Distribution of corticotropin-releasing factor and calcitonin gene-related peptide in the developing mouse cerebellum

Chronic stress, neuroinflammation, and depression: an overview of pathophysiological mechanisms and emerging anti-inflammatories

In a subset of patients, chronic exposure to stress is an etiological risk factor for neuroinflammation and depression. Neuroinflammation affects up to 27% of patients with MDD and is associated with a more severe, chronic, and treatment-resistant trajectory. Inflammation is not unique to depression and has transdiagnostic effects suggesting a shared etiological risk factor underlying psychopathologies and metabolic disorders. Research supports an association but not necessarily a causation with depression. Putative mechanisms link chronic stress to dysregulation of the HPA axis and immune cell glucocorticoid resistance resulting in hyperactivation of the peripheral immune system. The chronic extracellular release of DAMPs and immune cell DAMP-PRR signaling creates a feed forward loop that accelerates peripheral and central inflammation. Higher plasma levels of inflammatory cytokines, most consistently interleukin IL-1β, IL-6, and TNF-α, are correlated with greater depressive symptomatology. Cytokines sensitize the HPA axis, disrupt the negative feedback loop, and further propagate inflammatory reactions. Peripheral inflammation exacerbates central inflammation (neuroinflammation) through several mechanisms including disruption of the blood-brain barrier, immune cellular trafficking, and activation of glial cells. Activated glial cells release cytokines, chemokines, and reactive oxygen and nitrogen species into the extra-synaptic space dysregulating neurotransmitter systems, imbalancing the excitatory to inhibitory ratio, and disrupting neural circuitry plasticity and adaptation. In particular, microglial activation and toxicity plays a central role in the pathophysiology of neuroinflammation. Magnetic resonance imaging (MRI) studies most consistently show reduced hippocampal volumes. Neural circuitry dysfunction such as hypoactivation between the ventral striatum and the ventromedial prefrontal cortex underlies the melancholic phenotype of depression. Chronic administration of monoamine-based antidepressants counters the inflammatory response, but with a delayed therapeutic onset. Therapeutics targeting cell mediated immunity, generalized and specific inflammatory signaling pathways, and nitro-oxidative stress have enormous potential to advance the treatment landscape. Future clinical trials will need to include immune system perturbations as biomarker outcome measures to facilitate novel antidepressant development. In this overview, we explore the inflammatory correlates of depression and elucidate pathomechanisms to facilitate the development of novel biomarkers and therapeutics.

Dual Function of NRP1 in Axon Guidance and Subcellular Target Recognition in Cerebellum

Subcellular target recognition in the CNS is the culmination of a multiple-step program including axon guidance, target recognition, and synaptogenesis. In cerebellum, basket cells (BCs) innervate the soma and axon initial segment (AIS) of Purkinje cells (PCs) to form the pinceau synapse, but the underlying mechanisms remain incompletely understood. Here, we demonstrate that neuropilin-1 (NRP1), a Semaphorin receptor expressed in BCs, controls both axonal guidance and subcellular target recognition. We show that loss of Semaphorin 3A function or specific deletion of NRP1 in BCs alters the stereotyped organization of BC axon and impairs pinceau synapse formation. Further, we identified NRP1 as a trans-synaptic binding partner of the cell adhesion molecule neurofascin-186 (NF186) expressed in the PC AIS during pinceau synapse formation. These findings identify a dual function of NRP1 in both axon guidance and subcellular target recognition in the construction of GABAergic circuitry.

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.

Cellular expression and subcellular localization of secretogranin II in the mouse hippocampus and cerebellum

Secretogranin II (SgII), or chromogranin C, is thought to participate in the sorting and packaging of peptide hormones and neuropeptides into secretory granules and large dense-core vesicle (LDCVs), and also functions as a precursor of neuropeptide secretoneurin. Although SgII is widely distributed in the brain and is predominantly localized at terminals of mossy fibers in the hippocampus and cerebellum and climbing fibers in the cerebellum, its cellular expression and ultrastructural localization remain largely unknown. In the present study, we addressed this issue in the adult mouse brain by multiple-labeling fluorescence in situ hybridization and immunofluorescence and by preembedding and postembedding immunoelectron microscopies. SgII was expressed in various neurons, distributed as either tiny puncta or coarse aggregates in the neuropil, and intensely accumulated in perikarya of particular neurons, such as parvalbumin-positive interneurons and mossy cells in the hippocampus and Purkinje cells in the cerebellum. Coarse aggregates were typical of terminals of mossy fibers and climbing fibers. In these terminals, numerous immunogold particles were clustered on individual LDCVs, and one or two particles also fell within small synaptic vesicle-accumulating portions. SgII was further detected as tiny puncta in neural elements lacking LDCVs, such as parallel fibers of cerebellar granule cells, somatodendritic elements of various neurons and Bergmann glia. Thus, SgII is present in LDCV and non-LDCV compartments of various neural cells. The wide subcellular localization of SgII may reflect diverse release sites of neuropeptides and secretorneurin, or suggests its role in the sorting and packaging of molecules other than neuropeptides in non-LDCV compartments.

