Origins of cerebellar mossy and climbing fibers immunoreactive for corticotropin-releasing factor in the rabbit

Cerebellum Lecture: the Cerebellar Nuclei-Core of the Cerebellum

The cerebellum is a key player in many brain functions and a major topic of neuroscience research. However, the cerebellar nuclei (CN), the main output structures of the cerebellum, are often overlooked. This neglect is because research on the cerebellum typically focuses on the cortex and tends to treat the CN as relatively simple output nuclei conveying an inverted signal from the cerebellar cortex to the rest of the brain. In this review, by adopting a nucleocentric perspective we aim to rectify this impression. First, we describe CN anatomy and modularity and comprehensively integrate CN architecture with its highly organized but complex afferent and efferent connectivity. This is followed by a novel classification of the specific neuronal classes the CN comprise and speculate on the implications of CN structure and physiology for our understanding of adult cerebellar function. Based on this thorough review of the adult literature we provide a comprehensive overview of CN embryonic development and, by comparing cerebellar structures in various chordate clades, propose an interpretation of CN evolution. Despite their critical importance in cerebellar function, from a clinical perspective intriguingly few, if any, neurological disorders appear to primarily affect the CN. To highlight this curious anomaly, and encourage future nucleocentric interpretations, we build on our review to provide a brief overview of the various syndromes in which the CN are currently implicated. Finally, we summarize the specific perspectives that a nucleocentric view of the cerebellum brings, move major outstanding issues in CN biology to the limelight, and provide a roadmap to the key questions that need to be answered in order to create a comprehensive integrated model of CN structure, function, development, and evolution.

Adaptive Balance in Posterior Cerebellum

Vestibular and optokinetic space is represented in three-dimensions in vermal lobules IX-X (uvula, nodulus) and hemisphere lobule X (flocculus) of the cerebellum. Vermal lobules IX-X encodes gravity and head movement using the utricular otolith and the two vertical semicircular canals. Hemispheric lobule X encodes self-motion using optokinetic feedback about the three axes of the semicircular canals. Vestibular and visual adaptation of this circuitry is needed to maintain balance during perturbations of self-induced motion. Vestibular and optokinetic (self-motion detection) stimulation is encoded by cerebellar climbing and mossy fibers. These two afferent pathways excite the discharge of Purkinje cells directly. Climbing fibers preferentially decrease the discharge of Purkinje cells by exciting stellate cell inhibitory interneurons. We describe instances adaptive balance at a behavioral level in which prolonged vestibular or optokinetic stimulation evokes reflexive eye movements that persist when the stimulation that initially evoked them stops. Adaptation to prolonged optokinetic stimulation also can be detected at cellular and subcellular levels. The transcription and expression of a neuropeptide, corticotropin releasing factor (CRF), is influenced by optokinetically-evoked olivary discharge and may contribute to optokinetic adaptation. The transcription and expression of microRNAs in floccular Purkinje cells evoked by long-term optokinetic stimulation may provide one of the subcellular mechanisms by which the membrane insertion of the GABAA receptors is regulated. The neurosteroids, estradiol (E2) and dihydrotestosterone (DHT), influence adaptation of vestibular nuclear neurons to electrically-induced potentiation and depression. In each section of this review, we discuss how adaptive changes in the vestibular and optokinetic subsystems of lobule X, inferior olivary nuclei and vestibular nuclei may contribute to the control of balance.

Corticotropin-releasing factor depolarizes rat lateral vestibular nuclear neurons through activation of CRF receptors 1 and 2

Corticotropin-releasing factor (CRF) is a neuropeptide mainly synthesized in the hypothalamic paraventricular nucleus and has been traditionally implicated in stress and anxiety. Intriguingly, genetic or pharmacological manipulation of CRF receptors affects locomotor activity as well as motor coordination and balance in rodents, suggesting an active involvement of the central CRFergic system in motor control. Yet little is known about the exact role of CRF in central motor structures and the underlying mechanisms. Therefore, in the present study, we focused on the effect of CRF on the lateral vestibular nucleus (LVN) in the brainstem vestibular nuclear complex, an important center directly contributing to adjustment of muscle tone for both postural maintenance and the alternative change from the extensor to the flexor phase during locomotion. The results show that CRF depolarizes and increases the firing rate of neurons in the LVN. Tetrodotoxin does not block the CRF-induced depolarization and inward current on LVN neurons, suggesting a direct postsynaptic action of the neuropeptide. The CRF-induced depolarization on LVN neurons was partly blocked by antalarmin or antisauvagine-30, selective antagonists for CRF receptors 1 (CRFR1) and 2 (CRFR2), respectively. Furthermore, combined application of antalarmin and antisauvagine-30 totally abolished the CRF-induced depolarization. Immunofluorescence results show that CRFR1 and CRFR2 are co-localized in the rat LVN. These results demonstrate that CRF excites the LVN neurons by co-activation of both CRFR1 and CRFR2, suggesting that via the direct modulation on the LVN, the central CRFergic system may actively participate in the central vestibular-mediated postural and motor control.

