Hyperexcitability of inferior colliculus and acoustic startle reflex with age-related hearing loss

Hearing loss-related altered neuronal activity in the inferior colliculus

Hearing loss is well known to cause plastic changes in the central auditory system and pathological changes such as tinnitus and hyperacusis. Impairment of inner ear functions is the main cause of hearing loss. In aged individuals, not only inner ear dysfunction but also senescence of the central nervous system is the cause of malfunction of the auditory system. In most cases of hearing loss, the activity of the auditory nerve is reduced, but that of the successive auditory centers is increased in a compensatory way. It has been reported that activity changes occur in the inferior colliculus (IC), a critical nexus of the auditory pathway. The IC integrates the inputs from the brainstem and drives the higher auditory centers. Since abnormal activity in the IC is likely to affect auditory perception, it is crucial to elucidate the neuronal mechanism to induce the activity changes of IC neurons with hearing loss. This review outlines recent findings on hearing-loss-induced plastic changes in the IC and brainstem auditory neuronal circuits and discusses what neuronal mechanisms underlie hearing-loss-induced changes in the activity of IC neurons. Considering the different causes of hearing loss, we discuss age-related hearing loss separately from other forms of hearing loss (non-age-related hearing loss). In general, the main plastic change of IC neurons caused by both age-related and non-age-related hearing loss is increased central gain. However, plastic changes in the IC caused by age-related hearing loss seem to be more complex than those caused by non-age-related hearing loss.

Low-intensity noise exposure takes an essential part in the mechanism of late-onset hereditary hearing loss caused by Abcc1 mutation

With the development of the social economy, we are exposed to increasing noise in our daily lives. Our previous work found an ABCC1(NM_004996.3:c.A1769G, NP_004987.2:p.N590S) variant which cosegregated with the patients in an autosomal dominant non-syndromic hearing loss family. At present, the specific mechanism of deafness caused by ABCC1 mutation is still not clear. Using the knock-in mouse model simulating human ABCC1 mutation, we found that the occurrence of family-related phenotypes was likely attributed to the combination of the mouse genotype and low-intensity noise. GSH and GSSG are important physiological substrates of ABCC1. The destruction of GSH-GSSG balance in the cochleae of both Abcc1N591S/+ mice and Abcc1N591S/N591S mice during low-intensity noise exposure may result in irreversible damage to the hair cells of the cochleae, consequently leading to hearing loss in mice. The findings offered a potential novel idea for the prevention and management of hereditary hearing loss within this family.

Map plasticity following noise exposure in auditory cortex of rats: implications for disentangling neural correlates of tinnitus and hyperacusis

Introduction

Both tinnitus and hyperacusis, likely triggered by hearing loss, can be attributed to maladaptive plasticity in auditory perception. However, owing to their co-occurrence, disentangling their neural mechanisms proves difficult. We hypothesized that the neural correlates of tinnitus are associated with neural activities triggered by low-intensity tones, while hyperacusis is linked to responses to moderate- and high-intensity tones.

Methods

To test these hypotheses, we conducted behavioral and electrophysiological experiments in rats 2 to 8 days after traumatic tone exposure.

Results

In the behavioral experiments, prepulse and gap inhibition tended to exhibit different frequency characteristics (although not reaching sufficient statistical levels), suggesting that exposure to traumatic tones led to acute symptoms of hyperacusis and tinnitus at different frequency ranges. When examining the auditory cortex at the thalamocortical recipient layer, we observed that tinnitus symptoms correlated with a disorganized tonotopic map, typically characterized by responses to low-intensity tones. Neural correlates of hyperacusis were found in the cortical recruitment function at the multi-unit activity (MUA) level, but not at the local field potential (LFP) level, in response to moderate- and high-intensity tones. This shift from LFP to MUA was associated with a loss of monotonicity, suggesting a crucial role for inhibitory synapses.

Discussion

Thus, in acute symptoms of traumatic tone exposure, our experiments successfully disentangled the neural correlates of tinnitus and hyperacusis at the thalamocortical recipient layer of the auditory cortex. They also suggested that tinnitus is linked to central noise, whereas hyperacusis is associated with aberrant gain control. Further interactions between animal experiments and clinical studies will offer insights into neural mechanisms, diagnosis and treatments of tinnitus and hyperacusis, specifically in terms of long-term plasticity of chronic symptoms.

