Neuroinflammatory Dynamics Underlie Memory Impairments after Repeated Social Defeat.

Repeated social defeat (RSD) is a murine stressor that recapitulates key physiological, immunological, and behavioral alterations observed in humans exposed to chronic psychosocial stress. Psychosocial stress promotes prolonged behavioral adaptations that are associated with neuroinflammatory signaling and impaired neuroplasticity. Here, we show that RSD promoted hippocampal neuroinflammatory activation that was characterized by proinflammatory gene expression and by microglia activation and monocyte trafficking that was particularly pronounced within the caudal extent of the hippocampus. Because the hippocampus is a key area involved in neuroplasticity, behavior, and cognition, we hypothesize that stress-induced neuroinflammation impairs hippocampal neurogenesis and promotes cognitive and affective behavioral deficits. We show here that RSD caused transient impairments in spatial memory recall that resolved within 28 d. In assessment of neurogenesis, the number of proliferating neural progenitor cells (NPCs) and the number of young, developing neurons were not affected initially after RSD. Nonetheless, the neuronal differentiation of NPCs that proliferated during RSD was significantly impaired when examined 10 and 28 d later. In addition, social avoidance, a measure of depressive-like behavior associated with caudal hippocampal circuitry, persisted 28 d after RSD. Treatment with minocycline during RSD prevented both microglia activation and monocyte recruitment. Inhibition of this neuroinflammatory activation in turn prevented impairments in spatial memory after RSD but did not prevent deficits in neurogenesis nor did it prevent the persistence of social avoidance behavior. These findings show that neuroinflammatory activation after psychosocial stress impairs spatial memory performance independent of deficits in neurogenesis and social avoidance. SIGNIFICANCE STATEMENT: Repeated exposure to stress alters the homeostatic environment of the brain, giving rise to various cognitive and mood disorders that impair everyday functioning and overall quality of life. The brain, previously thought of as an immune-privileged organ, is now known to communicate extensively with the peripheral immune system. This brain-body communication plays a significant role in various stress-induced inflammatory conditions, also characterized by psychological impairments. Findings from this study implicate neuroimmune activation rather than impaired neurogenesis in stress-induced cognitive deficits. This idea opens up possibilities for novel immune interventions in the treatment of cognitive and mood disturbances, while also adding to the complexity surrounding the functional implications of adult neurogenesis.

Interleukin 1 type 1 receptor restore: a genetic mouse model for studying interleukin 1 receptor-mediated effects in specific cell types.

Interleukin-1 (IL-1) mediates diverse neurophysiological and neuropathological effects in the CNS through type I IL-1 receptor (IL-1R1). However, identification of IL-1R1-expressing cell types and cell-type-specific functions of IL-1R1 remains challenging. In this study, we created a novel genetic mouse model in which IL-1R1 gene expression is disrupted by an intronic insertion of a loxP flanked disruptive sequence that can be deleted by Cre recombinase, resulting in restored IL-1R1 gene expression under its endogenous promoters. A second mutation was introduced at stop codon of the IL-1R1 gene to allow tracking of the restored IL-1R1 protein by a 3HA tag and IL-1R1 mRNA by tdTomato fluorescence. These animals were designated as IL-1R1(r/r) and exhibited an IL-1R1 knock-out phenotype. We used IL-1R1 globally restored mice (IL-1R1(GR/GR)) as an IL-1R1 reporter and observed concordant labeling of IL-1R1 mRNA and protein in brain endothelial cells. Two cell-type-specific IL-1R1 restore lines were generated: Tie2Cre-IL-1R1(r/r) and LysMCre-IL-1R1(r/r). Brain endothelial COX-2 expression, CNS leukocyte infiltration, and global microglia activation induced by intracerebroventricular injection of IL-1ß were not observed in IL-1R1(r/r) or LysMCre-IL-1R1(r/r) mice, but were restored in Tie2Cre-IL-1R1(r/r) mice. These results reveal IL-1R1 expression in endothelial cells alone is sufficient to mediate these central IL-1-induced responses. In addition, ex vivo IL-1ß stimulation increased IL-1ß expression in bone marrow cells in wild-type, Tie2Cre-IL-1R1(r/r), and LysMCre-IL-1R1(r/r), but not IL-1R1(r/r) mice. These results demonstrate this IL-1R1 restore model is a valuable tool for studying cell-type-specific functions of IL-1R1.

