Effects of chronic high-rate electrical stimulation on the cochlea and eighth nerve in the deafened guinea pig

Emerging biotechnologies and biomedical engineering technologies for hearing reconstruction

Hearing impairment is a global health problem that affects social communications and the economy. The damage and loss of cochlear hair cells and spiral ganglion neurons (SGNs) as well as the degeneration of neurites of SGNs are the core causes of sensorineural hearing loss. Biotechnologies and biomedical engineering technologies provide new hope for the treatment of auditory diseases, which utilizes biological strategies or tissue engineering methods to achieve drug delivery and the regeneration of cells, tissues, and even organs. Here, the advancements in the applications of biotechnologies (including gene therapy and cochlear organoids) and biomedical engineering technologies (including drug delivery, electrode coating, electrical stimulation and bionic scaffolds) in the field of hearing reconstruction are presented. Moreover, we summarize the challenges and provide a perspective on this field.

Short-term overstimulation affects peripheral but not central excitability in an animal model of cochlear implantation

Objective: A smallbut persistent proportion of individuals do not gain the expected benefit from cochlear implants(CI). A step-change in the understanding of factors affecting outcomes could come through data science. This study evaluates clinical data capture to assess the quality and utility of Cl user's health records for data science, by assessing the recording of otitis media. Otitis media was selected as it is associated with the development of sensorineural hearing loss and may affect cochlear implant outcomes.Methods: A retrospective service improvement project ·evaluating the medical records of 594 people with a Cl under the care of the University of Southampton Auditory Implant Service between 2014 and 2020.Results: The clinicalrecords are suitable for data science research. Of the cohort studied 20% of Adults and more than 40% of the paediatric cases have a history of middle ear inflammation.Discussion: Data science has potentialto improve cochlear implant outcomes and improve understanding of the mechanisms underlying poor performance, through retrospective secondary analysis of real-world data.Conclusion: Implant centres and the British Cochlear Implant Group National Hearing Implant Registry are urged to consider the importance of consistently and accurate recording of patient data over time for each Cl user. Data where links to hearing loss have been identified, such as middle ear inflammation, may be particularly valuable in future analyses and to inform clinical trials.

Closing the Gap between the Auditory Nerve and Cochlear Implant Electrodes: Which Neurotrophin Cocktail Performs Best for Axonal Outgrowth and Is Electrical Stimulation Beneficial?

Neurotrophins promote neurite outgrowth of auditory neurons and may help closing the gap to cochlear implant (CI) electrodes to enhance electrical hearing. The best concentrations and mix of neurotrophins for this nerve regrowth are unknown. Whether electrical stimulation (ES) during outgrowth is beneficial or may direct axons is another open question. Auditory neuron explant cultures of distinct cochlear turns of 6-7 days old mice were cultured for four days. We tested different concentrations and combinations of BDNF and NT-3 and quantified the numbers and lengths of neurites with an advanced automated analysis. A custom-made 24-well electrical stimulator based on two bulk CIs served to test different ES strategies. Quantification of receptors trkB, trkC, p75^NTR, and histological analysis helped to analyze effects. We found 25 ng/mL BDNF to perform best, especially in basal neurons, a negative influence of NT-3 in combined BDNF/NT-3 scenarios, and tonotopic changes in trk and p75^NTR receptor stainings. ES largely impeded neurite outgrowth and glia ensheathment in an amplitude-dependent way. Apical neurons showed slight benefits in neurite numbers and length with ES at 10 and 500 µA. We recommend BDNF as a potent drug to enhance the man-machine interface, but CIs should be better activated after nerve regrowth.

Cochlear implants: Causes, effects and mitigation strategies for the foreign body response and inflammation

Cochlear implants provide effective auditory rehabilitation for patients with severe to profound sensorineural hearing loss. Recent advances in cochlear implant technology and surgical approaches have enabled a greater number of patients to benefit from this technology, including those with significant residual low frequency acoustic hearing. Nearly all cochleae implanted with a cochlear implant electrode array develop an inflammatory and fibrotic response. This tissue reaction can have deleterious consequences for implant function, residual acoustic hearing, and the development of the next generation of cochlear prosthetics. This article reviews the current understanding of the inflammatory/foreign body response (FBR) after cochlear implant surgery, its impact on clinical outcome, and therapeutic strategies to mitigate this response. Findings from both in human subjects and animal models across a variety of species are highlighted. Electrode array design, surgical techniques, implant materials, and the degree and type of electrical stimulation are some critical factors that affect the FBR and inflammation. Modification of these factors and various anti-inflammatory pharmacological interventions have been shown to mitigate the inflammatory/FBR response. Ongoing and future approaches that seek to limit surgical trauma and curb the FBR to the implanted biomaterials of the electrode array are discussed. A better understanding of the anatomical, cellular and molecular basis of the inflammatory/FBR response after cochlear implantation has the potential to improve the outcome of current cochlear implants and also facilitate the development of the next generation of neural prostheses.

Consecutive Treatment with Brain-Derived Neurotrophic Factor and Electrical Stimulation Has a Protective Effect on Primary Auditory Neurons

Degeneration of neurons, such as the inner ear spiral ganglion neurons (SGN), may be decelerated or even stopped by neurotrophic factor treatment, such as brain-derived neurotrophic factor (BDNF), as well as electrical stimulation (ES). In a clinical setting, drug treatment of the SGN could start directly during implantation of a cochlear implant, whereas electrical stimulation begins days to weeks later. The present study was conducted to determine the effects of consecutive BDNF and ES treatments on SGN density and electrical responsiveness. An electrode drug delivery device was implanted in guinea pigs 3 weeks after deafening and five experimental groups were established: two groups received intracochlear infusion of artificial perilymph (AP) or BDNF; two groups were treated with AP respectively BDNF in addition to ES (AP + ES, BDNF + ES); and one group received BDNF from the day of implantation until day 34 followed by ES (BDNF ⇨ ES). Electrically evoked auditory brainstem responses were recorded. After one month of treatment, the tissue was harvested and the SGN density was assessed. The results show that consecutive treatment with BDNF and ES was as successful as the simultaneous combined treatment in terms of enhanced SGN density compared to the untreated contralateral side but not in regard to the numbers of protected cells.

Stem Cell Based Drug Delivery for Protection of Auditory Neurons in a Guinea Pig Model of Cochlear Implantation

Background: The success of a cochlear implant (CI), which is the standard therapy for patients suffering from severe to profound sensorineural hearing loss, depends on the number and excitability of spiral ganglion neurons (SGNs). Brain-derived neurotrophic factor (BDNF) has a protective effect on SGNs but should be applied chronically to guarantee their lifelong survival. Long-term administration of BDNF could be achieved using genetically modified mesenchymal stem cells (MSCs), but these cells should be protected – by ultra-high viscous (UHV-) alginate (‘alginate-MSCs’) – from the recipient immune system and from uncontrolled migration.

Methods: Brain-derived neurotrophic factor-producing MSCs were encapsulated in UHV-alginate. Four experimental groups were investigated using guinea pigs as an animal model. Three of them were systemically deafened and (unilaterally) received one of the following: (I) a CI; (II) an alginate-MSC-coated CI; (III) an injection of alginate-embedded MSCs into the scala tympani followed by CI insertion and alginate polymerization. Group IV was normal hearing, with CI insertion in both ears and a unilateral injection of alginate-MSCs. Using acoustically evoked auditory brainstem response measurements, hearing thresholds were determined before implantation and before sacrificing the animals. Electrode impedance was measured weekly. Four weeks after implantation, the animals were sacrificed and the SGN density and degree of fibrosis were evaluated.

Results: The MSCs survived being implanted for 4 weeks in vivo. Neither the alginate-MSC injection nor the coating affected electrode impedance or fibrosis. CI insertion with and without previous alginate injection in normal-hearing animals resulted in increased hearing thresholds within the high-frequency range. Low-frequency hearing loss was additionally observed in the alginate-injected and implanted cochleae, but not in those treated only with a CI. In deafened animals, the alginate-MSC coating of the CI significantly prevented SGN from degeneration, but the injection of alginate-MSCs did not.

