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Noise-induced hearing loss is a common type of hearing loss. The effects of laser therapy have been investigated from various perspectives, including in wound healing, inflammation reduction, and nerve regeneration, as well as in hearing research. A promising feature of the laser is its capability to penetrate soft tissue; depending on the wavelength, laser energy can penetrate into the deepest part of the body without damaging non-target soft tissues. Based on this idea, we developed bilateral transtympanic laser therapy, which uses simultaneous laser irradiation in both ears, and evaluated the effects of bilateral laser therapy on cochlear damage caused by noise overexposure. Thus, the purpose of this research was to assess the benefits of simultaneous bilateral laser therapy compared with unilateral laser therapy and a control. Eighteen Sprague-Dawley rats were exposed to narrow-band noise at 115 dB SPL for 6 h. Multiple auditory brainstem responses were measured after each laser irradiation, and cochlear hair cells were counted after the 15th such irradiation. The penetration depth of the 808 nm laser was also measured after sacrifice. Approximately 5% of the laser energy reached the contralateral cochlea. Both bilateral and unilateral laser therapy decreased the hearing threshold after noise overstimulation in the rat model. The bilateral laser therapy group showed faster functional recovery at all tested frequencies compared with the unilateral laser therapy group. However, there was no difference in the endpoint ABR results or final hair cell survival, which was analyzed histologically.

A certain degree of noise can cause hearing problems without a permanent change in the hearing threshold, which is called hidden hearing loss and results from partial loss of auditory synapses. Photobiomodulation (PBM) enhances neural growth and connections in the peripheral nervous systems. In this study, we assessed whether PBM could rescue cochlear synaptopathy after acoustic overexposure in rat. PBM was performed for 7 days after noise exposure. The auditory brainstem responses (ABRs) were acquired before and after noise exposure using a tone and a paired‐click stimulus. Auditory response to paired click sound with short time interval was performed to evaluate auditory temporal processing ability. In the result, hearing threshold recovered 2 weeks after noise exposure in both groups. Peak wave 1 amplitude of the ABR and ABR recovery threshold did not recover in the noise only group, whereas it fully recovered in the noise + PBM group. The number of synaptic ribbons was significantly different in the control and noise only groups, while there was no difference between the control and noise + PBM group. These results indicate that PBM rescued peak wave 1 amplitude and maintained the auditory temporal processing ability resulting from a loss of synaptic ribbons after acoustic overexposure.

Hearing loss is very common and economically burdensome. No accepted therapeutic modality for sensorineural hearing loss is yet available; most clinicians emphasize rehabilitation, placing hearing aids and cochlear implants. Photobiomodulation (PBM) employs light energy to enhance or modulate the activities of specific organs, and is a popular non-invasive therapy used to treat skin lesions and neurodegenerative disorders. Efforts to use PBM to improve hearing have been ongoing for several decades. Initial in vitro studies using cell lines and ex vivo culture techniques have now been supplanted by in vivo studies in animals; PBM protects the sensory epithelium and triggers neural regeneration. Many reports have used PBM to treat tinnitus. In this brief review, we introduce PBM applications in hearing research, helpful protocols, and relevant background literature.

In the present study, we measured the spontaneous post synaptic currents (sPSCs) at the post synaptic principle cells of the medial nucleus of the trapezoid body (MNTB) in early postnatal mice after exposure to 1850 MHz radiofrequency electromagnetic fields (RF-EMF). sPSC frequencies and amplitudes were significantly increased in the RF-EMF exposed group. Moreover, the number of synaptic vesicles in the calyx of Held was significantly increased in presynaptic nerve terminals. Following RF-EMF exposure, the number of docking synaptic vesicles in the active zone increased, thereby expanding the total length of the presynaptic active zone in the calyx of Held. These data suggest that the increased sPSCs are a result of greater synaptic vesicle release from presynaptic nerves. However, we found no morphological changes in the inner hair cell ribbon synapses. Further, there were no significant changes in the hearing threshold of the auditory brainstem response at postnatal day 15. Our results indicate that exposure to RF-EMF at an early postnatal stage might directly affect brainstem auditory circuits, but it does not seem to alter general sound perception.

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Objectives. The relationship of estrogen (the primary female sex hormone) with hearing function has been studied in both humans and animals. However, whether estrogen levels affect hearing remains uncertain. Therefore, in this study, we investigated changes in the vulnerability of hearing to acoustic overexposure in ovariectomized female rats. Methods. Eighteen 8-week-old female Sprague-Dawley rats were separated into four groups as follows: sham ovariectomy (OP), OP only, and OP treated with low (10 µg/kg) or high doses (100 µg/kg) of estrogen. Rats in the estrogen replacement groups were given two intraperitoneal injections. Hearing thresholds were measured before noise exposure, and at 1 day and 2 weeks after exposure. Results. The hearing thresholds of the sham OP and OP-only groups were not significantly different. However, both estrogen groups showed a lower threshold shift than the OP-only group. Histological immunostaining analyses showed that hair cell loss in the 32 kHz region was more severe in the sham OP group than in the OP-only group. Furthermore, there was little or no hair cell loss in either estrogen replacement group and significantly more hair cell loss in the OP-only group. Conclusion. These results suggest that estrogen replacement may reduce the vulnerability of hearing to noise exposure in menopausal women.

We analyzed the effects of photobiomodulation (PBM) of various wavelengths on regeneration of the facial nerve using in vitro and in vivo experimental models. We assessed the antioxidative effect of PBM in geniculate ganglion neurons irradiated with a diode laser at 633 nm, 780 nm and 804 nm. Wavelengths of 633 and 780 nm but not 804 nm inhibited cell death by oxidative stress. We assessed the effects of PBM on functional and morphologic recovery in rats divided into control, facial nerve damage (FND) and FND irradiated with a 633 nm or 804 nm lasers. Injured rats treated with 633-nm light had better facial palsy scores, larger axon diameter and higher expression of Schwann cells compared with the FND group. No positive results were observed in rats irradiated at 804-nm light. These findings indicate that 633-nm PBM promotes accelerated nerve regeneration and improved functional recovery in an injured facial nerve.