Evidence of Tinnitus Development Due to Stress: An Experimental Study in Rats.

OBJECTIVES/HYPOTHESIS: Tinnitus can develop due to, or be aggravated by, stress in a rat model. To investigate stress as a possible causal factor in the development of tinnitus, we designed an animal study that included tinnitus behavior and excitatory/inhibitory neurotransmitter expression after noise exposure as well as restraint stress. STUDY DESIGN: An experimental animal study. METHODS: Wistar rats were grouped according to single or double exposure to noise and restraint stress. The noise exposure (NE) group was subjected to 110 dB sound pressure level (SPL) of 16 kHz narrow-band noise (NBN) for 1 hour, and the restraint stress (RS) group was restrained for 1 hour with or without noise exposure. Gap prepulse inhibition of the acoustic startle (GPIAS) reflex was measured at an NBN of 16 kHz to investigate tinnitus development. Various immunohistopathologic and molecular biologic studies were undertaken to evaluate possible mechanisms of tinnitus development after noise and/or restraint stress. RESULTS: The RS-only group showed a reduced GPIAS response, which is a reliable sign of tinnitus development. In the double-stimulus groups, more tinnitus-development signs of reduced GPIAS responses were observed. The expression of γ-aminobutyric acid A receptor α1 (GABAAR α1) in the hippocampus decreased in the NE│RS group. Increased N-methyl-d-aspartate receptor1 intensities in the NE│RS group and decreased GABAAR α1 intensities in the RS and NE│RS groups were observed in the CA3 region of the hippocampus. CONCLUSIONS: Tinnitus appeared to develop after stress alone in this animal study. An imbalance in excitatory and inhibitory neurotransmitters in the hippocampus may be related to the development of tinnitus after acute NE and/or stress. LEVEL OF EVIDENCE: NA Laryngoscope, 131:2332-2340, 2021.

Neuroprotective Effect of Valproic Acid on Salicylate-Induced Tinnitus.

High-dose salicylate induces temporary moderate hearing loss and the perception of a high-pitched tinnitus in humans and animals. Previous studies demonstrated that high doses of salicylate increase N-methyl-d-aspartate (NMDA) receptor levels, resulting in a rise in Ca2+ influx and induction of excitotoxicity. Glutamate excitotoxicity is associated with failure in the maintenance of calcium homeostasis, mitochondrial dysfunction, and production of reactive oxygen species (ROS). Valproic acid (VPA) is widely used for the management of bipolar disorder, epilepsy, and migraine headaches, and is known to regulate NMDA receptor activity. In this study, we examined the beneficial effects of VPA in a salicylate-induced tinnitus model in vitro and in vivo. Cells were pretreated with VPA followed by salicylate treatment. The expression levels of NMDA receptor subunit NR2B, phosphorylated cAMP response element-binding protein-an apoptosis marker, and intracellular levels of ROS were measured using several biochemical techniques. We observed increased expression of NR2B and its related genes TNFα and ARC, increased intracellular ROS levels, and induced expression of cleaved caspase-3. These salicylate-induced changes were attenuated in the neuronal cell line SH-SY5Y and rat cortical neurons after VPA pretreatment. Together, these results provide evidence of the beneficial effects of VPA in a salicylate-induced temporary hearing loss and tinnitus model.

Axonal sprouting in the dorsal cochlear nucleus affects gap­prepulse inhibition following noise exposure.

One of the primary theories of the pathogenesis of tinnitus involves maladaptive auditory­somatosensory plasticity in the dorsal cochlear nucleus (DCN), which is assumed to be due to axonal sprouting. Although a disrupted balance between auditory and somatosensory inputs may occur following hearing damage and may induce tinnitus, examination of this phenomenon employed a model of hearing damage that does not account for the causal relationship between these changes and tinnitus. The present study aimed to investigate changes in auditory­somatosensory innervation and the role that axonal sprouting serves in this process by comparing results between animals with and without tinnitus. Rats were exposed to a noise­inducing temporary threshold shift and were subsequently divided into tinnitus and non­tinnitus groups based on the results of gap prepulse inhibition of the acoustic startle reflex. DCNs were collected from rats divided into three sub­groups according to the number of weeks (1, 2 or 3) following noise exposure, and the protein levels of vesicular glutamate transporter 1 (VGLUT1), which is associated with auditory input to the DCN, and VGLUT2, which is in turn primarily associated with somatosensory inputs, were assessed. In addition, factors related to axonal sprouting, including growth­associated protein 43 (GAP43), postsynaptic density protein 95, synaptophysin, α­thalassemia/mental retardation syndrome X­linked homolog (ATRX), growth differentiation factor 10 (GDF10), and leucine­rich repeat and immunoglobulin domain­containing 1, were measured by western blot analyses. Compared to the non­tinnitus group, the tinnitus group exhibited a significant decrease in VGLUT1 at 1 week and a significant increase in VGLUT2 at 3 weeks post­exposure. In addition, rats in the tinnitus group exhibited significant increases in GAP43 and GDF10 protein expression levels in their DCN at 3 weeks following noise exposure. Results from the present study provided further evidence that changes in the neural input distribution to the DCN may cause tinnitus and that axonal sprouting underlies these alterations.

Comparisons of electromagnetic and piezoelectric floating-mass transducers in human cadaveric temporal bones.

Electromagnetic floating-mass transducers for implantable middle-ear hearing devices (IMEHDs) afford the advantages of a simple surgical implantation procedure and easy attachment to the ossicles. However, their shortcomings include susceptibility to interference from environmental electromagnetic fields, relatively high current consumption, and a limited ability to output high-frequency vibrations. To address these limitations, a piezoelectric floating-mass transducer (PFMT) has recently been developed. This paper presents the results of a comparative study of these two types of vibration transducer developed for IMEHDs. The differential electromagnetic floating-mass transducer (DFMT) and the PFMT were implanted in two different sets of three cadaveric human temporal bones. The resulting stapes displacements were measured and compared on the basis of the ASTM standard for describing the output characteristics of IMEHDs. The experimental results show that the PFMT can produce significantly higher equivalent sound pressure levels above 3 kHz, due to the flat response of the PFMT, than can the DFMT. Thus, it is expected that the PFMT can be utilized to compensate for high-frequency sensorineural hearing loss.