Physiological properties of afferents and synaptic reorganization in the rat cerebellum degranulated by postnatal X-irradiation

TrkB is necessary for pruning at the climbing fibre-Purkinje cell synapse in the developing murine cerebellum

TrkB, the cognate receptor for brain-derived neurotrophic factor and neurotrophin-4, has been implicated in regulating synapse formation in the central nervous system. Here we asked whether TrkB plays a role in the maturation of the climbing fibre-Purkinje cell (CF-PC) synapse. In rodent cerebellum, Purkinje cells are initially innervated by multiple climbing fibres that are subsequently culled to assume the mature mono-innervated state, and whose contacts translocate from the soma to the dendrites. By employing transgenic mice hypomorphic or null for TrkB expression, our results indicated that perturbation of TrkB in the immature cerebellum resulted in ataxia, that Purkinje cells remained multiply innervated by climbing fibres beyond the normal developmental time frame, and that synaptic transmission at the parallel fibre-Purkinje cell synapse remained functionally unaltered. Mechanistically, we present evidence that attributes the persistence of multiple climbing fibre innervation to an obscured discrimination of relative strengths among competing climbing fibres. Soma-to-dendrite translocation of climbing fibre terminals was unaffected. Thus, TrkB regulates pruning but not translocation of nascent CF-PC synaptic contacts.

Olivocerebellar climbing fibers in the granuloprival cerebellum: morphological study of individual axonal projections in the X-irradiated rat

Elimination of cerebellar granule cells early during postnatal development produces abnormal neural organization that retains immature characteristics in the adult, including innervation of each Purkinje cell by multiple climbing fibers from the inferior olive. To elucidate mechanisms underlying development of the olivocerebellar projection, we studied light-microscopic morphology of single olivocerebellar axons labeled with biotinylated dextran amine in adult rats rendered agranular by a single postnatal X-irradiation. Each reconstructed olivocerebellar axon gave off approximately 12 climbing fibers, approximately twice as many as in normal rats. Terminal arborizations of climbing fibers made irregular tufts in most areas, whereas they were arranged vertically in a few mildly affected areas. Each climbing fiber terminal arborization innervated only part of the dendritic arbor of a Purkinje cell, and multiple climbing fibers innervated a single Purkinje cell. These climbing fibers originated either from the same olivocerebellar axon (pseudomultiple innervation) or from distinct axons (true multiple innervation). Abundant non-climbing fiber thin collaterals projected to all cortical layers. Although the longitudinal pattern of the zonal olivocerebellar projection was generally observed, lateral branching, including bilateral projections, was relatively frequent. These results suggest that the granule cell-parallel fiber system induces several important features of olivocerebellar projection: (1) organization of the climbing fiber terminal arborization tightly surrounding Purkinje cell dendrites, (2) elimination of pseudo- and true multiple innervations establishing one-to-one innervation, (3) retraction of non-climbing fiber thin collaterals from the molecular layer, and (4) probable refinement of the longitudinal projection domains by removing aberrant transverse branches.

Cerebellum as a target for toxic substances

The Purkinje cells and the granule cells are the most important targets in cerebellum for toxic substances. The Purkinje cells are among the largest neuron in the brain and are very sensitive to ischaemia, bilirubin, ethanol and diphenylhydantoin. The granule cells are small and seem to be sensitive to loss of intracellular glutathione. Granule cells are sensitive to methyl halides, thiophene, methyl mercury, 2-chloropropionic acid and trichlorfon. The Purkinje cells appear in the rat brain on pre-natal day 14-16, whereas the granule cells appear post-natally. Both cells are sensitive to excitotoxic chemicals and also to an effect on DNA or its repair mechanisms.

