Strain Comparison in Rats Differentiates Strain-Specific from More General Correlates of Noise-Induced Hearing Loss and Tinnitus

Time-dependent effects of acoustic trauma and tinnitus on extracellular levels of amino acids in the inferior colliculus of rats

Chronic tinnitus is a debilitating condition with very few management options. Acoustic trauma that causes tinnitus has been shown to induce neuronal hyperactivity in multiple brain areas in the auditory pathway, including the inferior colliculus. This neuronal hyperactivity could be attributed to an imbalance between excitatory and inhibitory neurotransmission. However, it is not clear how the levels of neurotransmitters, especially neurotransmitters in the extracellular space, change over time following acoustic trauma and the development of tinnitus. In the present study, a range of amino acids were measured in the inferior colliculus of rats during acoustic trauma as well as at 1 week and 5 months post-trauma using in vivo microdialysis and high-performance liquid chromatography. Amino acid levels in response to sound stimulation were also measured at 1 week and 5 months post-trauma. It was found that unilateral exposure to a 16 kHz pure tone at 115 dB SPL for 1 h caused immediate hearing loss in all the animals and chronic tinnitus in 58 % of the animals. Comparing to the sham condition, extracellular levels of GABA were significantly increased at both the acute and 1 week time points after acoustic trauma. However, there was no significant difference in any of the amino acid levels measured between sham, tinnitus positive and tinnitus negative animals at 5 months post-trauma. There was also no clear pattern in the relationship between neurochemical changes and sound frequency/acoustic trauma/tinnitus status, which might be due to the relatively poorer temporal resolution of the microdialysis compared to electrophysiological responses.

Sheep as a large animal model for hearing research: comparison to common laboratory animals and humans

Sensorineural hearing loss (SNHL), caused by pathology in the cochlea, is the most common type of hearing loss in humans. It is generally irreversible with very few effective pharmacological treatments available to prevent the degenerative changes or minimise the impact. Part of this has been attributed to difficulty of translating "proof-of-concept" for novel treatments established in small animal models to human therapies. There is an increasing interest in the use of sheep as a large animal model. In this article, we review the small and large animal models used in pre-clinical hearing research such as mice, rats, chinchilla, guinea pig, rabbit, cat, monkey, dog, pig, and sheep to humans, and compare the physiology, inner ear anatomy, and some of their use as model systems for SNHL, including cochlear implantation surgeries. Sheep have similar cochlear anatomy, auditory threshold, neonatal auditory system development, adult and infant body size, and number of birth as humans. Based on these comparisons, we suggest that sheep are well-suited as a potential translational animal model that bridges the gap between rodent model research to the clinical use in humans. This is especially in areas looking at changes across the life-course or in specific areas of experimental investigation such as cochlear implantation and other surgical procedures, biomedical device development and age-related sensorineural hearing loss research. Combined use of small animals for research that require higher throughput and genetic modification and large animals for medical translation could greatly accelerate the overall translation of basic research in the field of auditory neuroscience from bench to clinic.

Universal Recommendations on Planning and Performing the Auditory Brainstem Responses (ABR) with a Focus on Mice and Rats

Translational audiology research aims to transfer basic research findings into practical clinical applications. While animal studies provide essential knowledge for translational research, there is an urgent need to improve the reproducibility of data derived from these studies. Sources of variability in animal research can be grouped into three areas: animal, equipment, and experimental. To increase standardization in animal research, we developed universal recommendations for designing and conducting studies using a standard audiological method: auditory brainstem response (ABR). The recommendations are domain-specific and are intended to guide the reader through the issues that are important when applying for ABR approval, preparing for, and conducting ABR experiments. Better experimental standardization, which is the goal of these guidelines, is expected to improve the understanding and interpretation of results, reduce the number of animals used in preclinical studies, and improve the translation of knowledge to the clinic.

Effects of whole-body vibration on reproductive physiology in a rat model of whole-body vibration

Findings from epidemiological studies suggest that occupational exposure to whole-body vibration (WBV) may increase the risk of miscarriage and contribute to a reduction in fertility rates in both men and women. However, workers exposed to WBV may also be exposed to other risk factors that contribute to reproductive dysfunction. The goal of this experiment was to examine the effects of WBV on reproductive physiology in a rat model. Male and female rats were exposed to WBV at the resonant frequency of the torso (31.5 Hz, 0.3 g amplitude) for 4 hr/day for 10 days. WBV exposure resulted in a significant reduction in number of developing follicles, and decrease in circulating estradiol concentrations, ovarian luteinizing hormone receptor protein levels, and marked changes in transcript levels for several factors involved in follicular development, cell cycle, and steroidogenesis. In males, WBV resulted in a significant reduction in spermatids and circulating prolactin levels, elevation in number of males having higher circulating testosterone concentrations, and marked alterations in levels of transcripts associated with oxidative stress, inflammation, and factors involved in regulating the cell cycle. Based upon these findings data indicate that occupational exposure to WBV contributes to adverse alterations in reproductive physiology in both genders that may lead to reduction in fertility.

Comparing the electrophysiological effects of traumatic noise exposure between rodents

Noise-induced hearing deficits are important health problems in the industrialized world. As the underlying physiological dysfunctions are not well understood, research in suitable animal models is urgently needed. Three rodent species (Mongolian gerbil, rat and mouse) were studied to compare the temporal dynamics of noise-induced hearing loss after identical procedures of noise exposure. Auditory brainstem responses (ABRs) were measured before, during and up to eight weeks after noise exposure for threshold determination and ABR waveform analysis. Trauma induction with stepwise increasing sound pressure level was interrupted by five interspersed ABR measurements. Comparing short- and long-term dynamics underlying the following noise-induced hearing loss revealed diverging time courses between the three species. Hearing loss occurred early on during noise exposure in all three rodent species at or above trauma frequency. Initial noise level (105 dB SPL) was most effective in rats while the delayed level-increase to 115 dB SPL affected mice much stronger. Induced temporary threshold shifts in rats and mice were larger in animals with lower pre-trauma ABR thresholds. The increase in activity (gain) along the auditory pathway was derived by comparing the amplitudes of short- and long-latency ABR waveform components. Directly after trauma, significant effects were found for rats (decreasing gain) and mice (increasing gain) while gerbils revealed high individual variability in gain changes. Taken together, our comparative study revealed pronounced species-specific differences in the development of noise-induced hearing loss and the related processing along the auditory pathway.