Tom J. H. Ruigrok

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.

Microcircuitry and function of the inferior olive

The inferior olive, which provides the climbing fibers to Purkinje cells in the cerebellar cortex, has been implicated in various functions, such as learning and timing of movements, and comparing intended with achieved movements. For example, climbing-fiber activity could transmit error signals during eye-blink conditioning or adaptation of the vestibulo-ocular reflex, or it could carry motor command signals beating on the rhythm of the oscillating and synchronous firing of ensembles of olivary neurons, or both. In this review, we approach the controversial issue of olivocerebellar function from the perspective of the unique organization of the microcircuitry of the olivary neuropil. The characteristic glomeruli are formed by a core of long dendritic or axonal spines, each of which is innervated by both an inhibitory terminal derived from the hindbrain and an excitatory terminal derived from either an ascending or descending input. The dendritic spines, which originate from dendrites with varicosities carrying dendritic lamellar bodies, are coupled by gap junctions. By drawing a comparison with a computational model by Segev and Rall,which might be applicable to the typical olivary spine with its unique morphological features and combined excitatory and inhibitory input, we propose that the microcircuitry of the inferior olive is capable of functioning both in motor learning and motor timing, but does not directly compare intended with achieved movements.

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.

A new combination of WGA-HRP anterograde tracing and GABA immunocytochemistry applied to afferents of the cat inferior olive at the ultrastructural level ☆

In order to identify cerebellar terminals in the cat inferior olive which contain gamma-aminobutyric acid (GABA), a technique was developed combining anterograde transport of wheatgerm agglutinine-conjugated horseradish peroxidase (WGA-HRP) with gold-immunocytochemistry. With this technique both the HRP reaction product and the immunogold labelling can be visualized in a single ultrathin section. Our results suggest that most, if not all of the WGA-HRP-labelled cerebellar terminals in the rostral medial accessory olive (MAO) and the rostral principal olive (PO) are GABAergic. In an additional experiment the GABAergic innervation of the rostral MAO was studied in combination with WGA-HRP anterograde tracing from the rostral mesencephalon. In this case the WGA-HRP-labelled terminals were never found to be GABA-positive.

Topography of cerebellar nuclear projections to the brain stem in the rat.

The organization of the cerebellum is characterized by a number of parallel and parasagittally ordered olivocorticonuclear modules; as such, the cerebellar nuclei basically function as output system of these modules. The present study provides a comprehensive and detailed description of the organization of the connections from the cerebellar nuclei to the brain stem in the rat. Thirteen small injections with the anterograde tracer Phaseolus vulgaris leucoagglutinin or biotinylated dextran amine which were centered on various aspects of the cerebellar nuclear complex are described and are illustrated with serial plots detailing the distribution of labeled varicosities throughout the brain stem. In every case at least 1,000 an up to 36,000 varicosities were plotted. All injections resulted in some or heavy labeling concentrated within specific regions of the contralateral inferior olivary complex and, usually, in some labeling of the contralateral ventrolateral thalamus. However, apart from these two areas it is shown that the cerebellar projections are generally very widespread and may be found throughout the entire brain stem. Below, only a survey of main projection areas will be given. Terminal arborizations originating from the rostral part of the medial cerebellar nucleus are mostly found in the caudal half of the brain stem with emphasis on the vestibular nuclear complex, whereas its caudal part rather connects to midbrain areas. Terminals that originate from the dorsolateral protuberance of the medial cerebellar nucleus are distributed more evenly throughout the brain stem and are mostly confined to reticular areas. The interstitial cell groups, interspersed between the medial and both interposed cerebellar nuclei, provide major projections to the ipsilateral vestibular nuclear complex and contralateral mesodiencephalic regions. However, reticular areas are also targeted over a large rostrocaudal range. The medial part of the posterior interposed nucleus sends most projections to the caudomedial red nucleus, prerubral regions and parvicellular reticular formation, all contralateral to the injection site. Projections that originate from more laterally placed injections are directed, apart from the inferior olivary complex, to the rostral half of the contralateral brain stem, where most labeled varicosities are found in the superior colliculus and zona incerta. The anterior interposed nucleus specifically targets the inferior olive, the red nucleus, the pontine reticulotegmental nucleus, the prectectum and the ventrolateral thalamic nucleus. More laterally placed injections also project to the ipsilateral parvicellular reticular formation and deep layers of the spinal trigeminal complex. The latter areas are more specifically targeted by the dorsolateral hump. In addition, its projections are found in the red nucleus and pretectum but do not seem to reach the ventrolateral thalamus. Projections from the lateral cerebellar nucleus are all characterized by a widespread distribution of terminals. Especially, the caudal aspect of the nucleus sends, apart from projections to the deep mesencephalic nucleus, red nucleus, periaquaductal gray, pretectum, prerubral area, and several thalamic regions, prominent projections to the caudal brain stem which terminate in the inferior olive and gigantocellular reticular formation. Projections from the ventral, parvicellular part of the nucleus are mostly, but not exclusively, directed to the rostral half of the brain stem and mainly terminate in the pararubral area, accessory oculomotor nuclei, pretectal areas, zona incerta, and in the parafascicular and ventrolateral thalamic nuclei. We conclude that the impact of the cerebellar nuclei on the brain stem is widespread; projections from different regions of the same cerebellar nucleus may show important differences in distribution of labeled terminals. On the other hand, injections placed in different cerebellar nuclei may result in a simila

