Response of primary auditory neurons to stimulation with infrared light in vitro.

OBJECTIVE: Infrared light can be used to modulate the activity of neuronal cells through thermally-evoked capacitive currents and thermosensitive ion channel modulation. The infrared power threshold for action potentials has previously been found to be far lower in the in vivo cochlea when compared with other neuronal targets, implicating spiral ganglion neurons (SGNs) as a potential target for infrared auditory prostheses. However, conflicting experimental evidence suggests that this low threshold may arise from an intermediary mechanism other than direct SGN stimulation, potentially involving residual hair cell activity. APPROACH: Patch-clamp recordings from cultured SGNs were used to explicitly quantify the capacitive and ion channel currents in an environment devoid of hair cells. Neurons were irradiated by a 1870 nm laser with pulse durations of 0.2-5.0 ms and powers up to 1.5 W. A Hodgkin-Huxley-type model was established by first characterising the voltage dependent currents, and then incorporating laser-evoked currents separated into temperature-dependent and temperature-gradient-dependent components. This model was found to accurately simulate neuronal responses and allowed the results to be extrapolated to stimulation parameter spaces not accessible during this study. MAIN RESULTS: The previously-reported low in vivo SGN stimulation threshold was not observed, and only subthreshold depolarisation was achieved, even at high light exposures. Extrapolating these results with our Hodgkin-Huxley-type model predicts an action potential threshold which does not deviate significantly from other neuronal types. SIGNIFICANCE: This suggests that the low-threshold response that is commonly reported in vivo may arise from an alternative mechanism, and calls into question the potential usefulness of the effect for auditory prostheses. The step-wise approach to modelling optically-evoked currents described here may prove useful for analysing a wider range of cell types where capacitive currents and conductance modulation are dominant.

Comparison of electrode impedance measures between a dexamethasone-eluting and standard Cochlear™ Contour Advance® electrode in adult cochlear implant recipients.

OBJECTIVE: To compare the difference in electrode impedance across discrete time points to 24 months post-activation for two groups of adult cochlear implant recipients, one using an investigational perimodiolar (Contour Advance®) array augmented with 40% concentration weight per weight (w/w) dexamethasone (the Drug Eluting Electrode, 'DEE' Group), and the other the commercially available Contour Advance ('Control' Group). DESIGN: Ten adult subjects were implanted with the DEE and fourteen with the Control. Electrode impedances were measured intra-operatively, one-week post-surgery, at initial activation (approximately two-weeks post-surgery), and at approximately one, three, six, 12 and 24 months post-activation. Two different impedance measurements were obtained: 1) in MP1+2 mode using Custom Sound programming software; and 2) 4-point impedance measures utilising BP+2 stimulation mode with recording on non-stimulating electrodes. Data were analysed with respect to both impedance averaged across all electrodes, and impedance for electrodes grouped into basal, middle and apical sections. RESULTS: Group mean MP1+2 impedance for the DEE was significantly lower than for the Control at all post-operative time points examined, and for each of the basal, middle and apical cochlear regions. Group mean 4-point impedance was significantly lower for the DEE than the Control in the basal region at six, 12 and 24 months post-activation and in the middle region at 12- and 24-months post-activation. The pattern of change in MP1+2 impedance differed significantly in the early post-operative period prior to device activation. A significant 4.8 kOhm reduction in impedance between surgery and one-week was observed for the DEE group but not for the Control. A 2.0 kOhm increase between the one and two week post-operative time points was observed for the Control but not for the DEE group. CONCLUSION: While rates of adoption of different surgical approaches differed between the groups and this may have had a confounding effect, the results suggest that passive elution of dexamethasone from the investigational device was associated with a change in the intracochlear environment following surgical implantation of the electrode array, as evidenced by the lower electrode impedance measures.

Protecting against electrode insertion trauma using dexamethasone.

OBJECTIVE: To compare the benefits of a dexamethasone-eluting array for hearing preservation and cochlear histopathology in low trauma (soft-surgery) and high trauma models of cochlear implant surgery. METHODS: Adult guinea pigs were implanted with an intra-cochlear array using two different surgical procedures: either a soft-surgery approach or following generation of electrode insertion trauma (high trauma). Two methods of dexamethasone delivery were evaluated: elution from an electrode array alone, and elution from a cochlear implant electrode array in combination with a pre-operative systemic injection. All electrode arrays were implanted for a period of 4 weeks. Outcome measures at 4 weeks post-implantation included auditory brainstem response (ABR) thresholds, histological analysis of spiral ganglion neuron density, fibrotic tissue, new bone growth, and cochlear damage. RESULTS: Animals exposed to high surgical trauma showed greater hearing loss than those in the low trauma model, irrespective of the presence of dexamethasone. Whilst the area of intra-cochlear fibrotic tissue growth post-implantation was also independent of dexamethasone administration, new bone growth was significantly reduced in its presence. Our high trauma model effectively obliterated the organ of Corti and significantly reduced spiral ganglion neuron densities in the lower basal turn. This trauma-induced reduction in spiral ganglion neuron survival decreased with the inclusion of a dexamethasone-eluting array. A pre-operative systemic injection of dexamethasone did not significantly improve any outcome measures beyond those provided with a dexamethasone-eluting array alone. CONCLUSION: Dexamethasone-eluting intra-cochlear arrays may inhibit osteoneogenesis, and reduce spiral ganglion neuron loss following traumatic cochlear implantation.

