In vitro effect of organic and inorganic mercury on the serotonergic system

A Hypothesis and Evidence That Mercury May be an Etiological Factor in Alzheimer's Disease

Mercury is one of the most toxic elements and causes a multitude of health problems. It is ten times more toxic to neurons than lead. This study was created to determine if mercury could be causing Alzheimer's disease (AD) by cross referencing the effects of mercury with 70 factors associated with AD. The results found that all these factors could be attributed to mercury. The hallmark changes in AD include plaques, beta amyloid protein, neurofibrillary tangles, phosphorylated tau protein, and memory loss-all changes that can be caused by mercury. Neurotransmitters such as acetylcholine, serotonin, dopamine, glutamate, and norepinephrine are inhibited in patients with Alzheimer's disease, with the same inhibition occurring in mercury toxicity. Enzyme dysfunction in patients with Alzheimer's disease include BACE 1, gamma secretase, cyclooxygenase-2, cytochrome-c-oxidase, protein kinases, monoamine oxidase, nitric oxide synthetase, acetyl choline transferase, and caspases, all which can be explained by mercury toxicity. Immune and inflammatory responses seen in patients with Alzheimer's disease also occur when cells are exposed to mercury, including complement activation, cytokine expression, production of glial fibrillary acid protein antibodies and interleukin-1, transforming growth factor, beta 2 microglobulins, and phosphodiesterase 4 stimulation. Genetic factors in patients with Alzheimer's disease are also associated with mercury. Apolipoprotein E 4 allele increases the toxicity of mercury. Mercury can inhibit DNA synthesis in the hippocampus, and has been associated with genetic mutations of presenilin 1 and 2, found in AD. The abnormalities of minerals and vitamins, specifically aluminum, calcium, copper, iron, magnesium, selenium, zinc, and vitamins B1, B12, E, and C, that occur in patients with Alzheimer's disease, also occur in mercury toxicity. Aluminum has been found to increase mercury's toxicity. Likewise, similar biochemical factors in AD are affected by mercury, including changes in blood levels of homocysteine, arachidonic acid, DHEA sulfate, glutathione, hydrogen peroxide, glycosamine glycans, acetyl-L carnitine, melatonin, and HDL. Other factors seen in Alzheimer's disease, such as increased platelet activation, poor odor identification, hypertension, depression, increased incidences of herpes virus and chlamydia infections, also occur in mercury exposure. In addition, patients diagnosed with Alzheimer's disease exhibit higher levels of brain mercury, blood mercury, and tissue mercury in some studies. The greatest exogenous sources of brain mercury come from dental amalgams. Conclusion: This review of the literature strongly suggests that mercury can be a cause of Alzheimer's Disease.

Insights into the Potential Role of Mercury in Alzheimer's Disease

Mercury (Hg), which is a non-essential element, is considered a highly toxic pollutant for biological systems even when present at trace levels. Elevated Hg exposure with the growing release of atmospheric pollutant Hg and rising accumulations of mono-methylmercury (highly neurotoxic) in seafood products have increased its toxic potential for humans. This review aims to highlight the potential relationship between Hg exposure and Alzheimer's disease (AD), based on the existing literature in the field. Recent reports have hypothesized that Hg exposure could increase the potential risk of developing AD. Also, AD is known as a complex neurological disorder with increased amounts of both extracellular neuritic plaques and intracellular neurofibrillary tangles, which may also be related to lifestyle and genetic variables. Research reports on AD and relationships between Hg and AD indicate that neurotransmitters such as serotonin, acetylcholine, dopamine, norepinephrine, and glutamate are dysregulated in patients with AD. Many researchers have suggested that AD patients should be evaluated for Hg exposure and toxicity. Some authors suggest further exploration of the Hg concentrations in AD patients. Dysfunctional signaling pathways in AD and Hg exposure appear to be interlinked with some driving factors such as arachidonic acid, homocysteine, dehydroepiandrosterone (DHEA) sulfate, hydrogen peroxide, glucosamine glycans, glutathione, acetyl-L carnitine, melatonin, and HDL. This evidence suggests the need for a better understanding of the relationship between AD and Hg exposure, and potential mechanisms underlying the effects of Hg exposure on regional brain functions. Also, further studies evaluating brain functions are needed to explore the long-term effects of subclinical and untreated Hg toxicity on the brain function of AD patients.

