A Cre-dependent, anterograde transsynaptic viral tracer for mapping output pathways of genetically marked neurons

Sex differences in neural projections of fear memory processing in mice and humans

It remains unexplored in the field of fear memory whether functional neuronal connectivity between two brain areas is necessary for one sex but not the other. Here, we show that chemogenetic silencing of centromedial (CeM)-Tac2 fibers in the lateral posterior BNST (BNSTpl) decreased fear memory consolidation in male mice but not females. Optogenetic excitation of CeM-Tac2 fibers in the BNSTpl exhibited enhanced inhibitory postsynaptic currents in males compared to females. In vivo calcium imaging analysis revealed a sex-dimorphic fear memory engram in the BNSTpl. Furthermore, in humans, the single-nucleotide polymorphism (SNP) in the Tac2 receptor (rs2765) (TAC3R) decreased CeM-BNST connectivity in a fear task, impaired fear memory consolidation, and increased the expression of the TAC3R mRNA in AA-carrier men but not in women. These sex differences in critical neuronal circuits underlying fear memory formation may be relevant to human neuropsychiatric disorders with fear memory alterations such as posttraumatic stress disorder.

A Systematic Review of Direct Outputs from the Cerebellum to the Brainstem and Diencephalon in Mammals

The cerebellum is involved in many motor, autonomic and cognitive functions, and new tasks that have a cerebellar contribution are discovered on a regular basis. Simultaneously, our insight into the functional compartmentalization of the cerebellum has markedly improved. Additionally, studies on cerebellar output pathways have seen a renaissance due to the development of viral tracing techniques. To create an overview of the current state of our understanding of cerebellar efferents, we undertook a systematic review of all studies on monosynaptic projections from the cerebellum to the brainstem and the diencephalon in mammals. This revealed that important projections from the cerebellum, to the motor nuclei, cerebral cortex, and basal ganglia, are predominantly di- or polysynaptic, rather than monosynaptic. Strikingly, most target areas receive cerebellar input from all three cerebellar nuclei, showing a convergence of cerebellar information at the output level. Overall, there appeared to be a large level of agreement between studies on different species as well as on the use of different types of neural tracers, making the emerging picture of the cerebellar output areas a solid one. Finally, we discuss how this cerebellar output network is affected by a range of diseases and syndromes, with also non-cerebellar diseases having impact on cerebellar output areas.

A Novel Retrograde AAV Variant for Functional Manipulation of Cortical Projection Neurons in Mice and Monkeys

Retrograde adeno-associated viruses (AAVs) are capable of infecting the axons of projection neurons and serve as a powerful tool for the anatomical and functional characterization of neural networks. However, few retrograde AAV capsids have been shown to offer access to cortical projection neurons across different species and enable the manipulation of neural function in non-human primates (NHPs). Here, we report the development of a novel retrograde AAV capsid, AAV-DJ8R, which efficiently labeled cortical projection neurons after local administration into the striatum of mice and macaques. In addition, intrastriatally injected AAV-DJ8R mediated opsin expression in the mouse motor cortex and induced robust behavioral alterations. Moreover, AAV-DJ8R markedly increased motor cortical neuron firing upon optogenetic light stimulation after viral delivery into the macaque putamen. These data demonstrate the usefulness of AAV-DJ8R as an efficient retrograde tracer for cortical projection neurons in rodents and NHPs and indicate its suitability for use in conducting functional interrogations.

ATLAS: A rationally designed anterograde transsynaptic tracer

Neural circuits, which constitute the substrate for brain processing, can be traced in the retrograde direction, from postsynaptic to presynaptic cells, using methods based on introducing modified rabies virus into genetically marked cell types. These methods have revolutionized the field of neuroscience. However, similarly reliable, transsynaptic, and non-toxic methods to trace circuits in the anterograde direction are not available. Here, we describe such a method based on an antibody-like protein selected against the extracellular N-terminus of the AMPA receptor subunit GluA1 (AMPA.FingR). ATLAS (Anterograde Transsynaptic Label based on Antibody-like Sensors) is engineered to release the AMPA.FingR and its payload, which can include Cre recombinase, from presynaptic sites into the synaptic cleft, after which it binds to GluA1, enters postsynaptic cells through endocytosis and subsequently carries its payload to the nucleus. Testing in vivo and in dissociated cultures shows that ATLAS mediates monosynaptic tracing from genetically determined cells that is strictly anterograde, synaptic, and non-toxic. Moreover, ATLAS shows activity dependence, which may make tracing active circuits that underlie specific behaviors possible.

Anatomical identification of a corticocortical top-down recipient inhibitory circuitry by enhancer-restricted transsynaptic tracing

Despite the importance of postsynaptic inhibitory circuitry targeted by mid/long-range projections (e.g., top-down projections) in cognitive functions, its anatomical properties, such as laminar profile and neuron type, are poorly understood owing to the lack of efficient tracing methods. To this end, we developed a method that combines conventional adeno-associated virus (AAV)-mediated transsynaptic tracing with a distal-less homeobox (Dlx) enhancer-restricted expression system to label postsynaptic inhibitory neurons. We called this method "Dlx enhancer-restricted Interneuron-SpECific transsynaptic Tracing" (DISECT). We applied DISECT to a top-down corticocortical circuit from the secondary motor cortex (M2) to the primary somatosensory cortex (S1) in wild-type mice. First, we injected AAV1-Cre into the M2, which enabled Cre recombinase expression in M2-input recipient S1 neurons. Second, we injected AAV1-hDlx-flex-green fluorescent protein (GFP) into the S1 to transduce GFP into the postsynaptic inhibitory neurons in a Cre-dependent manner. We succeeded in exclusively labeling the recipient inhibitory neurons in the S1. Laminar profile analysis of the neurons labeled via DISECT indicated that the M2-input recipient inhibitory neurons were distributed in the superficial and deep layers of the S1. This laminar distribution was aligned with the laminar density of axons projecting from the M2. We further classified the labeled neuron types using immunohistochemistry and in situ hybridization. This post hoc classification revealed that the dominant top-down M2-input recipient neuron types were somatostatin-expressing neurons in the superficial layers and parvalbumin-expressing neurons in the deep layers. These results demonstrate that DISECT enables the investigation of multiple anatomical properties of the postsynaptic inhibitory circuitry.

Investigations of brain-wide functional and structural networks of dopaminergic and CamKIIα-positive neurons in VTA with DREADD-fMRI and neurotropic virus tracing technologies

BACKGROUND

The ventral tegmental area (VTA) contains heterogeneous cell populations. The dopaminergic neurons in VTA play a central role in reward and cognition, while CamKIIα-positive neurons, composed mainly of glutamatergic and some dopaminergic neurons, participate in the reward learning and locomotor activity behaviors. The differences in brain-wide functional and structural networks between these two neuronal subtypes were comparatively elucidated.

METHODS

In this study, we applied a method combining Designer Receptors Exclusively Activated by Designer Drugs (DREADD) and fMRI to assess the cell type-specific modulation of whole-brain neural networks. rAAV encoding the cre-dependent hM3D was injected into the right VTA of DAT-cre or CamKIIα-cre transgenic rats. The global brain activities elicited by DREADD stimulation were then detected using BOLD-fMRI. Furthermore, the cre-dependent antegrade transsynaptic viral tracer H129ΔTK-TT was applied to label the outputs of VTA neurons.

