Chronic neurotrophin delivery promotes ectopic neurite growth from the spiral ganglion of deafened cochleae without compromising the spatial selectivity of cochlear implants

Chronic Electro-Acoustic Stimulation May Interfere With Electric Threshold Recovery After Cochlear Implantation in the Aged Guinea Pig

OBJECTIVES

Electro-acoustic stimulation (EAS) combines electric stimulation via a cochlear implant (CI) with residual low-frequency acoustic hearing, with benefits for music appreciation and speech perception in noise. However, many EAS CI users lose residual acoustic hearing, reducing this benefit. The main objectives of this study were to determine whether chronic EAS leads to more hearing loss compared with CI surgery alone in an aged guinea pig model, and to assess the relationship of any hearing loss to histology measures. Conversely, it is also important to understand factors impacting efficacy of electric stimulation. If one contributor to CI-induced hearing loss is damage to the auditory nerve, both acoustic and electric thresholds will be affected. Excitotoxicity from EAS may also affect electric thresholds, while electric stimulation is osteogenic and may increase electrode impedances. Hence, secondary objectives were to assess how electric thresholds are related to the amount of residual hearing loss after CI surgery, and how EAS affects electric thresholds and impedances over time.

DESIGN

Two groups of guinea pigs, aged 9 to 21 months, were implanted with a CI in the left ear. Preoperatively, the animals had a range of hearing losses, as expected for an aged cohort. At 4 weeks after surgery, the EAS group (n = 5) received chronic EAS for 8 hours a day, 5 days a week, for 20 weeks via a tether system that allowed for free movement during stimulation. The nonstimulated group (NS; n = 6) received no EAS over the same timeframe. Auditory brainstem responses (ABRs) and electrically evoked ABRs (EABRs) were recorded at 3 to 4 week intervals to assess changes in acoustic and electric thresholds over time. At 24 weeks after surgery, cochlear tissue was harvested for histological evaluation, only analyzing animals without electrode extrusions (n = 4 per ear).

RESULTS

Cochlear implantation led to an immediate worsening of ABR thresholds peaking between 3 and 5 weeks after surgery and then recovering and stabilizing by 5 and 8 weeks. Significantly greater ABR threshold shifts were seen in the implanted ears compared with contralateral, non-implanted control ears after surgery. After EAS and termination, no significant additional ABR threshold shifts were seen in the EAS group compared with the NS group. A surprising finding was that NS animals had significantly greater recovery in EABR thresholds over time, with decreases (improvements) of -51.8 ± 33.0 and -39.0 ± 37.3 c.u. at 12 and 24 weeks, respectively, compared with EAS animals with EABR threshold increases (worsening) of +1.0 ± 25.6 and 12.8 ± 44.3 c.u. at 12 and 24 weeks. Impedance changes over time did not differ significantly between groups. After exclusion of cases with electrode extrusion or significant trauma, no significant correlations were seen between ABR and EABR thresholds, or between ABR thresholds with histology measures of inner/outer hair cell counts, synaptic ribbon counts, stria vascularis capillary diameters, or spiral ganglion cell density.

CONCLUSIONS

The findings do not indicate that EAS significantly disrupts acoustic hearing, although the small sample size limits this interpretation. No evidence of associations between hair cell, synaptic ribbon, spiral ganglion cell, or stria vascularis with hearing loss after cochlear implantation was seen when surgical trauma is minimized. In cases of major trauma, both acoustic thresholds and electric thresholds were elevated, which may explain why CI-only outcomes are often better when trauma and hearing loss are minimized. Surprisingly, chronic EAS (or electric stimulation alone) may negatively impact electric thresholds, possibly by prevention of recovery of the auditory nerve after CI surgery. More research is needed to confirm the potentially negative impact of chronic EAS on electric threshold recovery.

Gene Electrotransfer via Conductivity-Clamped Electric Field Focusing Pivots Sensori-Motor DNA Therapeutics: "A Spoonful of Sugar Helps the Medicine Go Down"

Viral vectors and lipofection-based gene therapies have dispersion-dependent transduction/transfection profiles that thwart precise targeting. The study describes the development of focused close-field gene electrotransfer (GET) technology, refining spatial control of gene expression. Integration of fluidics for precise delivery of "naked" plasmid deoxyribonucleic acid (DNA) in sucrose carrier within the focused electric field enables negative biasing of near-field conductivity ("conductivity-clamping"-CC), increasing the efficiency of plasma membrane molecular translocation. This enables titratable gene delivery with unprecedently low charge transfer. The clinic-ready bionics-derived CC-GET device achieved neurotrophin-encoding miniplasmid DNA delivery to the cochlea to promote auditory nerve regeneration; validated in deafened guinea pig and cat models, leading to improved central auditory tuning with bionics-based hearing. The performance of CC-GET is evaluated in the brain, an organ problematic for pulsed electric field-based plasmid DNA delivery, due to high required currents causing Joule-heating and damaging electroporation. Here CC-GET enables safe precision targeting of gene expression. In the guinea pig, reporter expression is enabled in physiologically critical brainstem regions, and in the striatum (globus pallidus region) delivery of a red-shifted channelrhodopsin and a genetically-encoded Ca2+ sensor, achieved photoactivated neuromodulation relevant to the treatment of Parkinson's Disease and other focal brain disorders.

