Biomedical subjects

P Strata

Publications and source records attributed to P Strata.

At least 37 records · Page 2Linked to original sources

PubMed1997

The early phase of horizontal optokinetic responses in the pigmented rat and the effects of lesions of the visual cortex.

Horizontal optokinetic responses of pigmented rats were studied both in intact animals and in animals that had received lesions of the visual area of the cerebral cortex. In response to uniform velocity stimulation, there was an initial phase of rapid acceleration, larger than that reported in earlier studies, followed by a period of fairly uniform acceleration until the eye velocity approached that of the stimulus. As reported previously, responses to monocular stimulation were highly asymmetric, with the responses to nasotemporal stimulation being much weaker than those to temporonasal stimulation. Responses to sinusoidal stimulation were also studied. No significant effect of cortical lesions on the responses was seen.

Animals↗

PubMed1997

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.

Animals↗

PubMed1997

Intrinsic properties and environmental factors in the regeneration of adult cerebellar axons.

The success of axon regeneration in the adult mammalian brain depends on the presence of growth-permissive environmental conditions as well as on specific properties of the affected neurons. To investigate the relative contribution of extrinsic cues and intrinsic determinants to reparative processes we have investigated the regenerative properties of olivocerebellar and Purkinje cell axons. When these axon populations are severed in the cerebellar white matter and confronted with embryonic neural grafts of cerebellar or extracerebellar origin, the former vigorously regenerate into the transplant, whereas the latter invariably fail to do so (Rossi et al., 1995). The same response occurs when dissociated Schwann cells are implanted in the lesion site: Purkinje cell axons fail to regrow, whereas olivocerebellar fibres regenerate for considerable distances. Within the graft, regenerating fibres follow tortuous courses along Schwann cell bundles and sometimes end with poorly developed terminal plexuses. Some of them, however, succeed in crossing the graft and grow further into the host cortex, where they break into fine terminal branches confined to the granular layer. The remarkable regenerative response of olivocerebellar axons revealed by these experiments might be an intrinsic reaction of the affected neurons to axon injury or it might be elicited by growth promoting cues derived from the grafts. To elucidate this point we have undertaken the investigation of cellular changes occurring in adult inferior olivary neurons following the transection of the inferior cerebellar peduncle. Our results show that axotomy induces a series of cellular changes, or reparative and regressive character, which ultimately lead to cell death. Interestingly, however, these modifications are not uniformly distributed throughout the whole inferior olive. (i) Neuronal atrophy and degeneration progress more rapidly in the PO and DAO than in the MAO. (ii) A subpopulation of inferior olivary neurons become reactive for NADPH-diaphorase histochemistry, and their preferential localisation in the MAO suggests that this modification is related to the longer survival of these cells after axotomy. (iii) The developmentally regulated calcitonin gene-related peptide (CGRP) is reexpressed by a subset of neurons in the caudal nuclear compartments. These results further emphasise the conclusion that the dissimilar regenerative response of Purkinje cell and olivocerebellar axons confronted with permissive environmental conditions is due to different intrinsic properties of these neuronal populations. The reexpression of developmentally regulated substances by axotomised inferior olivary neurons suggests that their reparative reaction is triggered by axon injury. However, the pattern of growth of regenerating olivocerebellar axons is strongly conditioned by environmental constraints, which, in the present experimental conditions, do not allow them to reattain denervated Purkinje cells.

Animals↗

PubMed1997

Olivocerebellar axon regeneration and target reinnervation following dissociated Schwann cell grafts in surgically injured cerebella of adult rats.

The ability of Schwann cells to induce the regeneration of severed olivocerebellar and Purkinje cell axons across an injury up to their deafferented targets was tested by transplanting freshly dissociated cells from newborn rat sciatic nerves into surgically lesioned adult cerebella. The grafted glial cells consistently filled the lesion gap and migrated into the host parenchyma. Transected olivocerebellar axons vigorously regenerated into the graft, where their growth pattern and direction followed the arrangement of Schwann cell bundles. Although some of these axons terminated within the transplant, many of them rejoined the cerebellar parenchyma beyond the lesion. Here, their fate depended on the territory encountered. No growth occurred in the white matter. Numerous fibres penetrated into the granular layer and formed terminal branches that remained confined within this layer. A few of them, however, regenerated up to the molecular layer and formed climbing fibres on Purkinje cell dendrites. By contrast, the growth of transected Purkinje cell axons into the grafts was very poor. These results underscore the different intrinsic responsiveness of Purkinje cell and olivocerebellar axons to the growth-promoting action of Schwann cells, and show that the development and outcome of the regenerative phenomena is strongly conditioned by the spatial organization and specific features of the environmental cues encountered by the outgrowing axons along the course they follow. However, Schwann cells effectively bridge the lesion gap, induce the regeneration of olivocerebellar axons, and direct their growth up to the deafferented host cortex, where some of them succeed in reinnervating their natural targets.

