Objectives
Assess post-cranial irradiation
short-term threshold shift
short-term peripheral auditory histopathology
the mouse as an experimental model
Methods
Adult mice were exposed to single-dose radiation of 10 – 60 Gy. Pre- and post-irradiation (baseline, 2 – 8 days) audiometric brainstem response data were recorded with analysis of cochlear ultrastructure.
Results
Significant threshold shift occurred at all test frequencies in mice exposed to ≥ 20 Gy at 4 – 6 days post-irradiation. Ultrastructurally in Rosenthal’s canal and the spiral lamina, neuronal density and extracellular matrix decreased dramatically. There was overall preservation of hair cells, stria vascularis, and vasculature. No difference within Gy group was noted in the frequency or severity of pathology along the length of the cochlea.
Conclusions
The initial impact of radiation in the first week post-exposure focuses on spiral ganglion cell bodies and peripheral projections, resulting in significant threshold shift for irradiation dosages ≥ 20 Gy. This study demonstrates that the mouse is a viable model for study of short-term peripheral auditory effects using single-dose cranial irradiation. Additionally, with access to a precise animal irradiator, the mouse may be used as an experimental model for a fractionated irradiation dosage of 10 Gy, simulating stereotactic therapeutic cranial irradiation.
Hearing sensitivity in captive Atlantic bottlenose dolphins was assessed using a portable electrophysiologic data collection system, a transducer attached to the pan region of the mandible, and non-invasive recording electrodes. The auditory steady-state response (ASSR) was evoked using sinusoidal amplitude-modulated tones at half octave steps from 20–160 kHz and utilized to determine the upper frequency limit of hearing (i.e., the frequency at which threshold was ≤ 120 dB re 1 μPa). An auditory brainstem response (ABR) was then recorded to a moderate-amplitude click (peak-equivalent sound pressure level of 122 dB re 1 μPa) and examined to determine if relationships existed between the upper frequency limit of hearing and the waveform characteristics of the click-evoked ABR. The ASSR and click-evoked ABR were measured in 6 bottlenose dolphins with varying hearing sensitivity and frequency range of hearing. A significant relationship existed between click-evoked ABR wave amplitudes and the upper frequency limit of hearing. Test times for assessment using frequency-specific ASSR and click-evoked ABR were ∼45 minutes and 1 minute, respectively. With further definition of normative data, measurement of click-evoked ABRs could form the basis of an expedited electrophysiologic method for hearing screening in marine mammals.
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