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

Impact of varying maternal dietary folate intake on cerebellar cortex histomorphology and cell density in offspring rats

The cerebellum has a long, protracted developmental period that spans from the embryonic to postnatal periods; as a result, it is more sensitive to intrauterine and postnatal insults like nutritional deficiencies. Folate is crucial for foetal and early postnatal brain development; however, its effects on cerebellar growth and development are unknown. The aim of this study was to examine the effects of maternal folate intake on the histomorphology and cell density of the developing cerebellum. Twelve adult female rats (rattus norvegicus) were randomly assigned to one of four premixed diet groups: standard (2 mg/kg), folate-deficient (0 mg/kg), folate-supplemented (8 mg/kg) or folate supra-supplemented (40 mg/kg). The rats started their diets 14 days before mating and consumed them throughout pregnancy and lactation. On postnatal days 1, 7, 21 and 35, five pups from each group were sacrificed, and their brains were processed for light microscopic analysis. Histomorphology and cell density of the external granule, molecular, Purkinje and internal granule layers were obtained. The folate-deficient diet group had smaller, dysmorphic cells and significantly lower densities of external granule, molecular, Purkinje and internal granule cells. Although the folate-enriched groups had greater cell densities than the controls, the folate-supplemented group had considerably higher cell densities than the supra-supplemented group. The folate supra-supplemented group had ectopic Purkinje cells in the internal granule cell layer. These findings imply that a folate-deficient diet impairs cellular growth and reduces cell density in the cerebellar cortex. On the other hand, folate supplementation increases cell densities, but there appears to be an optimal dose of supplementation since excessive folate levels may be detrimental.

Involvement of the cerebellum in migraine

Migraine is a disabling neurological disease characterized by moderate or severe headaches and accompanied by sensory abnormalities, e.g., photophobia, allodynia, and vertigo. It affects approximately 15% of people worldwide. Despite advancements in current migraine therapeutics, mechanisms underlying migraine remain elusive. Within the central nervous system, studies have hinted that the cerebellum may play an important sensory integrative role in migraine. More specifically, the cerebellum has been proposed to modulate pain processing, and imaging studies have revealed cerebellar alterations in migraine patients. This review aims to summarize the clinical and preclinical studies that link the cerebellum to migraine. We will first discuss cerebellar roles in pain modulation, including cerebellar neuronal connections with pain-related brain regions. Next, we will review cerebellar symptoms and cerebellar imaging data in migraine patients. Lastly, we will highlight the possible roles of the neuropeptide calcitonin gene-related peptide (CGRP) in migraine symptoms, including preclinical cerebellar studies in animal models of migraine.

Cerebro-Cerebellar Networks in Migraine Symptoms and Headache

The cerebellum is associated with the biology of migraine in a variety of ways. Clinically, symptoms such as fatigue, motor weakness, vertigo, dizziness, difficulty concentrating and finding words, nausea, and visual disturbances are common in different types of migraine. The neural basis of these symptoms is complex, not completely known, and likely involve activation of both specific and shared circuits throughout the brain. Posterior circulation stroke, or neurosurgical removal of posterior fossa tumors, as well as anatomical tract tracing in animals, provided the first insights to theorize about cerebellar functions. Nowadays, with the addition of functional imaging, much progress has been done on cerebellar structure and function in health and disease, and, as a consequence, the theories refined. Accordingly, the cerebellum may be useful but not necessary for the execution of motor, sensory or cognitive tasks, but, rather, would participate as an efficiency facilitator of neurologic functions by improving speed and skill in performance of tasks produced by the cerebral area to which it is reciprocally connected. At the subcortical level, critical regions in these processes are the basal ganglia and thalamic nuclei. Altogether, a modulatory role of the cerebellum over multiple brain regions appears compelling, mainly by considering the complexity of its reciprocal connections to common neural networks involved in motor, vestibular, cognitive, affective, sensory, and autonomic processing—all functions affected at different phases and degrees across the migraine spectrum. Despite the many associations between cerebellum and migraine, it is not known whether this structure contributes to migraine initiation, symptoms generation or headache. Specific cerebellar dysfunction via genetically driven excitatory/inhibitory imbalances, oligemia and/or increased risk to white matter lesions has been proposed as a critical contributor to migraine pathogenesis. Therefore, given that neural projections and functions of many brainstem, midbrain and forebrain areas are shared between the cerebellum and migraine trigeminovascular pathways, this review will provide a synopsis on cerebellar structure and function, its role in trigeminal pain, and an updated overview of relevant clinical and preclinical literature on the potential role of cerebellar networks in migraine pathophysiology.

Undisturbed climbing fiber pruning in the cerebellar cortex of CX3 CR1-deficient mice

Pruning, the elimination of excess synapses is a phenomenon of fundamental importance for correct wiring of the central nervous system. The establishment of the cerebellar climbing fiber (CF)-to-Purkinje cell (PC) synapse provides a suitable model to study pruning and pruning-relevant processes during early postnatal development. Until now, the role of microglia in pruning remains under intense investigation. Here, we analyzed migration of microglia into the cerebellar cortex during early postnatal development and their possible contribution to the elimination of CF-to-PC synapses. Microglia enrich in the PC layer at pruning-relevant time points giving rise to the possibility that microglia are actively involved in synaptic pruning. We investigated the contribution of microglial fractalkine (CX₃ CR1) signaling during postnatal development using genetic ablation of the CX₃ CR1 receptor and an in-depth histological analysis of the cerebellar cortex. We found an aberrant migration of microglia into the granule and the molecular layer. By electrophysiological analysis, we show that defective fractalkine signaling and the associated migration deficits neither affect the pruning of excess CFs nor the development of functional parallel fiber and inhibitory synapses with PCs. These findings indicate that CX₃ CR1 signaling is not mandatory for correct cerebellar circuit formation. MAIN POINTS: Ablation of CX₃ CR1 results in a transient migration defect in cerebellar microglia. CX₃ CR1 is not required for functional pruning of cerebellar climbing fibers. Functional inhibitory and parallel fiber synapse development with Purkinje cells is undisturbed in CX₃ CR1-deficient mice.

Predisposition to Alcohol Drinking and Alcohol Consumption Alter Expression of Calcitonin Gene-Related Peptide, Neuropeptide Y, and Microglia in Bed Nucleus of Stria Terminalis in a Subnucleus-Specific Manner

Excessive alcohol consumption is often linked to anxiety states and has a major relay center in the anterior part of bed nucleus of stria terminalis (BNST). We analyzed the impact of (i) genetic predisposition to high alcohol preference and consumption, and (ii) alcohol intake on anterior BNST, namely anterolateral (AL), anteromedial (AM), and anteroventral (lateral + medial subdivisions: AVl, AVm) subnuclei. We used two rat lines selectively bred for low- and high-alcohol preference and consumption, named Sardinian alcohol-non preferring (sNP) and -preferring (sP), respectively, the latter showing also inherent anxiety-related behaviors. We analyzed the modulation of calcitonin gene-related peptide (CGRP; exerting anxiogenic effects in BNST), neuropeptide Y (NPY; exerting mainly anxiolytic effects), and microglia activation (neuroinflammation marker, thought to increase anxiety). Calcitonin gene-related peptide-immunofluorescent fibers/terminals did not differ between alcohol-naive sP and sNP rats. Fiber/terminal NPY-immunofluorescent intensity was lower in BNST-AM and BNST-AVm of alcohol-naive sP rats. Activation of microglia (revealed by morphological analysis) was decreased in BNST-AM and increased in BNST-AVm of alcohol-naive sP rats. Prolonged (30 consecutive days), voluntary alcohol intake under the homecage 2-bottle “alcohol vs. water” regimen strongly increased CGRP intensity in BNST of sP rats in a subnucleus-specific manner: in BNST-AL, BNST-AVm, and BNST-AM. CGRP area sum, however, decreased in BNST-AM, without changes in other subnuclei. Alcohol consumption increased NPY expression, in a subnucleus-specific manner, in BNST-AL, BNST-AVl, and BNST-AVm. Alcohol consumption increased many size/shapes parameters in microglial cells, indicative of microglia de-activation. Finally, microglia density was increased in ventral anterior BNST (BNST-AVl, BNST-AVm) by alcohol consumption. In conclusion, genetic predisposition of sP rats to high alcohol intake could be in part mediated by anterior BNST subnuclei showing lower NPY expression and differential microglia activation. Alcohol intake in sP rats produced complex subnucleus-specific changes in BNST, affecting CGRP/NPY expression and microglia and leading to hypothesize that these changes might contribute to the anxiolytic effects of voluntarily consumed alcohol repeatedly observed in sP rats.

