In vivo light-induced activation of neural circuitry in transgenic mice expressing channelrhodopsin-2

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.

Flexible Fiber Probe for Efficient Neural Stimulation and Detection

Functional probes are a leading contender for the recognition and manipulation of nervous behavior and are characterized by substantial scientific and technological potential. Despite the recent development of functional neural probes, a flexible biocompatible probe unit that allows for long-term simultaneous stimulation and signaling is still an important task. Here, a category of flexible tiny multimaterial fiber probes (<0.3 g) is described in which the metal electrodes are regularly embedded inside a biocompatible polymer fiber with a double-clad optical waveguide by thermal drawing. Significantly, this arrangement enables great improvement in mechanical properties, achieves high optical transmission (>90%), and effectively minimizes the impedance (by up to one order of magnitude) of the probe. This ability allows to realize long-term (at least 10 weeks) simultaneous optical stimulation and neural recording at the single-cell level in behaving mice with signal-to-noise ratio (SNR = 30 dB) that is more than 6 times that of the benchmark probe such as an all-polymer fiber.

Transient, Consequential Increases in Extracellular Potassium Ions Accompany Channelrhodopsin2 Excitation

Channelrhodopsin2 (ChR2) optogenetic excitation is widely used to study neurons, astrocytes, and circuits. Using complementary approaches in situ and in vivo, we found that ChR2 stimulation leads to significant transient elevation of extracellular potassium ions by ~5 mM. Such elevations were detected in ChR2-expressing mice, following local in vivo expression of ChR2(H134R) with adeno-associated viruses (AAVs), in different brain areas and when ChR2 was expressed in neurons or astrocytes. In particular, ChR2-mediated excitation of striatal astrocytes was sufficient to increase medium spiny neuron (MSN) excitability and immediate early gene expression. The effects on MSN excitability were recapitulated in silico with a computational MSN model and detected in vivo as increased action potential firing in awake, behaving mice. We show that transient, physiologically consequential increases in extracellular potassium ions accompany ChR2 optogenetic excitation. This coincidental effect may be important to consider during astrocyte studies employing ChR2 to interrogate neural circuits and animal behavior.

Using multiple approaches, Octeau et al. discover that optogenetic excitation of ChR2-expressing cells leads to significant transient extracellular potassium ion elevations that increase neuronal excitability and immediate early gene expression in neurons following in vivo stimulation.

Plasticity in oligodendrocyte lineage progression: An OPC puzzle on our nerves

Myelin deposition in the central nervous system has been shown to be responsive to experience, with sensory enrichment increasing myelination and sensory or social deprivation decreasing myelination. This process is referred to as "adaptive myelination" or "myelin plasticity" and signifies an essential component of new learning. However, whether these experience-driven adaptations are driven by (a) underlying changes in the generation of myelinating cells, (b) altered interactions between myelin sheath and axon, or (c) a combination of the above remains unclear. It has been suggested that myelination largely follows an "innate" and automatic programme, allowing for a predictable pattern of central nervous system myelin deposition over time. Adaptive myelination is thought to account for more nuanced alterations that do not dramatically shift this pattern, but ultimately drive functional responses. This makes the study of myelin plasticity particularly difficult, as it necessitates being able to clearly and specifically draw boundaries between the innate and adaptive programme. Thus, the field requires a holistic understanding of the remit of innate myelin development, prior to investigation of adaptive myelination. This review will collate literature regarding different aspects of oligodendrocyte and myelin development (namely, oligodendrocyte proliferation, differentiation, death and myelin sheath formation) in an innate context, before discussing how these parameters are proposed to change under adaptive conditions. It is the hope that this review will highlight the need for a comprehensive and integrated approach towards studying both innate and adaptive forms of myelination.

Noisy Light Augments the Na+ Current in Somatosensory Pyramidal Neurons of Optogenetic Transgenic Mice

In previous reports, we developed a method to apply Brownian optogenetic noise-photostimulation (BONP, 470 nm) up to 0.67 mW on the barrel cortex of in vivo ChR2 transgenic mice. In such studies, we found that the BONP produces an increase in the evoked field potentials and the neuronal responses of pyramidal neurons induced by somatosensory mechanical stimulation. Here we extended such findings by examining whether the same type of BONP augments the Na⁺ current amplitude elicited by voltage-clamp ramps of dissociated pyramidal neurons from the somatosensory cortex of ChR2 transgenic and wild type mice. We found that in all neurons from the ChR2 transgenic mice, but none of the wild type mice, the peak amplitude of a TTX-sensitive Na⁺ current and its inverse of latency exhibited inverted U-like graphs as a function of the BONP level. It means that an intermediate level of BONP increases both the peak amplitude of the Na⁺ current and its inverse of latency. Our research suggests that the impact of BONP on the Na⁺ channels of pyramidal neurons could be associated with the observed augmentation-effects in our previous in vivo preparation. Moreover, it provides caution information for the use of an appropriate range of light intensity, <0.67 mW, which could avoid opto non-genetics (also termed “optonongenetic”) related responses due to light-induced temperature changes.

Familiarity Detection and Memory Consolidation in Cortical Assemblies

Humans have a large capacity of recognition memory (Dudai, 1997), a fundamental property of higher-order brain functions such as abstraction and generalization (Vogt and Magnussen, 2007). Familiarity is the first step towards recognition memory. We have previously demonstrated using unsupervised neural network simulations that familiarity detection of complex patterns emerges in generic cortical microcircuits with bidirectional synaptic plasticity. It is therefore meaningful to conduct similar experiments on biological neuronal networks to validate these results. Studies of learning and memory in dissociated rodent neuronal cultures remain inconclusive to date. Synchronized network bursts (SNBs) that occur spontaneously and periodically have been speculated to be an intervening factor. By optogenetically stimulating cultured cortical networks with random dot movies (RDMs), we were able to reduce the occurrence of SNBs, after which an ability for familiarity detection emerged: previously seen patterns elicited higher firing rates than novel ones. Differences in firing rate were distributed over the entire network, suggesting that familiarity detection is a system level property. We also studied the change in SNB patterns following familiarity encoding. Support vector machine (SVM) classification results indicate that SNBs may be facilitating memory consolidation of the learned pattern. In addition, using a novel network connectivity probing method, we were able to trace the change in synaptic efficacy induced by familiarity encoding, providing insights on the long-term impact of having SNBs in the cultures.

