Relationships between neuronal birthdates and cytoarchitecture in the rat inferior olivary complex

Neurogenic timing of the inferior olive subdivisions is related to the olivocerebellar projection topography

The olivocerebellar projection is organized into an intricate topographical connection from the inferior olive (IO) subdivisions to the longitudinally-striped compartments of cerebellar Purkinje Cells (PCs), to play an essential role in cerebellar coordination and learning. However, the central mechanisms for forming topography need to be clarified. IO neurons and PCs are generated during overlapping periods of a few days in embryonic development. Therefore, we examined whether their neurogenic timing is specifically involved in the olivocerebellar topographic projection relationship. First, we mapped neurogenic timing in the entire IO by using the neurogenic-tagging system of neurog2-CreER (G2A) mice and specific labeling of IO neurons with FoxP2. IO subdivisions were classified into three groups depending on their neurogenic timing range. Then, we examined the relationships in the neurogenic-timing gradient between IO neurons and PCs by labeling topographic olivocerebellar projection patterns and PC neurogenic timing. Early, intermediate, and late groups of IO subdivisions projected to late, intermediate, and early groups of the cortical compartments, respectively, except for a few particular areas. The results indicated that the olivocerebellar topographic relationship is essentially arranged according to the reverse neurogenic-timing gradients of the origin and target.

Uncoupling axon guidance and neuronal migration in Robo3-deficient inferior olivary neurons

Inferior olivary (IO) neurons are born in the dorsal hindbrain and migrate tangentially toward the ventral midline. During their dorsoventral migration, IO neurons extend long leading processes that cross the midline, transform into axons, and project into the contralateral cerebellum. In absence of the axon guidance receptor Robo3, IO axons fail to cross the midline and project to the ipsilateral cerebellum. Remarkably, the IO cell bodies still reach the midline where they form a nucleus of abnormal cytoarchitecture. The mechanisms underlying the migration of Robo3-deficient IO neurons are unknown. Here, we used three-dimensional imaging and transgenic mice to label subsets of IO neurons and study their migratory behavior in Robo3 knockout. We show that IO migration is delayed in absence of Robo3. Strikingly, Robo3-deficient IO neurons progress toward the midline in a direction opposite to their axons. This occurs through a change of polarity and the generation of a second leading process at the rear of the cell. These results suggest that Robo3 receptor controls the establishment of neuronal polarity and the coupling of axonogenesis and cell body migration in IO neurons.

Inhibitory effects of draxin on axonal outgrowth and migration of precerebellar neurons

The rhombic lip, a dorsal stripe of the neuroepithelium lining the edge of the fourth ventricle, is the site of origin of precerebellar neurons (PCN), which migrate tangentially towards the floor plate. After reaching the floor plate, they project their axons to the cerebellum. Although previous studies have shown that the guidance molecules Netrin/DCC and Slit/Robo have critical roles in PCN migration, the molecular mechanisms underlying this process remain poorly understood. Here, we report that draxin, a repulsive axon guidance protein, is involved in PCN development. We found that draxin is expressed in the rhombic lip and migratory stream of some PCN in the developing hindbrain of mice. In addition, draxin inhibited neurite outgrowth and nuclei migration from rhombic lip explants. These results suggest that draxin functions as a repulsive guidance cue for PCN migration. However, we observed no significant differences in PCN distribution between draxin(-/-) and wild type embryos. Thus, draxin and other axon guidance cues may have redundant roles in PCN migration.

Architecture and development of olivocerebellar circuit topography

The cerebellum has a simple tri-laminar structure that is comprised of relatively few cell types. Yet, its internal micro-circuitry is anatomically, biochemically, and functionally complex. The most striking feature of cerebellar circuit complexity is its compartmentalized topography. Each cell type within the cerebellar cortex is organized into an exquisite map; molecular expression patterns, dendrite projections, and axon terminal fields divide the medial-lateral axis of the cerebellum into topographic sagittal zones. Here, we discuss the mechanisms that establish zones and highlight how gene expression and neural activity contribute to cerebellar pattern formation. We focus on the olivocerebellar system because its developmental mechanisms are becoming clear, its topographic termination patterns are very precise, and its contribution to zonal function is debated. This review deconstructs the architecture and development of the olivocerebellar pathway to provide an update on how brain circuit maps form and function.

