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.
Optogenetic control of neural activity enables innovative approaches to improve functional restoration of diseased sensory and motor systems. For clinical translation to succeed, optogenetic ...stimulation needs to closely match the coding properties of the targeted neuronal population and employ optimally operating emitters. This requires the customization of channelrhodopsins, emitters and coding strategies. Here, we provide a framework to parametrize optogenetic neural control and apply it to the auditory pathway that requires high temporal fidelity of stimulation. We used a viral gene transfer of ultrafast targeting-optimized Chronos into spiral ganglion neurons (SGNs) of the cochlea of mice. We characterized the light-evoked response by in vivo recordings from individual SGNs and neurons of the anteroventral cochlear nucleus (AVCN) that detect coincident SGN inputs. Our recordings from single SGNs demonstrated that their spike probability can be graded by adjusting the duration of light pulses at constant intensity, which optimally serves efficient laser diode operation. We identified an effective pulse width of 1.6 ms to maximize encoding in SGNs at the maximal light intensity employed here (∼35 mW). Alternatively, SGNs were activated at lower energy thresholds using short light pulses (<1 ms). An upper boundary of optical stimulation rates was identified at 316 Hz, inducing a robust spike rate adaptation that required a few tens of milliseconds to recover. We developed a semi-stochastic stimulation paradigm to rapidly (within minutes) estimate the input/output function from light to SGN firing and approximate the time constant of neuronal integration in the AVCN. By that, our data pave the way to design the sound coding strategies of future optical cochlear implants.
•We demonstrate that spike probability of the spiral ganglion neurons (SGN) and anteroventral cochlear nucleus neurons can be gradually “dialed” by adjusting the width of light pulses like what can be achieved with acoustic clicks of different sound pressure levels. Varying duration rather than light intensity is an important objective as laser diodes operate most efficiently with large driver currents and short pulses.•Under our experimental condition (light intensity ∼35 mW), we identified an optimal pulse width of 1.6 ms for maximizing information coding in SGNs.•The lowest energy requirement to activate SGNs was achieved using short light pulses (<1 ms).•We identified a spike failure induced during high rate optical stimulation (>300 Hz) that likely was due to depolarization block and required a few tens of milliseconds to recover. This is another important constraint that future optogenetic coding strategies need to consider.•Our data reveal that the timing of optogenetically evoked SGN spikes varied little across iterations for a given neuron but substantially within an SGN population, reminiscent of what is observed acoustically. This insight indicates that optogenetic cochlear implants will likely overcome the problem of overly synchronized activity in the auditory nerve faced by current state of the art electrical cochlear implants.•We established a new framework to rapidly determine the input/output function from light to firing of the spiral ganglion neurons using a semi-stochastic stimulus. This stimulus allowed us to approximate the time constant of neuronal integration in the neurons of the anteroventral cochlear nucleus, which recode the firing transmitted from the spiral ganglion neurons.
Optogenetic tools, providing non‐invasive control over selected cells, have the potential to revolutionize sensory prostheses for humans. Optogenetic stimulation of spiral ganglion neurons (SGNs) in ...the ear provides a future alternative to electrical stimulation used in cochlear implants. However, most channelrhodopsins do not support the high temporal fidelity pertinent to auditory coding because they require milliseconds to close after light‐off. Here, we biophysically characterized the fast channelrhodopsin Chronos and revealed a deactivation time constant of less than a millisecond at body temperature. In order to enhance neural expression, we improved its trafficking to the plasma membrane (Chronos‐ES/TS). Following efficient transduction of SGNs using early postnatal injection of the adeno‐associated virus AAV‐PHP.B into the mouse cochlea, fiber‐based optical stimulation elicited optical auditory brainstem responses (oABR) with minimal latencies of 1 ms, thresholds of 5 μJ and 100 μs per pulse, and sizable amplitudes even at 1,000 Hz of stimulation. Recordings from single SGNs demonstrated good temporal precision of light‐evoked spiking. In conclusion, efficient virus‐mediated expression of targeting‐optimized Chronos‐ES/TS achieves ultrafast optogenetic control of neurons.
Synopsis
Here we biophysically characterized and molecularly improved the fast gating blue‐light activated channelrhodopsin Chronos. Employing the potent viral vector AAV‐PHP.B we postnatally expressed the improved Chronos in cochlear neurons and achieved ultrafast neural control.
Biophysical characterization of the fast channelrhodopsin Chronos revealed a deactivation time constant of less than a millisecond at body temperature.
Molecular engineering of Chronos via adding trafficking sequences enhanced plasma membrane abundance of the opsin and use of postnatal AAV‐PHP.B carrying Chronos into the mouse cochlea enabled efficient expression in spiral ganglion neurons.
