Organization of spinocerebellar projection map in three types of agranular cerebellum: Purkinje cells vs. granule cells as organizer element

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.

Differential spatiotemporal development of Purkinje cell populations and cerebellum-dependent sensorimotor behaviors

Distinct populations of Purkinje cells (PCs) with unique molecular and connectivity features are at the core of the modular organization of the cerebellum. Previously, we showed that firing activity of PCs differs between ZebrinII-positive and ZebrinII-negative cerebellar modules (Zhou et al., 2014; Wu et al., 2019). Here, we investigate the timing and extent of PC differentiation during development in mice. We found that several features of PCs, including activity levels, dendritic arborization, axonal shape and climbing fiber input, develop differentially between nodular and anterior PC populations. Although all PCs show a particularly rapid development in the second postnatal week, anterior PCs typically have a prolonged physiological and dendritic maturation. In line herewith, younger mice exhibit attenuated anterior-dependent eyeblink conditioning, but faster nodular-dependent compensatory eye movement adaptation. Our results indicate that specific cerebellar regions have unique developmental timelines which match with their related, specific forms of cerebellum-dependent behaviors.

Eph/ephrin Function Contributes to the Patterning of Spinocerebellar Mossy Fibers Into Parasagittal Zones

Purkinje cell microcircuits perform diverse functions using widespread inputs from the brain and spinal cord. The formation of these functional circuits depends on developmental programs and molecular pathways that organize mossy fiber afferents from different sources into a complex and precisely patterned map within the granular layer of the cerebellum. During development, Purkinje cell zonal patterns are thought to guide mossy fiber terminals into zones. However, the molecular mechanisms that mediate this process remain unclear. Here, we used knockout mice to test whether Eph/ephrin signaling controls Purkinje cell-mossy fiber interactions during cerebellar circuit formation. Loss of ephrin-A2 and ephrin-A5 disrupted the patterning of spinocerebellar terminals into discrete zones. Zone territories in the granular layer that normally have limited spinocerebellar input contained ectopic terminals in ephrin-A2 ^-/-;ephrin-A5 ^-/- double knockout mice. However, the overall morphology of the cerebellum, lobule position, and Purkinje cell zonal patterns developed normally in the ephrin-A2 ^-/-;ephrin-A5 ^-/- mutant mice. This work suggests that communication between Purkinje cell zones and mossy fibers during postnatal development allows contact-dependent molecular cues to sharpen the innervation of sensory afferents into functional zones.

Cellular Mechanisms Involved in Cerebellar Microzonation

In the last 50 years, our vision of the cerebellum has vastly evolved starting with Voogd's (1967) description of extracerebellar projections' terminations and how the projection maps transformed the presumptive homogeneity of the cerebellar cortex into a more complex center subdivided into transverse and longitudinal distinct functional zones. The picture became still more complex with Richard Hawkes and colleagues' (Gravel et al., 1987) discovery of the biochemical heterogeneity of Purkinje cells (PCs), by screening their molecular identities with monoclonal antibodies. Antigens were expressed in a parasagittal pattern with subsets of PCs either possessing or lacking the respective antigens, which divided the cerebellar cortex into precise longitudinal compartments that are congruent with the projection maps. The correlation of these two maps in adult cerebellum shows a perfect matching of developmental mechanisms. This review discusses a series of arguments in favor of the essential role played by PCs in organizing the microzonation of the cerebellum during development (the "matching" hypothesis).

Insights into cerebellar development and connectivity

The cerebellum has a well-established role in controlling motor functions such coordination, balance, posture, and skilled learning. There is mounting evidence that it might also play a critical role in non-motor functions such as cognition and emotion. It is therefore not surprising that cerebellar defects are associated with a wide array of diseases including ataxia, dystonia, tremor, schizophrenia, dyslexia, and autism spectrum disorder. What is intriguing is that a seemingly uniform circuit that is often described as being "simple" should carry out all of these behaviors. Analyses of how cerebellar circuits develop have revealed that such descriptions massively underestimate the complexity of the cerebellum. The cerebellum is in fact highly patterned and organized around a series of parasagittal stripes and transverse zones. This topographic architecture partitions all cerebellar circuits into functional modules that are thought to enhance processing power during cerebellar dependent behaviors. What are arguably the most remarkable features of cerebellar topography are the developmental processes that produce them. This review is concerned with the genetic and cellular mechanisms that orchestrate cerebellar patterning. We place a major focus on how Purkinje cells control multiple aspects of cerebellar circuit assembly. Using this model, we discuss evidence for how "zebra-like" patterns in Purkinje cells sculpt the cerebellum, how specific genetic cues mediate the process, and how activity refines the patterns into an adult map that is capable of executing various functions. We also discuss how defective Purkinje cell patterning might impact the pathogenesis of neurological conditions.

Regional differences in Purkinje cell morphology in the cerebellar vermis of male mice

Regional differences in dendritic architecture can influence connectivity and dendritic signal integration, with possible consequences for neuronal computation. In the cerebellum, analyses of Purkinje cells (PCs), which are functionally critical as they provide the sole output of the cerebellar cortex, have suggested that the cerebellar cortex is not uniform in structure as traditionally assumed. However, the limitations of traditional staining methods and microscopy capabilities have presented difficulties in investigating possible local variations in PC morphology. To address this question, we used male mice expressing green fluorescent protein selectively in PCs. Using Neurolucida 360 with confocal image stacks, we reconstructed dendritic arbors of PCs residing in lobule V (anterior) and lobule IX (posterior) of the vermis. We then analyzed morphologies of individual arbors and the structure of the assembled "jungle," comparing these features across anatomical locations and age groups. Strikingly, we found that in lobule IX, half of the reconstructed PCs had two primary dendrites emanating from their soma, whereas fewer than a quarter showed this characteristic in lobule V. Furthermore, PCs in lobule V showed more efficient spatial occupancy compared to lobule IX, as well as greater packing density and increased arbor overlap in the adult. When analyzing complete ensembles of PC arbors, we also observed "hot spots" of increased dendritic density in lobule V, whereas lobule IX showed a more homogeneous spread of dendrites. These differences suggest that input patterns and/or physiology of PCs could likewise differ along the vermis, with possible implications for cerebellar function.

