Ultrastructural evidence for compensatory sprouting of climbing and mossy afferents to the cerebellar hemisphere after ipsilateral pedunculotomy in the newborn rat

Spatiotemporal dynamics of lesion-induced axonal sprouting and its relation to functional architecture of the cerebellum

Neurodegenerative lesions induce sprouting of new collaterals from surviving axons, but the extent to which this form of axonal remodelling alters brain functional structure remains unclear. To understand how collateral sprouting proceeds in the adult brain, we imaged post-lesion sprouting of cerebellar climbing fibres (CFs) in mice using in vivo time-lapse microscopy. Here we show that newly sprouted CF collaterals innervate multiple Purkinje cells (PCs) over several months, with most innervations emerging at 3–4 weeks post lesion. Simultaneous imaging of cerebellar functional structure reveals that surviving CFs similarly innervate functionally relevant and non-relevant PCs, but have more synaptic area on PCs near the collateral origin than on distant PCs. These results suggest that newly sprouted axon collaterals do not preferentially innervate functionally relevant postsynaptic targets. Nonetheless, the spatial gradient of collateral innervation might help to loosely maintain functional synaptic circuits if functionally relevant neurons are clustered in the lesioned area.

Neurodegenerative lesions induce sprouting from surviving axons, but the patterns of re-innervation of these collaterals in relation to existing functional networks remains unclear. Here the authors performed long term in vivo imaging in mice, of sprouts from cerebellar climbing fibers after a lesion, and describe the patterns of connectivity relative to functionally active zones.

Reinnervation of late postnatal Purkinje cells by climbing fibers: neosynaptogenesis without transient multi-innervation

Synaptic partner selection and refinement of projections are important in the development of precise and functional neuronal connections. We investigated the formation of new synaptic connections in a relatively mature system to test whether developmental events can be recapitulated at later stages (i.e., after the mature synaptic organization has been established), using a model of postlesional reinnervation in the olivo-cerebellar pathway. During the development of this pathway, synaptic connections between climbing fibers (CFs) and Purkinje cells (PCs) are diffuse and redundant before synapse elimination refines the pattern. The regression of CFs during the first 2 postnatal weeks in the rat leads to mono-innervation of each PC. After unilateral transection of the rat olivo-cerebellar pathway and intracerebellar injection of BDNF 24 h after lesion, axons from the remaining inferior olive can sprout into the deafferented hemicerebellum and establish new contacts with denervated PCs at later developmental stages. We found that these contacts are first established on somatic thorns before the CFs translocate to the PC dendrites, recapitulating the morphological steps of normal CF-PC synaptogenesis, but on a relatively mature PC. However, electrophysiology of PC reinnervation by transcommissural CFs in these animals showed that each PC is reinnervated by only one CF. This mono-innervation contrasts with the reinnervation of grafted immature PCs in the same cerebellum. Our results provide evidence that relatively mature PCs do not receive several olivary afferents during late reinnervation, suggesting a critical role of the target cell state in the control of CF-PC synaptogenesis. Thus, synapse exuberance and subsequent elimination are not a prerequisite to reach a mature relationship between synaptic partners.

Developmental neural plasticity and its cognitive benefits: olivocerebellar reinnervation compensates for spatial function in the cerebellum

The adult mammalian central nervous system displays limited reinnervation and recovery from trauma. However, during development, post-lesion plasticity may generate alternative paths, thus providing models to investigate reinnervation and repair. After unilateral transection of the neonatal rat olivocerebellar path (pedunculotomy), axons from the remaining inferior olive reinnervate the denervated hemicerebellum. Unfortunately, reinnervation to the cerebellar hemisphere is incomplete; therefore, its capacity to mediate hemispheric function (navigation) is unknown. We studied sensorimotor control and spatial cognition of rats with and without transcommissural reinnervation using simple (bridge and ladder) and complex (wire) locomotion tests and the Morris water maze (hidden, probe and cued paradigms). Although pedunculotomized animals completed locomotory tasks more slowly than controls, all groups performed equally in the cued maze, indicating that lesioned animals could orientate to and reach the platform. In animals pedunculotomized on day 3 (Px3), which develop olivocerebellar reinnervation, final spatial knowledge was as good as controls, although they learned more erratically, failing to retain all information from one day to the next. By contrast, animals pedunculotomized on day 11 (Px11), which do not develop reinnervation, did not learn the task, taking less direct routes and more time to reach the platform than controls. In the probe test, control and Px3, but not Px11, animals swam directly to the remembered location. Furthermore, the amount of transcommissural reinnervation to the denervated hemisphere correlated directly with spatial performance. These results show that transcommissural olivocerebellar reinnervation is associated with spatial learning, i.e. even partial circuit repair confers significant functional benefit.

Post-lesion transcommissural olivocerebellar reinnervation improves motor function following unilateral pedunculotomy in the neonatal rat

In the adult mammalian central nervous system, reinnervation and recovery from trauma are limited. During development, however, post-lesion plasticity may generate alternate paths providing models to investigate reinnervation and repair. Sometimes, these paths are maladaptive, although the relationship between dysfunction and anatomical abnormality remains unknown. After unilateral transection of the neonatal rat olivocerebellar path (pedunculotomy), axons from the remaining inferior olive reinnervate Purkinje cells in the denervated hemicerebellum with appropriate topography and synaptic function. However, whether this new pathway confers beneficial behavioural effects remains unknown. We studied the behavioural sequelae in rats with and without transcommissural reinnervation using righting and vestibular-drop reflexes, simple locomotion (bridge), complex locomotion (wire) and motor coordination (rotarod) tests. In animals pedunculotomised on day 3 (Px3), which develop olivocerebellar reinnervation, dynamic postural adjustments and complex motor skills develop normally, whereas simple gait is broad-based and slightly delayed. In contrast, Px11 animals, which do not develop reinnervation, have delayed maturation of postural reflexes, gait and complex locomotor skills. In addition, when compared to control animals, their performance in locomotory tasks was slower and the complex task impaired. On the rotarod, control and Px3 animals learned to coordinate their gait and walked for longer at 10 and 20 rpm than Px11 animals. These results show that transcommissural olivocerebellar reinnervation is associated with almost normal motor development and the ability to synchronise gait at slow and moderate speeds, i.e. this reinnervation confers significant behavioural function and is therefore truly compensatory.

Spatio-temporal changes in neurofilament proteins immunoreactivity following kainate-induced cerebellar lesion in rats

1. Spatio-temporal changes in phosphorylated (pNFP) and nonphosphorylated (npNFP) neurofilament proteins were assessed immunocytochemicaly in adult rat cerebellum, 2-30 days following unilateral injection of kainic acid (KA) or physiological saline (s.c.). 2. Analysis of the staining intensity and pattern demonstrated that injection of both KA and physiological saline elicited significant and long-lasting increase of pNFP and npNFP immunoreactivity, at the ipsilateral, and to lesser extent at the contralateral side of lesion. 3. Kainate intoxication induced abundant expression of pNFP and npNFP in cerebellar white matter, as well as in all layers of perilesioned cortex. Higher pNFP expression was evidenced in the Purkinje cell layer, particularly at cell bodies, initial segments, and proximal dendrites, which normally do not contain pNFP. In addition, synaptophysin immunocytochemistry was used as a marker of synaptogenesis and plasticity. 4. Spatio-temporal pattern of NFP and synaptophysin expression suggests that perilesioned cortex undergoes dynamic changes following brain demage and possess a reparative capacity to abridge the consequences of brain trauma.

Reparative mechanisms in the cerebellar cortex

In the adult brain, different neuronal populations display different degrees of plasticity. Here, we describe the highly different plastic properties of inferior olivary neurones and Purkinje cells. Olivary neurones show a basal expression of growth-associated proteins, such as GAP-43 and Krox24/EGR-1, and remarkable remodelling capabilities of their terminal arbour. They also regenerate their transected neurites into growth-permissive territories and may reinnervate the lost target. Sprouting and regrowing olivary axons are able to follow specific positional information cues to establish new connections according to the original projection map. In addition, they set a strong cell body reaction to injury, which in specific olivary subsets is regulated by inhibitory target-derived cues. In contrast, Purkinje cells do not have a constitutive level of growth-associated genes, and show little cell body reaction, no axonal regeneration after axotomy, and weak sprouting capabilities. Block of myelin-derived signals allows terminal arbour remodelling, but not regeneration, while selective over-expression of GAP-43 induces axonal sprouting along the axonal surface and at the level of the lesion. We suggest that the high constitutive intrinsic plasticity of the inferior olive neurones allows their terminal arbour to sustain the activity-dependent ongoing competition with the parallel fibres in order to maintain the post-synaptic territory, and possibly underlies mechanisms of learning and memory. Such a plasticity is used also as a reparative mechanism following axotomy. In contrast, in Purkinje cells, poor intrinsic regenerative capabilities and myelin-derived signals stabilise the mature connectivity and prevent axonal regeneration after lesion.

