Effect of selenium on fluoride-induced changes in synaptic plasticity in rat hippocampus

Focus on cognitive impairment induced by excessive fluoride: An update review

Fluorosis is a global public health concern. Prolonged exposure to excessive fluoride causes fluoride accumulation in the hippocampus, resulting in cognitive dysfunction. Cell death is necessary for maintaining tissue function and morphology, and changes in the external morphology of nerve cells and the function of many internal organelles are typical features of cell death; however, it is also a typical feature of cognitive impairment caused by fluorosis. However, the pathogenesis of cognitive impairment caused by different degrees of fluoride exposure varies. Herein, we provide an overview of cognitive impairment caused by excessive fluoride exposure in different age groups, and the underlying mechanisms for cognitive impairment in various model organisms. The mechanisms underlying these impairments include oxidative stress, synaptic and neurotransmission dysfunction, disruption of mitochondrial and energy metabolism, and calcium channel dysregulation. This study aims to provide potential insights that serve as a reference for subsequent research on the cognitive function caused by excessive fluoride.

Regulating effect of miR-132-3p on the changes of MAPK pathway in rat brains and SH-SY5Y cells exposed to excessive fluoride by targeting expression of MAPK1

BACKGROUND

Although the changes of mitogen-activated protein kinase (MAPK) pathway in the central nervous system (CNS) induced by excessive fluoride has been confirmed by our previous findings, the underlying mechanism(s) of the action remains unclear. Here, we investigate the possibility that microRNAs (miRNAs) are involved in the aspect.

METHODS

As a model of chronic fluorosis, SD rats received different concentrations of fluoride in their drinking water for 3 or 6 months and SH-SY5Y cells were exposed to fluoride. Literature reviews and bioinformatics analyses were used to predict and real-time PCR to measure the expression of 12 miRNAs; an algorithm-based approach was applied to identify multiply potential target-genes and pathways; the dual-luciferase reporter system to detect the association of miR-132-3p with MAPK1; and fluorescence in situ hybridization to detect miR-132-3p localization. The miR-132-3p inhibitor or mimics or MAPK1 silencing RNA were transfected into cultured cells. Expression of protein components of the MAPK pathway was assessed by immunofluorescence or Western blotting.

RESULTS

In the rat hippocampus exposed with high fluoride, ten miRNAs were down-regulated and two up-regulated. Among these, miR-132-3p expression was down-regulated to the greatest extent and MAPK1 level (selected from the 220 genes predicted) was corelated with the alteration of miR-132-3p. Furthermore, miR-132-3p level was declined, whereas the protein levels MAPK pathway components were increased in the rat brains and SH-SY5Y cells exposed to high fluoride. MiR-132-3p up-regulated MAPK1 by binding directly to its 3'-untranslated region. Obviously, miR-132-3p mimics or MAPK1 silencing RNA attenuated the elevated expressions of the proteins components of the MAPK pathway induced by fluorosis in SH-SY5Y cells, whereas an inhibitor of miR-132-3p just played the opposite effect.

CONCLUSION

MiR-132-3p appears to modulate the changes of MAPK signaling pathway in the CNS associated with chronic fluorosis.

Unraveling molecular characteristic of fluoride neurotoxicity on U87 glial-like cells: insights from transcriptomic and proteomic approach

The potential of fluoride (F) as a neurotoxicant in humans is still controversial in the literature. However, recent studies have raised the debate by showing different mechanism of F-induced neurotoxicity, as oxidative stress, energy metabolism and inflammation in the central nervous system (CNS). In the present study, we investigated the mechanistic action of two F concentration (0.095 and 0.22 μg/ml) on gene and protein profile network using a human glial cell in vitro model over 10 days of exposure. A total of 823 genes and 2,084 genes were modulated after exposure to 0.095 and 0.22 μg/ml F, respectively. Among them, 168 were found to be modulated by both concentrations. The number of changes in protein expression induced by F were 20 and 10, respectively. Gene ontology annotations showed that the main terms were related to cellular metabolism, protein modification and cell death regulation pathways, such as the MAP kinase (MAPK) cascade, in a concentration independent manner. Proteomics confirmed the changes in energy metabolism and also provided evidence of F-induced changes in cytoskeleton components of glial cells. Our results not only reveal that F has the potential to modulate gene and protein profiles in human U87 glial-like cells overexposed to F, but also identify a possible role of this ion in cytoskeleton disorganization.

