The effects of reduced climbing and parallel fibre input on Purkinje cell dendritic growth

Ablation of TrkB from Enkephalinergic Precursor-Derived Cerebellar Granule Cells Generates Ataxia

In ataxia disorders, motor incoordination (ataxia) is primarily linked to the dysfunction and degeneration of cerebellar Purkinje cells (PCs). In spinocerebellar ataxia 6 (SCA6), for example, decreased BDNF-TrkB signalling appears to contribute to PC dysfunction and ataxia. However, abnormal BDNF-TrkB signalling in granule cells (GCs) may contribute to PC dysfunction and incoordination in ataxia disorders, as TrkB receptors are also present in GCs that provide extensive input to PCs. This study investigated whether dysfunctional BDNF-TrkB signalling restricted to a specific subset of cerebellar GCs can generate ataxia in mice. To address this question, our research focused on TrkbPenk-KO mice, in which the TrkB receptor was removed from enkephalinergic precursor-derived cerebellar GCs. We found that deleting Ntrk2, encoding the TrkB receptor, eventually interfered with PC function, leading to ataxia symptoms in the TrkbPenk-KO mice without affecting their cerebellar morphology or levels of selected synaptic markers. These findings suggest that dysfunctional BDNF-TrkB signalling in a subset of cerebellar GCs alone is sufficient to trigger ataxia symptoms and may contribute to motor incoordination in disorders like SCA6.

Maturation of Purkinje cell firing properties relies on neurogenesis of excitatory neurons

Preterm infants that suffer cerebellar insults often develop motor disorders and cognitive difficulty. Excitatory granule cells, the most numerous neuron type in the brain, are especially vulnerable and likely instigate disease by impairing the function of their targets, the Purkinje cells. Here, we use regional genetic manipulations and in vivo electrophysiology to test whether excitatory neurons establish the firing properties of Purkinje cells during postnatal mouse development. We generated mutant mice that lack the majority of excitatory cerebellar neurons and tracked the structural and functional consequences on Purkinje cells. We reveal that Purkinje cells fail to acquire their typical morphology and connectivity, and that the concomitant transformation of Purkinje cell firing activity does not occur either. We also show that our mutant pups have impaired motor behaviors and vocal skills. These data argue that excitatory cerebellar neurons define the maturation time-window for postnatal Purkinje cell functions and refine cerebellar-dependent behaviors.

Interactions Between Purkinje Cells and Granule Cells Coordinate the Development of Functional Cerebellar Circuits

Cerebellar development has a remarkably protracted morphogenetic timeline that is coordinated by multiple cell types. Here, we discuss the intriguing cellular consequences of interactions between inhibitory Purkinje cells and excitatory granule cells during embryonic and postnatal development. Purkinje cells are central to all cerebellar circuits, they are the first cerebellar cortical neurons to be born, and based on their cellular and molecular signaling, they are considered the master regulators of cerebellar development. Although rudimentary Purkinje cell circuits are already present at birth, their connectivity is morphologically and functionally distinct from their mature counterparts. The establishment of the Purkinje cell circuit with its mature firing properties has a temporal dependence on cues provided by granule cells. Granule cells are the latest born, yet most populous, neuronal type in the cerebellar cortex. They provide a combination of mechanical, molecular and activity-based cues that shape the maturation of Purkinje cell structure, connectivity and function. We propose that the wiring of Purkinje cells for function falls into two developmental phases: an initial phase that is guided by intrinsic mechanisms and a later phase that is guided by dynamically-acting cues, some of which are provided by granule cells. In this review, we highlight the mechanisms that granule cells use to help establish the unique properties of Purkinje cell firing.

Climbing Fiber Development Is Impaired in Postnatal Car8 wdl Mice

The cerebellum is critical for an array of motor functions. During postnatal development, the Purkinje cells (PCs) guide afferent topography to establish the final circuit. Perturbing PC morphogenesis or activity during development can result in climbing fiber (CF) multi-innervation or mis-patterning. Structural defects during circuit formation typically have long-term effects on behavior as they contribute to the phenotype of movement disorders such as cerebellar ataxia. The Car8 ^wdl mouse is one model in which early circuit destruction influences movement. However, although the loss of Car8 leads to the mis-wiring of afferent maps and abnormal PC firing, adult PC morphology is largely intact and there is no neurodegeneration. Here, we sought to uncover how defects in afferent connectivity arise in Car8 ^wdl mutants to resolve how functional deficits persist in motor diseases with subtle neuropathology. To address this problem, we analyzed CF development during the first 3 weeks of life. By immunolabeling CF terminals with VGLUT2, we found evidence of premature CF synapse elimination and delayed translocation from PC somata at postnatal day (P) 10 in Car8 ^wdl mice. Surprisingly, by P15, the wiring normalized, suggesting that CAR8 regulates the early but not the late stages of CF development. The data support the hypothesis of a defined sequence of events for cerebellar circuits to establish function.

Profound morphological and functional changes of rodent Purkinje cells between the first and the second postnatal weeks: a metamorphosis?

Between the first and the second postnatal week, the development of rodent Purkinje cells is characterized by several profound transitions. Purkinje cells acquire their typical dendritic "espalier" tree morphology and form distal spines. During the first postnatal week, they are multi-innervated by climbing fibers and numerous collateral branches sprout from their axons, whereas from the second postnatal week, the regression of climbing fiber multi-innervation begins, and Purkinje cells become innervated by parallel fibers and inhibitory molecular layer interneurons. Furthermore, their periods of developmental cell death and ability to regenerate their axon stop and their axons become myelinated. Thus a Purkinje cell during the first postnatal week looks and functions differently from a Purkinje cell during the second postnatal week. These fundamental changes occur in parallel with a peak of circulating thyroid hormone in the mouse. All these features suggest to some extent an interesting analogy with amphibian metamorphosis.

Developmental plasticity of the dendritic compartment: focus on the cytoskeleton

Plasticity, the ability to undergo lasting changes in response to a stimulus, is an important attribute of neurons. It allows proper development and underlies learning, memory, and the recovery of the nervous system after severe injuries. Often, an outcome of neuronal plasticity is a structural plasticity manifested as a change of neuronal morphology. In this chapter, we focus on the structural plasticity of dendritic arbors and spines during development. Dendrites receive and compute synaptic inputs from other neurons. The number of dendrites and their branching pattern are strictly correlated with the function of a particular neuron and the geometry of the connections it receives. The development of proper dendritic tree morphology depends on the interplay between genetic programming and extracellular signals. Spines are tiny actin-rich dendritic protrusions that harbor excitatory synapses. No consensus has been reached regarding how dendritic spines form, and several models of spine morphogenesis exist. Nevertheless, most researchers agree that spinogenesis is an important target for structural plasticity. In this chapter, we discuss examples of such plasticity and describe the principles and molecular mechanisms underlying this process, focusing mostly on the regulation of the cytoskeleton during dendrito- and spinogenesis.

