Publications

Rodents continuously move their heads and whiskers in a coordinated manner while perceiving objects through whisker-touch. Studies in head-fixed rodents showed that the ventroposterior medial (VPM) and posterior medial (POm) thalamic nuclei code for whisker kinematics, with POm involvement reduced in awake animals. To examine VPM and POm involvement in coding head and whisker kinematics in awake, head-free conditions, we recorded thalamic neuronal activity and tracked head and whisker movements in male mice exploring an open arena. Using optogenetic tagging, we found that in freely moving mice, both nuclei equally coded whisker kinematics and robustly coded head kinematics. The fraction of neurons coding head kinematics increased after whisker trimming, ruling out whisker-mediated coding. Optogenetic activation of thalamic neurons evoked overt kinematic changes and increased the fraction of neurons leading changes in head kinematics. Our data suggest that VPM and POm integrate head and whisker information and can influence head kinematics during tactile perception.

Although aggression is associated with several psychiatric disorders, there is no effective treatment nor a rigorous definition for \u201cpathological aggression\u201d. Mice make a valuable model for studying aggression. They have a dynamic social structure that depends on the habitat and includes reciprocal interactions between the mice's aggression levels, social dominance hierarchy (SDH), and resource allocation. Nevertheless, the classical behavioral tests for territorial aggression and SDH in mice are reductive and have limited ethological and translational relevance. Recent work has explored the use of semi-natural environments to simultaneously study dominance-related behaviors, resource allocation, and aggressive behavior. Semi-natural setups allow experimental control of the environment combined with manipulations of neural activity. We argue that these setups can help bridge the translational gap in aggression research toward discovering neuronal mechanisms underlying maladaptive aggression.

Information is transmitted between brain regions through the release of neurotransmitters from long-range projecting axons. Understanding how the activity of such long-range connections contributes to behavior requires efficient methods for reversibly manipulating their function. Chemogenetic and optogenetic tools, acting through endogenous G-protein-coupled receptor pathways, can be used to modulate synaptic transmission, but existing tools are limited in sensitivity, spatiotemporal precision or spectral multiplexing capabilities. Here we systematically evaluated multiple bistable opsins for optogenetic applications and found that the Platynereis dumerilii ciliary opsin (PdCO) is an efficient, versatile, light-activated bistable G-protein-coupled receptor that can suppress synaptic transmission in mammalian neurons with high temporal precision in vivo. PdCO has useful biophysical properties that enable spectral multiplexing with other optogenetic actuators and reporters. We demonstrate that PdCO can be used to conduct reversible loss-of-function experiments in long-range projections of behaving animals, thereby enabling detailed synapse-specific functional circuit mapping.

Oxytocin is essential in shaping social behavior across the lifespan. While the role of oxytocin signaling in parental care has been widely investigated, little is known about its function in social behavior during early life. This is partly due to the lack of precise technologies for studying the developing brain. Here, we studied the role of oxytocin in pup social behavior under acute separation from and reunion with the mother. We show that the activity of oxytocin neurons was increased by acute maternal separation and returned to baseline after reunion. Behaviorally, maternally-separated pups emitted more ultrasonic vocalizations upon reunion, which were further modulated by nipple attachment behavior. These effects were attenuated by blocking the oxytocin receptor during maternal separation. To investigate the role of oxytocin neurons with higher precision, we established a method for transcranial optogenetic silencing of neuronal activity in untethered and freely behaving pups. Using this approach, we found that silencing of oxytocin neurons during maternal separation disrupted vocal behavior during separation and reunion in a sex-specific manner. Our findings reveal an important role of oxytocin in context-dependent vocal communication in pups, offering new insights into the mechanisms of social behavior during early life.Competing Interest StatementOY is listed as an inventor on a patent application (US20210403518A1) filed with the US Patent Office regarding type II bistable opsins, and serves as a consultant for Modulight.bio.

The medial prefrontal cortex (mPFC) mediates a variety of complex cognitive functions via its vast and diverse connections with cortical and subcortical structures. Understanding the patterns of synaptic connectivity that comprise the mPFC local network is crucial for deciphering how this circuit processes information and relays it to downstream structures. To elucidate the synaptic organization of the mPFC, we developed a high-throughput optogenetic method for mapping large-scale functional synaptic connectivity in acute brain slices. We show that in male mice, mPFC neurons that project to the basolateral amygdala (BLA) display unique spatial patterns of local-circuit synaptic connectivity, which distinguish them from the general mPFC cell population. When considering synaptic connections between pairs of mPFC neurons, the intrinsic properties of the postsynaptic cell and the anatomical positions of both cells jointly account for ~7.5% of the variation in the probability of connection. Moreover, anatomical distance and laminar position explain most of this fraction in variation. Our findings reveal the factors determining connectivity in the mPFC and delineate the architecture of synaptic connections in the BLA-projecting subnetwork.

Fasting initiates a multitude of adaptations to allow survival. Activation of the hypothalamicpituitaryadrenal (HPA) axis and subsequent release of glucocorticoid hormones is a key response that mobilizes fuel stores to meet energy demands ¹⁵. Despite the importance of the HPA axis response, the neural mechanisms that drive its activation during energy deficit are unknown. Here, we show that fasting-activated hypothalamic agouti-related peptide (AgRP)-expressing neurons trigger and are essential for fasting-induced HPA axis activation. AgRP neurons do so through projections to the paraventricular hypothalamus (PVH), where, in a mechanism not previously described for AgRP neurons, they presynaptically inhibit the terminals of tonically active GABAergic afferents from the bed nucleus of the stria terminalis (BNST) that otherwise restrain activity of corticotrophin-releasing hormone (CRH)-expressing neurons. This disinhibition of PVH ^Crh neurons requires γ-aminobutyric acid (GABA)/GABA-B receptor signalling and potently activates the HPA axis. Notably, stimulation of the HPA axis by AgRP neurons is independent of their induction of hunger, showing that these canonical hunger neurons drive many distinctly different adaptations to the fasted state. Together, our findings identify the neural basis for fasting-induced HPA axis activation and uncover a unique means by which AgRP neurons activate downstream neurons: through presynaptic inhibition of GABAergic afferents. Given the potency of this disinhibition of tonically active BNST afferents, other activators of the HPA axis, such as psychological stress, may also work by reducing BNST inhibitory tone onto PVH ^Crh neurons.

Optogenetic techniques have been developed to allow control over the activity of selected cells within a highly heterogeneous tissue, using a combination of genetic engineering and light. Optogenetics employs natural and engineered photoreceptors, mostly of microbial origin, to be genetically introduced into the cells of interest. As a result, cells that are naturally light-insensitive can be made photosensitive and addressable by illumination and precisely controllable in time and space. The selectivity of expression and subcellular targeting in the host is enabled by applying control elements such as promoters, enhancers and specific targeting sequences to the employed photoreceptor-encoding DNA. This powerful approach allows precise characterization and manipulation of cellular functions and has motivated the development of advanced optical methods for patterned photostimulation. Optogenetics has revolutionized neuroscience during the past 15 years and is primed to have a similar impact in other fields, including cardiology, cell biology and plant sciences. In this Primer, we describe the principles of optogenetics, review the most commonly used optogenetic tools, illumination approaches and scientific applications and discuss the possibilities and limitations associated with optogenetic manipulations across a wide variety of optical techniques, cells, circuits and organisms.

Physical exercise is known to augment brain functioning, improving memory and cognition. However, while some of the physiological effects of physical activity on the brain are known, little is known about its effects on the neural code. Using calcium imaging in freely behaving mice, we study how voluntary exercise affects the quality and long-term stability of hippocampal place codes. We find that running accelerates the emergence of a more informative spatial code in novel environments and increases code stability over days and weeks. Paradoxically, although runners demonstrated an overall more stable place code than their sedentary peers, their place code changed faster when controlling for code quality level. A model-based simulation shows that the combination of improved code quality and faster representational drift in runners, but neither of these effects alone, could account for our results. Thus, exercise may enhance hippocampal function via a more informative and dynamic place code.

This Review provides a comprehensive overview of presynaptic applications of optogenetic tools, including the associated challenges, current limitations and future directions for this approach. Optogenetic actuators enable highly precise spatiotemporal interrogation of biological processes at levels ranging from the subcellular to cells, circuits and behaving organisms. Although their application in neuroscience has traditionally focused on the control of spiking activity at the somatodendritic level, the scope of optogenetic modulators for direct manipulation of presynaptic functions is growing. Presynaptically localized opsins combined with light stimulation at the terminals allow light-mediated neurotransmitter release, presynaptic inhibition, induction of synaptic plasticity and specific manipulation of individual components of the presynaptic machinery. Here, we describe presynaptic applications of optogenetic tools in the context of the unique cell biology of axonal terminals, discuss their potential shortcomings and outline future directions for this rapidly developing research area.

Focal cortical epilepsies are frequently refractory to available anticonvulsant drug therapies. One key factor contributing to this state is the limited availability of animal models that allow to reliably study focal cortical seizures and how they recruit surrounding brain areas in vivo. In this study, we selectively expressed the inhibitory chemogenetic receptor, hM4D, in GABAergic neurons in focal cortical areas using viral gene transfer. GABAergic silencing using Clozapine-N-Oxide (CNO) demonstrated reliable induction of local epileptiform events in the electroencephalogram signal of awake freely moving mice. Anesthetized mice experiments showed consistent induction of focal epileptiform-events in both the barrel cortex (BC) and the medial prefrontal cortex (mPFC), accompanied by high-frequency oscillations, a known characteristic of human seizures. Epileptiform-events showed propagation indication with favored propagation pathways: from the BC on 1 hemisphere to its counterpart and from the BC to the mPFC, but not vice-versa. Lastly, sensory whisker-pad stimulation evoked BC epileptiform events post-CNO, highlighting the potential use of this model in studying sensory-evoked seizures. Combined, our results show that targeted chemogenetic inhibition of GABAergic neurons using hM4D can serve as a novel, versatile, and reliable model of focal cortical epileptic activity suitable for systematically studying cortical ictogenesis in different cortical areas.

