Temporal characteristics of the cochlear response after noise exposure

Vocalization-induced middle ear muscle reflex and auditory fovea do not contribute to the unimpaired auditory sensitivity after intense noise exposure in the CF-FM bat, Hipposideros pratti

Behaviors and auditory physiological responses of some species of echolocating bats remain unaffected after exposure to intense noise, but information on the underlying mechanisms remains limited. Here, we studied whether the vocalization-induced middle ear muscle (MEM) contractions (MEM reflex) and auditory fovea contributed to the unimpaired auditory sensitivity of constant frequency-frequency modulation (CF-FM) bats after exposure to broad-band intense noise. The vocalizations of the CF-FM bat, Hipposideros pratti, were inhibited through anesthesia to eliminate the vocalization-induced MEM reflex. First, the anesthetized bats were exposed to intense broad-band noise, and the findings showed that the bats could still maintain their auditory sensitivities. However, auditory sensitivities were seriously impaired in CBA/Ca mice exposed to intense noise under anesthesia. This indicated that the unimpaired auditory sensitivity in H. pratti after exposure to intense noise under anesthesia was not due to anesthetization. The bats were further exposed to low-frequency band-limited noise, whose passband did not overlap with echolocation call frequencies. The results showed that the auditory responses to sound frequencies within the noise spectrum and one-half octave higher than the spectrum were also unimpaired. Taken together, the results indicate that both vocalization-induced MEM reflex and auditory fovea do not contribute to the unimpaired auditory sensitivity in H. pratti after exposure to intense noise. The possible mechanisms underlying the unimpaired auditory sensitivity after echolocating bats were exposed to intense noise are discussed.

Gastrodin Ameliorates Learning and Memory Impairments Caused by Long-Term Noise Exposure

The developing brain is significantly affected by long-term exposure to noise at an early age, leading to functional disorders such as learning and memory impairments. Gastrodin (GAS), a natural organic compound, is an extraction of phenolic glycoside from the rhizome of Gastrodia elata. Clinically, GAS is extensively utilised for the treatment of neurological disorders. This study aimed to explore the effect and mechanism of GAS on noise exposure-induced learning and memory impairments. Rats aged 21 days were exposed to a 90 dB noise environment for 4 weeks and divided into the noise group, the noise + GAS group, and the control group to establish a noise exposure model. After noise exposure treatment, the improvement effect of GAS on the memory of rats was evaluated by Y-maze and Morris water maze. Enzyme-linked immunosorbent assay was utilised to determine the effect of GAS on neurotransmitter levels in the hippocampal tissue of noise-exposed rats. Western blot was applied for the detection of the protein levels of neurotrophic factors. The GAS treatment significantly improved spatial memory and increased the levels of key neurotransmitters (norepinephrine, dopamine and serotonin) and neurotrophic factors (neurotrophin-3 and brain-derived neurotrophic factor) in the hippocampal tissues of noise-exposed rats. These alterations correlate with enhanced cognitive functions, suggesting a neuroprotective effect of GAS against noise-induced cognitive impairments. This study supports the potential of GAS to treat noise-induced learning and memory impairments by modulating neurotransmitter secretion and enhancing the expression levels of neurotrophic factors. These findings offer potential therapeutic avenues for cognitive impairments induced by noise exposure.

Chronic stress induced loudness hyperacusis, sound avoidance and auditory cortex hyperactivity

Hyperacusis, a debilitating loudness intolerance disorder, has been linked to chronic stress and adrenal insufficiency. To investigate the role of chronic stress, rats were chronically treated with corticosterone (CORT) stress hormone. Chronic CORT produced behavioral evidence of loudness hyperacusis, sound avoidance hyperacusis, and abnormal temporal integration of loudness. CORT treatment did not disrupt cochlear or brainstem function as reflected by normal distortion product otoacoustic emissions, compound action potentials, acoustic startle reflexex, and auditory brainstem responses. In contrast, the evoked response from the auditory cortex was enhanced up to three fold after CORT treatment. This hyperactivity was associated with a significant increase in glucocorticoid receptors in auditory cortex layers II/III and VI. Basal serum CORT levels remained normal after chronic CORT stress whereas reactive serum CORT levels evoked by acute restraint stress were blunted (reduced) after chronic CORT stress; similar changes were observed after chronic, intense noise stress. Taken together, our results show for the first time that chronic stress can induce hyperacusis and sound avoidance. A model is proposed in which chronic stress creates a subclinical state of adrenal insufficiency that establishes the necessary conditions for inducing hyperacusis.

Altered resting-state functional connectivity of the anterior cingulate cortex in rats post noise exposure

Aims

We aimed to find where and how noise‐induced cochlear hearing loss affects the central nervous system during the early state and identify the neural substrate for aberrant patterns that mediating noise‐related anxiety−/depression‐ like behaviors.

Methods

Broad band noise with 122 dB for 2 hours was conducted to induce hearing loss. We defined 0 day (N0D) and 10 days (N10D) post noise as the acute and sub‐acute period. Behavioral tests (Open field test and light/dark test) and resting‐state fMRI were computed to evaluate emotional conditions and aberrant neural activity. Functional connectivity analysis using the anterior cingulate cortex as a seed was computed to reveal the spatial distribution beyond auditory network during both periods.