Animals↗

PubMed1996

Increase of B-50/GAP-43 immunoreactivity in uninjured muscle nerves of MDX mice.

Lack of dystrophin in mdx mice leads to muscle fibre degeneration followed by the formation of new myofibres. This degeneration-regeneration event occurs in clusters. It is accompanied by inflammation and remodelling of the intramuscular terminal nerve fibres. Since the growth-associated protein B-50/GAP-43 has been shown to be involved in axonal outgrowth and synaptic remodelling following neuronal injury, we have investigated the presence of B-50 in gastrocnemius and quadriceps muscles of mdx mice. Using immunocytochemistry we demonstrate increased presence of B-50 in terminal nerve branches at motor endplates of mdx mice, particularly in the clusters of de- and regenerating myofibres. In comparison, the control mice displayed no B-50 immunoreactivity in nerve fibres contacting motor endplates. Our findings indicate that during axonal remodelling and collateral sprouting the B-50 level in the terminal axon arbours is increased although there is no direct injury to the motoneurons. We suggest that the degenerating target and/or the inflammatory reaction induces the increased B-50 level in the motoaxons. The increased B-50 may be important for sprouting of the nerve fibres and re-establishment of synaptic contacts, and in addition, for maturation and survival of the newly formed myofibres.

Acetylcholinesterase↗

PubMed1996

Postsynaptic currents and short-term synaptic plasticity in Purkinje cells grafted onto an uninjured adult cerebellar cortex.

It has been shown recently that embryonic Purkinje cells grafted extraparenchymally into an intact cerebellum, in the absence of any sign of damage, are able to migrate into the host molecular layer where they receive a climbing fibre innervation. Using the same technique, we investigated the development of the electrophysiological properties of the synapses between the grafted cells and their main afferents. Purkinje cells either in the graft or having migrated into the molecular layer of the host were recorded using the whole-cell patch-clamp method in acutely prepared slices 17-112 days after grafting. Spontaneous postsynaptic currents with a single-exponential decay and mediated by GABAA receptors were very similar to those described in normal Purkinje cells. Excitatory postsynaptic currents (EPSCs) evoked by climbing fibre and by parallel fibre stimulation were blocked by an alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)/kainate antagonist, and displayed the linear current-voltage relation typical of postnatal Purkinje cells. The attainment of normal functional properties by the adult axons at the newly formed synaptic sites was shown by the expression of short-term facilitation of parallel fibre EPSCs and of short-term depression of climbing fibre EPSCs. The grafted Purkinje cells showed climbing fibre polyinnervation 17-20 days after grafting which evolved to monoinnervation at 23-45 days, confirming the completion of the developmental programme up to maturation. Our experiments support the view that the adult intact brain is able to accept and integrate an additional number of neurons which show fully mature electrophysiological properties which are electrophysiologically indistinguishable from those of the host neurons.

Animals↗

PubMed1996

Topographically organized climbing fibre sprouting in the adult rat cerebellum.

Adaptive recovery following brain injury requires the topography of projection maps to be restored. In the adult mammalian brain, the regeneration of severed axons does not normally occur and repair mainly relies on collateral reinnervation from uninjured neurons. Although reinnervation can be target specific at the single cell level, it is not known if the new connections are organized correctly. The normal olivocerebellar projection had precise topography in which subnuclei of the inferior olive terminate as climbing fibres on chemically defined bands of cerebellar Purkinje cells. This precision has been exploited to determine the topography of climbing fibre sprouting following an inferior olive lesion in the adult rat. Collateral reinnervation was found to respect the boundaries between the Purkinje cell compartments. Thus, topographical cues are available in the adult during post-lesion plasticity to guide the restoration of the olivocerebellar projection map.

Animals↗

PubMed1995

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.

Afferent Pathways↗

PubMed1995

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Bibliometrics↗

PubMed1995

Influence of eye motion on adaptive modifications of the vestibulo-ocular reflex in the rat.

While sustained retinal slip is assumed to be the basic conditioning stimulus in adaptive modifications of the vestibulo-ocular reflex (VOR) gain, several observations suggest that eye motion-related signals might also be involved. We oscillated pigmented rats over periods of 20 min around the vertical axis, at 0.3 Hz and 20 degrees/s peak velocity, in different retinal slip and/or eye motion conditions in order to modify their VOR gain. The positions of both eyes were recorded by means of a phase-detection coil system with the head restrained. The main findings came from the comparison of two basic conditions--including their respective controls--in which one or both eyes were reversibly immobilised by threads sutured to the eyes. In the first condition the animals were rotated in the light with one eye immobilised and the other eye free to move but covered. Rotation in the light in this open-loop condition immediately elicited high-gain compensatory eye movements of the non-impeded, covered eye. At the end of this training procedure, the VOR gain increased by 43.2%. In the second condition, both eyes were immobilised and one eye was covered. The result was an increase in the VOR gain of 26.3%. These two conditions were similar as to the visuo-vestibular drive during the exposure, but different as to the resulting--and allowed--eye motion, showing that the condition where the larger eye movements occurred yielded the larger VOR gain change. Our data support the idea proposed by Collewijn and Grootendorst (1979, p. 779) and Collewijn (1981, p. 146) that "[retinal] slip and eye movements seem to be relevant signals for the adaptation of the rabbit's visuo-vestibular oculomotor reflexes".(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptation, Physiological↗