Epha4 controls the midline crossing and contralateral axonal projections of inferior olive neurons

The guidance of axonal projections to ipsilateral and contralateral regions is essential for integration of bilateral sensory information and coordination of movement. In the development of olivocerebellar projections, newborn neurons of inferior olivary (IO) nuclei ventrally migrate from the hindbrain rhombic lip to the floor plate (FP). The cell bodies of IO neurons cannot cross the FP but their axons can, and thus IO neurons project their axons only to the contralateral cerebellar cortex. The molecular mechanisms determining the contralateral axonal projections of IO neurons, however, are obscure. The IO neurons and their axons express EphA4, whereas the FP expresses an EphA4 ligand, EphrinB3, from embryonic day 12.5. Therefore, we tested whether EphA4-deficient mice (EphA4(-/-) ) would show impairment in the development of olivocerebellar projections. We found that, in EphA4(-/-) embryos, some of the IO neurons projected their axons to the ipsilateral cerebellar cortex because the cell bodies of the IO neurons abnormally crossed the FP. Furthermore, even in adults, EphA4(-/-) cerebella were bilaterally innervated by unilateral IO subnuclei. These observations indicate that EphA4 is involved in the contralateral axonal projections of IO neurons by preventing their cell bodies from crossing the midline FP.

Origin of climbing fiber neurons and the definition of rhombic lip

Wilhelm His identified the rhombic lip in human embryos a century ago and thought that all mossy and climbing fiber neurons are derived from it. This has been accepted for more than one hundred years. However, recent genetic fate mapping studies have demonstrated that mossy fiber and climbing fiber neurons have distinct progenitor pools and originate from different subdomains in the ventricular zone of the caudal hindbrain. The majority of climbing fiber neurons has been found to derive from more medial parts of the caudal hindbrain although its dorsal boundary is unclear currently. Subsequently, several new definitions for the rhombic lip have been made according to the expression domain of different transcription factors, such as Math1, Wnt1 and Olig3, which have different ventral boundaries and make the area of rhombic lip confused. Therefore, a precise definition for rhombic lip is still lacking for the main reason that the origin of the entire inferior olivary nuclei is still unclear. Further genetic fate mapping is needed to determine the origin of all climbing fiber neurons which will help to give the rhombic lip an unambiguous definition.

Neural RNA-binding protein Musashi1 controls midline crossing of precerebellar neurons through posttranscriptional regulation of Robo3/Rig-1 expression

Precisely regulated spatiotemporal gene expression is essential for the establishment of neural circuits. In contrast to the increasing evidence for transcriptional regulation of axon guidance cues and receptors, the role of posttranscriptional regulation in axon guidance, especially in vivo, remains poorly characterized. Here, we demonstrate that the expression of Slit receptor Robo3/Rig-1, which plays crucial roles in axonal midline crossing, is regulated by a neural RNA-binding protein Musashi1 (Msi1). Msi1 binds to Robo3 mRNA through RNA recognition motifs and increases the protein level of Robo3 without affecting its mRNA level. In Msi1-deficient precerebellar neurons, Robo3 protein, but not its mRNA, is dramatically reduced. Moreover, similar to defects in Robo3-deficient mice, axonal midline crossing and neuronal migration of precerebellar neurons are severely impaired in Msi1-deficient mice. Together, these findings indicate that Msi1-mediated posttranscriptional regulation of Robo3 controls midline crossing of precerebellar neurons.

Molecular mechanisms controlling midline crossing by precerebellar neurons

Precerebellar neurons of the inferior olive (IO) and lateral reticular nucleus (LRN) migrate tangentially from the rhombic lip toward the floor plate following parallel pathways. This process is thought to involve netrin-1 attraction. However, whereas the cell bodies of LRN neurons cross the midline, IO neurons are unable to do so. In many systems and species, axon guidance and cell migration at the midline are controlled by Slits and their receptor Robos. We showed previously that precerebellar axons and neurons do not cross the midline in the absence of the Robo3 receptor. To determine whether this signaling by Slits and the two other Robo receptors, Robo1 and Robo2, also regulates precerebellar neuron behavior at the floor plate, we studied the phenotype of Slit1/2 and Robo1/2/3 compound mutants. Our results showed that many IO neurons can cross the midline in absence of Slit1/2 or Robo1/2, supporting a role for midline repellents in guiding precerebellar neurons. We also show that these molecules control the development of the lamellation of the inferior olivary complex. Last, the analysis of Robo1/2/3 triple mutants suggests that Robo3 inhibits Robo1/2 repulsion in precrossing LRN axons but not in IO axons in which it has a dominant and distinct function.