Temporal Sharpening of Sensory Responses by Layer V in the Mouse Primary Somatosensory Cortex

The timing of stimulus-evoked spikes encodes information about sensory stimuli. Here we studied the neural circuits controlling this process in the mouse primary somatosensory cortex. We found that brief optogenetic activation of layer V pyramidal cells just after whisker deflection modulated the membrane potential of neurons and interrupted their long-latency whisker responses, increasing their accuracy in encoding whisker deflection time. In contrast, optogenetic inhibition of layer V during either passive whisker deflection or active whisking decreased accuracy in encoding stimulus or touch time, respectively. Suppression of layer V pyramidal cells increased reaction times in a texture discrimination task. Moreover, two-color optogenetic experiments revealed that cortical inhibition was efficiently recruited by layer V stimulation and that it mainly involved activation of parvalbumin-positive rather than somatostatin-positive interneurons. Layer V thus performs behaviorally relevant temporal sharpening of sensory responses through circuit-specific recruitment of cortical inhibition.

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.

Fabrication and ex vivo evaluation of activated carbon-Pt microparticle based glutamate biosensor

As one of the most abundant neurotransmitters in the brain and the spinal cord, glutamate plays many important roles in the nervous system. Precise information about the level of glutamate in the extracellular space of living brain tissue may provide new insights on fundamental understanding of the role of glutamate in neurological disorders as well as neurophysiological phenomena. Electrochemical sensor has emerged as a promising solution that can satisfy the requirement for highly reliable and continuous monitoring method with good spatiotemporal resolution for characterization of extracellular glutamate concentration. Recently, we published a method to create a simple printable glutamate biosensor using platinum nanoparticles. In this work, we introduce an even simpler and lower cost conductive polymer composite using commercially available activated carbon with platinum microparticles to easily fabricate highly sensitive glutamate biosensor using direct ink writing method. The fabricated biosensors are functionality superior than previously reported with the sensitivity of 5.73 ± 0.078 nA μM^-1 mm^-2, detection limit of 0.03 μM, response time less than or equal to 1 s, and a linear range from 1 μM up to 925 μM. In this study, we utilize astrocyte cell culture to demonstrate our biosensor's ability to monitor glutamate uptake process. We also demonstrate direct measurement of glutamate release from optogenetic stimulation in mouse primary visual cortex (V1) brain slices.

Towards the optical cochlear implant: optogenetic approaches for hearing restoration

Cochlear implants (CIs) are considered the most successful neuroprosthesis as they enable speech comprehension in the majority of half a million CI users suffering from sensorineural hearing loss. By electrically stimulating the auditory nerve, CIs constitute an interface re-connecting the brain and the auditory scene, providing the patient with information regarding the latter. However, since electric current is hard to focus in conductive environments such as the cochlea, the precision of electrical sound encoding-and thus quality of artificial hearing-is limited. Recently, optogenetic stimulation of the cochlea has been suggested as an alternative approach for hearing restoration. Cochlear optogenetics promises increased spectral selectivity of artificial sound encoding, hence improved hearing, as light can conveniently be confined in space to activate the auditory nerve within smaller tonotopic ranges. In this review, we discuss the latest experimental and technological developments of cochlear optogenetics and outline the remaining challenges on the way to clinical translation.

Brain-State-Dependent Modulation of Neuronal Firing and Membrane Potential Dynamics in the Somatosensory Thalamus during Natural Sleep

The thalamus plays a central role in sleep rhythms in the mammalian brain and, yet, surprisingly little is known about its function and interaction with local cortical oscillations during NREM sleep (NREM). We investigated the neuronal correlates of cortical barrel activity in the two corresponding thalamic nuclei, the ventral posterior medial (VPM), and the posterior medial (Pom) nuclei during natural NREM in mice. Our data reveal (1) distinct modulations of VPM and Pom activity throughout NREM episodes, (2) a thalamic nucleus-specific phase-locking to cortical slow and spindle waves, (3) cell-specific subthreshold spindle oscillations in VPM neurons that only partially overlap with cortical spindles, and (4) that spindle features evolve throughout NREM episodes and vary according to the post-NREM state. Taken together, our results suggest that, during natural sleep, the barrel cortex exerts a leading role in the generation and transfer of slow rhythms to the somatosensory thalamus and reciprocally for spindle oscillations.

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.

Alpha-synuclein targets GluN2A NMDA receptor subunit causing striatal synaptic dysfunction and visuospatial memory alteration

Parkinson's disease is a progressive neurodegenerative disorder characterized by altered striatal dopaminergic signalling that leads to motor and cognitive deficits. Parkinson's disease is also characterized by abnormal presence of soluble toxic forms of α-synuclein that, when clustered into Lewy bodies, represents one of the pathological hallmarks of the disease. However, α-synuclein oligomers might also directly affect synaptic transmission and plasticity in Parkinson's disease models. Accordingly, by combining electrophysiological, optogenetic, immunofluorescence, molecular and behavioural analyses, here we report that α-synuclein reduces N-methyl-d-aspartate (NMDA) receptor-mediated synaptic currents and impairs corticostriatal long-term potentiation of striatal spiny projection neurons, of both direct (D1-positive) and indirect (putative D2-positive) pathways. Intrastriatal injections of α-synuclein produce deficits in visuospatial learning associated with reduced function of GluN2A NMDA receptor subunit indicating that this protein selectively targets this subunit both in vitro and ex vivo. Interestingly, this effect is observed in spiny projection neurons activated by optical stimulation of either cortical or thalamic glutamatergic afferents. We also found that treatment of striatal slices with antibodies targeting α-synuclein prevents the α-synuclein-induced loss of long-term potentiation and the reduced synaptic localization of GluN2A NMDA receptor subunit suggesting that this strategy might counteract synaptic dysfunction occurring in Parkinson's disease.