PubMed1994

Exposure to kainic acid mimics the effects of axotomy in cerebellar Purkinje cells of the adult rat.

We have investigated the long-term structural changes which affect Purkinje cells exposed to a single dose of kainic acid. Following intraparenchymal injection of the excitotoxin in the cerebellar cortex (1 microliter of a 1 mg/ml solution), Purkinje cells which survived within the lesioned area or close to its edges showed remarkable axonal abnormalities, involving the formation of torpedoes, hypertrophy of recurrent collaterals and atrophy of the corticofugal portion of the axon. In addition, their dendritic trees were often affected by conspicuous regressive alterations. The climbing fibres contacting these Purkinje cells were characterized by thick perisomatic plexuses, whereas their peridendritic branches were atrophic. The dendrites innervated by such atrophic olivary arbours were studded with huge numbers of newly formed spines. These alterations were already present a few days after kainic acid administration and persisted for the total period of observation of 6 months after the lesion. The remarkable similarity between the abnormalities of Purkinje cells exposed to kainic acid and those observed after axotomy indicates that in these two conditions common mechanisms determine analogous long-lasting modifications in the affected neurons. It is proposed that kainic acid-induced intracellular calcium overload disrupts cytoskeletal components and impairs axonal transport, thus depriving the affected Purkinje cells of retrograde trophic influences from their target neurons. As a consequence the affected neurons undergo long-lasting regressive modifications and compensatory remodelling phenomena.

Animals↗

PubMed1994

Embryonic Purkinje cells grafted on the surface of the adult uninjured rat cerebellum migrate in the host parenchyma and induce sprouting of intact climbing fibres.

By grafting solid pieces of cerebellar anlage onto the surface of the adult rat cerebellum, we have investigated the problem of the interactions between embryonic and adult neurons in an intact brain. A few days after grafting, embryonic astrocytic processes crossed the graft--host interface and radiated into the recipient molecular layer. Several grafted Purkinje cells also migrated into the host brain along such processes as well as adult Bergmann glia. Adult climbing fibres, labelled by means of Phaseolus vulgaris leucoagglutinin (PHA-L), sprouted new collateral branches which terminated on embryonic Purkinje cells at both extra- and intraparenchymal levels. No sign of activation of host astroglia or microglia was evident in the host cerebellum in relation to these processes. Embryonic Purkinje cells which migrated into the host cerebellum developed an adult-like morphology. Intraparenchymal grafts of neocortical embryonic tissue induced conspicuous growth of host olivary axons, characterized by a pattern which was different from that observed following cerebellar grafts. By contrast, when neocortical tissue was placed onto the surface of the recipient cerebellum, graft--host interactions were limited and climbing fibre sprouting was rarely seen. These results show that (i) supernumerary Purkinje cells can penetrate and settle in the adult intact cerebellar cortex, (ii) adult climbing fibres are able to innervate these new targets in the absence of any injury or activation of non-neuronal cells of the adult brain, and (iii) in the absence of damage to the adult brain, the plasticity of adult olivary axons is specifically elicited and controlled by embryonic Purkinje cells.

Animals↗

PubMed1994

Vergence compensation during binocularly- and monocularly-evoked horizontal optokinetic nystagmus in the pigmented rat.

During horizontal optokinetic nystagmus evoked by binocular stimulation in the rat, the slow phases are well-conjugate. The fast phases in the adducting eye are on average about 2 deg greater in amplitude than those of the abducting eye. This causes a transient convergence which is compensated for by a divergent drift within the 100 msec following the fast phase. The amplitudes of these convergence-divergence components fluctuates somewhat from one fast phase to another and their relative amplitudes may differ. As a consequence differences in vergence between successive slow phases may occur. Such differences are usually of small amplitude, but may be as large as 5 deg. When optokinetic nystagmus is evoked by monocular stimulation, the slow phase velocities are different in the two eyes, giving a disjunctive component which is compensated for by a difference in the relative amplitudes and velocities of the fast phases in the two eyes. However, the divergent drift immediately following the fast phases is very similar whatever form of stimulation is employed. It is suggested that during monocularly-evoked optokinetic nystagmus the oculomotor system compensates for the disjunctive component arising during the slow phases by giving a different balance to the pulses of innervation of two eyes, resulting in fast phases of different amplitude.