Antioxidant supplementation upregulates calbindin expression in cerebellar Purkinje cells of rat pups subjected to post natal exposure to sodium arsenite

Optimal cytoplasmic calcium (Ca²⁺) levels have been associated with adequate cell functioning and neuronal survival. Altered intracellular Ca²⁺ levels following impaired Ca²⁺ homeostasis could induce neuronal degeneration or even cell death. There are reports of arsenite induced oxidative stress and the associated disturbances in intracellular calcium homeostasis. The present study focused on determining the strategies that would modulate tissue redox status and calcium binding protein (CaBP) (Calbindin D28k-CB) expression affected adversely by sodium arsenite (NaAsO₂) exposure (postnatal) of rat pups. NaAsO₂ alone or along with antioxidants (AOXs) (alpha lipoic acid or curcumin) was administered by intraperitoneal (i.p.) route from postnatal day (PND) 1-21 (covering rapid brain growth period - RBGP) to experimental groups and animals receiving sterile water by the same route served as the controls. At the end of the experimental period, the animals were subjected to euthanasia and the cerebellar tissue obtained therefrom was processed for immunohistochemical localization and western blot analysis of CB protein. CB was diffusely expressed in cell body as well as dendritic processes of Purkinje cells (PCs) along the PC Layer (PCL) in all cerebellar folia of the control and the experimental animals. The multilayered pattern of CB +ve cells along with their downregulated expression and low packing density was significantly evident in the arsenic (iAs) alone exposed group as against the controls and AOX supplemented groups. The observations are suggestive of AOX induced restoration of CaBP expression in rat cerebellum following early postnatal exposure to NaAsO₂.

Comparison of the role of metabotropic glutamate receptor subtype 1 in developmental refinement of neuronal connectivity between the cerebellum and the sensory thalamus

Developmental refinement of neuronal connectivity is crucial for proper brain function. In the early phase of development, input fibers arrive at their target areas guided by specific molecular cues and form abundant immature synapses. Then, functionally important synapses are preserved and strengthened by neural activity while unnecessary synapses are eliminated. Afferent synapses in the sensory thalamus, such as from retina to lateral geniculate nucleus, and climbing fiber (CF)-Purkinje cell (PC) synapses in the cerebellum are valuable models for studying this developmental refinement of synaptic connectivity because only a limited number of input fibers innervate a given postsynaptic thalamocortical (TC) neuron or PC. The metabotropic glutamate receptor subtype 1 (mGluR1) is required for the refinement of both afferent-TC neuron and CF-PC synapses. However, mGluR1 functions differently at these synapses. While mGluR1 is critical for elimination of surplus CF-PC synapses in the cerebellum, retinogeniculate synapses require mGluR1 for maintenance of mature connectivity.

Molecular layer interneurons of the cerebellum: developmental and morphological aspects

During the past 25 years, our knowledge on the development of basket and stellate cells (molecular layer interneurons [MLIs]) has completely changed, not only regarding their origin from the ventricular zone, corresponding to the primitive cerebellar neuroepithelium, instead of the external granular layer, but above all by providing an almost complete account of the genetic regulations (transcription factors and other genes) involved in their differentiation and synaptogenesis. Moreover, it has been shown that MLIs' precursors (dividing neuroblasts) and not young postmitotic neurons, as in other germinal neuroepithelia, leave the germinative zone and migrate all along a complex and lengthy path throughout the presumptive cerebellar white matter, which provides suitable niches exerting epigenetic influences on their ultimate neuronal identities. Recent studies carried out on the anatomical-functional properties of adult MLIs emphasize the importance of these interneurons in regulating PC inhibition, and point out the crucial role played by electrical synaptic transmission between MLIs as well as ephaptic interactions between them and Purkinje cells at the pinceaux level, in the regulation of this inhibition.

Specific labeling of climbing fibers shows early synaptic interactions with immature Purkinje cells in the prenatal cerebellum

During development, growing axons must locate target cells to form synapses. This is not easy, since target cells are also growing and even actively migrating. In some brain regions, such axons have been reported to wait for the timing when target cells become mature, without invading their target region. However, in the cerebellum climbing fibers (CFs), major afferent axons, arrive near their target neurons, Purkinje cells, when the neurons are still actively migrating. We, therefore, examined whether synaptic contacts are established at such early stages. To specifically label CFs, we introduced by in utero electroporation a mixture of genes encoding for Ptf1a-enhancer-driven Cre recombinase and Cre-dependent fluorescent protein into the mouse hindbrain at embryonic day (E) 10.5 and observed them during development. The earliest stages at which labeled CFs were observed in the cerebellar primordium were E15.5-E16.5. These fibers were fasciculated in the dorsal region and entered the cerebellar primordium. Some fibers defasciculated and reached the caudal region. At E17.5 and E18.5, fasciculated fibers were also found in the mantle region, and some grew toward the surface of the primordium to penetrate a mass of Purkinje cells. Interestingly, as early as E16.5, labeled fibers were found to run in close apposition to Purkinje cell dendrites and to express a presynaptic marker. These observations suggest that CFs form synapses with Purkinje cells as soon as the fibers enter the cerebellum.

Embryonic stages in cerebellar afferent development

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

Cerebellum neurotransmission during postnatal development: [Pt(O,O'-acac)(γ-acac)(DMS)] vs cisplatin and neurotoxicity

Several chemotherapeutic drugs are known to cause neurotoxicity. Platinum-based agents in use or in clinical trials display neurotoxic potential accompanied by neurological complications; recent studies have identified a large number of behavioural issues in paediatric oncology patients. To understand the toxicity of platinum drugs at the molecular and cellular levels, this study compares the possible cytotoxic effects of an older platinum compound, cisplatin and a new platinum compound, [Pt(O,O'-acac)(γ-acac)(DMS)], on the CNS of postnatally developing rats, which is much more vulnerable to injury than the CNS of adult rats. Since several drugs interact with neurotransmitters during neuronal maturation, we performed immunostainings with antibodies raised against markers of glutamate and GABA, the major neurotransmitters in the cerebellum. After a single injection of cisplatin at postnatal day 10 (PD10), the labelling of Purkinje cells with the neurotransmitter markers evidenced alterations between PD11 and PD30, i.e. atrophy of the dendrite tree, changes in the distribution of synaptic contacts of parallel and climbing fibres, delay in the elimination of transient synapses on cell soma and severely impaired pinceau formation at the axon hillock. After treatment with [Pt(O,O'-acac)(γ-acac)(DMS)], the sole relevant change concerned the timing of climbing fibres elimination; the transient synapses disappearance on the Purkinje cell soma was delayed in some cells; instead, the growth of Purkinje cell dendrite tree was normal as was the formation of inhibitory synaptic contacts on these neurons. These findings add new evidence not only on the lower neurotoxicity of [Pt(O,O'-acac)(γ-acac)(DMS)] vs cisplatin but also on the involvement of neurotransmitters and relative synaptic connections in the maturation of central nerve tissue.

