Prenatal lipopolysaccharide does not accelerate progressive dopamine neuron loss in the rat as a result of normal aging

Perinatal Inflammation Reprograms Neuroendocrine, Immune, and Reproductive Functions: Profile of Cytokine Biomarkers

Viral and bacterial infections causing systemic inflammation are significant risk factors for developing body. Inflammatory processes can alter physiological levels of regulatory factors and interfere with developmental mechanisms. The brain is the main target for the negative impact of inflammatory products during critical ontogenetic periods. Subsequently, the risks of various neuropsychiatric diseases such as Alzheimer's and Parkinson's diseases, schizophrenia, and depression are increased in the offspring. Inflammation-induced physiological disturbances can cause immune and behavioral disorders, reproductive deficiencies, and infertility. The influence of maternal immune stress is mediated by the regulation of pro-inflammatory cytokines such as interleukin (IL)-1β, IL-6, monocyte chemotactic protein 1, leukemia-inhibiting factor, and tumor necrosis factor-alpha secretion in the maternal-fetal system. The increasing number of patients with neuronal and reproductive disorders substantiates the identification of biomarkers for these disorders targeted at their therapy.

Maternal Immune Activation and the Development of Dopaminergic Neurotransmission of the Offspring: Relevance for Schizophrenia and Other Psychoses

Prenatal infections have been linked to the development of schizophrenia (SCZ) and other neurodevelopmental disorders in the offspring, and work in animal models indicates that this is to occur through the maternal inflammatory response triggered by infection. Several studies in animal models demonstrated that acute inflammatory episodes are sufficient to trigger brain alterations in the adult offspring, especially in the mesolimbic dopamine (DA) system, involved in the pathophysiology of SCZ and other disorders involving psychosis. In the current review, we synthesize the literature on the clinical studies implicating prenatal infectious events in the development of SCZ. Then, we summarize evidence from animal models of maternal immune activation (MIA) and the behavioral and molecular alterations relevant for the function of the DAergic system. Furthermore, we discuss the evidence supporting the involvement of maternal cytokines, such as interleukin 6 (IL-6) and leptin (a hormone with effects on inflammation) in mediating the effects of MIA on the fetal brain, leading to the long-lasting effects on the offspring. In particular, IL-6 has been involved in mediating the effects of MIA animal models in the offspring through actions on the placenta, induction of IL-17a, or triggering the decrease in non-heme iron (hypoferremia). Maternal infection is very likely interacting with additional genetic and environmental risk factors in the development of SCZ; systematically investigating how these interactions produce specific phenotypes is the next step in understanding the etiology of complex psychiatric disorders.

Brain changes in a maternal immune activation model of neurodevelopmental brain disorders

The developing brain is sensitive to a variety of insults. Epidemiological studies have identified prenatal exposure to infection as a risk factor for a range of neurological disorders, including autism spectrum disorder and schizophrenia. Animal models corroborate this association and have been used to probe the contribution of gene-environment interactions to the etiology of neurodevelopmental disorders. Here we review the behavior and brain phenotypes that have been characterized in MIA offspring, including the studies that have looked at the interaction between maternal immune activation and genetic risk factors for autism spectrum disorder or schizophrenia. These phenotypes include behaviors relevant to autism, schizophrenia, and other neurological disorders, alterations in brain anatomy, and structural and functional neuronal impairments. The link between maternal infection and these phenotypic changes is not fully understood, but there is increasing evidence that maternal immune activation induces prolonged immune alterations in the offspring's brain which could underlie epigenetic alterations which in turn may mediate the behavior and brain changes. These concepts will be discussed followed by a summary of the pharmacological interventions that have been tested in the maternal immune activation model.

Valproate is protective against 6-OHDA-induced dopaminergic neurodegeneration in rodent midbrain: A potential role of BDNF up-regulation

BACKGROUND/PURPOSE

The main purpose of this study was to extend previously reported showing potent neuroprotective effect of valproic acid (VPA) in primary midbrain neuro-glial cultures to investigate whether VPA could protect dopamine (DA) neurons in vivo against 6-hydroxydopamine (6-OHDA)-induced neurodegeneration and to determine the underlying mechanism.

METHODS

Male adult rats received a daily intraperitoneal injection of VPA or saline for two weeks before and after injection of 5, 10, or 15 μg of 6-OHDA into the brain. All rats were evaluated for motor function by rotarod performance. Brain samples were prepared for immunohistochemical staining and for determination of levels of dopamine, dopamine metabolites, and neurotrophic factors.

RESULTS

6-OHDA injection showed significant and dose-dependent damage of dopaminergic neurons and decrease of striatal dopamine content. Rats in the VPA-treated group were markedly protected from the loss of dopaminergic neurons and showed improvements in motor performance, compared to the control group at the moderate 6-OHDA dose (10 μg). VPA-treated rats also showed significantly increased brain-derived neurotrophic factor (BDNF) levels in the striatum and substantia nigra compared to the levels in control animals.

