Neonatal noradrenaline depletion prevents the transition of Bergmann glia in the developing cerebellum

The association of DNA methylation and brain volume in healthy individuals and schizophrenia patients

Both methylation and brain volume patterns hold important biological information for the development and prognosis of schizophrenia (SZ). A combined study to probe the association between them provides a new perspective to understanding SZ. Genomic methylation of peripheral blood and regional brain volumes derived from magnetic resonance imaging were analyzed using parallel independent component analyses in this study. Nine methylation components and five brain volumetric components were extracted for 94 SZ patients and 106 healthy controls. After controlling for age, sex, race, and substance use, a component comprised primarily of bilateral cerebellar volumes was significantly correlated to a methylation component from 14 CpG sites in 13 genes. Both patients and healthy controls demonstrated similar associations, but patients had significantly smaller cerebellar volumes and dysmethylation in the associated epigenetic component compared to controls. The 13 genes are enriched in cellular growth and proliferation with some genes involved in neuronal growth and cerebellum development (GATA4, ADRA1D, EPHA3, and KCNK10), and these genes are prominently associated with neurological and psychological disorders. Such findings suggest that the methylation pattern of the genes coding for cellular growth may influence the cerebellar development through regulating gene expression, and the alteration in the methylation of these genes in SZ patients may contribute to the cerebellar volume reduction observed in patients.

Astrocytic beta(2)-adrenergic receptors: from physiology to pathology

Evidence accumulates for a key role of the beta(2)-adrenergic receptors in the many homeostatic and neuroprotective functions of astrocytes, including glycogen metabolism, regulation of immune responses, release of neurotrophic factors, and the astrogliosis that occurs in response to neuronal injury. A dysregulation of the astrocytic beta(2)-adrenergic-pathway is suspected to contribute to the physiopathology of a number of prevalent and devastating neurological conditions such as multiple sclerosis, Alzheimer's disease, human immunodeficiency virus encephalitis, stroke and hepatic encephalopathy. In this review we focus on the physiological functions of astrocytic beta(2)-adrenergic receptors, and their possible impact in disease states.

Oligodendrocyte ablation affects the coordinated interaction between granule and Purkinje neurons during cerebellum development

Oligodendrocytes (OLs) are the glial cells of the central nervous system (CNS) classically known to be devoted to the formation of myelin sheaths around most axons of the vertebrate brain. We have addressed the role of these cells during cerebellar development, by ablating OLs in vivo. Previous analyses had indicated that OL ablation during the first six postnatal days results into a striking cerebellar phenotype, whose major features are a strong reduction of granule neurons and aberrant Purkinje cells development. These two cell types are highly interconnected during cerebellar development through the production of molecules that help their proliferation, differentiation and maintenance. In this article, we present data showing that OL ablation has major effects on the physiology of Purkinje (PC) and granule cells (GC). In particular, OL ablation results into a reduction of sonic hedgehog (Shh), Brain Derived Neurotrophic Factor (BDNF), and Reelin (Rln) expression. These results indicate that absence of OLs profoundly alters the normal cerebellar developmental program.

Neuron specific alpha-adrenergic receptor expression in human cerebellum: implications for emerging cerebellar roles in neurologic disease

Recent data suggest novel functional roles for cerebellar involvement in a number of neurologic diseases. Function of cerebellar neurons is known to be modulated by norepinephrine and adrenergic receptors. The distribution of adrenergic receptor subtypes has been described in experimental animals, but corroboration of such studies in the human cerebellum, necessary for drug treatment, is still lacking. In the present work we studied cell-specific localizations of alpha1 adrenergic receptor subtype mRNA (alpha 1a, alpha 1b, alpha 1d), and alpha2 adrenergic receptor subtype mRNA (alpha 2a, alpha 2b, alpha 2c) by in situ hybridization on cryostat sections of human cerebellum (cortical layers and dentate nucleus). We observed unique neuron-specific alpha1 adrenergic receptor and alpha2 adrenergic receptor subtype distribution in human cerebellum. The cerebellar cortex expresses mRNA encoding all six alpha adrenergic receptor subtypes, whereas dentate nucleus neurons express all subtype mRNAs, except alpha 2a adrenergic receptor mRNA. All Purkinje cells label strongly for alpha 2a and alpha 2b adrenergic receptor mRNA. Additionally, Purkinje cells of the anterior lobe vermis (lobules I to V) and uvula/tonsil (lobules IX/HIX) express alpha 1a and alpha 2c subtypes, and Purkinje cells in the ansiform lobule (lobule HVII) and uvula/tonsil express alpha 1b and alpha 2c adrenergic receptor subtypes. Basket cells show a strong signal for alpha 1a, moderate signal for alpha 2a and light label for alpha 2b adrenergic receptor mRNA. In stellate cells, besides a strong label of alpha 2a adrenergic receptor mRNA in all and moderate label of alpha 2b message in select stellate cells, the inner stellate cells are also moderately positive for alpha 1b adrenergic receptor mRNA. Granule and Golgi cells express high levels of alpha 2a and alpha 2b adrenergic receptor mRNAs. These data contribute new information regarding specific location of adrenergic receptor subtypes in human cerebellar neurons. We discuss our observations in terms of possible modulatory roles of adrenergic receptor subtypes in cerebellar neurons responding to sensory and autonomic input signals, and review species differences in cerebellar adrenergic receptor expression.