Cerebellar Learning Properties Are Modulated by the CRF Receptor

Corticotropin-releasing factor (CRF) and its type 1 receptor (CRFR₁) play an important role in the responses to stressful challenges. Despite the well established expression of CRFR₁ in granular cells (GrCs), its role in procedural motor performance and memory formation remains elusive. To investigate the role of CRFR₁ expression in cerebellar GrCs, we used a mouse model depleted of CRFR₁ in these cells. We detected changes in the cellular learning mechanisms in GrCs depleted of CRFR₁ in that they showed changes in intrinsic excitability and long-term synaptic plasticity. Analysis of cerebella transcriptome obtained from KO and control mice detected prominent alterations in the expression of calcium signaling pathways components. Moreover, male mice depleted of CRFR₁ specifically in GrCs showed accelerated Pavlovian associative eye-blink conditioning, but no differences in baseline motor performance, locomotion, or fear and anxiety-related behaviors. Our findings shed light on the interplay between stress-related central mechanisms and cerebellar motor conditioning, highlighting the role of the CRF system in regulating particular forms of cerebellar learning.SIGNIFICANCE STATEMENT Although it is known that the corticotropin-releasing factor type 1 receptor (CRFR₁) is highly expressed in the cerebellum, little attention has been given to its role in cerebellar functions in the behaving animal. Moreover, most of the attention was directed at the effect of CRF on Purkinje cells at the cellular level and, to this date, almost no data exist on the role of this stress-related receptor in other cerebellar structures. Here, we explored the behavioral and cellular effect of granular cell-specific ablation of CRFR₁ We found a profound effect on learning both at the cellular and behavioral levels without an effect on baseline motor skills.

Inferior olive CRF plays a role in motor performance under challenging conditions

A well-coordinated stress response is pivotal for an organisms' survival. Corticotropin-releasing factor (CRF) is an essential component of the emotional and neuroendocrine stress response, however its role in cerebellar functions is poorly understood. Here, we explore the role of CRF in the inferior olive (IO) nucleus, which is a major source of input to the cerebellum. Using a CRF reporter line, in situ hybridization and immunohistochemistry, we demonstrate very high levels of the CRF neuropeptide expression throughout the IO sub-regions. By generating and characterizing IO-specific CRF knockdown and partial IO-CRF knockout, we demonstrate that reduction in IO-CRF levels is sufficient to induce motor deficiency under challenging conditions, irrespective of basal locomotion or anxiety-like behavior. Furthermore, we show that chronic social defeat stress induces a persistent decrease in IO-CRF levels, and that IO-CRF mRNA is upregulated shortly following stressful situations that demand a complex motor response. Taken together our results indicate a role for IO-CRF in challenge-induced motor responses.

Motor Control: CRF Regulates Coordination and Gait

The function of the olivo-cerebellar tract is not restricted to the supervision of plasticity in the cerebellar cortex. There is growing evidence that the climbing fibers also tune motor commands. A novel study unravels a role of corticotropin-releasing factor (CRF) in motor coordination and gait control.

A novel somatostatin-immunoreactive mossy fiber pathway associated with HSP25-immunoreactive purkinje cell stripes in the mouse cerebellum