CONCLUSION

Our studies demonstrate that VPA exerts neuroprotective effects in a rat model of 6-OHDA-induced Parkinson's disease (PD), likely in part by up-regulation BDNF.

Distinct alterations in motor & reward seeking behavior are dependent on the gestational age of exposure to LPS-induced maternal immune activation

The dopaminergic system is involved in motivation, reward and the associated motor activities. Mesodiencephalic dopaminergic neurons in the ventral tegmental area (VTA) regulate motivation and reward, whereas those in the substantia nigra (SN) are essential for motor control. Defective VTA dopaminergic transmission has been implicated in schizophrenia, drug addiction and depression whereas dopaminergic neurons in the SN are lost in Parkinson's disease. Maternal immune activation (MIA) leading to in utero inflammation has been proposed to be a risk factor for these disorders, yet it is unclear how this stimulus can lead to the diverse disturbances in dopaminergic-driven behaviors that emerge at different stages of life in affected offspring. Here we report that gestational age is a critical determinant of the subsequent alterations in dopaminergic-driven behavior in rat offspring exposed to lipopolysaccharide (LPS)-induced MIA. Behavioral analysis revealed that MIA on gestational day 16 but not gestational day 12 resulted in biphasic impairments in motor behavior. Specifically, motor impairments were evident in early life, which were resolved by adolescence, but subsequently re-emerged in adulthood. In contrast, reward seeking behaviors were altered in offspring exposed MIA on gestational day 12. These changes were not due to a loss of dopaminergic neurons per se in the postnatal period, suggesting that they reflect functional changes in dopaminergic systems. This highlights that gestational age may be a key determinant of how MIA leads to distinct alterations in dopaminergic-driven behavior across the lifespan of affected offspring.

Inflammation and the neural diathesis-stress hypothesis of schizophrenia: a reconceptualization

An interaction between external stressors and intrinsic vulnerability is one of the longest standing pathoaetiological explanations for schizophrenia. However, novel lines of evidence from genetics, preclinical studies, epidemiology and imaging have shed new light on the mechanisms that may underlie this, implicating microglia as a key potential mediator. Microglia are the primary immune cells of the central nervous system. They have a central role in the inflammatory response, and are also involved in synaptic pruning and neuronal remodeling. In addition to immune and traumatic stimuli, microglial activation occurs in response to psychosocial stress. Activation of microglia perinatally may make them vulnerable to subsequent overactivation by stressors experienced in later life. Recent advances in genetics have shown that variations in the complement system are associated with schizophrenia, and this system has been shown to regulate microglial synaptic pruning. This suggests a mechanism via which genetic and environmental influences may act synergistically and lead to pathological microglial activation. Microglial overactivation may lead to excessive synaptic pruning and loss of cortical gray matter. Microglial mediated damage to stress-sensitive regions such as the prefrontal cortex and hippocampus may lead directly to cognitive and negative symptoms, and account for a number of the structural brain changes associated with the disorder. Loss of cortical control may also lead to disinhibition of subcortical dopamine-thereby leading to positive psychotic symptoms. We review the preclinical and in vivo evidence for this model and consider the implications this has for treatment, and future directions.

Maternal Omega-3 Supplement Improves Dopaminergic System in Pre- and Postnatal Inflammation-Induced Neurotoxicity in Parkinson's Disease Model

Evidence suggests that idiopathic Parkinson's disease (PD) is the consequence of a neurodevelopmental disruption, rather than strictly a consequence of aging. Thus, we hypothesized that maternal supplement of omega-3 polyunsaturated fatty acids (ω-3 PUFA) may be associated with neuroprotection mechanisms in a self-sustaining cycle of neuroinflammation and neurodegeneration in lipopolysaccharide (LPS)-model of PD. To test this hypothesis, behavioral and neurochemical assay were performed in prenatally LPS-exposed offspring at postnatal day 21. To further determine whether prenatal LPS exposure and maternal ω-3 PUFAs supplementation had persisting effects, brain injury was induced on PN 90 rats, following bilateral intranigral LPS injection. Pre- and postnatal inflammation damage not only affected dopaminergic neurons directly, but it also modified critical features, such as activated microglia and astrocyte cells, disrupting the support provided by the microenvironment. Unexpectedly, our results failed to show any involvement of caspase-dependent and independent apoptosis pathway in neuronal death mechanisms. On the other hand, learning and memory deficits detected with a second toxic exposure were significantly attenuated in maternal ω-3 PUFAs supplementation group. In addition, ω-3 PUFAs promote beneficial effect on synaptic function, maintaining the neurochemical integrity in remaining neurons, without necessarily protect them from neuronal death. Thus, our results suggest that ω-3 PUFAs affect the functional ability of the central nervous system in a complex way in a multiple inflammation-induced neurotoxicity animal model of PD and they disclose new ways of understanding how these fatty acids control responses of the brain to different challenges.

