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.

Olov Oscarsson's Description of Afferent Pathways to the Cerebellum: Excellent Physiology, Base for Anatomy, and Road Toward Understanding Function

Olov Oscarsson's review on the functional organization of spinocerebellar paths is a prime demonstration of the great skills and huge knowledge base of the electrophysiologists of his era working on communication systems in the brain. Oscarsson describes and characterizes in detail no less than ten different communication lines between the spinal cord and the cerebellum. As such, his work proved to be a highly fertile basis for ongoing physiological and anatomical research. However, even after 50 years of continuing cerebellar research, many questions are still open and even care must be taken that the differentiation in spinocerebellar paths, so carefully demonstrated by Oscarsson, is not lost in present-day research.

Sympathetic components in left and right human cervical vagus nerve: implications for vagus nerve stimulation

Cervical vagus nerve stimulation is in a great variety of clinical situations indicated as a form of treatment. It is textbook knowledge that at the cervical level the vagus nerve contains many different fiber classes. Yet, recently, several reports have shown that this nerve also may contain an additional class of potentially noradrenergic fibers, suggested to denote efferent sympathetic fibers. As such, the nature and presence of these fibers should be considered when choosing a stimulation protocol. We have studied human vagus material extracted from dissection room cadavers in order to further confirm the presence of this class of fibers, to study their origin and direction within the nerve and to determine their distribution and variability between subjects and pairs of left and right nerves of the same individual. Sections were studied with immunohistochemical techniques using antibodies against tyrosine hydroxylase (TH: presumed to indicate noradrenergic fibers), myelin basic protein and neurofilament. Our results show that at least part of the TH-positive fibers derive from the superior cervical ganglion or sympathetic trunk, do not follow a cranial but take a peripheral course through the nerve. The portion of TH-positive fibers is highly variable between individuals but also between the left and right pairs of the same individual. TH-positive fibers can distribute and wander throughout the fascicles but maintain a generally clustered appearance. The fraction of TH-positive fibers generally diminishes in the left cervical vagus nerve when moving in a caudal direction but remains more constant in the right nerve. These results may help to determine optimal stimulation parameters for cervical vagus stimulation in clinical settings.

A disynaptic basal ganglia connection to the inferior olive: potential for basal ganglia influence on cerebellar learning

Recent studies have shown that the cerebellum and the basal ganglia are interconnected at subcortical levels. However, a subcortical basal ganglia connection to the inferior olive (IO), being the source of the olivocerebellar climbing fiber system, is not known. We have used classical tracing with CTb, retrograde transneuronal infection with wildtype rabies virus, conditional tracing with genetically modified rabies virus, and examination of material made available by the Allen Brain Institute, to study potential basal ganglia connections to the inferior olive in rats and mice. We show in both species that parvalbumin-positive, and therefore GABAergic, neurons in the entopeduncular nucleus, representing the rodent equivalent of the internal part of the globus pallidus, innervate a group of cells that surrounds the fasciculus retroflexus and that are collectively known as the area parafascicularis prerubralis. As these neurons supply a direct excitatory input to large parts of the inferior olivary complex, we propose that the entopeduncular nucleus, as a main output station of the basal ganglia, provides an inhibitory influence on olivary excitability. As such, this connection may influence olivary involvement in cerebellar learning and/or could be involved in transmission of reward properties that have recently been established for olivocerebellar signaling.

Input and output organization of the mesodiencephalic junction for cerebro-cerebellar communication

Most studies investigating the impact of the cerebral cortex (CC) onto the cerebellum highlight the role of the pons, which provides the mossy fibers to the cerebellum. However, cerebro‐cerebellar communication may also be mediated by the nuclei of the mesodiencephalic junction (MDJ) that project to the inferior olive (IO), which in turn provides the climbing fibers to the molecular layer. Here, we uncover the precise topographic relations of the inputs and outputs of the MDJ using multiple, classical, and transneuronal tracing methods as well as analyses of mesoscale cortical injections from Allen Mouse Brain. We show that the caudal parts of the CC predominantly project to the principal olive via the rostral MDJ and that the rostral parts of the CC predominantly project to the rostral medial accessory olive via the caudal MDJ. Moreover, using triple viral tracing technology, we show that the cerebellar nuclei directly innervate the neurons in the MDJ that receive input from CC and project to the IO. By unraveling these topographic and prominent, mono‐ and disynaptic projections through the MDJ, this work establishes that cerebro‐cerebellar communication is not only mediated by the pontine mossy fiber system, but also by the climbing fiber system.

Multizonal Cerebellar Influence Over Sensorimotor Areas of the Rat Cerebral Cortex

The cerebral cortex requires cerebellar input for optimizing sensorimotor processing. However, how the sensorimotor cortex uses cerebellar information is far from understood. One critical and unanswered question is how cerebellar functional entities (zones or modules) are connected to distinct parts of the sensorimotor cortices. Here, we utilized retrograde transneuronal infection of rabies virus (RABV) to study the organization of connections from the cerebellar cortex to M1, M2, and S1 of the rat cerebral cortex. RABV was co-injected with cholera toxin β-subunit (CTb) into each of these cortical regions and a survival time of 66-70 h allowed for third-order retrograde RABV infection of Purkinje cells. CTb served to identify the injection site. RABV+ Purkinje cells throughout cerebellar zones were identified by reference to the cerebellar zebrin pattern. All injections, including those into S1, resulted in multiple, zonally arranged, strips of RABV+ Purkinje cells. M1 injections were characterized by input from Purkinje cells in the vermal X-zone, medial paravermis (C1- and Cx-zones), and lateral hemisphere (D2-zone); M2 receives input from D2- and C3-zones; connections to S1 originate from X-, Cx-, C3-, and D2-zones. We hypothesize that individual domains of the sensorimotor cortex require information from a specific combination of cerebellar modules.

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.

