Optogenetic characterization of targeting-optimized Chronos and f-Chrimson variants for auditory encoding in mice

by Artur Johann Georg Mittring
Cumulative thesis
Date of Examination:2025-01-23
Date of issue:2024-12-03
Advisor:Prof. Dr. Tobias Moser
Referee:Prof. Dr. Tobias Moser
Referee:Prof. Dr. Silvio O. Rizzoli
Referee:Prof. Dr. Tobias Brügmann
Sponsor:Hertha Sponer College of the Multiscale Bioimaging Cluster of Excellence Göttingen
Sponsor:Else-Kröner-Fresenius-Promotionskolleg für Medizinstudierende Göttingen
Persistent Address: http://dx.doi.org/10.53846/goediss-10900

 

 

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Abstract

English

Electrical cochlear implants bypass the physiological micromechanics and synaptic sound encoding by sensory hair cells in the cochlea and provide state-of-the-art treatment of sensorineural hearing loss. To overcome the bottlenecks of this neuro-prosthesis (e.g. limited frequency discrimination due to spread of electrical excitation, limited intensity resolution due to low dynamic ranges, overly synchronized activation of the auditory nerve), the future optical cochlear implant provides an innovative alternative by directly stimulating spiral ganglion neurons with light. For clinical translation to succeed, adequate optogenetic stimulus specifications have to be identified, integrated in proper coding strategies and customized for suitable channelrhodopsins. To those ends, here, the light encoding of targeting-optimized Chronos, f-Chrimson, vf-Chrimson and targeting-optimized vf-Chrimson has been characterized after performing gene-therapy on mice by injecting virus-mediated cell-type specific vectors. In vivo juxtacellular single cell recordings from spiral ganglion neurons and anteroventral cochlear nucleus neurons were conducted employing a deterministic and a newly introduced semi-stochastic stimulation paradigm to rapidly arrive at the input-output function from stimulation to neural firing. By varying the light pulse duration, gradual activation of the first and second order neuron of the auditory pathway could be achieved in a similar manner as achieved with different intensities of acoustic clicks at constant duration. Maximum light encoding for targeting-optimized Chronos was observed at 1.6 ms as this light pulse duration showed the highest percentage of responsive neurons, the highest spike rate, a significantly lower adaptation ratio (at maximal light intensity, ~35 mW) and the largest interval of encoded intensities at 100 pps. Meeting the requirements for energy-saving optical medical devices operating with brief light pulses, the energy threshold for spiral ganglion neurons transduced with targeting-optimized Chronos decreased with lower pulse durations. Without reaching the range of encodable intensities achieved with acoustic stimulation, the single cell dynamic range of spiral ganglion neurons expressing blue-shifted targeting-optimized Chronos appeared to be higher than that mediated by the tested red-shifted channelrhodopsins except f-Chrimson. For low stimulation rates (≤ 100 pps), the temporal precision of light-induced firing was higher for neurons expressing targeting-optimized Chronos than for acoustic stimulation at the level of single cells. When considering an artificial population of spiral ganglion neurons, however, the temporal precision increased to near-physiological levels. For lower stimulation rates, this overcomes the problem of overly synchronized activation of the auditory nerve in current electrical cochlear implants. At higher repetition rates (> 300 pps), all characterized channelrhodopsins revealed declining spike rates. By assessing the recovery time needed to regain light-induced action potentials in spiral ganglion neurons expressing targeting-optimized Chronos after adaptation, we postulate a neural depolarization block to be responsible for interim spike failure. By that, this work informs the preclinical development and design of coding strategies of the prospective optical cochlear implant.
Keywords: Optogenetics; Optical cochlear implant; Neural network; Neuroprosthetic; Chronos; Auditory nerve; Auditory brainstem; Neural stimulation; Channelrhodopsin; Cochlear implant; Dynamic range; Temporal coding; Spiral ganglion; f-Chrimson; Chrimson; vf-Chrimson; Targeting-optimized