Stimulation of the inferior olivary complex alters the distribution of the type 1 corticotropin releasing factor receptor in the adult rat cerebellar cortex

In a previous study, it was shown that populations of climbing fibers, derived from the inferior olivary complex (IOC) contain the peptide corticotropin releasing factor (CRF) and that the expression of this peptide in climbing fibers could be modulated by the level of activity in olivary afferents. The intent of this study was to determine if there was comparable plasticity in the distribution of the type 1 CRF receptor (CRF-R1) in the cerebellum of the rat. Our results indicate that CRF-R1 was localized primarily to Purkinje cell somata and their primary dendrites and granule cells. In addition, scattered immunolabeling was present over the somata of Golgi cells, basket cells and stellate cells, as well as Bergmann glial cells and their processes. IOC stimulation for 30 min at 1 Hz increased CRF-R1 expression in molecular layer interneurons and processes of Bergmann glial cells. Little to no effect on CRF receptor distribution was observed in Purkinje cells, granule cells, or Golgi cells. IOC stimulation at 5 Hz however, increased CRF-R1 expression in the processes of Bergmann glial cells while decreasing its expression in basket, stellate and, to some extent, in Purkinje cells. The present results suggest that there is activity-dependent plasticity in CRF-R1 expression that must be considered in defining the mechanism by which the CRF family of peptides modulates activity in cerebellar circuits. The present results also suggest that the primary targets of CRF released from climbing fibers are Bergmann glial cells and interneurons in the molecular layer. Further, interneurons responded with a decrease in receptor expression following more intense levels of stimulation suggesting the possibility of internalization of the receptor. In contrast, Bergmann glial cells showed an increased expression in receptor expression. These data suggest that CRF released from climbing fibers may modulate the physiological properties of basket and stellate cells as well as having a heretofore unidentified and potentially unique effect on Bergmann glia.

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.

A novel diuretic hormone receptor in Drosophila: evidence for conservation of CGRP signaling

The Drosophila orphan G protein-coupled receptor encoded by CG17415 is related to members of the calcitonin receptor-like receptor (CLR) family. In mammals, signaling from CLR receptors depend on accessory proteins, namely the receptor activity modifying proteins (RAMPs) and receptor component protein (RCP). We tested the possibility that this Drosophila CLR might also require accessory proteins for proper function and we report that co-expression of the mammalian or Drosophila RCP or mammalian RAMPs permitted neuropeptide diuretic hormone 31 (DH31) signaling from the CG17415 receptor. RAMP subtype expression did not alter the pharmacological profile of CG17415 activation. CG17415 antibodies revealed expression within the principal cells of Malpighian tubules, further implicating DH31 as a ligand for this receptor. Immunostaining in the brain revealed an unexpected convergence of two distinct DH signaling pathways. In both the larval and adult brain, most DH31 receptor-expressing neurons produce the neuropeptide corazonin, and also express the CRFR-related receptor CG8422, which is a receptor for the neuropeptide diuretic hormone 44 (DH44). There is extensive convergence of CRF and CGRP signaling within vertebrates and we report a striking parallel in Drosophila involving DH44 (CRF) and DH31 (CGRP). Therefore, it appears that both the molecular details as well as the functional organization of CGRP signaling have been conserved.

Induction of galanin receptor-1 (GalR1) expression in external granule cell layer of post-natal mouse cerebellum