Optogenetic Control of the Peripheral Nervous System

The peripheral nervous system (PNS) is highly complicated and heterogenous. Conventional neuromodulatory approaches have revealed numerous essential biological functions of the PNS and provided excellent tools to treat a large variety of human diseases. Yet growing evidence indicated the importance of cell-type-specific neuromodulation in the PNS in not only biological research using animal models but also potential human therapies. Optogenetics is a recently developed neuromodulatory approach combining optics and genetics that can effectively stimulate or silence neuronal activity with high spatial and temporal precision. Here, I review research regarding optogenetic manipulations for cell-type-specific control of the PNS, highlighting the advantages and challenges of current optogenetic tools, and discuss their potential future applications.

Optogenetics in the Era of Cerebral Organoids

The human brain has been deemed the most complex organ and has captivated neuroscientists for decades. Most studies of this organ have relied on reductionist model systems. Although all model systems are essentially wrong, cerebral organoids so far represent the closest recapitulation of human brain development and disease both in terms of cell diversity and organization. The optogenetic technique can be used in this context to study the functional neuroanatomy of the brain, to examine the neural circuits, and to determine the etiology of neurological disorders. In this opinion article, we suggest ways in which optogenetics can be combined with cerebral organoids to allow unprecedented precision and accuracy in studying normal and aberrant neurodevelopmental processes and, as well, neurodegenerative diseases.

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.

Corticostriatal Transmission Is Selectively Enhanced in Striatonigral Neurons with Postnatal Loss of Tsc1

mTORC1 is a central signaling hub that integrates intra- and extracellular signals to regulate a variety of cellular metabolic processes. Mutations in regulators of mTORC1 lead to neurodevelopmental disorders associated with autism, which is characterized by repetitive, inflexible behaviors. These behaviors may result from alterations in striatal circuits that control motor learning and habit formation. However, the consequences of mTORC1 dysregulation on striatal neuron function are largely unknown. To investigate this, we deleted the mTORC1 negative regulator Tsc1 from identified striatonigral and striatopallidal neurons and examined how cell-autonomous upregulation of mTORC1 activity affects their morphology and physiology. We find that loss of Tsc1 increases the excitability of striatonigral, but not striatopallidal, neurons and selectively enhances corticostriatal synaptic transmission. These findings highlight the critical role of mTORC1 in regulating striatal activity in a cell type- and input-specific manner, with implications for striatonigral pathway dysfunction in neuropsychiatric disease.

Task-Demand-Dependent Neural Representation of Odor Information in the Olfactory Bulb and Posterior Piriform Cortex

In awake rodents, the neural representation of olfactory information in the olfactory bulb is largely dependent on brain state and behavioral context. Learning-modified neural plasticity has been observed in mitral/tufted cells, the main output neurons of the olfactory bulb. Here, we propose that the odor information encoded by mitral/tufted cell responses in awake mice is highly dependent on the behavioral task demands. We used fiber photometry to record calcium signals from the mitral/tufted cell population in awake, head-fixed male mice under different task demands. We found that the mitral/tufted cell population showed similar responses to two distinct odors when the odors were presented in the context of a go/go task, in which the mice received a water reward regardless of the identity of the odor presented. However, when the same odors were presented in a go/no-go task, in which one odor was rewarded and the other was not, then the mitral cell population responded very differently to the two odors, characterized by a robust reduction in the response to the nonrewarded odor. Thus, the representation of odors in the mitral/tufted cell population depends on whether the task requires discrimination of the odors. Strikingly, downstream of the olfactory bulb, pyramidal neurons in the posterior piriform cortex also displayed a task-demand-dependent neural representation of odors, but the anterior piriform cortex did not, indicating that these two important higher olfactory centers use different strategies for neural representation.SIGNIFICANCE STATEMENT The most important task of the olfactory system is to generate a precise representation of odor information under different brain states. Whether the representation of odors by neurons in olfactory centers such as the olfactory bulb and the piriform cortex depends on task demands remains elusive. We find that odor representation in the mitral/tufted cells of the olfactory bulb depends on whether the task requires odor discrimination. A similar neural representation is found in the posterior piriform cortex but not the anterior piriform cortex, indicating that these higher olfactory centers use different representational strategies. The task-demand-dependent representational strategy is likely important for facilitating information processing in higher brain centers responsible for decision making and encoding of salience.

Context-dependent limb movement encoding in neuronal populations of motor cortex

Neuronal networks of the mammalian motor cortex (M1) are important for dexterous control of limb joints. Yet it remains unclear how encoding of joint movement in M1 depends on varying environmental contexts. Using calcium imaging we measured neuronal activity in layer 2/3 of the M1 forelimb region while mice grasped regularly or irregularly spaced ladder rungs during locomotion. We found that population coding of forelimb joint movements is sparse and varies according to the flexibility demanded from individual joints in the regular and irregular context, even for equivalent grasping actions across conditions. This context-dependence of M1 encoding emerged during task learning, fostering higher precision of grasping actions, but broke apart upon silencing of projections from secondary motor cortex (M2). These findings suggest that M1 exploits information from M2 to adapt encoding of joint movements to the flexibility demands of distinct familiar contexts, thereby increasing the accuracy of motor output.

Network activity in primary motor cortex (M1) controls dexterous limb movements. Here, the authors show that the M1 population code varies according to contextual motor demands that are conveyed via the secondary motor cortex (M2).

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.

Leptin modulates olfactory discrimination and neural activity in the olfactory bulb

AIM

Leptin is an important peptide hormone that regulates food intake and plays a crucial role in modulating olfactory function. Although a few previous studies have investigated the effect of leptin on odor perception and discrimination in rodents, research on the neural basis underlying the behavioral changes is lacking. Here we study how leptin affects behavioral performance during a go/no-go task and how it modulates neural activity of mitral/tufted cells in the olfactory bulb, which plays an important role in odor information processing and representation.