Embryonic stages in cerebellar afferent development

The cerebellum is important for motor control, cognition, and language processing. Afferent and efferent fibers are major components of cerebellar circuitry and impairment of these circuits causes severe cerebellar malfunction, such as ataxia. The cerebellum receives information from two major afferent types – climbing fibers and mossy fibers. In addition, a third set of afferents project to the cerebellum as neuromodulatory fibers. The spatiotemporal pattern of early cerebellar afferents that enter the developing embryonic cerebellum is not fully understood. In this review, we will discuss the cerebellar architecture and connectivity specifically related to afferents during development in different species. We will also consider the order of afferent fiber arrival into the developing cerebellum to establish neural connectivity.

Establishment of topographic circuit zones in the cerebellum of scrambler mutant mice

The cerebellum is organized into zonal circuits that are thought to regulate ongoing motor behavior. Recent studies suggest that neuronal birthdates, gene expression patterning, and apoptosis control zone formation. Importantly, developing Purkinje cell zones are thought to provide the framework upon which afferent circuitry is organized. Yet, it is not clear whether altering the final placement of Purkinje cells affects the assembly of circuits into topographic zones. To gain insight into this problem, we examined zonal connectivity in scrambler mice; spontaneous mutants that have severe Purkinje cell ectopia due to the loss of reelin-disabled1 signaling. We used immunohistochemistry and neural tracing to determine whether displacement of Purkinje cell zones into ectopic positions triggers defects in zonal connectivity within sensory-motor circuits. Despite the abnormal placement of more than 95% of Purkinje cells in scrambler mice, the complementary relationship between molecularly distinct Purkinje cell zones is maintained, and consequently, afferents are targeted into topographic circuits. These data suggest that although loss of disabled1 distorts the Purkinje cell map, its absence does not obstruct the formation of zonal circuits. These findings support the hypothesis that Purkinje cell zones play an essential role in establishing afferent topography.

Engrailed homeobox genes regulate establishment of the cerebellar afferent circuit map

The spatial organization of the cerebellar afferent map has remarkable correspondence to two aspects of intrinsic patterning within the cerebellum embodied by a series of lobules and Purkinje cell (PC)-striped gene expression. Using male and female mice, we tested whether the Engrailed (En) homeobox genes are a common genetic substrate regulating all three systems, since they are expressed in spatially restricted domains within the cerebellum and are critical for patterning PC gene expression and foliation. Indeed, we discovered that En1/2 are necessary for the precise targeting of mossy fibers to distinct lobules, as well as their subsequent resolution into discrete parasagittal bands. Moreover, each En gene coordinately regulates afferent targeting and the striped pattern of PC protein expression (e.g., ZebrinII/AldolaseC) independent of regulating foliation. We further found that En1/2, rather than the presence of a full complement of lobules, are critical for generating PC protein stripes and mossy fiber bands, and that PC striped gene expression is determined before afferent banding. Thus, the En transcription factors not only regulate cerebellum circuit topography, but they also link afferent and efferent neurons precisely enough that alterations in PC protein expression can be used as a read out for underlying defects in circuitry. In summary, our data suggest that En1/2 are master regulators of three-dimensional organization of the cerebellum and coordinately regulate morphology, patterned gene expression, and afferent topography.

Defects in the cerebella of conditional Neurod1 null mice correlate with effective Tg(Atoh1-cre) recombination and granule cell requirements for Neurod1 for differentiation

Neurod1 is a crucial basic helix-loop-helix gene for most cerebellar granule cells and mediates the differentiation of these cells downstream of Atoh1-mediated proliferation of the precursors. In Neurod1 null mice, granule cells die throughout the posterior two thirds of the cerebellar cortex during development. However, Neurod1 is also necessary for pancreatic beta-cell development, and therefore Neurod1 null mice are diabetic, which potentially influences cerebellar defects. Here, we report a new Neurod1 conditional knock-out mouse model created by using a Tg(Atoh1-cre) line to eliminate Neurod1 in the cerebellar granule cell precursors. Our data confirm and extend previous work on systemic Neurod1 null mice and show that, in the central lobules, granule cells can be eradicated in the absence of Neurod1. Granule cells in the anterior lobules are partially viable and depend on as yet unknown genes, but the Purkinje cells show defects not previously recognized. Interestingly, delayed and incomplete Tg(Atoh1-cre) upregulation occurs in the most posterior lobules; this leads to near normal expression of Neurod1 with a concomitant normal differentiation of granule cells, Purkinje cells, and unipolar brush cells in lobules IX and X. Our analysis suggests that Neurod1 negatively regulates Atoh1 to ensure a rapid transition from proliferative precursors to differentiating neurons. Our data have implications for research on medulloblastoma, one of the most frequent brain tumors of children, as the results suggest that targeted overexpression of Neurod1 under Atoh1 promoter control may initiate the differentiation of these tumors.

Naturally occurring parvovirus-associated feline hypogranular cerebellar hypoplasia-- A comparison to experimentally-induced lesions using immunohistology

Three cases of feline cerebellar hypoplasia are presented. At the time of examination, the ages of the cats ranged from 2 months to 1 year. Necropsy revealed cerebellar and pons hypoplasia. Polymerase chain reaction for parvoviral deoxyribonucleic acid was positive in cerebellar tissue. Cell-specific immunolabeling was used to characterize the lesions, which were characterized into 2 types. In type 1 lesions, the cortex was nearly agranular, with an extremely thin molecular layer; the Purkinje cells were randomly placed and oriented, and their stunted main dendrite produced a thorn-covered atrophic dendritic tree; the basket cell axons ran randomly and had dysmorphic endings; and myelinated fibers were severely reduced in folia axes. In type 2 lesions, the cortex was hypogranular; the Purkinje cells were linearly organized, but their main dendrite extended too far in the molecular layer before giving up smooth, bent secondary dendrites; many basket cells were located along the cerebellar surface, and their axons ran at right angle to the surface; myelinated fibers were moderately reduced. Defects in climbing fiber synapse translocation and elimination were evident in both types of lesion. This immunohistologic study allowed a comparison between lesions in these spontaneous cerebellar hypoplasia cases with those documented when using silver impregnation studies after perinatal experimental cerebellar damage. Such a comparison is consistent with viral infection that occurs before birth in all 3 cases. Progress in parvovirus biology knowledge suggests that viral NS1 protein cytotoxicity might explain degenerative changes in the Purkinje cells that were present, in addition to the development defect.

