Identification of Characters and Localization of Images Using Direct Multiple-Electrode Stimulation With a Suprachoroidal Retinal Prosthesis.

Purpose: Retinal prostheses provide vision to blind patients by eliciting phosphenes through electrical stimulation. This study explored whether character identification and image localization could be achieved through direct multiple-electrode stimulation with a suprachoroidal retinal prosthesis. Methods: Two of three retinitis pigmentosa patients implanted with a suprachoroidal electrode array were tested on three psychophysical tasks. Electrode patterns were stimulated to elicit perception of simple characters, following which percept localization was tested using either static or dynamic images. Eye tracking was used to assess the association between accuracy and eye movements. Results: In the character identification task, accuracy ranged from 2.7% to 93.3%, depending on the patient and character. In the static image localization task, accuracy decreased from near perfect to <20% with decreasing contrast (patient 1). Patient 2 scored up to 70% at 100% contrast. In the dynamic image localization task, patient 1 recognized the trajectory of the image up to speeds of 64 deg/s, whereas patient 2 scored just above chance. The degree of eye movement in both patients was related to accuracy and, to some extent, stimulus direction. Conclusions: The ability to identify characters and localize percepts demonstrates the capacity of the suprachoroidal device to provide meaningful information to blind patients. The variation in scores across all tasks highlights the importance of using spatial cues from phosphenes, which becomes more difficult at low contrast. The use of spatial information from multiple electrodes and eye-movement compensation is expected to improve performance outcomes during real-world prosthesis use in a camera-based system. (ClinicalTrials.gov number, NCT01603576.).

Time-dependent activity of primary auditory neurons in the presence of neurotrophins and antibiotics.

In vitro cultures provide a valuable tool in studies examining the survival, morphology and function of cells in the auditory system. Primary cultures of primary auditory neurons have most notably provided critical insights into the role of neurotrophins in cell survival and morphology. Functional studies have also utilized in vitro models to study neuronal physiology and the ion channels that dictate these patterns of activity. Here we examine what influence time-in-culture has on the activity of primary auditory neurons, and how this affects our interpretation of neurotrophin and antibiotic-mediated effects in this population. Using dissociated cell culture we analyzed whole-cell patch-clamp recordings of spiral ganglion neurons grown in the presence or absence of neurotrophins and/or penicillin and streptomycin for 1-3 days in vitro. Firing threshold decreased, and both action potential number and latency increased over time regardless of treatment, whilst input resistance was lowest where neurotrophins were present. Differences in firing properties were seen with neurotrophin concentration but were not consistently maintained over the 3 days in vitro. The exclusion of antibiotics from culture media influenced most firing properties at 1 day in vitro in both untreated and neurotrophin-treated conditions. The only difference still present at 3 days was an increase in input resistance in neurotrophin-treated neurons. These results highlight the potential of neurotrophins and antibiotics to influence neural firing patterns in vitro in a time-dependent manner, and advise the careful consideration of their impact on SGN function in future studies.

The Appearance of Phosphenes Elicited Using a Suprachoroidal Retinal Prosthesis.

Purpose: Phosphenes are the fundamental building blocks for presenting meaningful visual information to the visually impaired using a bionic eye device. The aim of this study was to characterize the size, shape, and location of phosphenes elicited using a suprachoroidal retinal prosthesis. Methods: Three patients with profound vision loss due to retinitis pigmentosa were implanted with a suprachoroidal electrode array, which was used to deliver charge-balanced biphasic constant-current pulses at various rates, amplitudes, and durations to produce phosphenes. Tasks assessing phosphene appearance, location, overlap, and the patients' ability to recognize phosphenes were performed using a custom psychophysics setup. Results: Phosphenes were reliably elicited in all three patients, with marked differences in the reported appearances between patients and between electrodes. Phosphene shapes ranged from simple blobs to complex forms with multiple components in both space and time. Phosphene locations within the visual field generally corresponded to the retinotopic position of the stimulating electrodes. Overlap between phosphenes elicited from adjacent electrodes was observed with one patient, which reduced with increasing electrode separation. In a randomized recognition task, two patients correctly identified the electrode being stimulated for 57.2% and 23% of trials, respectively. Conclusions: Phosphenes of varying complexity were successfully elicited in all three patients, indicating that the suprachoroidal space is an efficacious site for electrically stimulating the retina. The recognition scores obtained with two patients suggest that a suprachoroidal implant can elicit phosphenes containing unique information. This information may be useful when combining phosphenes into more complex and meaningful images that provide functional vision.