Galanin is a modulator of fast transmission in adult brain and recent evidence suggests that it also acts as a trophic factor during neurogenesis and neural injury and repair. Previous studies in our laboratory have identified galanin mRNA in Purkinje cells of adult and developing rat (but not adult mouse) cerebellum; and galanin-binding sites in adult mouse (but not rat) cerebellum. The post-natal development of the cerebellum provides a unique and convenient model for the investigation of developmental processes and to learn more about putative cerebellar galanin systems, the current study examined the presence and distribution of galanin-like-immunoreactivity (- LI), [(125)I]-galanin binding sites and galanin receptor-1 (GalR1) mRNA in post-natal mouse cerebellum. Using autoradiography and in situ hybridization, [(125)I]-galanin binding sites and GalR1 mRNA were first detected on post-natal day 10 (P10) in the external germinal layer of all lobes and high levels were maintained until P14. Quantitative real-time PCR assays detected GalR1 mRNA in whole cerebellum across the post-natal period, with a strong induction and peak of expression at P10. Assessment of galanin levels in whole cerebellum by radioimmunoassay revealed relatively similar concentrations from P5 to P20 and in adult mice (80-170 pg/mg protein), with a significantly higher concentration (250 pg/mg, p < 0.01) detected at P3. These concentrations were some four- to six-fold lower than those in adult forebrain samples. Using immunohistochemistry, galanin-like-immuno-reactivity was observed in prominent fibrous elements within the white matter tracts of the cerebellum at P3-5 and in more punctate elements in the internal granule cell layer and associated with the Purkinje cell layer at P12 and P20. Increased levels of GalR1 mRNA and galanin binding (attributed to GalR1) in the external granule cell layer at P10-12/(14) coincide with granule cell migration from the external to the inner granule cell layer and together with demonstrated effects of other neuropeptide-receptor systems suggest a role for GalR1 signalling in regulating this or related developmental processes.

[125I]-Galanin binding in brain of wildtype, and galanin- and GalR1-knockout mice: Strain and species differences in GalR1 density and distribution

Widespread production of knockout and transgenic mice has led to an increased use of mice as animal models for studies of normal- and patho-physiology. Hence, the precise mapping of central transmitter/peptide systems in the mouse has become essential for the interpretation of functional studies and for the correct correlation with findings obtained in the rat, primates and/or human. In this regard, the current study reports the autoradiographic localization of [(125)I]-galanin (GAL) binding sites in brain of the common C57BL/6J and 129OlaHsd mouse strains, as well as in GAL and galanin receptor-1 (GalR1) knockout (KO) mice. In C57BL/6J and 129OlaHsd mice, [(125)I]-GAL binding sites were detected throughout the brain, including moderate-high relative densities in the basal ganglia (caudate putamen, nucleus [n.] accumbens, olfactory tubercle, substantia nigra), limbic regions (septum, bed n. stria terminalis, ventral hippocampus, amygdala), cingulate, retrosplenial, entorhinal cortex, centro-lateral/medial thalamic n., preoptic/lateral hypothalamus, midbrain (superior colliculus, periaqueductal gray), pons/medulla oblongata (parabrachial, pontine reticular and solitary tract n.) and cerebellar cortex. [(125)I]-GAL binding levels were low or absent in main olfactory bulb, neocortex, ventrolateral/geniculate thalamic n., dorsal hippocampus, inferior colliculus and cranial motor n. In simultaneous determinations, relative [(125)I]-GAL binding site densities in brain were generally lower in C57BL/6J than in 129OlaHsd mice, while the density and distribution of central binding in the GAL-KO mouse was essentially identical to that in its background-129OlaHsd strain. In contrast, no specific [(125)I]-GAL binding was detected in any region of GalR1-KO mouse brain, revealing that under the experimental conditions used, the peptide ligand binding is predominantly (exclusively) to the GalR1 subtype. This evaluation of GAL receptor site distribution in mouse brain has revealed similarities and some differences with the equivalent system in rat and provides a valuable reference for future comparative studies of central GAL transmission.

The postnatal developmental expression pattern of urocortin in the rat olivocerebellar system

Urocortin belongs to the family of corticotropin-releasing factor (CRF)-like peptides, which play an important role in sensorimotor coordination. CRF induces locomotor activity, and urocortin has an inhibitory effect. Here, we document the regional and subcellular localization of urocortin in the developing rat cerebellum to compare it with CRF. During the first postnatal week, urocortin immunoreactivity (UCN-ir), within the white matter and cerebellar cortex, was strongest in vermal lobules I, II, IX, and X, closely followed by lobules IV, V, and VIII; lobules VI and VII showed the weakest labeling. Cortical immunoreactivity was in the form of puncta that encircled Purkinje cell somata. By postnatal day (PD) 12, UCN-ir had increased appreciably in all lobules. In Purkinje cells, labeling was spread throughout their somata and proximal dendrites. By PD 15, labeling in lobules I-IV appeared to wane, yet still prevailed in the central and posterior lobules. This anterior-to-posterior gradient persisted through to adulthood. The study shows that urocortin and CRF have similar regional distribution profiles during development, suggesting synergistic roles within the vestibulocerebellum. The onset of the adult distributional pattern of urocortin at the stage when rats are capable of fluent walking patterns further strengthens the correlation between CRF-like peptides and postural control. An important difference between urocortin and CRF is the localization of urocortin, and not CRF, within Purkinje cells, implying that urocortin probably has an additional role in modulating the signals emanating from the cerebellar cortex to the deep cerebellar nuclei.

