Contralateral suppression of distortion-product otoacoustic emissions declines with age: a comparison of findings in CBA mice with human listeners.
OBJECTIVE
The auditory efferent system plays presumed roles in enhancing signals in noise, maintaining the cochlea for optimal acoustic signal processing, and may have a protective role in preserving auditory function in the face of ototoxic events. The objective of the study was to measure age-related changes of the medial olivocochlear efferent system in mice by comparing distortion-product otoacoustic emissions generated with and without contralateral white noise stimulation. Consistent with prior work, distortion-product otoacoustic emissions were typically reduced in magnitude when white noise was presented to the contralateral ear. This contralateral suppression is attributed to activation of the medial olivocochlear efferent system, which has an inhibitory effect on the cochlear hair cell system. By studying contralateral suppression on cochlear output in subjects of different ages, it is possible to describe aging effects on the medial olivocochlear system.


METHODS
CBA mice were divided into three age groups: young adult, middle-aged, and old-aged animals (21, 13, and 22 animals per group, respectively), and auditory brainstem responses were obtained before distortion-product otoacoustic emission testing to assess overall hearing abilities.


METHODS
2f1-f2 distortion-product otoacoustic emission recordings were obtained from individual subjects (anesthetized with ketamine/xylazine) in each age group under two conditions: 1) in quiet and 2) in the presence of a contralaterally applied wideband noise.


RESULTS
Principal findings were that distortion-product otoacoustic emission levels decreased with age for mice in a way similar to humans, when correcting for the absolute difference in life spans. In addition, contralateral suppression declined in middle-aged and old-aged groups relative to the young adults for mice in a manner similar to humans. The contralateral suppression decline at low frequencies preceded that of the decline in distortion-product otoacoustic emissions with age.


CONCLUSIONS
Functional decline of the medial olivocochlear efferent system with age precedes outer hair cell degeneration. Loss of medial olivocochlear suppressive function may play a role in the development of presbycusis in both clinical cases and animal models.
Jacobson
Michael
M
Division of Otolaryngology, University of Rochester School of Medicine and Dentistry, 601 Elmwood Avenue, Rochester, NY 14642-8629, USA.
Kim
SungHee
S
Romney
Joshua
J
Zhu
Xiaoxia
X
Frisina
Robert D
RD
D000367:Age Factors; D000375:Aging / Q000502:physiology*; D000818:Animals; D003051:Cochlea / Q000502:physiology*; D016057:Evoked Potentials, Auditory, Brain Stem; D005260:Female; D008297:Male; D051379:Mice; D008808:Mice, Inbred CBA; D017084:Otoacoustic Emissions, Spontaneous; D011304:Presbycusis / Q000503:physiopathology