Maternal care effects on the development of a sexually dimorphic motor system: the role of spinal oxytocin

Maternal licking in rats affects the development of the spinal nucleus of the bulbocavernosus (SNB), a sexually dimorphic motor nucleus that controls penile reflexes involved with copulation. Reduced maternal licking results in decreased motoneuron number, size, and dendritic length in the adult SNB, as well as deficits in adult male copulatory behavior. Our previous findings that licking-like tactile stimulation influences SNB dendritic development and upregulates Fos expression in the lumbosacral spinal cord suggest that afferent signaling is changed by differences in maternal stimulation. Oxytocin afferents from the hypothalamus are a possible candidate, given previous research that has shown oxytocin is released following sensory stimulation, oxytocin modulates excitability in the spinal cord, and is a pro-erectile modulator of male sex behavior. In this experiment, we used immunofluorescence and immediate early gene analysis to assess whether licking-like tactile stimulation of the perineum activated parvocellular oxytocinergic neurons in the hypothalamus in neonates. We also used enzyme immunoassay to determine whether this same stroking stimulation produced an increase in spinal oxytocin levels. We found that stroking increased Fos immunolabeling in small oxytocin-positive cells in the paraventricular nucleus of the hypothalamus, in comparison to unstroked or handled control pups. In addition, 60s of licking-like perineal stimulation produced a transient 89% increase in oxytocin levels in the lumbosacral spinal cord. Together, these results suggest that oxytocin afferent activity may contribute to the effects of early maternal care on the masculinization of the SNB and resultant male copulatory behavior.

Maternal care effects on SNB motoneuron development: the mediating role of sensory afferent distribution and activity

Maternal licking in rats affects the development of the spinal nucleus of the bulbocavernosus (SNB), a sexually dimorphic motor nucleus that controls penile reflexes involved with copulation. Reduced maternal licking produces decreased motoneuron number, size, and dendritic length in the rostral portion of the adult SNB as well as deficits in adult male copulatory behavior. Previous research suggests that decreases in perineal tactile stimulation may be responsible for these effects. To determine whether the regional effects of maternal licking on SNB morphology are driven by sensory afferent innervation of the lumbosacral spinal cord, we used WGA-HRP to reconstruct the location of sensory afferent fibers from the perineal skin. We found that these fibers are caudally concentrated relative to the area of the SNB dendritic field, with the rostral dendritic arbor receiving little perineal afferent innervation. We also assessed Fos expression following perineal tactile stimulation to determine whether it increased local spinal cord activity in the SNB dendritic field. Sixty seconds of licking-like perineal stimulation produced a transient 115% increase in Fos expression in the area of the SNB dendritic field. This effect was driven by a significant increase in Fos in the caudal portion of the SNB dendritic field, matching the pattern of perineal afferent fiber labeling. Perineal tactile stimulation also produced significantly greater Fos expression in male pups than in female pups. Together, these results suggest that perineal sensory afferent activity mediates the effects of early maternal care on the masculinization of the SNB and resultant male copulatory behavior.

Tactile stimulation during artificial rearing influences adult function and morphology in a sexually dimorphic neuromuscular system

Maternal licking of rat pups affects the development of the spinal nucleus of the bulbocavernosus (SNB), a sexually dimorphic motor nucleus that controls penile reflexes involved with copulation. Maternal licking influences SNB motoneurons, with reductions in licking producing decreased SNB number, size, and dendritic length in adulthood. Reduced maternal licking also produces deficits in adult male copulatory behavior. In this experiment, we used an artificial rearing paradigm to assess the potential role of tactile stimulation in mediating the effects of maternal licking on the SNB neuromuscular system. During artificial rearing, pups were stroked with a paintbrush to mimic maternal licking, receiving low, medium, or high levels of daily stimulation. In adulthood, ex copula penile reflex behavior was tested and the morphology of SNB motoneurons assessed. SNB motoneurons were retrogradely labeled with cholera toxin-conjugated HRP and dendritic arbor was reconstructed in three dimensions. Animals that received low levels of stimulation showed deficits in penile reflexes relative to maternally reared controls, including a longer latency to erection, fewer cup erections, and fewer erection clusters. SNB dendritic morphology was also shaped by stimulation condition, with animals that received low or medium levels of stimulation showing an average 27% reduction in dendritic length. In addition, several reflex behaviors were significantly correlated with dendritic length, including latency to first erection, percent of cup erections, and number of erection clusters. These results suggest that tactile stimulation provided by maternal licking mediates some of the effects of maternal care on the development of male copulatory behavior.

A promoter element with enhancer properties, and the orphan nuclear receptor RORalpha, are required for Purkinje cell-specific expression of a Gi/o modulator

The promoter and structural portion of the gene, Pcp-2(L7), has frequently been used to target expression of proteins to cerebellar Purkinje cells. In our continuing analysis of the transcription of this gene and how it relates to the G-protein and Ca2+ channel modulatory functions of the encoded protein, we have dissociated the promoter and structural gene and identified cooperative functions. A 0.9 kb fragment of the proximal promoter has positional properties of a classical enhancer, yet its function requires the presence of the structural gene. We demonstrate that RORalpha, the gene product of the mutant mouse locus called staggerer (Rora(sg)), binds to and activates expression through this promoter element using functional assays in vitro and in vivo. The structural gene has a repressive effect on gene expression outside Purkinje cells, and likely participates in the suppression of Pcp-2(L7) gene expression in the many other brain and non-neuronal cell types, besides Purkinje cells, known to express RORalpha. Additional studies in vivo show that while Pcp-2(L7) expression is dependent on RORalpha throughout the cerebellum, this dependence is greatest in the intermediate region between the vermis and far lateral hemispheres. Thus, in addition to its recently indicated role in Ca2+-mediated reciprocal cell-cell signaling in Purkinje cells, RORalpha may also contribute to functional differences in cerebellar subregions.

