Alzheimer type II astrocytic changes following sub-acute exposure to manganese and its prevention by antioxidant treatment

Mechanism of Alzheimer type II astrocyte development in hepatic encephalopathy

Type C hepatic encephalopathy (Type C HE) is a major and complex neurological condition that occurs following chronic liver failure. The molecular basis of Type C HE remains elusive. Type C HE is characterized by mental confusion, cognitive and motor disturbances. The presence of Alzheimer type II astrocytes (AT2A) is the key histopathological finding observed in Type C HE. However, nothing is currently known regarding AT2A development and its involvement in cognitive, and motor deficits in Type C HE. We, therefore, examined in rats the mechanisms by which liver failure contributes to the progression of AT2A, and its role in the development of cognitive and motor deficits in thioacetamide (TAA) model of Type C HE. We and others earlier reported increased oxidative/nitrosative stress (ONS), JNK1/2, and cMyc activation in ammonia-treated astrocyte cultures, as well as in brains from chronic liver failure. We now found increased levels of astrocytic glia maturation factor (GMF, a factor strongly implicated in neuroinflammation), as well as various inflammatory factors (IL-1β, TNF-α, IL-6, MMP-3, COX2, CXCL1, and PGE2), and reduced levels of GFAP and increased levels of aggregated nuclear protein Lamin A/C in rat brain cortex post-chronic liver failure. We also found increased levels of GMF and inflammatory factors (MMP-3, COX2, CXCL1, and PGE2) in astrocytes post-ammonia treatment in vitro. Additionally, pharmacological inhibition of upstream signaling of GMF (ONS, JNK1/2, and cMyc) or GMF inhibitors W-7 and trifluoperazine significantly reduced the levels of inflammatory factors, the number of AT2A cells, as well as the cognitive and motor deficits in TAA-treated rats. Increased levels of GMF were also identified in human post-mortem brain sections. These findings strongly suggest that increased levels of astrocytic GMF due to elevated levels of ONS, JNK1/2, and cMyc and the subsequent inflammation contribute to the development of AT2A and the consequent cognitive, and motor deficits in chronic liver failure.

Scientific opinion on the tolerable upper intake level for manganese

Following a request from the European Commission (EC), the EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA) was asked to deliver a scientific opinion on the tolerable upper intake level (UL) for manganese. Systematic reviews of the literature of human and animal data were conducted to assess evidence regarding excess manganese intake (including authorised manganese salts) and the priority adverse health effect, i.e. manganese-induced neurotoxicity. Available human and animal studies support neurotoxicity as a critical effect, however, data are not sufficient and suitable to characterise a dose-response relationship and identify a reference point for manganese-induced neurotoxicity. In the absence of adequate data to establish an UL, estimated background dietary intakes (i.e. manganese intakes from natural dietary sources only) observed among high consumers (95th percentile) were used to provide an indication of the highest level of intake where there is reasonable confidence on the absence of adverse effects. A safe level of intake of 8 mg/day was established for adults ≥ 18 years (including pregnant and lactating women) and ranged between 2 and 7 mg/day for other population groups. The application of the safe level of intake is more limited than an UL because the intake level at which the risk of adverse effects starts to increase is not defined.

Sodium P-aminosalicylic Acid Attenuates Manganese-Induced Neuroinflammation in BV2 Microglia by Modulating NF-κB Pathway

Exposure to high levels of manganese (Mn) leads to brain Mn accumulation, and a disease referred to as manganism. Activation of microglia plays an important role in Mn-induced neuroinflammation. Sodium p-aminosalicylic acid (PAS-Na) is a non-steroidal anti-inflammatory drug that inhibits Mn-induced neuroinflammation. The aim of the current study was to explore the role of NF-κB in the protective mechanism of PAS-Na on Mn-induced neuroinflammation in BV2 microglial experimental model. We treated BV2 microglia with 200 μM Mn for 24 h followed by 48 h treatment with graded concentrations of PAS-Na, using an NF-kB inhibitor, JSH-23, as a positive control. MTT results established that 200 and 400 μM PAS-Na treatment increased the Mn-induced cell viability reduction. NF-κB (P65) mRNA expression and the phosphorylation of p65 were increased in Mn-treated BV2 cell, and suppressed by PAS-Na, analogous to the effect of JSH-23 pretreatment. Furthermore, PAS-Na significantly reduced the contents of the inflammatory cytokine TNF-α and IL-1β, both of which were increased by Mn treatment. The current results show that PAS-Na attenuated Mn-induced inflammation by abrogating the activation of the NF-κB signaling pathways and reduced the release of pro-inflammatory cytokines.

Interaction of Manganese and Ammonia in the Brain of Hepatic Encephalopathy Rats

Background: Both ammonia and manganese (Mn) play a key role in the pathogenesis of hepatic encephalopathy (HE) and cause similar morphological and functional changes in astrocytes. Objectives: To investigate the interaction between brain Mn and ammonia in HE rats. Methods: Three rat models of minimal HE (MHE), chronic manganism (CHM), and chronic hyperammonemia (CHA) were constructed. A total of 48 Sprague-Dawley rats were divided into one control group (n = 6), MHE groups (n = 18, among which six rats were used to evaluate the MHE model), CHM groups (n = 12), and CHA groups (n = 12). The CHM, CHA, and the rest of MHE rats were randomly divided further into two subgroups, according to the MgSO4 treatment (oral administration of 496 mg/kg/day for seven weeks): MHE-7W and MHE + Mg-7W; CHM-7W and CHM + Mg-7W; and CHA-7W and CHA + Mg-7W, respectively. Rats’ blood ammonia, brain Mn, glutamine synthetase (GS), and glutamine (GLN) levels were measured and compared among groups. Results: Significantly higher brain Mn content in MHE-7W and CHM-7W rats, higher blood ammonia levels, brain GS activity, and GLN content were observed in MHE-7W, CHM-7W, and CHA-7W rats than in control rats. After MgSO4 treatment for seven weeks, significantly lower brain Mn content, blood ammonia levels, and GLN content were observed in MHE, CHM, and CHA rats. Conclusions: Our study showed that brain Mn accumulation could increase brain ammonia levels, while the accumulation of brain ammonia had no effect on the content of brain Mn.

The impact of manganese on neurotransmitter systems

BACKGROUND

Manganese (Mn) is a metal ubiquitously present in nature and essential for many living organisms. As a trace element, it is required in small amounts for the proper functioning of several important enzymes, and reports of Mn deficiency are indeed rare.

METHODS

This mini-review will cover aspects of Mn toxicokinetics and its impact on brain neurotransmission, as well as its Janus-faced effects on humans and other animal's health.

RESULTS

The estimated safe upper limit of intracellular Mn for physiological function is in anarrow range of 20-53 μM.Therefore, intake of higher levels of Mn and the outcomes, especially to the nervous system, have been well documented.

CONCLUSION

The metal affects mostly the brain by accumulating in specific areas, altering cognitive functions and locomotion, thus severely impacting the health of the exposed organisms.

Iron in neurodegenerative disorders: being in the wrong place at the wrong time?

