High-speed mapping of synaptic connectivity using photostimulation in Channelrhodopsin-2 transgenic mice

Development of wafer-scale multifunctional nanophotonic neural probes for brain activity mapping

Optical techniques, such as optogenetic stimulation and functional fluorescence imaging, have been revolutionary for neuroscience by enabling neural circuit analysis with cell-type specificity. To probe deep brain regions, implantable light sources are crucial. Silicon photonics, commonly used for data communications, shows great promise in creating implantable devices with complex optical systems in a compact form factor compatible with high volume manufacturing practices. This article reviews recent developments of wafer-scale multifunctional nanophotonic neural probes. The probes can be realized on 200 or 300 mm wafers in commercial foundries and integrate light emitters for photostimulation, microelectrodes for electrophysiological recording, and microfluidic channels for chemical delivery and sampling. By integrating active optical devices to the probes, denser emitter arrays, enhanced on-chip biosensing, and increased ease of use may be realized. Silicon photonics technology makes possible highly versatile implantable neural probes that can transform neuroscience experiments.

Plasticity of Response Properties of Mouse Visual Cortex Neurons Induced by Optogenetic Tetanization In Vivo

Heterosynaptic plasticity, along with Hebbian homosynaptic plasticity, is an important mechanism ensuring the stable operation of learning neuronal networks. However, whether heterosynaptic plasticity occurs in the whole brain in vivo, and what role(s) in brain function in vivo it could play, remains unclear. Here, we used an optogenetics approach to apply a model of intracellular tetanization, which was established and employed to study heterosynaptic plasticity in brain slices, to study the plasticity of response properties of neurons in the mouse visual cortex in vivo. We show that optogenetically evoked high-frequency bursts of action potentials (optogenetic tetanization) in the principal neurons of the visual cortex induce long-term changes in the responses to visual stimuli. Optogenetic tetanization had distinct effects on responses to different stimuli, as follows: responses to optimal and orthogonal orientations decreased, responses to null direction did not change, and responses to oblique orientations increased. As a result, direction selectivity of the neurons decreased and orientation tuning became broader. Since optogenetic tetanization was a postsynaptic protocol, applied in the absence of sensory stimulation, and, thus, without association of presynaptic activity with bursts of action potentials, the observed changes were mediated by mechanisms of heterosynaptic plasticity. We conclude that heterosynaptic plasticity can be induced in vivo and propose that it may play important homeostatic roles in operation of neural networks by helping to prevent runaway dynamics of responses to visual stimuli and to keep the tuning of neuronal responses within the range optimized for the encoding of multiple features in population activity.

Optogenetics 2.0: challenges and solutions towards a quantitative probing of neural circuits

To exploit the full potential of optogenetics, we need to titrate and tailor optogenetic methods to emulate naturalistic circuit function. For that, the following prerequisites need to be met: first, we need to target opsin expression not only to genetically defined neurons per se, but to specifically target a functional node. Second, we need to assess the scope of optogenetic modulation, i.e. the fraction of optogenetically modulated neurons. Third, we need to integrate optogenetic control in a closed loop setting. Fourth, we need to further safe and stable gene expression and light delivery to bring optogenetics to the clinics. Here, we review these concepts for the human and rodent brain.

Inferring causal connectivity from pairwise recordings and optogenetics

To understand the neural mechanisms underlying brain function, neuroscientists aim to quantify causal interactions between neurons, for instance by perturbing the activity of neuron A and measuring the effect on neuron B. Recently, manipulating neuron activity using light-sensitive opsins, optogenetics, has increased the specificity of neural perturbation. However, using widefield optogenetic interventions, multiple neurons are usually perturbed, producing a confound-any of the stimulated neurons can have affected the postsynaptic neuron making it challenging to discern which neurons produced the causal effect. Here, we show how such confounds produce large biases in interpretations. We explain how confounding can be reduced by combining instrumental variables (IV) and difference in differences (DiD) techniques from econometrics. Combined, these methods can estimate (causal) effective connectivity by exploiting the weak, approximately random signal resulting from the interaction between stimulation and the absolute refractory period of the neuron. In simulated neural networks, we find that estimates using ideas from IV and DiD outperform naïve techniques suggesting that methods from causal inference can be useful to disentangle neural interactions in the brain.

Optogenetic cortical spreading depolarization induces headache-related behaviour and neuroinflammatory responses some prolonged in familial hemiplegic migraine type 1 mice

BACKGROUND

Cortical spreading depolarization (CSD), the neurophysiological correlate of the migraine aura, can activate trigeminal pain pathways, but the neurobiological mechanisms and behavioural consequences remain unclear. Here we investigated effects of optogenetically-induced CSDs on headache-related behaviour and neuroinflammatory responses in transgenic mice carrying a familial hemiplegic migraine type 1 (FHM1) mutation.

METHODS

CSD events (3 in total) were evoked in a minimally invasive manner by optogenetic stimulation through the intact skull in freely behaving wildtype (WT) and FHM1 mutant mice. Related behaviours were analysed using mouse grimace scale (MGS) scoring, head grooming, and nest building behaviour. Neuroinflammatory changes were investigated by assessing HMGB1 release with immunohistochemistry and by pre-treating mice with a selective Pannexin-1 channel inhibitor.

RESULTS

In both WT and FHM1 mutant mice, CSDs induced headache-related behaviour, as evidenced by increased MGS scores and the occurrence of oculotemporal strokes, at 30 min. Mice of both genotypes also showed decreased nest building behaviour after CSD. Whereas in WT mice MGS scores had normalized at 24 h after CSD, in FHM1 mutant mice scores were normalized only at 48 h. Of note, oculotemporal stroke behaviour already normalized 5 h after CSD, whereas nest building behaviour remained impaired at 72 h; no genotype differences were observed for either readout. Nuclear HMGB1 release in the cortex of FHM1 mutant mice, at 30 min after CSD, was increased bilaterally in both WT and FHM1 mutant mice, albeit that contralateral release was more pronounced in the mutant mice. Only in FHM1 mutant mice, contralateral release remained higher at 24 h after CSD, but at 48 h had returned to abnormal, elevated, baseline values, when compared to WT mice. Blocking Panx1 channels by TAT-Panx₃₀₈ inhibited CSD-induced headache related behaviour and HMGB1 release.

CONCLUSIONS

CSDs, induced in a minimally invasive manner by optogenetics, investigated in freely behaving mice, cause various migraine relevant behavioural and neuroinflammatory phenotypes that are more pronounced and longer-lasting in FHM1 mutant compared to WT mice. Prevention of CSD-related neuroinflammatory changes may have therapeutic potential in the treatment of migraine.

Paired optogenetic and visual stimulation can change the orientation selectivity of visual cortex neurons

Synaptic plasticity is currently considered the main mechanism underlying the plastic modification of neural networks. The vast majority of studies of synaptic plasticity are carried out on reduced preparations, but the situation in vivo is fundamentally different from that in vitro. In this work, we used the Hebbian paradigm, which is known to induce long-term changes in synaptic strength in vitro, to manipulate the properties of a single pyramidal neuron in the mouse visual cortex. We have shown that optogenetic stimulation of a ChR2-expressing pyramidal neuron in the primary visual cortex of Thy-ChR2 mice paired with the presentation of a visual stimulus of non-optimal orientation induces long-term changes in the properties of the receptive field, manifested in alteration of the orientation selectivity of the cell. Non-paired stimulation did not lead to changes in the properties of the receptive field of the neuron during the experiment. Thus, we have demonstrated the role of associative plasticity in the dynamic organization of the receptive fields of neurons in the visual cortex.