Alterations in the Development of Rat Cerebellum and Impaired Behavior of Juvenile Rats after Neonatal 6-OHDA Treatment

The effects of neonatal systemic administration of the neurotoxin 6-hydroxydopamine (6-OHDA) on cerebellum development and behavior were studied in juvenile rats. The methods employed were immunohistochemistry, in situ hybridization, ligand binding, and behavioral testing. The results revealed, for the first time, that 6-OHDA treatment alters Bergmann glial cells and reduced the expression GABAA receptor subtypes alpha1 and alpha6 especially in granule cells. The Bergmann glial cells were abnormally located and structurally different (e.g., no intimate associations with Purkinje cells). Significant microglial activation was also observed. The animals showed impairment in behavior, especially in their orientation to a novel environment. Recent data on neuron-glia interactions support the conclusion that the observed structural changes in Bergmann glia and granular neurons disrupted the normal functioning of the Purkinje cells which then in turn resulted in the impaired sensory-motor coordination at least in juvenile rats. This paper is a summary of previously published work and some recent data in this field obtained at our laboratory.

Developmental neurotoxicity of chlorpyrifos: targeting glial cells

The pesticide chlorpyrifos (CPF) causes neurobehavioral damage, even at doses that do not elicit acute cholinergic toxicity. CPF disrupts the developing brain during glial proliferation and differentiation. Since glial cells play critical roles in brain development and function, we hypothesized that CPF neurotoxicity involves alteration of glial cell development. CPF effects in C6 glioma cells mirrored effects in the intact brain: inhibited DNA synthesis; interfered with adenylyl cyclase (AC) signaling; obstructed DNA binding to transcription factors involved in cell differentiation; and enhanced reactive oxygen species (ROS) formation. CPF was administered to prenatal and neonatal rats and examined for markers of astrocytes, oligodendrocytes, and neurons. Widespread effects were elicited by exposure during the peak period of gliogenesis. Males were preferentially targeted during postnatal exposures while females experienced delayed effects following gestational exposure, commensurate with behavioral outcomes. Alterations in glial cell development contribute to CPF neurotoxicity, extending vulnerability to myelination, synaptic plasticity, and architectural modeling, which continue into adolescence.

Cholinergic systems in brain development and disruption by neurotoxicants: nicotine, environmental tobacco smoke, organophosphates

Acetylcholine and other neurotransmitters play unique trophic roles in brain development. Accordingly, drugs and environmental toxicants that promote or interfere with neurotransmitter function evoke neurodevelopmental abnormalities by disrupting the timing or intensity of neurotrophic actions. The current review discusses three exposure scenarios involving acetylcholine systems: nicotine from maternal smoking during pregnancy, exposure to environmental tobacco smoke (ETS), and exposure to the organophosphate insecticide, chlorpyrifos (CPF). All three have long-term, adverse effects on specific processes involved in brain cell replication and differentiation, synaptic development and function, and ultimately behavioral performance. Many of these effects can be traced to the sequence of cellular events surrounding the trophic role of acetylcholine acting on its specific cellular receptors and associated signaling cascades. However, for chlorpyrifos, additional noncholinergic mechanisms appear to be critical in establishing the period of developmental vulnerability, the sites and type of neural damage, and the eventual outcome. New findings indicate that developmental neurotoxicity extends to late phases of brain maturation including adolescence. Novel in vitro and in vivo exposure models are being developed to uncover heretofore unsuspected mechanisms and targets for developmental neurotoxicants.