Brain iron deposits have been reported consistently in imaging and histologic examinations of patients with neurodegenerative disorders. While the origins of this finding have not been clarified yet, it is speculated that impaired iron homeostasis or deficient transport mechanisms result in the accumulation of this highly toxic metal ultimately leading to formation of reactive oxygen species and cell death. On the other hand, there are also those who support that iron is just an incidental finding, a by product of neuronal loss. A literature review has been performed in order to present the key findings in support of the iron hypothesis of neurodegeneration, as well as to identify conditions causing or resulting from iron overload and compare and contrast their features with the most prominent neurodegenerative disorders. There is an abundance of experimental and observational findings in support of the hypothesis in question; however, as neurodegeneration is a rare incident of commonly encountered iron-associated disorders of the nervous system, and this metal is found in non-neurodegenerative disorders as well, it is possible that iron is the result or even an incidental finding in neurodegeneration. Understanding the underlying processes of iron metabolism in the brain and particularly its release during cell damage is expected to provide a deeper understanding of the origins of neurodegeneration in the years to come.

Sodium P-aminosalicylic acid inhibits sub-chronic manganese-induced neuroinflammation in rats by modulating MAPK and COX-2

Excessive manganese (Mn) accumulation in the brain may induce an extrapyramidal disorder known as manganism. Inflammatory processes play a critical role in neurodegenerative diseases. Therapeutically, non-steroidal anti-inflammatory drugs or analogous anti-inflammatory therapies have neuroprotective effects. As a non-steroidal anti-inflammatory drug, p-aminosalicylic acid (PAS) has anti-inflammatory effects, which are mediated by decreased prostaglandins E2 (PGE2) levels. The aim of the current study was to investigate whether PAS-Na treatment prevents Mn-induced behavioral changes and neuroinflammation in vivo. Male Sprague-Dawley rats were intraperitoneally (i.p.) injected with MnCl₂·4H₂O (15mg/kg) for 12 weeks, followed by 6 weeks PAS-Na treatment. Sub-chronic Mn exposure increased Mn levels in the whole blood, cortex, hippocampus and thalamus, and induced learning and memory deficits, concomitant with astrocytes activation in the cortex, hippocampus and thalamus. Moreover inflammatory cytokine levels in serum and brain of Mn-treated group were increased, including IL-1β, IL-6, TNF-αand PGE2, especially in the hippocampus and thalamus. Furthermore, sub-chronic Mn exposure also increased inflammatory cytokines and COX-2 in transcription levels concomitant with increased MAPK signaling and COX-2 in the same selected brain regions. PAS-Na treatment at the highest doses also decreased Mn levels in the whole blood and selected brain tissues, and reversed the Mn-induced learning and memory deficits. PAS-Na inhibited astrocyte activation as well as the Mn-induced increase in inflammatory cytokine levels, reducing p38, ERK MAPK pathway and COX-2 activity. In contrast PAS-Na had no effects on the JNK MAPK pathway. These data establish the efficacy of PAS-Na not only as a chelating agent to mobilize whole blood Mn, but also as an anti-inflammatory agent.

Sodium para-aminosalicylate protected cultured basal ganglia astrocytes from manganese-induced DNA damages and alteration of amino acid neurotransmitter levels

Sodium para-aminosalicylate (PAS-Na) was first applied successfully in clinical treatment of two manganism patients with good prognosis. However, the mechanism of how PAS-Na protects against Mn-induced neurotoxicity is still elusive. The current study was conducted to explore the effects of PAS-Na on Mn-induced basal ganglia astrocyte injury, and the involvement of amino acid neurotransmitter in vitro. Basal ganglia astrocytes were exposed to 500 μM manganese chloride (MnCl2) for 24 hr, following by 50, 150, or 450 μM PAS-Na treatment for another 24 hr. MnCl2 significantly decreased viability of astrocytes and induced DNA damages via increasing the percentage of tail DNA and Olive tail moment of DNA. Moreover, Mn interrupted amino acid neurotransmitters by decreasing Gln levels and increasing Glu, Gly levels. In contrast, PAS-Na treatment reversed the aforementioned Mn-induced toxic effects on basal ganglia astrocytes. Taken together, our results demonstrated that excessive Mn exposure may induce toxic effects on basal ganglia astrocytes, while PAS-Na could protect basal ganglia astrocytes from Mn-induced neurotoxicity.

Hepatic Encephalopathy

This article reviews hepatic encephalopathy (HE) in companion animals. Clinical signs and categories of hepatic disease likely to cause HE are discussed. Ammonia has a key role in pathogenesis and current concepts in body ammonia metabolism are reviewed. Inflammation and manganese accumulation are also thought to be important in pathogenesis. Treatment of HE in acute and chronic cases is discussed along with the rationale for current treatment recommendations. Potential avenues for new treatments and human treatments, which may be transferable to companion animals, are reviewed.

Mitochondrial Redox Dysfunction and Environmental Exposures

SIGNIFICANCE

Mitochondria are structurally and biochemically diverse, even within a single type of cell. Protein complexes localized to the inner mitochondrial membrane synthesize ATP by coupling electron transport and oxidative phosphorylation. The organelles produce reactive oxygen species (ROS) from mitochondrial oxygen and ROS can, in turn, alter the function and expression of proteins used for aerobic respiration by post-translational and transcriptional regulation.

RECENT ADVANCES

New interest is emerging not only into the roles of mitochondria in disease development and progression but also as a target for environmental toxicants.

CRITICAL ISSUES

Dysregulation of respiration has been linked to cell death and is a major contributor to acute neuronal trauma, peripheral diseases, as well as chronic neurodegenerative diseases, such as Parkinson's disease and Alzheimer's disease.

FUTURE DIRECTIONS

Here, we discuss the mechanisms underlying the sensitivity of the mitochondrial respiratory complexes to redox modulation, as well as examine the effects of environmental contaminants that have well-characterized mitochondrial toxicity. The contaminants discussed in this review are some of the most prevalent and potent environmental contaminants that have been linked to neurological dysfunction, altered cellular respiration, and oxidation.

N-acetylcysteineamide protects against manganese-induced toxicity in SHSY5Y cell line

Manganese (Mn) is an essential trace element required for normal cellular functioning. However, overexposure of Mn can be neurotoxic resulting in the development of manganism, a syndrome that resembles Parkinson׳s disease. Although the pathogenetic basis of this disorder is unclear, several studies indicate that it is mainly associated with oxidative stress and mitochondrial energy failure. Therefore, this study is focused on (1) investigating the oxidative effects of Mn on neuroblastoma cells (SHSY5Y) and (2) elucidating whether a novel thiol antioxidant, N-acetylcysteineamide (NACA), provides any protection against Mn-induced neurotoxicity. Reactive oxygen species (ROS) were highly elevated after the exposure, indicating that mechanisms that induce oxidative stress were involved. Measures of oxidative stress parameters, such as glutathione (GSH), malondialdehyde (MDA), and activities of glutathione reductase (GR) and glutathione peroxidase (GPx) were altered in the Mn-treated groups. Loss of mitochondrial membrane potential, as assessed by flow cytometry and decreased levels of ATP, indicated that cytotoxicity was mediated through mitochondrial dysfunction. However, pretreatment with NACA protected against Mn-induced toxicity by inhibiting lipid peroxidation, scavenging ROS, and preserving intracellular GSH and mitochondrial membrane potential. NACA can potentially be developed into a promising therapeutic option for Mn-induced neurotoxicity. This article is part of a Special Issue entitled SI: Metals in neurodegeneration.