Trio controls the mature organization of neuronal clusters in the hindbrain

During the embryonic development of the hindbrain, movements of neuronal clusters allow the formation of mature "pools", in particular for inferior olivary (ION) and facial motor (fMN) nuclei. The cellular mechanisms of neuron clustering remain uncharacterized. We report that the absence of the Rho-guanine exchange factor Trio, which can activate both RhoG and Rac1 in vivo, prevents the proper formation of ION and fMN subnuclei. Rac1, but not RhoG, appears to be a downstream actor in Trio-induced lamellation. In addition, we report that Cadherin-11 is expressed by a subset of neurons through the overall period of ION and fMN parcellations, and defects observed in trio mutant mice are located specifically in Cadherin-11-expressing regions. Moreover, endogenous Cadherin-11 is found in a complex with Trio when lamellation occurs. Altogether, those results establish a link between Trio activity, the subsequent Rac1 activation, and neuronal clusters organization, as well as a possible recruitment of the Cadherin-11 adhesive receptor to form a complex with Trio.

And the Olive Said to the Cerebellum: Organization and Functional Significance of the Olivo-Cerebellar System

What is the role of the olive in cerebellar operations? And what is the role of the cerebellum in the sensory and motor functioning of the nervous system? It is clear that the olivo-cerebellar network plays a key role in the managing of vertebrate motor control, as lesions in this network cause motor deficits in humans and animals. However, it is increasingly becoming clear that the olivo-cerebellar system is involved in the control of more than simple motor behaviors. The elegant and almost geometric organization of the olivocerebellar network lends itself to performing these complex operations. In this review, the salient anatomical, physiological, and clinical features of this system are discussed. A computer-assisted visualization system is used to illustrate some of the anatomical points. The idea that the cerebellum and the olivo-cerebellar networks are perhaps in the upper echelons of the hierarchy of brain functioning, if such a hierarchy indeed exists, is discussed. NEUROSCIENTIST 13(6):616—625, 2007. DOI: 10.1177/1073858407299286

Precerebellar and vestibular nuclei of the short-beaked echidna (Tachyglossus aculeatus)

The monotremes are a unique group of living mammals, which diverged from the line leading to placental mammals at least 125 million years ago. We have examined the organization of pontine, inferior olivary, lateral reticular and vestibular nuclei in the brainstem of the short-beaked echidna (Tachyglossus aculeatus) to determine if the cyto- and chemoarchitecture of these nuclei are similar to that in placental mammals and marsupials. We have used Nissl staining in conjunction with enzyme-histochemistry for acetylcholinesterase, cytochrome oxidase and NADPH diaphorase as well as immunohistochemistry for non-phosphorylated neurofilament protein (SMI-32 antibody) and calcium binding proteins (parvalbumin, calbindin, calretinin). Homologies could be established between the arch shaped inferior olivary complex of the echidna and the principal, dorsal and medial accessory subdivisions of the therian inferior olivary complex. The pontine nuclei of the echidna included basilar and reticulotegmental components with similar cyto- and chemarchitectural features to therians and there were magnocellular and subtrigeminal components of the lateral reticular nucleus, also as seen in therians. Subdivisions and chemoarchitecture of the vestibular complex of the echidna were both similar to that region in rodents. In all three precerebellar nuclear groups studied and in the vestibular nucleus organization, the cyto- and chemoarchitecture of the echidna was very similar to that seen in therian mammals and no "primitive" or "reptilian" features were evident.

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.