There is no question that genetically encoded tools have revolutionized neuroscience. These include optically modulated tools for writing-in (optogenetics) and reading-out (calcium, voltage, and neurotransmitter indicators) neural activity as well as precision expression of these reagents using virally mediated delivery. With few exceptions, these powerful approaches are derived from naturally occurring molecules that are sourced from diverse organisms that span all kingdoms of life. Successful expression of genetic tools in standard neuroscience model organisms requires optimizing gene structure, taking into account differences in both protein translation and trafficking. Myriad approaches have resolved these two challenges, resulting in order-of-magnitude increases in functional expression. In this chapter, we focus on synthesizing prior experience in successfully enabling the transition of genes across kingdoms with a goal of facilitating the production of the next generation of molecular tools for neuroscience. We then provide a detailed protocol that allows expression and testing of novel genetically encoded tools in mammalian cell lines and primary cultured neurons.

Many organisms sense light using rhodopsins, photoreceptive proteins containing a retinal chromophore. Here we report the discovery, structure and biophysical characterization of bestrhodopsins, a microbial rhodopsin subfamily from marine unicellular algae, in which one rhodopsin domain of eight transmembrane helices or, more often, two such domains in tandem, are C-terminally fused to a bestrophin channel. Cryo-EM analysis of a rhodopsin-rhodopsin-bestrophin fusion revealed that it forms a pentameric megacomplex (~700 kDa) with five rhodopsin pseudodimers surrounding the channel in the center. Bestrhodopsins are metastable and undergo photoconversion between red- and green-absorbing or green- and UVA-absorbing forms in the different variants. The retinal chromophore, in a unique binding pocket, photoisomerizes from all-trans to 11-cis form. Heterologously expressed bestrhodopsin behaves as a light-modulated anion channel.

Neurons are brain cells that can represent and store information, and they help our bodies to respond to events happening around us. How is information represented, processed and stored by neurons? Which neurons are activated while we perform a specific behavior? These are fundamental scientific questions. One important experimental approach to answering these questions is to record the activity of neurons inside the brain. In 2001, scientists developed an approach that uses light and a fascinating protein that becomes brightly fluorescent when neurons are active. This engineered protein, called GCaMP, responds to the amount of calcium inside the neurons considering that once a neuron is active, calcium inside it increases. GCaMP is now commonly used in laboratories all over the world to study neurons activity. In this article, we explain how this tool works and what makes it so useful for studying the brain.

Neurophotonics was launched in 2014 coinciding with the launch of the BRAIN Initiative focused on development of technologies for advancement of neuroscience. For the last seven years, Neurophotonics agenda has been well aligned with this focus on neurotechnologies featuring new optical methods and tools applicable to brain studies. While the BRAIN Initiative 2.0 is pivoting towards applications of these novel tools in the quest to understand the brain, this status report reviews an extensive and diverse toolkit of novel methods to explore brain function that have emerged from the BRAIN Initiative and related large-scale efforts for measurement and manipulation of brain structure and function. Here, we focus on neurophotonic tools mostly applicable to animal studies. A companion report, scheduled to appear later this year, will cover diffuse optical imaging methods applicable to noninvasive human studies. For each domain, we outline the current state-of-the-art of the respective technologies, identify the areas where innovation is needed, and provide an outlook for the future directions.

Mammalian social interactions are orchestrated by a wide array of neural circuits. While some aspects of social behaviors are driven by subcortical circuits, and are considered to be highly conserved and hard-wired, others require dynamic and context-dependent modulation that integrates current state, past experience and goal-driven action selection. These cognitive social processes are known to be dependent on the integrity of the prefrontal cortex. However, the circuit mechanisms through which the prefrontal cortex supports complex social functions are still largely unknown, and it is unclear if and how they diverge from prefrontal control of behavior outside of the social domain. Here we review recent studies exploring the role of prefrontal circuits in mammalian social functions, and attempt to synthesize these findings to a holistic view of prefrontal control of sociability.

All-optical methods for imaging and manipulating brain networks with high spatial resolution are fundamental to study how neuronal ensembles drive behavior. Stimulation of neuronal ensembles using two-photon holographic techniques requires high-sensitivity actuators to avoid photodamage and heating. Moreover, two-photon-excitable opsins should be insensitive to light at wavelengths used for imaging. To achieve this goal, we developed a novel soma-targeted variant of the large-conductance blue-light-sensitive opsin CoChR (stCoChR). In the mouse cortex in vivo, we combined holographic two-photon stimulation of stCoChR with an amplified laser tuned at the opsin absorption peak and two-photon imaging of the red-shifted indicator jRCaMP1a. Compared to previously characterized blue-light-sensitive soma-targeted opsins in vivo, stCoChR allowed neuronal stimulation with more than 10-fold lower average power and no spectral crosstalk. The combination of stCoChR, tuned amplified laser stimulation, and red-shifted functional indicators promises to be a powerful tool for large-scale interrogation of neural networks in the intact brain.

Information is carried between brain regions through neurotransmitter release from axonal presynaptic terminals. Understanding the functional roles of defined neuronal projection pathways requires temporally precise manipulation of their activity. However, existing inhibitory optogenetic tools have low efficacy and off-target effects when applied to presynaptic terminals, while chemogenetic tools are difficult to control in space and time. Here, we show that a targeting-enhanced mosquito homolog of the vertebrate encephalopsin (eOPN3) can effectively suppress synaptic transmission through the G_i/o signaling pathway. Brief illumination of presynaptic terminals expressing eOPN3 triggers a lasting suppression of synaptic output that recovers spontaneously within minutes in vitro and in vivo. In freely moving mice, eOPN3-mediated suppression of dopaminergic nigrostriatal afferents induces a reversible ipsiversive rotational bias. We conclude that eOPN3 can be used to selectively suppress neurotransmitter release at presynaptic terminals with high spatiotemporal precision, opening new avenues for functional interrogation of long-range neuronal circuits in vivo.

Complex behavioral phenotyping techniques are becoming more prevalent in the field of behavioral neuroscience, and thus methods for manipulating neuronal activity must be adapted to fit into such paradigms. Here, we present a head-mounted, magnetically activated device for wireless optogenetic manipulation that is compact, simple to construct, and suitable for use in group-living mice in an enriched semi-natural arena over several days. Using this device, we demonstrate that repeated activation of oxytocin neurons in male mice can have different effects on pro-social and agonistic behaviors, depending on the social context. Our findings support the social salience hypothesis of oxytocin and emphasize the importance of the environment in the study of social neuromodulators. Our wireless optogenetic device can be easily adapted for use in a variety of behavioral paradigms, which are normally hindered by tethered light delivery or a limited environment.

Understanding the function of nitric oxide, a lipophilic messenger in physiological processes across nervous, cardiovascular and immune systems, is currently impeded by the dearth of tools to deliver this gaseous molecule in situ to specific cells. To address this need, we have developed iron sulfide nanoclusters that catalyse nitric oxide generation from benign sodium nitrite in the presence of modest electric fields. Locally generated nitric oxide activates the nitric oxide-sensitive cation channel, transient receptor potential vanilloid family member 1 (TRPV1), and the latency of TRPV1-mediated Ca(2+)responses can be controlled by varying the applied voltage. Integrating these electrocatalytic nanoclusters with multimaterial fibres allows nitric oxide-mediated neuronal interrogation in vivo. The in situ generation of nitric oxide in the ventral tegmental area with the electrocatalytic fibres evoked neuronal excitation in the targeted brain region and its excitatory projections. This nitric oxide generation platform may advance mechanistic studies of the role of nitric oxide in the nervous system and other organs.

Subdivisions of mouse whisker somatosensory thalamus project to cortex in a region-specific and layer-specific manner. However, a clear anatomical dissection of these pathways and their functional properties during whisker sensation is lacking. Here, we use anterograde trans-synaptic viral vectors to identify three specific thalamic subpopulations based on their connectivity with brainstem. The principal trigeminal nucleus innervates ventral posterior medial thalamus, which conveys whisker-selective tactile information to layer 4 primary somatosensory cortex that is highly sensitive to self-initiated movements. The spinal trigeminal nucleus innervates a rostral part of the posterior medial (POm) thalamus, signaling whisker-selective sensory information, as well as decision-related information during a goal-directed behavior, to layer 4 secondary somatosensory cortex. A caudal part of the POm, which apparently does not receive brainstem input, innervates layer 1 and 5A, responding with little whisker selectivity, but showing decision-related modulation. Our results suggest the existence of complementary segregated information streams to somatosensory cortices. 

