The impact of murine LRRK2 G2019S transgene overexpression on acute responses to inflammatory challenge.

The most common Parkinson's disease (PD) mutation is the gain-of-function LRRK2 G2019S variant, which has also been linked to inflammatory disease states. Yet, little is known of the role of G2019S in PD related complex behavioral or immune/hormonal processes in response to inflammatory/toxicant challenges. Hence, we characterized the behavioral, neuroendocrine-immune and central monoaminergic responses in G2019S overexpressing mutants following systemic interferon-gamma (IFN-γ) or lipopolysaccharide (LPS) administration. Although LPS markedly (and IFN-γ modestly in some cases) increased cytokine and corticosterone levels, while inducing pronounced sickness and home-cage activity deficits, the G2019S mutation had no effect on these parameters. No differences were observed with regards to brain microglia with the acute LPS injection, regardless of genotype. Nor did the G2019S mutation influence neurotransmitter levels within the medial prefrontal cortex or paraventricular nucleus of the hypothalamus. However, the LRRK2 G2019S transgenic mice did have altered monoamine levels within the striatum and hippocampus. Indeed, G2019S mice had altered basal levels and turnover of dopamine within the striatum, along with changes in hippocampal serotonin and norepinephrine activity in response to LPS and IFN-γ. The present findings suggest the importance of murine G2019S in hippocampal and striatal neurotransmission, but that the transgene didn't appear to be involved in functional behavioral and stress-like hormonal and cytokine changes provoked by inflammatory insults.

Age and Chronicity of Administration Dramatically Influenced the Impact of Low Dose Paraquat Exposure on Behavior and Hypothalamic-Pituitary-Adrenal Activity.

Little is known of the age-dependent and long-term consequences of low exposure levels of the herbicide and dopaminergic toxicant, paraquat. Thus, we assessed the dose-dependent effects of paraquat using a typical short-term (3 week) exposure procedure, followed by an assessment of the effects of chronic (16 weeks) exposure to a very low dose (1/10th of what previously induced dopaminergic neuronal damage). Short term paraquat treatment dose-dependently induced deficits in locomotion, sucrose preference and Y-maze performance. Chronic low dose paraquat treatment had a very different pattern of effects that were also dependent upon the age of the animal: in direct contrast to the short-term effects, chronic low dose paraquat increased sucrose consumption and reduced forced swim test (FST) immobility. Yet these effects were age-dependent, only emerging in mice older than 13 months. Likewise, Y-maze spontaneous alternations and home cage activity were dramatically altered as a function of age and paraquat chronicity. In both the short and long-term exposure studies, increased corticosterone and altered hippocampal glucocorticoid receptor (GR) levels were induced by paraquat, but surprisingly these effects were blunted in the older mice. Thus, paraquat clearly acts as a systemic stressor in terms of corticoid signaling and behavioral outcomes, but that paradoxical effects may occur with: (a) repeated exposure at; (b) very low doses; and (c) older age. Collectively, these data raise the possibility that repeated "hits" with low doses of paraquat in combination with aging processes might have promoted compensatory outcomes.

Ketamine modulates hippocampal neurogenesis and pro-inflammatory cytokines but not stressor induced neurochemical changes.

Considerable recent attention has focused on the rapid antidepressant effects observed in treatment resistant patients produced by the NMDA receptor antagonist, ketamine. Surprisingly, the effects of ketamine in the context of stressor exposure, as well as the consequences of its chronic use are unclear. Thus, we assessed the impact of acute and repeated ketamine treatment together with acute [restraint or lipopolysaccharide (LPS)] or chronic (unpredictable different psychogenic challenges) stressor exposure. Importantly, acute ketamine treatment did provoke an antidepressant-like effect in a forced swim test (FST) and this effect lasted for 8 days following repeated exposure to the drug. Although acute restraint and LPS individually provoked the expected elevation of plasma corticosterone and brain-region specific monoamine variations, ketamine had no influence on corticosterone and had, at best, sparse effects on the monoamine changes. Similarly, ketamine did not appreciably influence the stressor induced neurochemical and sucrose preference alterations, it did however, dose-dependently reverse the LPS induced elevation of the pro-inflammatory cytokines, interleukin-1ß (IL-1ß) and tumor necrosis factor-α (TNF-α). Likewise, repeated ketamine administration increased adult hippocampal neurogenesis. These data indicate that repeated ketamine administration had greater behavioral consequences than acute treatment and that the drug might be imparting antidepressant effects through its effects on neuroplasticity and inflammatory processes rather than the typical neurochemical/hormonal factors affected by stressors. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.

