Transneuronal tracing with neurotropic viruses reveals network macroarchitecture

MRI morphometry of the anterior and posterior cerebellar vermis and its relationship to sensorimotor and cognitive functions in children

INTRODUCTION

The human cerebellum emerges as a posterior brain structure integrating neural networks for sensorimotor, cognitive, and emotional processing across the lifespan. Developmental studies of the cerebellar anatomy and function are scant. We examine age-dependent MRI morphometry of the anterior cerebellar vermis, lobules I-V and posterior neocortical lobules VI-VII and their relationship to sensorimotor and cognitive functions.

METHODS

Typically developing children (TDC; n=38; age 9-15) and healthy adults (HAC; n=31; 18-40) participated in high-resolution MRI. Rigorous anatomically informed morphometry of the vermis lobules I-V and VI-VII and total brain volume (TBV) employed manual segmentation computer-assisted FreeSurfer Image Analysis Program [http://surfer.nmr.mgh.harvard.edu]. The neuropsychological scores (WASI-II) were normalized and related to volumes of anterior, posterior vermis, and TBV.

RESULTS

TBVs were age independent. Volumes of I-V and VI-VII were significantly reduced in TDC. The ratio of VI-VII to I-V (∼60%) was stable across age-groups; I-V correlated with visual-spatial-motor skills; VI-VII with verbal, visual-abstract and FSIQ.

CONCLUSIONS

In TDC neither anterior I-V nor posterior VI-VII vermis attained adult volumes. The "inverted U" developmental trajectory of gray matter peaking in adolescence does not explain this finding. The hypothesis of protracted development of oligodendrocyte/myelination is suggested as a contributor to TDC's lower cerebellar vermis volumes.

Glucose Metabolic Alteration of Cerebral Cortical Subareas in Rats with Renal Ischemia/Reperfusion Based on Small-Animal Positron Emission Tomography

OBJECTIVE

To investigate glucose metabolic alterations in cerebral cortical subareas using ¹⁸F-labeled glucose derivative fluorodeoxyglucose (FDG) micro-positron emission tomography (PET) scanning in a rat renal ischemia/reperfusion (RIR) model.

METHODS

Small-animal PET imaging in vivo was performed with ¹⁸F-labeled FDG as a PET tracer to identify glucose metabolic alterations in cerebral cortical subregions using a rat model of RIR.

RESULTS

We found that the average standardized uptake value (SUV_average) of the cerebral cortical subareas in the RIR group was significantly increased compared to the sham group (P<0.05). We also found that glucose uptake in different cortical subregions including the left auditory cortex, right medial prefrontal cortex, right para cortex, left retrosplenial cortex, right retrosplenial cortex, and right visual cortex was significantly increased in the RIR group (P<0.05), but there was no significant difference in the SUV_average of right auditory cortex, left medial prefrontal cortex, left para cortex, and left visual cortex between the two groups.

CONCLUSION

The ¹⁸F-FDG PET data suggests that RIR causes a profound shift in the metabolic machinery of cerebral cortex subregions.

The extended autonomic system, dyshomeostasis, and COVID-19

The pandemic viral illness COVID-19 is especially life-threatening in the elderly and in those with any of a variety of chronic medical conditions. This essay explores the possibility that the heightened risk may involve activation of the "extended autonomic system" (EAS). Traditionally, the autonomic nervous system has been viewed as consisting of the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system. Over the past century, however, neuroendocrine and neuroimmune systems have come to the fore, justifying expansion of the meaning of "autonomic." Additional facets include the sympathetic adrenergic system, for which adrenaline is the key effector; the hypothalamic-pituitary-adrenocortical axis; arginine vasopressin (synonymous with anti-diuretic hormone); the renin-angiotensin-aldosterone system, with angiotensin II and aldosterone the main effectors; and cholinergic anti-inflammatory and sympathetic inflammasomal pathways. A hierarchical brain network-the "central autonomic network"-regulates these systems; embedded within it are components of the Chrousos/Gold "stress system." Acute, coordinated alterations in homeostatic settings (allostasis) can be crucial for surviving stressors such as traumatic hemorrhage, asphyxiation, and sepsis, which throughout human evolution have threatened homeostasis; however, intense or long-term EAS activation may cause harm. While required for appropriate responses in emergencies, EAS activation in the setting of chronically decreased homeostatic efficiencies (dyshomeostasis) may reduce thresholds for induction of destabilizing, lethal vicious cycles. Testable hypotheses derived from these concepts are that biomarkers of EAS activation correlate with clinical and pathophysiologic data and predict outcome in COVID-19 and that treatments targeting specific abnormalities identified in individual patients may be beneficial.