Alcohol withdrawal produces changes in excitability, population discharge probability, and seizure threshold

BACKGROUND

Alcohol withdrawal syndrome (AWS) results from the sudden cessation of chronic alcohol use and is associated with high morbidity and mortality. Alcohol withdrawal-induced central nervous system (CNS) hyperexcitability results from complex, compensatory changes in synaptic efficacy and intrinsic excitability. These changes in excitability counteract the depressing effects of chronic ethanol on neural transmission and underlie symptoms of AWS, which range from mild anxiety to seizures and death. The development of targeted pharmacotherapies for treating AWS has been slow, due in part to the lack of available animal models that capture the key features of human AWS. Using a unique optogenetic method of probing network excitability, we examined electrophysiologic correlates of hyperexcitability sensitive to early changes in CNS excitability. This method is sensitive to pharmacologic treatments that reduce excitability and may represent a platform for AWS drug development.

METHODS

We applied a newly developed method, the optogenetic population discharge threshold (oPDT), which uses light intensity response curves to measure network excitability in chronically implanted mice. Excitability was tracked using the oPDT before, during, and after the chronic intermittent exposure (CIE) model of alcohol withdrawal (WD).

RESULTS

Alcohol withdrawal produced a dose-dependent leftward shift in the oPDT curve (denoting increased excitability), which was detectable in as few as three exposure cycles. This shift in excitability mirrored an increase in the number of spontaneous interictal spikes during withdrawal. In addition, Withdrawal lowered seizure thresholds and increased seizure severity in optogenetically kindled mice.

CONCLUSION

We demonstrate that the oPDT provides a sensitive measure of alcohol withdrawal-induced hyperexcitability. The ability to actively probe the progression of excitability without eliciting potentially confounding seizures promises to be a useful tool in the preclinical development of next-generation pharmacotherapies for AWS.

Differential Subcellular Distribution and Release Dynamics of Cotransmitted Cholinergic and GABAergic Synaptic Inputs Modify Dopaminergic Neuronal Excitability

We identified three types of monosynaptic cholinergic inputs spatially arranged onto medial substantia nigra dopaminergic neurons in male and female mice: cotransmitted acetylcholine (ACh)/GABA, GABA-only, and ACh only. There was a predominant GABA-only conductance along lateral dendrites and soma-centered ACh/GABA cotransmission. In response to repeated stimulation, the GABA conductance found on lateral dendrites decremented less than the proximally located GABA conductance, and was more effective at inhibiting action potentials. While soma-localized ACh/GABA cotransmission showed depression of the GABA component with repeated stimulation, ACh-mediated nicotinic responses were largely maintained. We investigated whether this differential change in inhibitory/excitatory inputs leads to altered neuronal excitability. We found that a depolarizing current or glutamate preceded by cotransmitted ACh/GABA was more effective in eliciting an action potential compared with current, glutamate, or ACh/GABA alone. This enhanced excitability was abolished with nicotinic receptor inhibitors, and modulated by T- and L-type calcium channels, thus establishing that activity of multiple classes of ion channels integrates to shape neuronal excitability.SIGNIFICANCE STATEMENT Our laboratory has previously discovered a population of substantia nigra dopaminegic neurons (DA) that receive cotransmitted ACh and GABA. This study used subcellular optogenetic stimulation of cholinergic presynaptic terminals to map the functional ACh and GABA synaptic inputs across the somatodendritic extent of substantia nigra DA neurons. We determined spatially clustered GABA-only inputs on the lateral dendrites while cotransmitted ACh and GABA clustered close to the soma. We have shown that the action of GABA and ACh in cotransmission spatially clustered near the soma play a critical role in enhancing glutamate-mediated neuronal excitability through the activation of T- and L-type voltage-gated calcium channels.

Tracking connectivity maps in human stem cell-derived neuronal networks by holographic optogenetics

Holographic optogenetic stimulation of human iPSC–derived neuronal networks was exploited to map precise functional connectivity motifs and their long-term dynamics during network development.

Neuronal networks derived from human induced pluripotent stem cells have been exploited widely for modeling neuronal circuits, neurological diseases, and drug screening. As these networks require extended culturing periods to functionally mature in vitro, most studies are based on immature networks. To obtain insights on long-term functional features, we improved a glia–neuron co-culture protocol within multi-electrode arrays, facilitating continuous assessment of electrical features in weekly intervals. By full-field optogenetic stimulation, we detected an earlier onset of neuronal firing and burst activity compared with spontaneous activity. Full-field stimulation enhanced the number of active neurons and their firing rates. Compared with full-field stimulation, which evoked synchronized activity across all neurons, holographic stimulation of individual neurons resulted in local activity. Single-cell holographic stimulation facilitated to trace propagating evoked activities of 400 individually stimulated neurons per multi-electrode array. Thereby, we revealed precise functional neuronal connectivity motifs. Holographic stimulation data over time showed increasing connection numbers and strength with culture age. This holographic stimulation setup has the potential to establish a profound functional testbed for in-depth analysis of human-induced pluripotent stem cell-derived neuronal networks.

Power-effective scanning with AODs for 3D optogenetic applications

Two-photon (2P) excitation is a cornerstone approach widely employed in neuroscience microscopy for deep optical access and sub-micrometric-resolution light targeting into the brain. However, besides structural and functional imaging, 2P optogenetic stimulations are less routinary, especially in 3D. This is because of the adopted scanning systems, often feebly effective, slow and mechanically constricted. Faster illumination can be achieved through acousto-optic deflectors (AODs) although their applicability to large volumes excitation has been limited by large efficiency drop along the optical axis. Here, we present a new AOD-based scheme for 2P 3D scanning that improves the power delivery between different illumination planes. We applied this approach to photostimulate an optogenetic actuator in zebrafish larvae, demonstrating the method efficiency observing increased activity responses and uniform activation probabilities from neuronal clusters addressed in the volume. This novel driving scheme can open to new AOD applications in neuroscience, allowing more effective 3D interrogation in large neuronal networks.

Isolation and Crystallization of the D156C form of Optogenetic ChR2

Channelrhodopsins (ChRs) are light-gated ion channels that are receiving increasing attention as optogenetic tools. Despite extensive efforts to gain understanding of how these channels function, the molecular events linking light absorption of the retinal cofactor to channel opening remain elusive. While dark-state structures of ChR2 or chimeric proteins have demonstrated the architecture of non-conducting states, there is a need for open- and desensitized-state structures to uncover the mechanistic principles underlying channel activity. To facilitate comprehensive structural studies of ChR2 in non-closed states, we report a production and purification procedure of the D156C form of ChR2, which displays prolonged channel opening compared to the wild type. We demonstrate considerable yields (0.45 mg/g fermenter cell culture) of recombinantly produced protein using S. cerevisiae, which is purified to high homogeneity both as opsin (retinal-free) and as functional ChR2 with added retinal. We also develop conditions that enable the growth of ChR2 crystals that scatter X-rays to 6 Å, and identify a molecular replacement solution that suggests that the packing is different from published structures. Consequently, our cost-effective production and purification pipeline opens the way for downstream structural studies of different ChR2 states, which may provide a foundation for further adaptation of this protein for optogenetic applications.

Feedback inhibition underlies new computational functions of cerebellar interneurons

The function of a feedback inhibitory circuit between cerebellar Purkinje cells and molecular layer interneurons (MLIs) was defined by combining optogenetics, neuronal activity recordings both in cerebellar slices and in vivo, and computational modeling. Purkinje cells inhibit a subset of MLIs in the inner third of the molecular layer. This inhibition is non-reciprocal, short-range (less than 200 μm) and is based on convergence of one to two Purkinje cells onto MLIs. During learning-related eyelid movements in vivo, the activity of a subset of MLIs progressively increases as Purkinje cell activity decreases, with Purkinje cells usually leading the MLIs. Computer simulations indicate that these relationships are best explained by the feedback circuit from Purkinje cells to MLIs and that this feedback circuit plays a central role in making cerebellar learning efficient.

