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

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.

Naturally occurring neuronal death during the development of the inferior olive in the chick

Naturally occurring neuronal death was found by in situ labelling of nuclear DNA fragmentation during the development of the chick inferior olive. Counting neuronal perikarya showed an evident loss of cells from embryonic day 18 to hatching. This reduction in neuronal numbers was followed by an increase of similar size from days 1-4 post-hatching. This biphasic evolution of the neuronal numbers is quite similar to that found in the inferior olive of rodents during the first two weeks of the postnatal life, a period also characterized by definitive synaptogenesis between climbing fibers and Pukinje cells in the cerebellum of the rodents. The similarity in the evolution of neuronal number in the inferior olive of both rodents and chicks, seems to indicate that definitive synaptogenesis between climbing fibers and Purkinje cells might occur from embryonic day 18 to postnatal day 3 in the chick cerebellum. Nevertheless, during the phase of cell loss the climbing fibers of chick have attained a more mature developmental stage than those of the rat. This difference suggests that naturally occurring neuronal death may be independent of the elimination of redundant axonic collaterals during the definitive climbing fibers-Purkinje cell synaptogenesis.

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.

Transient synaptic redundancy in the developing cerebellum and isostatic random stacking of hard spheres

We propose an automaton for the simulation of the distribution of the number of climbing fibers (CF) making synapses on each Purkinje cell (PC) at the maximum of the synaptic redundancy that exists transiently in the newborn cerebellum. This automaton is based on the hypothesis that the synaptic maximum is limited by topological constraints and can be described by an isostatic random stacking of hard spheres. There is convincing agreement between the simulated distribution of the number of CF axons per Purkinje cell and the distribution experimentally obtained by electrophysiological techniques.