Animals↗

PubMed1994

Cellular plasticity at the climbing fibre-Purkinje cell synapse as a model of plasticity in adulthood and ageing.

In the adult brain there is a high degree of plasticity which is documented here for climbing fibre terminal arborization impinging upon the proximal Purkinje cell dendrites. Such an arborization presents a more distal compartment, including 90% of branches and varicosities, whose integrity depends on the target and a more proximal compartment which survives to target degeneration. The same terminal arborization enlarges its territory of innervation through collateral sprouting when nearby Purkinje cells are deprived of their climbing fibres or when embryonic Purkinje cells are placed on the surface of an intact cerebellum. In the latter case axonal elongation occurs in the absence of degeneration products and of macrophage/microglia or astrocyte activation. In addition, the embryonic Purkinje cells are able to invade the intact host molecular layer where they develop dendritic arbours which become also innervated by climbing fibre collaterals.

Aging↗

PubMed1993

Effects of inferior olive inactivation and lesion on the activity of medial vestibular neurons in the rat.

In anaesthetized rats, the unitary activity from the medial vestibular nucleus had been recorded during horizontal sinusoidal rotation in the absence of visual stimulation. In the first series of experiments, the inferior olivary nuclei were selectively destroyed by means of 3-acetylpyridine. Unitary activity was recorded three to five days or one month after the lesion. A few days after the lesion, the average spontaneous activity, as well as the peak-to-peak amplitude of the modulation of the medial vestibular neurons during sinusoidal rotation, were significantly lower compared to those recorded in intact rats, and to those recorded one month after the lesion. In the second series of experiments, during reversible cooling of the inferior olive region of one side, in the contralateral medial vestibular nuclei 57% of units underwent a clear decrease in firing rate accompanied by a decrease in the amplitude of modulation. In rats whose inferior olivary nuclei had been destroyed by means of 3-acetylpyridine one month before, or whose cerebellum had been removed, there were few units that showed a decrease of the firing rate and modulation amplitude on cooling the same olivary region. Our experiments show that silencing the activity of the inferior olive causes a decrease both in the spontaneous firing rate and in the amplitude of the response of the vestibular neurons to natural labyrinthine stimulation. These results support the hypothesis that the inferior olive, by changing its firing rate, may regulate on-line the gain of reflexes which are under cerebellar control.

Animals↗

PubMed1993

Regressive modifications of climbing fibres following Purkinje cell degeneration in the cerebellar cortex of the adult rat.

The role of postsynaptic neurons in the maintenance of adult terminal axon arbours was investigated in the rat olivocerebellar system. The degeneration of Purkinje cells, the main target of olivary axons in the cerebellar cortex, was obtained by intraparenchymal application of kainate. The structural features of target-deprived climbing fibres, visualized by Phaseolus vulgaris leucoagglutinin tracing, were examined from two days to six months after the lesion. Following the degeneration of its Purkinje cell, the climbing fibre underwent remarkable regressive modifications involving the disappearance of most of the terminal arborization. Never the less, atrophic arbours still spanned through the molecular layer six months after the lesion. Morphometric evaluations showed that, one week after kainate application, total arbour length was already reduced to 52% of control, whereas the number of branches and of varicosities had both dropped around 40%. This retraction process progressed in the following stages to reach its maximum at about one month after the lesion, when total length was 30% of control and only 10% of branches and varicosities were still present. Only a slight tendency to a further decrease of the values could be detected at longer survival times. Branching pattern analysis revealed that such regressive phenomena mainly involved the distal compartment of the climbing fibres, the one made of fine varicose branchlets, while sparing the proximal thick branches. In addition, the whole process appeared to follow some rather strict guiding principles leading to an ordered branch retraction, from the periphery of the arbour inwards. Finally, in order to rule out the possibility that the observed changes could be due to a direct action of kainate on climbing fibres, we designed an alternative method of killing Purkinje cells by intraparenchymal injection of propidium iodide. The structural features of climbing fibres deprived of their target by such a procedure were very similar to those shown by arbours from time-matched kainate-lesioned animals at both qualitative and quantitative levels. Our results show that target deprivation induces remarkable structural modifications in the climbing fibre, leading to the retraction of most of the arbour. Never the less, the integrity of the Purkinje cell is not necessary for the maintenance of the whole arborization since its proximal compartment is maintained in the molecular layer for several months after target degeneration. It is proposed that the Purkinje cell, most likely by acting through a contact factor, directly controls the formation and the maintenance of the distal climbing fibre branches with their varicosities, which represent the presynaptic compartment of the axonal arbour.

Animals↗

PubMed1991

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.

Animals↗

PubMed1991

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.

Aged↗