Individual and interactive sex-specific effects of acute restraint and systemic IFN-γ treatment on neurochemistry.

Compelling evidence supports the involvement of the pro-inflammatory cytokines, IL-6, IFN-α and TNF-α in depression and related stress-associated pathologies. A role has also been suggested for the Th1-type cytokine, IFN-γ, with most mechanistic accounts focusing on the cytokine's capacity to induce indoleamine 2,3-dioxygenase (IDO), leading to diminished tryptophan and the generation of kynurenine metabolites. Beyond these IDO-dependent routes, there is surprisingly little evidence directly linking IFN-γ to alterations of brain regional monoamine activity and HPA axis functioning. Our specific aims in the present study were twofold: 1) assess the behavioural, plasma corticosterone and brain regional monoamine effects of acute systemic IFN-γ, with or without short duration restraint stress (15 min), and 2) determine the sex-specific nature of these effects. As predicted, IFN-γ stimulated monoaminergic activity within a number of stressor-sensitive limbic brain regions, most notably the paraventricular nucleus of the hypothalamus, central amygdala and prefrontal cortex. While several of these effects were sex-specific, there was little in the way of synergism between the cytokine and stressor treatments. Nonetheless, IFN-γ did synergistically interact with acute restraint stress to increase plasma corticosterone concentrations, and this effect was most pronounced in the male mice. These data are among the first to show that systemically administered IFN-γ can alone or in conjunction with psychologically relevant stressor, modify brain regional monoamine activity and the plasma corticosterone response.

The interactive effects of ketamine and magnesium upon depressive-like pathology.

Approximately one-third of patients with major depressive disorders (MDDs) are resistant to current treatment methods, and the majority of cases relapse at some point during therapy. This has resulted in novel treatments being adopted, including subanesthetic doses of ketamine, which affects aberrant neuroplastic circuits, glutamatergic signaling, and the production of brain-derived neurotrophic factor. Ketamine rapidly relieves depressive symptoms in treatment-resistant major depressive disorder patients with effects that last for up to 2 weeks even after a single administration. However, it is also a drug with an abusive potential and can have marked side effects. Hence, this study aimed at enhancing the antidepressant-like effects of ketamine (allowing for lower dosing regimens) by coadministering magnesium hydroaspartate (Mg(2+) normally affects the same receptors as ketamine) and also assessed whether an Mg(2+)-deficient diet would modify the impact of ketamine. It was found that a single 15 mg/kg dose of ketamine did indeed induce rapid antidepressant-like effects in the forced swim test but did not affect brain levels of the brain-derived neurotrophic factor. Contrary to our hypothesis, magnesium administration or deficiency did not influence the impact of ketamine on these outcomes. Thus, these data do not support the use of magnesium as an adjunct agent and instead suggest that further research involving other antidepressant and animal models is required to confirm the present findings.

Gender and brain regions specific differences in brain derived neurotrophic factor protein levels of depressed individuals who died through suicide.

Considerable evidence supports the view that depressive illness and suicidal behaviour stem from perturbations of neuroplasticity. Presently, we assessed whether depressed individuals who died by suicide displayed brain region-specific changes in brain derived neurotrophic factor (BDNF) and whether such effects varied by gender. Using postmortem samples from non-psychiatric controls and depressed individuals who died by suicide, BDNF protein levels were assessed within the hippocampus and frontopolar prefrontal cortex using Western blot. As expected, BDNF levels were reduced within the frontopolar prefrontal cortex among female depressed suicides; however, males showed no such effect. Contrastingly, within the hippocampus, depressed male but not female suicides displayed significant reductions of BDNF protein levels. Although the mechanisms driving the gender and brain region specific BDNF changes are unclear, our data do support the notion that complex alterations of neuroplasticity may be fundamentally involved in the illness.

