Induction of interleukin-1 associated with compensatory dopaminergic sprouting in the denervated striatum of young mice: model of aging and neurodegenerative disease

Differential inflammatory activation of IL-6 (−/−) astrocytes

IL-6 is a major immunomodulatory cytokine with neuroprotective activity. The absence of interleukin-6 (IL-6) results in increased vulnerability of dopaminergic neurons to the neurotoxicant, MPTP, and a compromised reactive microgliosis. To determine how astrogliosis may contribute to nigrostriatal degeneration in IL-6 (-/-) mice, the inflammatory profiles of astrocytes of IL-6 genotype were compared. Fourteen cytokines and four chemokines were simultaneously assayed in the supernatants of LPS-stimulated primary astrocyte cultures. In a time course of 6, 18 and 48 h and LPS stimulations of 0, 0.1, 1, 10 and 100 ng/ml, IL-6 (-/-) astrocytes secreted significantly greater amounts of the pro-inflammatory cytokines IL-1alpha, IL-1beta and TNFalpha than did IL-6 (+/+) cells. Elevated levels of IL-10 and IL-12p40 were only detected at 48 h post-stimulation with greater IL-10 in IL-6 (-/-) supernatants and greater IL-12p40 in IL-6 (+/+) supernatants. IL-6 (+/+) astrocytes produced more G-CSF and GM-CSF when compared with IL-6 (-/-) astrocytes. Chemokine levels were greater in supernatants of IL-6 (+/+) astrocytes than IL-6 (-/-) cells prior to 48 h post-stimulation. At that time, higher levels of MIP-1alpha were maintained in IL-6 (+/+) supernatant, while similar levels of MCP-1 in supernatants of both IL-6 (+/+) and IL-6 (-/-) cells were measured. Additionally, LPS (100 ng/ml) resulted in greater levels of KC and Rantes in IL-6 (-/-) astrocyte supernatants compared with IL-6 (+/+) supernatants at that time. These results suggest that the autocrine modulatory activities of IL-6 affect multiple cytokine secretory pathways, which could participate in neurodegenerative processes.

Glucocorticoid receptor–nitric oxide crosstalk and vulnerability to experimental parkinsonism: pivotal role for glia–neuron interactions

Inflammation and oxidative stress have been closely associated with the pathogenesis of neurodegenerative disorders, including Parkinson's disease (PD). The expression of inducible nitric oxide synthase (iNOS) in astrocytes and microglia and the production of large amounts of nitric oxide (NO) are thought to contribute to dopaminergic neuron demise. Increasing evidence, however, indicates that activated astroglial cells play key roles in neuroprotection and can promote recovery of CNS functions. Endogenous glucocorticoids (GCs) via glucocorticoid receptors (GRs) exert potent anti-inflammatory and immunosuppressive effects and are key players in protecting the brain against stimulation of innate immunity. Here we review our work showing that exposure to a dysfunctional GR from early embryonic life in transgenic (Tg) mice expressing GR antisense RNA represents a key vulnerability factor in the response of nigrostriatal dopaminergic neurons to the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and further report that exacerbation of dopaminergic neurotoxicity with no recovery is determined by failure of astroglia to exert neuroprotective effects. Aberrant iNOS gene expression and increased glia vulnerability to cell death characterized the response of GR-deficient mice to stimulation of innate immunity. More importantly, GR-deficient glial cells failed to protect fetal dopaminergic neurons against oxidative stress-induces cell death, whereas wild-type glia afforded neuroprotection. Thus, lack of iNOS/NO regulation by GCs can program an aberrant GR-NO crosstalk in turn responsible for loss of astroglia neuroprotective function in response to stimulation of innate immunity, pointing to glia and efficient GR-NO dialogue as pivotal factors orchestrating neuroprotection in experimental parkinsonism.

Neural mechanisms underlying motor dysfunction as detected by the tail suspension test in MPTP-treated C57BL/6 mice

Contradictory data on behavioral changes in MPTP-treated C57BL/6 mice have been reported, even though the toxin-treated mice have been widely used for non-clinical studies as an in vivo model of Parkinson's disease (PD). We found that the duration of immobility in the tail suspension test (TST) was significantly increased in MPTP-treated C57BL/6 mice as compared with control mice without a significant change in the locomotor activity (LA). Dopamine (DA) contents and protein levels of tyrosine hydroxylase and dopamine transporter in the striatum were profoundly decreased in the toxin-treated mice. These behavioral and neurobiochemical changes were almost completely inhibited by a pretreatment with deprenyl, a monoamine oxidase-B inhibitor. The stimulation of dopaminergic neurotransmission induced by L-dopa or a dopamine D2 receptor agonist ameliorated the increase in immobility time. Threshold level of striatal DA that produced the increase in immobility time in MPTP-treated mice was estimated to be between 11 and 27% of control level. We concluded that the increase in immobility time in the TST was induced by the nigrostriatal dopaminergic degeneration and was thought to be a consequence of motor dysfunction in this mouse model of PD.

Interleukine-1beta and interleukine-6 levels in striatum and other brain structures after MPTP treatment: influence of behavioral lateralization