Neuropeptide Y Signaling Regulates Recurrent Excitation in the Auditory Midbrain

Neuropeptides play key roles in shaping the organization and function of neuronal circuits. In the inferior colliculus (IC), which is in the auditory midbrain, Neuropeptide Y (NPY) is expressed by a class of GABAergic neurons that project locally and outside the IC. Most neurons in the IC have local axon collaterals; however, the organization and function of local circuits in the IC remain unknown. We previously found that excitatory neurons in the IC can express the NPY Y1 receptor (Y1R+) and application of the Y1R agonist, [Leu31, Pro34]-NPY (LP-NPY), decreases the excitability of Y1R+ neurons. As NPY signaling regulates recurrent excitation in other brain regions, we hypothesized that Y1R+ neurons form interconnected local circuits in the IC and that NPY decreases the strength of recurrent excitation in these circuits. To test this hypothesis, we used optogenetics to activate Y1R+ neurons in mice of both sexes while recording from other neurons in the ipsilateral IC. We found that nearly 80% of glutamatergic IC neurons express the Y1 receptor, providing extensive opportunities for NPY signaling to regulate local circuits. Additionally, Y1R+ neuron synapses exhibited modest short-term synaptic plasticity, suggesting that local excitatory circuits maintain their influence over computations during sustained stimuli. We further found that application of LP-NPY decreased recurrent excitation in the IC, suggesting that NPY signaling strongly regulates local circuit function in the auditory midbrain. Our findings show that Y1R+ excitatory neurons form interconnected local circuits in the IC, and their influence over local circuits is regulated by NPY signaling.SIGNIFICANCE STATEMENT Local networks play fundamental roles in shaping neuronal computations in the brain. The IC, localized in the auditory midbrain, plays an essential role in sound processing, but the organization of local circuits in the IC is largely unknown. Here, we show that IC neurons that express the Neuropeptide Y1 receptor (Y1R+ neurons) make up most of the excitatory neurons in the IC and form interconnected local circuits. Additionally, we found that NPY, which is a powerful neuromodulator known to shape neuronal activity in other brain regions, decreases the extensive recurrent excitation mediated by Y1R+ neurons in local IC circuits. Thus, our results suggest that local NPY signaling is a key regulator of auditory computations in the IC.

Neuropeptide Y signaling regulates recurrent excitation in the auditory midbrain

Neuropeptides play key roles in shaping the organization and function of neuronal circuits. In the inferior colliculus (IC), which is located in the auditory midbrain, Neuropeptide Y (NPY) is expressed by a large class of GABAergic neurons that project locally as well as outside the IC. The IC integrates information from numerous auditory nuclei making the IC an important hub for sound processing. Most neurons in the IC have local axon collaterals, however the organization and function of local circuits in the IC remains largely unknown. We previously found that neurons in the IC can express the NPY Y1 receptor (Y 1 R + ) and application of the Y 1 R agonist, [Leu 31 , Pro 34 ]-NPY (LP-NPY), decreases the excitability of Y 1 R + neurons. To investigate how Y 1 R + neurons and NPY signaling contribute to local IC networks, we used optogenetics to activate Y 1 R + neurons while recording from other neurons in the ipsilateral IC. Here, we show that 78.4% of glutamatergic neurons in the IC express the Y1 receptor, providing extensive opportunities for NPY signaling to regulate excitation in local IC circuits. Additionally, Y 1 R + neuron synapses exhibit modest short-term synaptic plasticity, suggesting that local excitatory circuits maintain their influence over computations during sustained stimuli. We further found that application of LP-NPY decreases recurrent excitation in the IC, suggesting that NPY signaling strongly regulates local circuit function in the auditory midbrain. Together, our data show that excitatory neurons are highly interconnected in the local IC and their influence over local circuits is tightly regulated by NPY signaling.

