Electrophysiological changes in auditory evoked potentials in rats with salicylate-induced tinnitus

The human neuroprotective placental protein composition suppressing tinnitus and restoring auditory brainstem response in a rodent model of sodium salicylate-induced ototoxicity

The effect of neuroprotective placental protein composition (NPPC) on the suppression of tinnitus and the restoration of the auditory brainstem response (ABR) characteristics was explored in tinnitus-induced rats. The animals were placed into two groups: (1) the study group, rats received sodium salicylate (SS) at the dose of 200 mg/kg twice a day for two weeks, and then 0.4 mg of the NPPC per day, between the 14th and 28th days, (2) the placebo group, rats received saline for two weeks, and then the NPPC alone between the 14th and 28th days. The gap pre-pulse inhibition of the acoustic startle (GPIAS), the pre-pulse inhibition (PPI), and the ABR assessments were performed on animals in both groups three times (baseline, day 14, and 28). The GPIAS value declined after 14 consecutive days of the SS injection, while NPPC treatment augmented the GPIAS score in the study group on the 28th day. The PPI outcomes revealed no significant changes, indicating hearing preservation after the SS and NPPC administrations. Moreover, some changes in ABR characteristics were observed following SS injection, including (1) higher ABR thresholds, (2) lowered waves I and II amplitudes at the frequencies of 6, 12, and 24 kHz and wave III at the 12 kHz, (3) elevated amplitude ratios, and (4) prolongation in brainstem transmission time (BTT). All the mentioned variables returned to their normal values after applying the NPPC. The NPPC use could exert positive therapeutic effects on the tinnitus-induced rats and improve their ABR parameters.

The Auditory Brainstem Response (ABR) Test, Supplementary to Behavioral Tests for Evaluation of the Salicylate-Induced Tinnitus

Tinnitus is a symptom of various disorders that affects the quality of life of millions people. Given the significance of the access to an objective and non-invasive method for tinnitus detection, in this study the auditory brainstem response (ABR) electrophysiological test was used to diagnose salicylate-induced tinnitus, in parallel with common behavioral tests. Wistar rats were divided into saline (n = 7), and salicylate (n = 7) groups for behavioral tests, and salicylate group (n = 5) for the ABR test. The rats were evaluated by pre-pulse inhibition (PPI), gap pre-pulse inhibition of the acoustic startle (GPIAS), and ABR tests, at baseline, 14 and 62 h after salicylate (350 mg/kg) or vehicle injection. The mean percentage of GPIAS test was significantly reduced following salicylate administration, which confirms the induction of tinnitus. The ABR test results showed an increase in the hearing threshold at click and 8, 12, and 16 kHz tones. Moreover, a decline was observed in the latency ratio of II-I waves in all tone burst frequencies with the highest variation in 12 and 16 kHz as well as a decrement in the latency ratio of III-I and IV-I only in 12 and 16 kHz. ABR test is able to evaluate the salicylate induced tinnitus pitch and confirm the results of behavioral tinnitus tests. GPIAS reflexive response is dependent on brainstem circuits and the auditory cortex while, ABR test can demonstrate the function of the auditory brainstem in more details, and therefore, a combination of these two tests can offer a more accurate tinnitus evaluation.

Graphical Abstract

Degenerate brainstem circuitry after combined physiochemical exposure to jet fuel and noise

Degenerate neural circuits exhibit "different" circuit properties yet produce similar circuit outcomes (many-to-one) which ensures circuit robustness and complexity. However, neuropathies may hijack degeneracy to yield robust and complex pathological circuits. The aim of the current study was to test the hypothesis that physiochemical exposure to combined jet fuel and noise might induce degeneracy in the brainstem. The auditory brainstem of pigmented rats was used as a model system. The animals were randomized into the following experimental groups: Fuel+Noise, fuel-only, noise-only, and control. Ascending volume conductance from various auditory brainstem regions were evaluated simultaneously with peripheral nervous system (PNS) input to brainstem circuitry. Data demonstrated normal PNS inputs for all groups. However, the Fuel+Noise exposure group produced different caudal brainstem circuit properties while rostral brainstem circuitry initiated outputs that were similar to that of control. This degenerative effect was specific to Fuel+Noise exposure, since neither noise-alone or fuel-alone produced the same result. Degeneracy in the auditory brainstem is consistent with perceptual abnormalities, such as poor speech discrimination (hear but not understand), tinnitus (ringing in the ear), hyperacusis (hypersensitivity to even low-level sound), and loudness intolerance. Therefore, a potential consequence of Fuel+Noise exposure among military and civilian populations may be evidenced as increased rates of super-threshold auditory perceptual abnormalities. This is particularly important because to date, the ototoxic profile of Fuel+Noise exposure has remained unresolved.

