Molecular profiling of neurons based on connectivity

Changes in nucleus accumbens core translatome accompanying incubation of cocaine craving

In the 'incubation of cocaine craving' model of relapse, rats exhibit progressive intensification (incubation) of cue-induced craving over several weeks of forced abstinence from cocaine self-administration. The expression of incubated craving depends on plasticity of excitatory synaptic transmission in nucleus accumbens core (NAcC) medium spiny neurons (MSN). Previously, we found that the maintenance of this plasticity and the expression of incubation depends on ongoing protein translation, and the regulation of translation is altered after incubation of cocaine craving. Here we used male and female rats that express Cre recombinase in either dopamine D1 receptor- or adenosine 2a (A2a) receptor-expressing MSN to express a GFP-tagged ribosomal protein in a cell-type specific manner, enabling us to use Translating Ribosome Affinity Purification (TRAP) to isolate actively translating mRNAs from both MSN subtypes for analysis by RNA-seq. We compared rats that self-administered saline or cocaine. Saline rats were assessed on abstinence day (AD) 1, while cocaine rats were assessed on AD1 or AD40-50. For both D1-MSN and A2a-MSN, there were few differentially translated genes between saline and cocaine AD1 groups. In contrast, pronounced differences in the translatome were observed between cocaine rats on AD1 and AD40-50, and this was far more robust in D1-MSN. Notably, all comparisons revealed sex differences in translating mRNAs. Sequencing results were validated by qRT-PCR for several genes of interest. This study, the first to combine TRAP-seq, transgenic rats, and a cocaine self-administration paradigm, identifies translating mRNAs linked to incubation of cocaine craving in D1-MSN and A2a-MSN of the NAcC.

Neurotensin-specific corticothalamic circuit regulates innate response conflict

Animals must simultaneously select and balance multiple action contingencies in ambiguous situations: for instance, evading danger during feeding. This has rarely been examined in the context of information selection; despite corticothalamic pathways that mediate sensory attention being relatively well characterized, neural mechanisms filtering conflicting actions remain unclear. Here, we develop a new loom/feed test to observe conflict between naturally induced fear and feeding and identify a novel anterior cingulate cortex (ACC) output to the ventral anterior and ventral lateral thalamus (VA/VL) that adjusts selectivity between these innate actions. Using micro-endoscopy and fiber photometry, we reveal that activity in corticofugal outputs was lowered during unbalanced/singularly occupied periods, as were the resulting decreased thalamic initiation-related signals for less-favored actions, suggesting that the integration of ACC-thalamic firing may directly regulate the output of behavior choices. Accordingly, the optoinhibition of ACC-VA/VL circuits induced high bias toward feeding at the expense of defense. To identify upstream "commander" cortical cells gating this output, we established dual-order tracing (DOT)-translating ribosome affinity purification (TRAP)-a scheme to label upstream neurons with transcriptome analysis-and found a novel population of neurotensin-positive interneurons (ACCNts). The photoexcitation of ACCNts cells indeed caused similarly hyper-selective behaviors. Collectively, this new "corticofugal action filter" scheme suggests that communication in multi-step cingulate circuits may critically influence the summation of motor signals in thalamic outputs, regulating bias between innate action types.

The Formation and Function of the VTA Dopamine System

The midbrain dopamine system is a sophisticated hub that integrates diverse inputs to control multiple physiological functions, including locomotion, motivation, cognition, reward, as well as maternal and reproductive behaviors. Dopamine is a neurotransmitter that binds to G-protein-coupled receptors. Dopamine also works together with other neurotransmitters and various neuropeptides to maintain the balance of synaptic functions. The dysfunction of the dopamine system leads to several conditions, including Parkinson's disease, Huntington's disease, major depression, schizophrenia, and drug addiction. The ventral tegmental area (VTA) has been identified as an important relay nucleus that modulates homeostatic plasticity in the midbrain dopamine system. Due to the complexity of synaptic transmissions and input-output connections in the VTA, the structure and function of this crucial brain region are still not fully understood. In this review article, we mainly focus on the cell types, neurotransmitters, neuropeptides, ion channels, receptors, and neural circuits of the VTA dopamine system, with the hope of obtaining new insight into the formation and function of this vital brain region.

Neuronal and Molecular Mechanisms Underlying Chronic Pain and Depression Comorbidity in the Paraventricular Thalamus

Patients with chronic pain often develop comorbid depressive symptoms, which makes the pain symptoms more complicated and refractory. However, the underlying mechanisms are poorly known. Here, in a repeated complete Freund's adjuvant (CFA) male mouse model, we reported a specific regulatory role of the paraventricular thalamic nucleus (PVT) glutamatergic neurons, particularly the anterior PVT (PVA) neurons, in mediating chronic pain and depression comorbidity (CDC). Our c-Fos protein staining observed increased PVA neuronal activity in CFA-CDC mice. In wild-type mice, chemogenetic activation of PVA glutamatergic neurons was sufficient to decrease the 50% paw withdrawal thresholds (50% PWTs), while depressive-like behaviors evaluated with immobile time in tail suspension test (TST) and forced swim test (FST) could only be achieved by repeated chemogenetic activation. Chemogenetic inhibition of PVA glutamatergic neurons reversed the decreased 50% PWTs in CFA mice without depressive-like symptoms and the increased TST and FST immobility in CFA-CDC mice. Surprisingly, in CFA-CDC mice, chemogenetically inhibiting PVA glutamatergic neurons failed to reverse the decrease of 50% PWTs, which could be restored by rapid-onset antidepressant S-ketamine. Further behavioral tests in chronic restraint stress mice and CFA pain mice indicated that PVA glutamatergic neuron inhibition and S-ketamine independently alleviate sensory and affective pain. Molecular profiling and pharmacological studies revealed the 5-hydroxytryptamine receptor 1D (Htr1d) in CFA pain-related PVT engram neurons as a potential target for treating CDC. These findings identified novel CDC neuronal and molecular mechanisms in the PVT and provided insight into the complicated pain neuropathology under a comorbid state with depression and related drug development.

Molecularly stratified hypothalamic astrocytes are cellular foci for obesity

Astrocytes safeguard the homeostasis of the central nervous system1,2. Despite their prominent morphological plasticity under conditions that challenge the brain's adaptive capacity3-5, the classification of astrocytes, and relating their molecular make-up to spatially devolved neuronal operations that specify behavior or metabolism, remained mostly futile6,7. Although it seems unexpected in the era of single-cell biology, the lack of a major advance in stratifying astrocytes under physiological conditions rests on the incompatibility of 'neurocentric' algorithms that rely on stable developmental endpoints, lifelong transcriptional, neurotransmitter, and neuropeptide signatures for classification6-8 with the dynamic functional states, anatomic allocation, and allostatic plasticity of astrocytes1. Simplistically, therefore, astrocytes are still grouped as 'resting' vs. 'reactive', the latter referring to pathological states marked by various inducible genes3,9,10. Here, we introduced a machine learning-based feature recognition algorithm that benefits from the cumulative power of published single-cell RNA-seq data on astrocytes as a reference map to stepwise eliminate pleiotropic and inducible cellular features. For the healthy hypothalamus, this walk-back approach revealed gene regulatory networks (GRNs) that specified subsets of astrocytes, and could be used as landmarking tools for their anatomical assignment. The core molecular censuses retained by astrocyte subsets were sufficient to stratify them by allostatic competence, chiefly their signaling and metabolic interplay with neurons. Particularly, we found differentially expressed mitochondrial genes in insulin-sensing astrocytes and demonstrated their reciprocal signaling with neurons that work antagonistically within the food intake circuitry. As a proof-of-concept, we showed that disrupting Mfn2 expression in astrocytes reduced their ability to support dynamic circuit reorganization, a time-locked feature of satiety in the hypothalamus, thus leading to obesity in mice. Overall, our results suggest that astrocytes in the healthy brain are fundamentally more heterogeneous than previously thought and topologically mirror the specificity of local neurocircuits.

