Functional sensory circuits built from neurons of two species

A central question for regenerative neuroscience is whether synthetic neural circuits, such as those built from two species, can function in an intact brain. Here, we apply blastocyst complementation to selectively build and test interspecies neural circuits. Despite approximately 10-20 million years of evolution, and prominent species differences in brain size, rat pluripotent stem cells injected into mouse blastocysts develop and persist throughout the mouse brain. Unexpectedly, the mouse niche reprograms the birth dates of rat neurons in the cortex and hippocampus, supporting rat-mouse synaptic activity. When mouse olfactory neurons are genetically silenced or killed, rat neurons restore information flow to odor processing circuits. Moreover, they rescue the primal behavior of food seeking, although less well than mouse neurons. By revealing that a mouse can sense the world using neurons from another species, we establish neural blastocyst complementation as a powerful tool to identify conserved mechanisms of brain development, plasticity, and repair.

Distributed X chromosome inactivation in brain circuitry is associated with X-linked disease penetrance of behavior

The precise anatomical degree of brain X chromosome inactivation (XCI) that is sufficient to alter X-linked disorders in females is unclear. Here, we quantify whole-brain XCI at single-cell resolution to discover a prevalent activation ratio of maternal to paternal X at 60:40 across all divisions of the adult brain. This modest, non-random XCI influences X-linked disease penetrance: maternal transmission of the fragile X mental retardation 1 (Fmr1)-knockout (KO) allele confers 55% of total brain cells with mutant X-active, which is sufficient for behavioral penetrance, while 40% produced from paternal transmission is tolerated. Local XCI mosaicism within affected maternal Fmr1-KO mice further specifies sensorimotor versus social anxiety phenotypes depending on which distinct brain circuitry is most affected, with only a 50%-55% mutant X-active threshold determining penetrance. Thus, our results define a model of X-linked disease penetrance in females whereby distributed XCI among single cells populating brain circuitries can regulate the behavioral penetrance of an X-linked mutation.

Sexual coordination in a whole-brain map of prairie vole pair bonding

Sexual bonds are central to the social lives of many species, including humans, and monogamous prairie voles have become the predominant model for investigating such attachments. We developed an automated whole-brain mapping pipeline to identify brain circuits underlying pair-bonding behavior. We identified bonding-related c-Fos induction in 68 brain regions clustered in seven major brain-wide neuronal circuits. These circuits include known regulators of bonding, such as the bed nucleus of the stria terminalis, paraventricular hypothalamus, ventral pallidum, and prefrontal cortex. They also include brain regions previously unknown to shape bonding, such as ventromedial hypothalamus, medial preoptic area, and the medial amygdala, but that play essential roles in bonding-relevant processes, such as sexual behavior, social reward, and territorial aggression. Contrary to some hypotheses, we found that circuits active during mating and bonding were largely sexually monomorphic. Moreover, c-Fos induction across regions was strikingly consistent between members of a pair, with activity best predicted by rates of ejaculation. A novel cluster of regions centered in the amygdala remained coordinated after bonds had formed, suggesting novel substrates for bond maintenance. Our tools and results provide an unprecedented resource for elucidating the networks that translate sexual experience into an enduring bond.

Neuronal activity drives IGF2 expression from pericytes to form long-term memory

Investigations of memory mechanisms have been, thus far, neuron centric, despite the brain comprising diverse cell types. Using rats and mice, we assessed the cell-type-specific contribution of hippocampal insulin-like growth factor 2 (IGF2), a polypeptide regulated by learning and required for long-term memory formation. The highest level of hippocampal IGF2 was detected in pericytes, the multi-functional mural cells of the microvessels that regulate blood flow, vessel formation, the blood-brain barrier, and immune cell entry into the central nervous system. Learning significantly increased pericytic Igf2 expression in the hippocampus, particularly in the highly vascularized stratum lacunosum moleculare and stratum moleculare layers of the dentate gyrus. Igf2 increases required neuronal activity. Regulated hippocampal Igf2 knockout in pericytes, but not in fibroblasts or neurons, impaired long-term memories and blunted the learning-dependent increase of neuronal immediate early genes (IEGs). Thus, neuronal activity-driven signaling from pericytes to neurons via IGF2 is essential for long-term memory.

Sexual coordination in a whole-brain map of prairie vole pair bonding

Sexual bonds are central to the social lives of many species, including humans, and monogamous prairie voles have become the predominant model for investigating such attachments. We developed an automated whole-brain mapping pipeline to identify brain circuits underlying pair-bonding behavior. We identified bonding-related c-Fos induction in 68 brain regions clustered in seven major brain-wide neuronal circuits. These circuits include known regulators of bonding, such as the bed nucleus of the stria terminalis, paraventricular hypothalamus, ventral pallidum, and prefrontal cortex. They also include brain regions previously unknown to shape bonding, such as ventromedial hypothalamus, medial preoptic area and the medial amygdala, but that play essential roles in bonding-relevant processes, such as sexual behavior, social reward and territorial aggression. Contrary to some hypotheses, we found that circuits active during mating and bonding were largely sexually monomorphic. Moreover, c-Fos induction across regions was strikingly consistent between members of a pair, with activity best predicted by rates of ejaculation. A novel cluster of regions centered in the amygdala remained coordinated after bonds had formed, suggesting novel substrates for bond maintenance. Our tools and results provide an unprecedented resource for elucidating the networks that translate sexual experience into an enduring bond.

A circuit from the locus coeruleus to the anterior cingulate cortex modulates offspring interactions in mice

Social sensitivity to other individuals in distress is crucial for survival. The anterior cingulate cortex (ACC) is a structure involved in making behavioral choices and is influenced by observed pain or distress. Nevertheless, our understanding of the neural circuitry underlying this sensitivity is incomplete. Here, we reveal unexpected sex-dependent activation of ACC when parental mice respond to distressed pups by returning them to the nest ("pup retrieval"). We observe sex differences in the interactions between excitatory and inhibitory ACC neurons during parental care, and inactivation of ACC excitatory neurons increased pup neglect. Locus coeruleus (LC) releases noradrenaline in ACC during pup retrieval, and inactivation of the LC-ACC pathway disrupts parental care. We conclude that ACC maintains sex-dependent sensitivity to pup distress under LC modulation. We propose that ACC's involvement in parenting presents an opportunity to identify neural circuits that support sensitivity to the emotional distress of others.