The basal interstitial nucleus (BIN) of the cerebellum provides diffuse ascending inhibitory input to the floccular granule cell layer

The basal interstitial nucleus (BIN) in the white matter of the vestibulocerebellum has been defined more than three decades ago, but has since been largely ignored. It is still unclear which neurotransmitters are being used by BIN neurons, how these neurons are connected to the rest of the brain and what their activity patterns look like. Here, we studied BIN neurons in a range of mammals, including macaque, human, rat, mouse, rabbit, and ferret, using tracing, immunohistological and electrophysiological approaches. We show that BIN neurons are GABAergic and glycinergic, that in primates they also express the marker for cholinergic neurons choline acetyl transferase (ChAT), that they project with beaded fibers to the glomeruli in the granular layer of the ipsilateral floccular complex, and that they are driven by excitation from the ipsilateral and contralateral medio-dorsal medullary gigantocellular reticular formation. Systematic analysis of codistribution of the inhibitory synapse marker VIAAT, BIN axons, and Golgi cell marker mGluR2 indicate that BIN axon terminals complement Golgi cell axon terminals in glomeruli, accounting for a considerable proportion ( > 20%) of the inhibitory terminals in the granule cell layer of the floccular complex. Together, these data show that BIN neurons represent a novel and relevant inhibitory input to the part of the vestibulocerebellum that controls compensatory and smooth pursuit eye movements.

Correction to: Cerebellar Modules and Their Role as Operational Cerebellar Processing Units: A Consensus paper

In the original version of this paper, the Title should have been written with "A Consensus paper" to read "Cerebellar Modules and Their Role as Operational Cerebellar Processing Units: A Consensus paper".

Comparative pathogenesis of rabies in bats and carnivores, and implications for spillover to humans

Bat-acquired rabies is becoming increasingly common, and its diagnosis could be missed partly because its clinical presentation differs from that of dog-acquired rabies. We reviewed the scientific literature to compare the pathogenesis of rabies in bats and carnivores-including dogs-and related this pathogenesis to differences in the clinical presentation of bat-acquired and dog-acquired rabies in human beings. For bat-acquired rabies, we found that the histological site of exposure is usually limited to the skin, the anatomical site of exposure is more commonly the face, and the virus might be more adapted for entry via the skin than for dog-acquired rabies. These factors could help to explain several differences in clinical presentation between individuals with bat-acquired and those with dog-acquired rabies. A better understanding of these differences should improve the recording of a patient's history, enable drawing up of a more sophisticated list of clinical characteristics, and therefore obtain an earlier diagnosis of rabies after contact with a bat or carnivore that has rabies.

Disynaptic Subthalamic Input to the Posterior Cerebellum in Rat

In the last decade, the interplay between basal ganglia and cerebellar functions has been increasingly advocated to explain their joint operation in both normal and pathological conditions. Yet, insight into the neuroanatomical basis of this interplay between both subcortical structures remains sparse and is mainly derived from work in primates. Here, in rodents, we have studied the existence of a potential disynaptic connection between the subthalamic nucleus (STN) and the cerebellar cortex as has been demonstrated earlier for the primate. A mixture of unmodified rabies virus (RABV: CVS 11) and cholera toxin B-subunit (CTb) was injected at places in the posterior cerebellar cortex of nine rats. The survival time was chosen to allow for disynaptic retrograde transneuronal infection of RABV. We examined the STN for neurons infected with RABV in all nine cases and related the results with the location of the RABV/CTb injection site, which ranged from the vermis of lobule VII, to the paravermis and hemispheres of the paramedian lobule and crus 2a. We found that cases with injection sites in the vermis of lobule VII showed prominent RABV labeling in the STN. In contrast, almost no subthalamic labeling was noted in cases with paravermal or hemispheral injection sites. We show circumstantial evidence that not only the pontine nuclei but also the pedunculotegmental nucleus may act as the intermediary in the connection from STN to cerebellar cortex. This finding implies that in the rat the STN links disynaptically to the vermal part of lobule VII of the cerebellar cortex, without any major involvement of the cerebellar areas that are linked to sensorimotor functions. As vermal lobule VII recently has been shown to process disynaptic input from the retrosplenial and orbitofrontal cortices, we hypothesize that in the rat the subthalamic input to cerebellar function might be used to influence more prominently non-motor functions of the cerebellum than motor functions. This latter aspect seems to contradict the primate results and could point to a more elaborate interaction between basal ganglia and cerebellum in more demanding motor tasks.

Visuo-vestibular information processing by unipolar brush cells in the rabbit flocculus

The unipolar brush cell (UBC) is a glutamatergic granular layer interneuron that is predominantly located in the vestibulocerebellum and parts of the vermis. In rat and rabbit, we previously found using juxtacellular labeling combined with spontaneous activity recording that cells with highly regular spontaneous activity belong to the UBC category. Making use of this signature, we recorded from floccular UBCs in both anesthetized and awake rabbits while delivering visuo-vestibular stimulation by using sigmoidal rotation of the whole animal. In the anesthetized rabbit, the activity of the presumed UBC units displayed a wide variety of modulation profiles that could be related to aspects of head velocity or acceleration. These modulation profiles could also be found in the awake rabbit where, in addition, they could also carry an eye position signal. Furthermore, units in the awake rabbit could demonstrate rather long response latencies of up to 0.5 s. We suggest that the UBCs recorded in this study mostly belong to the type I UBC category (calretinin-positive) and that they can play diverse roles in floccular visuo-vestibular information processing, such as transformation of velocity-related signals to acceleration-related signals.