Lipopolysaccharide-induced maternal inflammation affects the gonadotropin-releasing hormone neuron development in fetal mice

Recent studies provide evidence that prenatal immunological stress may affect the programming of reproductive health and sexual behavior in adult animals. The aim of this study was to investigate the influence of maternal inflammation, induced by an intraperitoneal (i.p.) injection of lipopolysaccharide (LPS, 45 µg/kg) on embryonic day 11.5 (E 11.5), on the development of the gonadotropin-releasing hormone (GnRH) system in mouse fetuses as well as on the proinflammatory cytokine level in pregnant mice and their fetuses. In the fetuses, the GnRH neuron migration from the olfactory pit to the forebrain was estimated on embryonic days 14.5 and 18.5. The levels of the proinflammatory cytokines interleukin (IL)-6, monocyte chemotactic protein (MCP)-1, tumor necrosis factor (TNF)-α and leukemia inhibitory factor (LIF) were measured with the cytometric bead and ELISA array method in the maternal and fetal blood, amniotic fluid and fetal cerebrospinal fluid (CSF). According to our data, activation of the immune system by LPS treatment on embryonic day 11.5 leads to an increased quantity of neurons in the nasal and olfactory bulb areas and a decreased quantity in the forebrain area on embryonic day 14.5. There was a slight decrease in the total number of neurons in the forebrain area on embryonic day 18.5. The levels of proinflammatory cytokines were significantly increased within 3 h after LPS treatment in the maternal and fetal blood, amniotic fluid and fetal CSF. IL-6-receptor immunoreactivity was detected on olfactory/vomeronasal axons. Thus, prenatal immunological stress delays the GnRH neuron migration in the nasal compartment of mouse fetuses, which may be mediated by the regulation of IL-6, MCP-1 and LIF secretion in the maternal-fetal system.

Short-term effects of an endotoxin on substantia nigra dopamine neurons

Inflammation has been implicated in the pathology of several neurodegenerative diseases, including Parkinson׳s disease (PD). Studies using the endotoxin lipopolysaccharide (LPS), a potent inflammogen, show that systemic insults can trigger prolonged microglial activation and pro-inflammatory cytokine production leading to degeneration of substantia nigra (SN) dopamine (DA) neurons, mimicking idiopathic PD. Because rapid effects of LPS on SN neurons had not been investigated previously, the focus of this study is to assess time-dependent alterations in SN neuroinflammation, DAergic neurons, and neuronal signaling cascades following LPS administration. LPS (5mg/kg, i.p.) or saline (0.9% NaCl) was administered to 8-month-old male mice. At 3h, 5h, and 12h post-injection, the morphology of the SN was assessed using antibodies directed against tyrosine hydroxylase (TH, DAergic marker), Iba-1 (pan-microglial marker), phospho-ERK, and phospho-CREB (signaling). LPS administration significantly reduced TH-immunoreactivity (ir) at all time-points with the greatest reduction observed at 12h post-injection. Reduced TH-ir was accompanied by a significant increase in activated microglia at all time-points following LPS. By 12h post-injection, LPS-treated mice exhibited activated as well as reactive microglia, which can result in neuronal damage. These data demonstrate that the initial reduction in TH-ir observed after an LPS injection was not concomitant with morphological alterations in microglial cells, even though a significant increase in phospho-ERK was observed in glial cells as soon as 3h post-injection. It is possible that the initial alteration in DA phenotype (TH reduction) may perpetuate an inflammatory response that persists and leads to further DAergic damage.

Do prenatal immune activation and maternal iron deficiency interact to affect neurodevelopment and early behavior in rat offspring?

Infection and iron deficiency are common during pregnancy and studies have described altered brain development in the offspring as a result of these individual maternal exposures. Both exposures have been identified as risk factors for schizophrenia yet they have never been modeled simultaneously. We developed a rat model of prenatal immune activation on a background of maternal iron deficiency to determine whether these factors interact to affect neurodevelopment and early behavior in offspring. Pregnant rats were placed on iron sufficient (IS) or iron deficient (ID) diets from E2 to P7, and administered LPS or saline on E15/16. Iron was reduced in liver, spleen, serum and placenta from ID dams by E15. LPS administration on E15 caused greater induction of serum interleukin-6 and tumor necrosis factor-α in ID dams compared to IS dams. Offspring (P0, P7) from ID dams had reduced iron in spleen, liver and brain compared to IS, which normalized by P21. Pups from ID dams showed differences in forelimb grasp and acoustic startle, whilst pups from LPS dams displayed differences in grip ability, geotaxis reflex, cliff avoidance and acoustic startle. Offspring from LPS dams displayed reduced locomotor activity at P7 and P60; offspring from ID dams showed no change. Our findings show effects of prenatal LPS and maternal iron deficiency were additive, such that offspring exposed to both insults displayed more neurodevelopmental abnormalities than offspring exposed to one alone. Yet surprisingly there was no interaction between factors, suggesting independent mechanisms of action.