Postnatal development and adult organisation of the olivocerebellar projection map in the hypogranular cerebellum of the rat

The olivocerebellar system is characterised by a precise topographical organisation, in which distinct subsets of inferior olivary axons project to neurochemically heterogeneous Purkinje cell subpopulations, arranged into parasagittally oriented compartments in the cerebellar cortex. Adult climbing fibres and Purkinje cells are linked by a one-to-one relationship, which is established during postnatal development after a transitory phase of multiple climbing fibre innervation. The elimination of redundant climbing fibre synapses is thought to be regulated by granule cell-mediated activity-dependent processes. In order to assess whether this developmental remodelling is also important for the construction of the mature olivocerebellar projection map, we examined the hypogranular cerebella of rats treated by means of methylazoxymethanol acetate (MAM) during early postnatal life, in which multiple climbing fibre innervation persists in the adult. In these animals we investigated the distribution of calcitonin gene-related peptide (CGRP)-immunoreactive olivocerebellar axons and arbours during early postnatal development, and the correspondence between climbing fibre strips and zebrin II-defined Purkinje cell bands in the adult. Our results show that: (1) the pattern of CGRP-immunoreactive climbing fibres observed during the first three postnatal weeks is not disrupted after granule cell degeneration; and (2) the alignment between olivocerebellar axon subsets and zebrin II+/- Purkinje cell compartments is normally achieved in adult rats. In contrast, the climbing fibre-Purkinje cell relationship is abnormal, and single arbours innervate restricted dendritic regions of several neighbouring target neurons. These results indicate that the normal distribution of olivocerebellar axon subsets to distinct cerebellar cortical compartments can be established independently from granule cell-mediated remodelling processes. Thus, the postnatal climbing fibre plasticity, which is needed to achieve the normal climbing fibre-Purkinje cell relationship, appears to be confined within the framework of a projection map established during earlier developmental phases.

The effect of X-radiation on cerebellar granule cells grown in culture. Ganglioside GM1 neuroprotective activity

In this paper we describe the effects of X-radiation on the viability of cerebellar granule cells grown in culture. Cell cultures were exposed to X-rays 2 h after plating and then grown for 1-7 days. Two days after X-ray exposure with a dose-range of 0.1-2 Gy (acute effect), a significant decrease in neuronal number was observed. The magnitude of the lethal effect was directly correlated to the dose of X-ray applied. When the interval between plating and irradiation was increased, the acute lethal effect of X-rays decreased. 3H-thymidine incorporation was maximal during the first 24 h in vitro and decreased to nearly blank levels, after 72 h. In some experiments, cells present in each culture dish were counted at day 2 and at day 7. We observed that the number of cells present in sham-irradiated cultures decreased from day 2 to day 7, reflecting cell death after several days in vitro. The cell loss observed in X-irradiated cultures was significantly greater as compared with sham-irradiated cultures, confirming the deleterious effect of X-ray on cell survival. This effect was completely prevented by GM1 (6.5, 10 and 30 microM) added 48 h after X-ray exposure, but not 1 h after plating. We conclude that X-rays induce two different effects: an acute effect related to impaired DNA synthesis which is very active during the first 24 h in vitro, and a long-term effect owing to a sublethal damage in the surviving neuronal population.

Different climbing fibres innervate separate dendritic regions of the same Purkinje cell in hypogranular cerebellum

Electrophysiological experiments have shown that in hypogranular cerebella the Purkinje cells are innervated by several climbing fibres. The aim of this paper is to provide morphological evidence for this multiple innervation and to describe the topographical distribution of the different climbing fibres onto the somadendritic region of the Purkinje cell. Experiments have been performed in hypogranular adult Wistar rats lesioned during the first postnatal week by methylazoxymethanol (MAM) or by X-irradiation. Purkinje cells were labelled by an anti-calbindin antibody, whereas climbing fibres were visualised by means of Phaseolus vulgaris leucoagglutinin. Purkinje cells showed variable degrees of abnormality and displacement. Climbing fibres made contact with the dendrites of all kinds of Purkinje cells, including those ectopically positioned whose dendrites branched in the white matter. This shows that Purkinje cells can develop dendritic branching in the absence of granule cells and maintain the capability of interacting with their proper afferents, even when they are severely affected and displaced. In four Purkinje cells we have been able to follow the course of two climbing fibre terminal arbourisations. Almost no terminal branches were present around the Purkinje cell soma, and the whole arbour covered the proximal two-thirds of the Purkinje cell dendritic tree. These arbourisations, after an initial common course along the primary dendrite, distributed to separate dendritic regions. The observation of a single labelled climbing fibre covering a limited region of the dendritic tree was more common. As this finding is never observed in control material, it is concluded that the remaining region is covered by another unlabelled climbing fibre belonging to a different inferior olive neurone. These results represent a morphological demonstration of multiple climbing fibre innervation of the adult Purkinje cell. The maintenance of polyinnervation in the adult, which is consequent to the loss of granule cells, is not associated with a defect in the peridendritic translocation of the olivary arbour. In addition, the strict segregation of the different climbing fibres to distinct territories of the Purkinje cell dendritic tree suggests that each terminal arbourisation acts as a functionally independent unit and prevents other competitors from invading its own target domain.