RESULTS

We found that DREADD stimulation of dopaminergic neurons induced significant BOLD signal changes in the VTA and several VTA-related regions including mPFC, Cg and Septum. More regions responded to selective activation of VTA CamKIIα-positive neurons, resulting in increased BOLD signals in VTA, Insula, mPFC, MC_R (Right), Cg, Septum, Hipp, TH_R, PtA_R, and ViC_R. Along with DREADD-BOLD analysis, further neuronal tracing identified multiple cortical (MC, mPFC) and subcortical (Hipp, TH) brain regions that are structurally and functionally connected by VTA dopaminergic and CamKIIα-positive neurons.

CONCLUSIONS

Our study dissects brain-wide structural and functional networks of two neuronal subtypes in VTA and advances our understanding of VTA functions.

Transneuronal Circuit Analysis with Pseudorabies Viruses

Our ability to understand the function of the nervous system is dependent upon defining the connections of its constituent neurons. Development of methods to define connections within neural networks has always been a growth industry in the neurosciences. Transneuronal spread of neurotropic viruses currently represents the best means of defining synaptic connections within neural networks. The method exploits the ability of viruses to invade neurons, replicate, and spread through the intimate synaptic connections that enable communication among neurons. Since the method was first introduced in the 1970s, it has benefited from an increased understanding of the virus life cycle, the function of viral genomes, and the ability to manipulate the viral genome in support of directional spread of virus and the expression of transgenes. In this article, we review these advances in viral tracing technology and the ways in which they may be applied for functional dissection of neural networks. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Retrograde infection of CNS circuits by peripheral injection of virus Basic Protocol 2: Transneuronal analysis by intracerebral injection Alternate Protocol 1: Transneuronal analysis with multiple recombinant strains Alternate Protocol 2: Conditional replication and spread of PRV Alternate Protocol 3: Conditional reporters of PRV infection and spread Alternate Protocol 4: Reporters of neural activity in polysynaptic circuits Support Protocol 1: Growing and titering a PRV viral stock Support Protocol 2: Immunohistochemical processing and detection Support Protocol 3: Dual-immunofluorescence localization.

Directed stepwise tracing of polysynaptic neuronal circuits with replication-deficient pseudorabies virus

Brain functions are accomplished by polysynaptic circuits formed by neurons wired together through multiple orders of synaptic connections. Polysynaptic connectivity has been difficult to examine due to a lack of methods of continuously tracing the pathways in a controlled manner. Here, we demonstrate directed, stepwise retrograde polysynaptic tracing by inducible reconstitution of replication-deficient trans-neuronal pseudorabies virus (PRVΔIE) in the brain. Furthermore, PRVΔIE replication can be temporally restricted to minimize its neurotoxicity. With this tool, we delineate a wiring diagram between the hippocampus and striatum-two major brain systems for learning, memory, and navigation-that consists of projections from specific hippocampal domains to specific striatal areas via distinct intermediate brain regions. Therefore, this inducible PRVΔIE system provides a tool for dissecting polysynaptic circuits underlying complex brain functions.

Identification of a GABAergic neural circuit governing leptin signaling deficiency-induced obesity

The hormone leptin is known to robustly suppress food intake by acting upon the leptin receptor (LepR) signaling system residing within the agouti-related protein (AgRP) neurons of the hypothalamus. However, clinical studies indicate that leptin is undesirable as a therapeutic regiment for obesity, which is at least partly attributed to the poorly understood complex secondary structure and key signaling mechanism of the leptin-responsive neural circuit. Here, we show that the LepR-expressing portal neurons send GABAergic projections to a cohort of α3-GABAA receptor expressing neurons within the dorsomedial hypothalamic nucleus (DMH) for the control of leptin-mediated obesity phenotype. We identified the DMH as a key brain region that contributes to the regulation of leptin-mediated feeding. Acute activation of the GABAergic AgRP-DMH circuit promoted food intake and glucose intolerance, while activation of post-synaptic MC4R neurons in the DMH elicited exactly opposite phenotypes. Rapid deletion of LepR from AgRP neurons caused an obesity phenotype which can be rescued by blockage of GABAA receptor in the DMH. Consistent with behavioral results, these DMH neurons displayed suppressed neural activities in response to hunger or hyperglycemia. Furthermore, we identified that α3-GABAA receptor signaling within the DMH exerts potent bi-directional regulation of the central effects of leptin on feeding and body weight. Together, our results demonstrate a novel GABAergic neural circuit governing leptin-mediated feeding and energy balance via a unique α3-GABAA signaling within the secondary leptin-responsive neural circuit, constituting a new avenue for therapeutic interventions in the treatment of obesity and associated comorbidities.

Monosynaptic restriction of the anterograde herpes simplex virus strain H129 for neural circuit tracing

Identification of synaptic partners is a fundamental task for systems neuroscience. To date, few reliable techniques exist for whole brain labeling of downstream synaptic partners in a cell-type-dependent and monosynaptic manner. Herein, we describe a novel monosynaptic anterograde tracing system based on the deletion of the gene UL6 from the genome of a cre-dependent version of the anterograde Herpes Simplex Virus 1 strain H129. Given that this knockout blocks viral genome packaging and thus viral spread, we reasoned that co-infection of a HSV H129 ΔUL6 virus with a recombinant adeno-associated virus expressing UL6 in a cre-dependent manner would result in monosynaptic spread from target cre-expressing neuronal populations. Application of this system to five nonreciprocal neural circuits resulted in labeling of neurons in expected projection areas. While some caveats may preclude certain applications, this system provides a reliable method to label postsynaptic partners in a brain-wide fashion.

D1 and D2 neurons in the nucleus accumbens enable positive and negative control over sugar intake in mice

Although the consumption of carbohydrates is needed for survival, their potent reinforcing properties drive obesity worldwide. In turn, sugar overconsumption reveals a major role for brain reward systems in regulating sugar intake. However, it remains elusive how different cell types within the reward circuitries control the initiation and termination of sugary meals. Here, we identified the distinct nucleus accumbens cell types that mediate the chemosensory versus postprandial properties of sweet sugars. Specifically, D1 neurons enhance sugar intake via specialized connections to taste ganglia, whereas D2 neurons mediate the termination of sugary meals via anatomical connections to circuits involved in appetite suppression. Consistently, D2, but not D1, neurons partially mediate the satiating effects of glucagon-like peptide 1 (GLP-1) agonists. Thus, these nucleus accumbens cell types function as a behavioral switch, enabling positive versus negative control over sugar intake. Our study contributes to unveiling the cellular and circuit substrates of sugar overconsumption.