Somatostatin 28 immunoreactivity (Sst28-ir) identifies a specific subset of mossy fiber terminals in the adult mouse cerebellum. By using double-labeling immunohistochemistry, we determined that Sst28-ir is associated with presynaptic mossy fiber terminal rosettes, and not Purkinje cells, Golgi cells, or unipolar brush cells. Sst28-ir mossy fibers are restricted to the central zone (lobules VI/VII) and nodular zone (lobules IX, X) of the vermis, and the paraflocculus and flocculus. Within each transverse zone the mossy fiber terminal fields form a reproducible array of parasagittal stripes. The boundaries of Sst28-ir stripes align with a specific array of Purkinje cell stripes revealed by using immunocytochemistry for the small heat shock protein HSP25. In the cerebellum of the homozygous weaver mouse, in which a subpopulation of HSP25-ir Purkinje cells are located ectopically, the corresponding Sst28-ir mossy fiber projection is also ectopic, suggesting a role for a specific Purkinje cell subset in afferent pattern formation. Likewise, in the scrambler mutant mouse, Sst28-ir mossy fibers show a very close association with HSP25-ir Purkinje cell clusters. HSP25 itself does not appear to be critical for normal patterning, however: in the KJR mouse, which does not express cerebellar HSP25, Sst28 expression appears to be normal. Likewise, the Purkinje cell patterning antigens zebrin II and HSP25 are expressed normally in both Sst- and Sst-receptor knockout mice, suggesting that somatostatinergic transmission is not necessary for Purkinje cell stripe formation.

Functional roles of neuropeptides in cerebellar circuits

Whereas the cerebellum contains 22 different types of neuropeptides as presently known, their expression is generally weak and diffusely dispersed in cerebellar tissues, which often makes their functional significance doubtful. Nevertheless, our knowledge about certain neuropeptides has advanced to the extent that we can figure out their unique functional roles in cerebellar circuits. Throughout the cerebellum, CRF is contained in climbing fibers and its spontaneous release is required for the induction of cerebellar long-term depression (LTD), a cellular mechanism of motor learning. Corticotropin-releasing factor (CRF) is also expressed in the paraventricular nucleus-pituitary system and amygdala-lower brainstem system, both of which are involved in coping responses to stress. In view that motor learning requires stressful efforts for correcting errors in repeated trials, CRF in climbing fibers may imply that the olivocerebellar system is part of a large CRF-operated functional system that acts to cope with various stressors. Orexin, on the other hand, is contained in beaded fibers, which, originating from the hypothalamus, project to various brainstem nuclei and also to the cerebellum, exclusively the flocculus. Currently available evidence suggests that, in fight-or-flight situations, orexinergic neurons switch the state of cardiovascular control systems including the flocculus to secure blood supply to working muscles. Considerable knowledge has also been accumulated about angiotensin II, galanin, and cerebellin, but there is still a gap in defining their unique functional roles in cerebellar circuits.

Spatial distribution of corticotropin-releasing factor immunopositive climbing fibers in the mouse cerebellum: analysis by whole mount immunohistochemistry

This study examined the spatial organization of corticotropin-releasing factor (CRF) immunopositive climbing fibers in the mouse cerebellum by whole mount immunohistochemistry. A striking pattern of parasagittal stripes of CRF staining was revealed. Cryosections of whole mount CRF stained cerebellum showed that anti-CRF immunostaining is restricted to climbing fibers in the molecular layer and does not penetrate deeper into the granular layer. The array of CRF stripes was reminiscent of zebrin II immunopositive Purkinje cell stripes in the anterior vermis and the hemispherical lobules. However, a direct comparison of the two distributions showed that the CRF-defined parasagittal stripes and transverse zones in the posterior vermis are different from those defined by the expression of zebrin II: in particular, CRF immunostaining revealed a transverse boundary between lobules VIb and VII and the presence of four CRF-immunopositive climbing fiber stripes in lobule VIII. Furthermore, an array of CRF stripes was seen in lobule X, the flocculus and the paraflocculus, despite uniform zebrin II expression in these areas. In these cases some, but not all, CRF-immunopositive stripes shared boundaries with Purkinje cell stripes that were visualized by the expression of heat shock protein 25. The results reveal a reproducible pattern of CRF-immunopositive climbing fiber innervation in the mouse cerebellum that bears a complex relationship to the stripes delineated by Purkinje cell compartmentation antigens.

Evidence for the presence of the type 2 corticotropin releasing factor receptor in the rodent cerebellum