Full-Spectrum Neuronal Diversity and Stereotypy through Whole Brain Morphometry

We conducted a large-scale study of whole-brain morphometry, analyzing 3.7 peta-voxels of mouse brain images at the single-cell resolution, producing one of the largest multi-morphometry databases of mammalian brains to date. We spatially registered 205 mouse brains and associated data from six Brain Initiative Cell Census Network (BICCN) data sources covering three major imaging modalities from five collaborative projects to the Allen Common Coordinate Framework (CCF) atlas, annotated 3D locations of cell bodies of 227,581 neurons, modeled 15,441 dendritic microenvironments, characterized the full morphology of 1,891 neurons along with their axonal motifs, and detected 2.58 million putative synaptic boutons. Our analysis covers six levels of information related to neuronal populations, dendritic microenvironments, single-cell full morphology, sub-neuronal dendritic and axonal arborization, axonal boutons, and structural motifs, along with a quantitative characterization of the diversity and stereotypy of patterns at each level. We identified 16 modules consisting of highly intercorrelated brain regions in 13 functional brain areas corresponding to 314 anatomical regions in CCF. Our analysis revealed the dendritic microenvironment as a powerful method for delineating brain regions of cell types and potential subtypes. We also found that full neuronal morphologies can be categorized into four distinct classes based on spatially tuned morphological features, with substantial cross-areal diversity in apical dendrites, basal dendrites, and axonal arbors, along with quantified stereotypy within cortical, thalamic and striatal regions. The lamination of somas was found to be more effective in differentiating neuron arbors within the cortex. Further analysis of diverging and converging projections of individual neurons in 25 regions throughout the brain reveals branching preferences in the brain-wide and local distributions of axonal boutons. Overall, our study provides a comprehensive description of key anatomical structures of neurons and their types, covering a wide range of scales and features, and contributes to our understanding of neuronal diversity and its function in the mammalian brain.

High-throughput confocal airy beam oblique light-sheet tomography of brain-wide imaging at single-cell resolution

Brain research is an area of research characterized by its cutting-edge nature, with brain mapping constituting a crucial aspect of this field. As sequencing tools have played a crucial role in gene sequencing, brain mapping largely depends on automated, high-throughput and high-resolution imaging techniques. Over the years, the demand for high-throughput imaging has scaled exponentially with the rapid development of microscopic brain mapping. In this paper, we introduce the novel concept of confocal Airy beam into oblique light-sheet tomography named CAB-OLST. We demonstrate that this technique enables the high throughput of brain-wide imaging of long-distance axon projection for the entire mouse brain at a resolution of 0.26 μm × 0.26 μm × 1.06 μm in 58 hours. This technique represents an innovative contribution to the field of brain research by setting a new standard for high-throughput imaging techniques.

Standard metadata for 3D microscopy

Recent advances in fluorescence microscopy techniques and tissue clearing, labeling, and staining provide unprecedented opportunities to investigate brain structure and function. These experiments' images make it possible to catalog brain cell types and define their location, morphology, and connectivity in a native context, leading to a better understanding of normal development and disease etiology. Consistent annotation of metadata is needed to provide the context necessary to understand, reuse, and integrate these data. This report describes an effort to establish metadata standards for three-dimensional (3D) microscopy datasets for use by the Brain Research through Advancing Innovative Neurotechnologies® (BRAIN) Initiative and the neuroscience research community. These standards were built on existing efforts and developed with input from the brain microscopy community to promote adoption. The resulting 3D Microscopy Metadata Standards (3D-MMS) includes 91 fields organized into seven categories: Contributors, Funders, Publication, Instrument, Dataset, Specimen, and Image. Adoption of these metadata standards will ensure that investigators receive credit for their work, promote data reuse, facilitate downstream analysis of shared data, and encourage collaboration.

Quantitative relationship between cerebrovascular network and neuronal cell types in mice

The cerebrovasculature and its mural cells must meet brain regional energy demands, but how their spatial relationship with different neuronal cell types varies across the brain remains largely unknown. Here we apply brain-wide mapping methods to comprehensively define the quantitative relationships between the cerebrovasculature, capillary pericytes, and glutamatergic and GABAergic neurons, including neuronal nitric oxide synthase-positive (nNOS+) neurons and their subtypes in adult mice. Our results show high densities of vasculature with high fluid conductance and capillary pericytes in primary motor sensory cortices compared with association cortices that show significant positive and negative correlations with energy-demanding parvalbumin+ and vasomotor nNOS+ neurons, respectively. Thalamo-striatal areas that are connected to primary motor sensory cortices also show high densities of vasculature and pericytes, suggesting dense energy support for motor sensory processing areas. Our cellular-resolution resource offers opportunities to examine spatial relationships between the cerebrovascular network and neuronal cell composition in largely understudied subcortical areas.

Cellular anatomy of the mouse primary motor cortex

An essential step toward understanding brain function is to establish a structural framework with cellular resolution on which multi-scale datasets spanning molecules, cells, circuits and systems can be integrated and interpreted1. Here, as part of the collaborative Brain Initiative Cell Census Network (BICCN), we derive a comprehensive cell type-based anatomical description of one exemplar brain structure, the mouse primary motor cortex, upper limb area (MOp-ul). Using genetic and viral labelling, barcoded anatomy resolved by sequencing, single-neuron reconstruction, whole-brain imaging and cloud-based neuroinformatics tools, we delineated the MOp-ul in 3D and refined its sublaminar organization. We defined around two dozen projection neuron types in the MOp-ul and derived an input-output wiring diagram, which will facilitate future analyses of motor control circuitry across molecular, cellular and system levels. This work provides a roadmap towards a comprehensive cellular-resolution description of mammalian brain architecture.

A multimodal cell census and atlas of the mammalian primary motor cortex

Here we report the generation of a multimodal cell census and atlas of the mammalian primary motor cortex as the initial product of the BRAIN Initiative Cell Census Network (BICCN). This was achieved by coordinated large-scale analyses of single-cell transcriptomes, chromatin accessibility, DNA methylomes, spatially resolved single-cell transcriptomes, morphological and electrophysiological properties and cellular resolution input-output mapping, integrated through cross-modal computational analysis. Our results advance the collective knowledge and understanding of brain cell-type organization1-5. First, our study reveals a unified molecular genetic landscape of cortical cell types that integrates their transcriptome, open chromatin and DNA methylation maps. Second, cross-species analysis achieves a consensus taxonomy of transcriptomic types and their hierarchical organization that is conserved from mouse to marmoset and human. Third, in situ single-cell transcriptomics provides a spatially resolved cell-type atlas of the motor cortex. Fourth, cross-modal analysis provides compelling evidence for the transcriptomic, epigenomic and gene regulatory basis of neuronal phenotypes such as their physiological and anatomical properties, demonstrating the biological validity and genomic underpinning of neuron types. We further present an extensive genetic toolset for targeting glutamatergic neuron types towards linking their molecular and developmental identity to their circuit function. Together, our results establish a unifying and mechanistic framework of neuronal cell-type organization that integrates multi-layered molecular genetic and spatial information with multi-faceted phenotypic properties.

