Hearing the light: neural and perceptual encoding of optogenetic stimulation in the central auditory pathway

Optogenetics provides a means to dissect the organization and function of neural circuits. Optogenetics also offers the translational promise of restoring sensation, enabling movement or supplanting abnormal activity patterns in pathological brain circuits. However, the inherent sluggishness of evoked photocurrents in conventional channelrhodopsins has hampered the development of optoprostheses that adequately mimic the rate and timing of natural spike patterning. Here, we explore the feasibility and limitations of a central auditory optoprosthesis by photoactivating mouse auditory midbrain neurons that either express channelrhodopsin-2 (ChR2) or Chronos, a channelrhodopsin with ultra-fast channel kinetics. Chronos-mediated spike fidelity surpassed ChR2 and natural acoustic stimulation to support a superior code for the detection and discrimination of rapid pulse trains. Interestingly, this midbrain coding advantage did not translate to a perceptual advantage, as behavioral detection of midbrain activation was equivalent with both opsins. Auditory cortex recordings revealed that the precisely synchronized midbrain responses had been converted to a simplified rate code that was indistinguishable between opsins and less robust overall than acoustic stimulation. These findings demonstrate the temporal coding benefits that can be realized with next-generation channelrhodopsins, but also highlight the challenge of inducing variegated patterns of forebrain spiking activity that support adaptive perception and behavior

All-optical electrophysiology in mammalian neurons using engineered microbial rhodopsins

All-optical electrophysiology—spatially resolved simultaneous optical perturbation and measurement of membrane voltage—would open new vistas in neuroscience research. We evolved two archaerhodopsin-based voltage indicators, QuasAr1 and QuasAr2, which show improved brightness and voltage sensitivity, have microsecond response times and produce no photocurrent. We engineered a channelrhodopsin actuator, CheRiff, which shows high light sensitivity and rapid kinetics and is spectrally orthogonal to the QuasArs. A coexpression vector, Optopatch, enabled cross-talk–free genetically targeted all-optical electrophysiology. In cultured rat neurons, we combined Optopatch with patterned optical excitation to probe back-propagating action potentials (APs) in dendritic spines, synaptic transmission, subcellular microsecond-timescale details of AP propagation, and simultaneous firing of many neurons in a network. Optopatch measurements revealed homeostatic tuning of intrinsic excitability in human stem cell–derived neurons. In rat brain slices, Optopatch induced and reported APs and subthreshold events with high signal-to-noise ratios. The Optopatch platform enables high-throughput, spatially resolved electrophysiology without the use of conventional electrodes

Independent two-color optogenetic excitation of neural populations

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Brain and Cognitive Sciences, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (pages 104-106).The optical modulation of neurons with channelrhodopsins, a class of genetically encoded light-gated ion channels, has enabled the spatiotemporally precise interrogation of the roles individual cell types play in neural circuit dynamics. A topic of great interest to the neuroscience community is the independent optical excitation of two distinct neuron populations with different wavelengths, which would enable the interrogation of emergent phenomena such as circuit dynamics, plasticity, and neuromodulation. Previous implementations have focused on maximizing spectral separation by driving one channelrhodopsin in the violet (405 nm) and the other in the yellow (590 nm), yet it has not been possible to achieve independent violet excitation without eliciting spikes from both populations, due to the intrinsic UV-blue light sensitivity of the retinal chromophore. This thesis designs and implements an improved two-color excitation scheme where effective light sensitivity is utilized to achieve independent optical excitation in blue (470 nm) and red (625 nm) channels. Zero post-synaptic crosstalk is demonstrated in acute murine slice, using two novel channeirhodopsins identified from a systematic screen of 80 naturally occurring, previously uncharacterized opsins in primary neuron culture. Gene88 is the first known yellow-peaked channelrhodopsin, with a peak 45 nm more red-shifted than any previous channelrhodopsin, while Gene90 has the fastest channel turn on, turn off, and recovery kinetics of any known channelrhodopsin. These opsins' novel properties enable the first known demonstration of post-synaptic crosstalk-free two-color excitation with temporally precise modulation of spatially inseparable neuron populations.by Nathan Cao Klapoetke.Ph. D

