The effects of the bacterial endotoxin lipopolysaccharide on synaptic transmission and plasticity in the CA1-subiculum pathway in vivo

The potential therapeutic role of itaconate and mesaconate on the detrimental effects of LPS-induced neuroinflammation in the brain

Despite advances in antimicrobial and anti-inflammatory treatment, inflammation and its consequences remain a major challenge in the field of medicine. Inflammatory reactions can lead to life-threatening conditions such as septic shock, while chronic inflammation has the potential to worsen the condition of body tissues and ultimately lead to significant impairment of their functionality. Although the central nervous system has long been considered immune privileged to peripheral immune responses, recent research has shown that strong immune responses in the periphery also affect the brain, leading to reactive microglia, which belong to the innate immune system and reside in the brain, and neuroinflammation. The inflammatory response is primarily a protective mechanism to defend against pathogens and tissue damage. However, excessive and chronic inflammation can have negative effects on neuronal structure and function. Neuroinflammation underlies the pathogenesis of many neurological and neurodegenerative diseases and can accelerate their progression. Consequently, targeting inflammatory signaling pathways offers potential therapeutic strategies for various neuropathological conditions, particularly Parkinson's and Alzheimer's disease, by curbing inflammation. Here the blood-brain barrier is a major hurdle for potential therapeutic strategies, therefore it would be highly advantageous to foster and utilize brain innate anti-inflammatory mechanisms. The tricarboxylic acid cycle-derived metabolite itaconate is highly upregulated in activated macrophages and has been shown to act as an immunomodulator with anti-inflammatory and antimicrobial functions. Mesaconate, an isomer of itaconate, similarly reduces the inflammatory response in macrophages. Nevertheless, most studies have focused on its esterified forms and its peripheral effects, while its influence on the CNS remained largely unexplored. Therefore, this study investigated the immunomodulatory and therapeutic potential of endogenously synthesized itaconate and its isomer mesaconate in lipopolysaccharide (LPS)-induced neuroinflammatory processes. Our results show that both itaconate and mesaconate reduce LPS-induced neuroinflammation, as evidenced by lower levels of inflammatory mediators, reduced microglial reactivity and a rescue of synaptic plasticity, the cellular correlate of learning and memory processes in the brain. Overall, this study emphasizes that both itaconate and mesaconate have therapeutic potential for neuroinflammatory processes in the brain and are of remarkable importance due to their endogenous origin and production, which usually leads to high tolerance.

Intermittent systemic exposure to lipopolysaccharide-induced inflammation disrupts hippocampal long-term potentiation and impairs cognition in aging male mice

Age-related cognitive decline, a common component of the brain aging process, is associated with significant impairment in daily functioning and quality of life among geriatric adults. While the complexity of mechanisms underlying cognitive aging are still being elucidated, microbial exposure and the multifactorial inflammatory cascades associated with systemic infections are emerging as potential drivers of neurological senescence. The negative cognitive and neurobiological consequences of a single pathogen-associated inflammatory experience, such as that modeled through treatment with lipopolysaccharide (LPS), are well documented. Yet, the brain aging impacts of repeated, intermittent inflammatory challenges are less well studied. To extend the emerging literature assessing the impact of infection burden on cognitive function among normally aging mice, here, we repeatedly exposed adult mice to intermittent LPS challenges during the aging period. Male 10-month-old C57BL6 mice were systemically administered escalating doses of LPS once every two weeks for 2.5 months. We evaluated cognitive consequences using the non-spatial step-through inhibitory avoidance task, and both spatial working and reference memory versions of the Morris water maze. We also probed several potential mechanisms, including cortical and hippocampal cytokine/chemokine gene expression, as well as hippocampal neuronal function via extracellular field potential recordings. Though there was limited evidence for an ongoing inflammatory state in cortex and hippocampus, we observed impaired learning and memory and a disruption of hippocampal long-term potentiation. These data suggest that a history of intermittent exposure to LPS-induced inflammation is associated with subtle but significantly impaired cognition among normally aging mice. The broader impact of these findings may have important implications for standard of care involving infections in aging individuals or populations at-risk for dementia.

IL-37 expression reduces acute and chronic neuroinflammation and rescues cognitive impairment in an Alzheimer's disease mouse model

The anti-inflammatory cytokine interleukin-37 (IL-37) belongs to the IL-1 family but is not expressed in mice. We used a human IL-37 (hIL-37tg) expressing mouse, which has been subjected to various models of local and systemic inflammation as well as immunological challenges. Previous studies reveal an immunomodulatory role of IL-37, which can be characterized as an important suppressor of innate immunity. Here, we examined the functions of IL-37 in the central nervous system and explored the effects of IL-37 on neuronal architecture and function, microglial phenotype, cytokine production and behavior after inflammatory challenge by intraperitoneal LPS-injection. In wild-type mice, decreased spine density, activated microglial phenotype and impaired long-term potentiation (LTP) were observed after LPS injection, whereas hIL-37tg mice showed no impairment. In addition, we crossed the hIL-37tg mouse with an animal model of Alzheimer’s disease (APP/PS1) to investigate the anti-inflammatory properties of IL-37 under chronic neuroinflammatory conditions. Our results show that expression of IL-37 is able to limit inflammation in the brain after acute inflammatory events and prevent loss of cognitive abilities in a mouse model of AD.