RORalpha, a pivotal nuclear receptor for Purkinje neuron survival and differentiation: from development to ageing

RORalpha (Retinoid-related Orphan Receptor) is a transcription factor belonging to the superfamily of nuclear receptors. The spontaneous staggerer (sg) mutation, which consists of a deletion in the Rora gene, has been shown to cause the loss of function of the RORalpha protein. The total loss of RORalpha expression leads to cerebellar developmental defects, particularly to a dramatic decreased survival of Purkinje cells and an early block in the differentiation process. This review focuses on recent studies which position RORalpha as a pivotal factor controlling Purkinje cell survival and differentiation, from development to ageing.

Maternal licking influences dendritic development of motoneurons in a sexually dimorphic neuromuscular system

Maternal licking of pups' perineal regions affects the development of the spinal nucleus of the bulbocavernosus (SNB), a sexually dimorphic motor nucleus in the lumbar spinal cord that controls penile reflexes involved with copulation. Maternal licking influences SNB motoneuron number, with reductions in licking resulting in fewer motoneurons. Reduced maternal licking also has functional consequences in adulthood, resulting in increased latency to ejaculation and postejaculatory intromission and longer interintromission intervals. In this experiment, we assessed the potential effect of maternal licking on the development of SNB dendritic morphology. To reduce maternal licking, dams were treated with intranasal application of zinc sulfate during the first two postnatal weeks, which interferes with their ability to detect the pup odors that drive the licking behavior. At either postnatal day (P) 28 (when SNB dendritic length is normally maximal) or P49 (when SNB dendritic morphology is normally mature), SNB motoneurons were retrogradely labeled with cholera toxin-conjugated HRP, and dendritic arbor was reconstructed in three dimensions. At P28, the dendritic arbor of reduced maternal licking pups was not different from controls; however at P49, reduced licking pups showed a 23% reduction in dendritic arbor in the SNB, an effect that was especially pronounced in the rostral end of the nucleus, where reductions reached 48%. These results suggest that reductions in perineal stimulation provided by maternal licking could affect adult male copulatory behavior via alterations in SNB motoneuron morphology, and thus support maternal licking as an important factor in normal neural and behavioral development.

Theoretical analysis of a synaptotropic dendrite growth mechanism

It is generally believed that the genome cannot encode explicit instructions to form each synaptic connection in the nervous system, but may provide general neurite growth mechanisms which will result in proper connectivity. Recent in vivo imaging has provided evidence for a synaptotropic growth mechanism, wherein synapses could influence dendrite growth by selectively stabilizing filopodia upon which they form. We undertook a theoretical investigation into the consequences of such a growth process. Discrete stochastic simulations demonstrate that the synaptotropic mechanism can result in decreased dendritic wiring length, is capable of searching for regions of high density pre-synaptic partners, and can recapitulate specific patterns of dendrite growth and connectivity. A mean-field analysis shows that growth by selective stabilization of filopodia can be approximated as a reaction-diffusion system, with a spatially varying diffusion constant that depends on the probability of synapse formation. Thus, growth will occur faster in regions of appropriate synaptic connections, and the net growth can be shown to climb a gradient of synaptic partner density. Synaptotropic growth thus presents a mechanism for the emergent development of connectivity based on local properties of the circuit elements, rather than explicit dependence on global guidance molecules or innate predetermined branching programs.

Development of a sexually dimorphic neuromuscular system in male rats after spinal transection: morphologic changes and implications for estrogen sites of action

The lumbar spinal cord of rats contains the sexually dimorphic, steroid-sensitive spinal nucleus of the bulbocavernosus (SNB). In male rats, SNB motoneurons exhibit a biphasic pattern of dendritic growth, having an initial period of exuberant growth followed by a period of retraction to mature lengths by 7 weeks of age. This growth is steroid dependent: dendrites fail to grow after castration, but growth is supported in castrates treated with estradiol. In this experiment, we examined whether supraspinal afferent input by means of descending spinal tracts to the SNB was involved in the normal postnatal development of SNB motoneurons, and whether the effect of estradiol on SNB dendritic growth could be explained by an indirect action of estradiol on supraspinal afferents. Motoneuron morphology was assessed in normal males, early- or late-postnatally transected males, castrated males left untreated or treated with estradiol, and transected castrates treated with estradiol. SNB motoneurons were retrogradely labeled with cholera toxin-horseradish peroxidase during both the growth and retraction phases of dendritic development and reconstructed in three dimensions. The removal of supraspinal afferents resulted in extremely local effects within the developing SNB arbor, as well as transient alterations in somal growth. Furthermore, spinal transection did not block the trophic effect of estradiol on supporting SNB dendritic growth, indicating that estrogens do not act by means of supraspinal input to support SNB motoneuron development.

Ultrastructural analysis of climbing fiber-Purkinje cell synaptogenesis in the rat cerebellum

Previous reports have described the transient expression of the neuropeptides calcitonin gene-related peptide and neuropeptide Y in selected subsets of rat olivocerebellar compartments during embryonic and postnatal development. Using these neuropeptides as endogenous markers for olivocerebellar fibers, the aim of this electron microscopic analysis was to reveal the synaptogenetic processes occurring between climbing fibers and their target Purkinje cells, from embryonic day 19 to postnatal day 16, the period during which Purkinje cells undergo intense emission and retraction of dendrites, and climbing fibers translocate their synapses along Purkinje cell membrane surfaces. The present findings provide the first direct evidence that climbing fiber synaptogenesis starts on embryonic day 19 and that these first synapses mainly involve the Purkinje cell embryonic dendrite rather than the Purkinje cell soma. At the same age, the presence of unlabeled synapses resembling calcitonin gene-related peptide-labeled synapses in the Purkinje cell plate makes it possible to conclude that climbing fibers form a major synaptic investment on embryonic Purkinje cells, a finding that strongly supports the hypothesis of an early differentiating role of climbing fibers on cerebellar development. Furthermore, during the period of intense dendritic remodeling of Purkinje cells, 'myelin figures' were often detected in Purkinje cell dendrites suggesting that they may at least in part represent real ultrastructural markers of membrane turnover that identifies the sites where Purkinje cell dendritic rearrangement is taking place. Finally the finding that the climbing fiber terminals apposed to degenerating dendrites did not generally show signs of degeneration leads us to suggests that climbing fiber translocation from a perisomatic to a dendritic location may be driven by the Purkinje cell dendritic remodeling.