Decades of research support the idea that associations between a conditioned stimulus (CS) and an unconditioned stimulus (US) are encoded in the lateral amygdala (LA) during fear learning. However, direct proof for the sources of CS and US information is lacking. Definitive evidence of the LA as the primary site for cue association is also missing. Here, we show that calretinin (Calr)-expressing neurons of the lateral thalamus (Calr ⁺LT neurons) convey the association of fast CS (tone) and US (foot shock) signals upstream from the LA in mice. Calr ⁺LT input shapes a short-latency sensory-evoked activation pattern of the amygdala via both feedforward excitation and inhibition. Optogenetic silencing of Calr ⁺LT input to the LA prevents auditory fear conditioning. Notably, fear conditioning drives plasticity in Calr ⁺LT neurons, which is required for appropriate cue and contextual fear memory retrieval. Collectively, our results demonstrate that Calr ⁺LT neurons provide integrated CSUS representations to the LA that support the formation of aversive memories.

A defining feature of sleep is reduced responsiveness to external stimuli, but the mechanisms mediating sensory-evoked arousal remain unclear. We hypothesized that reduced locus coeruleus (LC) norepinephrine (NE) activity during sleep mediates unresponsiveness, and its action promotes sensory-evoked awakenings. We tested this using electrophysiological, behavioral, pharmacological, and optogenetic techniques alongside auditory stimulation in freely behaving rats. We found that systemic reduction in NE signaling lowered probability of sound-evoked awakenings (SEAs). The level of tonic LC activity during sleep anticipated SEAs. Optogenetic LC activation promoted arousal as evident in sleep-wake transitions, EEG desynchronization, and pupil dilation. Minimal LC excitation before sound presentation increased SEA probability. Optogenetic LC silencing using a soma-targeted anion-conducting channelrhodopsin (stGtACR2) suppressed LC spiking and constricted pupils. Brief periods of LC opto-silencing reduced the probability of SEAs. Thus, LC-NE activity determines the likelihood of sensory-evoked awakenings, and its reduction during sleep constitutes a key factor mediating behavioral unresponsiveness.

Neurons in neocortical layer 1 (L1) are thought to regulate attentional processes through integration of longrange inputs and disinhibitory effects on the underlying cortex. A new study combines genetically targeted voltage imaging and optogenetics to elucidate the input-output transformations of the L1 network in the mouse somatosensory cortex, revealing unique features of sensory-evoked dynamics in L1 neurons.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Scheggia et al. show that a specific subpopulation of cortical neurons expressing somatostatin in the prefrontal cortex has a primary role in orchestrating the ability of mice to discriminate positive and negative affective states in others.The prefrontal cortex (PFC) is implicated in processing of the affective state of others through non-verbal communication. This social cognitive function is thought to rely on an intact cortical neuronal excitatory and inhibitory balance. Here combining in vivo electrophysiology with a behavioral task for affective state discrimination in mice, we show a differential activation of medial PFC (mPFC) neurons during social exploration that depends on the affective state of the conspecific. Optogenetic manipulations revealed a double dissociation between the role of interneurons in social cognition. Specifically, inhibition of mPFC somatostatin (SOM+), but not of parvalbumin (PV+) interneurons, abolishes affective state discrimination. Accordingly, synchronized activation of mPFC SOM+ interneurons selectively induces social discrimination. As visualized by in vivo single-cell microendoscopic Ca2+ imaging, an increased synchronous activity of mPFC SOM+ interneurons, guiding inhibition of pyramidal neurons, is associated with affective state discrimination. Our findings provide new insights into the neurobiological mechanisms of affective state discrimination.

Optogenetic technology uses light to control or modulate neuronal function, providing a means of interacting with neurons that is unchallenged in its temporal resolution, spatial resolution, and cell type specificity. Optogenetic experiments, particularly involving inhibitory optogenetic tools, often require prolonged illumination of neural tissue which may cause temperature changes. Numerous studies ensued, utilizing Channelrhodopsin-2 and other microbial rhodopsins in a variety of neural circuits and animal models. In neural circuits, connectivity and function are thought to be tightly linked. Neurons projecting to defined long-range targets have been shown to possess defined functional properties, and dissecting these roles has been a major application of optogenetic tools. Optogenetics is readily applicable to light-accessible preparations such as cultured neurons, brain slices, transparent organisms such as zebrafish larvae, or to the cortical surface of rodents, allowing for extensive flexibility in light delivery. Light needs to reach the target with sufficient irradiance to induce opsin activation in a sufficient number of neurons.

Mouse primary somatosensory barrel cortex (wS1) processes whisker sensory information, receiving input from two distinct thalamic nuclei. The first-order ventral posterior medial (VPM) somatosensory thalamic nucleus most densely innervates layer 4 (L4) barrels, whereas the higher-order posterior thalamic nucleus (medial part, POm) most densely innervates L1 and L5A. We optogenetically stimulated VPM or POm axons, and recorded evoked excitatory postsynaptic potentials (EPSPs) in different cell-types across cortical layers in wS1. We found that excitatory neurons and parvalbumin-expressing inhibitory neurons received the largest EPSPs, dominated by VPM input to L4 and POm input to L5A. In contrast, somatostatin-expressing inhibitory neurons received very little input from either pathway in any layer. Vasoactive intestinal peptide-expressing inhibitory neurons received an intermediate level of excitatory input with less apparent layer-specificity. Our data help understand how wS1 neocortical microcircuits might process and integrate sensory and higher-order inputs.

Motor neuron-specific microRNA-218 (miR-218) has recently received attention because of its roles in mouse development. However, miR-218 relevance to human motor neuron disease was not yet explored. Here, we demonstrate by neuropathology that miR-218 is abundant in healthy human motor neurons. However, in amyotrophic lateral sclerosis (ALS) motor neurons, miR-218 is down-regulated and its mRNA targets are reciprocally up-regulated (derepressed). We further identify the potassium channel Kv10.1 as a new miR-218 direct target that controls neuronal activity. In addition, we screened thousands of ALS genomes and identified six rare variants in the human miR-218-2 sequence. miR-218 gene variants fail to regulate neuron activity, suggesting the importance of this small endogenous RNA for neuronal robustness. The underlying mechanisms involve inhibition of miR-218 biogenesis and reduced processing by DICER. Therefore, miR-218 activity in motor neurons may be susceptible to failure in human ALS, suggesting that miR-218 may be a potential therapeutic target in motor neuron disease.

The prefrontal cortex (PFC) plays an important role in regulating social functions in mammals, and its dysfunction has been linked to social deficits in neurodevelopmental disorders. Yet little is known of how the PFC encodes social information and how social representations may be altered in such disorders. Here, we show that neurons in the medial PFC of freely behaving male mice preferentially respond to socially relevant olfactory cues. Population activity patterns in this region differed between social and nonsocial stimuli and underwent experience-dependent refinement. In mice lacking the autism-associated gene Cntnap2, both the categorization of sensory stimuli and the refinement of social representations were impaired. Noise levels in spontaneous population activity were higher in Cntnap2 knockouts and correlated with the degree to which social representations were disrupted. Our findings elucidate the encoding of social sensory cues in the medial PFC and provide a link between altered prefrontal dynamics and autism-associated social dysfunction.

Chemogenetic techniques allow selective manipulation of neurons by activating engineered actuator proteins with otherwise inert effector molecules. A recent study (Magnus et al. Science 2019;364: eaav5282) describes the coevolution of highly potent actuator effector pairs based on a clinically approved antismoking drug. These tools allow selective excitation or inhibition of neurons in the living brain with high specificity and no detectable side-effects.

We performed two-photon holographic stimulation of blue light-sensitive excitatory opsins using high- A nd low-repetition rate lasers and we demonstrate high-efficiency neural stimulation with few mW average power per cell in the intact mouse cortex.

Optogenetic silencing allows time-resolved functional interrogation of defined neuronal populations. However, the limitations of inhibitory optogenetic tools impose stringent constraints on experimental paradigms. The high light power requirement of light-driven ion pumps and their effects on intracellular ion homeostasis pose unique challenges, particularly in experiments that demand inhibition of a widespread neuronal population in vivo. Guillardia theta anion-conducting channelrhodopsins (GtACRs) are promising in this regard, due to their high single-channel conductance and favorable photon-ion stoichiometry. However, GtACRs show poor membrane targeting in mammalian cells, and the activity of such channels can cause transient excitation in the axon due to an excitatory chloride reversal potential in this compartment. Here, we address these problems by enhancing membrane targeting and subcellular compartmentalization of GtACRs. The resulting soma-targeted GtACRs show improved photocurrents, reduced axonal excitation and high light sensitivity, allowing highly efficient inhibition of neuronal activity in the mammalian brain.

Animals constantly evaluate their environment in order to avoid potential threats and obtain reward in the form of food, shelter and social interactions. In order to appropriately respond to sensory cues from the environment, the brain needs to form and store multiple cue-outcome associations. These can then be used to form predictions of the valence of sounds, smells and other sensory inputs arising from the surroundings. However, these associations must be subject to constant update, as the environment can rapidly change. Failing to adapt to such change can be detrimental to survival. Several systems in the mammalian brain have evolved to perform these important behavioral functions. Among these systems, the amygdala and prefrontal cortex are prominent players. Although the amygdala has been shown to form strong cue-outcome associations, the prefrontal cortex is essential for modifying these associations through extinction and reversal learning, and synaptic plasticity occurring in the strong reciprocal connections between these structures is thought to underlie both adaptive and maladaptive learning. Here we review the synaptic organization of the amygdala-prefrontal circuit, and summarize the physiological and behavioral evidence for its involvement in appetitive and aversive learning.