The Slit Receptor Rig-1/Robo3 Controls Midline Crossing by Hindbrain Precerebellar Neurons and Axons

During development, precerebellar neurons migrate dorsoventrally from the rhombic lip to the floor plate. Some of these neurons cross the midline while others stop. We have identified a role for the slit receptor Rig-1/Robo3 in directing this process. During their tangential migration, neurons of all major hindbrain precerebellar nuclei express high levels of Rig-1 mRNA. Rig-1 expression is rapidly downregulated as their leading process crosses the floor plate. Interestingly, most precerebellar nuclei do not develop normally in Rig-1-deficient mice, as they fail to cross the midline. In addition, inferior olivary neurons, which normally send axons into the contralateral cerebellum, project ipsilaterally in Rig-1 mutant mice. Similarly, neurons of the lateral reticular nucleus and basilar pons are unable to migrate across the floor plate and instead remain ipsilateral. These results demonstrate that Rig-1 controls the ability of both precerebellar neuron cell bodies and their axons to cross the midline.

Distinct roles of Rac1/Cdc42 and Rho/Rock for axon outgrowth and nucleokinesis of precerebellar neurons toward netrin 1

During embryonic development, tangentially migrating precerebellar neurons emit a leading process and then translocate their nuclei inside it (nucleokinesis). Netrin 1 (also known as netrin-1) acts as a chemoattractant factor for neurophilic migration of precerebellar neurons (PCN) both in vivo and in vitro. In the present work, we analyzed Rho GTPases that could direct axon outgrowth and/or nuclear migration. We show that the expression pattern of Rho GTPases in developing PCN is consistent with their involvement in the migration of PCN from the rhombic lips. We report that pharmacological inhibition of Rho enhances axon outgrowth of PCN and prevents nuclei migration toward a netrin 1 source, whereas inhibition of Rac and Cdc42 sub-families impair neurite outgrowth of PCN without affecting migration. We show, through pharmacological inhibition, that Rho signaling directs neurophilic migration through Rock activation. Altogether, our results indicate that Rho/Rock acts on signaling pathways favoring nuclear translocation during tangential migration of PCN. Thus, axon extension and nuclear migration of PCN in response to netrin 1 are not strictly dependent processes because: (1) distinct small GTPases are involved; (2) axon extension can occur when migration is blocked; and (3) migration can occur when axon outgrowth is impaired.

Ontogeny of the pre-Bötzinger complex in perinatal rats

The aim of this study was to provide a systematic examination of the ontogenesis of the mammalian respiratory rhythm generating center, the pre-Bötzinger complex (pre-BötC). A combination of immunohistochemical markers and electrophysiological recordings was used to determine the time of inception of the pre-BötC and the developmental changes during the perinatal period in rats spanning from embryonic day 15 (E15) to postnatal day 7. The first clear indication of neurons immunopositive for neurokinin-1 receptors (NK1Rs) and somatostatin expression, two proposed markers for pre-BötC neurons, was at approximately E17. Birth dating of neurons in the ventrolateral medulla using 5-bromo-2'-deoxyuridine demonstrated that NK1R-positive neurons populating the area of the pre-BötC during late E16-E18 are born at E12.5-E13.5, approximately 2 d later than adjacent NK1R-positive neurons in the ventrolateral medulla. Extracellular recordings of neuronal populations within the pre-BötC of perinatal medullary slice preparations demonstrated that the onset of rhythmical respiratory discharge commences at approximately E17. Application of substance P, a ligand for NK1R receptors, to the media bathing E17 medullary slice and brainstem-spinal cord preparations resulted in a marked increase in respiratory frequency. These data provide insights into the ontogeny of the pre-BötC, giving fundamental information on the genesis, settlement, and inception of rhythmic activity within the group of neurons proposed to be responsible for the respiratory rhythm generation. Furthermore, this provides the foundation for further analyses of cell lineage, the transcriptional control of respiratory neuronal development, and electrophysiological and pharmacological properties of the pre-BötC during the prenatal period.

Multiple influences on the migration of precerebellar neurons in the caudal medulla

