Reduced white matter maturation in the central auditory system of children living with HIV

Introduction

School-aged children experience crucial developmental changes in white matter (WM) in adolescence. The human immunodeficiency virus (HIV) affects neurodevelopment. Children living with perinatally acquired HIV (CPHIVs) demonstrate hearing and neurocognitive impairments when compared to their uninfected peers (CHUUs), but investigations into the central auditory system (CAS) WM integrity are lacking. The integration of the CAS and other brain areas is facilitated by WM fibers whose integrity may be affected in the presence of HIV, contributing to neurocognitive impairments.

Methods

We used diffusion tensor imaging (DTI) tractography to map the microstructural integrity of WM between CAS regions, including the lateral lemniscus and acoustic radiation, as well as between CAS regions and non-auditory regions of 11-year-old CPHIVs. We further employed a DTI-based graph theoretical framework to investigate the nodal strength and efficiency of the CAS and other brain regions in the structural brain network of the same population. Finally, we investigated associations between WM microstructural integrity outcomes and neurocognitive outcomes related to auditory and language processing. We hypothesized that compared to the CHUU group, the CPHIV group would have lower microstructural in the CAS and related regions.

Results

Our analyses showed higher mean diffusivity (MD), a marker of axonal maturation, in the lateral lemniscus and acoustic radiations, as well as WM between the CAS and non-auditory regions predominantly in frontotemporal areas. Most affected WM connections also showed higher axial and radial diffusivity (AD and RD, respectively). There were no differences in the nodal properties of the CAS regions between groups. The MD of frontotemporal and subcortical WM-connected CAS regions, including the inferior longitudinal fasciculus, inferior fronto-occipital fasciculus, and internal capsule showed negative associations with sequential processing in the CPHIV group but not in the CHUU group.

Discussion

The current results point to reduced axonal maturation in WM, marked by higher MD, AD, and RD, within and from the CAS. Furthermore, alterations in WM integrity were associated with sequential processing, a neurocognitive marker of auditory working memory. Our results provide insights into the microstructural integrity of the CAS and related WM in the presence of HIV and link these alterations to auditory working memory.

Cochlear synaptopathy impairs suprathreshold tone-in-noise coding in the cochlear nucleus

Hearing impairment without threshold elevations can occur when there is damage to high-threshold auditory nerve fiber synapses with cochlear inner hair cells. Instead, cochlear synaptopathy produces suprathreshold deficits, especially in older patients, which affect conversational speech. Since listening in noise at suprathreshold levels presents significant challenges to the ageing population, we examined the effects of synaptopathy on tone-in-noise coding on the central recipients of auditory nerve fibers - the cochlear nucleus neurons. To induce synaptopathy, guinea pigs received a unilateral sound overexposure to the left ears. A separate group received sham exposures. At 4 weeks post-exposure, thresholds had recovered but reduced ABR wave 1 amplitudes and auditory nerve synapse loss remained on the left side. Single unit responses were recorded from several cell types in the ventral cochlear nucleus to puretone and noise stimuli. Receptive fields and rate-level functions in the presence of continuous broadband noise were examined. The synaptopathy-inducing noise exposure did not affect mean unit tone-in-noise thresholds, nor the tone-in-noise thresholds in each animal, demonstrating equivalent tone-in-noise detection thresholds to sham animals. However, synaptopathy reduced single-unit responses to suprathreshold tones in the presence of background noise, particularly in the cochlear nucleus small cells. These data demonstrate that suprathreshold tone-in-noise deficits following cochlear synaptopathy are evident in the first neural station of the auditory brain, the cochlear nucleus neurons, and provide a potential target for assessing and treating listening-in-noise deficits in humans. KEY POINTS: Recording from multiple central auditory neurons can determine any tone-in-noise deficits in animals with quantified cochlear synapse damage. Using this technique, we found that tone-in-noise thresholds are not altered by cochlear synaptopathy, whereas coding of suprathreshold tones-in-noise is disrupted. Suprathreshold deficits occur in small cells and primary-like neurons of the cochlear nucleus. These data provide important insights into the mechanisms underlying difficulties associated with hearing in noisy environments. Abstract figure legend A) Unilateral noise exposure induces synaptopathy and temporary threshold shift (TTS) in guinea pigs. B) Auditory thresholds of single units in the cochlear nucleus are shifted in background noise, and this shift is not affected by synaptopathy. C) Suprathreshold firing rates are decreased by background sound, and this decrease correlates with cochlear synaptopathy. This article is protected by copyright. All rights reserved.