Photostimulation activates fast-spiking interneurons and pyramidal cells in the entorhinal cortex of Thy1-ChR2-YFP line 18 mice

The application of optogenetics in animals has provided new insights into both fundamental neuroscience and diseases of the nervous system. This is primarily due to the fact that optogenetics allows selectively activating or inhibiting particular types of neurons. One of the first transgenic mouse lines developed for the optogenetic experiment was Thy1-ChR2-YFP. Thy1 is an immunoglobulin superfamily member expressing in projection neurons, so it was assumed that channelrhodopsin-2 (ChR2) would be primarily expressed in projection neurons. However, the specificity of ChR2 expression under promoter Thy1 in different lines has to be clarified yet. Therefore, we aimed to determine the cell specificity of ChR2 expression in the entorhinal cortex of Thy1-ChR2-YFP line 18 mice. We have found that both pyramidal cells and fast-spiking interneurons in deep layers of the entorhinal cortex depolarized and fired in response to 470-nm photostimulation. To exclude the effect of synaptic activation of interneurons by pyramidal cells, we used a selective antagonist of AMPA receptors. Under these conditions, inhibitory postsynaptic currents decreased but did not disappear completely. Furthermore, gabazine inhibited these postsynaptic currents entirely, thus confirming the direct activation of interneurons by light. These data demonstrate that ChR2 is expressed in both pyramidal neurons and fast-spiking interneurons of the entorhinal cortex in Thy1-ChR2-YFP mice.

An Optogenetics-Inspired Flexible van der Waals Optoelectronic Synapse and its Application to a Convolutional Neural Network

Optogenetics refers to a technique that uses light to modulate neuronal activity with a high spatiotemporal resolution, which enables the manipulation of learning and memory functions in the human brain. This strategy of controlling neuronal activity using light can be applied for the development of intelligent systems, including neuromorphic and in-memory computing systems. Herein, a flexible van der Waals (vdW) optoelectronic synapse is reported, which is a core component of optogenetics-inspired intelligent systems. This synapse is fabricated on 2D vdW layered rhenium disulfide (ReS₂ ) that features an inherent photosensitive memory nature derived from the persistent photoconductivity (PPC) effect, successfully mimicking the dynamics of biological synapses. Based on first-principles calculations, the PPC effect is identified to originate from sulfur vacancies in ReS₂ that have an inherent tendency to form shallow defect states near the conduction band edges and under optical excitation lead to large lattice relaxation. Finally, the feasibility of applying the synapses in optogenetics-inspired intelligent systems is demonstrated via training and inference tasks for the CIFAR-10 dataset using a convolutional neural network composed of vdW optoelectronic synapse devices.

Optogenetic modeling of human neuromuscular circuits in Duchenne muscular dystrophy with CRISPR and pharmacological corrections

Duchenne muscular dystrophy (DMD) is caused by dystrophin gene mutations leading to skeletal muscle weakness and wasting. Dystrophin is enriched at the neuromuscular junction (NMJ), but how NMJ abnormalities contribute to DMD pathogenesis remains unclear. Here, we combine transcriptome analysis and modeling of DMD patient-derived neuromuscular circuits with CRISPR-corrected isogenic controls in compartmentalized microdevices. We show that NMJ volumes and optogenetic motor neuron–stimulated myofiber contraction are compromised in DMD neuromuscular circuits, which can be rescued by pharmacological inhibition of TGFβ signaling, an observation validated in a 96-well human neuromuscular circuit coculture assay. These beneficial effects are associated with normalization of dysregulated gene expression in DMD myogenic transcriptomes affecting NMJ assembly (e.g., MUSK) and axon guidance (e.g., SLIT2 and SLIT3). Our study provides a new human microphysiological model for investigating NMJ defects in DMD and assessing candidate drugs and suggests that enhancing neuromuscular connectivity may be an effective therapeutic strategy.

Role of Neuroinflammation and Blood-Brain Barrier Permutability on Migraine

Currently, migraine is treated mainly by targeting calcitonin gene-related peptides, although the efficacy of this method is limited and new treatment strategies are desired. Neuroinflammation has been implicated in the pathogenesis of migraine. In patients with migraine, peripheral levels of pro-inflammatory cytokines, such as interleukin-1β (IL-1β) and tumor necrosis factor-α, are known to be increased. Additionally, animal models of headache have demonstrated that immunological responses associated with cytokines are involved in the pathogenesis of migraine. Furthermore, these inflammatory mediators might alter the function of tight junctions in brain vascular endothelial cells in animal models, but not in human patients. Based on clinical findings showing elevated IL-1β, and experimental findings involving IL-1β and both the peripheral trigeminal ganglion and central trigeminal vascular pathways, regulation of the Il-1β/IL-1 receptor type 1 axis might lead to new treatments for migraine. However, the integrity of the blood-brain barrier is not expected to be affected during attacks in patients with migraine.

Stimulating GABAergic Neurons in the Nucleus Accumbens Core Alters the Trigeminal Neuropathic Pain Responses in a Rat Model of Infraorbital Nerve Injury

The nucleus accumbens core (NAcc) is an important component of brain reward circuitry, but studies have revealed its involvement in pain circuitry also. However, its effect on trigeminal neuralgia (TN) and the mechanism underlying it are yet to be fully understood. Therefore, this study aimed to examine the outcomes of optogenetic stimulation of NAcc GABAergic neurons in an animal model of TN. Animals were allocated into TN, sham, and control groups. TN was generated by infraorbital nerve constriction and the optogenetic virus was injected into the NAcc. In vivo extracellular recordings were acquired from the ventral posteromedial nucleus of the thalamus. Alterations of behavioral responses during stimulation "ON" and "OFF" conditions were evaluated. In vivo microdialysis was performed in the NAcc of TN and sham animals. During optogenetic stimulation, electrophysiological recordings revealed a reduction of both tonic and burst firing activity in TN animals, and significantly improved behavioral responses were observed as well. Microdialysis coupled with liquid chromatography/tandem mass spectrometry analysis revealed significant alterations in extracellular concentration levels of GABA, glutamate, acetylcholine, dopamine, and citrulline in NAcc upon optic stimulation. In fine, our results suggested that NAcc stimulation could modulate the transmission of trigeminal pain signals in the TN animal model.

Molecular Tools for Targeted Control of Nerve Cell Electrical Activity. Part I

In modern life sciences, the issue of a specific, exogenously directed manipulation of a cell’s biochemistry is a highly topical one. In the case of electrically excitable cells, the aim of the manipulation is to control the cells’ electrical activity, with the result being either excitation with subsequent generation of an action potential or inhibition and suppression of the excitatory currents. The techniques of electrical activity stimulation are of particular significance in tackling the most challenging basic problem: figuring out how the nervous system of higher multicellular organisms functions. At this juncture, when neuroscience is gradually abandoning the reductionist approach in favor of the direct investigation of complex neuronal systems, minimally invasive methods for brain tissue stimulation are becoming the basic element in the toolbox of those involved in the field. In this review, we describe three approaches that are based on the delivery of exogenous, genetically encoded molecules sensitive to external stimuli into the nervous tissue. These approaches include optogenetics (Part I) as well as chemogenetics and thermogenetics (Part II), which are significantly different not only in the nature of the stimuli and structure of the appropriate effector proteins, but also in the details of experimental applications. The latter circumstance is an indication that these are rather complementary than competing techniques.