Ultrastructural Changes of Caudate Nucleus in Mice Chronically Treated with Manganese

Manganese (Mn) is able to cross the blood-brain barrier and induces functional and structural alterations during the intoxication by this metal. Therefore, the effects of chronic administration of Mn in the caudate nucleus of mice were evaluated by electron microscopy. Male albino mice were injected intraperitoneally with MnCl2 (5 mg/kg/d) 5 d per week during 9 weeks. The control group received only 0.9% of NaCl solution. The caudate nuclei were extracted and subsequently processed to be observed on a conventional transmission electron microscope at 2, 4, 6, and 9 weeks after treatment. A high percentage of vacuolated and swollen mitochondria were found throughout all the analyzed periods. Myelin disarrangement and ultrastructural alterations related to edema were observed increased in Mn-treated mice at week 9. Granular degeneration of myelin at week 9 accompanied with deposition of electron dense granules in the neuropil was also observed. Edema in neuropil and glial cells was detected from week 2 to week 9 accompanied by swollen mitochondria. Neuronal bodies, synaptic terminals, and perivascular cells were found swollen. Decreased electron density in postsynaptic areas and decreased and dispersed synaptic vesicles in presynaptic areas were noted in Mn-treated animals. Some neurons from Mn-treated mice showed cisternae dilation of the Golgi apparatus. These results suggest that Mn-treatment produces structural alterations in the caudate nucleus that could be responsible for some of the neurotoxic effects of this metal.

Neurotoxicity of metals

Metals are frequently used in industry and represent a major source of toxin exposure for workers. For this reason governmental agencies regulate the amount of metal exposure permissible for worker safety. While essential metals serve physiologic roles, metals pose significant health risks upon acute and chronic exposure to high levels. The central nervous system is particularly vulnerable to metals. The brain readily accumulates metals, which under physiologic conditions are incorporated into essential metalloproteins required for neuronal health and energy homeostasis. Severe consequences can arise from circumstances of excess essential metals or exposure to toxic nonessential metal. Herein, we discuss sources of occupational metal exposure, metal homeostasis in the human body, susceptibility of the nervous system to metals, detoxification, detection of metals in biologic samples, and chelation therapeutic strategies. The neurologic pathology and physiology following aluminum, arsenic, lead, manganese, mercury, and trimethyltin exposures are highlighted as classic examples of metal-induced neurotoxicity.

Brain magnetic resonance in hepatic encephalopathy

The term hepatic encephalopathy (HE) covers a wide spectrum of neuropsychiatric abnormalities caused by portal-systemic shunting. The diagnosis requires demonstration of liver dysfunction or portal-systemic shunts and exclusion of other neurologic disorders. Most patients with this condition have liver dysfunction caused by cirrhosis, but it also occurs in patients with acute liver failure and less commonly, in patients with portal-systemic shunts that are not associated with hepatocellular disease. Various magnetic resonance (MR) techniques have improved our knowledge about the pathophysiology of HE. Proton MR spectroscopy and T1-weighted imaging can detect and quantify accumulations of brain products that are normally metabolized or eliminated such as glutamine and manganese. Other MR techniques such as T2-weighted and diffusion-weighted imaging can identify white matter abnormalities resulting from disturbances in cell volume homeostasis secondary to brain hyperammonemia. Partial or complete recovery of these abnormalities has been observed with normalization of liver function or after successful liver transplantation. MR studies have undoubtedly improved our understanding of the mechanisms involved in the pathogenesis of HE, and some findings can be considered biomarkers for monitoring the effects of therapeutic measures focused on correcting this condition.

Role of transcription factor yin yang 1 in manganese-induced reduction of astrocytic glutamate transporters: Putative mechanism for manganese-induced neurotoxicity

Astrocytes are the most abundant non-neuronal glial cells in the brain. Once relegated to a mere supportive role for neurons, contemporary dogmas ascribe multiple active roles for these cells in central nervous system (CNS) function, including maintenance of optimal glutamate levels in synapses. Regulation of glutamate levels in the synaptic cleft is crucial for preventing excitotoxic neuronal injury. Glutamate levels are regulated predominantly by two astrocytic glutamate transporters, glutamate transporter 1 (GLT-1) and glutamate aspartate transporter (GLAST). Indeed, the dysregulation of these transporters has been linked to several neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD) and Parkinson's disease (PD), as well as manganism, which is caused by overexposure to the trace metal, manganese (Mn). Although Mn is an essential trace element, its excessive accumulation in the brain as a result of chronic occupational or environmental exposures induces a neurological disorder referred to as manganism, which shares common pathological features with Parkinsonism. Mn decreases the expression and function of both GLAST and GLT-1. Astrocytes are commonly targeted by Mn, and thus reduction in astrocytic glutamate transporter function represents a critical mechanism of Mn-induced neurotoxicity. In this review, we will discuss the role of astrocytic glutamate transporters in neurodegenerative diseases and Mn-induced neurotoxicity.

Whole blood manganese concentrations in dogs with primary hepatitis

OBJECTIVES

Increased whole blood manganese concentrations have been reported in humans with primary liver disease. Due to the neurotoxic effects of manganese, altered manganese homeostasis has been linked to the development of hepatic encephalopathy. Whole blood manganese concentrations are increased in cases of canine congenital portosystemic shunts, but it remains unclear whether dogs with primary hepatopathies also have altered manganese homeostasis.

METHODS

Whole blood manganese concentrations were measured by graphite furnace atomic absorption spectrometry in 21 dogs with primary hepatitis, 65 dogs with a congenital portosystemic shunt, 31 dogs with non-hepatic illnesses and 18 healthy dogs.

RESULTS

The whole blood manganese concentrations were significantly different between dogs with primary hepatitis, dogs with non-hepatic illnesses and healthy dogs (P=0·002). Dogs with primary hepatitis had significantly increased whole blood manganese concentrations compared with healthy dogs (P<0·05) and dogs with non-hepatic illnesses (P<0·01). Dogs with primary hepatitis had significantly lower whole blood manganese concentration compared with dogs with congenital portosystemic shunts (P=0·0005).

CLINICAL SIGNIFICANCE

Dogs with primary hepatopathies have increased concentrations of whole blood manganese although these concentrations are not as high as those in dogs with congenital portosystemic shunts. The role of altered manganese homeostasis in canine hepatic encephalopathy is worthy of further study.

