Challenges for the application of optical stimulation in the cochlea for the study and treatment of hearing loss

ABSTRACT

Introduction: Electrical stimulation has long been the most effective strategy for evoking neural activity from bionic devices and has been used with great success in the cochlear implant to allow deaf people to hear speech and sound. Despite its success, the spread of electrical current stimulates a broad region of neural tissue meaning that contemporary devices have limited precision. Optical stimulation as an alternative has attracted much recent interest for its capacity to provide highly focused stimuli, and therefore, potentially improved sensory perception. Given its specificity of activation, optical stimulation may also provide a useful tool in the study of fundamental neuroanatomy and neurophysiological processes.

Areas covered: This review examines the advances in optical stimulation – infrared, nanoparticle-enhanced, and optogenetic-based – and its application in the inner ear for the restoration of auditory function following hearing loss.

Expert opinion: Initial outcomes suggest that optogenetic-based approaches hold the greatest potential and viability amongst optical techniques for application in the cochlea. The future success of this approach will be governed by advances in the targeted delivery of opsins to auditory neurons, improvements in channel kinetics, development of optical arrays, and innovation of opsins that activate within the optimal near-infrared therapeutic window.

KEYWORDS:

• Optogenetics
• hearing loss
• cochlea
• optical stimulation
• gene therapy

Article highlights

• Optogenetics enables optical stimulation of auditory neurons at lower energy levels than infrared neural stimulation or nanoparticle assisted near infrared stimulation and does not rely on potentially damaging thermal gradients

• Optogenetic neural stimulation relies on genetic modification of the target tissue with light sensitive ion channels known as opsins

• Optical stimulation has the potential to improve the precision of neural activation compared to electrical stimulation

• Long-term, cell specific expression of opsins in auditory neurons is feasible with viral gene transfer technologies

• Challenges include improving the efficiency and extent of gene transfer, development of opsins with fast kinetics and engineering light delivery technology to meet the target of improving the number of independent stimulating channels of a cochlear implant

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Declaration of interest

A Thompson is supported by the Photobiology Trust. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Additional information

Funding

The authors are supported by the National Health & Medical Research Council [project grants 1024350 and 1064375], Action on Hearing Loss [flexi grant F63], Melbourne Research Grant Support Scheme, and a Senior Wagstaff Fellowship in Otolaryngology (Royal Victorian Eye & Ear Hospital). The Bionics Institute acknowledges the support it receives from the Victorian Government through its Operational Infrastructure Support Program. A Thompson is supported by the Photobiology Trust.