Post-lesion transcommissural growth of olivary climbing fibres creates functional synaptic microzones

In the adult mammalian central nervous system, reinnervation and recovery from trauma is limited. During development, however, postlesion plasticity may generate alternate paths, providing models to investigate reinnervating axon-target interactions. After unilateral transection of the neonatal rat olivocerebellar path, axons from the ipsilateral inferior olive grow into the denervated hemicerebellum and develop climbing fibre (CF)-like arbors on Purkinje cells (PCs). However, the synaptic function and extent of PC reinnervation remain unknown. In adult rats pedunculotomized on postnatal day 3 the morphological and electrophysiological properties of reinnervating olivocerebellar axons were studied, using axonal reconstruction and patch-clamp PC recording of CF-induced synaptic currents. Reinnervated PCs displayed normal CF currents, and the frequency of PC reinnervation decreased with increasing laterality. Reinnervating CF arbors were predominantly normal but 6% branched within the molecular layer forming smaller secondary arbors. CFs arose from transcommissural olivary axons, which branched extensively near their target PCs to produce on average 36 CFs, which is six times more than normal. Axons terminating in the hemisphere developed more CFs than those terminating in the vermis. However, the precise parasagittal microzone organization was preserved. Transcommissural axons also branched, although to a lesser extent, to the deep cerebellar nuclei and terminated in a distribution indicative of the olivo-cortico-nuclear circuit. These results show that reinnervating olivocerebellar axons are highly plastic in the cerebellum, compensating anatomically and functionally for early postnatal denervation, and that this reparation obeys precise topographic constraints although axonal plasticity is modified by target (PC or deep nuclear neurons) interactions.

Abnormal cerebellar histogenesis in PEX2 Zellweger mice reflects multiple neuronal defects induced by peroxisome deficiency

The form and circuitry of the cerebellum develops by a complex process that requires integration of afferent-target interactions between multiple neuronal populations and migratory patterns established by neuron-glial interactions. Analysis of mice lacking the PEX2 peroxisome assembly gene, in which peroxisomal function is disrupted, reveals abnormal cerebellar histogenesis due to the disturbance of multiple cellular processes within neurons. Defects in cerebellar growth and the rostro-caudal foliation pattern reflect a reduced granule neuron population and abnormal Purkinje cell dendrite development. In granule neurons, there is increased apoptotic cell death and delayed movement from the EGL to IGL that reflects cell cycle, maturational and migrational abnormalities. The underlying Purkinje cells have stunted dendrite arbors with abnormal branching patterns, which may reflect altered inductive influences from the delayed granule neuron translocation. A delayed arborization of mutant olivary climbing fibers and their defective translocation from the perisomatic to the dendritic compartment of Purkinje cells results in numerous spines on the soma and proximal dendrites of Purkinje cells. Distal Purkinje cell dendritic spines also display abnormal morphology. These Purkinje cell dendritic abnormalities are seen in association with persistent and enlarged axonal spheroids, further indicating the presence of a degenerative process within the Purkinje cell. This PEX2(-/-) mouse model for the human peroxisomal biogenesis disorder Zellweger syndrome illustrates the complex interplay of abnormal developmental processes in the cerebellum and the importance of peroxisomal function for neuronal migration, proliferation, differentiation, and survival.

Climbing fiber development: do neurotrophins have a part to play?

The climbing fiber input to the cerebellum is crucial for its normal function but those factors which control the development of this precisely organized pathway are not fully elucidated. The neurotrophins are a family of peptides, which have many roles during development of the nervous system, including the cerebellum. Since the cerebellum and inferior olive express neurotrophins and their receptors, we propose that neurotrophins are involved in the regulation of climbing fiber development. Here we review the temporo-spatial expression of neurotrophins and their receptors at key ages during climbing fiber development and then examine evidence linking neurotrophins to climbing fiber development, including some of the intracellular pathways involved. During prenatal development the expression of neurotrophins in the hindbrain coupled with their function in neurogenesis and migration, is consistent with a role of NT3 in inferior olivary genesis. Subsequently, cerebellar expression of two neurotrophins, NT3 and NT4, is concurrent with olivary receptor expression and the time of olivary axonal outgrowth and this continues postnatally during early climbing fiber synaptogenesis on Purkinje cells. The expression-pattern of neurotrophins changes with age, with falling NGF, NT3 and NT4 but increasing granule cell BDNF. Importantly, olivary expression of neurotrophin receptors, and therefore climbing fiber responsiveness to neurotrophins, falls specifically during maturation of the climbing fiber-Purkinje cell synapse. The function of BDNF is less certain, but experimental studies indicate that it has a role in climbing fiber innervation of Purkinje cells, particularly synaptogenesis and synaptic plasticity. Its importance is highlighted by the overlap of BDNF signalling with several cellular pathways, which regulate climbing fiber maturation. From the data presented, we propose not only that neurotrophins are involved in climbing fiber development, but also that several act in a specific temporal order.

Distal extension of climbing fiber territory and multiple innervation caused by aberrant wiring to adjacent spiny branchlets in cerebellar Purkinje cells lacking glutamate receptor delta 2

Organized synapse formation on to Purkinje cell (PC) dendrites by parallel fibers (PFs) and climbing fibers (CFs) is crucial for cerebellar function. In PCs lacking glutamate receptor delta2 (GluRdelta2), PF synapses are reduced in number, numerous free spines emerge, and multiple CF innervation persists to adulthood. In the present study, we conducted anterograde and immunohistochemical labelings to investigate how CFs innervate PC dendrites under weakened synaptogenesis by PFs. In the GluRdelta2 knock-out mouse, CFs were distributed in the molecular layer more closely to the pial surface compared with the wild-type mouse. Serial electron microscopy demonstrated that CFs in the knock-out mouse innervated all spines protruding from proximal dendrites of PCs, as did those in the wild-type mouse. In the knock-out mouse, however, CF innervation extended distally to spiny branchlets, where nearly half of the spines were free of innervation in contrast to complete synapse formation by PFs in the wild-type mouse. Furthermore, from the end point of innervation, CFs aberrantly jumped to form ectopic synapses on adjacent spiny branchlets, whose proximal portions were often innervated by different CFs. Without GluRdelta2, CFs are thus able to expand their territory along and beyond dendritic trees of the target PC, resulting in persistent surplus CFs by innervating the distal dendritic segment. We conclude that GluRdelta2 is essential to restrict CF innervation to the proximal dendritic segment, by which territorized innervation by PFs and CFs is properly structured and the formation of excess CF wiring to adjacent PCs is suppressed.

Left Unilateral Inferior Pedunculotomy Prevents Neuronal Death During Postnatal Development of the Remaining Left Inferior Olivary Complex in the Rat

Neuronal death in the inferior olivary complex (IOC) was studied in control and unilaterally pedunculotomized newborn rats, from postnatal day 1 (P1) to P30, in order to test whether the approximately two-fold increase in available specific targets (i.e. Purkinje cells) that is theoretically provided by sectioning one inferior cerebellar peduncle to the developing climbing fibres of the remaining IOC could prevent the loss of inferior olivary neurons taking place during the first 2 weeks of postnatal life in the rat. Numerical estimation of the number of inferior olivary neurons in control and experimental rats showed that (i) in pedunculotomized rats, the number of inferior olivary neurons of the remaining inferior olivary complex was always greater than that encountered in control rats, (ii) the consistent decrease in the number of inferior olivary neurons observed in control animals between P2 and P8 was absent in cell counts of the pedunculotomized rats, and (iii) the increase in olivary cell number following the phase of cell decrease was also absent in pedunculotomized rats. It is concluded that the increase of available Purkinje cells during early postnatal development of the olivocerebellar projection prevents neuronal death in the remaining inferior olivary complex following pedunculotomy.