How the Cerebellum and Prefrontal Cortex Cooperate During Trace Eyeblinking Conditioning

Several data have demonstrated that during the widely used experimental paradigm for studying associative learning, trace eye blinking conditioning (TEBC), there is a strong interaction between cerebellum and medial prefrontal cortex (mPFC). Despite this evidence, the neural mechanisms underlying this interaction are still not clear. Here, we propose a neurophysiologically plausible computational model to address this issue. The model is constrained on the basis of two critical anatomo-physiological features: (i) the cerebello-cortical organization through two circuits, respectively, targeting M1 and mPFC; (ii) the different timing in the plasticity mechanisms of these parallel circuits produced by the granule cells time sensitivity according to which different subpopulations are active at different moments during conditioned stimuli. The computer simulations run with the model suggest that these features are critical to understand how the cooperation between cerebellum and mPFC supports motor areas during TEBC. In particular, a greater trace interval produces greater plasticity changes at the slow path synapses involving mPFC with respect to plasticity changes at the fast path involving M1. As a consequence, the greater is the trace interval, the stronger is the mPFC involvement. The model has been validated by reproducing data collected through recent real mice experiments.

The mind-body problem: Circuits that link the cerebral cortex to the adrenal medulla

Which regions of the cerebral cortex are the origin of descending commands that influence internal organs? We used transneuronal transport of rabies virus in monkeys and rats to identify regions of cerebral cortex that have multisynaptic connections with a major sympathetic effector, the adrenal medulla. In rats, we also examined multisynaptic connections with the kidney. In monkeys, the cortical influence over the adrenal medulla originates from 3 distinct networks that are involved in movement, cognition, and affect. Each of these networks has a human equivalent. The largest influence originates from a motor network that includes all 7 motor areas in the frontal lobe. These motor areas are involved in all aspects of skeletomotor control, from response selection to motor preparation and movement execution. The motor areas provide a link between body movement and the modulation of stress. The cognitive and affective networks are located in regions of cingulate cortex. They provide a link between how we think and feel and the function of the adrenal medulla. Together, the 3 networks can mediate the effects of stress and depression on organ function and provide a concrete neural substrate for some psychosomatic illnesses. In rats, cortical influences over the adrenal medulla and the kidney originate mainly from 2 motor areas and adjacent somatosensory cortex. The cognitive and affective networks, present in monkeys, are largely absent in rats. Thus, nonhuman primate research is essential to understand the neural substrate that links cognition and affect to the function of internal organs.

Innervated Properties of Acupuncture Points LI 4 and LR 3 in the Rat: Neural Pathway Tracing with Cholera Toxin Subunit B

Objective: Increasing evidence from acupuncture research suggests that the nervous system corresponds closely with classical acupuncture points. The aim of this research was to provide neuroanatomical evidence for revealing the innervated properties of different acupuncture points through comparing the sensory and motor pathways associated with Hegu (LI 4) and Taichong (LR 3) in rat extremities. Materials and Methods: Cholera toxin subunit B (CTB) was injected into LI 4 and LR 3 in different rats, and CTB neural labeling was examined using fluorescent immunohistochemistry and observed under fluorescent microscopy in the corresponding areas from the peripheral nervous system to the central nervous system, including the dorsal root ganglia (DRG), spinal cord, and brainstem. Results: When LI 4 was injected with CTB, CTB-labeled sensory neurons ranged from C-5 to T-1 DRG, and their transganglionic axons terminated in the C-5 to C-8 spinal dorsal horn as far as the cuneate nucleus, while labeled motor neurons were located in the C-7 to T-1 spinal ventral horn. In contrast, similar neural labeling was observed for LR 3 CTB injection, with an orderly arrangement in the L-3 to L-5 DRG, L-3 to L-5 spinal dorsal horn, gracile nucleus, and L-4 to L-6 spinal ventral horn. Conclusions: The present results provide further evidence to aid understanding of the differential innervation of acupuncture points LI 4 and LR 3. This innervation establishes its connection with the nervous system in a distinct segmental and regional pattern through the spinal sensory and motor pathways.

