First Entry Recovery of optoacoustic emissions after high-level noise exposure in the American bullfrog
PURPOSE
There were two main focuses in this study. First, the researchers wanted to see if cubic DPOAE is affected by exposure to too much noise. The second focus was to understand the processes of hair cell damage and recovery. To accomplish these goals, the authors of the study also needed to study and understand the morphology of amphibian papilla (AP) in the bullfrog, because the effects of noise on the papilla and noise-induced hearing loss in the bullfrog had not been previously studied. The authors were also interested in seeing if the American bullfrog would be a good model for future research in preventing hearing damage due to noise.
METHODS
The animals used in this experiment were adult bullfrogs, from the species R. catesbeiana.
To test in vivo sound exposure, noise stimuli were administered in a close acoustic system. Left or right ears of bullfrogs were administered 4 to 20 hours of high intensity (150-160 dB) pure tone at 800 Hz. This exposure damaged hair cells in the AP and eliminated DPOAEs. Exposure to high intensity pure tone for many hours was necessary to create consistent damage.
DPOAE measurements were taken before noise exposure and then 12, 24, 48, and 72 hours post exposure. They averaged three measurements at each frequency. To find the DPOAE threshold they recorded the lowest f2 level. They also checked for distortion levels and found that no distortion was present at the levels when DPOAE was recorded. Immunocytochemistry was performed on tissues in the ear. The tissues were studied with confocal microscopy and negative controls were used.
RESULTS
Results are presented in two major forms: images from immunolabeling and graphs. The study found that the largest difference in DPOAE after noise exposure was in regions that had maximal 2f1-f2 responses. To totally get rid of the DPOAE, a very intense signal lasting for 20 hours was required. The results of the study show that DPOAE levels in the American bullfrog are sensitive to hair cell damage. Even though they were sensitive to hair cell damage, the DPOAES did show recovery 3 to 5 days after exposure to intense sound. However, the bullfrogs did not recovery morphologically until 9 days later.
KEY RELEVANCE
This study is relevant for anyone interested in the physiological and morphological changes that occur due to high-intensity long term noise exposure. It is also relevant for people who are looking for a model organism for studies related to hearing.
Simmons, D., Lohr, R., Wotring, H., Burton, M., Hooper, R. and Baird, R. (2014). Recovery of otoacoustic emissions after high-level noise exposure in the American bullfrog. Journal of Experimental Biology, [online] 217(9), pp.1626-1636.
Second Entry Oncomodulin, an EF-Hand Ca2+ Buffer, is Critical for Maintaining Cochlear Function in Mice
PURPOSE
Before this study, it was known that Ca2+ levels in hair cells are controlled by proteins. There were also studies whose results showed that calcium homeostasis has a direct relationship to hearing loss. Oncomodulin is an EF hand domain that is a Ca2+ buffer. To study the importance of outer hair cell calcium buffering by oncomodulin, the researchers performed an experiment with a targeted deletion of the genes responsible for oncomodulin.
METHODS
This experiment was performed on mutant mice; the coding sequence for oncomodulin is on chromosome five in five exons. After performing gene deletion engineering, two of the oncomodulin heterozygote mutants were mated to create an oncomodulin homozygous mutant. The homozygous mutant was crossed with another mouse to create ActbCre; Ocmflox/flox mice. Histological analysis and immunocytochemistry was performed on cochlea from the mice.
RESULTS
The researchers did not find a change in the immunoreactivity of other EF hand buffers for calcium. The idea that the missing EF hand calcium buffers could be made up for by increasing mitochondrial calcium uptake or PMCA activity was disproved. The activity levels of these processes were not enough to make up for the missing oncomodulin function. This shows that the oncomodulin gene has a special role. This special role is that the oncomodulin gene helps the outer hair cells survive by supporting their electromotility.
KEY RELEVANCE
This study is relevant for anyone who is wondering about how the deletion of the oncomodulin gene affects hearing. This study is also relevant for anyone who is wondering about how oncomodulin functions as an EF hand buffer.
Tong, B., Hornak, A., Maison, S., Ohlemiller, K., Liberman, M. and Simmons, D. (2016). Oncomodulin, an EF-Hand Ca 2+ Buffer, Is Critical for Maintaining Cochlear Function in Mice. The Journal of Neuroscience, 36(5), pp.1631-1635.
Third Entry Inner ear morphological correlates of ultrasonic hearing in frogs
PURPOSE
The goal of this study was to understand the morphology of auditory organs in species of frogs. By studying the morphology, they wanted to obtain a better comprehension for the extrinsic and intrinsic hearing processes for high frequency hearing.