A whole-brain monosynaptic input connectome to neuron classes in mouse visual cortex

Identification of structural connections between neurons is a prerequisite to understanding brain function. Here we developed a pipeline to systematically map brain-wide monosynaptic input connections to genetically defined neuronal populations using an optimized rabies tracing system. We used mouse visual cortex as the exemplar system and revealed quantitative target-specific, layer-specific and cell-class-specific differences in its presynaptic connectomes. The retrograde connectivity indicates the presence of ventral and dorsal visual streams and further reveals topographically organized and continuously varying subnetworks mediated by different higher visual areas. The visual cortex hierarchy can be derived from intracortical feedforward and feedback pathways mediated by upper-layer and lower-layer input neurons. We also identify a new role for layer 6 neurons in mediating reciprocal interhemispheric connections. This study expands our knowledge of the visual system connectomes and demonstrates that the pipeline can be scaled up to dissect connectivity of different cell populations across the mouse brain.

Social trauma engages lateral septum circuitry to occlude social reward

In humans, traumatic social experiences can contribute to psychiatric disorders1. It is suggested that social trauma impairs brain reward function such that social behaviour is no longer rewarding, leading to severe social avoidance2,3. In rodents, the chronic social defeat stress (CSDS) model has been used to understand the neurobiology underlying stress susceptibility versus resilience following social trauma, yet little is known regarding its impact on social reward4,5. Here we show that, following CSDS, a subset of male and female mice, termed susceptible (SUS), avoid social interaction with non-aggressive, same-sex juvenile C57BL/6J mice and do not develop context-dependent social reward following encounters with them. Non-social stressors have no effect on social reward in either sex. Next, using whole-brain Fos mapping, in vivo Ca2+ imaging and whole-cell recordings, we identified a population of stress/threat-responsive lateral septum neurotensin (NTLS) neurons that are activated by juvenile social interactions only in SUS mice, but not in resilient or unstressed control mice. Optogenetic or chemogenetic manipulation of NTLS neurons and their downstream connections modulates social interaction and social reward. Together, these data suggest that previously rewarding social targets are possibly perceived as social threats in SUS mice, resulting from hyperactive NTLS neurons that occlude social reward processing.

Lateral septum modulates cortical state to tune responsivity to threat stimuli

Sudden unexpected environmental changes capture attention and, when perceived as potentially dangerous, evoke defensive behavioral states. Perturbations of the lateral septum (LS) can produce extreme hyperdefensiveness even to innocuous stimuli, but how this structure influences stimulus-evoked defensive responses and threat perception remains unclear. Here, we show that Crhr2-expressing neurons in mouse LS exhibit phasic activation upon detection of threatening but not rewarding stimuli. Threat-stimulus-driven activity predicts the probability but not vigor or type of defensive behavior evoked. Although necessary for and sufficient to potentiate stimulus-triggered defensive responses, LSCrhr2 neurons do not promote specific behaviors. Rather, their stimulation elicits negative valence and physiological arousal. Moreover, LSCrhr2 activity tracks brain state fluctuations and drives cortical activation and rapid awakening in the absence of threat. Together, our findings suggest that LS directs bottom-up modulation of cortical function to evoke preparatory defensive internal states and selectively enhance responsivity to threat-related stimuli.

Activity of a direct VTA to ventral pallidum GABA pathway encodes unconditioned reward value and sustains motivation for reward

Dopamine signaling from the ventral tegmental area (VTA) plays critical roles in reward-related behaviors, but less is known about the functions of neighboring VTA GABAergic neurons. We show here that a primary target of VTA GABA projection neurons is the ventral pallidum (VP). Activity of VTA-to-VP-projecting GABA neurons correlates consistently with size and palatability of the reward and does not change following cue learning, providing a direct measure of reward value. Chemogenetic stimulation of this GABA projection increased activity of a subset of VP neurons that were active while mice were seeking reward. Optogenetic stimulation of this pathway improved performance in a cue-reward task and maintained motivation to work for reward over days. This VTA GABA projection provides information about reward value directly to the VP, likely distinct from the prediction error signal carried by VTA dopamine neurons.

Rhesus macaque versus rat divergence in the corticospinal projectome

We used viral intersectional tools to map the entire projectome of corticospinal neurons associated with fine distal forelimb control in Fischer 344 rats and rhesus macaques. In rats, we found an extraordinarily diverse set of collateral projections from corticospinal neurons to 23 different brain and spinal regions. Remarkably, the vast weighting of this "motor" projection was to sensory systems in both the brain and spinal cord, confirmed by optogenetic and transsynaptic viral intersectional tools. In contrast, rhesus macaques exhibited far heavier and narrower weighting of corticospinal outputs toward spinal and brainstem motor systems. Thus, corticospinal systems in macaques primarily constitute a final output system for fine motor control, whereas this projection in rats exerts a multi-modal integrative role that accesses far broader CNS regions. Unique structural-functional correlations can be achieved by mapping and quantifying a single neuronal system's total axonal output and its relative weighting across CNS targets.

Distinct neural networks derived from galanin-containing nociceptors and neurotensin-expressing pruriceptors

Pain and itch are distinct sensations arousing evasion and compulsive desire for scratching, respectively. It's unclear whether they could invoke different neural networks in the brain. Here, we use the type 1 herpes simplex virus H129 strain to trace the neural networks derived from two types of dorsal root ganglia (DRG) neurons: one kind of polymodal nociceptors containing galanin (Gal) and one type of pruriceptors expressing neurotensin (Nts). The DRG microinjection and immunosuppression were performed in transgenic mice to achieve a successful tracing from specific types of DRG neurons to the primary sensory cortex. About one-third of nuclei in the brain were labeled. More than half of them were differentially labeled in two networks. For the ascending pathways, the spinothalamic tract was absent in the network derived from Nts-expressing pruriceptors, and the two networks shared the spinobulbar projections but occupied different subnuclei. As to the motor systems, more neurons in the primary motor cortex and red nucleus of the somatic motor system participated in the Gal-containing nociceptor-derived network, while more neurons in the nucleus of the solitary tract (NST) and the dorsal motor nucleus of vagus nerve (DMX) of the emotional motor system was found in the Nts-expressing pruriceptor-derived network. Functional validation of differentially labeled nuclei by c-Fos test and chemogenetic inhibition suggested the red nucleus in facilitating the response to noxious heat and the NST/DMX in regulating the histamine-induced scratching. Thus, we reveal the organization of neural networks in a DRG neuron type-dependent manner for processing pain and itch.

An inter-organ neural circuit for appetite suppression

Glucagon-like peptide-1 (GLP-1) is a signal peptide released from enteroendocrine cells of the lower intestine. GLP-1 exerts anorectic and antimotility actions that protect the body against nutrient malabsorption. However, little is known about how intestinal GLP-1 affects distant organs despite rapid enzymatic inactivation. We show that intestinal GLP-1 inhibits gastric emptying and eating via intestinofugal neurons, a subclass of myenteric neurons that project to abdominal sympathetic ganglia. Remarkably, cell-specific ablation of intestinofugal neurons eliminated intestinal GLP-1 effects, and their chemical activation functioned as a GLP-1 mimetic. GLP-1 sensing by intestinofugal neurons then engaged a sympatho-gastro-spinal-reticular-hypothalamic pathway that links abnormal stomach distension to craniofacial programs for food rejection. Within this pathway, cell-specific activation of discrete neuronal populations caused systemic GLP-1-like effects. These molecularly identified, delimited enteric circuits may be targeted to ameliorate the abdominal bloating and loss of appetite typical of gastric motility disorders.