Monitoring and Modulating Inflammation-Associated Alterations in Synaptic Plasticity: Role of Brain Stimulation and the Blood-Brain Interface

Inflammation of the central nervous system can be triggered by endogenous and exogenous stimuli such as local or systemic infection, trauma, and stroke. In addition to neurodegeneration and cell death, alterations in physiological brain functions are often associated with neuroinflammation. Robust experimental evidence has demonstrated that inflammatory cytokines affect the ability of neurons to express plasticity. It has been well-established that inflammation-associated alterations in synaptic plasticity contribute to the development of neuropsychiatric symptoms. Nevertheless, diagnostic approaches and interventional strategies to restore inflammatory deficits in synaptic plasticity are limited. Here, we review recent findings on inflammation-associated alterations in synaptic plasticity and the potential role of the blood–brain interface, i.e., the blood–brain barrier, in modulating synaptic plasticity. Based on recent findings indicating that brain stimulation promotes plasticity and modulates vascular function, we argue that clinically employed non-invasive brain stimulation techniques, such as transcranial magnetic stimulation, could be used for monitoring and modulating inflammation-induced alterations in synaptic plasticity.

Metformin Ameliorates Lipopolysaccharide-Induced Depressive-Like Behaviors and Abnormal Glutamatergic Transmission

Simple Summary

Metformin is a promising drug for diabetes and has been reported to have antidepressant effects in depression patients or patients with comorbid depression and other diseases. However, it is largely unclear how metformin ameliorates depressive-like behaviors. To this end, we injected mice with a bacterial endotoxin (lipopolysaccharide) to induce depressive-like behaviors such as increased immobility in the forced swimming test and tail suspension test. In this depression mouse model, metformin administration ameliorated depressive-like behaviors. Glutamate is a major excitatory signal for the communications between neurons in the brain. Dysfunction of glutamatergic neurotransmission is implicated in the pathogenesis of depression. Glutamatergic transmission was elevated in our depression mouse model. Metformin administration also recovered the glutamatergic transmission deficit in the model. Taken together, our results suggest metformin had antidepressant effects and can correct abnormal glutamatergic transmission in the lipopolysaccharide-induced depression mouse model. These findings provide new insights into the underlying mechanism by which metformin acts against depression.

Abstract

Metformin, a first-line drug for type 2 diabetes mellitus (T2DM), has been found to reduce depressive symptoms in patients with comorbid depression and other diseases. However, it is largely unclear how metformin ameliorates depressive-like behaviors. Here, we used lipopolysaccharide (LPS) to induce depressive-like behaviors in mice and found that LPS-treated mice exhibited increased immobility in the forced swimming test (FST) and tail suspension test (TST), as well as increased glutamatergic transmission. Furthermore, metformin administration in the LPS-treated mice ameliorated depressive-like behaviors and elevated glutamatergic transmission. Our results suggest that metformin has antidepressant effects and can correct abnormal glutamatergic transmission, providing an insight into the underlying mechanism by which metformin acts against depression.

Baicalin alleviates lipopolysaccharide-induced neuroglial activation and inflammatory factors activation in hippocampus of adult mice

Baicalin is a natural flavonoid that exerts a variety of pharmaceutical effects such as anti-inflammatory and antioxidant. Lipopolysaccharide (LPS) is an endotoxin that releases inflammatory cytokines and induces inflammatory response. This study was investigated the anti-inflammatory mechanism of baicalin against LPS-induced inflammatory response in the hippocampus. Adult mice were randomly grouped into control, LPS-treated, and LPS and baicalin co-treated animals. LPS (250 μg/kg/day) and baicalin (10 mg/kg/day) were administered intraperitoneally for 7 consecutive days. We measured neuroglia cells activation and inflammatory factors activation using Western blot analysis and immunofluorescence staining techniques. Ionized calcium binding adaptor molecule-1 (Iba-1) and glial fibrillary acidic protein (GFAP) are widely used as microglia and astrocyte markers, respectively. LPS treatment increased Iba-1 and GFAP expression, while baicalin co-treatment attenuated this overexpression. Nuclear factor-kappa B (NF-κB) is a key mediator of inflammation. Baicalin co-treatment alleviated LPS-induced increase of NF-κB in the hippocampus. In addition, LPS treatment upregulated pro-inflammatory cytokines including interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α). However, baicalin co-treatment prevented LPS-induced increases of IL-1β and TNF-α in the hippocampus. Results from the present study showed that baicalin suppresses LPS-induced neuroinflammation by regulating microglia and astrocyte activation and modulating inflammatory factors in the hippocampus. Thus, these results demonstrate that baicalin has neuroprotective effect by alleviates microglia and astrocyte activation and modulates inflammatory response by suppressing NF-κB expression in hippocampus with neuroinflammation caused by LPS.

Enduring Changes in Neuronal Function upon Systemic Inflammation Are NLRP3 Inflammasome Dependent

Neuroinflammation can be caused by various insults to the brain and represents an important pathologic hallmark of neurodegenerative diseases including Alzheimer's disease (AD). Infection-triggered acute systemic inflammation is able to induce neuroinflammation and may negatively affect neuronal morphology, synaptic plasticity, and cognitive function. In contrast to acute effects, persisting consequences for the brain on systemic immune stimulation remain largely unexplored. Here, we report an age-dependent vulnerability of wild-type (WT) mice of either sex toward a systemic immune stimulation by Salmonella typhimurium lipopolysaccharide (LPS). Decreased neuronal complexity three months after peripheral immune stimulation is accompanied by impairment in long-term potentiation (LTP) and spatial learning. Aged APP/PS1 mice reveal an increased sensitivity also to LPS of Escherichia coli, which had no effect in WT mice. We further report that these effects are mediated by NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome activation, since the genetic ablation and pharmacological inhibition using the NLRP3 inhibitor MCC950 rescue the morphological and electrophysiological phenotype.SIGNIFICANCE STATEMENT Acute peripheral immune stimulation has been shown to have both positive and negative effects on Aβ deposition. Improvements or worsening may be possible in acute inflammation. However, there is still no evidence of effects longer than a month after stimulation. The data are pointing to an important role of the NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome for mediating the long-term consequences of systemic immune stimulation, which in addition turns out to be age dependent.