Dopamine-dependent compensation maintains motor behavior in mice with developmental ablation of dopaminergic neurons

The loss of dopaminergic neurons in the substantia nigra pars compacta (SNc) and consequent depletion of striatal dopamine are known to underlie the motor deficits observed in Parkinson's disease (PD). Adaptive changes in dopaminergic terminals and in postsynaptic striatal neurons can compensate for significant losses of striatal dopamine, resulting in preservation of motor behavior. In addition, compensatory changes independent of striatal dopamine have been proposed based on PD therapies that modulate nondopaminergic circuits within the basal ganglia. We used a genetic strategy to selectively destroy dopaminergic neurons in mice during development to determine the necessity of these neurons for the maintenance of normal motor behavior in adult and aged mice. We find that loss of 90% of SNc dopaminergic neurons and consequent depletion of >95% of striatal dopamine does not result in changes in motor behavior in young-adult or aged mice as evaluated by an extensive array of motor behavior tests. Treatment of aged mutant mice with the dopamine receptor antagonist haloperidol precipitated motor behavior deficits in aged mutant mice, indicating that <5% of striatal dopamine is sufficient to maintain motor function in these mice. We also found that mutant mice exhibit an exaggerated response to l-DOPA compared with control mice, suggesting that preservation of motor function involves sensitization of striatal dopamine receptors. Our results indicate that congenital loss of dopaminergic neurons induces remarkable adaptions in the nigrostriatal system where limited amounts of dopamine in the dorsal striatum can maintain normal motor function.

Distinct activation profiles in microglia of different ages: a systematic study in isolated embryonic to aged microglial cultures

Microglia have been implicated in disease progression for several age-related brain disorders. However, while microglia's contribution to the progression of these disorders is accepted, the effect of aging on their endogenous cellular characteristics has received limited attention. In fact, a comprehensive study of how the structure and function of microglia changes as a function of developmental age has yet to be performed. Here, we describe the functional response characteristics of primary microglial cultures prepared from embryonic, neonatal (Neo), 2-3month-old, 6-8month-old, 9-11month-old, and 13-15month-old rats. Microglial morphology, glutamate (GLU) uptake, and release of trophic and inflammatory factors were assessed under basal conditions and in microglia activated with adenosine 5'-triphosphate (ATP) or lipopolysaccharide. We found that microglia from different age groups were both morphologically and functionally distinct. Upon activation by ATP, Neo microglia were the most reactive, upregulating nitric oxide, tumor necrosis factor-α, and brain-derived neurotrophic factor release as well as GLU uptake. This upregulation translated into neurotoxicity in microglia-neuron co-cultures that were not observed with microglia of different developmental ages. Interestingly, 13-15month-old microglia exhibited similar activation profiles to Neo microglia, whereas microglia from younger adults and embryos were activated less by ATP. Our data also identify age-dependent differences in purinergic receptor subtype expression that contribute to the regulation of neuronal survival. Combined, our data demonstrate that microglial activation and purinergic receptor profiles vary non-linearly with developmental age, a potentially important finding for studies examining the role of microglia in neurodegenerative disorders.

Neonatal systemic exposure to lipopolysaccharide enhances susceptibility of nigrostriatal dopaminergic neurons to rotenone neurotoxicity in later life

Brain inflammation via intracerebral injection with lipopolysaccharide (LPS) in early life has been shown to increase risks for the development of neurodegenerative disorders in adult rats. To determine if neonatal systemic LPS exposure has the same effects on enhancement of adult dopaminergic neuron susceptibility to rotenone neurotoxicity as centrally injected LPS does, LPS (2 μg/g body weight) was administered intraperitoneally into postnatal day 5 (P5) rats and when grown to P70, rats were challenged with rotenone, a commonly used pesticide, through subcutaneous minipump infusion at a dose of 1.25 mg/kg/day for 14 days. Systemically administered LPS can penetrate into the neonatal rat brain and cause acute and chronic brain inflammation, as evidenced by persistent increases in IL-1β levels, cyclooxygenase-2 expression and microglial activation in the substantia nigra (SN) of P70 rats. Neonatal LPS exposure resulted in suppression of tyrosine hydroxylase (TH) expression, but not actual death of dopaminergic neurons in the SN, as indicated by the reduced number of TH+ cells and unchanged total number of neurons (NeuN+) in the SN. Neonatal LPS exposure also caused motor function deficits, which were spontaneously recoverable by P70. A small dose of rotenone at P70 induced loss of dopaminergic neurons, as indicated by reduced numbers of both TH+ and NeuN+ cells in the SN, and Parkinson's disease (PD)-like motor impairment in P98 rats that had experienced neonatal LPS exposure, but not in those without the LPS exposure. These results indicate that although neonatal systemic LPS exposure may not necessarily lead to death of dopaminergic neurons in the SN, such an exposure could cause persistent functional alterations in the dopaminergic system and indirectly predispose the nigrostriatal system in the adult brain to be damaged by environmental toxins at an ordinarily nontoxic or subtoxic dose and develop PD-like pathological features and motor dysfunction.