Localization of low affinity nerve growth factor receptor in the rat inferior olivary complex during development and plasticity of climbing fibres

The rat olivocerebellar pathway has a precise topography from an inferior olive (IOC) to Purkinje cells in the contralateral hemicerebellum. While its development and plasticity have been documented, the molecular mechanisms underlying these events are not fully elucidated. Neurotrophins are a family of growth factors with diverse roles in development and neuronal plasticity, acting through a two-receptor system, including a low affinity receptor (LNGFR) which binds all neurotrophins with similar affinity. Since neurotrophins are present in the cerebellum during early postnatal development when LNGFR is synthesized in the IOC, they may act as target-derived trophic agents for climbing fibres during development and plasticity. To assess this, standard immunohistochemistry was used to document the distribution of LNGFR in the rat IOC during climbing fibre development and until cerebellar development was complete at postnatal day 28 (P28). LNGFR immunoreactivity (LNGFR-IR) was detected in the IOC from P0 until P15, however after P7 it diminished in intensity and distribution, a change which indicates a relationship between cerebellar neurotrophins and climbing fibre development. After denervation of the left hemicerebellum, there was an apparent increase in inferior olivary LNGFR-IR that was concurrent with climbing fibre re-innervation. Thus the results of this study support the hypothesis that neurotrophins are involved in climbing fibre development and suggest a possible contribution to the plasticity of the olivocerebellar pathway.

The role of some brain structures in the switching of the descending influences in operantly conditioned rats

A hypothesis was proposed according to which the switching of descending influences by the corticospinal and corticorubrospinal systems was associated with rubro-olivary projection involvement depending on the context of movement [Kennedy P. R. (1990) Trends Neurosci. 13, 474-479]. Our results confirmed and extended this hypothesis. It was shown that a preliminary transection of the dorsolateral funiculus (containing the rubrospinal tract) accelerated the compensatory rehabilitation process following lesions of the red nucleus and the ventrolateral thalamic nucleus in albino rats with learned instrumental reflexes on equilibrium. A preliminary lesion of the ventrolateral thalamic nucleus considerably hampered the switching process; nevertheless, performance of the reflexes suggested that the switching of cerebellar ascending influences to the cerebral cortex could be completed through other cerebellocortical pathways as well. Comparison of the results of electrolytic and chemical lesions of the red nucleus suggested a similar conclusion. It was established that the conditioning and recovery of already learned instrumental reflexes were impossible after complete neurotoxic destruction of the inferior olive. The data obtained emphasize the role of the inferior olive, ventrolateral thalamic nucleus and red nucleus in the switching of descending influences in operantly motor conditioned rats. Motor deficit and the compensatory rehabilitation process depended on the severity of inferior olive destruction combined with a high transection of the dorsolateral funiculus and a destroyed red nucleus. Long-lasting training improved compensation of motor deficit and stabilized instrumental reflexes to some extent in rats with incomplete destruction of the inferior olive. It has been suggested that these modifications occur because of collateral sprouting in the olivocerebellar system.

Granule neuron DNA damage following deafferentation in adult rats cerebellar cortex: a lesion model

Neuronal programmed cell death is regulated by a neurotrophic supply from targets and afferent inputs. The relative contribution of each component varies according to neuronal type and age. We have previously reported that primary cultures of cerebellar granule cells undergo apoptosis when deprived of depolarising KCl concentrations, suggesting a significant role of afferent inputs in the control of cerebellar granule cells survival. This issue was investigated by setting up various in vivo lesional paradigms in order to obtain partial or total deafferentation of the cerebellar granule layer in adult rats. At different times after surgery, cerebellar sections were subjected to TUNEL staining in order to detect possible DNA damage. One week after unilateral pedunculotomy, few scattered groups of apoptotic granule neurons were observed in the homolateral hemisphere. On the contrary, total deafferentation obtained by a new experimental paradigm based on an "L-cut" lesion induced massive and widespread apoptotic death in the granule layer of the deafferentated area. The time window of DNA fragmentation in granule layer was one to seven days after the "L-cut". Selective Purkinje cell deafferentation obtained by 3-acetylpyridine injection did not result in TUNEL staining in the cerebellar cortex. The current finding that mossy fiber axotomy induces granule cell apoptotic death points out for the first time the crucial role of afferent inputs in mature granule cell survival. Moreover, the in vivo lesional model described here may prove to be an useful tool for investigating cellular and molecular mechanisms of neuronal death triggered by deafferentation.

An ultrastructural study of granule cell/Purkinje cell synapses in tottering (tg/tg), leaner (tg(la)/tg(la)) and compound heterozygous tottering/leaner (tg/tg(la)) mice

Homozygous tottering (tg/tg) and compound heterozygous tottering/leaner (tg/tg(la)) mutant mice exhibit juvenile onset of three abnormal neurological phenotypes: (i) petit mal-like epilepsy; (ii) ataxia; and (iii) an intermittent myoclonus-like movement disorder. Homozygous leaner mice (tg(la)/tg(la)) exhibit early onset of ataxia (postnatal days 10-12), and also exhibit the myoclonus-like movement disorder and evidence of absence seizure activity; the myoclonus-like disorder is most evident in the first month of life, then diminishes in severity and frequency. The ultrastructure of the cerebellar molecular layer was examined in adult (six to eight months) and juvenile (20-25 days) mice of all three mutant genotypes. In mice of all three genotypes and both ages, Purkinje cell dendritic spines were observed to make multiple contacts with individual parallel fiber varicosities in all sections analysed. These multiple synaptic units were observed in both anterior and posterior vermis and hemispheres of the cerebellum, and ranged from two to nine spines/parallel fiber varicosity. Occasionally, one of the postsynaptic spines belonged to an ectopic spine emerging from the proximal region of a Purkinje cell dendrite. This increase in the multiple synaptic index of some parallel fiber varicosities was observed in juvenile tottering mice before the onset of the symptoms of the neurological disorders. This is highly suggestive that the onset of the neurological phenotype is not a primary cause of multiple Purkinje cell dendritic spines synapsing with single parallel fiber varicosities in these mice, but on the contrary, that it could be the cause of the ataxic symptoms.

Acute neuronal and vascular changes following unilateral cerebellar pedunculotomy in the neonatal rat

During development of the central nervous system (CNS) both deafferentation and axotomy induce increased neuronal death and result in a smaller brain with diminished function at maturity. Unilateral cerebellar pedunculotomy has been used as a model to study the relative importance of these 2 types of lesion on the survival of developing CNS neurons. Within the cerebellum, unilateral pedunculotomy causes deafferentation of the hemicerebellum and axotomy in the efferent pathway from the ipsilateral deep cerebellar nuclei. This results in a smaller hemicerebellum with normal cortical laminae but no extracerebellar outflow. In order to identify the sequence of events which leads to this altered structure and therefore to understand the relative importance of afferent versus target-derived trophic support, unilateral cerebellar pedunculotomy was performed on neonatal rat pups, aged between 1 and 3 days. The cerebella were analysed for histological and vascular changes after survival times of 0, 3, 6, 9, 12, 18, 21, 24 and 48 h. The results show that the effects of axotomy on the deep cerebellar nuclear neurons begin within 3 h of the lesion and apoptotic neuronal degeneration occurs within 48 h. However, the cerebellar cortical neurons continue to undergo normal histological development for at least 48 h after deafferentation. In addition, since ischaemia induces similar effects, a study of the vascular tree was made. The results indicate that the pedunculotomy does not alter the blood supply to the cerebellum, nor induce ischaemia of the cerebellar neurons. From this it may be hypothesised that target-derived trophic support is more crucial for the survival of immature neurons than is the trophic effect of afferent input.

Reestablishment of the olivocerebellar projection map by compensatory transcommissural reinnervation following unilateral transection of the inferior cerebellar peduncle in the newborn rat

It is unclear whether reparative processes in the injured mammalian brain are able to restore the topographic organisation of neuronal connections. To address this question, we have investigated the plasticity of the olivocerebellar system. This pathway has a precise topographic arrangement, in which subsets of inferior olivary neurons project to parasagittally oriented Purkinje cell compartments. Following unilateral transection of the inferior cerebellar peduncle in newborn rats, axons from the contralateral projection cross the cerebellar midline and reinnervate the deafferented hemicerebellum. By this experimental approach, we first analysed the behaviour of calcitonin gene-related peptide (CGRP)-immunoreactive climbing fibres. This marker is transiently expressed by a subset of developing inferior olivary axons, which terminate in the cerebellar cortex into several parasagittal strips. We show that transcommissural axons reestablish the original pattern of climbing fibre bands within a few days after lesion. Then, in adult animals injured at birth, we assessed whether the newly formed climbing fibre bands align with zebrin II+/- Purkinje cell compartments, as in normal conditions. The newly formed projection is organised in parasagittally oriented strips which mirror the distribution of their counterparts on the intact side and are precisely aligned to the heterogeneous Purkinje cell compartments. In addition, the patchy distribution of olivo-nuclear fibres suggests that specific reinnervation is also achieved in the deep nuclei. Thus, transcommissural olivocerebellar reinnervation is not random, but it is regulated by selective interactions between distinct subsets of olivocerebellar axons and target neurons aimed at reestablishing the correct projection map.