Caveats in Transneuronal Tracing with Unmodified Rabies Virus: An Evaluation of Aberrant Results Using a Nearly Perfect Tracing Technique

Apart from the genetically engineered, modified, strains of rabies virus (RABV), unmodified ‘fixed’ virus strains of RABV, such as the ‘French’ subtype of CVS11, are used to examine synaptically connected networks in the brain. This technique has been shown to have all the prerequisite characteristics for ideal tracing as it does not metabolically affect infected neurons within the time span of the experiment, it is transferred transneuronally in one direction only and to all types of neurons presynaptic to the infected neuron, number of transneuronal steps can be precisely controlled by survival time and it is easily detectable with a sensitive technique. Here, using the ‘French’ CVS 11 subtype of RABV in Wistar rats, we show that some of these characteristics may not be as perfect as previously indicated. Using injection of RABV in hind limb muscles, we show that RABV-infected spinal motoneurons may already show lysis 1 or 2 days after infection. Using longer survival times we were able to establish that Purkinje cells may succumb approximately 3 days after infection. In addition, some neurons seem to resist infection, as we noted that the number of RABV-infected inferior olivary neurons did not progress in the same rate as other infected neurons. Furthermore, in our hands, we noted that infection of Purkinje cells did not result in expected transneuronal labeling of cell types that are presynaptic to Purkinje cells such as molecular layer interneurons and granule cells. However, these cell types were readily infected when RABV was injected directly in the cerebellar cortex. Conversely, neurons in the cerebellar nuclei that project to the inferior olive did not take up RABV when this was injected in the inferior olive, whereas these cells could be infected with RABV via a transneuronal route. These results suggest that viral entry from the extracellular space depends on other factors or mechanisms than those used for retrograde transneuronal transfer. We conclude that transneuronal tracing with RABV may result in unexpected results, as not all properties of RABV seem to be ubiquitously valid.

Excitatory Cerebellar Nucleocortical Circuit Provides Internal Amplification during Associative Conditioning

Closed-loop circuitries between cortical and subcortical regions can facilitate precision of output patterns, but the role of such networks in the cerebellum remains to be elucidated. Here, we characterize the role of internal feedback from the cerebellar nuclei to the cerebellar cortex in classical eyeblink conditioning. We find that excitatory output neurons in the interposed nucleus provide efference-copy signals via mossy fibers to the cerebellar cortical zones that belong to the same module, triggering monosynaptic responses in granule and Golgi cells and indirectly inhibiting Purkinje cells. Upon conditioning, the local density of nucleocortical mossy fiber terminals significantly increases. Optogenetic activation and inhibition of nucleocortical fibers in conditioned animals increases and decreases the amplitude of learned eyeblink responses, respectively. Our data show that the excitatory nucleocortical closed-loop circuitry of the cerebellum relays a corollary discharge of premotor signals and suggests an amplifying role of this circuitry in controlling associative motor learning.

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Cerebellar nuclei provide modular corollary discharge to the cerebellar cortex

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Nucleocortical afferents have unique molecular and ultrastructural features

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Eyeblink conditioning induces structural plasticity of nucleocortical mossy fibers

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Nucleocortical afferents amplify the amplitude of conditioned eyeblink responses

Long-term follow-up of peptidergic and nonpeptidergic reinnervation of the epidermis following sciatic nerve reconstruction in rats

OBJECT

Peripheral nerve injuries are a commonly encountered clinical problem and often result in long-term functional deficits. The current gold standard for transected nerves is an end-to-end reconstruction, which results in the intermittent appearance of neuropathic pain.

METHODS

To improve our understanding of the relation between this type of reconstruction and neuropathic pain, the authors transected and immediately end-to-end reconstructed the sciatic nerve in rats. The effect of this procedure on neuropathic pain, as measured by thermal and mechanical hypersensitivity at 4 different time points (5, 10, 20, and 30 weeks), was related to the density of peptidergic and nonpeptidergic fiber innervation in the glabrous skin of rats' hind paws.

RESULTS

Thermal hypersensitivity occurring 20 weeks after reconstruction was accompanied by a significant increase in peptidergic epidermal fibers. However, the lesion-induced reduction in the density of nonpeptidergic epidermal fibers remained decreased at all experimental time points. Moreover, temporal collateral sprouting by undamaged saphenous nerve was visualized using the recently revised Evans blue extravasation technique. Strikingly, as the sciatic nerve repopulated rats' hind paw, the saphenous nerve withdrew to its original territory.

CONCLUSIONS

The authors conclude that the transient thermal hypersensitivity is related to increased density of epidermal peptidergic fibers, which mainly originate from regenerating fibers. Furthermore, a changed composition in the peptidergic and nonpeptidergic epidermal fibers is demonstrated following end-to-end reconstruction of the sciatic nerve.

Identifying Purkinje cells using only their spontaneous simple spike activity

BACKGROUND

We have extended our cerebellar cortical interneuron classification algorithm that uses statistics of spontaneous activity (Ruigrok et al., 2011) to include Purkinje cells. Purkinje cells were added because they do not always show a detectable complex spike, which is the accepted identification. The statistical measures used in the present study were obtained from morphologically identified interneurons and complex spike identified Purkinje cells, recorded from ketamine-xylazine anesthetized rats and rabbits, and from awake rabbits.

NEW METHOD

The new algorithm has an added decision step that classifies Purkinje cells using a combination of the median absolute difference from the median interspike interval (MAD) and the mean of the relative differences of successive interspike intervals (CV2). These measures reflect the high firing rate and intermediate regularity of Purkinje cell simple spike activity.

RESULTS

Of 86 juxtacellularly labeled interneurons and 110 complex spike-identified Purkinje cells, 61 interneurons and 95 Purkinje cells were correctly classified, 22 interneurons and 13 Purkinje cells were deemed unclassifiable, and 3 interneurons and 2 Purkinje cells were incorrectly classified.

COMPARISON WITH EXISTING METHODS

The new algorithm improves on our previous algorithm because it includes Purkinje cells. This algorithm is the only one for the cerebellum that does not presume anatomical knowledge of whether the cells are in the molecular layer or the granular layer.

CONCLUSIONS

These results strengthen the view that the new decision algorithm is useful for identifying neurons recorded at all cerebellar depths, particularly those neurons recorded in the rabbit vestibulocerebellum.