Paraquat and psychological stressor interactions as pertains to Parkinsonian co-morbidity.

A number of epidemiological and experimental studies have implicated the non-selective herbicide, paraquat, in the development of sporadic Parkinson's disease (PD). While preclinical research has focused mainly on elucidating the nigrostriatal effects of paraquat, relatively little data are available concerning non-motor brain systems and inflammatory immune processes (which have been implicated in PD). Hence, in the present study, we sought to take a multi-system approach to characterize the influence of paraquat upon extra-nigrostriatal brain regions, as well ascertain whether the impact of the pesticide might be enhanced in the context of chronic intermittent stressor exposure. Our findings support the contention that paraquat itself acted as a systemic stressor, with the pesticide increasing plasma corticosterone, as well as altering neurochemical activity in the locus coeruleus, paraventricular nucleus of the hypothalamus, nucleus accumbens, dorsal striatum, and central amygdala. However, with the important exception striatal dopamine turnover, the stressor treatment did not further augment these effects. Additionally, paraquat altered inter-cytokine correlations and, to a lesser extent, circulating cytokine levels, and concomitant stress exposure modulated some of these effects. Finally, paraquat provoked significant (albeit modest) reductions of sucrose preference and weight gain, hinting at possible anhendonic-like or sickness responses. These data suggest that, in addition to being a well known oxidative stress generator, paraquat can act as a systemic stressor affecting hormonal and neurochemical activity, but largely not interacting with a concomitant stressor regimen.

IFN-γ differentially modulates memory-related processes under basal and chronic stressor conditions.

Cytokines are inflammatory messengers that orchestrate the brain's response to immunological challenges, as well as possibly even toxic and psychological insults. We previously reported that genetic ablation of the pro-inflammatory cytokine, interferon-gamma (IFN-γ), attenuated some of the corticosteroid, cytokine, and limbic dopaminergic variations induced by 6 weeks of exposure to an unpredictable psychologically relevant stressor. Presently, we sought to determine whether a lack of IFN-γ would likewise modify the impact of chronic stress on hippocampus-dependent memory function and related neurotransmitter and neurotrophin signaling systems. As predicted, chronic stress impaired spatial recognition memory (Y-maze task) in the wild-type animals. In contrast, though the IFN-γ knockouts (KOs) showed memory disturbances in the basal state, under conditions of chronic stress these mice actually exhibited facilitated memory performance. Paralleling these findings, while overall the KOs displayed altered noradrenergic and/or serotonergic activity in the hippocampus and locus coeruleus, norepinephrine utilization in both of these memory-related brain regions was selectively increased among the chronically stressed KOs. However, contrary to our expectations, neither IFN-γ deletion nor chronic stressor exposure significantly affected nucleus accumbens dopaminergic neurotransmission or hippocampal brain-derived neurotrophic factor protein expression. These findings add to a growing body of evidence implicating cytokines in the often differential regulation of neurobehavioral processes in health and disease. Whereas in the basal state IFN-γ appears to be involved in sustaining memory function and the activity of related brain monoamine systems, in the face of ongoing psychologically relevant stress the cytokine may, in fact, act to restrict potentially adaptive central noradrenergic and spatial memory responses.

Use of induced pluripotent stem cell derived neurons engineered to express BDNF for modulation of stressor related pathology.