Polarized light microscopy in reproductive and developmental biology

The polarized light microscope reveals orientational order in native molecular structures inside living cells, tissues, and whole organisms. It is a powerful tool used to monitor and analyze the early developmental stages of organisms that lend themselves to microscopic observations. In this article, we briefly discuss the components specific to a traditional polarizing microscope and some historically important observations on: chromosome packing in the sperm head, the first zygote division of the sea urchin, and differentiation initiated by the first asymmetric cell division in the sand dollar. We then introduce the LC-PolScope and describe its use for measuring birefringence and polarized fluorescence in living cells and tissues. Applications range from the enucleation of mouse oocytes to analyzing the polarized fluorescence of the water strider acrosome. We end with new results on the birefringence of the developing chick brain, which we analyzed between developmental stages of days 12-20.

Synapse elimination in the developing cerebellum

Neural circuits in neonatal animals contain numerous redundant synapses that are functionally immature. During the postnatal period, unnecessary synapses are eliminated while functionally important synapses become stronger and mature. The climbing fiber (CF) to the Purkinje cell (PC) synapse is a representative model for the analysis of postnatal refinement of neuronal circuits in the central nervous system. PCs are initially innervated by multiple CFs with similar strengths around postnatal day 3 (P3). Only a single CF is selectively strengthened during P3–P7 (functional differentiation), and the strengthened CF undergoes translocation from soma to dendrites of PCs from P9 on (dendritic translocation). Following the functional differentiation, supernumerary CF synapses on the soma are eliminated, which proceeds in two distinct phases: the early phase from P7 to around P11 and the late phase from around P12 to P17. Here, we review our current understanding of cellular and molecular mechanisms of CF synapse elimination in the developing cerebellum.

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

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

Climbing fiber synapse elimination in cerebellar Purkinje cells

Innervation of Purkinje cells (PCs) by multiple climbing fibers (CFs) is refined into mono-innervation during the first three postnatal weeks of rodents' lives. In this review article, we will integrate the current knowledge on developmental process and mechanisms of CF synapse elimination. In the 'creeper' stage of CF innervation (postnatal day 0 (P0)∼), CFs creep among PC somata to form transient synapses on immature dendrites. In the 'pericellular nest' stage (P5∼), CFs densely surround and innervate PC somata. CF innervation is then displaced to the apical portion of PC somata in the 'capuchon' stage (P9∼), and translocate to dendrites in the 'dendritic' (P12∼) stage. Along with the developmental changes in CF wiring, functional and morphological distinctions become larger among CF inputs. PCs are initially innervated by more than five CFs with similar strengths (∼P3). During P3-7 only a single CF is selectively strengthened (functional differentiation), and it undergoes dendritic translocation from P9 on (dendritic translocation). Following the functional differentiation, perisomatic CF synapses are eliminated nonselectively; this proceeds in two distinct phases. The early phase (P7-11) is conducted independently of parallel fiber (PF)-PC synapse formation, while the late phase (P12-17) critically depends on it. The P/Q-type voltage-dependent Ca(2+) channel in PCs triggers selective strengthening of single CF inputs, promotes dendritic translocation of the strengthened CFs, and drives the early phase of CF synapse elimination. In contrast, the late phase is mediated by the mGluR1-Gαq-PLCβ4-PKCγ signaling cascade in PCs driven at PF-PC synapses, whose structural connectivity is stabilized and maintained by the GluRδ2-Cbln1-neurexin system.

Cav2.1 in cerebellar Purkinje cells regulates competitive excitatory synaptic wiring, cell survival, and cerebellar biochemical compartmentalization

In the adult cerebellum, each Purkinje cell (PC) is innervated by a single climbing fiber (CF) in proximal dendrites and 10(5)-10(6) parallel fibers (PFs) in distal dendrites. This organized wiring is established postnatally through heterosynaptic competition between PFs and CFs and homosynaptic competition among multiple CFs. Using PC-specific Cav2.1 knock-out mice (PC-Cav2.1 KO mice), we have demonstrated recently that postsynaptic Cav2.1 plays a key role in the homosynaptic competition by promoting functional strengthening and dendritic translocation of single "winner" CFs. Here, we report that Cav2.1 in PCs, but not in granule cells, is also essential for the heterosynaptic competition. In PC-Cav2.1 KO mice, the extent of CF territory was limited to the soma and basal dendrites, whereas PF territory was expanded reciprocally. Consequently, the proximal somatodendritic domain of PCs displayed hyperspiny transformation and fell into chaotic innervation by multiple CFs and numerous PFs. PC-Cav2.1 KO mice also displayed patterned degeneration of PCs, which occurred preferentially in aldolase C/zebrin II-negative cerebellar compartments. Furthermore, the mutually complementary expression of phospholipase Cβ3 (PLCβ3) and PLCβ4 was altered such that their normally sharp boundary was blurred in the PCs of PC-Cav2.1 KO mice. This blurring was caused by an impaired posttranscriptional downregulation of PLCβ3 in PLCβ4-dominant PCs during the early postnatal period. A similar alteration was noted in the banded expression of the glutamate transporter EAAT4 in PC-Cav2.1 KO mice. Therefore, Cav2.1 in PCs is essential for competitive synaptic wiring, cell survival, and the establishment of precise boundaries and reciprocity of biochemical compartments in PCs.

Developmental switching of perisomatic innervation from climbing fibers to basket cell fibers in cerebellar Purkinje cells

In early postnatal development, perisomatic innervation of cerebellar Purkinje cells (PCs) switches from glutamatergic climbing fibers (CFs) to GABAergic basket cell fibers (BFs). Here we examined the switching process in C57BL/6 mice. At postnatal day 7 (P7), most perisomatic synapses were formed by CFs on to somatic spines. The density of CF-spine synapses peaked at P9, when pericellular nest around PCs by CFs was most developed, and CF-spine synapses constituted 88% of the total perisomatic synapses. Thereafter, CF-spine synapses dropped to 63% at P12, 6% at P15, and <1% at P20, whereas BF synapses increased reciprocally. During the switching period, a substantial number of BF synapses existed as BF-spine synapses (37% of the total perisomatic synapses at P15), and free spines surrounded by BFs or Bergmann glia also emerged. By P20, BF-spine synapses and free spines virtually disappeared, and BF-soma synapses became predominant (88%), thus attaining the adult pattern of perisomatic innervation. Parallel with the presynaptic switching, postsynaptic receptor phenotype also switched from glutamatergic to GABAergic. In the active switching period, particularly at P12, fragmental clusters of AMPA-type glutamate receptor were juxtaposed with those of GABA(A) receptor. When examined with serial ultrathin sections, immunogold labeling for glutamate and GABA(A) receptors was often clustered beneath single BF terminals. These results suggest that a considerable fraction of somatic spines is succeeded from CFs to BFs and Bergmann glia in the early postnatal period, and that the switching of postsynaptic receptor phenotypes mainly proceeds under the coverage of BF terminals.