Cerebellar modules operate at different frequencies

Due to the uniform cyto-architecture of the cerebellar cortex, its overall physiological characteristics have traditionally been considered to be homogeneous. In this study, we show in awake mice at rest that spiking activity of Purkinje cells, the sole output cells of the cerebellar cortex, differs between cerebellar modules and correlates with their expression of the glycolytic enzyme aldolase C or zebrin. Simple spike and complex spike frequencies were significantly higher in Purkinje cells located in zebrin-negative than zebrin-positive modules. The difference in simple spike frequency persisted when the synaptic input to, but not intrinsic activity of, Purkinje cells was manipulated. Blocking TRPC3, the effector channel of a cascade of proteins that have zebrin-like distribution patterns, attenuated the simple spike frequency difference. Our results indicate that zebrin-discriminated cerebellar modules operate at different frequencies, which depend on activation of TRPC3, and that this property is relevant for all cerebellar functions.

DOI:http://dx.doi.org/10.7554/eLife.02536.001

The cerebellum, located at the back of the brain underneath the cerebral hemispheres, is best known for its role in the control of movement. Despite its small size, the cerebellum contains more than half of the brain's neurons. These are organized in a repeating pattern in which cells called Purkinje cells receive inputs from two types of fibers: climbing fibers, which ascend into the cerebellum from the brainstem; and parallel fibers, which run perpendicular to the climbing fibers. This gives rise to a characteristic ‘crystalline’ structure.

As a result of this uniform circuitry, it was widely believed was that all Purkinje cells throughout the cerebellum would function the same way. However, the presence of distinct patterns of gene expression in different regions suggests that this is not the case. Molecules called zebrins, for example, are found in some Purkinje cells but not others, and this gives rise to a pattern of zebrin-positive and zebrin-negative stripes. A number of other molecules have similar distributions, suggesting that these differences in molecular machinery could underlie differences in cellular physiology.

Zhou, Lin et al. have now provided one of the first direct demonstrations of such physiological differences by showing that zebrin-positive cells generate action potentials at lower frequencies than zebrin-negative cells. This pattern is seen throughout the cerebellum, and is evident even when the positive and negative cells are neighbors, which indicates that these differences do not simply reflect differences in the locations of the cells or differences in the inputs they receive from parallel fibers. Additional experiments revealed that the distinct firing rates are likely not generated by zebrin itself, but rather by proteins that are expressed alongside zebrin, most notably those that work through an ion channel called TRPC3.

By showing that cells arranged in the same type of circuit can nevertheless have distinct firing rates, the work of Zhou, Lin et al. has revealed an additional level of complexity in the physiology of the cerebellum. In addition to improving our understanding of how the brain controls movement, these findings might also be of interest to researchers studying the increasing number of neurological and psychiatric disorders in which cerebellar dysfunction has been implicated.

DOI:http://dx.doi.org/10.7554/eLife.02536.002

Collateralization of cerebellar output to functionally distinct brainstem areas. A retrograde, non-fluorescent tracing study in the rat

The organization of the cerebellum is characterized by a number of longitudinally organized connection patterns that consist of matching olivo-cortico-nuclear zones. These entities, referred to as modules, have been suggested to act as functional units. The various parts of the cerebellar nuclei (CN) constitute the output of these modules. We have studied to what extent divergent and convergent patterns in the output of the modules to four, functionally distinct brain areas can be recognized. Two retrograde tracers were injected in various combinations of the following nuclei: the red nucleus (RN), as a main premotor nucleus; the prerubral area, as a main supplier of afferents to the inferior olive (IO); the nucleus reticularis tegmenti pontis (NRTP), as a main source of cerebellar mossy fibers; and the IO, as the source of climbing fibers. For all six potential combinations three cases were examined. All nine cases with combinations that involved the IO did not, or hardly, resulted in double labeled neurons. In contrast, all other combinations resulted in at least 10% and up to 67% of double labeled neurons in cerebellar nuclear areas where both tracers were found. These results show that the cerebellar nuclear neurons that terminate within the studied areas represent basically two intermingled populations of projection cells. One population corresponds to the small nucleo-olivary neurons whereas the other consists of medium- to large-sized neurons which are likely to distribute their axons to several other areas. Despite some consistent differences between the output patterns of individual modules we propose that modular cerebellar output to premotor areas such as the RN provides simultaneous feedback to both the mossy fiber and the climbing fiber system and acts in concert with a designated GABAergic nucleo-olivary circuit. These features seem to form a basic characteristic of cerebellar operation.

Thermo-sensitive TRP channels in peripheral nerve injury: a review of their role in cold intolerance

One of the sensory complications of traumatic peripheral nerve injury is thermal intolerance, which manifests in humans mainly as cold intolerance. It has a major effect on the quality of life, and adequate therapy is not yet available. In order to better understand the pathophysiological background of thermal intolerance, we focus first on the various transient receptor potential (TRP) channels that are involved in temperature sensation, including their presence in peripheral nerves and in keratinocytes. Second, the role of thermo-sensitive TRP channels in cold and heat intolerance is described showing three different mechanisms that contribute to thermal intolerance in the skin: (a) an increased expression of TRP channels on nerve fibres and on keratinocytes, (b) a lower activation threshold of TRP channels and (c) the sprouting of non-injured nerve fibres. Finally, the data that are available on the effects of TRP channel agonists and antagonists and their clinical use are discussed. In conclusion, TRP channels play a major role in temperature sensation and in cold and heat intolerance. Unfortunately, the available pharmaceutical agents that successfully target TRP channels and counteract thermal intolerance are still very limited. Yet, our focus should remain on TRP channels since it is difficult to imagine a reliable treatment for thermal intolerance that will not involve TRP channels.

