impact of laser-induced shock wave exposure on cochlear histology and cellular survival in mice

Characterization of LISW Effects on Cochlear Pathophysiology in Mice

Hearing loss and tinnitus are among the most prevalent disabilities, reported in up to 62% of veterans1. Several blast-induced auditory complications, including sensorineural hearing loss (SNHL) and tympanic membrane perforation (TMP), have been reported in individuals exposed to blast overpressure2. Moreover, research on individuals exposed to blasts suggests that blast exposure frequently results in defects in auditory temporal resolution, even when the hearing thresholds are within normal range, which is known as &#34;hidden hearing loss (HHL)&#34;3. It is well established that there is a substantial loss of cochlear synapses between inner hair cells (IHCs) and auditory neurons (ANs) in blast-related cochlear pathology4. Synaptic degeneration results in impaired auditory processing and is a major contributing factor in the development of HHL5. Thus, auditory organs are fragile components containing complex and highly organized structures. However, the precise mechanism by which blast waves affect the inner ear at the cellular level remains unclear. This is because of the challenges in replicating the precise clinical and mechanical intricacies of blast injuries in laboratory settings and the complexity of blast-induced cochlear pathologies.
The primary component of a blast injury is the shock wave (SW), characterized by a rapid and high increase in peak pressure6. The complexity of blast injuries has been extensively investigated in numerous retrospective studies7,8,9. There are various devices for blast generation, such as compressed gas10, shock tubes11, and small-magnitude explosives12, at different levels of pressure. The pressure waveform of the SW generated by recently developed devices closely resembled that of an actual explosion. An important concept in establishing an animal model of blast-induced sensorineural hearing loss is to minimize concomitant injuries, other than auditory damage, to reduce animal death. Thus, blast injury studies have been developed in which shock tubes have been miniaturized and the output can be precisely controlled so that exposed animals rarely die. However, although these animal models usually develop complications, such as TMP, evaluation of cochlear function is difficult because of concurrent conductive hearing loss2. We previously performed an ear-protected animal study on blast injury using earplugs and found no incidence of TMP13. The earplugs could partially attenuate severe cochlear damage but not central auditory neurodegeneration or tinnitus development. Thus, earplugs protect the cochleae as well as the tympanic membrane. However, an animal model of blast-induced pure cochlear damage without TMP is required to study the cochlear pathophysiology caused by blast injuries.
We previously developed a topical blast injury model of the inner ear in rats and mice using a laser-induced shock wave (LISW)14,15. This method can be safely and easily performed at a standard laboratory level and has been used to generate models of lung and head blast injuries16,17. The energy of the LISW can be adjusted by changing the laser type and power, allowing control over the degree of cochlear damage. The LISW-induced cochlear injury model is valuable for studying the mechanisms of SNHL caused by blast injuries and investigating potential treatments. In this study, we describe detailed experimental protocols for creating a mouse model of blast-induced cochlear damage using LISW and demonstrate cochlear degeneration, including the loss of hair cells (HCs), cochlear synapses, and spiral ganglion neurons (SGNs), in an energy-dependent manner in mice following LISW exposure.

Author Spotlight: Development of a Laser-Induced Shock Wave Animal Model Without Tympanic Membrane Perforation

Research Application of Laser-Induced Shock Wave for Studying Blast-Induced Cochlear Injury

To evaluate pure sensory neuro cochlear dysfunction caused by blast exposure result tympanic membrane perforation. We develop an experimental animal model of blast induced cochlear injury using a razor induce shock wave. This method can avoid can tympanic membrane perforation and concomitant systemic injuries and reproduce the functional decline in the sensory neuro hearing loss component in an energy dependent manner after after LISW exposure.This animal model could be an ideal platform for stating the pathological mechanism and exploring potential treatment for the blast induced cochlear dysfunction.