Spread of activation and interaction between channels with multi-channel optogenetic stimulation in the mouse cochlea

For individuals with severe to profound hearing loss resulting from irreversibly damaged hair cells, cochlear implants can be used to restore hearing by delivering electrical stimulation directly to the spiral ganglion neurons. However, current spread lowers the spatial resolution of neural activation. Since light can be easily confined, optogenetics is a technique that has the potential to improve the precision of neural activation, whereby visible light is used to stimulate neurons that are modified with light-sensitive opsins. This study compares the spread of neural activity across the inferior colliculus of the auditory midbrain during electrical and optical stimulation in the cochlea of acutely deafened mice with opsin-modified spiral ganglion neurons (H134R variant of the channelrhodopsin-2). Monopolar electrical stimulation was delivered via each of four 0.2 mm wide platinum electrode rings at 0.6 mm centre-to-centre spacing, whereas 453 nm wavelength light was delivered via each of five 0.22 × 0.27 mm micro-light emitting diodes (LEDs) at 0.52 mm centre-to-centre spacing. Channel interactions were also quantified by threshold changes during simultaneous stimulation by pairs of electrodes or micro-LEDs at different distances between the electrodes (0.6, 1.2 and 1.8 mm) or micro-LEDs (0.52, 1.04, 1.56 and 2.08 mm). The spread of activation resulting from single channel optical stimulation was approximately half that of monopolar electrical stimulation as measured at two levels of discrimination above threshold (p<0.001), whereas there was no significant difference between optical stimulation in opsin-modified deafened mice and pure tone acoustic stimulation in normal-hearing mice. During simultaneous micro-LED stimulation, there were minimal channel interactions for all micro-LED spacings tested. For neighbouring micro-LEDs/electrodes, the relative influence on threshold was 13-fold less for optical stimulation compared electrical stimulation (p<0.05). The outcomes of this study show that the higher spatial precision of optogenetic stimulation results in reduced channel interaction compared to electrical stimulation, which could increase the number of independent channels in a cochlear implant. Increased spatial resolution and the ability to activate more than one channel simultaneously could lead to better speech perception in cochlear implant recipients.

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.

Emerging therapies for human hearing loss

INTRODUCTION

More than 5% of the world's population have a disabling hearing loss which can be managed by hearing aids or implanted electrical devices. However, outcomes are highly variable, and the sound perceived by recipients is far from perfect. Sparked by the discovery of progenitor cells in the cochlea and rapid progress in drug delivery to the cochlea, biological and pharmaceutical therapies are currently in development to improve the function of the cochlear implant or eliminate the need for it altogether.

AREAS COVERED

This review highlights progress in emerging regenerative strategies to restore hearing and adjunct therapies to augment the cochlear implant. Novel approaches include the reprogramming of progenitor cells to restore the sensory hair cell population in the cochlea, gene therapy and gene editing to treat hereditary and acquired hearing loss. A detailed review of optogenetics is also presented as a technique that could enable optical stimulation of the spiral ganglion neurons, replacing or complementing electrical stimulation.

EXPERT OPINION

Increasing evidence of substantial reversal of hearing loss in animal models, alongside rapid advances in delivery strategies to the cochlea and learnings from clinical trials will amalgamate into a biological or pharmaceutical therapy to replace or complement the cochlear implant.

Endoscopic-Assisted Drug Delivery for Inner Ear Regeneration

Sensorineural hearing loss is caused by irreversible loss of auditory hair cells and/or neurons and is increasing in prevalence. Hair cells and neurons do not regenerate after damage, but novel regeneration therapies based on small molecule drugs, gene therapy, and cell replacement strategies offer promising therapeutic options. Endogenous and exogenous regeneration techniques are discussed in context of their feasibility for hair cell and neuron regeneration. Gene therapy and treatment of synaptopathy represent promising future therapies. Minimally invasive endoscopic ear surgery offers a viable approach to aid in delivery of pharmacologic compounds, cells, or viral vectors to the inner ear for all of these techniques.

Viral-mediated transduction of auditory neurons with opsins for optical and hybrid activation

Optical stimulation is a paradigm-shifting approach to modulating neural activity that has the potential to overcome the issue of current spread that occurs with electrical stimulation by providing focused stimuli. But optical stimulation either requires high power infrared light or genetic modification of neurons to make them responsive to lower power visible light. This work examines optical activation of auditory neurons following optogenetic modification via AAV injection in two species (mouse and guinea pig). An Anc80 viral vector was used to express the channelrhodopsin variant ChR2-H134R fused to a fluorescent reporter gene under the control of the human synapsin-1 promoter. The AAV was administered directly to the cochlea (n = 33) or posterior semi-circular canal of C57BL/6 mice (n = 4) or to guinea pig cochleae (n = 6). Light (488 nm), electrical stimuli or the combination of these (hybrid stimulation) was delivered to the cochlea via a laser-coupled optical fibre and co-located platinum wire. Activation thresholds, spread of activation and stimulus interactions were obtained from multi-unit recordings from the central nucleus of the inferior colliculus of injected mice, as well as ChR2-H134R transgenic mice (n = 4). Expression of ChR2-H134R was examined by histology. In the mouse, transduction of auditory neurons by the Anc80 viral vector was most successful when injected at a neonatal age with up to 89% of neurons transduced. Auditory neuron transductions were not successful in guinea pigs. Inferior colliculus responses to optical stimuli were detected in a cochleotopic manner in all mice with ChR2-H134R expression. There was a significant correlation between lower activation thresholds in mice and higher proportions of transduced neurons. There was no difference in spread of activation between optical stimulation and electrical stimulation provided by the light/electrical delivery system used here (optical fibre with bonded 25 µm platinum/iridium wire). Hybrid stimulation, comprised of sub-threshold optical stimulation to 'prime' or raise the excitability of the neurons, lowered the threshold for electrical activation in most cases, but the impact on excitation width was more variable compared to transgenic mice. This study demonstrates the impact of opsin expression levels and expression pattern on optical and hybrid stimulation when considering optical or hybrid stimulation techniques for neuromodulation.