METHODS

A go/no-go odor discrimination task was used in the behavioral test. For in vivo studies, single unit recordings, local field potential recordings and fiber photometry recordings were used. For in vitro studies, we performed patch clamp recordings in the slice of the olfactory bulb.

RESULTS

Behaviorally, leptin affects performance and reaction time in a difficult odor-discrimination task. Leptin decreases the spontaneous firing of single mitral/tufted cells, decreases the odor-evoked beta and high gamma local field potential response, and has bidirectional effects on the odor-evoked responses of single mitral/tufted cells. Leptin also inhibits the population calcium activity in genetically identified mitral/tufted cells and granule cells. Furthermore, in vitro slice recordings reveal that leptin inhibits mitral cell activity through direct modulation of the voltage-sensitive potassium channel.

CONCLUSIONS

The behavioral reduction in odor discrimination observed after leptin administration is likely due to decreased neural activity in mitral/tufted cells, caused by modulation of potassium channels in these cells.

An active role for neurons in glioma progression: making sense of Scherer's structures

Perineuronal satellitosis, the microanatomical clustering of glioma cells around neurons in the tumor microenvironment, has been recognized as a histopathological hallmark of high-grade gliomas since the seminal observations of Scherer in the 1930s. In this review, we explore the emerging understanding that neuron‒glioma cell interactions regulate malignancy and that neuronal activity is a critical determinant of glioma growth and progression. Elucidation of the interplay between normal and malignant neural circuitry is critical to realizing the promise of effective therapies for these seemingly intractable diseases. Here, we review current knowledge regarding the role of neuronal activity in the glioma microenvironment and highlight critical knowledge gaps in this burgeoning research space.

Modulation of Neural Activity for Myelination in the Central Nervous System

Electrical stimulation has been playing a significant role in revealing various functions and mechanisms of the nervous system. It is no different for myelination, a process in which oligodendrocytes in the central nervous system (CNS) or Schwann Cells in the peripheral nerve system (PNS) wrap around axons to provide an insulating layer in vitro and in vivo. It has been widely recognized that the myelin sheath accelerates axon signal conduction and provides neuroprotection. Recent studies have begun to reveal its role in plasticity. The major mechanism that enables this process is activity-dependent myelination - the phenomenon where neuronal activity supports oligodendrocyte maturation and myelin sheath formation. In light of recent discoveries, a better understanding of this phenomenon has a potential to provide therapeutic targets for not only demyelinating diseases, but also psychiatric disorders. There is a growing need for experimental platforms capable of dissecting the effect of neural activity on myelination in health and disease. The effect of neural activity is commonly studied by comparing the myelination levels in cultures with neurons of low and high activity. Electrical stimulation is particularly well suited as a method of inducing neural activity in these systems. In this review, we describe in vitro platforms for studying activity-dependent myelination, which utilize neuron stimulation via electrical field. We also discuss stimulation profiles, as well as the alternatives to electrical stimulation in the context of regular, compartmentalized, and organotypic co-cultures.

Nanostructure Endows Neurotherapeutic Potential in Optogenetics: Current Development and Future Prospects

Optogenetics have evolved as a promising tool to control the processes at a cellular level via photons. Specially, it confers a specific control over cellular function through real-time cytomodulation even in freely moving animals. Neuronal stimulation is prerequisite for deep tissue light penetration or insertion of optrode for light illumination to the neurons that have been proven to be compromised due to poor light penetration and invasiveness of the procedure, respectively. In this review, the application of nanotechnology is being elaborated by the use of metal nanoparticles (AuNPs), upconversion nanocrystals (UCNPs), and quantum dots (CdSe) for targeting particular organs or tissues, and their potential to emit a specific light on excitation to overcome the limitations associated with earlier methods has been elucidated. The optothermal and magnetothermal properties, photoluminescence, and higher photostability of nanomaterials are explored in context of therapeutic applicability of optogenetics. The nanostructure characteristics and specific ion channel targeting have shown promising therapeutic potential against neurodegenerative disorders (Alzheimer's, Parkinson's, Huntington's), epilepsy, and blindness. This review compiles mechanical and optical characteristics of nanomaterials that endow superior optogenetic therapeutic potentials to cure immedicable infirmities.

Blue Light Promotes Neurite Outgrowth of Retinal Explants in Postnatal ChR2 Mice

Neurons in the adult mammalian CNS fails to regenerate after severe injury. However, it is known that an increase in neural activity occurs in mouse retinal ganglion cells (RGCs) after extrinsic stimulation and this can induce axon growth. In the present study, we applied an optogenetic approach using a mouse model, specifically involving channelrhodopsin-2 (ChR2) expression in RGCs. We investigated whether modulation of RGC neural activity exclusively by blue light stimulation is able to promote neurite outgrowth of postnatal retinal explants. The results showed that activation of RGCs expressing ChR2 by 20 Hz blue light for 1 h is a most effective way of enhancing neurite outgrowth in postnatal retinas. This is achieved via gap junctions that spread neural activity across the whole retina. Moreover, we found that activation of intrinsic photosensitive RGCs (ipRGCs) by blue light also contributes significantly to the promotion of neurite outgrowth in the same postnatal retinal explants. Our findings not only demonstrate that a short-term increase in RGC neural activity is sufficient to facilitate the neurite outgrowth of retinal explants, but also highlight the fact that the temporal pattern of neural activity in RGCs is a critical factor in regulating axon regeneration.

