Changes in Properties of Auditory Nerve Synapses following Conductive Hearing Loss

Mechanisms of age-related hearing loss at the auditory nerve central synapses and postsynaptic neurons in the cochlear nucleus

Sound information is transduced from mechanical vibration to electrical signals in the cochlea, conveyed to and further processed in the brain to form auditory perception. During the process, spiral ganglion neurons (SGNs) are the key cells that connect the peripheral and central auditory systems by receiving information from hair cells in the cochlea and transmitting it to neurons of the cochlear nucleus (CN). Decades of research in the cochlea greatly improved our understanding of SGN function under normal and pathological conditions, especially about the roles of different subtypes of SGNs and their peripheral synapses. However, it remains less clear how SGN central terminals or auditory nerve (AN) synapses connect to CN neurons, and ultimately how peripheral pathology links to structural alterations and functional deficits in the central auditory nervous system. This review discusses recent progress about the morphological and physiological properties of different subtypes of AN synapses and associated postsynaptic CN neurons, their changes during aging, and the potential mechanisms underlying age-related hearing loss.

Potential uses of auditory nerve stimulation to modulate immune responses in the inner ear and auditory brainstem

Bioelectronic medicine uses electrical stimulation of the nervous system to improve health outcomes throughout the body primarily by regulating immune responses. This concept, however, has yet to be applied systematically to the auditory system. There is growing interest in how cochlear damage and associated neuroinflammation may contribute to hearing loss. In conjunction with recent findings, we propose here a new perspective, which could be applied alongside advancing technologies, to use auditory nerve (AN) stimulation to modulate immune responses in hearing health disorders and following surgeries for auditory implants. In this article we will: (1) review the mechanisms of inflammation in the auditory system in relation to various forms of hearing loss, (2) explore nerve stimulation to reduce inflammation throughout the body and how similar neural-immune circuits likely exist in the auditory system (3) summarize current methods for stimulating the auditory system, particularly the AN, and (4) propose future directions to use bioelectronic medicine to ameliorate harmful immune responses in the inner ear and auditory brainstem to treat refractory conditions. We will illustrate how current knowledge from bioelectronic medicine can be applied to AN stimulation to resolve inflammation associated with implantation and disease. Further, we suggest the necessary steps to get discoveries in this emerging field from bench to bedside. Our vision is a future for AN stimulation that includes additional protocols as well as advances in devices to target and engage neural-immune circuitry for therapeutic benefits.

High calcium concentrations reduce cellular excitability of mouse MNTB neurons

Calcium, a universal intracellular signaling molecule, plays essential roles in neural functions. Historically, in most in vitro brain slice electrophysiology studies, the extracellular calcium concentration ([Ca2+]e) in artificial cerebrospinal fluid is of a wide range and typically higher than the physiological value. At high [Ca2+]e, synaptic transmission is generally enhanced. However, the effects and the underlying mechanisms of calcium on intrinsic neuronal properties are diverse. Using whole-cell patch clamp in acute brainstem slices obtained from mice of either sex, we investigated the effects and the underlying mechanisms of high [Ca2+]e on intrinsic neuronal properties of neurons in the medial nucleus of the trapezoid body (MNTB), an auditory brainstem component in the sound localization circuitry. Compared to the physiological [Ca2+]e (1.2 mM), high [Ca2+]e at 1.8 and 2.4 mM significantly reduced the cellular excitability of MNTB neurons, resulting in decreased spike firing rate, depolarized spike threshold, and decreased the ability to follow high frequency inputs. High extracellular magnesium concentrations at 1.8 and 2.4 mM produced similar but less robust effects, due to surface charge screening. Upon high calcium application, voltage-gated sodium channel currents remained largely unchanged. Calcium-sensing receptors were detected in MNTB neurons, but blocking these receptors did not eliminate the effects of high calcium on spontaneous spiking. We attribute the lack of significant effects in these last two experiments to the moderate changes in calcium we tested. Our results call for the use of physiological [Ca2+]e in brain slice experiments.