Correlation of Electrically Evoked Compound Action Potential Amplitude Growth Function Slope and Anamnestic Parameters in Cochlear Implant Patients-Identification of Predictors for the Neuronal Health Status

Cochlear implants (CI) are the treatment of choice in profoundly deaf patients. Measuring the electrically evoked compound action potential (ECAP) has become an important tool for verifying the function of the spiral ganglion neurons (SGN), which are the target cells of the CI stimulation. ECAP measurement is only possible after electrode insertion. No information about the neuronal health status is available before cochlear implantation. We investigated possible correlations between the ECAP amplitude growth function (AGF) slope and anamnestic parameters to identify possible predictors for SGN health status and therefore for CI outcome. The study included patients being implanted with various electrode array lengths. Correlation analysis was performed for the mean AGF slope of the whole array, for separate electrodes as well as for grouped electrodes of the apical, medial, and basal region, with duration of deafness, age at implantation, residual hearing (grouped for electrode length), and etiology. The mean ECAP AGF slopes decreased from apical to basal. They were not correlated to the length of the electrode array or any etiology. For the mean of the full array or when grouped for the apical, middle, and basal part, the ECAP AGF slope was negatively correlated to the duration of hearing loss and the age at implantation. Since a significant negative correlation of the ECAP AGF slope and age at cochlear implantation and duration of deafness was observed, this study supports the statement that early implantation of a CI is recommended for sensorineural hearing loss. Additional factors such as the cochlear coverage and insertion angle influence the ECAP AGF slope and performance of the patient and should be included in future multifactorial analysis to study predictive parameters for the CI outcome.

Hybrid optogenetic and electrical stimulation for greater spatial resolution and temporal fidelity of cochlear activation

OBJECTIVE

Compared to electrical stimulation, optogenetic stimulation has the potential to improve the spatial precision of neural activation in neuroprostheses, but it requires intense light and has relatively poor temporal kinetics. We tested the effect of hybrid stimulation, which is the combination of subthreshold optical and electrical stimuli, on spectral and temporal fidelity in the cochlea by recording multiunit activity in the inferior colliculus of channelrhodopsin (H134R variant) transgenic mice.

APPROACH

Pulsed light or biphasic electrical pulses were delivered to cochlear spiral ganglion neurons of acutely deafened mice, either as individual stimuli or as hybrid stimuli for which the timing of the electrical pulse had a varied delay relative to the start of the optical pulse. Response thresholds, spread of activation and entrainment data were obtained from multi-unit recordings from the auditory midbrain.

MAIN RESULTS

Facilitation occurred when subthreshold electrical stimuli were applied at the end of, or up to 3.75 ms after subthreshold optical pulses. The spread of activation resulting from hybrid stimulation was significantly narrower than electrical-only and optical-only stimulation (p < 0.01), measured at equivalent suprathreshold levels of loudness that are relevant to cochlear implant users. Furthermore, temporal fidelity, measured as maximum following rates to 300 ms pulse trains bursts up to 240 Hz, was 2.4-fold greater than optical-only stimulation (p < 0.05).

SIGNIFICANCE

By significantly improving spectral resolution of electrical- and optical-only stimulation and the temporal fidelity of optical-only stimulation, hybrid stimulation has the potential to increase the number of perceptually independent stimulating channels in a cochlear implant.

Cochlear implants and other inner ear prostheses: today and tomorrow

Cochlear implants (CIs) are implantable auditory prostheses designed to restore access to sound in deaf individuals via direct electrical stimulation of the auditory nerve. While CIs have been successful in restoring speech perception to many deaf patients, outcomes are variable and speech recognition in noise remains a problem. This chapter will review the factors underlying this variability, and discuss significant recent innovations to address these issues including neural health preservation, characterization, and regeneration, and other inner ear prostheses. The emerging role of central auditory plasticity will also be discussed. Together, these advances will point to the likely future directions for advancing the next generation of CIs and other inner ear prostheses.

Alginate-encapsulated brain-derived neurotrophic factor-overexpressing mesenchymal stem cells are a promising drug delivery system for protection of auditory neurons

The cochlear implant outcome is possibly improved by brain-derived neurotrophic factor treatment protecting spiral ganglion neurons. Implantation of genetically modified mesenchymal stem cells may enable the required long-term brain-derived neurotrophic factor administration. Encapsulation of mesenchymal stem cells in ultra-high viscous alginate may protect the mesenchymal stem cells from the recipient’s immune system and prevent their uncontrolled migration. Alginate stability and survival of mesenchymal stem cells in alginate were evaluated. Brain-derived neurotrophic factor production was measured and its protective effect was analyzed in dissociated rat spiral ganglion neuron co-culture. Since the cochlear implant is an active electrode, alginate–mesenchymal stem cell samples were electrically stimulated and alginate stability and mesenchymal stem cell survival were investigated. Stability of ultra-high viscous-alginate and alginate–mesenchymal stem cells was proven. Brain-derived neurotrophic factor production was detectable and spiral ganglion neuron survival, bipolar morphology, and neurite outgrowth were increased. Moderate electrical stimulation did not affect the mesenchymal stem cell survival and their viability was good within the investigated time frame. Local drug delivery by ultra-high viscous-alginate-encapsulated brain-derived neurotrophic factor–overexpressing mesenchymal stem cells is a promising strategy to improve the cochlear implant outcome.