Anatomical investigation of potential contacts between climbing fibers and cerebellar Golgi cells in the mouse

Climbing fibers (CFs) originating in the inferior olive (IO) constitute one of the main inputs to the cerebellum. In the mammalian cerebellar cortex each of them climbs into the dendritic tree of up to 10 Purkinje cells (PCs) where they make hundreds of synaptic contacts and elicit the so-called all-or-none complex spikes controlling the output. While it has been proven that CFs contact molecular layer interneurons (MLIs) via spillover mechanisms, it remains to be elucidated to what extent CFs contact the main type of interneuron in the granular layer, i.e., the Golgi cells (GoCs). This issue is particularly relevant, because direct contacts would imply that CFs can also control computations at the input stage of the cerebellar cortical network. Here, we performed a systematic morphological investigation of labeled CFs and GoCs at the light microscopic level following their path and localization through the neuropil in both the granular and molecular layer. Whereas in the molecular layer the appositions of CFs to PCs and MLIs were prominent and numerous, those to cell-bodies and dendrites of GoCs in both the granular layer and molecular layer were virtually absent. Our results argue against the functional significance of direct synaptic contacts between CFs and interneurons at the input stage, but support those at the output stage.

Spatiotemporal dynamics of re-innervation and hyperinnervation patterns by uninjured CGRP fibers in the rat foot sole epidermis after nerve injury

The epidermis is innervated by fine nerve endings that are important in mediating nociceptive stimuli. However, their precise role in neuropathic pain is still controversial. Here, we have studied the role of epidermal peptidergic nociceptive fibers that are located adjacent to injured fibers in a rat model of neuropathic pain. Using the Spared Nerve Injury (SNI) model, which involves complete transections of the tibial and common peroneal nerve while sparing the sural and saphenous branches, mechanical hypersensitivity was induced of the uninjured lateral (sural) and medial (saphenous) area of the foot sole. At different time points, a complete foot sole biopsy was taken from the injured paw and processed for Calcitonin Gene-Related Peptide (CGRP) immunohistochemistry. Subsequently, a novel 2D-reconstruction model depicting the density of CGRP fibers was made to evaluate the course of denervation and re-innervation by uninjured CGRP fibers. The results show an increased density of uninjured CGRP-IR epidermal fibers on the lateral and medial side after a SNI procedure at 5 and 10 weeks. Furthermore, although in control animals the density of epidermal CGRP-IR fibers in the footpads was lower compared to the surrounding skin of the foot, 10 weeks after the SNI procedure, the initially denervated footpads displayed a hyper-innervation. These data support the idea that uninjured fibers may play a considerable role in development and maintenance of neuropathic pain and that it is important to take larger biopsies to test the relationship between innervation of injured and uninjured nerve areas.

Organization of cerebral projections to identified cerebellar zones in the posterior cerebellum of the rat

The cerebrocerebellar connection makes use of two of the largest fiber tracts in the mammalian brain, i.e., the cerebral and medial cerebellar peduncles. Neuroanatomical approaches aimed to elucidate the organization of this important connection have been hindered by its multisynaptic nature, the complex organization of its components, and the dependency of conventional tracers on precisely placed injections. To overcome these problems, we used rabies virus (RV) as a retrograde transneuronal tracer. RV was injected simultaneously with cholera toxin β subunit (CTb) into selected areas of the cerebellar cortex of 18 male Wistar rats. A survival time of 48-50 h resulted in first- and second-order labeling of RV in combination with first-order labeling of CTb. The distribution of CTb-labeled neurons in the inferior olive established the zonal identity of the injection site. In this way, it was possible to examine the cortical distribution of neurons from which disynaptic cerebrocerebellar projections to specific cerebellar loci originate. The results show that this distribution covaries with the identity of the injected cerebellar lobule. More subtle changes were present when different zones of the same lobule were injected. The C1 zone of lobule VIII receives a more prominent projection from the somatosensory cortex compared with the C2/D zones. The laterally positioned D zones receive information from more rostral regions of the cerebral cortex. The vermis of lobule VII receives a prominent input from the retrosplenial and orbitofrontal cortices. Different injection sites also result in differences in laterality of the connections.

Ins and outs of cerebellar modules

The modular concept of cerebellar connections has been advocated in the lifetime work of Jan Voogd. In this concept, a cerebellar module is defined as the conglomerate of one or multiple and non-adjacent, parasagittally arranged zones of Purkinje cells, their specific projection to a well-defined region of the cerebellar nuclei, and the climbing fiber input to these zones by a well-defined region of the inferior olivary complex. The modular organization of these olivo-cortico-nuclear connections is further exemplified by matching reciprocal connections between inferior olive and cerebellar nuclei. Because the different regions of the cerebellar nuclei show highly specific output patterns, cerebellar modules have been suggested to constitute functional entities. This idea is strengthened by the observation that anatomically defined modules adhere to the distribution of chemical markers in the cerebellar cortex suggesting that modules not only differ in their input and output relations but also may differ in operational capabilities. Here, I will briefly review some recent data on the establishment of cerebellar modules in rats. Furthermore, some evidence will be shown suggesting that the other main afferent system (i.e., mossy fibers), at least to some extent, also adheres to the modular organization. Finally, using retrograde transneuronal tracing with rabies virus, some evidence will be provided that several cerebellar modules may be involved in the control of individual muscles.

Encoding of whisker input by cerebellar Purkinje cells

The cerebellar cortex is crucial for sensorimotor integration. Sensorimotor inputs converge on cerebellar Purkinje cells via two afferent pathways: the climbing fibre pathway triggering complex spikes, and the mossy fibre–parallel fibre pathway, modulating the simple spike activities of Purkinje cells. We used, for the first time, the mouse whisker system as a model system to study the encoding of somatosensory input by Purkinje cells.We show that most Purkinje cells in ipsilateral crus 1 and crus 2 of awake mice respond to whisker stimulation with complex spike and/or simple spike responses. Single-whisker stimulation in anaesthetised mice revealed that the receptive fields of complex spike and simple spike responses were strikingly different. Complex spike responses, which proved to be sensitive to the amplitude, speed and direction of whisker movement, were evoked by only one or a few whiskers. Simple spike responses, which were not affected by the direction of movement, could be evoked by many individual whiskers. The receptive fields of Purkinje cells were largely intermingled, and we suggest that this facilitates the rapid integration of sensory inputs from different sources. Furthermore, we describe that individual Purkinje cells, at least under anaesthesia, may be bound in two functional ensembles based on the receptive fields and the synchrony of the complex spike and simple spike responses. The ‘complex spike ensembles’ were oriented in the sagittal plane, following the anatomical organization of the climbing fibres, while the ‘simple spike ensembles’ were oriented in the transversal plane, as are the beams of parallel fibres.