Spreading depression as an innate antiseizure mechanism

Spreading depression (SD) is an intense and prolonged depolarization in the central nervous systems from insect to man. It is implicated in neurological disorders such as migraine and brain injury. Here, using an in vivo mouse model of focal neocortical seizures, we show that SD may be a fundamental defense against seizures. Seizures induced by topical 4-aminopyridine, penicillin or bicuculline, or systemic kainic acid, culminated in SDs at a variable rate. Greater seizure power and area of recruitment predicted SD. Once triggered, SD immediately suppressed the seizure. Optogenetic or KCl-induced SDs had similar antiseizure effect sustained for more than 30 min. Conversely, pharmacologically inhibiting SD occurrence during a focal seizure facilitated seizure generalization. Altogether, our data indicate that seizures trigger SD, which then terminates the seizure and prevents its generalization.

Opposed hemodynamic responses following increased excitation and parvalbumin-based inhibition

Understanding cellular contributions to hemodynamic activity is essential for interpreting blood-based brain mapping signals. Optogenetic studies examining cell-specific influences on local hemodynamics have reported that excitatory activity results in cerebral perfusion and blood volume increase, while inhibitory activity contributes to both vasodilation and vasoconstriction. How specific subpopulations of interneurons regulate the brain's blood supply is less examined. Parvalbumin interneurons are the largest subpopulation of GABAergic neurons in the brain, critical for brain development, plasticity, and long-distance excitatory neurotransmission. Despite their essential role in brain function, the contribution of parvalbumin neurons to neurovascular coupling has been relatively unexamined. Using optical intrinsic signal imaging and laser speckle contrast imaging, we photostimulated awake and anesthetized transgenic mice expressing channelrhodopsin under a parvalbumin promoter. Increased parvalbumin activity reduced local oxygenation, cerebral blood volume, and cerebral blood flow. These "negative" hemodynamic responses were consistent within and across mice and reproducible across a broad range of photostimulus parameters. However, the sign and magnitude of the hemodynamic response resulting from increased parvalbumin activity depended on the type and level of anesthesia used. Opposed hemodynamic responses following increased excitation or parvalbumin-based inhibition suggest unique contributions from different cell populations to neurovascular coupling.

Unraveling ChR2-driven stochastic Ca2+ dynamics in astrocytes: A call for new interventional paradigms

Optogenetic targeting of astrocytes provides a robust experimental model to differentially induce Ca2+ signals in astrocytes in vivo. However, a systematic study quantifying the response of optogenetically modified astrocytes to light is yet to be performed. Here, we propose a novel stochastic model of Ca2+ dynamics in astrocytes that incorporates a light sensitive component-channelrhodopsin 2 (ChR2). Utilizing this model, we investigated the effect of different light stimulation paradigms on cells expressing select variants of ChR2 (wild type, ChETA, and ChRET/TC). Results predict that depending on paradigm specification, astrocytes might undergo drastic changes in their basal Ca2+ level and spiking probability. Furthermore, we performed a global sensitivity analysis to assess the effect of variation in parameters pertinent to the shape of the ChR2 photocurrent on astrocytic Ca2+ dynamics. Results suggest that directing variants towards the first open state of the ChR2 photocycle (o1) enhances spiking activity in astrocytes during optical stimulation. Evaluation of the effect of Ca2+ buffering and coupling coefficient in a network of ChR2-expressing astrocytes demonstrated basal level elevations in the stimulated region and propagation of calcium activity to unstimulated cells. Buffering reduced the diffusion range of Ca2+ within the network, thereby limiting propagation and influencing the activity of astrocytes. Collectively, the framework presented in this study provides valuable information for the selection of light stimulation paradigms that elicit desired astrocytic activity using existing ChR2 constructs, as well as aids in the engineering of future application-oriented optogenetic variants.

Gamma frequency activation of inhibitory neurons in the acute phase after stroke attenuates vascular and behavioral dysfunction

Alterations in gamma oscillations occur in several neurological disorders, and the entrainment of gamma oscillations has been recently proposed as a treatment for neurodegenerative disease. Optogenetic stimulation enhances recovery in models of stroke when applied weeks after injury; however, the benefits of acute brain stimulation have not been investigated. Here, we report beneficial effects of gamma-frequency modulation in the acute phase, within 1 h, after stroke. Transgenic VGAT-ChR2 mice are subject to awake photothrombotic stroke in an area encompassing the forelimb sensory and motor cortex. Optogenetic stimulation at 40 Hz in the peri-infarct zone recovers neuronal activity 24 h after stroke in motor and parietal association areas, as well as blood flow over the first week after stroke. Stimulation significantly reduces lesion volume and improves motor function. Our results suggest that acute-phase modulation of cortical oscillatory dynamics may serve as a target for neuroprotection against stroke.

Patterned Optogenetic Stimulation Using a DMD Projector

Optical manipulation is a powerful way to control neural activity in vitro and in vivo with millisecond precision. Patterning of light provides the remarkable ability to simultaneously target spatially segregated neurons from a population. Commercially available projectors provide one of the simplest and most economical ways of achieving spatial light modulation at millisecond timescales. Here, we describe the protocol for constructing a projector-based spatio-temporal light patterning system integrated with a microscope on a typical electrophysiology rig. The set-up is well suited for applications requiring rapid, distinct, and combinatorial inputs, akin to brain activity. This equipment involved is fairly economical (<$5000 including all optical and mechanical components), and the set-up is easy to assemble and program.

Molecular Layer Interneurons: Key Elements of Cerebellar Network Computation and Behavior

Molecular layer interneurons (MLIs) play an important role in cerebellar information processing by controlling Purkinje cell (PC) activity via inhibitory synaptic transmission. A local MLI network, constructed from both chemical and electrical synapses, is organized into spatially structured clusters that amplify feedforward and lateral inhibition to shape the temporal and spatial patterns of PC activity. Several recent in vivo studies indicate that such MLI circuits contribute not only to sensorimotor information processing, but also to precise motor coordination and cognitive processes. Here, we review current understanding of the organization of MLI circuits and their roles in the function of the mammalian cerebellum.

In vivo voltage-sensitive dye imaging of mouse cortical activity with mesoscopic optical tomography

Significance: Cellular layering is a hallmark of the mammalian neocortex with layer and cell type-specific connections within the cortical mantle and subcortical connections. A key challenge in studying circuit function within the neocortex is to understand the spatial and temporal patterns of information flow between different columns and layers. Aim: We aimed to investigate the three-dimensional (3D) layer- and area-specific interactions in mouse cortex in vivo. Approach: We applied a new promising neuroimaging method-fluorescence laminar optical tomography in combination with voltage-sensitive dye imaging (VSDi). VSDi is a powerful technique for interrogating membrane potential dynamics in assemblies of cortical neurons, but it is traditionally used for two-dimensional (2D) imaging. Our mesoscopic technique allows visualization of neuronal activity in a 3D manner with high temporal resolution. Results: We first demonstrated the depth-resolved capability of 3D mesoscopic imaging technology in Thy1-ChR2-YFP transgenic mice. Next, we recorded the long-range functional projections between sensory cortex (S1) and motor cortex (M1) in mice, in vivo, following single whisker deflection. Conclusions: The results show that mesoscopic imaging technique has the potential to investigate the layer-specific neural connectivity in the mouse cortex in vivo. Combination of mesoscopic imaging technique with optogenetic control strategy is a promising platform for determining depth-resolved interactions between cortical circuit elements.