Retrograde monosynaptic tracing through an engineered human embryonic stem cell line reveals synaptic inputs from host neurons to grafted cells

Retrograde monosynaptic tracing with EnvA-pseudotyped rabies virus has been employed to identify the afferent and efferent connectivity of transplanted human embryonic stem (hES) cell-derived neurons in animal models. Due to the protracted development of transplanted human neurons in host animals, it is important that those transplanted cells express avian leukosis and sarcoma virus subgroup A receptor (TVA) and rabies glycoprotein G (Rgp) for a period of up to several months to enable identification of the synaptic inputs from host neurons to grafted neurons through this rabies virus-based method. Here, we report the generation of an engineered hES cell line through CRISPR/Cas9-mediated targeting to the AAVS1 locus of an EnvA-pseudotyped rabies virus-based tool for retrograde monosynaptic tracing. This engineered hES cell line, named H1-CAG-GTRgp, expresses GFP, TVA and Rgp. Upon transplantation of H1-CAG-GTRgp-derived neural progenitor cells (NPCs) into the rat brain after traumatic injury, the grafted neurons derived from H1-CAG-GTRgp cells expressed GFP, TVA, and Rgp stably for up to 6 months post-transplantation and received robust synaptic inputs from host neurons in the target regions of the orthotopic neural circuitry. The retrograde monosynaptic tracing hES cell line provides an efficient approach to analyze transplant connectivity for the comprehensive assessment of host-donor cell innervation.

Validation study of neurotrophin-3-releasing chitosan facilitation of neural tissue generation in the severely injured adult rat spinal cord

It was previously reported that a tube holding chitosan carriers loaded with neurotrophin-3 (NT-3), after insertion into a 5 mm long transection gap in the adult rat spinal cord, triggered de novo neural tissue generation and functional recovery. Here, we report an effort to validate these findings using stringent blinding methodologies, which are crucial for robustness in reproducing biomedical studies. Radio frequency identification (RFID) chips were utilized to label rats that were randomly assigned into three experimental groups: transection with chitosan-NT-3 implant (C-NT3), transection only (T-controls), and laminectomy only (S-controls), blinding the experimenters to the treatments. Three months after surgery, animals only known by their RFID were functionally, electrophysiologically, and anatomically assessed. The data were then collected into the proper groups and statistically analyzed. Neural tissue with nestin-, Tuj1-, and NeuN-positive cells was found bridging the transection gap in C-NT3 rats, but not in T-controls. Motor- and somatosensory-evoked potentials were detected in C-NT3 rats and S-controls, but not in T-controls. Hind limb movement was significantly better in C-NT3 rats compared with T-controls. Our validation study indicates that C-NT3 implants facilitate neural tissue generation, at least in part, by eliciting endogenous neurogenesis. Our data support the use of C-NT3 implants for tissue remodeling in the injured spinal cord.

An Anatomic Characterization of the Midbrain Near Response Neurons in the Macaque Monkey

Purpose

These experiments were designed to reveal the location of the premotor neurons that have previously been designated physiologically as the midbrain near response cells controlling vergence, lens accommodation, and pupillary constriction in response to target distance.

Methods

To identify this population, the fixed N2c strain of rabies virus was injected into the ciliary body of seven Macaca fascicularis monkeys. The virus was trans-synaptically transported to the brain. Following a 58- to 76-hour survival, animals were perfused with formalin fixative. After frozen sectioning, tissue was reacted to reveal the location of the infected populations by use of a monoclonal anti-rabies antibody. Another series of sections was processed to determine which of the rabies-positive cells were cholinergic motoneurons by use of an antibody to choline acetyl transferase.