Possible sites of action of the new calcitonin gene-related peptide receptor antagonists

Migraine is considered a neurovascular disease affecting more than 10% of the general population. Currently available drugs for the acute treatment of migraine are vasoconstrictors, which have limitations in their therapeutic use. The calcitonin gene-related peptide (CGRP) has a key role in migraine, where levels of CGRP are increased during acute migraine attacks. CGRP is expressed throughout the central and peripheral nervous system, consistent with control of vasodilatation and transmission of nociceptive information. In migraine, CGRP is released from the trigeminal system. At peripheral synapses CGRP results in vasodilatation via receptors on the smooth muscle cells. At central synapses, CGRP acts postjunctionally on second-order neurons to transmit pain centrally via the brainstem and midbrain to higher cortical pain regions. The recently developed CGRP-receptor antagonists have demonstrated clinical efficacy in the treatment of acute migraine attacks. A remaining question is their site of action. The CGRP-receptor components (calcitonin receptor-like receptor, receptor activity modifying protein 1 and receptor component protein) are found to colocalize in the smooth muscle cells of intracranial arteries and in large-sized neurons in the trigeminal ganglion. The CGRP receptor has also been localized within parts of the brain and the brainstem. The aim of this paper is to review recent localization studies of CGRP and its receptor components within the nervous system and to discuss whether these sites could be possible targets for the CGRP-receptor antagonists.

Pregnenolone sulfate increases glutamate release at neonatal climbing fiber-to-Purkinje cell synapses

Development of cerebellar Purkinje cells (PCs) is modulated by neuroactive steroids. Developing hippocampal pyramidal neurons retrogradely release a pregnenolone sulfate (PregS)-like neurosteroid that may contribute to glutamatergic synapse stabilization. We hypothesized that PregS could exert a similar effect on developing PCs. To test this hypothesis, we performed whole-cell patch-clamp recordings from PCs in acute cerebellar vermis slices from neonatal rats. PregS induced a robust (∼3000%) and reversible increase in AMPA receptor-mediated miniature excitatory postsynaptic current (AMPA-mEPSC) frequency without affecting the amplitude, time-to-rise, or half-width of these events. PregS also increased the frequency of GABA(A) receptor-mediated miniature postsynaptic currents but to a significantly lesser extent (<100%). The PregS-induced increase of AMPA-mEPSC frequency was not significantly decreased by antagonists of receptors (NMDA, glycine, α7 nicotinic acetylcholine and σ1) that have been shown to modulate glutamatergic transmission at PCs and/or mediate the actions of PregS on neurotransmitter release. Ca(2+) chelation experiments suggested that PregS acts by increasing presynaptic terminal [Ca(2+)](i), an effect that is independent of voltage-gated Ca(2+) channels, but is blocked by the antagonist of transient receptor potential (TRP) channels, La(3+). PregS also increased the amplitude of EPSCs evoked by climbing fiber (CF) stimulation and decreased the paired-pulse ratio of these events. Neither CF nor parallel fiber-evoked EPSCs were affected by PregS in slices from juvenile rats. These results suggest that glutamate release at CF-to-PC synapses is an important target of PregS in the neonatal cerebellar cortex, an effect that may play a role in the refinement of these synapses.

Cerebellar distribution of calcitonin gene-related peptide (CGRP) and its receptor components calcitonin receptor-like receptor (CLR) and receptor activity modifying protein 1 (RAMP1) in rat

Clinical and experimental results have revealed a fundamental role of calcitonin gene-related peptide (CGRP) in primary headaches. CGRP is widely expressed in neurons both in the central nervous system (CNS) and in peripheral sensory nerves. In the CNS there is a wide distribution of CGRP-containing neurons with the highest levels seen in striatum, amygdale and cerebellum. Moreover, in acute attacks of migraine there is evidence of cerebellar activation. To understand the role of CGRP, antibodies towards the CGRP receptor components calcitonin receptor-like receptor (CLR) and receptor activity modifying protein type 1 (RAMP1) have been developed. In the present study we therefore examined immunohistochemically the distribution of CGRP and its receptor components in the cerebellum. CGRP immunoreactivity was only found intracellularly in the cerebellar Purkinje cell bodies, whereas CLR and RAMP1 were detected on the surface of the Purkinje cell bodies and in their processes. The elaborate dendritic tree of Purkinje cell fibers was distinctly visualized with the RAMP1 antibody. In addition, profoundly stained fibers spanning from the molecular layer into the medulla was observed with the RAMP1 antibody. Judged from the high density of immunoreactive cells expressing CGRP, RAMP1 or CLR, and from the double staining of CGRP and RAMP1 it is likely that most, if not all, Purkinje cells express both the peptide and the receptor components. Double staining with RAMP1 and the glial cell markers glial fibrillary acidic protein (GFAP) and S-100 revealed an almost identical staining pattern of the antibodies in the area of the cell body surfaces. However, as judged by confocal microscopy, no double staining was present. Instead, it was discovered that the glial cells tightly surrounded the Purkinje cells which easily could be interpreted as co-localization in the epifluorescence microscope. Our observations demonstrate that there is a rich expression of CGRP and CGRP receptor elements in the cerebellum which points towards a functional role of CGRP in cerebellar Purkinje cells. Recent advances in the biology of the cerebellum indicate that there may be a role in nociception; hence a target of the recently discovered CGRP receptor antagonists that have demonstrated improvement in migraine pain and associated symptoms could be cerebellar CGRP receptors.

Calcitonin gene-related peptide (CGRP) stimulates purkinje cell dendrite growth in culture

Previous reports described the transient expression during development of Calcitonin Gene-Related Peptide (CGRP) in rodent cerebellar climbing fibers and CGRP receptor in astrocytes. Here, mixed cerebellar cultures were used to analyze the effects of CGRP on Purkinje cells growth. Our results show that CGRP stimulated Purkinje cell dendrite growth under cell culture conditions mimicking Purkinje cell development in vivo. The stimulation was not blocked by CGRP8-37, a specific antagonist, suggesting the activation of other related receptors. CGRP did not affect survival of Purkinje cells, granule cells or astrocytes. The selective expression of Receptor Component Protein (RCP) (a component of CGRP receptor family) in astrocytes points to a role of these cells as mediators of CGRP effect. Finally, in pure cerebellar astrocyte cultures CGRP induced a transient morphological differentiation from flat, polygonal to stellate form. It is concluded that CGRP influences Purkinje cell dendrite growth in vitro, most likely through the involvement of astrocytes.

Neuronal apoptosis in the developing cerebellum

The following study analysed apoptosis in proliferative cells and migrating neurons of the developing cerebellum. The external granular layer, Purkinje cell layer and internal granular layer in the developing mouse cerebellar cortex were analysed by active caspase-3 immunohistochemistry, Hoechst 33258 staining and Western blot analysis. Immunocytochemistry results indicated that the peak of apoptosis appeared at postnatal days P8, P5 and P9 in the external granular layer, Purkinje cell layer and internal granular layer, respectively. Subsequently, in each region, the rate of apoptosis decreased with increasing age. In contrast, Western blot results demonstrated the highest expression of activated caspase-3 in the cerebellum at P5, followed by a subsequent decline and disappearance of expression by P14. Activated caspase-8 was expressed maximally at P10, and subsequently disappeared by P30. The results of this study suggest that the key period of neuronal apoptosis in the cerebellar cortex is between P0 and P14, indicating that this developmental period could be susceptible to treatment for congenital neurodegenerative diseases.

CGRP receptor antagonism and migraine

Calcitonin gene-related peptide (CGRP) is expressed throughout the central and peripheral nervous systems, consistent with control of vasodilatation, nociception, motor function, secretion, and olfaction. alphaCGRP is prominently localized in primary spinal afferent C and ADelta fibers of sensory ganglia, and betaCGRP is the main isoform in the enteric nervous system. In the CNS there is a wide distribution of CGRP-containing neurons, with the highest levels occurring in striatum, amygdala, colliculi, and cerebellum. The peripheral projections are involved in neurogenic vasodilatation and inflammation, and central release induces hyperalgesia. CGRP is released from trigeminal nerves in migraine. Trigeminal nerve activation results in antidromic release of CGRP to cause non-endothelium-mediated vasodilatation. At the central synapses in the trigeminal nucleus caudalis, CGRP acts postjunctionally on second-order neurons to transmit pain signals centrally via the brainstem and midbrain to the thalamus and higher cortical pain regions. Recently developed CGRP receptor antagonists are effective at aborting acute migraine attacks. They may act both centrally and peripherally to attenuate signaling within the trigeminovascular pathway.