Functional interrogation of neural circuits with virally transmitted optogenetic tools

The vertebrate brain comprises a plethora of cell types connected by intertwined pathways. Optogenetics enriches the neuroscientific tool set for disentangling these neuronal circuits in a manner which exceeds the spatio-temporal precision of previously existing techniques. Technically, optogenetics can be divided in three types of optical and genetic combinations: (1) it is primarily understood as the manipulation of the activity of genetically modified cells (typically neurons) with light, i.e. optical actuators. (2) A second combination refers to visualizing the activity of genetically modified cells (again typically neurons), i.e. optical sensors. (3) A completely different interpretation of optogenetics refers to the light activated expression of a genetically induced construct. Here, we focus on the first two types of optogenetics, i.e. the optical actuators and sensors in an attempt to give an overview into the topic. We first cover methods to express opsins into neurons and introduce strategies of targeting specific neuronal populations in different animal species. We then summarize combinations of optogenetics with behavioral read out and neuronal imaging. Finally, we give an overview of the current state-of-the-art and an outlook on future perspectives.

Heterogeneous somatostatin-expressing neuron population in mouse ventral tegmental area

The cellular architecture of the ventral tegmental area (VTA), the main hub of the brain reward system, remains only partially characterized. To extend the characterization to inhibitory neurons, we have identified three distinct subtypes of somatostatin (Sst)-expressing neurons in the mouse VTA. These neurons differ in their electrophysiological and morphological properties, anatomical localization, as well as mRNA expression profiles. Importantly, similar to cortical Sst-containing interneurons, most VTA Sst neurons express GABAergic inhibitory markers, but some of them also express glutamatergic excitatory markers and a subpopulation even express dopaminergic markers. Furthermore, only some of the proposed marker genes for cortical Sst neurons were expressed in the VTA Sst neurons. Physiologically, one of the VTA Sst neuron subtypes locally inhibited neighboring dopamine neurons. Overall, our results demonstrate the remarkable complexity and heterogeneity of VTA Sst neurons and suggest that these cells are multifunctional players in the midbrain reward circuitry.

Heterogeneous somatostatin-expressing neuron population in mouse ventral tegmental area

The cellular architecture of the ventral tegmental area (VTA), the main hub of the brain reward system, remains only partially characterized. To extend the characterization to inhibitory neurons, we have identified three distinct subtypes of somatostatin (Sst)-expressing neurons in the mouse VTA. These neurons differ in their electrophysiological and morphological properties, anatomical localization, as well as mRNA expression profiles. Importantly, similar to cortical Sst-containing interneurons, most VTA Sst neurons express GABAergic inhibitory markers, but some of them also express glutamatergic excitatory markers and a subpopulation even express dopaminergic markers. Furthermore, only some of the proposed marker genes for cortical Sst neurons were expressed in the VTA Sst neurons. Physiologically, one of the VTA Sst neuron subtypes locally inhibited neighboring dopamine neurons. Overall, our results demonstrate the remarkable complexity and heterogeneity of VTA Sst neurons and suggest that these cells are multifunctional players in the midbrain reward circuitry.

Peri-Infarct Hot-Zones Have Higher Susceptibility to Optogenetic Functional Activation-Induced Spreading Depolarizations

BACKGROUND AND PURPOSE

Spreading depolarizations (SDs) are recurrent and ostensibly spontaneous depolarization waves that may contribute to infarct progression after stroke. Somatosensory activation of the metastable peri-infarct tissue triggers peri-infarct SDs at a high rate.

METHODS

We directly measured the functional activation threshold to trigger SDs in peri-infarct hot zones using optogenetic stimulation after distal middle cerebral artery occlusion in Thy1-ChR2-YFP mice.

RESULTS

Optogenetic activation of peri-infarct tissue triggered SDs at a strikingly high rate (64%) compared with contralateral homotopic cortex (8%; P=0.004). Laser speckle perfusion imaging identified a residual blood flow of 31±2% of baseline marking the metastable tissue with a propensity to develop SDs.

CONCLUSIONS

Our data reveal a spatially distinct increase in SD susceptibility in peri-infarct tissue where physiological levels of functional activation are capable of triggering SDs. Given the potentially deleterious effects of peri-infarct SDs, the effect of sensory overstimulation in hyperacute stroke should be examined more carefully.

Non-invasively triggered spreading depolarizations induce a rapid pro-inflammatory response in cerebral cortex

Cortical spreading depolarization (CSD) induces pro-inflammatory gene expression in brain tissue. However, previous studies assessing the relationship between CSD and inflammation have used invasive methods that directly trigger inflammation. To eliminate the injury confounder, we induced CSDs non-invasively through intact skull using optogenetics in Thy1-channelrhodopsin-2 transgenic mice. We corroborated our findings by minimally invasive KCl-induced CSDs through thinned skull. Six CSDs induced over 1 h dramatically increased cortical interleukin-1β (IL-1β), chemokine (C-C motif) ligand 2 (CCL2), and tumor necrosis factor-α (TNF-α) mRNA expression peaking around 1, 2 and 4 h, respectively. Interleukin-6 (IL-6) and intercellular adhesion molecule-1 (ICAM-1) were only modestly elevated. A single CSD also increased IL-1β, CCL2, and TNF-α, and revealed an ultra-early IL-1β response within 10 min. The response was blunted in IL-1 receptor-1 knockout mice, implicating IL-1β as an upstream mediator, and suppressed by dexamethasone, but not ibuprofen. CSD did not alter systemic inflammatory indices. In summary, this is the first report of pro-inflammatory gene expression after non-invasively induced CSDs. Altogether, our data provide novel insights into the role of CSD-induced neuroinflammation in migraine headache pathogenesis and have implications for the inflammatory processes in acute brain injury where numerous CSDs occur for days.

Light Up the Brain: The Application of Optogenetics in Cell-Type Specific Dissection of Mouse Brain Circuits

The exquisite intricacies of neural circuits are fundamental to an animal’s diverse and complex repertoire of sensory and motor functions. The ability to precisely map neural circuits and to selectively manipulate neural activity is critical to understanding brain function and has, therefore been a long-standing goal for neuroscientists. The recent development of optogenetic tools, combined with transgenic mouse lines, has endowed us with unprecedented spatiotemporal precision in circuit analysis. These advances greatly expand the scope of tractable experimental investigations. Here, in the first half of the review, we will present applications of optogenetics in identifying connectivity between different local neuronal cell types and of long-range projections with both in vitro and in vivo methods. We will then discuss how these tools can be used to reveal the functional roles of these cell-type specific connections in governing sensory information processing, and learning and memory in the visual cortex, somatosensory cortex, and motor cortex. Finally, we will discuss the prospect of new optogenetic tools and how their application can further advance modern neuroscience. In summary, this review serves as a primer to exemplify how optogenetics can be used in sophisticated modern circuit analyses at the levels of synapses, cells, network connectivity and behaviors.

Neurovascular coupling during optogenetic functional activation: Local and remote stimulus-response characteristics, and uncoupling by spreading depression

Neurovascular coupling is a fundamental response that links activity to perfusion. Traditional paradigms of neurovascular coupling utilize somatosensory stimulation to activate the primary sensory cortex through subcortical relays. Therefore, examination of neurovascular coupling in disease models can be confounded if the disease process affects these multisynaptic pathways. Optogenetic stimulation is an alternative to directly activate neurons, bypassing the subcortical relays. We employed minimally invasive optogenetic cortical activation through intact skull in Thy1-channelrhodopsin-2 transgenic mice, examined the blood flow changes using laser speckle imaging, and related these to evoked electrophysiological activity. Our data show that optogenetic activation of barrel cortex triggers intensity- and frequency-dependent hyperemia both locally within the barrel cortex (>50% CBF increase), and remotely within the ipsilateral motor cortex (>30% CBF increase). Intriguingly, activation of the barrel cortex causes a small (∼10%) but reproducible hypoperfusion within the contralateral barrel cortex, electrophysiologically linked to transhemispheric inhibition. Cortical spreading depression, known to cause neurovascular uncoupling, diminishes optogenetic hyperemia by more than 50% for up to an hour despite rapid recovery of evoked electrophysiological activity, recapitulating a unique feature of physiological neurovascular coupling. Altogether, these data establish a minimally invasive paradigm to investigate neurovascular coupling for longitudinal characterization of cerebrovascular pathologies.

