High frequency neural spiking and auditory signaling by ultrafast red-shifted optogenetics

Abstract

Optogenetics revolutionizes basic research in neuroscience and cell biology and bears potential for medical applications. We develop mutants leading to a unifying concept for the construction of various channelrhodopsins with fast closing kinetics. Due to different absorption maxima these channelrhodopsins allow fast neural photoactivation over the whole range of the visible spectrum. We focus our functional analysis on the fast-switching, red light-activated Chrimson variants, because red light has lower light scattering and marginal phototoxicity in tissues. We show paradigmatically for neurons of the cerebral cortex and the auditory nerve that the fast Chrimson mutants enable neural stimulation with firing frequencies of several hundred Hz. They drive spiking at high rates and temporal fidelity with low thresholds for stimulus intensity and duration. Optical cochlear implants restore auditory nerve activity in deaf mice. This demonstrates that the mutants facilitate neuroscience research and future medical applications such as hearing restoration.

Ultrafast optogenetic stimulation of the auditory pathway by targeting‐optimized Chronos

“…When increasing stimulus rate, oABR amplitudes declined (Fig G) and latencies prolonged (Fig G and I). However, in contrast to our previous reports on (i) ChR2: where potentials were found only up to 70 Hz (Hernandez et al , ) and (ii) CatCh: up to 200 Hz, and f‐Chrimson: up to 250 Hz (respectively Wrobel et al , ; Mager et al , ), we could detect sizable P 1 ‐N 1 up to stimulus rates of 500 Hz for Chronos (Fig G left, H) and 1,000 Hz for Chronos‐ES/TS (the highest stimulus rate tested in our experiments, Fig G and H). P 1 latency increased with higher stimulus rates in both cases.…”

“…To further validate the Chronos‐ES/TS‐mediated SGN stimulation and evaluate the temporal fidelity of stimulation, we performed juxtacellular recordings from auditory nerve fibers (central axon of SGN) as described in (Hernandez et al , ; Mager et al , ). In brief, we targeted glass micropipettes to where the auditory nerve enters the anteroventral cochlear nucleus (AVCN) and searched for responses while stimulating the SGNs through the round window via an optical fiber‐coupled to a blue laser.…”

“…However, the temporal fidelity of ChR2mediated optogenetic control of SGN firing seemed limited; auditory brainstem responses broke down even below 100 Hz of stimulation. Higher temporal fidelity of optogenetic SGN stimulation might be achieved when using faster ChRs such as Chronos (Klapoetke et al, 2014), the newly engineered Chronos mutant ChroME (Mardinly et al, 2018), or fast Chrimson mutants (Mager et al, 2018).…”

Optimized Chronos sets the clock for optogenetic hearing restoration

“…Limited timing accuracy and fidelity of the “first‐generation” light‐gated actuators has been one of the principal challenges confronting optogenetics (Gunaydin et al , ). Considerable efforts have been invested to tackle this issue, fruitfully resulting in a new generation of opsins exhibiting faster kinetics and largely improved light‐driven neuronal firing fidelity (Klapoetke et al , ; Mager et al , ). The blue‐light activated Chronos, considered the fastest closing ChR (Klapoetke et al , ; Ronzitti et al , ) until the description of the red‐shifted very fast Chrimson (Mager et al , ), therefore seemed to be an obvious candidate for auditory optogenetics.…”

“…Considerable efforts have been invested to tackle this issue, fruitfully resulting in a new generation of opsins exhibiting faster kinetics and largely improved light‐driven neuronal firing fidelity (Klapoetke et al , ; Mager et al , ). The blue‐light activated Chronos, considered the fastest closing ChR (Klapoetke et al , ; Ronzitti et al , ) until the description of the red‐shifted very fast Chrimson (Mager et al , ), therefore seemed to be an obvious candidate for auditory optogenetics. Its utility for fast auditory signaling had already been demonstrated for stimulating neurons of the brainstem and the midbrain (Guo et al , ; Hight et al , ).…”