METHODS
Four species of frogs were collected, anaesthetized, and had their tissues collected and preserved. Immunohistochemistry was performed and was followed by confocal microscopy and data analysis. Both basilar and amphibian papillae were used in immunohistochemistry experiments and analyzed. From the images, they collected data regarding hair cell soma length and bundle length. Data from the left and right ear of each frog was combined. Univariate ANOVA and pair-wise comparisons Tukey’s post-hoc test were used for statistical analysis.
RESULTS
By studying amphibian papilla, it was found that in all species except for R. pipiens, there is an inverse relationship between the hair cells’ soma length and bundle length, as well as the frequency that the cells are tuned to. Soma length had the same general trend in all three areas of amphibian papilla. Bundle length did have a significant difference in some regions of the amphibian papilla. In the middle amphibian papilla, the three frogs who had ultrasonic hearing had the lowest values for bundle length. Regarding basilar papilla, the three species with ultrasonic hearing, along with O. chloronota, had smaller basilar papilla organs and sensory epithelia than the R. pipiens and A. daorum species. The three species with ultrasonic hearing and O. chloronota had soma lengths that were significantly decreased in size compared to those of R. pipiens and A. daorum. The bundle lengths of the species with ultrasonic hearing and O. chloronata were also significantly decreased in size compared to those of R. pipiens and A. daorum. It is important to note that in this instance, there was no statistical difference between the bundle lengths of O. chloronata and A. daorum. By finding these morphological differences, it is hypothesized that these contrasts in morphology are what allow some species to be sensitive to ultrasonic sounds. The scientists also believe that some frogs were able to acquire ultrasonic hearing not through sudden changes, but through slow changes in the frog auditory organs.
RELEVANCE
This study is relevant for anyone who is interested in ultrasonic hearing in frogs. This study would show them the basic differences for why some frogs have ultrasonic sensitivity and others do not. This study is also relevant for anyone who is interested in studying how morphology affects high frequency hearing in animals.
Arch, V., Simmons, D., Quiñones, P., Feng, A., Jiang, J., Stuart, B., Shen, J., Blair, C.
and Narins, P. (2012). Inner ear morphological correlates of ultrasonic hearing
in frogs. Hearing Research, 283(1-2), pp.70-79.
Fourth Entry Little effect of natural noise on high-frequency hearing in frogs, Odorrana tormota
PURPOSE
The purpose of this study was to see if noisy background noise in the habitat of the frog species O. tormota affects the response of high frequency sensitive auditory neurons. Since frogs communicate through sound, it is important to see if the background noise in their environments impedes their ability to communicate through high frequency sounds.
METHODS
Male frogs were wild caught, anesthetized, wrapped with wet gauze, and placed on a paraffin platform on a vibration-isolated table in a soundproof and anechoic electromagnetically shielded room (Liu et al.). The noise present in their natural habitat near the Tao Hua Creek waterfall was recorded by a ¼ inch wideband omnidirectional microphone and a digital audio recorder. The amplitude spectrum of the noises was analyzed.
Pure tone stimuli were amplified and placed 50 cm from the frog eardrum. Auditory evoked near field potentials (AENFPs) were measured in the torus semicircularis through the use of microelectrodes. AENFPs were created with pure tone stimuli and recorded while background noise was also present. AENFPs were also created with pure tone stimuli and recorded while natural noise (at a higher decibel than background noise) was also present. The neural response of the torus semircularis was recorded. The results were analyzed using a one-way ANOVA.
RESULTS
This study found that there is not a significant difference in the amplitudes and thresholds of auditory evoked near field potentials (AENFPs) when there is 35 dB background noise and 65 dB natural noise. This shows that O. tormota are likely unaffected by the natural noise that occurs at up to 65 dB. The highest level of natural noise tested was at 85 dB and did cause an effect. Natural noise at 85 dB decreased the amplitudes of the AENFPs in the midbrain.
RELEVANCE
How the animal kingdom communicates is very fascinating to many people. This study is relevant for anyone who is interested in how animals communicate with each other. It is especially relevant for people who are wondering how factors in the animals’ environment affect their communication, especially factors that occur naturally, such as the rushing water from a waterfall or the noises from other animals.
Liu, J., Yang, H., Hu, G., Li, S., Xu, Z., Qi, Z. and Shen, J. (2015). Little effect of natural noise on high-frequency hearing in frogs, Odorrana tormota. Journal of Comparative Physiology A, 201(10), pp.1029-1034.