Leptin and interleukin-6 alter the function of mesolimbic dopamine neurons in a rodent model of prenatal inflammation

Maternal inflammation during critical stages of gestation is thought to underlie the link between prenatal infection and several neurodevelopmental psychiatric disorders in the offspring, including schizophrenia. Increased activity of mesolimbic dopamine (DA) neurons, a hallmark of psychosis, is found in offspring of rodents exposed to a prenatal inflammatory challenge but it is unclear how this effect is elicited. Using an experimental model of localized aseptic inflammation with turpentine oil (TURP) we sought to establish whether circulating interleukin-6 (IL-6) and leptin play a role in the effects of prenatal inflammation on DA neurons. Both mediators are involved in the systemic inflammatory response to immunogens, with IL-6 mediating the early phase, followed by leptin in the late phase of the response. Maternal treatment with TURP at gestational day (GD) 15 enhanced the locomotor response to the DA indirect agonist, amphetamine (AMPH), increased the expression of tyrosine hydroxylase (TH), an enzyme involved in DA synthesis, DA levels and the expression of the post-synaptic protein spinophilin in the nucleus accumbens (NAcc) in the adult offspring. All of these alterations were totally abolished by co-treating the pregnant dams with a neutralizing IL-6 antiserum. Neutralization of maternal leptin prevented the enhanced behavioral sensitization and elevation of DA and spinophilin in the NAcc but spared other changes regulated by IL-6, such as increased NAcc TH levels and acute locomotor response to AMPH. Our results provide novel evidence to suggest that prenatal surges in both maternal circulating IL-6 and leptin contribute to the appearance of sensitized DA function in the adult offspring.

Effect of pesticides on the aggregation of mutant huntingtin protein

The classical reports on neurodegeneration concentrate on studying disruption of signalling cascades. Although it is now well recognized that misfolding and aggregation of specific proteins are associated with a majority of these diseases, their role in aggravating the symptoms is not so well understood. Huntington's disease (HD) is a neurodegenerative disorder that results from damage to complex II of mitochondria. In this work, we have studied the effect of mitochondrial complex I inhibitors, viz. 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and rotenone, and complex II inhibitor, viz. 3-nitropropionic acid, on the aggregation of mutant huntingtin (mthtt) protein, whose misfolding and aggregation results in cellular abnormalities characteristic of HD. All three inhibitors were found to accelerate the aggregation of mthtt in vitro, although the amounts of aggregates formed were different in all cases. Thus, apart from their effect on mitochondrial viability, these neurotoxins are capable of interfering with the protein aggregation process and thus, hastening the onset of the disease.

Prenatal lipopolysaccharide exposure increases anxiety-like behaviors and enhances stress-induced corticosterone responses in adult rats

Maternal infection during pregnancy may affect fetal brain development and lead to neurological and mental disorders. Previously, we used lipopolysaccharide [LPS, 33 μg/kg, intraperitoneal injection] exposure on gestation day 10.5 to mimic maternal bacterial infection in rats and found reduced dopaminergic and serotoninergic neurons in the offspring. In the present study, we examined the anxiety and stress responses of the affected offspring and the neurophysiological changes in their brains. Our results show that LPS rats displayed more anxiety-like behaviors and heightened stress responses. Dopamine (DA) in the nucleus accumbens and serotonin (5-HT) in the medial prefrontal cortex and the hippocampus were significantly reduced in LPS rats. Their glucocorticoid receptors in the dorsal hippocampus and the 5-HT(1A) receptors in the dorsal and ventral hippocampus were also reduced. In addition, chronic but not acute fluoxetine treatment reversed the behavioral changes and increased hippocampal 5-HT(1A) receptor expression. This study demonstrates that LPS exposure during a critical time of embryonic development could produce long-term reduction of DA and 5-HT and other neurophysiological changes; such alterations may be associated with the increases in stress response and anxiety-like behaviors in the offspring.

Contributions of central and systemic inflammation to the pathophysiology of Parkinson's disease

Idiopathic Parkinson's disease (PD) represents a complex interaction between the inherent vulnerability of the nigrostriatal dopaminergic system, a possible genetic predisposition, and exposure to environmental toxins including inflammatory triggers. Evidence now suggests that chronic neuroinflammation is consistently associated with the pathophysiology of PD. Activation of microglia and increased levels of pro-inflammatory mediators such as TNF-α, IL-1β and IL-6, reactive oxygen species and eicosanoids has been reported after post-mortem analysis of the substantia nigra from PD patients and in animal models of PD. It is hypothesised that chronically activated microglia secrete high levels of pro-inflammatory mediators which damage neurons and further activate microglia, resulting in a feed forward cycle promoting further inflammation and neurodegeneration. Moreover, nigrostriatal dopaminergic neurons are more vulnerable to pro-inflammatory and oxidative mediators than other cell types because of their low intracellular glutathione concentration. Systemic inflammation has also been suggested to contribute to neurodegeneration in PD, as lymphocyte infiltration has been observed in brains of PD patients and in animal models of PD, substantiating the current theory of a fundamental role of inflammation in neurodegeneration. We will examine the current evidence in the literature which offers insight into the premise that both central and systemic inflammation may contribute to neurodegeneration in PD. We will discuss the emerging possibility of the use of diagnostic tools such as imaging technologies for PD patients. Finally, we will present the immunomodulatory therapeutic strategies that are now under investigation and in clinical trials as potential neuroprotective drugs for PD.