Morphology, molecular codes, and circuitry produce the three-dimensional complexity of the cerebellum

The most noticeable morphological feature of the cerebellum is its folded appearance, whereby fissures separate its anterior-posterior extent into lobules. Each lobule is molecularly coded along the medial-lateral axis by parasagittal stripes of gene expression in one cell type, the Purkinje cells (PCs). Additionally, within each lobule distinct combinations of afferents terminate and supply the cerebellum with synchronized sensory and motor information. Strikingly, afferent terminal fields are organized into parasagittal domains, and this pattern bears a close relationship to PC molecular coding. Thus, cerebellum three-dimensional complexity obeys a basic coordinate system that can be broken down into morphology and molecular coding. In this review, we summarize the sequential stages of cerebellum development that produce its laminar structure, foliation, and molecular organization. We also introduce genes that regulate morphology and molecular coding, and discuss the establishment of topographical circuits within the context of the two coordinate systems. Finally, we discuss how abnormal cerebellar organization may result in neurological disorders like autism.

The Lurcher mouse: Fresh insights from an old mutant

The Lurcher mouse was first discovered in 1954 as a spontaneously occurring autosomal dominant mutation that caused the degeneration of virtually all cerebellar Purkinje cells and most olivary neurons and granule cells. More recent molecular studies revealed that Lurcher is a gain of function mutation in the delta2 glutamate receptor (GluRdelta2) that converts an alanine to threonine in the highly conserved third hydrophobic segment of GluRdelta2. The mutation converts the receptor into a constitutively leaky cation channel. The GluRdelta2 receptor is predominantly expressed in cerebellar Purkinje cells and in the heterozygous Lurcher mutant (+/Lc). Purkinje cells die due to the mutation in the GluRdelta2 receptor, while olivary neurons and granule cells degenerate due to the loss of their Purkinje cell targets. The purpose of the review is to provide highlights from 5 decades of research on the Lurcher mutant that have provided insights into the developmental mechanisms that regulate cell number during development, cerebellar pattern formation, cerebellar physiology, and the role of the cerebellum in CNS function.

The stop signal revised: immature cerebellar granule neurons in the external germinal layer arrest pontine mossy fiber growth

During the formation of neuronal circuits, afferent axons often enter target regions before their target cells are mature and then make temporary contacts with nonspecific targets before forming synapses on specific target cells. The regulation of these different steps of afferent-target interactions is poorly understood. The cerebellum is a good model for addressing these aspects, because cerebellar development is well defined and identified neurons in the circuitry can be purified and combined in vitro. Previous reports from our laboratory showed that cultured granule neurons specifically arrest the extension of their pontine mossy fiber afferents, leading us to propose that granule cells arrested growth of their afferents as a prelude to synaptogenesis. However, we knew little about the differentiation state of the cultured granule cells that mediate afferent arrest. In this study, we better define the purified granule cell fraction by marker expression and morphology, and demonstrate that only freshly plated granule cells in the precursor and premigratory state arrest mossy fiber outgrowth. Mature granule cells, in contrast, support extension, defasciculation, and synapse formation, as in vivo. In addition, axonal tracing in vivo during the first postnatal week indicates that immature mossy fibers extend into the Purkinje cell layer but never into the external germinal layer (EGL), where precursors of granule cell targets reside. We found that the stop-growing signals are dependent on heparin-binding factors, and we propose that such signals in the EGL restrict the extension of mossy fiber afferents and prevent invasion of proliferative regions.

From clusters to stripes: The developmental origins of adult cerebellar compartmentation

Many aspects of the adult cerebellum are organized into parasagittal stripes, including several types of neurons and prominent afferent and efferent projections. Purkinje cells are the best-studied example of parasagittal organization in the cerebellum and, in particular, zebrin II/aldolase C is the stereotypical molecular marker of Purkinje cell stripe heterogeneity in the adult. Zebrin II is a member of the so-called 'late-onset' class of parasagittal markers, which are first expressed shortly after the birth of the mouse and do not reach maturity until 2-3 weeks postnatal. In contrast, 'early-onset' pattern markers are expressed in ordered Purkinje cell clusters in the embryonic cerebellum but become expressed homogeneously shortly after birth. The approximately 10 day temporal gap between the patterned expression of early and late markers has impeded the identification of putative genealogical relationships between clusters and stripes. This review will describe Purkinje cell patterns and their transitions, and critically discuss the evidence for genealogical relationships between early and late patterns.

Naturally occurring neuronal death during the postnatal development of Purkinje cells and their precerebellar afferent projections

Naturally occurring neuronal death plays a substantial developmental role in the building of the neural circuitries. The neuronal death caused by different cerebellar mutations is mostly of an apoptotic nature. Apart from the identity of the intrinsic mechanisms of the mutations, adult cerebellar mutants are a powerful tool to causally study the development of the cerebellar connectivity. Thus, studies on adult cerebellar neuronal cell death occurring in mouse mutants elucidate: (i) the dependence of the postsynaptic neurons on their partners, (ii) the 'en cascade' postsynaptic transneuronal degeneration after target-deprivation, and (iii) the close relationship between the molecular modular organization of the cerebellar cortex and dying Purkinje cells. Neuronal cell death has been extensively studied in developing olivocerebellar system. However, less data are available on the occurrence of naturally occurring neuronal death during the in vivo normal development of the Purkinje cells and the mossy fiber system neurons. The developmental role of neuronal death during the establishment and refinement of the olivocerebellar projection is currently discussed. Moreover, the occurrence of neuronal death during the development of the basilar pontine nuclei and its role in the acquisition of the adult pontocerebellar projection is still poorly understood. In the present review, we correlate the dates of Purkinje cells death with the inferior olivary and basilar pontine neuronal apoptosis, discussing their developmental relationships during the elaboration of the fine-grained maps of the cerebellar afferent connections.

Mechanisms underlying reorganization of fractured tactile cerebellar maps after deafferentation in developing and adult rats

Our previous studies showed that fractured tactile cerebellar maps in rats reorganize after deafferentation during development and in adulthood while maintaining a fractured somatotopy. Several months after deafferentation of the infraorbital branch of the trigeminal nerve, the missing upper lip innervation is replaced in the tactile maps in the granule cell layer of crus IIa. The predominant input into the denervated area is always the upper incisor representation. This study examined whether this reorganization was caused by mechanisms intrinsic to the cerebellum or extrinsic, i.e., occurring in somatosensory structures afferent to the cerebellum. We first compared normal and deafferented maps and found that the expansion of the upper incisor is not caused by a preexisting bias in the strength or abundance of upper incisor input in normal animals. We then mapped tactile representations before and immediately after denervation. We found that the pattern of reorganization observed in the cerebellum several months later is not caused by unmasking of a silent or weaker upper incisor representation. Both results indicate that the reorganization is not a result of subsequent growth or sprouting mechanism within the cerebellum itself. Finally, we compared postlesion maps in the cerebellum and the somatosensory cortex. We found that the upper incisor representation significantly expands in both regions and that this expansion is correlated, suggesting that reorganization in the cerebellum is a passive consequence of reorganization in afferent cerebellar pathways. This result has important developmental and functional implications.