Changes in the expression of the extracellular matrix molecules tenascin-C and tenascin-R after 3-acetylpyridine-induced lesion of the olivocerebellar system of the adult rat

In the central nervous system of rodents, the extracellular matrix glycoproteins tenascin-C and tenascin-R are expressed predominantly by astrocytes and oligodendrocytes respectively. Both molecules support neurite outgrowth from several neuronal cell types when presented as uniform substrates. When offered as a sharp boundary with a permissive substrate, however, both molecules prevent neurite elongation. On the basis of these observations it has been suggested that tenascin-C and tenascin-R may be relevant in determining the cellular response after injury in the adult rodent central nervous system. To investigate whether tenascin-C and tenascin-R may play important functional roles in the lesioned central nervous system, we have analysed their expression in the olivocerebellar system of the adult rat after 3-acetylpyridine-induced degeneration of nerve cells in the inferior olivary nucleus. Tenascin-C mRNA was not detectable at any time in the unlesioned or lesioned inferior olivary nucleus by in situ hybridization. In the cerebellar cortex, tenascin-C mRNA in Golgi epithelial cells was down-regulated 3 days after the lesion and returned to control values 80 days after the lesion. Tenascin-R mRNA was expressed by distinct neural cell types in the unlesioned olivocerebellar system. After a lesion, the density of cells containing tenascin-R transcripts increased significantly in the inferior olivary nucleus and in the white matter of the cerebellar cortex. Immunohistochemical and immunochemical investigations confirmed these observations at the protein level. Our data thus suggest differential functions of tenascin-C and tenascin-R in the injured central nervous system.

Reciprocal trophic interactions between climbing fibres and Purkinje cells in the rat cerebellum

In the adult cerebellum both the climbing fibre arbour and the Purkinje cell are very plastic and each element is able to exert a remarkable action on the other one. The adult phenotype of the Purkinje cell is strictly dependent on the presence of its climbing fibre arbour. When the climbing fibre is missing, the Purkinje cell undergoes a hyperspiny transformation and becomes hyperinnervated by the parallel fibres. However, this change is fully reversible. The climbing fibre-deprived Purkinje cell is able to elicit sprouting of nearby located intact climbing fibres and the new arbour is able to fully restore synaptic connections which appear normal both morphologically and functionally. Multiple climbing fibre innervation of a single Purkinje cell persists in the adult hypogranular cerebellum. The different fibres are distributed to separate dendritic regions, suggesting a local competition between the different arbours for their territory. It is postulated that in the intact rat, an activity dependent mechanism of the parallel fibre favours the predominance of one arbour with the elimination of its competitors. When the Purkinje cell is deleted, the climbing fibre arbour becomes heavily atrophic and reduced in size. The analysis of the pattern of this atrophy indicates that the climbing fibre arbour is made by two compartments: a proximal one, whose survival depends on the integrity of the inferior olive, and a distal one, which represents the true pre-synaptic site, which strictly depends on the target. The climbing fibre terminal arbour is able to extend its territory of innervation not only when adult intact climbing fibres are confronted with nearby denervated Purkinje cells, but also when an embryonic cerebellum is grafted onto the surface of an adult unlesioned cerebellum. In this case, collaterals of intact climbing fibre arbours elongate through the pial surface, enter the graft to innervate the Purkinje cells. This growth is likely under the influence of a tropic signal released by the embryonic Purkinje cells. This suggests that the sprouting observed in the adult rat following a subtotal inferior olive lesion is also triggered by a similar factor. The axonal elongation and the consequent synaptogenesis are likely guided by local cues. In this condition, the distribution of the new collateral reinnervation occurs within its projectional map. In addition, when the inferior cerebellar peduncle is sectioned at birth, the climbing fibres of the non-deafferented hemicerebellum emit collaterals which cross the midline and innervate cerebellar strips which are symmetrically positioned relative to the intact side. In the grafting experiments, both the migrated and non-migrated Purkinje cells show the typical electrophysiological properties of the mature cerebellum. These data show that the disappearance of neuronal elements is not a necessary prerequisite to allow new neurones to become fully morphologically and functionally integrated into an adult brain. The reciprocal trophic influence between the climbing fibres and the Purkinje cells shown in the present series of experiments are likely operative in the adult brain not only in pathological conditions and they could give a basic contribution to the synaptic plasticity underlying learned behaviour.

Ipsilaterally located olivocerebellar projection neurons of the chick

The use of horseradish peroxidase (HRP) and 1,1'-Dioctadecyl-3,3,3',3'- tetramethyl-indocarbocyanine perchlorate (DiI) as retrograde tracers, applied in vitro within the olivocerebellar tract of both embryos (9 to 21 days old) and postnatal (3-60 days old) chickens, has allowed the observation of a small population of neurons located ipsilaterally to the placement of the tracer. These neurons, whose morphology indicated that they belong to the inferior olive rather than to the reticular formation or the raphe nuclei, followed the same developmental steps as normally placed inferior olivary neurons. Furthermore, pedunculotomy experiments made on 3-day-old chickens demonstrated that ipsilateral neurons sent their axons through the cerebellar peduncle. In contrast to the completely crossed arrangement of the olivocerebellar projection, the present results show the existence, as in the rat, of a few ipsilateral inferior olivary neurons whose significance is unclear.

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.

Morphological evidence for the presence of ipsilateral inferior olivary neurons during postnatal development of the olivocerebellar projection in the rat

The presence of ipsilateral inferior olivary neurons during postnatal development of the olivocerebellar projection in the rat was investigated by two in vitro axonal tracing methods and by the axotomy of one olivocerebellar tract. The experiments were carried out before (P1), during (P5-P10) and after (P20) the period of multiple innervation of Purkinje cells by climbing fibers. According to present results: (1) ipsilateral inferior olivary neurons are distributed, on all analyzed days, throughout the entire inferior olive; (2) cell counts after axotomy experiments demonstrated that they represent a small population of inferior olivary neurons, whose number oscillated between 271 +/- 30 in young animals (pedunculotomized at P1 and killed at P7) and 26 +/- 12 in older ones (pedunculotomized at P20 and killed at P40). This experiment confirmed that most of these neurons are eliminated during the regressive events that take place during normal development of the olivocerebellar projection; and (3) few ipsilateral inferior olivary neurons, however, survive at P40, but their significance is still unclear.

Topographic spinocerebellar mossy fiber projections are maintained in the lurcher mutant

A variety of recent studies of cerebellar development have focused attention on the role of Purkinje cells as organizing elements for the topography of afferent fiber connectivity in the cerebellum. We have investigated the involvement of Purkinje and granule cells in the maintenance of topographic spinocerebellar mossy fiber projections by analyzing the distribution of spinocerebellar mossy fiber terminals in lurcher (+/Lc) mutant mice. Purkinje cells in the +/Lc mutant degenerate starting after the first week of postnatal development because of an intrinsic genetic defect. The loss of their Purkinje cell targets also results in the death of 90% of the granule cells. We examined the distribution of spinocerebellar mossy fiber terminals in the juvenile and adult +/Lc mutant to determine how the pattern of afferent projections is affected by the loss of Purkinje cells shortly after innervation of the cerebellum. Labeling of spinocerebellar mossy fiber terminals with WGA-HRP in the P38 and adult +/Lc mutant showed that, despite the loss of almost all Purkinje cells and 90% of the granule cells, spinocerebellar mossy fibers project to the appropriate folia and segregate into relatively normal parasagittal bands. While we cannot rule out the possibility that Purkinje cells may be involved in the initial establishment of topographic maps, our results indicate that Purkinje cells are not necessary for the maintenance of the normal spinocerebellar mossy fiber topographic map.