Neurotrophin gene therapy to promote survival of spiral ganglion neurons after deafness

Hearing impairment is a major health and economic concern worldwide. Currently, the cochlear implant (CI) is the standard of care for remediation of severe to profound hearing loss, and in general, contemporary CIs are highly successful. But there is great variability in outcomes among individuals, especially in children, with many CI users deriving much less or even marginal benefit. Much of this variability is related to differences in auditory nerve survival, and there has been substantial interest in recent years in exploring potential therapies to improve survival of the cochlear spiral ganglion neurons (SGN) after deafness. Preclinical studies using osmotic pumps and other approaches in deafened animal models to deliver neurotrophic factors (NTs) directly to the cochlea have shown promising results, especially with Brain-Derived Neurotrophic Factor (BDNF). More recent studies have focused on the use of NT gene therapy to force expression of NTs by target cells within the cochlea. This could provide the means for a one-time treatment to promote long-term NT expression and improve neural survival after deafness. This review summarizes the evidence for the efficacy of exogenous NTs in preventing SGN degeneration after hearing loss and reviews the animal research to date suggesting that NT gene therapy can elicit long-term NT expression in the cochlea, resulting in significantly improved SGN and radial nerve fiber survival after deafness. In addition, we discuss NT gene therapy in other non-auditory applications and consider some of the remaining issues with regard to selecting optimal vectors, timing of treatment, and place/method of delivery, etc. that must be resolved prior to considering clinical application.

Sustained-Release Triamcinolone Acetonide Hydrogels Reduce Hearing Threshold Shifts in a Model for Cochlear Implantation with Hearing Preservation

INTRODUCTION

In recent years, the preservation of residual hearing has become a major factor in patients undergoing cochlear implantation (CI). In studies attempting to pharmaceutically improve hearing preservation rates, glucocorticoids (GCs) applied perioperatively in many institutions have emerged as a promising treatment regimen. Although dexamethasone is most commonly used and has been applied successfully by various research groups, recently pharmacological properties have been reported to be relatively unsuitable for topical delivery to the inner ear. Consequently other glucocorticoids merit further evaluation. The aim of this study was therefore to evaluate the otoprotective effects of the topical application of a sustained-release triamcinolone acetonide (TAAC) hydrogel in CI with hearing preservation.

METHODS

Normal-hearing pigmented guinea pigs were randomized into a group receiving a single dose of a 6% TAAC poloxamer 407 hydrogel, a group receiving a 30% TAAC hydrogel and a control group. All hydrogel applications were performed 1 day prior to CI. After a cochleostomy was drilled, a specifically designed silicone electrode was inserted into the scala tympani for 5 mm. Frequency-specific compound action potentials of the auditory nerve (0.5-32 kHz) were measured pre- and directly postoperatively as well as on days 3, 7, 14, 21, and 28. Finally, temporal bones were harvested for histological evaluation.

RESULTS

Application of the TAAC hydrogels resulted in significantly reduced hearing threshold shifts in low, middle and high frequencies and improved spiral ganglion cell survival in the second turn of the cochlea. Outer hair cell numbers in the basal and second turn of the cochlea were slightly reduced after TAAC application.

CONCLUSION

In summary, we were able to demonstrate functional benefits of a single preoperative application of a TAAC hydrogel in a guinea pig model for CI, which persisted until the end of the observational period, that is, 28 days after surgery.

New molecular therapies for the treatment of hearing loss

An estimated 466 million people suffer from hearing loss worldwide. Sensorineural hearing loss is characterized by degeneration of key structures of the sensory pathway in the cochlea such as the sensory hair cells, the primary auditory neurons and their synaptic connection to the hair cells - the ribbon synapse. Various strategies to protect or regenerate these sensory cells and structures are the subject of intensive research. Yet despite recent advances in our understandings of the capacity of the cochlea for repair and regeneration there are currently no pharmacological or biological interventions for hearing loss. Current research focusses on localized cochlear drug, gene and cell-based therapies. One of the more promising drug-based therapies is based on neurotrophic factors for the repair of the ribbon synapse after noise exposure, as well as preventing loss of primary auditory neurons and regrowth of the auditory neuron fibers after severe hearing loss. Drug therapy delivery technologies are being employed to address the specific needs of neurotrophin and other therapies for hearing loss that include the need for high doses, long-term delivery, localised or cell-specific targeting and techniques for their safe and efficacious delivery to the cochlea. Novel biomaterials are enabling high payloads of drugs to be administered to the cochlea with subsequent slow-release properties that are proving to be beneficial for treating hearing loss. In parallel, new gene therapy technologies are addressing the need for cell specificity and high efficacy for the treatment of both genetic and acquired hearing loss with promising reports of hearing recovery. Some biomaterials and cell therapies are being used in conjunction with the cochlear implant ensuring therapeutic benefit to the primary neurons during electrical stimulation. This review will introduce the auditory system, hearing loss and the potential for repair and regeneration in the cochlea. Drug delivery to the cochlea will then be reviewed, with a focus on new biomaterials, gene therapy technologies, cell therapy and the use of the cochlear implant as a vehicle for drug delivery. With the current pre-clinical research effort into therapies for hearing loss, including clinical trials for gene therapy, the future for the treatment for hearing loss is looking bright.

Laminin-coated electrodes improve cochlear implant function and post-insertion neuronal survival

The benefits of Cochlear implant (CI) technology depend among other factors on the proximity of the electrode array to the spiral ganglion neurons. Laminin, a component of the extracellular matrix, regulates Schwann cell proliferation and survival as well as reorganization of actin fibers within their cytoskeleton, which is necessary for myelination of peripheral axons. In this study we explore the effectiveness of laminin-coated electrodes in promoting neuritic outgrowth from auditory neurons towards the electrode array and the ability to reduce acoustic and electric auditory brainstem response (i.e. aABR and eABR) thresholds. In vitro: Schwann cells and neurites are attracted towards laminin-coated surfaces with longer neuritic processes in laminin-coated dishes compared to uncoated dishes. In vivo: Animals implanted with laminin-coated electrodes experience significant decreases in eABR and aABR thresholds at selected frequencies compared to the results from the uncoated electrodes group. At 1 month post implantation there were a greater number of spiral ganglion neurons and neuritic processes projecting into the scala tympani of animals implanted with laminin-coated electrodes compared to animals with uncoated electrodes. These data suggest that Schwann cells are attracted towards laminin-coated electrodes and promote neuritic outgrowth/ guidance and promote the survival of spiral ganglion neurons following electrode insertion trauma.

