Comparison of distortion product otoacoustic emissions in 28 inbred strains of mice

Cochlear function was evaluated in a longitudinal study of 28 inbred strains of mice at 3 and 5 mo of age using measures of distortion product otoacoustic emissions (DPOAEs) in response to a federal initiative to develop rapid mouse phenotyping methodologies. DP-grams at f2 frequencies ranging from 6.3 to 54.2 kHz were obtained in about 3 min/ear by eliciting 2f1 − f2 DPOAEs in 0.1-octave steps of f2 with primary tones at L1 = L2 = 55, 65, and 75 dB SPL. CBA/CaJ mice exhibited average levels of ∼26 dB SPL and this strain was selected as the normal reference strain against which the others were compared. Based upon the configurations of their DP-grams, the 28 mouse strains could be categorized into four distinct groups. That is, nine of the strains including the CBA were designated as the CBA-like group because these mice displayed robust DPOAE levels across frequency. In contrast, the remaining three groups all exhibited irregular DP-gram patterns. Specifically, eight of the remaining 19 strains showed a progressive high- to low-frequency reduction in DPOAE levels that was typical of age-related hearing loss (AHL) associated with mouse strains homozygous for the ahl allele and were labeled as AHL-like strains. Seven strains demonstrating relatively even patterns of reduced DPOAE levels across the frequency-test range were designated as Flat-loss strains. Finally, the remaining four strains exhibited no measurable DPOAEs at either 3 or 5 mo of age and thus were classified as Absent strains. Extending the f2 test frequencies up to ∼54 kHz led to the detection of very early-onset reductions in cochlear function in non-CBA-like groups so that all strains could be categorized by 3 mo of age. Predictably, the AHL-like strains showed more pronounced DPOAE losses at 5 mo than at 3 mo. A similar deterioration in DPOAE levels was not apparent for the Flat-loss strains. Both the AHL-like and Flat-loss strains showed considerably more variability in DPOAE levels than did the CBA-like strains. Together, these findings indicate that DP-grams adequately reveal both frequency-specific loss patterns and details of inbred strain variability.