Online ascorbate sensing reveals oxidative injury occurrence in inferior colliculus in salicylate-induced tinnitus animal model

Tinnitus is a widespread and serious clinical and social problem. Although oxidative injury has been suggested to be one of pathological mechanisms in auditory cortex, whether this mechanism could be applied to inferior colliculus remains unclear. In this study, we used an online electrochemical system (OECS) integrating in vivo microdialysis with selective electrochemical detector to continuously monitor the dynamics of ascorbate efflux, an index of oxidative injury, in inferior colliculus of living rats during sodium salicylate-induced tinnitus. We found that OECS with a carbon nanotubes (CNTs)-modified electrode as the detector selectively responses to ascorbate, which is free from the interference from sodium salicylate and MK-801 that were used to induce tinnitus animal model and investigate the N-methyl-d-aspartate (NMDA) receptor mediated excitotoxicity, respectively. With the OECS, we found that the extracellular ascorbate level in inferior colliculus significantly increases after salicylate administration and such increase was suppressed by immediate injection of NMDA receptor antagonist MK-801. In addition, we found that salicylate administration significantly increases the spontaneous and sound stimuli evoked neural activity in inferior colliculus and that the increases were inhibited by the injection of MK-801. These results suggest that oxidative injury may occur in inferior colliculus following salicylate-induced tinnitus, which is closely relevant to the NMDA-mediated neuronal excitotoxicity. This information is useful for understanding the neurochemical processes in inferior colliculus involved in tinnitus and its related brain diseases.

KCNQ4 potassium channel subunit deletion leads to exaggerated acoustic startle reflex in mice

The potassium voltage-gated channel subfamily Q member 4 (KCNQ4) subunit forms channels responsible for M-current, a muscarine-sensitive potassium current regulating neuronal excitability. In contrast to other KCNQ subunits, its expression is restricted to the cochlear outer hair cells, the auditory brainstem and other brainstem nuclei in a great overlap with structures involved in startle reflex. We aimed to show whether startle reflexis affected by the loss of KCNQ4 subunit and whether these alterations are similar to the ones caused by brainstem hyperexcitability. Young adult KCNQ4 knockout mice and wild-type littermates, as well as mice expressing hM3D chemogenetic actuator in the pontine caudal nucleus and neurons innervating it were used for testing acoustic startle. The acoustic startle reflex was significantly increased in knockout mice compared with wild-type littermates. When mice expressing human M3 muscarinic (hM3D) in nuclei related to startle reflex were tested, a similar increase of the first acoustic startle amplitude and a strong habituation of the further responses was demonstrated. We found that the acoustic startle reflex is exaggerated and minimal habituation occurs in KCNQ4 knockout animals. These changes are distinct from the effects of the hyperexcitability of nuclei involved in startle. One can conclude that the exaggerated startle reflex found with the KCNQ4 subunit deletion is the consequence of both the cochlear damage and the changes in neuronal excitability of startle networks.

Low Intensity Noise Exposure Enhanced Auditory Loudness and Temporal Processing by Increasing Excitability of DCN

Sound stimulation is generally used for tinnitus and hyperacusis treatment. Recent studies found that long-term noise exposure can change synaptic and firing properties in the central auditory system, which will be detected by the acoustic startle reflex. However, the perceptual consequences of long-term low-intensity sound exposure are indistinct. This study will detect the effects of moderate-level noise exposure (83 dB SPL) on auditory loudness, and temporal processing was evaluated using CBA/CaJ mice. C-Fos staining was used to detect neural activity changes in the central auditory pathway. With two weeks of 83 dB SPL noise exposure (8 hours per day), no persistent threshold shift of the auditory brainstem response (ABR) was identified. On the other hand, noise exposure enhanced the acoustic startle response (ASR) and gap-induced prepulse inhibition significantly (gap-PPI). Low-level noise exposure, according to the findings, can alter temporal acuity. Noise exposure increased the number of c-Fos labeled neurons in the dorsal cochlear nucleus (DCN) and caudal pontine reticular nucleus (PnC) but not at a higher level in the central auditory nuclei. Our results suggested that noise stimulation can change acoustical temporal processing presumably by increasing the excitability of auditory brainstem neurons.