Sensory information is encoded within the brain in distributed spatiotemporal patterns of neuronal activity. Understanding how these patterns influence behavior requires a method to measure and to bidirectionally perturb with high spatial resolution the activity of the multiple neuronal cell types engaged in sensory processing. Here, we combined two-photon holography to stimulate neurons expressing blue light-sensitive opsins (ChR2 and GtACR2) with two-photon imaging of the red-shifted indicator jRCaMP1a in the mouse neocortex in vivo. We demonstrate efficient control of neural excitability across cell types and layers with holographic stimulation and improved spatial resolution by opsin somatic targeting. Moreover, we performed simultaneous two-photon imaging of jRCaMP1a and bidirectional two-photon manipulation of cellular activity with negligible effect of the imaging beam on opsin excitation. This all-optical approach represents a powerful tool to causally dissect how activity patterns in specified ensembles of neurons determine brain function and animal behavior. Forli et al. developed an all-optical method to image and bidirectionally manipulate brain networks with high spatial resolution and minimal crosstalk in the intact mammalian brain. They validate the method across cell types and layers in the mouse neocortex.

We combined two-photon holographic stimulation of blue light-sensitive opsins with two-photon imaging of red-shifted fluorescence indicators in the intact mouse neocortex to perform cellular-resolution control of neural excitability with minimal crosstalk between imaging and stimulation.

The bed nucleus of the stria terminalis (BNST) is critical in mediating states of anxiety, and its dysfunction has been linked to stress-related mental disease. Although the anxiety-related role of distinct subregions of the anterior BNST was recently reported, little is known about the contribution of the posterior BNST (pBNST) to the behavioral and neuroendocrine responses to stress. Previously, we observed abnormal expression of corticotropin-releasing factor receptor type 2 (CRFR2) to be associated with post-traumatic stress disorder (PTSD)-like symptoms. Here, we found that CRFR2-expressing neurons within the pBNST send dense inhibitory projections to other stress-related brain regions (for example, the locus coeruleus, medial amygdala and paraventricular nucleus), implicating a prominent role of these neurons in orchestrating the neuroendocrine, autonomic and behavioral response to stressful situations. Local CRFR2 activation by urocortin 3 depolarized the cells, increased the neuronal input resistance and increased firing of action potentials, indicating an enhanced excitability. Furthermore, we showed that CRFR2-expressing neurons within the pBNST are critically involved in the modulation of the behavioral and neuroendocrine response to stress. Optogenetic activation of CRFR2 neurons in the pBNST decreased anxiety, attenuated the neuroendocrine stress response, ameliorated stress-induced anxiety and impaired the fear memory for the stressful event. Moreover, activation following trauma exposure reduced the susceptibility for PTSD-like symptoms. Optogenetic inhibition of pBNST CRFR2 neurons yielded opposite effects. These data indicate the relevance of pBNST activity for adaptive stress recovery.

Science is ideally suited to connect people from different cultures and thereby foster mutual understanding. To promote international life science collaboration, we have launched \u201cThe Science Bridge\u201d initiative. Our current project focuses on partnership between Western and Middle Eastern neuroscience communities. Science is ideally suited to connect people from different cultures and thereby foster mutual understanding. To promote international life science collaboration, Lissek et al. have launched \u201cThe Science Bridge\u201d initiative. Theirs current project focuses on partnership between Western and Middle Eastern neuroscience communities.

Optogenetic techniques allow versatile, cell type-specific light-based control of cellular activity in diverse set of cells, circuits, and brain structures. Optogenetic actuators are genetically encoded light-sensitive membrane proteins that can be selectively introduced into cellular circuits in the living brain using a variety of genetic approaches. Gene targeting approaches used in optogenetic studies vary greatly in their specificity, their spatial coverage, the level of transgene expression and their potential adverse effects on neuronal cell health. Here, we describe the major gene targeting approaches utilized in optogenetics and provide a simple set of guidelines through which these approaches can be evaluated when designing an in vitro or in vivo optogenetic study.

Despite decades of research on neurobiological mechanisms of psychostimulant addiction, the only effective treatment for many addicts is contingency management, a behavioral treatment that uses alternative non-drug reward to maintain abstinence. However, when contingency management is discontinued, most addicts relapse to drug use. The brain mechanisms underlying relapse after cessation of contingency management are largely unknown, and, until recently, an animal model of this human condition did not exist. Here we used a novel rat model, in which the availability of a mutually exclusive palatable food maintains prolonged voluntary abstinence from intravenous methamphetamine self-administration, to demonstrate that the activation of monosynaptic glutamatergic projections from anterior insular cortex to central amygdala is critical to relapse after the cessation of contingency management. We identified the anterior insular cortex-to-central amygdala projection as a new addiction-and motivation-related projection and a potential target for relapse prevention.

Reversible silencing of neuronal activity is a powerful approach for isolating the roles of specific neuronal populations in circuit dynamics and behavior. In contrast with neuronal excitation, for which the majority of studies have used a limited number of optogenetic and chemogenetic tools, the number of genetically encoded tools used for inhibition of neuronal activity has vastly expanded. Silencing strategies vary widely in their mechanism of action and in their spatial and temporal scales. Although such manipulations are commonly applied, the design and interpretation of neuronal silencing experiments present unique challenges, both technically and conceptually. Here, we review the most commonly used tools for silencing neuronal activity and provide an in-depth analysis of their mechanism of action and utility for particular experimental applications. We further discuss the considerations that need to be given to experimental design, analysis, and interpretation of collected data. Finally, we discuss future directions for the development of new silencing approaches in neuroscience.

Sodium pumping rhodopsins (NaRs) are a unique member of the microbial-type I rhodopsin family which actively transport Na⁺ and H⁺ depending on ionic condition. In this study, we surveyed 12 different NaRs from various sources of eubacteria for their electrophysiological as well as spectroscopic properties. In mammalian cells several of these NaRs exhibited a Na⁺ based pump photocurrent and four interesting candidates were chosen for further characterization. Voltage dependent photocurrent amplitudes revealed a membrane potential-sensitive turnover rate, indicating the presence of an electrically-charged intermediate(s) in the photocycle reaction. The NaR from Salinarimonas rosea DSM21201 exhibited a red-shifted absorption spectrum, and slower kinetics compared to the first described sodium pump, KR2. Although the ratio of Na⁺ to H⁺ ion transport varied among the NaRs we tested, the NaRs from Flagellimonas sp_DIK and Nonlabens sp_YIK_SED-11 showed significantly higher Na⁺ selectivity when compared to KR2. All four further investigated NaRs showed a functional expression in dissociated hippocampal neuron culture and hyperpolarizing activity upon light-stimulation. Additionally, all four NaRs allowed optical inhibition of electrically-evoked neuronal spiking. Although efficiency of silencing was 35 times lower than silencing with the enhanced version of the proton pump AR3 from Halorubrum sodomense, our data outlines a new approach for hyperpolarization of excitable cells without affecting the intracellular and extracellular proton environment.

Key mitochondrial functions such as ATP production, Ca²⁺ uptake and release, and substrate accumulation depend on the proton electrochemical gradient (ΔμH+) across the inner membrane. Although several drugs can modulate ΔμH+, their effects are hardly reversible, and lack cellular specificity and spatial resolution. Although channelrhodopsins are widely used to modulate the plasma membrane potential of excitable cells, mitochondria have thus far eluded optogenetic control. Here we describe a toolkit of optometabolic constructs based on selective targeting of channelrhodopsins with distinct functional properties to the inner mitochondrial membrane of intact cells. We show that our strategy enables a light-dependent control of the mitochondrial membrane potential (Δψm) and coupled mitochondrial functions such as ATP synthesis by oxidative phosphorylation, Ca²⁺ dynamics, and respiratory metabolism. By directly modulating Δψm, the mitochondriatargeted opsinswere used to control complex physiological processes such as spontaneous beats in cardiac myocytes and glucose-dependent ATP increase in pancreatic β-cells. Furthermore, our optometabolic tools allow modulation of mitochondrial functions in single cells and defined cell regions.

Fear-related disorders are thought to reflect strong and persistent fear memories. The basolateral amygdala (BLA) and the medial prefrontal cortex (mPFC) form strong reciprocal synaptic connections that play a key role in acquisition and extinction of fear memories. While synaptic contacts of BLA cells onto mPFC neurons are likely to play a crucial role in this process, the BLA connects with several additional nuclei within the fear circuit that could relay fear-associated information to the mPFC, and the contribution of direct monosynaptic BLAmPFC inputs is not yet clear. Here we establish an optogenetic stimulation protocol that induces synaptic depression in BLAmPFC synapses. In behaving mice, optogenetic high-frequency stimulation of BLA inputs to mPFC interfered with retention of cued associations, attenuated previously acquired cue-associated responses in mPFC neurons and facilitated extinction. Our findings demonstrate the contribution of BLA inputs to mPFC in forming and maintaining cued fear associations.

The ability to plan and execute appropriately timed responses to external stimuli is based on a well-orchestrated balance between movement initiation and inhibition. In impulse control disorders involving the prefrontal cortex (PFC) [1], this balance is disturbed, emphasizing the critical role that PFC plays in appropriately timing actions [2-4]. Here, we employed optogenetic and electro-physiological techniques to systematically analyze the functional role of five key subareas of the rat medial PFC (mPFC) and orbitofrontal cortex (OFC) in action control [5-9]. Inactivation of mPFC subareas induced drastic changes in performance, namely an increase (prelimbic cortex, PL) or decrease (infralimbic cortex, IL) of premature responses. Additionally, electrophysiology revealed a significant decrease in neuronal activity of a PL subpopulation prior to premature responses. In contrast, inhibition of OFC subareas (mainly the ventral OFC, i.e., VO) significantly impaired the ability to respond rapidly after external cues. Consistent with these findings, mPFC activity during response preparation predicted trial outcomes and reaction times significantly better than OFC activity. These data support the concept of opposing roles of IL and PL in directing proactive behavior and argue for an involvement of OFC in predominantly reactive movement control. By attributing defined roles to rodent PFC sections, this study contributes to a deeper understanding of the functional heterogeneity of this brain area and thus may guide medically relevant studies of PFC-associated impulse control disorders in this animal model for neural disorders [10-12].