Euflammation attenuates peripheral inflammation-induced neuroinflammation and mitigates immune-to-brain signaling.

Peripheral inflammation can trigger a number of neuroinflammatory events in the CNS, such as activation of microglia and increases of proinflammatory cytokines. We have previously identified an interesting phenomenon, termed "euflammation", which can be induced by repeated subthreshold infectious challenges. Euflammation causes innate immune alterations without overt neuroimmune activation. In the current study, we examined the protective effect of euflammation against peripheral inflammation-induced neuroinflammation and the underlying mechanisms. When Escherichia coli or lipopolysaccharide (LPS) was injected inside or outside the euflammation induction locus (EIL), sickness behavior, global microglial activation, proinflammatory cytokine production in the brain, expression of endothelial cyclooxygenase II and induction of c-fos expression in the paraventricular nucleus of the hypothalamus were all attenuated in the euflammatory mice compared with those in the control unprimed mice. Euflammation also modulated innate immunity outside the EIL by upregulating receptors for pathogen-associated molecular patterns in spleen cells. In addition, euflammation attenuated CNS activation in response to an intra-airpouch (outside the EIL) injection of LPS without suppressing the cytokine expression in the airpouch. Collectively, our study demonstrates that signaling of peripheral inflammation to the CNS is modulated dynamically by peripheral inflammatory kinetics. Specifically, euflammation can offer effective protection against both bacterial infection and endotoxin induced neuroinflammation.

Molecular mechanisms of repeated social defeat-induced glucocorticoid resistance: Role of microRNA.

Glucocorticoid (GC) resistance is a severe problem associated with various inflammatory diseases. Previous studies have shown that repeated social stress induces GC resistance in innate immune cells, but the underlying molecular mechanisms have not been fully elucidated. Therefore, the purpose of this study was to examine potential underlying molecular mechanism(s) of repeated social defeat (RSD) stress on GC resistance in splenic macrophages. It was hypothesized that mRNA expression of receptors for GC and nuclear translocating-associated regulators in splenic macrophages would be affected by RSD, and that these changes would be associated with epigenetic modification. The data showed that the mRNA expression of GC and mineralocorticoid receptors were significantly decreased in splenic macrophages by RSD. RSD also induced a significantly decreased mRNA expression in FK506-binding protein 52 (FKBP52), consequently resulting in a significantly increased ratio of FKBP51 to FKBP52. Moreover, DNA methyltransferases 3a and 3b showed a significant decrease in their mRNA expression in the RSD group as did mRNA expression of histone deacetyltransferase 2. The RSD group also showed a significantly reduced quantity of methylated DNA in splenic macrophages. Based on microRNA (miRNA) profiling data, it was determined that RSD induced significantly increased expression of 9 different miRNAs that were predicted to interact with mRNAs of the GC receptor (6 miRNAs), mineralocorticoid receptor (3 miRNAs) and FKBP52 (2 miRNAs). Spearman correlation analysis revealed significantly strong correlations between the expression of 2 miRNAs and their target mRNA expression for GC receptors. Among these miRNAs, we verified direct effects of miRNA-29b and -340 overexpression on mRNA expression of GC receptors in L929 cells. The overexpression of miRNA-29b or -340 in L929 cells significantly reduced LPS-induced overexpression of GC receptors. In conclusion, this study provides evidence that epigenetic regulation, such as DNA methylation and miRNA expression, may play a role in the RSD-induced GC resistance that we have observed in splenic macrophages.

The prebiotics 3'Sialyllactose and 6'Sialyllactose diminish stressor-induced anxiety-like behavior and colonic microbiota alterations: Evidence for effects on the gut-brain axis.