Impact of Laser-Induced Shock Wave Exposure on Cochlear Histology and Cellular Survival in Mice

One month after LISW exposure, conduct a pathological examination of the mouse's cochlea. Start hemoperfusion with lactated Ringer's solution, followed by transcardiac perfusion with 4%paraformaldehyde. After decapitation of the mouse, remove the cochlea and perfuse directly with 4%paraformaldehyde at four degrees Celsius overnight.After acquiring the entire image of the cochlea, using ImageJ software, compute the cochlear frequency map to precisely localize the specific cochlear regions at different frequencies. Using the given formula, calculate the survival rate of the hair cells at each frequency. After acquiring high-resolution Z-stack images of the inner hair cells, calculate the number of synapses.Visualize the hematoxylin and eosin-stained cochlea sections and count the number of spiral ganglion neurons in the middle turn of the Rosenthal's canal to calculate the spiral ganglion neuron survival per control. Compared to the control, fluorescent immunostaining showed intact cochlear structures with no significant loss of hair cells or nerve fibers across all groups. However, quantitative assessments indicated that the survival of outer hair cells was significantly lower in the 2.5 joules per square centimeter group.Meanwhile, inner hair cell survival was consistent among groups. Synaptic ribbon counts were notably reduced in higher-frequency groups. This was paralleled by a decrease in spiral ganglion neuron density in the 2.5 joules per square centimeter group, indicating that cochlear degeneration is dependent on LISW energy exposure.

impact-laser-induced-shock-wave-exposure-on-cochlear-histology

Research

JoVE Journal

Medicine

The ear is the organ most susceptible to explosion overpressure, and cochlear injuries frequently occur after blast exposure. Blast exposure can lead to sensorineural hearing loss (SNHL), which is an irreversible hearing loss that negatively affects the quality of life. Detailed blast-induced cochlear pathologies, such as the loss of hair cells, spiral ganglion neurons, cochlear synapses, and disruption of stereocilia, have been previously documented. However, determining cochlear sensorineural deterioration after a blast injury is challenging because animals exposed to blast overpressure usually experience tympanic membrane perforation (TMP), which causes concurrent conductive hearing loss. To evaluate pure sensorineural cochlear dysfunction, we developed an experimental animal model of blast-induced cochlear injury using a laser-induced shock wave. This method avoids TMP and concomitant systemic injuries and reproduces the functional decline in the SNHL component in an energy-dependent manner after LISW exposure. This animal model could be a platform for elucidating the pathological mechanisms and exploring potential treatments for blast-induced cochlear dysfunction.

Here, we describe experimental protocols for creating an animal model of blast-induced cochlear injury using laser-induced shock wave (LISW). Exposure of the temporal bone to LISW allows the reproduction of blast-induced cochlear pathophysiology. This animal model could be a platform for elucidating cochlear pathology and exploring potential treatments for blast injuries.

The ear is the organ most susceptible to explosion overpressure, and cochlear injuries frequently occur after blast exposure. Blast exposure can lead to ...

Evaluating the Impact of Laser-Induced Shock Wave Exposure on Cochlear Function and Auditory Thresholds in Mice

To begin, prepare a laser-induced shockwave, or LISW exposure assembly. Irradiate the exposure assembly with a 532 nanometer Q-switched NDYAG laser to generate the LISW behind the target. Focus the laser pulse through a plano-convex lens to a three millimeter diameter spot on the laser target.Use the LISW irradiation to generate plasma at the bonding surface of the two materials, and vaporize the rubber, leaving vaporized rubber in the cavity. Carefully shave the postauricular regions of the anesthetized mouse to avoid retaining the trapped air in the fur. Fix the mouse on a plate with their postauricular regions positioned upwards towards the LISW focal area.To ensure acoustic impedance matching, use an ultrasound conductive gel between the laser target and the skin surface. Attach a black rubber target percutaneously to the postauricular region of the mouse ear. Apply a single LISW pulse to the cochlea via the temporal bone at three energy densities.Insert a ground electrode under the caudal region of the tail. Then, insert a stainless steel needle electrode for electroencephalogram recording under the ear canal and frontal region of the ear. Present the stimulation sound over a small earphone and measure the sound pressure level near the tympanic membrane of the mouse.Generate output burst stimuli from a sound generator at 37 cycles, and amplify the sound pressure from a 20 decibel sound pressure level to 80 decibel SPL in five decibel SPL steps. Measure the auditory brainstem response, or ABR peak one amplitude, to evaluate the cochlear functions. Then, ABR peak analysis software automatically analyzes the ABR waveforms with respect to the hearing thresholds and ABR P1 amplitude.An auditory function assessment using ABR post LISW exposure revealed significant ABR threshold shifts at one day, persisting for one month at higher energy levels. ABR thresholds in the two joules per square centimeters group showed recovery to pre-exposure levels, indicating energy dependent effects. After one day and one month of LISW exposure, significant decreases in ABR P1 amplitudes were observed in all LISW irradiated groups across frequencies.