Firing frequency and entrainment maintained in primary auditory neurons in the presence of combined BDNF and NT3.

Primary auditory neurons rely on neurotrophic factors for development and survival. We previously determined that exposure to brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT3) alters the activity of hyperpolarization-activated currents (Ih) in this neuronal population. Since potassium channels are sensitive to neurotrophins, and changes in Ih are often accompanied by a shift in voltage-gated potassium currents (IK), this study examined IK with exposure to both BDNF and NT3 and the impact on firing entrainment during high frequency pulse trains. Whole-cell patch-clamp recordings revealed significant changes in action potential latency and duration, but no change in firing adaptation or total outward IK. Dendrotoxin-I (DTX-I), targeting voltage-gated potassium channel subunits KV1.1 and KV1.2, uncovered an increase in the contribution of DTX-I sensitive currents with exposure to neurotrophins. No difference in Phrixotoxin-1 (PaTX-1) sensitive currents, mediated by KV4.2 and KV4.3 subunits, was observed. Further, no difference was seen in firing entrainment. These results show that combined BDNF and NT3 exposure influences the contribution of KV1.1 and KV1.2 to the low voltage-activated potassium current (IKL). Whilst this is accompanied by a shift in spike latency and duration, both firing frequency and entrainment to high frequency pulse trains are preserved.

Cell-based neurotrophin treatment supports long-term auditory neuron survival in the deaf guinea pig.

The cochlear implant provides auditory cues to profoundly deaf patients by electrically stimulating the primary auditory neurons (ANs) of the cochlea. However, ANs degenerate in deafness; the preservation of a robust AN target population, in combination with advances in cochlear implant technology, may provide improved hearing outcomes for cochlear implant patients. The exogenous delivery of neurotrophins such as brain-derived neurotrophic factor (BDNF) and neurotrophin-3 is well known to support AN survival in deafness, and cell-based therapies provide a potential clinically viable option for delivering neurotrophins into the deaf cochlea. This study utilized cells that were genetically modified to express BDNF and encapsulated in alginate microspheres, and investigated AN survival in the deaf guinea pig following (a) cell-based neurotrophin treatment in conjunction with chronic electrical stimulation from a cochlear implant, and (b) long-term cell-based neurotrophin delivery. In comparison to deafened controls, there was significantly greater AN survival following the cell-based neurotrophin treatment, and there were ongoing survival effects for at least six months. In addition, functional benefits were observed following cell-based neurotrophin treatment and chronic electrical stimulation, with a statistically significant decrease in electrically evoked auditory brainstem response thresholds observed during the experimental period. This study demonstrates that cell-based therapies, in conjunction with a cochlear implant, shows potential as a clinically transferable means of providing neurotrophin treatment to support AN survival in deafness. This technology also has the potential to deliver other therapeutic agents, and to be used in conjunction with other biomedical devices for the treatment of a variety of neurodegenerative conditions.

Treating hearing disorders with cell and gene therapy.

Hearing loss is an increasing problem for a substantial number of people and, with an aging population, the incidence and severity of hearing loss will become more significant over time. There are very few therapies currently available to treat hearing loss, and so the development of new therapeutic strategies for hearing impaired individuals is of paramount importance to address this unmet clinical need. Most forms of hearing loss are progressive in nature and therefore an opportunity exists to develop novel therapeutic approaches to slow or halt hearing loss progression, or even repair or replace lost hearing function. Numerous emerging technologies have potential as therapeutic options. This paper details the potential of cell- and gene-based therapies to provide therapeutic agents to protect sensory and neural cells from various insults known to cause hearing loss; explores the potential of replacing lost sensory and nerve cells using gene and stem cell therapy; and describes the considerations for clinical translation and the challenges that need to be overcome.

Drug delivery to the inner ear.

Bionic devices electrically activate neural populations to partially restore lost function. Of fundamental importance is the functional integrity of the targeted neurons. However, in many conditions the ongoing pathology can lead to continued neural degeneration and death that may compromise the effectiveness of the device and limit future strategies to improve performance. The use of drugs that can prevent nerve cell degeneration and promote their regeneration may improve clinical outcomes. In this paper we focus on strategies of delivering neuroprotective drugs to the auditory system in a way that is safe and clinically relevant for use in combination with a cochlear implant. The aim of this approach is to prevent neural degeneration and promote nerve regrowth in order to improve outcomes for cochlear implant recipients using techniques that can be translated to the clinic.