Conclusion: Brain-derived neurotrophic factor-producing MSCs encapsulated in UHV-alginate prevent SGNs from degeneration in the form of coating on the CI surface, but not in the form of an injection. No increase in fibrosis or impedance was detected. Further research and development aimed at verifying long-term mechanical and biological properties of coated electrodes in vitro and in vivo, in combination with chronic electrical stimulation, is needed before the current concept can be tested in clinical trials.

[Reactions in the organ of Corti to electrical stimulation : StED technology for detecting changes]

Increasing numbers of cochlear implant patients have residual hearing. Despite surgical and pharmacological efforts to preserve residual hearing, a significant number of these patients suffer a late, unexplained loss of residual hearing. Surgical trauma can be excluded as the cause. To investigate this phenomenon and because cells in their native environment react differently to stimuli (such as electrical current) than isolated cells, whole-organ explants from cochleae may be a better model. For early detection of synaptic changes in the organ of Corti, a high-resolution microscopic technique such as stimulated emission depletion (StED) can be used. The aim of this study was establishment of a qualitative and quantitative technique to determinate changes in the organ of Corti and its synapses after electrical stimulation. Explanted organs of Corti from postnatal rats (P2-4) were cultured on a coverslip for 24 h and subsequently exposed to biphasic pulsed electrical stimulation (amplitude 0.44-2.0 mA, pulse width 400 μs, interpulse delay 120 μs, repetition 1 kHz) for another 24 h. For visualization, the cytoskeleton and the ribbon synapses were stained immunocytochemically. For an early detectable response to electrical stimulation, the number of synapses was quantified. Organs of Corti without electrical stimulation served as a reference. Initial research has shown that electrical stimulation can cause changes in ribbon synapses and that StED can detect these alterations. The herein established model could be of great importance for identification of molecular changes in the organ of Corti in response to electrical or other stimuli.

Influence of In Vitro Electrical Stimulation on Survival of Spiral Ganglion Neurons

Patients scheduled for cochlear implantation often retain residual hearing in the low frequencies. Unfortunately, some patients lose their residual hearing following implantation and the reasons for this are not well understood. Evidence suggests that electrotoxicity could be one of the factors responsible for this late adverse effect. Therefore, the aim of this study was to investigate the survival of spiral ganglion neurons (SGN) subjected to in vitro electrical stimulation (ES). A stimulation setup was developed to provide defined electrical fields at given points of the chamber. SGN isolated from Sprague Dawley rats (P3-4) were dissociated and cultured in the chamber for 24 h prior to biphasic, pulsed electrical field exposure for another 24 h. The current varied in the range of 0 to 2 mA and the pulse width from 10 to 400 μs. Neurite growth and survival were evaluated with respect to the charge density at the position of the cells. Non-exposed SGN cultures served as control. Charge densities below 2.2 μC·cm^-2·phase^-1 appeared to have no effect on SGN survival and neurite outgrowth. Charge densities above 4.9 μC·cm^-2·phase^-1 were detrimental to almost all cells in culture. After fitting results to a sigmoidal dose response curve, a LD₅₀ of 2.9 μC·cm^-2·phase^-1 was calculated. This screening regarding survival and outgrowth of SGN provides parameters that could be used to further investigate the effect of ES on SGN and to develop possible protection strategies, which could potentially rescue residual hearing in the implanted patients.

Semaphorin 3A induces acute changes in membrane excitability in spiral ganglion neurons in vitro

The development and survival of spiral ganglion neurons (SGNs) are dependent on multiple trophic factors as well as membrane electrical activity. Semaphorins (Sema) constitute a family of membrane-associated and secreted proteins that have garnered significant attention as a potential SGN "navigator" during cochlea development. Previous studies using mutant mice demonstrated that Sema3A plays a role in the SGN pathfinding. The mechanisms, however, by which Sema3A shapes SGNs firing behavior are not known. In these studies, we found that Sema3A plays a novel role in regulating SGN resting membrane potential and excitability. Using dissociated SGN from pre-hearing (P3-P5) and post-hearing mice (P12-P15), we recorded membrane potentials using whole-cell patch clamp recording techniques in apical and basal SGN populations. Recombinant Sema3A was applied to examine the effects on intrinsic membrane properties and action potentials evoked by current injections. Apical and basal SGNs from newborn mice treated with recombinant Sema3A (100 ng/ml) displayed a higher resting membrane potential, higher threshold, decreased amplitude, and prolonged latency and duration of spikes. Although a similar phenomenon was observed in SGNs from post-hearing mice, the resting membrane potential was essentially indistinguishable before and after Sema3A exposure. Sema3A-mediated changes in membrane excitability were associated with a significant decrease in K⁺ and Ca²⁺ currents. Sema3A acts through linopirdine-sensitive K⁺ channels in apical, but not in the basal SGNs. Therefore, Sema3A induces differential effects in SGN membrane excitability that are dependent on age and location, and constitutes an additional early and novel effect of Sema3A SGNs in vitro.

Local inner ear application of dexamethasone in cochlear implant models is safe for auditory neurons and increases the neuroprotective effect of chronic electrical stimulation

Dexamethasone (DEX) can reduce fibrous tissue growth as well as loss of residual hearing which may occur after cochlear implantation. Little is known about the effect of local inner ear DEX treatment on the spiral ganglion neurons (SGN), which are the target of the electrical stimulation with a cochlear implant (CI). Three different clinically relevant strategies of DEX-delivery into the inner ear were used. DEX was either eluted from the electrode carriers' silicone, released from a reservoir by passive diffusion, or actively applied using a pump based system. The effect of the locally applied DEX on SGN density, size and function was evaluated. DEX did not affect the SGN density compared to the relevant control groups. Simultaneously applied with chronic electrical stimulation (ES), DEX increased the neuroprotective effect of ES in the basal region and the hearing threshold tended to decrease. The EABR thresholds did not correlate with the relevant SGN density. When correlating the SGN number with fibrosis, no dependency was observed. DEX concentrations as applied in these animal models are safe for inner ear delivery in terms of their effect on SGN density. Additionally, DEX tends to improve the neuroprotective effect of chronic electrical stimulation by increasing the number of surviving neurons. This is an important finding in regard to clinical applications of DEX for local treatment of the inner ear in view of cochlear implantation and other applications.

Nanofibrous scaffolds for the guidance of stem cell-derived neurons for auditory nerve regeneration

Impairment of spiral ganglion neurons (SGNs) of the auditory nerve is a major cause for hearing loss occurring independently or in addition to sensory hair cell damage. Unfortunately, mammalian SGNs lack the potential for autonomous regeneration. Stem cell based therapy is a promising approach for auditory nerve regeneration, but proper integration of exogenous cells into the auditory circuit remains a fundamental challenge. Here, we present novel nanofibrous scaffolds designed to guide the integration of human stem cell-derived neurons in the internal auditory meatus (IAM), the foramen allowing passage of the spiral ganglion to the auditory brainstem. Human embryonic stem cells (hESC) were differentiated into neural precursor cells (NPCs) and seeded onto aligned nanofiber mats. The NPCs terminally differentiated into glutamatergic neurons with high efficiency, and neurite projections aligned with nanofibers in vitro. Scaffolds were assembled by seeding GFP-labeled NPCs on nanofibers integrated in a polymer sheath. Biocompatibility and functionality of the NPC-seeded scaffolds were evaluated in vivo in deafened guinea pigs (Cavia porcellus). To this end, we established an ouabain-based deafening procedure that depleted an average 72% of SGNs from apex to base of the cochleae and caused profound hearing loss. Further, we developed a surgical procedure to implant seeded scaffolds directly into the guinea pig IAM. No evidence of an inflammatory response was observed, but post-surgery tissue repair appeared to be facilitated by infiltrating Schwann cells. While NPC survival was found to be poor, both subjects implanted with NPC-seeded and cell-free control scaffolds showed partial recovery of electrically-evoked auditory brainstem thresholds. Thus, while future studies must address cell survival, nanofibrous scaffolds pose a promising strategy for auditory nerve regeneration.