I will describe our work aimed at developing optogenetic techniques for functional mapping and manipulation of synaptic connectivity within local microcircuits and long-range connections between distant brain regions.

Major resources are now available to develop tools and technologies aimed at dissecting the circuitry and computations underlying behavior, unraveling the underpinnings of brain disorders, and understanding the neural substrates of cognition. Scientists from around the world shared their views around new tools and technologies to drive advances in neuroscience.

Social encounters are associated with varying degrees of emotional arousal and stress. The mechanisms underlying adequate socioemotional balance are unknown. The medial amygdala (MeA) is a brain region associated with social behavior in mice. Corticotropin-releasing factor receptor type-2 (CRF-R2) and its specific ligand urocortin-3 (Ucn3), known components of the behavioral stress response system, are highly expressed in the MeA. Here we show that mice deficient in CRF-R2 or Ucn3 exhibit abnormally low preference for novel conspecifics. MeA-specific knockdown of Crfr2 (Crhr2) in adulthood recapitulated this phenotype. In contrast, pharmacological activation of MeA CRF-R2 or optogenetic activation of MeA Ucn3 neurons increased preference for novel mice. Furthermore, chemogenetic inhibition of MeA Ucn3 neurons elicited pro-social behavior in freely behaving groups of mice without affecting their hierarchal structure. These findings collectively suggest that the MeA Ucn3-CRF-R2 system modulates the ability of mice to cope with social challenges.

We investigated the efficacy of optogenetic inhibition at presynaptic terminals using halorhodopsin, archaerhodopsin and chloride-conducting channelrhodopsins. Precisely timed activation of both archaerhodopsin and halorhodpsin at presynaptic terminals attenuated evoked release. However, sustained archaerhodopsin activation was paradoxically associated with increased spontaneous release. Activation of chloride-conducting channelrhodopsins triggered neurotransmitter release upon light onset. Thus, the biophysical properties of presynaptic terminals dictate unique boundary conditions for optogenetic manipulation.

It is commonly assumed, but has rarely been demonstrated, that sex differences in behaviour arise from sexual dimorphism in the underlying neural circuits. Parental care is a complex stereotypic behaviour towards offspring that is shared by numerous species. Mice display profound sex differences in offspring-directed behaviours. At their first encounter, virgin females behave maternally towards alien pups while males will usually ignore the pups or attack them. Here we show that tyrosine hydroxylase (TH)-expressing neurons in the anteroventral periventricular nucleus (AVPV) of the mouse hypothalamus are more numerous in mothers than in virgin females and males, and govern parental behaviours in a sex-specific manner. In females, ablating the AVPV TH⁺ neurons impairs maternal behaviour whereas optogenetic stimulation or increased TH expression in these cells enhance maternal care. In males, however, this same neuronal cluster has no effect on parental care but rather suppresses inter-male aggression. Furthermore, optogenetic activation or increased TH expression in the AVPV TH⁺ neurons of female mice increases circulating oxytocin, whereas their ablation reduces oxytocin levels. Finally, we show that AVPV TH⁺ neurons relay a monosynaptic input to oxytocin-expressing neurons in the paraventricular nucleus. Our findings uncover a previously unknown role for this neuronal population in the control of maternal care and oxytocin secretion, and provide evidence for a causal relationship between sexual dimorphism in the adult brain and sex differences in parental behaviour.

Invivo experimental evidence indicates that acute neuronal activation increases Aβ release from presynaptic terminals, whereas long-term effects ofchronic synaptic activation on Aβ pathology remain unclear. To address this issue, we adopted optogenetics and transduced stabilized step-function opsin, a channelrhodopsin engineered to elicit a long-lasting neuronal hyperexcitability, into the hippocampal perforant pathway of APP transgenic mice. Invivo microdialysis revealed a ~24% increase in the hippocampal interstitial fluid Aβ42 levels immediately after acute light activation. Five months of chronic optogenetic stimulation increased Aβ burden specifically in the projection area of the perforant pathway (i.e., outer molecular layer of the dentate gyrus) of the stimulated side by ~2.5-fold compared with that in the contralateral side. Epileptic seizures were observed during the course of chronic stimulation, which might have partly contributed to the Aβ pathology. These findings implicate functional abnormalities of specific neuronal circuitry in Aβ pathology and Alzheimer disease.

Patients with psychiatric disease are diagnosed by psychiatrists based on the information of non-quantitative objective parameters, including behavioral phenotypes. However, how any neural mechanism affects such behavioral phenotypes in patients is still unclear. Recent functional studies suggested the alteration in brain neural/network activity responds to subjected stimuli in some brain regions of psychiatric patients, indicating that excitatory/inhibitory (E/I) imbalance occurs in local neural circuits responsible for regional activities. Moreover, in human genetics, a large number of genetic variations, including single nucleotide variation (SNV) and copy number variation (CNV), have been found in psychiatric patients. Such variations must be causes of a psychiatric behavioral phenotype, while understanding of the relationship between genetic variations and neural mechanisms underlying psychiatric behavior remains poor due to the heterogeneity in genetic variations. Functional and molecular analyses with SNV and CNV suggest the mutations of synaptic genes might contribute to the abnormal neural activity due to synaptic dysfunction. To overcome the sparse knowledge of psychiatric neural phenotypes, we can choose two ways: one is to detect the abnormalities of neural function in animal models with the genetic variations found in human genetics, which means construct validity of an animal model, and another is to reproduce the behavioral phenotypes seen in psychiatric disorders by artificially controlling neural functions, referred to as face validity. Analyzing the neural activity in animal models with construct and face validities would help us understand the neural state in psychiatric patients.

Optogenetics is a technical methodology that allows direct light-based manipulation of genetically specified cells. Optogenetic methods have provided novel insights into the role of defined neuronal populations in brain function and animal behavior. An expanding palette of single-component optogenetic tools provides powerful interventional strategies for modulating the function of targeted neurons in awake, behaving mammals and for detailed interrogation of circuit physiology in vitro. Although several genetic methods can be utilized for delivering these genes into target cell populations, the use of viral vectors for delivery of optogenetic tools has several important advantages. In recent years, techniques for viral vector-mediated delivery of optogenetic tools have improved and expanded significantly. These techniques now allow modular use of optogenetic tools in defined cell types and circuits and dovetail well with genetic mouse models and recombinase-based driver lines. Here, we review the use of viral vectors for delivering genes encoding optogenetic tools into the rodent brain and provide a detailed protocol for viral transduction of mouse cortical neurons and chronic implantation of a fiberoptic connector for light delivery in vivo

We characterize an inhibitory circuit motif in the Drosophila olfactory system, parallel inhibition, which differs from feedforward or feedback inhibition. Excitatory and GABAergic inhibitory projection neurons (ePNs and iPNs) each receive input from antennal lobe glomeruli and send parallel output to the lateral horn, a higher center implicated in regulating innate olfactory behavior. Ca²⁺ imaging of specific lateral horn neurons as an olfactory readout revealed that iPNs selectively suppressed food-related odor responses, but spared signal transmission from pheromone channels. Coapplying food odorant did not affect pheromone signal transmission, suggesting that the differential effects likely result from connection specificity of iPNs, rather than a generalized inhibitory tone. Ca²⁺ responses in the ePN axon terminals show no detectable suppression by iPNs, arguing against presynaptic inhibition as a primary mechanism. The parallel inhibition motif may provide specificity in inhibition to funnel specific olfactory information, such as food and pheromone, into distinct downstream circuits

Tomosyn, a syntaxin-binding protein, is known to inhibit vesicle priming and synaptic transmission via interference with the formation of SNARE complexes. Using a lentiviral vector, we specifically overexpressed tomosyn1 in hippocampal dentate gyrus neurons in adult mice. Mice were then subjected to spatial learning and memory tasks and electrophysiological measurements from hippocampal slices. Tomosyn1-overexpression significantly impaired hippocampus-dependent spatial memory while tested in the Morris water maze. Further, tomosyn1-overexpressing mice utilize swimming strategies of lesser cognitive ability in the Morris water maze compared with control mice. Electrophysiological measurements at mossy fiber-CA3 synapses revealed impaired paired-pulse facilitation in the mossy fiber of tomosyn1-overexpressing mice. This study provides evidence for novel roles for tomosyn1 in hippocampus-dependent spatial learning and memory, potentially via decreased synaptic transmission in mossy fiber-CA3 synapses. Moreover, it provides new insight regarding the role of the hippocampal dentate gyrus and mossy fiber-CA3 synapses in swimming strategy preference, and in learning and memory.