Neurons destined to form several precerebellar nuclei are generated in the dorsal neuroepithelium (rhombic lip) of caudal hindbrain. They form two ventrally directed migratory streams, which behave differently. While neurons in the superficial migration migrate in a subpial position and cross the midline to settle into the contralateral hindbrain, neurons in the olivary migration travel deeper in the parenchyma and stop ipsilaterally against the floor plate. In the present study, we compared the behavior of the two neuronal populations in an organotypic culture system that preserves several aspects of their in vivo environment. Both migrations occurred in mouse hindbrain explants dissected at E11.5 even when the floor plate was ablated at the onset of the culture period, indicating that they could rely on dorsoventral cues already distributed in the neural tube. Nevertheless, the local constraints necessary for the superficial migration were more specific than for the olivary migration. Distinct chemoattractive and chemorespulsive signal were found to operate on the migrations. The floor plate exhibited a strong chemoattractive influence on both migrations, which deviated from their normal path in the direction of ectopic floor plate fragments. It was also found to produce a short-range stop signal and to induce inferior olive aggregation. The ventral neural tube was also found to inhibit or slow down the migration of olivary neurons. Interestingly, while ectopic sources of netrin were found to influence both migrations, this effect was locally modulated and affected differentially the successive phases of migration. Consistent with this observation, while neurons in the superficial migration expressed the Dcc-netrin receptor, the migrating olivary neurons did not express Dcc before they reached the midline. Our observations provide a clearer picture of the hierarchy of environmental cues that influence the morphogenesis of these precerebellar nuclei.

Netrin 1 acts as an attractive or as a repulsive cue for distinct migrating neurons during the development of the cerebellar system

Netrin 1 is a long-range diffusible factor that exerts chemoattractive or chemorepulsive effects on developing axons growing to or away from the neural midline. Here we used tissue explants to study the action of netrin 1 in the migration of several cerebellar and precerebellar cell progenitors. We show that netrin 1 exerts a strong chemoattractive effect on migrating neurons from the embryonic lower rhombic lip at E12-E14, which give rise to precerebellar nuclei. Netrin 1 promotes the exit of postmitotic migrating neurons from the embryonic lower rhombic lip and upregulates the expression of TAG-1 in these neurons. In addition, in the presence of netrin 1, the migrating neurons are not isolated but are associated with thick fascicles of neurites, typical of the neurophilic way of migration. In contrast, the embryonic upper rhombic lip, which contains tangentially migrating granule cell progenitors, did not respond to netrin 1. Finally, in the postnatal cerebellum, netrin 1 repels both the parallel fibres and migrating granule cells growing out from explants taken from the external germinal layer. The developmental patterns of expression in vivo of netrin 1 and its receptors are consistent with the notion that netrin 1 secreted in the midline acts as chemoattractive cue for precerebellar neurons migrating circumferentially along the extramural stream. Similarly, the pattern of expression in the postnatal cerebellum suggests that netrin 1 could regulate the tangential migration of postmitotic premigratory granule cells. Thus, molecular mechanisms considered as primarily involved in axonal guidance appear also to steer neuronal cell migration.

Morphogenesis of the optic tectum in the medaka (Oryzias latipes): a morphological and molecular study, with special emphasis on cell proliferation

We analyzed the medaka optic tectum (OT) morphogenesis by using 5-bromo-2'-deoxyuridine (BrdU) immunohistochemistry (with a new method we developed for pulse-labeling embryos) and in situ hybridization with three probes, two for recently cloned homeobox genes (Ol-Prx3 [Paired-Related-Homeobox3] and Ol-Gsh1 [Genetic-Screen-Homeobox1]) and one for Ol-tailless. The tectal anlage first appears as a sheet of proliferating cells expressing Ol-Gsh1 and Ol-tailless but not Ol-Prx3. Cells subsequently cease to proliferate in a superficial and rostral zone and begin to express Ol-Prx3. When tectal lamination begins, the proliferative zone (mpz) becomes restricted to a crescent at the OT medial, caudal, and lateral margin. This mpz functions throughout the fish's entire life. It produces cells that are added at the OT's edge as radial rows, spanning every layer of the OT. The cells of the mpz continue to express Ol-tailless in the adult, whereas Ol-Gsh1 expression is turned off. When superficial layers form, Ol-Prx3 expression becomes restricted to the underlying deep layer, where it persists in the adult. Ol-Prx3 seems to be a marker for the differentiation of a subset of deep cells and allows analysis of tectal lamination, whereas Ol-tailless and Ol-Gsh1 could be involved in the control of tectal cell proliferation. This study constitutes a first step toward molecular approach to OT development in anamniotes. We compare and discuss the expression patterns of the homologs of the genes studied, and more generally the morphogenetic patterns of the medaka tectum, with those encountered in other cortical structures and in other vertebrate groups.

