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. 2023 Jul 11:17:1196477.
doi: 10.3389/fnint.2023.1196477. eCollection 2023.

Balance beam crossing times are slower after noise exposure in rats

Dylan Bartikofsky  1 , Mikayla Jade Hertz  1 , David S Bauer  2 , Richard Altschuler  1   2 , W Michael King  2 , Courtney Elaine Stewart  1
Affiliations

Balance beam crossing times are slower after noise exposure in rats

Dylan Bartikofsky et al. Front Integr Neurosci. .

Abstract

Introduction: The vestibular system integrates signals related to vision, head position, gravity, motion, and body position to provide stability during motion through the environment. Disruption in any of these systems can reduce agility and lead to changes in ability to safely navigate one's environment. Causes of vestibular decline are diverse; however, excessive noise exposure can lead to otolith organ dysfunction. Specifically, 120 decibel (dB) sound pressure level (SPL) 1.5 kHz-centered 3-octave band noise (1.5 kHz 3OBN) causes peripheral vestibular dysfunction in rats, measured by vestibular short-latency evoked potential (VsEP) and reduced calretinin-immunolabeling of calyx-only afferent terminals in the striolar region of the saccule. The present study examined the functional impact of this noise exposure condition, examining changes in motor performance after noise exposure with a balance beam crossing task.

Methods: Balance beam crossing time in rats was assessed for 19 weeks before and 5 weeks after noise exposure. Balance beam crossings were scored to assess proficiency in the task. When animals were proficient, they received a single exposure to 120 dB SPL 3-octave band noise.

Results: During the initial training phase slower crossing times and higher scores, including multiple failures were observed. This was followed by a period of significant improvement leading to proficiency, characterized by fast and stable crossing times and consistently low scores. After noise exposure, crossing times were significantly elevated from baseline for 4-weeks. A total of 5 weeks after noise exposure, crossing times improved, and though still trending higher than baseline, they were no longer significantly different from baseline.

Discussion: These findings show that the noise-induced peripheral vestibular changes we previously observed at cellular and electro-physiological levels also have an impact at a functional level. It has been previously shown that imbalance is associated with slower walking speed in older adults and aged rats. These findings in noise-exposed rats may have implications for people who experience noisy environments and for seniors with a history of noise exposure who also experience balance disorders and may be at increased fall risk.

Keywords: balance; mobility; motor impairment; noise exposure; noise-induced vestibular loss; vestibular.

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Conflict of interest statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