The 'creeper stage' in cerebellar climbing fiber synaptogenesis precedes the 'pericellular nest'--ultrastructural evidence with parvalbumin immunocytochemistry

In perinatal rats, neurons in the dorsal cap of the inferior olivary complex transiently express parvalbumin-immunoreactivity (PA-IR). Their terminal axonic fields, particularly in the flocculonodular lobe, appear as precise bands of fine and convoluted immunostained fibers extending over the Purkinje plate and the nascent molecular layer. Thicker PA-IR fibers, corresponding to vestibular fibers, are observed only under the Purkinje plate. Electron microscopic analysis of the PA-IR climbing fibers within the bands allowed us to study their synaptogenesis with Purkinje cells. At birth (P0), thin PA-IR climbing fiber axons creep over these immature neurons following the contours of their perikarya and dendrites. They establish a few synaptic contacts, some of them of mature appearance, upon the smooth surface of Purkinje cell apical dendrites. PA-IR axonal growth cones are observed in the upper portion of the molecular layer. This precocious stage of climbing fiber/Purkinje cell synaptogenesis has been named here the 'creeper' stage. After the regression of the Purkinje cell apical dendrites (by P5), PA-IR climbing fibers are perisomatically located and synapse on Purkinje cell somatic protrusions, forming the classical pericellular 'nests'. The presence of mature synapses at P0 indicates the precocity of climbing fiber/Purkinje cell synaptogenesis and suggests its fetal onset. Therefore, this process of synaptogenesis occurs in two steps: (i) an early transient one, simultaneous with the initiation of the formation of the olivocerebellar map, that could be involved in the maintenance of the nascent topography of the projection and (ii) a latter step which concerns the refinement of the projection within a given PA-IR band through the regression of multiple innervation of Purkinje cells by climbing fibers, step which brings the required synaptic specificity to the adult olivocerebellar system.

Co-cultures of inferior olive and cerebellum: electrophysiological evidence for multiple innervation of Purkinje cells by olivary axons

Slices of inferior olive (IO) and cerebellum were co-cultured for several weeks by means of the roller tube technique. Recordings were carried out intracellularly from Purkinje cells (PCs) which were identified morphologically by intracellular injection of the fluorescent dye Lucifer yellow, or by immunohistochemical stainings with antibodies raised against the 28 kD Ca(2+)-binding protein calbindin. Following stimulation of olivary tissue, an all-or-none full complex spike response was recorded in some PCs consisting of a fast rising spike followed by a depolarizing potential. In other PCs, graded stimulation of the olivary explant induced synaptic potentials which were characterized by step-wise variation in their amplitude and resembled the ones occurring spontaneously. In contrast, only smoothly graded synaptic potentials were observed in cerebellar mono-cultures. These results indicate that some of the PCs in olivo-cerebellar co-cultures are innervated by several olivary neurons.

Extent of multiple innervation of cerebellar Purkinje cells by climbing fibers in adult X-irradiated rats. Comparison of different schedules of irradiation during the first postnatal week

The multiple innervation of cerebellar Purkinje cells (PCs) by climbing fibers (CFs) that is transient in normal developing rats can be experimentally maintained in cerebella which have been degranulated by repetitive postnatal X-irradiation restricted to the first postnatal week. Since the involution of redundant CFs occurs essentially between postnatal days 5 and 10, and given that postirradiation effects last 2-3 days, the question arose to know whether it is possible to further delimit a 'critical period' of irradiation within the first week. An estimate of the extent of multiple innervation of PCs by CFs was made in adult rats that had been irradiated according to 5 different schedules: in two groups, rats received X-rays applied repetitively during the first postnatal week (PN0-7 groups); in the 3 other groups, X-rays were delivered either during the first part of the week (early group PN1-3) or during the last part of the week (late groups PN4-7). In addition, two daily doses were tested (150 and 200 r). The CF pathway was electrically stimulated in anesthetized rats at the level of the inferior olive or in the cerebellar white matter. Intracellular recordings of spontaneous and evoked CF responses in PCs allowed to estimate the number of afferent CFs and to calculate the mean value (m) per PC for each group. The majority of recorded cells was located in lobules VII and VIII and similar results were obtained in these two lobules.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a multiple innervation of cerebellar Purkinje cells by climbing fibers in adult ferrets infected at birth by a mink enteritis virus

