Noise-induced changes in rat head stability and orientation correlate with dysfunction of otolith irregular afferents

Exposure to intense noise damages both the cochlea and vestibular end organs. Our group previously reported attenuated vestibular short-latency evoked potentials (VsEP) and reduced numbers of calretinin-positive (CR+) calyces in the saccule following noise exposure. Here, we examined rats resting head orientation with respect to gravity as well as head stability following a 4-hour exposure to 120 dB SPL noise. We also assessed how behavioral changes are related to changes in VsEP waveforms and calretinin expression in the utricle and saccule to elucidate potential underlying mechanisms. We found significant reductions in the P2N2 and N2P3 amplitudes following noise exposure. The number of CR+ calyces in both saccule and utricle were also significantly reduced. The size of the reduction in N2P3 amplitude was significantly correlated to the number of CR+ calyces. Animals with a greater loss of CR+ calyces in the utricle following noise showed significant decreases in the average speed of y-axis rotational head motion, while those with a fewer loss of CR+ calyces showed significant increases. In addition, animals with larger noise-induced changes in VsEP and CR+ calyces held their heads motionless longer following noise exposure. We hypothesize that noise exposure is inherently destructive to an animals head stability and thereby manifests as an increase in average head speed in mildly to moderately affected animals. But when the damage was large enough, animals exhibited reduced duration and head motion speed as a behavioral adaptation. The noise exposure also significantly altered the pitch angle of head orientation in animals who had the largest reduction in CR+ calyces in the saccule, suggesting that saccular irregular afferents, including those that are CR+, are critical in control of head and body posture.