Morphology of single olivocerebellar axons labeled with biotinylated dextran amine in the rat

Cerebellum Lecture: the Cerebellar Nuclei-Core of the Cerebellum

The cerebellum is a key player in many brain functions and a major topic of neuroscience research. However, the cerebellar nuclei (CN), the main output structures of the cerebellum, are often overlooked. This neglect is because research on the cerebellum typically focuses on the cortex and tends to treat the CN as relatively simple output nuclei conveying an inverted signal from the cerebellar cortex to the rest of the brain. In this review, by adopting a nucleocentric perspective we aim to rectify this impression. First, we describe CN anatomy and modularity and comprehensively integrate CN architecture with its highly organized but complex afferent and efferent connectivity. This is followed by a novel classification of the specific neuronal classes the CN comprise and speculate on the implications of CN structure and physiology for our understanding of adult cerebellar function. Based on this thorough review of the adult literature we provide a comprehensive overview of CN embryonic development and, by comparing cerebellar structures in various chordate clades, propose an interpretation of CN evolution. Despite their critical importance in cerebellar function, from a clinical perspective intriguingly few, if any, neurological disorders appear to primarily affect the CN. To highlight this curious anomaly, and encourage future nucleocentric interpretations, we build on our review to provide a brief overview of the various syndromes in which the CN are currently implicated. Finally, we summarize the specific perspectives that a nucleocentric view of the cerebellum brings, move major outstanding issues in CN biology to the limelight, and provide a roadmap to the key questions that need to be answered in order to create a comprehensive integrated model of CN structure, function, development, and evolution.

Pax3 induces target-specific reinnervation through axon collateral expression of PSA-NCAM

Damaged or dysfunctional neural circuits can be replaced after a lesion by axon sprouting and collateral growth from undamaged neurons. Unfortunately, these new connections are often disorganized and rarely produce clinical improvement. Here we investigate how to promote post-lesion axonal collateral growth, while retaining correct cellular targeting. In the mouse olivocerebellar path, brain-derived neurotrophic factor (BDNF) induces correctly-targeted post-lesion cerebellar reinnervation by remaining intact inferior olivary axons (climbing fibers). In this study we identified cellular processes through which BDNF induces this repair. BDNF injection into the denervated cerebellum upregulates the transcription factor Pax3 in inferior olivary neurons and induces rapid climbing fiber sprouting. Pax3 in turn increases polysialic acid-neural cell adhesion molecule (PSA-NCAM) in the sprouting climbing fiber path, facilitating collateral outgrowth and pathfinding to reinnervate the correct targets, cerebellar Purkinje cells. BDNF-induced reinnervation can be reproduced by olivary Pax3 overexpression, and abolished by olivary Pax3 knockdown, suggesting that Pax3 promotes axon growth and guidance through upregulating PSA-NCAM, probably on the axon's growth cone. These data indicate that restricting growth-promotion to potential reinnervating afferent neurons, as opposed to stimulating the whole circuit or the injury site, allows axon growth and appropriate guidance, thus accurately rebuilding a neural circuit.

Complex spikes perturb movements and reveal the sensorimotor map of Purkinje cells

Computations that are performed by the cerebellar cortex are transmitted via simple spikes of Purkinje cells (P-cells) to downstream structures, but because P-cells are many synapses away from muscles, we do not know the relationship between modulation of simple spikes and control of behavior. Here, we recorded the spiking activities of hundreds of P-cells in the oculomotor vermis of marmosets during saccadic eye movements and found that following the presentation of a visual stimulus, the olivary input to a P-cell coarsely described the direction and amplitude of the visual stimulus as well as the upcoming movement. Occasionally, the complex spike occurred just before saccade onset, suppressing the P-cell's simple spikes and disrupting its output during that saccade. Remarkably, this brief suppression of simple spikes altered the saccade's trajectory by pulling the eyes toward the part of the visual space that was preferentially encoded by the olivary input to that P-cell. Thus, there is an alignment between the sensory space encoded by the complex spikes and the behavior conveyed by the simple spikes: a reduction in simple spikes is a signal to bias the ongoing movement toward the part of the sensory space preferentially encoded by the olivary input to that P-cell.

Afferent convergence to a shared population of interneuron AMPA receptors

Precise alignment of pre- and postsynaptic elements optimizes the activation of glutamate receptors at excitatory synapses. Nonetheless, glutamate that diffuses out of the synaptic cleft can have actions at distant receptors, a mode of transmission called spillover. To uncover the extrasynaptic actions of glutamate, we localized AMPA receptors (AMPARs) mediating spillover transmission between climbing fibers and molecular layer interneurons in the cerebellar cortex. We found that climbing fiber spillover generates calcium transients mediated by Ca2+-permeable AMPARs at parallel fiber synapses. Spillover occludes parallel fiber synaptic currents, indicating that separate, independently regulated afferent pathways converge onto a common pool of AMPARs. Together these findings demonstrate a circuit motif wherein glutamate 'spill-in' from an unconnected afferent pathway co-opts synaptic receptors, allowing activation of postsynaptic AMPARs even when canonical glutamate release is suppressed.

Complex spikes perturb movements, revealing the sensorimotor map of Purkinje cells

The cerebellar cortex performs computations that are critical for control of our actions, and then transmits that information via simple spikes of Purkinje cells (P-cells) to downstream structures. However, because P-cells are many synapses away from muscles, we do not know how their output affects behavior. Furthermore, we do not know the level of abstraction, i.e., the coordinate system of the P-cell's output. Here, we recorded spiking activities of hundreds of P-cells in the oculomotor vermis of marmosets during saccadic eye movements and found that following the presentation of a visual stimulus, the olivary input to a P-cell encoded a probabilistic signal that coarsely described both the direction and the amplitude of that stimulus. When this input was present, the resulting complex spike briefly suppressed the P-cell's simple spikes, disrupting the P-cell's output during that saccade. Remarkably, this brief suppression altered the saccade's trajectory by pulling the eyes toward the part of the visual space that was preferentially encoded by the olivary input to that P-cell. Thus, analysis of behavior in the milliseconds following a complex spike unmasked how the P-cell's output influenced behavior: the preferred location in the coordinates of the visual system as conveyed probabilistically from the inferior olive to a P-cell defined the action in the coordinates of the motor system for which that P-cell's simple spikes directed behavior.

Neural Circuits of Inputs and Outputs of the Cerebellar Cortex and Nuclei

This article is dedicated to the memory of Masao Ito. Masao Ito made numerous important contributions revealing the function of the cerebellum in motor control. His pioneering contributions to cerebellar physiology began with his discovery of inhibition and disinhibition of target neurons by cerebellar Purkinje cells, and his discovery of the presence of long-term depression in parallel fiber-Purkinje cell synapses. Purkinje cells formed the nodal point of Masao Ito's landmark model of motor control by the cerebellum. These discoveries became the basis for his ideas regarding the flocculus hypothesis, the adaptive motor control system, and motor learning by the cerebellum, inspiring many new experiments to test his hypotheses. This article will trace the achievements of Ito and colleagues in analyzing the neural circuits of the input-output organization of the cerebellar cortex and nuclei, particularly with respect to motor control. The article will discuss some of the important issues that have been solved and also those that remain to be solved for our understanding of motor control by the cerebellum.