Chapter 5 Cholinergic innervation and receptors in the cerebellum

Publisher Summary This chapter discusses the physiological and behavioral effects of acetylcholin (ACh) in the cerebellum and cholinergic innervation of the cerebellum. The overall levels of ACh and choline acetyltransferase (ChAT) are relatively low in the cerebellum as compared to other parts of the brain, whereas the amount of acetylcholinesterase activity is relatively high. The chapter describes the source and ultrastructural characteristics of ChAT-immunoreactive fibers in the cerebellum of the rat, and the distribution of muscarinic and nicotinic receptors in the cerebellum of the rat, rabbit, cat, and monkey to define which of the cerebellar afferents may use ACh as a neurotransmitter, what target structures are they, and which cholinergic receptor mediates the actions of these pathways. In addition to cholinergic mossy fibers, the rat cerebellum is innervated by beaded ChAT-immunoreactive fibers. It is demonstrated that these fibers originate in the pedunculopontine tegmental nucleus—the lateral paragigantocellular nucleus—and in various raphe nuclei to a lesser extent.

Mesodiencephalic and cerebellar terminals terminate upon the same dendritic spines in the glomeruli of the cat and rat inferior olive: An ultrastructural study using a combination of [3H]-leucine and wheat germ agglutinin coupled horseradish peroxidase anterograde tracing

The mesodiencephalic and cerebellar afferents in the rostral medial accessory and principal olive of the cat and rat were studied following anterograde transport of tritiated leucine combined with anterograde transport of wheat germ agglutinin coupled horseradish peroxidase in the same animals. In all studied areas at least one-third of the labelled glomeruli appeared to contain both mesodiencephalic and cerebellar terminals. In many of these cases it was found that the terminals from both afferent systems contacted the same dendritic spines. Therefore, these olivary spines may be, as will be discussed, well suited for being involved in a timing process.

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

Single Purkinje Cell Can Innervate Multiple Classes of Projection Neurons in the Cerebellar Nuclei of the Rat: A Light Microscopic and Ultrastructural Triple-Tracer Study in the Rat

Two different populations of projection neurons are intermingled in the cerebellar nuclei. One group consists of small, γ‐aminobutyric acid‐containing (GABAergic) neurons that project to the inferior olive, and the other group consists of larger, non‐GABAergic neurons that provide an input to one or more, usually premotor, centers in the brainstem, such as the red nucleus, the thalamus, and the superior colliculus. All cerebellar nuclear neurons are innervated by GABAergic Purkinje cells. In this study, we investigated whether individual Purkinje cells of the C1 zone of the paramedian lobe of the rat innervate both groups of projection neurons in the anterior interposed nucleus. Two different, retrogradely transported tracers, either cholera toxin β subunit (CTb) or wheat germ agglutinin coupled to horseradish peroxidase (WGA‐HRP) and a gold lectin tracer were injected into the red nucleus and the inferior olive, respectively, whereas Purkinje cell axons were anterogradely labeled with biotinylated dextran amine (BDA) injected into the paramedian lobule.

Cerebellum and Precerebellar Nuclei

This chapter is a systematic review of the gross anatomy, the histology of the cerebellar cortex and its nuclei, and of the connections of the cerebellum. Where possible, decriptions are based on observations on the human cerebellum, but most data are derived from animal experiments. Sections on the physiology of the cerebellar cortex, its relation to the cerebellar nuclei and the inferior olive are included. The functional localization within the cerebellum is discussed in sections on its skeletomotor, oculomotor, and non-motor functions, integrating morphological data with the results of imaging of the human cerebellum.