Memory enhancing effect of Nigella Sativa hydro-alcoholic extract on lipopolysaccharide-induced memory impairment in rats

In this study, the effects of Nigella Sativa (NS) hydro-alcoholic extract on lipopolysaccharide (LPS)-induced learning and memory impairments, hippocampal cytokine levels, and brain tissues oxidative damage were investigated in rats. The rats were grouped and treated: (1) control (saline), (2) LPS (1 mg/kg i.p.), and (3-5) 100, 200, or 400 mg/kg NS hydro-alcoholic extract 30 min before LPS injection. The treatment was started since 6 days before the behavioral experiments and continued during the behavioral tests (LPS injection 2 h before each behavioral experiment). Finally, the brains were removed for biochemical assessments. In Morris water maze (MWM) test, LPS increased the escape latency and traveled path compared to control group, whereas all doses of NS hydro-alcoholic extract decreased them compared to LPS group. In passive avoidance (PA) test, the latency to enter the dark compartment in LPS group was shorter than control group while in all treated groups it was longer than LPS group. LPS increased tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), malondialdehyde (MDA), and nitric oxide (NO) metabolites, and decreased thiol content, superoxide dismutase (SOD), and catalase (CAT) in the hippocampal tissues compared to control group while NS hydro-alcoholic extract decreased MDA and NO metabolites and increased thiol content, SOD, and CAT compared to LPS group. Findings of the current study indicated that the hydro-alcoholic extract of NS improved the LPS-induced learning and memory impairments induced by LPS in rats by improving hippocampal cytokine levels and brain tissues oxidative damage.

Pharmacological Tools to Activate Microglia and their Possible use to Study Neural Network Patho-physiology

BACKGROUND

Microglia are the resident immunocompetent cells of the CNS and also constitute a unique cell type that contributes to neural network homeostasis and function. Understanding microglia cell-signaling not only will reveal their diverse functions but also will help to identify pharmacological and non-pharmacological tools to modulate the activity of these cells.

METHODS

We undertook a search of bibliographic databases for peer-reviewed research literature to identify microglial activators and their cell-specificity. We also looked for their effects on neural network function and dysfunction.

RESULTS

We identified several pharmacological targets to modulate microglial function, which are more or less specific (with the proper control experiments). We also identified pharmacological targets that would require the development of new potent and specific modulators. We identified a wealth of evidence about the participation of microglia in neural network function and their alterations in pathological conditions.

CONCLUSION

The identification of specific microglia-activating signals provides experimental tools to modulate the activity of this heterogeneous cell type in order to evaluate its impact on other components of the nervous system, and it also helps to identify therapeutic approaches to ease some pathological conditions related to microglial dysfunction.

Lipopolysaccharide-Induced Spatial Memory and Synaptic Plasticity Impairment Is Preventable by Captopril

Introduction. Renin-angiotensin system has a role in inflammation and also is involved in many brain functions such as learning, memory, and emotion. Neuroimmune factors have been proposed as the contributors to the pathogenesis of memory impairments. In the present study, the effect of captopril on spatial memory and synaptic plasticity impairments induced by lipopolysaccharide (LPS) was investigated. Methods. The rats were divided and treated into control (saline), LPS (1 mg/kg), LPS-captopril (LPS-Capto; 50 mg/kg captopril before LPS), and captopril groups (50 mg/kg) before saline. Morris water maze was done. Long-term potentiation (LTP) from CA1 area of hippocampus was assessed by 100 Hz stimulation in the ipsilateral Schaffer collateral pathway. Results. In the LPS group, the spent time and traveled path to reach the platform were longer than those in the control, while, in the LPS-Capto group, they were shorter than those in the LPS group. Moreover, the slope and amplitude of field excitatory postsynaptic potential (fEPSP) decreased in the LPS group, as compared to the control group, whereas, in the LPS-Capto group, they increased compared to the LPS group. Conclusion. The results of the present study showed that captopril improved the LPS-induced memory and LTP impairments induced by LPS in rats. Further investigations are required in order to better understand the exact responsible mechanism(s).

Lipolysaccharide-Induced Neuroinflammation Is Associated with Alzheimer-Like Amyloidogenic Axonal Pathology and Dendritic Degeneration in Rats

Chronic neuroinflammation is thought to play an etiological role in Alzheimer's disease (AD), which is characterized pathologically by amyloid and tau formation, as well as neuritic dystrophy and synaptic degeneration. The causal relationship between these pathological events is a topic of ongoing research and discussion. Recent data from transgenic AD models point to a tight spatiotemporal link between neuritic and amyloid pathology, with the obligatory enzyme for β-amyloid (Aβ) production, namely β-secretase-1 (BACE1), is overexpressed in axon terminals undergoing dystrophic change. However, the axonal pathology inherent with BACE1 elevation seen in transgenic AD mice may be secondary to increased soluble Aβ in these genetically modified animals. Here we explored the occurrence of the AD-like axonal and dendritic pathology in adult rat brain affected by LPS-induced chronic neuroinflammation. Unilateral intracerebral LPS injection induced prominent inflammatory response in glial cells in the ipsilateral cortex and hippocampal formation. BACE1 protein levels were elevated the ipsilateral hippocampal lysates in the LPS treated animals relative to controls. BACE1 immunoreactive dystrophic axons appeared in the LPS-treated ipsilateral cortex and hippocampal formation, colocalizing with increased β-amyloid precursor protein and Aβ antibody (4G8) immunolabeling. Quantitative Golgi studies revealed reduction of dendritic branching points and spine density on cortical layer III and hippocampal CA3 pyramidal neurons in the LPS-treated ipsilateral cerebrum. These findings suggest that Alzheimer-like amyloidogenic axonal pathology and dendritic degeneration occur in wildtype mammalian brain in partnership with neuroinflammation following LPS injection.