MPTP (N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) induces diminution of the dopamine in nigrostriatal pathway and cognitive deficits in mice. MPTP treatment also increases pro-inflammatory cytokine production in substantia nigra and striatum. Since, pro-inflammatory cytokines influence striatal dopamine content and provoke cognitive impairments, the cognitive defects induced by MPTP may be partly due to brain cytokine induction in other structures than nigrostriatal pathway. Furthermore, behavioral lateralization, as assessed by paw preference, influences cytokine production at the periphery and in the central nervous system. Behavioral lateralization may thus influence brain cytokine levels after MPTP. In order to address these issues, mice selected for paw preference were injected with 25 mg/kg MPTP i.p. for five consecutive days after which striatal dopamine and DOPAC contents were measured by HPLC and IL-1beta and IL-6 quantified by ELISA in the striatum, cerebral cortex, hippocampus and hypothalamus. The results showed that MPTP treatment induced dramatic loss of DA in striatum, simultaneously, IL-6 levels decreased in the striatum and increased in hippocampus and hypothalamus, while IL-1beta levels decreased in the striatum, cerebral cortex and hippocampus. Interestingly, striatal dopamine turnover under basal conditions as well as striatal IL-1beta and IL-6 levels under basal conditions and after MPTP depended on behavioral lateralization. Left pawed mice showed a higher decrease in dopamine turnover and lower cytokine levels as compared to right pawed animals. Behavioral lateralization also influenced IL-6 hippocampal levels under basal conditions and IL-1beta cortical levels after MPTP. From these results, it can be concluded that MPTP-induced cognitive defects are accompanied by an alteration of pro-inflammatory cytokine levels in brain structures other than those involved in the nigrostriatal pathway. In addition, MPTP-induced dopamine decrease is influenced by behavioral lateralization, possibly through an effect on brain cytokine levels.

Perinatal inflammatory cytokine challenge results in distinct neurobehavioral alterations in rats: implication in psychiatric disorders of developmental origin

Maternal stress, viral infection, and obstetric complications, which trigger cytokine signaling, are hypothesized to be involved in schizophrenia and its related disorders. The etiologic contribution of individual cytokines to such psychiatric disorders, however, remains to be evaluated. To estimate the impact of peripheral cytokine challenge on neurobehavioral development, we examined effects of four proinflammatory cytokines on rat neonates and their later behavioral performance. Sublethal doses of interleukin-1 alpha, interleukin-2, interleukin-6, or interferon-gamma were subcutaneously administered to rat pups for 9 days. These animals displayed alterations in physical development, including lower weight gain and/or accelerated eyelid opening. In addition, behavioral abnormalities related to fear/anxiety levels and sensorimotor gating emerged at different developmental stages, depending on the cytokine species administered. During juvenile stages, neonatal interleukin-2 treatment increased exploratory locomotor activity, whereas other cytokine treatments did not. At the post-puberty stage, however, the interleukin-2-induced abnormal motor activity became undetectable, whereas interleukin-1 alpha-treated rats developed abnormalities in startle response, prepulse inhibition (PPI), and social interaction. Subchronic treatment of an anti-psychotic drug, clozapine, ameliorated the impairment of prepulse inhibition without altering startle responses. These animal experiments illustrate that, during early postnatal development, inflammatory cytokine challenge in the periphery can induce future psycho-behavioral and/or cognitive impairments with various latencies, although the pathologic mechanisms underlying these abnormalities remain to be determined.

Double-knockout mice for alpha- and beta-synucleins: effect on synaptic functions

An abundant presynaptic protein, alpha-synuclein, is centrally involved in the pathogenesis of Parkinson's disease. However, conflicting data exist about the normal function of alpha-synuclein, possibly because alpha-synuclein is redundant with the very similar beta-synuclein. To investigate the functions of synucleins systematically, we have now generated single- and double-knockout (KO) mice that lack alpha- and/or beta-synuclein. We find that deletion of synucleins in mice does not impair basic brain functions or survival. We detected no significant changes in the ultrastructure of synuclein-deficient synapses, in short- or long-term synaptic plasticity, or in the pool size or replenishment of recycling synaptic vesicles. However, protein quantitations revealed that KO of synucleins caused selective changes in two small synaptic signaling proteins, complexins and 14-3-3 proteins. Moreover, we found that dopamine levels in the brains of double-KO but not single-KO mice were decreased by approximately 20%. In contrast, serotonin levels were unchanged, and dopamine uptake and release from isolated nerve terminals were normal. These results show that synucleins are not essential components of the basic machinery for neurotransmitter release but may contribute to the long-term regulation and/or maintenance of presynaptic function.

Glial responses associated with dopaminergic striatal reinnervation following lesions of the rat substantia nigra

Lesioning of dopaminergic substantia nigra pars compacta (SNpc) neurons leads to depletion of dopamine (DA) and dopaminergic axons in the dorsal striatum, followed by subsequent compensatory sprouting of dopaminergic fibers and striatal reinnervation. In this study, the response of striatal glia (microglia and astroglia) was compared with the degeneration and regeneration of dopaminergic axons following SNpc lesions. Following partial SNpc lesions, density of dopamine transporter (DAT) immunoreactive (-ir) terminals in the dorsal striatum returned to normal within 16 weeks of injury, suggesting that dopaminergic reinnervation of the striatum was complete. In conjunction, the glial responses in the dorsal striatum consisted of two peaks. The first peak in glial density occurred immediately after lesioning, peaking at 7 days, implying that it was likely to be associated with removal of debris from degenerating terminals. The second glial response commenced 8 weeks after lesioning and peaked some time after 16 weeks. The time of onset of the second peak suggests that it may be associated with the establishment of synapses rather than with axonal guidance.

Microglial activation precedes dopamine terminal pathology in methamphetamine-induced neurotoxicity

Previous studies have demonstrated methamphetamine (METH)-induced toxicity to dopaminergic and serotonergic axons in rat striatum. Although several studies have identified the nature of reactive astrogliosis in this lesion model, the response of microglia has not been examined in detail. In this investigation, we characterized the temporal relationship of reactive microgliosis to neuropathological alterations of dopaminergic axons in striatum following exposure to methamphetamine. Adult male Sprague-Dawley rats were administered a neurotoxic regimen of methamphetamine and survived 12 h, or 1, 2, 4, and 6 days after treatment. Immunohistochemical methods were used to evaluate reactive changes in microglia throughout the brain of methamphetamine-treated rats, with a particular focus upon striatum. Pronounced morphological changes, indicative of reactive microgliosis, were evident in the brains of all methamphetamine-treated animals and were absent in saline-treated control animals. These included hyperplastic changes in cell morphology that substantially increased the size and staining intensity of reactive microglia. Quantitative analysis of reactive microglial changes in striatum demonstrated that these changes were most robust within the ventrolateral region and were maximal 2 days after methamphetamine administration. Analysis of tissue also revealed that microglial activation preceded the appearance of pathological changes in striatal dopamine fibers. Reactive microgliosis was also observed in extra-striatal regions (somatosensory and piriform cortices, and periaqueductal gray). These data demonstrate a consistent, robust, and selective activation of microglia in response to methamphetamine administration that, at least in striatum, precedes the appearance of morphological indicators of axon pathology. These observations raise the possibility that activated microglia may contribute to methamphetamine-induced neurotoxicity.