Advancements in the Quest to Map, Monitor, and Manipulate Neural Circuitry

Neural circuits and the cells that comprise them represent the functional units of the brain. Circuits relay and process sensory information, maintain homeostasis, drive behaviors, and facilitate cognitive functions such as learning and memory. Creating a functionally-precise map of the mammalian brain requires anatomically tracing neural circuits, monitoring their activity patterns, and manipulating their activity to infer function. Advancements in cell-type-specific genetic tools allow interrogation of neural circuits with increased precision. This review provides a broad overview of recombination-based and activity-driven genetic targeting approaches, contemporary viral tracing strategies, electrophysiological recording methods, newly developed calcium, and voltage indicators, and neurotransmitter/neuropeptide biosensors currently being used to investigate circuit architecture and function. Finally, it discusses methods for acute or chronic manipulation of neural activity, including genetically-targeted cellular ablation, optogenetics, chemogenetics, and over-expression of ion channels. With this ever-evolving genetic toolbox, scientists are continuing to probe neural circuits with increasing resolution, elucidating the structure and function of the incredibly complex mammalian brain.

Lighting Up Neural Circuits by Viral Tracing

Neurons are highly interwoven to form intricate neural circuits that underlie the diverse functions of the brain. Dissecting the anatomical organization of neural circuits is key to deciphering how the brain processes information, produces thoughts, and instructs behaviors. Over the past decades, recombinant viral vectors have become the most commonly used tracing tools to define circuit architecture. In this review, we introduce the current categories of viral tools and their proper application in circuit tracing. We further discuss some advances in viral tracing strategy and prospective innovations of viral tools for future study.

Trans-Seq maps a selective mammalian retinotectal synapse instructed by Nephronectin

The mouse visual system serves as an accessible model to understand mammalian circuit wiring. Despite rich knowledge in retinal circuits, the long-range connectivity map from distinct retinal ganglion cell (RGC) types to diverse brain neuron types remains unknown. In this study, we developed an integrated approach, called Trans-Seq, to map RGCs to superior collicular (SC) circuits. Trans-Seq combines a fluorescent anterograde trans-synaptic tracer, consisting of codon-optimized wheat germ agglutinin fused to mCherry, with single-cell RNA sequencing. We used Trans-Seq to classify SC neuron types innervated by genetically defined RGC types and predicted a neuronal pair from αRGCs to Nephronectin-positive wide-field neurons (NPWFs). We validated this connection using genetic labeling, electrophysiology and retrograde tracing. We then used transcriptomic data from Trans-Seq to identify Nephronectin as a determinant for selective synaptic choice from αRGC to NPWFs via binding to Integrin α8β1. The Trans-Seq approach can be broadly applied for post-synaptic circuit discovery from genetically defined pre-synaptic neurons.

Strategies for Targeting Neural Circuits: How to Manipulate Neurons Using Virus Vehicles

Viral strategies are the leading methods for mapping neural circuits. Viral vehicles combined with genetic tools provide the possibility to visualize entire functional neural networks and monitor and manipulate neural circuit functions by high-resolution cell type- and projection-specific targeting. Optogenetics and chemogenetics drive brain research forward by exploring causal relationships among different brain regions. Viral strategies offer a fresh perspective for the analysis of the structure-function relationship of the neural circuitry. In this review, we summarize current and emerging viral strategies for targeting neural circuits and focus on adeno-associated virus (AAV) vectors.

The Effects of Light and the Circadian System on Rhythmic Brain Function

Life on earth has evolved under the influence of regularly recurring changes in the environment, such as the 24 h light/dark cycle. Consequently, organisms have developed endogenous clocks, generating 24 h (circadian) rhythms that serve to anticipate these rhythmic changes. In addition to these circadian rhythms, which persist in constant conditions and can be entrained to environmental rhythms, light drives rhythmic behavior and brain function, especially in nocturnal laboratory rodents. In recent decades, research has made great advances in the elucidation of the molecular circadian clockwork and circadian light perception. This review summarizes the role of light and the circadian clock in rhythmic brain function, with a focus on the complex interaction between the different components of the mammalian circadian system. Furthermore, chronodisruption as a consequence of light at night, genetic manipulation, and neurodegenerative diseases is briefly discussed.

Adult Neurogenesis under Control of the Circadian System

The mammalian circadian system is a hierarchically organized system, which controls a 24-h periodicity in a wide variety of body and brain functions and physiological processes. There is increasing evidence that the circadian system modulates the complex multistep process of adult neurogenesis, which is crucial for brain plasticity. This modulatory effect may be exercised via rhythmic systemic factors including neurotransmitters, hormones and neurotrophic factors as well as rhythmic behavior and physiology or via intrinsic factors within the neural progenitor cells such as the redox state and clock genes/molecular clockwork. In this review, we discuss the role of the circadian system for adult neurogenesis at both the systemic and the cellular levels. Better understanding of the role of the circadian system in modulation of adult neurogenesis can help develop new treatment strategies to improve the cognitive deterioration associated with chronodisruption due to detrimental light regimes or neurodegenerative diseases.

A distinct hypothalamus-to-β cell circuit modulates insulin secretion

The central nervous system has long been thought to regulate insulin secretion, an essential process in the maintenance of blood glucose levels. However, the anatomical and functional connections between the brain and insulin-producing pancreatic β cells remain undefined. Here, we describe a functional transneuronal circuit connecting the hypothalamus to β cells in mice. This circuit originates from a subpopulation of oxytocin neurons in the paraventricular hypothalamic nucleus (PVN^OXT), and it reaches the islets of the endocrine pancreas via the sympathetic autonomic branch to innervate β cells. Stimulation of PVN^OXT neurons rapidly suppresses insulin secretion and causes hyperglycemia. Conversely, silencing of these neurons elevates insulin levels by dysregulating neuronal signaling and secretory pathways in β cells and induces hypoglycemia. PVN^OXT neuronal activity is triggered by glucoprivation. Our findings reveal that a subset of PVN^OXT neurons form functional multisynaptic circuits with β cells in mice to regulate insulin secretion, and their function is necessary for the β cell response to hypoglycemia.

Optophysiology: Illuminating cell physiology with optogenetics

Optogenetics combines light and genetics to enable precise control of living cells, tissues, and organisms with tailored functions. Optogenetics has the advantages of noninvasiveness, rapid responsiveness, tunable reversibility, and superior spatiotemporal resolution. Following the initial discovery of microbial opsins as light-actuated ion channels, a plethora of naturally occurring or engineered photoreceptors or photosensitive domains that respond to light at varying wavelengths has ushered in the next chapter of optogenetics. Through protein engineering and synthetic biology approaches, genetically-encoded photoswitches can be modularly engineered into protein scaffolds or host cells to control a myriad of biological processes, as well as to enable behavioral control and disease intervention in vivo. Here, we summarize these optogenetic tools on the basis of their fundamental photochemical properties to better inform the chemical basis and design principles. We also highlight exemplary applications of opsin-free optogenetics in dissecting cellular physiology (designated "optophysiology"), and describe the current progress, as well as future trends, in wireless optogenetics, which enables remote interrogation of physiological processes with minimal invasiveness. This review is anticipated to spark novel thoughts on engineering next-generation optogenetic tools and devices that promise to accelerate both basic and translational studies.