Corticotropin releasing factor (CRF), localized in afferent inputs to the cerebellum, binds to two receptors defined as the Type 1 (CRF-R1) and the Type 2 (CRF-R2alpha). CRF-R1 has been localized to the cerebellum, as has a truncated isoform of CRF-R2alpha. Evidence for the presence of the full length isoform of CRF-R2alpha in the cerebellum is conflicting. We used RT-PCR, immunohistochemical, and physiologic techniques to resolve this conflict. RT-PCR data show low levels of CRF-R2alpha in the vermis and hemisphere of the cerebellum. These observations were confirmed by the Gene Expression Nervous System Atlas (GENSAT) database. A CRF-R2alpha antibody was used to determine the cellular distribution of the receptor in the cerebellum. The vast majority of the receptors are localized to Bergmann glial cells located throughout the cerebellum, as well as astrocytes in the granule cell layer. Neuronal labeling is present in sub-populations of Purkinje cells, Golgi cells, basket cells, and cerebellar nuclear neurons. Physiologic data show that urocortin II, which binds selectively to CRF-R2alpha, increases the firing rate of both Purkinje cells and nuclear neurons; this response can be blocked by the CRF-R2alpha-specific antagonist, antisauvagine-30. The present results confirm that CRF-R2alpha is present in the cerebellum and functions in circuits that modulate the firing rate of Purkinje cells and cerebellar nuclear neurons. A comparative analysis showed that the patterns of distribution of CRF-R1, CRF-R2alpha and CRF-R2alpha-tr are distinct. These data indicate that the CRF family of peptides modulates cerebellar output by binding to multiple CRF receptors.

Presynaptic localization of a truncated isoform of the type 2 corticotropin releasing factor receptor in the cerebellum

It is now well established that corticotropin releasing factor is present in two major excitatory afferent systems to the cerebellum, namely climbing fibers and mossy fibers. Two major classes of corticotropin releasing factor receptors, each with unique binding characteristics, have been identified as type 1 and type 2. In this study we used an antibody made to the n-terminus of the type 2 corticotropin releasing factor receptor. Characterization of this antibody showed that it strongly labeled a protein with a molecular weight of 16-32 kDa and only faintly labels a 62-83 kDa protein. The lower molecular weight protein corresponds to the weight of a recently described truncated isoform of this receptor that is designated corticotropin releasing factor-type 2alpha-truncated isoform. We carried out transfection paradigms using corticotropin releasing factor-type 2alpha-truncated isoform constructs and confirmed that the antibody recognized the truncated isoform of the type 2 corticotropin releasing factor receptor. Further, light and electron microscopic studies were carried out in mice and rats to define the distribution of the truncated receptor. Immunoreactivity is evident in the basal region of many, but not all Purkinje cell bodies and their initial axonal segments, as well as the initial axonal segments of isolated Golgi cells, and cerebellar nuclear neurons. In addition, punctate elements in the molecular layer were immunolabeled. The localization of the receptor to the initial segment of Purkinje cells was confirmed with electron microscopy. Further, the punctate labeling in the molecular layer was localized to parallel fibers and their terminals. In conclusion, evidence has been presented to show that distinct isoforms of the corticotropin releasing factor receptor are present in the cerebellum. The complex interactions between corticotropin releasing factor and other members of the corticotropin releasing factor family of peptides with both pre- and postsynaptic receptors support a growing concept that corticotropin releasing factor plays an important role in modulating activity in cerebellar circuits and ultimately in controlling motor behavior.

Nucleus prepositus

The cytoarchitecture and the histochemistry of nucleus prepositus hypoglossi and its afferent and efferent connections to oculomotor structures are described. The functional significance of the afferent connections of the nucleus is discussed in terms of current knowledge of the firing behavior of prepositus neurons in alert animals. The efferent connections of the nucleus and the results of lesion experiments suggest that it plays a role in a variety of functions related to the control of gaze.

Oculomotor cerebellum

The anatomical, physiological, and behavioral evidence for the involvement of three regions of the cerebellum in oculomotor behavior is reviewed here: (1) the oculomotor vermis and paravermis of lobules V, IV, and VII; (2) the uvula and nodulus; (3) flocculus and ventral paraflocculus. No region of the cerebellum controls eye movements exclusively, but each receives sensory information relevant for the control of multiple systems. An analysis of the microcircuitry suggests how sagittal climbing fiber zones bring visual information to the oculomotor vermis; convey vestibular information to the uvula and nodulus, while optokinetic space is represented in the flocculus. The mossy fiber projections are more heterogeneous. The importance of the inferior olive in modulating Purkinje cell responses is discussed.