Compartmentalized Input-Output Organization of Lugaro Cells in the Cerebellar Cortex

Purkinje cells (PCs) are principal cerebellar neurons, and several classes of interneurons modulate their activity. Lugaro cells (LCs) are one such inhibitory interneuron with distinctive cytology and location, but still most enigmatic among cerebellar neurons. Here we serendipitously produced a novel transgenic mouse line, where a half of Yellow Cameleon (YC)(+) cells in the cerebellar cortex were judged to be LCs, and YC(+) LCs were estimated to constitute one-third of the total LC populations. Neurochemically, two-thirds of YC(+) LCs were dually GABAergic/glycinergic, with the rest being GABAergic. Beneath the PC layer, they extended a sheet of somatodendritic meshwork interconnected with neighboring LCs by adherens junctions, and received various inputs from climbing fibers, mossy fibers, granule cell axons, recurrent PC axons, Golgi cell axons, LC axons, and serotonergic fibers. Intriguingly, somatodendritic elements of individual LCs preferentially extended within a given cerebellar compartment defined by aldolase C expression. In turn, YC(+) LCs projected a dense lattice of ascending and transverse axons to the molecular layer, and innervated molecular layer interneurons (basket and stellate cells) and Golgi cells, but not PCs. Of note, ascending axons profusely innervated individual targets within a cerebellar compartment, while transverse axons ran across several compartments and innervated targets sparsely. This unique circuit configuration highlights that LCs integrate various excitatory, inhibitory, and modulatory inputs coming to the belonging cerebellar compartment and that, as an interneuron-selective interneuron, LCs can effectively disinhibit cerebellar cortical activities in a compartment-dependent manner through inhibition of inhibitory interneurons selectively targeting PCs and granule cells.

Cerebellar Development and Circuit Maturation: A Common Framework for Spinocerebellar Ataxias

Spinocerebellar ataxias (SCAs) affect the cerebellum and its afferent and efferent systems that degenerate during disease progression. In the cerebellum, Purkinje cells (PCs) are the most vulnerable and their prominent loss in the late phase of the pathology is the main characteristic of these neurodegenerative diseases. Despite the constant advancement in the discovery of affected molecules and cellular pathways, a comprehensive description of the events leading to the development of motor impairment and degeneration is still lacking. However, in the last years the possible causal role for altered cerebellar development and neuronal circuit wiring in SCAs has been emerging. Not only wiring and synaptic transmission deficits are a common trait of SCAs, but also preventing the expression of the mutant protein during cerebellar development seems to exert a protective role. By discussing this tight relationship between cerebellar development and SCAs, in this review, we aim to highlight the importance of cerebellar circuitry for the investigation of SCAs.

Diverse Neuron Properties and Complex Network Dynamics in the Cerebellar Cortical Inhibitory Circuit

Neuronal inhibition can be defined as a spatiotemporal restriction or suppression of local microcircuit activity. The importance of inhibition relies in its fundamental role in shaping signal processing in single neurons and neuronal circuits. In this context, the activity of inhibitory interneurons proved the key to endow networks with complex computational and dynamic properties. In the last 50 years, the prevailing view on the functional role of cerebellar cortical inhibitory circuits was that excitatory and inhibitory inputs sum spatially and temporally in order to determine the motor output through Purkinje cells (PCs). Consequently, cerebellar inhibition has traditionally been conceived in terms of restricting or blocking excitation. This assumption has been challenged, in particular in the cerebellar cortex where all neurons except granule cells (and unipolar brush cells in specific lobules) are inhibitory and fire spontaneously at high rates. Recently, a combination of electrophysiological recordings in vitro and in vivo, imaging, optogenetics and computational modeling, has revealed that inhibitory interneurons play a much more complex role in regulating cerebellar microcircuit functions: inhibition shapes neuronal response dynamics in the whole circuit and eventually regulate the PC output. This review elaborates current knowledge on cerebellar inhibitory interneurons [Golgi cells, Lugaro cells (LCs), basket cells (BCs) and stellate cells (SCs)], starting from their ontogenesis and moving up to their morphological, physiological and plastic properties, and integrates this knowledge with that on the more renown granule cells and PCs. We will focus on the circuit loops in which these interneurons are involved and on the way they generate feed-forward, feedback and lateral inhibition along with complex spatio-temporal response dynamics. In this perspective, inhibitory interneurons emerge as the real controllers of cerebellar functioning.

Neurons of the inferior olive respond to broad classes of sensory input while subject to homeostatic control

KEY POINTS

Purkinje cells in the cerebellum integrate input from sensory organs with that from premotor centres. Purkinje cells use a variety of sensory inputs relaying information from the environment to modify motor control. Here we investigated to what extent the climbing fibre inputs to Purkinje cells signal mono- or multi-sensory information, and to what extent this signalling is subject to recent history of activity. We show that individual climbing fibres convey multiple types of sensory information, together providing a rich mosaic projection pattern of sensory signals across the cerebellar cortex. Moreover, firing probability of climbing fibres following sensory stimulation depends strongly on the recent history of activity, showing a tendency to homeostatic dampening.

ABSTRACT

Cerebellar Purkinje cells integrate sensory information with motor efference copies to adapt movements to behavioural and environmental requirements. They produce complex spikes that are triggered by the activity of climbing fibres originating in neurons of the inferior olive. These complex spikes can shape the onset, amplitude and direction of movements and the adaptation of such movements to sensory feedback. Clusters of nearby inferior olive neurons project to parasagittally aligned stripes of Purkinje cells, referred to as 'microzones'. It is currently unclear to what extent individual Purkinje cells within a single microzone integrate climbing fibre inputs from multiple sources of different sensory origins, and to what extent sensory-evoked climbing fibre responses depend on the strength and recent history of activation. Here we imaged complex spike responses in cerebellar lobule crus 1 to various types of sensory stimulation in awake mice. We find that different sensory modalities and receptive fields have a mild, but consistent, tendency to converge on individual Purkinje cells, with climbing fibres showing some degree of input-specificity. Purkinje cells encoding the same stimulus show increased events with coherent complex spike firing and tend to lie close together. Moreover, whereas complex spike firing is only mildly affected by variations in stimulus strength, it depends strongly on the recent history of climbing fibre activity. Our data point towards a mechanism in the olivo-cerebellar system that regulates complex spike firing during mono- or multi-sensory stimulation around a relatively low set-point, highlighting an integrative coding scheme of complex spike firing under homeostatic control.