Selenium protects neonates against neurotoxicity from prenatal exposure to manganese

Manganese (Mn) exposure can affect brain development. Whether Selenium (Se) can protect neonates against neurotoxicity from Mn exposure remains unclear. We investigated this issue in 933 mother-newborn pairs in Shanghai, China, from 2008 through 2009. Umbilical cord serum concentrations of Mn and Se were measured and Neonatal Behavioral Neurological Assessment (NBNA) tests were conducted. The scores <37 were defined as the low NBNA. The median concentrations of cord serum Mn and Se were 4.0 µg/L and 63.1 µg/L, respectively. After adjusting for potential confounders, the interaction between Se and Mn was observed. Cord blood Mn levels had different effects on NBNA scores stratified by different cord blood Se levels. With Se Collapse

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MESH Headings

• Analysis of Variance
• Female
• Humans
• Infant, Newborn
• Male
• Manganese/toxicity
• Maternal Exposure/adverse effects
• Neuroprotective Agents/pharmacology
• Pregnancy
• Prenatal Exposure Delayed Effects/prevention & control
• Selenium/pharmacology

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Affiliation(s)

Xin Yang MOE-Shanghai Key Lab of Children’s Environmental Health, Xinhua Hospital Affiliated to Shanghai Jiao Tong University School of Medicine, Shanghai, China
YiXiao Bao Department of Pediatrics, Xinhua Hospital Affiliated to Shanghai Jiao Tong University School of Medicine, Shanghai, China
HuanHuan Fu MOE-Shanghai Key Lab of Children’s Environmental Health, Xinhua Hospital Affiliated to Shanghai Jiao Tong University School of Medicine, Shanghai, China
LuanLuan Li MOE-Shanghai Key Lab of Children’s Environmental Health, Xinhua Hospital Affiliated to Shanghai Jiao Tong University School of Medicine, Shanghai, China
TianHong Ren MOE-Shanghai Key Lab of Children’s Environmental Health, Xinhua Hospital Affiliated to Shanghai Jiao Tong University School of Medicine, Shanghai, China
XiaoDan Yu MOE-Shanghai Key Lab of Children’s Environmental Health, Xinhua Hospital Affiliated to Shanghai Jiao Tong University School of Medicine, Shanghai, China

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Evaluation of neurobehavioral and neuroinflammatory end-points in the post-exposure period in rats sub-acutely exposed to manganese

Manganese (Mn) can cause manganism, a neurological disorder similar to Parkinson' Disease (PD). The neurobehavioral and neuroinflammatory end-points in the Mn post exposure period have not been studied yet. Rats were injected on alternate days with 8 doses of MnCl2 (25mg/kg) or saline, then euthanized 1, 10, 30 or 70 days following the last dose. Whole-blood (WB) (p<0.05), urine (p<0.05) and brain cortical (p<0.0001) Mn levels were significantly increased 24h after the last dose. Decreases in the rats' ambulation were noted 1, 10 and 30 days after the last Mn dose (p<0.001; p<0.05; p<0.001, respectively) and also in the rearing activity at the four time-points (p<0.05). Cortical glial fibrillary acid protein immunoreactivity (GFAP-ir) was significantly increased at 1, 10, 30 (p<0.0001) and 70 (p<0.001) days after the last Mn dose, as well as tumor necrosis α (TNF-α) levels (p<0.05) but just on day 1. Taken together, the results show that, during the 70-day clearance phase of Mn, the recovery is not immediate as behavioral alterations and neuroinflammation persist long after Mn is cleared from the cortical brain compartment.

Manganese neurotoxicity and the role of reactive oxygen species

Manganese (Mn) is an essential dietary nutrient, but an excess or accumulation can be toxic. Disease states, such as manganism, are associated with overexposure or accumulation of Mn and are due to the production of reactive oxygen species, free radicals, and toxic metabolites; alteration of mitochondrial function and ATP production; and depletion of cellular antioxidant defense mechanisms. This review focuses on all of the preceding mechanisms and the scientific studies that support them as well as providing an overview of the absorption, distribution, and excretion of Mn and the stability and transport of Mn compounds in the body.

Role of astrocytes in manganese mediated neurotoxicity

Astrocytes are responsible for numerous aspects of metabolic support, nutrition, control of the ion and neurotransmitter environment in central nervous system (CNS). Failure by astrocytes to support essential neuronal metabolic requirements plays a fundamental role in the pathogenesis of brain injury and the ensuing neuronal death. Astrocyte-neuron interactions play a central role in brain homeostasis, in particular via neurotransmitter recycling functions. Disruption of the glutamine (Gln)/glutamate (Glu) -γ-aminobutyric acid (GABA) cycle (GGC) between astrocytes and neurons contributes to changes in Glu-ergic and/or GABA-ergic transmission, and is associated with several neuropathological conditions, including manganese (Mn) toxicity. In this review, we discuss recent advances in support of the important roles for astrocytes in normal as well as neuropathological conditions primarily those caused by exposure to Mn.

Therapeutic efficacy of silymarin from milk thistle in reducing manganese-induced hepatic damage and apoptosis in rats

Oxidative stress has been proposed as a possible mechanism involved in manganese (Mn) toxicity. Using natural antioxidants against metal-induced hepatotoxicity is a modern approach. The present study investigated the beneficial role of silymarin, a natural flavonoid, in Mn-induced hepatotoxicity focusing on histopathology and biochemical approaches. Male Wistar rats were exposed orally to manganese chloride (20 mg/mL) for 30 days followed by intraperitoneal cotreatment with silymarin (100 mg/kg). Exposure to Mn resulted in a significant elevation of the plasma marker enzyme activities and bilirubin level related to liver dysfunction of reactive oxygen species (ROS) production and hepatic oxidative stress indices. This metal reduced the activities of superoxide dismutase, catalase and glutathione peroxidase and nonenzymatic antioxidant levels such as reduced glutathione, total sulfhydryl groups and vitamin C. In addition, it caused hepatic hemorrhage, cellular degeneration and necrosis of hepatocytes as indicated by liver histopathology and DNA fragmentation studies. Coadministration of silymarin alleviated Mn oxidative damage effects by inhibiting ROS generation. Histological studies also supported the beneficial role of silymarin against Mn-induced hepatic damages. Combining all, results suggested that silymarin could protect hepatic tissues against Mn-induced oxidative stress probably through its antioxidant activity. Therefore, its supplementation could provide a new approach for the reduction in hepatic complication due to Mn poisoning.

Manganese and the brain

Manganese (Mn) is an essential trace metal that is pivotal for normal cell function and metabolism. Its homeostasis is tightly regulated; however, the mechanisms of Mn homeostasis are poorly characterized. While a number of proteins such as the divalent metal transporter 1, the transferrin/transferrin receptor complex, the ZIP family metal transporters ZIP-8 and ZIP-14, the secretory pathway calcium ATPases SPCA1 and SPCA2, ATP13A2, and ferroportin have been suggested to play a role in Mn transport, the degree that each of them contributes to Mn homeostasis has still to be determined. The recent discovery of SLC30A10 as a crucial Mn transporter in humans has shed further light on our understanding of Mn transport across the cell. Although essential, Mn is toxic at high concentrations. Mn neurotoxicity has been attributed to impaired dopaminergic (DAergic), glutamatergic and GABAergic transmission, mitochondrial dysfunction, oxidative stress, and neuroinflammation. As a result of preferential accumulation of Mn in the DAergic cells of the basal ganglia, particularly the globus pallidus, Mn toxicity causes extrapyramidal motor dysfunction. Firstly described as "manganism" in miners during the nineteenth century, this movement disorder resembles Parkinson's disease characterized by hypokinesia and postural instability. To date, a variety of acquired causes of brain Mn accumulation can be distinguished from an autosomal recessively inherited disorder of Mn metabolism caused by mutations in the SLC30A10 gene. Both, acquired and inherited hypermanganesemia, lead to Mn deposition in the basal ganglia associated with pathognomonic magnetic resonance imaging appearances of hyperintense basal ganglia on T1-weighted images. Current treatment strategies for Mn toxicity combine chelation therapy to reduce the body Mn load and iron (Fe) supplementation to reduce Mn binding to proteins that interact with both Mn and Fe. This chapter summarizes our current understanding of Mn homeostasis and the mechanisms of Mn toxicity and highlights the clinical disorders associated with Mn neurotoxicity.