Distinct Contributions of Whisker Sensory Cortex and Tongue-Jaw Motor Cortex in a Goal-Directed Sensorimotor Transformation

The neural circuits underlying goal-directed sensorimotor transformations in the mammalian brain are incompletely understood. Here, we compared the role of primary tongue-jaw motor cortex (tjM1) and primary whisker sensory cortex (wS1) in head-restrained mice trained to lick a reward spout in response to whisker deflection. Two-photon microscopy combined with microprisms allowed imaging of neuronal network activity across cortical layers in transgenic mice expressing a genetically encoded calcium indicator. Early-phase activity in wS1 encoded the whisker sensory stimulus and was necessary for detection of whisker stimuli. Activity in tjM1 encoded licking direction during task execution and was necessary for contralateral licking. Pre-stimulus activity in tjM1, but not wS1, was predictive of lick direction and contributed causally to small preparatory jaw movements. Our data reveal a shift in coding scheme from wS1 to tjM1, consistent with the hypothesis that these areas represent cortical start and end points for this goal-directed sensorimotor transformation.

Stereotactic system for accurately targeting deep brain structures in awake head-fixed mice

Deep brain nuclei, such as the amygdala, nucleus basalis, and locus coeruleus, play a crucial role in cognition and behavior. Nonetheless, acutely recording electrical activity from these structures in head-fixed awake rodents has been very challenging due to the fact that head-fixed preparations are not designed for stereotactic accuracy. We overcome this issue by designing the DeepTarget, a system for stereotactic head fixation and recording, which allows for accurately directing recording electrodes or other probes into any desired location in the brain. We then validated it by performing intracellular recordings from optogenetically tagged amygdalar neurons followed by histological reconstruction, which revealed that it is accurate and precise to within ~100 μm. Moreover, in another group of mice we were able to target both the mammillothalamic tract and subthalamic nucleus. This approach can be adapted to any type of extracellular electrode, fiber optic, or other probe in cases where high accuracy is needed in awake, head-fixed rodents.NEW & NOTEWORTHY Accurate targeting of recording electrodes in awake head-restrained rodents is currently beyond our reach. We developed a device for stereotactic implantation of a custom head bar and a recording system that together allow the accurate and precise targeting of any brain structure, including deep and small nuclei. We demonstrated this by performing histology and intracellular recordings in the amygdala of awake mice. The system enables the targeting of any probe to any location in the awake brain.

Mapping Structure-Function Relationships in the Brain

Mapping the structural and functional connectivity of the brain is a major focus of systems neuroscience research and will help to identify causally important changes in neural circuitry responsible for behavioral dysfunction. Several methods for examining brain activity in humans have been extended to rodent and monkey models in which molecular and genetic manipulations exist for linking to human disease. In this review, which is part of a special issue focused on bridging brain connectivity information across species and spatiotemporal scales, we address mapping brain activity and neural connectivity in rodents using optogenetics in conjunction with either functional magnetic resonance imaging or optical intrinsic signal imaging. We chose to focus on these techniques because they are capable of reporting spontaneous or evoked hemodynamic activity most closely linked to human neuroimaging studies. We discuss the capabilities and limitations of blood-based imaging methods, usage of optogenetic techniques to map neural systems in rodent models, and other powerful mapping techniques for examining neural connectivity over different spatial and temporal scales. We also discuss implementing strategies for mapping brain connectivity in humans with both basic and clinical applications, and conclude with how cross-species mapping studies can be utilized to influence preclinical imaging studies and clinical practices alike.

Potential of Engineered Bacteriorhodopsins as Photoactivated Biomaterials in Modulating Neural Stem Cell Behavior

Bacteriorhodopsin (BR), a light-sensitive bacterial proton pump, has been demonstrated the capacity for regulating the neural activity in mammalian cells. Because of the difficulty in production and purification in large quantities, the BR proteins have neither been directly employed to biomedical applications nor verified the functionality by protein administration. Previously, we have invented a highly expressible bacteriorhodopsin (HEBR) and established the massive production protocol. In the current study, we mass-produced the two types of HEBR proteins that have normal or abnormal activity on the proton pumping, and then we treated murine neural stem cells (NSCs) with these HEBR proteins. We discovered that the cell behaviors including growth, metabolism, mitochondrial inner membrane potential, and differentiation were obviously affected in NSCs after the treatment of HEBR proteins. Particularly, these effects induced by HEBR proteins were correlated to their proton pump activity and could be altered by cell culture substrate materials. Current findings suggest that the engineered light-sensitive HEBR protein can serve as a biological material to directly influence the multiple behaviors of mammalian cells, which is further modified by the cell culture substrate material, revealing the versatile potential of HEBR protein in biomaterial applications.

Loss of Adaptive Myelination Contributes to Methotrexate Chemotherapy-Related Cognitive Impairment

Activity-dependent myelination is thought to contribute to adaptive neurological function. However, the mechanisms by which activity regulates myelination and the extent to which myelin plasticity contributes to non-motor cognitive functions remain incompletely understood. Using a mouse model of chemotherapy-related cognitive impairment (CRCI), we recently demonstrated that methotrexate (MTX) chemotherapy induces complex glial dysfunction for which microglial activation is central. Here, we demonstrate that remote MTX exposure blocks activity-regulated myelination. MTX decreases cortical Bdnf expression, which is restored by microglial depletion. Bdnf-TrkB signaling is a required component of activity-dependent myelination. Oligodendrocyte precursor cell (OPC)-specific TrkB deletion in chemotherapy-naive mice results in impaired cognitive behavioral performance. A small-molecule TrkB agonist rescues both myelination and cognitive impairment after MTX chemotherapy. This rescue after MTX depends on intact TrkB expression in OPCs. Taken together, these findings demonstrate a molecular mechanism required for adaptive myelination that is aberrant in CRCI due to microglial activation.

Optogenetic approaches to study the mammalian brain

Optogenetics has revolutionized neurobiological research by allowing to disentangle intricate neuronal circuits at a spatio-temporal precision unmatched by other techniques. Here, we review current advances of optogenetic applications in mammals, especially focusing on freely moving animals. State-of-the-art strategies allow the targeted expression of opsins in neuronal subpopulations, defined either by genetic cell type or neuronal projection pattern. Optogenetic manipulations of these subpopulations become particularly powerful when combined with behavioral paradigms and neurophysiological readout techniques. Thereby, specific roles can be assigned to identified cells. All-optical approaches with the opportunity to write complex three dimensional patterns into neuronal networks have recently emerged. While clinical implications of the new tool set seem tempting, we emphasize here the role of optogenetics for basic research.