Diesel exhaust activates and primes microglia: air pollution, neuroinflammation, and regulation of dopaminergic neurotoxicity

BACKGROUND

Air pollution is linked to central nervous system disease, but the mechanisms responsible are poorly understood.

OBJECTIVES

Here, we sought to address the brain-region-specific effects of diesel exhaust (DE) and key cellular mechanisms underlying DE-induced microglia activation, neuroinflammation, and dopaminergic (DA) neurotoxicity.

METHODS

Rats were exposed to DE (2.0, 0.5, and 0 mg/m3) by inhalation over 4 weeks or as a single intratracheal administration of DE particles (DEP; 20 mg/kg). Primary neuron-glia cultures and the HAPI (highly aggressively proliferating immortalized) microglial cell line were used to explore cellular mechanisms.

RESULTS

Rats exposed to DE by inhalation demonstrated elevated levels of whole-brain IL-6 (interleukin-6) protein, nitrated proteins, and IBA-1 (ionized calcium-binding adaptor molecule 1) protein (microglial marker), indicating generalized neuroinflammation. Analysis by brain region revealed that DE increased TNFα (tumor necrosis factor-α), IL-1β, IL-6, MIP-1α (macrophage inflammatory protein-1α) RAGE (receptor for advanced glycation end products), fractalkine, and the IBA-1 microglial marker in most regions tested, with the midbrain showing the greatest DE response. Intratracheal administration of DEP increased microglial IBA-1 staining in the substantia nigra and elevated both serum and whole-brain TNFα at 6 hr posttreatment. Although DEP alone failed to cause the production of cytokines and chemokines, DEP (5 μg/mL) pretreatment followed by lipopolysaccharide (2.5 ng/mL) in vitro synergistically amplified nitric oxide production, TNFα release, and DA neurotoxicity. Pretreatment with fractalkine (50 pg/mL) in vitro ameliorated DEP (50 μg/mL)-induced microglial hydrogen peroxide production and DA neurotoxicity.

CONCLUSIONS

Together, these findings reveal complex, interacting mechanisms responsible for how air pollution may cause neuroinflammation and DA neurotoxicity.

Neonatal exposure to lipopolysaccharide enhances vulnerability of nigrostriatal dopaminergic neurons to rotenone neurotoxicity in later life

Brain inflammation in early life has been proposed to play important roles in the development of neurodegenerative disorders in adult life. To test this hypothesis, we used a neonatal rat model of lipopolysaccharide (LPS) exposure (1000 EU/g body weight, intracerebral injection on P5) to produce brain inflammation. By P70, when LPS-induced behavioral deficits were spontaneously recovered, animals were challenged with rotenone, a commonly used pesticide, through subcutaneous mini-pump infusion at a dose of 1.25 mg/kg per day for 14 days. This rotenone treatment regimen ordinarily does not produce toxic effects on behaviors in normal adult rats. Our results show that neonatal LPS exposure enhanced the vulnerability of nigrostriatal dopaminergic neurons to rotenone neurotoxicity in later life. Rotenone treatment resulted in motor neurobehavioral impairments in rats with the neonatal LPS exposure, but not in those without the neonatal LPS exposure. Rotenone induced losses of tyrosine hydroxylase immunoreactive neurons in the substantia nigra and decreased mitochondrial complex I activity in the striatum of rats with neonatal LPS exposure, but not in those without this exposure. Neonatal LPS exposure with later exposure to rotenone decreased retrogradely labeled nigrostriatal dopaminergic projecting neurons. The current study suggests that perinatal brain inflammation may enhance adult susceptibility to the development of neurodegenerative disorders triggered later on by environmental toxins at an ordinarily non-toxic or sub-toxic dose. Our model may be useful for studying mechanisms involved in the pathogenesis of nonfamilial Parkinson's disease and the development of potential therapeutic treatments.