Calcitonin gene-related peptide (CGRP) triggers Ca2+ responses in cultured astrocytes and in Bergmann glial cells from cerebellar slices

The neuropeptide calcitonin gene-related peptide (CGRP) is transiently expressed in cerebellar climbing fibers during development while its receptor is mainly expressed in astrocytes, in particular Bergmann glial cells. Here, we analyzed the effects of CGRP on astrocytic calcium signaling. Mouse cultured astrocytes from cerebellar or cerebral cortex as well as Bergmann glial cells from acutely isolated cerebellar slices were loaded with the Ca(2+) sensor Fura-2. CGRP triggered transient increases in intracellular Ca(2+) in astrocytes in culture as well as in acute slices. Responses were observed in the concentration range of 1 nm to 1 mm, in both the cell body and its processes. The calcium transients were dependent on release from intracellular stores as they were blocked by thapsigargin but not by the absence of extracellular calcium. In addition, after CGRP application a further delayed transient increase in calcium activity could be observed. Finally, cerebellar astrocytes from neonatal mice expressed receptor component protein, a component of the CGRP receptor, as revealed by immunofluorescence and confocal microscopy. It is thus proposed that the CGRP-containing afferent fibers in the cerebellum (the climbing fibers) modulate calcium in astrocytes by releasing the neuropeptide during development and hence possibly influence the differentiation of Purkinje cells.

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.

Intrinsic versus extrinsic determinants during the development of Purkinje cell dendrites

The peculiar shape and disposition of Purkinje cell (PC) dendrites, planar and highly branched, offers an optimal model to analyze cellular and molecular regulators for the acquisition of neuronal dendritic trees. During the first 2 weeks after the end of the proliferation period, PCs undergo a 2-phase remodeling process of their dendrites. The first phase consists in the complete retraction of the primitive but extensive dendritic tree, together with the formation of multiple filopodia-like processes arising from the cell body. In the second phase, there is a progressive disappearance of the somatic processes along with rapid growth and branching of the mature dendrite. Mature Purkinje cell dendrites bear two types of spiny protrusions, named spine and thorn. The spines are numerous, elongated, located at the distal dendritic compartment and form synapses with parallel fibers, whereas the thorns are shorter, rounded, emerge from the proximal compartment and synapse with climbing fibers. Different culture models and mutant mice analyses suggest the identification of intrinsic versus extrinsic determinants of the Purkinje cell dendritic development. The early phase of dendritic remodeling might be cell autonomous and regulated by specific transcription factors such as retinoid-related orphan receptor alpha (RORalpha). Afferent fibers, trophic factors and hormones regulate the orientation and growth of the mature dendritic tree contributing, with still unknown intrinsic factors, to sculpt its general architecture. The formation of spines appears as an intrinsic phenomenon independent of their presynaptic partner, the parallel fibers, and confined to the distal compartment by inhibitory influences of the climbing fibers along the proximal compartment.

Stwl modifies chromatin compaction and is required to maintain DNA integrity in the presence of perturbed DNA replication

Hydroxyurea, a well-known DNA replication inhibitor, induces cell cycle arrest and intact checkpoint functions are required to survive DNA replication stress induced by this genotoxic agent. Perturbed DNA synthesis also results in elevated levels of DNA damage. It is unclear how organisms prevent accumulation of this type of DNA damage that coincides with hampered DNA synthesis. Here, we report the identification of stonewall (stwl) as a novel hydroxyurea-hypersensitive mutant. We demonstrate that Stwl is required to prevent accumulation of DNA damage induced by hydroxyurea; yet, Stwl is not involved in S/M checkpoint regulation. We show that Stwl is a heterochromatin-associated protein with transcription-repressing capacities. In stwl mutants, levels of trimethylated H3K27 and H3K9 (two hallmarks of silent chromatin) are decreased. Our data provide evidence for a Stwl-dependent epigenetic mechanism that is involved in the maintenance of the normal balance between euchromatin and heterochromatin and that is required to prevent accumulation of DNA damage in the presence of DNA replication stress.

Ethanol-related increases in degenerating bodies in the Purkinje neuron dendrites of aging rats

Chronic ethanol consumption in aging rats results in regression of Purkinje neuron (PN) dendritic arbors ([Pentney, 1995 Measurements of dendritic pathlengths provide evidence that ethanol-induced lengthening of terminal dendritic segments may result from dendritic regression. Alcohol Alcohol. 30, 87-96]), loss of synapses (Dlugos and Pentney, 1997), dilation of the smooth endoplasmic reticulum (SER), and the formation of degenerating bodies within PN dendrites ([Dlugos, C.A., 2006a. Ethanol-Related Smooth Endoplasmic Reticulum Dilation in Purkinje Dendrites of Aging Rats. Alcohol., Clin. Exp. Res. 30, 883-891,Dlugos, C.A., 2006b. Smooth endoplasmic reticulum dilation and degeneration in Purkinje neuron dendrites of aging ethanol-fed female rats. Cerebellum. 5, 155-162]). Dilation of the SER and the formation of degenerating bodies may be a predictor of dendritic regression. Ethanol-induced effects on mitochondria may be involved as mitochondria cooperate with the SER to maintain calcium homeostasis. The purpose of this study was to determine whether degenerating body number and mitochondrial density and structure are altered by chronic ethanol treatment in PN dendrites. Male, Fischer 344 rats, 12 months of age, were fed an ethanol or pair-fed liquid diet, or rat chow for a period of 10, 20, or 40 weeks (15 rats/treatment; 45 rats/treatment duration). Ethanol-fed rats received 35% of their calories as ethanol. At the end of treatment, all animals were euthanized, perfused, and tissue prepared for electron microscopy. The densities of degenerating bodies and mitochondria, mitochondrial areas, and the distance between the SER and the mitochondria were measured. Results showed that there was an ethanol-related increase in degenerating bodies compared to controls at 40 weeks. Ethanol-induced alterations to mitochondria were absent. Correlation of the present results with those of previous studies suggest that degenerating bodies may be formed from membrane reabsorption during dendritic regression or from degenerating SER whose function has been compromised by dilation.

TrkB is necessary for pruning at the climbing fibre-Purkinje cell synapse in the developing murine cerebellum

TrkB, the cognate receptor for brain-derived neurotrophic factor and neurotrophin-4, has been implicated in regulating synapse formation in the central nervous system. Here we asked whether TrkB plays a role in the maturation of the climbing fibre-Purkinje cell (CF-PC) synapse. In rodent cerebellum, Purkinje cells are initially innervated by multiple climbing fibres that are subsequently culled to assume the mature mono-innervated state, and whose contacts translocate from the soma to the dendrites. By employing transgenic mice hypomorphic or null for TrkB expression, our results indicated that perturbation of TrkB in the immature cerebellum resulted in ataxia, that Purkinje cells remained multiply innervated by climbing fibres beyond the normal developmental time frame, and that synaptic transmission at the parallel fibre-Purkinje cell synapse remained functionally unaltered. Mechanistically, we present evidence that attributes the persistence of multiple climbing fibre innervation to an obscured discrimination of relative strengths among competing climbing fibres. Soma-to-dendrite translocation of climbing fibre terminals was unaffected. Thus, TrkB regulates pruning but not translocation of nascent CF-PC synaptic contacts.