Quantification of stroke lesion volume using epidural EEG in a cerebral ischaemic rat model

Precise monitoring of the brain after a stroke is essential for clinical decision making. Due to the non-invasive nature and high temporal resolution of electroencephalography (EEG), it is widely used to evaluate real-time cortical activity. In this study, we investigated the stroke-related EEG biomarkers and developed a predictive model for quantifying the structural brain damage in a focal cerebral ischaemic rat model. We enrolled 31 male Sprague-Dawley rats and randomly assigned them to mild stroke, moderate stroke, severe stroke, and control groups. We induced photothrombotic stroke targeting the right auditory cortex. We then acquired EEG signal responses to sound stimuli (frequency linearly increasing from 8 to 12 kHz with 750 ms duration). Power spectral analysis revealed a significant correlation of the relative powers of alpha, theta, delta, delta/alpha ratio, and (delta + theta)/(alpha + beta) ratio with the stroke lesion volume. The auditory evoked potential analysis revealed a significant association of amplitude and latency with stroke lesion volume. Finally, we developed a multiple regression model combining EEG predictors for quantifying the ischaemic lesion (R2 = 0.938, p value < 0.001). These findings demonstrate the potential application of EEG as a valid modality for monitoring the brain after a stroke.

Auditory Brainstem Responses (ABR) of Rats during Experimentally Induced Tinnitus: Literature Review

Tinnitus is a subjective phantom sound perceived only by the affected person and a symptom of various auditory and non-auditory conditions. The majority of methods used in clinical and basic research for tinnitus diagnosis are subjective. To better understand tinnitus-associated changes in the auditory system, an objective technique measuring auditory sensitivity-the auditory brainstem responses (ABR)-has been suggested. Therefore, the present review aimed to summarize ABR's features in a rat model during experimentally induced tinnitus. PubMed, Web of Science, Science Direct, and Scopus databanks were searched using Medical Subject Heading (MeSH) terms: auditory brainstem response, tinnitus, rat. The search identified 344 articles, and 36 of them were selected for the full-text analyses. The experimental protocols and results were evaluated, and the gained knowledge was synthesized. A high level of heterogeneity between the studies was found regarding all assessed areas. The most consistent finding of all studies was a reduction in the ABR wave I amplitude following exposure to noise and salicylate. Simultaneously, animals with salicylate-induced but not noise-induced tinnitus had an increased amplitude of wave IV. Furthermore, the present study identified a need to develop a consensus experimental ABR protocol applied in future tinnitus studies using the rat model.

Auditory Brainstem Changes in Timing may Underlie Hyperacusis in a Salicylate-induced Acute Rat Model

Hyperacusis, an exaggerated, sometimes painful perception of loudness even for soft sounds, is a poorly understood distressing condition. While the involvement of modified gain of central auditory neurons and the influence of nonauditory brain regions are well-documented, the issue of where in the auditory system these abnormalities arise remains open, particularly when hyperacusis comes without sensorineural hearing loss. Here we used acute intraperitoneal administration of sodium salicylate (150 mg/kg) in rats, enough to produce > 10-dB decrease in acoustic startle threshold with mild hearing loss at low frequencies (<10 kHz). Anesthesia, necessary for middle-ear-reflex (MEMR) threshold measurements, abolished the olivocochlear efferent reflex but not the MEMR acting on frequencies < 10 kHz, and its mean threshold increased from 55 dB SPL in controls to 80 dB SPL in salicylate-treated animals 60-90 minutes after injection, with an about 3-dB increase in acoustic energy reaching the cochlea. The mean latencies of auditory brainstem-evoked responses (ABR) conspicuously decreased after salicylate, by 0.25 millisecond at 6 kHz at every level, a frequency-dependent effect absent above 12 kHz. A generic model of loudness based upon cross-frequency coincidence detection predicts that with such timing changes, a transient sound may seem as loud at <40 dB SPL as it does in controls at >60 dB SPL. Candidate circuits able to act at the same time on the startle reflex, the MEMR and ABRs may be serotoninergic, as salicylate is known to increase brain serotonin and 5-HT neurons participate in MEMR and ABR circuits.