The entire trajectories of single pontocerebellar axons and their lobular and longitudinal terminal distribution patterns in multiple aldolase C-positive compartments of the rat cerebellar cortex

The mammalian cerebellar cortex is compartmentalized, both anatomically and histochemically, into multiple parasagittal bands. To characterize the multiple zonal patterns of pontocerebellar mossy fiber projection, single neurons in the basilar pontine nucleus (BPN) were labeled by injecting biotinylated dextran amine into the BPN, and the entire axonal trajectory of single labeled neurons (n = 25) was reconstructed in relation to aldolase C compartments of Purkinje cells in rats. Single pontocerebellar axons, after passing through the contralateral middle cerebellar peduncle, ran transversely in the deep cerebellar white matter toward and often across the midline, and on their ways, gave rise to 2-10 primary collaterals at almost right angles in specific lobules only contralaterally or bilaterally with contralateral predominance. Each primary collateral further branched in a parasagittal plane to form a strip-shaped longitudinal termination zone with rosette-type swellings clustered in aldolase C-positive compartments in a single or multiple lobules, mainly in compartment 4+//5+, 5+//6+, and 6+//7+. Axons arising from the central, rostral, and lateral part of the BPN projected with multiple branches, mainly to simple lobule, crus II and paramedian lobule, to crus I and dorsal paraflocculus, and to ventral paraflocculus and lobule IXc, respectively. The results showed the pontocerebellar projection is closely related to lobular and compartmental organization of the cerebellum. A comparison of single axon morphologies of different mossy fiber systems indicates that the projection pattern of single pontocerebellar neurons with multiple collaterals innervating different longitudinal compartments arranged in a mediolateral direction represents a general feature of mossy fiber projection.

Cerebellar Modules and Their Role as Operational Cerebellar Processing Units: A Consensus paper [corrected]

The compartmentalization of the cerebellum into modules is often used to discuss its function. What, exactly, can be considered a module, how do they operate, can they be subdivided and do they act individually or in concert are only some of the key questions discussed in this consensus paper. Experts studying cerebellar compartmentalization give their insights on the structure and function of cerebellar modules, with the aim of providing an up-to-date review of the extensive literature on this subject. Starting with an historical perspective indicating that the basis of the modular organization is formed by matching olivocorticonuclear connectivity, this is followed by consideration of anatomical and chemical modular boundaries, revealing a relation between anatomical, chemical, and physiological borders. In addition, the question is asked what the smallest operational unit of the cerebellum might be. Furthermore, it has become clear that chemical diversity of Purkinje cells also results in diversity of information processing between cerebellar modules. An additional important consideration is the relation between modular compartmentalization and the organization of the mossy fiber system, resulting in the concept of modular plasticity. Finally, examination of cerebellar output patterns suggesting cooperation between modules and recent work on modular aspects of emotional behavior are discussed. Despite the general consensus that the cerebellum has a modular organization, many questions remain. The authors hope that this joint review will inspire future cerebellar research so that we are better able to understand how this brain structure makes its vital contribution to behavior in its most general form.

Roles for neuronal and glial autophagy in synaptic pruning during development

The dendritic protrusions known as spines represent the primary postsynaptic location for excitatory synapses. Dendritic spines are critical for many synaptic functions, and their formation, modification, and turnover are thought to be important for mechanisms of learning and memory. At many excitatory synapses, dendritic spines form during the early postnatal period, and while many spines are likely being formed and removed throughout life, the net number are often gradually "pruned" during adolescence to reach a stable level in the adult. In neurodevelopmental disorders, spine pruning is disrupted, emphasizing the importance of understanding its governing processes. Autophagy, a process through which cytosolic components and organelles are degraded, has recently been shown to control spine pruning in the mouse cortex, but the mechanisms through which autophagy acts remain obscure. Here, we draw on three widely studied prototypical synaptic pruning events to focus on two governing principles of spine pruning: 1) activity-dependent synaptic competition and 2) non-neuronal contributions. We briefly review what is known about autophagy in the central nervous system and its regulation by metabolic kinases. We propose a model in which autophagy in both neurons and non-neuronal cells contributes to spine pruning, and how other processes that regulate spine pruning could intersect with autophagy. We further outline future research directions to address outstanding questions on the role of autophagy in synaptic pruning.

Encoding of error and learning to correct that error by the Purkinje cells of the cerebellum

The primary output cells of the cerebellar cortex, Purkinje cells, make kinematic predictions about ongoing movements via high-frequency simple spikes, but receive sensory error information about that movement via low-frequency complex spikes (CS). How is the vector space of sensory errors encoded by this low-frequency signal? Here we measured Purkinje cell activity in the oculomotor vermis of animals during saccades, then followed the chain of events from experience of visual error, generation of CS, modulation of simple spikes, and ultimately change in motor output. We found that while error direction affected the probability of CS, error magnitude altered its temporal distribution. Production of CS changed the simple spikes on the next trial, but regardless of the actual visual error, this change biased the movement only along a vector that was parallel to the Purkinje cell's preferred error. From these results, we inferred the anatomy of a sensory-to-motor adaptive controller that transformed visual error vectors into motor-corrections.

Single axonal morphology and termination to cerebellar aldolase C stripes characterize distinct spinocerebellar projection systems originating from the thoracic spinal cord in the mouse

The spinocerebellar projection has an essential role in sensorimotor coordination of limbs and the trunk. Multiple groups of spinocerebellar projections have been identified in retrograde labeling studies. In this study, we aimed at characterizing projection patterns of these groups using a combination of anterograde labeling of the thoracic spinal cord and aldolase C immunostaining of longitudinal stripes of the cerebellar cortex in the mouse. We reconstructed 22 single spinocerebellar axons, wholly in the cerebellum and brain stem and partly, in the spinal cord. They were classified into three groups, (a) non-crossed axons of Clarke's column neurons (NCC, 8 axons), (b) non-crossed axons of marginal Clarke's column neurons (NMCC, 7 axons), and (c) crossed axons of neurons in the medial ventral horn (CMVH, 7 axons), based on previous retrograde labeling studies. While NCC axons projected mainly to multiple bilateral stripes in vermal lobules II-IV and VIII-IX, and the ipsilateral medial cerebellar nucleus, NMCC axons projected mainly to ipsilateral stripes in paravermal lobules II-V and copula pyramidis, and the anterior interposed nucleus. CMVH axons projected bilaterally to multiple stripes in lobules II-V with a small number of terminals but had abundant collaterals in the spinal cord and medullary reticular nuclei as well as in the vestibular and cerebellar nuclei. The results indicate that, while CMVH axons overlap with propriospinal and spinoreticular projections, NCC and NMCC axons are primarily spinocerebellar axons, which seem to be involved in relatively more proximal and distal sensorimotor controls, respectively.

Synaptic excitation by climbing fibre collaterals in the cerebellar nuclei of juvenile and adult mice

KEY POINTS

The inferior olive sends instructive motor signals to the cerebellum via the climbing fibre projection, which sends collaterals directly to large premotor neurons of the mouse cerebellar nuclei (CbN cells). Optogenetic activation of inferior olivary axons in vitro evokes EPSCs in CbN cells of several hundred pA to more than 1 nA. The inputs are three-fold larger at younger ages, 12 to 14 days old, than at 2 months old, suggesting a strong functional role for this pathway earlier in development. The EPSCs are multipeaked, owing to burst firing in several olivary afferents that fire asynchronously. The convergence of climbing fibre collaterals onto CbN cells decreases from ∼40 to ∼8, which is consistent with the formation of closed-loop circuits in which each CbN neuron receives input from 4-7 collaterals from inferior olivary neurons as well as from all 30-50 Purkinje cells that are innervated by those olivary neurons.