Ameliorative effects of different doses of selenium against fluoride-triggered apoptosis and oxidative stress-mediated renal injury in rats through the activation of Nrf2/HO-1/NQO1 signaling pathway

Excess fluoride (F) exposure can cause oxidative stress in the kidney. As an antioxidant, selenium (Se) can potentially protect the kidney from F-induced injury in rats. Hence, the histopathological, renal biochemical, oxidative stress, and apoptotic-related indices upon exposure to 100 mg/L sodium fluoride (NaF) and various doses of sodium selenite (Na₂SeO₃; 0.5, 1, and 2 mg/L) were assessed. Our results demonstrated that F-mediated renal structural damage and apoptosis elevated the content of serum creatinine (SCr), inhibited the activity of catalase (CAT) in serum, and increased the production of reactive oxygen species (ROS) in kidney and malondialdehyde (MDA) in serum. Interestingly, 1 mg/L dietary supplementation of Se tangibly mitigated these injuries. Furthermore, F could also change the gene and protein expression of the nuclear factor erythroid 2-related factor 2 (Nrf2), heme oxygenase-1 (HO-1), and NAD(P)H quinone oxidoreductase1 (NQO1). Concomitantly, the different concentrations of Se notably alleviated their expression. Taken together, 1-2 mg/L Se ameliorated F-induced renal injury through oxidative stress and apoptosis-related routes. The recorded ameliorative effects might be related to the activation of the Nrf2/HO-1/NQO1 signaling pathway.

Fluoride Induced Neurobehavioral Impairments in Experimental Animals: a Brief Review

Fluoride is one of the major toxicants in the environment and is often found in drinking water at higher concentrations. Living organisms including humans exposed to high fluoride levels are found to develop mild-to-severe detrimental pathological conditions called fluorosis. Fluoride can cross the hematoencephalic barrier and settle in various brain regions. This accumulation affects the structure and function of both the central and peripheral nervous systems. The neural ultrastructure damages are reflected in metabolic and cognitive activities. Hindrances in synaptic plasticity and signal transmission, early neuronal apoptosis, functional alterations of the intercellular signaling pathway components, improper protein synthesis, dyshomeostasis of the transcriptional and neurotrophic factors, oxidative stress, and inflammatory responses are accounted for the fluoride neurotoxicity. Fluoride causes a decline in brain functions that directly influence the overall quality of life in both humans and animals. Animal studies are widely used to explore the etiology of fluoride-induced neurotoxicity. A good number of these studies support a positive correlation between fluoride intake and toxicity phenotypes closely associated with neurotoxicity. However, the experimental dosages highly surpass the normal environmental concentrations and are difficult to compare with human exposures. The treatment procedures are highly dependent on the dosage, duration of exposure, sex, and age of specimens among other factors which make it difficult to arrive at general conclusions. Our review aims to explore fluoride-induced neuronal damage along with associated histopathological, behavioral, and cognitive effects in experimental models. Furthermore, the correlation of various molecular mechanisms upon fluoride intoxication and associated neurobehavioral deficits has been discussed. Since there is no well-established mechanism to prevent fluorosis, phytochemical-based alleviation of its characteristic indications has been proposed as a possible remedial measure.

Role of oxidative stress-mediated cell death and signaling pathways in experimental fluorosis

Free radicals and other oxidants have enticed the interest of researchers in the fields of biology and medicine, owing to their role in several pathophysiological conditions, including fluorosis (Fluoride toxicity). Radical species affect cellular biomolecules such as nucleic acids, proteins, and lipids, resulting in oxidative stress. Reactive oxygen species-mediated oxidative stress is a common denominator in fluoride toxicity. Fluorosis is a global health concern caused by excessive fluoride consumption over time. Fluoride alters the cellular redox homeostasis, and its toxicity leads to the activation of cell death mechanisms like apoptosis, autophagy, and necroptosis. Even though a surfeit of signaling pathways is involved in fluorosis, their toxicity mechanisms are not fully understood. Thus, this review aims to understand the role of reactive species in fluoride toxicity with an outlook on the effects of fluoride in vitro and in vivo models. Also, we emphasized the signal transduction pathways and the mechanism of cell death implicated in fluoride-induced oxidative stress.

Domain-specific effects of prenatal fluoride exposure on child IQ at 4, 5, and 6-12 years in the ELEMENT cohort

OBJECTIVE

Prenatal exposure to fluoride has been associated with adverse neurodevelopmental outcomes. However, the neuropsychological profile of fluoride's developmental neurotoxicity at low levels and the stability of this relationship across childhood has not been characterized. We investigated the longitudinal and domain specific effect of prenatal fluoride exposure on IQ among children ages 4, 5, and 6-12 years in the Early Life Exposures in Mexico to Environmental Toxicants (ELEMENT) cohort.