Inhibitory Neuron Activity Contributions to Hemodynamic Responses and Metabolic Load Examined Using an Inhibitory Optogenetic Mouse Model

Hemodynamic signals are routinely used to noninvasively assess brain function in humans and animals. This work examined the contribution of inhibitory neuron activity on hemodynamic responses captured by changes in blood flow, volume and oxygenation in the cortex of lightly anesthetized mice. Because cortical activity is not commonly initiated by inhibitory neurons, experiments were conducted to examine the neuronal activity properties elicited by photo-stimulation. We observed comparable increases in neuronal activity evoked by forelimb and photo-stimulation; however, significantly larger increases in blood flow and volume were produced by photo-stimulation of inhibitory neurons compared with forelimb stimulation. Following blockade of glutamate and GABA-A receptors to reduce postsynaptic activity contributions, neuronal activity was reliably modulated and hemodynamic changes persisted, though slightly reduced. More importantly, photo-stimulation-evoked changes in blood flow and volume were suppressed by 75–80% with the administration of a nitric oxide synthase inhibitor, suggesting that inhibitory neurons regulate blood flow mostly via nitric oxide. Lastly, forelimb and photo-stimulation of excitatory neurons produced local decreases in blood oxygenation, while large increases were generated by photo-stimulation of inhibitory neurons. Estimates of oxygen metabolism suggest that inhibitory neuron activity has a small impact on tissue metabolic load, indicating a mismatch between the metabolic demand and blood flow regulation properties of inhibitory and excitatory neurons.

All-Optical Volumetric Physiology for Connectomics in Dense Neuronal Structures

All-optical physiology (AOP) manipulates and reports neuronal activities with light, allowing for interrogation of neuronal functional connections with high spatiotemporal resolution. However, contemporary high-speed AOP platforms are limited to single-depth or discrete multi-plane recordings that are not suitable for studying functional connections among densely packed small neurons, such as neurons in Drosophila brains. Here, we constructed a 3D AOP platform by incorporating single-photon point stimulation and two-photon high-speed volumetric recordings with a tunable acoustic gradient-index (TAG) lens. We demonstrated the platform effectiveness by studying the anterior visual pathway (AVP) of Drosophila. We achieved functional observation of spatiotemporal coding and the strengths of calcium-sensitive connections between anterior optic tubercle (AOTU) sub-compartments and >70 tightly assembled 2-μm bulb (BU) microglomeruli in 3D coordinates with a single trial. Our work aids the establishment of in vivo 3D functional connectomes in neuron-dense brain areas.

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All-optical volumetric physiology = precise stimulation + fast volumetric recording

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Precise single-photon point stimulation among genetically defined neurons

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3D two-photon imaging by an acoustic gradient-index lens for dense neural structures

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Observation of 3D functional connectivity in Drosophila anterior visual pathway

Optogenetically-Induced Population Discharge Threshold as a Sensitive Measure of Network Excitability

Network excitability is governed by synaptic efficacy, intrinsic excitability, and the circuitry in which these factors are expressed. The complex interplay between these factors determines how circuits function and, at the extreme, their susceptibility to seizure. We have developed a sensitive, quantitative estimate of network excitability in freely behaving mice using a novel optogenetic intensity-response procedure. Synchronous activation of deep sublayer CA1 pyramidal cells produces abnormal network-wide epileptiform population discharges (PDs) that are nearly indistinguishable from spontaneously-occurring interictal spikes (IISs). By systematically varying light intensity, and therefore the magnitude of the optogenetically-mediated current, we generated intensity-response curves using the probability of PD as the dependent variable. Manipulations known to increase excitability, such as sub-convulsive doses (20 mg/kg) of the chemoconvulsant pentylenetetrazol (PTZ), produced a leftward shift in the curve compared to baseline. The anti-epileptic drug levetiracetam (LEV; 40 mk/kg), in combination with PTZ, produced a rightward shift. Optogenetically-induced PD threshold (oPDT) baselines were stable over time, suggesting the metric is appropriate for within-subject experimental designs with multiple pharmacological manipulations.

Orofacial Movements Involve Parallel Corticobulbar Projections from Motor Cortex to Trigeminal Premotor Nuclei

How do neurons in orofacial motor cortex (MCtx) orchestrate behaviors? We show that focal activation of MCtx corticobulbar neurons evokes behaviorally relevant concurrent movements of the forelimb, jaw, nose, and vibrissae. The projections from different locations in MCtx form gradients of boutons across premotor nuclei spinal trigeminal pars oralis (SpVO) and interpolaris rostralis (SpVIr). Furthermore, retrograde viral tracing from muscles that control orofacial actions shows that these premotor nuclei segregate their outputs. In the most dramatic case, both SpVO and SpVIr are premotor to forelimb and vibrissa muscles, while only SpVO is premotor to jaw muscles. Functional confirmation of the superimposed control by MCtx was obtained through selective optogenetic activation of corticobulbar neurons on the basis of their preferential projections to SpVO versus SpVIr. We conclude that neighboring projection neurons in orofacial MCtx form parallel pathways to distinct pools of trigeminal premotor neurons that coordinate motor actions into a behavior.

Toward the Optical Cochlear Implant

When hearing fails, cochlear implants (CIs) provide open speech perception to most of the currently half a million CI users. CIs bypass the defective sensory organ and stimulate the auditory nerve electrically. The major bottleneck of current CIs is the poor coding of spectral information, which results from wide current spread from each electrode contact. As light can be more conveniently confined, optical stimulation of the auditory nerve presents a promising perspective for a fundamental advance of CIs. Moreover, given the improved frequency resolution of optical excitation and its versatility for arbitrary stimulation patterns the approach also bears potential for auditory research. Here, we review the current state of the art focusing on the emerging concept of optogenetic stimulation of the auditory pathway. Developing optogenetic stimulation for auditory research and future CIs requires efforts toward viral gene transfer to the neurons, design and characterization of appropriate optogenetic actuators, as well as engineering of multichannel optical implants.

Functional Principles of Posterior Septal Inputs to the Medial Habenula

The medial habenula (MHb) is an epithalamic hub contributing to expression and extinction of aversive states by bridging forebrain areas and midbrain monoaminergic centers. Although contradictory information exists regarding their synaptic properties, the physiology of the excitatory inputs to the MHb from the posterior septum remains elusive. Here, combining optogenetics-based mapping with ex vivo and in vivo physiology, we examine the synaptic properties of posterior septal afferents to the MHb and how they influence behavior. We demonstrate that MHb cells receive sparse inputs producing purely glutamatergic responses via calcium-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), heterotrimeric GluN2A-GluN2B-GluN1 N-methyl-D-aspartate (NMDA) receptors, and inhibitory group II metabotropic glutamate receptors. We describe the complex integration dynamics of these components by MHb cells. Finally, we combine ex vivo data with realistic afferent firing patterns recorded in vivo to demonstrate that efficient optogenetic septal stimulation in the MHb induces anxiolysis and promotes locomotion, contributing long-awaited evidence in favor of the importance of this septo-habenular pathway.