Auditory Deprivation during Development Alters Efferent Neural Feedback and Perception

Auditory experience plays a critical role in hearing development. Developmental auditory deprivation because of otitis media, a common childhood disease, produces long-standing changes in the central auditory system, even after the middle ear pathology is resolved. The effects of sound deprivation because of otitis media have been mostly studied in the ascending auditory system but remain to be examined in the descending pathway that runs from the auditory cortex to the cochlea via the brainstem. Alterations in the efferent neural system could be important because the descending olivocochlear pathway influences the neural representation of transient sounds in noise in the afferent auditory system and is thought to be involved in auditory learning. Here, we show that the inhibitory strength of the medial olivocochlear efferents is weaker in children with a documented history of otitis media relative to controls; both boys and girls were included in the study. In addition, children with otitis media history required a higher signal-to-noise ratio on a sentence-in-noise recognition task than controls to achieve the same criterion performance level. Poorer speech-in-noise recognition, a hallmark of impaired central auditory processing, was related to efferent inhibition, and could not be attributed to the middle ear or cochlear mechanics.SIGNIFICANCE STATEMENT Otitis media is the second most common reason children go to the doctor. Previously, degraded auditory experience because of otitis media has been associated with reorganized ascending neural pathways, even after middle ear pathology resolved. Here, we show that altered afferent auditory input because of otitis media during childhood is also associated with long-lasting reduced descending neural pathway function and poorer speech-in-noise recognition. These novel, efferent findings may be important for the detection and treatment of childhood otitis media.

Developmental and activity-dependent modulation of coupling distance between release site and Ca2+ channel

Synapses are junctions between a presynaptic neuron and a postsynaptic cell specialized for fast and precise information transfer. The presynaptic terminal secretes neurotransmitters via exocytosis of synaptic vesicles. Exocytosis is a tightly regulated reaction that occurs within a millisecond of the arrival of an action potential. One crucial parameter in determining the characteristics of the transmitter release kinetics is the coupling distance between the release site and the Ca2+ channel. Still, the technical limitations have hindered detailed analysis from addressing how the coupling distance is regulated depending on the development or activity of the synapse. However, recent technical advances in electrophysiology and imaging are unveiling their different configurations in different conditions. Here, I will summarize developmental- and activity-dependent changes in the coupling distances revealed by recent studies.

Induction of Activity-Dependent Plasticity at Auditory Nerve Synapses

Exposure to nontraumatic noise in vivo drives long-lasting changes in auditory nerve synapses, which may influence hearing, but the induction mechanisms are not known. We mimicked activity in acute slices of the cochlear nucleus from mice of both sexes by treating them with high potassium, after which voltage-clamp recordings from bushy cells indicated that auditory nerve synapses had reduced EPSC amplitude, quantal size, and vesicle release probability (P r). The effects of high potassium were prevented by blockers of nitric oxide (NO) synthase and protein kinase A. Treatment with the NO donor, PAPA-NONOate, also decreased P r, suggesting NO plays a central role in inducing synaptic changes. To identify the source of NO, we activated auditory nerve fibers specifically using optogenetics. Strobing for 2 h led to decreased EPSC amplitude and P r, which was prevented by antagonists against ionotropic glutamate receptors and NO synthase. This suggests that the activation of AMPA and NMDA receptors in postsynaptic targets of auditory nerve fibers drives release of NO, which acts retrogradely to cause long-term changes in synaptic function in auditory nerve synapses. This may provide insight into preventing or treating disorders caused by noise exposure.SIGNIFICANCE STATEMENT Auditory nerve fibers undergo long-lasting changes in synaptic properties in response to noise exposure in vivo, which may contribute to changes in hearing. Here, we investigated the cellular mechanisms underlying induction of synaptic changes using high potassium and optogenetic stimulation in vitro and identified important signaling pathways using pharmacology. Our results suggest that auditory nerve activity drives postsynaptic depolarization through AMPA and NMDA receptors, leading to the release of nitric oxide, which acts retrogradely to regulate presynaptic neurotransmitter release. These experiments revealed that auditory nerve synapses are unexpectedly sensitive to activity and can show dramatic, long-lasting changes in a few hours that could affect hearing.