Time course of cerebellar morphological development in postnatal ferrets: ontogenetic and comparative perspectives

We provide the first systematic description of the morphological ontogenesis of the ferret cerebellum and compare its relative time-course to that of the rat cerebellum. Overall cerebellar size, foliation, and thickness of cortical layers were quantified and Purkinje cell morphology was characterized at 24 timepoints in ferrets from postnatal day (P)1 to P63. The ferret cerebellum was substantially larger than that of the rat and had a much longer developmental period. In ferrets, Purkinje cells were dispersed into a monolayer by P9, the formation of folia declined abruptly around P20, and the external granular layer peaked in thickness around P22 and disappeared by P56. Timepoints of corresponding relative developmental maturity of the quantified architectural features of rat and ferret cerebella were determined and their relationship was analyzed by linear regression. The time-conversion equation derived, describing the relationship between cerebellar morphogenesis in the two species, had a determination coefficient (r2) of 0.95. Conspicuously, the equation predicted with high accuracy the timing of structural changes in individual Purkinje cells in the ferret cerebellum. The conversion equation should be useful for precise quantitative translation of data between studies of ferret and rat cerebellum and for comparisons between development of motor and sensory structures and functions in ferrets. The degree of similarity in the time-courses of cerebellar development in two distantly related mammals makes explicit in quantitative terms how remarkably conserved the cerebellum is in phylogenesis. Therefore, the methodology should be applicable to precise quantitative conversions of cerebellar developmental time-courses also between other species.

Differential neuronal and glial expression of GluR1 AMPA receptor subunit and the scaffolding proteins SAP97 and 4.1N during rat cerebellar development

In neurons, AMPA glutamate receptors are developmentally regulated and selectively targeted to synaptic sites. Astroglial cells also express AMPA receptors, but their developmental pattern of expression and targeting mechanisms are unknown. In this study we investigated by immunocytochemistry at the light and electron microscopy level the expression of GluR1 and its scaffolding proteins SAP97 (synapse-associated protein) and 4.1N during cerebellar development. In cerebellar cortex the GluR1 AMPA receptor subunit is expressed exclusively in Bergmann glia in the adult rodent. Interestingly, we observed that GluR1 was expressed postsynaptically at the climbing fibers (CF) synapse at early ages during Purkinje cell dendritic growth and before the complete ensheathment of CF/Purkinje cell synapses by Bergmann glia. However, its expression changed from neurons to Bergmann glia once these glial cells had completed their enwrapping process. In contrast, GluR2/3 and GluR4 AMPAR subunits were stably expressed in both Purkinje cells (GluR2/3) and Bergmann glia (GluR4) throughout postnatal development. Our data indicate that GluR1 expression undergoes a developmental switch from neurons to glia and that this appears to correlate with the degree of Purkinje cell dendritic growth and their enwrapping by Bergmann glia. SAP97 and 4.1N were developmentally regulated in the same pattern as GluR1. Therefore, SAP97 and 4.1N may play a role in the transport and insertion of GluR1 at CF/Purkinje cell synapses during early ages and at Bergmann glia plasma membrane in the adult. The parallel fiber (PF)/Purkinje cell synapse contained GluR2/3 but lacked GluR1, SAP97, and 4.1N at the time of PF synaptogenesis.

Reorganization of the rat cerebellar cortex during postnatal development following cisplatin treatment

We examined the effects of the antitumor agent cisplatin on the development and plasticity of cerebellar cytoarchitecture. Since knowledge of the parallel and climbing fiber-Purkinje cell system is important in order to determine the architectural basis of cerebellar function, we used immunofluorescence for vesicular glutamate transporters (VGluT1 and VGluT2) to evaluate the trend of synaptogenesis of parallel and climbing fibers on Purkinje cells in the cerebellum vermis after a single injection of cisplatin to 10-day-old rats, i.e., during a crucial period of cerebellar development. The temporal and spatial patterns of VGluT1 and VGluT2 immunoreactivity after the early cisplatin injury provided evidence that remodeling of excitatory afferents and Purkinje cell dendrites occurs. After an early slow down of Purkinje cell dendrite growth, 7 days following the treatment, the extension of the molecular layer was reduced, as was parallel fiber innervation, but VGluT1 immunoreactive fibers contacted Purkinje cell dendrite branches extending within the external granular layer. VGluT2 immunopositive climbing fiber varicosities were still largely present on the soma and stem dendrites of Purkinje cells. Twenty days after the cisplatin injection, the thickness of the VGluT1 immunopositive molecular layer was reduced. VGluT2 climbing fiber varicosities were found on the remodeled Purkinje cell dendrites, as in controls, although at a lower density. Alterations in the immunoreactivity for polysialic acid neural cell adhesion molecule (PSA-NCAM) during the recovery phase suggest that this molecule plays a fundamental role not only during development, but also in the reorganization of neuroarchitecture. The changes were restricted to the neocerebellar vermis and were likely dependent on the different timing of lobule formation. The results of these investigations reveal the existence of vulnerability windows of the cerebellum to exposure to experimental or environmental cytotoxic agents during a critical period in development.

Postnatal development of a large auditory nerve terminal: the endbulb of Held in cats

The endbulbs of Held are formed by the ascending branches of myelinated auditory nerve fibers and represent one of the largest synaptic endings in the brain. Most of the developmental changes in structure occur during the first 30 postnatal days of age. The neonatal endbulb begins as a flattened expansion with many filopodia, resembling a growth cone and characterized by numerous puncta adherentia and synapses associated with small postsynaptic densities; the most impressive feature of the ending at this age is its highly irregular plasma membrane that interdigitates with that of the postsynaptic spherical bushy cell. During these first 30 days, the number of puncta adherentia diminishes, postsynaptic densities nearly double in size, intermembraneous cisternae emerge, and plasma membranes flatten. These features endow the endbulb with an adult-like appearance. On the other hand, synaptic vesicle density increases progressively from approximately 50/microm2 at birth to 100/microm2 at adulthood. Mitochondria size remains constant over this developmental period but mitochondrial volume fraction increases until 60 days postnatal. Although many features of endbulb morphology stabilize by 30 days, other features suggest that endbulb development continues into the third month of age. Many of these observations correlate with the maturation of physiological response properties and suggest issues for further study.

Differential expression of NMDA and AMPA/KA receptor subunits in the inferior olive of postnatal rats

We have employed immunohistochemistry to determine the expression patterns of receptor subunits of N-methyl-d-aspartate (NMDA-NR1 and NR2A/B) and alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid/kainic acid (AMPA/KA-GluR1, GluR2, GluR2/3, GluR4, and GluR5/6/7) in the inferior olive of postnatal rats up to adulthood. Immunoreactivity for distinct receptor subunits was predominantly localized in the soma and dendrites of neurons. Semi-quantification showed that the overall immunoreactivity in the inferior olive of adults was intense for GluR1, moderate for NR1 and NR2A/B, and low for GluR2, GluR2/3, GluR4, and GluR5/6/7. At P7, GluR1 was restricted to the dorsomedial cell column, subnucleus beta, principal nucleus and ventrolateral protrusion while the other subunits were found in all subnuclei of the inferior olive. The immunoreactivities for all glutamate receptor subunits ranged from low to moderate. As the rats matured, the immunoreactivity of GluR4 decreased after the second postnatal week, while those of the other subunits showed a general trend of increase, reaching adult level during the third postnatal week. Double immunofluorescence revealed that all NR1-containing neurons exhibited NR2A/B immunoreactivity, indicating that native NMDA receptors comprise of hetero-oligomeric combinations of NR1 and NR2A/B. Furthermore, co-localization of NMDA and AMPA/KA receptor subunits was demonstrated in individual neurons of the inferior olive. All NR1-containing neurons exhibited GluR1 immunoreactivity, and all NR2A/B-containing neurons showed GluR5/6/7 immunoreactivity. Our data suggest that NMDA and AMPA/KA receptors are involved in glutamate-mediated neurotransmission, contributing to synaptic plasticity and reorganization of circuitry in the inferior olive during postnatal development.