Acute stress and hippocampal output: exploring dorsal CA1 and subicular synaptic plasticity simultaneously in anesthetized rats

The Cornu Ammonis-1 (CA1) subfield and subiculum (SUB) serve as major output structures of the hippocampal formation. Exploring forms of synaptic plasticity simultaneously within these two output regions may improve understanding of the dynamics of hippocampal circuitry and information transfer between hippocampal and cortical brain regions. Using a novel dual-channel electrophysiological preparation in urethane-anesthetized adult male Sprague-Dawley rats in vivo, we examined the effects of acute restraint stress (30 min) on short- and long-term forms of synaptic plasticity in both CA1 and SUB by stimulating the CA3 region. Paired-pulse facilitation was disrupted in SUB but not CA1 in the dual-channel experiments following exposure to acute stress. Disruptions in CA1 PPF were evident in subsequent single-channel experiments with a more anterior recording site. Acute stress disrupted long-term potentiation induced by high-frequency stimulation (10 bursts of 20 pulses at 200 Hz) in both CA1 and SUB. Low-frequency stimulation (900 pulses at 1 Hz) did not alter CA1 plasticity while a late-developing potentiation was evident in SUB that was disrupted following exposure to acute stress. These findings highlight differences in the sensitivity to acute stress for distinct forms of synaptic plasticity within synapses in hippocampal output regions. The findings are discussed in relation to normal and aberrant forms of hippocampal-cortical information processing.

Cytokines induced by long-term potentiation (LTP) recording: a potential explanation for the lack of correspondence between learning/memory performance and LTP

The relationship between learning/memory performance and long-term potentiation (LTP) induction is ambiguous. Although a large body of data supports a strong correspondence between learning/memory performance and LTP, many studies have also provided evidence to the contrary. In this study, we found that 2-month-old senescence-accelerated mice/prone 8 (SAMP8 mice) displayed both impaired performance in a Morris Water Maze (MWM) and enhanced LTP compared to senescence-accelerated mice/resistance 1 (SAMR1). BALB/c mice challenged with Complete Freund's Adjuvant (CFA) performed better in the shuttle-box test but displayed impaired LTP compared to intact animals. It is interesting that BALB/c mice challenged with Incomplete Freund's Adjuvant (IFA) performed better than intact animals, with no LTP impairment. Cytokine analysis showed no significant differences between the interleukin-6 (IL-6), interleukin-10 (IL-10) or TNF-α content in the intact hippocampal tissues of either the SAMR1 and SAMP8 mice or the immune-challenged BALB/c and intact animals. Further analysis demonstrated that the increase in cytokine content was higher in the hippocampal tissues used for LTP recording in the SAMR1 and CFA-challenged animals compared to the SAMP8 and intact BALB/c mice. A correlation analysis demonstrated that pro-inflammatory cytokines (IL-6 and TNF-α) displayed a negative correlation with LTP, while an anti-inflammatory cytokine (IL-10) displayed a positive correlation with LTP. These results suggest that pro-inflammatory cytokines induced by LTP manipulation in experiments (e.g., via tissue injury caused by electrode insertion) may be one of the factors contributing to the observed lack of correspondence between memory/learning ability and LTP.

Acute stress, but not corticosterone, disrupts short- and long-term synaptic plasticity in rat dorsal subiculum via glucocorticoid receptor activation

The subiculum (SUB) serves as the major output structure of the hippocampus; therefore, exploring synaptic plasticity within this region is of great importance for understanding the dynamics of hippocampal circuitry and hippocampal-cortical interactions. Previous research has shown exposure to acute stress dramatically alters synaptic plasticity within the hippocampus proper. Using in vivo electrophysiological recordings in urethane-anesthetized adult male Sprague-Dawley rats, we tested the effects of either acute restraint stress (30 min) or corticosterone (CORT) injections (3 mg/kg; s.c.) on short- and long-term forms of synaptic plasticity in the Cornu Ammonis 1-SUB pathway. Paired-pulse facilitation and two forms of long-term plasticity (long-term potentiation and late-developing potentiation) were significantly reduced after exposure to acute stress but not CORT treatment. Measurements of plasma CORT confirmed similar levels of circulating hormone in animals exposed to either acute stress or CORT treatment. The disruptive effects of acute stress on both short- and long-term forms of synaptic plasticity are mediated by glucocorticoid receptor (GR) activation as these disruptions were blocked by pre-treatment with the selective GR antagonist RU38486 (10 mg/kg; s.c.). The present results highlight the susceptibility of subicular plasticity to acute stress and provide evidence that GR activation is necessary but not sufficient for mediating these alterations.

Long-term changes of spine dynamics and microglia after transient peripheral immune response triggered by LPS in vivo

Background

An episode of peripheral immune response may create long-lasting alterations in the neural network. Recent studies indicate a glial involvement in synaptic remodeling. Therefore it is postulated that both synaptic and glial changes could occur under the peripheral inflammation.

Results

We tested this possibility by in vivo two-photon microscopy of dendritic spines after induction of a peripheral immune response by lipopolysaccharide (LPS) treatment of mice.

We observed that the spines were less stable in LPS-treated mice. The accumulation of spine changes gradually progressed and remained low over a week after LPS treatment but became significantly larger at four weeks. Over eight weeks after LPS treatment, the fraction of eliminated spines amounted to 20% of the initial population and this persistent destabilization resulted in a reduction of the total spine density.

We next evaluated glial activation by LPS administration. Activation of microglia was confirmed by a persistent increase of Iba1 immunoreactivity. Morphological changes in microglia were observed two days after LPS administration and were partially recovered within one week but sustained over a long time period.

