Emily N. Mangano

Proinflammatory cytokines differentially influence adult hippocampal cell proliferation depending upon the route and chronicity of administration

Disturbances of hippocampal plasticity, including impaired dendritic branching and reductions of neurogenesis, are provoked by stressful insults and may occur in depression. Although corticoids likely contribute to stressor-induced reductions of neurogenesis, other signaling messengers, including pro-inflammatory cytokines might also be involved. Accordingly, the present investigation assessed whether three proinflammatory cytokines, namely interleukin-1β (IL-1β), IL-6, and tumor necrosis factor-α (TNF-α) (associated with depression) influenced cellular proliferation within the hippocampus. In this regard, systemic administration of TNF-α reduced 5-bromo-2-deoxyuridine (BrdU) labeling within the hippocampus, whereas IL-1β and IL-6 had no such effect. However, repeated but not a single intra-hippocampal infusion of IL-6 and IL-1β actually increased cellular proliferation and IL-6 infusion also enhanced microglial staining within the hippocampus. Yet, no changes in doublecortin expression were apparent, suggesting that the cytokine did not influence the birth of cells destined to become neurons. Essentially, the route of administration and chronicity of cytokine administration had a marked influence upon the nature of hippocampal alterations provoked, suggesting that cytokines may differentially regulate hippocampal plasticity in neuropsychiatric conditions.

Inflammatory mechanisms of neurodegeneration in toxin-based models of Parkinson's disease.

Parkinsons disease (PD) has been associated with exposure to a variety of environmental agents, including pesticides, heavy metals, and organic pollutants; and inflammatory processes appear to constitute a common mechanistic link among these insults. Indeed, toxin exposure has been repeatedly demonstrated to induce the release of oxidative and inflammatory factors from immunocompetent microglia, leading to damage and death of midbrain dopamine (DA) neurons. In particular, proinflammatory cytokines such as tumor necrosis factor-α and interferon-γ, which are produced locally within the brain by microglia, have been implicated in the loss of DA neurons in toxin-based models of PD; and mounting evidence suggests a contributory role of the inflammatory enzyme, cyclooxygenase-2. Likewise, immune-activating bacterial and viral agents were reported to have neurodegenerative effects themselves and to augment the deleterious impact of chemical toxins upon DA neurons. The present paper will focus upon the evidence linking microglia and their inflammatory processes to the death of DA neurons following toxin exposure. Particular attention will be devoted to the possibility that environmental toxins can activate microglia, resulting in these cells adopting a “sensitized” state that favors the production of proinflammatory cytokines and damaging oxidative radicals.

Interferon-γ plays a role in paraquat-induced neurodegeneration involving oxidative and proinflammatory pathways

Exposure to environmental contaminants, particularly pesticides, may be an important etiological factor in Parkinsons disease (PD); and evidence suggests a role for microglia-dependent inflammatory and oxidative processes in nigrostriatal pathology induced by such toxins. Yet, the events mediating microglial activation and their effects are not fully known. To this end, we hypothesized that the proinflammatory cytokine, interferon-gamma (IFN-γ), may be a prime factor in the pathogenesis of PD, given its critical role in regulating microglial responses to pathogens. Indeed, the present investigation demonstrated that genetic deletion of IFN-γ protected substantia nigra pars compacta (SNc) dopamine (DA) neurons from the toxic effects of the pesticide, paraquat, and normalized changes in inflammatory and oxidative factors within this brain region. Specifically, IFN-γ knockout prevented the paraquat-induced morphological signs of microglial activation and expression of key nicotinamide adenine dinucleotide phosphate (NADPH) oxidase subunits, while also preventing time-dependent changes in proinflammatory enzymes (inducible nitric oxide synthase [iNOS], cyclooxygenase-2 [COX-2]), cytokines (interleukin-1β [IL-1β], tumor necrosis factor-α [TNF-α]), and signaling factors (c-Jun N-terminal kinase [JNK], p38 MAP kinase [p38], Signal transducer and activator of transcription-1 [STAT1], nuclear factor kappa B [NF-κB]). Moreover, paraquat transiently suppressed substantia nigra pars compacta expression of trophic and proneuroplastic factors (cyclic-AMP response element binding protein [CREB], brain-derived neurotrophic factor [BDNF]), and IFN-γ deficiency again reversed these effects. These data suggest that IFN-γ is important for paraquat-induced neurodegeneration and the accompanying oxidative, inflammatory, and trophic changes that characterize the response to the toxin. Targeting IFN-γ could thus have therapeutic implications for PD and other neurodegenerative conditions that involve multiple inflammatory pathways.

