Stereociliary anomaly in the guinea pig: effects of hair bundle rotation on cochlear sensitivity

Histological analysis of cochleas from 100 albino guinea pigs (Hartley stra in) obtained from Charles River Laboratories revealed an apparently congeni tal anomaly in 24% of animals, with roughly equal prevalence in males and f emales. In affected animals, 15-50% of the first-row outer hair cells (OHCs ) showed distinctly abnormal orientation of the W-shaped stereociliary arra y. These abnormal hair bundles could be rotated by up to 180 degrees from t he normal quasi-radial orientation. Second- and third-row OHCs appeared nor mal in all cases. Cochlear sensitivity was assayed in a subset of animals v ia compound action potentials (CAPs): CAP thresholds in affected animals we re, on average, elevated by 5-10 dB with respect to normal controls. If the contributions of individual OHCs to cochlear 'amplification' add linearly, and if the total OHC contribution corresponds to similar to 45 dB of 'gain ', a quantitative correlation of the degree of stereociliary rotation and t he degree of threshold shift in these ears suggests that first-row OHCs mak e a larger contribution to the cochlear amplifier than either of the other OHC rows. (C) 1999 Elsevier Science B.V. All rights reserved.