Conclusions

These results indicate long-lasting aggravating effects of a single transient peripheral immune response on both spines and microglia. The parallel persistent alterations of both spine turnover and the state of microglia in vivo suggest the presence of a pathological mechanism that sustains the enhanced remodeling of neural networks weeks after peripheral immune responses. This pathological mechanism may also underlie long-lasting cognitive dysfunctions after septic encephalopathy in human patients.

NOS2 gene deficiency protects from sepsis-induced long-term cognitive deficits

To date, long-term consequences of septic encephalopathy on cerebral metabolism, cognition, learning, and memory capabilities and factors involved are poorly understood. In this study, we used a murine sepsis model to demonstrate that bacterial lipopolysaccharide (LPS) causes long-term cognitive deficits in mice. Two months after LPS treatment, wild-type mice committed more working and reference memory errors than controls. The behavioral impairment was independent of the cerebral glucose uptake as evidenced by (18)F-Fluordeoxyglucose small animal positron emission tomography. In contrast, mice deficient for the inducible nitric oxide synthase gene (NOS2-/-) did not show any cognitive changes when challenged with LPS. Immunohistochemical analysis demonstrated that LPS did not lead to neuronal cell death but caused sustained microglial activation in wild-type as compared to NOS2-/- mice. Expression analysis showed that LPS-treated NOS2-/- mice had lower brain mRNA levels for proinflammatory factors compared with wild-type mice. Expression analysis demonstrated distinct changes in the content of synaptic proteins in wild-type mice, which were not observed in the NOS2-/- mice. Together, this data set outlines the importance of the NOS2 activation for long-term cerebral changes after severe sepsis.

Lipopolysaccharide dose dependently impairs rapid toxin (LiCl)-induced gustatory conditioning: a taste reactivity examination of the conditioned taste aversion

There is much debate on how immune activation affects cognitive processing. Research has shown that stimulation of the immune system can significantly impair, have no adverse effects, or enhance learning and memory processes in animals. The present experiment evaluated the effects of the bacterial endotoxin, lipopolysaccharide (LPS) on the acquisition of a rapidly acquired conditioned taste aversion using a toxin-containing food. Male Long Evans rats were fitted with intraoral cannulae and habituated to the taste reactivity procedure. Rats received two conditioning days, 72 h apart, in which they were injected systemically with LPS (200, 100, or 50 microg/kg) or NaCl (0.9% vehicle) and 90 min later placed in the taste reactivity test chamber. Rats were given 5 brief (1 min) intraoral infusions of either a LiCl-adulterated sucrose solution (0.15M LiCl+0.3M sucrose) or NaCl-sucrose solution (0.15M NaCl+0.3M sucrose) across a 1h period. On the test day (72 h after the last conditioning trial), rats were given a 2 min intraoral infusion of the respective taste in a drug-free state. Individual taste reactivity responses were recorded and analyzed. Results demonstrate that rats treated with LPS dose-dependently increased ingestive responding to the LiCl-sucrose flavor while at the same time showing reduced rejection response frequency on the two conditioning days. LPS treatment did not alter taste reactivity responding to the NaCl-sucrose solution. On the test day, the LPS groups again displayed a dose dependent increase in ingestive responses and a decrease in rejection responses to the LiCl-sucrose taste. The present results suggest that LPS-induced immune system activation, significantly impairs the rapid acquisition of a conditioned taste aversion.

Controlling hippocampal output: the central role of subiculum in hippocampal information processing

The subiculum has a central position between the hippocampus proper and entorhinal and other cortices, as well as a range of subcortical structures. The functional role of subiculum within the hippocampal formation circuit remains largely unexplored and a theoretical and experimental consensus on its functions has yet to emerge. Presented here is a simple and speculative model of the functions of the subiculum, based partly on anatomical, behavioural and neurophysiological considerations. The model suggests, firstly, that the subiculum acts to amplify hippocampal output, given the prominent bursting behaviour of its neurons and, secondly, that there is a dorso-ventral segregation of function within the subiculum. The dorsal component appears principally concerned with the processing of information about space, movement and memory, whereas the ventral component appears to play a major regulatory role in the inhibition of the HPA axis.

The subiculum: what it does, what it might do, and what neuroanatomy has yet to tell us

The subiculum is a pivotal but under-investigated structure positioned between the hippocampus proper and entorhinal and other cortices, as well as a range of subcortical structures. The subiculum has a range of electrophysiological and functional properties which are quite distinct from its input areas; given the widespread set of cortical and subcortical areas with which it interacts, it is able to influence activity in quite disparate brain regions. The rules governing plasticity of synaptic transmission in the hippocampal-subicular axis are poorly understood; this axis appears to share some properties in common with the hippocampus proper, but behaves quite differently in other respects. Equally, its functional properties are not well understood; it plays an important but ill-defined role in spatial navigation, mnemonic processing and control of the response to stress. Here, I review investigations of synaptic plasticity in the hippocampal-subicular pathway, recordings of subicular neurons in the freely moving behaving animal, the effects of behavioural and other stressors on subicular synaptic plasticity, and anatomical data on the dorso-ventral organization of the subiculum in relation to the hypothalamic-pituitary-adrenal (HPA) axis. I argue that there is a dorso-ventral segregation of function within the subiculum: the dorsal component appears principally concerned with the processing of information about space, movement and memory, whereas the ventral component appears to play a major regulatory role in the inhibition of the HPA axis.