Results

At earlier time points, only cholinergic cells in the preganglionic Edinger-Westphal nucleus ipsilateral to the injection were labeled. At later time points, an additional population of noncholinergic, premotor cells was present. These were most numerous at the caudal end of the supraoculomotor area, where they formed a bilateral band, oriented mediolaterally immediately above the oculomotor nucleus. Rostral to this, a smaller bilateral population was located near the midline within the supraoculomotor area.

Conclusions

Most lens preganglionic motoneurons are multipolar cells making up a continuous column within the Edinger-Westphal nucleus. A population of premotor cells that likely represents the midbrain near response cells is located in the supraoculomotor area. These cells are bilaterally distributed relative to the eye they control, and are most numerous caudally.

Corticosterone Signaling and a Lateral Habenula-Ventral Tegmental Area Circuit Modulate Compulsive Self-Injurious Behavior in a Rat Model

Self-injurious behavior (SIB) is commonly observed in patients with neuropsychiatric disorders, as well as in nonclinical populations with stress-related mental-health problems. However, the exact circuitry mechanisms underlying SIB have remained poorly understood. Here, with bilateral injection of muscimol into the entopeduncular nucleus (EP), we established a rat model of SIB. Following the muscimol injection, the male rats exhibited in a dose-dependent manner stereotypic self-biting behavior that lasted for hours and often resulted in wounds of various severities. The SIB was associated with an elevated level of serum corticosterone and could be exacerbated by enhancing the corticosterone signaling and, conversely, alleviated by inhibiting the corticosterone signaling. Activity mapping using c-fos immunostaining, combined with connectivity mapping using herpes simplex virus-based anterograde tracing from the EP and pseudorabies virus-based retrograde tracing from the masseter muscle, revealed the potential involvement of many brain areas in SIB. In particular, the lateral habenula (LHb) and the ventral tegmental area (VTA), the two connected brain areas involved in stress response and reward processing, showed a significant increase in neuronal activation during SIB. Furthermore, suppressing the LHb activity or modulating the GABAergic transmission in the VTA could significantly reduce the occurrence of SIB. These results demonstrate the importance of stress hormone signaling and the LHb-VTA circuit in modulating SIB resulting from EP malfunction, and suggest potential targets for therapeutic intervention of SIB and related disorders.SIGNIFICANCE STATEMENT Self-injurious behavior (SIB) occurs in ∼4% of the general population, with substantially higher occurrence among adolescents and patients of neuropsychiatric disorders. Stress has been linked to the occurrence of SIB, yet the underlying mechanisms have remained unclear. Using a rat model of SIB induced by disruption of activity in the entopeduncular nucleus (EP), we found that the behavior is regulated by stress and linked to corticosterone signaling. Viral tracing and c-fos immunostaining revealed the involvement of various subcortical areas, especially the EP-lateral habenula (LHb)-ventral tegmental area (VTA) circuit, in SIB. Furthermore, regulating activity in the LHb or the VTA alleviates SIB. These results may have implications in the development of new strategies for treating SIB.

Roles of Multiple Globus Pallidus Territories of Monkeys and Humans in Motivation, Cognition and Action: An Anatomical, Physiological and Pathophysiological Review

The globus pallidus (GP) communicates with widespread cortical areas that support various functions, including motivation, cognition and action. Anatomical tract-tracing studies revealed that the anteroventral GP communicates with the medial prefrontal and orbitofrontal cortices, which are involved in motivational control; the anterodorsal GP communicates with the lateral prefrontal cortex, which is involved in cognitive control; and the posterior GP communicates with the frontal motor cortex, which is involved in action control. This organization suggests that distinct subdivisions within the GP play specific roles. Neurophysiological studies examining GP neurons in monkeys during behavior revealed that the types of information coding performed within these subdivisions differ greatly. The anteroventral GP is characterized by activities related to motivation, such as reward seeking and aversive avoidance; the anterodorsal GP is characterized by activity that reflects cognition, such as goal decision and action selection; and the posterior GP is characterized by activity associated with action preparation and execution. Pathophysiological studies have shown that GABA-related substances or GP lesions result in abnormal activity in the GP, which causes site-specific behavioral and motor symptoms. The present review article discusses the anatomical organization, physiology and pathophysiology of the three major GP territories in nonhuman primates and humans.