Seeing the Forest and Its Trees Together: Implementing 3D Light Microscopy Pipelines for Cell Type Mapping in the Mouse Brain

The brain is composed of diverse neuronal and non-neuronal cell types with complex regional connectivity patterns that create the anatomical infrastructure underlying cognition. Remarkable advances in neuroscience techniques enable labeling and imaging of these individual cell types and their interactions throughout intact mammalian brains at a cellular resolution allowing neuroscientists to examine microscopic details in macroscopic brain circuits. Nevertheless, implementing these tools is fraught with many technical and analytical challenges with a need for high-level data analysis. Here we review key technical considerations for implementing a brain mapping pipeline using the mouse brain as a primary model system. Specifically, we provide practical details for choosing methods including cell type specific labeling, sample preparation (e.g., tissue clearing), microscopy modalities, image processing, and data analysis (e.g., image registration to standard atlases). We also highlight the need to develop better 3D atlases with standardized anatomical labels and nomenclature across species and developmental time points to extend the mapping to other species including humans and to facilitate data sharing, confederation, and integrative analysis. In summary, this review provides key elements and currently available resources to consider while developing and implementing high-resolution mapping methods.

A novel H129-based anterograde monosynaptic tracer exhibits features of strong labeling intensity, high tracing efficiency, and reduced retrograde labeling

Background

Viral tracers are important tools for mapping brain connectomes. The feature of predominant anterograde transneuronal transmission offers herpes simplex virus-1 (HSV-1) strain H129 (HSV1-H129) as a promising candidate to be developed as anterograde viral tracers. In our earlier studies, we developed H129-derived anterograde polysynaptic tracers and TK deficient (H129-dTK) monosynaptic tracers. However, their broad application is limited by some intrinsic drawbacks of the H129-dTK tracers, such as low labeling intensity due to TK deficiency and potential retrograde labeling caused by axon terminal invasion. The glycoprotein K (gK) of HSV-1 plays important roles in virus entry, egress, and virus-induced cell fusion. Its deficiency severely disables virus egress and spread, while only slightly limits viral genome replication and expression of viral proteins. Therefore, we created a novel H129-derived anterograde monosynaptic tracer (H129-dgK) by targeting gK, which overcomes the limitations of H129-dTK.

Methods

Using our established platform and pipeline for developing viral tracers, we generated a novel tracer by deleting the gK gene from the H129-G4. The gK-deleted virus (H129-dgK-G4) was reconstituted and propagated in the Vero cell expressing wildtype H129 gK (gK_wt) or the mutant gK (gK_mut, A40V, C82S, M223I, L224V, V309M), respectively. Then the obtained viral tracers of gK_mut pseudotyped and gK_wt coated H129-dgK-G4 were tested in vitro and in vivo to characterize their tracing properties.

Results

H129-dgK-G4 expresses high levels of fluorescent proteins, eliminating the requirement of immunostaining for imaging detection. Compared to the TK deficient monosynaptic tracer H129-dTK-G4, H129-dgK-G4 labeled neurons with 1.76-fold stronger fluorescence intensity, and visualized 2.00-fold more postsynaptic neurons in the downstream brain regions. gK_mut pseudotyping leads to a 77% decrease in retrograde labeling by reducing axon terminal invasion, and thus dramatically improves the anterograde-specific tracing of H129-dgK-G4. In addition, assisted by the AAV helper trans-complementarily expressing gK_wt, H129-dgK-G4 allows for mapping monosynaptic connections and quantifying the circuit connectivity difference in the Alzheimer’s disease and control mouse brains.

Conclusions

gK_mut pseudotyped H129-dgK-G4, a novel anterograde monosynaptic tracer, overcomes the limitations of H129-dTK tracers, and demonstrates desirable features of strong labeling intensity, high tracing efficiency, and improved anterograde specificity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13024-021-00508-6.

Application of AAV1 for Anterograde Transsynaptic Circuit Mapping and Input-Dependent Neuronal Cataloging

Viruses that spread transsynaptically provide a powerful means to study interconnected circuits in the brain. Here we describe the use of adeno-associated virus, serotype 1 (AAV1), as a tool to achieve robust, anterograde transsynaptic spread in a variety of unidirectional pathways. A protocol for performing intracranial AAV1 injections in mice is presented, along with additional guidance for planning experiments, sourcing materials, and optimizing the approach to achieve the most successful outcomes. By following the methods presented here, researchers will be able to reveal postsynaptically connected neurons downstream of a given AAV1 injection site and access these input-defined cells for subsequent mapping, recording, and manipulation to characterize their anatomical and functional properties. © 2022 Wiley Periodicals LLC. Basic Protocol: Stereotaxic injection of AAV1 for anterograde transsynaptic spread.

Contralateral Projection of Anterior Cingulate Cortex Contributes to Mirror-Image Pain

Long-term limb nerve injury often leads to mirror-image pain (MIP), an abnormal pain sensation in the limb contralateral to the injury. Although it is clear that MIP is mediated in part by central nociception processing, the underlying mechanisms remain poorly understood. The anterior cingulate cortex (ACC) is a key brain region that receives relayed peripheral nociceptive information from the contralateral limb. In this study, we induced MIP in male mice, in which a unilateral chronic constrictive injury of the sciatic nerve (CCI) induced a decreased nociceptive threshold in both hind limbs and an increased number of c-Fos-expressing neurons in the ACC both contralateral and ipsilateral to the injured limb. Using viral-mediated projection mapping, we observed that a portion of ACC neurons formed monosynaptic connections with contralateral ACC neurons. Furthermore, the number of cross-callosal projection ACC neurons that exhibited c-Fos signal was increased in MIP-expressing mice, suggesting enhanced transmission between ACC neurons of the two hemispheres. Moreover, selective inhibition of the cross-callosal projection ACC neurons contralateral to the injured limb normalized the nociceptive sensation of the uninjured limb without affecting the increased nociceptive sensation of the injured limb in CCI mice. In contrast, inhibition of the non-cross-callosal projection ACC neurons contralateral to the injury normalized the nociceptive sensation of the injured limb without affecting the MIP exhibited in the uninjured limb. These results reveal a circuit mechanism, namely, the cross-callosal projection of ACC between two hemispheres, that contributes to MIP and possibly other forms of contralateral migration of pain sensation.SIGNIFICANCE STATEMENT Mirror-image pain (MIP) refers to the increased pain sensitivity of the contralateral body part in patients with chronic pain. This pathology requires central processing, yet the mechanisms are less known. Here, we demonstrate that the cross-callosal projection neurons in the anterior cingulate cortex (ACC) contralateral to the injury contribute to MIP exhibited in the uninjured limb, but do not affect nociceptive sensation of the injured limb. In contrast, the non-cross-callosal projection neurons in the ACC contralateral to the injury contribute to nociceptive sensation of the injured limb, but do not affect MIP exhibited in the uninjured limb. Our study depicts a novel cross-callosal projection of ACC that contributes to MIP, providing a central mechanism for MIP in chronic pain state.