A specialized channel for encoding auditory transients in the magnocellular division of the human medial geniculate nucleus

We test the hypothesis that there exists a generalized magnocellular system in the brain optimized for temporal processing. In the visual system, it is well known that the magnocellular layers in the lateral geniculate nucleus (LGN) are strongly activated by transients and quickly habituate. However, little is known about the perhaps analogous magnocellular division of the medial geniculate nucleus (MGN), the auditory relay in the thalamus. We measured the functional responses of the MGN in 11 subjects who passively listened to sustained and transient nonlinguistic sounds, using functional MRI. We observed that voxels in the ventromedial portion of the MGN, corresponding to the magnocellular division, exhibited a robust preference to transient sounds, consistently across subjects, whereas the remainder of the MGN did not discriminate between sustained and transient sounds. We conclude that the magnocellular neurons in the MGN parallel the magnocellular neurons in its visual counterpart, LGN, and constitute an information stream specialized for encoding auditory dynamics.

Invoking the influence of emotion in central auditory processing to improve the treatment of speech impairments

Objectives:

To explore the benefits of invoking unconscious sentiment to improve the treatment of stuttering and misarticulation.

Methods:

This cross-sectional study of 80 participants with speech issues (44 patients with misarticulation and 36 with stuttering) who underwent comprehensive speech and hearing evaluations to confirm and diagnose speech difficulties. Speech and language pathologists then calculated either the percentage of correctly pronounce sounds in misarticulation cases or stuttering severity index-4 scores in cases of stuttering following the use of therapeutic stimuli recorded with familiar and non-familiar voices of similar linguistic and phonetic complexity. Descriptive and inferential statistics were used to compare the data collected following the use of familiar and unfamiliar stimuli.

Results:

The analysis showed that the number of dysfluencies in cases of stuttering were significantly fewer when employing familiar voices than unfamiliar voices (3% errors vs 12% errors; Z= -5.16 p<0.001). Additionally, the percentages of correct pronouncing of target sounds in cases of articulation disorders were prominently higher when using familiar voices compared with unfamiliar voices (88% PCC vs 66% PCC; Z= -5.65, p<0.001)

Conclusion:

This study confirms the utility of invoking emotion in improving speech therapy and maximizing therapeutic outcomes. This study also recommends engaging families and friends in providing speech services to the speech-impaired population to improve patient progress.

Noise Induced Hearing Loss and Tinnitus-New Research Developments and Remaining Gaps in Disease Assessment, Treatment, and Prevention

Long-term noise exposure often results in noise induced hearing loss (NIHL). Tinnitus, the generation of phantom sounds, can also result from noise exposure, although understanding of its underlying mechanisms are limited. Recent studies, however, are shedding light on the neural processes involved in NIHL and tinnitus, leading to potential new and innovative treatments. This review focuses on the assessment of NIHL, available treatments, and development of new pharmacologic and non-pharmacologic treatments based on recent studies of central auditory plasticity and adaptive changes in hearing. We discuss the mechanisms and maladaptive plasticity of NIHL, neuronal aspects of tinnitus triggers, and mechanisms such as tinnitus-associated neural changes at the cochlear nucleus underlying the generation of tinnitus after noise-induced deafferentation. We include observations from recent studies, including our own studies on associated risks and emerging treatments for tinnitus. Increasing knowledge of neural plasticity and adaptive changes in the central auditory system suggest that NIHL is preventable and transient abnormalities may be reversable, although ongoing research in assessment and early detection of hearing difficulties is still urgently needed. Since no treatment can yet reverse noise-related damage completely, preventative strategies and increased awareness of hearing health are essential.