A multidimensional concept for mercury neuronal and sensory toxicity in fish - From toxicokinetics and biochemistry to morphometry and behavior

BACKGROUND

Neuronal and sensory toxicity of mercury (Hg) compounds has been largely investigated in humans/mammals with a focus on public health, while research in fish is less prolific and dispersed by different species. Well-established premises for mammals have been governing fish research, but some contradictory findings suggest that knowledge translation between these animal groups needs prudence [e.g. the relative higher neurotoxicity of methylmercury (MeHg) vs. inorganic Hg (iHg)]. Biochemical/physiological differences between the groups (e.g. higher brain regeneration in fish) may determine distinct patterns. This review undertakes the challenge of identifying sensitive cellular targets, Hg-driven biochemical/physiological vulnerabilities in fish, while discriminating specificities for Hg forms.

SCOPE OF REVIEW

A functional neuroanatomical perspective was conceived, comprising: (i) Hg occurrence in the aquatic environment; (ii) toxicokinetics on central nervous system (CNS)/sensory organs; (iii) effects on neurotransmission; (iv) biochemical/physiological effects on CNS/sensory organs; (v) morpho-structural changes on CNS/sensory organs; (vi) behavioral effects. The literature was also analyzed to generate a multidimensional conceptualization translated into a Rubik's Cube where key factors/processes were proposed.

MAJOR CONCLUSIONS

Hg neurosensory toxicity was unequivocally demonstrated. Some correspondence with toxicity mechanisms described for mammals (mainly at biochemical level) was identified. Although the research has been dispersed by numerous fish species, 29 key factors/processes were pinpointed.

GENERAL SIGNIFICANCE

Future trends were identified and translated into 25 factors/processes to be addressed. Unveiling the neurosensory toxicity of Hg in fish has a major motivation of protecting ichtyopopulations and ecosystems, but can also provide fundamental knowledge to the field of human neurodevelopment.

Aerobic Fitness and Neurocognitive Function Scores in Young Faroese Adults and Potential Modification by Prenatal Methylmercury Exposure

BACKGROUND

Exposure to methylmercury was shown to decrease neural stem cell populations, whereas aerobic fitness has beneficial effects on the adult brain that relies on improved neurogenesis in the hippocampus.

OBJECTIVES

We examined the association between aerobic fitness and neurocognitive outcomes at young adult age, along with the potential moderating effect of prenatal exposure to methylmercury.

METHODS

At age 22 years, 262 members of a Faroese birth cohort, established in 1986-1987, underwent a graded exercise test of aerobic fitness to measure maximal oxygen uptake (VO_2Max). Their prenatal methylmercury exposure had been assessed from the mercury concentration in cord blood. We estimated cross-sectional associations between VO_2Max and multiple measures of neurocognitive function. In addition, we compared groups with low and high prenatal methylmercury exposure.

RESULTS

A 1 standard deviation (SD) increase in VO_2Max was associated with better scores on short-term memory and cognitive processing speed by 0.21 SD (95% CI: -0.04, 0.46) and 0.28 SD (95% CI: 0.02, 0.54), respectively. In the group with lower prenatal methylmercury exposure, a 1 SD increase in VO_2Max was associated with increased scores on cognitive processing speed by 0.45 SD (95% CI: 0.08, 0.81) and with a slightly lesser benefit in short-term memory. No such association was observed in the group with high prenatal methylmercury exposure.

CONCLUSIONS

Higher aerobic capacity was associated with better performance in short-term memory and processing speed. However, prenatal methylmercury exposure seemed to attenuate these positive associations.

Behavioral and neurochemical sensitization to amphetamine following early postnatal administration of methylmercury (MeHg)

Perinatal exposure to methylmercury (MeHg) in rodents has been linked to changes in sensitivity to dopaminergic agents later in life. In an effort to determine the behavioral and neurochemical response to the indirect dopaminergic and serotonergic agonist amphetamine following neonatal exposure to MeHg, male BALB/c mice were administered MeHg during critical periods of neural development and challenged with amphetamine as adults. Mice were observed 15, 30 and 60 min after a single amphetamine injection (7.5 mg/kg i.p.) for presence of stereotypic and self-injurious behaviors, abnormal posture, and hyperthermia. Mice treated with 2 or 4 mg/kg MeHg on alternate days 3-15 of life demonstrated an increase in body temperature and the appearance of stereotypic and self-injurious behaviors not observed when amphetamine was administered to either vehicle-exposed mice or those treated with an equivalent total amount of MeHg administered on postnatal days 13 and 15. Neurochemical analysis of MeHg- and vehicle-exposed mice challenged with amphetamine or saline revealed alterations in dopaminergic and serotonergic activity which corresponded to the sensitized behavioral response to amphetamine. These observations demonstrate a critical window for MeHg exposure affecting the later appearance of amphetamine-induced self-injurious behavior and support the hypothesis that early exposure to environmental neurotoxicants may predispose individuals to engage in aberrant, intrusive behaviors later in life.