There are extensive bidirectional interactions between the gut microbiota and the central nervous system (CNS), and studies demonstrate that stressor exposure significantly alters gut microbiota community structure. We tested whether oligosaccharides naturally found in high levels in human milk, which have been reported to impact brain development and enhance the growth of beneficial commensal microbes, would prevent stressor-induced alterations in gut microbial community composition and attenuate stressor-induced anxiety-like behavior. Mice were fed standard laboratory diet, or laboratory diet containing the human milk oligosaccharides 3'Sialyllactose (3'SL) or 6'Sialyllactose (6'SL) for 2 weeks prior to being exposed to either a social disruption stressor or a non-stressed control condition. Stressor exposure significantly changed the structure of the colonic mucosa-associated microbiota in control mice, as indicated by changes in beta diversity. The stressor resulted in anxiety-like behavior in both the light/dark preference and open field tests in control mice. This effect was associated with a reduction in immature neurons in the dentate gyrus as indicated by doublecortin (DCX) immunostaining. These effects were not evident in mice fed milk oligosaccharides; stressor exposure did not significantly change microbial community structure in mice fed 3'SL or 6'SL. In addition, 3'SL and 6'SL helped maintain normal behavior on tests of anxiety-like behavior and normal numbers of DCX+ immature neurons. These studies indicate that milk oligosaccharides support normal microbial communities and behavioral responses during stressor exposure, potentially through effects on the gut microbiota-brain axis.

Kinetic characteristics of euflammation: the induction of controlled inflammation without overt sickness behavior.

We found recently that controlled progressive challenge with subthreshold levels of E. coli can confer progressively stronger resistance to future reinfection-induced sickness behavior to the host. We have termed this type of inflammation "euflammation". In this study, we further characterized the kinetic changes in the behavior, immunological, and neuroendocrine aspects of euflammation. Results show euflammatory animals only display transient and subtle sickness behaviors of anorexia, adipsia, and anhedonia upon a later infectious challenge which would have caused much more severe and longer lasting sickness behavior if given without prior euflammatory challenges. Similarly, infectious challenge-induced corticosterone secretion was greatly ameliorated in euflammatory animals. At the site of E.coli priming injections, which we termed euflammation induction locus (EIL), innate immune cells displayed a partial endotoxin tolerant phenotype with reduced expression of innate activation markers and muted inflammatory cytokine expression upon ex vivo LPS stimulation, whereas innate immune cells outside EIL displayed largely opposite characteristics. Bacterial clearance function, however, was enhanced both inside and outside EIL. Finally, sickness induction by an infectious challenge placed outside the EIL was also abrogated. These results suggest euflammation could be used as an efficient method to "train" the innate immune system to resist the consequences of future infectious/inflammatory challenges.

ß-Adrenergic receptor mediated increases in activation and function of natural killer cells following repeated social disruption.

Natural killer (NK) cells are specialized innate lymphocytes important in the early defense against tumor and virus bearing cells. Many factors influence the immune system's effectiveness against pathogens, including stress. Social disruption (SDR) "primes" macrophages/monocytes and dendritic cells thereby enhancing their anti-microbial function. What remains unclear is whether similar responses are evident in NK cells. Current studies investigated the cellular distribution and activation/inhibitory phenotypes of NK cells in the spleen, lung, and blood of C57BL/6 male mice following SDR. Furthermore, cytolytic activity and anti-viral cytokine production of splenic NK cells were determined. Lastly, ß-adrenergic receptor (ß-AR) signaling was investigated to determine possible mechanisms behind the SDR-induced NK cell alterations. Results indicated NK cells from SDR mice have increased expression of CD16 and CD69 and reduced NKG2a and Ly49a expression on splenic CD3-/DX5+ NK cells indicative of an activated phenotype, both immediately and 14h post-SDR. Administration of propranolol (10mg/kg; non-selective ß-adrenergic receptor antagonist) was shown to block these "priming" effects at the 14h time-point. In the lung, SDR had similar effects on activation and inhibitory receptors 14h post-SDR, however no alterations were evident in the blood besides increased NK cells directly after SDR. Additionally, splenic NK cells from SDR mice had increased CD107a surface expression, cytolytic activity, and IFN-γ production was increased upon costimulation with IgG and IL-2 ex vivo. Collectively, these data suggest that social stress "primes" NK cells in the spleen and lung to be more proficient in their cytolytic and anti-viral/tumor effecter functions through ß-adrenergic receptor dependent signaling.