State and rate-of-change encoding in parallel mesoaccumbal dopamine pathways

The nervous system uses fast- and slow-adapting sensory detectors in parallel to enable neuronal representations of external states and their temporal dynamics. It is unknown whether this dichotomy also applies to internal representations that have no direct correlation in the physical world. Here we find that two distinct dopamine (DA) neuron subtypes encode either a state or its rate-of-change. In mice performing a reward-seeking task, we found that the animal's behavioral state and rate-of-change were encoded by the sustained activity of DA neurons in medial ventral tegmental area (VTA) DA neurons and transient activity in lateral VTA DA neurons, respectively. The neural activity patterns of VTA DA cell bodies matched DA release patterns within anatomically defined mesoaccumbal pathways. Based on these results, we propose a model in which the DA system uses two parallel lines for proportional-differential encoding of a state variable and its temporal dynamics.

Neuroanatomical, electrophysiological, and morphological characterization of melanin-concentrating hormone cells coexpressing cocaine- and amphetamine-regulated transcript

Melanin-concentrating hormone (MCH) cells in the hypothalamus regulate fundamental physiological functions like energy balance, sleep, and reproduction. This diversity may be ascribed to the neurochemical heterogeneity among MCH cells. One prominent subpopulation of MCH cells coexpresses cocaine- and amphetamine-regulated transcript (CART), and as MCH and CART can have opposing actions, MCH/CART+ and MCH/CART- cells may differentially modulate behavioral outcomes. However, it is not known if there are differences in the cellular properties underlying their functional differences; thus, we compared the neuroanatomical, electrophysiological, and morphological properties of MCH cells in male and female Mch-cre;L10-Egfp reporter mice. Half of MCH cells expressed CART and were most prominent in the medial hypothalamus. Whole-cell patch-clamp recordings revealed differences in their passive and active membrane properties in a sex-dependent manner. Female MCH/CART+ cells had lower input resistances, but male cells largely differed in their firing properties. All MCH cells increased firing when stimulated, but their firing frequency decreases with sustained stimulation. MCH/CART+ cells showed stronger spike rate adaptation than MCH/CART- cells. The kinetics of excitatory events at MCH cells also differed by cell type, as the rising rate of excitatory events was slower at MCH/CART+ cells. By reconstructing the dendritic arborization of our recorded cells, we found no sex differences, but male MCH/CART+ cells had less dendritic length and fewer branch points. Overall, distinctions in topographical division and cellular properties between MCH cells add to their heterogeneity and help elucidate their response to stimuli or effect on modulating their respective neural networks.

Sequential appetite suppression by oral and visceral feedback to the brainstem

The termination of a meal is controlled by dedicated neural circuits in the caudal brainstem. A key challenge is to understand how these circuits transform the sensory signals generated during feeding into dynamic control of behaviour. The caudal nucleus of the solitary tract (cNTS) is the first site in the brain where many meal-related signals are sensed and integrated1-4, but how the cNTS processes ingestive feedback during behaviour is unknown. Here we describe how prolactin-releasing hormone (PRLH) and GCG neurons, two principal cNTS cell types that promote non-aversive satiety, are regulated during ingestion. PRLH neurons showed sustained activation by visceral feedback when nutrients were infused into the stomach, but these sustained responses were substantially reduced during oral consumption. Instead, PRLH neurons shifted to a phasic activity pattern that was time-locked to ingestion and linked to the taste of food. Optogenetic manipulations revealed that PRLH neurons control the duration of seconds-timescale feeding bursts, revealing a mechanism by which orosensory signals feed back to restrain the pace of ingestion. By contrast, GCG neurons were activated by mechanical feedback from the gut, tracked the amount of food consumed and promoted satiety that lasted for tens of minutes. These findings reveal that sequential negative feedback signals from the mouth and gut engage distinct circuits in the caudal brainstem, which in turn control elements of feeding behaviour operating on short and long timescales.

Functional genomic mechanisms of opioid action and opioid use disorder: a systematic review of animal models and human studies

In the past two decades, over-prescription of opioids for pain management has driven a steep increase in opioid use disorder (OUD) and death by overdose, exerting a dramatic toll on western countries. OUD is a chronic relapsing disease associated with a lifetime struggle to control drug consumption, suggesting that opioids trigger long-lasting brain adaptations, notably through functional genomic and epigenomic mechanisms. Current understanding of these processes, however, remain scarce, and have not been previously reviewed systematically. To do so, the goal of the present work was to synthesize current knowledge on genome-wide transcriptomic and epigenetic mechanisms of opioid action, in primate and rodent species. Using a prospectively registered methodology, comprehensive literature searches were completed in PubMed, Embase, and Web of Science. Of the 2709 articles identified, 73 met our inclusion criteria and were considered for qualitative analysis. Focusing on the 5 most studied nervous system structures (nucleus accumbens, frontal cortex, whole striatum, dorsal striatum, spinal cord; 44 articles), we also conducted a quantitative analysis of differentially expressed genes, in an effort to identify a putative core transcriptional signature of opioids. Only one gene, Cdkn1a, was consistently identified in eleven studies, and globally, our results unveil surprisingly low consistency across published work, even when considering most recent single-cell approaches. Analysis of sources of variability detected significant contributions from species, brain structure, duration of opioid exposure, strain, time-point of analysis, and batch effects, but not type of opioid. To go beyond those limitations, we leveraged threshold-free methods to illustrate how genome-wide comparisons may generate new findings and hypotheses. Finally, we discuss current methodological development in the field, and their implication for future research and, ultimately, better care.