Bacterial endotoxin-induced behavioral alterations in two variations of the Morris water maze

Several studies report that lipopolysaccharide (LPS) or interleukin-1beta (IL-1beta) may affect behavior in a variety of learning tasks, including the Morris water maze (MWM), though the nature of these effects varies with testing parameters. The present study used C57BL/6J mice to evaluate the effect of a single intraperitoneal LPS injection 4 h prior to day 1 of testing, LPS before each day of testing, or saline prior to each test day, on performance in two variations of the MWM. In the first experiment, one that utilized a standard hidden platform, LPS clearly affected performance, as shown by increased latencies and greatly decreased swimming speeds. However, a modest effect on distance swam was only present during later test days. These data show a clear deficit in performance (driven by decreased swim speed), and some evidence for learning decrements on later test days. To explore to what degree the effects of LPS in the water maze were the result of alterations in performance factors such as motor behavior, a second experiment was conducted in which a highly visible jet-black platform was utilized. Despite eliminating the need for spatial learning, mice administered LPS still exhibited significantly increased latency scores and decreased swim speed. However, there was no difference between treatment groups in distance swam. These results reinforce the idea that, even when present, potential learning effects of LPS may sometimes be difficult to untangle from performance effects unrelated to learning, and underscore the need for utilizing behavioral tests that offer suitable control for LPS-induced performance effects.

Chronic LPS exposure produces changes in intrinsic membrane properties and a sustained IL-beta-dependent increase in GABAergic inhibition in hippocampal CA1 pyramidal neurons

Chronic inflammation has been reported to be a significant factor in the induction and progression of a number of chronic neurological disorders including Alzheimer's disease and Down syndrome. It is believed that inflammation may promote synaptic dysfunction, an effect that is mediated in part by pro-inflammatory cytokines such as interleukin-1beta (IL-1beta). However, the role of IL-1beta and other cytokines in synaptic transmission is still poorly understood. In this study, we have investigated how synaptic transmission and neuronal excitability in hippocampal pyramidal neurons are affected by chronic inflammation induced by exposing organotypic slices to the bacterial cell-wall product lipopolysaccharide (LPS). We report that CA1 pyramidal neurons recorded in whole cell from slices previously exposed to LPS for 7 days had resting membrane potential and action potential properties similar to those of the controls. However, they had significantly lower membrane resistance and a more elevated action potential threshold, and displayed a slower frequency of action potential discharge. Moreover, the amplitude of pharmacologically isolated postsynaptic gamma-aminobutyric acid (GABA)ergic potentials, but not excitatory glutamatergic postsynaptic potentials, was significantly larger following chronic LPS exposure. Interestingly, co-incubation of the IL-1 receptor antagonist (IL-1Ra) concurrently with LPS prevented the increase in GABAergic transmission, but not the reduction in intrinsic neuronal excitability. Finally, we confirmed that LPS dramatically increased IL-1beta, and IL-1beta-dependent IL-6 levels in the culture medium for 2 days before returning to baseline. We conclude that CA1 pyramidal neurons in slices chronically exposed to LPS show a persistent decrease in excitability due to a combined decrease in intrinsic membrane excitability and an enhancement in synaptic GABAergic input, the latter being dependent on IL-1beta. Therefore, chronic inflammation in hippocampus produces IL-1beta-dependent and -independent effects in neuronal and synaptic function that could contribute significantly to cognitive disturbances.

Peripheral lipopolysaccharide administration impairs two-way active avoidance conditioning in C57BL/6J mice

Peripheral administration of lipopolysaccharide (LPS) or interleukin-1 (IL-1) may lead to alterations of CNS function and behavioral changes designated "sickness behavior." Further, some experiments show evidence of LPS- and cytokine-mediated alterations in learning and memory. The current series of experiments examined the effects of a single or repeated intraperitoneal LPS injections, at a number of doses and time points before or after test sessions, on behavior in a two-way active avoidance conditioning paradigm. Subjects were able to avoid the mild shock stimulus, escape it, or fail to respond to it. Subjects treated with LPS at many, but not all, of the time points sampled showed impaired learning, by exhibiting significantly fewer avoidance responses than controls. Furthermore, an LPS-induced increase in non-cued inter-trial interval crossings was observed during the later days of testing, suggesting that a greater percentage of their avoidance responses was not conditioned and their behavior was less efficient. Taken together, the results suggest that LPS-treated animals showed a diminished association between conditioned stimulus (CS) and unconditioned stimulus (US). These results support the theory that peripheral immune stimuli may induce deleterious effects on learning, and extend the work to a negatively reinforced operant procedure.

Cyclooxygenase inhibition attenuates endotoxin-induced spatial learning deficits, but not an endotoxin-induced blockade of long-term potentiation

Peripheral administration of lipopolysaccharide (LPS), a potent bacterial endotoxin, can cause a variety of central effects, including production of cytokines and cyclooxygenases in the brain, as well as peripheral increases in corticosterone. These, in turn, may contribute to neuroimmune-induced neurocognitive deficits. We show here LPS causes deficits in hippocampal-dependent spatial learning in the water maze but that treatment with ibuprofen, a broad-spectrum cyclooxygenase inhibitor, reverses the deficits induced in spatial learning by LPS. We also show that LPS causes an impairment in the induction of long-term potentiation in the dentate gyrus in vivo, a major contemporary model of learning and memory. No differences were found in corticosterone levels in trunk blood but we find a decrease in brain-derived neurotrophic factor (BDNF) expression in LPS group compared to saline controls. Paradoxically compared to the behavioral findings treatment with ibuprofen does not attenuate the LPS-induced impairment in LTP or BDNF concentration in tetanized tissue.