Transient ipsilateral innervation of the cerebellum by developing olivocerebellar neurons. A retrograde double-labelling study with fast blue and diamidino yellow

In neonatal rats the injection of Fast Blue and Diamidino Yellow retrograde fluorescent tracers, each into separate cerebellar hemispheres, reveals the presence of double-labelled neurons positioned bilaterally in the inferior olivary complex during the early postnatal period (postnatal day 0 to postnatal day 5). This suggests that those neurons whose axons are able to take up both tracers project to both hemicerebellar during this period of postnatal development. Double-labelled neurons were observed in one- and five-day-old injected postnatal rats, but were absent in older animals (10 and 30 days old). The presence of these neurons coincides with a transient period of poly-innervation of Purkinje cells by climbing fibres. They may thus be participating in transitory interactions preceding the formation of definitive climbing fibre synaptic arrangements in the cerebellar cortex. The technique employed is unable to clearly define the pathway of this transient olivocerebellar projection into the ipsilateral cerebellum; however, in direct evidence--like the topographic distribution of double-labelled neurons relative to tracer injection sites, and the small number of single-labelled neurons within the ipsilateral olivary complex, together with previous data on the axonogenesis of olivary neurons [Bourrat and Sotelo (1988) Devl Brain Res. 39, 19-37]--suggests that these fibres reach the cerebellum through the contralateral inferior cerebellar peduncle and give rise to collaterals, some of which subsequently decussate again within the cerebellum. These fibres probably represent transient collaterals of the normally contralateral olivocerebellar fibres that cross the cerebellar midline and reach mirror-image loci within the ipsilateral hemicerebellum.

Cerebellar Purkinje cells provide target support over a limited spatial range: evidence from lurcher chimeric mice

The distribution of Purkinje cells, granule cells, and olivary neurons was quantitatively analyzed in a lurcher +/Lc in equilibrium C3H/HeJ chimera in which the surviving wild type Purkinje cells were unilaterally distributed in the left hemicerebella. The left hemisphere of this mouse contains 7600 Purkinje cells, approximately 10% of the number of Purkinje cells in inbred C3H/HeJ mice. The right hemisphere contains 300 Purkinje cells, all of which are found within 200 microns of the midline. As in other +/Lc in equilibrium wild type chimeras, the ratio of granule cells to Purkinje cells is increased in the left hemisphere, reflecting increased granule cell survival. In the right hemisphere, however, the number of granule cells is reduced to that found in +/Lc mutants. In the inferior olive, almost twice as many neurons are found in the right nucleus as opposed to the left nucleus. As the projections of olivary neurons are crossed, the number of olivary neurons is increased in the nuclei that project to the cerebellar hemisphere containing Purkinje cells compared to the olivary nuclei that project to the cerebellar hemisphere with almost no Purkinje cells. The preferential survival of granule cells and olivary neurons that either occupy or project to the hemicerebellum containing Purkinje cells suggests that the availability of trophic support from target Purkinje cell neurons is spatially restricted.

Early dendritic development of Purkinje cells in the rat cerebellum. A light and electron microscopic study using axonal tracing in 'in vitro' slices

The early stages in the formation of Purkinje cell dendritic arbors have been analyzed using the horseradish peroxidase (HRP) 'in vitro' axonal tracing method, from embryonic day 19 (E19) to postnatal day 6 (P6). These stages comprise the transition from the bipolar Purkinje cell, at the end of its migration, to the phase of stellate cell with disoriented dendrites. Postmigratory Purkinje cells in the cortical plate exhibit poorly elaborated bipolar shapes, here named 'simple-fusiform' cells. They constitute the vast majority of labeled cells up to P0, and thereafter they decrease in number until P4. As a result of continuous outgrowth of new primary dendrites emerging from the apical pole but also from the basal and lateral aspects of the cell bodies, the Purkinje cells enter the 'complex-fusiform' phase, which peaks by P1 and slowly disappears by P6. The disappearance of 'complex-fusiform' cells is the result of an intense regressive process with resorption or retraction of the long dendrites that reaches a maximum by P3. We have called this stage: the Purkinje cell with 'regressive-atrophic' dendrites. This regression marks the initiation of the phase of the stellate cell, characterized by the explosive outgrowth of shorter perisomatic protrusions emerging in all directions. By P6, almost all the labeled Purkinje cells have attained this phase. The ultrastructural study of the labeled Purkinje cells has revealed that the transient dendrites of the fusiform cells have all the cytologic features of mature dendrites, particularly cytoskeletal elements (microtubules) and free polyribosomes. More importantly, axon terminals of unknown origin establish a few, constantly present, mature-like synaptic contacts on the dendritic shafts and spinous protrusions from P0, the earliest studied age. Their frequency increases on the Purkinje cells which enter the phase of stellate cell. Our results emphasize that the transformation of bipolar postmigratory Purkinje cells into the stellate cell stage results from a complex cascade of alternating creative and destructive processes, taking place in parallel with the formation and regression of mature synaptic contacts, between the remodelling dendritic arbors and unidentified afferent inputs. Purkinje cells, in all the different transitional stages, are present side by side in the same folial regions, at least until P4, and receive a similar contingent of synaptic input. This indicates that the dendritic remodelling is not driven by the synaptic inputs, but obeys either neural interactions that lead Purkinje cells to assume their monocellular layer configuration, or an internal clock depending on the Purkinje cell birthdate, or an interplay between these two kinds of mechanisms.

Reinnervation of cerebellar Purkinje cells by climbing fibres surviving a subtotal lesion of the inferior olive in the adult rat. I. Development of new collateral branches and terminal plexuses

Cerebellar climbing fibres react by collateral sprouting after subtotal lesions of the inferior olive, and the newly formed branches are able to reinnervate neighbouring denervated Purkinje cells. In the present paper, we used the Phaseolus vulgaris leucoagglutinin (PHA-L) tracing technique to label the climbing fibres and study their plasticity in detail at the light microscopical level. The specific objectives were to study the time course and morphological aspects of their sprouting, to estimate their extent of growth, and to compare the newly formed terminal plexuses with normal climbing fibres. Intraperitoneal injection of 3-acetylpyridine induced degeneration of the majority of the olivary neurones, which terminate as climbing fibres in the cerebellar cortex. Regularly, small numbers of neurones survived in the inferior olive. In the cerebellar cortex scattered surviving climbing fibres were found, which were devoid of any sign of injury. Already 3 days after the lesion, surviving climbing fibres had emitted collateral branches, which elongated for some distance through the molecular layer and ended with a number of varicosities and very fine branchlets. By 7 days, it was possible to recognize new developing arbours which grew in the molecular layer with the same orientation as normal climbing fibres. At longer survival times, extensive terminal arbours had developed and double labelling experiments confirmed that they terminated around the proximal dendrites of Purkinje cells. The newly formed terminal plexuses resembled, in all essential aspects, normal climbing fibres. In addition, from 1 month onward, it was evident that every surviving climbing fibre was able to form several new terminal plexuses reinnervating a number of neighbouring Purkinje cells. The result of this process was the formation of large clusters of newly formed plexuses around the parental arborization. Quantitative estimates indicated that the domain of innervation of single surviving climbing fibres could be increased by more than six times. It is concluded that climbing fibres surviving a subtotal olivary lesion are capable of extensive sprouting, axonal growth, and formation of new terminal plexuses, which resemble normal climbing fibres. Previous electrophysiological evidence indicates that this reinnervation is functional. The high specificity with which sprouting olivary axons reinnervate the proximal Purkinje cell dendrites suggests the existence of precise interactions between the growing fibres and their target. This example of "homotypic" collateral sprouting and reinnervation may thus provide a useful model for the study of nerve-target interactions.