Chronos enabled synchronized optically driven firing in spiral ganglion neurons for stimulation rates of up to hundreds of Hz as required for future optical cochlear implants.
Biophysical characterization and molecular engineering of the fast channelrhodopsin Chronos results in a new optogentics tool for controlling fast neural circuitries with high temporal fidelity.
Auditory nerve fibers (ANFs) transmit acoustic information from the sensory hair cells to the cochlear nuclei. In experimental and clinical audiology, probing the whole ANF population remains a ...difficult task, as the ANFs differ greatly in their threshold and onset response to sound. Thus, low spontaneous rate (SR) fibers, which have rather higher thresholds, delay and larger jitter in their first spike latency are not detectable in the far-field compound action potential of the auditory nerve. Here, we developed a new protocol of acoustic stimulation together with electrophysiological signal processing to track the steady state activity of ANFs. Mass potentials at the round window were recorded in response to repetitive 300-ms bursts of 1/3 octave band noise centered on a frequency probe. Analysis was assessed during the last 200-ms of the response to capture the steady-state response of ANFs. To eliminate the microphonic component reflecting the sensory cells activity, repetitive pairs of sounds of opposite polarities were used. The spectral analysis was calculated on the average of two consecutive responses, and the neural gain was calculated by dividing point-by-point the spectrum to sound over unstimulated condition. In response to low-sound-level stimulation, neural gain predominated in the low-frequency cochlear regions, while a second component of responses centered on higher cochlear frequency regions appeared beyond 30 dB SPL. At 60 dB SPL, neural gain showed a bimodal shape, with a notch near 5.6 kHz. In addition to correlate with the functional mapping of ANFs along the tonotopic axis, the deletion of low-SR fibers leads to a reduction in the high-frequency response, where the low-SR fibers are preferentially located. Thus, mass potentials at the round window may provide a useful tool to probe the SR-based distribution of ANFs in humans and in other species in which direct single-unit recordings are difficult to achieve or not feasible.
Optogenetic stimulation of spiral ganglion neurons (SGNs) in the ear provides a future alternative to electrical stimulation used in current cochlear implants. Here, we employed fast and very fast ...variants of the red‐light‐activated channelrhodopsin (ChR) Chrimson (f‐Chrimson and vf‐Chrimson) to study their utility for optogenetic stimulation of SGNs in mice. The light requirements were higher for vf‐Chrimson than for f‐Chrimson, even when optimizing membrane expression of vf‐Chrimson by adding potassium channel trafficking sequences. Optogenetic time and intensity coding by single putative SGNs were compared with coding of acoustic clicks. vf‐Chrimson enabled putative SGNs to fire at near‐physiological rates with good temporal precision up to 250 Hz of stimulation. The dynamic range of SGN spike rate coding upon optogenetic stimulation was narrower than for acoustic clicks but larger than reported for electrical stimulation. The dynamic range of spike timing, on the other hand, was more comparable for optogenetic and acoustic stimulation. In conclusion, f‐Chrimson and vf‐Chrimson are promising candidates for optogenetic stimulation of SGNs in auditory research and future cochlear implants.
Synopsis
Identifying suitable channelrhodopsins is crucial for future optogenetic restoration of sound encoding by optical cochlear implants. Here, fast and very fast light‐activated Chrimsons were compared for their utility to optogenetically encode timing and intensity information in the auditory nerve.
Very fast Chrimson increases temporal fidelity but confers lower light sensitivity of optogenetic auditory nerve fiber stimulation compared with fast Chrimson.
Adding trafficking sequences of the inwardly rectifying potassium channel 2.1 improved plasma membrane expression of very fast Chrimson enabling shorter stimulus durations
The dynamic range, based on the discharge rate, of optogenetic auditory nerve fiber stimulation was narrower than that of acoustic stimulation.
The dynamic range, based on temporal precision of spiking, of optogenetic auditory nerve fiber stimulation was broader than that based on discharge rate.
Identifying suitable channelrhodopsins is crucial for future optogenetic restoration of sound encoding by optical cochlear implants. Here, fast and very fast light‐activated Chrimson were compared for their utility to optogenetically encode timing and intensity information in the auditory nerve.