Effects of intraperitoneal lipopolysaccharide on Morris maze performance in year-old and 2-month-old female C57BL/6J mice

Several studies have shown that systemic lipopolysaccharide (LPS) or interleukin-1beta (IL-1beta) may affect performance in various learning tasks, including the Morris water maze. In the current study, female C57BL/6J mice, either 2 months or 1 year of age, were given 5 days of testing followed by 3 days of rest, and then three additional days of testing. Mice either received a single LPS injection on day 1 and saline on days 2-5, LPS injections on days 1-5, or saline injections on days 1-5. Daily LPS administration significantly prolonged latency for the animals to find the platform, and decreased their swimming speed. Year-old mice treated with LPS each day also exhibited significantly higher levels of thigmotaxis in the maze. Despite effects on latency and swim speed, no effect of LPS treatment was observed for distance traveled to the platform or other measures that clearly indicate disruption of learning in the maze. On the other hand, age was a significant factor affecting both latency and distance, with older animals swimming greater distances to find the platform. Additionally, older animals were more adversely affected by daily LPS treatment. In this study, although LPS-induced performance impairments in the Morris water maze were noted, particularly in older animals, these effects were not clearly indicative of learning impairment per se.

Combination treatment of neonatal rats with hypoxia-ischemia and endotoxin induces long-lasting memory and learning impairment that is associated with extended cerebral damage

OBJECTIVE

We assessed the long-term effects of perinatal hypoxia-ischemia and endotoxin on attention and short- and long-term memory in neonatal rats with the use of behavioral tasks and brain histologic results.

STUDY DESIGN

Four hours after injections of lipopolysaccharide (1 mg/kg, intraperitoneally) or saline solution, 7-day-old Wistar rat pups were subjected to unilateral hypoxia-ischemia for 1 hour. We studied 4 groups: controls (n = 43 rats), lipopolysaccharide alone (n = 12 rats), hypoxia-ischemia alone (n = 29 rats), and combined lipopolysaccharide + hypoxia-ischemia treatment (n = 34 rats). Seven to 16 weeks after the treatment, we measured attention with a choice reaction time task, short-term memory with an 8-arm radial maze task, and long-term memory with a water maze task. At 19 weeks of age, the brain was removed, fixed, and sectioned coronally; and the volume of each part was measured.

RESULTS

A loss of volume in the hippocampus was observed in the lipopolysaccharide, hypoxia-ischemia, and lipopolysaccharide + hypoxia-ischemia groups; a loss of striatum was observed in the hypoxia-ischemia and lipopolysaccharide + hypoxia-ischemia groups, but loss of cortex was observed only in the lipopolysaccharide + hypoxia-ischemia group. The lipopolysaccharide, hypoxia-ischemia, and lipopolysaccharide + hypoxia-ischemia groups showed significantly poorer performance (attention deficit) than controls in the choice reaction time task. Correct choices decreased, and error increased in the lipopolysaccharide + hypoxia-ischemia group compared with the other groups in the radial maze task, which shows short-term memory impairment. Swimming distance was significantly greater in the hypoxia-ischemia and lipopolysaccharide + hypoxia-ischemia groups than in the other 2 groups in the water maze test, which shows long-term memory impairment.

CONCLUSION

Combined lipopolysaccharide and hypoxia-ischemia treatment synergistically induced short-term memory impairment that is associated with loss of cortical volume.

Lipopolysaccharide retards development of amygdala kindling but does not affect fully-kindled seizures in rats

Seizures are common sequel to brain insults in cases such as stroke, trauma and infection where there is a certain neuroinflammation. Intracerebroventricular (i.c.v.) administration of lipopolysaccharide (LPS) induces an inflammatory state in brain that is used as a model of neuroinflammation. We studied the effect of LPS (0.25 and 2.5 microg/rat, i.c.v.) on development of electrical kindling of the amygdala and on fully-kindled seizures. LPS, at the doses used, had no effect on fully-kindled seizures and afterdischarge (AD) duration at 0.5, 2 or 4h after administration. However, daily injection of LPS (2.5 microg/rat) retarded acquisition of kindled behavioral seizures. This antiepileptogenic effect could be due to the release of inflammatory mediators from microglia and the related morphological and functional changes in synaptic neurotransmission.

The Cdkn1a gene (p21Waf1/Cip1) is an inflammatory response gene in the mouse central nervous system

We used high-density cDNA microarray analysis to examine changes in the gene expression profile of the hippocampus of C57BL/6 mice following intraperitoneal injection of lipopolysaccharide (LPS). Three hours after injection, the greatest increase in RNA expression was found for an expressed sequence tag subsequently identified as the Cdkn1a gene, coding for the cyclin-dependent kinase inhibitor p21(Waf1/Cip1). Northern blot hybridization confirmed the induction of Cdkn1a mRNA in the central nervous system (CNS), and also revealed similar increases in kidney, liver and heart. Induction of Cdkn1a expression was transient, reaching maximal levels in the CNS 3-6 h after LPS administration, and returning to untreated levels by 24 h. Combined use of laser capture microdissection and quantitative reverse transcription-polymerase chain reaction showed that there was a similar change in Cdkn1a expression for the pyramidal cell layer as for total hippocampus.

Analysis of recordings of single-unit firing and population activity in the dorsal subiculum of unrestrained, freely moving rats

We examined neuronal activity in the dorsal subiculum of unrestrained, male adult Wistar rats, which were implanted with a moveable eight-electrode microdrive. The subiculum is the primary hippocampal formation output area and is comparatively uninvestigated neurophysiologically. We compared subicular unit activity and the subicular EEG while rats occupied a small, restricted environment and also correlated neuronal activity with the ongoing behavior of the animal. Units were separated using their electrophysiological characteristics into bursting units, regular spiking units, theta-modulated units, and fast spiking units. The bursting and regular spiking unit classes are similar to hippocampal CA1 units, whereas the fast spiking units appear to be interneurons. Bursting units were variable in their behavior: some units bursted regularly, and others bursted only occasionally. Theta-modulated units have not been described before; these were similar to regular spiking units in all respects except that they increased their firing significantly when theta oscillations were present in the simultaneous EEG record. Subicular EEG was similar to hippocampal EEG, with theta oscillations dominating "alert, moving" behaviors, while large amplitude irregular activity (LIA), which included sharp waves, predominated when theta oscillations were not present, mainly during "alert, still" and "quiet" behaviors. A relatively small proportion of subicular recordings (approximately 32%) were phase-locked to theta; this is a smaller proportion than in areas from which the subiculum takes major inputs. The relative lack of entrainment of subicular neurons by this important intrinsic rhythm is suggestive of a limit to which theta might be capable of affecting both subicular and hippocampal information processing more generally.