Tyrosine hydroxylase and dopamine transporter expression following 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced neurodegeneration of the mouse nigrostriatal pathway

Administration of the neurotoxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to C57BL/6 mice targets nigrostriatal dopaminergic neurons, leading to cell death and the depletion of striatal dopamine. After MPTP lesioning in young adult mice, surviving nigrostriatal dopaminergic neurons display robust and reproducible return of striatal dopamine weeks to months after injury. Thus, the mouse provides an excellent model with which to investigate the mechanisms underlying neuroplasticity of the nigrostriatal system following neurotoxic injury. The purpose of this study was to analyze proteins and mRNA transcripts of genes involved in dopamine biosynthesis (tyrosine hydroxylase; TH) and uptake (dopamine transporter; DAT) with regard to time course (7-90 days) after MPTP lesioning. Molecular analysis using immunohistochemistry and Western immunoblotting techniques demonstrated an increase in striatal TH by 30-60 days postlesioning that returned to near-control (prelesioned) levels by 60-90 days. In situ hybridization histochemistry indicated that this increase in TH protein might be due in part to increased TH mRNA expression in surviving nigrostriatal dopaminergic neurons. Analysis of TH protein at 7, 30, 60, and 90 days postlesioning with two-dimensional polyacrylamide gel electrophoresis in conjunction with Western immunoblotting revealed altered TH protein isoforms migrating at isoelectric points different from those of the native isoform. In contrast to TH protein, which returned to prelesioned levels by 60 days, DAT protein analysis showed that increased expression of striatal DAT protein did not return to near-prelesion levels until 90 days postlesioning. These results suggest that TH and DAT may differ in their time course of expression in surviving dopaminergic neurons and may play a role in mediating the return of striatal dopamine.

Identification of brain-derived neurotrophic factor in nestin-expressing astroglial cells in the neostriatum of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated mice

Up-regulation of nestin expression was significantly induced in the caudate-putamen of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice in our previous observation [Brain Res 925 (2002) 9]. We hypothesized that the nestin-expressing cells might play an important role in the pathogenesis of parkinsonian model, and characterization of these nestin-expressing cells was studied by RT-PCR, immunohistochemistry and semi-quantitative analysis for various markers of glial fibrillary acid protein (GFAP), S-100, neuronal nuclear specific protein (NeuN), beta-tubulin, Ki-67 and brain-derived neurotrophic factor (BDNF) expression in MPTP-treated C57/BL mice. Firstly, significant increasing in both nestin protein and mRNA was found in MPTP-treated mice. Up-regulation of nestin expression started at day 1, peaked at day 3, and gradually went down at days 7-21 in the neostriatum after MPTP treatment. Secondly, double immunofluorescence indicated that almost all of nestin-positive cells exhibited GFAP (98%) or S-100 (96%)-immunoreactivity, whereas NeuN or beta-tubulin was hardly detected in these nestin-positive cells. Thirdly, a minor population (7.0%) of nestin-positive cells showed Ki-67 (cell proliferation marker)-immunoreactivity, showing some of them went into cell mitotic state. Finally but more interestingly, a major population (86%) of nestin-expressing cells also exhibited immunoreactivity for BDNF, one neurotrophic factor. These results present time-dependent up-regulation of nestin expression in neostriatum, the proliferative and neurotrophic properties of nestin-expressing astroglial cells in MPTP-treated C57/BL mice. Taken together with previous observations, this study suggests that nestin-expressing activated astroglial cells, possibly partially through synthesizing and releasing neurotrophic factors such as BDNF in the basal ganglia, may play important roles in protection of nigrostriatal dopamine neurons and in the pathogenesis of Parkinson's disease in mammals.

Glucocorticoid receptor deficiency increases vulnerability of the nigrostriatal dopaminergic system: critical role of glial nitric oxide

Glucocorticoids (GCs) exert via glucocorticoid receptors (GRs) potent anti-inflammatory and immunosuppressive effects. Emerging evidence indicates that an inflammatory process is involved in dopaminergic nigro-striatal neuronal loss in Parkinson's disease. We here report that the GR deficiency of transgenic (Tg) mice expressing GR antisense RNA from early embryonic life has a dramatic impact in "programming" the vulnerability of dopaminergic neurons to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The GR deficiency of Tg mice exacerbates MPTP-induced toxicity to dopaminergic neurons, as revealed by both severe loss of tyrosine hydroxylase positive nigral neurons and sharp decreases in striatal levels of dopamine and its metabolites. In addition, the late increase in dopamine oxidative metabolism and ascorbic acid oxidative status in GR-deficient mice was far greater than in wild-type (Wt) mice. Inducible nitric oxide synthase (iNOS) was sharply increased in activated astrocytes, macrophages/microglia of GR-deficient as compared with Wt mice. Moreover, GR-deficient microglia produced three- to fourfold higher nitrite levels than Wt mice; these increases preceded the loss of dopaminergic function and were resistant to GR the inhibitory effect of GC, pointing to peroxynitrites as candidate neurotoxic effectors. The iNOS inhibitor N6-(1-iminoethyl)-L-lysine normalized vulnerability of Tg mice, thus establishing a novel link between genetic impairment of GR function and vulnerability to MPTP.