Genetic dissection of the glutamatergic neuron system in cerebral cortex

Diverse types of glutamatergic pyramidal neurons mediate the myriad processing streams and output channels of the cerebral cortex1,2, yet all derive from neural progenitors of the embryonic dorsal telencephalon3,4. Here we establish genetic strategies and tools for dissecting and fate-mapping subpopulations of pyramidal neurons on the basis of their developmental and molecular programs. We leverage key transcription factors and effector genes to systematically target temporal patterning programs in progenitors and differentiation programs in postmitotic neurons. We generated over a dozen temporally inducible mouse Cre and Flp knock-in driver lines to enable the combinatorial targeting of major progenitor types and projection classes. Combinatorial strategies confer viral access to subsets of pyramidal neurons defined by developmental origin, marker expression, anatomical location and projection targets. These strategies establish an experimental framework for understanding the hierarchical organization and developmental trajectory of subpopulations of pyramidal neurons that assemble cortical processing networks and output channels.

A Genetically Defined Compartmentalized Striatal Direct Pathway for Negative Reinforcement

The striosome compartment within the dorsal striatum has been implicated in reinforcement learning and regulation of motivation, but how striosomal neurons contribute to these functions remains elusive. Here, we show that a genetically identified striosomal population, which expresses the Teashirt family zinc finger 1 (Tshz1) and belongs to the direct pathway, drives negative reinforcement and is essential for aversive learning in mice. Contrasting a "conventional" striosomal direct pathway, the Tshz1 neurons cause aversion, movement suppression, and negative reinforcement once activated, and they receive a distinct set of synaptic inputs. These neurons are predominantly excited by punishment rather than reward and represent the anticipation of punishment or the motivation for avoidance. Furthermore, inhibiting these neurons impairs punishment-based learning without affecting reward learning or movement. These results establish a major role of striosomal neurons in behaviors reinforced by punishment and moreover uncover functions of the direct pathway unaccounted for in classic models.

Brain-wide mapping of c-fos expression in the single prolonged stress model and the effects of pretreatment with ACH-000029 or prazosin

Post-traumatic stress disorder (PTSD) is a mental health condition that is triggered by a stressful event, with symptoms including exaggerated startle response, intrusive traumatic memories and nightmares. The single prolonged stress (SPS) is a multimodal stress protocol that comprises a sequential exposure to physical restraint, forced swimming, predator scent and ether anesthesia. This procedure generates behavioral and neurobiological alterations that resemble clinical findings of PTSD, and thus it is commonly used to model the disease in rodents. Here, we applied c-fos mapping to produce a comprehensive view of stress-activated brain regions in mice exposed to SPS alone or to SPS after oral pretreatment with the serotonin-noradrenaline receptor dual modulator ACH-000029 or the α1-adrenergic blocker prazosin. The SPS protocol evoked c-fos expression in several brain regions that control the stress-anxiety response, including the central and medial amygdala, the bed nucleus of the stria terminalis, the pallidum, the paraventricular hypothalamus, the intermediodorsal, paraventricular and central medial thalamic nuclei, the periaqueductal gray, the lateral habenula and the cuneiform nucleus. These effects were partially blocked by pretreatment with prazosin but completely prevented by ACH-000029. Collectively, these findings contribute to the brain-wide characterization of neural circuits involved in PTSD-related stress responses. Furthermore, the identification of brain areas regulated by ACH-000029 and prazosin revealed regions in which SPS-induced activation may depend on the combined or isolated action of the noradrenergic and serotonergic systems. Finally, the dual regulation of serotonin and α1 receptors by ACH-000029 might represent a potential pharmacotherapy that can be applied in the peri-trauma or early post-trauma period to mitigate the development of symptoms in PTSD patients.

Whole-Brain Profiling of Cells and Circuits in Mammals by Tissue Clearing and Light-Sheet Microscopy

Tissue clearing and light-sheet microscopy have a 100-year-plus history, yet these fields have been combined only recently to facilitate novel experiments and measurements in neuroscience. Since tissue-clearing methods were first combined with modernized light-sheet microscopy a decade ago, the performance of both technologies has rapidly improved, broadening their applications. Here, we review the state of the art of tissue-clearing methods and light-sheet microscopy and discuss applications of these techniques in profiling cells and circuits in mice. We examine outstanding challenges and future opportunities for expanding these techniques to achieve brain-wide profiling of cells and circuits in primates and humans. Such integration will help provide a systems-level understanding of the physiology and pathology of our central nervous system.

The evolution of brain structure captured in stereotyped cell count and cell type distributions

The stereotyped features of brain structure, such as the distribution, morphology and connectivity of neuronal cell types across brain areas, are those most likely to explain the remarkable capacity of the brain to process information and govern behaviors. Recent advances in anatomical methods, including the simple but versatile isotropic fractionator and several whole-brain labeling, clearing and microscopy methods, have opened the door to an exciting new era in comparative brain anatomy, one that has the potential to transform our understanding of the brain structure-function relationship by representing the evolution of brain complexity in quantitative anatomical features shared across species and species-specific or clade-specific. Here we discuss these methods and their application to mapping brain cell count and cell type distributions-two particularly powerful neural correlates of vertebrate cognitive and behavioral capabilities.

The serotonergic and alpha-1 adrenergic receptor modulator ACH-000029 ameliorates anxiety-like behavior in a post-traumatic stress disorder model

Post-traumatic stress disorder (PTSD) is a severe chronic mental illness that develops in individuals exposed to life-threatening trauma and is characterized by hyperarousal, flashbacks and nightmares. The serotonergic (5-HT) and noradrenergic (NE) systems are deeply involved in the pathogenesis of PTSD. We have previously reported a novel anxiolytic compound, ACH-000029, that modulates 5-HT and α1-adrenergic receptors and induces acute anxiolytic-like effects in rodents. Here, we investigated the potential of ACH-000029 to prevent anxiety-like behavior in the single prolonged stress (SPS) PTSD model. Mice were subjected to the SPS procedure, followed by a 7-day treatment with ACH-000029 and, for comparison, with the α1-adrenergic antagonist prazosin. Animals were behaviorally assessed using social interaction, elevated plus maze and open field tests. Interestingly, treatment with ACH-000029 but not with prazosin ameliorated the SPS-induced sociability impairment and anxiety-like behavior. The brain-wide c-fos mapping, used as a surrogate for brain activity, indicated the brain structures that were altered by SPS and putatively involved in the anxiolytic-like effect of ACH-000029. The SPS protocol produced long-lasting impairment of regions involved in stress-anxiety response, such as the amygdala, prefrontal cortex, globus pallidus and superior colliculus. ACH-000029 treatment reversed the SPS-induced c-fos changes in the globus pallidus, lateral septum and entorhinal cortex and exclusively modulated c-fos levels in subregions from the retrosplenial cortex, cerebellum, superior colliculus and ventromedial hypothalamus. These results support the hypothesis that the dual regulation of 5-HT and α1-adrenergic receptors is required to alleviate PTSD symptoms and suggest a possible role of ACH-000029 as a PTSD treatment.