Floor plate and netrin-1 are involved in the migration and survival of inferior olivary neurons

During their circumferential migration, the nuclei of inferior olivary neurons translocate within their axons until they reach the floor plate where they stop, although their axons have already crossed the midline to project to the contralateral cerebellum. Signals released by the floor plate, including netrin-1, have been implicated in promoting axonal growth and chemoattraction during axonal pathfinding in different midline crossing systems. In the present study, we report experiments that strongly suggest that the floor plate could also be involved in the migration of inferior olivary neurons. First, we show that the pattern of expression of netrin receptors DCC (for deleted in colorectal cancer), neogenin (a DCC-related protein), and members of the Unc5 family in wild-type mice is consistent with a possible role of netrins in directing the migration of precerebellar neurons from the rhombic lips. Second, we have studied mice deficient in netrin-1 production. In these mice, the number of inferior olivary neurons is remarkably decreased. Some of them are located ectopically along the migration stream, whereas the others are located medioventrally and form an atrophic inferior olivary complex: most subnuclei are missing. However, axons of the remaining olivary cell bodies located in the vicinity of the floor plate still succeed in entering their target, the cerebellum, but they establish an ipsilateral projection instead of the normal contralateral projection. In vitro experiments involving ablations of the midline show a fusion of the two olivary masses normally located on either side of the ventral midline, suggesting that the floor plate may function as a specific stop signal for inferior olivary neurons. These results establish a requirement for netrin-1 in the migration of inferior olivary neurons and suggest that it may function as a specific guidance cue for the initial steps of the migration from the rhombic lips and then later in the development of the normal crossed projection of the inferior olivary neurons. They also establish a requirement for netrin-1, either directly or indirectly, for the survival of inferior olivary neurons.

Increased inferior olivary neuron and cerebellar granule cell numbers in transgenic mice overexpressing the human Bcl-2 gene

Neuron-target interactions during development are critical for determining the final numbers of neurons in the nervous system. To investigate the role of Purkinje cells and programmed cell death in the regulation of afferent neuron numbers, we have counted olivary neurons and granule cells in two lines of transgenic mice (NSE73a and NSE71) that overexpress a human gene for bcl-2 (Hu-bcl-2) in Purkinje cells and olivary neurons, but not in granule cells. Bcl-2 overexpression in vivo reduces naturally occurring neuronal cell death and cell death following axotomy, target removal, or ischemia. Olivary neuron numbers in NSE73a and NSE71 transgenic mice are significantly increased compared to controls by 28% and 27%, respectively, while granule cell numbers are only increased in NSE73a mice (29% above controls). We have previously shown that Purkinje cell number is increased by 43% in NSE73a transgenics and by 23% in NSE71 transgenics. The ratio of Purkinje cells to olivary neurons is not significantly different between the control and transgenic mice, while the ratio of granule cells to Purkinje cells is significantly decreased in the NSE71 transgenic mice compared to controls and NSE73a transgenics. The increased numbers of olivary neurons suggest that bcl-2 overexpression rescues these neurons from programmed cell death. The increase in granule cell number in only one transgenic line is discussed with respect to hypotheses that Purkinje cells regulate both granule cell progenitor proliferation and the survival of differentiated granule cells.

Localisation of insulin-like growth factor-I (IGF-I) immunoreactivity in the olivocerebellar system of developing and adult rats

The molecular mechanisms which underlie the development of the olivocerebellar topography are not fully understood. Insulin-like growth factor-I (IGF-I) is a growth factor known to play important roles in neural development and it has been identified within the cerebellum and the inferior olive. To assess the contribution of IGF-I to the development of climbing fibre topography, the distribution of IGF-I-like immunoreactivity (IGF-I IR) was identified in the cerebellar cortex and inferior olive of rats, 0, 3, 5, 7, 10, 15, 21, 28 and 90 days old. In the cerebellar cortex, IGF-I IR was localised solely to Purkinje cells and its distribution was spatially and temporally regulated in a manner which coincides with climbing fibre development. At birth, weak IGF-I IR was detected in a few Purkinje cells in the ventral vermis. More Purkinje cells became positive until at postnatal day 7(P7) all Purkinje cells displayed IGF-I IR. Subsequently, a subpopulation of Purkinje cells lost their reactivity for IGF-I to leave IGF-I-positive cells organised into sagittal bands by P15. IGF-I IR was also seen in all subdivisions of the inferior olive between birth and P10 in a distribution which paralleled the maturation of the inferior olive. The Purkinje cell and inferior olivary IGF-I IR parallels climbing fibre development and thus the results of this study support the hypothesis that IGF-I is involved in the development of climbing fibre topography.