Auditory processing remains sensitive to environmental experience during adolescence in a rodent model

Elevated neural plasticity during development contributes to dramatic improvements in perceptual, motor, and cognitive skills. However, malleable neural circuits are vulnerable to environmental influences that may disrupt behavioral maturation. While these risks are well-established prior to sexual maturity (i.e., critical periods), the degree of neural vulnerability during adolescence remains uncertain. Here, we induce transient hearing loss (HL) spanning adolescence in gerbils, and ask whether behavioral and neural maturation are disrupted. We find that adolescent HL causes a significant perceptual deficit that can be attributed to degraded auditory cortex processing, as assessed with wireless single neuron recordings and within-session population-level analyses. Finally, auditory cortex brain slices from adolescent HL animals reveal synaptic deficits that are distinct from those typically observed after critical period deprivation. Taken together, these results show that diminished adolescent sensory experience can cause long-lasting behavioral deficits that originate, in part, from a dysfunctional cortical circuit.

Homeostatic regulation through strengthening of neuronal network-correlated synaptic inputs

Homeostatic regulation is essential for stable neuronal function. Several synaptic mechanisms of homeostatic plasticity have been described, but the functional properties of synapses involved in homeostasis are unknown. We used longitudinal two-photon functional imaging of dendritic spine calcium signals in visual and retrosplenial cortices of awake adult mice to quantify the sensory deprivation-induced changes in the responses of functionally identified spines. We found that spines whose activity selectively correlated with intrinsic network activity underwent tumor necrosis factor alpha (TNF-α)-dependent homeostatic increases in their response amplitudes, but spines identified as responsive to sensory stimulation did not. We observed an increase in the global sensory-evoked responses following sensory deprivation, despite the fact that the identified sensory inputs did not strengthen. Instead, global sensory-evoked responses correlated with the strength of network-correlated inputs. Our results suggest that homeostatic regulation of global responses is mediated through changes to intrinsic network-correlated inputs rather than changes to identified sensory inputs thought to drive sensory processing.

Noise Exposure Potentiates Exocytosis From Cochlear Inner Hair Cells

Noise-induced hearing loss has gained relevance as one of the most common forms of hearing impairment. The anatomical correlates of hearing loss, principally cell damage and/or death, are relatively well-understood histologically. However, much less is known about the physiological aspects of damaged, surviving cells. Here we addressed the functional consequences of noise exposure on the capacity of inner hair cells (IHCs) to release synaptic vesicles at synapses with spiral ganglion neurons (SGNs). Mice of either sex at postnatal day (P) 15–16 were exposed to 1–12 kHz noise at 120 dB sound pressure level (SPL), for 1 h. Exocytosis was measured by tracking changes in membrane capacitance (ΔCm) from IHCs of the apical cochlea. Upon IHC depolarization to different membrane potentials, ΔC_m showed the typical bell-shaped curve that mirrors the voltage dependence of Ca²⁺ influx, in both exposed and unexposed cells. Surprisingly, from IHCs at 1-day after exposure (d.a.e.), we found potentiation of exocytosis at the peak of the bell-shaped curve. The increase in exocytosis was not accompanied by changes in whole-cell Ca²⁺ influx, suggesting a modification in coupling between Ca²⁺ channels and synaptic vesicles. Consistent with this notion, noise exposure also changed the Ca²⁺-dependence of exocytosis from linear to supralinear. Noise exposure did not cause loss of IHCs, but did result in a small reduction in the number of IHC-SGN synapses at 1-d.a.e. which recovered by 14-d.a.e. In contrast, a strong reduction in auditory brainstem response wave-I amplitude (representing synchronous firing of SGNs) and distortion product otoacoustic emissions (reflecting outer hair cell function) indicated a profound hearing loss at 1- and 14-d.a.e. To determine the role of glutamate release in the noise-induced potentiation of exocytosis, we evaluated vesicular glutamate transporter-3 (Vglut3) knock-out (KO) mice. Unlike WT, IHCs from Vglut3^KO mice showed a noise-induced reduction in ΔC_m and Ca²⁺ influx with no change in the Ca²⁺-dependence of exocytosis. Together, these results indicate that traumatic noise exposure triggers changes of IHC synaptic function including a Vglut3-dependent potentiation of exocytosis.