A light microscope-based double retrograde tracer strategy to chart central neuronal connections

This protocol describes a double retrograde tracing method to chart divergent projections in the CNS using light microscope techniques. It is based on immunohistochemical visualization of retrograde transport of cholera toxin b-subunit (CTb) and silver enhancement of a gold-lectin conjugate. Production of the gold-lectin is explained in detail, and a technique is offered to record through the injection pipettes, to help guide accurate placement of injections. Visualization of the two tracers results in light brown staining of CTb-labeled neurons and labeling by black particles of gold-lectin-containing neurons. Both types of label are easily recognized in the same neuron. The labeling is permanent and is well suited for studies in which large areas of the brain need to be surveyed. The whole procedure (excluding survival time) takes approximately 5-7 d to complete.

A fluorescence-based double retrograde tracer strategy for charting central neuronal connections

Microspheres (beads) tagged with different fluorescent markers can be used for double retrograde axonal tracing of CNS connections. They have several advantages over other double tracer techniques, including ease-of-use, high transport efficiency, distinctive cell labeling and the ability to produce well-defined injection sites. In this protocol we describe the basic procedure for their use, some common problems and how these can be overcome. The protocol, including animal surgery, preparation and delivery of tracer can be completed in approximately 0.5 d. Subsequent histological processing (excluding survival time) can be completed in 0.5-1 d.

Precise spatial relationships between mossy fibers and climbing fibers in rat cerebellar cortical zones

Classically, mossy fiber and climbing fiber terminals are regarded as having very different spatial distributions in the cerebellar cortex. However, previous anatomical studies have not studied these two major cerebellar inputs with sufficient resolution to confirm this assumption. Here, we examine the detailed pattern of collateralization of both types of cerebellar afferent using small injections of the bidirectional tracer cholera toxin b subunit into the posterior cerebellum. The cortical and zonal location of these injections was characterized by mapping climbing fiber field potentials, the distribution of retrogradely labeled olivary neurons, and the intrinsic zebrin pattern of Purkinje cells. Labeled climbing fiber collaterals were distributed as longitudinal strips and were always accompanied by clusters of labeled mossy fiber rosettes in the subjacent granular layer. Two- and three-dimensional reconstructions and quantitative analysis showed that mossy fibers also collateralized to other stripe-like regions usually below Purkinje cells with the same zebrin-positive or zebrin-negative characteristics as that of the injection site and associated climbing fiber collaterals. The distribution of retrogradely labeled neurons in two major sources of mossy fibers, the lateral reticular and basilar pontine nuclei, revealed interlobular and some interzonal differences. These data indicate that nonadjacent cerebellar zones, sharing the same climbing fiber input and zebrin identity, also share a common mossy fiber input. Other cerebellar cortical regions that receive collaterals from the same mossy fibers usually also have the same zebrin signature. Together with the distribution of neurons in precerebellar centers, the findings suggest a revision of the modular hypothesis for information processing in the cerebellar cortex.

Zonal organization of the mouse flocculus: physiology, input, and output

The zones of the flocculus have been mapped in many species with a noticeable exception, the mouse. Here, the functional map of the mouse was constructed via extracellular recordings followed by tracer injections of biotinylated-dextran-amine and immunohistochemistry for heat-shock protein-25. Zones were identified based on the Purkinje cell complex spike modulation occurring in response to optokinetic stimulation. In zones 1 and 3 Purkinje cells responded best to rotation about a horizontal axis oriented at 135 degrees ipsilateral azimuth, whereas in zones 2 and 4 they responded best to rotation about the vertical axis. The tracing experiments showed that Purkinje cells of zone 1 projected to the parvicellular part of lateral cerebellar nucleus and superior vestibular nucleus, while Purkinje cells of zone 3 projected to group Y and the superior vestibular nucleus. Purkinje cells of zones 2 and 4 projected to the magnocellular and parvicellular parts of the medial vestibular nucleus, while some also innervated the lateral vestibular nucleus or nucleus prepositus hypoglossi. The climbing fiber inputs to Purkinje cells in zones 1 and 3 were derived from neurons in the ventrolateral outgrowth of the contralateral inferior olive, whereas those in zones 2 and 4 were derived from the contralateral caudal dorsal cap. Purkinje cells in zones 1 and 2, but not in zones 3 and 4, were positively labeled for heat-shock protein-25. The present study illustrates that Purkinje cells in the murine flocculus are organized in discrete zones with specific functions, specific input - output relations, and a specific histochemical signature.

Organization of pontocerebellar projections to identified climbing fiber zones in the rat

The organization of pontocerebellar projections to the paravermis and hemisphere of the posterior cerebellum of the rat was studied in relation to the organization of climbing fibers. Small injections of cholera toxin subunit B were placed in the cerebellar cortex at locations predetermined by evoked climbing fiber potentials from selected body parts or based on coordinates. The injection site was characterized with respect to the zebrin pattern and by the distribution of retrogradely labeled neurons in the inferior olive. The following zones were studied: hindlimb-related zones C1 and C2 of lobule VIII; forelimb-related zones C1, C2, and D0/D1 of the paramedian lobule; and face-related zones A2 of the paramedian lobule and C2 and D0 of crus 2B. The results show that the distribution of pontine neurons is closely related to the climbing fiber somatotopy. Injections centered on face-related zones result in distribution of pontine neurons within the pontine core region. Forelimb regions surround this core, whereas hindlimb regions are mostly supplied by caudal pontine regions and by a single patch of more rostrally located neurons. This distribution fits well with published data on the somatotopy of the corticopontine projection from the rat primary somatosensory cortex. However, apart from differences in the participation of ipsilaterally projecting cells, the distribution of pontine neurons does not change significantly when the injection covers different zones of the same lobule such as C1 and C2 of lobule VIII; C1, C2, and D0/D1 of the paramedian lobule; A2 of the paramedian lobule; and C2 and D0 of crus 2B.