Inflammatory priming of the substantia nigra influences the impact of later paraquat exposure: Neuroimmune sensitization of neurodegeneration

Activation of microglia along with the release of inflammatory cytokines and oxidative factors often accompanies toxin-induced degeneration of substantia nigra pars compacta (SNc) dopamine (DA) neurons. Multiple toxin exposure may synergistically influence microglial-dependent DA neuronal loss and, in fact, pre-treatment with one toxin may sensitize DA neurons to the impact of subsequent insults. Thus, we assessed whether priming SNc neurons with the inflammatory agent, lipopolysaccharide (LPS), influenced the impact of later exposure to the pesticide, paraquat, which has been reported to provoke DA loss. Indeed, LPS infusion into the SNc sensitized DA neurons to the neurodegenerative effects of a series of paraquat injections commencing 2 days later. In contrast, LPS pre-treatment actually protected against some of neurodegenerative effects of paraquat when the pesticide was administered 7 days after the endotoxin. These sensitization and de-sensitization effects were associated with altered expression of reactive microglia expressing inducible immunoproteasome subunits, as well as variations of fibroblast growth factor and a time-dependent infiltration of peripheral immune cells. Circulating levels of the inflammatory cytokines, interleukin (IL)-6, IL-2, tumor necrosis factor-alpha and interferon-gamma were also time-dependently elevated following intra-SNc LPS infusion. These data suggest that inflammatory priming may influence DA neuronal sensitivity to subsequent environmental toxins by modulating the state of glial and immune factors, and these findings may be important for neurodegenerative conditions, such as Parkinsons disease (PD).

Behavior and Pro-Inflammatory Cytokine Variations Among Submissive and Dominant Mice Engaged in Aggressive Encounters: Moderation by Corticosterone Reactivity

Psychosocial stressors contribute to the pathophysiology of affective disorders and variations of cytokine functioning have been implicated in this process. The present investigation demonstrated, in mice, the impact of stressful aggressive encounters on activity levels, plasma corticosterone and cytokine concentrations, and on cytokine mRNA expression within the prefrontal cortex (PFC) and hippocampus. As glucocorticoids have been tied to cytokine variations, mice were subdivided into low or high corticosterone responders, defined in terms of circulating hormone levels 75 min post-confrontation. Interestingly, stressor-induced effects among low and high responders varied as a function of whether mice were submissive or dominant during the aggressive bout. Agonistic encounters elicited subsequent hyperactivity, particularly among low corticosterone responders and among dominant mice. Plasma levels of corticosterone and interleukin (IL)-6 concomitantly increased after aggressive encounters and varied with dominance status and with the low versus high corticosterone response. Among the low responders corticosterone and IL-6 increases were modest and only apparent among submissive mice, whereas among high responders these elevations were more pronounced and comparable in submissive and dominant mice. Aggressive episodes also increased IL-1β and IL-6 mRNA brain expression. The IL-1β rise was greater in the PFC and hippocampus of submissive mice that were low responders. Among high responders IL-1β and IL-6 increased in both groups, although in the PFC this effect was specific to dominant mice. The data are discussed in terms of their relevance to the impact of aggressive encounters on affective behaviors, and to the role that cytokines might play in this regard.