Distribution of corticotropin-releasing factor-immunoreactive neurons in the central nervous system of the domestic chicken and Japanese quail

In birds, as in mammals, corticotropin-releasing factor (CRF) is present in a number of extrahypothalamic brain regions, indicating that CRF may play a role in physiological and behavioral responses other than the control of adrenocorticotropin hormone release by the pituitary. To provide a foundation for investigation of the roles of CRF in the control of avian behavior, the distribution of CRF immunoreactivity was determined throughout the central nervous system of the domestic chicken (Gallus domesticus) and Japanese quail (Coturnix japonica). The distribution of CRF-immunoreactive (-ir) perikarya and fibers in the chicken and quail brain was found to be more extensive than previously reported, notably in the telencephalon. Numerous CRF-ir perikarya and fibers were present in the hyperstriatum, hippocampus, neostriatum, lobus parolfactorius, and archistriatum, as well as in the nucleus taeniae, nucleus accumbens, and bed nucleus of the stria terminalis, which exhibited the strongest immunolabeling in the telencephalon. The presence of dense populations of CRF-ir perikarya in the medial lobus parolfactorius, nucleus of the stria terminalis, and paleostriatum ventrale, apparently giving rise to CRF-ir projections to the mesencephalic reticular formation, the parabrachial/pericerulear region, and the dorsal vagal complex, suggests that these telencephalic areas may constitute part of the avian "central extended amygdala." These results have important implications for understanding the role of extrahypothalamic CRF systems in emotional responses in birds.

Interaction between the hypothalamic-pituitary-adrenal axis and behavioural compensation following unilateral vestibular deafferentation

Vestibular compensation is defined as the process of behavioural recovery that occurs following the loss of sensory input from one or both vestibular labyrinths. The visual and postural instability resulting from the vestibular damage must alter the homeostasis of the subject; however, very little research has been conducted that investigates the interaction between vestibular compensation and the adaptive stress response of the body, i.e. the hypothalamic-pituitary-adrenal (HPA) axis. The aim of this review is to describe and evaluate the experimental evidence indicating a link between vestibular compensation and the body's response to stress, via the HPA axis.

Frequency-dependent expression of corticotropin releasing factor in the rat's cerebellum

Corticotropin releasing factor (CRF), localized in extrinsic afferents in the mammalian cerebellum, is defined as a neuromodulator within cerebellar circuits, and appears to be an essential element in the generation of long term depression, a proposed mechanism for motor learning. These physiological studies are based on exogenous application of CRF and do not address potential mechanisms that may influence endogenous release of the peptide. In the present study, immunohistochemistry was used to analyze changes in the lobular distribution of CRF-like immunoreactivity (LIR). In addition radioimmunoassay (RIA) was used to quantify changes in levels of the peptide in the cerebellum following stimulation of the inferior cerebellar peduncle (ICP) at 10 or 40 Hz or the inferior olivary nucleus (ION) at 1, 5, 10, or 20 Hz. Results indicate that there is a greater distribution of CRF-like-immunolabeled climbing fibers, mossy fibers, and astrocytes in all lobules of the cerebellum that is directly related to stimulation frequency. Maximal effects were elicited with 40 Hz ICP and 5-10 Hz ION stimulation. Quantitatively, the RIA studies indicate that there is a significant increase in CRF levels in the vermis, hemispheres and flocculus that correlates closely with stimulation frequency. In conclusion, stimulation of cerebellar afferents induces a significant change in the distribution and levels of CRF-LIR in climbing fibers, mossy fibers and glial cells. This suggests that the modulatory effects ascribed to CRF may influence a greater number of target neurons when levels of activity in afferent systems is increased.

Topological relationship between corticotropin-releasing factor-immunoreactive cerebellar afferents and tyrosine hydroxylase-immunoreactive Purkinje cells in a hereditary ataxic mutant, rolling mouse Nagoya

Using immunohistochemistry we examined the distribution of corticotropin-releasing factor-positive cerebellar afferents and the topological relationship between their projections and the distribution of tyrosine hydroxylase-positive Purkinje cells in an ataxic mutant, rolling mouse Nagoya. In the mutants, some climbing fibers were more intensely stained for corticotropin-releasing factor, but their zonal distribution remained the same as in non-ataxic littermates (control mice). These climbing fibers arose from the dorsal accessory nucleus, the ventral lamella of principal nucleus, the dorsomedial cell group, the subnucleus A, the beta subnucleus and the ventrolateral protrusion of the inferior olive, since perikarya in these olivary subdivisions were more intensely stained for corticotropin-releasing factor than in controls. Some mossy fiber rosettes in the vermal lobules, the simple lobule, the crus I of ansiform lobule, the copula pyramidis and the flocculus also exhibited corticotropin-releasing factor immunoreactivity and were more densely stained in the mutants than in controls. Double immunostaining for corticotropin-releasing factor and tyrosine hydroxylase in the mutant cerebellum revealed that the distribution of tyrosine hydroxylase-positive Purkinje cells corresponded to terminal fields of corticotropin-releasing factor-positive climbing fibers but not corticotropin-releasing factor-positive mossy fibers. This study indicated an increased corticotropin-releasing factor immunoreactivity in some climbing or mossy fibers in the cerebellum of rolling mouse Nagoya. We also found that the distribution of tyrosine hydroxylase-positive Purkinje cells corresponded to terminal fields of corticotropin-releasing factor-positive climbing fibers in the mutant cerebellum. As the transcription of the tyrosine hydroxylase gene is facilitated by Ca2+, abnormal tyrosine hydroxylase expression in the mutant Purkinje cells may indicate functional abnormality by alterations in intracellular Ca2+ concentrations. Therefore, we suggest that an increased level of corticotropin-releasing factor in a specific population of climbing fibers may alter the function of their target Purkinje cells.