Late granule cell genesis in quail cerebellum

Proliferation of avian cerebellar neurons, including granule cells, is thought to be completed during embryonic life, and aspects of cell addition in cerebellar lobules in posthatching life are unknown. The present study tested the hypothesis that cell genesis in late embryonic and posthatching stages of quail cerebellum occurs in parallel with the performance of motor programs. After exposure to bromodeoxyuridine, short (20 hours) and long survival time points were selected to investigate survival and migration of labeled cells. Quantitative analysis of the lobular distribution of labeled cells was performed with the stereological disector method. External granular layer (EGL) proliferation did not cease after hatching, indicating that there is an extended posthatching period, lasting until P20, when cells can be added into the internal granular layer, modifying the cerebellar circuitry and function. Indeed, long survival experiments suggested that EGL-labeled cells migrated into the internal granular layer and survived for a prolonged time, although many of the progenitor cells remained in the EGL for days. Double-labeling experiments revealed that most of the late-generated granule cells were NeuN positive, but only few expressed nitric oxide synthase. In addition to granule cells, the white matter and a glutamic acid decarboxylase (GAD)-positive cell population in the molecular layer around Purkinje somata showed bromodeoxyuridine labeling. Although all lobules showed significant posthatching proliferation, an anteroposterior gradient was evident. The index of granule cell production and survival supports a spatiotemporal pattern, in correlation with the functional division of cerebellum into anterior and posterior domains.

Developmental regulation of nicotinic acetylcholine receptor-mediated [3H]norepinephrine release from rat cerebellum

Presynaptic modulation of synaptic transmission is the primary function of central nicotinic acetylcholine receptors (nAChRs) in developing and adult brain. nAChR activation regulates release of various neurotransmitters, including norepinephrine (NA). Given evidence that NA may serve a critical functional role in cerebellar development, we have undertaken studies to determine whether nAChRs modulate NA release in developing cerebellum. In vitro experiments using cerebellar slices examined the effects of nAChR stimulation on release of radiolabeled NA ([3H]NA). Our data indicate the presence of functional nAChRs on NA terminals in immature cerebellum and subsequent developmental regulation of receptor properties. During postnatal week one, the maximally effective dose of nicotine released 35.0 +/- 1.2% of cerebellar [3H]NA stores. There was a subsequent decline in maximal nicotine-stimulated NA release until postnatal day 30, when Emax values were statistically indistinguishable from adult. Although the efficacy of nicotine changed substantially throughout development, EC50 values did not differ significantly (EC50 = 4.4-12.0 micro m). Pharmacological analysis indicated that this developmental shift in maximum nicotine effect reflects a change in the properties of the nAChRs. These data support recent findings of a possible functional role of nAChRs in regulating cerebellar ontogeny, and provides further support for the role of NA as a neurotrophic factor during development.

Chlorpyrifos targets developing glia: effects on glial fibrillary acidic protein

The organophosphate pesticide, chlorpyrifos (CPF), is a developmental neurotoxicant. In cell cultures, CPF affects gliotypic cells to a greater extent than neuronotypic cells, suggesting that glial development is a specific target. We administered CPF to developing rats and examined the levels of glial fibrillary acidic protein (GFAP), an astrocytic marker. Prenatal CPF exposure (gestational days 17-20) elicited an increase in GFAP levels in fetal brain, but the effect was seen only at high doses that elicited maternal and fetal systemic toxicity. Early postnatal (PN) CPF treatment (PN1-4) elicited effects only in the cerebellum of male rats; GFAP was suppressed initially (PN5) and showed a rebound elevation (PN10) before returning to normal values by PN30. In contrast, when we administered CPF during the peak of gliogenesis and glial cell differentiation (PN11-14), GFAP was initially decreased across all brain regions and in both sexes; in males, subsequent elevations were seen on PN30, with the largest effect in the striatum; females also showed an increase in striatal GFAP. Our results indicate that CPF disrupts the pattern of glial development in vivo, with the maximum effect corresponding to the peak period of gliogenesis and glial cell differentiation. As glia are responsible for axonal guidance, synaptogenesis and neuronal nutrition, glial targeting suggests that these late-occurring developmental processes are vulnerable to CPF, extending the critical period for susceptibility into stages of synaptic plasticity, myelination, and architectural modeling of the developing brain.