Distinct Cortical-Thalamic-Striatal Circuits through the Parafascicular Nucleus

The thalamic parafascicular nucleus (PF), an excitatory input to the basal ganglia, is targeted with deep-brain stimulation to alleviate a range of neuropsychiatric symptoms. Furthermore, PF lesions disrupt the execution of correct motor actions in uncertain environments. Nevertheless, the circuitry of the PF and its contribution to action selection are poorly understood. We find that, in mice, PF has the highest density of striatum-projecting neurons among all sub-cortical structures. This projection arises from transcriptionally and physiologically distinct classes of PF neurons that are also reciprocally connected with functionally distinct cortical regions, differentially innervate striatal neurons, and are not synaptically connected in PF. Thus, mouse PF contains heterogeneous neurons that are organized into parallel and independent associative, limbic, and somatosensory circuits. Furthermore, these subcircuits share motifs of cortical-PF-cortical and cortical-PF-striatum organization that allow each PF subregion, via its precise connectivity with cortex, to coordinate diverse inputs to striatum.

Hippocampal brain-derived neurotrophic factor determines recruitment of anatomically connected networks after stress in diabetic mice

Diabetes increases adrenal steroids in humans and animal models, but potential interactions with psychological stress remain poorly understood. Diabetic rodents exhibit anxiety and reductions in hippocampal brain-derived neurotrophic factor (BDNF) expression, and these studies investigated whether loss of BDNF-driven hippocampal activity promotes anxiety and disinhibits the HPA axis. Mice with genetic obesity and diabetes (db/db) received intrahippocampal injections of lentivirus for BDNF overexpression (db/db-BDNFOE), and Wt mice received lentiviral constructs for BDNF knockdown (Wt-BDNFKD). Behavioral anxiety and glucocorticoid responses to acute restraint were compared with mice that received a fluorescent reporter (Wt-GFP, db/db-GFP). These experiments revealed that changes in hippocampal BDNF were necessary and sufficient for behavioral anxiety and HPA axis disinhibition. To examine patterns of stress-induced regional activity, we used algorithmic detection of cFos and automated segmentation of forebrain regions to generate maps of functional covariance, which were subsequently aligned with anatomical connectivity weights from the Brain Architecture Management database. db/db-GFP mice exhibited reduced activation of the hippocampal ventral subiculum (vSub) and anterior bed nucleus of stria terminalis (aBNST), and increases in the paraventricular hypothalamus (PVH), relative to Wt-GFP. BDNFKD recapitulated this pattern in Wt mice, and BDNFOE normalized activation of the vSub > aBNST > PVH pathway in db/db mice. Analysis of forebrain activation revealed largely overlapping patterns of network disruption in db/db-GFP and Wt-BDNFKD mice, implicating BDNF-driven hippocampal activity as a determinant of stress vulnerability in both the intact and diabetic brain.

Radial Glial Lineage Progression and Differential Intermediate Progenitor Amplification Underlie Striatal Compartments and Circuit Organization

The circuitry of the striatum is characterized by two organizational plans: the division into striosome and matrix compartments, thought to mediate evaluation and action, and the direct and indirect pathways, thought to promote or suppress behavior. The developmental origins of these organizations and their developmental relationships are unknown, leaving a conceptual gap in understanding the cortico-basal ganglia system. Through genetic fate mapping, we demonstrate that striosome-matrix compartmentalization arises from a lineage program embedded in lateral ganglionic eminence radial glial progenitors mediating neurogenesis through two distinct types of intermediate progenitors (IPs). The early phase of this program produces striosomal spiny projection neurons (SPNs) through fate-restricted apical IPs (aIPSs) with limited capacity; the late phase produces matrix SPNs through fate-restricted basal IPs (bIPMs) with expanded capacity. Notably, direct and indirect pathway SPNs arise within both aIPS and bIPM pools, suggesting that striosome-matrix architecture is the fundamental organizational plan of basal ganglia circuitry.

The BRAIN Initiative Cell Census Consortium: Lessons Learned toward Generating a Comprehensive Brain Cell Atlas

A comprehensive characterization of neuronal cell types, their distributions, and patterns of connectivity is critical for understanding the properties of neural circuits and how they generate behaviors. Here we review the experiences of the BRAIN Initiative Cell Census Consortium, ten pilot projects funded by the U.S. BRAIN Initiative, in developing, validating, and scaling up emerging genomic and anatomical mapping technologies for creating a complete inventory of neuronal cell types and their connections in multiple species and during development. These projects lay the foundation for a larger and longer-term effort to generate whole-brain cell atlases in species including mice and humans.

Brain-wide Maps Reveal Stereotyped Cell-Type-Based Cortical Architecture and Subcortical Sexual Dimorphism

The stereotyped features of neuronal circuits are those most likely to explain the remarkable capacity of the brain to process information and govern behaviors, yet it has not been possible to comprehensively quantify neuronal distributions across animals or genders due to the size and complexity of the mammalian brain. Here we apply our quantitative brain-wide (qBrain) mapping platform to document the stereotyped distributions of mainly inhibitory cell types. We discover an unexpected cortical organizing principle: sensory-motor areas are dominated by output-modulating parvalbumin-positive interneurons, whereas association, including frontal, areas are dominated by input-modulating somatostatin-positive interneurons. Furthermore, we identify local cell type distributions with more cells in the female brain in 10 out of 11 sexually dimorphic subcortical areas, in contrast to the overall larger brains in males. The qBrain resource can be further mined to link stereotyped aspects of neuronal distributions to known and unknown functions of diverse brain regions.

aMAP is a validated pipeline for registration and segmentation of high-resolution mouse brain data

The validation of automated image registration and segmentation is crucial for accurate and reliable mapping of brain connectivity and function in three-dimensional (3D) data sets. While validation standards are necessarily high and routinely met in the clinical arena, they have to date been lacking for high-resolution microscopy data sets obtained from the rodent brain. Here we present a tool for optimized automated mouse atlas propagation (aMAP) based on clinical registration software (NiftyReg) for anatomical segmentation of high-resolution 3D fluorescence images of the adult mouse brain. We empirically evaluate aMAP as a method for registration and subsequent segmentation by validating it against the performance of expert human raters. This study therefore establishes a benchmark standard for mapping the molecular function and cellular connectivity of the rodent brain.