FIGURE 1
FIGURE 1
Experimental timeline. Rats are trained twice per week over a 19-week period. At the end of training, rats are noise exposed (red arrow) and balance beam crossing resumes on the following day (1 day post-noise). After noise exposure, 10 trials per day continue to be collected twice per week for 5 weeks or 31 days.
FIGURE 2
FIGURE 2
The balance beam apparatus. (A) A 90 cm high conduit with 2 IR sensor assemblies spaced 90 cm apart. (B) A top-down view of the start and stop sensors (S1 and S2). Each sensor assembly has an infrared beam spaced 30 cm from a sensor. Rats are placed 55 cm behind S1 at the start of the task. Each trial is initiated when the rat walks through S1 disrupting the light-sensor connection. Each trial is terminated when the rat walks through S2 or the rat refuses to cross for more than 10 s after walking through the S1 sensor.
FIGURE 3
FIGURE 3
Balance beam crossing times improve with training. During the initial 12-week training period (yellow box), all weeks are different (^) from more than one proficient timepoint (green box). Weeks −16, −11, and −9 are also significantly different from other training timepoint (*). Week −16 is significantly slower than most other timepoint. This may be because there were 27 trials that took longer than 3 s, including six failures due to crossing times that exceeded 10 s. In weeks −11 and −9, crossing times were significantly faster and fall below the cut-off time for proficiency, suggesting improvement in training. Taken together, these data show that extended training does produce a measurable improvement in crossing time and underscores the importance of training prior to experimental treatments. The final week of baseline data (blue dashed box) was used as the baseline for crossing times for 5 weeks after noise exposure (Figure 5). The data from Figure 5 are also plotted for comparison with the training data. Although control rats continue to show stable and proficient balance beam crossing times, noise exposed rats’ crossing times become slower, reaching mean crossing times similar to training timepoint.
FIGURE 4
FIGURE 4
Balance beam crossing scores improve with training. During the initial 12-week training period (yellow box), all weeks except for week −8 are significantly different from more than one other training and more than one proficient timepoint (*). Crossing scores are less variable, even in early training than crossing times, and are more consistent from week to week. By the end of the training period (weeks −11 to −8), crossing scores are more consistent and lower, falling below the cut-off for proficiency, suggesting improvement in training. The final week of baseline data (blue dashed box) was used as the baseline for crossing scores for 5 weeks after noise exposure (Figure 6). The data from Figure 6 are also plotted for comparison with the training data. Although control rats balance beam crossing scores remain lower than noise exposed rats during the post-treatment timepoint, noise exposed rats’ mean balance beam crossing scores do not exceed the cut-off for proficiency. This may suggest that crossing scores are a less sensitive measure of noise-induced motor impairment than balance beam crossing times.
FIGURE 5
FIGURE 5
Crossing times are elevated after noise exposure. After noise exposure (red circles), mean balance beam crossing times are significantly elevated from baseline (red asterisks; n = 12; ***p < 0.001). Mean control balance beam crossing times (black circles) are not different from baseline at any timepoint (n = 8). At baseline, noise and control crossing times are not significantly different. However, during weeks 1–4, control and noise mean balance beam crossing times are significantly different (black asterisks; ***p < 0.001; *p < 0.05). Mean noise values are produced from a total of 240 individual data points per experimental week. Mean control values are produced from a total of 160 individual data points per experimental week. Mean values are plotted with standard error of the mean (SEM).
FIGURE 6
FIGURE 6
Crossing scores are unaffected by noise exposure. Control (black circles, n = 8) and noise-exposed (red circles, n = 12) mean balance beam crossing scores are not significantly elevated from baseline (n = 12). Mean noise values are produced from a total of 240 individual data points per experimental week. Mean control values are produced from a total of 160 individual data points per experimental week. Mean values are plotted with standard error of the mean (SEM).
FIGURE 7
FIGURE 7
Number of stops during balance beam crosses are not significantly affected by noise exposure stops in control (black circles, n = 8) and noise-exposed (red circles, n = 12) groups are not significantly elevated from baseline (n = 12). Mean noise values are produced from a total of 240 individual data points per experimental week. Mean control values are produced from a total of 160 individual data points per experimental week. Mean values are plotted with standard error of the mean (SEM).
FIGURE 8
FIGURE 8
Individual rats’ balance beam crossing times. Mean balance beam crossing times averaged from 20 trials per week are plotted for each rat that was noise exposed (n = 12). For comparison, mean control balance beam crossing times for weeks −1 to 5 are plotted as a horizontal gray dashed line and mean baseline (week −1) times are plotted for noise-exposed rats. Mean non-noise exposed rats are extremely similar to control rats. After noise exposure, most noise exposed rats’ mean crossing times are greater than the mean. The highest balance beam crossing time is seen 2 weeks after noise exposure. After this, balance beam crossing times show some improvement and looks more similar to baseline in most rats by week 5.
FIGURE 9
FIGURE 9
Individual rats’ balance beam crossing scores. Mean balance beam crossing scores averaged from 20 trials per week are plotted for each rat that was noise exposed (n = 12). For comparison, mean control balance beam crossing scores for weeks −1 to 5 are plotted as a horizontal gray dashed line and mean baseline (week −1) scores are plotted for noise-exposed rats. Mean non-noise exposed and baseline noise-exposed rats are similar. After noise exposure, the distribution of crossing scores does not change significantly with the exception of one outlier data point that is related to increased stopping (Figure 10).
FIGURE 10
FIGURE 10
Individual rats’ balance beam crossing stops. Stops during balance beam crossing were totaled for each experimental day and 2 days per experimental week were averaged for each noise exposed rat (n = 12). For comparison, mean control stops during balance beam crossing for weeks −1 to 5 are plotted as a horizontal gray dashed line and mean baseline (week −1) stops are plotted for noise-exposed rats. Mean non-noise exposed and baseline noise-exposed rats are similar. After noise exposure, the distribution of stops during balance beam crossing do not change significantly with the exception of one outlier data point.
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