Abnormal cerebellar histogenesis in PEX2 Zellweger mice reflects multiple neuronal defects induced by peroxisome deficiency

The form and circuitry of the cerebellum develops by a complex process that requires integration of afferent-target interactions between multiple neuronal populations and migratory patterns established by neuron-glial interactions. Analysis of mice lacking the PEX2 peroxisome assembly gene, in which peroxisomal function is disrupted, reveals abnormal cerebellar histogenesis due to the disturbance of multiple cellular processes within neurons. Defects in cerebellar growth and the rostro-caudal foliation pattern reflect a reduced granule neuron population and abnormal Purkinje cell dendrite development. In granule neurons, there is increased apoptotic cell death and delayed movement from the EGL to IGL that reflects cell cycle, maturational and migrational abnormalities. The underlying Purkinje cells have stunted dendrite arbors with abnormal branching patterns, which may reflect altered inductive influences from the delayed granule neuron translocation. A delayed arborization of mutant olivary climbing fibers and their defective translocation from the perisomatic to the dendritic compartment of Purkinje cells results in numerous spines on the soma and proximal dendrites of Purkinje cells. Distal Purkinje cell dendritic spines also display abnormal morphology. These Purkinje cell dendritic abnormalities are seen in association with persistent and enlarged axonal spheroids, further indicating the presence of a degenerative process within the Purkinje cell. This PEX2(-/-) mouse model for the human peroxisomal biogenesis disorder Zellweger syndrome illustrates the complex interplay of abnormal developmental processes in the cerebellum and the importance of peroxisomal function for neuronal migration, proliferation, differentiation, and survival.

Decreased marble burying behavior in female mice lacking neuromedin-B receptor (NMB-R) implies the involvement of NMB/NMB-R in 5-HT neuron function

Neuromedin B (NMB) is a mammalian bombesin-like peptide distributed widely in the central nervous system. This peptide exerts its function via the NMB receptor (NMB-R). Female NMB-R-deficient mice were used to study the role that NMB/NMB-R may play in 5-HT neuron function since this relationship was suggested in previous in vitro studies. As 5-HT neurons are thought to modulate marble burying behavior, a role for NMB-R in this behavior was assessed. Relative to wild-type mice, NMB-R-deficient mice showed decreased marble burying behavior. However, depletion of 5-HT by treatment with p-chlorophenylalanine (p-CPA) increased burying behavior in NMB-R-deficient mice suggesting that increased levels of 5-HT in the brain cause a decrease in burying behavior in NMB-R-deficient mice. While HPLC analysis showed that 5-HT content in the whole brain does not differ between NMB-R-deficient and wild-type mice, an immunohistochemical analysis of brain sections showed that 5-HT expression in the dorsal raphe (DR) nucleus is elevated in NMB-R-deficient mice. Furthermore, a quantitative RT-PCR analysis revealed that 5-HT(1A)-receptor gene expression is downregulated in NMB-R-deficient mice at the whole brain level. These behavioral and biological results suggest that NMB/NMB-R may modulate 5-HT neuronal activity by affecting DR function.

Modulation of 5-HT system in mice with a targeted disruption of neuromedin B receptor

To assess the role of neuromedin B receptor (NMB-R) on the modulation of serotonergic (5-HT) system, the function of the 5-HT system was examined in mice lacking the NMB-R gene. Immunohistochemical analysis of brain sections revealed that 5-HT expression level in the dorsal raphe neurons was elevated in NMB-R-deficient mice compared with wild-type mice. Although restraint stress enhanced 5-HT expression in these neurons in wild-type mice, this treatment did not affect 5-HT expression level in NMB-R-deficient mice, indicating the modulation of 5-HT system in the mutant mice. Since 5-HT system is involved in responses to stress and anxiety, we characterized stress response in these mice. The number of c-Fos expressing cells in the paraventricular nucleus of the hypothalamus was higher in NMB-R-deficient mice than in wild-type mice in both basal and stressed conditions. Moreover, the plasma corticosterone level under restraint stress was elevated in NMB-R-deficient mice compared to wild-type mice. In the forced swimming tests, the duration of immobility was longer in mutant mice than in wild-type mice. These data show dysregulated response to stress in NMB-R-deficient mice. However, behavior related to anxiety, assessed by elevated plus-maze and light-dark box, was not affected in NMB-R-deficient mice. NMB-R is known to be expressed in dorsal raphe neurons, and our data suggest that NMB-R has an important role in fine tuning of subsets of 5-HT neurons in this nucleus, and impairment of this system leads to the dysregulated response to stress.