Time-course of the expression of inflammatory cytokines and matrix metalloproteinases in the striatum and mesencephalon of mice injected with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, a dopaminergic neurotoxin

Injection of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice results in a retrograde nigrostriatal dopaminergic pathway denervation and subsequent tissue reorganization. Since the role of inflammatory mediators after MPTP remains unclear, proinflammatory cytokine and matrix metalloproteinase (MMP) expression were evaluated by comparative RT-PCR during denervation and tissue reorganization following a single-dose of MPTP (40 mg/kg, s.c.) in young (8-week-old) mice. The time-course of denervation/reorganization was assessed through [(3)H]GBR-12935 binding on dopamine transporter and tyrosine hydroxylase immunohistochemistry. In the striatum, TNF-alpha, IL-1alpha, IL-1beta, IL-6 and MMP-9 mRNA expression peaked on day 1. In the ventral mesencephalon, cytokines (TNF-alpha, IL-1alpha, IL-1beta) and MMP-9 mRNA expression peaked on day 3. During tissue reorganization (day 6 through 16), the only change observed in the striatum consisted of IL-1alpha mRNA and protein overexpression together with MMP-2 downregulation. Whereas the early expression of proinflammatory cytokines and MMP might participate in the retrograde nigrostriatal denervation, the late component of IL-1alpha expression suggests a possible role for this cytokine in the subsequent striatal reorganization.

Human neural stem cell transplantation in the MPTP-lesioned mouse

Human neural stem cells have exhibited a remarkable versatility to respond to environmental signals. Their characterization in models of neurotoxic injury may provide insight into human disease treatment paradigms. This study investigates the survival and migration of transplanted human stem cells and tyrosine hydroxylase immunoreactivity in the parkinsonian 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned mouse model, using antisera recognizing human nuclear protein (hNuc) and tyrosine hydroxylase (TH). Our results indicate long-term (up to 90 days) survival of human stem cell xenograft in the MPTP-lesioned mouse and the presence of hNuc-immunoreactive cells at sites distal to the transplant core. Few TH-positive cells are identified in the striatum by immunoperoxidase staining and using immunofluorescent double labeling, infrequent TH-immunoreactive, transplanted cells are identified.

Grid performance test to measure behavioral impairment in the MPTP-treated-mouse model of parkinsonism

Behavioral impairments in mice following administration of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) require large depletions in striatal dopamine content and are often transient. In this paper, we describe a simple and inexpensive test that measures long-term behavioral deficits in mice treated with moderate doses of MPTP. These measures are significantly correlated with the loss of striatal dopamine and immunoreactivity of the dopamine transporter, vesicular monoamine transporter and tyrosine hydroxylase. In addition, behavioral impairments on the measures were reversed following L-DOPA administration. Employment of this test will allow for more efficacious use of mice in PD research, as well as provide more sensitive measures of behavioral improvement following potential therapeutic or neuroprotective interventions.

D2 Dopamine receptor blockade results in sprouting of DA axons in the intact animal but prevents sprouting following nigral lesions

Recently it was demonstrated that sprouting of dopaminergic neurons and a microglial and astrocyte response follows both partial lesions of the substantia nigra pars compacta and blockade of the D2 dopamine receptor. We therefore studied the effects of the combination of these two treatments (lesioning and D2 dopamine receptor blockade). Haloperidol administration caused a 57% increase in dopaminergic terminal tree size (measured as terminal density per substantia nigra pars compacta neuron) and an increase of glia in the striatum. Following small to medium nigral lesions (less than 60%), terminal tree size increased by 51% on average and returned density of dopaminergic terminals to normal. In contrast, administration of haloperidol for 16 weeks following lesioning resulted in reduced dopaminergic terminal density and terminal tree size (13%), consistent with absent or impaired sprouting. Glial cell numbers increased but were less than with lesions alone. When haloperidol was administered after the striatum had been reinnervated through sprouting (16-32 weeks after lesioning), terminal tree size increased up to 150%, similar to the effect of haloperidol in normal animals. By examining the effect of administering haloperidol at varying times following a lesion, we concluded that a switch in the effect of D2 dopamine receptor blockade occurred after dopaminergic synapses began to form in the striatum. We postulate that when synapses are present, D2 dopamine receptor blockade results in increased terminal density, whereas prior to synapse formation D2 dopamine receptor blockade causes attenuation of a sprouting response. We speculate that D2 dopamine receptors located on growth cones 'push' neurites toward their targets, and blockade of these receptors could lead to attenuation of sprouting.

Immunity and schizophrenia: autoimmunity, cytokines, and immune responses

As is evident from the present account, there is no single or persuasive argument that signals emanating from the immune system are directly involved in the etiology of schizophrenia. We do not even know if we are dealing with a single disorder with a single causality; almost certainly we are not. The precise etiology of schizophrenia, as with so many neurological disorders, remains obscure. However, there is abundant evidence in schizophrenia of mutual dysregulation of neuronal function and immune system activity. Although this evidence is not always consistent, a pattern emerges suggesting aspects of immune activity being involved in the pathology of neuronal development that characterizes schizophrenia. Exposure to infective agents, HLA associations, autoimmune associations, disturbances in lymphocyte populations, and cytokine imbalances with a skew toward Th2 activity are supportive of this view. That the evidence is not always consistent is a testament to the complexity and heterogeneity of the disorder, to confounding by antipsychotics that themselves are immunomodulatory, and to the multifaceted nature, with all its checks and balances, of the immune system itself.