Manganese and ammonia interactions in the brain of cirrhotic rats: effects on brain ammonia metabolism

Hepatic encephalopathy is a major complication of cirrhosis. Ammonia and manganese have been associated with hepatic encephalopathy underlying mechanisms. Motor impairment and brain edema are common signs of hepatic encephalopathy. In the present study a model of liver damage in rats was combined with ammonia and manganese exposure to evaluate the role of these substances separately and their interactions on brain glutamine, water content and motor coordination. Additionally, we explored brain levels of each substance -Mn and ammonia- in the presence or absence of the other. Liver damage was induced by bile duct ligation. Rats were exposed to MnCl2 in drinking water (1 mg Mn/ml) and to ammonia in chow pellets containing 20% ammonium acetate (w/w). As expected, manganese and ammonia levels increased in the brain of cirrhotic rats exposed to these substances; in these animals, glutamine brain levels also increased and positively correlated with tissue water content in cortex. A three way-ANOVA showed that manganese favored ammonia and glutamine accumulation in brain, and possibly their subsequent deleterious effects, as evidenced by the fact that manganese and ammonia accumulation in the brain of cirrhotic rats severely affected motor function. These results suggest that even when controlling ammonia levels in cirrhotic patients, reduction of manganese intake is also a potential strategy to be considered in clinical practice.

Role of glial cells in manganese neurotoxicity

The objectives of this focused review are to (i) provide a systematic overview of recent advances pertaining to the role of glia, namely microglia and astrocytes, in the neuropathology associated with excessive exposure to manganese (Mn), (ii) highlight possible mechanisms and factors involved in Mn-modulated, glia-derived neuroinflammation, and (iii) discuss the implications of excessive neuroinflammation on neuronal injury within the context of Mn overexposure. As this is not meant to be a comprehensive review on the topic of Mn neurotoxicity, the reader may wish to refer to several broader and more comprehensive reviews. After a brief introduction to Mn neurotoxicity, we first discuss the role of glial cells in neurodegeneration. Next, we review existing in vitro and in vivo studies that implicate Mn as a modulator of glial activation and ensuing neuroinflammation. This is followed by an examination of recognized and potential mechanisms that are involved in the modulation of glial inflammatory output by Mn; here the common pathways activated by Mn in glial and neuronal cells, including outcomes of such activation, are also addressed. We finish with a discussion of the implications of Mn-modulated glial activation for neuronal survival and with a list of data gaps in the topic that need to be filled in the future.

Protective effect of Melissa officinalis aqueous extract against Mn-induced oxidative stress in chronically exposed mice

Manganese (Mn) is an essential element for biological systems; however occupational exposure to high levels of this metal may lead to neurodegenerative disorders, resembling Parkinson's disease (PD). While its mechanisms of neurotoxicity have yet to be fully understood, oxidative stress plays a critical role. Thus, the main goal of this study was to investigate the efficacy of aqueous extract of Melissa officinalis in attenuating Mn-induced brain oxidative stress in mice. Sixteen male mice were randomly divided into two groups and treated for 3 months: the first group consumed tap water (control group) and the second group was treated with Mn (50 mg/kg/day for habituation during the first 15 days followed by 100 mg/kg/day for additional 75 days) in the drinking water. After 3 months both groups were sub divided (n=4 per group) and treated for additional 3 months with Mn and/or M. officinalis in the drinking water. The first group (control) was treated with water and served as control; the second group (M. officinalis) was treated with M. officinalis (100 mg/kg/day); the third group was treated with Mn (100 mg/kg/day); the fourth group (Mn+M. officinalis) was treated with both Mn and M. officinalis (100 mg/kg/day each). Mn-treated mice showed a significant increase in thiobarbituric acid reactive species (TBARS) levels (a marker of oxidative stress) in both the hippocampus and striatum. These changes were accompanied by a decrease in total thiol content in the hippocampus and a significant increase in antioxidant enzyme activity (superoxide dismutase and catalase) in the hippocampus, striatum, cortex and cerebellum. Co-treatment with M. officinalis aqueous extract in Mn-treated mice significantly inhibited the antioxidant enzyme activities and attenuated the oxidative damage (TBARS and decreased total thiol levels). These results establish that M. officinalis aqueous extract possesses potent antioxidative properties, validating its efficacy in attenuating Mn-induced oxidative stress in the mouse brain.

Protective effects of antioxidants and anti-inflammatory agents against manganese-induced oxidative damage and neuronal injury

Exposure to excessive manganese (Mn) levels leads to neurotoxicity, referred to as manganism, which resembles Parkinson's disease (PD). Manganism is caused by neuronal injury in both cortical and subcortical regions, particularly in the basal ganglia. The basis for the selective neurotoxicity of Mn is not yet fully understood. However, several studies suggest that oxidative damage and inflammatory processes play prominent roles in the degeneration of dopamine-containing neurons. In the present study, we assessed the effects of Mn on reactive oxygen species (ROS) formation, changes in high-energy phosphates and associated neuronal dysfunctions both in vitro and in vivo. Results from our in vitro study showed a significant (p<0.01) increase in biomarkers of oxidative damage, F(2)-isoprostanes (F(2)-IsoPs), as well as the depletion of ATP in primary rat cortical neurons following exposure to Mn (500 μM) for 2h. These effects were protected when neurons were pretreated for 30 min with 100 of an antioxidant, the hydrophilic vitamin E analog, trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), or an anti-inflammatory agent, indomethacin. Results from our in vivo study confirmed a significant increase in F(2)-IsoPs levels in conjunction with the progressive spine degeneration and dendritic damage of the striatal medium spiny neurons (MSNs) of mice exposed to Mn (100mg/kg, s.c.) 24h. Additionally, pretreatment with vitamin E (100mg/kg, i.p.) or ibuprofen (140 μg/ml in the drinking water for two weeks) attenuated the Mn-induced increase in cerebral F(2)-IsoPs? and protected the MSNs from dendritic atrophy and dendritic spine loss. Our findings suggest that the mediation of oxidative stress/mitochondrial dysfunction and the control of alterations in biomarkers of oxidative injury, neuroinflammation and synaptodendritic degeneration may provide an effective, multi-pronged therapeutic strategy for protecting dysfunctional dopaminergic transmission and slowing of the progression of Mn-induced neurodegenerative processes.