Axonal plasticity and functional recovery after spinal cord injury in mice deficient in both glial fibrillary acidic protein and vimentin genes

The lack of axonal regeneration in the injured adult mammalian spinal cord leads to permanent functional disabilities. The inability of neurons to regenerate their axon is appreciably due to an inhospitable environment made of an astrocytic scar. We generated mice knock-out for glial fibrillary acidic protein and vimentin, the major proteins of the astrocyte cytoskeleton, which are upregulated in reactive astrocytes. These animals, after a hemisection of the spinal cord, presented reduced astroglial reactivity associated with increased plastic sprouting of supraspinal axons, including the reconstruction of circuits leading to functional restoration. Therefore, improved anatomical and functional recovery in the absence of both proteins highlights the pivotal role of reactive astrocytes in axonal regenerative failure in adult CNS and could lead to new therapies of spinal cord lesions.

Degeneration of pontine mossy fibres during cerebellar development in weaver mutant mice

In weaver mutant mice, substitution of an amino acid residue in the pore region of GIRK2, a subtype of the G-protein-coupled inwardly rectifying K+ channel, changes the properties of the homomeric channel to produce a lethal depolarized state in cerebellar granule cells and dopaminergic neurons in substantia nigra. Degeneration of these types of neurons causes strong ataxia and Parkinsonian phenomena in the mutant mice, respectively. On the other hand, the mutant gene is also expressed in various other brain regions, in which the mutant may have effects on neuronal survival. Among these regions, we focused on the pontine nuclei, the origin of the pontocerebellar mossy fibres, projecting mainly into the central region of the cerebellar cortex. The results of histological analysis showed that by P9 the number of neurons in the nuclei was reduced in the mutant to about one half and by P18 to one third of those in the wild type, whereas until P7 the number were about the same in wild-type and weaver mutant mice. Three-dimensional reconstruction of the nuclei showed a marked reduction in volume and shape of the mutant nuclei, correlating well with the decrease in neuronal number. In addition, DiI (a lipophilic tracer dye) tracing experiments revealed retraction of pontocerebellar mossy fibres from the cerebellar cortex after P5. From these results, we conclude that projecting neurons in the pontine nuclei, as well as cerebellar granule cells and dopaminergic neurons in substantia nigra, strongly degenerate in weaver mutant mice, resulting in elimination of pontocerebellar mossy fibres during cerebellar development.

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

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

Regional differences in the Purkinje cells settled pattern: a comparative autoradiographic study in control and homozygous weaver mice

To determine whether Purkinje cells located in the vermis and the lateral hemispheres of weaver mice homozygotes are distributed according to precise neurogenetic gradients, [3H]thymidine autoradiography was applied on sections of homozygous weaver mice and normal controls on postnatal day 90. The experimental animals were the offspring of pregnant dams injected with [3H]thymidine on embryonic days 11-12, 12-13, 13-14, and 14-15. The results indicate that, at the level of the vermis, neurogenetic gradients were similar for wild-type and homozygous weaver in each lobe studied of the cerebellar cortex. The same was found for the lobulus simplex and for the ansiform and paramedian lobules when the lateral hemisphere was considered. In the vermis of both experimental groups, the anterior and inferior lobes have more late-generated Purkinje cells than the central and posterior lobes, while in the lateral hemisphere, the lobulus simplex and the ansiform lobule present more early generated Purkinje cells than the paramedian lobule. In weaver homozygotes, the most important deficit of Purkinje cells, in the region of the vermis, was observed in the central lobe; depletion was less observable in the anterior lobe and least observable in the posterior and inferior lobes. In the lateral hemispheres, the most important loss of Purkinje cells was observed in the paramedian lobule, followed by the lobulus simplex. The ansiform lobule presented values that showed no statistical difference between control and homozygous weaver. When Purkinje cells were registered in the entire sections, no significant differences were observed between the two experimental groups. This was due to a considerable volume of the weaver homozygote cerebellar tissue, which has no counterpart in the control mice, compensating for the neuronal loss observed in the other studied areas of the lateral hemisphere.

Motor performance and regional brain metabolism of spontaneous murine mutations with cerebellar atrophy

Three spontaneous mutations with cerebellar atrophy were evaluated for motor coordination and regional brain metabolism, as assessed by cytochrome oxidase (CO) activity. Despite similar neuropathological characteristics, the behavioral phenotype of Lurcher is less severe than that of staggerer, possibly caused by the slower onset of their neuronal degeneration. Although fewer cerebellar cells degenerate in hot-foot than in Lurcher, their motor deficits are more severe, indicating the presence of dysfunctional cells. CO activity in the deep cerebellar nuclei was increased in Lurcher and staggerer but unchanged in hot-foot, probably due to the severe loss of GABAergic input from Purkinje cells in the first two mutants but not the third. Altered CO activity in cerebellar-related pathways was linearly correlated with motor performance, indicating that the activity of this enzyme is associated not only with neuronal activity but also with motor performance.

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

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

Selective disruption of "late onset" sagittal banding patterns by ectopic expression of engrailed-2 in cerebellar Purkinje cells

To explore the role of Engrailed proteins in development of the cerebellum, Engrailed-2 (En-2) was ectopically expressed in cerebellar Purkinje cells from the late embryonic stage into adulthood. The fundamental organization of Purkinje cell sagittal zones as revealed by the "early onset" markers L7-beta-gal and cadherin-8 was found to be virtually identical to that in wild type. In contrast, "late onset" sagittal banding patterns revealed by Purkinje cell markers zebrin I, zebrin II, and 9-O-acetyl GD3 Ganglioside (P-Path), and the granule cell marker NADPH-diaphorase, were disrupted. In general, although some evidence of banding was still detectable, boundaries defined by the latter markers were poorly defined, and the patterns overall took on a diffuse appearance. In parallel with the changes in late onset markers, anterograde tracing of spinocerebellar axons revealed a general diffusion of the mossy fiber projection pattern in lobule VIII and the anterior lobe. These observations suggest that at least two separate mediolateral boundary systems exist in the cerebellum, and these are differentially affected by ectopic En-2 expression. Alternatively, one boundary system exists that remains primarily intact in the mutant, but recognition of this system by a set of late developmental events is perturbed.