Increased vulnerability of dopaminergic neurons in MPTP-lesioned interleukin-6 deficient mice

To test the hypothesis that neuroinflammation contributes to dopaminergic neuron death in the MPTP-lesioned mouse, we compared nigrostriatal degeneration in interleukin (IL)-6 (+/+) with IL-6 (-/-) mice. In the absence of IL-6, a single injection of MPTP (30 mg/kg) resulted in significantly greater striatal dopamine depletion than that measured in IL-6 (+/+) mice. The observed dopamine depletion was MPTP dose dependent. This loss of striatal dopamine and a significantly greater loss of TH+ cells in the substantia nigra pars compacta in IL-6 (-/-) mice as compared with control IL-6 (+/+) mice, suggest that IL-6 is neuroprotective in the MPTP-lesioned nigrostriatal system. Co-localization experiments identified striatal astrocytes as the source of IL-6 in IL-6 (+/+) mice at 1 and 7 days postinjection of MPTP. The increased sensitivity of dopaminergic neurons to neurotoxicant in the absence of IL-6, is compatible with a neuroprotective activity of IL-6 in the injured nigrostriatal system.

The role of interleukin-1, interleukin-6, and glia in inducing growth of neuronal terminal arbors in mice

After injury to the substantia nigra pars compacta (SNpc), remaining neurons sprout to ensure normal dopamine delivery to the striatum. The consequent striatal reinnervation is highly regulated, with remaining cells sprouting so that density of dopamine terminals returns to normal. Sprouting as a result of injury is accompanied by a strong glial response; however, it is difficult to know whether this response is as a result of the injury or whether it is aiding in the sprouting. The two cytokines interleukin-1 (IL-1) and interleukin-6 (IL-6) are important modulators of the glia response. This study demonstrates their role in regulating the sprouting of dopaminergic neurons and the associated glia response as a means to examine the role of glia in sprouting. Sprouting was induced by 6-hydroxydopamine lesions of the SNpc and by haloperidol treatment (in the absence of injury). In wild-type animals, sprouting in association with microglial and astrocyte proliferation followed partial lesions of the SNpc and haloperidol treatment. Neither treatment evoked sprouting or glia proliferation in the type I IL-1 receptor-deficient mice, whereas in IL-6-deficient mice, both treatments resulted in glial proliferation but not sprouting. We conclude that IL-1 plays a role in modulating glia proliferation and thereby guidance and trophic factors for new fibers, whereas IL-6 may be important in triggering the outgrowth of new fibers. This study demonstrates that these cytokines play an important role in plasticity and regeneration that is separate from the inflammatory response associated with brain injury.

Effects of long-term treatment with dopamine receptor agonists and antagonists on terminal arbor size

This study demonstrates that pharmacological manipulation of the dopamine (DA) receptors can modulate the size of the axonal tree of substantia nigra pars compacta (SNpc) neurons in mice. Pharmacological blockade or genetic ablation of the D2 receptor (D2R) resulted in sprouting of DA SNpc neurons whereas treatment with a D2 agonist resulted in pruning of the terminal arbor of these neurons. Agents such as cocaine, that indirectly stimulate D2R, also resulted in reduced terminal arbor. Specific D1 agonists or antagonists had no effect on the density of DA terminals in the striatum. We conclude that the D2 receptor has a central role in regulating the size of the terminal arbor of nigrostriatal neurons. These findings have implications relating to the use of dopaminergic agonists in the management of Parkinson's disease and in controlling plasticity following injury, loss or transplantation of DA neurons.

Guidance of dopaminergic neuritic growth by immature astrocytes in organotypic cultures of rat fetal ventral mesencephalon

Astrocytes, with their many functions in producing and controlling the environment in the brain, are of great interest when it comes to studying regeneration after injury and neurodegenerative diseases such as in grafting in Parkinson's disease. This study was performed to investigate astrocytic guidance of growth derived from dopaminergic neurons using organotypic cultures of rat fetal ventral mesencephalon. Primary cultures were studied at different time points starting from 3 days up to 28 days. Cultures were treated with either interleukin-1 beta (IL-1 beta), which has stimulating effects on astrocytic proliferation, or the astrocytic inhibitor cytosine arabinoside (Ara-C). Tyrosine hydroxylase (TH)-immunohistochemistry was used to visualize dopaminergic neurons, and antibodies against glial fibrillary acidic protein (GFAP) and S100 beta were used to label astrocytes. The results revealed that a robust TH-positive nerve fiber production was seen already at 3 days in vitro. These neurites had disappeared by 5 days. This early nerve fiber outgrowth was not guided by direct interactions with glial cells. Later, at 7 days in vitro, a second wave of TH-positive neuritic outgrowth was clearly observed. GFAP-positive astrocytic processes guided these neurites. TH-positive neurites arborized overlying S100 beta-positive astrocytes in an area distal to the GFAP-positive astrocytic processes. Treatment with IL-1 beta resulted in an increased area of TH-positive nerve fiber network. In cultures treated with Ara-C, neither astrocytes nor outgrowth of dopaminergic neurites were observed. In conclusion, this study shows that astrocytes play a major role in long-term dopaminergic outgrowth, both in axonal elongation and branching of neurites. The long-term nerve fiber growth is preceded by an early transient outgrowth of dopamine neurites.

Significant up-regulation of nestin protein in the neostriatum of MPTP-treated mice. Are the striatal astrocytes regionally activated after systemic MPTP administration?

We are interested in the possible role of central glial cells in pathogenesis of Parkinson's disease of mammals. Parkinsonism model was induced by systemic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration, and the reactive glial cells were examined by immunocytochemical visualization of nestin protein in the brains and spinal cords of C57 mice. Abundant nestin-like immunoreactivity was predominately found in the caudate putamen of MPTP-treated mice and about 481-fold of nestin-like immunoreactive cells increased compared with that of control animals, indicating that significant up-regulation of nestin protein occurred in these regions. Majority of nestin-like immunoreactive cells characterized with astrocytic profiles of multiple, radical and hypotrophic processes, and showed a distribution and dynamic patterns similar to that of glial fibrillary acid protein (GFAP)-immunoreactive cells in the caudate putamen. Double immunofluorescence confirmed that 100% of nestin-like immunoreactive cells exhibited GFAP-immunoreactivity while nestin/GFAP double-labeled cells constituted about 84% of total GFAP-immunoreactive cells in the caudate putamen, indicating these nestin-like immunoreactive cells belong to a reactive population of the astrocytes. On the other hand, no obvious changes of nestin- or GFAP-like immunoreactivities were detected in the globus pallidus, the substantia nigra and the ventral tegmental area after MPTP-treatment. The results have provided morphological evidence for the regional activation of astrocytic glial cells following systemic MPTP administration, suggesting that a large population of reactive striatal astrocytes might play an important role in initial pathogenesis or acute stage of Parkinson's disease in mammals.