Progressive atrophy of cerebellar Purkinje cell dendrites during aging of the heterozygous staggerer mouse (Rora(+/sg))

Staggerer (Rora(sg/sg)) is an autosomal mutation in an orphan nuclear hormone receptor gene, RORalpha, that acts intrinsically within the Purkinje cells and causes dysgenesis of the cerebellar cortex. Purkinje cell number is severely reduced, and the surviving cells are small with poorly developed dendrites. In contrast, the cytoarchitecture of the cerebellar cortex of the heterozygous staggerer (Rora(+/sg)) appears to be normal. However, quantitative studies have revealed a premature loss of Purkinje cells with advancing age. Most of the loss (25--30%) is complete by 13 months with little change thereafter. To address the question of whether all Purkinje cells, even the surviving ones, are affected by aging even though their cell bodies remain intact, we studied the evolution with age of the dendritic arbor through a semi-quantitative analysis of Golgi-impregnated Purkinje cells. A total of ten different morphological parameters were measured in 4-, 12- and 22-month-old wild type and heterozygous Rora(+/sg) mice. While the effects of the aging process are apparent in the wild type cerebellum, they are considerably accelerated in the Rora(+/sg) mouse. By 12 months the Rora(+/sg) Purkinje cell dendrite is as atrophic as a wild type dendrite from a 22-month-old and the dendritic regression continues well beyond the period of cell death in the heterozygous Rora(+/sg) mouse.

Cytological compartmentalization in the staggerer cerebellum, as revealed by calbindin immunohistochemistry for Purkinje cells

The staggerer mouse carries a deletion in a gene encoding the nuclear hormone receptor RORalpha, which leads to severe impairments in phenotypic differentiation of cerebellar Purkinje cells. We previously found parasagittal compartments in the mature staggerer cerebellum, as defined by different transcription levels of Purkinje cell-specific molecules including calbindin. In the present study, we developed a hightiter anti-calbindin antibody to examine morphological features of the staggerer Purkinje cells. Immunohistochemistry for calbindin revealed compartmentalized Purkinje cell populations with different cell sizes, alignments, cell densities, and dendritic arborization, as well as different immunoreactivities, corresponding to the "transcriptional" compartments. Based on these immunohistochemical and cytological characteristics, the rostral cerebellum was clearly subdivided into three to seven parasagittal zones (Zones I-VII). Purkinje cells in Zones I and III were associated with the strongest calbindin immunoreactivities and exhibited morphological features reminiscent of the wild-type cells, i.e., large flask-shaped cell bodies, monolayer alignment, and arborized dendrites. Purkinje cells in Zone V were also labeled strongly, but they were small in cell size, ectopic and possessed long unbranched dendrites. On the other hand, Purkinje cells in Zones II, IV, and VI were very low in calbindin immunoreactivity and marked by small cell size, ectopia, poorly-developed dendrites and low cell density. Considering that this unique cytological compartmentalization emerges as the result of RORalpha gene mutation, it is suggested that normal cytodifferentiation of Purkinje cells is governed by both RORalpha-dependent and -independent mechanisms, and further that the latter mechanism might exert unevenly along the mediolateral cerebellar axis.

Distribution of metabotropic glutamate receptor type 1a in Purkinje cell dendritic spines is independent of the presence of presynaptic parallel fibers

The metabotropic glutamate receptor type 1a (mGluR1a) is expressed at a high level in the molecular layer of the cerebellar cortex, where it is localized mostly in dendritic spines of Purkinje cells, innervated by parallel fibers. Treatment with methylazoxymethanol (MAM) of mouse pups at postnatal days (PND) 0 + 1 or 5 + 6 results in the partial loss of granule cells, the extent of which depends on the age of the animal at the time of injection. As a consequence of hypogranularity, the number of parallel fibers is decreased to such an amount that many of the postsynaptic Purkinje cell dendritic spines are devoid of axonal input, and only a limited number of spines participate in the formation of parallel fiber synapses, or, infrequently, in heterologous or heterotopic synapses with other presynaptic partners. At PND 30, 50% of the spines in the cerebella of mice treated with MAM at PND 0 + 1 was not contacted by any presynaptic element, compared to 5% in controls or 15% in the cerebella of mice treated with MAM at PND 5 + 6. The localization of mGluR1a was visualized by immunocytochemistry on ultrathin sections: approximately 80% of all Purkinje cell dendritic spines were immunopositive in controls and in both groups of MAM-treated mice, indicating that mGluR1a was present in Purkinje dendritic spines even when the corresponding synaptic input was absent. This observation indicates that the expression and subcellular distribution of mGluR1a are inherent, genetically determined properties of Purkinje cells.

Conditional ablation of cerebellar astrocytes in postnatal transgenic mice

Astrocytes have been proposed to have multiple roles in the development and maintenance of the vertebrate CNS. To facilitate documentation of these roles, we designed a transgene to enable their ablation at selectable times. The transgene consists of the coding region for the herpes simplex virus-thymidine kinase (HSV-TK) under the control of the human glial fibrillary acidic protein gene promoter. The HSV-TK is innocuous but converts the antiherpetic agent ganciclovir (GCV) to a toxic product that interferes with DNA replication in proliferating cells. In a developmental study, transgenic mice were treated with GCV during the first postnatal week, with evaluation at P19. Treated mice displayed severe ataxia. Histological examination revealed disrupted astrocyte development, particularly in the cerebellum, with marked secondary effects on other cell types. Cerebellar defects included a loss in the numbers of astrocytes and an overall reduction in cerebellar size and disruption of the normally well defined cellular layers. Radial glia were disordered, Purkinje cells were ectopically distributed and displayed abnormal dendritic trees, and granule cells were markedly depleted. These effects were more severe in animals treated on postnatal day 1 versus treatment at day 5. A major factor causing granule cell death was excitotoxicity attributable to activation of NMDA receptors. These results suggest a critical role for astrocytes in cerebellar development.