Differential Effects of Exercise on fMRI of the Midbrain Ascending Arousal Network Nuclei in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) and Gulf War Illness (GWI) in a Model of Postexertional Malaise (PEM)

BACKGROUND

Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS), Gulf War Illness (GWI) and control subjects underwent fMRI during difficult cognitive tests performed before and after submaximal exercise provocation (Washington 2020). Exercise caused increased activation in ME/CFS but decreased activation for GWI in the dorsal midbrain, left Rolandic operculum and right middle insula. Midbrain and isthmus nuclei participate in threat assessment, attention, cognition, mood, pain, sleep, and autonomic dysfunction.

METHODS

Activated midbrain nuclei were inferred by a re-analysis of data from 31 control, 36 ME/CFS and 78 GWI subjects using a seed region approach and the Harvard Ascending Arousal Network.

RESULTS

Before exercise, control and GWI subjects showed greater activation during cognition than ME/CFS in the left pedunculotegmental nucleus. Post exercise, ME/CFS subjects showed greater activation than GWI ones for midline periaqueductal gray, dorsal and median raphe, and right midbrain reticular formation, parabrachial complex and locus coeruleus. The change between days (delta) was positive for ME/CFS but negative for GWI, indicating reciprocal patterns of activation. The controls had no changes.

CONCLUSIONS

Exercise caused the opposite effects with increased activation in ME/CFS but decreased activation in GWI, indicating different pathophysiological responses to exertion and mechanisms of disease. Midbrain and isthmus nuclei contribute to postexertional malaise in ME/CFS and GWI.

Changes in the Gene Expression Profiles of the Inferior Colliculus Following Unilateral Cochlear Ablation in Adult Rats

This study aimed to explore gene expression changes in the inferior colliculus (IC) after single-sided deafness (SSD). Forty 8-week-old female Sprague-Dawley rats were used. Twenty rats underwent right-side cochlear ablation, and IC tissues were harvested after 2 weeks (SSD 2-week group). Twenty rats underwent a sham operation and were sacrificed after 2 weeks (control group). Both sides of the IC were analyzed using a gene expression array. Pathway analyses were performed on genes that were differentially expressed compared with their levels in the control group. The expression levels of genes involved in the candidate pathways were confirmed using reverse transcription polymerase chain reaction (RT-PCR). Among the genes with ≥ 1.5-fold changes in expression levels and P < 0.05, there were 7 and 9 genes with increased and decreased expression, respectively, in the ipsilateral IC and 10 and 12 genes with increased and decreased expression, respectively, in the contralateral IC. The pathway analysis did not identify significantly related pathway. In the bilateral analysis, a total of 14 genes were ≥ 1.3-fold downregulated in both the ipsilateral and contralateral IC in the SSD 2-week group compared with their expression in the control group. Pathway analyses of these 14 genes included 7 genes, namely, amine compound solute carrier (Slc)5a7; Slc18a3; Slc6a5; synaptic vesicle glycoprotein 2C (Sv2c); S100 calcium binding protein A10 (S100a10); a gene with sequence similarity to family 111, member A (Fam111a); and peripherin (Prph), that were related to the acetylcholine neurotransmitter release cycle, SLC transporters, and the neurotransmitter release cycle pathways. RT-PCR showed reduced expression of Slc5a7, Sv2c, and Prph in the contralateral IC and Slc18a3 and Slc6a5 in the ipsilateral IC of the SSD 2-week group compared with that in the control group.

Metabolic changes in the brain and blood of rats following acoustic trauma, tinnitus and hyperacusis