Electropermeabilization of adherent cells with cochlear implant electrical stimulation in vitro.

Cochlear implant stimulation creates a reduction in electrode impedance that returns to pre-stimulation levels following cessation of stimulation and is presumed to be associated with the fibrous tissue covering over the electrode array. This study assessed the possibility that transitory impedance reduction originates from a change in the membrane permeability of cells on the electrode (electropermeabilization). These changes can be recorded using the dye propidium iodide, which fluoresces upon entry into the leaky cell. The in vitro model used showed impedance reduction and dye uptake into adherent cells overlying planar gold electrodes stimulated with as little as 5 min of clinically relevant cochlear implant stimulation. The delayed additions of propidium iodide showed a similar dye uptake to those groups with concurrent dye addition, suggesting the electropermeabilization was not reversible. Further understanding of the mechanisms behind these impedance and cell permeability changes with cochlear implant electrical stimulation may provide opportunities for creating long-lasting reductions in electrode impedance.

Directing human induced pluripotent stem cells into a neurosensory lineage for auditory neuron replacement.

Emerging therapies for sensorineural hearing loss include replacing damaged auditory neurons (ANs) using stem cells. Ultimately, it is important that these replacement cells can be patient-matched to avoid immunorejection. As human induced pluripotent stem cells (hiPSCs) can be obtained directly from the patient, they offer an opportunity to generate patient-matched neurons for transplantation. Here, we used an established neural induction protocol to differentiate two hiPSC lines (iPS1 and iPS2) and one human embryonic stem cell line (hESC; H9) toward a neurosensory lineage in vitro. Immunocytochemistry and qRT-PCR were used to analyze the expression of key markers involved in AN development at defined time points of differentiation. The hiPSC- and hESC-derived neurosensory progenitors expressed the dorsal hindbrain marker (PAX7), otic placodal marker (PAX2), proneurosensory marker (SOX2), ganglion neuronal markers (NEUROD1, BRN3A, ISLET1, ßIII-tubulin, Neurofilament kDa 160), and sensory AN markers (GATA3 and VGLUT1) over the time course examined. The hiPSC- and hESC-derived neurosensory progenitors had the highest expression levels of the sensory neural markers at 35 days in vitro. Furthermore, the neurons generated from this assay were found to be electrically active. While all cell lines analyzed produced functional neurosensory-like progenitors, variabilities in the levels of marker expression were observed between hiPSC lines and within samples of the same cell line, when compared with the hESC controls. Overall, these findings indicate that this neural assay was capable of differentiating hiPSCs toward a neurosensory lineage but emphasize the need for improving the consistency in the differentiation of hiPSCs into the required lineages.

Gold-nanorod-assisted near-infrared stimulation of primary auditory neurons.

Infrared stimulation offers an alternative to electrical stimulation of neuronal tissue, with potential for direct, non-contact activation at high spatial resolution. Conventional methods of infrared neural stimulation (INS) rely on transient heating due to the absorption of relatively intense laser beams by water in the tissue. However, the water absorption also limits the depth of penetration of light in tissue. Therefore, the use of a near-infrared laser at 780 nm to stimulate cultured rat primary auditory neurons that are incubated with silica-coated gold nanorods (Au NRs) as an extrinsic absorber is investigated. The laser-induced electrical behavior of the neurons is observed using whole-cell patch clamp electrophysiology. The nanorod-treated auditory neurons (NR-ANs) show a significant increase in electrical activity compared with neurons that are incubated with non-absorbing silica-coated gold nanospheres and control neurons with no gold nanoparticles. The laser-induced heating by the nanorods is confirmed by measuring the transient temperature increase near the surface of the NR-ANs with an open pipette electrode. These findings demonstrate the potential to improve the efficiency and increase the penetration depth of INS by labeling nerves with Au NRs and then exposing them to infrared wavelengths in the water window of tissue.

Challenges for stem cells to functionally repair the damaged auditory nerve.

INTRODUCTION: In the auditory system, a specialized subset of sensory neurons are responsible for correctly relaying precise pitch and temporal cues to the brain. In individuals with severe-to-profound sensorineural hearing impairment these sensory auditory neurons can be directly stimulated by a cochlear implant, which restores sound input to the brainstem after the loss of hair cells. This neural prosthesis therefore depends on a residual population of functional neurons in order to function effectively. AREAS COVERED: In severe cases of sensorineural hearing loss where the numbers of auditory neurons are significantly depleted, the benefits derived from a cochlear implant may be minimal. One way in which to restore function to the auditory nerve is to replace these lost neurons using differentiated stem cells, thus re-establishing the neural circuit required for cochlear implant function. Such a therapy relies on producing an appropriate population of electrophysiologically functional neurons from stem cells, and on these cells integrating and reconnecting in an appropriate manner in the deaf cochlea. EXPERT OPINION: Here we review progress in the field to date, including some of the key functional features that stem cell-derived neurons would need to possess and how these might be enhanced using electrical stimulation from a cochlear implant.