The endotoxin-induced neuroinflammation model of Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by the progressive loss of dopaminergic (DA) neurons in the substantia nigra. Although the exact cause of the dopaminergic neurodegeneration remains elusive, recent postmortem and experimental studies have revealed an essential role for neuroinflammation that is initiated and driven by activated microglial and infiltrated peripheral immune cells and their neurotoxic products (such as proinflammatory cytokines, reactive oxygen species, and nitric oxide) in the pathogenesis of PD. A bacterial endotoxin-based experimental model of PD has been established, representing a purely inflammation-driven animal model for the induction of nigrostriatal dopaminergic neurodegeneration. This model, by itself or together with genetic and toxin-based animal models, provides an important tool to delineate the precise mechanisms of neuroinflammation-mediated dopaminergic neuron loss. Here, we review the characteristics of this model and the contribution of neuroinflammatory processes, induced by the in vivo administration of bacterial endotoxin, to neurodegeneration. Furthermore, we summarize the recent experimental therapeutic strategies targeting endotoxin-induced neuroinflammation to elicit neuroprotection in the nigrostriatal dopaminergic system. The potential of the endotoxin-based PD model in the development of an early-stage specific diagnostic biomarker is also emphasized.

Effects of prenatal infection on brain development and behavior: a review of findings from animal models

Epidemiological studies with human populations indicate associations between maternal infection during pregnancy and increased risk in offspring for central nervous system (CNS) disorders including schizophrenia, autism and cerebral palsy. Since 2000, a large number of studies have used rodent models of systemic prenatal infection or prenatal immune activation to characterize changes in brain function and behavior caused by the prenatal insult. This review provides a comprehensive summary of these findings, and examines consistencies and trends across studies in an effort to provide a perspective on our current state of understanding from this body of work. Results from these animal modeling studies clearly indicate that prenatal immune activation can cause both acute and lasting changes in behavior and CNS structure and function in offspring. Across laboratories, studies vary with respect to the type, dose and timing of immunogen administration during gestation, species used, postnatal age examined and specific outcome measure quantified. This makes comparison across studies and assessment of replicability difficult. With regard to mechanisms, evidence for roles for several acute mediators of effects of prenatal immune activation has emerged, including circulating interleukin-6, increased placental cytokines and oxidative stress in the fetal brain. However, information required to describe the complete mechanistic pathway responsible for acute effects of prenatal immune activation on fetal brain is lacking, and no studies have yet addressed the issue of how acute prenatal exposure to an immunogen is transduced into a long-term CNS change in the postnatal animal. Directions for further research are discussed.

Docosahexaenoic acid confers neuroprotection in a rat model of perinatal hypoxia-ischemia potentiated by Escherichia coli lipopolysaccharide-induced systemic inflammation

OBJECTIVE

Lipopolysaccharide pretreatment potentiates hypoxic ischemic injury. We hypothesized that docosahexaenoic acid pretreatment would improve function and reduce brain volume loss in this rat model of perinatal brain injury and inflammation.

STUDY DESIGN

Seven-day-old rats were divided into 3 groups: intraperitoneal docosahexaenoic acid 1 mg/kg and lipopolysaccharide 0.1 mg/kg, 25% albumin and lipopolysaccharide, and normal saline. Injections were given 2.5 hours before carotid ligation, followed by 90 minutes 8% O2. Rats underwent sensorimotor function testing and brain volume loss assessment on postnatal day 14.

RESULTS

Docosahexaenoic acid pretreatment improved vibrissae forepaw placing scores compared with albumin/lipopolysaccharide (mean+/-standard deviation weighted score/20: 17.72+/-0.92 docosahexaenoic acid/lipopolysaccharide vs 13.83+/-0.82 albumin/lipopolysaccharide; P<.007). Albumin/lipopolysaccharide rats scores were worse than those of the normal saline/normal saline rats (13.83+/-0.82 vs 17.21+/-0.71; P=.076). No significant differences in brain volume loss were observed among groups.

CONCLUSION

Lipopolysaccharide inflammatory stimulation in conjunction with hypoxic ischemic resulted in poorer function than hypoxic ischemic alone. Docosahexaenoic acid pretreatment had significantly improved function in neonatal rats exposed to lipopolysaccharide and hypoxic ischemic.

Prenatal lipopolysaccharide reduces motor activity after an immune challenge in adult male offspring

Prenatal lipopolysaccharide (LPS) exposure causes reproductive, behavioral and neurochemical injuries in both the mother and pups. Previous investigations by our group showed that prenatal LPS administration (100 microg/kg, i.p.) on gestational day 9.5 impaired the male offspring's social behavior in infancy and adulthood. In the present study, we investigated whether these social behavioral changes were associated with motor activity impairment. Male rat pups treated prenatally with LPS or not were tested for reflexological development and open field general activity during infancy. In adulthood, animals were tested for open field general activity, haloperidol-induced catalepsy and apomorphine-induced stereotypy; striatal dopamine levels and turnover were also measured. Moreover, LPS-treated or untreated control pups were challenged with LPS in adulthood and observed for general activity in the open field. In relation to the control group, the motor behavior of prenatally treated male pups was unaffected at basal levels, both in infancy and in adulthood, but decreased general activity was observed in adulthood after an immune challenge. Also, striatal dopamine and metabolite levels were decreased in adulthood. In conclusion, prenatal LPS exposure disrupted the dopaminergic system involved with motor function, but this neurochemical effect was not accompanied by behavioral impairment, probably due to adaptive plasticity processes. Notwithstanding, behavioral impairment was revealed when animals were challenged with LPS, resulting in enhanced sickness behavior.