Beta-adrenoceptor signaling in the developing brain: sensitization or desensitization in response to terbutaline

Beta(2)-adrenoceptor agonists are commonly used to arrest preterm labor but they also penetrate the placenta to stimulate fetal beta-adrenergic receptors (betaAR), and have been implicated in subsequent neurobehavioral deficits. We administered terbutaline to pregnant rats on gestational days (GD) 17-20 and during two postnatal (PN) periods, PN2-5 and PN11-14, that correspond to third trimester human neurological development. We then examined betaAR binding sites and adenylyl cyclase (AC) signaling in fetal brain or neonatal brain regions. Although fetal terbutaline administration evoked betaAR downregulation, the ability of isoproterenol to stimulate AC was enhanced instead of desensitized. Sensitization occurred at post-receptor signaling proteins, as augmented responses were also seen for stimulants that bypass the receptors to work on G-proteins (NaF) or that stimulate AC directly (forskolin and Mn(2+)). When terbutaline was given on PN2-5, betaAR downregulation was obtained in brainstem, forebrain and cerebellum, but desensitization of the AC response was seen only in the forebrain; the desensitization was heterologous, reflecting decrements in total AC activity rather than specific loss of the betaAR response. With treatment on PN11-14, only the cerebellum showed betaAR downregulation and induction at the level of post-receptor signaling proteins maintained the betaAR-mediated AC response. Our results indicate that, unlike the adult, betaAR signaling in the fetus and neonate is resistant to homologous desensitization by beta-agonists, and in fact, displays heterologous sensitization that sustains or enhances the overall response. The inability to desensitize betaAR responses may lead to disruption of neural cell development as a consequence of tocolytic therapy.

Does the developmental neurotoxicity of chlorpyrifos involve glial targets? Macromolecule synthesis, adenylyl cyclase signaling, nuclear transcription factors, and formation of reactive oxygen in C6 glioma cells

The widespread use of chlorpyrifos (CPF) has raised major concerns about its potential to cause fetal or neonatal neurobehavioral damage, even at doses that do not evoke acute toxicity. CPF has been shown to inhibit replication of brain cells, to elicit alterations in neurotrophic signaling governing cell differentiation and apoptosis, and to evoke oxidative stress. However, the specific cell types targeted by CPF have not been clarified, an issue of vital importance in establishing the boundaries of the critical period in which the developing brain is vulnerable. In the current study, we evaluated the effects of CPF on C6 glioma cells, a well-established glial model. In undifferentiated C6 cells, CPF inhibited DNA synthesis in a concentration-dependent manner, with greater potency than had been seen previously with neuronal cell lines. Just as found after in vivo CPF treatment or with neuronal cell lines, the effects on cell replication were independent of cholinergic stimulation, as cholinergic antagonists did not block CPF-induced inhibition. CPF interfered with cell signaling mediated through adenylyl cyclase at the level of G-protein function; the effects again were greater in undifferentiated C6 cells but were still detectable in differentiating cells. In contrast, differentiation enhanced the ability of CPF to elicit the formation of reactive oxygen species and to evoke deficits in Sp1, a nuclear transcription factor essential for differentiation. These results indicate that glial-type cells are targeted by CPF through the same multiple mechanisms that have been demonstrated for the effects of CPF on brain development in vivo. Because glial development continues long after the conclusion of neurogenesis, and given that CPF targets events in both glial cell replication and the later stages of differentiation, the vulnerable period for developmental neurotoxicity of CPF is likely to extend well into childhood.

Microglial response to the neurotoxicity of 6-hydroxydopamine in neonatal rat cerebellum

Depletion of noradrenaline in newborn rats by 6-hydroxydopamine (6-OHDA) affects the postnatal development and reduces the granular cell area in the neocerebellum (lobules V-VII). During the first postnatal month, Bergmann glial fibers guide the migration of immature granule cells to the internal granule cell layer. Microglia and Bergmann glia may play an important role in this process, but the exact mechanism behind this phenomenon is not known. We studied the effect of systemic administration of 6-OHDA on the expression and localization on microglia and Bergmann glia in the neonatal cerebellum by immunohistochemistry. In the neocerebellum, 6-OHDA treatment caused a significant increase in the number of activated microglia. The increase was observed mainly in the granule cell layer and the cerebellar medulla. Bergmann glial cells in treated brains were abnormally located, did not form intimate associations with Purkinje cells, and the glial fibers were structurally different. Our findings indicate that a noradrenergic influence may be necessary for the normal maturation and migration of granule cells, and abnormal migration may be the result of Bergmann glia destruction and the activation of microglia. Activated microglia in the granule cell layer may be used as a marker for an injured cerebellar area.