Chronic Conductive Hearing Loss Is Associated With Speech Intelligibility Deficits in Patients With Normal Bone Conduction Thresholds

OBJECTIVES

The main objective of this study is to determine whether chronic sound deprivation leads to poorer speech discrimination in humans.

DESIGN

We reviewed the audiologic profile of 240 patients presenting normal and symmetrical bone conduction thresholds bilaterally, associated with either an acute or chronic unilateral conductive hearing loss of different etiologies.

RESULTS

Patients with chronic conductive impairment and a moderate, to moderately severe, hearing loss had lower speech recognition scores on the side of the pathology when compared with the healthy side. The degree of impairment was significantly correlated with the speech recognition performance, particularly in patients with a congenital malformation. Speech recognition scores were not significantly altered when the conductive impairment was acute or mild.

CONCLUSIONS

This retrospective study shows that chronic conductive hearing loss was associated with speech intelligibility deficits in patients with normal bone conduction thresholds. These results are as predicted by a recent animal study showing that prolonged, adult-onset conductive hearing loss causes cochlear synaptopathy.

Association of Conductive Hearing Loss with the Structural Changes in the Organ of Corti

OBJECTIVE

The aim of the study was to evaluate the association of conductive hearing loss (CHL) with the structural changes in the organ of Corti.

METHODS

Twenty ears of 10 healthy adult Wistar albino rats were included in the study. The right ears (n = 10) of the animals served as controls (group 1), and no surgical intervention was performed in these ears. A tympanic membrane perforation without annulus removal was performed under operative microscope on the left ears (n = 5) in 5 of 10 animals (group 2). A tympanic membrane perforation with annulus removal was performed under operative microscope on the left ears (n = 5) of the remaining 5 animals (group 3). Auditory brainstem response testing was performed in the animals before the interventions. After 3 months, the animals were sacrificed, their temporal bones were removed, and inner ears were investigated using scanning electron microscopy (SEM). The organ of Corti was evaluated from the cochlear base to apex in the modiolar axis, and the parameters were scored semiquantitatively.

RESULTS

In group 1, the pre- and post-intervention hearing thresholds were similar (p > 0.05). In group 2, a hearing decrease of at least 5 dB was encountered in all test frequencies (p > 0.05). In group 3, at the frequency range of 2-32 kHz, there was a significant hearing loss after 3 months (p < 0.01). After 3 months, the hearing thresholds in group 2 and 3 were higher than group 1 (p < 0.01). The hearing threshold in group 3 was higher than group 2 (p < 0.01). On SEM evaluation, the general cell morphology and stereocilia of the outer hair cells were preserved in all segments of the cochlea in group 1 with a mean SEM score of 0.2. There was segmental degeneration in the general cell morphology and outer hair cells in group 2 with a mean SEM score of 2.2. There was widespread degeneration in the general cell morphology and outer hair cells in group 3 with a mean SEM score of 3.2. The SEM scores of group 2 and 3 were significantly higher than group 1 (p < 0.05). The SEM scores of group 3 were significantly higher than group 2 (p < 0.05).

