Auditory Assessment

There are several methods to test hearing function in mice. The most comprehensive approach is often the better method of phenotypic characterization, incorporating several types of testing. However, in mice genetically engineered to express mutations in a specific region or response, there may be one test in particular that is more highly sensitive to the area of interest. Researchers should be aware of the basis behind each testing system and coordinate the choice of test with the specific gene in question.

Exposure to an augmented acoustic environment (AAE) can be used to assess the amount of hearing loss evident in transgenic mice as compared to control mice. In AAE, mice are exposed beginning at approximately four weeks of age to a 70 dB SPL broadband noise. Exposure should be for at least 12 h per night and should continue for no less than 30 days sequentially. After AAE exposure, auditory performance is measured using one or more of several tests.32

The simplest auditory assessment exam is the startle reflex. In this test, the amplitude of response to immediate intense sounds is measured, with no prepulse present. The startle stimulus is a 100 dB 4 kHz tone burst, played in 10 ms duration with a 1 ms rise/fall time. Animals to be tested are placed in a startle chamber so that responses can be measured via transduction of movements into voltage units. Consequential voltage units are displayed on a digital storage oscilloscope that shows a spike-like voltage change in reaction to an acoustic startle response from the animal in the chamber. Amplitude of the response is defined as the largest peak-to-peak voltage deflection in the first 30 ms following onset of startle stimulus.22

By the addition of prepulse inhibition (PPI) to the simple startle reflex, researchers can evaluate behavioral responses to tones of moderate intensity. A 70 dB tone presented 100 ms prior to the intense startle stimulus results in a reduced startle amplitude. Using PPI, the neural pathways that involve the auditory brainstem and other central regions can be evaluated. The magnitude of the PPI response reflects the behavioral salience of the prepulse tones.32

The auditory-evoked brainstem response (ABR) can be used to measure auditory thresholds in mice by testing for cochlear sensitivity. The assay is based on the emission of synchronized neural discharges from the auditory nerve and brainstem in response to short acoustic stimuli. The procedure consists of anesthetizing mice and placing on a heating pad, then implanting with stainless steel electrodes below each ear superficial to the auditory nerve. Click stimuli tones, randomized over 8 to 32 kHz, are presented through earphones over both ears. The loudness of each tone is varied in 5 dB steps. The auditory threshold is defined as the lowest stimulus intensity at which a normalized ABR wave can be identified.

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