Molecular Profiling of VGluT1 AND VGluT2 Ventral Subiculum to Nucleus Accumbens Shell Projections

The nucleus accumbens shell is a critical node in reward circuitry, encoding environments associated with reward. Long-range inputs from the ventral hippocampus (ventral subiculum) to the nucleus accumbens shell have been identified, yet their precise molecular phenotype remains to be determined. Here we used retrograde tracing to identify the ventral subiculum as the brain region with the densest glutamatergic (VGluT1-Slc17a7) input to the shell. We then used circuit-directed translating ribosome affinity purification to examine the molecular characteristics of distinct glutamatergic (VGluT1, VGluT2-Slc17a6) ventral subiculum to nucleus accumbens shell projections. We immunoprecipitated translating ribosomes from this population of projection neurons and analysed molecular connectomic information using RNA sequencing. We found differential gene enrichment across both glutamatergic projection neuron subtypes. In VGluT1 projections, we found enrichment of Pfkl, a gene involved in glucose metabolism. In VGluT2 projections, we found a depletion of Sparcl1 and Dlg1, genes known to play a role in depression- and addiction-related behaviours. These findings highlight potential glutamatergic neuronal-projection-specific differences in ventral subiculum to nucleus accumbens shell projections. Together these data advance our understanding of the phenotype of a defined brain circuit.

Applications of nanobodies in brain diseases

Nanobodies are antibody fragments derived from camelids, naturally endowed with properties like low molecular weight, high affinity and low immunogenicity, which contribute to their effective use as research tools, but also as diagnostic and therapeutic agents in a wide range of diseases, including brain diseases. Also, with the success of Caplacizumab, the first approved nanobody drug which was established as a first-in-class medication to treat acquired thrombotic thrombocytopenic purpura, nanobody-based therapy has received increasing attention. In the current review, we first briefly introduce the characterization and manufacturing of nanobodies. Then, we discuss the issue of crossing of the brain-blood-barrier (BBB) by nanobodies, making use of natural methods of BBB penetration, including passive diffusion, active efflux carriers (ATP-binding cassette transporters), carrier-mediated influx via solute carriers and transcytosis (including receptor-mediated transport, and adsorptive mediated transport) as well as various physical and chemical methods or even more complicated methods such as genetic methods via viral vectors to deliver nanobodies to the brain. Next, we give an extensive overview of research, diagnostic and therapeutic applications of nanobodies in brain-related diseases, with emphasis on Alzheimer's disease, Parkinson's disease, and brain tumors. Thanks to the advance of nanobody engineering and modification technologies, nanobodies can be linked to toxins or conjugated with radionuclides, photosensitizers and nanoparticles, according to different requirements. Finally, we provide several perspectives that may facilitate future studies and whereby the versatile nanobodies offer promising perspectives for advancing our knowledge about brain disorders, as well as hopefully yielding diagnostic and therapeutic solutions.

Virally encoded connectivity transgenic overlay RNA sequencing (VECTORseq) defines projection neurons involved in sensorimotor integration

Behavior arises from concerted activity throughout the brain. Consequently, a major focus of modern neuroscience is defining the physiology and behavioral roles of projection neurons linking different brain areas. Single-cell RNA sequencing has facilitated these efforts by revealing molecular determinants of cellular physiology and markers that enable genetically targeted perturbations such as optogenetics, but existing methods for sequencing defined projection populations are low throughput, painstaking, and costly. We developed a straightforward, multiplexed approach, virally encoded connectivity transgenic overlay RNA sequencing (VECTORseq). VECTORseq repurposes commercial retrogradely infecting viruses typically used to express functional transgenes (e.g., recombinases and fluorescent proteins) by treating viral transgene mRNA as barcodes within single-cell datasets. VECTORseq is compatible with different viral families, resolves multiple populations with different projection targets in one sequencing run, and identifies cortical and subcortical excitatory and inhibitory projection populations. Our study provides a roadmap for high-throughput identification of neuronal subtypes based on connectivity.

Genetically encoded intrabodies as high-precision tools to visualize and manipulate neuronal function

Basic neuroscience research employs numerous forms of antibodies as key reagents in diverse applications. While the predominant use of antibodies is as immunolabeling reagents, neuroscientists are making increased use of intracellular antibodies or intrabodies. Intrabodies are recombinant antibodies genetically encoded for expression within neurons. These can be used to target various cargo (fluorescent proteins, reporters, enzymes, etc.) to specific molecules and subcellular domains to report on and manipulate neuronal function with high precision. Intrabodies have the advantages inherent in all genetically encoded recombinant antibodies but represent a distinct subclass in that their structure allows for their expression and function within cells. The high precision afforded by the ability to direct their expression to specific cell types, and the selective binding of intrabodies to targets within these allows intrabodies to offer unique advantages for neuroscience research, given the tremendous molecular, cellular and morphological complexity of brain neurons. Intrabodies expressed within neurons have been used for a variety of purposes in basic neuroscience research. Here I provide a general background to intrabodies and their development, and examples of their emerging utility as valuable basic neuroscience research tools.

A Small Virus to Deliver Small Antibodies: New Targeted Therapies Based on AAV Delivery of Nanobodies

Nanobodies are camelid-derived single-domain antibodies that present some advantages versus conventional antibodies, such as a smaller size, and higher tissue penetrability, stability, and hydrophilicity. Although nanobodies can be delivered as proteins, in vivo expression from adeno-associated viral (AAV) vectors represents an attractive strategy. This is due to the fact that AAV vectors, that can provide long-term expression of recombinant genes, have shown an excellent safety profile, and can accommodate genes for one or several nanobodies. In fact, several studies showed that AAV vectors can provide sustained nanobody expression both locally or systemically in preclinical models of human diseases. Some of the pathologies addressed with this technology include cancer, neurological, cardiovascular, infectious, and genetic diseases. Depending on the indication, AAV-delivered nanobodies can be expressed extracellularly or inside cells. Intracellular nanobodies or “intrabodies” carry out their function by interacting with cell proteins involved in disease and have also been designed to help elucidate cellular mechanisms by interfering with normal cell processes. Finally, nanobodies can also be used to retarget AAV vectors, when tethered to viral capsid proteins. This review covers applications in which AAV vectors have been used to deliver nanobodies, with a focus on their therapeutic use.

Advances in Gene Delivery Methods to Label and Modulate Activity of Upper Motor Neurons: Implications for Amyotrophic Lateral Sclerosis

The selective degeneration of both upper motor neurons (UMNs) and lower motor neurons (LMNs) is the pathological hallmark of amyotrophic lateral sclerosis (ALS). Unlike the simple organisation of LMNs in the brainstem and spinal cord, UMNs are embedded in the complex cytoarchitecture of the primary motor cortex, which complicates their identification. UMNs therefore remain a challenging neuronal population to study in ALS research, particularly in the early pre-symptomatic stages of animal models. A better understanding of the mechanisms that lead to selective UMN degeneration requires unequivocal visualization and cellular identification of vulnerable UMNs within the heterogeneous cortical neuronal population and circuitry. Here, we review recent novel gene delivery methods developed to cellularly identify vulnerable UMNs and modulate their activity in various mouse models. A critical overview of retrograde tracers, viral vectors encoding reporter genes and transgenic reporter mice used to visualize UMNs in mouse models of ALS is provided. Functional targeting of UMNs in vivo with the advent of optogenetic and chemogenetic technology is also discussed. These exciting gene delivery techniques will facilitate improved anatomical mapping, cell-specific gene expression profiling and targeted manipulation of UMN activity in mice. These advancements in the field pave the way for future work to uncover the precise role of UMNs in ALS and improve future therapeutic targeting of UMNs.