METHODS

We measured the average of maternal urinary fluoride at each trimester of pregnancy adjusted for creatinine (MUF_CRE). Children were administered the McCarthy Scales of Children's Abilities at ages 4 (N = 386) and 5 (N = 308), and the Wechsler Abbreviated Scale of Intelligence at age 6-12 (N = 278). We used generalized estimating equation (GEE) models to estimate the population averaged effect of MUF_CRE concentration on longitudinal General Cognitive Index (GCI)/Full-Scale IQ (FSIQ), Verbal IQ (VIQ), and Performance IQ (PIQ) scores (N = 348). We tested for possible interactions between MUF_CRE and child sex as well as for MUF_CRE and time point on children's IQ. All models controlled for relevant available covariates.

RESULTS

The mean/median MUF_CRE concentration was 0.90/0.83 mg/L (SD = 0.39; IQR, 0.64-1.11 mg/L). A 0.5 mg/L increase in MUF_CRE predicted an average 2.12-point decrease in GCI/FSIQ (95% CI: -3.49, -0.75) and 2.63-point decrease in PIQ (95% CI: -3.87, -1.40). MUF_CRE was marginally associated with VIQ across time (B = -1.29, 95% CI: -2.60, 0.01). No interactions between MUF_CRE and child sex or MUF_CRE and time were observed.

CONCLUSION

The negative association between prenatal fluoride exposure and longitudinal IQ was driven by decrements in non-verbal intelligence (i.e. PIQ), suggesting that visual-spatial and perceptual reasoning abilities may be more impacted by prenatal fluoride exposure as compared to verbal abilities.

Environmental Toxicity Assessment of Sodium Fluoride and Platinum-Derived Drugs Co-Exposure on Aquatic Organisms

Pharmaceuticals are widely acknowledged to be a threat to aquatic life. Over the last two decades, the steady use of biologically active chemicals for human health has been mirrored by a rise in the leaking of these chemicals into natural environments. The aim of this work was to detect the toxicity of sodium fluoride (NaF) exposure and platinum-derived drugs in an ecological setting on aquatic organism development. From 24 to 96 h post-fertilization, zebrafish embryos were treated to dosages of NaF 10 mg/L⁻¹ + cisplatin (CDDP) 100 μM, one with NaF 10 mg/L⁻¹ + carboplatin (CARP) 25 μM, one with NaF 10 mg/L⁻¹ + CDDP 100 μM + CARP 25 μM. Fluoride exposure in combination with Cisplatin and Carboplatin (non-toxic concentration) had an effect on survival and hatching rate according to this study. Additionally, it significantly disturbed the antioxidant defense system and increased ROS in zebrafish larvae. NaF 10 mg/L⁻¹ associated with CDDP 100 μM and CARP 25 μM, increased the production of apoptosis-related proteins (caspase 3, bax, and bcl-2) and the downregulation of acetylcholinesterase (AChE) activity, while no effect was seen for the single exposure.

Prolonged fluoride exposure alters neurotransmission and oxidative stress in the zebrafish brain

Fluoride is an essential chemical found in dental preparations, pesticides and drinking water. Excessive fluoride exposure is related to toxicological and neurological disruption. Zebrafish are used in translational approaches to understand neurotoxicity in both biomedical and environmental areas. However, there is no complete knowledge about the cumulative effects of fluoride on neurotransmission systems. Therefore, the aim of this study was to evaluate whether prolonged exposure to sodium fluoride (NaF) alters cholinergic and glutamatergic systems and oxidative stress homeostasis in the zebrafish brain. Adult zebrafish were used, divided into four experimental groups, one control group and three groups exposed to NaF at 30, 50 and 100 mg.L^-1 for a period of 30 days. After NaF at 30 mg.L^-1 exposure, there were significant decreases in acetylcholinesterase (29.8%) and glutamate uptake (39.3%). Furthermore, thiobarbituric acid-reactive species were decreased at NaF 50 mg.L^-1 (32.7%), while the group treated with NaF at 30 mg.L^-1 showed an increase in dichlorodihydrofluorescein oxidation (41.4%). NaF at 30 mg.L^-1 decreased both superoxide dismutase (55.3%) and catalase activities (26.1%). The inhibitory effect observed on cholinergic and glutamatergic signalling mechanisms could contribute to the neurodegenerative events promoted by NaF in the zebrafish brain.