Evidence of spinocerebellar mossy fiber segregation in the juvenile staggerer cerebellum

Developmental and experimental studies of climbing fiber and mossy fiber connectivity in the cerebellum have suggested that Purkinje cells are the critical organizing elements for connectivity patterns. This hypothesis is supported by evidence that spinocerebellar mossy fiber projections are abnormally diffuse in P25 sg/sg mutant mice in which the differentiation of a reduced number of sg/sg Purkinje cells is blocked due to a cell autonomous defect. However, mossy fiber distribution may be disrupted in sg/sg mutants not because of the Purkinje cell deficits, but because of the death of virtually all granule cells following the 4th postnatal week. To test this hypothesis, we have analyzed the distribution of wheat germ agglutinin-horseradish peroxidase (WGA-HRP)-labeled spinocerebellar mossy fiber terminals in sg/sg mutants at the end of the period of granule cell genesis (postnatal day [P] 12-P13) and before massive granule cell death (P16). Two percent WGA-HRP was injected into the lower thoracic/upper lumbar region of the spinal cord of eight homozygous sg/sg mutants (P12-P16) and five controls (+/sg and +/+). We have found that spinocerebellar mossy fibers segregate into distinct terminal fields in the anterior cerebellar lobules of P12 to P16 sg/sg mutants, although the medial-lateral distribution of spinocerebellar mossy fiber projections is different from controls. The results from this study and previous analysis of sg/sg mutants support the hypothesis that topographic cues are expressed in the early postnatal staggerer mutant, but mossy fiber terminals become disorganized or retract as granule cells die in the older staggerer mutant. J. Comp. Neurol. 378:354-362, 1997.

The Engrailed-2 homeobox gene and patterning of spinocerebellar mossy fiber afferents

The mouse Engrailed-2 gene, En-2, appears to be involved in cerebellar pattern formation. Homozygous null mutants for En-2 have abnormal foliation patterns in the posterior half of the cerebellum and there are changes in Purkinje and granule cell gene expression in some posterior folia, possibly reflecting changes in cell identity. We have examined the distribution of spinocerebellar mossy fiber terminals in homozygous En-2hd null mutants to determine if En-2 is involved in regulating the pattern of afferent connectivity in the cerebellum. Spinocerebellar mossy fiber terminals were labeled following WGA-HRP injections in the lumbar region of 5 homozygous En-2hd mutants and 4 heterozygous controls. The distribution of spinocerebellar mossy fiber terminals was consistently altered in lobules VIII and IX of the En-2hd mutants. The principal changes were a reduction in the number of mossy fiber terminal fields in the dorsal aspect of lobule VIII and the dorsal midline field in lobule IX was fused into a single compartment. The results suggest that the deletion of En-2 expression does not transform lobule identity, at least with respect to afferent fiber positional information cues. However, the changes in foliation and afferent connectivity in the En-2 mutant support a broad role for the En-2 gene in cerebellar patterning.

Partial ablation of the neonatal external granular layer disrupts mossy fiber topography in the adult rat cerebellum

The spinocerebellar projection in the rat is compartmentalized in an array of parasagittal bands of mossy fiber terminals. These bands align reproducibly with bands of Purkinje cells that differentially express zebrin II. To investigate whether this alignment is obligatory, Purkinje cell and mossy fiber compartmentation has been compared in the rat cerebellum where the cytoarchitecture was contorted by neonatal administration of methylazoxymethanol. Methylazoxymethanol ablates many granule cell precursors, leaving a much reduced external granular layer, and adult rats that received a single methylazoxymethanol injection at birth showed varying degrees of abnormal cerebellar foliation. Zebrin II immunocytochemistry nevertheless revealed no fundamental abnormality in the Purkinje cell compartments. However, despite the normal Purkinje cell compartmentation being retained, the spinocerebellar mossy fiber-Purkinje cell topography is disrupted by methylazoxymethanol treatment. The normal precise array of parasagittal mossy fiber terminal fields becomes blurred across the lobule, and the normal clear banding is difficult to follow. These data suggest that, despite the early topography being dependent on the Purkinje cells, the granule cell-mossy fiber interactions also regulate the topography of the spinocerebellar projection.

A behavioral study of the development of hereditary cerebellar ataxia in the shaker rat mutant

shaker Mutant rats were first identified by their abnormal motor behaviors and degeneration of cerebellar Purkinje cells and brainstem inferior olivary neurons. After 6 generations of inbreeding 77% of shaker rat mutants are mildly ataxic (identified as mild shaker mutants) and 23% are ataxic and exhibit a whole body tremor (strong shaker mutants) by 3 months of age. This study of shaker mutants from birth to 3 months of age was designed to: (1) compare the somatic and motor development of shaker mutants with age matched normal rats; (2) identify the temporal onset of motor deficits; and (3) correlate qualitative differences in Purkinje cell degeneration between 3-month-old mild and strong shaker rat mutants. Shaker mutant rats consistently weighed less than age-matched control animals. Analysis of motor-development using the hindlimb splay test demonstrated the distance between hindpaws was significantly greater in shaker mutant rats than in controls starting at 42 postnatal days (PND) of age. Hindlimb stride width was greater for shaker than control rats at 42 PNDs. However, after 42 PNDS shaker mutant average hindlimb width was narrower than controls. Forelimb stride width was consistently narrower in shaker mutants than in normal rats. Hindlimb placement was impaired in shaker rat mutants after 15 PND. Forelimb placement, cliff avoidance and surface righting were only transiently impaired in shaker mutants. Mid-air righting, performance of a geotaxic response, and climbing and jumping postural reactions were similar in shaker and normal rats. The spatial extent of Purkinje cell survival/degeneration correlated with differences in abnormal motor activity seen in 3-month-old mild and strong shaker mutants. In mild shaker rat mutants, Purkinje cells appeared to have degenerated randomly throughout the cortex. In strong shaker mutants most Purkinje cells in the anterior lobe had degenerated. In the posterior lobe Purkinje cell degeneration appeared to be numerically significant, but many surviving cells were present. Although Purkinje cell loss was not numerically quantified in this study, a strong association between the extent and type of spatial loss of Purkinje cells, and the severity of clinical signs, appears to exist.