Brachial plexopathy as a complication of COVID-19.

COVID-19 affects a wide spectrum of organ systems. We report a 52-year-old man with hypertension and newly diagnosed diabetes mellitus who presented with hypoxic respiratory failure due to COVID-19 and developed severe brachial plexopathy. He was not treated with prone positioning respiratory therapy. Associated with the flaccid, painfully numb left upper extremity was a livedoid, purpuric rash on his left hand and forearm consistent with COVID-19-induced microangiopathy. Neuroimaging and electrophysiological data were consistent with near diffuse left brachial plexitis with selective sparing of axillary, suprascapular and pectoral fascicles. Given his microangiopathic rash, elevated D-dimers and paucifascicular plexopathy, we postulate a patchy microvascular thrombotic plexopathy. Providers should be aware of this significant and potentially under-recognised neurologic complication of COVID-19.

BACE1 partial deletion induces synaptic plasticity deficit in adult mice.

BACE1 is the first enzyme involved in APP processing, thus it is a strong therapeutic target candidate for Alzheimer's disease. The observation of deleterious phenotypes in BACE1 Knock-out (KO) mouse models (germline and conditional) raised some concerns on the safety and tolerability of BACE1 inhibition. Here, we have employed a tamoxifen inducible BACE1 conditional Knock-out (cKO) mouse model to achieve a controlled partial depletion of BACE1 in adult mice. Biochemical and behavioural characterization was performed at two time points: 4-5 months (young mice) and 12-13 months (aged mice). A ~50% to ~70% BACE1 protein reduction in hippocampus and cortex, respectively, induced a significant reduction of BACE1 substrates processing and decrease of Aßx-40 levels at both ages. Hippocampal axonal guidance and peripheral nerve myelination were not affected. Aged mice displayed a CA1 long-term potentiation (LTP) deficit that was not associated with memory impairment. Our findings indicate that numerous phenotypes observed in germline BACE1 KO reflect a fundamental role of BACE1 during development while other phenotypes, observed in adult cKO, may be absent when partially rather than completely deleting BACE1. However, we demonstrated that partial depletion of BACE1 still induces CA1 LTP impairment, supporting a role of BACE1 in synaptic plasticity in adulthood.

Influence of prenatal stress on behavioral, endocrine, and cytokine responses to adulthood bacterial endotoxin exposure.

Prior research suggests that prenatal stress, among other effects, can lead to hyper-reactivity of the offspring's hypothalamic-pituitary-adrenal (HPA) axis and alterations in immune function. These stress-induced changes have been linked to a greater propensity to develop depression or anxiety disorders. Furthermore, prenatally stressed offspring may be more susceptible to certain diseases. The immune alterations induced by prenatal stress exposure may disrupt the normal communication between the immune system, endocrine system, and central nervous system, potentially making prenatally stressed individuals more vulnerable to the negative aspects of immune activation, including cytokine-induced cognitive deficits and anxiety. The present study investigated whether prenatal stress would exaggerate these detrimental effects of peripheral immune activation. We hypothesized that prenatally stressed subjects would be hypersensitive to endotoxin administration and would therefore show exaggerated learning deficits, increased anxiety-like behavior, and increased peripheral and central interleukin-1beta (IL-1beta) levels. The observed results only partially supported our hypotheses, as prenatally stressed subjects showed evidence, albeit modest, of increased anxiety-like behavior following endotoxin administration relative to non-stressed controls. While prenatal stress exposure or lipopolysaccharide (LPS) administration independently impaired learning, the data failed to support the hypothesis that prenatally stressed subjects would show exaggerated cognitive deficits, engendered via enhanced peripheral and central IL-1beta levels, following immune activation. Collectively, the data suggest that although prenatal stress exposure led to increases in anxiety-like behavior following endotoxin exposure, it did not appear to increase susceptibility to LPS-induced cognitive decline or elevations in proinflammatory cytokine production.