Abnormal Purkinje cell dendrites in lurcher chimeric mice result from a deafferentation-induced atrophy

Previous studies of Purkinje cell dendrites in lurcher<-->wild-type mouse chimeras (lurcher chimeras) have documented the surprising occurrence of unusual atrophic dendritic morphologies among the wild-type cells of the mosaic cerebella. We have hypothesized that these aberrant morphologies arise from a process of developmental deafferentation that is due to the unique loss of mutant Purkinje cells in these chimeras. These earlier studies left unanswered the question of whether the abnormal dendrites were the result of a blocked developmental process (agenesis) or regressive events that deform a previously well-developed dendritic arbor (atrophy). Using a set of simple morphometric measures, we now examine wild-type Purkinje cells in young lurcher chimeras. At postnatal day 20, normal Purkinje cell development is nearly but not fully complete. In lurcher chimeras, the morphologies of the wild-type Purkinje cell dendrites are similar to those in wild-type controls of the same age. This means that they are larger in height, width, and cross-section than their counterparts in adult lurcher chimeras. The younger cells exhibit almost none of the atrophic morphologies described in mature animals. We conclude that the aberrant morphologies found in adult lurcher chimeras arise from atrophy rather than through a failure in development. Furthermore, consideration of the details of the wild-type dendrites in the lurcher chimeras leads to the proposal that the height and width of the Purkinje cell dendritic tree are controlled by two independent mechanisms.

Motoneuron development after deafferentation. I. dorsal rhizotomy does not alter growth in the spinal nucleus of the bulbocavernosus (SNB)

The spinal nucleus of the bulbocavernosus (SNB) and the dorsolateral nucleus (DLN) are sexually dimorphic motor nuclei in the rat lumbar spinal cord. During postnatal development, SNB and DLN motoneurons grow substantially in measures of soma size, dendritic length, and radial dendritic extent. SNB motoneurons exhibit a biphasic pattern of dendritic growth, where there is an initial period of exuberant growth followed by a period of retraction to mature lengths by 7 weeks. In this experiment, we examined whether primary afferent input to the SNB nucleus was necessary for the normal postnatal growth of SNB motoneurons. We partially deafferented the SNB via unilateral dorsal rhizotomy of lumbosacral dorsal roots in male rats at 1 week of age. Using cholera toxin horseradish peroxidase (BHRP) to visualize SNB motoneurons, we examined SNB motoneuron morphology at 4 and 7 weeks of age. SNB motoneurons in rhizotomized males developed normally; measures of dendritic length in rhizotomized males were typically exuberant at 4 weeks of age, and declined significantly to mature lengths by 7 weeks of age. In addition, dorsal rhizotomy did not alter the development of SNB motoneuron soma size or radial dendritic extent. These results are discussed in reference to sensorimotor connections in the SNB, the extent of the deafferentation, and dendrodendritic interactions.

Insulin-like growth factor I is an afferent trophic signal that modulates calbindin-28kD in adult Purkinje cells

Recent evidence suggests that Purkinje cells are specific targets of insulin-like growth factor I (IGF-I) through their entire life span. During development, Purkinje cell numbers and their calbindin-28kD content increase after IGF-I treatment in culture. In the adult, part of the IGF-I present in the cerebellum is transported from the inferior olive, and modulates Purkinje cell function. We investigated whether IGF-I produced by inferior olive neurons and transported to the contralateral cerebellum through climbing fibers may modulate the levels of calbindin-28kD in the cerebellum of adult animals. Twenty-four hr after injection of an antisense oligonucleotide of IGF-I into the inferior olive, both IGF-I and calbindin-28kD levels in the contralateral cerebellar lobe were significantly reduced, while the number of calbindin-positive Purkinje cells was unchanged. The effect of the antisense on IGF-I levels was fully reversed 3 days after its injection into the inferior olive, with a postinhibitory rebound observed at this time, while calbindin-28kD levels slowly returned to control values. A control oligonucleotide did not produce any change in either IGF-I or calbindin-28kD content in the cerebellum. These results indicate that normal levels of IGF-I in the inferior olive are necessary to maintain appropriate levels of IGF-I in the cerebellum and of calbindin-28kD in the Purkinje cell. These results also extend our previous findings on the existence of an olivo-cerebellar IGF-I-containing pathway with trophic influence on the adult Purkinje cell.

Acutely isolated and cultured cells from the electrosensory lateral line lobe of a gymnotiform teleost

The present study established the morphological and immunocytochemical criteria necessary to identify neuronal and nonneuronal cells after dissociating select regions of the medullary electrosensory lateral line lobe of adult weakly electric fish (Apteronotus leptorhynchus). Cells dissociated from the pyramidal cell body layers of the centromedial and lateral segments exhibited similar characteristics in the acutely dissociated preparation and up to 14 days in culture. Basilar and nonbasilar pyramidal cells were tentatively identified according to a bipolar or monopolar process extension, and polymorphic cells by the extension of three or more processes and positive immunoreactivity for gamma-aminobutyric acid. Nonneuronal cells were identified by the pattern of process arborization and positive immunolabel for gamma-aminobutyric acid or glial fibrillary acidic protein. Neuronal cells increased in total number over the first 4 days and could appear for the first time on any day in culture. Individual pyramidal cells could maintain their morphology from the time of dissociation and over several days in culture. Pyramidal cell processes were phenotypically similar to apical and basal dendrites found in situ but were reduced in size and in the degree of process branching. These results indicate that dissociated adult apteronotid neurons can maintain a morphology sufficiently similar to that found in situ as to allow tentative identification, opening up a wide range of possibilities for studying the electrophysiological and regenerative properties of electrosensory neurons.

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.

Purkinje cell dendritic arbors in chick embryos following chronic treatment with an N-methyl-D-aspartate receptor antagonist

The normal development of Purkinje cell dendrites is dependent on afferent innervation. To investigate the role of neuronal activity in Purkinje cell dendritic development, chick embryos were chronically treated with a potent, selective, and systemically active competitive N-methyl-D-aspartate (NMDA) receptor antagonist, NPC 12626. The NMDA receptor was chosen as a target for pharmacological blockade because of the importance of the NMDA receptor in synaptic plasticity and stabilization in development. Chick embryos were given daily injections of NPC 12626 (25 to 100 mg/kg) from embryonic day 14 (E14) to E17. The initial injections of NPC 12626 dramatically blocked embryo movements, but activity levels partially recovered following subsequent injections. Embryo movements were reduced by 24% at the end of the experiment. Embryos were killed on E18, and their brains processed for Golgi-Cox staining. The morphology of Golgi-stained Purkinje cells in drug-treated embryos was similar to control embryos. Morphometric analysis showed, however, that chronic treatment with NPC 12626 resulted in a 19% reduction in Purkinje cell dendritic tree area and a 13% reduction in the number of dendritic branch points. The overall width and height of the drug-treated dendritic trees were not significantly different from controls, suggesting that NPC 12626 reduced Purkinje cell dendritic area by interfering with branch formation. The volume of the granule cell layer and the heights of the molecular and external granule cell layers was not reduced, suggesting that NPC 12626 treatment did not simply delay development. These results suggest that activation of the NMDA receptor may mediate the afferent-target interactions in the cerebellum that regulate the elaboration of Purkinje cell dendrites.

