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

Abstract

Objective. Optogenetic stimulation of the auditory nerve offers the ability to overcome the limitations of cochlear implants through spatially precise stimulation, but cannot achieve the temporal precision nor temporal fidelity required for good hearing outcomes. Auditory midbrain recordings have indicated a combined (hybrid) stimulation approach may permit improvements in the temporal precision without sacrificing spatial precision by facilitating electrical activation thresholds. However, previous research has been conducted in undeafened or acutely deafened animal models, and the impact of chronic deafness remains unclear. Our study aims to compare the temporal precision of auditory nerve responses to optogenetic, electrical, and combined stimulation in acutely and chronically deafened animals. 
Methods. We directly compare the temporal fidelity (measured as percentage of elicited responses) and precision (i.e., stability of response size and timing) of electrical, optogenetic, and hybrid stimulation (varying sub-threshold or supra-threshold optogenetic power levels combined with electrical stimuli) through compound action potential and single-unit recordings of the auditory nerve in transgenic mice expressing the opsin ChR2-H134R in auditory neurons. Recordings were conducted immediately or 2-3 weeks following aminoglycoside deafening when there was evidence of auditory nerve degeneration. 
Main results. Results showed that responses to electrical stimulation had significantly greater temporal precision than optogenetic stimulation (p < 0.001 for measures of response size and timing). This temporal precision could be maintained with hybrid stimulation, but only when the optogenetic stimulation power used was below or near activation threshold and worsened with increasing optical power. Chronically deafened mice showed poorer facilitation of electrical activation thresholds with concurrent optogenetic stimulation than acutely deafened mice. Additionally, responses in chronically deafened mice showed poorer temporal fidelity, but improved temporal precision to optogenetic and hybrid stimulation compared to acutely deafened mice. 
Significance. These findings show that the improvement to temporal fidelity and temporal precision provided by a hybrid stimulation paradigm can also be achieved in chronically deafened animals, albeit at higher levels of concurrent optogenetic stimulation levels.

Combined optogenetic and electrical stimulation of the sciatic nerve for selective control of sensory fibers

“…Optogenetic stimulation rates are limited by opsin kinetics, with the H134R variant used in this study exhibiting a relatively slow closing time of 18 ms that limits the rate at which stimuli can be applied ( Nagel et al, 2005 ). Long-term high rate optogenetic stimulation is less reliable compared to electrical stimulation, and the response size decreases over the duration of the pulse trains, as shown in the auditory system ( Hart et al, 2020 ; Thompson et al, 2020 ; Ajay et al, 2023 ). However, when optogenetic stimulation is combined with electrical stimulation, the reliability of firing increases ( Hart et al, 2020 ; Thompson et al, 2020 ) and can be ‘tuned’ to have response properties that are more like electrical responses or more like optical responses by adjusting the ratio of optical and electrical components of the combined stimulus ( Ajay et al, 2023 ).…”

“…Long-term high rate optogenetic stimulation is less reliable compared to electrical stimulation, and the response size decreases over the duration of the pulse trains, as shown in the auditory system ( Hart et al, 2020 ; Thompson et al, 2020 ; Ajay et al, 2023 ). However, when optogenetic stimulation is combined with electrical stimulation, the reliability of firing increases ( Hart et al, 2020 ; Thompson et al, 2020 ) and can be ‘tuned’ to have response properties that are more like electrical responses or more like optical responses by adjusting the ratio of optical and electrical components of the combined stimulus ( Ajay et al, 2023 ). Reliable control of neuromodulation would be essential to treat a condition such as pain, so keeping the optical stimuli near threshold helps to maintain the responses during repetitive stimulation.…”

“…The potential for thermal damage with long-term high intensity light has been identified in the retina and the brain ( Stujenske et al, 2015 ; Montazeri et al, 2019 ; Owen et al, 2019 ), but has not been well studied in the periphery. Furthermore, neural responses to light are less reliable compared to electrical stimulation ( Hart et al, 2020 ; Thompson et al, 2020 ; Ajay et al, 2023 ). Some of these issues may be resolved with rapid development and discovery of new opsins with improved temporal properties sensitivities ( Yizhar et al, 2011 ), but an alternative solution could be achieved by combining optogenetics with electrical stimulation to generate a hybrid response.…”

“…The greatest influence of combined stimulation on reducing electrical activation thresholds in the hybrid response occurred when the electrical stimulus was delayed relative to the optical stimulus, even when the electrical stimulus occurred after the end of the optical stimulus ( Hart et al, 2020 ; Thompson et al, 2020 ), in line with the closing kinetics of the ChR2-H134R opsin ( Berndt et al, 2011 ). Compared to optical-only stimulation, combined stimulation also increases the reliability of firing with improved phase locking at higher stimulation rates ( Thompson et al, 2020 ; Ajay et al, 2023 ).…”

Impact of opsin kinetics on high-rate stimulation of the auditory nerve in mice

“…Compared to transgenic animals, delity at 100 pps for H134R was signi cantly lower, possibly as a result of the poorer quality and/or quantity of opsin expression in transduced animal models 5 . Studies have shown that improving tra cking to the plasma membrane improves activation thresholds and the reliability of responses at high stimulation rates.…”