Pain and itch processing by subpopulations of molecularly diverse spinal and trigeminal projection neurons

A remarkable molecular and functional heterogeneity of the primary sensory neurons and dorsal horn interneurons transmits pain- and or itch-relevant information, but the molecular signature of the projection neurons that convey the messages to the brain is unclear. Here, using retro-TRAP (translating ribosome affinity purification) and RNA sequencing, we reveal extensive molecular diversity of spino- and trigeminoparabrachial projection neurons. Among the many genes identified, we highlight distinct subsets of Cck+ -, Nptx2+ -, Nmb+ -, and Crh+ -expressing projection neurons. By combining in situ hybridization of retrogradely labeled neurons with Fos-based assays, we also demonstrate significant functional heterogeneity, including both convergence and segregation of pain- and itch-provoking inputs into molecularly diverse subsets of NK1R- and non-NK1R-expressing projection neurons.

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.

Precise Mapping of Single Neurons by Calibrated 3D Reconstruction of Brain Slices Reveals Topographic Projection in Mouse Visual Cortex

Recent breakthroughs in neuroanatomical tracing methods have helped unravel complicated neural connectivity in whole-brain tissue at single-cell resolution. However, in most cases, analysis of brain images remains dependent on highly subjective and sample-specific manual processing, preventing precise comparison across sample animals. In the present study, we introduce AMaSiNe, software for automated mapping of single neurons in the standard mouse brain atlas with annotated regions. AMaSiNe automatically calibrates misaligned and deformed slice samples to locate labeled neuronal positions from multiple brain samples into the standardized 3D Allen Mouse Brain Reference Atlas. We exploit the high fidelity and reliability of AMaSiNe to investigate the topographic structures of feedforward projections from the lateral geniculate nucleus to the primary visual area by reconstructing rabies-virus-injected brain slices in 3D space. Our results demonstrate that distinct organization of neural projections can be precisely mapped using AMaSiNe.

Insights Into Translatomics in the Nervous System

Most neurological disorders are caused by abnormal gene translation. Generally, dysregulation of elements involved in the translational process disrupts homeostasis in neurons and neuroglia. Better understanding of how the gene translation process occurs requires detailed analysis of transcriptomic and proteomic profile data. However, a lack of strictly direct correlations between mRNA and protein levels limits translational investigation by combining transcriptomic and proteomic profiling. The much better correlation between proteins and translated mRNAs than total mRNAs in abundance and insufficiently sensitive proteomics approach promote the requirement of advances in translatomics technology. Translatomics which capture and sequence the mRNAs associated with ribosomes has been effective in identifying translational changes by genetics or projections, ribosome stalling, local translation, and transcript isoforms in the nervous system. Here, we place emphasis on the main three translatomics methods currently used to profile mRNAs attached to ribosome-nascent chain complex (RNC-mRNA). Their prominent applications in neurological diseases including glioma, neuropathic pain, depression, fragile X syndrome (FXS), neurodegenerative disorders are outlined. The content reviewed here expands our understanding on the contributions of aberrant translation to neurological disease development.

Specificity in sociogenomics: Identifying causal relationships between genes and behavior

There has been rapid growth in the use of transcriptomic analyses to study the interplay between gene expression and behavior. Experience can modify gene expression in the brain, leading to changes in internal state and behavioral displays, while gene expression variation between species is thought to specify many innate behavior differences. However, providing a causal association between a gene and a given behavior remains challenging as it is difficult to determine when and where a gene contributes to the function of a behaviorally-relevant neuronal population. Moreover, given that there are fewer genetic tools available for non-traditional model organisms, transcriptomic approaches have been largely limited to profiling of bulk tissue, which can obscure the contributions of subcortical brain regions implicated in multiple behaviors. Here, we discuss how emerging single cell technologies combined with methods offering additional spatial and connectivity information can give us insight about the genetic profile of specific cells involved in the neural circuit of target social behaviors. We also emphasize how these techniques are broadly adaptable to non-traditional model organisms. We propose that, ultimately, a combination of these approaches applied throughout development will be key to discerning how genes shape the formation of social behavior circuits.

Cellular birthdate predicts laminar and regional cholinergic projection topography in the forebrain

The basal forebrain cholinergic system projects broadly throughout the cortex and constitutes a critical source of neuromodulation for arousal and attention. Traditionally, this system was thought to function diffusely. However, recent studies have revealed a high degree of spatiotemporal specificity in cholinergic signaling. How the organization of cholinergic afferents confers this level of precision remains unknown. Here, using intersectional genetic fate mapping, we demonstrate that cholinergic fibers within the mouse cortex exhibit remarkable laminar and regional specificity and that this is organized in accordance with cellular birthdate. Strikingly, birthdated cholinergic projections within the cortex follow an inside-out pattern of innervation. While early born cholinergic populations target deep layers, late born ones innervate superficial laminae. We also find that birthdate predicts cholinergic innervation patterns within the amygdala, hippocampus, and prefrontal cortex. Our work reveals previously unappreciated specificity within the cholinergic system and the developmental logic by which these circuits are assembled.

Recombinant Antibodies in Basic Neuroscience Research

Basic neuroscience research employs antibodies as key reagents to label, capture, and modulate the function of proteins of interest. Antibodies are immunoglobulin proteins. Recombinant antibodies are immunoglobulin proteins whose nucleic acid coding regions, or fragments thereof, have been cloned into expression plasmids that allow for unlimited production. Recombinant antibodies offer many advantages over conventional antibodies including their unambiguous identification and digital archiving via DNA sequencing, reliable expression, ease and reliable distribution as DNA sequences and as plasmids, and the opportunity for numerous forms of engineering to enhance their utility. Recombinant antibodies exist in many different forms, each of which offers potential advantages and disadvantages for neuroscience research applications. I provide an overview of recombinant antibodies and their development. Examples of their emerging use as valuable reagents in basic neuroscience research are also discussed. Many of these examples employ recombinant antibodies in innovative experimental approaches that cannot be pursued with conventional antibodies. © 2020 Wiley Periodicals LLC.

Circuit and molecular architecture of a ventral hippocampal network

The ventral hippocampus (vHPC) is a critical hub in networks that process emotional information. While recent studies have indicated that ventral CA1 (vCA1) projection neurons are functionally dissociable, the basic principles of how the inputs and outputs of vCA1 are organized remain unclear. Here, we used viral and sequencing approaches to define the logic of the extended vCA1 circuit. Using high-throughput sequencing of genetically barcoded neurons (MAPseq) to map the axonal projections of thousands of vCA1 neurons, we identify a population of neurons that simultaneously broadcast information to multiple areas known to regulate the stress axis and approach-avoidance behavior. Through molecular profiling and viral input-output tracing of vCA1 projection neurons, we show how neurons with distinct projection targets may differ in their inputs and transcriptional signatures. These studies reveal new organizational principles of vCA1 that may underlie its functional heterogeneity.