Deletion of FMR1 in Purkinje Cells Enhances Parallel Fiber LTD, Enlarges Spines, and Attenuates Cerebellar Eyelid Conditioning in Fragile X Syndrome

Absence of functional FMRP causes Fragile X syndrome. Abnormalities in synaptic processes in the cerebral cortex and hippocampus contribute to cognitive deficits in Fragile X patients. So far, the potential roles of cerebellar deficits have not been investigated. Here, we demonstrate that both global and Purkinje cell-specific knockouts of Fmr1 show deficits in classical delay eye-blink conditioning in that the percentage of conditioned responses as well as their peak amplitude and peak velocity are reduced. Purkinje cells of these mice show elongated spines and enhanced LTD induction at the parallel fiber synapses that innervate these spines. Moreover, Fragile X patients display the same cerebellar deficits in eye-blink conditioning as the mutant mice. These data indicate that a lack of FMRP leads to cerebellar deficits at both the cellular and behavioral levels and raise the possibility that cerebellar dysfunctions can contribute to motor learning deficits in Fragile X patients.

Modulation of cutaneous reflexes in hindlimb muscles during locomotion in the freely walking rat: A model for studying cerebellar involvement in the adaptive control of reflexes during rhythmic movements

This study aims to demonstrate stepphase-dependent modulation in the gain of cutaneously triggered reflexes in the freely locomoting rat. Electromyographic recordings of biceps femoris (mainly involved in knee flexion) and gastrocnemius (mainly involved in ankle extension) muscles were continuously monitored during locomotion and cutaneous reflexes were induced by subcutaneously placed stimulation electrodes in the lateral malleolal region. The results show that the reflex responses in both muscles during locomotion were generally reduced compared to reflexes induces in rest. For the biceps femoris reduction of reflex gain was highest during the stance phase whereas for the gastrocnemius the period of highest depression was found during the swing phase. We conclude that stepphase-dependent modulation of peripheral reflexes can be measured in freely locomoting rats and generally concur with previous studies in cat and man that this type of modulation may be functionally important for maintaining and adjusting gait. Moreover, although the mechanism of inducing and maintaining this modulation is not fully known, it is now open to experimental investigation in rodents.

Between in and out: linking morphology and physiology of cerebellar cortical interneurons

We used the juxtacellular recording and labeling technique of Pinault (1996) in the uvula/nodulus of the ketamine anesthetized rat in an attempt to link different patterns of spontaneous activity with different types of morphologically identified cerebellar cortical interneurons. Cells displaying a somewhat irregular, syncopated cadence of spontaneous activity averaging 4-10 Hz could, upon successful entrainment and visualization, be morphologically identified as Golgi cells. Spontaneously firing cells with a highly or fairly regular firing rate of 10-35 Hz turned out to be unipolar brush cells. We also found indications that other types of cerebellar cortical neurons might also be distinguished on the basis of the characteristics of their spontaneous firing. Comparison of the interspike interval histograms of spontaneous activity obtained in the anaesthetized rat with those obtained in the awake rabbit points to a way whereby the behaviorally related modulation of specific types of interneurons can be studied. In particular, the spontaneous activity signatures of Golgi cells and unipolar brush cells anatomically identified in the uvula/nodulus of the anaesthetized rat are remarkably similar to the spontaneous activity patterns of some units we have recorded in the flocculus of the awake rabbit. The spontaneous activity patterns of at least some types of cerebellar interneurons clearly have the potential to serve as identifying signatures in behaving animals.

Topography of olivo-cortico-nuclear modules in the intermediate cerebellum of the rat

This study provides a detailed anatomical description of the relation between olivo-cortico-nuclear modules of the intermediate cerebellum of the rat and the intrinsic zebrin pattern of the Purkinje cells. Strips of climbing fibers were labeled using small injections of biotinylated dextran amine into either the medial or dorsal accessory olives, while, in some cases, simultaneous retrograde labeling of Purkinje cells was obtained using gold-lectin injections into selected parts of the interposed nuclei. Our data are represented in a new, highly detailed, cortical surface reconstruction of the zebrin pattern and in relation to the collateral labeling of the climbing fibers to the cerebellar nuclei. We show that the somatotopic regions of the dorsal accessory olive behave differently in their projections to essentially zebrin-negative regions that represent the C1 and C3 zones of the anterior and posterior parts of the cortex. The rostral part of the medial accessory olive projects to zebrin-positive areas, in particular to the P4+ band of the anterior lobe and lobule VI and to the P5+ band of the posterior lobe, indicating that C2 has two noncontiguous representations in the SL and crus 1. By relating the areas of overlap that resulted from the injections in the accessory olives, i.e., labeling of climbing fiber strips and patches of climbing fiber nuclear collaterals, with the results from the injections in the interposed nuclei, i.e., retrograde labeling of Purkinje cells and of inferior olivary neurons, direct verification of the concept of modular cerebellar connections was obtained.