Mutant huntingtin enhances activation of dendritic Kv4 K+ channels in striatal spiny projection neurons

Huntington's disease (HD) is initially characterized by an inability to suppress unwanted movements, a deficit attributable to impaired synaptic activation of striatal indirect pathway spiny projection neurons (iSPNs). To better understand the mechanisms underlying this deficit, striatal neurons in ex vivo brain slices from mouse genetic models of HD were studied using electrophysiological, optical and biochemical approaches. Distal dendrites of iSPNs from symptomatic HD mice were hypoexcitable, a change that was attributable to increased association of dendritic Kv4 potassium channels with auxiliary KChIP subunits. This association was negatively modulated by TrkB receptor signaling. Dendritic excitability of HD iSPNs was rescued by knocking-down expression of Kv4 channels, by disrupting KChIP binding, by restoring TrkB receptor signaling or by lowering mutant-Htt (mHtt) levels with a zinc finger protein. Collectively, these studies demonstrate that mHtt induces reversible alterations in the dendritic excitability of iSPNs that could contribute to the motor symptoms of HD.

Neuronal activity in vivo enhances functional myelin repair

In demyelinating diseases such as Multiple Sclerosis (MS), demyelination of neuronal fibers impairs impulse conduction and causes axon degeneration. While neuronal activity stimulates oligodendrocyte production and myelination in normal conditions, it remains unclear whether the activity of demyelinated axons restores their loss-of-function in a harmful environment. To investigate this question, we established a model to induce a moderate optogenetic stimulation of demyelinated axons in the corpus callosum at the level of the motor cortex in which cortical circuit activation and locomotor effects were reduced in adult freely moving mice. We demonstrate that a moderate activation of demyelinated axons enhances the differentiation of oligodendrocyte precursor cells onto mature oligodendrocytes, but only under a repeated stimulation paradigm. This activity-dependent increase in the oligodendrocyte pool promotes an extensive remyelination and functional restoration of conduction, as revealed by ultrastructural analyses and compound action potential recordings. Our findings reveal the need of preserving an appropriate neuronal activity in the damaged tissue to promote oligodendrocyte differentiation and remyelination, likely by enhancing axon-oligodendroglia interactions. Our results provide new perspectives for translational research using neuromodulation in demyelinating diseases.

A light in the dark: state of the art and perspectives in optogenetics and optopharmacology for restoring vision

In the last decade, innovative therapeutic strategies against inherited retinal degenerations (IRDs) have emerged. In particular, chemical- and opto-genetics approaches or a combination of them have been identified for modulating neuronal/optical activity in order to restore vision in blinding diseases. The ‘chemical-genetics approach’ (optopharmacology) uses small molecules (exogenous photoswitches) for restoring light sensitivity by activating ion channels. The ‘opto-genetics approach’ employs light-activated photosensitive proteins (exogenous opsins), introduced by viral vectors in injured tissues, to restore light response. These approaches offer control of neuronal activities with spatial precision and limited invasiveness, although with some drawbacks. Currently, a combined therapeutic strategy (optogenetic pharmacology) is emerging. This review describes the state of the art and provides an overview of the future perspectives in vision restoration.

Selective remodeling of glutamatergic transmission to striatal cholinergic interneurons after dopamine depletion

The widely held view that the pathophysiology of Parkinson's disease arises from an under-activation of the direct pathway striatal spiny neurons (dSPNs) has gained support from a recently described weakening of the glutamatergic projection from the parafascicular nucleus (PfN) to dSPNs in experimental parkinsonism. However, the impact of the remodeling of the thalamostriatal projection cannot be fully appreciated without considering its impact on cholinergic interneurons (ChIs) that themselves preferentially activate indirect pathway spiny neurons (iSPNs). To study this thalamostriatal projection, we virally transfected with Cre-dependent channelrhodopsin-2 (ChR2) the PfN of Vglut2-Cre mice that were dopamine-depleted with 6-hydroxydopamine (6-OHDA). In parallel, we studied the corticostriatal projection to ChIs in 6-OHDA-treated transgenic mice expressing ChR2 under the Thy1 promoter. We found the 6-OHDA lesions failed to affect short-term synaptic plasticity or the size of unitary responses evoked optogenetically in either of these projections. However, we found that NMDA-to-AMPA ratios at PfN synapses-that were significantly larger than NMDA-to-AMPA ratios at cortical synapses-were reduced by 6-OHDA treatment, thereby impairing synaptic integration at PfN synapses onto ChIs. Finally, we found that application of an agonist of the D5 dopamine receptors on ChIs potentiated NMDA currents without affecting AMPA currents or short-term plasticity selectively at PfN synapses. We propose that dopamine depletion leads to an effective de-potentiation of NMDA currents at PfN synapses onto ChIs which degrades synaptic integration. This selective remodeling of NMDA currents at PfN synapses may counter the selective weakening of PfN synapses onto dSPNs in parkinsonism.

Strong, weak and neuron type dependent lateral inhibition in the olfactory bulb

In many sensory systems, different sensory features are transmitted in parallel by several different types of output neurons. In the mouse olfactory bulb, there are only two output neuron types, the mitral and tufted cells (M/T), which receive similar odor inputs, but they are believed to transmit different odor characteristics. How these two neuron types deliver different odor information is unclear. Here, by combining electrophysiology and optogenetics, it is shown that distinct inhibitory networks modulate M/T cell responses differently. Overall strong lateral inhibition was scarce, with most neurons receiving lateral inhibition from a handful of unorganized surrounding glomeruli (~5% on average). However, there was a considerable variability between different neuron types in the strength and frequency of lateral inhibition. Strong lateral inhibition was mostly found in neurons locked to the first half of the respiration cycle. In contrast, weak inhibition arriving from many surrounding glomeruli was relatively more common in neurons locked to the late phase of the respiration cycle. Proximal neurons could receive different levels of inhibition. These results suggest that there is considerable diversity in the way M/T cells process odors so that even neurons that receive the same odor input transmit different odor information to the cortex.