Motor, cognitive, and affective areas of the cerebral cortex influence the adrenal medulla

Modern medicine has generally viewed the concept of "psychosomatic" disease with suspicion. This view arose partly because no neural networks were known for the mind, conceptually associated with the cerebral cortex, to influence autonomic and endocrine systems that control internal organs. Here, we used transneuronal transport of rabies virus to identify the areas of the primate cerebral cortex that communicate through multisynaptic connections with a major sympathetic effector, the adrenal medulla. We demonstrate that two broad networks in the cerebral cortex have access to the adrenal medulla. The larger network includes all of the cortical motor areas in the frontal lobe and portions of somatosensory cortex. A major component of this network originates from the supplementary motor area and the cingulate motor areas on the medial wall of the hemisphere. These cortical areas are involved in all aspects of skeletomotor control from response selection to motor preparation and movement execution. The second, smaller network originates in regions of medial prefrontal cortex, including a major contribution from pregenual and subgenual regions of anterior cingulate cortex. These cortical areas are involved in higher-order aspects of cognition and affect. These results indicate that specific multisynaptic circuits exist to link movement, cognition, and affect to the function of the adrenal medulla. This circuitry may mediate the effects of internal states like chronic stress and depression on organ function and, thus, provide a concrete neural substrate for some psychosomatic illness.

Contribution of the Cerebellum in Cue-Dependent Force Changes During an Isometric Precision Grip Task

The “raspberry task” represents a precision grip task that requires continuous adjustment of grip forces and pull forces. During this task, subjects use a specialised grip rod and have to increase the pull force linearly while the rod is locked. The positions of the fingers are unrestrained and freely selectable. From the finger positions and the geometry of the grip rod, a physical lever was derived which is a comprehensive measurement of the subject’s grip behaviour. In this study, the involvement of the cerebellum in establishing cued force changes (CFC) was examined. The auditory stimulus was associated with a motor behaviour that has to be readjusted during an ongoing movement that already started. Moreover, cerebellar involvement on grip behaviour was examined. The results show that patients presenting with degenerating cerebellar disease (CBL) were able to elicit CFC and were additionally able to optimise grip behaviour by minimising the lever. Comparison of the results of CBL with a control group of healthy subjects showed, however, that the CFC incidence was significantly lower and the reduction of the lever was less in CBL. Hence, the cerebellum is involved not only in the classical conditioning of reflexes but also in the association of sensory stimuli with complex changes in motor behaviour. Furthermore, the cerebellum is involved in the optimisation of grip behaviour during ongoing movements. Recent studies lead to the assumption that the cerebello-reticulo-spinal pathway might be important for the reduced optimisation of grip behaviour in CBL.

The functional logic of corticostriatal connections

Unidirectional connections from the cortex to the matrix of the corpus striatum initiate the cortico-basal ganglia (BG)-thalamocortical loop, thought to be important in momentary action selection and in longer-term fine tuning of behavioural repertoire; a discrete set of striatal compartments, striosomes, has the complementary role of registering or anticipating reward that shapes corticostriatal plasticity. Re-entrant signals traversing the cortico-BG loop impact predominantly frontal cortices, conveyed through topographically ordered output channels; by contrast, striatal input signals originate from a far broader span of cortex, and are far more divergent in their termination. The term 'disclosed loop' is introduced to describe this organisation: a closed circuit that is open to outside influence at the initial stage of cortical input. The closed circuit component of corticostriatal afferents is newly dubbed 'operative', as it is proposed to establish the bid for action selection on the part of an incipient cortical action plan; the broader set of converging corticostriatal afferents is described as contextual. A corollary of this proposal is that every unit of the striatal volume, including the long, C-shaped tail of the caudate nucleus, should receive a mandatory component of operative input, and hence include at least one area of BG-recipient cortex amongst the sources of its corticostriatal afferents. Individual operative afferents contact twin classes of GABAergic striatal projection neuron (SPN), distinguished by their neurochemical character, and onward circuitry. This is the basis of the classic direct and indirect pathway model of the cortico-BG loop. Each pathway utilises a serial chain of inhibition, with two such links, or three, providing positive and negative feedback, respectively. Operative co-activation of direct and indirect SPNs is, therefore, pictured to simultaneously promote action, and to restrain it. The balance of this rival activity is determined by the contextual inputs, which summarise the external and internal sensory environment, and the state of ongoing behavioural priorities. Notably, the distributed sources of contextual convergence upon a striatal locus mirror the transcortical network harnessed by the origin of the operative input to that locus, thereby capturing a similar set of contingencies relevant to determining action. The disclosed loop formulation of corticostriatal and subsequent BG loop circuitry, as advanced here, refines the operating rationale of the classic model and allows the integration of more recent anatomical and physiological data, some of which can appear at variance with the classic model. Equally, it provides a lucid functional context for continuing cellular studies of SPN biophysics and mechanisms of synaptic plasticity.

