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

Tinnitus Generation and Behavioral Changes Caused by Chronic Stress: A Behavioral and Brain Study in a Rat Model

OBJECTIVES

This study explores the connection between chronic stress and tinnitus, a phantom auditory perception, using an animal model.

METHODS

Rats were subjected to 2 h of daily restraint stress for 10 days. Tinnitus was assessed on the last day of stress exposure using the gap response of pre-pulse inhibition acoustic reflex, measured at 60 dB background sound level at 8, 16, and 20 kHz. Chronic stress-exposed rats were categorized into two groups: tinnitus (RTG) and non-tinnitus (RNTG). Various tests, including hearing assessments (distortion product otoacoustic emissions and auditory brainstem response), behavioral evaluations (elevated plus maze test and forced swimming test), and immunohistochemical studies in the auditory and limbic brain regions, were conducted to understand the relationship between chronic stress, tinnitus, and behavioral changes.

RESULTS

Following chronic restraint stress, 64.3% of the rats exhibited tinnitus with no audiometric changes. EPM and FST indicated an increase of anxiety- and depression-related behavior in RTG. Immunohistochemical analyses identified specific alterations in the expression of neurotransmitter receptors within brain regions implicated in tinnitus. Specifically, we observed a decrease in γ-aminobutyric acid A receptor α1 expression and an increase in glutamate receptor (N-methyl-D-aspartate receptor subunit 1 and receptor subunit 2B) expression in specific brain region. These changes suggest a reorganization of neural circuits associated with the tinnitus generation and behavioral changes of the rats after chronic stress exposure.

CONCLUSION

Chronic stress alone can be a causal factor for the generation of tinnitus and behavioral changes through altered neural activities in tinnitus-related brain networks.

LEVEL OF EVIDENCE

NA Laryngoscope, 135:873-881, 2025.

Stress-Induced Tinnitus in a Rat Model: Transcriptomics of the Prefrontal Cortex and Hippocampus

OBJECTIVES

The molecular mechanisms by which stress leads to the development of tinnitus are not yet well understood. This study aimed to identify brain changes in a stress-induced tinnitus (ST) animal model through transcriptome analysis of the prefrontal lobe and hippocampus.

METHODS

Twenty Sprague-Dawley rats were subjected to restraint stress for 2 h. Following the gap prepulse inhibition of the acoustic startle (GPIAS) reflex test to assess tinnitus development, the prefrontal lobes and hippocampi of the brains were harvested from 15 rats: five with evident tinnitus (ST), five with noticeable non-tinnitus (stress-induced non-tinnitus; SNT), and five without stress (control group). Comparative RNA-seq analysis was conducted to examine gene expression profiles.

RESULTS

In comparison to the control group, the ST group exhibited 971 and 463 differentially expressed genes (DEGs) in the prefrontal lobe and hippocampus, respectively (FDR < 0.05). The SNT group showed a largely similar gene expression to the control group. Enrichment analysis of the prefrontal lobe revealed the downregulation of gene sets associated with neurotransmitter and synapse-related functions and the upregulation of cell cycle-related gene sets in the ST group. In the hippocampus, there were significantly downregulated gene sets associated with steroid production and upregulated gene sets related to the extracellular matrix in the ST group. Immune-related gene sets were upregulated in both the prefrontal lobe and hippocampus.

CONCLUSION

Our research presents evidence that differences in genetic expression in the prefrontal lobe and hippocampus after exposure to stress play a significant role in the development of tinnitus.

LEVEL OF EVIDENCE

NA Laryngoscope, 135:882-888, 2025.

Photobiomodulation therapy in improvement of harmful neural plasticity in sodium salicylate-induced tinnitus

Tinnitus is a common annoying symptom without effective and accepted treatment. In this controlled experimental study, photobiomodulation therapy (PBMT), which uses light to modulate and repair target tissue, was used to treat sodium salicylate (SS)-induced tinnitus in a rat animal model. Here, PBMT was performed simultaneously on the peripheral and central regions involved in tinnitus. The results were evaluated using objective tests including gap pre-pulse inhibition of acoustic startle (GPIAS), auditory brainstem response (ABR) and immunohistochemistry (IHC). Harmful neural plasticity induced by tinnitus was detected by doublecortin (DCX) protein expression, a known marker of neural plasticity. PBMT parameters were 808 nm wavelength, 165 mW/cm2 power density, and 99 J/cm2 energy density. In the tinnitus group, the mean gap in noise (GIN) value of GPIAS test was significantly decreased indicated the occurrence of an additional perceived sound like tinnitus and also the mean ABR threshold and brainstem transmission time (BTT) were significantly increased. In addition, a significant increase in DCX expression in the dorsal cochlear nucleus (DCN), dentate gyrus (DG) and the parafloccular lobe (PFL) of cerebellum was observed in the tinnitus group. In PBMT group, a significant increase in the GIN value, a significant decrease in the ABR threshold and BTT, and also significant reduction of DCX expression in the DG were observed. Based on our findings, PBMT has the potential to be used in the management of SS-induced tinnitus.

The association between stress, emotional states, and tinnitus: a mini-review

Extensive literature supporting the view of tinnitus induced stress in patients is available. However, limited evidence has been produced studying the opposite, that is, does stress cause tinnitus? The hypothalamus pituitary adrenal axis, one of the main neuroendocrine systems involved in stress response, is commonly disturbed in tinnitus patients. Patients with chronic tinnitus have been shown to develop abnormal responses to psycho-social stress, where the hypothalamus pituitary adrenal axis response is weaker and delayed, suggesting chronic stress contributes to the development of chronic tinnitus. The sympathetic branch of the autonomic nervous system also plays a major role in stress response and its chronic hyperactivity seems to be involved in developing tinnitus. Psycho-social stress has been shown to share the same probability of developing tinnitus as occupational noise and contributes to worsening tinnitus. Additionally, exposure to high stress levels and occupational noise doubles the likelihood of developing tinnitus. Interestingly, short-term stress has been shown to protect the cochlea in animals, but chronic stress exposure has negative consequences. Emotional stress also worsens pre-existing tinnitus and is identified as an important indicator of tinnitus severity. Although there is limited body of literature, stress does seem to play a vital role in the development of tinnitus. This review aims to highlight the association between stress, emotional states, and the development of tinnitus while also addressing the neural and hormonal pathways involved.