ABSTRACT

The inferior olive conveys instructive signals to the cerebellum that drive sensorimotor learning. Inferior olivary neurons transmit their signals via climbing fibres, which powerfully excite Purkinje cells, evoking complex spikes and depressing parallel fibre synapses. Additionally, however, these climbing fibres send collaterals to the cerebellar nuclei (CbN). In vivo and in vitro data suggest that climbing fibre collateral excitation is weak in adult mice, raising the question of whether the primary role of this pathway may be developmental. We therefore examined climbing fibre collateral input to large premotor CbN cells over development by virally expressing channelrhodopsin in the inferior olive. In acute cerebellar slices from postnatal day (P)12-14 mice, light-evoked EPSCs were large (> 1 nA at -70 mV). The amplitude of these EPSCs decreased over development, reaching a plateau of ∼350 pA at P20-60. Trains of EPSCs (5 Hz) depressed strongly throughout development, whereas convergence estimates indicated that the total number of functional afferents decreased with age. EPSC waveforms consisted of multiple peaks, probably resulting from action potential bursts in single collaterals and variable times to spike threshold in converging afferents. Activating climbing fibre collaterals evoked well-timed increases in firing probability in CbN neurons, especially in younger mice. The initially strong input, followed by the decrement in synaptic strength coinciding with the pruning of climbing fibres in the cerebellar cortex, implicates the climbing fibre collateral pathway in early postnatal development. Additionally, the persistence of substantial synaptic input at least to P60 suggests that this pathway may function in cerebellar processing into adulthood.

Climbing fibers mediate vestibular modulation of both "complex" and "simple spikes" in Purkinje cells

Climbing and mossy fibers comprise two distinct afferent paths to the cerebellum. Climbing fibers directly evoke a large multispiked action potential in Purkinje cells termed a "complex spike" (CS). By logical exclusion, the other class of Purkinje cell action potential, termed "simple spike" (SS), has often been attributed to activity conveyed by mossy fibers and relayed to Purkinje cells through granule cells. Here, we investigate the relative importance of climbing and mossy fiber pathways in modulating neuronal activity by recording extracellularly from Purkinje cells, as well as from mossy fiber terminals and interneurons in folia 8-10. Sinusoidal roll-tilt vestibular stimulation vigorously modulates the discharge of climbing and mossy fiber afferents, Purkinje cells, and interneurons in folia 9-10 in anesthetized mice. Roll-tilt onto the side ipsilateral to the recording site increases the discharge of both climbing fibers (CSs) and mossy fibers. However, the discharges of SSs decrease during ipsilateral roll-tilt. Unilateral microlesions of the beta nucleus (β-nucleus) of the inferior olive blocks vestibular modulation of both CSs and SSs in contralateral Purkinje cells. The blockage of SSs occurs even though primary and secondary vestibular mossy fibers remain intact. When mossy fiber afferents are damaged by a unilateral labyrinthectomy (UL), vestibular modulation of SSs in Purkinje cells ipsilateral to the UL remains intact. Two inhibitory interneurons, Golgi and stellate cells, could potentially contribute to climbing fiber-induced modulation of SSs. However, during sinusoidal roll-tilt, only stellate cells discharge appropriately out of phase with the discharge of SSs. Golgi cells discharge in phase with SSs. When the vestibularly modulated discharge is blocked by a microlesion of the inferior olive, the modulated discharge of CSs and SSs is also blocked. When the vestibular mossy fiber pathway is destroyed, vestibular modulation of ipsilateral CSs and SSs persists. We conclude that climbing fibers are primarily responsible for the vestibularly modulated discharge of both CSs and SSs. Modulation of the discharge of SSs is likely caused by climbing fiber-evoked stellate cell inhibition.

Dendritic translocation of climbing fibers: a new face of old phenomenon

During postnatal development of the cerebellum, the number of climbing fibers that innervate individual Purkinje cells decreases from many to one. This is one of the most characterized models of activity-dependent refinement of synaptic circuitry in the mammalian brain. As surplus climbing fibers are eliminated, subcellular location of climbing fiber terminals moves from the soma to the dendrites of Purkinje cells. The role of this dendritic translocation in climbing fiber elimination has been under debate for a long time, but recent studies have significantly changed the view. Traditionally, dendritic translocation was considered neither sufficient nor necessary for climbing fiber elimination, but experimental evidence obtained over the past 5 years indicates crucial roles of dendritic translocation for selectively maintaining one fiber while removing all the others. Here, I provide an overview of (i) several key findings that have contributed for developing the traditional and current views of dendritic translocation, (ii) how our novel experimental approach supports the current view, and (iii) a remaining question that is yet to be answered.

High frequency synchrony in the cerebellar cortex during goal directed movements

The cerebellum is involved in sensory-motor integration and cognitive functions. The origin and function of the field potential oscillations in the cerebellum, especially in the high frequencies, have not been explored sufficiently. The primary objective of this study was to investigate the spatio-temporal characteristics of high frequency field potentials (150–350 Hz) in the cerebellar cortex in a behavioral context. To this end, we recorded from the paramedian lobule in rats using micro electro-corticogram (μ-ECoG) electrode arrays while the animal performed a lever press task using the forelimb. The phase synchrony analysis shows that the high frequency oscillations recorded at multiple points across the paramedian cortex episodically synchronize immediately before and desynchronize during the lever press. The electrode contacts were grouped according to their temporal course of phase synchrony around the time of lever press. Contact groups presented patches with slightly stronger synchrony values in the medio-lateral direction, and did not appear to form parasagittal zones. The size and location of these patches on the cortical surface are in agreement with the sensory evoked granular layer patches originally reported by Welker's lab (Shambes et al., 1978). Spatiotemporal synchrony of high frequency field potentials has not been reported at such large-scales previously in the cerebellar cortex.

How the cerebellum may monitor sensory information for spatial representation

The cerebellum has already been shown to participate in the navigation function. We propose here that this structure is involved in maintaining a sense of direction and location during self-motion by monitoring sensory information and interacting with navigation circuits to update the mental representation of space. To better understand the processing performed by the cerebellum in the navigation function, we have reviewed: the anatomical pathways that convey self-motion information to the cerebellum; the computational algorithm(s) thought to be performed by the cerebellum from these multi-source inputs; the cerebellar outputs directed toward navigation circuits and the influence of self-motion information on space-modulated cells receiving cerebellar outputs. This review highlights that the cerebellum is adequately wired to combine the diversity of sensory signals to be monitored during self-motion and fuel the navigation circuits. The direct anatomical projections of the cerebellum toward the head-direction cell system and the parietal cortex make those structures possible relays of the cerebellum influence on the hippocampal spatial map. We describe computational models of the cerebellar function showing that the cerebellum can filter out the components of the sensory signals that are predictable, and provides a novelty output. We finally speculate that this novelty output is taken into account by the navigation structures, which implement an update over time of position and stabilize perception during navigation.

Dendritic planarity of Purkinje cells is independent of Reelin signaling

The dendritic planarity of Purkinje cells is critical for cerebellar circuit formation. In the absence of Crk and CrkL, the Reelin pathway does not function resulting in partial Purkinje cell migration and defective dendritogenesis. However, the relationships among Purkinje cell migration, dendritic development and Reelin signaling have not been clearly delineated. Here, we use synchrotron X-ray microscopy to obtain 3-D images of Golgi-stained Purkinje cell dendrites. Purkinje cells that failed to migrate completely exhibited conical dendrites with abnormal 3-D arborization and reduced dendritic complexity. Furthermore, their spines were fewer in number with a distorted morphology. In contrast, Purkinje cells that migrated successfully displayed planar dendritic and spine morphologies similar to normal cells, despite reduced dendritic complexity. These results indicate that, during cerebellar formation, Purkinje cells migrate into an environment that supports development of dendritic planarity and spine formation. While Reelin signaling is important for the migration process, it does not make a direct major contribution to dendrite formation.