Strategies for targeting primate neural circuits with viral vectors

Understanding how the brain works requires understanding how different types of neurons contribute to circuit function and organism behavior. Progress on this front has been accelerated by optogenetics and chemogenetics, which provide an unprecedented level of control over distinct neuronal types in small animals. In primates, however, targeting specific types of neurons with these tools remains challenging. In this review, we discuss existing and emerging strategies for directing genetic manipulations to targeted neurons in the adult primate central nervous system. We review the literature on viral vectors for gene delivery to neurons, focusing on adeno-associated viral vectors and lentiviral vectors, their tropism for different cell types, and prospects for new variants with improved efficacy and selectivity. We discuss two projection targeting approaches for probing neural circuits: anterograde projection targeting and retrograde transport of viral vectors. We conclude with an analysis of cell type-specific promoters and other nucleotide sequences that can be used in viral vectors to target neuronal types at the transcriptional level.

Viral vector-based tools advance knowledge of basal ganglia anatomy and physiology

Viral vectors were originally developed to deliver genes into host cells for therapeutic potential. However, viral vector use in neuroscience research has increased because they enhance interpretation of the anatomy and physiology of brain circuits compared with conventional tract tracing or electrical stimulation techniques. Viral vectors enable neuronal or glial subpopulations to be labeled or stimulated, which can be spatially restricted to a single target nucleus or pathway. Here we review the use of viral vectors to examine the structure and function of motor and limbic basal ganglia (BG) networks in normal and pathological states. We outline the use of viral vectors, particularly lentivirus and adeno-associated virus, in circuit tracing, optogenetic stimulation, and designer drug stimulation experiments. Key studies that have used viral vectors to trace and image pathways and connectivity at gross or ultrastructural levels are reviewed. We explain how optogenetic stimulation and designer drugs used to modulate a distinct pathway and neuronal subpopulation have enhanced our mechanistic understanding of BG function in health and pathophysiology in disease. Finally, we outline how viral vector technology may be applied to neurological and psychiatric conditions to offer new treatments with enhanced outcomes for patients.

Avian sarcoma leukosis virus receptor-envelope system for simultaneous dissection of multiple neural circuits in mammalian brain

Pathway-specific gene delivery is requisite for understanding complex neuronal systems in which neurons that project to different target regions are locally intermingled. However, conventional genetic tools cannot achieve simultaneous, independent gene delivery into multiple target cells with high efficiency and low cross-reactivity. In this study, we systematically screened all receptor-envelope pairs resulting from the combination of four avian sarcoma leukosis virus (ASLV) envelopes (EnvA, EnvB, EnvC, and EnvE) and five engineered avian-derived receptors (TVA950, TVB(S3), TVC, TVB(T), and DR-46TVB) in vitro. Four of the 20 pairs exhibited both high infection rates (TVA-EnvA, 99.6%; TVB(S3)-EnvB, 97.7%; TVC-EnvC, 98.2%; and DR-46TVB-EnvE, 98.8%) and low cross-reactivity (<2.5%). Next, we tested these four receptor-envelope pairs in vivo in a pathway-specific gene-transfer method. Neurons projecting into a limited somatosensory area were labeled with each receptor by retrograde gene transfer. Three of the four pairs exhibited selective transduction into thalamocortical neurons expressing the paired receptor (>98%), with no observed cross-reaction. Finally, by expressing three receptor types in a single animal, we achieved pathway-specific, differential fluorescent labeling of three thalamic neuronal populations, each projecting into different somatosensory areas. Thus, we identified three orthogonal pairs from the list of ASLV subgroups and established a new vector system that provides a simultaneous, independent, and highly specific genetic tool for transferring genes into multiple target cells in vivo. Our approach is broadly applicable to pathway-specific labeling and functional analysis of diverse neuronal systems.