Encapsulated cell device approach for combined electrical stimulation and neurotrophic treatment of the deaf cochlea

Profound hearing impairment can be overcome by electrical stimulation (ES) of spiral ganglion neurons (SGNs) via a cochlear implant (CI). Thus, SGN survival is critical for CI efficacy. Application of glial cell line-derived neurotrophic factor (GDNF) has been shown to reduce SGN degeneration following deafness. We tested a novel method for local, continuous GDNF-delivery in combination with ES via a CI. The encapsulated cell (EC) device contained a human ARPE-19 cell-line, genetically engineered for secretion of GDNF. In vitro, GDNF delivery was stable during ES delivered via a CI. In the chronic in vivo part, cats were systemically deafened and unilaterally implanted into the scala tympani with a CI and an EC device, which they wore for six months. The implantation of control devices (same cell-line not producing GDNF) had no negative effect on SGN survival. GDNF application without ES led to an unexpected reduction in SGN survival, however, the combination of GDNF with initial, short-term ES resulted in a significant protection of SGNs. A tight fibrous tissue formation in the scala tympani of the GDNF-only group is thought to be responsible for the increased SGN degeneration, due to mechanisms related to an aggravated foreign body response. Furthermore, the fibrotic encapsulation of the EC device led to cell death or cessation of GDNF release within the EC device during the six months in vivo. In both in vitro and in vivo, fibrosis was reduced by CI stimulation, enabling the neuroprotective effect of the combined treatment. Thus, fibrous tissue growth limits treatment possibilities with an EC device. For a stable and successful long-term neurotrophic treatment of the SGN via EC devices in human CI users, it would be necessary to make changes in the treatment approach (provision of anti-inflammatories), the EC device surface (reduced cell adhesion) and the ES (initiation prior to fibrosis formation).

Fabrication and evaluation of an improved polymer-based cochlear electrode array for atraumatic insertion

An atraumatic cochlear electrode array has become indispensable to high-performance cochlear implants such as electric acoustic stimulation (EAS), wherein the preservation of residual hearing is significant. For an atraumatic implantation, we propose and demonstrate a new improved design of a cochlear electrode array based on liquid crystal polymer (LCP), which can be fabricated by precise batch processes and a thermal lamination process, in contrast to conventional wire-based cochlear electrode arrays. Using a thin-film process of LCP-film-mounted silicon wafer and thermal press lamination, we devise a multi-layered structure with variable layers of LCP films to achieve a sufficient degree of basal rigidity and a flexible tip. A peripheral blind via and self-aligned silicone elastomer molding process can reduce the width of the array. Measuring the insertion and extraction forces in a human scala tympani model, we investigate five human temporal bone insertion trials and record electrically evoked auditory brainstem responses (EABR) acutely in a guinea pig model. The diameters of the finalized electrode arrays are 0.3 mm (tip) and 0.75 mm (base). The insertion force with a displacement of 8 mm from a round window and the maximum extraction force are 2.4 mN and 34.0 mN, respectively. The electrode arrays can be inserted from 360° to 630° without trauma at the basal turn. The EABR data confirm the efficacy of the array. A new design of LCP-based cochlear electrode array for atraumatic implantation is fabricated. Verification indicates that foretells the development of an atraumatic cochlear electrode array and clinical implant.

Neuronal Survival, Morphology and Outgrowth of Spiral Ganglion Neurons Using a Defined Growth Factor Combination

Objectives

The functionality of cochlear implants (CI) depends, among others, on the number and excitability of surviving spiral ganglion neurons (SGN). The spatial separation between the SGN, located in the bony axis of the inner ear, and the CI, which is inserted in the scala tympani, results in suboptimal performance of CI patients and may be decreased by attracting the SGN neurites towards the electrode contacts. Neurotrophic factors (NTFs) can support neuronal survival and neurite outgrowth.

Methods

Since brain-derived neurotrophic factor (BDNF) is well known for its neuroprotective effect and ciliary neurotrophic factor (CNTF) increases neurite outgrowth, we evaluated if the combination of BDNF and CNTF leads to an enhanced neuronal survival with extended neurite outgrowth. Both NTFs were added in effective high concentrations (BDNF 50ng/ml, CNTF 100ng/ml), alone and in combination, to cultured dissociated SGN of neonatal rats for 48 hours.

Results

The neuronal survival and neurite outgrowth were significantly higher in SGN treated with the combination of the two NTFs compared to treatment with each factor alone. Additionally, with respect to the morphology, the combination of BDNF and CNTF leads to a significantly higher number of bipolar neurons and a decreased number of neurons without neurites in culture.

Conclusion

The combination of BDNF and CNTF shows a great potential to increase the neuronal survival and the number of bipolar neurons in vitro and to regenerate retracted nerve fibers.

Neurotrophins and cochlear implants: a solution to sensorineural deafness?

OBJECTIVES

To review current trends for treating sensorineural deafness by enhancing spiral ganglion neuron (SGN) survival using neurotrophins combined with cochlear implants and identify areas for future research and development.

METHODS

A literature search was undertaken on PubMed and Google scholar using terms: neurotrophins, cochlear implants (CIs), and sensorineural to identify the most recent and significant publications. The abstracts were read to identify relevant papers; these were accessed in full and analysed for this review.

RESULTS

Neurotrophins have a known role in cochlear development and the maintenance of SGNs. So far experiments using osmotic pumps to deliver neurotrophins have been successful for short-term enhanced survival of SGN's following aminoglycoside ototoxicity in animal models. They have demonstrated the re-sprouting of radial nerve fibres from SGN's towards the source of delivery. In addition electrical stimulation, gene and cell-based therapy have increased SGN survival to varying degrees.

DISCUSSION

Osmotic pumps carry a high risk of infection therefore CIs coated in a drug containing polymer or hydrogel are a realistic alternative for sustained delivery of neurotrophins. Increased SGN survival combined with neuronal re-growth raises the possibility for CIs to stimulate discrete SGN populations. Unfortunately, the duration of treatment needed for long-term survival still remains unclear and further work is needed. Nevertheless the combination of regenerative medicine to CI technology presents a novel approach to developing CI technology.

Targeted delivery of brain-derived neurotrophic factor for the treatment of blindness and deafness

Neurodegenerative causes of blindness and deafness possess a major challenge in their clinical management as proper treatment guidelines have not yet been found. Brain-derived neurotrophic factor (BDNF) has been established as a promising therapy against neurodegenerative disorders including hearing and visual loss. Unfortunately, the blood–retinal barrier and blood–cochlear barrier, which have a comparable structure to the blood–brain barrier prevent molecules of larger sizes (such as BDNF) from exiting the circulation and reaching the targeted cells. Anatomical features of the eye and ear allow use of local administration, bypassing histo-hematic barriers. This paper focuses on highlighting a variety of strategies proposed for the local administration of the BDNF, like direct delivery, viral gene therapy, and cell-based therapy, which have been shown to successfully improve development, survival, and function of spiral and retinal ganglion cells. The similarities and controversies for BDNF treatment of posterior eye diseases and inner ear diseases have been analyzed and compared. In this review, we also focus on the possibility of translation of this knowledge into clinical practice. And finally, we suggest that using nanoparticulate drug-delivery systems may substantially contribute to the development of clinically viable techniques for BDNF delivery into the cochlea or posterior eye segment, which, ultimately, can lead to a long-term or permanent rescue of auditory and optic neurons from degeneration.