Microzonal projection and climbing fiber remodeling in single olivocerebellar axons of newborn rats at postnatal days 4-7

An adult olivocerebellar axon ramifies into about seven climbing fibers that innervate single Purkinje cells arranged in a longitudinal microzone. To clarify the developmental basis of this projection, individual olivocerebellar axons were labeled with biotinylated dextran amine injected into the inferior olive in rats at postnatal days 4-7. The entire trajectories of single olivocerebellar axons and single terminal arbors of climbing fibers were reconstructed from serial sections of the cerebellum and medulla. Single axons ramified into climbing fibers that terminated in a narrow band-shaped area comparable to the adult microzone. This indicated that olivocerebellar microzones are predetermined. Terminal arbors of climbing fibers were remodeled from loose creeper type, through intermediate transitional type, into dense nest type. Each olivocerebellar axon had some 100 nascent climbing fibers in the creeper stage, whereas each axon had about 10 climbing fibers and about as many atrophic climbing fibers in the nest stage. This decrease indicated that overabundant nascent climbing fibers degenerate concomitantly with the remodeling of remaining climbing fibers. Atrophic terminal arbors and non-climbing fiber thin collaterals were considered the intermediate forms of degenerating climbing fibers. This remodeling and degeneration of climbing fibers may be related to the electrophysiological regression of climbing fiber-Purkinje cell synapses. The remodeling of climbing fibers occurred earliest in lobules VIII (caudal part) and IXa-b, and then in lobules IXc and X. The more developed granular layer in these areas compared to other areas suggests that the cortical environment triggers climbing fiber remodeling.

The effects of microgravity on the development of surface righting in rats

The active interaction of neonatal animals with their environment has been shown to be a decisive factor in the postnatal development of sensory systems, which demonstrates a critical period in their maturation. The direct demonstration of such a dependence on the rearing environment has not been demonstrated for motor system function. Nor has the role of gravity in mammalian motor system development been investigated. Here we report the results of two space flight missions examining the effect of removing gravity on the development of surface righting. Since the essential stimulus that drives this synergy, gravitation, was missing, righting did not occur while the animals were in the microgravity environment. We hypothesize that this absence of contextual motor experience arrested the maturation of the motor tactics for surface righting. Such effects were permanent in rats spending 16 days (from postnatal day (P), P14 to P30), but were transient in animals spending nine days (from P15 to P24) in microgravity. Thus, active, contextual interaction with the environment during a critical period of development is necessary for the postnatal maturation of motor tactics as exemplified by surface righting, and such events must occur within a particular time period. Further, Earth's gravitational field is not assumed by the developing motor system. Rather, postnatal motor system development is appropriate to the gravitational field in which the animal is reared.

Early expression of AMPA receptors and lack of NMDA receptors in developing rat climbing fibre synapses

Whether nascent glutamatergic synapses acquire their AMPA receptors constitutively or via a regulated pathway triggered by pre-existing NMDA receptor activation is still an open issue. Here, we provide evidence that some glutamatergic synapses develop without expressing NMDA receptors. Using immunocytochemistry, we showed that synapses between developing rat climbing fibres and Purkinje cells expressed GluR2-containing AMPA receptors as soon as they were formed (i.e. on embryonic day 19) but never carried detectable NMDA receptors. This was confirmed by electrophysiological recordings. Excitatory synaptic currents were recorded in Purkinje cells as early as P0. However, no NMDA receptor-mediated component was found in either spontaneous or evoked synaptic responses. In addition, we ruled out a possible role of extrasynaptic NMDA receptors by showing that AMPA receptor clustering at nascent climbing fibre synapses was not modified by chronic in utero NMDA receptor blockade.

The effects of ethanol on the developing cerebellum and eyeblink classical conditioning

In rats, developmental ethanol exposure has been used to model the central nervous system deficits associated with human fetal alcohol syndrome. Binge-like ethanol exposure of neonatal rats depletes cells in the cerebellum, including Purkinje cells, granule cells, and deep nuclear cells, and produces deficits in simple tests of motor coordination. However, the extent to which anatomical damage is related to behavioral deficits has been difficult to estimate. Eyeblink classical conditioning is known to engage a discrete brain stem-cerebellar circuit, making it an ideal test of cerebellar functional integrity after developmental ethanol exposure. Eyeblink conditioning is a simple form of motor learning in which a neutral stimulus (such as a tone) comes to elicit an eyeblink when repeatedly paired with a stimulus that evokes an eyeblink prior to training (such as mild periorbital stimulation). In eyeblink conditioning, one of the deep cerebellar nuclei, the interpositus nucleus, as well as specific Purkinje cell populations, are sites of convergence for tone conditioned stimulus and somatosensory unconditioned stimulus information, and, together with brain stem nuclei, provide the necessary and sufficient substrate for the learned response. A series of studies have shown that eyeblink conditioning is impaired in both weanling and adult rats given binge-like exposure to ethanol as neonates. In addition, interpositus nucleus neurons from ethanol-exposed rats showed impaired activation during eyeblink conditioning. These deficits are accompanied by a permanent reduction In the deep cerebellar nuclear cell population. Because particular cerebellar cell populations are utilized in well-defined ways during eyeblink conditioning, conclusions regarding the underlying neural substrates of behavioral change after developmental ethanol exposure are greatly strengthened.

Cellular and molecular control of dendritic growth and development of cerebellar Purkinje cells

Purkinje cells are the principal neurons of the cerebellar cortex and are characterized by a large and highly branched dendritic tree. For this reason, they have for a long time been an attractive model system to study the regulation of dendritic growth and differentiation. In this article, I will first review studies on different aspects of Purkinje cell dendritic development and then go on to present studies which have aimed at experimentally altering Purkinje cell dendritic development. Some of the cellular and molecular mechanisms which have been shown by these studies to be important determinants of Purkinje cell dendritic development will be discussed, in particular the role of the parallel fiber input, of hormones, and of neuronal growth factors. The organotypic slice culture method will be introduced as an important experimental tool to study Purkinje cell dendritic development under controlled conditions. Using cerebellar slice cultures, protein kinase C (PKC) has been identified as a major determinant of Purkinje cell dendritic development and the contribution of specific isoforms of PKC will be discussed. Finally, it will be shown that Purkinje cell dendritic development in slice cultures does not depend on the activation of glutamate receptors and appears to be independent of the presence of the neurotrophin BDNF. These studies indicate that the initial outgrowth of the Purkinje cell dendritic tree can occur in the absence of signals derived from afferent fibers, but is under control of PKC signaling.