Chronic intracochlear electrical stimulation at high charge densities results in platinum dissolution but not neural loss or functional changes in vivo

OBJECTIVE

Although there are useful guidelines defining the boundary between damaging and non-damaging electrical stimulation they were derived from acute studies using large surface area electrodes in direct contact with cortical neurons. These parameters are a small subset of the parameters used by neural stimulators. More recently, histological examination of cochleae from patients that were long-term cochlear implant users have shown evidence of particulate platinum (Pt). The pathophysiological effect of Pt within the cochlea is unknown. We examined the response of the cochlea to stimulus levels beyond those regarded as safe, and to evaluate the pathophysiological response of the cochlea following chronic stimulation at charge densities designed to induce Pt corrosion in vivo.

APPROACH

19 guinea pigs were systemically deafened and implanted with a cochlear electrode array containing eight Pt electrodes of 0.05, 0.075 or 0.2 mm² area. Animals were electrically stimulated continuously for 28 d using charge balanced current pulses at charge densities of 400, 267 or 100 µC/cm²/phase. Electrically-evoked auditory brainstem responses (EABRs) were recorded to monitor neural function. On completion of stimulation electrodes were examined using scanning electron microscopy (SEM) and cochleae examined histology. Finally, analysis of Pt was measured using energy dispersive x-ray spectroscopy (EDS) and inductively coupled plasma mass spectrometry (ICP-MS).

MAIN RESULTS

Compared with unstimulated control electrodes and electrodes stimulated at 100 µC/cm²/phase, stimulation at 267 or 400 µC/cm²/phase resulted in significant Pt corrosion. Cochleae stimulated at these high charge densities contained particulate Pt. The extent of the foreign body response depended on the level of stimulation; cochleae stimulated at 267 or 400 µC/cm²/phase exhibited an extensive tissue response that included a focal region of necrosis close to the electrode. Despite chronic stimulation at high charge densities there was no loss of auditory neurons (ANs) in stimulated cochleae compared with their contralateral controls. Indeed, we report a statistically significant increase in AN density proximal to electrodes stimulated at 267 or 400 µC/cm²/phase. Finally, there was no evidence of a reduction in AN function associated with chronic stimulation at 100, 267 or 400 µC/cm²/phase as evidenced by stable EABR thresholds over the stimulation program.

SIGNIFICANCE

Chronic electrical stimulation of Pt electrodes at 267 or 400 µC/cm²/phase evoked a vigorous tissue response and produced Pt corrosion products that were located close to the electrode. Despite these changes at the electrode/tissue interface there was no evidence of neural loss or a reduction in neural function.

The development of neural stimulators: a review of preclinical safety and efficacy studies

OBJECTIVE

Given the rapid expansion of the field of neural stimulation and the rigorous regulatory approval requirements required before these devices can be applied clinically, it is important that there is clarity around conducting preclinical safety and efficacy studies required for the development of this technology.

APPROACH

The present review examines basic design principles associated with the development of a safe neural stimulator and describes the suite of preclinical safety studies that need to be considered when taking a device to clinical trial.

MAIN RESULTS

Neural stimulators are active implantable devices that provide therapeutic intervention, sensory feedback or improved motor control via electrical stimulation of neural or neuro-muscular tissue in response to trauma or disease. Because of their complexity, regulatory bodies classify these devices in the highest risk category (Class III), and they are therefore required to go through a rigorous regulatory approval process before progressing to market. The successful development of these devices is achieved through close collaboration across disciplines including engineers, scientists and a surgical/clinical team, and the adherence to clear design principles. Preclinical studies form one of several key components in the development pathway from concept to product release of neural stimulators. Importantly, these studies provide iterative feedback in order to optimise the final design of the device. Key components of any preclinical evaluation include: in vitro studies that are focussed on device reliability and include accelerated testing under highly controlled environments; in vivo studies using animal models of the disease or injury in order to assess efficacy and, given an appropriate animal model, the safety of the technology under both passive and electrically active conditions; and human cadaver and ex vivo studies designed to ensure the device's form factor conforms to human anatomy, to optimise the surgical approach and to develop any specialist surgical tooling required.

SIGNIFICANCE

The pipeline from concept to commercialisation of these devices is long and expensive; careful attention to both device design and its preclinical evaluation will have significant impact on the duration and cost associated with taking a device through to commercialisation. Carefully controlled in vitro and in vivo studies together with ex vivo and human cadaver trials are key components of a thorough preclinical evaluation of any new neural stimulator.

NANOCI-Nanotechnology Based Cochlear Implant With Gapless Interface to Auditory Neurons

Cochlear implants (CI) restore functional hearing in the majority of deaf patients. Despite the tremendous success of these devices, some limitations remain. The bottleneck for optimal electrical stimulation with CI is caused by the anatomical gap between the electrode array and the auditory neurons in the inner ear. As a consequence, current devices are limited through 1) low frequency resolution, hence sub-optimal sound quality and 2), large stimulation currents, hence high energy consumption (responsible for significant battery costs and for impeding the development of fully implantable systems). A recently completed, multinational and interdisciplinary project called NANOCI aimed at overcoming current limitations by creating a gapless interface between auditory nerve fibers and the cochlear implant electrode array. This ambitious goal was achieved in vivo by neurotrophin-induced attraction of neurites through an intracochlear gel-nanomatrix onto a modified nanoCI electrode array located in the scala tympani of deafened guinea pigs. Functionally, the gapless interface led to lower stimulation thresholds and a larger dynamic range in vivo, and to reduced stimulation energy requirement (up to fivefold) in an in vitro model using auditory neurons cultured on multi-electrode arrays. In conclusion, the NANOCI project yielded proof of concept that a gapless interface between auditory neurons and cochlear implant electrode arrays is feasible. These findings may be of relevance for the development of future CI systems with better sound quality and performance and lower energy consumption. The present overview/review paper summarizes the NANOCI project history and highlights achievements of the individual work packages.