Optogenetics has rapidly become a standard method in neuroscience research. Although significant progress has been made in the development of molecular tools, refined techniques for combined light delivery and recording in vivo are still lacking. For example, simultaneous intracellular recording and light stimulation have only been possible by using two separate positioning systems. To overcome this limitation, we have developed a glass pipette holder which contains an additional port for the insertion of an optical fiber into the pipette. This device, which we called " optopatcher" allows whole cell patch-clamp recording simultaneously with direct projection of light from the recording pipette. The holder spares the use of an additional manipulator and, importantly, enables accurate, stable and reproducible illumination. In addition, replacement of standard pipettes is done as easily as with the available commercial holders. Here we used the optopatcher in vivo to record the membrane potential of neurons from different cortical layers in the motor cortex of transgenic mice expressing channelrhodopsin-2 under the Thy1 promoter. We demonstrate the utility of the optopatcher by recording LFP and intracellular responses to light stimulation.

Optogenetics with microbial opsin genes has enabled high-speed control of genetically specified cell populations in intact tissue. However, it remains a challenge to independently control subsets of cells within the genetically targeted population. Although spatially precise excitation of target molecules can be achieved using two-photon laser-scanning microscopy (TPLSM) hardware, the integration of two-photon excitation with optogenetics has thus far required specialized equipment or scanning and has not yet been widely adopted. Here we take a complementary approach, developing opsins with custom kinetic, expression and spectral properties uniquely suited to scan times typical of the raster approach that is ubiquitous in TPLSM laboratories. We use a range of culture, slice and mammalian in vivo preparations to demonstrate the versatility of this toolbox, and we quantitatively map parameter space for fast excitation, inhibition and bistable control. Together these advances may help enable broad adoption of integrated optogenetic and TPLSM technologies across experimental fields and systems.

Cognitive and social deficits lie at the core of many neuropsychiatric diseases and are among the many behavioral symptoms not amenable to pharmacological intervention. Despite significant advances in identifying genes potentially involved in the pathogenesis of complex psychiatric conditions such as autism and schizophrenia, knowledge of the physiological functions that are affected (and are therefore potential targets for clinical intervention) is scarce. In psychiatric disorders with a strong genetic component, animal models have provided important links between disease-related genes and behavioral impairment. Social dysfunction, for instance, is commonly observed in transgenic rodent disease models. However, the causal relationships between the behavioral and physiological abnormalities in these models are not well-understood. Optogenetic techniques have evolved to provide a wide range of experimental paradigms in which neural circuit activity can be perturbed with high spatial and temporal precision, enabling causal investigation of the function of defined physiological events in neuronal subgroups. With optogenetics, researchers have begun to elucidate the basic neural mechanisms of social behaviors and of disease-relevant social and cognitive dysfunction. The synthesis of optogenetic technology with genetic animal models will allow forward- and reverse-engineering approaches to investigating the neural correlates of psychiatric disease. This review outlines the neural systems known to be involved in social behavior, illustrates how optogenetic technology has been applied to analyze this circuitry, and imagines how it might be further developed in future studies to elucidate these complex circuits both from a basic science perspective and in the context of psychiatric disease.

Channelrhodopsins (ChRs) are light-gated cation channels derived from algae that Have shown experimental utility in optogenetics; for example, neurons expressing ChRs can be optically controlled with high temporal precision within systems as complex as freely moving mammals. Although ChRs have been broadly applied to neuroscience research, little is known about the molecular mechanisms by which these unusual and powerful proteins operate. Here we present the crystal structure of a ChR (a C1C2 chimaera between ChR1 and ChR2 from Chlamydomonas reinhardtii) at 2.3-Ã.. resolution. The structure reveals the essential molecular architecture of ChRs, including the retinal-binding pocket and cation conduction pathway. This integration of structural and electrophysiological analyses provides insight into the molecular basis for the remarkable function of ChRs, and paves the way for the precise and principled design of ChR variants with novel properties.

Diverse optogenetic tools have allowed versatile control over neural activity. Many depolarizing and hyperpolarizing tools have now been developed in multiple laboratories and tested across different preparations, presenting opportunities but also making it difficult to draw direct comparisons. This challenge has been compounded by the dependence of performance on parameters such as vector, promoter, expression time, illumination, cell type and many other variables. As a result, it has become increasingly complicated for end users to select the optimal reagents for their experimental needs. For a rapidly growing field, critical figures of merit should be formalized both to establish a framework for further development and so that end users can readily understand how these standardized parameters translate into performance. Here we systematically compared microbial opsins under matched experimental conditions to extract essential principles and identify key parameters for the conduct, design and interpretation of experiments involving optogenetic techniques.

Currently there is no general approach for achieving specific optogenetic control of genetically defined cell types in rats, which provide a powerful experimental system for numerous established neurophysiological and behavioral paradigms. To overcome this challenge we have generated genetically restricted recombinase-driver rat lines suitable for driving gene expression in specific cell types, expressing Cre recombinase under the control of large genomic regulatory regions (200-300 kb). Multiple tyrosine hydroxylase (Th)::. Cre and choline acetyltransferase (Chat)::. Cre lines were produced that exhibited specific opsin expression in targeted cell types. We additionally developed methods for utilizing optogenetic tools in freely moving rats and leveraged these technologies to clarify the causal relationship between dopamine (DA) neuron firing and positive reinforcement, observing that optical stimulation of DA neurons in the ventral tegmental area (VTA) of Th::. Cre rats is sufficient to support vigorous intracranial self-stimulation (ICSS). These studies complement existing targeting approaches by extending the generalizability of optogenetics to traditionally non-genetically-tractable but vital animal models.

The capture and utilization of light is an exquisitely evolved process. The single-component microbial opsins, although more limited than multicomponent cascades in processing, display unparalleled compactness and speed. Recent advances in understanding microbial opsins have been driven by molecular engineering for optogenetics and by comparative genomics. Here we provide a Primer on these light-activated ion channels and pumps, describe a group of opsins bridging prior categories, and explore the convergence of molecular engineering and genomic discovery for the utilization and understanding of these remarkable molecular machines.

Severe behavioural deficits in psychiatric diseases such as autism and schizophrenia have been hypothesized to arise from elevations in the cellular balance of excitation and inhibition (E/I balance) within neural microcircuitry. This hypothesis could unify diverse streams of pathophysiological and genetic evidence, but has not been susceptible to direct testing. Here we design and use several novel optogenetic tools to causally investigate the cellular E/I balance hypothesis in freely moving mammals, and explore the associated circuit physiology. Elevation, but not reduction, of cellular E/I balance within the mouse medial prefrontal cortex was found to elicit a profound impairment in cellular information processing, associated with specific behavioural impairments and increased high-frequency power in the 30-80 Hz range, which have both been observed in clinical conditions in humans. Consistent with the E/I balance hypothesis, compensatory elevation of inhibitory cell excitability partially rescued social deficits caused by E/I balance elevation. These results provide support for the elevated cellular E/I balance hypothesis of severe neuropsychiatric disease-related symptoms.

Cortico-thalamo-cortical circuits mediate sensation and generate neural network oscillations associated with slow-wave sleep and various epilepsies. Cortical input to sensory thalamus is thought to mainly evoke feed-forward synaptic inhibition of thalamocortical (TC) cells via reticular thalamic nucleus (nRT) neurons, especially during oscillations. This relies on a stronger synaptic strength in the cortico-nRT pathway than in the cortico-TC pathway, allowing the feed-forward inhibition of TC cells to overcome direct cortico-TC excitation. We found a systemic and specific reduction in strength in GluA4-deficient (Gria4 ) mice of one excitatory synapse of the rhythmogenic cortico-thalamo-cortical system, the cortico-nRT projection, and observed that the oscillations could still be initiated by cortical inputs via the cortico-TC-nRT-TC pathway. These results reveal a previously unknown mode of cortico-thalamo-cortical transmission, bypassing direct cortico-nRT excitation, and describe a mechanism for pathological oscillation generation. This mode could be active under other circumstances, representing a previously unknown channel of cortico-thalamo-cortical information processing.

Genetically encoded, single-component optogenetic tools have made a significant impact on neuroscience, enabling specific modulation of selected cells within complex neural tissues. As the optogenetic toolbox contents grow and diversify, the opportunities for neuroscience continue to grow. In this review, we outline the development of currently available single-component optogenetic tools and summarize the application of various optogenetic tools in diverse model organisms.

Both observational and perturbational technologies are essential for advancing the understanding of brain function and dysfunction. But while observational techniques have greatly advanced in the last century, techniques for perturbation that are matched to the speed and heterogeneity of neural systems have lagged behind. The technology of optogenetics represents a step toward addressing this disparity. Reliable and targetable single-component tools (which encompass both light sensation and effector function within a single protein) have enabled versatile new classes of investigation in the study of neural systems. Here we provide a primer on the application of optogenetics in neuroscience, focusing on the single-component tools and highlighting important problems, challenges, and technical considerations.

Optogenetics is a technique for controlling subpopulations of neurons in the intact brain using light. This technique has the potential to enhance basic systems neuroscience research and to inform the mechanisms and treatment of brain injury and disease. Before launching large-scale primate studies, the method needs to be further characterized and adapted for use in the primate brain. We assessed the safety and efficiency of two viral vector systems (lentivirus and adeno-associated virus), two human promoters (human synapsin (hSyn) and human thymocyte-1 (hThy-1)) and three excitatory and inhibitory mammalian codon-optimized opsins (channelrhodopsin-2, enhanced Natronomonas pharaonis halorhodopsin and the step-function opsin), which we characterized electrophysiologically, histologically and behaviorally in rhesus monkeys (Macaca mulatta). We also introduced a new device for measuring in vivo fluorescence over time, allowing minimally invasive assessment of construct expression in the intact brain. We present a set of optogenetic tools designed for optogenetic experiments in the non-human primate brain.