It has been increasingly recognized that tinnitus is likely to be generated by complex network changes. Acoustic trauma that causes tinnitus induces significant changes in multiple metabolic pathways in the brain. However, it is not clear whether those metabolic changes in the brain could also be reflected in blood samples and whether metabolic changes could discriminate acoustic trauma, hyperacusis and tinnitus. We analyzed brain and serum metabolic changes in rats following acoustic trauma or a sham procedure using metabolomics. Hearing levels were recorded before and after acoustic trauma and behavioral measures to quantify tinnitus and hyperacusis were conducted at 4 weeks following acoustic trauma. Tissues from 11 different brain regions and serum samples were collected at about 3 months following acoustic trauma. Among the acoustic trauma animals, eight exhibited hyperacusis-like behavior and three exhibited tinnitus-like behavior. Using Gas chromatography-mass spectrometry and multivariate statistical analysis, significant metabolic changes were found in acoustic trauma animals in both the brain and serum samples with a number of metabolic pathways significantly perturbated. Furthermore, metabolic changes in the serum were able to differentiate sham from acoustic trauma animals, as well as sham from hyperacusis animals, with high accuracy. Our results suggest that serum metabolic profiling in combination with machine learning analysis may be a promising approach for identifying biomarkers for acoustic trauma, hyperacusis and potentially, tinnitus.

Hyperexcitability of the Nucleus Accumbens Is Involved in Noise-Induced Hyperacusis

Reduced tolerance to sound stimuli (hyperacusis) is commonly seen in tinnitus patients. Dysfunction of limbic systems, such as the nucleus accumbens (NAc), may be involved in emotional reactions to the sound stimuli in tinnitus patients. To study the functional changes in the NAc in hyperacusis, we have examined the neural activity changes of the NAc using c-Fos staining in an animal model of hyperacusis. The c-Fos staining was also examined in the medial geniculate nucleus (MGN), a central auditory pathway which has neural projections to the NAc. Postnatal rats (14 days) were exposed to loud noise (115 dB SPL, 4 hours for two consecutive days) to induce hyperacusis (n = 4). Rats without noise exposure were used as the controls (n = 4). After P35, rats in both groups were put in a behavioral training for sound detection. After they were trained to detect sound stimuli, their reaction time to noise bursts centered at 2 kHz (40-110 dB SPL) was measured. Rats in the noise group showed a significantly shorter reaction time than those in the control group to the noise bursts at high intensities, suggesting the noise exposure induced hyperacusis behavior. The c-Fos expressions in the NAc and the MGNs of the noise group were significantly higher than those of the control group. Our results suggested that early-age noise exposure caused hyperactivity in the NAc and the MGNs which may induce the loudness increase in these rats.

Exercise alters brain activation in Gulf War Illness and Myalgic Encephalomyelitis/Chronic Fatigue Syndrome

Gulf War Illness affects 25-30% of American veterans deployed to the 1990-91 Persian Gulf War and is characterized by cognitive post-exertional malaise following physical effort. Gulf War Illness remains controversial since cognitive post-exertional malaise is also present in the more common Myalgic Encephalomyelitis/Chronic Fatigue Syndrome. An objective dissociation between neural substrates for cognitive post-exertional malaise in Gulf War Illness and Myalgic Encephalomyelitis/Chronic Fatigue Syndrome would represent a biological basis for diagnostically distinguishing these two illnesses. Here, we used functional magnetic resonance imaging to measure neural activity in healthy controls and patients with Gulf War Illness and Myalgic Encephalomyelitis/Chronic Fatigue Syndrome during an N-back working memory task both before and after exercise. Whole brain activation during working memory (2-Back > 0-Back) was equal between groups prior to exercise. Exercise had no effect on neural activity in healthy controls yet caused deactivation within dorsal midbrain and cerebellar vermis in Gulf War Illness relative to Myalgic Encephalomyelitis/Chronic Fatigue Syndrome patients. Further, exercise caused increased activation among Myalgic Encephalomyelitis/Chronic Fatigue Syndrome patients within the dorsal midbrain, left operculo-insular cortex (Rolandic operculum) and right middle insula. These regions-of-interest underlie threat assessment, pain, interoception, negative emotion and vigilant attention. As they only emerge post-exercise, these regional differences likely represent neural substrates of cognitive post-exertional malaise useful for developing distinct diagnostic criteria for Gulf War Illness and Myalgic Encephalomyelitis/Chronic Fatigue Syndrome.