Anterograde transneuronal tracing and genetic control with engineered yellow fever vaccine YFV-17D

Transneuronal viruses are powerful tools for tracing neuronal circuits or delivering genes to specific neurons in the brain. While there are multiple retrograde viruses, few anterograde viruses are available. Further, available anterograde viruses often have limitations such as retrograde transport, high neuronal toxicity or weak signals. We developed an anterograde viral system based on a live attenuated vaccine for yellow fever-YFV-17D. Replication- or packaging-deficient mutants of YFV-17D can be reconstituted in the brain, leading to efficient synapse-specific and anterograde-only transneuronal spreading, which can be controlled to achieve either monosynaptic or polysynaptic tracing. Moreover, inducible transient replication of YFV-17D mutant is sufficient to induce permanent transneuronal genetic modifications without causing neuronal toxicity. The engineered YFV-17D systems can be used to express fluorescent markers, sensors or effectors in downstream neurons, thus providing versatile tools for mapping and functionally controlling neuronal circuits.

Architectures of neuronal circuits

[Figure: see text].

Principles of odor coding in vertebrates and artificial chemosensory systems

The biological olfactory system is the sensory system responsible for the detection of the chemical composition of the environment. Several attempts to mimic biological olfactory systems have led to various artificial olfactory systems using different technical approaches. Here we provide a parallel description of biological olfactory systems and their technical counterparts. We start with a presentation of the input to the systems, the stimuli, and treat the interface between the external world and the environment where receptor neurons or artificial chemosensors reside. We then delineate the functions of receptor neurons and chemosensors as well as their overall I-O relationships. Up to this point, our account of the systems goes along similar lines. The next processing steps differ considerably: while in biology the processing step following the receptor neurons is the "integration" and "processing" of receptor neuron outputs in the olfactory bulb, this step has various realizations in electronic noses. For a long period of time, the signal processing stages beyond the olfactory bulb, i.e., the higher olfactory centers were little studied. Only recently there has been a marked growth of studies tackling the information processing in these centers. In electronic noses, a third stage of processing has virtually never been considered. In this review, we provide an up-to-date overview of the current knowledge of both fields and, for the first time, attempt to tie them together. We hope it will be a breeding ground for better information, communication, and data exchange between very related but so far little connected fields.

Top-down control of conditioned overconsumption is mediated by insular cortex Nos1 neurons

Associative learning allows animals to adapt their behavior in response to environmental cues. For example, sensory cues associated with food availability can trigger overconsumption even in sated animals. However, the neural mechanisms mediating cue-driven non-homeostatic feeding are poorly understood. To study this, we recently developed a behavioral task in which contextual cues increase feeding even in sated mice. Here, we show that an insular cortex to central amygdala circuit is necessary for conditioned overconsumption, but not for homeostatic feeding. This projection is marked by a population of glutamatergic nitric oxide synthase-1 (Nos1)-expressing neurons, which are specifically active during feeding bouts. Finally, we show that activation of insular cortex Nos1 neurons suppresses satiety signals in the central amygdala. The data, thus, indicate that the insular cortex provides top-down control of homeostatic circuits to promote overconsumption in response to learned cues.

Convergence Circuit Mapping: Genetic Approaches From Structure to Function

Understanding the complex neural circuits that underpin brain function and behavior has been a long-standing goal of neuroscience. Yet this is no small feat considering the interconnectedness of neurons and other cell types, both within and across brain regions. In this review, we describe recent advances in mouse molecular genetic engineering that can be used to integrate information on brain activity and structure at regional, cellular, and subcellular levels. The convergence of structural inputs can be mapped throughout the brain in a cell type-specific manner by antero- and retrograde viral systems expressing various fluorescent proteins and genetic switches. Furthermore, neural activity can be manipulated using opto- and chemo-genetic tools to interrogate the functional significance of this input convergence. Monitoring neuronal activity is obtained with precise spatiotemporal resolution using genetically encoded sensors for calcium changes and specific neurotransmitters. Combining these genetically engineered mapping tools is a compelling approach for unraveling the structural and functional brain architecture of complex behaviors and malfunctioned states of neurological disorders.

Comparative Ultrastructural Analysis of Thalamocortical Innervation of the Primary Motor Cortex and Supplementary Motor Area in Control and MPTP-Treated Parkinsonian Monkeys

The synaptic organization of thalamic inputs to motor cortices remains poorly understood in primates. Thus, we compared the regional and synaptic connections of vGluT2-positive thalamocortical glutamatergic terminals in the supplementary motor area (SMA) and the primary motor cortex (M1) between control and MPTP-treated parkinsonian monkeys. In controls, vGluT2-containing fibers and terminal-like profiles invaded layer II-III and Vb of M1 and SMA. A significant reduction of vGluT2 labeling was found in layer Vb, but not in layer II-III, of parkinsonian animals, suggesting a potential thalamic denervation of deep cortical layers in parkinsonism. There was a significant difference in the pattern of synaptic connectivity in layers II-III, but not in layer Vb, between M1 and SMA of control monkeys. However, this difference was abolished in parkinsonian animals. No major difference was found in the proportion of perforated versus macular post-synaptic densities at thalamocortical synapses between control and parkinsonian monkeys in both cortical regions, except for a slight increase in the prevalence of perforated axo-dendritic synapses in the SMA of parkinsonian monkeys. Our findings suggest that disruption of the thalamic innervation of M1 and SMA may underlie pathophysiological changes of the motor thalamocortical loop in the state of parkinsonism.

Avian adeno-associated virus as an anterograde transsynaptic vector

BACKGROUND

Retrograde and anterograde transsynaptic viral vectors are useful tools for studying the input and output organization of neuronal circuitry, respectively. While retrograde transsynaptic viral vectors are widely used, viral vectors that show anterograde transsynaptic transduction are not common.

NEW METHOD

We chose recombinant avian adeno-associated virus (A3V) carrying the mCherry gene and injected it into the eyeball, cochlear duct, and midbrain auditory center of chickens. We observed different survival times to examine the virus transcellular transport and the resulting mCherry expression. To confirm the transcellular transduction mode, we co-injected A3V and cholera toxin B subunit.

RESULTS

Injecting A3V into the eyeball and cochlea labeled neurons in the visual and auditory pathways, respectively. Second-, and third-order labeling occurred approximately two and seven days, respectively, after injection into the midbrain. The distribution of labeled neurons strongly suggests that A3V transport is preferentially anterograde and transduces postsynaptic neurons.

COMPARISON WITH EXISTING METHOD(S)

A3V displays no extrasynaptic leakage and moderate speed of synapse passage, which is better than other viruses previously reported. Compared with AAV1&9, which have been shown to pass one synapse anterogradely, A3V passes several synapses in the anterograde direction.

CONCLUSIONS

A3V would be a good tool to study the topographic organization of projection axons and their target neurons.