Development of the spinocerebellar projection in the prenatal mouse

An abundance of information is available concerning the spinocerebellar projection in adult mammals. However, only a few studies have attempted a developmental analysis of this important projection system in early postnatal and/or prenatal animals. The present study provides an analysis of the development of the projection from the spinal cord to the cerebellum in fetal mice using anterograde tracing techniques in an in vitro preparation. After applications of biocytin to the caudal cervical spinal cord, anterogradely labelled fibers were present in the brainstem of embryonic day 12 (E12/13) mice, however, there was no indication of label in the cerebellum. At E13/14, labelled fibers were evident in the rostrolateral portions of the cerebellum/isthmus region. By E15/16, labelled spinocerebellar fibers had progressed farther into the cerebellum and were seen crossing the midline in a very superficial position. At older ages, the number of crossing fibers increased, and they became more ventrally positioned within the cerebellum. At E17/18 and E18/19, labelled spinocerebellar fibers were observed to branch and invade deeper portions of the cerebellum including the cerebellar nuclei. However, at E18/19, there was no indication of the parasagittal organization characteristic of this projection in the adult animal. The results of this study indicate that spinocerebellar fibers are present within the cerebellum significantly earlier than the development and differentiation of their primary targets, the granule cells. Furthermore, these data suggest that spinocerebellar fibers may form associations with cerebellar nuclear cells during fetal development.

Chemical heterogeneity in adult rat cerebellar Purkinje cells as revealed by zebrin I and low-affinity nerve growth factor receptor immunocytochemical expression following injury

Cerebellar Purkinje cells in rat express low-affinity nerve growth factor receptor during development, but rarely in normal adult animals. However, after either mechanical injury or colchicine treatment during adulthood, these cells re-express low-affinity nerve growth factor receptor-immunoreactive protein. Two Purkinje cell subpopulations were defined in normal adult cerebellum by the presence or the absence of zebrin I antigen. Nevertheless, it remains an open question as to whether low-affinity nerve growth factor receptor-immunoreactive protein can be expressed by all damaged Purkinje cells, independent of their location and their staining with antibodies against intrinsic molecular markers that reveal Purkinje cell heterogeneity, such as zebrin I. In this study, a serial-section immunocytochemical mapping of the expression zebrin I and low-affinity nerve growth factor receptor, using specific monoclonal antibodies, we carried out in colchicine-treated rats. After mechanical damage of the cerebellar cortex, co-localization of these antigens at the cellular level was also analysed in thin adjacent sections, and by using a combined immunocytochemical staining method in individual sections. The findings revealed the existence of three sub-sets of Purkinje cells: (1) two complementary groups distinctly immunoreactive to one antibody, but not to the other and (2) a third group that contained double-labelled cells. In contrast, co-expression of both antigens was never observed following mechanical lesions. The seemingly independent response to mechanical injury of Purkinje cells located in different zebrin-defined compartments, indicates that particular subpopulations of Purkinje cells may respond differentially to traumatic injury.

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.

Developing mossy fiber terminal fields in the rat cerebellar cortex may segregate because of Purkinje cell compartmentation and not competition

Many mossy fiber afferent projections to the rat cerebellar cortex terminate in parasagittal bands. In particular, the anterior lobe vermis of the cerebellum contains alternating bands of mossy fibers from the spinal cord and external cuneate nuclei. The cerebellar cortical efferents, the Purkinje cells, are also organized in parasagittal bands. These can be revealed by immunochemical staining for the antigen zebrin II, which is selectively expressed by bands of Purkinje cells. In some cases, the boundaries between mossy fiber terminal fields align with identified transitions between zebrin+/- sets of Purkinje cells, whereas others are located within apparently homogeneous Purkinje cell compartments. Two theories can explain the terminal-field topography: In one view, mossy fiber terminals segregate during development, because growth cones from different sources compete for common territory. Alternatively, mossy fiber growth cones directly recognize chemically distinct target territories, and activity-dependent mechanisms play only minor roles. To explore these issues, two sets of experiments were performed. First, the terminal-field map of the neonatal spinocerebellar projection was compared to the Purkinje cell compartmentation as revealed by anticalbindin immunocytochemistry. Second, subsets of spinocerebellar mossy fiber afferents were ablated early in postnatal development, and the consequences for the neighboring cuneocerebellar terminal fields were mapped in the adult with reference to the zebrin II+/- compartments. These experiments revealed no evidence that competitive interactions constrain the mossy fiber terminal-field distribution but, rather, suggest that the organization of the mossy fiber projections follows the compartmentation of the Purkinje cells.

Use of voltage-sensitive dyes and optical recordings in the central nervous system

Understanding the spatio-temporal features of the information processing occurring in any complex neural structure requires the monitoring and analysis of the activity in populations of neurons. Electrophysiological and other mapping techniques have provided important insights into the function of neural circuits and neural populations in many systems. However, there remain limitations with these approaches. Therefore, complementary techniques which permit the monitoring of the spatio-temporal activity in neuronal populations are of continued interest. One promising approach to monitor the electrical activity in populations of neurons or on multiple sites of a single neuron is with voltage-sensitive dyes coupled with optical recording techniques. This review concentrates on the use of voltage-sensitive dyes and optical imaging as tools to study the activity in neuronal populations in the central nervous system. Focusing on 'fast' voltage-sensitive dyes first, several technical issues and developments in optical imaging will be reviewed. These will include more recent developments in voltage-sensitive dyes as well as newer developments in optical recording technology. Second, studies using voltage-sensitive dyes to investigate information processing questions in the central nervous system and in the invertebrate nervous system will be reviewed. Some emphasis will be placed on the cerebellum, but the major goal is to survey how voltage-sensitive dyes and optical recordings have been utilized in the central nervous system. The review will include optical studies on the visual, auditory, olfactory, somatosensory, auditory, hippocampal and brainstem systems, as well as single cell studies addressing information processing questions. Discussion of the intrinsic optical signals is also included. The review attempts to show how voltage-sensitive dyes and optical recordings can be used to obtain high spatial and temporal resolution monitoring of neuronal activity.