Neuroinflammation in Parkinson's disease: its role in neuronal death and implications for therapeutic intervention

Parkinson's disease (PD) is the second most common neurodegenerative disease, after Alzheimer's disease. The potential causes of PD remain uncertain, but recent studies suggest neuroinflammation and microglia activation play important roles in PD pathogenesis. Major unanswered questions include whether protein aggregates cause the selective loss of dopaminergic neurons in the substantia nigra that underlies the clinical symptoms and whether neuroinflammation is a consequence or a cause of nigral cell loss. Within the microenvironment of the brain, glial cells play a critical role in homeostatic mechanisms that promote neuronal survival. Microglia have a specialized immune surveillance role and mediate innate immune responses to invading pathogens by secreting a myriad of factors that include, cytokines, chemokines, prostaglandins, reactive oxygen and nitrogen species, and growth factors. Some of these factors have neuroprotective and trophic activities and aid in brain repair processes; while others enhance oxidative stress and trigger apoptotic cascades in neurons. Therefore, pro- and anti-inflammatory responses must be in balance to prevent the potential detrimental effects of prolonged or unregulated inflammation-induced oxidative stress on vulnerable neuronal populations. In this review, we discuss potential triggers of neuroinflammation and review the strongest direct evidence that chronic neuroinflammation may have a more important role to play in PD versus other neurodegenerative diseases. Alternatively, we propose that genetic deficiency is not the only way to reduce protective factors in the brain which may function to keep microglial responses in check or regulate the sensitivity of DA neurons. If chronic inflammation can be shown to decrease the levels of neuroprotective factors in the midbrain, in essence genetic haploinsufficiency of protective factors such as Parkin or RGS10 may result from purely environmental triggers (aging, chronic systemic disease, etc.), increasing the vulnerability to inflammation-induced nigral DA neuron death and predisposing an individual to development of PD. Lastly, we review the latest epidemiological and experimental evidence supporting the potential use of anti-inflammatory and immunomodulatory drugs as neuroprotective agents to delay the progressive nigrostriatal degeneration that leads to motor dysfunction in PD.

The blood-brain barrier in neurodegenerative disease: a rhetorical perspective

Recent studies suggest that the function of the blood-brain barrier (BBB) is not static under normal physiologic conditions and is likely altered in neurodegenerative disease. Prevailing thinking about CNS function, and neurodegenerative disease in particular, is neurocentric excluding the impact of factors outside the CNS. This review challenges this perspective and discusses recent reports suggesting the involvement of peripheral factors including toxins and elements of adaptive immunity that may not only play a role in pathogenesis, but also progression of neurodegenerative diseases. Central to this view is neuroinflammation. Several studies indicate that the neuroinflammatory changes that accompany neurodegeneration affect the BBB or its function by altering transport systems, enhancing immune cell entry, or influencing the BBB's role as a signaling interface. Such changes impair the BBB's normal homeostatic function and affect neural activity. Moreover, recent studies reveal that alterations in BBB and its transporters affect the entry of drugs used to treat neurodegenerative diseases. Incorporating BBB compromise and dysfunction into our view of neurodegenerative disease leads to the inclusion of peripheral mediators in its pathogenesis and progression. In addition, this changing view of the BBB raises interesting new therapeutic possibilities for drug delivery as well as treatment strategies designed to reinstate normal barrier function.

Docosahexaenoic acid pretreatment confers neuroprotection in a rat model of perinatal cerebral hypoxia-ischemia

OBJECTIVE

We hypothesized that pretreatment with docosahexaenoic acid (DHA), a potentially neuroprotective polyunsaturated fatty acid, would improve function and reduce brain damage in a rat model of perinatal hypoxia-ischemia.

STUDY DESIGN

Seven-day-old rats were divided into 3 treatment groups that received intraperitoneal injections of DHA 1, 2.5, or 5 mg/kg as DHA-albumin complex and 3 controls that received 25% albumin, saline, or no injection. Subsequently, rats underwent right carotid ligation followed by 90 minutes of 8% oxygen. Rats underwent sensorimotor testing (vibrissae-stimulated forepaw placing) and morphometric assessment of right-sided tissue loss on postnatal day 14.

RESULTS

DHA pretreatment improved forepaw placing response to near-normal levels (9.5 +/- 0.9 treatment vs 7.1 +/- 2.2 controls; normal = 10; P < .0001). DHA attenuated hemisphere damage compared with controls (P = .0155), with particular benefit in the hippocampus with 1 mg/kg (38% protection vs albumin controls).

CONCLUSION

DHA pretreatment improves functional outcome and reduces volume loss after hypoxia-ischemia in neonatal rats.