Mapping of Brain Activity by Automated Volume Analysis of Immediate Early Genes

Understanding how neural information is processed in physiological and pathological states would benefit from precise detection, localization, and quantification of the activity of all neurons across the entire brain, which has not, to date, been achieved in the mammalian brain. We introduce a pipeline for high-speed acquisition of brain activity at cellular resolution through profiling immediate early gene expression using immunostaining and light-sheet fluorescence imaging, followed by automated mapping and analysis of activity by an open-source software program we term ClearMap. We validate the pipeline first by analysis of brain regions activated in response to haloperidol. Next, we report new cortical regions downstream of whisker-evoked sensory processing during active exploration. Last, we combine activity mapping with axon tracing to uncover new brain regions differentially activated during parenting behavior. This pipeline is widely applicable to different experimental paradigms, including animal species for which transgenic activity reporters are not readily available.

Whole-Brain Mapping of Neuronal Activity in the Learned Helplessness Model of Depression

Some individuals are resilient, whereas others succumb to despair in repeated stressful situations. The neurobiological mechanisms underlying such divergent behavioral responses remain unclear. Here, we employed an automated method for mapping neuronal activity in search of signatures of stress responses in the entire mouse brain. We used serial two-photon tomography to detect expression of c-FosGFP - a marker of neuronal activation - in c-fosGFP transgenic mice subjected to the learned helplessness (LH) procedure, a widely used model of stress-induced depression-like phenotype in laboratory animals. We found that mice showing "helpless" behavior had an overall brain-wide reduction in the level of neuronal activation compared with mice showing "resilient" behavior, with the exception of a few brain areas, including the locus coeruleus, that were more activated in the helpless mice. In addition, the helpless mice showed a strong trend of having higher similarity in whole-brain activity profile among individuals, suggesting that helplessness is represented by a more stereotypic brain-wide activation pattern. This latter effect was confirmed in rats subjected to the LH procedure, using 2-deoxy-2[18F]fluoro-D-glucose positron emission tomography to assess neural activity. Our findings reveal distinct brain activity markings that correlate with adaptive and maladaptive behavioral responses to stress, and provide a framework for further studies investigating the contribution of specific brain regions to maladaptive stress responses.

Dopamine neurons projecting to the posterior striatum form an anatomically distinct subclass

Combining rabies-virus tracing, optical clearing (CLARITY), and whole-brain light-sheet imaging, we mapped the monosynaptic inputs to midbrain dopamine neurons projecting to different targets (different parts of the striatum, cortex, amygdala, etc) in mice. We found that most populations of dopamine neurons receive a similar set of inputs rather than forming strong reciprocal connections with their target areas. A common feature among most populations of dopamine neurons was the existence of dense ‘clusters’ of inputs within the ventral striatum. However, we found that dopamine neurons projecting to the posterior striatum were outliers, receiving relatively few inputs from the ventral striatum and instead receiving more inputs from the globus pallidus, subthalamic nucleus, and zona incerta. These results lay a foundation for understanding the input/output structure of the midbrain dopamine circuit and demonstrate that dopamine neurons projecting to the posterior striatum constitute a unique class of dopamine neurons regulated by different inputs.

DOI:http://dx.doi.org/10.7554/eLife.10032.001

Most neurons send their messages to recipient neurons by releasing a substance called a ‘neurotransmitter’ that binds to receptors on the target cell. The sites of this type of signal transmission are called synapses. Some small populations of neurons modulate the activity of hundreds or thousands of these synapses all across the brain by releasing ‘neuromodulators’ that affect how they work. These neuromodulators are essential because they broadcast information that is likely to be useful to many brain regions, like a ‘news channel’ for the brain.

One important neuromodulator in the mammalian brain is dopamine, which contributes to motivation, learning, and the control of movement. Clusters of cells deep in the brain release dopamine, and people with Parkinson's disease gradually lose these cells. This makes it increasingly difficult for their brains to produce the correct amount of dopamine, and results in symptoms such as tremors and stiff muscles.

Individual dopamine neurons typically send information to a single part of the brain. This suggests that dopamine neurons with different targets might have different roles. To explore this possibility, Menegas et al. classified dopamine neurons in the mouse brain into eight types based on the areas to which they project, and then mapped which neurons send input signals to each type. These inputs are likely to shape the activity of each type (that is, their ‘message’ to the rest of the brain). The mapping revealed that most dopamine neurons do not receive substantial input from the area to which they project (i.e., they do not form ‘closed loops’). Instead, most of their input comes from a common set of brain regions, including a particularly large number of inputs from the ventral striatum.

However, Menegas et al. found one exception. Dopamine neurons that target part of the brain called the posterior striatum receive relatively little input from the ventral striatum. Their input comes instead from a set of other brain structures, and in particular from a region called the subthalamic nucleus. Electrical stimulation of the subthalamic nucleus can help to relieve the symptoms of Parkinson's disease. Therefore, the results presented by Menegas et al. suggest that this population of dopamine neurons might be particularly relevant to Parkinson's disease and that focusing future studies on them could ultimately be beneficial for patients.

DOI:http://dx.doi.org/10.7554/eLife.10032.002

Mapping social behavior-induced brain activation at cellular resolution in the mouse

Understanding how brain activation mediates behaviors is a central goal of systems neuroscience. Here, we apply an automated method for mapping brain activation in the mouse in order to probe how sex-specific social behaviors are represented in the male brain. Our method uses the immediate-early-gene c-fos, a marker of neuronal activation, visualized by serial two-photon tomography: the c-fos-GFP+ neurons are computationally detected, their distribution is registered to a reference brain and a brain atlas, and their numbers are analyzed by statistical tests. Our results reveal distinct and shared female and male interaction-evoked patterns of male brain activation representing sex discrimination and social recognition. We also identify brain regions whose degree of activity correlates to specific features of social behaviors and estimate the total numbers and the densities of activated neurons per brain areas. Our study opens the door to automated screening of behavior-evoked brain activation in the mouse.