Reinnervation of cerebellar Purkinje cells by climbing fibres surviving a subtotal lesion of the inferior olive in the adult rat. II. Synaptic organization on reinnervated Purkinje cells

A salient feature of the cerebellar Purkinje cells is the highly ordered distribution of their excitatory afferents on the dendritic tree. Climbing fibres synapse exclusively on the proximal dendrites, whereas parallel fibres articulate with the distal branches, the so-called spiny branchlets. This input organization is lost following the removal of climbing fibres. Such denervation results in the formation of a large number of new spines on the proximal dendrites, and these become contacted by sprouting parallel fibres, which thereby extend their domain of innervation. We have previously shown that the climbing fibres surviving a subtotal lesion of the inferior olive sprout and reinnervate neighbouring Purkinje cells. In the present ultrastructural study, we have investigated the features of Purkinje cells reinnervated by sprouting climbing fibres. The objectives were to examine the fine morphology of the newly formed synapses and to determine whether the modifications of Purkinje cell morphology and afferent organization are reversed by this reinnervation. Surviving climbing fibres were labelled by the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) and immunohistochemically visualized by means of the gold-substituted silver peroxidase technique, 2 and 6 months after 3-acetylpyridine lesions of the inferior olive in adult rats. Sprouting climbing fibres and newly formed arborizations were identified in the light microscope, isolated, and cut in serial ultrathin sections for electron microscopic analysis. The labelled boutons belonging to newly formed terminal plexuses exhibited the typical morphological features of climbing fibre terminals, i.e., a high number of round synaptic vesicles and a few small mitochondria. Most frequently they formed asymmetric synapses on stubby thorns protruding from the proximal Purkinje cell dendrites. In some instances, however, the postsynaptic element consisted of long slender spines or spines showing an atypical morphology. A number of labelled boutons was also in contact with the perikarya of reinnervated Purkinje cells, either articulating with spines or synapsing directly on the smooth somatic surface. The proximal dendrites of denervated Purkinje cells were characterized by large numbers of spines, which were frequently postsynaptic to parallel fibres. By contrast, Purkinje cells reinnervated by the sprouting climbing fibres generally showed a lower number of spines on their proximal dendrites, indicating a reversal of this morphological change. The aberrant parallel fibre input was also decreased on reinnervated dendrites or had completely disappeared. Nevertheless, some reinnervated Purkinje cells showed the persistence of some parallel fibre synapses on their proximal dendrites. On occasion, climbing fibre and parallel fibre boutons synapsed on the same spine.

Effects of unilateral climbing fibre deafferentation on cytochrome oxidase activity in the developing rat cerebellum

In a previous study, we found a relationship between climbing fibre synaptogenesis and cytochrome oxidase activity in Purkinje cells during normal development of the rat cerebellum. To determine whether removal of a major depolarizing afferent would alter the level of cytochrome oxidase activity in a post-synaptic neuron, climbing fibre input to Purkinje cells in the right hemicerebellum was interrupted by unilateral pedunculotomy in postnatal day 1 rat pups. After survival to postnatal day 5 (P5) or postnatal day 10 (P10), the cytochrome oxidase reactivity of mitochondria, packing density of mitochondria and perikaryal area of Purkinje cell somata were quantified at the electron microscopic level and compared with the same parameters in both sham-operated animals and normal controls. We found that the areal and numerical densities of darkly reactive mitochondria were lower in deafferented cells than those in the sham-operated animals. Cells of sham-operated animals, however, had higher densities of darkly reactive mitochondria than those in normal animals of the same age group, indicating that cell shrinkage or retarded growth had an effect on the levels of cytochrome oxidase activity in the operated animals. In addition, both operated groups had higher numerical densities of mitochondria than cells of normal animals, reflecting the decreased cell size of the sham and deafferented groups. From these data, we concluded that neonatal destruction of climbing fibres leads to lower levels of cytochrome oxidase activity in Purkinje cell somata that survived to both P5 and P10. The data from the P5 animals was more striking than that from P10, perhaps reflecting the increased number of synaptic interactions of Purkinje cells at P10. We also concluded that destruction of excitatory input did not lead to changes in the total area or number of mitochondria in a post-synaptic neuron, indicating that there was a conversion from darkly to lightly reactive mitochondria in the partially deafferented neurons; however, this may also reflect the smaller cell size of the deafferented group. Thus, our results further substantiate the close relationship between the levels of cytochrome oxidase activity in Purkinje cell somata and the type of input that they receive or fail to receive.

Cell loss in the inferior olive of the staggerer mutant mouse is an indirect effect of the gene

Staggerer (sg) is an autosomal recessive mutation in mouse that causes severe cerebellar atrophy. In this mutant, the Purkinje cell (PC) number is reduced by about 75% and the remaining Purkinje cells have a reduced dendritic arbor and an ectopic location. Previous analysis of staggerer chimeras has demonstrated that the Purkinje cell phenotypes are all direct consequences of the cell-autonomous action of the staggerer gene. The two major afferents to the Purkinje cell are also affected. Virtually all of the granule cells die by the end of the first postnatal month. This death, however, has been shown to be an indirect consequence of mutant gene action. The second major afferent system is from the cells of the inferior olive that project to the main trunks of the Purkinje cell dendrite via the climbing fiber system. Quantitative studies of cell number in the inferior olive have shown that the number of cells is reduced by about 62% in adult sg/sg mutants. We report here the results of our quantitative analysis of three staggerer chimeras. beta-glucuronidase activity was used as an independent cell marker. Our findings demonstrate that inferior olive cell death in staggerer mutant mice is an indirect effect of staggerer gene action. Thus, as for the granule cells, the loss of olivary neurons most likely results from a target related cell death.

Brain plasticity after corpus callosum transection in the newborn rat

The importance of afferent fibers in the final shaping of the central nervous system has been well established in several papers. In order to study how much one can influence cortical development by changing afferent input to the cortex cerebri, we transected corpus callosum in rats at 2 or 3 postnatal days. Equal numbers of lesioned and control animals were transcardially perfused with mixed aldehydes at postnatal day 6 (P-6) or P-14. Brain tissue was processed for Golgi impregnation, Nissl stain, and quantitative electron microscopic analysis. Morphometric image analysis (MOP 3) of the primary sensory area did not show significant differences with respect to cortical depth and total and apical dendritic length. The cross-sectional area and maximal diameter of neuronal cell bodies were smaller in P-14-lesioned animals. Ultrastructural analysis of the cortex revealed that synaptic density was not reduced in lesioned animals. In conclusion, early in development, rat brain can compensate for major changes in the commissural afferent system.

Development of parasagittal zonation in the rat cerebellar cortex: MabQ113 antigenic bands are created postnatally by the suppression of antigen expression in a subset of Purkinje cells

Monoclonal antibody mabQ113 recognizes a polypeptide antigen that, in the adult cerebellum, is confined to a subset of Purkinje cells that are clustered together to form parasagittal bands interposed by similar nonimmunoreactive bands. The Purkinje cell compartments are congruent with bands of climbing fibers projecting from subregions of the inferior olivary complex (IOC). The array of mabQ113 parasagittal bands appears late in the development of the cortex. Weak mabQ113 immunoreactivity is first seen at postnatal day 6 (P6) in the Purkinje cells of the posterior lobe of the vermis. From the earliest stages there are signs of differential expression of the mabQ113 antigen in clusters of Purkinje cells: four mabQ113+ clusters are clearly present in the posterior lobe of the vermis at P6-P7. Their relation to the adult band display remains uncertain. During the next few days immunoreactivity spreads rostrally throughout the rest of the vermis and laterally to include the Purkinje cells in the hemispheres, until by P12 all the Purkinje cells in the cerebellum are mabQ113+. Nevertheless, signs of the adult band display are seen already in the vermis where the cells destined to become the vermal mabQ113+ bands (P1+, P2+ and P3+) stain more intensely than their neighbours. Following the stage of global mab113 epitope expression, bands are created by the selective suppression of immunoreactivity by Purkinje cells in the P- regions. By P15 the mabQ113+ and mabQ113- bands are clearly differentiated in the vermis and selective staining has begun to appear in the hemispheres also. The band pattern matures gradually during the third and fourth postnatal weeks until the adult appearance is attained by P30. The cerebellar afferent projections were lesioned to explore the interplay of cerebellar input and mabQ113 expression. The olivocerebellar projection was lesioned bilaterally by using 3-acetylpyridine in the adult and unilaterally in the newborn by electrolytic lesion and unilateral inferior cerebellar pedunculectomy. Mossy fibers from the dorsal and ventral spinocerebellar tracts were lesioned surgically both in adults and in newborn and trigeminal projections to the cerebellum were removed in the newborn by unilateral ablation of the spinal trigeminal nucleus. The consequences of total blockage of vibrissal and hindlimb inputs were also explored in both adults and neonates. None of these treatments led to a modification in the pattern of mabQ113 epitope expression.(ABSTRACT TRUNCATED AT 400 WORDS)

Cerebellar afferents in early postnatal rats: a retrograde fluorescence study

The development of afferent projections to the cerebellum was studied by making small (20 nl) injections of True blue into the cerebella of neonatal rats under general anaesthesia. Neurones in all main precerebellar nuclei were labelled even in rats less than one day old. With the exception of the disappearance of small numbers of ipsilateral olivocerebellar projections, no qualitative differences were seen after injections in animals one or two weeks older.