CONCLUSION

CHL may be associated with an inner ear damage. The severity of damage appears to be associated with severity and duration of CHL. Early correction of CHL is advocated in order to reverse or prevent progression of the inner ear damage, which will enhance the success rates of hearing restoration surgeries. Subjective differences and compliance of the hearing aid users may be due to the impact of CHL on inner ear structures.

Ultrastructural maturation of the endbulb of Held active zones comparing wild-type and otoferlin-deficient mice

Endbulbs of Held are located in the anteroventral cochlear nucleus and present the first central synapses of the auditory pathway. During development, endbulbs mature functionally to enable rapid and powerful synaptic transmission with high temporal precision. This process is accompanied by morphological changes of endbulb terminals. Loss of the hair cell-specific protein otoferlin (Otof) abolishes neurotransmission in the cochlea and results in the smaller endbulb of Held terminals. Thus, peripheral hearing impairment likely also leads to alterations in the morphological synaptic vesicle (SV) pool size at individual endbulb of Held active zones (AZs). Here, we investigated endbulb AZs in pre-hearing, young, and adult wild-type and Otof^−/− mice. During maturation, SV numbers at endbulb AZs increased in wild-type mice but were found to be reduced in Otof^−/− mice. The SV population at a distance of 0–15 nm was most strongly affected. Finally, overall SV diameters decreased in Otof^−/− animals during maturation.

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Maturation of wt endbulb of Held active zones leads to more synaptic vesicles

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At endbulbs of otoferlin knockout mice, synaptic vesicles decline with age

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Mainly two distinct synaptic vesicle populations are affected

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Synaptic vesicles sizes are reduced in six-month-old otoferlin knockout animals

Biased auditory nerve central synaptopathy is associated with age-related hearing loss

KEY POINTS

Sound information is transmitted by different subtypes of spiral ganglion neurons (SGN) from the ear to the brain. Selective damage of SGN peripheral synapses (cochlear synaptopathy) is widely recognized as one of the primary mechanisms of hearing loss, whereas the mechanisms at the SGN central synapses remain unclear. We report that different subtypes of SGN central synapses converge at different ratios onto individual target cochlear nucleus neurons with distinct physiological properties, and show biased morphological and physiological changes during age-related hearing loss (ARHL). The results reveal a new dimension in cochlear nucleus neural circuitry that systematically reassembles and processes auditory information from different SGN subtypes, which is altered during ageing and probably contributes to the development of ARHL. In addition to known cochlear synaptopathy, the present study shows that SGN central synapses are also pathologically changed during ageing, which collectively helps us better understand the structure and function of SGNs during ARHL.

ABSTRACT

Sound information is transmitted from the cochlea to the brain by different subtypes of spiral ganglion neurons (SGN), which show varying degrees of vulnerability under pathological conditions. Selective cochlear synaptopathy, the preferential damage of certain subtypes of SGN peripheral synapses, has been recognized as one of the main mechanisms of hearing loss. The organization and function of the auditory nerve (AN) central synapses from different subtypes of SGNs remain unclear, including how different AN synapses reassemble onto individual neurons in the cochlear nucleus, as well as how they differentially change during hearing loss. Combining immunohistochemistry with electrophysiology, we investigated the convergence pattern and subtype-specific synaptopathy of AN synapses at the endbulb of Held, as well as the response properties of their postsynaptic bushy neurons in CBA/CaJ mice of either sex under normal hearing and age-related hearing loss (ARHL). We found that calretinin-expressing (type I_a ) and non-calretinin-expressing (type I_b /I_c ) endbulbs converged along a continuum of different ratios onto individual bushy neurons with varying physiological properties. Endbulbs degenerated during ageing in parallel with ARHL. Furthermore, the degeneration was more severe in non-calretinin-expressing synapses, which correlated with a gradual decrease in bushy neuron subpopulation predominantly innervated by these inputs. These synaptic and cellular changes were profound in middle-aged mice when their hearing thresholds were still relatively normal and prior to severe ARHL. Our findings suggest that biased AN central synaptopathy and the correlated shift in cochlear nucleus neuronal composition play significant roles in weakened auditory input and altered central auditory processing during ARHL.