Bromodeoxyuridine: a diagnostic tool in biology and medicine, Part III. Proliferation in normal, injured and diseased tissue, growth factors, differentiation, DNA replication sites and in situ hybridization

This paper is a continuation of parts I (history, methods and cell kinetics) and II (clinical applications and carcinogenesis) published previously (Dolbeare, 1995 Histochem. J. 27, 339, 923). Incorporation of bromodeoxyuridine (BrdUrd) into DNA is used to measure proliferation in normal, diseased and injured tissue and to follow the effect of growth factors. Immunochemical detection of BrdUrd can be used to determine proliferative characteristics of differentiating tissues and to obtain birth dates for actual differentiation events. Studies are also described in which BrdUrd is used to follow the order of DNA replication in specific chromosomes, DNA replication sites in the nucleus and to monitor DNA repair. BrdUrd incorporation has been used as a tool for in situ hybridization experiments.

Neurogenesis of subpopulations of rat lumbar dorsal root ganglion neurons including neurons projecting to the dorsal column nuclei

The time of birth of subpopulations of dorsal root ganglion (DRG) neurons was studied with immunohistochemistry for 5-bromodeoxyuridine (BrdU). Pregnant rats were injected with BrdU i.p. to label the neurons on one of the embryonic days (E) E11-E16. When they were adults, the rats were given injections of Fluoro-Gold (FG) into the gracile nucleus to identify DRG neurons projecting to this structure. Following a 5 day survival period, the animals were perfused with aldehyde fixative. Sections from the L3-L5 DRGs were processed for BrdU immunohistochemistry followed by either immunostaining for the antineurofilament antibody RT97, as marker of the light neuronal subpopulation, or histochemical staining for the B4 isolectin from Griffonia simplicifolia I, as marker of the small dark subpopulation. The results indicated that the DRG neurons were generated between E12 and E16. The RT97+ neurons were generated on E12-E15, with a peak at E13. FG+ neurons, the majority of which were RT97+, were also generated on E12-E15. The B4+ neurons were generated on E13-E16, with a peak around E14. The overall pattern of neurogenesis of the DRG neurons showed that the RT97+ neurons were produced prior to the B4+ neurons. These findings are in agreement with earlier observations that the large DRG neurons are generated earlier than the small dark neurons. Our findings also suggest the existence of a third neuronal subpopulation that might be produced at the latest period of DRG neurogenesis at E15-E16.

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

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

Neurogenetical segregation of the vestibulospinal neurons in the rat

The time of origin of the vestibulospinal projection neurons was determined by a double-labeling method using 5-bromodeoxyuridine (BrdU), the thymidine analogue, and Fluoro-Gold (FG), a retrograde fluorescent tracer. Rat fetuses were exposed to BrdU in utero to label the vestibular neurons on one of the embryonic (E) days between E12 and E15. Upon reaching adulthood, the rats were given unilateral injections of FG into the cervical cord to identify the spinal projection neurons. Brainstem sections were immunohistochemically processed for BrdU and then examined for neurons that were both BrdU-positive and FG-positive in the vestibular nuclei. In the lateral vestibular nucleus (LVe), most of the vestibulospinal neurons were generated on E12. In the inferior vestibular nucleus (IVe), the vestibulospinal neurons were produced almost equally on both E12 and E13. In the medial vestibular nucleus (MVe), the vestibulospinal neurons were generated consistently on days between E12 and E14 with a mild peak on E13. The present results thus demonstrate that genesis of the vestibulospinal neurons occurs sequentially in the following order: firstly in the LVe, secondly in the IVe, and finally in the MVe. The different sequential generation of vestibulospinal neurons among the LVe, MVe and IVe may reflect the fact that the vestibulospinal projections are differentially organized depending on the nature of each subnucleus.