Neonatal endotoxin exposure impairs avoidance learning and attenuates endotoxin-induced sickness behavior and central IL-1beta gene transcription in adulthood.

Infection during infancy, a time of critical neural development, may have long-term implications. Infection or exposure to an immune stimulus such as lipopolysaccharide (LPS) early in life leads to alterations in the reactivity of the hypothalamic-pituitary-adrenal axis (HPA) and febrile response in adulthood. Relatively few studies have assessed the behavioral and cognitive alterations induced by perinatal immune challenge. The data indicate that neonatal immune activation may alter adulthood behavior with, or sometimes without, subsequent adulthood exposure, depending on the study. The current study investigated the behavioral effects and IL-1beta transcription following intraperitoneal LPS administration on postnatal days (PNDs) 4 and 5, and subsequent LPS or saline administration in adulthood. Alterations in anxiety, motor behavior, and learning were assessed in male and female subjects. The results indicate that neonatal endotoxin exposure attenuated the LPS-induced decrease in motor behavior in female, but not male, subjects. Furthermore, perinatal immune activation disrupted avoidance learning in male, but not female, subjects in the absence of adulthood LPS administration. In addition, for male subjects, neonatal LPS exposure diminished central IL-1beta gene transcription following adulthood LPS administration. These findings indicate that perinatal endotoxin exposure may lead to alterations in the behavioral response to adulthood LPS administration, and provide evidence that early immune activation alone may trigger alterations in adulthood learning ability.

Age increases vulnerability to bacterial endotoxin-induced behavioral decrements.

Peripheral lipopolysaccharide (LPS) or proinflammatory cytokines produce alterations in learning, memory, and other behaviors. Additionally, research has demonstrated that factors such as dose, route of administration, species, strain, gender, and age are important modulatory factors in the effects of endotoxin exposure. Previous research from our laboratory and others indicate that LPS-induced behavioral deficits are greater in older subjects. The current study examined avoidance learning in a negatively reinforced operant procedure (i.e., two-way active avoidance conditioning) following single or repeated intraperitoneal LPS injections in 2- and 12-month-old male C57BL/6J mice. LPS-treated subjects show impaired acquisition of the task regardless of the age of the subject, as these animals performed significantly fewer avoidance responses than controls. However, the effects of LPS administration were more pronounced in the 12-month-old animals, particularly for the subjects given repeated LPS injections. These results support the hypothesis that endotoxin exposure is capable of altering performance in this task in a way that may reflect deficits in learning, and provide evidence that increased age may exacerbate these deleterious behavioral effects.

Prostacyclin mediates endothelial COX-2-dependent neuroprotective effects during excitotoxic brain injury.

In a previous study, we found that intracerebral administration of excitotoxin (RS)-(tetrazole-5yl) glycine caused increased neural damage in the brain in an endothelial COX-2 deleted mouse line (Tie2Cre COX-2(flox/flox) ). In this study, we investigated whether prostacyclin might mediate this endothelial COX-2-dependent neuroprotection. Administration of excitotoxin into the striatum induced the production of prostacyclin (PGI2) in wild type, but not in endothelial COX-2 deleted mice. Inhibition of PGI2 synthase exacerbated brain lesions induced by the excitotoxin in wild type, but not in endothelial COX-2 deleted mice. Administration of a PGI2 agonist reduced neural damage in both wild type and endothelial COX-2 deleted mice. Increased PGI2 synthase expression was found in infiltrating neutrophils. In an ex vivo assay, PGI2 reduced the excitotoxin-induced calcium influx into neurons, suggesting a cellular mechanism for PGI2 mediated neuroprotection. These results reveal that PGI2 mediates endothelial COX-2 dependent neuroprotection.