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.

Soma-Targeted Imaging of Neural Circuits by Ribosome Tethering

Neuroscience relies on techniques for imaging the structure and dynamics of neural circuits, but the cell bodies of individual neurons are often obscured by overlapping fluorescence from axons and dendrites in surrounding neuropil. Here, we describe two strategies for using the ribosome to restrict the expression of fluorescent proteins to the neuronal soma. We show first that a ribosome-tethered nanobody can be used to trap GFP in the cell body, thereby enabling direct visualization of previously undetectable GFP fluorescence. We then design a ribosome-tethered GCaMP for imaging calcium dynamics. We show that this reporter faithfully tracks somatic calcium dynamics in the mouse brain while eliminating cross-talk between neurons caused by contaminating neuropil. In worms, this reporter enables whole-brain imaging with faster kinetics and brighter fluorescence than commonly used nuclear GCaMPs. These two approaches provide a general way to enhance the specificity of imaging in neurobiology.

Heterogeneity in striatal dopamine circuits: Form and function in dynamic reward seeking

The striatal dopamine system has long been studied in the context of reward learning, motivation, and movement. Given the prominent role dopamine plays in a variety of adaptive behavioral states, as well as diseases like addiction, it is essential to understand the full complexity of dopamine neurons and the striatal systems they target. A growing number of studies are uncovering details of the heterogeneity in dopamine neuron subpopulations. Here, we review that work to synthesize current understanding of dopamine system heterogeneity across three levels, anatomical organization, functions in behavior, and modes of action, wherein we focus on signaling profiles and local mechanisms for modulation of dopamine release. Together, these studies reveal new and emerging dimensions of the striatal dopamine system, informing its contribution to dynamic motivational and decision-making processes.

Single-Cell Analysis of Neuroinflammatory Responses Following Intracranial Injection of G-Deleted Rabies Viruses

Viral vectors are essential tools for the study of neural circuits, with glycoprotein-deleted rabies viruses being widely used for monosynaptic retrograde tracing to map connectivity between specific cell types in the nervous system. However, the use of rabies virus is limited by the cytotoxicity and the inflammatory responses these viruses trigger. While components of the rabies virus genome contribute to its cytotoxic effects, the function of other neuronal and non-neuronal cells within the vicinity of the infected host neurons in either effecting or mitigating virally-induced tissue damage are still being elucidated. Here, we analyzed 60,212 single-cell RNA profiles to assess both global and cell-type-specific transcriptional responses in the mouse dorsal raphe nucleus (DRN) following intracranial injection of glycoprotein-deleted rabies viruses and axonal infection of dorsal raphe serotonergic neurons. Gene pathway analyses revealed a down-regulation of genes involved in metabolic processes and neurotransmission following infection. We also identified several transcriptionally diverse leukocyte populations that infiltrate the brain and are distinct from resident immune cells. Cell type-specific patterns of cytokine expression showed that antiviral responses were likely orchestrated by Type I and Type II interferon signaling from microglia and infiltrating CD4+ T cells, respectively. Additionally, we uncovered transcriptionally distinct states of microglia along an activation trajectory that may serve different functions, which range from surveillance to antigen presentation and cytokine secretion. Intercellular interactions inferred from transcriptional data suggest that CD4+ T cells facilitate microglial state transitions during the inflammatory response. Our study uncovers the heterogeneity of immune cells mediating neuroinflammatory responses and provides a critical evaluation of the compatibility between rabies-mediated connectivity mapping and single-cell transcriptional profiling. These findings provide additional insights into the distinct contributions of various cell types in mediating different facets of antiviral responses in the brain and will facilitate the design of strategies to circumvent immune responses to improve the efficacy of viral gene delivery.

Canine Adenovirus 2: A Natural Choice for Brain Circuit Dissection

Canine adenovirus-2 (CAV) is a canine pathogen that has been used in a variety of applications, from vaccines against more infectious strains of CAV to treatments for neurological disorders. With recent engineering, CAV has become a natural choice for neuroscientists dissecting the connectivity and function of brain circuits. Specifically, as a reliable genetic vector with minimal immunogenic and cytotoxic reactivity, CAV has been used for the retrograde transduction of various types of projection neurons. Consequently, CAV is particularly useful when studying the anatomy and functions of long-range projections. Moreover, combining CAV with conditional expression and transsynaptic tracing results in the ability to study circuits with cell- and/or projection-type specificity. Lastly, with the well-documented knowledge of viral transduction, new innovations have been developed to increase the transduction efficiency of CAV and circumvent its tropism, expanding the potential of CAV for circuit analysis.

Aversion hot spots in the dopamine system

Through the development of optogenetics and other viral vector-based technologies, our view of the dopamine system has substantially advanced over the last decade. In particular, progress has been made in the reclassification of dopamine neurons based on subtypes displaying specific projections, which are associated with different features at the anatomical, molecular and behavioral level. Together, these discoveries have raised the possibility that individual groups of dopamine cells make a unique contribution to the processing of reward and aversion. Here, we review recent studies that have identified non-canonical dopamine pathways that are excited in response to aversive stimuli, including dopamine projections to the ventromedial shell of the nucleus accumbens, prefrontal cortex, tail of the striatum, and amygdala.

Classification of Midbrain Dopamine Neurons Using Single-Cell Gene Expression Profiling Approaches

Dysfunctional dopamine (DA) signaling has been associated with a broad spectrum of neuropsychiatric disorders, prompting investigations into how midbrain DA neuron heterogeneity may underpin this variety of behavioral symptoms. Emerging literature indeed points to functional heterogeneity even within anatomically defined DA clusters. Recognizing the need for a systematic classification scheme, several groups have used single-cell profiling to catalog DA neurons based on their gene expression profiles. We aim here not only to synthesize points of congruence but also to highlight key differences between the molecular classification schemes derived from these studies. In doing so, we hope to provide a common framework that will facilitate investigations into the functions of DA neuron subtypes in the healthy and diseased brain.

Connect-seq to superimpose molecular on anatomical neural circuit maps

The mouse brain contains about 75 million neurons interconnected in a vast array of neural circuits. The identities and functions of individual neuronal components of most circuits are undefined. Here we describe a method, termed "Connect-seq," which combines retrograde viral tracing and single-cell transcriptomics to uncover the molecular identities of upstream neurons in a specific circuit and the signaling molecules they use to communicate. Connect-seq can generate a molecular map that can be superimposed on a neuroanatomical map to permit molecular and genetic interrogation of how the neuronal components of a circuit control its function. Application of this method to hypothalamic neurons controlling physiological responses to fear and stress reveals subsets of upstream neurons that express diverse constellations of signaling molecules and can be distinguished by their anatomical locations.