The role of dopamine receptors in regulating the size of axonal arbors

Factors that regulate terminal arbor size of substantia nigra pars compacta (SNpc) neurons during development and after injury are not well understood. This study examined the role of dopamine receptors in regulating arbor size. Terminal arbors were examined in mice with targeted deletion of the D1 or D2 dopamine receptor [D1(-/-) and D2(-/-) mice, respectively]. Terminal trees were also examined after treatment with receptor blockers and after partial SNpc lesions. Immunohistochemistry was performed, and the number of SNpc neurons and dopaminergic terminals in the striatum was estimated. The number of dopaminergic SNpc neurons were reduced in D1(-/-) and D2(-/-) mice. Density of dopaminergic terminals was unchanged in D1(-/-) mice and increased in D2 (-/-) mice. Steady-state striatal DA and DOPAC levels revealed that dopamine activity was enhanced in D2(-/-) mice but reduced in D1(-/-) mice. Two months after partial SNpc lesions, striatal terminal density was normal in both wild-type and D1(-/-) mice but reduced in D2(-/-) mice. Administration of DA receptor antagonists resulted in larger terminal arbors in D1(-/-) and wild-type mice, whereas D2(-/-) mice showed no change in terminal density. Functional blockade of the D2R during development or in the adult brain results in increased axonal sprouting. Partial SNpc lesions resulted in compensatory sprouting, only in mice with functional D2R. These results suggest that individual dopaminergic axons in D2(-/-) mice have reached maximal arbor size. We conclude that the D2 receptor may play a role in modulating the extent of the terminal arbor of SNpc neurons.

Global ischemia induces expression of acid-sensing ion channel 2a in rat brain

Acid-sensing ion channels (ASICs) are ligand-gated cation channels that respond to acidic stimuli. They are expressed throughout the mammalian nervous system. In the peripheral nervous system, ASICs act as nociceptors, responding to the tissue acidosis that accompanies ischemic and inflammatory conditions. The function of ASICs in the central nervous system is not known. In this article, the authors present evidence that transient global ischemia induces ASIC 2a protein expression in neurons that survive ischemia. Western blot analysis with an anti-ASIC 2a antibody revealed up-regulation of an 80 kD protein in ischemic rat brain. Immunohistochemical analysis showed that ASIC 2a protein expression increased in neurons of the hippocampus and cortex. Klenow fragment-mediated labeling of DNA strand breaks determined that ASIC 2a induction did not occur in cells with detectable DNA damage. The current results suggest a possible role for ASICs in mediating a cellular response to ischemia.

Long-term induction of Fos-related antigen-2 after methamphetamine-, methylenedioxymethamphetamine-, 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine- and trimethyltin-induced brain injury

A long-term induction of Fos-related antigens has been shown in neurons after brain injury, suggesting that Fos-related antigens are involved in enhancing the transcription of genes related to the process of regeneration and repair. In the present study, we report that levels of Fos-related antigen-2 are elevated in several models of chemically induced brain injury. Trimethyltin, which causes degeneration of neurons primarily in the hippocampus and other limbic regions, results in a five-fold induction of Fos-related antigen-2 immunoreactivity in neurons in the pyramidal and dentate layers of the hippocampus starting at seven days post-treatment and persisting for 60days. Methamphetamine and methylenedioxymethamphetamine, agents which cause degeneration of dopaminergic nerve terminals in the striatum of the mouse, cause an increase in Fos-related antigen-2 immunoreactivity which begins at three days post-treatment and returns to basal levels by days 5 and 15, respectively. Treatment with 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine elevated levels of Fos-related antigen-2 in the mouse striatum at three days post-treatment. This abbreviated time-course of Fos-related antigen-2 induction is consistent with less severe insult (terminal damage) relative to trimethyltin (cell death), but induction occurs during the period of regeneration and repair in both models. Dexfenfluramine, a non-neurotoxic amphetamine, does not induce Fos-related antigen-2 expression. Decreasing core temperature of the mouse, which blocks amphetamine-induced neurotoxicity, also blocks Fos-related antigen-2 induction. In summary, Fos-related antigen-2 is induced in models of both cell death and terminal degeneration, suggesting that this transcription factor may serve as a universal signal transduction molecule involved in the regulation of genes related to regeneration and repair in the CNS.

Spontaneous long-term compensatory dopaminergic sprouting in MPTP-treated mice

The present study sought to determine whether severe 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication elicits spontaneous long-term compensatory sprouting in mice. Animals, once treated, were kept without further treatment for 0.5, 1, 5, or 7 months. The stability of the nigral degeneration was checked by evaluation of the number of tyrosine hydroxylase immunoreactive (TH-IR) neurons, whereas sprouting was assessed using both [(3)H]-dopamine (DA) uptake by striatal synaptosomes and optical density of TH-immunolabeled fibers in the striatum as markers. At 0.5 month after MPTP intoxication (80 mg/kg, i.p.), we observed comparable decreases of 83% in DA uptake, 83.3% in TH fiber density, and 74% in the number of TH-IR neurons compared to age-matched saline-treated animals. From 5 months onwards, both DA uptake and striatal TH fiber density increased significantly (50% and 34.9% at 5 months, 65% and 67.4% at 7 months, respectively) in comparison with age-matched saline-treated animals, although the number of TH-IR neurons remained stable (73% of degeneration at 7 months). These results indicate clearly that spontaneous long-term compensatory dopaminergic sprouting is a phenomenon that is not restricted to situations of partial nigral degeneration but can, on the contrary, constitute a response even to severe stable MPTP-induced nigral degeneration.