Acute Noise Exposure Is Associated With Intrinsic Apoptosis in Murine Central Auditory Pathway

Noise that is capable of inducing the hearing loss (NIHL) has a strong impact on the inner ear structures and causes early and most obvious pathophysiological changes in the auditory periphery. Several studies indicated that intrinsic apoptotic cell death mechanisms are the key factors inducing cellular degeneration immediately after noise exposure and are maintained for days or even weeks. In addition, studies demonstrated several changes in the central auditory system following noise exposure, consistent with early apoptosis-related pathologies. To clarify the underlying mechanisms, the present study focused on the noise-induced gene and protein expression of the pro-apoptotic protease activating factor-1 (APAF1) and the anti-apoptotic B-cell lymphoma 2 related protein a1a (BCL2A1A) in the cochlear nucleus (CN), inferior colliculus (IC) and auditory cortex (AC) of the murine central auditory pathway. The expression of Bcl2a1a mRNA was upregulated immediately after trauma in all tissues investigated, whereas the protein levels were significantly reduced at least in the auditory brainstem. Conversely, acute noise has decreased the expression of Apaf1 gene along the auditory pathway. The changes in APAF1 protein level were not statistically significant. It is tempting to speculate that the acoustic overstimulation leads to mitochondrial dysfunction and induction of apoptosis by regulation of proapoptotic and antiapoptotic proteins. The inverse expression pattern on the mRNA level of both genes might reflect a protective response to decrease cellular damage. Our results indicate the immediate presence of intrinsic apoptosis following noise trauma. This, in turn, may significantly contribute to the development of central structural deficits. Auditory pathway-specific inhibition of intrinsic apoptosis could be a therapeutic approach for the treatment of acute (noise-induced) hearing loss to prevent irreversible neuronal injury in auditory brain structures and to avoid profound deficits in complex auditory processing.

The effect of obstructive sleep apnea syndrome on the central auditory system

Background/aim: Obstructive sleep apnea syndrome (OSAS) is a disease characterized by repeated hypoxia attacks during sleep. The effect of hypoxia on the central nervous system is a well-known entity. In this study we aimed to investigate the effect of OSAS on the central auditory system. Materials and methods: Twenty-one OSAS patients diagnosed by polysomnography (PSG) and 10 control subjects were included in the study. After a thorough otorhinolaryngology examination, all subjects underwent pure tone audiometry (250 to 8000 Hz frequency). The subjects with normal otoscopic examination and hearing threshold were included in the study. All participants underwent speech discrimination analyses and auditory time processing and sequencing tests, i.e. frequency pattern test (FPT) and duration pattern test (DPT). Results: Although hearing was normal in the OSAS patients, significant loss was observed in the speech discrimination rates compared to the control group (P < 0.05). Significant disruption was also detected in the FPT and SPT in the OSAS patients (P < 0.05). Conclusion: Repeated hypoxic episodes in OSAS resulted in statistically significant impairments in the central auditory pathways, even if the hearing threshold was within normal limits.

Cellular and Molecular Underpinnings of Neuronal Assembly in the Central Auditory System during Mouse Development

During development, the organization of the auditory system into distinct functional subcircuits depends on the spatially and temporally ordered sequence of neuronal specification, differentiation, migration and connectivity. Regional patterning along the antero-posterior axis and neuronal subtype specification along the dorso-ventral axis intersect to determine proper neuronal fate and assembly of rhombomere-specific auditory subcircuits. By taking advantage of the increasing number of transgenic mouse lines, recent studies have expanded the knowledge of developmental mechanisms involved in the formation and refinement of the auditory system. Here, we summarize several findings dealing with the molecular and cellular mechanisms that underlie the assembly of central auditory subcircuits during mouse development, focusing primarily on the rhombomeric and dorso-ventral origin of auditory nuclei and their associated molecular genetic pathways.

Inner Hair Cell Loss Disrupts Hearing and Cochlear Function Leading to Sensory Deprivation and Enhanced Central Auditory Gain