Auditory brainstem development and plasticity

During development and adulthood, the normal activity of the auditory nerve plays a critical role in the maintenance of both fundamental structural, molecular, and functional parameters of auditory nerve synapses, and the postsynaptic excitatory or inhibitory neurons within the cochlear nucleus (CN). In addition, normal activity within the synaptic circuits of the CN is key to developing and maintaining appropriate synapse connectivity as well as the initiation of binaural sound processing in the superior olivary complex (SOC). Development plays a critical role in the proper neuronal connectivity and establishes a topographic map along the entire auditory pathway. Furthermore, evidence shows that neurons and synaptic circuits in the auditory brainstem are not hard-wired, but instead are plastic in response to hearing deficits. Whether this plasticity in response to hearing loss is compensatory or pathological is still unknown.

Mechanisms and Functional Consequences of Presynaptic Homeostatic Plasticity at Auditory Nerve Synapses

Multiple forms of homeostasis influence synaptic function under diverse activity conditions. Both presynaptic and postsynaptic forms of homeostasis are important, but their relative impact on fidelity is unknown. To address this issue, we studied auditory nerve synapses onto bushy cells in the cochlear nucleus of mice of both sexes. These synapses undergo bidirectional presynaptic and postsynaptic homeostatic changes with increased and decreased acoustic stimulation. We found that both young and mature synapses exhibit similar activity-dependent changes in short-term depression. Experiments using chelators and imaging both indicated that presynaptic Ca²⁺ influx decreased after noise exposure, and increased after ligating the ear canal. By contrast, Ca²⁺ cooperativity was unaffected. Experiments using specific antagonists suggest that occlusion leads to changes in the Ca²⁺ channel subtypes driving neurotransmitter release. Furthermore, dynamic-clamp experiments revealed that spike fidelity primarily depended on changes in presynaptic depression, with some contribution from changes in postsynaptic intrinsic properties. These experiments indicate that presynaptic Ca²⁺ influx is homeostatically regulated in vivo to enhance synaptic fidelity.SIGNIFICANCE STATEMENT Homeostatic mechanisms in synapses maintain stable function in the face of different levels of activity. Both juvenile and mature auditory nerve synapses onto bushy cells modify short-term depression in different acoustic environments, which raises the question of what the underlying presynaptic mechanisms are and the relative importance of presynaptic and postsynaptic contributions to the faithful transfer of information. Changes in short-term depression under different acoustic conditions were a result of changes in presynaptic Ca²⁺ influx. Spike fidelity was affected by both presynaptic and postsynaptic changes after ear occlusion and was only affected by presynaptic changes after noise-rearing. These findings are important for understanding regulation of auditory synapses under normal conditions and also in disorders following noise exposure or conductive hearing loss.

Sound-localisation performance in patients with congenital unilateral microtia and atresia fitted with an active middle ear implant

OBJECTIVE

This study assessed the safety and sound-localisation ability of the Vibrant Soundbridge (VSB) (Med-EL, Innsbruck, Austria) in patients with unilateral microtia and atresia (MA).

METHODS

This was a single-centre retrospective research study. Twelve subjects with unilateral conductive hearing loss (UCHL) caused by ipsilateral MA were recruited, each of whom underwent VSB implantation and auricular reconstruction. The bone-conduction (BC) threshold was measured postoperatively, and the accuracy of sound localisation was evaluated at least 6 months after surgery. Horizontal sound-localisation performance was investigated with the VSB activated and inactivated, at varying sound stimuli levels (65, 70 and 75 dB SPL). Localisation benefit was analysed via the mean absolute error (MAE).