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Medial habenular (MHb) neurons receive sparse inputs from the posterior septum (PS)

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PS afferents to the MHb function in a purely glutamatergic mode

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Excitatory ionotropic and inhibitory metabotropic receptors convey PS inputs in the MHb

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PS activation in the MHb increases locomotion and induces anxiolysis

Electromagnetic Regulation of Cell Activity

The ability to observe the effects of rapidly and reversibly regulating cell activity in targeted cell populations has provided numerous physiologic insights. Over the last decade, a wide range of technologies have emerged for regulating cellular activity using optical, chemical, and, more recently, electromagnetic modalities. Electromagnetic fields can freely penetrate cells and tissue and their energy can be absorbed by metal particles. When released, the absorbed energy can in turn gate endogenous or engineered receptors and ion channels to regulate cell activity. In this manner, electromagnetic fields acting on external nanoparticles have been used to exert mechanical forces on cell membranes and organelles to generate heat and interact with thermally activated proteins or to induce receptor aggregation and intracellular signaling. More recently, technologies using genetically encoded nanoparticles composed of the iron storage protein, ferritin, have been used for targeted, temporal control of cell activity in vitro and in vivo. These tools provide a means for noninvasively modulating gene expression, intracellular organelles, such as endosomes, and whole-cell activity both in vitro and in freely moving animals. The use of magnetic fields interacting with external or genetically encoded nanoparticles thus provides a rapid noninvasive means for regulating cell activity.

Determinants of Optogenetic Cortical Spreading Depolarizations

Cortical spreading depolarization (SD) is the electrophysiological event underlying migraine aura, and a critical contributor to secondary damage after brain injury. Experimental models of SD have been used for decades in migraine and brain injury research; however, they are highly invasive and often cause primary tissue injury, diminishing their translational value. Here we present a non-invasive method to trigger SDs using light-induced depolarization in transgenic mice expressing channelrhodopsin-2 in neurons (Thy1-ChR2-YFP). Focal illumination (470 nm, 1-10 mW) through intact skull using an optical fiber evokes power-dependent steady extracellular potential shifts and local elevations of extracellular [K+] that culminate in an SD when power exceeds a threshold. Using the model, we show that homozygous mice are significantly more susceptible to SD (i.e., lower light thresholds) than heterozygous ChR2 mice. Moreover, we show SD susceptibility differs significantly among cortical divisions (motor, whisker barrel, sensory, visual, in decreasing order of susceptibility), which correlates with relative channelrhodopsin-2 expression. Furthermore, the NMDA receptor antagonist MK-801 blocks the transition to SD without diminishing extracellular potential shifts. Altogether, our data show that the optogenetic SD model is highly suitable for examining physiological or pharmacological modulation of SD in acute and longitudinal studies.

Optical imaging and modulation of neurovascular responses

The cerebral microvasculature consists of pial vascular networks, parenchymal descending arterioles, ascending venules and parenchymal capillaries. This vascular compartmentalization is vital to precisely deliver blood to balance continuously varying neural demands in multiple brain regions. Optical imaging techniques have facilitated the investigation of dynamic spatial and temporal properties of microvascular functions in real time. Their combination with transgenic animal models encoding specific genetic targets have further strengthened the importance of optical methods for neurovascular research by allowing for the modulation and monitoring of neuro vascular function. Image analysis methods with three-dimensional reconstruction are also helping to understand the complexity of microscopic observations. Here, we review the compartmentalized cerebral microvascular responses to global perturbations as well as regional changes in response to neural activity to highlight the differences in vascular action sites. In addition, microvascular responses elicited by optical modulation of different cell-type targets are summarized with emphasis on variable spatiotemporal dynamics of microvascular responses. Finally, long-term changes in microvascular compartmentalization are discussed to help understand potential relationships between CBF disturbances and the development of neurodegenerative diseases and cognitive decline.

Optogenetic stimulation of complex spatio-temporal activity patterns by acousto-optic light steering probes cerebellar granular layer integrative properties

Optogenetics provides tools to control afferent activity in brain microcircuits. However, this requires optical methods that can evoke asynchronous and coordinated activity within neuronal ensembles in a spatio-temporally precise way. Here we describe a light patterning method, which combines MHz acousto-optic beam steering and adjustable low numerical aperture Gaussian beams, to achieve fast 2D targeting in scattering tissue. Using mossy fiber afferents to the cerebellar cortex as a testbed, we demonstrate single fiber optogenetic stimulation with micron-scale lateral resolution, >100 µm depth-penetration and 0.1 ms spiking precision. Protracted spatio-temporal patterns of light delivered by our illumination system evoked sustained asynchronous mossy fiber activity with excellent repeatability. Combining optical and electrical stimulations, we show that the cerebellar granular layer performs nonlinear integration, whereby sustained mossy fiber activity provides a permissive context for the transmission of salient inputs, enriching combinatorial views on mossy fiber pattern separation.

Longitudinal Optogenetic Motor Mapping Revealed Structural and Functional Impairments and Enhanced Corticorubral Projection after Contusive Spinal Cord Injury in Mice

Current evaluation of impairment and repair after spinal cord injury (SCI) is largely dependent on behavioral assessment and histological analysis of injured tissue and pathways. Here, we evaluated whether transcranial optogenetic mapping of motor cortex could reflect longitudinal structural and functional damage and recovery after SCI. In Thy1-Channelrhodopsin2 transgenic mice, repeated motor mappings were made by recording optogenetically evoked electromyograms (EMGs) of a hindlimb at baseline and 1 day and 2, 4, and 6 weeks after mild, moderate, and severe spinal cord contusion. Injuries caused initial decreases in EMG amplitude, losses of motor map, and subsequent partial recoveries, all of which corresponded to injury severity. Reductions in map size were positively correlated with motor performance, as measured by Basso Mouse Scale, rota-rod, and grid walk tests, at different time points, as well as with lesion area at spinal cord epicenter at 6 weeks post-SCI. Retrograde tracing with Fluoro-Gold showed decreased numbers of cortico- and rubrospinal neurons, with the latter being negatively correlated with motor map size. Combined retro- and anterograde tracing and immunostaining revealed more neurons activated in red nucleus by cortical stimulation and enhanced corticorubral axons and synapses in red nucleus after SCI. Electrophysiological recordings showed lower threshold and higher amplitude of corticorubral synaptic response after SCI. We conclude that transcranial optogenetic motor mapping is sensitive and efficient for longitudinal evaluation of impairment and plasticity of SCI, and that spinal cord contusion induces stronger anatomical and functional corticorubral connection that may contribute to spontaneous recovery of motor function.

Effective Connectivity Measured Using Optogenetically Evoked Hemodynamic Signals Exhibits Topography Distinct from Resting State Functional Connectivity in the Mouse

Brain connectomics has expanded from histological assessment of axonal projection connectivity (APC) to encompass resting state functional connectivity (RS-FC). RS-FC analyses are efficient for whole-brain mapping, but attempts to explain aspects of RS-FC (e.g., interhemispheric RS-FC) based on APC have been only partially successful. Neuroimaging with hemoglobin alone lacks specificity for determining how activity in a population of cells contributes to RS-FC. Wide-field mapping of optogenetically defined connectivity could provide insights into the brain's structure-function relationship. We combined optogenetics with optical intrinsic signal imaging to create an efficient, optogenetic effective connectivity (Opto-EC) mapping assay. We examined EC patterns of excitatory neurons in awake, Thy1-ChR2 transgenic mice. These Thy1-based EC (Thy1-EC) patterns were evaluated against RS-FC over the cortex. Compared to RS-FC, Thy1-EC exhibited increased spatial specificity, reduced interhemispheric connectivity in regions with strong RS-FC, and appreciable connection strength asymmetry. Comparing the topography of Thy1-EC and RS-FC patterns to maps of APC revealed that Thy1-EC more closely resembled APC than did RS-FC. The more general method of Opto-EC mapping with hemoglobin can be determined for 100 sites in single animals in under an hour, and is amenable to other neuroimaging modalities. Opto-EC mapping represents a powerful strategy for examining evolving connectivity-related circuit plasticity.