Developmental dynamics of Purkinje cells and dendritic spines in rat cerebellar cortex

Quantitative morphological changes of the developing Purkinje cells were studied from 6 to 90 postnatal (PN) days in the IVth lobule of vermis in the cerebellum of rats. The soma size (mean diameter) of Purkinje cells increased rapidly between 6 PN (on average 10 microns) and 18 PN (about 17 microns) days; it did not change between 18 and 25 PN days, but increased moderately again between 25 and 48 PN days (22-23 microns) and stabilized on the same value. In contrast, the number of Purkinje cells/100 microns (the "linear density") decreased rapidly from 6 to 18 PN days. The molecular layer area belonging to 1 Purkinje cell increased rapidly from 6 to 25 PN days (from about 370 to 6,200 microns 2) and less rapidly between PN days 30 to 48 (up to 9,300 microns 2), followed by a moderate decrease at PN day 90 (about 6,600 microns 2). The volume belonging to 1 Purkinje cell dendritic arbor was about 5,500 microns 3 at PN day 6, 93,000 microns 3 at PN day 25, and 100,000 microns 3 at PN day 90. The numerical density of dendritic spines in the molecular layer showed a biphasic curve: a rapid increase from PN days 6 to 21 followed by a significant but short decrease at PN day 25, moderate rise from PN days 25 to 48, and a subsequent decline between PN days 48 and 90. The number of spines belonging to 1 Purkinje cell showed two developmental "peaks": the first peak at 21 PN days was moderate (5.6 x 10(4) spines/Purkinje cell) while the second maximum at 48 PN days was more significant (1.2 x 10(5) spines/Purkinje cell), which then declined to 6.3 x 10(4) spines/Purkinje cell at PN day 90. It is suggested that the temporary overproduction and the following decline in the number of Purkinje dendritic spines during the development of the cerebellar cortex may be the morphological indicator of the dynamics of synaptogenetic and of synaptic stabilization processes.

The effect of influenza C virus on the Purkinje cells of chick embryo cerebellum

Intra-amniotic inoculation of influenza C virus resulted in observable and quantitatively measurable changes in the Purkinje cells of chick embryo cerebellum. Purkinje cells were visualized by the Golgi-Cox procedure and prepared for statistical and computer evaluation from camera lucida drawings. Four computer-generated measurements (the area of the dendritic arbor, the perimeter of the dendritic tree, and the height and width of the cell's arborization) and two manually counted measurements (total number of branches and the number of first order branches) were made. Analysis of Purkinje cells from influenza C virus-infected embryos showed disturbances in dendritic arborization patterns and misalignment in the arrangement of the cells in the Purkinje cell layer compared to control cells. Statistical evaluation of Purkinje cell arborization showed significant decreases in all measured parameters for the influenza C virus-infected members when compared with the members of the uninfected control group.

Cell-cell interactions influence survival and differentiation of purified Purkinje cells in vitro

To determine the role of cell-cell interactions in Purkinje cell survival and dendritic differentiation, perinatal mouse Purkinje cells were purified, and their development was analyzed in vitro. In isolation at low density, Purkinje cell survival was poor, improved by neuronal contacts, either with purified granule neurons or with Purkinje cells themselves. Moreover, coculture with specific cell populations led to widely different degrees of Purkinje cell differentiation. Purified Purkinje cells cultured alone or with an inappropriate afferent, the mossy fibers, did not progress beyond immature forms. With astroglia, Purkinje cells had thin smooth processes. Proper Purkinje cell differentiation was driven only by coculture with granule cells, resulting in dendrites with spines receiving synapses. These results suggest that Purkinje cell differentiation is regulated by local epigenetic factors, provided in large part by the granule neuron.

Development of the dendritic fields of layer 3 pyramidal cells in the kitten's visual cortex

The cat's visual cortex is immature at birth and undergoes extensive postnatal development. For example, cells of layers 2 and 3 do not complete migration until about 3 weeks after birth. Despite the importance of dendritic growth for synaptic and functional development, there have been few studies of dendritic development in the cat's visual cortex to correlate with numerous studies of functional and synaptic development. Accordingly, we used the Golgi method to study the development of the dendrites of layer 3 pyramidal cells in the visual cortex of a series of cats ranging in age from 2 days to 3 years. Blocks of visual cortex were impregnated by the Golgi-Kopsch method and sectioned in the tangential plane. Layer 3 pyramidal cells were drawn with a camera lucida and analyzed by Sholl diagrams and vector addition. In kittens < 1 week old, these cells were very immature, with only an apical dendrite and no basal dendrites. Basal dendrites appeared during the second week. By 2 weeks, all of the basal dendrites had emerged from the soma, but they had few branches and were tipped with growth cones. By 4 weeks, they had finished branching but continued to grow in length until, by 5 weeks, they reached their adult size. Examination of the basal dendritic fields in the tangential plane revealed that their dendritic fields were more elongated at 2 weeks than at later ages, perhaps because of their smaller size. The distribution of dendritic field orientations was uniform at all ages except 3 and 4 weeks, when there was a preponderance of fields oriented in the rostrocaudal direction. Because dendritic growth and branching occurred very rapidly over a period that precedes and overlaps with the peak periods of synaptogenesis and of sensitivity to the effects of early visual experience, they may depend on afferent visual activity. The early emergence of primary dendrites, however, suggests that this process is independent of afferent activity. The coincident timing of dendritic branching with the presence of dendritic growth cones suggests that branching may occur at growth cones.

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.

The dorsal cochlear nucleus of the adult lurcher mouse is specifically invaded by embryonic grafted Purkinje cells

The fate of embryonic Purkinje cells grafted over the brainstem surface of the adult Lurcher mouse was analyzed using anti-calbindin (CaBP) immunocytochemistry. Purkinje cells are able to migrate specifically into the molecular layer of the host dorsal cochlear nucleus (DCoN) and develop dendritic trees that are practically isoplanar, suggesting synaptic interactions with the parallel fibres of the DCoN. These results provide a new argument in favour of the homology between the cerebellum and the DCoN.