Manganese accumulation in the CNS and associated pathologies

Manganese (Mn) is an essential metal for life. It is a key constituent of clue enzymes in the central nervous system, contributing to antioxidant defenses, energetic metabolism, ammonia detoxification, among other important functions. Until now, Mn transport mechanisms are partially understood; however, it is known that it shares some mechanisms of transport with iron. CNS is susceptible to Mn toxicity because it possesses mechanisms that allow Mn entry and favor its accumulation. Cases of occupational Mn exposure have been extensively reported in the literature; however, there are other ways of exposure, such as long-term parental nutrition and liver failure. Manganism and hepatic encephalopathy are the most common pathologies associated with the effects of Mn exposure. Both pathologies are associated with motor and psychiatric disturbances, related in turn to mechanisms of damage such as oxidative stress and neurotransmitters alterations, the dopaminergic system being one of the most affected. Although manganism and Parkinson's disease share some characteristics, they differ in many aspects that are discussed here. The mechanisms for Mn transport and its participation in manganism and hepatic encephalopathy are also considered in this review. It is necessary to find an effective therapeutic strategy to decrease Mn levels in exposed individuals and to treat Mn long term effects. In the case of patients with chronic liver failure it would be worthwhile to test a low-Mn diet in order to ameliorate symptoms of hepatic encephalopathy possibly related to Mn accumulation.

Manganese induces oxidative stress, redox state unbalance and disrupts membrane bound ATPases on murine neuroblastoma cells in vitro: protective role of silymarin

Manganese (Mn) is an essential trace element required for ubiquitous enzymatic reactions. Chronic overexposure to this metal may promote potent neurotoxic effects. The mechanism of Mn toxicity is not well established, but several studies indicate that oxidative stress play major roles in the Mn-induced neurodegenerative processes. Silymarin (SIL) has antioxidant properties and stabilizes intracellular antioxidant defense systems. The aim of this study was to evaluate the toxic effects of MnCl₂ on the mouse neuroblastoma cell lines (Neuro-2A), to characterize the toxic mechanism associated with Mn exposure and to investigate whether SIL could efficiently protect against neurotoxicity induced by Mn. A significant increase in LDH release activity was observed in Neuro-2A cells associated with a significant decrease in cellular viability upon 24 h exposure to MnCl₂ at concentrations of 200 and 800 μM (P < 0.05) when compared with control unexposed cells. In addition, exposure cells to MnCl₂ (200 and 800 μM), increases oxidant biomarkers and alters enzymatic and non enzymatic antioxidant systems. SIL treatment significantly reduced the levels of LDH, nitric oxide, reactive oxygen species and the oxidants/antioxidants balance in Neuro-2A cells as compared to Mn-exposed cells. These results suggested that silymarin is a powerful antioxidant through a mechanism related to its antioxidant activity, able to interfere with radical-mediated cell death. SIL may be useful in diseases known to be aggravated by reactive oxygen species and in the development of novel treatments for neurodegenerative disorders such as Alzheimer or Parkinson diseases.

Preweaning Mn exposure leads to prolonged astrocyte activation and lasting effects on the dopaminergic system in adult male rats

Little is known about the effects of manganese (Mn) exposure over neurodevelopment and whether these early insults result in effects lasting into adulthood. To determine if early Mn exposure produces lasting neurobehavioral and neurochemical effects, we treated neonate rats with oral Mn (0, 25, or 50 mg Mn/kg/d over PND 1-21) and evaluated (1) behavioral performance in the open arena in the absence (PND 97) and presence (PND 98) of a d-amphetamine challenge, (2) brain dopamine D1 and D2-like receptors and dopamine transporter densities in the prefrontal cortex, striatum, and nucleus accumbens (PND 107), and (3) astrocyte marker glial fibrillary acidic protein (GFAP) levels in these same brain regions (PND 24 and 107). We found that preweaning Mn exposure did not alter locomotor activity or behavior disinhibition in adult rats, though Mn-exposed animals did exhibit an enhanced locomotor response to d-amphetamine challenge. Preweaning Mn exposure led to increased D1 and D2 receptor levels in the nucleus accumbens and prefrontal cortex, respectively, compared with controls. We also found increased GFAP expression in the prefrontal cortex in Mn-exposed PND 24 weanlings, and increased GFAP levels in prefrontal cortex, medial striatum and nucleus accumbens of adult (PND 107) rats exposed to preweaning Mn, indicating an effect of Mn exposure on astrogliosis that persisted and/or progressed to other brain regions in adult animals. These data show that preweaning Mn exposure leads to lasting molecular and functional impacts in multiple brain regions of adult animals, long after brain Mn levels returned to normal.

Aquaporin-4 in manganese-treated cultured astrocytes

Manganese in excess is neurotoxic and causes CNS injury resembling that of Parkinson's disease. In brain, astrocytes predominantly take up and accumulate manganese and are thus vulnerable to its toxicity. Manganese was shown to induce cell swelling in cultured astrocytes, and oxidative/nitrosative stress (ONS) mediates such swelling. As aquaporin-4 (AQP4) is important in the mechanism of astrocyte swelling, we examined the effect of manganese on AQP4 protein levels in cultured astrocytes. Treatment of cultures with manganese increased AQP4 protein in the plasma membrane (PM), whereas total cellular AQP4 protein and mRNA levels were unchanged, suggesting that increased AQP4 levels is due to its increased stability and/or increased trafficking to the PM and not to its neosynthesis. AQP4 gene silencing by small interfering ribonucleic acid resulted in a marked reduction in astrocyte swelling by manganese. Antioxidants, as well as an inhibitor of nitric oxide synthase, diminished the increase in AQP4 protein expression, suggesting a role of ONS in the mechanism of AQP4 increase. As ONS is known to activate mitogen-activated protein kinases (MAPKs) and MAPK activation has been implicated in astrocyte swelling, we examined the effect of manganese on the activation of MAPKs and found an increased phosphorylation of extracellular signal-regulated kinase (ERK)1/2, C-Jun amino-terminal kinase (JNK)1/2/3, and p38-MAPK. Inhibitors of ERK1/2 and p38-MAPK (but not of JNK) blocked (40-60%) the manganese-induced increase in AQP4 protein content and astrocyte swelling, suggesting the involvement of these kinases in the increased AQP4 content. Inhibition of oxidative stress or MAPKs may represent potential strategies for counteracting AQP4-related neurological complications associated with manganese toxicity.

Silymarin, a natural antioxidant, protects cerebral cortex against manganese-induced neurotoxicity in adult rats

Manganese (Mn) is an essential element for biological systems, nevertheless occupational exposure to high levels of Mn can lead to neurodegenerative disorders, characterized by serious oxidative and neurotoxic effects with similarities to Parkinson's disease. The aim of this study was to investigate the potential effects of silymarin (SIL), an antioxidant flavonoid, against manganese chloride induced neurotoxicity both in vivo (cerebral cortex of rats) and in vitro (Neuro2a cells). Twenty-eight male Wistar rats were randomly divided into four groups: the first group (C) received vehicle solution (i.p.) served as controls. The second group (Mn) received orally manganese chloride (20 mg/ml). The third group (Mn + SIL) received both Mn and SIL. The fourth group (SIL) received only SIL (100 mg/kg/day, i.p.). Animals exposed to Manganese chloride showed a significant increase in TBARS, NO, AOPP and PCO levels in cerebral cortex. These changes were accompanied by a decrease of enzymatic (SOD, CAT, GPx) and non-enzymatic (GSH, NpSH, Vit C) antioxidants. Co-administration of silymarin to Mn-treated rats significantly improved antioxidant enzyme activities and attenuated oxidative damages observed in brain tissue. The potential effect of SIL to prevent Mn induced neurotoxicity was also reflected by the microscopic study, indicative of its neuroprotective effects. We concluded that silymarin possesses neuroprotective potential, thus validating its use in alleviating manganese-induced neurodegenerative effects.