Results

Anxiety−/depression‐like behaviors were found in rats with noise exposure. Between‐group analysis revealed that N0D rats displayed widespread reductions in functional connectivity, spanning primary somatosensory cortex, medial geniculate body, inferior colliculus, cingulate cortex, cerebellar lobule comparing with N10D rats and a similar pattern was also occurred in comparison with the control group.

Conclusion

Taken together, an “acoustic‐causing” network accounting for distress and gating of noise exposure related anxiety/depression was proposed.

Aberrant Functional Network of Small-World in Sudden Sensorineural Hearing Loss With Tinnitus

Few researchers investigated the topological properties and relationships with cognitive deficits in sudden sensorineural hearing loss (SNHL) with tinnitus. To explore the topological characteristics of the brain connectome following SNHL from the global level and nodal level, we recruited 36 bilateral SNHL patients with tinnitus and 37 well-matched healthy controls. Every subject underwent pure tone audiometry tests, neuropsychological assessments, and MRI scanning. AAL atlas was employed to divide a brain into 90 cortical and subcortical regions of interest, then investigated the global and nodal properties of “small world” network in SNHL and control groups using a graph-theory analysis. The global characteristics include small worldness, cluster coefficient, characteristic path length, local efficiency, and global efficiency. Node properties include degree centrality, betweenness centrality, nodal efficiency, and nodal clustering coefficient. Interregional connectivity analysis was also computed among 90 nodes. We found that the SNHL group had significantly higher hearing thresholds and cognitive impairments, as well as disrupted internal connections among 90 nodes. SNHL group displayed lower AUC of cluster coefficient and path length lambda, but increased global efficiency. The opercular and triangular parts of the inferior frontal gyrus, rectus gyrus, parahippocampal gyrus, precuneus, and amygdala showed abnormal local features. Some of these connectome alterations were correlated with cognitive ability and the duration of SNHL. This study may prove potential imaging biomarkers and treatment targets for future studies.

Effect of antiepileptic drug levetiracetam on cochlear function

BACKGROUND

Levetiracetam (LEV, 5-100 mg/kg) has been shown to prevent audiogenic seizures in a dose-dependent manner. This chemical is known to bind to synaptic vesicle protein 2A and inhibit l-type calcium channels, affecting neurotransmitter release. We hypothesize that the drug prevents audiogenic seizures partially by affecting cochlear neural response.

METHODS

To test this hypothesis, rats were given 1000, 500, 50, or 0 mg/kg (saline control) LEV-injection. Distortion product otoacoustic emissions (DPOAE), reflecting outer hair cell (OHC) function, and cochlear compound action potentials (CAP), reflecting cochlear neural output, were recorded and compared pre- and post-LEV.

RESULTS

1000 mg/kg LEV-injection did not significantly affect DPOAE. The high dose LEV-injection, however, significantly reduced CAP amplitude resulting threshold shift (TS), prolonged CAP latency, and enhanced CAP forward masking. CAP latency and forward masking were significantly affected at the 500 mg/kg dose, but CAP-TS remained unchanged after LEV-injection. Interestingly, CAP latency wassignificantly prolonged, at least at the low stimulation levels, although the amplitude of CAP remained constant after a clinical dose of LEV-injection (50 mg/kg).

DISCUSSION

Since the clinical dose of LEV-injection does not reduce CAP amplitude, the reduction of cochlear neural output is unlikely to be the underlying mechanism of LEV in the treatment of audiogenic seizure. The delayed cochlear neural response may be partially related to the prevention of audiogenic seizure. However, neuropharmacological changes in the central nervous system must play a major role in the treatment of audiogenic seizure, as it does in the treatment of focal epilepsy.

The increase in the degree of neural forward masking of cochlea following salicylate application

BACKGROUND

High doses of salicylate are known to reduce cochlear response amplitude and raise threshold. However, its effect on the cochlear forward masking, reflecting temporal resolution, is still unclear.

METHODS

The neural forward masking of cochlea was evaluated using double-tone stimulation. The first tone burst (5ms) was named the "masker" and the second tone burst (5 ms) was named the "probe". The frequency and intensity of the masker and probe were equal, and the masker-probe interval varied from 2 to 32 ms. The reduction (%) of the probe-evoked cochlear compound action potential caused by the addition of the masker tone was used to represent cochlear forward masking. The data obtained before and 2 h following the injection of sodium salicylate (250 mg/kg) were compared.

RESULTS

The neural forward masking of cochlea in the normal rats increased as the masker-probe interval decreased. At 16 kHz, for example, it increased from ~5% to 32ms masker-probe interval to ~85% at 2ms masker-probe interval. Two hours post salicylate injection, the neural forward masking was significantly enhanced except at 32 ms masker-probe interval. Interestingly, this enhancement was only observed in the limited frequency range of 16-30 kHz.

DISCUSSION

The enhancement of forward masking of cochlea following salicylate administration may reflect defective neurotransmitter release. This frequency-dependent injury in the cochlea may lead to the development of central plasticity observed after salicylate administration, likely through the increase in central gain, leading to perceptual consequences.