The use of light-activated modulation techniques, such as optogenetics, is growing in popularity for enabling basic neuroscience research. It is also being explored for advancing more applied applications like therapeutic neuromodulation. However, current hardware systems are generally limited to acute measurements or require external tethering of the system to the light source. This paper presents an implantable prototype for use in techniques that modulate neurological state through optically-activated channels and compounds. The prototype system employs a three chip custom IC architecture to manage information flow into the neural substrate, while also handling power dissipation and providing a chronic barrier to the tissue interface. In addition to covering the details of the IC architecture, we discuss system level design constraints and solutions, and in-vitro test results using our prototype system with an optogenetic model. Potential technical limitations for the broader adoption of these techniques will also be considered.

Neural activity in the noradrenergic locus coeruleus correlates with periods of wakefulness and arousal. However, it is unclear whether tonic or phasic activity in these neurons is necessary or sufficient to induce transitions between behavioral states and to promote long-term arousal. Using optogenetic tools in mice, we found that there is a frequency-dependent, causal relationship among locus coeruleus firing, cortical activity, sleep-to-wake transitions and general locomotor arousal. We also found that sustained, high-frequency stimulation of the locus coeruleus at frequencies of 5 Hz and above caused reversible behavioral arrests. These results suggest that the locus coeruleus is finely tuned to regulate organismal arousal and that bursts of noradrenergic overexcitation cause behavioral attacks that resemble those seen in people with neuropsychiatric disorders.

Biological processes are governed by extensive networks of dynamic molecular interactions. Yet, establishing a spatial and temporal map of these interactions and their direct relationship to specific cell functions has remained a challenge. Here, we implement sensitized emission Fö rster resonance energy transfer (FRET) stoichiometry under total internal reflection fluorescence (TIRF) microscopy. We demonstrate through quantitative analysis and modeling that evanescent fields must be precisely matched between FRET excitation wavelengths to isolate dynamic interactions between bimolecular FRET pairs that are not entirely membrane-delimited. We then use TIRF-FRET to monitor the behavior of individual insulin-containing secretory granules at the plasma membrane of living cells, while simultaneously tracking the dynamic interaction between the GTPase Rab27A and its effector Slp4A, on those same granules. Notably, insulin granules that underwent exocytosis demonstrated a specific increase in Rab27A-GTP/Slp4A FRET in the 5 s before membrane fusion, which coincided temporally with an increase in granule displacement and mobility. These results demonstrate an initial spatiotemporal mapping of a dynamic protein-protein interaction on individual secretory granules that is linked to a specific granule behavior in living cells.

Regulation of exocytosis by voltage-gated K⁺ channels has classically been viewed as inhibition mediated by K⁺ fluxes. We recently identified a new role for Kv2.1 in facilitating vesicle release from neuroendocrine cells, which is independent of K⁺ flux. Here, we show that Kv2.1-induced facilitation of release is not restricted to neuroendocrine cells, but also occurs in the somatic-vesicle release from dorsal-root-ganglion neurons and is mediated by direct association of Kv2.1 with syntaxin. We further show in adrenal chromaffin cells that facilitation induced by both wild-type and non-conducting mutant Kv2.1 channels in response to long stimulation persists during successive stimulation, and can be attributed to an increased number of exocytotic events and not to changes in single-spike kinetics. Moreover, rigorous analysis of the pools of released vesicles reveals that Kv2.1 enhances the rate of vesicle recruitment during stimulation with high Ca ²⁺, without affecting the size of the readily releasable vesicle pool. These findings place a voltage-gated K⁺ channel among the syntaxin-binding proteins that directly regulate pre-fusion steps in exocytosis.

Channelrhodopsins such as channelrhodopsin-2 (ChR2) can drive spiking with millisecond precision in a wide variety of cells, tissues and animal species. However, several properties of this protein have limited the precision of optogenetic control. First, when ChR2 is expressed at high levels, extra spikes (for example, doublets) can occur in response to a single light pulse, with potential implications as doublets may be important for neural coding. Second, many cells cannot follow ChR2-driven spiking above the gamma (∼40 Hz) range in sustained trains, preventing temporally stationary optogenetic access to a broad and important neural signaling band. Finally, rapid optically driven spike trains can result in plateau potentials of 10 mV or more, causing incidental upstates with information-processing implications. We designed and validated an engineered opsin gene (ChETA) that addresses all of these limitations (profoundly reducing extra spikes, eliminating plateau potentials and allowing temporally stationary, sustained spike trains up to at least 200 Hz).

Synchronized oscillations and inhibitory interneurons have important and interconnected roles within cortical microcircuits. In particular, interneurons defined by the fast-spiking phenotype and expression of the calcium-binding protein parvalbumin have been suggested to be involved in gamma (30-80Hz) oscillations, which are hypothesized to enhance information processing. However, because parvalbumin interneurons cannot be selectively controlled, definitive tests of their functional significance in gamma oscillations, and quantitative assessment of the impact of parvalbumin interneurons and gamma oscillations on cortical circuits, have been lacking despite potentially enormous significance (for example, abnormalities in parvalbumin interneurons may underlie altered gamma-frequency synchronization and cognition in schizophrenia and autism). Here we use a panel of optogenetic technologies in mice to selectively modulate multiple distinct circuit elements in neocortex, alone or in combination. We find that inhibiting parvalbumin interneurons suppresses gamma oscillations in vivo, whereas driving these interneurons (even by means of non-rhythmic principal cell activity) is sufficient to generate emergent gamma-frequency rhythmicity. Moreover, gamma-frequency modulation of excitatory input in turn was found to enhance signal transmission in neocortex by reducing circuit noise and amplifying circuit signals, including inputs to parvalbumin interneurons. As demonstrated here, optogenetics opens the door to a new kind of informational analysis of brain function, permitting quantitative delineation of the functional significance of individual elements in the emergent operation and function of intact neural circuitry.

Priming is the process by which vesicles become available for fusion at nerve terminals and is modulated by numerous proteins and second messengers. One of the prominent members of this diverse family is tomosyn. Tomosyn has been identified as a syntaxin-binding protein; it inhibits vesicle priming, but its mode of action is not fully understood. The inhibitory activity of tomosyn depends on its N-terminal WD40-repeat domain and is regulated by the binding of its SNARE motif to syntaxin. Here, we describe new physiological information on the function of tomosyn and address possible interpretations of these results in the framework of the recently described crystal structure of the yeast tomosyn homolog Sro7. We also present possible molecular scenarios for vesicle priming and the involvement of tomosyn in these processes.

Here we describe bi-stable channelrhodopsins that convert a brief pulse of light into a stable step in membrane potential. These molecularly engineered probes nevertheless retain millisecond-scale temporal precision. Photocurrents can be precisely initiated and terminated with different colors of light, but operate at vastly longer time scales than conventional channelrhodopsins as a result of modification at the C128 position that extends the lifetime of the open state. Because of their enhanced kinetic stability, these step-function tools are also effectively responsive to light at orders of magnitude lower intensity than wild-type channelrhodopsins. These molecules therefore offer important new capabilities for a broad range of in vivo applications.

In neurons and neuroendocrine cells, docked vesicles need to undergo priming to become fusion competent. Priming is a multi-step process that was shown to be associated with vesicle immobilization. However, it is not known whether vesicle immobilization is sufficient to acquire complete fusion competence. To extend our understanding of the physical manifestation of vesicle priming, we took advantage of tomosyn, a SNARE-related protein that specifically inhibits vesicle priming, and measured its effect on vesicle dynamics in live chromatin cells using total internal reflection fluorescence microscopy. We show here that while in control cells vesicles undergo immobilization before fusion, vesicle immobilization is attenuated in tomosyn overexpressing cells. This in turn increases the turnover rate of vesicles near the membrane and attenuates the fusion of newcomer vesicles. Moreover, the release probability of immobile vesicles in tomosyn cells is significantly reduced, suggesting that immobilization is an early and necessary step in priming but is insufficient, as further molecular processes are needed to acquire complete fusion competence. Using tomosyn as a molecular tool we provide a mechanistic link between functional docking and priming and suggest that functional docking is the first step in vesicle priming, followed by molecular modifications that do not translate into changes in vesicle mobility.

The introduction of two microbial opsin-based tools, channelrhodopsin-2 (ChR2) and halorhodopsin (NpHR), to neuroscience has generated interest in fast, multimodal, cell type-specific neural circuit control. Here we describe a cation-conducting channelrhodopsin (VChR1) from Volvox carteri that can drive spiking at 589 nm, with excitation maximum red-shifted ∼70 nm compared with ChR2. These results demonstrate fast photostimulation with yellow light, thereby defining a functionally distinct third category of microbial rhodopsin proteins.

Tomosyn is a cytoplasmic protein that was shown to bind to Syntaxin1 and SNAP-25 through an R-SNARE domain, forming a complex that is almost identical in structure to the neuronal SNARE complex. Tomosyn inhibits exocytosis in various cell types and these effects were attributed to direct competition between tomosyn's SNARE domain and Synaptobrevin/VAMP. In the present study, we investigated the contribution of different domains of tomosyn to its activity. We show that a tomosyn mutant that lacks the entire SNARE domain is a potent inhibitor of vesicle priming, similar to the full-length tomosyn. The SNARE domain of tomosyn failed to inhibit exocytosis, indicating that this domain is not required for the inhibition. In contrast, over-expression of a N-terminally truncated mutant did not lead to inhibition of exocytosis although this mutant still bound to Syntaxin. Our results indicate that tomosyn can inhibit exocytosis independently of its SNARE interaction with Syntaxin and that the integrity of the WD40-domain is crucial for tomosyn's inhibitory function. Furthermore, we demonstrate that the entire N-terminal region of tomosyn, the WD40-repeats and the linker, is required for tomosyn's inhibitory effect.