Adaptive changes in the nigrostriatal pathway in response to increased 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced neurodegeneration in the mouse

Although several adaptive mechanisms have been identified that mask the existence of Parkinson's disease and delay the onset and aggravation of motor symptoms, the timescale and implications of this compensatory process remain an enigma. In order to examine: (i) the nature of the dopaminergic adaptive mechanisms that come into action; (ii) their sequential activation in relation to the severity of degeneration; and (iii) their efficacy with regard to the maintenance of a normal level of basal ganglia activity, we analysed the brains of mice treated daily with 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP, 4 mg/kg, i.p.) and killed at 5-day intervals from day 0 (D0) to D20. Our results demonstrate the sequential activation of two compensatory mechanisms: (i) an increase in striatal tyrosine hydroxylase (TH) protein content attested by the persistence of TH immunolabelling up to D15, contrasting with the decrease observed in both the number of nigral TH-immunoreactive neurons (-70.2%) and striatal dopamine content (-38.4%); (ii) a downregulation of DA uptake in surviving terminals at D20 (73.4% of nigral degeneration). At this point, the failure of adaptive mechanisms to maintain striatal dopaminergic homeostasis is also illustrated by an increase in the cytochrome oxidase activity of substantia nigra pars reticulata, a marker of neuronal function. It has been postulated that an increase in dopamine release per pulse could constitute an adaptive mechanism. The data we present from our MPTP mice model infirm this hypothesis. This study explores the link between the degree of nigral degeneration and the sequential activation of dopaminergic compensatory mechanisms in the nigrostriatal pathway and, in so doing, proposes a rethink of the paradigm applied to these mechanisms.

Axonal sprouting following lesions of the rat substantia nigra

Parkinson's disease is characterized by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta. Symptoms do not appear until most nigral neurons are lost, implying that compensatory mechanisms are present. Sprouting has been proposed as one of these mechanisms. This study quantified the extent of compensatory axonal sprouting following injury of dopaminergic neurons within the substantia nigra pars compacta. Specifically, the extent of the axonal arbour and axonal varicosity morphology was measured after partial destruction (with 6-hydroxydopamine) of the substantia nigra of the adult male rat. Four months later, the substantia nigra was injected with the anterograde neuronal tracer dextran-biotin to trace the full extent of individual axons. An unbiased estimate of neuron number was performed in each animal. This demonstrated nigral neuronal loss ranging from 10 to 90% on the side that received the injection whilst a 7% reduction was observed in the side contralateral to the lesion. Coincident with this loss, some nigral neurons lose tyrosine hydroxylase expression. Vigorous axonal sprouting was observed in the terminal arbours of lesioned animals and was associated with an increased axonal varicosity size. Axonal varicosities and branching points were primarily confined to the dorsal 1.5mm of the caudate-putamen, an area predominantly innervated by nigral neurons. It appears that dopaminergic neurons were responsible for this sprouting because the density of dopamine transporter immunoreactive varicosities in the caudate-putamen was maintained until about a 70% loss of neurons. It was concluded that substantial compensation in the form of sprouting and new dopaminergic synapse formation occurs following lesions of the substantia nigra pars compacta.

Striatal dopaminergic correlates of stable parkinsonism and degree of recovery in old-world primates one year after MPTP treatment

Despite widespread use of the primate 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of Parkinson's disease, there is a paucity of data concerning the relationship between striatal dopaminergic function and behavior over time. This study examines the relationship between markers of dopamine neuron integrity and dopaminergic metabolic activity in striatal subregions with the degree of parkinsonian disability in 32 monkeys treated with MPTP one year earlier. Based on the parkinsonian summary score during the month following MPTP treatment, each monkey was assigned to one of four severity categories. We called these categories "Severe", "Moderate", "Mild" and "Asymptomatic". Monkeys in the Severe category were behaviorally stable, and loss of dopamine concentration was greater than 98% in all subregions of striatum one year after MPTP treatment. This value was not significantly different from the level of depletion, reported previously, at one to two months after MPTP in Severe monkeys, and apparently this loss of striatal dopamine is beyond the level from which effective compensations can occur. The parkinsonian disabilities in monkeys of other severity groups (Moderate, Mild, Asymptomatic) improved significantly over the year, despite having mean dopamine depletion of 75-99% in different subregions of striatum at one to two months after MPTP treatment. At one year after MPTP treatment, the mean dopamine depletions in different subregions of caudate nucleus and putamen had diminished in Asymptomatics (21-81%), Milds (35-96%), and Moderates (86-97%). Dopamine loss in nucleus accumbens was relatively spared compared with most striatal subregions, yet in Severe monkeys the decrease in this region reached 96%. In addition, at one year after MPTP treatment, there was a significant linear relationship between parkinsonian behavioral severity category and dopamine concentration, and homovanillic acid concentration and homovanillic acid/dopamine ratio in the striatum. The re-establishment of dopamine levels and homovanillic acid/dopamine ratios was most pronounced in putamen, ventromedial caudate nucleus and nucleus accumbens. Thus the small difference in striatal dopamine loss that distinguishes monkeys with widely different behavior at one to two months after MPTP increases over time. We suggest that the milder the initial loss, the greater capacity there is for regeneration or sprouting of dopamine terminals, which is reflected in marked increases in dopamine levels and modest elevations of metabolic activity (homovanillic acid/dopamine ratio). With greater initial losses, there is less capacity to increase terminal density, which is reflected later by smaller increases in striatal dopamine levels and more marked increases in metabolic activity. It appears that 5-10% of normal striatal dopamine levels is sufficient for overtly normal motor performance in non-human primates.