Cerebellar afferents originating from the medullary reticular formation that are different from mossy, climbing or monoaminergic fibers in the rat

Integration of cortical Purkinje cell inputs and brain stem inputs is essential in generating cerebellar outputs to the cerebellar nuclei (CN). Currently, collaterals of climbing and mossy fiber axons, noradrenergic, serotoninergic and cholinergic axons, and collaterals of rubrospinal axons are known to innervate the CN from the brain stem. We investigated whether other afferents to the CN from the medulla exist in the rat. Retrograde labeling revealed the presence of neurons that project to the CN but not to the cerebellar cortex in the median reticular formation in the rostrodorsal medulla (tentatively named 'caudal raphe interpositus area', CRI). Anterograde tracer injection into the CRI labeled abundant axonal terminals in the CN, mainly in the ventral parvocellular part of the posterior interposed and lateral nucleus. Axonal reconstruction showed that a single CRI axon projected to the CN with 170-1086 varicosities, more broadly and densely than collaterals of a mossy or climbing fiber axon. CRI axons had no or a few collaterals that projected to the granular and Purkinje cell layers of the cerebellar cortex with some small terminals, indicating that these axons are different from mossy fiber axons. CRI axons also had collaterals that projected to the medial vestibular nucleus and an ascending branch that was not reconstructed. The location of the CRI, electron microscopic observations, and immunostaining results all indicated that CRI axons are not monoaminergic. We conclude that CRI axons form a type of afferent projection to the CN that is different from mossy, climbing or monoaminergic fibers.

Formation and reformation of climbing fibre synapses in the cerebellum: a similar story?

The assembly of neural circuits involves multiple sequential steps, in particular the formation and maturation of synaptic connections. This often prolonged process involves several stages including the appropriate morphological and physiological maturation of each synaptic partner as well as their mutual interaction in order to ensure correct cellular and subcellular targeting. Understanding the processes involved becomes critical if neural circuits are to be appropriately reassembled following lesion, atrophy or neurodegeneration. We study the climbing fibre to Purkinje cell synapse as an example of a neural circuit which undergoes initial synaptic formation, selective stabilisation and elimination of redundant connections, in order to better understand the relative roles of each synaptic partner in the process of synaptogenesis and post-lesion synapse reformation. In particular, we are interested in the molecules which may underlie these processes. Here, we present data showing that the maturational state of both the target Purkinje cell and the climbing fibre axon influence their capacity for synapse formation. The climbing fibre retains some ability to recapitulate developmental processes irrespective of its maturational state. In contrast, the experience of synaptic formation and selective stabilisation/elimination permanently changes the Purkinje cell so that it cannot be repeated. Thus, if the climbing fibre-Purkinje cell synapse is recreated after the period of normal maturation, the process of synaptic competition, involving the gradual weakening of one climbing fibre synapse and stabilisation of another, no longer takes place. Moreover, we show that these processes of synaptic competition can only proceed during a specific developmental phase. To understand why these changes occur, we have investigated the role of molecules involved in the development of the olivocerebellar path and show that brain-derived neurotrophic factor, through activation of its receptor TrkB, as well as polysialated neural cell adhesion molecule and the transcription factor RORα regulate these processes.

Modulated discharge of Purkinje and stellate cells persists after unilateral loss of vestibular primary afferent mossy fibers in mice

Cerebellar Purkinje cells are excited by two afferent pathways: climbing and mossy fibers. Climbing fibers evoke large "complex spikes" (CSs) that discharge at low frequencies. Mossy fibers synapse on granule cells whose parallel fibers excite Purkinje cells and may contribute to the genesis of "simple spikes" (SSs). Both afferent systems convey vestibular information to folia 9c-10. After making a unilateral labyrinthectomy (UL) in mice, we tested how the discharge of CSs and SSs was changed by the loss of primary vestibular afferent mossy fibers during sinusoidal roll tilt. We recorded from cells identified by juxtacellular neurobiotin labeling. The UL preferentially reduced vestibular modulation of CSs and SSs in folia 8-10 contralateral to the UL. The effects of a UL on Purkinje cell discharge were similar in folia 9c-10, to which vestibular primary afferents project, and in folia 8-9a, to which they do not project, suggesting that vestibular primary afferent mossy fibers were not responsible for the UL-induced alteration of SS discharge. UL also induced reduced vestibular modulation of stellate cell discharge contralateral to the UL. We attribute the decreased modulation to reduced vestibular modulation of climbing fibers. In summary, climbing fibers modulate CSs directly and SSs indirectly through activation of stellate cells. Whereas vestibular primary afferent mossy fibers cannot account for the modulated discharge of SSs or stellate cells, the nonspecific excitation of Purkinje cells by parallel fibers may set an operating point about which the discharges of SSs are sculpted by climbing fibers.

New insights on vertebrate olivo-cerebellar climbing fibers from computerized morphological reconstructions

Characterization of neuronal connectivity is essential to understanding the architecture of the animal nervous system. Specific labeling and imaging techniques can visualize axons and dendrites of single nerve cells. Two-dimensional manual drawing has long been used to describe the morphology of labeled neuronal elements. However, quantitative morphometry, which is essential to understanding functional significance, cannot be readily extracted unless the detailed neuronal geometry is comprehensively reconstructed in three-dimensional space. We have recently applied an accurate and robust digital reconstruction system to cerebellar climbing fibers, which form highly dense and complex terminal arbors as one of the strongest presynaptic endings in the vertebrate nervous system. Resulting statistical analysis has shown how climbing fibers morphology is special in comparison to other axonal terminals. While thick primary branches may convey excitation quickly and faithfully to the far ends, thin tendril branches, which have a larger bouton density, form the majority of presynaptic outputs. This data set, now publicly available from NeuroMorpho.Org for further modeling and analysis, may constitute the first detailed and comprehensive digital reconstruction of the complete axonal terminal field with identified branch types and full accounting of boutons for any neuronal class in the vertebrate brain.

In vivo single branch axotomy induces GAP-43-dependent sprouting and synaptic remodeling in cerebellar cortex

Plasticity in the central nervous system in response to injury is a complex process involving axonal remodeling regulated by specific molecular pathways. Here, we dissected the role of growth-associated protein 43 (GAP-43; also known as neuromodulin and B-50) in axonal structural plasticity by using, as a model, climbing fibers. Single axonal branches were dissected by laser axotomy, avoiding collateral damage to the adjacent dendrite and the formation of a persistent glial scar. Despite the very small denervated area, the injured axons consistently reshape the connectivity with surrounding neurons. At the same time, adult climbing fibers react by sprouting new branches through the intact surroundings. Newly formed branches presented varicosities, suggesting that new axons were more than just exploratory sprouts. Correlative light and electron microscopy reveals that the sprouted branch contains large numbers of vesicles, with varicosities in the close vicinity of Purkinje dendrites. By using an RNA interference approach, we found that downregulating GAP-43 causes a significant increase in the turnover of presynaptic boutons. In addition, silencing hampers the generation of reactive sprouts. Our findings show the requirement of GAP-43 in sustaining synaptic stability and promoting the initiation of axonal regrowth.

