Peripheral maps and synapse elimination in the cerebellum of the rat. II. Representation of peripheral inputs through the climbing fiber pathway in the posterior vermis of X-irradiated adult rats

The interscutularis muscle connectome

The complete connectional map (connectome) of a neural circuit is essential for understanding its structure and function. Such maps have only been obtained in Caenorhabditis elegans. As an attempt at solving mammalian circuits, we reconstructed the connectomes of six interscutularis muscles from adult transgenic mice expressing fluorescent proteins in all motor axons. The reconstruction revealed several organizational principles of the neuromuscular circuit. First, the connectomes demonstrate the anatomical basis of the graded tensions in the size principle. Second, they reveal a robust quantitative relationship between axonal caliber, length, and synapse number. Third, they permit a direct comparison of the same neuron on the left and right sides of the same vertebrate animal, and reveal significant structural variations among such neurons, which contrast with the stereotypy of identified neurons in invertebrates. Finally, the wiring length of axons is often longer than necessary, contrary to the widely held view that neural wiring length should be minimized. These results show that mammalian muscle function is implemented with a variety of wiring diagrams that share certain global features but differ substantially in anatomical form. This variability may arise from the dominant role of synaptic competition in establishing the final circuit.

Conventionally, the organization of a neural circuit is studied by sparsely labeling its constituent neurons and pooling data from multiple samples. If significant variation exists among circuits, this approach may not answer how each neuron integrates into the circuit's functional organization. An alternative is to solve the complete wiring diagram (connectome) of each instantiation of the circuit, which would enable the identification and characterization of each neuron and its relationship with all others. We obtained six connectomes from the same muscle in adult transgenic mice expressing fluorescent protein in motor axons. Certain quantitative features were found to be common to each connectome, but the branching structure of each axon was unique, including the left and right copies of the same neuron in the same animal. We also found that axonal arbor length is often not minimized, contrary to expectation. Thus mammalian muscle function is implemented with a variety of wiring diagrams that share certain global features but differ substantially in anatomical form, even within a common genetic background.

Reconstruction of the complete set of motor axons innervating a muscle reveals wiring variability, even among corresponding neurons on the left and right sides of the same animal.

Developmental modifications of olivocerebellar topography: the granuloprival cerebellum reveals multiple routes from the inferior olive

Correct function of neural circuits depends on highly organized neuronal connections, refined from less precise projections through synaptic elimination, collateral regression, or neuronal death. We examined regressive phenomena that define olivocerebellar topography during maturation from Purkinje cell polyinnervation to monoinnervation. We used bilateral retrograde tracing to determine the source of olivocerebellar afferents to posterior vermis lobules VII-VIII in a model of retained immature Purkinje cell polyinnervation, the granuloprival cerebellum. In controls, labelled neurons were found only in the contralateral inferior olive (ION) clustered in a small ventromedial locus that is congruent with known olivocerebellar topography. In granuloprival animals, olivary labelling appeared more dispersed and was present in homologous ipsilateral regions. Double-labelled neurons were never seen. Retrograde tracing following unilateral olivocerebellar transection in adult granuloprival rats revealed: 1) the origin of the normal (remaining) path projecting through the contralateral inferior peduncle was more localized than in irradiated nonpedunculotomized rats, 2) a small double-crossed path, and 3) a projection that ascends the peduncle ipsilateral to the ION of origin, part of which crosses the midline within the cerebellum. Electrophysiological and immunohistochemical assessment in the neonatal cerebellum revealed that transcommissural paths are not present during development but sprout within the irradiated cerebellum. Therefore, the olivocerebellar projection in the granuloprival rat, as a model of the immature path, shows parasagittal organization similar to that of controls in its normally crossed path but possesses additional abnormal projections. Thus, maturation of olivocerebellar topography involves removal of whole developmental paths to define laterality plus synapse elimination within largely predefined parasagittal zones.

HRP injection in lobule VI-VII of the cerebellar cortex reveals a bilateral inferior olive projection in granuloprival rats

In immature rats, Purkinje cells receive synapses from multiple climbing fibers. During development, this multi-innervation regresses and only one climbing fiber innervates each Purkinje cell in the adult. The multi-innervation of immature rats is maintained in the adult if the precursors of the cerebellar granule cells are destroyed by early postnatal X-irradiation. The present study was undertaken to determine the origin of climbing fibers projecting to lobule VI-VII of the cerebellum in X-irradiated granuloprival rats. Olivary neurons were labelled by retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase, which was injected by iontophoresis in the right vermis of lobule VI-VII. Three-dimensional reconstructions of the inferior olive were made for granuloprival and control rats. No significant variation in the shape and dimension of the olive was observed between the two groups. Labeled cells were found in the middle part of the median accessory olive (MAO). In control rats, stained cells were found only in the contralateral MAO, whereas in the granuloprival rats they were located in both the contralateral and the ipsilateral MAO. Homologous zones were marked in control and granuloprival rats in the middle part of MAO. In granuloprival rats, there was a symmetry in the distribution of the stained cells in the ipsi- and contralateral MAO along the three axes. Therefore, polyinnervation involves homologous regions of both inferior olivary nuclei.

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.

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.

Synapse elimination in the central nervous system: functional significance and cellular mechanisms

Recent research into the developmental elimination of supernumerary synapses has increased understanding of this process. In this review we discuss synapse elimination both at the neuromuscular junction and in the central nervous system, considering some possible underlying mechanisms suggested by recent studies. In addition a well-described example of central nervous system synapse elimination, the climbing fiber-Purkinje cell synapse of the cerebellum, is used to explore the functional significance of synaptic regression during brain development.