Atrial natriuretic peptide-like immunoreactivity in neurons and astrocytes of human cerebellum and inferior olivary complex

Atrial natriuretic peptide (ANP) has previously been localized in areas of mammalian brain associated with olfaction, cardiovascular function, and fluid/electrolyte homeostasis. Despite the presence of several types of natriuretic peptide receptors in mammalian cerebellum, neither intrinsic nor extrinsic sources of the natriuretic peptides have been described. In this report we describe the immunohistochemical localization of both intrinsic and extrinsic sources for ANP in human cerebellum. ANP-like immunoreactivity (ANP-LIR) was observed in climbing fibers in the cerebellar molecular layer that probably originated from isolated immunopositive neurons of the inferior olivary complex. Intrinsic sources of ANP-LIR included small subpopulations of protoplasmic and fibrous astrocytes and Bergmann glia, as well as Golgi and Lugaro neurons of the granule cell layer. These results suggest that, in addition to its presumptive roles in local vasoregulation, ANP may serve as a modulator of the activity of Purkinje neurons.

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].

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.

The temporal and spatial development of CRF binding sites in the postnatal mouse cerebellum

This study describes the distribution and relative level of labeling of binding sites for corticotrophin releasing factor (CRF) in the postnatal mouse cerebellum. At birth low levels of labeling are present throughout the cerebellum. However, this labeling is most densely distributed in the caudal and lateral aspects of the cerebellum. By P3 CRF binding sites are present throughout the cerebellum, although the greatest level of labeling is in the posterior lobe of the vermis, especially lobules IX and X; this correlates with the early differential pattern of CRF distribution in cerebellar afferents within these same lobules. At P10, the adult pattern of distribution and level of labeling begins to emerge. The presence of CRF and CRF binding sites at birth, and during postnatal growth, suggests that this peptide could play a role in the regulation of developmental events within the cerebellum.

Many major CNS axon projections develop normally in the absence of semaphorin III

The semaphorins constitute a large gene family of transmembrane and secreted molecules, many of which are expressed in the nervous system. Genetic studies in Drosophila have revealed a role for semaphorins in axon guidance and synapse formation, and several in vitro studies in mice have demonstrated a dramatic chemorepellent effect of semaphorin III (Sema III) on the axons of several populations of neurons. To investigate the function of Sema III during in vivo axon guidance in the mammalian CNS, we studied the development of axonal projections in mutant mice lacking Sema III. Projections were studied for which either the in vitro evidence suggests a role for Sema III in axon guidance (e.g., cerebellar mossy fibers, thalamocortical axons, or cranial motor neurons) or the in vivo expression suggests a role for Sema III in axon guidance (e.g., cerebellar Purkinje cells, neocortex). We find that many major axonal projections, including climbing fiber, mossy fiber, thalamocortical, and basal forebrain projections and cranial nerves, develop normally in the absence of Sema III. Despite its in vitro function and in vivo expression, it appears as if Sema III is not absolutely required for the formation of many major CNS tracts. Such data are consistent with recent models suggesting that axon guidance is controlled by a balance of forces resulting from multiple guidance cues. Our data lead us to suggest that if Sema III functions in part to guide the formation of major axonal projections, then it does so in combination with both other semaphorins and other families of guidance molecules.

Peptide localization in the mouse inferior olive

The inferior olivary complex is the sole source of climbing fibers to the cerebellar cortex. Physiologically these afferents have been shown to have a powerful excitatory effect on their target neurons, namely Purkinje cells. Thus, any modulation of olivary firing rate or responsiveness will alter Purkinje cell firing and ultimately cerebellar function. Neuropeptides have been shown to modulate neuronal activity in several systems. The intent of the present study is to determine the olivary distribution of five peptides previously shown to be present and functional in cerebellar circuitry including cholecystokinin, calcitonin gene-related peptide, corticotropin releasing factor, enkephalin and substance P. These studies were carried out in the adult C57BL/6J mouse using the peroxidase anti-peroxidase immunohistochemical technique. All five peptides labeled varicosities of varying sizes. Varicosities labeled for cholecystokinin, calcitonin gene-related peptide and corticotropin releasing factor were densely distributed throughout the inferior olive. In contrast, varicosities immunostained for substance P and enkephalin, were more restricted in their distribution. The overlap in the distribution of these peptides suggests that they may be colocalized with each other as well as with excitatory or inhibitory amino acids known to be present in afferents to the inferior olive. Because of the extensive distribution of the peptides, it is likely that they are derived from multiple brainstem sources. These findings serve as baseline data for future physiological studies designed to address the functional role of peptides in olivary circuitry.