Mapping sensory circuits by anterograde transsynaptic transfer of recombinant rabies virus

Primary sensory neurons convey information from the external world to relay circuits within the CNS, but the identity and organization of the neurons that process incoming sensory information remains sketchy. Within the CNS, viral tracing techniques that rely on retrograde transsynaptic transfer provide a powerful tool for delineating circuit organization. Viral tracing of the circuits engaged by primary sensory neurons has, however, been hampered by the absence of a genetically tractable anterograde transfer system. In this study, we demonstrate that rabies virus can infect sensory neurons in the somatosensory system, is subject to anterograde transsynaptic transfer from primary sensory to spinal target neurons, and can delineate output connectivity with third-order neurons. Anterograde transsynaptic transfer is a feature shared by other classes of primary sensory neurons, permitting the identification and potentially the manipulation of neural circuits processing sensory feedback within the mammalian CNS.

Specific relationship between excitatory inputs and climbing fiber receptive fields in deep cerebellar nuclear neurons

Many mossy fiber pathways to the neurons of the deep cerebellar nucleus (DCN) originate from the spinal motor circuitry. For cutaneously activated spinal neurons, the receptive field is a tag indicating the specific motor function the spinal neuron has. Similarly, the climbing fiber receptive field of the DCN neuron reflects the specific motor output function of the DCN neuron. To explore the relationship between the motor information the DCN neuron receives and the output it issues, we made patch clamp recordings of DCN cell responses to tactile skin stimulation in the forelimb region of the anterior interposed nucleus in vivo. The excitatory responses were organized according to a general principle, in which the DCN cell responses became stronger the closer the skin site was located to its climbing fiber receptive field. The findings represent a novel functional principle of cerebellar connectivity, with crucial importance for our understanding of the function of the cerebellum in movement coordination.

Why do primordial germ cells migrate through an embryo and what does it mean for biological evolution?

An explanation of the role of primordial germ cell (PGC) migration during embryogenesis is proposed. According to the hypothesis, various PGCs during their migrations through an early embryo are contacting with anlagen of organs and acquiring nonidentical organ specificities. An individual PGC gets such an organ specificity, which corresponds to specificity of the first anlage with which this PGC has the first contact. As a result, the cellular descendants of PGCs (oocytes or spermatocytes) will express nonidentical organ-specific receptors, hence becoming functionally heterogeneous. Therefore, each clone of germ cells becomes capable of recognizing specifically the molecular signals that correspond only to "its" organ of the body. Such signals are produced by the body's organ when it functions in an extreme mode. Signals from the "exercising" organ of the body are delivered to the gonad only via the brain retransmitter, which is composed of neurons grouped as virtual organs of a homunculus. Homunculi are so-called somatotopic maps of the skeletomotor and other parts of the body represented in the brain. Signals, as complexes of regulatory RNAs and proteins, are transported from the "exercising" organ of the body to the corresponding virtual organ of the homunculus where they are processed and then forwarded to the gonad. The organ-specific signal will be selectively recognized by certain gametocytes according to their organ specificity, and then it will initiate the directed epimutation in the gametocyte genome. The nonrandomness of the gene order in chromosomes, that is the synteny and genetic map, is controlled by the so-called creatron that consolidates the soma and germline into a united system, providing the possibility of evolutionary responses of an organism to environmental influences.

