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

Abstract

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.

Auditory nerve responses to combined optogenetic and electrical stimulation in chronically deaf mice

“…Unfortunately, such techniques do not achieve complete transfection of the nerve, leaving some neurons that do not express the opsin and are incapable of optogenetic and hybrid responses. Indeed, a previous study demonstrated that hybrid stimulation did not have any consistent effect on the spatial precision, although subthreshold optogenetic stimulation did reduce the electrical activation threshold [46]. Techniques to improve transfection rates of gene therapy are likely to benefit not only optogenetic stimulation but also hybrid stimulation.…”

Is there an unmet medical need for improved hearing restoration?

“…For example, the combination of electrical and infrared stimulation of the auditory nerve in deaf white cats reduced the required radiant (light) energy (Richter et al , 2014 ). Another study combining electrical and optogenetic stimulation in mice expressing a ChR2 variant showed that sub‐threshold optical stimulation was able to lower the threshold for subsequent electrical stimulation (Richardson et al , 2021 ).…”

Hybrid Optogenetic and Electrical Stimulation of Retinal Ganglion Cells for Artificial Vision

“…As a new frontier of optogenetic based therapies emerges, we envisage a technique such as an opto-electrical hybrid stimulation could contribute to improved delivery of arti cial vision and serve as a pioneer for the intersection between gene therapy and bionics. The e cacy of hybrid stimulation has already been demonstrated in both the cochlea [24][25][26][27] and sciatic nerve 28 . In the cochlea, hybrid stimulation reduced the electrical stimulation current required, thus signi cantly reducing the spread of activation compared to electrical-only stimulation 27 .…”