Clinical application of neurotrophic factors: the potential for primary auditory neuron protection.

Sensorineural hearing loss, as a result of damage to or destruction of the sensory epithelia within the cochlea, is a common cause of deafness. The subsequent degeneration of the neural elements within the inner ear may impinge upon the efficacy of the cochlear implant. Experimental studies have demonstrated that neurotrophic factors can prevent this degeneration in animal models of deafness, and can even provide functional benefits. Neurotrophic factor therapy may therefore provide similar protective effects in humans, resulting in improved speech perception outcomes among cochlear implant patients. There are, however, numerous issues pertaining to delivery techniques and treatment regimes that need to be addressed prior to any clinical application. This review considers these issues in view of the potential therapeutic application of neurotrophic factors within the auditory system.

Netrin-1 as a guidance molecule in the postnatal rat cochlea.

During synaptogenesis a number of growth factors and peptides control the guidance of auditory neuron (spiral ganglion neuron, SGN) axons to their target cells. Furthermore, evidence suggests that these factors exert their actions at discrete times and sites during development. This study demonstrates that the guidance molecule netrin-1 is expressed in the early postnatal rat cochlea, but shows decreasing expression with increasing age. These results suggest that netrin-1 may be involved in guiding axonal growth from SGNs for the onset of innervation, but is not required for maintenance of synaptic connections.

Delayed neurotrophin treatment supports auditory neuron survival in deaf guinea pigs.

As key factors in the development and maintenance of the auditory system, neurotrophins can prevent auditory neuron degeneration when applied within three to five days of deafening. We tested each of the neurotrophins BDNF, NT-3, NT-4/5 and NGF for their ability to support auditory neuron survival following a two-week period of deafness in guinea pigs, when approximately 15% auditory neuron degeneration has already occurred. Although delayed, the treatment with each neurotrophin prevented further degeneration with similar efficacy.

Regulation of axonal growth and guidance by the neurotrophin family of neurotrophic factors.

1. The neurotrophins play an important role during development to stimulate and guide axonal growth for the establishment of a correctly wired and functional neural system. Neurotrophins can also regulate adult nervous system plasticity by promoting neuronal survival and stimulating nerve regrowth following injury. 2. Therefore, the potential exists for these neurotrophic factors to be used as therapeutic agents for the treatment of neurodegenerative disorders. However, in order to realize the full capacity of neurotrophic factors as therapeutic agents, it is important to understand the mechanisms by which they elicit their survival and regenerative effects. 3. The present paper reviews some of the ways in which neurotrophins regulate axonal growth and guidance.

BDNF-induced survival of auditory neurons in vivo: Cessation of treatment leads to accelerated loss of survival effects.

Neurotrophic factors are important for the development and maintenance of the auditory system. They have also been shown to act as survival factors for auditory neurons in animal deafness models. Studies have demonstrated recently that these neurotrophic factors not only maintain survival of auditory neurons, but that these surviving neurons retain functionality. It remains to be determined, however, if a single administration of a neurotrophic factor is sufficient to maintain auditory neuron survival after loss of hair cells, or if sustained delivery is required. This study investigated the longevity of the survival effects of BDNF on auditory neurons in deafened guinea pigs. Briefly, the left cochleae of deafened guinea pigs were infused with BDNF for 28 days via a mini-osmotic pump, and neuronal survival was analyzed at various stages after the completion of treatment. BDNF treatment prevented the degeneration of auditory neurons that normally is seen after a loss of hair cells, supporting previous studies. Our results indicate, however, that cessation of BDNF treatment leads to an accelerated decline in auditory neuron survival as compared to that observed in deafened, untreated cochleae. These findings indicate that much work remains to be done to establish a technique for the long-term survival of auditory neurons in the deaf ear.

Role of trophic factors in the development, survival and repair of primary auditory neurons.

1. Neurotrophic factors have been identified as crucial for the development of the auditory system and have also been proven to be important for continued survival and maintenance of auditory neural connections. 2. In addition, both in vitro and in vivo studies have demonstrated that these trophic molecules can prevent the secondary wave of auditory neuron degeneration normally seen following the loss of hair cells. 3. Furthermore, neurotrophic factors have been reported to enhance neuronal excitation and to improve the efficacy of synaptic transmission. 4. As such, these molecules are strong candidates to be used as therapeutic agents in conjunction with the cochlear implant, or even to repair and/or regenerate damaged or lost auditory nerve and sensory cells.