The distribution of corticotropin-releasing factor (CRF), CRF binding sites and CRF1 receptor mRNA in the mouse cerebellum

The purpose of the present study is to determine the distribution of CRF containing afferents, and correlate these findings with the distribution of CRF binding sites and the neuronal localization of mRNA for the CRF1 receptor in the cerebellum of a single species, the mouse. Corticotropin releasing factor (CRF) has been localized within climbing fibers and mossy fibers throughout the cerebellar cortex of the mouse using immunohistochemistry. CRF immunoreactive, axonal varicosities also are present within all four of the cerebellar nuclei. 125I-labeled CRF binding sites are evident throughout all three layers of the cerebellar cortex (molecular, Purkinje and granule cell layers), but are not seen within the cerebellar nuclei. In situ hybridization histochemistry was employed using an antisense riboprobe corresponding to the full length sequence of the rat mRNA for the CRF1 receptor. Positive signal is present throughout the cerebellum in Purkinje cells and the granule cell layer. CRF1 receptor mRNA also is expressed within all four of the cerebellar nuclei. Further experiments are required to reconcile the lack of CRF binding sites in the cerebellar nuclei with the positive mRNA receptor expression and the presence of immunoreactive axonal varicosities. In previous physiological experiments, iontophoretic application of CRF enhances spontaneous as well as quisqualate-induced activity of Purkinje cells in slice preparations of the mouse cerebellum. When the results of the anatomical techniques are compared to the physiological data, there is convergent evidence to suggest that CRF influences the firing rate or responsiveness of Purkinje cells directly via release of the peptide from the climbing fiber system and indirectly via the mossy fiber-granule cell-parallel fiber circuit. Taken together, these anatomical and physiological data provide strong evidence to suggest that, in the adult cerebellum, CRF functions as a neuromodulator.

Modulation by acute stress of chloride permeation across microdissected vestibular neurons membranes: different results in two rabbit strains and CRF involvement

Free hand isolation of adult rabbit vestibular Deiters' neurons and dissection of their single membranes allows the study of their ionic permeability characteristics in a microchambers device. In the case of hare-like rabbits, the dissection of such membranes presents evidence of a high basal permeation of labelled chloride, possibly related to mechanical disturbance of the plasma membrane-related cytoskeleton and activation of chloride channels. This did not apply to the laboratory strain of white New Zealand rabbits. However, membranes from hare-like rabbits which were stressed by being rotated on a platform before the experiment, behaved like those from the New Zealand strain. Vice versa, habituation to handling day after day of New Zealand rabbits resulted in a chloride permeation equal to that of unstressed hare-like rabbits. We propose that the stressful conditions result in the release of neurochemical messages to the vestibular Deiters' cells which influence their electrophysiological behavior. The corticotropin releasing factor (CRF), a stress-related peptide present in the climbing fibers, actually blocks the basal chloride permeation across the Deiters' membranes and this effect is partially reversed by its receptor antagonist, alpha-helical CRF [9-41].