Population dynamics and entrainment of basal ganglia pacemakers are shaped by their dendritic arbors

The theory of phase oscillators is an essential tool for understanding population dynamics of pacemaking neurons. GABAergic pacemakers in the substantia nigra pars reticulata (SNr), a main basal ganglia (BG) output nucleus, receive inputs from the direct and indirect pathways at distal and proximal regions of their dendritic arbors, respectively. We combine theory, optogenetic stimulation and electrophysiological experiments in acute brain slices to ask how dendritic properties impact the propensity of the various inputs, arriving at different locations along the dendrite, to recruit or entrain SNr pacemakers. By combining cable theory with sinusoidally-modulated optogenetic activation of either proximal somatodendritic regions or the entire somatodendritic arbor of SNr neurons, we construct an analytical model that accurately fits the empirically measured somatic current response to inputs arising from illuminating the soma and various portions of the dendritic field. We show that the extent of the dendritic tree that is illuminated generates measurable and systematic differences in the pacemaker’s phase response curve (PRC), causing a shift in its peak. Finally, we show that the divergent PRCs correctly predict differences in two major features of the collective dynamics of SNr neurons: the fidelity of population responses to sudden step-like changes in inputs; and the phase latency at which SNr neurons are entrained by rhythmic stimulation, which can occur in the BG under both physiological and pathophysiological conditions. Our novel method generates measurable and physiologically meaningful spatial effects, and provides the first empirical demonstration of how the collective responses of SNr pacemakers are determined by the transmission properties of their dendrites. SNr dendrites may serve to delay distal striatal inputs so that they impinge on the spike initiation zone simultaneously with pallidal and subthalamic inputs in order to guarantee a fair competition between the influence of the monosynaptic direct- and polysynaptic indirect pathways.

The substantia nigra pars reticulata (SNr) is a main output nucleus of the basal ganglia (BG), where inputs from the competing direct and indirect pathways converge onto the same neurons. Interestingly, these inputs are differentially distributed with direct and indirect pathway projections arriving at distal and proximal regions of the dendritic arbor, respectively. We employ a novel method combining theory with electrophysiological experiments and optogenetics to study the distinct effects of inputs arriving at different locations along the dendrite. Our approach represents a useful compromise between complexity and reduction in modelling. Our work addresses the question of high fidelity encoding of inputs by networks of neurons in the new context of pacemaking neurons, which are driven to fire by their intrinsic dynamics rather than by a network state. We provide the first empirical demonstration that dendritic delays can introduce latencies in the responses of a population of neurons that are commensurate with synaptic delays, suggesting a new role for SNr dendrites with implications for BG function.

Potential Application of Optogenetic Stimulation in the Treatment of Pain and Migraine Headache: A Perspective from Animal Studies

Optogenetic manipulation is uniquely useful in unraveling the functional organization of neuronal circuits in the central nervous system by enabling reversible gain- or loss-of-function of discrete populations of neurons within restricted brain regions. This state-of-the-art technology can produce circuit-specific neuromodulation by overexpressing light-sensitive proteins (opsins) in particular cell types of interest. Here, we discuss the principle of optogenetic manipulation and its application in pain research using animal models, and we also discuss how to potentially use optogenetic stimulation in the treatment of migraine headache in the future.

The Astrocyte-Neuron Interface: An Overview on Molecular and Cellular Dynamics Controlling Formation and Maintenance of the Tripartite Synapse

Astrocytes are known to provide trophic support to neurons and were originally thought to be passive space-filling cells in the brain. However, recent advances in astrocyte development and functions have highlighted their active roles in controlling brain functions by modulating synaptic transmission. A bidirectional cross talk between astrocytic processes and neuronal synapses define the concept of tripartite synapse. Any change in astrocytic structure/function influences neuronal activity which could lead to neurodevelopmental and neurodegenerative disorders. In this chapter, we briefly overview the methodologies used in deciphering the mechanisms of dynamic interplay between astrocytes and neurons.

Neuromodulation-dependent effect of gated high-frequency, LFMS-like electric field stimulation in mouse cortical slices

Low-field magnetic stimulation (LFMS) is a gated high-frequency non-invasive brain stimulation method (500 Hz gated at 2 Hz) with a proposed antidepressant effect. However, it has remained unknown how such stimulation paradigms modulate neuronal network activity and how the induced changes depend on network state. Here we examined the immediate and outlasting effects of the gated high-frequency electric field associated with LFMS on the cortical activity as a function of neuromodulatory tone that defines network state. We used a sham-controlled study design to investigate effects of stimulation (20 min of 0.5 s trains of 500 Hz charge-balanced pulse stimulation patterned at 0.5 Hz) on neural activity in mouse medial prefrontal cortex in vitro. Bath application of cholinergic and noradrenergic agents enabled us to examine the stimulation effects as a function of neuromodulatory tone. The stimulation attenuated the increase in firing rate of layer V cortical neurons during the post-stimulation period in the presence of cholinergic activation. The same stimulation had no significant immediate or outlasting effect in the absence of exogenous neuromodulators or in the presence of noradrenergic activation. These results provide electrophysiological insights into the neuromodulatory-dependent effects of gated high-frequency stimulation. More broadly, our results are the first to provide a mechanistic demonstration of how behavioral states and arousal levels may modify the effects of non-invasive brain stimulation.

Diminishing fear: Optogenetic approach toward understanding neural circuits of fear control

Understanding complex behavioral processes, both learned and innate, requires detailed characterization of the principles governing signal flow in corresponding neural circuits. Previous studies were hampered by the lack of appropriate tools needed to address the complexities of behavior-driving micro- and macrocircuits. The development and implementation of optogenetic methodologies revolutionized the field of behavioral neuroscience, allowing precise spatiotemporal control of specific, genetically defined neuronal populations and their functional connectivity both in vivo and ex vivo, thus providing unprecedented insights into the cellular and network-level mechanisms contributing to behavior. Here, we review recent pioneering advances in behavioral studies with optogenetic tools, focusing on mechanisms of fear-related behavioral processes with an emphasis on approaches which could be used to suppress fear when it is pathologically expressed. We also discuss limitations of these methodologies as well as review new technological developments which could be used in future mechanistic studies of fear behavior.

