Low-frequency tone pips elicit exaggerated startle reflexes in C57BL/6J mice with hearing loss

Abstract

The strength of the acoustic startle reflex (ASR) as a function of age was studied in adult C57BL/6J and CBA/CaJ mice, because altered ASR levels are a potential behavioral consequence of the neural reorganization that accompanies the early-onset hearing loss of the C57BL, in contrast to the normal-hearing CBA. For C57BL mice at 14-36 weeks of age, compared with 7-week-old mice, high-frequency thresholds measured with the auditory brainstem response (ABR) were less sensitive by about 25-30 dB while the hearing loss at low frequencies was 10-15 dB, but by 60 weeks losses of 45-50 dB were present across the entire spectrum. Their ASR amplitudes for 16 kHz tone pips were highest at 7 weeks and then declined with age, but, for 4 kHz tones the ASR increased in strength at 18 weeks and beyond to levels above that of the younger mice. This hyperreactivity persisted even in 60-week-old mice. The ASR for 16 kHz stimuli was positively correlated with hearing sensitivity, but the ASR for 4 kHz stimuli was positively correlated with hearing loss for mice that were 18-36 weeks of age. Furthermore, ASR amplitudes for 4 kHz stimuli were positively correlated with the 16 kHz ASR in young C57BL mice but negatively correlated in older mice. There were no similar ASR or ABR changes in adult CBA mice through 19 weeks of age. Correlations between ASR and ABR scores were always weakly positive, and correlations between 4 kHz and 16 kHz ASR amplitudes were always strongly positive. The ASR data in older C57BL mice with hearing loss are consistent with reports describing their increased neural representation of low-frequency sounds and reinforce the value of this strain for studying the functional consequences that accompany age-related cochlear degeneration.

Figure 1

Mean ABR thresholds (±SEM) for C57BL/6J mice (A) and CBA/CaJ mice (B) with increasing age. The numbers of C57BL mice at each age are 5, 8, 8, 7, and 7, and of CBA mice, 8, 8, and 8, from youngest to oldest. Note the (1) early hearing loss in the C57BL strain across the spectrum, but especially at high frequencies; (2) small additional loss of sensitivity across the next three age groups; and (3) final large and widespread loss in sensitivity in the oldest group. The CBA mice showed no hearing loss between 10 and 19 weeks of age.

Figure 2

Mean ASR amplitudes (±SEM) across frequency and level, (A–C) for six ages of C57BL mice, 7–60 weeks, (D–F) for three ages of CBA mice, 10, 14, and 19 weeks. The ASR is measured in arbitrary voltage units, a linear function of the force exerted on the accelerometer by the startle flinch. In the C57BL mice, note the (1) overall age-related decrement in the ASR to the 16 kHz tone pips; (2) initial decrement at 14 weeks and subsequent recovery at 18 weeks in the ASR to the 4 kHz and 8 kHz stimuli; and (3) continuing exaggerated ASR at 4 kHz that persisted in the oldest age group. There were no changes in the ASR for the CBA mice.

Figure 3

Mean ASR thresholds (± SEM) as a function of age and stimulus frequency in C57BL (A) and CBA mice (B). The threshold was the lowest level at which the ASR differed (p < 0.05, one-tail) from the activity baseline. In the C57BL mice, the 16 kHz ASR threshold increased with age, except for a slight drop or plateau from 14 to 24 weeks of age. The only other significant effects of age were for 8 kHz, for the loss of sensitivity at 14 weeks followed by a recovery at 18 and 20 weeks of age. There were no threshold changes in the CBA mice. Compared with the CBA mice, the C57BL mice had lower thresholds for the 4 kHz stimuli but higher thresholds for the 16 kHz stimuli.

Figure 4

Scatterplots showing the relationship for the C57BL mice between the mean ABR threshold (across all frequencies) and the maximum mean ASR value at any level. Each point represents the data from one mouse, and the mice appear once for each of the three ASR frequencies. Note the negative slope of the regression lines at 16 kHz for all ages and for all the ASR frequencies at 14 weeks of age, these showing that higher ASR values accompanied greater hearing sensitivity. All other slopes were positive, showing the tendency for the mice with greater hearing loss to respond more vigorously to-low-frequency stimuli within the older groups. (Pearson product–moment correlation coefficients are provided in Table 2.)