Exposure to Hg2+ and Pb2+ changes NTPDase and ecto-5'-nucleotidase activities in central nervous system of zebrafish (Danio rerio)

Neurotransmission can be affected by exposure to heavy metals, such as mercury and lead. ATP is a signaling molecule that can be metabolized by a group of enzymes called ecto-nucleotidases. Here we investigated the effects of mercury chloride (HgCl(2)) and lead acetate (Pb(CH(3)COO)(2)) on NTPDase (nucleoside triphosphate diphosphohydrolase) and ecto-5'-nucleotidase activities in zebrafish brain membranes. In vitro exposure to HgCl(2) decreased ATP and ADP hydrolysis in an uncompetitive mechanism and AMP hydrolysis in a non-competitive manner. Pb(CH(3)COO)(2) inhibited ATP hydrolysis in an uncompetitive manner, but not ADP and AMP hydrolysis. In vivo exposure of zebrafish to HgCl(2) or Pb(CH(3)COO)(2) (20mug/L, during 24, 96h and 30 days) caused differential effects on nucleotide hydrolysis. HgCl(2), during 96h, inhibited the hydrolysis of ATP, ADP and AMP. After 30 days of exposure to HgCl(2), ATP hydrolysis returned to the control levels, ADP hydrolysis was strongly increased and AMP hydrolysis remained inhibited. Exposure to Pb(CH(3)COO)(2) during 96h caused a significant decrease only on ATP hydrolysis. After 30 days, Pb(CH(3)COO)(2) promoted the inhibition of ATP, ADP and AMP hydrolysis. Semi-quantitative RT-PCR analysis showed no changes in the expression of NTPDase1 and 5'-nucleotidase, following 30 days of exposure to both metals. This study demonstrated that Hg(2+) and Pb(2+) affect the ecto-nucleotidase activities, an important enzymatic pathway for the control of purinergic signaling.

Relationship between platelet monoamine oxidase-B (MAO-B) activity and mercury exposure in fish consumers from the Lake St. Pierre region of Que., Canada

Mercury (Hg) is a widespread neurotoxic compound that bio-accumulates in fish and marine mammals. Monoamine oxidase (MAO; EC 1.4.3.4) regulates biogenic amine concentration in the brain and peripheral tissue and has been shown to be a molecular target of Hg compounds in animal models. Blood platelet monoamine oxidase-B (MAO-B) activity may reflect MAO function in the central nervous tissue. Therefore, the objective of this study was to evaluate the relationship between platelet MAO-B and Hg exposure in fish-eating adults (n=127) living along the St. Lawrence River (Lake St. Pierre, Que., Canada). Hg concentrations were determined in blood and hair samples. A significant negative association was observed between platelet MAO-B activity and blood-Hg (r=-0.193, p=0.029) but not with hair-Hg levels (r=-0.125, p=0.169). Multiple linear regression analysis demonstrated that blood-Hg (beta=-4.6, p=0.011) and heavy smoking (beta=-8.5, p=0.001) were associated with reduced platelet MAO activity in the total population. In addition, this reduction in MAO-B activity appeared to be associated with blood-Hg concentrations above 3.4 microg/L (75th percentile). Possible gender related differences were also observed and are discussed. Our results suggest that MAO-B activity in blood platelets may be a useful tool to assess biochemical effects of Hg exposure in human populations. These changes in platelet MAO-B may reflect enzymatic changes in nervous tissue and should be further investigated as a surrogate marker of neurotoxicity.

Serotonin production in lymphocytes and mercury intolerance

Patients with suspected illness due to mercury in dental amalgam were classified as tolerant or intolerant depending on their psychosomatic responses following in vivo epicutaneous provocation with low doses (patch test doses) of metallic mercury and phenylmercuric acetate. Ten intolerant patients and nine tolerant patients plus seven healthy amalgam-free and metal non-allergic controls were recruited to the study. Peripheral blood lymphocytes were exposed in vitro to three concentration of mercuric chloride (0.92, 1.83 and 3.68 microM) with and without 10 microg phytohaemagglutinine (PHA)/ml and the release of serotonin into the supernatant was measured. Lymphocytes exposed only to HgCl(2) showed no significant dose-dependent increase of serotonin, but the response of the tolerant patients was significantly higher compared with the controls. No other differences were found. Co-culture with mercuric chloride and PHA showed a statistically significant dose-dependant release of serotonin, but no differences between the three clinical groups could be detected. Thus, our results could not validate the concept of mercury tolerance and intolerance.