Unipolar brush cell axons form a large system of intrinsic mossy fibers in the postnatal vestibulocerebellum

The unipolar brush cells (UBCs), a class of neurons recently identified in the granular layer of the vestibulocerebellum, receive excitatory synaptic input from mossy fibers (MFs) in the form of a giant glutamatergic synapse. UBCs are provided with axons that bear synaptic endings situated at the center of glomeruli, similar to cerebellar MF afferents. A single MF stimulus evokes a prolonged train of action potentials in the UBC (Rossi et al., 1995), which is presumably distributed to postsynaptic targets. Knowledge of the synaptic connections of UBC axons is essential to define the role of these cells in the integration of vestibular signals in the cerebellar circuitry. To evaluate these connections, the nodulus (folium X) was isolated from vermal slices of postnatal day 8 mice, cultured for 2-4 or 15-30 days in vitro, and studied by electron and fluorescence microscopy. The peak of degeneration of extrinsic MF terminals, which have been severed from the parent cell bodies, was observed at 2 days in vitro (DIV). Quantification of degenerating and nondegenerating (e.g., intrinsic) MF terminals indicated that about half of the MF terminals were provided by local UBC axons synapsing on dendrites of granule cells and other UBCs. The proportion of nondegenerating vs. degenerating MF terminals terminating on UBCs also indicated that approximately two-thirds of the intrinsic MFs are involved in UBC-UBC connections. In long-term cultures, the granular layer appeared well preserved and the UBC axons formed an extensive system of MF collaterals. It is suggested that UBCs may act by spatially amplifying vestibular inputs carried by extrinsic MFs.

Corticotropin releasing factor in the embryonic mouse cerebellum

Corticotropin releasing factor (CRF) is a 41 amino acid peptide that has been localized throughout the mouse cerebellum on postnatal day (P0). The wide-spread distribution of CRF within this brain region at birth suggests that it likely is present during embryonic stages of development. Thus, the intent of this study was to use immunohistochemical techniques to determine when CRF is first present in the cerebellar anlage, to analyze its distribution within the developing cerebellum, and to correlate these findings with early events in cerebellar ontogeny. CRF can first be detected in the cerebellum on embryonic day (E) 10 in scattered puncta that appear to approximate cell bodies throughout the cerebellar plate. Between E11 and E14 the number of puncta increase in the intermediate zone and more dorsal aspect of the cerebellum and decrease in the ventricular zone. At E14, in addition to the puncta, lightly immunolabeled cell bodies are observed in the ventricular zone. Just prior to birth at E17, CRF-immunoreactive varicosities distribute along the multitiered Purkinje cell layer and the intermediate zone. The CRF-positive cell bodies increase in number and intensity of staining. The majority remain within the ventricular zone, although a few also are present in the intermediate zone; it is postulated that these may be glial cells or neurons that are transiently expressing CRF. In conclusion, CRF-positive punctate elements derived from an as yet unknown source are present in the embryonic cerebellum just prior to and during the birth of Purkinje cells and nuclear neurons. The presence of this peptide at this critical stage of cerebellar development and its continued expression throughout the postnatal period of ontogeny suggests that CRF may play an important developmental role.

Developmental expression of corticotropin-releasing factor in the postnatal murine cerebellum

Corticotropin-releasing factor (CRF) is present in climbing and mossy fibers and both have a distinct pattern of distribution in the adult cerebellar cortex. The intent of this developmental study is to determine when the lobular pattern of CRF distribution emerges, and to analyze the morphogenesis of CRF immunoreactive climbing and mossy fibers in individual cerebellar lobules. Between postnatal day (P)0 and P3, CRF-immunoreactive (IR) punctate elements are present throughout the cerebellum. By P3, there is a decrease in the density of staining in the white matter and punctate elements become concentrated within the developing cortex. Between P3 and P7 CRF-IR, varicosities circumscribe Purkinje cell bodies, and are present in the internal and external granule cell layers. Between P10 and P12, there is a major reduction in the density of CRF-IR puncta, especially in the internal and external granule cell layers. Varicosities remain around Purkinje cell bodies and some extend into the molecular layer. During this interval, CRF-IR profiles are first evident in axonal configurations characteristic of developing climbing fibers, although there are lobular differences in the degree of maturation of this afferent system. Axonal enlargements characteristic of immature mossy fibers can first be seen at P10 in lobules IX and X; they cannot be differentiated until P12-14 in more rostral or lateral lobules. CRF-IR fibers in lobules IX and X, the vestibulocerebellum, develop into mature climbing and mossy fibers before any other area of the cerebellum. In other lobules of the cerebellum the gradient of maturation for these axonal phenotypes is from medial to lateral and posterior to anterior. Between P10 and P12, CRF-IR climbing fibers are present in all lobules of the cerebellum. After P12, few climbing fibers are observed in the anterior lobe of the cerebellum at midvermal levels; those present are only faintly immunolabeled. Based on its early expression and uniform distribution between P0 and P10, CRF could have a role in cerebellar development. After this age, as climbing and mossy fiber terminal phenotypes mature, and the differential adult patterns of distribution emerge, CRF likely begins to function as a neuromodulator as has been shown in the adult cerebellum.