Neuronal death and synapse elimination in the olivocerebellar system: III. Cell counts in the inferior olive of developing rats X-irradiated from birth

The change with age of cell number in the developing inferior olivary nucleus (ION) of the normal rat, compared to the time course of the regression of the polyneuronal innervation of Purkinje cells by olivary axons (i.e., the climbing fibers), suggests that the involution of the redundant olivocerebellar contacts is caused by a reduction of axonal branching rather than by degeneration of the parent cells, this being also suggested by the normal size of the olivary population in adult rodents whose Purkinje cells retain polyneuronal innervation. However, the similar size of the adult ION population does not necessarily imply that the development history is the same in normal and multiply innervated adult rodents. Therefore, cell counts were performed in developing rats which had been repeatedly X-irradiated from birth until postnatal day 14 and which retained polyneuronal innervation. The results show that, although less marked than during normal development, the evolution of the ION population is also characterized by a phase of cell loss followed by a slow increase. However, the number of cells in X-irradiated rats is higher than in their controls from birth to postnatal day 15 but becomes identical at 20 days and later. These data confirm that cell death in the ION does not play a major role in the shaping of olivocerebellar connections.

The dentato-olivary projection in the rat as a presumptive GABAergic link in the olivo-cerebello-olivary loop. An ultrastructural study

It is here shown that autoradiographically labelled axon terminals of the dentato-olivary projection form a heterogeneous population. However, a majority of them constitute an even class of synapses, characterized by their small axonal size, their content in pleimorphic vesicles, and the establishment of symmetric synapses on small dendrites, about 5% of which are linked through a gap junction. The same material, used for immunocytochemistry of GABA with the postembedding technique, discloses that a majority of boutons with cytological features similar to the dentato-olivary terminals are GABA-immunoreactive, especially those synapsing on dendrites linked by gap junctions. The cerebello-olivary projection, despite its heterogeneity, thus appears as part of the GABAergic system which governs the synaptic modulation of the electrotonic coupling between olivary neurons.

Morphology of Purkinje cell axon terminals in intracerebellar nuclei following inferior olive lesion

We have examined the ultrastructural changes of axons and synaptic boutons in the intracerebellar nuclei of the rat at 3 days to one year after inferior olive lesion performed by means of electrocoagulation or 3-acetylpyridine injection. A large number of preterminal segments and axons terminals undergoes remarkable ultrastructural changes after total or subtotal olivary lesion. Large membrane bound vacuoles and clusters of small synaptic vesicles characterize a good number of these terminals at 3 days up to one month after the lesion. Tightly packed tubules and cisternae of smooth endoplasmic reticulum appear during the first week in an increasing number of axon terminals. Boutons with large whorled bodies formed by smooth membranes increase in number during the second half of the first month and further increase in density until the sixth month. They are still present in large amounts at one year. Immunoreactivity for 3',5'-guanosine-phosphate-dependent protein kinase, which is specific for Purkinje neurons, can be detected in the axons and synaptic terminals displaying the ultrastructural changes described above. These results are discussed in relation to a possible trophic action of the climbing fibers on the Purkinje cells. We suggest that, at least in part, these alterations may be the consequence of the intense Purkinje cell hyperactivity which is present for up to one month from inferior olive lesion.

The effect of unilateral cerebellar pedunculotomy on the vascular development of the neonatal rat cerebellum

After cerebellar pedunculotomy the density of the blood vessel network in the cerebellar cortex was not different from that in the control animals. But the pattern of the blood vessels was different, being less organized in the operated animals.

Compensatory climbing fiber innervation after unilateral pedunculotomy in the newborn rat: origin and topographic organization

In neonatal rats the unilateral transection of the cerebellar peduncles causes a fast and complete degeneration of the contralateral inferior olive. Axons from the remaining olive recross the cerebellar midline and partially innervate the deprived hemicortex. Analysis of the topographic organization of this compensatory projection studied with the axonal tracing method provided the following results: Retrograde tracing experiments revealed that the bulk of compensatory afferents originates from neurons in the ipsilateral medial accessory olive, especially from its medial region, whereas afferents from the principal olive and the dorsal accessory olive contribute to a much lesser degree. In case of incomplete neonatal pedunculotomy, neurons with a similar location in the ipsilateral intact olive still contribute to the innervation of the partially deprived hemicortex, along with the atrophic contralateral olive. Moreover, these experiments revealed important information about the organization of the compensation. Although its specificity was not totally maintained, the mediolateral distribution of sprouted afferents in the cerebellum matched the caudorostral disposition of parent neurons in the olive, as in the case in normal olivocerebellar projection. Anterograde studies showed that compensatory fibers recrossing the cerebellar midline spread throughout the whole extent of the deprived cortex and terminate solely in the molecular layer as typical climbing fibers. The latter were not homogeneously distributed, their density being markedly reduced according to a mediolateral gradient. Compensatory projection followed a sagittal striped pattern, as does the normal climbing fiber projection. Moreover, if the cortex is divided broadly into vermal, intermediate, and hemispheral regions, an apparent reciprocity seems to exist concerning the relative involvement of the various cortical subdivision in both hemicerebella. Our present results indicate that the immature olivocerebellar system is capable of anatomical plasticity, although to a limited extent. More important, they suggest that a certain degree of specificity is maintained during the process of sprouting, resulting in a topographical arrangement of the transcommissural climbing fiber projection. This indicates, in turn, that cues which guide the growth of olivocerebellar fibers during normal development could also direct the compensatory innervation.

Afferent influences on brain stem auditory nuclei of the chicken: cessation of amino acid incorporation as an antecedent to age-dependent transneuronal degeneration

Previous studies of the avian auditory system have revealed that removal of the peripheral receptor (the cochlea) leads to a transneuronal degeneration of auditory relay neurons in nucleus magnocellularis (NM) of the brain stem. An early manifestation of the degeneration which can be observed within 12 hours is a decrease of histochemical staining for RNA (Nissl staining); such a decrease could reflect an alteration in protein synthetic activity within the NM neurons. The present study evaluates this possibility by determining whether the cochlea removal led to an alteration incorporation of protein precursors in the target neurons which exhibit transneuronal degeneration and if so, how early the changes appeared. The cochlea was removed unilaterally in seventeen 10-day-old chicks and two 66-week-old mature chickens, and incorporation of protein precursors was evaluated in the neurons of NM at 0.5, 1.5, 3, 6, 12, and 24 hours following the cochlea removal. Each chick received an intravenous injection of 3H leucine, and was allowed to survive for 30 minutes after the injection of precursor. The brains were then prepared for autoradiography. The extent of incorporation by neurons in NM was determined by counting grains overlying each cell body and determining grain density/micrometers2 of neuron cross-sectional area. We found that auditory relay neurons whose synaptic inputs have been silenced exhibit dramatic decreases in protein synthesis within 30 minutes after removal of the cochlea; leucine incorporation was reduced by about 50%. In chicks sacrificed 3 to 24 hours after removal of the cochlea, some neurons (about 1/3) were entirely unlabeled despite heavy labeling of their neighbors and heavy labeling of all NM neurons on the opposite side of the brain. The remaining neurons exhibited about a 15% reduction in incorporation in comparison with the cells in the contralateral (control) NM. While the decreases in incorporation were apparent at all survival intervals, there was no consistent decrease in Nissl staining until 6 hours after cochlea removal. There were no changes in protein precursor incorporation following removal of the cochlea in adult birds, a result which is in keeping with the relative absence of transneuronal degeneration following removal of the cochlea at maturity. The results suggest a very rapid transneuronal regulation of protein metabolism within target neurons in young animals, perhaps by activity-related events.