Effects of mercury on serotonin concentration in the brain of tilapia, Oreochromis mossambicus

In order to know the effect of mercury pollution on the serotonergic system of fish, serotonin concentrations in a discrete brain region of tilapia, Oreochromis mossambicus, were examined. Serotonin concentration was measured using a high performance liquid chromatography system with electrochemical detector. In male fish, the concentrations of serotonin were 1.468 +/- 0.350, 0.811 +/- 0.190 and 0.330 +/- 0.061 micrograms/g wet tissue in hypothalamus, telencephalon and optic lobe, respectively. The serotonin content was significantly different between each region; the hypothalamus had a higher content than that of the telencephalon and optic lobe. The serotonin concentration in female hypothalamus was 1.102 +/- 0.112 micrograms/g wet tissue which was significantly lower than that in males. However, serotonin concentration in the telencephalon and optic lobe showed no difference between male and female. After exposure to 0.015 and 0.03 ppm HgCl2 for 6 months beginning 7 days posthatching, male sample fish showed a significantly dose-dependent decrease in serotonin concentration in the hypothalamus. But a similar phenomenon was not found in other regions of the brain. These results suggest that exposure to HgCl2 results in an attenuated development of the serotonergic system in the hypothalamus of fish.

Effect of HgCl2 on acetylcholine, carbachol, and glutamate currents of Aplysia neurons

1. Using conventional two-microelectrode voltage-clamp techniques we studied the effects of inorganic mercury (HgCl2) on acetylcholine-, carbachol-, and glutamate-activated currents on Aplysia neurons. Hg2+ was applied with microperfusion. 2. Acetylcholine and carbachol activated an inward, sodium-dependent current in the anterior neurons of the pleural ganglion. The medial neurons gave a biphasic current to acetylcholine and carbachol, which was outward at resting membrane potential. The faster component was Cl- dependent and reversed at about -60 mV, while the slower component was K+ dependent and reversed at greater than -80 mV. 3. Hg2+ (0.1-10 microM) caused a dramatic increase in the acetylcholine- and carbachol-induced inward current in anterior neurons and the fast Cl- current in medial neurons. With only a 1-min preapplication of Hg2+, the acetylcholine- or carbachol-activated sodium or chloride currents were increased to 300% and the effect was only partly reversible. The threshold concentration was 0.1 microM Hg2+. 4. Contrary to the effects on sodium and chloride currents, concentrations of 0.1-10 microM Hg2+ caused a complete and irreversible blockade of K(+)-dependent acetylcholine and carbachol currents. The block of the potassium current was relatively fast and increased with time. The concentration of HgCl2 that gave a half-maximal blockade of the carbachol-activated potassium current was 0.89 microM. The chloride-dependent current elicited by glutamate on medial neurons was increased by HgCl2 as well. 5. These results suggest that actions at agonist-activated channels must be considered as contributing to mercury neurotoxicity. It is possible that the toxic actions of Hg2+ on synaptic transmission at both pre- and postsynaptic sites are important factors in the mechanism of Hg2+ toxicity.

Psychometric evidence that mercury from silver dental fillings may be an etiological factor in depression, excessive anger, and anxiety

Scores on the Beck Depression Inventory were compared for 25 women who had silver dental fillings (amalgams) and for 23 women without amalgams. Women with amalgams had significantly higher scores and reported more symptoms of fatigue and insomnia. Anger scores from the State-Trait Anger Expression Inventory showed that the women with amalgams had statistically significantly higher mean scores on expressing anger without provocation and experiencing more intense angry feelings. The women without amalgams scored significantly higher on controlling anger, which suggested they invested more energy in monitoring and preventing the experience and expression of anger. Anxiety scores from the State-Trait Anxiety Inventory showed the women with amalgams scored significantly less pleasant, satisfied, happy, secure, and steady, and had a more difficult time making decisions. They had significantly higher Trait Anxiety scores. The women with amalgams also had significantly higher levels of mercury in the oral cavity before and after chewing gum. The study suggests that amalgam mercury may be an etiological factor in depression, excessive anger, and anxiety because mercury can produce such symptoms perhaps by affecting the neurotransmitters in the brain.

Evidence that mercury from silver dental fillings may be an etiological factor in smoking

The smoking habits of 119 subjects without silver/mercury dental fillings were compared to 115 subjects with amalgams. The amalgam group had 2.5-times more smokers per group than the non-amalgam group, which was highly significant. Because mercury decreases dopamine, serotonin, norepinephrine, and acetylcholine in the brain, and nicotine has just the opposite effect on these neurotransmitters, this may help explain why persons with dental amalgams smoke more than persons without amalgams.