Combined cell and gene-based therapeutic strategies offer potential in the treatment of neurodegenerative and psychiatric conditions that have been associated with structural brain disturbances. In the present investigation, we used a novel virus-free re-programming method to generate induced pluripotent stem cells (iPSCs), and then subsequently transformed these cells into neural cells which over-expressed brain derived neurotrophic factor (BDNF). Importantly, the infusion of iPSC derived neural cells (as a cell replacement and gene delivery tool) and BDNF (as a protective factor) influenced neuronal outcomes. Specifically, intracerebroventricular transplantation of iPSC-derived neural progenitors that over-expressed BDNF reversed the impact of immune (lipopolysaccharide) and chronic stressor challenges upon subventricular zone adult neurogenesis, and the iPSC-derived neural progenitor cells alone blunted the stressor-induced corticosterone response. Moreover, our findings indicate that mature dopamine producing neurons can be generated using iPSC procedures and appear to be viable when infused in vivo. Taken together, these data could have important implications for using gene-plus-cell replacement methods to modulate stressor related pathology.

Neuroplasticity and the next wave of antidepressant strategies.

Depression is a common chronic psychiatric disorder that is also often co-morbid with numerous neurological and immune diseases. Accumulating evidence indicates that disturbances of neuroplasticity occur with depression, including reductions of hippocampal neurogenesis and cortical synaptogenesis. Improper trophic support stemming from stressor-induced reductions of growth factors, most notably brain derived neurotrophic factor (BDNF), likely drives such aberrant neuroplasticity. We posit that psychological and immune stressors can interact upon a vulnerable genetic background to promote depression by disturbing BDNF and neuroplastic processes. Furthermore, the chronic and commonly relapsing nature of depression is suggested to stem from "faulty wiring" of emotional circuits driven by neuroplastic aberrations. The present review considers depression in such terms and attempts to integrate the available evidence indicating that the efficacy of current and "next wave" antidepressant treatments, whether used alone or in combination, is at least partially tied to their ability to modulate neuroplasticity. We particularly focus on the N-methyl-D-aspartate (NMDA) antagonist, ketamine, which already has well documented rapid antidepressant effects, and the trophic cytokine, erythropoietin (EPO), which we propose as a potential adjunctive antidepressant agent.

Antidepressant-like effects of erythropoietin: a focus on behavioural and hippocampal processes.

Depression is a chronic and debilitating condition with a significant degree of relapse and treatment resistance that could stem, at least in part, from disturbances of neuroplasticity. This has led to an increased focus on treatment strategies that target brain derived neurotrophic factor (BDNF), synaptic plasticity and adult neurogenesis. In the current study we aimed to assess whether erythropoietin (EPO) would have antidepressant-like effects given its already established pro-trophic actions. In particular, we assessed whether EPO would diminish the deleterious effects of a social stressor in mice. Indeed, EPO induced anxiolytic and antidepressant-like responses in a forced swim test, open field, elevated-plus maze, and a novelty test, and appeared to blunt some of the negative behavioural effects of a social stressor. Furthermore, EPO promoted adult hippocampal neurogenesis, an important feature of effective antidepressants. Finally, a separate study using the mTOR inhibitor rapamycin revealed that antagonizing this pathway prevented the impact of EPO upon forced swim performance. These data are consistent with previous findings showing that the mTOR pathway and its neurogenic and synaptogenic effects might mediate the behavioral consequences of antidepressant agents. Our findings further highlight EPO as a possible adjunct treatment for affective disorders, as well as other stressor associated disorders of impaired neuroplasticity.

Cytokines as potential biomarkers for Parkinson's disease: a multiplex approach.

Cytokines, which are immunological messengers facilitating both intra- and inter-system communication, are considered central players in the neuroinflammatory cascades associated with the neurodegenerative process in Parkinson's disease (PD) and other neurological disorders. They have also been implicated in depression and other cognitive (e.g., memory impairment, dementia) and affective disturbances (e.g., anxiety) that show high co-morbidity with neurodegenerative diseases. As such, cytokines may hold great promise as serological biomarkers in PD, with potential applications ranging from early diagnosis and disease staging, to prognosis, drug discovery, and tracking the response to treatment. Subclassification or risk stratification in PD could be based (among other things) on reliably determined cytokine panel profiles or "signatures" of particular co-morbid disease states or at-risk groups (e.g., PD alone, PD with depression and/or dementia). Researchers and clinicians seeking to describe cytokine variations in health vs. disease will benefit greatly from technologies that allow a high degree of multiplexing and thus permit the simultaneous determination of a large roster of cytokines in single small-volume samples. The need for such highly paralleled assays is underscored by the fact that cytokines do not act in isolation but rather against a backdrop of complementary and antagonistic cytokine effects; ascribing valence to the actions of any one cytokine thus requires specific knowledge about the larger cytokine milieu. This chapter provides a technological overview of the major cytokine multiplex assay platforms before discussing the implications of such tools for biomarker discovery and related applications in PD and its depressive and cognitive co-morbidities.

