaptation occurs in response to chronic gravitational loading.  We used linear vestibular evoked potentials (VsEPs) to characterize gravity receptor function in rats following 14 days of 2G centrifugation.  VsEP response peak latencies (P1 to N2), amplitudes (P1/N1 to P2/N2), and thresholds were quantified and compared between experimental and control animals.  One experimental group consisted of animals placed on the centrifuge on postnatal day 7 i.e. P7 (P21 test group).  The second experimental group was placed on the centrifuge on P14 (P28 test group).   The age matched control groups were at 1G (stationary) or 1.03G (rotational).  There were no significant changes in the latencies and amplitudes among the experimental and control groups.  However, VsEP thresholds (in dBre: 1.0 g/ms) were significantly different.   For the P21 group, thresholds for the 2G animals (-8.70 ± 3.52dB p=0.006) were significantly higher than the controls (-12.19 ± 2.44dB (pooled)).  For the P28 group, thresholds for the 2G animals (-8.75 ± 2.99dB p=0.04) were significantly higher than stationary controls (-12.00 ± 4.34dB).  These results tend to support the hypothesis that functional adaptation to 2G occurs in the mammalian vestibular system.  The results suggest that the adaptive process may include a reduction in general sensitivity of the vestibular periphery.  This contrasts with previous studies in the chicken, which demonstrated adaptive changes in VsEP central response components, but no reduction in VsEP thresholds (Jones et. al. 2000).  Although, the reduction in sensitivity seen here appears to be consistent it is not dramatic.  

     (Supported by NASA:  NAG5-4607 (TAJ).)