Factors associated with hearing loss in a normal-hearing guinea pig model of Hybrid cochlear implants

The Hybrid cochlear implant (CI), also known as Electro-Acoustic Stimulation (EAS), is a new type of CI that preserves residual acoustic hearing and enables combined cochlear implant and hearing aid use in the same ear. However, 30-55% of patients experience acoustic hearing loss within days to months after activation, suggesting that both surgical trauma and electrical stimulation may cause hearing loss. The goals of this study were to: 1) determine the contributions of both implantation surgery and EAS to hearing loss in a normal-hearing guinea pig model; 2) determine which cochlear structural changes are associated with hearing loss after surgery and EAS. Two groups of animals were implanted (n = 6 per group), with one group receiving chronic acoustic and electric stimulation for 10 weeks, and the other group receiving no direct acoustic or electric stimulation during this time frame. A third group (n = 6) was not implanted, but received chronic acoustic stimulation. Auditory brainstem response thresholds were followed over time at 1, 2, 6, and 16 kHz. At the end of the study, the following cochlear measures were quantified: hair cells, spiral ganglion neuron density, fibrous tissue density, and stria vascularis blood vessel density; the presence or absence of ossification around the electrode entry was also noted. After surgery, implanted animals experienced a range of 0-55 dB of threshold shifts in the vicinity of the electrode at 6 and 16 kHz. The degree of hearing loss was significantly correlated with reduced stria vascularis vessel density and with the presence of ossification, but not with hair cell counts, spiral ganglion neuron density, or fibrosis area. After 10 weeks of stimulation, 67% of implanted, stimulated animals had more than 10 dB of additional threshold shift at 1 kHz, compared to 17% of implanted, non-stimulated animals and 0% of non-implanted animals. This 1-kHz hearing loss was not associated with changes in any of the cochlear measures quantified in this study. The variation in hearing loss after surgery and electrical stimulation in this animal model is consistent with the variation in human patients. Further, these findings illustrate an advantage of a normal-hearing animal model for quantification of hearing loss and damage to cochlear structures without the confounding effects of chemical- or noise-induced hearing loss. Finally, this study is the first to suggest a role of the stria vascularis and damage to the lateral wall in implantation-induced hearing loss. Further work is needed to determine the mechanisms of implantation- and electrical-stimulation-induced hearing loss.

Genetic, cellular, and functional evidence for Ca2+ inflow through Cav1.2 and Cav1.3 channels in murine spiral ganglion neurons

Spiral ganglion neurons (SGNs) of the eighth nerve serve as the bridge between hair cells and the cochlear nucleus. Hair cells use Cav1.3 as the primary channel for Ca(2+) inflow to mediate transmitter release. In contrast, SGNs are equipped with multiple Ca(2+) channels to mediate Ca(2+)-dependent functions. We examined directly the role of Cav1.3 channels in SGNs using Cav1.3-deficient mice (Cav1.3(-/-)). We revealed a surprising finding that SGNs functionally express the cardiac-specific Cav1.2, as well as neuronal Cav1.3 channels. We show that evoked action potentials recorded from SGNs show a significant decrease in the frequency of firing in Cav1.3(-/-) mice compared with wild-type (Cav1.3(+/+)) littermates. Although Cav1.3 is the designated L-type channel in neurons, whole-cell currents recorded in isolated SGNs from Cav1.3(-/-) mice showed a surprising remnant current with sensitivity toward the dihydropyridine (DHP) agonist and antagonist, and a depolarization shift in the voltage-dependent activation compared with that in the Cav1.3(+/+) mice. Indeed, direct measurement of the elementary properties of Ca(2+) channels, in Cav1.3(+/+) neurons, confirmed the existence of two DHP-sensitive single-channel currents, with distinct open probabilities and conductances. We demonstrate that the DHP-sensitive current in Cav1.3(-/-) mice is derived from Cav1.2 channel activity, providing for the first time, to our knowledge, functional data for the expression of Cav1.2 currents in neurons. Finally, using shRNA gene knockdown methodology, and histological analyses of SGNs from Cav1.2(+/-) and Cav1.3(+/-) mice, we were able to establish the differential roles of Cav1.2 and Cav1.3 in SGNs.

Safety considerations for deep brain stimulation: review and analysis

Deep brain stimulation has emerged rapidly as an effective therapy for movement disorders. Deep brain stimulation includes an implanted brain electrode and a pacemaker-like implanted pulse generator. The clinical application of deep brain stimulation proceeded in the absence of clear understandings of its mechanisms of action or extensive preclinical studies of safety and efficacy. Post mortem studies suggest that there is a loss of neurons in proximity to the active electrode, but the resulting lesions are not sufficient to treat the disorder and efficacy requires continued stimulation. Overall complication rates can exceed 25%, and permanent neurologic sequelae result in 4-6% of cases. As the application of deep brain stimulation expands, it is critical to understand the origin of adverse events and the delivery of nondamaging stimulation.

Intracochlear electrical stimulation suppresses apoptotic signaling in rat spiral ganglion neurons after deafening in vivo

OBJECTIVE

To establish the intracellular consequences of electrical stimulation to spiral ganglion neurons after deafferentation. Here we use a rat model to determine the effect of both low and high pulse rate acute electrical stimulation on activation of the proapoptotic transcription factor Jun in deafferented spiral ganglion neurons in vivo.

STUDY DESIGN

Experimental animal study.

SETTING

Hearing research laboratories of the University of Iowa Departments of Biology and Otolaryngology.

METHODS

A single electrode was implanted through the round window of kanamycin-deafened rats at either postnatal day 32 (P32, n = 24) or P60 (n = 22) for 4 hours of stimulation (monopolar, biphasic pulses, amplitude twice electrically evoked auditory brainstem response [eABR] threshold) at either 100 or 5000 Hz. Jun phosphorylation was assayed by immunofluorescence to quantitatively assess the effect of electrical stimulation on proapoptotic signaling.

RESULTS

Jun phosphorylation was reliably suppressed by 100 Hz stimuli in deafened cochleae of P32 but not P60 rats. This effect was not significant in the basal cochlear turns. Stimulation frequency may be consequential: 100 Hz was significantly more effective than was 5 kHz stimulation in suppressing phospho-Jun.

CONCLUSIONS

Suppression of Jun phosphorylation occurs in deafferented spiral ganglion neurons after only 4 hours of electrical stimulation. This finding is consistent with the hypothesis that electrical stimulation can decrease spiral ganglion neuron death after deafferentation.

Effect of monopolar and bipolar electric stimulation on survival and size of human spiral ganglion cells as studied by postmortem histopathology

The spiral ganglion cell (SGC) is the target of electrical stimulation in cochlear implants. This study is designed to test the hypothesis that chronic electrical stimulation tends to preserve SGCs in implanted hearing-impaired ears. A total of 26 pairs of temporal bones were studied from 26 individuals who in life suffered bilateral profound hearing impairment that was symmetric (in degree of impairment and etiology) across ears and then underwent unilateral cochlear implantation. The subjects were divided in two groups by stimulus configuration: bipolar (n = 16) or monopolar (n = 10). The temporal bones were prepared for histological review by standard methods and two measures of SGC status were made by cochlear segment: count and maximal cross-sectional area. Within-subject comparison of the measures between the implanted-stimulated and the unimplanted ears showed: (1) for both stimulus configurations, the mean (across subjects and segments) of the count difference (implanted ear - unimplanted ear) was significantly less than zero; (2) the mean (across subject) count difference for cochlear segments I, II and III (segments with electrode contacts in the implanted ear) was significantly less negative than the mean difference for cochlear segment IV (no electrode in implanted ear) for bipolar but not for monopolar stimulation; (3) neither implantation-stimulation nor stimulus configuration significantly influenced the measures of maximum cross-sectional cell area. The SGC count results are consistent with the hypothesis that implantation results in a propensity across the whole cochlea for SGCs to degenerate and with chronic bipolar stimulation ameliorating this propensity in those cochlear segments with electrodes present.