Localization of synapsin-I and PSD-95 in developing postnatal rat cerebellar cortex

The comparative localization of two prominent synaptic proteins, synapsin-I (Syn-I) and PSD-95, was investigated in slices of developing (P3-P21) rat cerebellar cortex using double- or triple-label fluorescence immunohistochemistry and confocal microscopy. During the first postnatal week, Syn-I and PSD-95 immunoreactive (IR) puncta were strongly concentrated in the Purkinje cell layer (PCL) where they circumscribed irregularly shaped PC somata, forming pericellular nests that likely correspond to early climbing fiber synapses. PSD-95 and Syn-I puncta also were found along the shafts and at the tips of growing PC dendrite branches labeled with calbindin. During the second postnatal week, synaptic puncta were lost from the PC layer, while many new puncta were added to the molecular layer (ML). At P10, about half of the PCs were circumscribed by PSD-95 or Syn-I puncta, whereas at P14 no PCs were circumscribed. By P14, PSD-95 and Syn-I became most strongly localized to many small puncta in the ML and to large clusters at mossy fiber rosettes in the glomerular layer (GL) where PSD-95 often encircled Syn-I clusters. Some large clusters in the GL contained only PSD-95 or Syn-I, but not both, suggesting differential growth or remodeling of pre- and post-synaptic structures. No PSD-95 staining of pre-synaptic terminal pinceau was observed during the first 3 weeks of postnatal development. Thus, in relation to PCs, there is a developmental shift in PSD-95 localization whereby, first, it is concentrated on PC cell bodies and short dendrites (P3-P7), then it is lost on PC cell bodies (P7-14) and becomes localized almost exclusively to PC dendrites (P14-P21).

Binding sites of gamma-secretase inhibitors in rodent brain: distribution, postnatal development, and effect of deafferentation

gamma-Secretase is a multimeric complex consisted of presenilins (PSs) and three other proteins. PSs appear to be key contributors for the enzymatic center, the potential target of a number of recently developed gamma-secretase inhibitors. Using radiolabeled and unlabeled inhibitors as ligands, this study was aimed to determine the in situ distribution of gamma-secretase in the brain. Characterization using PS-1 knock-out mouse embryos revealed 50 and 80% reductions of gamma-secretase inhibitor binding density in the heterozygous (PS-1(+/-)) and homozygous (PS-1-/-) embryos, respectively, relative to the wild type (PS-1(+/+)). The pharmacological profile from competition binding assays suggests that the ligands may target at the N- and C-terminal fragments of PS essential for gamma-secretase activity. In the adult rat brain, the binding sites existed mostly in the forebrain, the cerebellum, and discrete brainstem areas and were particularly abundant in areas rich in neuronal terminals, e.g., olfactory glomeruli, CA3-hilus area, cerebellar molecular layer, and pars reticulata of the substantia nigra. In the developing rat brain, diffuse and elevated expression of binding sites occurred at the early postnatal stage relative to the adult. The possible association of binding sites with neuronal terminals in the adult brain was further investigated after olfactory deafferentation. A significant decrease with subsequent recovery of binding sites was noted in the olfactory glomeruli after chemical damage of the olfactory epithelium. The findings in this study support a physiological role of PS or gamma-secretase complex in neuronal and synaptic development and plasticity.

Reparative mechanisms in the cerebellar cortex

In the adult brain, different neuronal populations display different degrees of plasticity. Here, we describe the highly different plastic properties of inferior olivary neurones and Purkinje cells. Olivary neurones show a basal expression of growth-associated proteins, such as GAP-43 and Krox24/EGR-1, and remarkable remodelling capabilities of their terminal arbour. They also regenerate their transected neurites into growth-permissive territories and may reinnervate the lost target. Sprouting and regrowing olivary axons are able to follow specific positional information cues to establish new connections according to the original projection map. In addition, they set a strong cell body reaction to injury, which in specific olivary subsets is regulated by inhibitory target-derived cues. In contrast, Purkinje cells do not have a constitutive level of growth-associated genes, and show little cell body reaction, no axonal regeneration after axotomy, and weak sprouting capabilities. Block of myelin-derived signals allows terminal arbour remodelling, but not regeneration, while selective over-expression of GAP-43 induces axonal sprouting along the axonal surface and at the level of the lesion. We suggest that the high constitutive intrinsic plasticity of the inferior olive neurones allows their terminal arbour to sustain the activity-dependent ongoing competition with the parallel fibres in order to maintain the post-synaptic territory, and possibly underlies mechanisms of learning and memory. Such a plasticity is used also as a reparative mechanism following axotomy. In contrast, in Purkinje cells, poor intrinsic regenerative capabilities and myelin-derived signals stabilise the mature connectivity and prevent axonal regeneration after lesion.

Corticotropin-releasing factor and urocortin differentially modulate rat Purkinje cell dendritic outgrowth and differentiation in vitro

The precise outgrowth and arborization of dendrites is crucial for their function as integrators of signals relayed from axons and, hence, the functioning of the brain. Proper dendritic differentiation is particularly resonant for Purkinje cells as the intrinsic activity of this cell-type is governed by functionally distinct regions of its dendritic tree. Activity-dependent mechanisms, driven by electrical signaling and trophic factors, account for the most active period of dendritogenesis. An as yet unexplored trophic modulator of Purkinje cell dendritic development is corticotropin-releasing factor (CRF) and family member, urocortin, both of which are localized in climbing fibers. Here, we use rat organotypic cerebellar slice cultures to investigate the roles of CRF and urocortin on Purkinje cell dendritic development. Intermittent exposure (12 h per day for 10 days in vitro) of CRF and urocortin induced significantly more dendritic outgrowth (45% and 70%, respectively) and elongation (25% and 15%, respectively) compared with untreated cells. Conversely, constant exposure to CRF and urocortin significantly inhibited dendritic outgrowth. The trophic effects of CRF and urocortin are mediated by the protein kinase A and mitogen-activating protein kinase pathways. The study shows unequivocally that CRF and urocortin are potent regulators of dendritic development. However, their stimulatory or inhibitory effects are dependent upon the degree of expression of these peptides. Furthermore, the effects of CRF and urocortin on neuronal differentiation and re-modeling may provide a cellular basis for pathologies such as major depression, which show perturbations in the expression of these stress peptides.

Functional differentiation of multiple climbing fiber inputs during synapse elimination in the developing cerebellum

We studied how physiological properties of cerebellar climbing fiber (CF) to Purkinje cell (PC) synapses change during developmental transition from multiple to mono CF innervation onto each PC. From P3 to P6, differences in the strengths of multiple CFs became larger. Around P10, each PC was either monoinnervated by one strong CF (CF-mono) or multiply innervated by one strong CF (CF-multi-S) plus a few weaker CFs (CF-multi-W). We show that simultaneous release of multiple vesicles per site occurs normally from CF-multi-S, CF-mono, and mature CFs, but less frequently from CF-multi-W and neonatal CFs. We also present evidence suggesting that weaker CFs with lower probability of multivesicular release would be withdrawn preferentially. The results suggest that differentiation into strong and weak CFs with high and low probabilities of multivesicular release precedes developmental CF synapse elimination.

Sodium imaging of climbing fiber innervation fields in developing mouse Purkinje cells

Maturation of specific neuronal connections in the mature nervous system includes elimination of redundant synapses formed earlier during development. In the cerebellum of adult animals, each Purkinje cell (PC) is innervated by a single climbing fiber (CF). In early postnatal development each PC is innervated by multiple CFs and elimination of synapses formed by supernumerary CFs occurs until monoinnervation is established at around postnatal day 20 (P20) in mice. It is not clear whether multiple CFs, or only a single CF, translocate from the cell body of immature PCs to the developing dendrite and, in case several CFs translocate, whether they share or segregate their innervation fields. To localize CF innervation fields, we imaged changes in postsynaptic sodium concentration resulting from CF-mediated postsynaptic currents. We found that more than one CF translocates from an innervation field on the cell body of the PC to the developing dendrite and that these CFs share rather than segregate their innervation fields. We concluded that both the soma and the proximal dendrite of the PC are territories of competition for the developing CFs and that the overlapping of their termination fields may be the prerequisite for a local process of elimination of all but one CF, as previously demonstrated in the developing neuromuscular junction.