Pharmacokinetic principles in the inner ear: Influence of drug properties on intratympanic applications

Local drug delivery to the ear has gained wide clinical acceptance, with the choice of drug and application protocol in humans largely empirically-derived. Here, we review the pharmacokinetics underlying local therapy of the ear using the drugs commonly used in clinical practice as examples. Based on molecular properties and perilymph measurements interpreted through computer simulations we now better understand the principles underlying entry and distribution of these and other drugs in the ear. From our analysis, we have determined that dexamethasone-phosphate, a pro-drug widely-used clinically, has molecular and pharmacokinetic properties that make it ill-suited for use as a local therapy for hearing disorders. This polar form of dexamethasone, used as a more soluble agent in intravenous preparations, passes less readily through lipid membranes, such as those of the epithelia restricting entry at the round window membrane and stapes. Once within the inner ear, dexamethasone-phosphate is cleaved to the active form, dexamethasone, which is less polar, passes more readily through lipid membranes of the blood-perilymph barrier and is rapidly eliminated from perilymph without distributing to apical cochlear regions. Dexamethasone-phosphate therefore provides only a brief exposure of the basal regions of the cochlea to active drug. Other steroids, such as triamcinolone-acetonide, exhibit pharmacokinetic properties more appropriate to the ear and merit more detailed consideration.

Myelin-induced inhibition in a spiral ganglion organ culture - Approaching a natural environment in vitro

The performance of a cochlear implant depends on the defined interaction between afferent neurons of the spiral ganglion and the inserted electrode. Neurite outgrowth can be induced by neurotrophins such as brain-derived neurotrophic factor (BDNF) via tropomyosin kinase receptor B (TrkB). However, neurotrophin signaling through the p75 neurotrophin receptor (p75) inhibits neurite outgrowth in the presence of myelin. Organotypic cultures derived from postnatal (P3-5) mice were used to study myelin-induced inhibition in the cochlear spiral ganglion. Neurite outgrowth was analyzed and quantified utilizing an adapted Sholl analysis. Stimulation of neurite outgrowth was quantified after application of BDNF, the selective TrkB agonist 7,8-dihydroxyflavone (7,8-DHF) and a selective inhibitor of the Rho-associated kinase (Y27632), which inhibits the p75 pathway. Myelin-induced inhibition was assessed by application of myelin-associated glycoprotein (MAG-Fc) to stimulate the inhibitory p75 pathway. Inhibition of neurite outgrowth was achieved by the selective TrkB inhibitor K252a. Stimulation of neurite outgrowth was observed after treatment with BDNF, 7,8 DHF and a combination of BDNF and Y27632. The 7,8-DHF-induced growth effects could be inhibited by K252a. Furthermore, inhibition of neurite outgrowth was observed after supplementation with MAG-Fc. Myelin-induced inhibition could be overcome by 7,8-DHF and the combination of BDNF and Y27632. In this study, myelin-induced inhibition of neurite outgrowth was established in a spiral ganglion model. We reveal that 7,8-DHF is a viable novel compound for the stimulation of neurite outgrowth in a myelin-induced inhibitory environment. The combination of TrkB stimulation and ROCK inhibition can be used to overcome myelin inhibition.

Structural and Ultrastructural Changes to Type I Spiral Ganglion Neurons and Schwann Cells in the Deafened Guinea Pig Cochlea

Sensorineural hearing loss is commonly caused by damage to cochlear sensory hair cells. Coinciding with hair cell degeneration, the peripheral fibres of type I spiral ganglion neurons (SGNs) that normally form synaptic connections with the inner hair cell gradually degenerate. We examined the time course of these degenerative changes in type I SGNs and their satellite Schwann cells at the ultrastructural level in guinea pigs at 2, 6, and 12 weeks following aminoglycoside-induced hearing loss. Degeneration of the peripheral fibres occurred prior to the degeneration of the type I SGN soma and was characterised by shrinkage of the fibre followed by retraction of the axoplasm, often leaving a normal myelin lumen devoid of axoplasmic content. A statistically significant reduction in the cross-sectional area of peripheral fibres was evident as early as 2 weeks following deafening (p < 0.001, ANOVA). This was followed by a decrease in type I SGN density within Rosenthal's canal that was statistically significant 6 weeks following deafening (p < 0.001, ANOVA). At any time point examined, few type I SGN soma were observed undergoing degeneration, implying that once initiated, soma degeneration was rapid. While there was a significant reduction in soma area as well as changes to the morphology of the soma, the ultrastructure of surviving type I SGN soma appeared relatively normal over the 12-week period following deafening. Satellite Schwann cells exhibited greater survival traits than their type I SGN; however, on loss of neural contact, they reverted to a non-myelinating phenotype, exhibiting an astrocyte-like morphology with the formation of processes that appeared to be searching for new neural targets. In 6- and 12-week deafened cochlea, we observed cellular interaction between Schwann cell processes and residual SGNs that distorted the morphology of the SGN soma. Understanding the response of SGNs, Schwann cells, and the complex relationship between them following aminoglycoside deafening is important if we are to develop effective therapeutic techniques designed to rescue SGNs.