Metal toxicity, liver disease and neurodegeneration

Hepatocerebral disorders are serious neuropsychiatric conditions that result from liver failure. These disorders are characterized neuropathologically by varying degrees of neuronal cell death in basal ganglia, cerebellum, and spinal cord, and include clinical entities such as Wilson's Disease, post-shunt myelopathy, hepatic encephalopathy, and acquired non-Wilsonian hepatocerebral degeneration. Morphologic changes to astrocytes (Alzheimer type II astrocytosis) are a major feature of hepatocerebral disorders. Neurological symptoms include Parkinsonism, cognitive dysfunction, and ataxia. Pathophysiologic mechanisms responsible for cerebral dysfunction and neuronal cell death in hepatocerebral disorders include ammonia toxicity and neurotoxic effects of metals such as copper, manganese, and iron. Molecular mechanisms of neurotoxicity include oxidative/nitrosative stress, glutamate (NMDA)-receptor-mediated excitotoxicity, and neuroinflammatory mechanisms. However, neuronal cell death in hepatocerebral disorders is limited by adaptive mechanisms that may include NMDA-receptor down-regulation, the synthesis of neuroprotective steroids and hypothermia. Management and treatment of hepatocerebral disorders include chelation therapy (Wilson's Disease), the use of ammonia-lowering agents (lactulose, antibiotics, ornithine aspartate) and liver transplantation.

A possible neuroprotective action of a vinylic telluride against Mn-induced neurotoxicity

Manganese (Mn) is a metal required by biological systems. However, environmental or occupational exposure to high levels of Mn can produce a neurological disorder called manganism, which has similarities to Parkinson's disease. Diethyl-2-phenyl-2-tellurophenyl vinylphosphonate (DPTVP) is an organotellurium compound with a high antioxidant activity, especially in the brain. The present study was designed to investigate the effects of long-term low-dose exposure to Mn in drinking water on behavioral and biochemical parameters in rats and to determine the effectiveness of vinylic telluride in attenuating the effects of Mn. After 4 months of treatment with MnCl(2) (13.7 mg/kg), rats exhibited clear signs of neurobehavioral toxicity, including a decrease in the number of rearings in the open field and altered motor performance in rotarod. The administration of DPTVP (0.150 micromol/kg, ip, 2 weeks) improved the motor performance of Mn-treated rats, indicating that the compound could be reverting Mn neurotoxicity. Ex vivo, we observed that Mn concentrations in the Mn-treated group were highest in the striatum, consistent with a statistically significant decrease in mitochondrial viability and [(3)H]glutamate uptake, and increased lipid peroxidation. Mn levels in the hippocampus and cortex were indistinguishable from controls, and no significant differences were noted in the ex vivo assays in these areas. Treatment with DPTVP fully reversed the biochemical parameters altered by Mn. Furthermore, DPTVP treatment was also associated with a reduction in striatal Mn levels. Our results demonstrate that DPTVP has neuroprotective activity against Mn-induced neurotoxicity, which may be attributed to its antioxidant activity and/or its effect on striatal Mn transport.

Region specific increase in the antioxidant enzymes and lipid peroxidation products in the brain of rats exposed to lead

The objective of this study is to determine the effect of lead (pb) on antioxidant enzymes and lipid peroxidation products in different regions of rat brain. Wistar male rats were treated with lead acetate (500 ppm) through drinking water for a period of 8 weeks. Control animals were maintained on sodium acetate. Treated and control rats were sacrificed at intervals of 1st, 4th and 8th week and the whole brains were dissected on ice into four regions namely the cerebellum, the hippocampus, the frontal cortex and the brain stem. Antioxidant enzymes namely catalase and superoxide dismutase in all the four regions of brain were determined. In addition, lipid peroxidation products were also estimated. The results indicated a gradual increase in the activity of antioxidant enzymes in different regions of the brain and this response was time-dependent. However, the increase was more in the cerebellum and the hippocampus compared to other regions of the brain. The lipid peroxidation products also showed a similar trend suggesting increased effect of lead in these two regions of the brain. The data indicated a region-specific oxidative stress in the brain exposed to lead.

Morphological changes of rat astrocytes induced by liver damage but not by manganese chloride exposure

Liver cirrhosis is a common cause of death around the world. One of its more severe complications is hepatic encephalopathy. As a consequence of liver impairment, manganese (Mn) and other substances accumulate in the brain. Astrocytic morphological changes have been found in postmortem brains of cirrhotic patients. In this study we used a model of cirrhosis induced by bile duct ligation and Mn accumulation by exposing rats to MnCl(2) (1 mg Mn/ml) in their drinking water. Four experimental groups were used: Sham, Sham plus Mn treatment, BDL (bile duct ligated) and BDL plus Mn treatment. Brain Mn was measured by atomic absorption spectrophotometry in cortex, striatum and globus pallidus. Altered and normal astrocytes were counted in the same brain areas. Brain Mn was highest in rats of the BDLMn group. An increased number of altered astrocytes was found only in BDL groups, Mn did not modify this effect. No changes were found in the total number of astrocytes. According to our results, biliary obstruction induced an increase in the number of altered astrocytes since early stages of cirrhosis and Mn did not affect this effect.

Acquired hepatocerebral degeneration

Cirrhosis and its co-morbidities may cause a variety of neurological complications, the most common being bouts of toxic metabolic encephalopathy. A proportion of patients with chronic liver disease develop acquired hepatocerebral degeneration (AHD), a chronic progressive neurological syndrome characterized by parkinsonism, ataxia and other movement disorders. This article reviews the clinical spectrum, pathophysiology, neuroimaging features and differential diagnosis of AHD along with emerging treatment options.

Manganese and its role in Parkinson's disease: from transport to neuropathology

The purpose of this review is to highlight recent advances in the neuropathology associated with Mn exposures. We commence with a discussion on occupational manganism and clinical aspects of the disorder. This is followed by novel considerations on Mn transport (see also chapter by Yokel, this volume), advancing new hypotheses on the involvement of several transporters in Mn entry into the brain. This is followed by a brief description of the effects of Mn on neurotransmitter systems that are putative modulators of dopamine (DA) biology (the primary target of Mn neurotoxicity), as well as its effects on mitochondrial dysfunction and disruption of cellular energy metabolism. Next, we discuss inflammatory activation of glia in neuronal injury and how disruption of synaptic transmission and glial-neuronal communication may serve as underlying mechanisms of Mn-induced neurodegeneration commensurate with the cross-talk between glia and neurons. We conclude with a discussion on therapeutic aspects of Mn exposure. Emphasis is directed at treatment modalities and the utility of chelators in attenuating the neurodegenerative sequelae of exposure to Mn. For additional reading on several topics inherent to this review as well as others, the reader may wish to consult Aschner and Dorman (Toxicological Review 25:147-154, 2007) and Bowman et al. (Metals and neurodegeneration, 2009).