The absence of reactive astrocytosis is indicative of a unique inflammatory process in Parkinson's disease

Virtually any neurological disorder leads to activation of resident microglia and invasion of blood-borne macrophages, which are accompanied by an increase in number and change in phenotype of astrocytes, a phenomenon generally termed reactive astrocytosis. One of the functions attributed to activation of astrocytes is thought to involve restoration of tissue damage. Hitherto, the role of astrocytes in the inflammatory reaction occurring in Parkinson's disease has not received much attention. In the present study, we examined the inflammatory events in autopsies of the substantia nigra and putamen from Parkinson's disease patients using age-matched autopsies from normal patients as controls. In the substantia nigra, activation of microglia was consistently observed in all Parkinson's disease autopsies as verified from immunohistochemical detection of CR3/43 and ferritin. Activation of resident microglia was not observed in the putamen. No differences were observed between controls and Parkinson's disease autopsies from the substantia nigra and putamen, in terms of distribution, cellular density or cellular morphology of astrocytes stained for glial fibrillary acidic protein or metallothioneins I and II, the latter sharing high affinity for metal ions and known to be induced in reactive astrocytes, possibly to exert anti-oxidative effects. Together, these findings indicate that the inflammatory process in Parkinson's disease is characterized by activation of resident microglia without reactive astrocytosis, suggesting that the progressive loss of dopaminergic neurons in Parkinson's disease is an ongoing neurodegenerative process with a minimum of involvement of the surrounding nervous tissue. The absence of reactive astrocytosis in Parkinson's disease contrasts what follows in virtually any other neurological disorder and may indicate that the inflammatory process in Parkinson's disease is a unique phenomenon.

Neurotrophic factors in neurodegenerative disorders: model of Parkinson's disease

Neurotrophic factors are compounds that enhance neuronal survival and differentiation. Most of these compounds exert their pharmacological actions on selective types of neurons, and therefore, are considered promising new therapeutic agents for the treatment of different neurodegenerative disorders characterized by selective degeneration of certain neuronal groups. Those compounds have been used in humans for several neurological disorders including amyotrophic lateral sclerosis--ciliary derived neurotrophic factor (CNTF) and brain derived neurotrophic factor (BDNF), Alzheimer's disease and peripheral neuropathy--nerve growth factor (NGF) and Parkinson's disease (PD)--glial derived neurotrophic factor (GDNF). In spite of well founded clinical experiments by previous experimental work in animal models some of these trials have been negative. For instance, animal models of PD have shown that several neurotrophic factors, including GDNF and other compounds, reduce apoptosis and increase resistance of dopamine neurons to neurotoxins in vitro. These compounds prevent or recover the damage to dopamine neurons of rodents and primates produced by chemical or mechanical acute lesions including 6-OH-DA, MPTP, methamphetamine and axotomy. The differences between the promising results obtained in experimental models and the lack of clinical results or excessive toxicity found in humans could be attributed to the following reasons: (a) Lack of relevance between the pathogenesis of the experimental lesion and the corresponding neurodegenerative disorder. (b) Poor correlation between results obtained in acute, self-limited, selective deficit produced to experimental animals and those available in more complex, chronic and progressive disorders involving patients. (c) Inadequate delivery of the active product to the target area in the human brain. (d) Poor information from acute experiments in animals which does not predict long-term effects of chronic infusion in humans. Further experimental work, therefore, is needed to transfer these neurotrophic factors to the clinic.

Inflammatory mediators and stroke: new opportunities for novel therapeutics

Contrary to previous dogmas, it is now well established that brain cells can produce cytokines and chemokines, and can express adhesion molecules that enable an in situ inflammatory reaction. The accumulation of neutrophils early after brain injury is believed to contribute to the degree of brain tissue loss. Support for this hypothesis has been drawn from many studies where neutrophil-depletion blockade of endothelial-leukocyte interactions has been achieved by various techniques. The inflammation reaction is an attractive pharmacologic opportunity, considering its rapid initiation and progression over many hours after stroke and its contribution to evolution of tissue injury. While the expression of inflammatory cytokines that may contribute to ischemic injury has been repeatedly demonstrated, cytokines may also provide "neuroprotection" in certain conditions by promoting growth, repair, and ultimately, enhanced functional recovery. Significant additional basic work is required to understand the dynamic, complex, and time-dependent destructive and protective processes associated with inflammation mediators produced after brain injury. The realization that brain ischemia and trauma elicit robust inflammation in the brain provides fertile ground for discovery of novel therapeutic agents for stroke and neurotrauma. Inhibition of the mitogen-activated protein kinase (MAPK) cascade via cytokine suppressive anti-inflammatory drugs, which block p38 MAPK and hence the production of interleukin-1 and tumor necrosis factor-alpha, are most promising new opportunities. However, spatial and temporal considerations need to be exercised to elucidate the best opportunities for selective inhibitors for specific inflammatory mediators.

Reovirus type 3 clone 9 increases interleukin-1alpha level in the brain of neonatal, but not adult, mice

Reovirus Type 3 clone 9 (T3C9)-induced lethal encephalitis is age dependent. We examined the effects of T3C9 inoculated into neonatal and adult mice by intracerebral, intramuscular, or peroral routes and the effect of lipopolysaccharide (LPS) on IL-1alpha levels in the blood and the brain. In parallel, we measured mice survival to T3C9 challenge, primary replication, and growth in and spread to the brain. The results show that T3C9 infection increased IL-1alpha only in the brain of neonatal mice, whereas LPS enhanced IL-1alpha in the brain and in the blood in both neonatal and adult mice. In neonatal mice, a T3C9-induced IL-1alpha increase coincided with viral replication-induced nervous tissue injury and preceded death. Anti-IL-1alpha antibody partially protected neonatal mice against T3C9 peroral challenge, further suggesting that this cytokine is involved in the mechanisms leading to lethal encephalitis. In adult mice, T3C9 was not lethal and did not modify IL-1alpha levels although it slowly replicated in nervous tissues when inoculated directly into the brain. Together, these results suggest that differences in nervous tissue response to T3C9 replication between newborn and adult mice could account in part for the age-dependent susceptibility to T3C9-induced lethal encephalitis.