Age-related changes in auditory potentials of Mongolian gerbil.

The Mongolian gerbil is being evaluated as an animal model of age-related hearing loss (presbyacusis). Part of this evaluation involves estimating auditory thresholds from evoked potentials arising from the auditory nerve and brainstem. The gerbils are born and reared in an environment where the ambient noise level is less than 40 dBA. Some animals are followed longitudinally (8, 19, 23.5 and 36 months), others are studied at 6-8 months (controls), or at 36 months (cross-sectional). Physiological responses are obtained with the animals anesthetized with ketamine and xylazine and transdermal electrodes attached to the head. Auditory signals are tone pips with center frequencies from 1 to 16 kHz in octave steps. Signal levels are varied from 10 to 80 dB SPL in 10 dB steps. For animals (N = 48) in the age range of 6-8 months, mean auditory thresholds were about 20 dB SPL between 2.0 and 8.0 kHz, 25 dB at 16 kHz and 30 dB at 1.0 kHz. By age 22-24 months (N = 15) thresholds had increased by about 10 dB at nearly all frequencies. By age 36 months (N = 37 ears, 32 animals) threshold increases were about 30-35 dB at 8 and 16 kHz, were 25 dB at 4 kHz and 2 kHz, and were 19 dB at 1 kHz. These hearing losses in 36-month gerbil are qualitatively similar to human data for 60-65-year-old males and 70-year-old females. Individual differences in hearing loss were large with the range exceeding 65 dB. While some animals (26/37) had a high-frequency sloping loss, others (11/37) had a bimodal audiometric shape where the hearing loss was smallest at 4 kHz and increased by at least 10 dB at adjacent frequencies.