Spatial and temporal pattern of Purkinje cell degeneration in shaker mutant rats with hereditary cerebellar ataxia

Temporal-spatial patterns of surviving Purkinje cells were studied quantitatively in a rat mutant (shaker) with differential hereditary cerebellar ataxia and Purkinje cell degeneration. Shaker rat mutants are characterized behaviorally as mild if they are ataxic or as strong if they have ataxia and tremor. Purkinje cells degenerate in both mild and strong shaker mutants, but the temporal and spatial patterns of cell death are strikingly different. In mild shaker mutants, Purkinje cell death is temporally restricted, with 31-46% of the Purkinje cells in lobules I-IX dying by 3 months of age. Very few Purkinje cells degenerate after this age. Purkinje cell death is spatially random. In lobules I-IX, every second, third, or fourth Purkinje cell degenerates. Purkinje cells in lobule X do not degenerate. In strong shaker mutants, Purkinje cell degeneration is temporally protracted and spatially restricted. By 3 months of age, most Purkinje cells in lobules I-VIa, -b, and -d have degenerated. Numerous Purkinje cells in the paravermis of lobules VIIb-VIII have also degenerated. Surviving Purkinje cells in the vermis and lateral hemisphere of lobules VIIb-VIII are aligned in parasagittally oriented stripes or transversely oriented bands. Purkinje cells continue to degenerate in localized areas of the posterior lobe such that, by 18 months of age, surviving Purkinje cells are limited primarily to lobules VIc, VIIa, IXd, and X. Quantitative analysis indicates that none of the Purkinje cells in these lobules degenerate.

Topographic spinocerebellar mossy fiber projections are maintained in the lurcher mutant

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

Spatial patterns of functional activation of the cerebellum investigated using high field (4 T) MRI

Using single and multislice functional MRI at high field strength (4 T) we studied cerebellar activation in 12 subjects making a series of alternating wrist flexion and extension movements against constant inertial loads. Three spatial patterns of activation were observed: (i) parasagittal bands of activity localized primarily in the ipsilateral intermediate and lateral zones of the cerebellar hemispheres, (ii) medio-lateral bands which in some subjects followed the contour of individual folia and (iii) fragmented regions of activation covering extensive areas of the cerebellum. Bilateral activation of the cerebellum was observed in all subjects with measurable activity. Mean statistically significant activation intensity ranged from 2.34 to 13.54% above baseline.

Abnormal embryonic cerebellar development and patterning of postnatal foliation in two mouse Engrailed-2 mutants

The cerebellum is an ideal system to study pattern formation in the central nervous system because of its simple cytoarchitecture and regular organization of folds and neural circuitry. Engrailed-2 (En-2) is expressed in a spatially restricted broad band around the mesencephalic-metencephalic junction, a region from which the cerebellum is derived. Mice homozygous for a targeted deletion of the En-2 homeobox, En-2hd, previously have been shown to have an altered adult cerebellar foliation pattern. To address whether the En-2hd allele was hypomorphic, we generated a putative null mutation that makes an N-terminal deletion (ntd). Mice homozygous for this new mutation, En-2ntd, display an identical cerebellar patterning defect, suggesting that both alleles represent null alleles. We also examined the developmental profile of En-2 homozygous mutant cerebellar foliation. This revealed a complex phenotype of general developmental delay and abnormal formation of specific fissures with the most severe morphological disruptions being limited to the posterior region of the cerebellum. The expression of two transgenes, which express lacZ in lobe-specific patterns in the cerebellum, also was found to be altered in En-2 homozygotes, suggesting possible lobe transformations. Finally, during embryogenesis there was a clear delay in fusion of the cerebellar rudiments at the midline by 15.5 d.p.c. This and the expression pattern of En-2 suggests that although cerebellar foliation is largely a postnatal process, the patterning of the cerebellum may begin during embryogenesis and that En-2 plays a critical role in this early process.

The olivocerebellar projection in normal (+/+), heterozygous weaver (wv/+), and homozygous weaver (wv/wv) mutant mice: comparison of terminal pattern and topographic organization

Olivocerebellar organization and topography were analyzed in adult normal (+/+), heterozygous weaver (wv/+), and homozygous weaver (wv/wv) mutant mice. The two genotypes (wv/+ and wv/wv) of the weaver mutant present a gradation of abnormal cerebellar morphology. Purkinje cell (PC) ectopia ranges from mild (wv/+) to moderate (wv/wv), and regional PC loss is also graded in the two types. To determine olivocerebellar organization and topography, tritiated amino acids were placed into different regions of the inferior olivary complex (IO) in normal, heterozygous, and homozygous weaver mice. Despite some PC loss and ectopia, olivocerebellar fiber (OCF) terminals in both homozygous and heterozygous weaver mice have an orthogonal distribution and topography similar to that seen in normal mice. Differences in OCF termination, such as an increased density of OCF terminal label in the lower portion of the molecular layer, the PC, and granule cell layers, are seen in homozygous weaver mice. In some heterozygous weaver and normal cases, multiple injections labeling most IO cells on one side of the IO resulted in continuous OCF terminal labeling in many regions of the contralateral cerebellar cortex, suggesting that all PCs receive OCF input. Retrograde analysis involving injections of horseradish peroxidase conjugated to wheat germ agglutinin into different mediolateral cerebellar regions in homozygous weaver mice further demonstrates a generally normal olivocerebellar topography.

Evidence of early topographic organization in the embryonic olivocerebellar projection: a model system for the study of pattern formation processes in the central nervous system

Many projection systems within the peripheral and central nervous system are topographically organized, and it has become increasingly clear that interactions which occur during development determine the projection patterns these systems exhibit in the adult. The olivocerebellar system was chosen as a model system for this study of afferent pattern formation because it has several characteristics which lend themselves to a study of this type. Applications of horseradish peroxidase were made to both the cerebellar primordium and to the inferior olive of embryonic and neonatal mice using an in vitro perfusion system to support the tissue during the transport period. Fibers labeled after restricted olivary applications are limited to particular mediolateral regions of the cerebellum. Similarly, olivary cells retrogradely labeled after discrete cerebellar applications are restricted to particular olivary subdivisions. The results indicate that the olivocerebellar projection displays elements of topographic organization as early as E15 and that the pattern displayed is roughly comparable to that of the adult mammal. The observed trajectories of olivocerebellar fibers and their concomitant association with both Purkinje and cerebellar nuclear cells during embryonic development suggests a role for either or both cell types in the pattern formation process.