Improved Auditory Nerve Survival with Nanoengineered Supraparticles for Neurotrophin Delivery into the Deafened Cochlea

Cochlear implants electrically stimulate spiral ganglion neurons (SGNs) in order to provide speech cues to severe-profoundly deaf patients. In normal hearing cochleae the SGNs depend on endogenous neurotrophins secreted by sensory cells in the organ of Corti for survival. SGNs gradually degenerate following deafness and consequently there is considerable interest in developing clinically relevant strategies to provide exogenous neurotrophins to preserve SGN survival. The present study investigated the safety and efficacy of a drug delivery system for the cochlea using nanoengineered silica supraparticles. In the present study we delivered Brain-derived neurotrophic factor (BDNF) over a period of four weeks and evaluated SGN survival as a measure of efficacy. Supraparticles were bilaterally implanted into the basal turn of cochleae in profoundly deafened guinea pigs. One ear received BDNF-loaded supraparticles and the other ear control (unloaded) supraparticles. After one month of treatment the cochleae were examined histologically. There was significantly greater survival of SGNs in cochleae that received BDNF supraparticles compared to the contralateral control cochleae (repeated measures ANOVA, p = 0.009). SGN survival was observed over a wide extent of the cochlea. The supraparticles were well tolerated within the cochlea with a tissue response that was localised to the site of implantation in the cochlear base. Although mild, the tissue response was significantly greater in cochleae treated with BDNF supraparticles compared to the controls (repeated measures ANOVA, p = 0.003). These data support the clinical potential of this technology particularly as the supraparticles can be loaded with a variety of therapeutic drugs.

Effects of brain-derived neurotrophic factor (BDNF) on the cochlear nucleus in cats deafened as neonates

Many previous studies have shown significant neurotrophic effects of intracochlear delivery of BDNF in preventing degeneration of cochlear spiral ganglion (SG) neurons after deafness in rodents and our laboratory has shown similar results in developing cats deafened prior to hearing onset. This study examined the morphology of the cochlear nucleus (CN) in a group of neonatally deafened cats from a previous study in which infusion of BDNF elicited a significant improvement in survival of the SG neurons. Five cats were deafened by systemic injections of neomycin sulfate (60 mg/kg, SQ, SID) starting one day after birth, and continuing for 16-18 days until auditory brainstem response (ABR) testing demonstrated profound bilateral hearing loss. The animals were implanted unilaterally at about 1 month of age using custom-designed electrodes with a drug-delivery cannula connected to an osmotic pump. BDNF (94 μg/ml; 0.25 μl/hr) was delivered for 10 weeks. The animals were euthanized and studied at 14-23 weeks of age. Consistent with the neurotrophic effects of BDNF on SG survival, the total CN volume in these animals was significantly larger on the BDNF-treated side than on the contralateral side. However, total CN volume, both ipsi- and contralateral to the implants in these deafened juvenile animals, was markedly smaller than the CN in normal adult animals, reflecting the severe effects of deafness on the central auditory system during development. Data from the individual major CN subdivisions (DCN, Dorsal Cochlear Nucleus; PVCN, Posteroventral Cochlear Nucleus; AVCN, Anteroventral Cochlear Nucleus) also were analyzed. A significant difference was observed between the BDNF-treated and control sides only in the AVCN. Measurements of the cross-sectional areas of spherical cells showed that cells were significantly larger in the AVCN ipsilateral to the implant than on the contralateral side. Further, the numerical density of spherical cells was significantly lower in the AVCN ipsilateral to the implant than on the contralateral side, consistent with the larger AVCN volume observed with BDNF treatment. Together, findings indicate significant neurotrophic effects of intracochlear BDNF infusion on the developing CN.

Effect of current focusing on the sensitivity of inferior colliculus neurons to amplitude-modulated stimulation

In multichannel cochlear implants (CIs), current is delivered to specific electrodes along the cochlea in the form of amplitude-modulated pulse trains, to convey temporal and spectral cues. Our previous studies have shown that focused multipolar (FMP) and tripolar (TP) stimulation produce more restricted neural activation and reduced channel interactions in the inferior colliculus (IC) compared with traditional monopolar (MP) stimulation, suggesting that focusing of stimulation could produce better transmission of spectral information. The present study explored the capability of IC neurons to detect modulated CI stimulation with FMP and TP stimulation compared with MP stimulation. The study examined multiunit responses of IC neurons in acutely deafened guinea pigs by systematically varying the stimulation configuration, modulation depth, and stimulation level. Stimuli were sinusoidal amplitude-modulated pulse trains (carrier rate of 120 pulses/s). Modulation sensitivity was quantified by measuring modulation detection thresholds (MDTs), defined as the lowest modulation depth required to differentiate the response of a modulated stimulus from an unmodulated one. Whereas MP stimulation showed significantly lower MDTs than FMP and TP stimulation (P values <0.05) at stimulation ≤2 dB above threshold, all stimulation configurations were found to have similar modulation sensitivities at 4 dB above threshold. There was no difference found in modulation sensitivity between FMP and TP stimulation. The present study demonstrates that current focusing techniques such as FMP and TP can adequately convey amplitude modulation and are comparable to MP stimulation, especially at higher stimulation levels, although there may be some trade-off between spectral and temporal fidelity with current focusing stimulation.