Automated reconstruction of neuronal morphology based on local geometrical and global structural models

Digital reconstruction of neurons from microscope images is an important and challenging problem in neuroscience. In this paper, we propose a model-based method to tackle this problem. We first formulate a model structure, then develop an algorithm for computing it by carefully taking into account morphological characteristics of neurons, as well as the image properties under typical imaging protocols. The method has been tested on the data sets used in the DIADEM competition and produced promising results for four out of the five data sets.

Compartmentalization of the deep cerebellar nuclei based on afferent projections and aldolase C expression

The distribution of aldolase C (zebrin II)-positive and -negative Purkinje cells (PCs) can be used to define about 20 longitudinally extended compartments in the cerebellar cortex of the rat, which may correspond to certain aspects of cerebellar functional localization. An equivalent compartmental organization may exist in the deep cerebellar nuclei (DCN). This DCN compartmentalization is primarily represented by the afferent projection pattern in the DCN. PC projections and collateral nuclear projections of olivocerebellar climbing fiber axons have a relatively localized terminal arbor in the DCN. Projections of these axons make a closed olivo-cortico-nuclear circuit to connect a longitudinal stripe-shaped cortical compartment to a small subarea in the DCN, which can be defined as a DCN compartment. The actual DCN compartmentalization, which has been revealed by systematically mapping these projections, is quite different from the cortical compartmentalization. The stripe-shaped alternation of aldolase C-positive and -negative narrow longitudinal compartments in the cerebellar cortex is transformed to the separate clustering of positive and negative compartments in the caudoventral and rostrodorsal DCN, respectively. The distinctive projection of aldolase C-positive and -negative PCs to the caudoventral and rostrodorsal DCN underlies this transformation. Accordingly, the medial cerebellar nucleus is divided into the rostrodorsal aldolase C-negative and caudoventral aldolase C-positive parts. The anterior and posterior interposed nuclei generally correspond to the aldolase C-negative and -positive parts, respectively. DCN compartmentalization is important for understanding functional localization in the DCN since it is speculated that aldolase C-positive and -negative compartments are generally associated with somatosensory and other functions, respectively.

Precerebellar cell groups in the hindbrain of the mouse defined by retrograde tracing and correlated with cumulative Wnt1-cre genetic labeling

The precerebellar nuclei are hindbrain and spinal cord centers that send fibers to the cerebellum. The neurons of the major hindbrain precerebellar nuclei are derived from the rhombic lip. Wnt1, a developmentally important gene involved in intercellular signaling, is expressed in the developing rhombic lip. We sought to investigate the relationship between the cell clusters expressing Wnt1 and the precerebellar nuclei in the hindbrain. We therefore defined the hindbrain precerebellar nuclei by retrograde tracing, following cerebellar injections of HRP, and compared these results with the cell clusters expressing Wnt1 in newborn mice. We found that 39 distinct hindbrain nuclei project to the cerebellum. Of these nuclei, all but three (namely the oral pontine reticular nucleus, the caudal pontine reticular nucleus, and the subcoeruleus nucleus) contain neurons expressing Wnt1. This shows a high degree of overlap between the precerebellar nuclei and the nuclei that express Wnt1. However, it should be noted that neurons expressing Wnt1 are also found in the superior olivary complex, which is a basal plate derivative lacking cerebellar projections.

Neural circuitry and plasticity mechanisms underlying delay eyeblink conditioning

Pavlovian eyeblink conditioning has been used extensively as a model system for examining the neural mechanisms underlying associative learning. Delay eyeblink conditioning depends on the intermediate cerebellum ipsilateral to the conditioned eye. Evidence favors a two-site plasticity model within the cerebellum with long-term depression of parallel fiber synapses on Purkinje cells and long-term potentiation of mossy fiber synapses on neurons in the anterior interpositus nucleus. Conditioned stimulus and unconditioned stimulus inputs arise from the pontine nuclei and inferior olive, respectively, converging in the cerebellar cortex and deep nuclei. Projections from subcortical sensory nuclei to the pontine nuclei that are necessary for eyeblink conditioning are beginning to be identified, and recent studies indicate that there are dynamic interactions between sensory thalamic nuclei and the cerebellum during eyeblink conditioning. Cerebellar output is projected to the magnocellular red nucleus and then to the motor nuclei that generate the blink response(s). Tremendous progress has been made toward determining the neural mechanisms of delay eyeblink conditioning but there are still significant gaps in our understanding of the necessary neural circuitry and plasticity mechanisms underlying cerebellar learning.

The DIADEM data sets: representative light microscopy images of neuronal morphology to advance automation of digital reconstructions

The comprehensive characterization of neuronal morphology requires tracing extensive axonal and dendritic arbors imaged with light microscopy into digital reconstructions. Considerable effort is ongoing to automate this greatly labor-intensive and currently rate-determining process. Experimental data in the form of manually traced digital reconstructions and corresponding image stacks play a vital role in developing increasingly more powerful reconstruction algorithms. The DIADEM challenge (short for DIgital reconstruction of Axonal and DEndritic Morphology) successfully stimulated progress in this area by utilizing six data set collections from different animal species, brain regions, neuron types, and visualization methods. The original research projects that provided these data are representative of the diverse scientific questions addressed in this field. At the same time, these data provide a benchmark for the types of demands automated software must meet to achieve the quality of manual reconstructions while minimizing human involvement. The DIADEM data underwent extensive curation, including quality control, metadata annotation, and format standardization, to focus the challenge on the most substantial technical obstacles. This data set package is now freely released ( http://diademchallenge.org ) to train, test, and aid development of automated reconstruction algorithms.

The mysterious microcircuitry of the cerebellar nuclei

The microcircuitry of cerebellar cortex and, in particular, the physiology of its main element, the Purkinje neuron, has been extensively investigated and described. However, activity in Purkinje neurons, either as single cells or populations, does not directly mediate the cerebellar effects on the motor effector systems. Rather, the result of the entire cerebellar cortical computation is passed to the relatively small cerebellar nuclei that act as the final, integrative processing unit in the cerebellar circuitry. The nuclei ultimately control the temporal and spatial features of the cerebellar output. Given this key role, it is striking that the internal organization and the connectivity with afferent and efferent pathways in the cerebellar nuclei are rather poorly known. In the present review, we discuss some of the many critical shortcomings in the understanding of cerebellar nuclei microcircuitry: the extent of convergence and divergence of the cerebellar cortical pathway to the various cerebellar nuclei neurons and subareas, the possible (lack of) conservation of the finely-divided topographical organization in the cerebellar cortex at the level of the nuclei, as well as the absence of knowledge of the synaptic circuitry within the cerebellar nuclei. All these issues are important for predicting the pattern-extraction and encoding capabilities of the cerebellar nuclei and, until resolved, theories and models of cerebellar motor control and learning may err considerably.