Sub-millisecond optogenetic control of neuronal firing with two-photon holographic photoactivation of Chronos

Optogenetic neuronal network manipulation promises to unravel a long-standing mystery in neuroscience: how does microcircuit activity relate causally to behavioral and pathological states? The challenge to evoke spikes with high spatial and temporal complexity necessitates further joint development of light-delivery approaches and custom opsins. Two-photon (2P) light-targeting strategies demonstrated in-depth generation of action potentials in photosensitive neurons both in vitro and in vivo, but thus far lack the temporal precision necessary to induce precisely timed spiking events. Here, we show that efficient current integration enabled by 2P holographic amplified laser illumination of Chronos, a highly light-sensitive and fast opsin, can evoke spikes with submillisecond precision and repeated firing up to 100 Hz in brain slices from Swiss male mice. These results pave the way for optogenetic manipulation with the spatial and temporal sophistication necessary to mimic natural microcircuit activity.National Institutes of Health (U.S.) (Grant 1-U01-NS090501-01

Synthetic Physiology: Strategies for Adapting Tools from Nature for Genetically Targeted Control of Fast Biological Processes [Chapter 18]

The life and operation of cells involve many physiological processes that take place over fast timescales of milliseconds to minutes. Genetically encoded technologies for driving or suppressing specific fast physiological processes in intact cells, perhaps embedded within intact tissues in living organisms, are critical for the ability to understand how these physiological processes contribute to emergent cellular and organismal functions and behaviors. Such “synthetic physiology” tools are often incredibly complex molecular machines, in part because they must operate at high speeds, without causing side effects. We here explore how synthetic physiology molecules can be identified and deployed in cells, and how the physiology of these molecules in cellular contexts can be assessed and optimized. For concreteness, we discuss these methods in the context of the “optogenetic” light-gated ion channels and pumps that we have developed over the past few years as synthetic physiology tools and widely disseminated for use in neuroscience for probing the role of specific brain cell types in neural computations, behaviors, and pathologies. We anticipate that some of the insights revealed here may be of general value for the field of synthetic physiology, as they raise issues that will be of importance for the development and use of high-performance, high-speed, side-effect free physiological control tools in heterologous expression systems.National Institutes of Health (U.S.) (NIH Director's New Innovator Award DP2OD002002)National Institutes of Health (U.S.) (NIH grant 1R01DA029639)National Institutes of Health (U.S.) (NIH grant 1RC1MH088182)National Institutes of Health (U.S.) (NIH grant 1RC2DE020919)National Institutes of Health (U.S.) (NIH grant 1R01NS067199)National Institutes of Health (U.S.) (NIH grant 1R43NS070453)National Science Foundation (U.S.) (NSF CAREER Award)National Science Foundation (U.S.) (NSF grant EFRI 0835878)National Science Foundation (U.S.) (NSF Grant DMS 0848804)National Science Foundation (U.S.) (NSF Grant DMS 1042134

All-optical electrophysiology in mammalian neurons using engineered microbial rhodopsins

All-optical electrophysiology—spatially resolved simultaneous optical perturbation and measurement of membrane voltage—would open new vistas in neuroscience research. We evolved two archaerhodopsin-based voltage indicators, QuasAr1 and 2, which show improved brightness and voltage sensitivity, microsecond response times, and produce no photocurrent. We engineered a novel channelrhodopsin actuator, CheRiff, which shows improved light sensitivity and kinetics, and spectral orthogonality to the QuasArs. A co-expression vector, Optopatch, enabled crosstalk-free genetically targeted all-optical electrophysiology. In cultured neurons, we combined Optopatch with patterned optical excitation to probe back-propagating action potentials in dendritic spines, synaptic transmission, sub-cellular microsecond-timescale details of action potential propagation, and simultaneous firing of many neurons in a network. Optopatch measurements revealed homeostatic tuning of intrinsic excitability in human stem cell-derived neurons. In brain slice, Optopatch induced and reported action potentials and subthreshold events, with high signal-to-noise ratios. The Optopatch platform enables high-throughput, spatially resolved electrophysiology without use of conventional electrodes