Temporary Neurotrophin Treatment Prevents Deafness-Induced Auditory Nerve Degeneration and Preserves Function

After substantial loss of cochlear hair cells, exogenous neurotrophins prevent degeneration of the auditory nerve. Because cochlear implantation, the current therapy for profound sensorineural hearing loss, depends on a functional nerve, application of neurotrophins is being investigated. We addressed two questions important for fundamental insight into the effects of exogenous neurotrophins on a degenerating neural system, and for translation to the clinic. First, does temporary treatment with brain-derived neurotrophic factor (BDNF) prevent nerve degeneration on the long term? Second, how does a BDNF-treated nerve respond to electrical stimulation? Deafened guinea pigs received a cochlear implant, and their cochleas were infused with BDNF for 4 weeks. Up to 8 weeks after treatment, their cochleas were analyzed histologically. Electrically evoked compound action potentials (eCAPs) were recorded using stimulation paradigms that are informative of neural survival. Spiral ganglion cell (SGC) degeneration was prevented during BDNF treatment, resulting in 1.9 times more SGCs than in deafened untreated cochleas. Importantly, SGC survival was almost complete 8 weeks after treatment cessation, when 2.6 times more SGCs were observed. In four eCAP characteristics (three involving alteration of the interphase gap of the biphasic current pulse and one involving pulse trains), we found large and statistically significant differences between normal-hearing and deaf controls. Importantly, for BDNF-treated animals, these eCAP characteristics were near normal, suggesting healthy responsiveness of BDNF-treated SGCs. In conclusion, clinically practicable short-term neurotrophin treatment is sufficient for long-term survival of SGCs, and it can restore or preserve SGC function well beyond the treatment period. Significance statement: Successful restoration of hearing in deaf subjects by means of a cochlear implant requires a healthy spiral ganglion cell population. Deafness-induced degeneration of these cells can be averted with neurotrophic factors. In the present study in deafened guinea pigs, we investigated the long-term effects of temporary (i.e., clinically practicable) treatment with brain-derived neurotrophic factor (BDNF). We show that, after treatment cessation, the neuroprotective effect remains for at least 8 weeks. Moreover, for the first time, it is shown that the electrical responsiveness of BDNF-treated spiral ganglion cells is preserved during this period as well. These findings demonstrate that treatment of the auditory nerve with neurotrophic factors may be relevant for cochlear implant users.

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.

Evaluation of focused multipolar stimulation for cochlear implants in long-term deafened cats

OBJECTIVE

Focused multipolar (FMP) stimulation has been shown to produce restricted neural activation using intracochlear stimulation in animals with a normal population of spiral ganglion neurons (SGNs). However, in a clinical setting, the widespread loss of SGNs and peripheral fibres following deafness is expected to influence the effectiveness of FMP.

APPROACH

We compared the efficacy of FMP stimulation to both monopolar (MP) and tripolar (TP) stimulation in long-term deafened cat cochleae (n = 8). Unlike our previous study, these cochleae contained <10% of the normal SGN population adjacent to the electrode array. We also evaluated the effect of electrode position on stimulation modes by using either modiolar facing or lateral wall facing half-band electrodes. The spread of neural activity across the inferior colliculus, a major nucleus within the central auditory pathway, was used as a measure of spatial selectivity.

MAIN RESULTS

In cochleae with significant SGN degeneration, we observed that FMP and TP stimulation resulted in greater spatial selectivity than MP stimulation (p < 0.001). However, thresholds were significantly higher for FMP and TP stimulation compared to MP stimulation (p < 0.001). No difference between FMP and TP stimulation was found in any measures. The high threshold levels for FMP stimulation was significantly reduced without compromising spatial selectivity by varying the degree of current focusing (referred as 'partial-FMP' stimulation). Spatial selectivity of all stimulation modes was unaffected by the electrode position. Finally, spatial selectivity in long-term deafened cochleae was significantly less than that of cochleae with normal SGN population (George S S et al 2014 J. Neural Eng. 11 065003).

SIGNIFICANCE

The present results indicate that the greater spatial selectivity of FMP and TP stimulation over MP stimulation is maintained in cochleae with significant neural degeneration and is not adversely affected by electrode position. The greater spatial selectivity of FMP and TP stimulation would be expected to result in improved clinical performance.

Gene therapy boosts the bionic ear

Close-field electroporation gene delivery using the cochlear implant electrode array enhances the bionic ear (Pinyon et al., this issue).

Examining the electro-neural interface of cochlear implant users using psychophysics, CT scans, and speech understanding

This study examines the relationship between focused-stimulation thresholds, electrode positions, and speech understanding in deaf subjects treated with a cochlear implant (CI). Focused stimulation is more selective than monopolar stimulation, which excites broad regions of the cochlea, so may be more sensitive as a probe of neural survival patterns. Focused thresholds are on average higher and more variable across electrodes than monopolar thresholds. We presume that relatively high focused thresholds are the result of larger distances between the electrodes and the neurons. Two factors are likely to contribute to this distance: (1) the physical position of electrodes relative to the modiolus, where the excitable auditory neurons are normally located, and (2) the pattern of neural survival along the length of the cochlea, since local holes in the neural population will increase the distance between an electrode and the nearest neurons. Electrode-to-modiolus distance was measured from high-resolution CT scans of the cochleae of CI users whose focused-stimulation thresholds were also measured. A hierarchical set of linear models of electrode-to-modiolus distance versus threshold showed a significant increase in threshold with electrode-to-modiolus distance (average slope = 11 dB/mm). The residual of these models was hypothesized to reflect neural survival in each subject. Consonant-Nucleus-Consonant (CNC) word scores were significantly correlated with the within-subject variance of threshold (r(2) = 0.82), but not with within-subject variance of electrode distance (r(2) = 0.03). Speech understanding also significantly correlated with how well distance explained each subject's threshold data (r(2) = 0.63). That is, subjects with focused thresholds that were well described by electrode position had better speech scores. Our results suggest that speech understanding is highly impacted by individual patterns of neural survival and that these patterns manifest themselves in how well (or poorly) electrode position predicts focused thresholds.