Changes in dendritic morphology of rat spinal motoneurons during development and after unilateral target deletion

During normal development, motoneuron dendrites in the spinal nucleus of the bulbocavernosus (SNB) grow exuberantly to almost twice their adult length and then retract. In this study, we retrogradely labeled SNB motoneurons with cholera toxin B-conjugated horseradish peroxidase (BHRP) to examine the maturation of SNB dendritic arbors in more detail, particularly with regard to its spatial distribution and reorganization. The number and orientation of SNB motoneuron primary processes did not change over the first ten weeks of life. In contrast, total dendritic length, radial extent and arbor area increased significantly through the first four postnatal weeks and declined thereafter. The declines in length and extent were restricted to particular portions of the arbor, specifically the dorsal, ipsi- and contralateral projections. Estimates of the degree of overlap between the dendritic arbors from both sides of the SNB reflected these changes, with overlap initially increasing and then decreasing as the SNB established its adult dendritic morphology. To determine if dendritic interactions facilitated by this arbor overlap might be involved in regulating the normal retraction of SNB dendrites, we reduced SNB motoneuron numbers unilaterally by target muscle removal on the day of birth. Somal size, number and orientation of primary processes developed normally in unilateral muscle-extirpated animals. The dendritic morphology of surviving SNB motoneurons in unilateral muscle extirpated males was altered, with significant increases in dendritic length, extent and arbor area relative to those of normal males. These results indicate that substantial changes in dendritic organization of SNB motoneurons occur in normal development and may be influenced by interactions between dendrites from the two halves of the SNB.

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.

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.

Chronic exposure to low concentrations of hydrogen sulfide produces abnormal growth in developing cerebellar Purkinje cells

Hydrogen sulfide (H2S) may produce deleterious effects on the developing central nervous system. The dendritic fields of developing cerebellar Purkinje cells were analyzed to determine the effects of chronic exposure to low concentrations of H2S during perinatal development. Treatment with two concentrations (20 and 50 ppm) of H2S produced severe alterations in the architecture and growth characteristics of the Purkinjec cell dendritic fields. The architectural modifications included longer branches, an increase in the vertex path length and variations in the number of branches in particular areas of the dendritic field. The treated cells also exhibited a nonsymmetrical growth pattern at a time when random terminal branching is normally occurring. These findings suggest that developing neurons exposed to low concentrations of H2S are at risk of severe deficits.

Retrograde modulation of dendritic geometry in the vertebrate brain during development

The neurons of the chick's isthmo-optic nucleus (ION) are known to innervate the retina. We here show that removing the retinal primordia causes the ION dendritic trees to be much less polarized than normal. Our observations were made at 11 embryonic days, which is before the isthmo-optic neurons become dependent on the retina for survival. Other parameters such as neuronal size were unchanged, so the effect seems to have been specific to dendritic shape. Our interpretation is that early target removal eliminates a retrograde signal that normally enhances dendritic polarization.

Studies of the dendritic tree of wild-type cerebellar Purkinje cells in lurcher chimeric mice

Naturally occurring mutations are valuable tools for the study of neural development, especially when used in conjunction with the techniques of chimeric mouse production. In this study we examine the response of Purkinje cell dendrites to the altered developmental environment found in the lurcher in equilibrium with wild-type chimera. Lurcher (+/Lc) is an autosomal dominant gene that causes the cell-autonomous degeneration of all Purkinje cells of +/Lc genotype. Thus, in +/Lc in equilibrium with +/+ chimeras, only wild-type Purkinje cells survive to maturity. The number of these survivors can vary from less than 10,000 to greater than 100,000. Previous work has shown that the final ratio of presynaptic granule cells to postsynaptic Purkinje cells is increased in lurcher chimeras. On average, therefore, one might expect that each remaining Purkinje cell would experience an increased supply of afferents, and our hypothesis was that dendritic growth and/or sprouting might occur as a result. This proved incorrect and, indeed, the Purkinje cells in the lurcher chimeras show changes of a predominantly atrophic nature. Unusual morphologies are found, including variable branching density, failure of the distal dendrite to reach the pial surface, loss of isoplanarity, and the frequent appearance of large caliber, primary or secondary dendritic branches ending abruptly in "stub ends." Quantitative analysis of Golgi-Cox impregnated material reveals that in lurcher chimeras the Purkinje cell dendritic arbor is reduced by more than 60% compared to wild-type animals. We present possible explanations for this finding and consider several potential implications.

Olivocerebellar fiber maturation in normal and lurcher mutant mice: defective development in lurcher

Olivocerebellar fiber maturation was examined in normal and lurcher mutant mice between postnatal day 5 (P5) and P15, using the anterograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) from the inferior olive. Immunocytochemistry for the Purkinje cell marker PEP-19 was used to demonstrate Purkinje cell development in the same material. In mutant and normal animals, a regional developmental variation is observed such that, when compared at a given age, cortex lining the vermal fissures appears developmentally advanced over cortex in the cerebellar hemispheres. In the primary fissure of the normal animals, the first recognizable Purkinje cell dendrites appear on P6, and the olivocerebellar fibers first enter the climbing stage of their development on P9. In lurcher animals Purkinje cell development proceeds on this schedule, but olivocerebellar fibers are never observed to enter the molecular layer. These afferents maintain dense perisomatic nests around Purkinje cells, even in P13-15 lurchers. Examination of P14 lurchers by transmission electron microscopy indicates that the olivocerebellar fibers form synapses on Purkinje cell somatic spines and that the basket cell axons fail to form their typical perisomal nests around Purkinje cells. In addition, parallel fibers can be observed to synapse on dendritic spines on the Purkinje cell primary dendrites. We interpret these results as indicating a recognition defect between olivocerebellar fibers and Purkinje cell dendrites. An analysis of this defect in lurcher may reveal how the normal transformation of olivocerebellar fibers, from perisomal to dendritic terminals, is achieved.