Nucleus Accumbens Tac1-Expressing Neurons Mediate Stress-Induced Anhedonia-like Behavior in Mice

Major depressive disorder (MDD) presents with two primary symptoms: depressed mood and anhedonia, which suggests that distinct neuronal circuits may regulate MDD. However, the underlying circuits of these individual symptoms linked to depression remain elusive. Herein, we identify a discrete circuit of tachykinin precursor 1 (Tac1)-expressing neurons in the nucleus accumbens (NAc) lateral shell, which project to ventral pallidum and contribute to stress-induced anhedonia-like behavior. Selective inhibition and activation of Tac1^NAc neurons bidirectionally modulate stress susceptibility, revealing that Tac1 neurons in the NAc are critical for regulating anhedonia-like behaviors. We find that a subpopulation of VP neurons receives inhibitory inputs from Tac1^NAc neurons and exhibits decreased excitability in susceptible mice. Furthermore, the inhibition of the neurokinin 1 receptor promotes susceptibility to social stress. Overall, our study reveals a discrete circuit regulating anhedonia-like behavior in mice.

HSV-1 H129-Derived Anterograde Neural Circuit Tracers: Improvements, Production, and Applications

Anterograde viral tracers are powerful and essential tools for dissecting the output targets of a brain region of interest. They have been developed from herpes simplex virus 1 (HSV-1) strain H129 (H129), and have been successfully applied to map diverse neural circuits. Initially, the anterograde polysynaptic tracer H129-G4 was used by many groups. We then developed the first monosynaptic tracer, H129-dTK-tdT, which was highly successful, yet improvements are needed. Now, by inserting another tdTomato expression cassette into the H129-dTK-tdT genome, we have created H129-dTK-T2, an updated version of H129-dTK-tdT that has improved labeling intensity. To help scientists produce and apply our H129-derived viral tracers, here we provide the protocol describing our detailed and standardized procedures. Commonly-encountered technical problems and their solutions are also discussed in detail. Broadly, the dissemination of this protocol will greatly support scientists to apply these viral tracers on a large scale.

The gut-brain axis: Identifying new therapeutic approaches for type 2 diabetes, obesity, and related disorders

BACKGROUND

The gut-brain axis, which mediates bidirectional communication between the gastrointestinal system and central nervous system (CNS), plays a fundamental role in multiple areas of physiology including regulating appetite, metabolism, and gastrointestinal function. The biology of the gut-brain axis is central to the efficacy of glucagon-like peptide-1 (GLP-1)-based therapies, which are now leading treatments for type 2 diabetes (T2DM) and obesity. This success and research to suggest a much broader role of gut-brain circuits in physiology and disease has led to increasing interest in targeting such circuits to discover new therapeutics. However, our current knowledge of this biology is limited, largely because the scientific tools have not been available to enable a detailed mechanistic understanding of gut-brain communication.

SCOPE OF REVIEW

In this review, we provide an overview of the current understanding of how sensory information from the gastrointestinal system is communicated to the central nervous system, with an emphasis on circuits involved in regulating feeding and metabolism. We then describe how recent technologies are enabling a better understanding of this system at a molecular level and how this information is leading to novel insights into gut-brain communication. We also discuss current therapeutic approaches that leverage the gut-brain axis to treat diabetes, obesity, and related disorders and describe potential novel approaches that have been enabled by recent advances in the field.

MAJOR CONCLUSIONS

The gut-brain axis is intimately involved in regulating glucose homeostasis and appetite, and this system plays a key role in mediating the efficacy of therapeutics that have had a major impact on treating T2DM and obesity. Research into the gut-brain axis has historically largely focused on studying individual components in this system, but new technologies are now enabling a better understanding of how signals from these components are orchestrated to regulate metabolism. While this work reveals a complexity of signaling even greater than previously appreciated, new insights are already being leveraged to explore fundamentally new approaches to treating metabolic diseases.

Multiplexing viral approaches to the study of the neuronal circuits

Neural circuits are composed of multitudes of elaborately interconnected cell types. Understanding neural circuit function requires not only cell-specific knowledge of connectivity, but the ability to record and manipulate distinct cell types independently. Recent advances in viral vectors promise the requisite specificity to perform true "circuit-breaking" experiments. However, such new avenues of multiplexed, cell-specific investigation raise new technical issues: one must ensure that both the viral vectors and their transgene payloads do not overlap with each other in both an anatomical and a functional sense. This review describes benefits and issues regarding the use of viral vectors to analyse the function of neural circuits and provides a resource for the design and implementation of such multiplexing experiments.

The Medullary Targets of Neurally Conveyed Sensory Information from the Rat Hepatic Portal and Superior Mesenteric Veins

Vagal and spinal sensory endings in the wall of the hepatic portal and superior mesenteric veins (PMV) provide the brain with chemosensory information important for energy balance and other functions. To determine their medullary neuronal targets, we injected the transsynaptic anterograde viral tracer HSV-1 H129-772 (H129) into the PMV wall or left nodose ganglion (LNG) of male rats, followed by immunohistochemistry (IHC) and high-resolution imaging. We also determined the chemical phenotype of H129-infected neurons, and potential vagal and spinal axon terminal appositions in the dorsal motor nucleus of the vagus (DMX) and the nucleus of the solitary tract (NTS). PMV wall injections generated H129-infected neurons in both nodose ganglia and in thoracic dorsal root ganglia (DRGs). In the medulla, cholinergic preganglionic parasympathetic neurons in the DMX were virtually the only targets of chemosensory information from the PMV wall. H129-infected terminal appositions were identified on H129-infected somata and dendrites in the DMX, and on H129-infected DMX dendrites that extend into the NTS. Sensory transmission via vagal and possibly spinal routes from the PMV wall therefore reaches DMX neurons via axo-somatic appositions in the DMX and axo-dendritic appositions in the NTS. However, the dearth of H129-infected NTS neurons indicates that sensory information from the PMV wall terminates on DMX neurons without engaging NTS neurons. These previously underappreciated direct sensory routes into the DMX enable a vago-vagal and possibly spino-vagal reflexes that can directly influence visceral function.

Comprehensive Analysis of Long-Range Connectivity from and to the Posterior Parietal Cortex of the Mouse

The posterior parietal cortex (PPC) is a major multimodal association cortex implicated in a variety of higher order cognitive functions, such as visuospatial perception, spatial attention, categorization, and decision-making. The PPC is known to receive inputs from a collection of sensory cortices as well as various subcortical areas and integrate those inputs to facilitate the execution of functions that require diverse information. Although many recent works have been performed with the mouse as a model system, a comprehensive understanding of long-range connectivity of the mouse PPC is scarce, preventing integrative interpretation of the rapidly accumulating functional data. In this study, we conducted a detailed neuroanatomic and bioinformatic analysis of the Allen Mouse Brain Connectivity Atlas data to summarize afferent and efferent connections to/from the PPC. Then, we analyzed variability between subregions of the PPC, functional/anatomical modalities, and species, and summarized the organizational principle of the mouse PPC. Finally, we confirmed key results by using additional neurotracers. A comprehensive survey of the connectivity will provide an important future reference to comprehend the function of the PPC and allow effective paths forward to various studies using mice as a model system.