Manganese-enhanced MRI of layer-specific activity in the visual cortex from awake and free-moving rats

Cortical responses to visual stimulation have been studied extensively in the rodent, but often require post-stimulation ex vivo examination of the tissue. Here, we test the hypothesis that visual stimulus-dependent cortical activity from awake and free-moving rats can be encoded following systemically administered MnCl(2), and activity subsequently readout using manganese-enhanced MRI (MEMRI), a technique that can be performed without sacrificing the animal. Unanesthetized Sprague-Dawley rats, with or without systemic injection of MnCl(2), were maintained for 8 h in either a visually stimulating environment or darkness. To identify vision-dependent changes in cortical activity, animals were anesthetized and cortices were examined by 3D RARE MEMRI. Mean signal intensities in sub-cortical regions (e.g., superior colliculus and the lateral geniculate), and cortical regions (primary and accessory visual cortices) were compared. Cortex linearization was performed to aid in layer-specific signal intensity comparisons. Manganese administration alone globally increased signal intensity in the brain (P<0.0001). In visually stimulated and unstimulated rats, layer-specific analysis revealed that stimulated rats had on average significantly (P<0.05) higher signal intensities in layers IV and V of the primary visual cortex, as well as in deeper portions of the superficial superior colliculus, relative to dark adapted rats. Such differences went undetected without layer-specific analysis. We demonstrate, for the first time, the feasibility of layer-specific stimulus-dependant non-invasive MEMRI readout after encoding activity in awake and free moving rats. Future MEMRI studies are envisioned that measure the effects on cortical activity of sensory stimulation, as well as normal development, disease, plasticity, and therapy in longitudinal studies.

MR imaging findings in hepatic encephalopathy

The term hepatic encephalopathy (HE) includes a spectrum of neuropsychiatric abnormalities occurring in patients with liver dysfunction. Most cases are associated with cirrhosis and portal hypertension or portal-systemic shunts, but the condition can also be seen in patients with acute liver failure and, rarely, with portal-systemic bypass and no associated intrinsic hepatocellular disease. Although HE is a clinical condition, several neuroimaging techniques, particularly MR imaging, may eventually be useful for the diagnosis because they can identify and measure the consequences of central nervous system (CNS) increase in substances that under normal circumstances, are efficiently metabolized by the liver. Classic MR imaging abnormalities include high signal intensity in the globus pallidum on T1-weighted images, likely a reflection of increased tissue concentrations of manganese, and an elevated glutamine/glutamate peak coupled with decreased myo-inositol and choline signals on proton MR spectroscopy, representing disturbances in cell-volume homeostasis secondary to brain hyperammonemia. Recent data have shown that white matter abnormalities, also related to increased CNS ammonia concentration, can also be detected with several MR imaging techniques such as magnetization transfer ratio measurements, fast fluid-attenuated inversion recovery sequences, and diffusion-weighted images. All these MR imaging abnormalities, which return to normal with restoration of liver function, probably reflect the presence of mild diffuse brain edema, which seems to play an essential role in the pathogenesis of HE. It is likely that MR imaging will be increasingly used to evaluate the mechanisms involved in the pathogenesis of HE and to assess the effects of therapeutic measures focused on correcting brain edema in these patients.

Increased APLP1 expression and neurodegeneration in the frontal cortex of manganese-exposed non-human primates

Chronic manganese (Mn) exposure produces a neurological syndrome with psychiatric, cognitive, and parkinsonian features. Gene expression profiling in the frontal cortex of Cynomologous macaques receiving 3.3-5.0 mg Mn/kg weekly for 10 months showed that 61 genes were increased and four genes were decreased relative to controls from a total of 6766 genes. Gene changes were associated with cell cycle regulation, DNA repair, apoptosis, ubiquitin-proteasome system, protein folding, cholesterol homeostasis, axonal/vesicular transport, and inflammation. Amyloid-beta (Abeta) precursor-like protein 1, a member of the amyloid precursor protein family, was the most highly up-regulated gene. Immunohistochemistry confirmed increased amyloid precursor-like protein 1 protein expression and revealed the presence of diffuse Abeta plaques in Mn-exposed frontal cortex. Cortical neurons and white matter fibers from Mn-exposed animals accumulated silver grains indicative of on-going degeneration. Cortical neurons also exhibited nuclear hypertrophy, intracytoplasmic vacuoles, and apoptosis stigmata. p53 immunolabeling was increased in the cytoplasm of neurons and in the nucleus and processes of glial cells in Mn-exposed tissue. In summary, chronic Mn exposure produces a cellular stress response leading to neurodegenerative changes and diffuse Abeta plaques in the frontal cortex. These changes may explain the subtle cognitive deficits previously demonstrated in these same animals.

Aquaporin-4 in hepatic encephalopathy

Brain edema is a critical component of hepatic encephalopathy (HE) associated with acute liver failure and such edema appears to be principally due to astrocyte swelling (cytotoxic edema). Ammonia is believed to represent a major factor responsible for astrocyte swelling, although the mechanisms by which ammonia causes such swelling are not completely understood. Recent studies have implicated potential role of oxidative stress, and the mitochondrial permeability transition (mPT). While it is not known how oxidative stress and the mPT cause astrocyte swelling, it is reasonable to suggest that these events may affect one or more plasma membrane proteins involved in water and ion homeostasis in astrocytes. One such protein strongly implicated in brain edema in other neurological conditions is the water channel protein aquaporin-4 (AQP-4), which is abundantly expressed in astrocytes. This article summarizes the potential role of AQP-4 in brain edema in in vivo models of HE, as well as in ammonia-induced cell swelling in cultured astrocytes. The involvement of AQP-4 in the effects of manganese, another toxin implicated in HE, will also be discussed.

The neurosteroid system: an emerging therapeutic target for hepatic encephalopathy

Both acute and chronic liver failure induce cerebral complications known as hepatic encephalopathy (HE) and thought to selectively involve brain astrocytes. Alterations of astrocytic-neuronal cross talk occurs affecting brain function. In acute liver failure, astrocyte undergo swelling, which results in increased intracranial pressure and may lead to brain herniation. In chronic liver failure, Alzheimer-type II astrocytosis is a characteristic change. Neurosteroids (NS) synthesized in the brain mainly by astrocytes independent of peripheral steroidal sources (adrenals and gonads) are suggested to play a role in HE. NS bind and modulate different types of membrane receptors. Effects on the gamma amino butyric acid (GABA)-A receptor complex are the most extensively studied. For example, the NS tetrahydroprogesterone (allopregnanolone), and tetrahydrodeoxycorticosterone (THDOC) are potent positive allosteric modulators of GABA-A receptors. As a consequence of modulation of these receptors, NS are well-known to modulate inhibitory neurotransmission in the central nervous system. Some NS bind to intracellular receptors, and in this way may also regulate gene expression. In HE, it has been well documented that neurotransmission and gene expression alterations occur during the progression of the disease. This review summarizes findings of relevance for the involvement of NS in human and experimental HE.