Morphometric analysis of the neuronal numbers and densities of the inferior olivary complex in the donkey (Equus asinus)

The morphometric interrelations between the compartments of the inferior olivary complex (IOC) in the donkey (Equus asinus) were ascertained by examining serial sections throughout the entire length of the IOC for both sides. Nissl-stained celloidin sections of four brainstems of donkeys were used. The IOC consisted of three major nuclei and four small cell groups. The total neuronal count in both sides of the IOC was 202,040±8480 cells. The medial accessory olivary nucleus (MAO) had the largest relative area (46%) and the highest number of neurons (90,800±7600). The dorsal accessory olivary nucleus (DAO) had the second largest relative area (33%), while the principal olivary nucleus (PO) had the lowest relative area (21%). However, the total neuron count in the PO was larger (60,840±1840) than DAO (50,360±4040). The average neuronal density was 2700±400 cells/mm(3). The numerical values of the current study of the IOC in the donkey were similar to those of other mammals.

Remodeling of monoplanar Purkinje cell dendrites during cerebellar circuit formation

Dendrite arborization patterns are critical determinants of neuronal connectivity and integration. Planar and highly branched dendrites of the cerebellar Purkinje cell receive specific topographical projections from two major afferent pathways; a single climbing fiber axon from the inferior olive that extend along Purkinje dendrites, and parallel fiber axons of granule cells that contact vertically to the plane of dendrites. It has been believed that murine Purkinje cell dendrites extend in a single parasagittal plane in the molecular layer after the cell polarity is determined during the early postnatal development. By three-dimensional confocal analysis of growing Purkinje cells, we observed that mouse Purkinje cells underwent dynamic dendritic remodeling during circuit maturation in the third postnatal week. After dendrites were polarized and flattened in the early second postnatal week, dendritic arbors gradually expanded in multiple sagittal planes in the molecular layer by intensive growth and branching by the third postnatal week. Dendrites then became confined to a single plane in the fourth postnatal week. Multiplanar Purkinje cells in the third week were often associated by ectopic climbing fibers innervating nearby Purkinje cells in distinct sagittal planes. The mature monoplanar arborization was disrupted in mutant mice with abnormal Purkinje cell connectivity and motor discoordination. The dendrite remodeling was also impaired by pharmacological disruption of normal afferent activity during the second or third postnatal week. Our results suggest that the monoplanar arborization of Purkinje cells is coupled with functional development of the cerebellar circuitry.

Projection patterns of single mossy fiber axons originating from the dorsal column nuclei mapped on the aldolase C compartments in the rat cerebellar cortex

Although cerebellar mossy fibers are the most abundant cerebellar afferents and are deeply involved in cerebellar function, the organization of their projection has remained obscure, particularly in relation to cerebellar compartmentalization. The dorsal column nuclei (DCN) are a major source of cerebellar mossy fibers and possess distinct somatotopic representations of specific somatosensory submodalities. We reconstructed individual dextran-labeled DCN axons completely from serial sections and mapped their terminals on the longitudinal cerebellar compartments that were visualized by aldolase C immunostaining to clarify their projection pattern. Individual axons branched and formed about 100 rosette terminals in the cerebellar cortex, but infrequently projected to the cerebellar nuclei (1 out of 15 axons). Cortical terminals were clustered in multiple areas in the vermis and pars intermedia mostly, but not exclusively, ipsilateral to the origin of the axon. The gracile, cuneate, and external cuneate nuclei (ECuN) mainly projected to the copula pyramidis and lobule V, paramedian and simple lobules, and lobules I-V and VIII-IX, respectively, although there was some overlap. The majority of terminals were located within aldolase C negative or lightly positive compartments. However, terminals of a single axon can be located on aldolase C-negative as well as on aldolase C-positive compartments. In particular, the rostral ECuN, which is responsive to shoulder movements, projected consistently to lobule IX, which were mostly aldolase C-positive. In sum, DCN-cerebellar axons project to multiple compartments with terminals clustered mainly in the conventional spinocerebellar region with a coarse topography, which shows some relationship to the cortical compartments defined by aldolase C.

Topsy turvy: functions of climbing and mossy fibers in the vestibulo-cerebellum

The cerebellum's role in sensory-motor control and adaptation is undisputed. However, a key hypothesis pertaining to the function of cerebellar circuitry lacks experimental support. It is universally assumed that the discharge of mossy fibers accounts for modulation of Purkinje cell "simple spikes" (SSs). This assumption acts as a prism through which all other functions of cerebellar circuitry are viewed. The vestibulo-cerebellum (nodulus and uvula) receives a large, unilateral, vestibular primary afferent mossy fiber projection. We can test its role in modulating Purkinje cell SSs by recording the modulated activity of both mossy fiber terminals and Purkinje cell SSs evoked by identical natural vestibular stimulation. Sinusoidal rotation about the longitudinal axis (roll) modulates the activity of vestibular primary afferent mossy and climbing fibers as well as Purkinje cell SSs and complex spikes (CSs). Remarkably, vestibular primary afferent mossy fibers discharge 180 degrees out of phase with SSs. This indicates that mossy fibers cannot account for SS modulation unless an inhibitory synapse is interposed between mossy fibers or vestibular climbing fibers and Purkinje cells. The authors review several experiments that address the relative contributions of mossy and climbing fiber afferents to the modulation of SSs. They conclude that climbing fibers, not mossy fibers, are primarily responsible for the modulation of SSs as well as CSs and they propose revised functions for these two afferent systems.

Automation of Hessian-Based Tubularity Measure Response Function in 3D Biomedical Images

The blood vessels and nerve trees consist of tubular objects interconnected into a complex tree- or web-like structure that has a range of structural scale 5 μm diameter capillaries to 3 cm aorta. This large-scale range presents two major problems; one is just making the measurements, and the other is the exponential increase of component numbers with decreasing scale. With the remarkable increase in the volume imaged by, and resolution of, modern day 3D imagers, it is almost impossible to make manual tracking of the complex multiscale parameters from those large image data sets. In addition, the manual tracking is quite subjective and unreliable. We propose a solution for automation of an adaptive nonsupervised system for tracking tubular objects based on multiscale framework and use of Hessian-based object shape detector incorporating National Library of Medicine Insight Segmentation and Registration Toolkit (ITK) image processing libraries.

Development of an anatomical technique for visualizing the mode of climbing fiber innervation in Purkinje cells and its application to mutant mice lacking GluRδ2 and Ca(v)2.1

In the adult cerebellum, a single climbing fiber (CF) innervates proximal dendrites of Purkinje cells (PCs). This monoinnervation is established by the developmental elimination of surplus CFs through homosynaptic competition among multiply innervating CFs and heterosynaptic competition between CFs and parallel fibers, i.e., granule cell axons innervating distal PC dendrites. Although the developmental process of CF monoinnervation and defects in it in mutant and experimental animal models have been extensively studied by electrophysiological techniques, for quite some time this subject was poorly understood from a morphological perspective due to a lack of neuroanatomical methods that could distinguish CFs with different neuronal origins. Soon after the identification of type 2 vesicular glutamate transporter (VGluT2) that selectively detects CF terminals in the molecular layer, we developed a novel method of combined anterograde tracer labeling and VGluT2 immunohistochemistry. This method enables us to identify the mode (mono vs. multiple) of CF innervation and the site of multiple innervation. Since then, we have applied this method to various kinds of gene-manipulated mice manifesting ataxia and other cerebellar phenotypes. In this review, we summarize experimental procedures for the combined tracer/VGluT2 labeling method, and then introduce what we have learned by applying this method in studies on the role of GluRδ2 and Ca(v)2.1 in CF monoinnervation. This method has provided informative anatomical correlates to electrophysiological data and vice versa, and will extend our knowledge of the molecular and cellular mechanisms for the development, plasticity, degeneration, and repair of the CF-PC projection system.