Immune-to-central nervous system communication and its role in modulating pain and cognition: Implications for cancer and cancer treatment

This paper reviews the nature of communication from the immune system to the brain and some implications of this communication for phenomena that are not ordinarily considered to be modulated by immune function. Pro-inflammatory cytokines released by activated immune cells signal the brain by both blood-borne and neural routes, leading to alterations in neural activity. The cascade of altered neural activity includes the induction of pro-inflammatory cytokines within the brain and spinal cord. The cytokines in the brain, specifically in the hippocampus, interfere with the consolidation of memory, while the cytokines within the spinal cord exaggerate pain. Activation of this immune-to-central nervous system pathway, with the consequent production of cytokines within the central nervous system, may be involved in the mediation of a number of phenomena that occur during cancer and cancer treatment.

Early life environment: does it have implications for predisposition to disease?

Early life environmental factors have been associated with altered predisposition to a variety of pathologies. A considerable literature examines pre- and postnatal factors associated with increased risk of cardiovascular, metabolic (i.e. insulin resistance, hyperlipidemia) and psychiatric disease, and the importance of hormonal programming. The brain is exquisitely sensitive to environmental inputs during development and the stress responsiveness of the hypothalamic-pituitary-adrenal (HPA) axis has been shown to be both up- and down-regulated by early life exposure to limited nutrition, stress, altered maternal behaviors, synthetic steroids and inflammation. It has been suggested that peri-natal programming of HPA axis regulation might therefore contribute to metabolic and psychiatric disease etiology. In addition, glucocorticoids play modulatory roles regulating many aspects of immune function, notably controlling both acute and chronic inflammatory responses. Neuroendocrine-immune communication is bidirectional, and therefore it is expected that environmental factors altering HPA regulation have implications for stress effects on immune function and predisposition to inflammation. The impact of pre- and postnatal factors altering immune function, stress responsivity and predisposition to inflammatory disease are reviewed. It is also examined whether the early 'immune environment' might similarly influence predisposition to disease and alter neuroendocrine function. Evidence indicating a role for early life inflammation and infection as an important factor programming the neuroendocrine-immune axis and altering predisposition to disease is considered.

Short-term synaptic plasticity

Synaptic transmission is a dynamic process. Postsynaptic responses wax and wane as presynaptic activity evolves. This prominent characteristic of chemical synaptic transmission is a crucial determinant of the response properties of synapses and, in turn, of the stimulus properties selected by neural networks and of the patterns of activity generated by those networks. This review focuses on synaptic changes that result from prior activity in the synapse under study, and is restricted to short-term effects that last for at most a few minutes. Forms of synaptic enhancement, such as facilitation, augmentation, and post-tetanic potentiation, are usually attributed to effects of a residual elevation in presynaptic [Ca(2+)]i, acting on one or more molecular targets that appear to be distinct from the secretory trigger responsible for fast exocytosis and phasic release of transmitter to single action potentials. We discuss the evidence for this hypothesis, and the origins of the different kinetic phases of synaptic enhancement, as well as the interpretation of statistical changes in transmitter release and roles played by other factors such as alterations in presynaptic Ca(2+) influx or postsynaptic levels of [Ca(2+)]i. Synaptic depression dominates enhancement at many synapses. Depression is usually attributed to depletion of some pool of readily releasable vesicles, and various forms of the depletion model are discussed. Depression can also arise from feedback activation of presynaptic receptors and from postsynaptic processes such as receptor desensitization. In addition, glial-neuronal interactions can contribute to short-term synaptic plasticity. Finally, we summarize the recent literature on putative molecular players in synaptic plasticity and the effects of genetic manipulations and other modulatory influences.

Lipopolysaccharide causes deficits in spatial learning in the watermaze but not in BDNF expression in the rat dentate gyrus

We investigated the effects of a single injection and a daily injection of lipopolysaccharide (LPS) on spatial learning and brain-derived neurotrophic factor (BDNF) expression in the rat dentate gyrus. LPS is derived from the cell wall of Gram-negative bacteria and is a potent endotoxin that causes the release of cytokines such as interleukin-1 and tumour necrosis factor. LPS is thought to activate both the neuroimmune and neuroendocrine systems; it also blocks long-term potentiation in the hippocampus. Here, we examined the effects of LPS on a form of hippocampal-dependent learning-spatial learning in the water maze. Rats were injected with LPS intraperitoneally (100 microg/kg) and trained in the water maze. The first group of rats were injected on day 1 of training, 4 h prior to learning the water maze task. Groups 2 and 3 were injected daily, again 4 h prior to the water-maze task; group 2 with LPS and group 3 with saline. A number of behavioural variables were recorded by a computerised tracking system for each trial. The behavioural results showed a single injection of LPS (group 1) impaired escape latency in both the acquisition and retention phases of the study, whereas a daily injection of LPS did not significantly impair acquisition or retention. BDNF expression was analysed in the dentate gyrus of all animals. No significant differences in BDNF expression were found between the three groups.