Rat subthalamic nucleus and zona incerta share extensively overlapped representations of cortical functional territories

The subthalamic nucleus (STN) and the zona incerta (ZI) are two major structures of the subthalamus. The STN has strong connections between the basal ganglia and related nuclei. The ZI has strong connections between brainstem reticular nuclei, sensory nuclei, and nonspecific thalamic nuclei. Both the STN and ZI receive heavy projections from a subgroup of layer V neurons in the cerebral cortex. The major goal of this study was to investigate the following two questions about the cortico-subthalamic projections using the lentivirus anterograde tracing method in the rat: 1) whether cortical projections to the STN and ZI have independent functional organizations or a global organization encompassing the entire subthalamus as a whole; and 2) how the cortical functional zones are represented in the subthalamus. This study revealed that the subthalamus receives heavy projections from the motor and sensory cortices, that the cortico-subthalamic projections have a large-scale functional organization that encompasses both the STN and two subdivisions of the ZI, and that the group of cortical axons that originate from a particular area of the cortex sequentially innervate and form separate terminal fields in the STN and ZI. The terminal zones formed by different cortical functional areas have highly overlapped and fuzzy borders, as do the somatotopic representations of the sensorimotor cortex in the subthalamus. The present study suggests that the layer V neurons in the wide areas of the sensorimotor cortex simultaneously control STN and ZI neurons. Together with other known afferent and efferent connections, possible new functionality of the STN and ZI is discussed.

Experience-induced Arc/Arg3.1 primes CA1 pyramidal neurons for metabotropic glutamate receptor-dependent long-term synaptic depression

A novel experience induces the Arc/Arg3.1 gene as well as plasticity of CA1 neural networks. To understand how these are linked, we briefly exposed GFP reporter mice of Arc transcription to a novel environment. Excitatory synaptic function of CA1 neurons with recent in vivo Arc induction (ArcGFP+) was similar to neighboring noninduced neurons. However, in response to group 1 metabotropic glutamate receptor (mGluR) activation, ArcGFP+ neurons preferentially displayed long-term synaptic depression (mGluR-LTD) and robust increases in dendritic Arc protein. mGluR-LTD in ArcGFP+ neurons required rapid protein synthesis and Arc, suggesting that dendritic translation of Arc underlies the priming of mGluR-LTD. In support of this idea, novelty exposure increased Arc messenger RNA in CA1 dendrites and promoted mGluR-induced translation of Arc in hippocampal synaptoneurosomes. Repeated experience suppressed synaptic transmission onto ArcGFP+ neurons and occluded mGluR-LTD ex vivo. mGluR-LTD priming in neurons with similar Arc activation history may contribute to encoding a novel environment.

Mapping brain circuitry with a light microscope

The beginning of the 21st century has seen a renaissance in light microscopy and anatomical tract tracing that together are rapidly advancing our understanding of the form and function of neuronal circuits. The introduction of instruments for automated imaging of whole mouse brains, new cell type–specific and trans-synaptic tracers, and computational methods for handling the whole-brain data sets has opened the door to neuroanatomical studies at an unprecedented scale. We present an overview of the present state and future opportunities in charting long-range and local connectivity in the entire mouse brain and in linking brain circuits to function.

Sensorimotor assessment of the unilateral 6-hydroxydopamine mouse model of Parkinson's disease

Parkinson's disease (PD), the second most common neurodegenerative disorder, is characterized by marked impairments in motor function caused by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta (SNc). Animal models of PD have traditionally been based on toxins, such as 6-hydroxydopamine (6-OHDA) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), that selectively lesion dopaminergic neurons. Motor impairments from 6-OHDA lesions of SNc neurons are well characterized in rats, but much less work has been done in mice. In this study, we compare the effectiveness of a series of drug-free behavioral tests in assessing sensorimotor impairments in the unilateral 6-OHDA mouse model, including six tests used for the first time in this PD mouse model (the automated treadmill "DigiGait" test, the challenging beam test, the adhesive removal test, the pole test, the adjusting steps test, and the test of spontaneous activity) and two tests used previously in 6-OHDA-lesioned mice (the limb-use asymmetry "cylinder" test and the manual gait test). We demonstrate that the limb-use asymmetry, challenging beam, pole, adjusting steps, and spontaneous activity tests are all highly robust assays for detecting sensorimotor impairments in the 6-OHDA mouse model. We also discuss the use of the behavioral tests for specific experimental objectives, such as simple screening for well-lesioned mice in studies of PD cellular pathophysiology or comprehensive behavioral analysis in preclinical therapeutic testing using a battery of sensorimotor tests.

Introduction of green fluorescent protein (GFP) into hippocampal neurons through viral infection

Expression of green fluorescent protein (GFP), its more fluorescent mutant forms (e.g., EGFP [enhanced GFP]), or their fusion protein derivatives, affords a number of informative possibilities in cellular neuroscience. EGFP is a soluble protein and appears to be homogeneously distributed within the cytosol of neurons when expressed. Thus, it reveals the structure of the neuron, including the cell body, and axonal and dendritic arbors. It is also sufficiently bright to reveal detailed structures such as axonal boutons and dendritic spines. When expressed as a fusion protein, EGFP can provide information about the distribution characteristics of the proteins within neurons. Furthermore, during single-cell electrophysiological studies, such expression can direct the investigator to record from a cell carrying a foreign gene. In this protocol, we describe the use of the Sindbis pseudovirus expression system to deliver GFP to neurons. Sindbis is a member of the alphaviruses, which are plus-stranded RNA viruses. This protocol uses the DH(26S) strain, which preferentially infects neurons over glia (50:1). Two infection methods are given: one for dissociated hippocampal cultured neurons and one for organotypic hippocampal slices.

Fluorescent Arc/Arg3.1 indicator mice: a versatile tool to study brain activity changes in vitro and in vivo

The brain-specific immediate early gene Arc/Arg3.1 is induced in response to a variety of stimuli, including sensory and behavior-linked neural activity. Here we report the generation of transgenic mice, termed TgArc/Arg3.1-d4EGFP, expressing a 4-h half-life form of enhanced green fluorescent protein (d4EGFP) under the control of the Arc/Arg3.1 promoter. We show that d4EGFP-mediated fluorescence faithfully reports Arc/Arg3.1 induction in response to physiological, pathological and pharmacological stimuli, and that this fluorescence permits electrical recording from activated neurons in the live mouse. Moreover, the fluorescent Arc/Arg3.1 indicator revealed activity changes in circumscribed brain areas in distinct modes of stress and in a mouse model of Alzheimer's disease. These findings identify the TgArc/Arg3.1-d4EGFP mouse as a versatile tool to monitor Arc/Arg3.1 induction in neural circuits, both in vitro and in vivo.