Effects of brain-derived neurotrophic factor (BDNF) and electrical stimulation on survival and function of cochlear spiral ganglion neurons in deafened, developing cats

Both neurotrophic support and neural activity are required for normal postnatal development and survival of cochlear spiral ganglion (SG) neurons. Previous studies in neonatally deafened cats demonstrated that electrical stimulation (ES) from a cochlear implant can promote improved SG survival but does not completely prevent progressive neural degeneration. Neurotrophic agents combined with an implant may further improve neural survival. Short-term studies in rodents have shown that brain-derived neurotrophic factor (BDNF) promotes SG survival after deafness and may be additive to trophic effects of stimulation. Our recent study in neonatally deafened cats provided the first evidence of BDNF neurotrophic effects in the developing auditory system over a prolonged duration Leake et al. (J Comp Neurol 519:1526-1545, 2011). Ten weeks of intracochlear BDNF infusion starting at 4 weeks of age elicited significant improvement in SG survival and larger soma size compared to contralateral. In the present study, the same deafening and BDNF infusion procedures were combined with several months of ES from an implant. After combined BDNF + ES, a highly significant increase in SG numerical density (>50 % improvement re: contralateral) was observed, which was significantly greater than the neurotrophic effect seen with ES-only over comparable durations. Combined BDNF + ES also resulted in a higher density of myelinated radial nerve fibers within the osseous spiral lamina. However, substantial ectopic and disorganized sprouting of these fibers into the scala tympani also occurred, which may be deleterious to implant function. EABR thresholds improved (re: initial thresholds at time of implantation) on the chronically stimulated channels of the implant. Terminal electrophysiological studies recording in the inferior colliculus (IC) revealed that the basic cochleotopic organization was intact in the midbrain in all studied groups. In deafened controls or after ES-only, lower IC thresholds were correlated with more selective activation widths as expected, but no such correlation was seen after BDNF + ES due to much greater variability in both measures.

Human cochlea: anatomical characteristics and their relevance for cochlear implantation

This is a review of the anatomical characteristics of human cochlea and the importance of variations in this anatomy to the process of cochlear implantation (CI). Studies of the human cochlea are essential to better comprehend the physiology and pathology of man's hearing. The human cochlea is difficult to explore due to its vulnerability and bordering capsule. Inner ear tissue undergoes quick autolytic changes making investigations of autopsy material difficult, even though excellent results have been presented over time. Important issues today are novel inner ear therapies including CI and new approaches for inner ear pharmacological treatments. Inner ear surgery is now a reality, and technical advancements in the design of electrode arrays and surgical approaches allow preservation of remaining structure/function in most cases. Surgeons should aim to conserve cochlear structures for future potential stem cell and gene therapies. Renewal interest of round window approaches necessitates further acquaintance of this complex anatomy and its variations. Rough cochleostomy drilling at the intricate "hook" region can generate intracochlear bone-dust-inducing fibrosis and new bone formation, which could negatively influence auditory nerve responses at a later time point. Here, we present macro- and microanatomic investigations of the human cochlea viewing the extensive anatomic variations that influence electrode insertion. In addition, electron microscopic (TEM and SEM) and immunohistochemical results, based on specimens removed at surgeries for life-threatening petroclival meningioma and some well-preserved postmortal tissues, are displayed. These give us new information about structure as well as protein and molecular expression in man. Our aim was not to formulate a complete description of the complex human anatomy but to focus on aspects clinically relevant for electric stimulation, predominantly, the sensory targets, and how surgical atraumaticity best could be reached.

Lipidic nanocapsule drug delivery: neuronal protection for cochlear implant optimization

Objective

Sensorineural hearing loss leads to the progressive degeneration of spiral ganglion cells (SGC). Next to postoperative fibrous tissue growth, which should be suppressed to assure a close nerve–electrode interaction, the density of healthy SGC is one factor that influences the efficiency of cochlear implants (CI), the choice of treatment for affected patients. Rolipram, a phosphodiesterase-4 inhibitor, has proven neuroprotective and anti-inflammatory effects and might also reduce SGC degeneration and fibrosis, but it has to pass the cellular membrane to be biologically active.

Methods

Lipidic nanocapsules (LNC) can be used as biodegradable drug carriers to increase the efficacy of conventional application methods. We examined the biological effects of rolipram and LNC’s core encapsulated rolipram on SGC and dendritic cell (DC) tumor necrosis factor-α (TNF-α) production in vitro and on SGC survival in systemically-deafened guinea pigs in vivo.

Results

Our results prove that rolipram does not have a beneficial effect on cultured SGC. Incorporation of rolipram in LNC increased the survival of SGC significantly. In the DC study, rolipram significantly inhibited TNF-α in a dose-dependent manner. The rolipram-loaded LNC provided a significant cytokine inhibition as well. In vivo data do not confirm the in vitro results.

Conclusion

By transporting rolipram into the SGC cytoplasm, LNC enabled the neuroprotective effect of rolipram in vitro, but not in vivo. This might be due to dilution of test substances by perilymph or an inadequate release of rolipram based on differing in vivo and in vitro conditions. Nevertheless, based on in vitro results, proving a significantly increased neuronal survival when using LNC-rolipram compared to pure rolipram and pure LNC application, we believe that the combination of rolipram and LNC can potentially reduce neuronal degeneration and fibrosis after CI implantation. We conclude that rolipram is a promising drug that can be used in inner ear therapy and that LNC have potential as an inner ear drug-delivery system. Further experiments with modified conditions might reveal in vivo biological effects.

Spiral ganglion neuron survival and function in the deafened cochlea following chronic neurotrophic treatment

Cochlear implants electrically stimulate residual spiral ganglion neurons (SGNs) to provide auditory cues for the severe-profoundly deaf. However, SGNs gradually degenerate following cochlear hair cell loss, leaving fewer neurons available for stimulation. Providing an exogenous supply of neurotrophins (NTs) has been shown to prevent SGN degeneration, and when combined with chronic intracochlear electrical stimulation (ES) following a short period of deafness (5 days), may also promote the formation of new neurons. The present study assessed the histopathological response of guinea pig cochleae treated with NTs (brain-derived neurotrophic factor and neurotrophin-3) with and without ES over a four week period, initiated two weeks after deafening. Results were compared to both NT alone and artificial perilymph (AP) treated animals. AP/ES treated animals exhibited no evidence of SGN rescue compared with untreated deafened controls. In contrast, NT administration showed a significant SGN rescue effect in the lower and middle cochlear turns (two-way ANOVA, p < 0.05) compared with AP-treated control animals. ES in combination with NT did not enhance SGN survival compared with NT alone. SGN function was assessed by measuring electrically-evoked auditory brainstem response (EABR) thresholds. EABR thresholds following NT treatment were significantly lower than animals treated with AP (two-way ANOVA, p = 0.033). Finally, the potential for induced neurogenesis following the combined treatment was investigated using a marker of DNA synthesis. However, no evidence of neurogenesis was observed in the SGN population. The results indicate that chronic NT delivery to the cochlea may be beneficial to cochlear implant patients by increasing the number of viable SGNs and decreasing activation thresholds compared to chronic ES alone.

Nerve maintenance and regeneration in the damaged cochlea

Following the onset of sensorineural hearing loss, degeneration of mechanosensitive hair cells and spiral ganglion cells (SGCs) in humans and animals occurs to variable degrees, with a trend for greater neural degeneration with greater duration of deafness. Emergence of the cochlear implant prosthesis has provided much needed aid to many hearing impaired patients and has become a well-recognized therapy worldwide. However, ongoing peripheral nerve fiber regression and subsequent degeneration of SGC bodies can reduce the neural targets of cochlear implant stimulation and diminish its function. There is increasing interest in bio-engineering approaches that aim to enhance cochlear implant efficacy by preventing SGC body degeneration and/or regenerating peripheral nerve fibers into the deaf sensory epithelium. We review the advancements in maintaining and regenerating nerves in damaged animal cochleae, with an emphasis on the therapeutic capacity of neurotrophic factors delivered to the inner ear after an insult. Additionally, we summarize the histological process of neuronal degeneration in the inner ear and describe different animal models that have been employed to study this mechanism. Research on enhancing the biological infrastructure of the deafened cochlea in order to improve cochlear implant efficacy is of immediate clinical importance.