Re-establishment of anxiety in stress-sensitized mice is caused by monocyte trafficking from the spleen to the brain.

BACKGROUND: Persistent anxiety-like symptoms may have an inflammatory-related pathophysiology. Our previous work using repeated social defeat (RSD) in mice showed that recruitment of peripheral myeloid cells to the brain is required for the development of anxiety. Here, we aimed to determine if 1) RSD promotes prolonged anxiety through redistribution of myeloid cells and 2) prior exposure to RSD sensitizes the neuroimmune axis to secondary subthreshold stress. METHODS: Mice were subjected to RSD and several immune and behavioral parameters were determined .5, 8, or 24 days later. In follow-up studies, control and RSD mice were subjected to subthreshold stress at 24 days. RESULTS: Repeated social defeat-induced macrophage recruitment to the brain corresponded with development and maintenance of anxiety-like behavior 8 days after RSD, but neither remained at 24 days. Nonetheless, social avoidance and an elevated neuroinflammatory profile were maintained at 24 days. Subthreshold social defeat in RSD-sensitized mice increased peripheral macrophage trafficking to the brain that promoted re-establishment of anxiety. Moreover, subthreshold social defeat increased social avoidance in RSD-sensitized mice compared with naïve mice. Stress-induced monocyte trafficking was linked to redistribution of myeloid progenitor cells in the spleen. Splenectomy before subthreshold stress attenuated macrophage recruitment to the brain and prevented anxiety-like behavior in RSD-sensitized mice. CONCLUSIONS: These data indicate that monocyte trafficking from the spleen to the brain contributes re-establishment of anxiety in stress-sensitized mice. These findings show that neuroinflammatory mechanisms promote mood disturbances following stress-sensitization and outline novel neuroimmune interactions that underlie recurring anxiety disorders such as posttraumatic stress disorder.

Prolonged restraint stress increases IL-6, reduces IL-10, and causes persistent depressive-like behavior that is reversed by recombinant IL-10.

Altered inflammatory cytokine profiles are often observed in individuals suffering from major depression. Recent clinical work reports on elevated IL-6 and decreased IL-10 in depression. Elevated IL-6 has served as a consistent biomarker of depression and IL-10 is proposed to influence depressive behavior through its ability to counterbalance pro-inflammatory cytokine expression. Clinical and animal studies suggest a role for IL-10 in modifying depressive behavior. Murine restraint stress (RST) is regularly employed in the study of behavioral and biological symptoms associated with depressive disorders. While responses to acute RST exposure have been widely characterized, few studies have examined the ongoing and longitudinal effects of extended RST and fewer still have examined the lasting impact during the post-stress period. Consistent with clinical data, we report that a protocol of prolonged murine RST produced altered cytokine profiles similar to those observed in major depressive disorder. Parallel to these changes in circulating cytokines, IL-10 mRNA expression was diminished in the cortex and hippocampus throughout the stress period and following cessation of RST. Moreover, chronic RST promoted depressive-like behavior throughout the 28-day stress period and these depressive-like complications were maintained weeks after cessation of RST. Because of the correlation between IL-10 suppression and depressive behavior and because many successful antidepressant therapies yield increases in IL-10, we examined the effects of IL-10 treatment on RST-induced behavioral changes. Behavioral deficits induced by RST were reversed by exogenous administration of recombinant IL-10. This work provides one of the first reports describing the biological and behavioral impact following prolonged RST and, taken together, this study provides details on the correlation between responses to chronic RST and those seen in depressive disorders.

Controlled progressive innate immune stimulation regimen prevents the induction of sickness behavior in the open field test.