Critical period plasticity of axonal arbors of layer 2/3 pyramidal neurons in rat somatosensory cortex: layer-specific reduction of projections into deprived cortical columns

We examined the effect of whisker trimming during early postnatal development on the morphology of axonal arbors in rat somatosensory cortex. Axonal arbors from populations of layer 2/3 pyramidal neurons in the D2 column were labeled by lentivirus-mediated expression of green fluorescent protein. Axonal projection patterns were compared between untrimmed control animals and animals with all whiskers in A-, B-, and C-rows trimmed (D- and E-rows left intact) from postnatal days 7 to 15 (termed from here on DE-pairing). Control animals had approximately symmetrical horizontal projections toward C- and E-row columns in both supra- and infragranular layers. Following DE-pairing, the density of axons in supragranular layers projecting from the labeled neurons in the D2 column was higher in E- than in C-row columns. This asymmetry resulted primarily from a reduction in projection density toward the deprived C-row columns. In contrast, no change was observed in infragranular layers. The results indicate that DE-pairing during early postnatal development results in reduced axonal projection from nondeprived into deprived columns and that cortical neurons are capable of structural rearrangements at subsets of their axonal arbors.

Stereotaxic gene delivery in the rodent brain

Stereotaxic surgery has been an invaluable tool in systems neuroscience, applied in many experiments for the creation of site-targeted lesions, injection of anatomical tracers or implantation of electrodes or microdialysis probes. In this protocol, we describe stereotaxic surgery optimized for gene delivery by recombinant adeno-associated viruses and lentiviruses in mice and rats. This method allows the manipulation of gene expression in the rodent brain with excellent spatiotemporal control; essentially any brain region of choice can be targeted and cells (or a subpopulation of cells) in that region can be stably genetically altered at any postnatal developmental stage up to adulthood. Many aspects of the method, its versatility, ease of application and high reproducibility, make it an attractive approach for studying genetic, cellular and circuit functions in the brain. The entire protocol can be completed in 1-2 hours.

Select overexpression of homer1a in dorsal hippocampus impairs spatial working memory

Long Homer proteins forge assemblies of signaling components involved in glutamate receptor signaling in postsynaptic excitatory neurons, including those underlying synaptic transmission and plasticity. The short immediate-early gene (IEG) Homer1a can dynamically uncouple these physical associations by functional competition with long Homer isoforms. To examine the consequences of Homer1a-mediated "uncoupling" for synaptic plasticity and behavior, we generated forebrain-specific tetracycline (tet) controlled expression of Venus-tagged Homer1a (H1aV) in mice. We report that sustained overexpression of H1aV impaired spatial working but not reference memory. Most notably, a similar impairment was observed when H1aV expression was restricted to the dorsal hippocampus (HP), which identifies this structure as the principal cortical area for spatial working memory. Interestingly, H1aV overexpression also abolished maintenance of CA3-CA1 long-term potentiation (LTP). These impairments, generated by sustained high Homer1a levels, identify a requirement for long Homer forms in synaptic plasticity and temporal encoding of spatial memory.

Two-photon targeted patching (TPTP) in vivo

Two-photon-excited fluorescence laser-scanning microscopy (2PLSM) has provided a wealth of information about the spatiotemporal properties of biological processes at the single cell and population level. Because such nonlinear optical methods allow for imaging deep within biological tissue, 2PLSM can be combined with patch-clamp techniques to obtain electrophysiological recordings from specific fluorescently labeled cells in vivo. Here a protocol referred to as two-photon targeted patching (TPTP) describes a method that may be used to record from cells in the intact animal labeled by virtually any type of fluorophore. We target neurons that have been optically and genetically identified using green fluorescent protein (GFP) expressed under the control of a specific promoter. TPTP when combined with genetic approaches therefore permits electrophysiological recordings from specified neurons and their compartments, including dendrites. This technique may be repeated in the same preparation many times over the course of several hours and is equally applicable to non-neuronal cell types.

Silencing and un-silencing of tetracycline-controlled genes in neurons

To identify the underlying reason for the controversial performance of tetracycline (Tet)-controlled regulated gene expression in mammalian neurons, we investigated each of the three components that comprise the Tet inducible systems, namely tetracyclines as inducers, tetracycline-transactivator (tTA) and reverse tTA (rtTA), and tTA-responsive promoters (P_tets). We have discovered that stably integrated P_tet becomes functionally silenced in the majority of neurons when it is inactive during development. P_tet silencing can be avoided when it is either not integrated in the genome or stably-integrated with basal activity. Moreover, long-term, high transactivator levels in neurons can often overcome integration-induced P_tet gene silencing, possibly by inducing promoter accessibility.

Viral vectors: a wide range of choices and high levels of service

Viruses are intracellular parasites with simple DNA or RNA genomes. Virus life revolves around three steps: infection of a host cell, replication of its genome within the host cell environment, and formation of new virions; this process is often but not always associated with pathogenic effects against the host organism. Since the mid-1980s, the main goal of viral vectorology has been to develop recombinant viral vectors for long-term gene delivery to mammalian cells, with minimal associated toxicity. Today, several viral vector systems are close to achieving this aim, providing stable transgenic expression in many different cell types and tissues. Here we review application characteristics of four vector systems, derived from adeno-associated viruses, adenoviruses, retroviruses and herpes simplex virus-1, for in vivo gene delivery. We discuss the transfer capacity of the expression vectors, the stability of their transgenic expression, the tropism of the recombinant viruses, the likelihood of induction of immunotoxicity, and the ease (or difficulty) of the virus production. In the end, we discuss applications of these vectors for delivery of three molecular systems for conditional mutagenesis, two for inducible transcriptional control of transgenic expression (the tet and the dimerizer systems), and the third one for inducible control of endogenous gene expression based on RNA interference.

Postsynaptic excitability is necessary for strengthening of cortical sensory responses during experience-dependent development

Sensory experience is necessary for normal cortical development. This has been shown by sensory deprivation and pharmacological perturbation of the cortex. Because these manipulations affect the cortical network as a whole, the role of postsynaptic cellular properties during experience-dependent development is unclear. Here we addressed the developmental role of somatodendritic excitability, which enables postsynaptic spike timing-dependent forms of plasticity, in rat somatosensory cortex. We used short interfering RNA (siRNA)-based knockdown of Na+ channels to suppress the somatodendritic excitability of small numbers of layer 2/3 pyramidal neurons in the barrel cortex, without altering the ascending sensory pathway. In vivo recordings from siRNA-expressing cells revealed that this manipulation interfered with the normal developmental strengthening of sensory responses. The sensory responsiveness of neighboring cortical neurons was unchanged, indicating that the cortical network was unchanged. We conclude that somatodendritic excitability of the postsynaptic neuron is needed for the regulation of synaptic strength in the developing sensory cortex.