RESULTS

There was no statistical difference in mean BC threshold of impaired ears measured preoperatively and postoperatively. When compared with VSB-inactivated condition, the MAE increased significantly in unilateral MA patients in the VSB-activated condition. Besides, sound-localisation performance worsened remarkably when sound was presented at 70 dB SPL and 75 dB SPL. Regarding the side of signal location, the average MAE with the VSB device was much higher than that without the VSB when sound was from the normal-hearing ear. However, no significant difference was observed when sound was located from the impaired ear.

CONCLUSION

This study demonstrates that in patients with unilateral MA, the VSB device does not affect inner-ear function. Sound-localisation ability is not improved, but deteriorated at follow-up. Our results suggest that the VSB-aided localisation abilities may be related to the thresholds between the ears, plasticity of auditory system and duration of use of VSB.

Early-Stage Ocular Hypertension Alters Retinal Ganglion Cell Synaptic Transmission in the Visual Thalamus

Axonopathy is a hallmark of many neurodegenerative diseases including glaucoma, where elevated intraocular pressure (ocular hypertension, OHT) stresses retinal ganglion cell (RGC) axons as they exit the eye and form the optic nerve. OHT causes early changes in the optic nerve such as axon atrophy, transport inhibition, and gliosis. Importantly, many of these changes appear to occur prior to irreversible neuronal loss, making them promising points for early diagnosis of glaucoma. It is unknown whether OHT has similarly early effects on the function of RGC output to the brain. To test this possibility, we elevated eye pressure in mice by anterior chamber injection of polystyrene microbeads. Five weeks post-injection, bead-injected eyes showed a modest RGC loss in the peripheral retina, as evidenced by RBPMS antibody staining. Additionally, we observed reduced dendritic complexity and lower spontaneous spike rate of On-αRGCs, targeted for patch clamp recording and dye filling using a Opn4-Cre reporter mouse line. To determine the influence of OHT on retinal projections to the brain, we expressed Channelrhodopsin-2 (ChR2) in melanopsin-expressing RGCs by crossing the Opn4-Cre mouse line with a ChR2-reporter mouse line and recorded post-synaptic responses in thalamocortical relay neurons in the dorsal lateral geniculate nucleus (dLGN) of the thalamus evoked by stimulation with 460 nm light. The use of a Opn4-Cre reporter system allowed for expression of ChR2 in a narrow subset of RGCs responsible for image-forming vision in mice. Five weeks following OHT induction, paired pulse and high-frequency stimulus train experiments revealed that presynaptic vesicle release probability at retinogeniculate synapses was elevated. Additionally, miniature synaptic current frequency was slightly reduced in brain slices from OHT mice and proximal dendrites of post-synaptic dLGN relay neurons, assessed using a Sholl analysis, showed a reduced complexity. Strikingly, these changes occurred prior to major loss of RGCs labeled with the Opn4-Cre mouse, as indicated by immunofluorescence staining of ChR2-expressing retinal neurons. Thus, OHT leads to pre- and post-synaptic functional and structural changes at retinogeniculate synapses. Along with RGC dendritic remodeling and optic nerve transport changes, these retinogeniculate synaptic changes are among the earliest signs of glaucoma.

Effects of Non-traumatic Noise and Conductive Hearing Loss on Auditory System Function

The effects of traumatic noise-exposure and deafening on auditory system function have received a great deal of attention. However, lower levels of noise as well as temporary conductive hearing loss also have consequences on auditory physiology and hearing. Here we review how abnormal acoustic experience at early ages affects the ascending and descending auditory pathways, as well as hearing behavior.