Abnormal ipsilateral functional vibrissae projection onto Purkinje cells multiply innervated by climbing fibers in the rat

We have previously shown that synapse elimination occurring in the climbing fiber (CF)-Purkinje cell (PC) relationships during normal postnatal development is likely involved in the refinement of vibrissae projections onto the cerebellar cortex. In normal adult rats, CF-mediated vibrissae projections onto cerebellar Purkinje cells of the vermis of lobule VII are strictly contralateral and located in a narrow microzone whereas they are widely distributed in rats whose PCs remained multiply innervated by CFs due to postnatal irradiation. Given the proximity of this microzone to the midline, the question arose as to whether this synapse elimination process could participate in the segregation of ipsilateral and contralateral projections. In the present study, we compared the topographical map of the ipsilateral and contralateral CF-mediated projections of the third row of vibrissae onto the vermal PCs of lobule VII in adult normal rats and in polyinnervated rats. Using intracellular electrophysiological recordings, we examined the responsiveness of PCs to mechanical stimulation of vibrissae, and positioned responsive cells on an averaged planar map of lobule VII. In normal rats no ipsilateral responses were found, while in irradiated rats ipsilateral responses were distributed evenly from the midline to 700 microns apart. These results suggest that synapse elimination participates in the segregation of ipsi and contralateral mystacial inputs to the vermis.

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.

Enlargement of olivo-cerebellar microzones in the agranular cerebellum of adult rats

Previous work on normal adult rat showed that the vibrissae project, through the climbing fiber (CF) system, onto the Purkinje cells (PCs) of the contralateral cerebellar hemivermis of lobule VII. The highly elaborated CF projections from a given row of vibrissae delimit a narrow parasagittal zone which can be regarded as a functional olivo-cerebellar microzone. Interestingly, the adult one-to-one relationship between PCs and CFs is preceded by a transient phase during which each PC receives synaptic inputs from several CF collaterals which will be eliminated but one, when granule cells begin to establish synapses on PCs. Therefore, the question arose as to whether this synaptic elimination process could participate in the refinement of the topographical organization of CF projections and could contribute to the formation of such precise peripheral maps onto the cerebellum. In the present study, the topographical map of the CF-mediated projection of mystacial vibrissae onto the vermal PCs of lobule VII was determined in adult rats whose cerebellar PCs remain polyinnervated by olivary CFs due to degranulation by postnatal X-irradiation. Using intracellular recordings, we examined the responsiveness of PCs in lobule VII during mechanical stimulation of the 3rd row of contralateral vibrissae, and positioned cells responding to the stimulation on an averaged planar map of lobule VII. Comparison of the results to those obtained in our previous work on normal rats showed that the activated cells were more numerous and more diffusely distributed.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Representation of vibrissae inputs through the climbing fiber pathway in lobule VII of the adult rat cerebellar vermis

The present study gives a detailed description of the functional characteristics and of the topographic distribution of Purkinje cell (PC) responses, mediated through the climbing fiber pathway and elicited by mechanical stimulation of two different rows (A and C) of vibrissae in a circumscribed region of the posterior vermis of the rat cerebellum. Experiments were carried out on normal adult rats under barbiturate anesthesia. PCs were recorded in an area extending 1500 microns laterally to the midline in the vermal part of lobule VII contralateral to the stimulation. Using micromapping techniques and computer analysis, we located the cells on the map of the unfolded PC layer. Taking into account the mean latency of the responses and the probability of discharge of PCs, restricted areas of projection were found. For each row, these areas formed two longitudinal patches located between midline and plane 1100 microns and separated by a non-responsive plane at 500 microns. Cells having the best characteristics of responses to the stimulation of row C were located mainly in plane 200 microns. Cells giving the best responses to the stimulation of row A were located mainly in the posterior part of plane 200 microns which was therefore a zone of convergence for both rows.

Peripheral maps and synapse elimination in the cerebellum of the rat. I. Representation of peripheral inputs through the climbing fiber pathway in the posterior vermis of the normal adult rat

The present study gives a detailed description of the functional characteristics and of the topographic distribution of responses, mediated through the climbing fiber pathway and elicited by electrical stimulation of several peripheral inputs, in a circumscribed region of the posterior vermis of the rat cerebellum. Experiments were carried out on normal adult rats under urethane anaesthesia. Purkinje cells (PCs) which responded to the electric stimulation of the contralateral snout, of the ipsilateral and contralateral hindlimb or forepad, and of the tail were recorded in an area extending 1000 microns laterally to the midline in the vermal part of lobules VII and VIII. Using precise micromapping techniques and computer analysis, we located the cells on the map of the unfolded PC layer. Taking into account the mean latency of the responses and the probability of discharge of the PCs, restricted areas of projection were found for the snout, the forepads and the tail. Zones of short-latency responses form compact patches of less than 1 mm2. There was some overlap of projection zones from tail and snout and from forepads and snout. In these zones, there was a convergence of several peripheral inputs on some of the PCs tested. No precise projection of the hindlimbs could be detected in the same lobules. These results fit well with the hypothesis already proposed that the representation of peripheral inputs through the climbing fiber pathway is fractured into a mosaic of patches, which are partly overlapping, and in which remote parts of the body are represented in adjacent areas.