Membrane resistivity estimated for the Purkinje neuron by means of a passive computer model

A multicompartment passive electrotonic computer model is constructed for the cerebellar Purkinje cell of the guinea-pig. The model has 1089 coupled compartments to accurately represent the morphology of the Purkinje cell. In order that the calculated behavior of the model fit the published electrophysiological observations of somatic and dendritic input conductance, the neural membrane resistivity must be spatially non-uniform. The passive electrical parameter values for which the model best fits the observations of input conductances, pulse attenuation and current-clamp voltage transients are rm,dend = 45,740 omega cm2, rm,soma = 760 omega cm2, ri = 225 omega cm and cm = 1.16 microF/cm2 (the membrane and cytoplasm specific resistivities and membrane specific capacitance, respectively). The model with these parameter values is electrically compact, with electrotonic length X = 0.33 and dendritic dominance ratio p = 0.44. Analysis of the calculated voltage transient of the multicompartment model by the methods of equivalent-cylinder cable theory is shown to result in very different and unreliable conclusions. The significance for neuronal function of the estimated electrical parameter values is discussed. The possible effect of active conductances on these conclusions is assessed.

Afferent influences on brain stem auditory nuclei of the chicken: time course and specificity of dendritic atrophy following deafferentation

The time course and specificity of the changes in dendritic morphology following deafferentation were examined in nucleus laminaris of young chickens. The dendrites of nucleus laminaris neurons are segregated into dorsal and ventral domains, which are innervated separately from the ipsilateral and contralateral nucleus magnocellularis, respectively. Transection of the crossed dorsal cochlear tract deafferents the ventral dendrites of nucleus laminaris bilaterally without interrupting the matching input to the dorsal dendrites. In 10-day-old chicks, atrophy of the ventral dendrites began immediately after transecting the tract; the ventral dendrites were 10% shorter by 1 hour and 16% shorter by 2 hours after deafferentation. The length of the ventral dendrites progressively decreased over the next 2 weeks, resulting in at least a 60% loss of ventral dendrite 16 days after surgery. The dorsal dendrites of the same cells, whose afferents remained intact, did not change in length during the time course of this study. However, 16 days after the lesion, spines appeared on the normally smooth dorsal and ventral dendrites. The time course of dendritic atrophy and its restriction to the deafferented postsynaptic surface are related to possible mechanisms by which afferents regulate and maintain their target neurons.

Long-term effect of mossy fiber degeneration in the rat

Mossy fiber-deafferentated rats (20) were permitted to survive from 34 to to 120 days and subsequently examined following Golgi-Cox preparation or after processing for electron microscopy. The primary response to mossy fiber deafferentation was transneuronal degeneration of the granule cell system. Morphological evidence is provided that suggests that the mossy fiber varicosity plays an important role in the fragmentation and removal of the granule cell digitiform dendrite. Computer-assisted image analysis of Golgi-impregnated Purkinje cells indicated significant losses in both smooth branch and spiny branchlet numbers following loss of the mossy fiber input. Ultrastructural examination revealed that a secondary transneuronal degeneration occurred within the dendritic arborization of both Purkinje cells and molecular layer interneurons. Although an overall reduction in the number of dendritic spines occurred along the terminal branchlets following mossy fiber deafferentation, several of the existing spines underwent marked changes in length, with some elongating to more than twice their size. By increasing the length of their spines, denervated Purkinje cells may acquire new synaptic contacts.

Pontocerebellar system linking the two hemispheres by intracerebellar branching

Origin and percentage of bilaterally projecting pontocerebellar neurons whose axons branch within the cerebellum and link the two cerebellar hemispheres were studied in cats using double retrograde fluorescent tracing and lesion techniques. These pontocerebellar neurons, which can be viewed as a separate component of the pontocerebellar system, account for 5-10% of the pontine neurons projecting to the lateral hemisphere. The system is mainly composed of neurons the axons of which recross within the cerebellum. The pontocerebellar interhemispheric system described here is likely to be the basis for the previously described intracerebellar commissural system. The bilateral distribution of these fibers may provide an important substrate for the coordination of bilaterally performed movements.

Postnatal development of the inferior olivary complex in the rat. II. Topographic organization of the immature olivocerebellar projection

The state of organization of the olivocerebellar projection in newborn and 5-day-old rats has been analyzed by autoradiography of anterogradely transported 3H-leucine, as well as by retrograde transport of horseradish peroxidase. The efferent axons of the inferior olivary neurons are already present and already highly organized in the cerebellum of newborn rats. Most of the autoradiographic labelling subsequent to the injection of 3H-leucine into the inferior olive is seen in the subcortical medullary zone. Labelled axons only partially invade the gray matter, where they reach the zone occupied by randomly distributed Purkinje cells. At this immature stage, olivocerebellar projections are already entirely crossed and distributed according to a pattern which is similar to the adult. At the fifth postnatal day olivocerebellar projections have moved from the medullary zone toward the interface between the molecular and the granular layers where Purkinje cells have arranged in a monolayer. Evidence for translocation of climbing fibers from their perisomatic to their peridendritic position is already distinct in these young cerebella. Combination of anterograde and retrograde fiber system tracing experiments discloses the following crossed topography of olivocerebellar projections: The caudal half of the medial accessory olive projects mainly to the vermis of the posterior lobe, whereas its rostral half projects to the flocculus, paraflocculus, and the intermediate cortex. The principal olive, ventral and dorsal lamellae, supplies climbing fiber inputs to the hemispheric cortex. The caudal half of the dorsal accessory olive projects to the lateral portion of the vermis of the anterior lobe, whereas neurons in its rostral half send their axons toward the intermediate cortex. This topographic arrangement is, therefore, similar to that reported for adult mammals. The present results, alone or when compared with those obtained during other studies on the synaptogenesis between climbing fibers and Purkinje cells, allow the following conclusions: The climbing fibers enter the cerebellar cortex before Purkinje cells have reached the developmental phase compatible with synaptogenesis. They wait in the medullary white matter until appropriate maturation of their cellular targets. Olivocerebellar topography is roughly similar in newborn, 5-day -old, and adult rats. Synaptogenesis between climbing fibers and Purkinje cells, which is known not to start before the second postnatal day, is not necessary for the establishment of the topographic organization of the olivocerebellar projection.

Rat cerebellar afferents after unilateral pedunculotomy. A retrograde fluorescent double-labelling study

Cerebellar afferents from the lateral reticular nucleus (LRN) and inferior olive (IO) were investigated using retrograde fluorescent labelling in rats subjected to a unilateral inferior cerebellar pedunculotomy at 6-8 days of age. The remaining (ipsilateral) IO of the operated group was essentially the same as in control animals but the remaining (contralateral) LRN in the operated group had a greater percentage of neurones with collaterals to both sides of the cerebellum.

Differentiation of cerebellar anlage heterotopically transplanted to adult rat brain: a light and electron microscopic study

Pieces of cerebellar primordia from (days 14 or 15 of gestation) E14 or E15 rat embryos were dissected out and transplanted into a cavity of the occipital cortex and underlying hippocampus, over the superior colliculus of 2-month-old rats. The host animals were allowed to survive for 2 to 3 months. The cytoarchitectonic and the synaptic organizations were analyzed in 16 of such transplants. Only 4 of the implants established connections with the host brain through several thin peduncles composed of myelinated fibers. The remaining 12 implants survived in an extraparenchymal situation. Independently of its partial linking to the host brain, the graft grew and developed a cerebellar structure composed of nuclear and cortical regions. The latter exhibited normal lamination and foliation, and contained the five categories of neurons which characterize normal cerebellar cortex. Electron microscopic examination disclosed that the synaptic connections normally present in the cerebellar cortex were also formed in the implants with the exception of climbing fibers, which were absent. The cerebellar interneurons kept their normal topographic distribution and gave origin to numerous synapses which maintained their own specificity. Some mossy fibers were present in the granule cell layer at the center of typical glomeruli. However, abnormal synaptic arrangements were also observed within the neuropil of this granule cell layer. They consisted of pseudoglomerular formations composed of clusters of tightly packed small axon terminals covered by granule cell dendrites. The origin of these boutons was not established. Since they did not correspond to the classes of presynaptic elements normally synapsing on these dendrites, they constitute a new example of cerebellar heterologous synapses. Their presence could be related to changes in the cellular environment due to the rarity of mossy afferents. HRP tracing experiments, carried out in extraparenchymal transplants, have allowed us to determine that the corticonucleocortical loop of normal cerebellum is also developed in the implants. Nuclear neurons are at the origin of the mossy fibers involved in glomerular formations, whereas Purkinje cells project to the nuclear region. The establishment of these reciprocal connections could determine the functional stabilization of both kinds of cerebellar neurons and thus the long survival of extraparenchymal grafts. These results allow the conclusion that the presence of extracerebellar afferents is not necessary for the organotypic and synaptotypic differentiation of cerebellar anlage.