Cochlear infrastructure for electrical hearing

Although the cochlear implant is already the world's most successful neural prosthesis, opportunities for further improvement abound. Promising areas of current research include work on improving the biological infrastructure in the implanted cochlea to optimize reception of cochlear implant stimulation and on designing the pattern of electrical stimulation to take maximal advantage of conditions in the implanted cochlea. In this review we summarize what is currently known about conditions in the cochlea of deaf, implanted humans and then review recent work from our animal laboratory investigating the effects of preserving or reinnervating tissues on psychophysical and electrophysiological measures of implant function. Additionally we review work from our human laboratory on optimizing the pattern of electrical stimulation to better utilize strengths in the cochlear infrastructure. Histological studies of human temporal bones from implant users and from people who would have been candidates for implants show a range of pathologic conditions including spiral ganglion cell counts ranging from approximately 2% to 92% of normal and partial hair cell survival in some cases. To duplicate these conditions in a guinea pig model, we use a variety of deafening and implantation procedures as well as post-deafening therapies designed to protect neurons and/or regenerate neurites. Across populations of human patients, relationships between nerve survival and functional measures such as speech have been difficult to demonstrate, possibly due to the numerous subject variables that can affect implant function and the elapsed time between functional measures and postmortem histology. However, psychophysical studies across stimulation sites within individual human subjects suggest that biological conditions near the implanted electrodes contribute significantly to implant function, and this is supported by studies in animal models comparing histological findings to psychophysical and electrophysiological data. Results of these studies support the efforts to improve the biological infrastructure in the implanted ear and guide strategies which optimize stimulation patterns to match patient-specific conditions in the cochlea.

Combining cell-based therapies and neural prostheses to promote neural survival

Cochlear implants provide partial restoration of hearing for profoundly deaf patients by electrically stimulating spiral ganglion neurons (SGNs); however, these neurons gradually degenerate following the onset of deafness. Although the exogenous application of neurotrophins (NTs) can prevent SGN loss, current techniques to administer NTs for long periods of time have limited clinical applicability. We have used encapsulated choroid plexus cells (NTCells; Living Cell Technologies, Auckland, New Zealand) to provide NTs in a clinically viable manner that can be combined with a cochlear implant. Neonatal cats were deafened and unilaterally implanted with NTCells and a cochlear implant. Animals received chronic electrical stimulation (ES) alone, NTs alone, or combined NTs and ES (ES + NT) for a period of as much as 8 months. The opposite ear served as a deafened unimplanted control. Chronic ES alone did not result in increased survival of SGNs or their peripheral processes. NT treatment alone resulted in greater SGN survival restricted to the upper basal cochlear region and an increased density of SGN peripheral processes. Importantly, chronic ES in combination with NTs provided significant SGN survival throughout a wider extent of the cochlea, in addition to an increased peripheral process density. Re-sprouting peripheral processes were observed in the scala media and scala tympani, raising the possibility of direct contact between peripheral processes and a cochlear implant electrode array. We conclude that cell-based therapy is clinically viable and effective in promoting SGN survival for extended durations of cochlear implant use. These findings have important implications for the safe delivery of therapeutic drugs to the cochlea.

The spiral ganglion: connecting the peripheral and central auditory systems

In mammals, the initial bridge between the physical world of sound and perception of that sound is established by neurons of the spiral ganglion. The cell bodies of these neurons give rise to peripheral processes that contact acoustic receptors in the organ of Corti, and the central processes collect together to form the auditory nerve that projects into the brain. In order to better understand hearing at this initial stage, we need to know the following about spiral ganglion neurons: (1) their cell biology including cytoplasmic, cytoskeletal, and membrane properties, (2) their peripheral and central connections including synaptic structure; (3) the nature of their neural signaling; and (4) their capacity for plasticity and rehabilitation. In this report, we will update the progress on these topics and indicate important issues still awaiting resolution.

Brain-derived neurotrophic factor promotes cochlear spiral ganglion cell survival and function in deafened, developing cats

Postnatal development and survival of spiral ganglion (SG) neurons depend on both neural activity and neurotrophic support. Our previous studies showed that electrical stimulation from a cochlear implant only partially prevents SG degeneration after early deafness. Thus, neurotrophic agents that might be combined with an implant to improve neural survival are of interest. Recent studies reporting that brain-derived neurotrophic factor (BDNF) promotes SG survival after deafness have been conducted in rodents and limited to relatively short durations. Our study examined longer duration BDNF treatment in deafened cats that may better model the slow progression of SG degeneration in human cochleae, and this is the first study of BDNF in the developing auditory system. Kittens were deafened neonatally, implanted at 4-5 weeks with intracochlear electrodes containing a drug-delivery cannula, and BDNF or artificial perilymph was infused for 10 weeks from a miniosmotic pump. In BDNF-treated cochleae, SG cells grew to normal size and were significantly larger than cells on the contralateral side. However, their morphology was not completely normal, and many neurons lacked or had thinned perikaryl myelin. Unbiased stereology was employed to estimate SG cell density, independent of cell size. BDNF was effective in promoting significantly improved survival of SG neurons in these developing animals. BDNF treatment also resulted in higher density and larger size of myelinated radial nerve fibers, sprouting of fibers into the scala tympani, and improvement of electrically evoked auditory brainstem response thresholds. BDNF may have potential therapeutic value in the developing auditory system, but many serious obstacles currently preclude clinical application.

The effect of cochlear-implant-mediated electrical stimulation on spiral ganglion cells in congenitally deaf white cats

It has long been observed that loss of auditory receptor cells is associated with the progressive degeneration of spiral ganglion cells. Chronic electrical stimulation via cochlear implantation has been used in an attempt to slow the rate of degeneration in cats neonatally deafened by ototoxic agents but with mixed results. The present study examined this issue using white cats with a history of hereditary deafness as an alternative animal model. Nineteen cats provided new data for this study: four normal-hearing cats, seven congenitally deaf white cats, and eight congenitally deaf white cats with unilateral cochlear implants. Data from additional cats were collected from the literature. Electrical stimulation began at 3 to 4 or 6 to 7 months after birth, and cats received stimulation for approximately 7 h a day, 5 days a week for 12 weeks. Quantitative analysis of spiral ganglion cell counts, cell density, and cell body size showed no marked improvement between cochlear-implanted and congenitally deaf subjects. Average ganglion cell size from cochlear-implanted and congenitally deaf cats was statistically similar and smaller than that of normal-hearing cats. Cell density from cats with cochlear implants tended to decrease within the upper basal and middle cochlear turns in comparison to congenitally deaf cats but remained at congenitally deaf levels within the lower basal and apical cochlear turns. These results provide no evidence that chronic electrical stimulation enhances spiral ganglion cell survival, cell density, or cell size compared to that of unstimulated congenitally deaf cats. Regardless of ganglion neuron status, there is unambiguous restoration of auditory nerve synapses in the cochlear nucleus of these cats implanted at the earlier age.

Spiral ganglion cell survival after round window membrane application of brain-derived neurotrophic factor using gelfoam as carrier

Several studies have shown that treatment with various neurotrophins protects spiral ganglion cells (SGCs) from degeneration in hair-cell deprived cochleas. In most of these studies the neurotrophins are delivered by means of intracochlear delivery methods. Recently, other application methods that might be more suited in cochlear implant patients have been developed. We have examined if round window membrane application of gelfoam infiltrated with a neurotrophin resulted in SGC survival in deafened guinea pigs. Two weeks after deafening, gelfoam cubes infiltrated with 6 μg of brain-derived neurotrophic factor (BDNF) were deposited onto the round window membrane of the right cochleas. Electric pulses were delivered through an electrode positioned within the round window niche to electrically evoke auditory brainstem responses (eABRs). Two or four weeks after deposition of the gelfoam all cochleas were histologically examined. We found that local BDNF treatment enhances the survival of SGCs in the basal cochlear turn after two and four weeks. The treatment had no effect on SGC size or shape. In animals treated with BDNF, eABR amplitudes were smaller than in normal-hearing control animals and similar to those in deafened controls. We conclude that BDNF delivered by means of local gelfoam application provides a protective effect, which is limited compared to intracochlear delivery methods.