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 Parkinson's 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-1ß [IL-1ß], 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.

The effects of paraquat on regional brain neurotransmitter activity, hippocampal BDNF and behavioural function in female mice.

Accumulating evidence implicates pesticides such as paraquat in the development of Parkinson's disease (PD). Indeed, paraquat exposure is associated with an increased risk of PD and when administered to rodents the pesticide recapitulates many of the neuropathological and behavioural features of the disease. However, it is unclear whether any sexual dimorphism exists in the in vivo murine response to paraquat intoxication, since most studies have used exclusively males. Accordingly, we sought to determine the impact of repeated paraquat exposure on a range of neural and behavioural outcomes in female C57BL/6J mice. The present investigation revealed that the female mice were largely resistant to the paraquat-induced nigrostriatal dopamine changes and locomotor deficits that were reported previously in males. Similarly, in contrast to the reductions of hippocamapal brain-derived neurotrophic factor (BDNF) previously reported in paraquat treated male mice, the herbicide actually increased levels of the trophic factor in females. Yet, similar to our previous findings in males, paraquat increased norepinephrine utilization within the hippocampus and prefrontal cortex of the female mice. However, these changes did not translate into anxiety- or- depression-like behaviours in the open field test, as the females actually seemed to show enhanced exploration. Consistent with reports of a greater incidence of PD in males, these data suggest that female mice may be less susceptible than males to the nigrostriatal dopaminergic and motor effects of environmental toxins. The augmented hippocampal BDNF and noradrenergic changes observed could conceivably act to buffer female mice against some of the deleterious behavioural effects of parquat.

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

Parkinson's 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-gamma deficiency modifies the effects of a chronic stressor in mice: Implications for psychological pathology.

Pro-inflammatory cytokines promote behavioral and neurochemical variations similar to those evident following stressor exposure, and have been implicated in promoting depressive illness. Indeed, immunotherapeutic application of the cytokine, interferon-alpha, promoted depressive illness in cancer and hepatitis C patients. We assessed the possibility that another interferon cytokine family member, interferon-gamma (IFN-gamma), might contribute to the behavioral and biochemical alterations provoked by a chronic stressor regimen that has been used to model neuropsychiatric pathology in rodents. As predicted, IFN-gamma-deficient mice displayed basal differences in behavior (e.g., reduced open field exploration) and altered neurochemical activity (e.g., increased noradrenergic and serotonergic activity within the central amygdala), relative to their wild-type counterparts. Moreover, stressor-induced elevations of corticosterone and the pro-inflammatory cytokine, tumor necrosis factor-alpha, were attenuated in IFN-gamma-deficient mice. Similarly, the IFN-gamma null mice were refractory to the chronic stressor-induced alterations of dopamine metabolism (within the prefrontal cortex, paraventricular nucleus of the hypothalamus and central amygdala) evident in wild-type mice. Yet, the chronic stressor provoked signs of anxiety (e.g., reduced open field exploration) and depression-like behavior (e.g., increased forced swim immobility, reduced consumption of a palatable solution) among both wild-type and IFN-gamma knockout mice alike, suggesting a dissociation of behavioral functioning from the stressor-induced alterations of immunological, hormonal and dopaminergic activity. Together, these data suggest a complex neurobehavioral phenotype, wherein IFN-gamma deletion engenders a state of heightened basal emotionality coupled with increased monoaminergic activity in the amygdala. At the same time, however, IFN-gamma deficiency appears to blunt some of the neurochemical, corticoid and cytokine alterations ordinarily associated with chronic stressor exposure.

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.