Critical role and its underlying molecular events of the plasminogen receptor, S100A10 in malignant tumor and non-tumor diseases

S100A10 is a small molecular weight protein expressed in the cytoplasm of many cells and one of the members of the S100 protein family that binds calcium and forms the largest subgroup of EF-hand proteins. The regulatory processes of S100A10 are complicated. S100A10 participates in the regulation of a variety of tumor and non-tumor diseases through cascade reactions with multitudinous signaling molecules. In malignant tumors, such as acute promyelocytic leukemia (APL) and lung cancer, S100A10 is likely involved in their progression, including invasion and metastasis through the regulation of plasmin production and subsequent plasmin-dependent stimulation of other proteases, such as matrix metalloproteinase (MMP)-2 and -9. Both the plasmin and MMPs are capable of inducing degradation of the extracellular matrix (ECM) and basement membrane, which is a critical step for tumor progression. In non-tumor diseases, the distribution of S100A10 in the brain and its interaction with 5-hydroxytryptamine 1B (5-HT1B) receptor, an important mediator in the central nervous system that maintains a dynamic balance of the neurotransmitters, correlates with depression-like behavior. S100A10 also participates in inflammatory responses through the regulation of peripheral macrophage migration to the inflammatory sites, which depends on the generation of plasmin and other proteinases at the surface of macrophages. Considerable attention should be paid to understand the significant role of S100A10 in the modulation of malignant tumor and non-tumor diseases.

Control of non-homeostatic feeding in sated mice using associative learning of contextual food cues

Feeding is a complex motivated behavior controlled by a distributed neural network that processes sensory information to generate adaptive behavioral responses. Accordingly, studies using appetitive Pavlovian conditioning confirm that environmental cues that are associated with food availability can induce feeding even in satiated subjects. However, in mice, appetitive conditioning generally requires intensive training and thus can impede molecular studies that often require large numbers of animals. To address this, we developed and validated a simple and rapid context-induced feeding (Ctx-IF) task in which cues associated with food availability can later lead to increased food consumption in sated mice. We show that the associated increase in food consumption is driven by both positive and negative reinforcement and that spaced training is more effective than massed training. Ctx-IF can be completed in ~1 week and provides an opportunity to study the molecular mechanisms and circuitry underlying non-homeostatic eating. We have used this paradigm to map brain regions that are activated during Ctx-IF with cFos immunohistochemistry and found that the insular cortex, and other regions, are activated following exposure to cues denoting the availability of food. Finally, we show that inhibition of the insular cortex using GABA agonists impairs performance of the task. Our findings provide a novel assay in mice for defining the functional neuroanatomy of appetitive conditioning and identify specific brain regions that are activated during the development of learned behaviors that impact food consumption.

Informing disease modelling with brain-relevant functional genomic annotations

The past decade has seen a surge in the number of disease/trait-associated variants, largely because of the union of studies to share genetic data and the availability of electronic health records from large cohorts for research use. Variant discovery for neurological and neuropsychiatric genome-wide association studies, including schizophrenia, Parkinson's disease and Alzheimer's disease, has greatly benefitted; however, the translation of these genetic association results to interpretable biological mechanisms and models is lagging. Interpreting disease-associated variants requires knowledge of gene regulatory mechanisms and computational tools that permit integration of this knowledge with genome-wide association study results. Here, we summarize key conceptual advances in the generation of brain-relevant functional genomic annotations and amongst tools that allow integration of these annotations with association summary statistics, which together provide a new and exciting opportunity to identify disease-relevant genes, pathways and cell types in silico. We discuss the opportunities and challenges associated with these developments and conclude with our perspective on future advances in annotation generation, tool development and the union of the two.

A toolbox of nanobodies developed and validated for use as intrabodies and nanoscale immunolabels in mammalian brain neurons

Nanobodies (nAbs) are small, minimal antibodies that have distinct attributes that make them uniquely suited for certain biomedical research, diagnostic and therapeutic applications. Prominent uses include as intracellular antibodies or intrabodies to bind and deliver cargo to specific proteins and/or subcellular sites within cells, and as nanoscale immunolabels for enhanced tissue penetration and improved spatial imaging resolution. Here, we report the generation and validation of nAbs against a set of proteins prominently expressed at specific subcellular sites in mammalian brain neurons. We describe a novel hierarchical validation pipeline to systematically evaluate nAbs isolated by phage display for effective and specific use as intrabodies and immunolabels in mammalian cells including brain neurons. These nAbs form part of a robust toolbox for targeting proteins with distinct and highly spatially-restricted subcellular localization in mammalian brain neurons, allowing for visualization and/or modulation of structure and function at those sites.

A Viral Receptor Complementation Strategy to Overcome CAV-2 Tropism for Efficient Retrograde Targeting of Neurons

Retrogradely transported neurotropic viruses enable genetic access to neurons based on their long-range projections and have become indispensable tools for linking neural connectivity with function. A major limitation of viral techniques is that they rely on cell-type-specific molecules for uptake and transport. Consequently, viruses fail to infect variable subsets of neurons depending on the complement of surface receptors expressed (viral tropism). We report a receptor complementation strategy to overcome this by potentiating neurons for the infection of the virus of interest-in this case, canine adenovirus type-2 (CAV-2). We designed AAV vectors for expressing the coxsackievirus and adenovirus receptor (CAR) throughout candidate projection neurons. CAR expression greatly increased retrograde-labeling rates, which we demonstrate for several long-range projections, including some resistant to other retrograde-labeling techniques. Our results demonstrate a receptor complementation strategy to abrogate endogenous viral tropism and thereby facilitate efficient retrograde targeting for functional analysis of neural circuits.

Regulation of Energy Expenditure by Brainstem GABA Neurons

Homeostatic control of core body temperature is essential for survival. Temperature is sensed by specific neurons, in turn eliciting both behavioral (i.e., locomotion) and physiologic (i.e., thermogenesis, vasodilatation) responses. Here, we report that a population of GABAergic (Vgat-expressing) neurons in the dorsolateral portion of the dorsal raphe nucleus (DRN), hereafter DRN^Vgat neurons, are activated by ambient heat and bidirectionally regulate energy expenditure through changes in both thermogenesis and locomotion. We find that DRN^Vgat neurons innervate brown fat via a descending projection to the raphe pallidus (RPa). These neurons also densely innervate ascending targets implicated in the central regulation of energy expenditure, including the hypothalamus and extended amygdala. Optogenetic stimulation of different projection targets reveals that DRN^Vgat neurons are capable of regulating thermogenesis through both a "direct" descending pathway through the RPa and multiple "indirect" ascending pathways. This work establishes a key regulatory role for DRN^Vgat neurons in controlling energy expenditure.

In vivo nuclear capture and molecular profiling identifies Gmeb1 as a transcriptional regulator essential for dopamine neuron function

Midbrain dopamine (mDA) neurons play a central role in reward signaling and are widely implicated in psychiatric and neurodegenerative disorders. To understand how mDA neurons perform these functions, it is important to understand how mDA-specific genes are regulated. However, cellular heterogeneity in the mammalian brain presents a major challenge to obtaining this understanding. To this end, we developed a virus-based approach to label and capture mDA nuclei for transcriptome (RNA-Seq), and low-input chromatin accessibility (liDNase-Seq) profiling, followed by predictive modeling to identify putative transcriptional regulators of mDA neurons. Using this method, we identified Gmeb1, a transcription factor predicted to regulate expression of Th and Dat, genes critical for dopamine synthesis and reuptake, respectively. Gmeb1 knockdown in mDA neurons resulted in downregulation of Th and Dat, as well as in severe motor deficits. This study thus identifies Gmeb1 as a master regulator of mDA gene expression and function, and provides a general method for identifying cell type-specific transcriptional regulators.