Orientation of spiral ganglion neurite extension in electrical fields of charge-balanced biphasic pulses and direct current in vitro

Cochlear implants function by stimulating residual spiral ganglion neurons (SGNs) with electric current. Promoting the neurites of SGNs to grow towards an electrode array could improve the outcome of cochlear implantation. Several studies have suggested that DC electrical fields (EFs) can affect the direction of neurite extension. To investigate the impact of steady DC EFs, pulsed DC EFs and charge-balanced biphasic pulsed EFs on the direction of SGN neurite extension, we established an SGN culturing and imaging system. SGN explants of newborn Sprague-Dawley rats were cultured on slides. Slides were positioned into the monitoring system with neurite terminals in a certain orientation and location, while the migration of SGN growth cones was monitored by taking serial photos. Deflection angles of neurites at a certain period were measured. In steady or pulsed DC EFs, neurites on laminin-coated slides turned towards the cathode, while those on PDL-coated slides turned towards the anode. The neurites in charge-balanced biphasic pulsed EFs had an obvious orientation of turning away from the electrode rings. This orientation diminished with increasing distance from the electrode rings and followed the nonuniform characteristics of charge-balanced biphasic pulsed EFs.

Synaptic plasticity after chemical deafening and electrical stimulation of the auditory nerve in cats

The effects of deafness on brain structure and function have been studied using animal models of congenital deafness that include surgical ablation of the organ of Corti, acoustic trauma, ototoxic drugs, and hereditary deafness. This report describes the morphologic plasticity of auditory nerve synapses in response to ototoxic deafening and chronic electrical stimulation of the auditory nerve. Normal kittens were deafened by neonatal administration of neomycin that eliminated auditory receptor cells. Some of these cats were raised deaf, whereas others were chronically implanted with cochlear electrodes at 2 months of age and electrically stimulated for up to 12 months. The large endings of the auditory nerve, endbulbs of Held, were studied because they hold a key position in the timing pathway for sound localization, are readily identifiable, and exhibit deafness-associated abnormalities. Compared with those of normal hearing cats, synapses of ototoxically deafened cats displayed expanded postsynaptic densities, a 35.4% decrease in synaptic vesicle (SV) density, and a reduction in the somatic size of spherical bushy cells (SBCs). In comparison with normal hearing cats, ototoxically deafened cats that received cochlear stimulation had endbulbs that expressed postsynaptic densities (PSDs) that were statistically identical in size, showed a 48.1% reduction in SV density, and whose target SBCs had a 25.5% reduction in soma area. These results demonstrate that electrical stimulation via a cochlear implant in chemically deafened cats preserves PSD size but not other aspects of synapse morphology. This determination further suggests that the effects of ototoxic deafness are not identical to those of hereditary deafness.

Effects of isoflurane on auditory evoked potentials in the cochlea and brainstem of guinea pigs

Electrophysiological recordings of the auditory system are commonly performed in deeply anesthetized animals. This study evaluated the effects of various concentrations of the volatile anesthetic isoflurane (1-3%) on the compound action potential (CAP), cochlear microphonic (CM) and auditory brainstem response (ABR). Recordings were initiated in the awake, lightly restrained animal. Anesthesia was induced with a single dose of Hypnorm (fentanyl and fluanisone). After tracheostomy increasing isoflurane concentrations were applied in N(2)O/O(2) via controlled ventilation. Data were compared to recordings in the awake animal using repeated measures ANOVA and Dunnett's post hoc test. On average, isoflurane dose-dependently suppressed the amplitude and increased the latency of the CAP. CM amplitude was suppressed. These effects were most profound at high frequencies and were typically significant at isoflurane concentrations of 2.5% and 3%. Amplitude and latency of the second negative peak of the CAP (N(2)) were affected to a greater extent compared to the first peak (N(1)). On average, isoflurane dose-dependently reduced the amplitude and increased the latency of the ABR. These effects were typically significant at an isoflurane concentration of 2%. Effects on peak IV and V were more pronounced compared to the early peaks I and III.

Functional evaluation of a cell replacement therapy in the inner ear

HYPOTHESIS

Cell replacement therapy in the inner ear will contribute to the functional recovery of hearing loss.

BACKGROUND

Cell replacement therapy is a potentially powerful approach to replace degenerated or severely damaged spiral ganglion neurons. This study aimed at stimulating the neurite outgrowth of the implanted neurons and enhancing the potential therapeutic of inner ear cell implants.

METHODS

Chronic electrical stimulation (CES) and exogenous neurotrophic growth factor (NGF) were applied to 46 guinea pigs transplanted with embryonic dorsal root ganglion (DRG) neurons 4 days postdeafening. The animals were evaluated with the electrically evoked auditory brainstem responses (EABRs) at experimental Days 7, 11, 17, 24, and 31. The animals were euthanized at Day 31, and the inner ears were dissected for immunohistochemistry investigation.

RESULTS

Implanted DRG cells, identified by enhanced green fluorescent protein fluorescence and a neuronal marker, were found close to Rosenthal canal in the adult inner ear for up to 4 weeks after transplantation. Extensive neurite projections clearly, greater than in nontreated animals, were observed to penetrate the bony modiolus and reach the spiral ganglion region in animals supplied with CES and/or NGF. There was, however, no significant difference in the thresholds of EABRs between DRG-transplanted animals supplied with CES and/or NGF and DRG-transplanted animals without CES or NGF supplement.

CONCLUSION

The results suggest that CES and/or NGF can stimulate neurite outgrowth from implanted neurons, although based on EABR measurement, these interventions did not induce functional connections to the central auditory pathway. Additional time or novel approaches may enhance functional responsiveness of implanted cells in the adult cochlea.

Effects of delayed treatment with combined GDNF and continuous electrical stimulation on spiral ganglion cell survival in deafened guinea pigs

Electrical stimulation (ES) of spiral ganglion cells (SGC) via a cochlear implant is the standard treatment for profound sensor neural hearing loss. However, loss of hair cells as the morphological correlate of sensor neural hearing loss leads to deafferentation and death of SGC. Although immediate treatment with ES or glial cell line-derived neurotrophic factor (GDNF) can prevent degeneration of SGC, only few studies address the effectiveness of delayed treatment. We hypothesize that both interventions have a synergistic effect and that even delayed treatment would protect SGC. Therefore, an electrode connected to a pump was implanted into the left cochlea of guinea pigs 3 weeks after deafening. The contralateral untreated cochleae served as deafened intraindividual controls. Four groups were set up. Control animals received intracochlear infusion of artificial perilymph (AP/-). The experimental groups consisted of animals treated with AP in addition to continuous ES (AP/ES) or treated with GDNF alone (GDNF/-) or GDNF combined with continuous ES (GDNF/ES). Acoustically and electrically evoked auditory brain stem responses were recorded. All animals were killed 48 days after deafening; their cochleae were histologically evaluated. Survival of SGC increased significantly in the GDNF/- and AP/ES group compared with the AP/- group. A highly significant increase in SGC density was observed in the GDNF/ES group compared with the control group. Additionally, animals in the GDNF/ES group showed reduced EABR thresholds. Thus, delayed treatment with GDNF and ES can protect SGC from degeneration and may improve the benefits of cochlear implants.

Polypyrrole-coated electrodes for the delivery of charge and neurotrophins to cochlear neurons

Sensorineural hearing loss is associated with gradual degeneration of spiral ganglion neurons (SGNs), compromising hearing outcomes with cochlear implant use. Combination of neurotrophin delivery to the cochlea and electrical stimulation from a cochlear implant protects SGNs, prompting research into neurotrophin-eluting polymer electrode coatings. The electrically conducting polypyrrole/para-toluene sulfonate containing neurotrophin-3 (Ppy/pTS/NT3) was applied to 1.7 mm2 cochlear implant electrodes. Ppy/pTS/NT3-coated electrode arrays stored 2 ng NT3 and released 0.1 ng/day with electrical stimulation. Guinea pigs were implanted with Ppy/pTS or Ppy/pTS/NT3 electrode arrays two weeks after deafening via aminoglycosides. The electrodes of a subgroup of these guinea pigs were electrically stimulated for 8 h/day for 2 weeks. There was a loss of SGNs in the implanted cochleae of guinea pigs with Ppy/pTS-coated electrodes indicative of electrode insertion damage. However, guinea pigs implanted with electrically stimulated Ppy/pTS/NT3-coated electrodes had lower electrically-evoked auditory brainstem response thresholds and greater SGN densities in implanted cochleae compared to non-implanted cochleae and compared to animals implanted with Ppy/pTS-coated electrodes (p<0.05). Ppy/pTS/NT3 did not exacerbate fibrous tissue formation and did not affect electrode impedance. Drug-eluting conducting polymer coatings on cochlear implant electrodes present a clinically viable method to promote preservation of SGNs without adversely affecting the function of the cochlear implant.