Tomosyn, a soluble R-SNARE protein identified as a binding partner of the Q-SNARE syntaxin 1A, is thought to be critical in setting the level of fusion-competent SNARE complexes for neurosecretion. To date, there has been no direct evaluation of the dynamics in which tomosyn transits through tomosyn-SNARE complexes or of the extent to which tomosyn-SNARE complexes are regulated by secretory demand. Here, we employed biochemical and optical approaches to characterize the dynamic properties of tomosyn-syntaxin 1A complexes in live adrenal chromaffin cells. We demonstrate that secretagogue stimulation results in the rapid translocation of tomosyn from the cytosol to plasma membrane regions and that this translocation is associated with an increase in the tomosyn-syntaxin 1A interaction, including increased cycling of tomosyn into tomosyn-SNARE complexes. The secretagogue-induced interaction was strongly reduced by pharmacological inhibition of the Rho-associated coiled-coil forming kinase, a result consistent with findings demonstrating secretagogue-induced activation of RhoA. Stimulation of chromaffin cells with lysophosphatidic acid, a nonsecretory stimulus that strongly activates RhoA, resulted in effects on tomosyn similar to that of application of the secretagogue. In PC-12 cells overexpressing tomosyn, secretagogue stimulation in the presence of lysophosphatidic acid resulted in reduced evoked secretory responses, an effect that was eliminated upon inhibition of Rho-associated coiled-coil forming kinase. Moreover, this effect required an intact interaction between tomosyn and syntaxin 1A. Thus, modulation of the tomosyn-syntaxin 1A interaction in response to secretagogue activation is an important mechanism allowing for dynamic regulation of the secretory response.

Ras signaling to its downstream effectors appears to include combinations of extracellular-signal-regulated Ras activation at the plasma membrane (PM) and endomembranes, dynamic lateral segregation in the PM, and translocation of Ras from the PM to intracellular compartments. These processes are governed by the C-terminal polybasic farnesyl domain in K-Ras 4B and by the cysteine-palmitoylated C-terminal farnesyl domains in H-Ras and N-Ras. K-Ras 4B has no palmitoylated cysteines. Depalmitoylation/repalmitoylation of H-/N-Ras proteins promotes their cellular redistribution and signaling by mechanisms as yet unknown, possibly involving chaperones. Palmitoylation of H-/N-Ras also promotes their association with 'rasosomes', randomly diffusing nanoparticles that apparently provide a means by which multiple copies of activated Ras and its signal can spread rapidly. Ubiquitination of H-Ras evidently targets it to the endosomes. The polybasic farnesyl domain of K-Ras 4B was shown to act as a target for Ca⁺⁺/ calmodulin, which sequesters the active protein from the PM, thereby facilitating its trafficking to Golgi apparatus and early endosomes. Protein kinase C-dependent phosphorylation of S181 in K-Ras 4B was shown to provide a regulated farnesyl-electrostatic switch on K-Ras 4B, which promotes its translocation to the mitochondria. All these translocation events are characterized by nonconventional trafficking of the farnesyl-modified Ras proteins and seem to govern the selectivity and probably also the robustness of the Ras signal. In this review, we discuss the various modifications and interactions of the farnesylated C-terminus, the trafficking of Ras proteins in the PM and between the PM and the endomembranes, and the relevance of the subcellular localization of Ras for Ras function.

1. Ras signaling and oncogenesis depend on the dynamic interplay of Ras with distinctive plasma membrane (PM) microdomains and various intracellular compartments. Such interaction is dictated by individual elements in the carboxy-terminal domain of the Ras proteins, including a farnesyl isoprenoid group, sequences in the hypervariable region (hvr)-linker, and palmitoyl groups in H/N-Ras isoforms. 2. The farnesyl group acts as a specific recognition unit that interacts with prenyl-binding pockets in galectin-1 (Gal-1), galectin-3 (Gal-3), and cGMP phosphodiesterase δ. This interaction appears to contribute to the prolongation of Ras signals in the PM, the determination of Ras effector usage, and perhaps also the transport of cytoplasmic Ras. Gal-1 promotes H-Ras signaling to Raf at the expense of phosphoinositide 3-kinase (PI3-K) and Ral guanine nucleotide exchange factor (RalGEF), while galectin-3 promotes K-Ras signaling to both Raf and PI3-K. 3. The hvr-linker and the palmitates of H-Ras and N-Ras determine the micro- and macro-localizations of these proteins in the PM and in the Golgi, as well as in 'rasosomes', randomly moving nanoparticles that carry palmitoylated Ras proteins and their signal through the cytoplasm. 4. The dynamic compartmentalization of Ras proteins contributes to the spatial organization of Ras signaling, promotes redistribution of Ras, and provides an additional level of selectivity to the signal output of this regulatory GTPase.

The coupling of voltage-gated Ca²⁺ channel (VGCC) to exocytotic proteins suggests a regulatory function for the channel in depolarization-evoked exocytosis. To explore this possibility we have examined catecholamine secretion in PC12 and chromaffin cells. We found that replacing Ca²⁺ with La³⁺ or other lanthanide ions supported exocytosis in divalent ion-free solution. Cd²⁺, nifedipine, or verapamil inhibited depolarization-evoked secretion in La³⁺, indicating specific binding of La³⁺ at the pore of L-type VGCC, probably at the poly-glutamate (EEEE) locus. Lanthanide efficacy was stringently dependent on ionic radius with La³⁺ > Ce³⁺ > Pr³⁺, consistent with a size-selective binding interface of trivalent cations at the channel pore. La³⁺ inward currents were not detected and the highly sensitive La³⁺/fura-2 imaging assay (∼1 pm) detected no La³⁺ entry, cytosolic La³⁺ build-up, or alterations in cytosolic Ca ². These results provide strong evidence that occupancy of the pore of the channel by an impermeable cation leads to a conformational change that is transmitted to the exocytotic machinery upstream of intracellular cation build-up (intracellular Ca²⁺ concentration). Our model allows for a tight temporal and spatial coupling between the excitatory stimulation event and vesicle fusion. It challenges the conventional view that intracellular Ca ²⁺ ion build-up via VGCC permeation is required to trigger secretion and establishes the VGCC as a plausible Ca²⁺ sensor protein in the process of neuroendocrine secretion.

Spatiotemporal modulation of Ras signaling from different intracellular compartments requires mechanisms allowing Ras and its signals to navigate across cells. Here, we describe one mechanism by which clusters of palmitoylated H-Ras and N-Ras isoforms but not nonpalmitoylated K-Ras diffuse through the cytoplasm, independently of ATP, on fast, randomly moving, small cytosolic nanoparticles ("rasosomes"). Rasosomes forced to diffuse out of live cells and trapped by Ras antibody beads appear as round structures of 80- to 100-nm diameter. Association of H-Ras with rasosomes requires Ras palmitoylation and the hypervariable sequence (hvr) upstream of the palmitoylated cysteines. H-Ras hvr mutants that fail to interact with rasosomes are biologically inactive. Epidermal growth factor stimulation rapidly increases active H-Ras-GTP and phosphorylated extracellular signal-regulated kinase (ERK) on rasosomes. Similarly, rasosomes carrying H-Ras(G12V) but not H-Ras are loaded with active ERK. Thus, the rasosome represents a hitherto unknown particle that enables Ras signal information to spread rapidly across cells.

Yeast artificial chromosomes (YACs) that contain human DNA backbone undergo DNA double-strand breaks (DSBs) and recombination during yeast meiosis at rates similar to the yeast native chromosomes. Surprisingly, YACs containing DNA covering a recombination hot spot in the mouse major histocompatibility complex class III region do not show meiotic DSBs and undergo meiotic recombination at reduced levels. Moreover, segregation of these YACs during meiosis is seriously compromised. In meiotic yeast cells carrying the mutations sir2 or sir4, but not sir3, these YACs show DSBs, suggesting that a unique chromatin structure of the YACs, involving Sir2 and Sir4, protects the YACs from the meiotic recombination machinery. We speculate that the paucity of DSBs and recombination events on these YACs during yeast meiosis may reflect the refractory nature of the corresponding region in the mouse genome.

Neurotransmitter release is a multistep process that is coordinated by a large number of synaptic proteins and depends on proper protein-protein interactions. Using morphological, capacitance, and amperometric measurements, we investigated the effect of tomosyn, a Syntaxin-binding protein, on the different kinetic components of exocytosis in adrenal chromaffin cells. Overexpression of tomosyn decreased the release probability and led to a 50% reduction in the number of fusion-competent vesicles. The number of docked vesicles and the fusion kinetics of single vesicles were not altered suggesting that tomosyn inhibits the priming step. Interestingly, this inhibition is partially relieved at elevated calcium concentration. Calcium ramp experiments supported the latter finding and indicated that the reduction in secretion is caused by a shift in the calcium-dependence of release. These results indicate that secretion is not entirely blocked but occurs at higher calcium concentrations. We suggest that tomosyn inhibits the priming step and impairs the efficiency of vesicle pool refilling in a calcium-dependent manner.