Sleep Regulation by Neurotensinergic Neurons in a Thalamo-Amygdala Circuit

A crucial step in understanding the sleep-control mechanism is to identify sleep neurons. Through systematic anatomical screening followed by functional testing, we identified two sleep-promoting neuronal populations along a thalamo-amygdala pathway, both expressing neurotensin (NTS). Rabies-mediated monosynaptic retrograde tracing identified the central nucleus of amygdala (CeA) as a major source of GABAergic inputs to multiple wake-promoting populations; gene profiling revealed NTS as a prominent marker for these CeA neurons. Optogenetic activation and inactivation of NTS-expressing CeA neurons promoted and suppressed non-REM (NREM) sleep, respectively, and optrode recording showed they are sleep active. Further tracing showed that CeA GABAergic NTS neurons are innervated by glutamatergic NTS neurons in a posterior thalamic region, which also promote NREM sleep. CRISPR/Cas9-mediated NTS knockdown in either the thalamic or CeA neurons greatly reduced their sleep-promoting effect. These results reveal a novel thalamo-amygdala circuit for sleep generation in which NTS signaling is essential for both the upstream glutamatergic and downstream GABAergic neurons.

An Excitatory Circuit in the Perioculomotor Midbrain for Non-REM Sleep Control

The perioculomotor (pIII) region of the midbrain was postulated as a sleep-regulating center in the 1890s but largely neglected in subsequent studies. Using activity-dependent labeling and gene expression profiling, we identified pIII neurons that promote non-rapid eye movement (NREM) sleep. Optrode recording showed that pIII glutamatergic neurons expressing calcitonin gene-related peptide alpha (CALCA) are NREM-sleep active; optogenetic and chemogenetic activation/inactivation showed that they strongly promote NREM sleep. Within the pIII region, CALCA neurons form reciprocal connections with another population of glutamatergic neurons that express the peptide cholecystokinin (CCK). Activation of CCK neurons also promoted NREM sleep. Both CALCA and CCK neurons project rostrally to the preoptic hypothalamus, whereas CALCA neurons also project caudally to the posterior ventromedial medulla. Activation of each projection increased NREM sleep. Together, these findings point to the pIII region as an excitatory sleep center where different subsets of glutamatergic neurons promote NREM sleep through both local reciprocal connections and long-range projections.

α1- and β3-Adrenergic Receptor-Mediated Mesolimbic Homeostatic Plasticity Confers Resilience to Social Stress in Susceptible Mice

BACKGROUND

Homeostatic plasticity in mesolimbic dopamine (DA) neurons plays an essential role in mediating resilience to social stress. Recent evidence implicates an association between stress resilience and projections from the locus coeruleus (LC) to the ventral tegmental area (VTA) (LC→VTA) DA system. However, the precise circuitry and molecular mechanisms of the homeostatic plasticity in mesolimbic DA neurons mediated by the LC→VTA circuitry, and its role in conferring resilience to social defeat stress, have not been described.

METHODS

In a well-established chronic social defeat stress model of depression, using projection-specific electrophysiological recordings and optogenetic, pharmacological, and molecular profiling techniques, we investigated the functional role and molecular basis of an LC→VTA circuit in conferring resilience to social defeat stress.

RESULTS

We found that LC neurons projecting to the VTA exhibit enhanced firing activity in resilient, but not susceptible, mice. Optogenetically mimicking this firing adaptation in susceptible mice reverses their depression-related behaviors, and induces reversal of cellular hyperactivity and homeostatic plasticity in VTA DA neurons projecting to the nucleus accumbens. Circuit-specific molecular profiling studies reveal that α1- and β3-adrenergic receptors are highly expressed in VTA→nucleus accumbens DA neurons. Pharmacologically activating these receptors induces similar proresilient effects at the ion channel and cellular and behavioral levels, whereas antagonizing these receptors blocks the proresilient effect of optogenetic activation of LC→VTA circuit neurons in susceptible mice.

CONCLUSIONS

These findings reveal a key role of the LC→VTA circuit in mediating homeostatic plasticity in stress resilience and reveal α1- and β3-adrenergic receptors as new molecular targets for therapeutically promoting resilience.

Molecular tools to elucidate factors regulating alcohol use

Alcohol use disorder (AUD) is a pervasive societal problem, marked by high levels of alcohol intake and recidivism. Despite these common disease traits, individuals diagnosed with AUD display a range of disordered drinking and alcohol-related behaviors. The diversity in disease presentation, as well as the established polygenic nature of the disorder and complex neurocircuitry, speaks to the variety of neurochemical changes resulting from alcohol intake that may differentially regulate alcohol-related behaviors. Investigations into the molecular adaptations responsible for maladaptive alcohol-related behavioral outcomes require an ever-evolving set of molecular tools to elucidate with increasing precision how alcohol alters behavior through neurochemical changes. This review highlights recent advances in molecular methodology, addressing how incorporation of these cutting-edge techniques not only may enhance current knowledge of the molecular bases of AUD, but also may facilitate identification of improved treatment targets that may be therapeutic in specific subpopulations of AUD individuals.

The common features of different brain activities

The term "brain activity" refers to a wide range of mental faculties that can be assessed either on anatomical/functional micro-, meso- and macro- spatiotemporal scales of observation, or on intertwined cortical levels with mutual interactions. Our aim is to show that every brain activity encompassed in a given anatomical or functional level necessarily displays a counterpart in others, i.e., they are "dual". "Duality" refers to the case where two seemingly different physical systems turn out to be equivalent. We describe a method, based on novel topological findings, that makes different manifolds (standing for different brain activities) able to scatter, collide and combine, in order that they merge, condense and stitch together in a quantifiable way. We develop a computational tool which explains how, despite their local cortical functional differences, all mental processes, from perception to emotions, from cognition to mind wandering, may be reduced to a single, general brain activity that takes place in dimensions higher than the classical three-dimensional plus time. This framework permits a topological duality among different brain activities and neuro-techniques, because it holds for all the types of spatio-temporal nervous functions, independent of their cortical location, inter- and intra-level relationships, strength, magnitude and boundaries.