There are three times as many outer hair cells (OHC) as inner hair cells (IHC), yet IHC transmit virtually all acoustic information to the brain as they synapse with 90–95% of type I auditory nerve fibers. Here we review a comprehensive series of experiments aimed at determining how loss of the IHC/type I system affects hearing by selectively destroying these cells in chinchillas using the ototoxic anti-cancer agent carboplatin. Eliminating IHC/type I neurons has no effect on distortion product otoacoustic emission or the cochlear microphonic potential generated by OHC; however, it greatly reduces the summating potential produced by IHC and the compound action potential (CAP) generated by type I neurons. Remarkably, responses from remaining auditory nerve fibers maintain sharp tuning and low thresholds despite innervating regions of the cochlea with ~80% IHC loss. Moreover, chinchillas with large IHC lesions have surprisingly normal thresholds in quiet until IHC losses exceeded 80%, suggesting that only a few IHC are needed to detect sounds in quiet. However, behavioral thresholds in broadband noise are elevated significantly and tone-in-narrow band noise masking patterns exhibit greater remote masking. These results suggest the auditory system is able to compensate for considerable loss of IHC/type I neurons in quiet but not in difficult listening conditions. How does the auditory brain deal with the drastic loss of cochlear input? Recordings from the inferior colliculus found a relatively small decline in sound-evoked activity despite a large decrease in CAP amplitude after IHC lesion. Paradoxically, sound-evoked responses are generally larger than normal in the auditory cortex, indicative of increased central gain. This gain enhancement in the auditory cortex is associated with decreased GABA-mediated inhibition. These results suggest that when the neural output of the cochlea is reduced, the central auditory system compensates by turning up its gain so that weak signals once again become comfortably loud. While this gain enhancement is able to restore normal hearing under quiet conditions, it may not adequately compensate for peripheral dysfunction in more complex sound environments. In addition, excessive gain increases may convert recruitment into the debilitating condition known as hyperacusis.

Distribution of neuronal KCC2a and KCC2b isoforms in mouse CNS

The neuronal K-Cl cotransporter KCC2 maintains the low intracellular chloride concentration required for the fast hyperpolarizing actions of inhibitory neurotransmitters in mature central nervous system (CNS). The KCC2 gene produces two isoforms, KCC2a and KCC2b, that differ in their N-termini. Increase of KCC2b in the cortex underlies the developmental shift in γ-aminobutyric acid (GABA)ergic responses, whereas the physiological role of KCC2a is still poorly characterized. The two KCC2 isoforms show equal distribution in mouse brainstem neurons at birth; however their postnatal expression patterns, and the subcellular localization of KCC2a, have not yet been described. Here, we compared the pattern of KCC2a and KCC2b expression in different regions of postnatal mouse CNS by immunohistochemistry by using isoform-specific antibodies. Tissue from KCC2a isoform-specific knockout mice was used as a negative control. KCC2b expression increased postnatally and was widely expressed in adult brain. KCC2a immunoreactivity was low or absent in most parts of the adult cortex, hippocampus, thalamus, and cerebellar cortex. Both isoforms were widely present in the developing and mature hypothalamus, a large part of the brainstem, and the spinal cord. A notable exception was the lack of KCC2a staining in the brainstem auditory system. At the subcellular level, the isoforms were only partially colocalized. In neuronal somas, KCC2b immunoreactivity was concentrated at the plasma membrane, whereas KCC2a signal was not. Moreover, although both isoforms were expressed in microtubule-associated protein (MAP)2-positive dendrites, they appeared in non-overlapping dendritic compartments. The results, together with those of previous studies, suggest that KCC2a and KCC2b have overlapping roles in neonatal neurons but presumably different roles in mature neurons.

Whole-cell patch-clamp recording reveals subthreshold sound-evoked postsynaptic currents in the inferior colliculus of awake bats

The inferior colliculus receives excitatory and inhibitory input from parallel auditory pathways that differ in discharge patterns, latencies, and binaural properties. Processing in the inferior colliculus may depend on the temporal sequence in which excitatory and inhibitory synaptic inputs are activated and on the resulting balance between excitation and inhibition. To explore this issue at the cellular level, we used the novel approach of whole-cell patch-clamp recording in the midbrain of awake bats (Eptesicus fuscus) to record EPSCs or IPSCs. Sound-evoked EPSCs were recorded in most neurons. These EPSCs were frequently preceded by an IPSC, followed by an IPSC, or both. These findings help explain the large latency range and transient responses that characterize inferior colliculus neurons. The EPSC was sometimes followed by long-lasting oscillatory currents, suggesting that a single brief sound sets up a pattern of altered excitability that persists far beyond the duration of the initial sound. In three binaural neurons, ipsilateral sound evoked a large IPSC that partially or totally canceled the EPSC evoked by contralateral sound. In one binaural neuron with ipsilaterally evoked IPSCs, contralaterally evoked IPSCs occurred in response to frequencies above and below the neuron's best frequency. Thus, both monaural and binaural interactions can occur at single inferior colliculus neurons. These results show that whole-cell patch-clamp recording offers a powerful means of understanding how subthreshold processes determine the responses of auditory neurons.