Brainstem origin of corticotropin-releasing factor afferents to the nucleus interpositus anterior of the cat

Corticotropin-releasing factor (CRF) has been described within varicosities that have a uniform distribution throughout the cerebellar nuclei of the cat. To date, however, no data are available as to the source of these nuclear afferents. Thus, a double-label technique was used to identify brainstem neurons which give rise to the CRF-containing afferents in the nucleus interpositus anterior (NIA) of the cat's cerebellum. Injections of fluorescent-tagged microspheres, which are retrogradely transported by cells with axons in the injection site, were made into lateral and medial aspects of the nucleus. The same sections were also processed for CRF immunohistochemistry. The primary source of CRF afferents to the NIA are the medial and dorsal accessory olivary nuclei. In addition to the inferior olive, several other brainstem nuclei also provide CRF afferents to the cerebellar nuclei. The medial aspect of the NIA receives afferents from the lateral reticular nucleus, external cuneate nucleus, perihypoglossal nucleus, medial vestibular nucleus and inferior central raphe nucleus. Additional afferents to more lateral aspects of the NIA are derived from the lateral reticular nucleus, external cuneate nucleus, and the magnocellular, lateral and gigantocellular tegmental areas. The brainstem nuclei that give rise to the CRF projection to the NIA receive input primarily from the spinal cord and likely relay information related to the status of an ongoing movement. A previous physiological study by Bishop has shown that CRF enhances the excitatory activity of nuclear neurons. CRF released from these afferents likely would enhance nuclear cell activity and thus provide a stronger or more prolonged effect on their respective target neurons in the brainstem.

Developmental profile of corticotropin releasing hormone messenger RNA in the rat inferior olive

Corticotropin releasing hormone is a neurotransmitter in the inferior olive complex of marsupials and mammals. The ontogeny of corticotropin releasing hormone gene expression in the rat inferior olive has not been described. Using in-situ hybridization histochemistry in 25 animals, we established the developmental profile of the peptide's messenger ribonucleic acid in the postnatal rat. CRH-messenger RNA was first detectable in two of four animals on the fifth postnatal day. Subsequently, gene expression increased linearly with age: by day 14, CRH was expressed in all olivary complex nuclei, and the distribution and relative abundance on day 18 were comparable to those in the adult. The developmental profile of CRH-mRNA in the rat inferior olive differs from those in the mouse and opossum, and from the pattern in the rat hypothalamus, suggesting species- and site-specificity of the peptide's function.

Distribution of corticotropin-releasing factor (CRF) binding sites in the opossum cerebellum

Corticotropin-releasing factor (CRF) has been demonstrated in the hypothalamic-pituitary-adrenal axis and studied in extrahypothalamic sites throughout the brain. Localization of CRF immunoreactivity, CRF mRNA, and CRF receptors within specific brain areas supports an extrahypothalamic function for CRF. Previous reports have revealed the localization of several peptides, including CRF, in the cerebellar cortex and nuclei of the North American opossum (Didelphis marsupialis virginiana); climbing fibers, mossy fibers, and a beaded plexus of axons in the Purkinje cell layer demonstrate CRF immunoreactivity. CRF also is localized within neurons in the inferior olivary complex and other brainstem nuclei which are known to project to the cerebellar cortex. Physiological recordings indicate CRF potentiates the excitatory effects of both aspartate and glutamate, the putative transmitters of the major afferent inputs to the cerebellum. The present study reports that specific CRF binding sites are present in all lobules of the opossum cerebellar cortex, with the greatest density in vermal lobules V through X, the flocculus, and the paraflocculus. The cerebellar nuclei do not appear to be labeled. CRF binding sites are present over all neuronal layers of the cerebellar cortex. The presence of CRF immunoreactivity in climbing fibers, mossy fibers, and a beaded axonal plexus, and CRF binding sites within the cerebellar cortex, as well as the fact that CRF potentiates the excitatory effects of both aspartate and glutamate, indicate that this peptide may function as a neuromodulator in the cerebellum of the North American opossum. An attempt is made to correlate the distribution of CRF-IR neurons and fibers, CRF mRNA, and CRF receptors to the extrahypothalamic function of CRF as it relates to the olivocerebellar pathway.