Optogenetic stimulation of type I spiral ganglion neurons (SGNs) promises an alternative to the electrical stimulation by current cochlear implants (CIs) for improved hearing restoration by future ...optical CIs (oCIs). Most of the efforts in using optogenetic stimulation in the cochlea so far used early postnatal injection of viral vectors carrying blue-light activated channelrhodopsins (ChRs) into the cochlea of mice. However, preparing clinical translation of the oCI requires (
) reliable and safe transduction of mature SGNs of further species and (
) use of long-wavelength light to avoid phototoxicity. Here, we employed a fast variant of the red-light activated channelrhodopsin Chrimson (f-Chrimson) and different AAV variants to implement optogenetic SGN stimulation in Mongolian gerbils. We compared early postnatal (p8) and adult (>8 weeks) AAV administration, employing different protocols for injection of AAV-PHP.B and AAV2/6 into the adult cochlea. Success of the optogenetic manipulation was analyzed by optically evoked auditory brainstem response (oABR) and immunohistochemistry of mid-modiolar cryosections of the cochlea. In order to most efficiently evaluate the immunohistochemical results a semi-automatic procedure to identify transduced cells in confocal images was developed. Our results indicate that the rate of SGN transduction is significantly lower for AAV administration into the adult cochlea compared to early postnatal injection. SGN transduction upon AAV administration into the adult cochlea was largely independent of the chosen viral vector and injection approach. The higher the rate of SGN transduction, the lower were oABR thresholds and the larger were oABR amplitudes. Our results highlight the need to optimize viral vectors and virus administration for efficient optogenetic manipulation of SGNs in the adult cochlea for successful clinical translation of SGN-targeting gene therapy and of the oCI.
In the last 15 years, optogenetics has revolutionized the life sciences and enabled studies of complex biological systems such as the brain. Applying optogenetics also has great potential for ...restorative medicine, such as hearing restoration, by stimulating genetically modified spiral ganglion neurons of the cochlea with light. To this end, opsins with short closing kinetics are required, given the high firing rates and utmost temporal precision of spiking in these neurons. Chronos is the fastest native blue channelrhodopsin (ChR) reported so far with a closing kinetics bellow 1 ms at body temperature and an interesting candidate for the development of the future optogenetic cochlear implants. This book chapter explains in more details the development and application of Chronos with optimized membrane targeting for temporally precise optical stimulation of spiral ganglion neurons. In addition, the generation of adeno-associated virus (AAV) and AAV delivery to the cochlea of postnatal mice and the procedure to record optically evoked auditory brainstem responses are described.
Sound-evoked compound action potential (CAP), which captures the synchronous activation of the auditory nerve fibers (ANFs), is commonly used to probe deafness in experimental and clinical settings. ...All ANFs are believed to contribute to CAP threshold and amplitude: low sound pressure levels activate the high-spontaneous rate (SR) fibers, and increasing levels gradually recruit medium- and then low-SR fibers. In this study, we quantitatively analyze the contribution of the ANFs to CAP 6 days after 30-min infusion of ouabain into the round window niche. Anatomic examination showed a progressive ablation of ANFs following increasing concentration of ouabain. CAP amplitude and threshold plotted against loss of ANFs revealed three ANF pools: 1) a highly ouabain-sensitive pool, which does not participate in either CAP threshold or amplitude, 2) a less sensitive pool, which only encoded CAP amplitude, and 3) a ouabain-resistant pool, required for CAP threshold and amplitude. Remarkably, distribution of the three pools was similar to the SR-based ANF distribution (low-, medium-, and high-SR fibers), suggesting that the low-SR fiber loss leaves the CAP unaffected. Single-unit recordings from the auditory nerve confirmed this hypothesis and further showed that it is due to the delayed and broad first spike latency distribution of low-SR fibers. In addition to unraveling the neural mechanisms that encode CAP, our computational simulation of an assembly of guinea pig ANFs generalizes and extends our experimental findings to different species of mammals. Altogether, our data demonstrate that substantial ANF loss can coexist with normal hearing threshold and even unchanged CAP amplitude.
Artificial control of neuronal activity enables the study of neural circuits and restoration of neural functions. Direct, rapid, and sustained photocontrol of intact neurons could overcome the ...limitations of established electrical stimulation such as poor selectivity. We have developed fast photoswitchable ligands of glutamate receptors (GluRs) to enable neuronal control in the auditory system. The new photoswitchable ligands induced photocurrents in untransfected neurons upon covalently tethering to endogenous GluRs and activating them reversibly with visible light pulses of a few milliseconds. As a proof of concept of these molecular prostheses, we applied them to the ultrafast synapses of auditory neurons of the cochlea that encode sound and provide auditory input to the brain. This drug-based method afforded the optical stimulation of auditory neurons of adult gerbils at hundreds of hertz without genetic manipulation that would be required for their optogenetic control. This indicates that the new photoswitchable ligands are also applicable to the spatiotemporal control of fast spiking interneurons in the brain.
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