Peripheral immune activation by bacterial mimics or live replicating pathogens is well known to induce central nervous system activation. Sickness behavior alterations are often associated with inflammation-induced increases in peripheral proinflammatory cytokines (eg, interleukin [IL]-1ß and IL-6). However, most researchers have used acute high dose endotoxin/bacterial challenges to observe these outcomes. Using this methodology may pose inherent risks in the translational interpretation of the experimental data in these studies. Studies using Escherichia coli have yet to establish the full kinetics of repeated E. coli peripheral injections. Therefore, we sought to examine the effects of repeated low dose E. coli on sickness behavior and local peripheral inflammation in the open field test. Results from the current experiments showed a behavioral dose response, where increased amounts of E. coli resulted in correspondingly increased sickness behavior. Furthermore, animals that received a subthreshold dose (ie, one that did not cause sickness behavior) of E. coli 24 hours prior were able to withstand a larger dose of E. coli on the second day (a dose that would normally cause sickness behavior in mice without prior exposure) without inducing sickness behavior. In addition, animals that received escalating subthreshold doses of E. coli on days 1 and 2 behaviorally tolerated a dose of E. coli 25 times higher than what would normally cause sickness behavior if given acutely. Lastly, increased levels of E. coli caused increased IL-6 and IL-1ß protein expression in the peritoneal cavity, and this increase was blocked by administering a subthreshold dose of E. coli 24 hours prior. These data show that progressive challenges with subthreshold levels of E. coli may obviate the induction of sickness behavior and proinflammatory cytokine expression.

Neural and behavioral responses to low-grade inflammation.

Neural and behavioral responses after peripheral immune challenge have been observed in numerous studies. The majority of these studies have utilized relatively high doses of lipopolysaccharide (LPS) as the immune stimulant. Little attention has been given to the effects of LPS dose ranges that simulate low grade-inflammation. The current studies were designed to characterize neural and behavioral responses following low-dose LPS stimulation. Results show burrowing and open field activity was significantly impaired following a single i.p. injection of 10, but not 1, µg/kg of LPS. In addition, following repeated 1 µg/kg LPS administration for 10 days, animals showed the progressive development of motor deficits over time. To correlate behavior with CNS activity, cFos activation was determined in the paraventricular nucleus, nucleus of the solitary tract, central amygdaloid nucleus, and ventrolateral medulla. Data revealed there was a dose-dependent activation in all brain areas examined, but only the PVN showed significant activation by low-dose LPS. Additionally, animals that received 1 µg/kg of LPS for 8 days had PVN cFos activation similar to animals that received a single 10 µg/kg LPS injection. These data demonstrate neural and behavior responses can be induced by low-grade inflammation and chronic exposure to sub-threshold levels of LPS can precipitate significantly heightened neural and behavioral responses.

The effects of age on lipopolysaccharide-induced cognitive deficits and interleukin-1ß expression.

An acute LPS challenge immediately following day 1 of shuttlebox training triggered exacerbated central IL-1ß production and disrupted memory consolidation and/or further acquisition of the task in 18-month-old mice, compared to 4-month-old controls. These deficits cannot be attributed to alterations in sickness behavior. The findings suggest that age and immune activation combine to impair learning and memory consolidation processes, and that increased central IL-1ß production may play a role.

Systemic lipopolysaccharide plus MPTP as a model of dopamine loss and gait instability in C57Bl/6J mice.

In most environmental models of Parkinson's disease (PD), a single neurodegenerative agent is introduced to cause nigrostriatal dopamine depletion. However, cell loss in human PD often might derive, at least in part, from multiple toxins or vulnerabilities, any one of which alone does not inevitably lead to chronic dopamine depletion. In the present research, male C57BL/6J mice were systemically administered the inflammatory bacterial endotoxin, lipopolysaccharide (LPS) and the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) alone or in combination and the behavior as well as striatal dopamine levels were compared to saline-treated mice. Mice in the combination (LPS+MPTP) group, but not in the single-factor groups, showed both dopamine depletion and parkinsonian symptoms, i.e., reduced stride length, at 4 months post-injection. MPTP alone acutely reduced striatal dopamine levels but this effect was transient as striatal dopamine recovered to normal levels after time (4 months). The LPS-only group showed no dopamine depletion or reduced stride length. These data are consistent with the view that nigrostriatal dopamine neurons might succumb after time to multiple toxic agents that independently may have only a transient, adverse effect.