Prolonged Exposure of CBA/Ca Mice to Moderately Loud Noise Can Cause Cochlear Synaptopathy but Not Tinnitus or Hyperacusis as Assessed With the Acoustic Startle Reflex

Hearing loss changes the auditory brain, sometimes maladaptively. When deprived of cochlear input, central auditory neurons become more active spontaneously and begin to respond more strongly and synchronously to better preserved sound frequencies. This spontaneous and sound-evoked central hyperactivity has been postulated to trigger tinnitus and hyperacusis, respectively. Localized hyperactivity has also been observed after long-term exposure to noise levels that do not damage the cochlea. Adult animals exposed to bands of nondamaging noise exhibited suppressed spontaneous and sound-evoked activity in the area of primary auditory cortex (A1) stimulated by the exposure band but had increased spontaneous and evoked activity in neighboring A1 areas. We hypothesized that the cortically suppressed frequencies should for some time after exposure be perceived as less loud than before (hypoacusis), whereas the hyperactivity outside of the exposure band might lead to frequency-specific hyperacusis or tinnitus. To investigate this, adult CBA/Ca mice were exposed for >2 months to 8 to 16 kHz noise at 70 or 75 dB sound pressure level and tested for hypo-/hyperacusis and tinnitus using tone and gap prepulse inhibition of the acoustic startle reflex. Auditory brainstem responses and distortion product otoacoustic emissions showed evidence of cochlear synaptopathy after exposure at 75 but not 70 dB, putting a lower bound on damaging noise levels for CBA/Ca mice. Contrary to hypothesis, neither exposure significantly shifted startle results from baseline. These negative findings nevertheless have implications for startle test methodology and for the putative role of central hyperactivity in hyperacusis and tinnitus.

Loss of the Cochlear Amplifier Prestin Reduces Temporal Processing Efficacy in the Central Auditory System

Active mechanical amplification of sound occurs in cochlear outer hair cells (OHCs) that change their length with oscillations of their membrane potential. Such length changes are the proposed cellular source of the cochlear amplifier, and prestin is the motor protein responsible for OHC electromotility. Previous findings have shown that mice lacking prestin displayed a loss of OHC electromotility, subsequent loss of distortion-product otoacoustic emissions, and a 40–60 dB increase in hearing thresholds. In this study we were interested in studying the functional consequences of the complete loss of cochlear amplification on neural coding of frequency selectivity, tuning, and temporal processing in the auditory midbrain. We recorded near-field auditory evoked potentials and multi-unit activity from the inferior colliculus (IC) of prestin (−/−) null and prestin (+/+) wild-type control mice and determined frequency response areas (FRAs), tuning sharpness, and gap detection to tone bursts and silent gaps embedded in broadband noise. We were interested in determining if the moderate to severe sensorineural hearing loss associated with the loss of motor protein prestin would also impair auditory midbrain temporal-processing measures, or if compensatory mechanisms within the brainstem could compensate for the loss of prestin. In prestin knockout mice we observed that there are severe impairments in midbrain tuning, thresholds, excitatory drive, and gap detection suggesting that brainstem and midbrain processing could not overcome the auditory processing deficits afforded by the loss of OHC electromotility mediated by the prestin protein.

Functional Change in the Caudal Pontine Reticular Nucleus Induced by Age-Related Hearing Loss

Increased acoustic startle responses (ASR), which represent reduced uncomfortable loudness level in humans, have been reported in middle-aged C57BL/6J mice with sensorineural hearing loss. Although neural plasticity changes in the central auditory system after the peripheral lesions were suggested to underlie this phenomenon, the neurological cause of exaggerated ASR is still not clear. In this study, the local field potentials and firing rates of the caudal pontine reticular nucleus (PnC), which plays a major role in the ASR pathway, were recorded in 2-month- and 6-month-old C57BL/6 J mice. Consistent with our previous studies, the amplitude of ASR increased, and the threshold of ASR decreased in the 6-month-old mice after developing 20–40 dB hearing loss. The PnC response induced by high-frequency stimuli (>20 kHz) decreased in the 6-month group, whereas the PnC response induced by low-frequency stimuli (<12 kHz) showed a significant increase in the 6-month group compared to the 2-month group. The enhancement of PnC response is similar to the ASR increase found in the 6-month-old C57 mice. Our results suggest that the high-frequency hearing loss caused an increase in PnC sensitivity in the C57 mice which may enhance ASRs.