Fine structure of the dorsal cap of the inferior olive and its GABAergic and non-GABAergic input from the nucleus prepositus hypoglossi in rat and rabbit

The dorsal cap of the inferior olive is involved in the control of eye movements and is excited by inputs from the midbrain. In the present study we attempted to determine the inhibitory input to this nucleus in rat and rabbit. The projection from the nucleus prepositus hypoglossi to the dorsal cap was studied in the light microscope by anterograde tracing of Phaseolus vulgaris-leucoagglutinin and lesion-induced depletion of glutamic acid decarboxylase immunoreactivity, and in the electron microscope by anterograde tracing of wheat germ agglutinin-coupled horseradish peroxidase combined with GABA immunocytochemistry. We show that the nucleus prepositus hypoglossi projects bilaterally to the dorsal cap, contralaterally to the ventrolateral outgrowth, and ipsilaterally to the medial accessory olive. After lesioning of the nucleus prepositus hypoglossi, the caudal dorsal cap was depleted of most of its glutamic acid decarboxylase-immunoreactive terminals while the rostral dorsal cap and the ventrolateral outgrowth were depleted of a minor part. Ultrastructural analysis indicates that the majority, but not all, of the terminals from the nucleus prepositus hypoglossi in the dorsal cap are GABA-positive. These GABA-positive and GABA-negative terminals form predominantly symmetric and asymmetric synapses; most of them synapse on dendrites outside and inside glomeruli, frequently in association with dendrodendritic gap junctions, while a small minority are axosomatic. None of the terminals from the nucleus prepositus hypoglossi was found to form a crest synapse, although synapses of this kind were predominantly formed by GABAergic terminals. This study shows that the dorsal cap receives a major inhibitory input from the nucleus prepositus hypoglossi, the terminals of which are located at strategic positions on the olivary neurons.

Early Development of Olivocerebellar Projections in the Fetal Rat Using CGRP Immunocytochemistry

The expression of calcitonin gene-related peptide (CGRP) immunoreactivity in certain inferior olivary neurons is transient and developmentally regulated. Labelled neurons begin to appear at embryonic day 16 (E16), and reach their maximal extent by postnatal day 2 (P2). The extinction of the labelling occurs between P13 and P16. Expression of CGRP immunoreactivity is also observed in a few cerebellar fibres from E17, when axons in the restiform bundle begin to enter medially the cerebellar parenchyma. Their maximal extent is reached by P6, and thereafter they slowly disappear following a precise pattern, although fibre extinction is not complete. The spatio-temporal changes in the olivary distribution of the labelled neurons and the changes in the cerebellar labelled fibres follow the known pattern of topographic arrangement of the olivocerebellar system in adult rats. Moreover, the developmental phases of the CGRP-labelled fibres in postnatal rats correspond to those known for climbing fibre phenotypic acquisition. Thus, CGRP immunocytochemistry identifies in the fetal rat a subset of inferior olivary neurons and their corresponding cerebellar climbing fibres. Using this approach, we have analysed some of the initial events leading to the formation of the olivocerebellar projection, and obtained the following information: (i) Olivocerebellar axons are not randomly distributed in the restiform bundle before they enter the cerebellum. (ii) In the presence of a large spectrum of choices at the surface of the rostral half of the cerebellar plate the labelled olivary axons begin to enter the cerebellum at a precise medial point to abut a region composed solely of migrating Purkinje cells, and establish contacts with their targets before these neurons reach their final cortical location. (iii) From E18 to E19, the bundle of labelled fibres loses its superficial location, being bypassed by migrating Purkinje cells, to occupy a region corresponding to the prospective white matter. This translocation is coincident with the occurrence of a second axonal entry point, somewhat more lateral than the previous one, and with the appearance of a new lateral stripe of labelled fibres. (iv) Both the early and the late appearing labelled stripes remain confined from the time of their formation in precise cerebellar territories, indicating that only some clusters of Purkinje cells are contacted by the CGRP fibres. The results obtained imply that there is neither a waiting period nor an initial phase of randomness in the formation of the olivocerebellar projection map. This absence of chaotic cerebellar invasion, and the high selectivity of the entry points, suggest that the orientation of CGRP-positive olivocerebellar fibres towards their targets is regulated by positional information shared between subsets of olivary neurons and clusters of Purkinje cells. The result of this process would be the formation of a precocious coarse topography that would need further refinement.