The organization of the corticonuclear and olivocerebellar climbing fiber projections to the rat cerebellar vermis: the congruence of projection zones and the zebrin pattern

The zonal organization of the corticonuclear and the olivocerebellar climbing fiber projections to the vermis of the cerebellum of the rat was compared to the pattern of zebrin-positive and zebrin-negative bands in material double-stained for zebrin II and for different anterograde tracers injected in subnuclei of the inferior olive, or retrograde tracers injected in the cerebellar and vestibular target nuclei of the Purkinje cells of the vermis. Projection zones A(1), A(X), X, B, C(X) in the vermis and A(2) (accessory A zone) and C(2) in the hemisphere were defined by their efferent corticonuclear and their afferent climbing fiber connections, and were found to share the same topographical framework with the zebrin pattern.

Collateralization of climbing and mossy fibers projecting to the nodulus and flocculus of the rat cerebellum

Collateralization of mossy and climbing fibers was investigated using cortical injections of cholera toxin b-subunit in the rat vestibulocerebellum. Injections were characterized by their retrograde labeling within the inferior olive. Collateral labeling was plotted using color-coded density profiles of the whole cerebellar cortex. Injections in the medial part of the nodulus resulted in olivary labeling that was restricted to the rostral part of the dorsal cap. Climbing fiber collaterals were found in medial and lateral nodular zones as well as in the ventral paraflocculus and adjacent flocculus. Injections in the intermediate part of the nodulus resulted in olivary labeling of the beta-subnucleus but could also involve the ventrolateral outgrowth. In the latter case, climbing fiber collaterals were found in the two floccular zones and in a small region in the lateral-most part of crus I. All nodular injections showed a bilaterally symmetric distribution of collateral mossy fiber rosettes that was mostly confined to the vestibulocerebellum and originated predominantly from the vestibular nuclei. Injections in the flocculus labeled the caudal part of the dorsal cap and/or the ventrolateral outgrowth. Mossy fiber rosettes were observed throughout the vestibulocerebellum but also included other regions of the cerebellar cortex in a bilaterally symmetric pattern corresponding with a more widespread precerebellar origin. Climbing fibers originating in the rostral dorsal cap, labeled from an injection in the ventral paraflocculus, collateralize to a medial and lateral zone in the nodulus. Climbing fiber collaterals were usually accompanied by subjacent labeling of mossy fiber rosettes. These results demonstrate that some nodular and floccular zones are related and, at least partially, share a common input.

Cerebellar LTD and learning-dependent timing of conditioned eyelid responses

Mammals can be trained to make a conditioned movement at a precise time, which is correlated to the interval between the conditioned stimulus and unconditioned stimulus during the learning. This learning-dependent timing has been shown to depend on an intact cerebellar cortex, but which cellular process is responsible for this form of learning remains to be demonstrated. Here, we show that protein kinase C-dependent long-term depression in Purkinje cells is necessary for learning-dependent timing of Pavlovian-conditioned eyeblink responses.

The distribution of climbing and mossy fiber collateral branches from the copula pyramidis and the paramedian lobule: congruence of climbing fiber cortical zones and the pattern of zebrin banding within the rat cerebellum

Individual cerebellar cortical zones defined by the somatotopy of climbing fiber responses and by their olivo-cortico-nuclear connections located in the paramedian lobule and the copula pyramidis of the rat cerebellum were microinjected with cholera toxin B subunit. Collateral branches of climbing and mossy fibers were mapped and related to the pattern of zebrin-positive and -negative bands of Purkinje cells. Climbing fiber collaterals from the copula distribute to the anterior lobe: from the paramedian lobule mainly to lobulus simplex and rostral crus I. Climbing fibers terminating in particular zones (X, A2, C1, CX, C2, C3, D1, and D2) in the paramedian lobule or the copula collateralize to one or two corresponding zones in lobulus simplex, crus I and II, the paraflocculus, and/or the anterior lobe. These zones can be defined by their relationship to the pattern of zebrin banding. Collaterals from mossy fibers, labeled from the same injection sites in the copula and paramedian lobule, often distribute bilaterally in a symmetrical pattern of multiple but ill-defined longitudinal strips in the anterior lobe and/or lobulus simplex. One or more of these longitudinal aggregates of mossy fiber collaterals was always found subjacent to the strip(s) of labeled climbing fiber collaterals arising from the same locus in the paramedian lobule or the copula. Corticonuclear projections focused on the target nucleus of each zone, although a bilateral plexus of thinner axons, presumably of mossy fiber collateral origin, was sometimes also present in several other regions of the cerebellar nuclei. Overall, these results suggest that climbing fiber zones and zebrin banding reflect a common organizational scheme within the cerebellar cortex.

Deformation of network connectivity in the inferior olive of connexin 36-deficient mice is compensated by morphological and electrophysiological changes at the single neuron level

Compensatory mechanisms after genetic manipulations have been documented extensively for the nervous system. In many cases, these mechanisms involve genetic regulation at the transcription or expression level of existing isoforms. We report a novel mechanism by which single neurons compensate for changes in network connectivity by retuning their intrinsic electrical properties. We demonstrate this mechanism in the inferior olive, in which widespread electrical coupling is mediated by abundant gap junctions formed by connexin 36 (Cx36). It has been shown in various mammals that this electrical coupling supports the generation of subthreshold oscillations, but recent work revealed that rhythmic activity is sustained in knock-outs of Cx36. Thus, these results raise the question of whether the olivary oscillations in Cx36 knock-outs simply reflect the status of wild-type neurons without gap junctions or the outcome of compensatory mechanisms. Here, we demonstrate that the absence of Cx36 results in thicker dendrites with gap-junction-like structures with an abnormally wide interneuronal gap that prevents electrotonic coupling. The mutant olivary neurons show unusual voltage-dependent oscillations and an increased excitability that is attributable to a combined decrease in leak conductance and an increase in voltage-dependent calcium conductance. Using dynamic-clamp techniques, we demonstrated that these changes are sufficient to transform a wild-type neuron into a knock-out-like neuron. We conclude that the absence of Cx36 in the inferior olive is not compensated by the formation of other gap-junction channels but instead by changes in the cytological and electroresponsive properties of its neurons, such that the capability to produce rhythmic activity is maintained.