Neonatal 6-hydroxydopamine treatment affects GABA(A) receptor subunit expression during postnatal development of the rat cerebellum

Neurotoxic elimination of noradrenergic terminals by 6-hydroxydopamine (6-OHDA) leads to alteration of the granule cell layer formation. We have studied the developmental expression of GABA(A) receptor subunits in rat cerebellum after neonatal administration of 6-OHDA during the first postnatal month of life. 6-OHDA was injected subcutaneously. The expression of GABA(A) receptor subunits was studied by in situ hybridization and immunohistochemistry. The alterations were observed in the neocerebellum - the part of the cerebellum which starts development postnatally. The migration of granule cells was delayed, and the total area of the granule cell layer in the neocerebellum from 6-OHDA-treated rats was reduced to 22.6+/-5% of the corresponding area from control rats. In situ hybridization with subunit-specific antisense oligonucleotide probes was performed for alpha1, alpha2, alpha3, alpha5, alpha6, beta1, beta2, gamma1 and gamma2 subunits of the GABA(A) receptor. In neocerebellum, 6-OHDA treatment caused a significant reduction in the alpha1, alpha6 and gamma2 subunit mRNA levels. The expression of the other subunits was not changed. It has been shown that in the postnatal cerebellum alpha1 and alpha6 subunits can be detected in granule cells only when the cells had migrated to their final destination. Our findings indicate that a noradrenergic influence may be necessary for the normal maturation and migration of cerebellar granule cells.

Chlorpyrifos releases norepinephrine from adult and neonatal rat brain synaptosomes

Exposure of developing animals to apparently subtoxic doses of chlorpyrifos (CPF) during a critical period of synaptogenesis has been shown to affect catecholaminergic synaptic development and neuronal activity separably from its inhibition of cholinesterase. We used rat brain synaptosome preparations to examine whether CPF has a direct effect on the release of norepinephrine (NE). Synaptosomes were preloaded with [3H]NE in the absence of CPF and were then exposed to the compound during subsequent neurotransmitter release. There was a robust increase in release at 50 microg/ml of CPF. The effect was not mediated through cholinergic receptors, as neither atropine nor mecamylamine interfered with the actions of CPF. Enhanced NE release was seen in synaptosomes derived from neonatal rat brain as well as adult rat brain, albeit with a smaller effect in neonates. Our results suggest that CPF interacts directly with presynaptic nerve terminals to influence neurotransmitter release; in the context of the immature brain, these effects can alter synaptic development through the trophic actions of catecholamines.

Neonatal chlorpyrifos exposure alters synaptic development and neuronal activity in cholinergic and catecholaminergic pathways

After routine home application of chlorpyrifos (CPF), infant and child exposures can exceed acceptable levels. We treated neonatal rats daily on postnatal days (PN) 1-4 (1 mg/kg) or days 11-14 (5 mg/kg), treatments that evoked no overt signs of toxicity. Effects on the development of cholinergic neuronal function were assessed using choline acetyltransferase (ChAT) activity and hemicholinium-3 (HC-3) binding as indices of synaptic proliferation and synaptic activity, respectively. In the forebrain, early CPF treatment caused a decrease in ChAT without affecting HC-3 binding; late treatment decreased HC-3 binding without affecting ChAT. In the brainstem, early treatment had no effect on either parameter but late treatment decreased both ChAT and HC-3 binding. Effects of CPF were not limited to development of cholinergic synapses but also involved catecholamine pathways. For norepinephrine or dopamine, either early or late CPF treatment evoked an increase in synaptic activity (transmitter turnover). The cerebellum, a region with sparse cholinergic innervation, was affected the most. Effects on catecholamine systems were unrelated to the magnitude or temporal pattern of cholinesterase inhibition. Our results suggest that CPF exposure during the postnatal period of synaptogenesis elicits widespread disruption of cholinergic and catecholaminergic pathways. As this is the period in which patterns of synaptic responsiveness is programmed by neural input, the period of developmental vulnerability to CPF is likely to extend into childhood.