Recurrent cortical circuits implement concentration-invariant odor coding

Animals rely on olfaction to find food, attract mates, and avoid predators. To support these behaviors, they must be able to identify odors across different odorant concentrations. The neural circuit operations that implement this concentration invariance remain unclear. We found that despite concentration-dependence in the olfactory bulb (OB), representations of odor identity were preserved downstream, in the piriform cortex (PCx). The OB cells responding earliest after inhalation drove robust responses in sparse subsets of PCx neurons. Recurrent collateral connections broadcast their activation across the PCx, recruiting global feedback inhibition that rapidly truncated and suppressed cortical activity for the remainder of the sniff, discounting the impact of slower, concentration-dependent OB inputs. Eliminating recurrent collateral output amplified PCx odor responses rendered the cortex steeply concentration-dependent and abolished concentration-invariant identity decoding.

Toward Cochlear Therapies

Sensorineural hearing impairment is the most common sensory disorder and a major health and socio-economic issue in industrialized countries. It is primarily due to the degeneration of mechanosensory hair cells and spiral ganglion neurons in the cochlea via complex pathophysiological mechanisms. These occur following acute and/or chronic exposure to harmful extrinsic (e.g., ototoxic drugs, noise...) and intrinsic (e.g., aging, genetic) causative factors. No clinical therapies currently exist to rescue the dying sensorineural cells or regenerate these cells once lost. Recent studies have, however, provided renewed hope, with insights into the therapeutic targets allowing the prevention and treatment of ototoxic drug- and noise-induced, age-related hearing loss as well as cochlear cell degeneration. Moreover, genetic routes involving the replacement or corrective editing of mutant sequences or defected genes are showing promise, as are cell-replacement therapies to repair damaged cells for the future restoration of hearing in deaf people. This review begins by recapitulating our current understanding of the molecular pathways that underlie cochlear sensorineural damage, as well as the survival signaling pathways that can provide endogenous protection and tissue rescue. It then guides the reader through to the recent discoveries in pharmacological, gene and cell therapy research towards hearing protection and restoration as well as their potential clinical application.

Optogenetic manipulation of medullary neurons in the locust optic lobe

The locust is a widely used animal model for studying sensory processing and its relation to behavior. Due to the lack of genomic information, genetic tools to manipulate neural circuits in locusts are not yet available. We examined whether Semliki Forest virus is suitable to mediate exogenous gene expression in neurons of the locust optic lobe. We subcloned a channelrhodopsin variant and the yellow fluorescent protein Venus into a Semliki Forest virus vector and injected the virus into the optic lobe of locusts ( Schistocerca americana). Fluorescence was observed in all injected optic lobes. Most neurons that expressed the recombinant proteins were located in the first two neuropils of the optic lobe, the lamina and medulla. Extracellular recordings demonstrated that laser illumination increased the firing rate of medullary neurons expressing channelrhodopsin. The optogenetic activation of the medullary neurons also triggered excitatory postsynaptic potentials and firing of a postsynaptic, looming-sensitive neuron, the lobula giant movement detector. These results indicate that Semliki Forest virus is efficient at mediating transient exogenous gene expression and provides a tool to manipulate neural circuits in the locust nervous system and likely other insects. NEW & NOTEWORTHY Using Semliki Forest virus, we efficiently delivered channelrhodopsin into neurons of the locust optic lobe. We demonstrate that laser illumination increases the firing of the medullary neurons expressing channelrhodopsin and elicits excitatory postsynaptic potentials and spiking in an identified postsynaptic target neuron, the lobula giant movement detector neuron. This technique allows the manipulation of neuronal activity in locust neural circuits using optogenetics.

Optogenetic entrainment of neural oscillations with hybrid fiber probes

OBJECTIVE

Optogenetic modulation of neural activity is a ubiquitous tool for basic investigation of brain circuits. While the majority of optogenetic paradigms rely on short light pulses to evoke synchronized activity of optically sensitized cells, many neurobiological processes are associated with slow local field potential (LFP) oscillations. Therefore, we developed a hybrid fiber probe capable of simultaneous electrophysiological recording and optical stimulation and used it to investigate the utility of sinusoidal light stimulation for evoking oscillatory neural activity in vivo across a broad frequency range.

APPROACH

We fabricated hybrid fiber probes comprising a hollow cylindrical array of 9 electrodes and a flexible optical waveguide integrated within the core. We implanted these probes in the hippocampus of transgenic Thy1-ChR2-YFP mice that broadly express the blue-light sensitive cation channel channelrhodopsin 2 (ChR2) in excitatory neurons across the brain. The effects of the sinusoidal light stimulation were characterized and contrasted with those corresponding to pulsed stimulation in the frequency range of physiological LFP rhythms (3-128 Hz).

MAIN RESULTS

Within hybrid probes, metal electrode surfaces were vertically aligned with the waveguide tip, which minimized optical stimulation artifacts in neurophysiological recordings. Sinusoidal stimulation resulted in reliable and coherent entrainment of LFP oscillations up to 70 Hz, the cutoff frequency of ChR2, with response amplitudes inversely scaling with the stimulation frequencies. Effectiveness of the stimulation was maintained for two months following implantation.

SIGNIFICANCE

Alternative stimulation patterns complementing existing pulsed protocols, in particular sinusoidal light stimulation, are a prerequisite for investigating the physiological mechanisms underlying brain rhythms. So far, studies applying sinusoidal stimulation in vivo were limited to single stimulation frequencies. We show the feasibility of sinusoidal stimulation in vivo to induce coherent LFP oscillations across the entire frequency spectrum supported by the gating dynamics of ChR2 and introduce a hybrid fiber probe tailored to continuous light stimulation.