Climbing fibers induce microRNA transcription in cerebellar Purkinje cells

The coordinated expression of as many as 100 proteins may be required to sustain simple changes in synaptic transmission. While each protein may be regulated separately, the translation of multiple proteins could be regulated by microRNAs. MicroRNAs are short non-coding RNAs that translationally repress cognate sequences in targeted mRNAs. If these targeted sequences are shared across several genes, then a single microRNA could, effectively regulate the activity of several genes in parallel. Here we investigate whether microRNA transcription is influenced by naturally evoked synaptic activity at the climbing fiber-Purkinje cell synapse in the mouse cerebellar flocculus. Mice received 24 h of binocular horizontal optokinetic stimulation (HOKS) evoking sustained increases in climbing fiber activity to Purkinje cells in one flocculus and decreases to Purkinje cells in the other. Increased climbing fiber activity increased transcription of 12 microRNAs in the flocculus. The transcription of one of these microRNAs, miR335, was proportional to duration of stimulation, increasing 18-fold after 24 h of HOKS. We localized miR335 transcripts to Purkinje cells using hybridization histochemistry. Transcripts of miR335 decayed to baseline within 3 h after HOKS was stopped. We identified mRNA targets for miR335 using multiple screens: sequence analysis, microinjection of miR335 inhibitors and identification of mRNAs whose transcription decreased during HOKS. Two genes, calbindin and 14-3-3-θ passed these screens. Our data suggest that microRNA transcription could provide an important synaptic or homeostatic mechanism for the regulation of proteins that contribute to Purkinje cell plasticity.

Role of presynaptic kainate receptors at parallel fiber-purkinje cell synapses in induction of cerebellar LTD: interplay with climbing fiber input

Until recently, except for A1 adenosine, N-methyl-d-aspartate, and cannabinoid receptors, little effort has been made to unravel possible roles of parallel fiber (PF) presynaptic receptors in long-term depression (LTD) of synaptic transmission at PF-Purkinje cell (PC) synapses. Presynaptic kainate (KA) receptors are also present on PFs and might also influence LTD induction by modulating glutamate (Glu) release at PF-PC synapses. This hypothesis was tested by comparing the efficacy of two pairing protocols in inducing LTD in adult wild-type and knock-out mice for the Glu receptor 6 (GluR6) subunit of KA receptors. Activation of presynaptic KA receptors was unnecessary for LTD induction when PF inputs were paired with climbing fiber (CF) stimulation but became crucial when CF input was replaced by direct depolarization of PCs. By comparing basal paired-pulse facilitation of PF-excitatory postsynaptic currents (EPSCs) and depolarization-induced suppression of excitation in adult wild-type and GluR6 knock-out mice, it was shown that the participation of PF presynaptic KA receptors in LTD induction was likely to mainly result from their tonic activation by basal extracellular Glu, rather than from their activation by retrograde release of Glu by PCs during pairing protocols. Finally, this study suggests that, in adult mice, CFs not only participate in LTD induction by depolarizing postsynaptic cells but also by activating postsynaptic mGluR1alpha metabotropic glutamate receptors at CF-PC synapses.

3D electron microscopic reconstruction of segments of rat cerebellar Purkinje cell dendrites receiving ascending and parallel fiber granule cell synaptic inputs

Growing physiological evidence suggests that there are functional differences between synapses made by the ascending and parallel fiber segments of the granule axon on cerebellar Purkinje cells. Supporting this view, our previous electron microscopic studies suggested that these synapses also contacted different regions of the Purkinje cell dendrite, and in particular that ascending segment synapses are made exclusively on the smallest diameter Purkinje cell dendrites. In the current study we used serial electron microscopic techniques to reconstruct Purkinje cell dendritic segments up to almost 10 mum in length. Using a combination of anatomical and immunological labeling techniques we identified the ascending or parallel fiber origins of the excitatory synaptic inputs onto dendritic spines, as well as the location of inhibitory synapses made directly on the dendritic shaft. The results confirmed that there are regions of the Purkinje cell dendrite receiving exclusively ascending or parallel fiber synapses and that ascending segment synapses are only found on small-diameter dendrites. In addition, we describe for the first time small-diameter dendritic regions contacted by both types of excitatory synapses. While our data suggest that the majority of inhibitory inputs to the Purkinje cell tree are associated with parallel fiber synaptic inputs, we also found inhibitory inputs on dendritic regions with mixed ascending and parallel fiber inputs, or exclusively parallel fiber inputs. The finding that ascending and parallel fiber inputs can be segregated on the Purkinje cell dendritic tree provides further evidence that these excitatory granule cell synaptic inputs may be functionally distinct.

Activity-dependent plasticity of developing climbing fiber-Purkinje cell synapses

Elimination of redundant synapses and strengthening of the surviving ones are crucial steps in the development of the nervous system. Both processes can be readily followed at the climbing fiber to Purkinje cell synapse in the cerebellum. Shortly after birth, around five equally strong climbing fiber synapses are established. Subsequently, one of these five synaptic connections starts to grow in size and synaptic strength, while the others degenerate and eventually disappear. Both the elimination of the redundant climbing fiber synapses and the strengthening of the surviving one depend on a combination of a genetically coded blueprint and synaptic activity. Recently, it has been shown that synaptic activity affects the synaptic strength of developing climbing fibers. Remarkably, the same pattern of paired activity of the presynaptic climbing fiber and the postsynaptic Purkinje cell resulted in strengthening of already "large" climbing fibers and weakening of already "weak" climbing fibers. In this review, we will integrate the current knowledge of synaptic plasticity of climbing fibers with that of other processes affecting climbing fiber development.

Projection of reconstructed single Purkinje cell axons in relation to the cortical and nuclear aldolase C compartments of the rat cerebellum

Although the overall topography of the cerebellar corticonuclear projection formed by Purkinje cell (PC) axons has been described, only a few studies have dealt with the organization of this projection at the level of individual PC axons. Thus, we reconstructed 65 single PC axons that were labeled with biotinylated dextran amine in the rat. We then analyzed the relationship between the projections of these PCs and the compartmentalization of the cerebellar cortex and nuclei based on the topography of olivocerebellar projection and aldolase C expression in PCs. After giving rise to short local recurrent collaterals near the soma, a PC axon formed a terminal arbor in a specific small area in the cerebellar nuclei (CN). The terminal arbors of vermal PCs were spread more widely than those of hemispheric PCs and sometimes extended to extracerebellar targets. PCs located in any of the aldolase C-positive (Groups I and II) and -negative (Groups III and IV) stripes consistently projected to the caudoventral and rostrodorsal parts of the CN, respectively, precisely in accordance with the compartmentalization of the cortex and nuclei. Mediolateral segregation and rostrocaudal convergence were seen between projections of separate PCs in a single aldolase C compartment. The results revealed a tight link between the projection patterns of individual PC axons, the topography of the olivocerebellar pathway, and the aldolase C expression pattern. Their overall correspondence seems to reflect a basic aspect of cerebellar organization, although some area-dependent variation in the relationship of these three entities was also present.