Emerging role of viral vectors for circuit-specific gene interrogation and manipulation in rodent brain

Over the past half century, novel tools have allowed the characterization of myriad molecular underpinnings of neural phenomena including synaptic function, neurogenesis and neurodegeneration, membrane excitability, and neurogenetics/epigenetics. More recently, transgenic mice have made possible cell type-specific explorations of these phenomena and have provided critical models of many neurological and psychiatric diseases. However, it has become clear that many critical areas of study require tools allowing the study and manipulation of individual neural circuits within the brain, and viral vectors have come to the forefront in driving these circuit-specific studies. Here, we present a surface-level review of the general classes of viral vectors used for study of the brain, along with their suitability for circuit-specific studies. We then cover in detail a new long-lasting, retrograde expressing form of herpes simplex virus termed LT-HSV that has become highly useful in circuit-based studies. We detail some of its current uses and propose a variety of future uses for this critical new tool, including circuit-based transgene overexpression, gene editing, and gene expression profiling.

Dopamine neuron glutamate cotransmission evokes a delayed excitation in lateral dorsal striatal cholinergic interneurons

Dopamine neurons have different synaptic actions in the ventral and dorsal striatum (dStr), but whether this heterogeneity extends to dStr subregions has not been addressed. We have found that optogenetic activation of dStr dopamine neuron terminals in mouse brain slices pauses the firing of cholinergic interneurons in both the medial and lateral subregions, while in the lateral subregion the pause is shorter due to a subsequent excitation. This excitation is mediated mainly by metabotropic glutamate receptor 1 (mGluR1) and partially by dopamine D1-like receptors coupled to transient receptor potential channel 3 and 7. DA neurons do not signal to spiny projection neurons in the medial dStr, while they elicit ionotropic glutamate responses in the lateral dStr. The DA neurons mediating these excitatory signals are in the substantia nigra (SN). Thus, SN dopamine neurons engage different receptors in different postsynaptic neurons in different dStr subregions to convey strikingly different signals.

Editorial note

This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Efficient Gene Transfer to the Central Nervous System by Single-Stranded Anc80L65

Adeno-associated viral vectors (AAVs) have demonstrated potential in applications for neurologic disorders, and the discovery that some AAVs can cross the blood-brain barrier (BBB) after intravenous injection has further expanded these opportunities for non-invasive brain delivery. Anc80L65, a novel AAV capsid designed from in silico reconstruction of the viral evolutionary lineage, has previously demonstrated robust transduction capabilities after local delivery in various tissues such as liver, retina, or cochlea, compared with conventional AAVs. Here, we compared the transduction efficacy of Anc80L65 with conventional AAV9 in the CNS after intravenous, intracerebroventricular (i.c.v.), or intraparenchymal injections. Anc80L65 was more potent at targeting the brain and spinal cord after intravenous injection than AAV9, and mostly transduced astrocytes and a wide range of neuronal subpopulations. Although the efficacy of Anc80L65 and AAV9 is similar after direct intraparenchymal injection in the striatum, Anc80L65's diffusion throughout the CNS was more extensive than AAV9 after i.c.v. infusion, leading to widespread EGFP expression in the cerebellum. These findings demonstrate that Anc80L65 is a highly efficient gene transfer vector for the murine CNS. Systemic injection of Anc80L65 leads to notable expression in the CNS that does not rely on a self-complementary genome. These data warrant further testing in larger animal models.

Cell-Type Identification in the Autonomic Nervous System

The autonomic nervous system controls various internal organs and executes crucial functions through sophisticated neural connectivity and circuits. Its dysfunction causes an imbalance of homeostasis and numerous human disorders. In the past decades, great efforts have been made to study the structure and functions of this system, but so far, our understanding of the classification of autonomic neuronal subpopulations remains limited and a precise map of their connectivity has not been achieved. One of the major challenges that hinder rapid progress in these areas is the complexity and heterogeneity of autonomic neurons. To facilitate the identification of neuronal subgroups in the autonomic nervous system, here we review the well-established and cutting-edge technologies that are frequently used in peripheral neuronal tracing and profiling, and discuss their operating mechanisms, advantages, and targeted applications.

Molecular profiling of reticular gigantocellularis neurons indicates that eNOS modulates environmentally dependent levels of arousal

Neurons of the medullary reticular nucleus gigantocellularis (NGC) and their targets have recently been a focus of research on mechanisms supporting generalized CNS arousal (GA) required for proper cognitive functions. Using the retro-TRAP method, we characterized transcripts enriched in NGC neurons which have projections to the thalamus. The unique expression and activation of the endothelial nitric oxide (eNOS) signaling pathway in these cells and their intimate connections with blood vessels indicate that these neurons exert direct neurovascular coupling. Production of nitric oxide (NO) within eNOS-positive NGC neurons increases after environmental perturbations, indicating a role for eNOS/NO in modulating environmentally appropriate levels of GA. Inhibition of NO production causes dysregulated behavioral arousal after exposure to environmental perturbation. Further, our findings suggest interpretations for associations between psychiatric disorders and mutations in the eNOS locus.

Functional analysis reveals differential effects of glutamate and MCH neuropeptide in MCH neurons

OBJECTIVES

Melanin-concentrating hormone (MCH) neurons in the lateral hypothalamus (LH) regulate food intake and body weight, glucose metabolism and convey the reward value of sucrose. In this report, we set out to establish the respective roles of MCH and conventional neurotransmitters in these neurons.

METHODS

MCH neurons were profiled using Cre-dependent molecular profiling technologies (vTRAP). MCHCre mice crossed to Vglut2fl/flmice or to DTRfl/flwere used to identify the role of glutamate in MCH neurons. We assessed metabolic parameters such as body composition, glucose tolerance, or sucrose preference.

RESULTS

We found that nearly all MCH neurons in the LH are glutamatergic and that a loss of glutamatergic signaling from MCH neurons from a glutamate transporter (VGlut2) knockout leads to a reduced weight, hypophagia and hyperkinetic behavior with improved glucose tolerance and a loss of sucrose preference. These effects are indistinguishable from those seen after ablation of MCH neurons. These findings are in contrast to those seen in mice with a knockout of the MCH neuropeptide, which show normal glucose preference and do not have improved glucose tolerance.

CONCLUSIONS

Overall, these data show that the vast majority of MCH neurons are glutamatergic, and that glutamate and MCH signaling mediate partially overlapping functions by these neurons, presumably by activating partially overlapping postsynaptic populations. The diverse functional effects of MCH neurons are thus mediated by a composite of glutamate and MCH signaling.

Genetic approaches to access cell types in mammalian nervous systems

Understanding brain circuit organization and function requires systematic dissection of its cellular components. With vast cell number and diversity, mammalian nervous systems present a daunting challenge for achieving specific and comprehensive cell type access-prerequisite to circuit analysis. Genetic approaches in the mouse have relied on germline engineering to access marker-defined cell populations. Combinatorial strategies that engage marker intersection, anatomy and projection pattern (e.g. antero-grade and retro-grade viral vectors), and developmental lineage substantially increase the specificity of cell type targeting. While increasing number of mouse cell types are becoming experimentally accessible, comprehensive coverage requires larger coordinated efforts with strategic infrastructural and fiscal planning. CRISPR-based genome editing may enable cell type access in other species, but issues of time, cost and ethics remain, especially for primates. Novel approaches that bypass the germline, such as somatic cell engineering and cell surface-based gene delivery, may reduce the barrier of genetic access to mammalian cell types.