New roles for the cerebellum in health and disease

The cerebellum has a well-established role in maintaining motor coordination and studies of cerebellar learning suggest that it does this by recognizing neural patterns, which it uses to predict optimal movements. Serious damage to the cerebellum impairs this learning and results in a set of motor disturbances called ataxia. However, recent work implicates the cerebellum in cognition and emotion, and it has been argued that cerebellar dysfunction contributes to non-motor conditions such as autism spectrum disorders (ASD). Based on human and animal model studies, two major questions arise. Does the cerebellum contribute to non-motor as well as motor diseases, and if so, how does altering its function contribute to such diverse symptoms? The architecture and connectivity of cerebellar circuits may hold the answers to these questions. An emerging view is that cerebellar defects can trigger motor and non-motor neurological conditions by globally influencing brain function. Furthermore, during development cerebellar circuits may play a role in wiring events necessary for higher cognitive functions such as social behavior and language. We discuss genetic, electrophysiological, and behavioral evidence that implicates Purkinje cell dysfunction as a major culprit in several diseases and offer a hypothesis as to how canonical cerebellar functions might be at fault in non-motor as well as motor diseases.

Cerebellar networks with basal ganglia: feasibility for tracking cerebello-pallidal and subthalamo-cerebellar projections in the human brain

Neuroanatomical studies using transneuronal virus tracers in macaque monkeys recently demonstrated that substantial interactions exist between basal ganglia and the cerebellum. To what extent these interactions are present in the human brain remains unclear; however, these connections are thought to provide an important framework for understanding cerebellar contributions to the manifestation of basal ganglia disorders, especially with respect to tremor genesis in movement disorders such as Parkinson's disease. Here, we tested the feasibility of assessing these connections in vivo and non-invasively in the human brain with diffusion magnetic resonance imaging and tractography. After developing a standardized protocol for manual segmentation of basal ganglia and cerebellar structures, masks for diffusion tractography were defined based on structural magnetic resonance images. We tested intra- and inter-observer stability and carried out tractography for dentato-pallidal and subthalamo-cerebellar projections. After robustly achieving connection probabilities per tract, the connectivity values and connectional fingerprints were calculated in a group of healthy volunteers. Probabilistic diffusion tractography was applicable to probe the inter-connection of the cerebellum and basal ganglia. Our data confirmed that dentato-thalamo-striato-pallidal and subthalamo-cerebellar connections also exist in the human brain at a level similar to those that were recently suggested by transneuronal tracing studies in non-human primates. Standardized segmentation protocols made these findings reproducible with high stability. We have demonstrated that diffusion tractography in humans in vivo is capable of revealing the structural bases of cerebellar networks with the basal ganglia. These findings support the role of the cerebellum as a satellite system of established cortico-basal ganglia networks in humans.

Cerebellar networks with the cerebral cortex and basal ganglia

The dominant view of cerebellar function has been that it is exclusively concerned with motor control and coordination. Recent findings from neuroanatomical, behavioral, and imaging studies have profoundly changed this view. Neuroanatomical studies using virus transneuronal tracers have demonstrated that cerebellar output reaches vast areas of the neocortex, including regions of prefrontal and posterior parietal cortex. Furthermore, it has recently become clear that the cerebellum is reciprocally connected with the basal ganglia, which suggests that the two subcortical structures are part of a densely interconnected network. Taken together, these findings elucidate the neuroanatomical substrate for cerebellar involvement in non-motor functions mediated by the prefrontal and posterior parietal cortex, as well as in processes traditionally associated with the basal ganglia.

Reverse Genetics of Mononegavirales: The Rabies Virus Paradigm

The neurotropic rabies virus (RABV) is a prototype member of the Mononegavirales order of viruses and is the most significant human pathogen of the Rhabdoviridae family. A reverse genetics system for RABV was established almost 20 years ago, providing a paradigm for other Mononegavirales members as well. The availability of engineered recombinant viruses opened a new era to study common aspects of Mononegavirales biology and specific aspects of the unique lifestyle and pathogenesis of individual members. Above all, the knowledge gained has allowed engineering of beneficial biomedical tools such as viral vectors, vaccines, and tracers. In this chapter, the development of the classical rabies virus reverse genetics approach is described, and some of the most exciting biomedical applications for recombinant RABV and other Mononegavirales are briefly addressed.