The innervation of calcitonin gene-related peptide to the Purkinje cells and granule cells in the developing mouse cerebellum

The present study analyzed the ontogeny of calcitonin gene-related peptide-like immunoreactive (CGRP-IR) structures in the mouse cerebellum. No CGRP-IR neurons were detected at any stage, but three types of CGRP-IR fibers were seen: (1) CGRP-IR dense fiber plexuses which appeared transiently in the developing cerebellum, (2) thin varicose fibers, and (3) mossy fiber-like fibers. The CGRP-IR dense fiber plexuses appeared in the developing Purkinje cell layer at postnatal day 2. From postnatal days 6 to 11, these fibers formed pericellular nests around Purkinje cells. After that stage, these fibers rapidly disappeared and no such plexuses were seen in the adult cerebellum. CGRP-IR fiber plexuses were not evenly distributed, and they had a parasagittal banded pattern in the frontal sections. These plexuses existed in the region of all vermis, crus 1 of the ansiform lobe, simplex lobule, and flocculus, while the other lobules were devoid of such fibers. Under electron microscopy, these CGRP-IR fibers were seen to make synaptic contacts with somatic spines of Purkinje cells, suggesting that CGRP-IR plexuses were closely related to the developing Purkinje cells. Mossy fiber-like CGRP-IR fibers appeared in the granular layer on postnatal day 2, and increased in number to reach a peak on postnatal day 12. Thereafter, they decreased slightly to reach a plateau on postnatal day 30. Under electron microscopy these CGRP-IR fibers were revealed to be the mossy fibers which regulated the granule cells. Thin varicose CGRP-IR fibers were rarely seen at birth, but on postnatal day 8, many fibers appeared in all layers and increased by postnatal day 30. They distributed equally throughout the cerebellar cortex with a slight predominance in density in the molecular and Purkinje cell layer. Immunoelectron microscopic analysis showed that these fibers made synaptic contacts with small dendrites in the molecular layer.

Expression of the Purkinje cell specific zebrin antigens in the cerebellum of the meander tail mutant mouse

The cerebellum of the meander tail mutant mouse is characterized by normal cytoarchitecture in the posterior lobe and agranular, abnormal cytoarchitecture in the anterior lobe. The Purkinje cells form a monolayer in the posterior lobe but are dispersed throughout the cortex of the anterior lobe. Examination of these cells with the zebrin antibodies demonstrates that in spite of the morphologic and laminar disorganization of these cells in the anterior lobe, they are organized into the appropriate number of correctly positioned immunopositive zebrin clusters.

Ontogenesis of cerebellar afferents identified by cholecystokinin-like immunoreactivity

The present account provides a developmental timetable for the maturation of cholecystokinin (CCK)-positive fibers in the cerebellar cortex and cerebellar nuclei of the opossum. CCK-positive fibers are in the cerebellar peduncle by postnatal day (PD) 1, however they wait until PD 7 to penetrate the cerebellar anlage. Between PD 7 and PD 20 the fibers wait again in the medullary core of the cerebellum. After PD 20, there are 2 distinct patterns of CCK localization within the overlying cortical layers. The first pattern develops between PD 20-26 when CCK puncta are present in restricted foci within the Purkinje cell layer of the anterior lobe vermis. They distribute in 4 parasagittal bands, 2 on either side of the midline, that extend from the primary fissure rostrally into the anterior lobe of the cerebellum. By PD 33 two additional parasagittal bands are present in the posterior lobe vermis. The vast majority of these CCK puncta are transient in nature as all but a few disappear by PD 84. Those that remain progress through a series of developmental stages characteristic of climbing fiber ontogeny. These climbing fibers persist in lobules V, VII and VIII of the adult cerebellum. Further, there is a transient expression of CCK-immunoreactivity within inferior olivary neurons. These observations support the interpretation that the transient population of CCK-IR puncta are immature climbing fiber axons derived from the inferior olive. The second pattern of CCK localization is evident between PD 30-33, the time when granule cells first can be recognized in a histologically distinct internal granule cell layer (IGL). Between PD 30 and PD 68 there is a differential pattern of distribution of CCK-IR profiles within the lobules of the cerebellum. Initially, CCK-IR axons are only present in the anterior vermis where they are aligned in register with the bands of CCK puncta in the Purkinje cell layer. CCK-IR puncta are not present in the posterior lobe vermis or hemispheres until later stages of development. Further, a sagittal organization is not evident in either of these latter 2 areas. Initially, CCK-IR profiles in the IGL cannot be identified as mossy fibers based on their terminal morphology. When they first enter the IGL they appear as punctate elements. Over time they become increasingly more complex in shape and between PD 68-84 develop morphological characteristics of adult mossy fiber rosettes. The cerebellar nuclei can be distinguished histologically by PD 18, but CCK-IR fibers are not evident among these neurons until PD 36 which corresponds to about the time they can be visualized in the IGL. In addition, CCK-IR cell bodies first appear in the cerebellar nuclei between PD 26-30; these are present in the adult.(ABSTRACT TRUNCATED AT 400 WORDS)

Olivary morphology and olivocerebellar topography in adult lurcher mutant mice

In adult lurcher mice, in which virtually all cerebellar Purkinje cells have degenerated as a direct consequence of mutant gene action, the inferior olivary complex suffers a severe retrograde transneuronal atrophy. Our analysis indicates a 63% cell loss in the lurcher inferior olive, homogeneously distributed between the medial and dorsal accessory, and principal olivary subdivisions. Olivary neurons are reduced in cross-sectional area by 30% in lurcher mice, compared to normal controls. All olivary subdivisions morphologically identifiable in normal mice are also found in the lurcher inferior olive. Analysis of olivocerebellar topography by retrograde transport of lectin-conjugated horseradish peroxidase and fluorogold, in both single and double labeling paradigms, reveals no abnormalities in the general organization of this highly ordered projection. This stability may be based on the initial establishment of the topographic pattern in late embryogenesis or early postnatal periods, prior to the onset of lurcher Purkinje cell degeneration, or, alternatively, the lurcher gene may not alter critical afferent and target characteristics at stages when the topographic relationship is being established. Once established, the olivocerebellar projection is apparently not dependent on the Purkinje cell for long-term maintenance of its general topographic organization.

Spinocerebellar projection in the meander tail mutant mouse: organization in the granular posterior lobe and the agranular anterior lobe

The cerebellum of the mutant mouse, meander tail, is characterized by normal cytoarchitecture posteriorly and abnormal, agranular cortex anteriorly. Anterograde WGA-HRP tracing analysis of the spinocerebellar projection reveals typical mossy fiber labeling posteriorly in lobule VIII. However, in the anterior cortex, a finer, more diffuse pattern of labeling is seen, unlike the distinct banded pattern of mossy fiber rosettes which characterizes the spinocerebellar projection in the normal animal.