Newborn ferrets were inoculated with Mink Enteritis virus (parvovirus). They developed a cerebellar hypoplasia and presented severe ataxia. Electrophysiological study by intracellular recordings in the cerebellar cortex demonstrates that in these ferrets, like in other mammals, Purkinje cells deprived from granule cell input during development remain multiply innervated by climbing fibers in the adult.

Stability of multiple innervation of Purkinje cells by climbing fibers in the agranular cerebellum of old rats X-irradiated at birth

In the cerebellum of old X-irradiated rats, the multiple innervation of Purkinje cells by climbing fibers, which normally regresses after birth, was still present up to 18 months. Furthermore, the mean number of synaptic contacts upon each Purkinje cell was not smaller than what is observed in young adults, indicating that, after irradiation, these redundant synapses are definitively stabilized.

Cyclic nucleotide-dependent protein kinases and some major substrates in the rat cerebellum after neonatal X-irradiation

The levels of cAMP-dependent protein kinase (type I), or cGMP-dependent protein kinase, or protein I, and of a 23,000 MW substrate for the cGMP-dependent protein kinase were measured in cerebella from normal rats and in the cerebella from rats in which a selective loss of interneurons in the cerebellar cortex had been produced by X-irradiation. A decrease was observed in the concentrations of cAMP-dependent protein kinase and of protein I, whereas an increase was observed in the concentrations of cGMP-dependent protein kinase and of the 23,000 MW substrate. The data, taken together with the results of other studies, support the interpretation that cAMP-dependent protein kinase and protein I are distributed throughout the cerebellum, but that cGMP-dependent protein kinase and the 23,000 MW substrate are highly concentrated in Purkinje cells.

Extent of multiple innervation of Purkinje cells by climbing fibers in the olivocerebellar system of weaver, reeler, and staggerer mutant mice

Multiple innervation of cerebellar Purkinje cells (PCs) by climbing fibers (CFs) has been described recently in adult weaver, reeler, and staggerer mutant mice, instead of the monoinnervation found in normal adults. In the present study, the extent of this multiple innervation was estimated by two methods, using both evoked and spontaneous activity of the olivocerebellar system. Concordant values were obtained: the mean number of CF collaterals per PC was between 3.5 and 4 in weaver and staggerer and close to 3.2 for the multiply innervated PCs of reeler mice. These values are of the same order of magnitude as those for the transient multiple innervation in developing rats (Mariani and Changeux, 1981a,b).

Degranulation of rat cerebellum induces selective variations in gene expression

Selective variations in the composition of poly(A)-containing mRNA were found to be induced in the rat cerebellum by X-irradiation. mRNA populations prepared from normal and X-irradiated rat cerebella at different stages of their development displayed equal efficiencies when translated in vitro in reticulocyte lysates. Specific differences were revealed, however, when the labeled translation products of both mRNA preparations were subjected to two-dimensional gel electrophoresis followed by fluorography of the dried gels. Of more than 100 polypeptide products, several showed marked intensity differences, indicating changes in the abundance of their directing mRNA species. These differences appear both in developing and in mature cerebellar mRNA, and the extent of modification in mRNA is much higher than the consequent changes in the composition of proteins in the irradiated cerebellum. The degranulation-induced modifications in levels of specific cerebellar mRNA species can be used to identify proteins whose biosynthesis depends on the presence of interneurons.