Topological Link-Vertex Analysis of the growth of Purkinje cell dendritic trees in normal, reeler, and weaver mice

The growth of Purkinje cell dendritic trees in normal, reeler, and weaver mice has been defined by using Link-Vertex Analysis. Growth probably occurs in three phases in normal and cortically located reeler trees. Phase I, completed by 7 days postnatum (dpn), establishes a rudimentary tree of 100 segments by random terminal branching. Phase II lasts from 7 to 20 dpn when some 690 and 450 segments are generated in normal and cortical reeler trees respectively. Phase II is initiated by an inductive stimulus mediated by a finite number of parallel fibres. Thereafter, dendritic trees develop a similar topology in normal and cortical reeler Purkinje cells through random interactions between parallel fibres and dendritic growth cones. We have defined this process with the aid of computer simulation techniques. Interactions appear to be restricted to a narrow growth front occupied by the highest centrifugally ordered terminals behind which adhesions occur at a greatly reduced frequency. The cortical reeler tree thus has fewer segments than normal because fewer parallel fibres are available, but it is surprisingly normal in most other respects. Phase III is a period of remodelling and extends from 20 dpn into adulthood when high-ordered terminals are eroded and middle-ordered terminals are added, with no change in total segment number in both normal and cortical reeler trees. Weaver and deeply placed reeler Purkinje cell dendritic trees are not influenced by parallel fibres. Accordingly, their growth is arrested at the end of Phase I, when both types of mutant tree have generated 100 segments by unrestrained random terminal branching. In the absence of parallel fibres, Phase II is not induced and remodelling, characteristic of Phase III, does not occur.

Electron microscopic and Golgi study in a case of hemimegalencephaly

Pathological findings in a case of hemimegalencephaly are presented. Hemispherectomy, performed because of intractable seizures, allowed an electron microscopic and Golgi study. Glial abnormalities consisted of hyperplasia of glia cells with giant astrocytes often containing several nuclei and proliferation of numerous Rosenthal fibers. Golgi stain showed many giant neurons with a perikaryon covered by perisomatic processes, and a complex dendritic tree. Glial abnormalities could be correlated with the firmness of the hemisphere and intense hypersignal on magnetic resonance imaging. Giant neurons were associated with an increase in size of the perikaryon and dendritic tree; this pattern suggests a polyploidy.

Fine structure of synaptogenesis in the vertebrate central nervous system

This article reviews studies of the formation of synaptic junctions in the vertebrate central nervous system. It is focused on electron microscopic investigations of synaptogenesis, although insights from other disciplines are interwoven where appropriate, as are findings from developing peripheral and invertebrate nervous systems. The first part of the review is concerned with the morphological maturation of synapses as described from both qualitative and quantitative perspectives. Next, epigenetic influences on synaptogenesis are examined, and later in the article the concept of epigenesis is integrated with that of hierarchy. It is suggested that the formation of synaptic junctions may take place as an ordered progression of epigenetically modulated events wherein each level of cellular affinity becomes subordinate to the one that follows. The ultimate determination of whether a synapse is maintained, modified or dissolved would be made by the changing molecular fabric of its junctional membranes. In closing, a hypothetical model of synaptogenesis is proposed, and an hierarchial order of events is associated with a speculative synaptogenic sequence. Key elements of this hypothesis are 1) epigenetic factors that facilitate generally appropriate interactions between neurites; 2) independent expression of surface specializations that contain sufficient information for establishing threshold recognition between interacting neurites; 3) exchange of molecular information that biases the course of subsequent junctional differentiation and ultimately results in 4) the stabilization of synaptic junctions into functional connectivity patterns.

Computer-assisted analysis of the developing Purkinje neuron. II. A comparative study of the dendritic development in differently aged kittens and in organotypic cultures explanted at the same ages

Golgi-prepared cerebella from 10-13-(model K) and 22-day-old (model C) kittens were analyzed and compared with 30-40 days in vitro (DIV) horseradish peroxidase (HRP)-stained organotypic cultures from kitten cerebella explanted at comparable ages (12 (model E) and 21 (model T) postnatal days). In addition, HRP-stained cerebella explanted from 1-day-old kittens were used as a reference (model N). Computer reconstructions and morphometric parameters (14 variables) allowed a comparative quantitative analysis of the Purkinje cell dendritic trees and axonal processes. The biplanar arrangement of the dendritic sheets observed in vivo at 10-13 days (K) was replaced by a mostly bipolar one in the in vitro Purkinje cells (E) explanted at the same age with their axon following the direction of either dendrite. The dendritic expansion observed in intact animals between 12 and 22 days was mostly due to an increase in the number of dendritic segments having shorter lengths rather than an increase in the number of rows. A discriminant analysis permitting the recognition of 3 populations of Purkinje cells in both groups reinforced the previous hypothesis that the dendritic shapes are modelled by specific afferents pre-existent in vivo.

Quantitative study of the Purkinje cell dendritic spines in the rat cerebellum

The number of spines on an individual Purkinje cell in the cerebellar cortex of the rat was determined by stereological methods. Investigations were based on thin section electron micrographs, freeze fracture replicas, and horseradish peroxidase labeled cells. Purkinje cell dendritic spines in our embedded material had a mean length of 1.4 +/- 0.05 micron and mean neck and head diameters of 0.22 +/- 0.01 micron and 0.45 +/- 0.02 micron, respectively. From these dimensions, an estimate of spine volume in embedded material of 0.132 micron 3 was obtained. The density of dendritic spines in our fixed material was 8.15 x 10(8) or 7.24 x 10(8) per microliters of molecular layer from volume fraction and density per mm2, respectively. The number of spines per linear micron of Purkinje cell spiny branchlet was 17.2 from freeze fracture and 17.6 from horseradish peroxidase labeled dendrites. These all indicate that there are between 154,000 and 175,000 spines on the dendritic tree of each Purkinje cell, considerably more than previously reported for the rat.

Effect of phenobarbital on Purkinje cell growth patterns in the rat cerebellum

The effects of low level phenobarbital administration (18 postcoitus to 21 days postnatal) on Purkinje cell growth and remodeling were studied from 3 to 20 weeks postnatal). The Purkinje cell dendritic trees were analyzed both metrically and topologically using the method of vertex analysis. The total segment length, mean terminal path length, and mean vertex path length were reduced in the treated cells. The pattern of segment frequency as related to equivalent orders was abnormal in the treated cells. The Va/Vb vertex ratios and the levels of trichotomy indicated that the treated cells underwent nonrandom remodeling, unlike the control cells which exhibited dichotomous, random terminal branching. These observations confirm that phenobarbital produces distinct long-term morphologic alterations in Purkinje cells.