Spectrally distinct channelrhodopsins for two-colour optogenetic peripheral nerve stimulation

Technologies for peripheral nerve stimulation have conventionally relied on the anatomic placement of electrodes adjacent to subsets of sensory fibres or motor fibres that selectively target an end effector. Here, we demonstrate the use of optogenetics to directly target the innervating fibres of an end effector by relying on retrograde transfection of adeno-associated virus serotype 6 to restrict axonal opsin expression to the desired fibre targets. By using an in vivo screen in rats, we identify the first channelrhodopsins as well as a halorhodopsin that respond to red light in the peripheral nerve. Combining two channelrhodopsins with spectrally distinct activation profiles allowed us to drive opposing muscle activity via two-colour illumination of the same mixed nerve. We also show halorhodopsin-mediated reductions in electrically evoked muscle tremor spectrally optimized for deep peripheral nerves. Our non-invasive peripheral neurostimulator with targeted multi-fascicle resolution enables scientific and clinical exploration, such as motor control in paralysis, biomimetic sensation feedback for amputees and targeted inhibition of muscle tremor.

Interaction of Intrinsic and Synaptic Currents Mediate Network Resonance Driven by Layer V Pyramidal Cells

Cortical oscillations modulate cellular excitability and facilitate neuronal communication and information processing. Layer 5 pyramidal cells (L5 PYs) drive low-frequency oscillations (<4 Hz) in neocortical networks in vivo. In vitro, individual L5 PYs exhibit subthreshold resonance in the theta band (4-8 Hz). This bandpass filtering of periodic input is mediated by h-current (Ih) and m-current (IM) that selectively suppress low-frequency input. It has remained unclear how these intrinsic properties of cells contribute to the emergent, network oscillation dynamics. To begin to close this gap, we studied the link between cellular and network mechanisms of network resonance driven by L5 PYs. We performed multielectrode array recordings of network activity in slices of medial prefrontal cortex from the Thy1-ChR2-eYFP line and activated the network by temporally patterned optogenetic suprathreshold stimulation. Networks driven by stimulation of L5 PYs exhibited resonance in the theta band. We found that Ih and IM play a role in resonant suprathreshold network response to depolarizing stimuli. The action of Ih in mediating resonance was dependent on synaptic transmission while that of IM was not. These results demonstrate how synergistic interaction of synaptic and intrinsic ion channels contribute to the response of networks driven by L5 PYs.

Wireless control of cellular function by activation of a novel protein responsive to electromagnetic fields

The Kryptopterus bicirrhis (glass catfish) is known to respond to electromagnetic fields (EMF). Here we tested its avoidance behavior in response to static and alternating magnetic fields stimulation. Using expression cloning we identified an electromagnetic perceptive gene (EPG) from the K. bicirrhis encoding a protein that responds to EMF. This EPG gene was cloned and expressed in mammalian cells, neuronal cultures and in rat’s brain. Immunohistochemistry showed that the expression of EPG is confined to the mammalian cell membrane. Calcium imaging in mammalian cells and cultured neurons expressing EPG demonstrated that remote activation by EMF significantly increases intracellular calcium concentrations, indicative of cellular excitability. Moreover, wireless magnetic activation of EPG in rat motor cortex induced motor evoked responses of the contralateral forelimb in vivo. Here we report on the development of a new technology for remote, non-invasive modulation of cell function.

Deep 2-photon imaging and artifact-free optogenetics through transparent graphene microelectrode arrays

Recent advances in optical technologies such as multi-photon microscopy and optogenetics have revolutionized our ability to record and manipulate neuronal activity. Combining optical techniques with electrical recordings is of critical importance to connect the large body of neuroscience knowledge obtained from animal models to human studies mainly relying on electrophysiological recordings of brain-scale activity. However, integration of optical modalities with electrical recordings is challenging due to generation of light-induced artifacts. Here we report a transparent graphene microelectrode technology that eliminates light-induced artifacts to enable crosstalk-free integration of 2-photon microscopy, optogenetic stimulation, and cortical recordings in the same in vivo experiment. We achieve fabrication of crack- and residue-free graphene electrode surfaces yielding high optical transmittance for 2-photon imaging down to ~ 1 mm below the cortical surface. Transparent graphene microelectrode technology offers a practical pathway to investigate neuronal activity over multiple spatial scales extending from single neurons to large neuronal populations.

A spherical aberration-free microscopy system for live brain imaging

The high-resolution in vivo imaging of mouse brain for quantitative analysis of fine structures, such as dendritic spines, requires objectives with high numerical apertures (NAs) and long working distances (WDs). However, this imaging approach is often hampered by spherical aberration (SA) that results from the mismatch of refractive indices in the optical path and becomes more severe with increasing depth of target from the brain surface. Whereas a revolving objective correction collar has been designed to compensate SA, its adjustment requires manual operation and is inevitably accompanied by considerable focal shift, making it difficult to acquire the best image of a given fluorescent object. To solve the problems, we have created an objective-attached device and formulated a fast iterative algorithm for the realization of an automatic SA compensation system. The device coordinates the collar rotation and the Z-position of an objective, enabling correction collar adjustment while stably focusing on a target. The algorithm provides the best adjustment on the basis of the calculated contrast of acquired images. Together, they enable the system to compensate SA at a given depth. As proof of concept, we applied the SA compensation system to in vivo two-photon imaging with a 25 × water-immersion objective (NA, 1.05; WD, 2 mm). It effectively reduced SA regardless of location, allowing quantitative and reproducible analysis of fine structures of YFP-labeled neurons in the mouse cerebral cortical layers. Interestingly, although the cortical structure was optically heterogeneous along the z-axis, the refractive index of each layer could be assessed on the basis of the compensation degree. It was also possible to make fully corrected three-dimensional reconstructions of YFP-labeled neurons in live brain samples. Our SA compensation system, called Deep-C, is expected to bring out the best in all correction-collar-equipped objectives for imaging deep regions of heterogeneous tissues.

GABAergic inhibition in dual-transmission cholinergic and GABAergic striatal interneurons is abolished in Parkinson disease

We report that half striatal cholinergic interneurons are dual transmitter cholinergic and GABAergic interneurons (CGINs) expressing ChAT, GAD65, Lhx7, and Lhx6 mRNAs, labeled with GAD and VGAT, generating monosynaptic dual cholinergic/GABAergic currents and an inhibitory pause response. Dopamine deprivation increases CGINs ongoing activity and abolishes GABAergic inhibition including the cortico-striatal pause because of high [Cl⁻]_i levels. Dopamine deprivation also dramatically increases CGINs dendritic arbors and monosynaptic interconnections probability, suggesting the formation of a dense CGINs network. The NKCC1 chloride importer antagonist bumetanide, which reduces [Cl⁻]_i levels, restores GABAergic inhibition, the cortico-striatal pause-rebound response, and attenuates motor effects of dopamine deprivation. Therefore, most of the striatal cholinergic excitatory drive is balanced by a concomitant powerful GABAergic inhibition that is impaired by dopamine deprivation. The attenuation by bumetanide of cardinal features of Parkinson’s disease paves the way to a novel therapeutic strategy based on a restoration of low [Cl⁻]_i levels and GABAergic inhibition.

Cholinergic interneurons of the striatum are involved reward-related behaviors and have been implicated in Parkinson’s disease. Here the authors report that half of cholinergic neurons co-release acetylcholine and GABA, and study the role of these neurons in a model of Parkinson’s Disease.