Intersectional targeting of defined neural circuits by adeno-associated virus vectors

The mammalian nervous system is a complex network of interconnected cells. We review emerging techniques that use the axonal transport of adeno-associated virus (AAV) vectors to dissect neural circuits. These intersectional approaches specifically target AAV-mediated gene expression to discrete neuron populations based on their axonal connectivity, including: (a) neurons with one defined output, (b) neurons with one defined input, (c) neurons with one defined input and one defined output, and (d) neurons with two defined inputs or outputs. The number of labeled neurons can be directly controlled to trace axonal projections and examine cellular morphology. These approaches can precisely target the expression of fluorescent reporters, optogenetic ion channels, chemogenetic receptors, disease-associated proteins, and other factors to defined neural circuits in mammals ranging from mice to macaques, and thereby provide a powerful new means to understand the structure and function of the nervous system.

Stress-related memories disrupt sociability and associated patterning of hippocampal activity: a role of hilar oxytocin receptor-positive interneurons

In susceptible individuals, memories of stressful experiences can give rise to debilitating socio-affective symptoms. This occurs even when the ability to retrieve such memories is limited, as seen in patients suffering from traumatic amnesia. We therefore hypothesized that the encoding, rather than retrieval, mechanisms of stress-related memories underlie their impact on social and emotional behavior. To test this hypothesis, we used combinations of stress-enhanced and state-dependent fear conditioning, which engage different encoding mechanisms for the formation of stress-related memories. We found that the encoding of stress-enhanced state-dependent memories robustly and sex specifically impairs sociability in male mice and disrupts the asymmetry of dentate gyrus (DG)/CA3 activity accompanying social interactions. These deficits were restored by chemogenetic inactivation of oxytocin receptor-positive interneurons localized in the hilus (Oxtr-HI), and by inactivation of dorsohippocampal efferents to the caudal lateral septum. Together, our data suggest that disrupted patterning of dorsohippocampal DG/CA3 activity underlies stress-induced sociability deficits, and that Oxtr-HI can be a cellular target for improving these deficits.

Can transsynaptic viral strategies be used to reveal functional aspects of neural circuitry?

Viruses have proved instrumental to elucidating neuronal connectivity relationships in a variety of organisms. Recent advances in genetic technologies have facilitated analysis of neurons directly connected to a defined starter population. These advances have also made viral transneuronal mapping available to the broader neuroscience community, where one-step rabies virus mapping has become routine. This method is commonly used to identify inputs onto defined cell populations, to demonstrate the quantitative proportion of inputs coming from specific brain regions, or to compare input patterns between two or more cell populations. Furthermore, the number of inputs labeled is often assumed to reflect the number of synaptic connections, and these viruses are commonly believed to label strong synapses more efficiently than weak synapses. While these maps are often interpreted to provide a quantitative estimate of the synaptic landscape onto starter cell populations, in fact very little is known about how transneuronal transmission takes place. We do not know how these viruses transmit between neurons, if they display biases in the cell types labeled, or even if transmission is synapse-specific. In this review, we discuss the experimental evidence against or in support of key concepts in viral tracing, focusing mostly on the use of one-step rabies input mapping and related methods. Does spread of these viruses occur specifically through synaptic connections, preferentially through synapses, or non-specifically? How efficient is viral transneuronal transmission, and is this efficiency equal in all cell types? And lastly, to what extent does viral labeling reflect functional connectivity?

Viral tools for neuroscience

Recombinant viruses are the workhorse of modern neuroscience. Whether one would like to understand a neuron's morphology, natural activity patterns, molecular composition, connectivity or behavioural and physiologic function, most studies begin with the injection of an engineered virus, often an adeno-associated virus or herpes simplex virus, among many other types. Recombinant viruses currently enable some combination of cell type-specific, circuit-selective, activity-dependent and spatiotemporally resolved transgene expression. Viruses are now used routinely to study the molecular and cellular functions of a gene within an identified cell type in the brain, and enable the application of optogenetics, chemogenetics, calcium imaging and related approaches. These advantageous properties of engineered viruses thus enable characterization of neuronal function at unprecedented resolution. However, each virus has specific advantages and disadvantages, which makes viral tool selection paramount for properly designing and executing experiments within the central nervous system. In the current Review, we discuss the key principles and uses of engineered viruses and highlight innovations that are needed moving forward.

Genome-wide translational profiling of amygdala Crh-expressing neurons reveals role for CREB in fear extinction learning

Fear and extinction learning are adaptive processes caused by molecular changes in specific neural circuits. Neurons expressing the corticotropin-releasing hormone gene (Crh) in central amygdala (CeA) are implicated in threat regulation, yet little is known of cell type-specific gene pathways mediating adaptive learning. We translationally profiled the transcriptome of CeA Crh-expressing cells (Crh neurons) after fear conditioning or extinction in mice using translating ribosome affinity purification (TRAP) and RNAseq. Differential gene expression and co-expression network analyses identified diverse networks activated or inhibited by fear vs extinction. Upstream regulator analysis demonstrated that extinction associates with reduced CREB expression, and viral vector-induced increased CREB expression in Crh neurons increased fear expression and inhibited extinction. These findings suggest that CREB, within CeA Crh neurons, may function as a molecular switch that regulates expression of fear and its extinction. Cell-type specific translational analyses may suggest targets useful for understanding and treating stress-related psychiatric illness.

A limbic circuit selectively links active escape to food suppression

Stress has pleiotropic physiologic effects, but the neural circuits linking stress to these responses are not well understood. Here, we describe a novel population of lateral septum neurons expressing neurotensin (LS^Nts) in mice that are selectively tuned to specific types of stress. LS^Nts neurons increase their activity during active escape, responding to stress when flight is a viable option, but not when associated with freezing or immobility. Chemogenetic activation of LS^Nts neurons decreases food intake and body weight, without altering locomotion and anxiety. LS^Nts neurons co-express several molecules including Glp1r (glucagon-like peptide one receptor) and manipulations of Glp1r signaling in the LS recapitulates the behavioral effects of LS^Nts activation. Activation of LS^Nts terminals in the lateral hypothalamus (LH) also decreases food intake. These results show that LS^Nts neurons are selectively tuned to active escape stress and can reduce food consumption via effects on hypothalamic pathways.

Hindbrain Double-Negative Feedback Mediates Palatability-Guided Food and Water Consumption

Hunger and thirst have distinct goals but control similar ingestive behaviors, and little is known about neural processes that are shared between these behavioral states. We identify glutamatergic neurons in the peri-locus coeruleus (periLC^VGLUT2 neurons) as a polysynaptic convergence node from separate energy-sensitive and hydration-sensitive cell populations. We develop methods for stable hindbrain calcium imaging in free-moving mice, which show that periLC^VGLUT2 neurons are tuned to ingestive behaviors and respond similarly to food or water consumption. PeriLC^VGLUT2 neurons are scalably inhibited by palatability and homeostatic need during consumption. Inhibition of periLC^VGLUT2 neurons is rewarding and increases consumption by enhancing palatability and prolonging ingestion duration. These properties comprise a double-negative feedback relationship that sustains food or water consumption without affecting food- or water-seeking. PeriLC^VGLUT2 neurons are a hub between hunger and thirst that specifically controls motivation for food and water ingestion, which is a factor that contributes to hedonic overeating and obesity.