Locus coeruleus stimulation potentiates Purkinje cell responses to afferent input: the climbing fiber system

In cerebellum, the evoked responses of the Purkinje cell to both excitatory and inhibitory afferent input have previously been shown to be enhanced by local iontophoresis of norepinephrine (NE). The influence of locus coeruleus (LC) conditioning stimulation on Purkinje cell responses to climbing fiber input was examined to determine whether endogenous NE, released from synaptic terminals, could exert similar potentiative effects. Stimulation of LC, at intensities which by themselves were subthreshold for directly affecting background activity, markedly enhanced complex spike excitation of Purkinje cells elicited by activation of climbing fiber inputs from sensorimotor cortex. Depressant responses observed after complex spike excitation were also augmented by the LC conditioning. Iontophoretic application of sotalol, a specific beta-adrenergic receptor blocker, reversibly antagonized this facilitation of climbing fiber-evoked responses. In addition, the potentiative effects of LC stimulation were not observed after destruction of NE-containing axons and terminals in cerebellum by 6-OHDA. These results suggest that noradrenergic input from the LC can enhance the efficacy of climbing fiber synaptic action on the Purkinje cell, and are thus consistent with the hypothesis of a 'modulatory' role rather than a specific information transfer function for NE in cerebellum.

Mossy fiber and Purkinje cell axon collateral arborization patterns in normal and X-irradiated rat cerebellum: a light microscopic study using horseradish peroxidase fiber filling techniques

The horseradish peroxidase neuronal staining technique was applied to reveal through anterograde and retrograde staining the detailed morphology of fiber arborization patterns in the cerebella of normal adult rats as well as those treated with neonatal X-irradiation. Morphological features of anterogradely-filled mossy fibers including the axonal arborization and glomerular-type synaptic specializations were found to be similar in all groups studied. In addition, retrogradely-filled Purkinje cell axon collaterals in the cerebella degranulated by X-irradiation exhibited much more elaborate branching and arborization of recurrent collaterals than those labeled in the normal cerebellar cortex. The presence of glomerular-type specializations in the degranulated cerebellum suggested that mossy fibers develop and maintain their normal features independent of major influences from granule cells. The hypertrophy observed after destruction of the cerebellar interneurons suggests the hypothesis that the growth of the normal Purkinje cell collateral system is normally suppressed by extrinsic factors within the neuronal circuit.

Heterologous synapses upon Purkinje cells in the cerebellum of the Reeler mutant mouse: an experimental light and electron microscopic study

The projections of the spinal cord upon the cerebellum of normal and Reeler mutant mice were compared by light and electron microscopic methods after hemicordotomy. In both genotypes this afferent system projects to the cerebellar cortex and to the roof nuclei. In the Reeler, there is an additional projection among the Purkinje cells and interneurons of the central cerebellar mass. In both normal and Reeler cerebellar cortex this mossy fiber system terminates as large glomeruli. In Reeler the spinal projection also gives rise to a smaller terminal which is distributed both to the cortex and the central cerebellar mass. In both genotypes the dendrites of granule cells and the somata and dendrites of Golgi cells are synaptic targets of the glomeruli of the cortical projection. In Reeler both the glomeruli and smaller terminals also form heterologous synaptic contacts with dendrite spines of heterotopic intracortical and subcortical Purkinje cells. In both genotypes the synapses are exclusively type I. A second class of heterologous synapse, a type I junction between axons of Golgi cells and Purkinje cell spines, is also recognized in electron micrographs. The present study is the first unequivocal demonstration by experimental hodologic method of heterologous synaptic junctions in the mammalian central nervous system. The existence of such junctions in the cytoarchitectonically anomalous cerebellum of this mutant emphasizes the critical role played by the cellular environment in shaping neural circuits in the developing nervous system.

Selective survival of beta 1-adenergic receptors in rat cerebellum following neonatal x-irradiation