Interferon-γ deficiency modifies the motor and co-morbid behavioral pathology and neurochemical changes provoked by the pesticide paraquat

In addition to nigrostriatal pathology and corresponding motor disturbances, Parkinsons disease (PD) is often characterized by co-morbid neuropsychiatric symptoms, most notably anxiety and depression. Separate lines of evidence indicate that inflammatory processes associated with microglial activation and cytokine release may be fundamental to the progression of both PD and its co-morbid psychiatric pathology. Accordingly, we assessed the contribution of the pro-inflammatory cytokine, interferon-gamma (IFN-gamma), to a range of PD-like pathology provoked by the ecologically relevant herbicide and dopamine (DA) toxin, paraquat. To this end, paraquat provoked overt motor impairment (reduced home-cage activity and impaired vertical climbing) and signs of anxiety-like behavior (reduced open field exploration) in wild-type but not IFN-gamma-deficient mice. Correspondingly, paraquat promoted somewhat divergent variations in neurochemical activity among wild-type and IFN-gamma null mice at brain sites important for both motor (striatum) and co-morbid affective pathologies (dorsal hippocampus, medial prefrontal cortex, and locus coeruleus). Specifically, the herbicide provoked a dosing regimen-dependent reduction in striatal DA levels that was prevented by IFN-gamma deficiency. In addition, the herbicide influenced serotonergic and noradrenergic activity within the dorsal hippocampus and medial prefrontal cortex; and elevated noradrenergic activity within the locus coeruleus. Although genetic ablation of IFN-gamma had relatively few effects on monoamine variations within the locus coeruleus and prefrontal cortex, loss of the pro-inflammatory cytokine did normalize the paraquat-induced noradrenergic alterations within the hippocampus. These findings further elucidate the functional implications of paraquat intoxication and suggest an important role for IFN-gamma in the striatal and motor pathology, as well as the co-morbid behavioral and hippocampal changes induced by paraquat.

Cyclooxygenase-2 deficiency modifies the neurochemical effects, motor impairment and co-morbid anxiety provoked by paraquat administration in mice

Parkinson’s disease and other motor disorders of midbrain basal ganglia dopaminergic functioning are often characterized by alterations of brainstem and limbic systems with accompanying co‐morbid anxiety and depressive symptoms. Accumulating evidence suggests that inflammatory processes may play an important role in such neurodegenerative and psychiatric pathology. In this regard, inhibition of the inflammatory enzyme cyclooxygenase‐2 (COX‐2) was reported to limit the impact of stressors as well as the neurodegenerative effects of dopaminergic toxins. The present investigation assessed the impact of the putative dopamine toxin paraquat (a widely used herbicide) upon motor functioning, behavioural indices of anxiety‐like states and central monoamine levels and whether these effects were altered in mice lacking COX‐2. Indeed, paraquat did induce motor impairment and altered dopamine utilization within the striatum, and COX‐2 deletion moderately attenuated these effects. Conversely, COX‐2 deficiency enhanced the impact of paraquat upon indices of anxiety (open field exploration) and on serotonergic, noradrenergic and dopaminergic alterations within two brain regions implicated in stressor‐related pathologies, namely the dorsal hippocampus and medial prefrontal cortex. These results suggest that COX‐2 might differentially influence the motor and psychiatric symptoms associated with environmental toxin exposure. Furthermore, these data indicate that the neurochemical impact of paraquat is not restricted to the nigrostriatal dopamine pathway but also involves stressor‐sensitive limbic regions. It is possible that COX‐2 may play a dual role by contributing to the motor impairment induced by paraquat, but acting to moderate the effects of paraquat upon processes aligned with anxiety and depression.

Granulocyte macrophage-colony stimulating factor protects against substantia nigra dopaminergic cell loss in an environmental toxin model of Parkinson's disease