Enhanced survival of spiral ganglion cells after cessation of treatment with brain-derived neurotrophic factor in deafened guinea pigs

Exogenous delivery of neurotrophic factors into the cochlea of deafened animals rescues spiral ganglion cells (SGCs) from degeneration. To be clinically relevant for human cochlear implant candidates, the protective effect of neurotrophins should persist after cessation of treatment and the treated SGCs should remain functional. In this study, the survival and functionality of SGCs were investigated after temporary treatment with brain-derived neurotrophic factor (BDNF). Guinea pigs in the experimental group were deafened, and 2 weeks later, the right cochleae were implanted with an electrode array and drug delivery cannula. BDNF was administered to the implanted cochleae during a 4-week period via a mini-osmotic pump. After completion of the treatment, the osmotic pumps were removed. Two weeks later, the animals were killed and the survival of SGCs was analyzed. To monitor the functionality of the auditory nerve, electrically evoked auditory brainstem responses (eABRs) were recorded in awake animals throughout the experiment. BDNF treatment resulted in enhanced survival of SGCs 2 weeks after cessation of the treatment and prevented the decreases in size and circularity that are seen in the untreated contralateral cochleae. The amplitude of the suprathreshold eABR response in BDNF-treated animals was significantly larger than in deafened control animals and comparable to that in normal-hearing control animals. The amplitude in the BDNF-treated group did not decrease significantly after cessation of treatment. The eABR latency in BDNF-treated animals was longer than normal and comparable to that in deafened control animals. These morphological and functional findings demonstrate that neurotrophic intervention had a lasting effect, which is promising for future clinical application of neurotrophic factors in implanted human cochleae.

Delayed electrical stimulation and BDNF application following induced deafness in rats

CONCLUSION

Under the condition of delayed intervention (30 days after deafening) following gentamicin+furosemide deafening in rats, we conclude that chronic intracochlear electrical stimulation (ES) and continuous intracochlear administration of brain-derived neurotrophic factor (BDNF) enhance spiral ganglion cell (SGC) body and peripheral process survival and improve auditory sensitivity. Moreover, the combination of ES and BDNF has a synergistic protective effect rather than an additive effect. Both SGC body and peripheral process influence the auditory sensitivity, and the latter appears to be more important.

OBJECTIVE

To determine the influence of delayed application of combined ES and neurotrophins on the survival of SGC body and peripheral processes after induced deafness in the rat. This study also explored the relationship between auditory sensitivity and SGC/peripheral process density.

MATERIALS AND METHODS

The left cochlea of profoundly deafened rats was implanted with an electrode and drug-delivery system 30 days after deafening. BDNF or artificial perilymph (AP) was delivered continuously for 28 days. Experimental animals received ES with or without BDNF (BDNF+ES and ES+AP), and control animals received BDNF or AP without ES (BDNF and AP). The right cochleae of the animals served as deafened untreated controls. Electrically evoked auditory brainstem responses (EABRs) were recorded immediately after surgery and every 7 days.

RESULTS

In the AP group, EABR thresholds demonstrated a systematic and rapid increase throughout the treatment period after the deafening procedure and electrode implantation. However, in the other three treatment groups, EABR thresholds showed a slow increase at the beginning and then slow decrease. The thresholds of the BDNF and ES+AP groups were significantly less than those of the AP group from day 7 to 28 and those of the BDNF+ES group were significantly less than those of other three groups from day 21 to 28, indicating that BDNF infusion and chronic ES have a synergistic effect rather than an additive effect. In terms of SGC and peripheral process density, the difference between the treated and control ears of BDNF, ES+AP, and BDNF+ES groups was clearly significant. Analysis of the SGC/peripheral process density of the left cochlea across the treatment groups demonstrated that SGC/peripheral process density of the BDNF and ES+AP groups was significantly greater than that of the AP group and the density of the BDNF+ES group was significantly greater than that of the other three groups, indicating that BDNF infusion and chronic ES have a synergistic effect rather than an additive effect. Finally, a functional formula was developed relating the last EABR threshold and SGC density and process density.

Novel drug delivery systems for inner ear protection and regeneration after hearing loss

BACKGROUND

A cochlear implant, the only current treatment for restoring auditory perception after severe or profound sensorineural hearing loss (SNHL), works by electrically stimulating spiral ganglion neurons (SGNs). However, gradual degeneration of SGNs associated with SNHL can compromise the efficacy of the device.

OBJECTIVE

To review novel drug delivery systems for preserving and/or regenerating sensory cells in the cochlea after SNHL.

METHODS

The effectiveness of traditional cochlear drug delivery systems is compared to newer techniques such as cell, polymer and gene transfer technologies. Special requirements for local drug delivery to the cochlea are discussed, such as protecting residual hearing and site-specific drug delivery for cell preservation and regeneration.

RESULTS/CONCLUSIONS

Drug delivery systems with the potential for immediate clinical translation, as well as those that will contribute to the future of hearing preservation or cochlear cellular regeneration, are identified.

Study on neural stem cell transplantation into natural rat cochlea via round window

OBJECTIVE

The aim of the study is to investigate the survival of neural stem cells (NSCs) in normal rat cochlea and their potential effect on auditory function and cochlea structures via round window transplantation.

METHODS

In comparison with the normal rats without any transplantation (group III), normal rat cochleae were transplanted with NSCs infected with adenovirus carrying green fluorescence protein (GFP) gene (group I) or the artificial perilymph (group II) via round windows. Auditory functions were monitored by thresholds of auditory brain stem responses (ABRs); the cochlea structures were examined by hematoxylin and eosin staining; survivals of implanted NSCs were determined by the expression of GFP; survivals of hair cells were accessed by whole mount preparation; and ultrastructures of hair cells were examined by scanning electron microscopy.

RESULT

There were significant differences in the click-ABR thresholds in rats among all 3 groups neither at pretransplantation nor at posttransplantation; there were no significant differences in these values before and after transplantation in the same rats from each group. After transplantation, the cochlea structures were normal in both group I and group II. Grafted NSCs were visualized by the GFP expression in every turn of the cochlea in all animals of group I. There were no significant differences in the losses of outer hair cells (OHCs) among 3 groups. The inner hair cells and most OHCs were normal in every turns of cochleae of all groups.

CONCLUSION

Neural stem cells survived in normal rat cochlea after transplantation via round window and showed no obvious effects on auditory functions and inner ear pathologic examination of the rat cochlea.

Longitudinal study of the ecap measured in children with cochlear implants

In children with cochlear implant (CI), the recording of the electrically evoked compound action potential (ECAP) of the auditory nerve represents an option to assess changes in auditory nerve responses and the interaction between the electrode bundle and the neural tissue over time. Aim: To study ECAP in children during the first year of CI use. Materials and methods: The ECAP characteristics have been analyzed in 13 children implanted younger than three years of age. Series study. Results: During the first year of CI use there was a significant statistical raise in the N1 peak amplitude, in basal electrodes, between the second and third return visits. There were not any significant differences obtained for N1 peak, latency, slope, p-NRT or recovery time, in the return visits. Conclusion: During the first year of CI use, the electrical stimulation provided by the intracochlear electrodes did not cause significant changes to ECAP characteristics, except for an increase in N1 peak amplitude.