Although beta 2-adrenergic receptors predominate in adult rat cerebellum, a small number of beta 1-receptors are also present. To investigate the cellular localization of beta 1- and beta 2-adrenergic receptors, the effects of intermittent neonatal X-irradiation focused on the cerebellum were determined on the densities of the two subtypes of beta-adrenergic receptor. This treatment destroys the late-maturing cerebellar interneurons including the granule, basket and stellate cells. A 60--70% loss of cerebellar mass was observed in 2-, 6- and 12-week-old animals. The large early-maturing Purkinje cells are resistant to the effects of X-irradiation. The total number of beta-adrenergic receptors per cerebellum, measured by Scatchard analysis of saturation isotherms of specific [125I]iodohydroxybenzylpindolol (IHYP) binding, was reduced by 70--80% in 2-, 6- and 12-week-old animals. The relative proportion of beta 1- and beta 2-receptors was determined by analyzing the inhibition of specific IHYP binding by the beta 1 selective antagonist practolol. The total number of beta 2-adrenergic receptors per cerebellum was reduced by 81--83% in 6- and 12-week-old X-irradiated rats. However, the number of beta 1-adrenergic receptors per cerebellum in 6- and 12-week-old X-irradiated rats was not significantly different from that in control animals. The results suggest that beta 2 receptors in the rat cerebellum are primarily associated with the small interneurons destroyed by neonatal X-irradiation. The beta 1 receptors may be located on a cell population which is unaffected by this treatment, possibly on cerebellar Purkinje cells.

Fate of the multiple innervation of cerebellar Purkinje cells by climbing fibers in immature control, x-irradiated and hypothyroid rats

The fate of the multiple innervation of Purkinje cells (PCs) by climbing fibers (CFs) was studied as a function of age in immature rats rendered agranular by X-irradiation, in immature hypothyroid rats, and compared to that in controls. This was done by examining in each group the intracellular activities of PCs mediated via CFs throughout maturation. From the third day in control rats, CF responses of PCs evoked by juxta fastigial region (JF) stimulation or occurring spontaneously already resembled the adult responses with, however, some important differences: (1) most of these responses were graded by steps with the intensity of the stimulation before day 13, due to the multiple innervation of PCs by CFs (see below); (2) immature CF responses exhibited a longer duration and their initial spike started near the peak of the EPSP instead of near the baseline later on. Finally, an anlage of CF response was already present in most PCs on day 2, and consisted of a single fast spike elicited near the peak of an underlying all-or-none EPSP. In the 3 groups of rats, CF EPSPs already closely resembled the adult ones as early as 3 days, although their total duration and especially their time to peak were longer. In control rats, these CF EPSPs reversed with depolarizing currents from day 3 and currents for reversal were much lower than in the adult. 'Dual' CF EPSPs of PCS37 were encountered in immature 7- to 10-day-old controls, and persisted in hypothyroid rats until the end of the third postnatal week. The mono- or the multiple innervation of PCs by CFs was ascertained in th 3 groups according to the graded or the all-or-none character of CF EPSPs, and the number of CFs impinging on a given PC was estimated by the number of steps in the response. In control rats, most of PCs were already multiply innervated by CFs as early as 3 days. The multiple innervation culminated on day 5 with an average number of 3.4 CFs for PC, and rapidly regressed later on, so that the adult-type monoinnervation was the rule after day 13. In hypothyroid rats, the establishment of the redundancy and its regression was delayed by 2--3 days. In X-irradiated rats, the settlement and the involution of the multiple innervation of PCs by CFs was exactly superimposed with that seen in controls until day 8. Later on, regression of the supernumerary contacts no longer occurred so that most PCs remained multiply innervated until adulthood. Finally, the first clear-cut IPSPs were detected in PCs on day 9 in control and X-irradiated rats and 2--3 days later in hypothyroid animals.

Multiple innervation of Purkinje cells by climbing fibers in the cerebellum of the adult staggerer mutant mouse

Intracellular recordings from Purkinje cells (PC) in the cerebellum of adult staggerer mutant mice revealed that the orthodromic response of PCs to juxtafastigial (JF) stimulation closely resembled a climbing fiber response (CFR). However, for most of the PCs studied, these responses were graded in a stepwise manner when the stimulus strength was increased. The underlying excitatory synaptic potentials (EPSPs) had the typical shape of EPSPs mediated through climbing fibers (CFs), but their size fluctuated in discrete steps, the highest one reaching the firing level. In the same PCs, the size of the spontaneous EPSPs fluctuated in a similar fashion and the frequency of each step was in the range of CF-mediated EPSPs. These results strongly suggest that in staggerer mice several CFs synapse with each PC instead of a single CF as in normal adults. Furthermore, the activation through some of these CFs does not reach the firing level of the corresponding PC.