Auditory central gain compensates for changes in cochlear output after prolonged low-level noise exposure

Remarkably, the central auditory system can modify the strength of its sound-evoked neural response based on prior acoustic experiences, a phenomenon referred to as central gain. Gain changes are well documented following traumatic noise exposure, but much less is known about central gain dynamics following prolonged exposure to low-level noise, a common acoustic experience in many urban and work environments. We recently reported that the neural output of the cochlea is reduced, while gain was enhanced in the inferior colliculus (IC) following a 5-week exposure to 75 dB noise. To determine if similar effects were present at even lower intensities, we exposed rats to a 65 dB noise expecting to see little to no change in the cochlea or IC. The exposure had little effect on distortion product otoacoustic emissions and did not cause any hair cell loss. However, the amplitude of the CAP, which reflects the neural output of cochlea, was depressed by 50-75%. Surprisingly, neural responses from the IC were enhanced up to 70%, mainly at frequencies within the noise exposure band. One-week post-exposure, CAP amplitudes returned to normal at frequencies within or above the exposure band, whereas responses evoked by frequencies below the exposure band were enhanced by more than 80%. In contrast, IC responses below the exposure frequency were depressed 10-20% whereas responses within the exposure frequency band were enhanced 10-20%. Thus, the central auditory system dynamically up- and down-regulates its gain to maintain supra-threshold neural responses within a narrow homeostatic range; a function that likely contributes to the prevention of sounds from being perceived as muffled or too loud.

Homeostatic plasticity and synaptic scaling in the adult mouse auditory cortex

It has been demonstrated that sensory deprivation results in homeostatic adjustments recovering neuronal activity of the deprived cortex. For example, deprived vision multiplicatively scales up mEPSC amplitudes in the primary visual cortex, commonly referred to as synaptic scaling. However, whether synaptic scaling also occurs in auditory cortex after auditory deprivation remains elusive. Using periodic intrinsic optical imaging in adult mice, we show that conductive hearing loss (CHL), initially led to a reduction of primary auditory cortex (A1) responsiveness to sounds. However, this was followed by a complete recovery of A1 activity evoked sounds above the threshold for bone conduction, 3 days after CHL. Over the same time course patch-clamp experiments in slices revealed that mEPSC amplitudes in A1 layers 2/3 pyramids scaled up multiplicatively in CHL mice. No recovery of sensory evoked A1 activation was evident in TNFα KO animals, which lack synaptic scaling. Additionally, we could show that the suppressive effect of sounds on visually evoked visual cortex activity completely recovered along with TNFα dependent A1 homeostasis in WT animals. This is the first demonstration of homeostatic multiplicative synaptic scaling in the adult A1. These findings suggest that mild hearing loss massively affects auditory processing in adult A1.

Changes in tonal audiometry in children with progressive sensorineural hearing loss and history of Neonatal Intensive Care Unit discharge. A 20 year long-term follow-up

OBJECTIVE

Newborns from Neonatal intensive care units (NICU) are at high-risk for sensorineural hearing loss (SNHL) a follow-up is needed for early diagnosis and intervention. Our objective here was to describe the features and changes of SNHL at different periods during a follow-up of almost 20 years.

METHODS

Risk factors for SNHL during development were analyzed. The audiological examination included: Brainstem auditory evoked potentials (BAEP), and Transient evoked otoacoustic emissions (TEOAE). At birth; tonal audiometry (between 125 and 8000 Hz), and tympanometry were performed at 5, 10, 15, and 20 years of age.

RESULTS

Sixty-five percent of cases presented bilateral absence of BAEP. At 5 years of age, the most frequent SNHL level was severe (42.5%), followed by moderate (22.5%), and profound level (20%), in all cases, the SNHL was symmetrical with a predominance of lesion for the high frequencies. Exchange transfusion was associated with a higher degree of SNHL (OR = 6.00, CI = 1.11-32.28, p < 0.02). In 55%, SNHL remained stable, but in 40% of the cases it was progressive. At the end of the study six cases with moderate loss progressed to the severe level and seven cases with severe level progressed to profound.

CONCLUSIONS

Forty percent of infants with SNHL discharged from NICU may present a progression in the hearing loss. Exchange transfusion was associated with a higher degree of SNHL. NICU graduates with SNHL merit a long-term audiological follow-up throughout their lifespan.