Parkinsons disease (PD) has been linked to exposure to a variety of chemical (e.g., pesticides) and inflammatory agents, which may act cumulatively over time. Finding novel means of limiting pathology associated with toxin exposure would have tremendous clinical importance. To this end, we assessed whether the hematopoietic trophic cytokine, granulocyte macrophage colony stimulating factor (GM-CSF), would inhibit the neurodegenerative effects of the pesticide, paraquat, administered either alone or following priming with the bacterial endotoxin, lipopolysaccharide (LPS). As previously observed, paraquat provoked a modest but significant neurodegenerative effect that was markedly augmented with LPS priming. Central infusion of GM-CSF into the substantia nigra pars compacta (SNc) prevented the loss of SNc dopamine neurons to a degree comparable to that of glial derived neurotrophic factor. Importantly, systemic administration of GM-CSF also had neuroprotective consequences, suggesting that the trophic cytokine can cross the blood brain barrier to promote neuronal survival. Indeed, GM-CSF acted to inhibit the LPS and paraquat induced microglial response, while augmenting astrocyte immunoreactivity within the SNc. Moreover, GM-CSF blunted the paraquat induced reduction of brain derived neurotrophic factor within the hippocampus, as well as in cultured mesencephalic neurons. Although paraquat reduced mesencephalic levels of the anti-apoptotic protein, Bcl-2, GM-CSF had no effect in this regard. Hence, GM-CSF appears to affect inflammatory and/or neuroplastic factors within the SNc that may be linked to neurodegeneration, as well as in other brain regions (hippocampus), which could be important for co-morbid non-motor symptoms in PD. These data suggest that peripheral GM-CSF administration might hold promise as a treatment of PD.

Lipopolysaccharide and a social stressor influence behaviour, corticosterone and cytokine levels: Divergent actions in cyclooxygenase-2 deficient mice and wild type controls

Administration of the endotoxin, lipopolysaccharide (LPS) diminished motor activity and increased plasma corticosterone as well as circulating levels of interleukin-1beta (IL-1beta), IL-6, tumor necrosis-factor-alpha (TNF-alpha) and IL-10. Among cyclooxygenase-2 (COX-2) knockout mice the behavioural, corticosterone and cytokine variations promoted by LPS were moderately (home cage activity, corticosterone, TNF-alpha) or largely (IL-6) reduced. However, if mice were exposed to a psychosocial stressor (social disruption associated with grouping mice with novel cage-mates after a period of isolation) coupled with LPS treatment, then the effects of the COX-2 deletion were absent, or there was a synergistic or additive elevation apparent (e.g., in the case of TNF-alpha, IL-6 and corticosterone). Evidently, COX-2 deletion may have either pro- or anti-inflammatory actions, depending upon the psychosocial context in which immune activation occurs.

An in vivo animal study assessing long-term changes in hypothalamic cytokines following perinatal exposure to a chemical mixture based on Arctic maternal body burden

BackgroundThe geographic distribution of environmental toxins is generally not uniform, with certain northern regions showing a particularly high concentration of pesticides, heavy metals and persistent organic pollutants. For instance, Northern Canadians are exposed to high levels of persistent organic pollutants like polychlorinated biphenyls (PCB), organochlorine pesticides (OCs) and methylmercury (MeHg), primarily through country foods. Previous studies have reported associations between neuronal pathology and exposure to such toxins. The present investigation assessed whether perinatal exposure (gestation and lactation) of rats to a chemical mixture (27 constituents comprised of PCBs, OCs and MeHg) based on Arctic maternal exposure profiles at concentrations near human exposure levels, would affect brain levels of several inflammatory cytokinesMethodsRats were dosed during gestation and lactation and cytokine levels were measured in the brains of offspring at five months of age. Hypothalamic cytokine protein levels were measured with a suspension-based array system and differences were determined using ANOVA and post hoc statistical tests.ResultsThe early life PCB treatment alone significantly elevated hypothalamic interleukin-6 (IL-6) levels in rats at five months of age to a degree comparable to that of the entire chemical mixture. Similarly, the full mixture (and to a lesser degree PCBs alone) elevated levels of the pro-inflammatory cytokine, IL-1b, as well as the anti-inflammatory cytokine, IL-10. The full mixture of chemicals also moderately increased (in an additive fashion) hypothalamic levels of the pro-inflammatory cytokines, IL-12 and tumor necrosis factor (TNF-α). Challenge with bacterial endotoxin at adulthood generally increased hypothalamic levels to such a degree that differences between the perinatally treated chemical groups were no longer detectable.ConclusionsThese data suggest that exposure at critical neurodevelopmental times to environmental chemicals at concentrations and combinations reflective of those observed in vulnerable population can have enduring consequences upon cytokines that are thought to contribute to a range of pathological states. In particular, such protracted alterations in the cytokine balance within the hypothalamus would be expected to favor marked changes in neuro-immune and hormonal communication that could have profound behavioral consequences.