Learning from stargazin: the mouse, the phenotype and the unexpected

The stargazin gene (also referred to as Cacng2) has been identified by forward genetics in a spontaneous mouse mutant with ataxic gait, upward head-elevating movements (hence the name stargazer for the mouse) and episodes of spike-wave discharges. Stargazin is related to the gamma-1 subunit of skeletal muscle voltage-dependent calcium channel (VDCC), and a deficit in its role as auxiliary VDCC subunit was proposed to underlie the epileptic phenotype of the mouse; yet, a conclusive demonstration of stargazin function in VDCC regulation is still lacking. In contrast, stargazin and its three closely related isoforms gamma-3, gamma-4 and gamma-8 were shown to function as auxiliary subunits for a very different ion channel - the AMPA-type glutamate receptor - prominently regulating early intracellular transport, synaptic targeting and anchoring, and ion channel functions of this major excitatory receptor in the brain.

Cellular mechanisms of motor control in the vibrissal system

In this article we discuss the experimental advantages that the vibrissal motor system offers for analysis of motor control and the specializations of this system related to the unique characteristics of whisker movements. Whisker movements are often rhythmic, fast, and bilateral. Movements of individual whiskers have simple characteristics, whereas, movements of the entire vibrissae array are complex and sophisticated. In the last few years, powerful methods for high precision tracking of whisker movements have become available. The whisker musculature is arranged to permit forward movements of individual whiskers and consists-depending on the species-mainly or exclusively of fast contracting, fast fatigable muscle fibers. Whisker motor neurons are located in the lateral facial nucleus and their cellular properties might contribute to the rhythmicity of whisking. Numerous structures provide input to the lateral facial nucleus, the most mysterious and important one being the putative central pattern generator (CPG). Although recent studies identified candidate structures for the CPG, the precise identity and the functional organization of this structure remains uncertain. The vibrissa motor cortex (VMC) is the largest motor representation in the rodent brain, and recent work has clarified its localization, subdivisions, cytoarchitectonics, and connectivity. Single-cell stimulation experiments in VMC allow determining the cellular basis of cortical motor control with unprecedented precision. The functional significance of whisker movements remains to be determined.

Stargazin reduces desensitization and slows deactivation of the AMPA-type glutamate receptors

The AMPA-type glutamate receptors mediate the majority of the fast excitatory synaptic transmission and critically contribute to synaptic plasticity in the brain, hence the existence of numerous trafficking proteins dedicated to regulation of their synaptic delivery and turnover. Stargazin (also termed gamma2) is a member of a recently identified protein family termed transmembrane AMPA receptor regulatory proteins (TARPs). TARPs physically associate with AMPA receptors and participate in their surface delivery and anchoring at the postsynaptic membrane. Here, we report that next to its trafficking roles, stargazin may also act as a positive allosteric modulator of AMPA receptor ion channel function. Coexpression of stargazin with AMPA receptor subunits, either in Xenopus oocytes or in human embryonic kidney 293 cells, significantly reduced receptor desensitization in response to glutamate. Receptor deactivation rates were also slowed, and the recovery from desensitization was accelerated. Structurally, based on the data showing a tight correlation between desensitization and the stability of the AMPA receptor intradimer interface, we propose that binding of stargazin may stabilize the receptor conformation. Functionally, our data suggest that AMPA receptors complexed with stargazin (and possibly also with other TARPs) at the postsynaptic membrane are significantly more responsive to synaptically released glutamate compared with AMPA receptors lacking stargazin/TARP interaction. The putative existence of such two states of synaptic AMPA receptors, with and without stargazin/TARP binding, may provide a novel mechanism for regulation of excitatory synaptic strength during development and/or in synaptic plasticity in the adult brain.

Monosynaptic pathway from rat vibrissa motor cortex to facial motor neurons revealed by lentivirus-based axonal tracing

The mammalian motor cortex typically innervates motor neurons indirectly via oligosynaptic pathways. However, evolution of skilled digit movements in humans, apes, and some monkey species is associated with the emergence of abundant monosynaptic cortical projections onto spinal motor neurons innervating distal limb muscles. Rats perform skilled movements with their whiskers, and we examined the possibility that the rat vibrissa motor cortex (VMC) projects monosynaptically onto facial motor neurons controlling the whisker movements. First, single injections of lentiviruses to VMC sites identified by intracortical microstimulations were used to label a distinct subpopulation of VMC axons or presynaptic terminals by expression of enhanced green fluorescent protein (GFP) or GFP-tagged synaptophysin, respectively. Four weeks after the injections, GFP and synaptophysin-GFP labeling of axons and putative presynaptic terminals was detected in the lateral portion of the facial nucleus (FN), in close proximity to motor neurons identified morphologically and by axonal back-labeling from the whisker follicles. The VMC projections were detected bilaterally, with threefold larger density of labeling in the contralateral FN. Next, multiple VMC injections were used to label a large portion of VMC axons, resulting in overall denser but still laterally restricted FN labeling. Ultrastructural analysis of the GFP-labeled VMC axons confirmed the existence of synaptic contacts onto dendrites and somata of FN motor neurons. These findings provide anatomical demonstration of monosynaptic VMC-to-FN pathway in the rat and show that lentivirus-based expression of GFP and GFP-tagged presynaptic proteins can be used as a high-resolution neuroanatomical tracing method.

Rap2-JNK removes synaptic AMPA receptors during depotentiation

The related small GTPases Ras and Rap1 are important for signaling synaptic AMPA receptor (-R) trafficking during long-term potentiation (LTP) and long-term depression (LTD), respectively. Rap2, which shares 60% identity to Rap1, is present at excitatory synapses, but its functional role is unknown. Here, we report that Rap2 activity, stimulated by NR2A-containing NMDA-R activation, depresses AMPA-R-mediated synaptic transmission via activation of JNK rather than Erk1/2 or p38 MAPK. Moreover, Rap2 controls synaptic removal of AMPA-Rs with long cytoplasmic termini during depotentiation. Thus, Rap2-JNK pathway, which opposes the action of the NR2A-containing NMDA-R-stimulated Ras-ERK1/2 signaling and complements the NR2B-containing NMDA-R-stimulated Rap1-p38 MAPK signaling, channels the specific signaling for depotentiating central synapses.