Fluoride exposure decreased neurite formation on cerebral cortical neurons of SD rats in vitro

Fluoride, a geochemical element, can damage the brain and result in dysfunction of the central nervous system. In recent years, fluoride-induced neurotoxicity has become one of research focuses of environmental toxicology. Our previous study showed that fluoride could induce the structural damages of the cerebral cortex and reduce the learning and memory abilities of mice offspring. However, the underlying mechanisms of these effects remain unclear. In this study, primary neurons were isolated from the cerebral cortices of postnatal 1-day SD rats. The primary cultured cerebral cortical neurons were adherent and the cellular network was obvious. Neurons were identified by Nissl's staining and were used for experiments. Different concentrations of sodium fluoride (0.5, 1.0, 1.5, 2.0 and 2.5 mM) were chosen to explore its toxic effects on neuron of SD rats in vitro. Results showed that neuronal morphology was obviously damaged in 2.0 and 2.5 mM, but was not adversely affected in 0.5 and 1 mM. Further studies revealed that the neurites of neuron were shrunken and even became fractured with the increase in NaF dose, which have been detected by scanning electron microscopy (SEM). Meanwhile, TEM showed marginated chromatin, widened nuclear gaps, damaged nuclei and swollen or even absent mitochondria in 1.5, 2 and 2.5 mM group. The cytoskeletal staining was consistent with the above results. The number of neurites of cerebral cortical neuron significantly decreased after fluoride exposure by immunofluorescent assay. In summary, high fluoride (1.5, 2 and 2.5 mM) concentrations exerted a significant toxic effect on the cellular morphologies and neural formation of primary cultured cortical neurons. These findings provide new insights into the roles of NaF in neuronal damage and can contribute to an improved understanding of fluoride-induced neurotoxicity.

Human cultured IMR-32 neuronal-like and U87 glial-like cells have different patterns of toxicity under fluoride exposure

BACKGROUND

Fluoride (F) is a naturally exists in nature but several studies have indicated it as an environmental toxicant to all leaving beings. Human F exposure has increased over the years since this ion has been used by industry on foods, beverages, toothpastes and on water supply. Although F is safe at optimal concentrations in water supply, human exposure to high levels could trigger neurofunctional deficits.

MATERIALS AND METHODS

In this study, human glial-like (U87) and neuronal-like (IMR-32) cells lineages were used to access F toxicity and CNS cell sensibility on both cell facing the same protocol. Cells were exposed to F over 3, 5 and 10 days on two different F concentrations. Fluoride exposed cells were evaluated by standard toxicity assays to cell viability, apoptosis, necrosis and general cell metabolism. Oxidative stress parameters were evaluated by ATP and ROS levels, lipid peroxidation, GSH/GSSG ratio and comet assay.

RESULTS

No changes were observed in IMR-32 at any given time while after 10 days of exposure to 0.22μg/mL, U87 glial-like cells showed signs of toxicity such as decreased cell viability by necrosis while general cell metabolism was increased. Oxidative stress parameters were next evaluated only on U87 glial-like cells after 10 days of exposure. F induced a decrease on ATP levels while no changes were observed on reactive oxygen species and lipid peroxidation. GSH/GSSG ratio was decreased followed by DNA damage both on 0.22μg/mL F.

CONCLUSIONS

Our results suggest an important differential behavior of the distinct types of cells exposed to the different fluoride concentrations, pointing that the U87 glial-like cells as more susceptible to damage triggered by this ion.

Probiotic alleviate fluoride-induced memory impairment by reconstructing gut microbiota in mice

Fluoride which is widespread in our environment and food due to its geological origin and industrial pollution has been identified as a developmental neurotoxicant. Gut-brain axis provides new insight into brain-derived injury. We previously found the psychoactive effects of a probiotic strain, Lactobacillus johnsonii BS15 against fluoride-induced memory dysfunction in mice by modulating the gut-brain axis. In this study, we aimed to detect the link between the reconstruction of gut microbiota and gut-brain axis through which probiotic alleviate fluoride-induced memory impairment. We also added an hour of water avoidance stress (WAS) before behavioral tests and sampling, aiming to demonstrate the preventive effects of the probiotic on fluoride-induced memory impairment after psychological stress. Mice were given fluoridated drinking water (sodium fluoride 100 ppm, corresponding to 37.8 ± 2.4 ppm F¯) for 70 days and administered with PBS or a probiotic strain, Lactobacillus johnsonii BS15 for 28 days prior to and throughout a 70 day exposure to sodium fluoride. Results showed that fluoride increases the hyperactivity of hypothalamic-pituitary-adrenal (HPA) axis and reduces the exploration ratio in novel object recognition (NOR) test and the spontaneous exploration during the T-maze test in mice following WAS, which were significantly improved by the probiotic. 16S rRNA sequencing showed a significant separation in ileal microbiota between the fluoride-treated mice and control mice. Lactobacillus was the main targeting bacteria and significantly reduced in fluoride-treated mice. BS15 reconstructed the fluoride-post microbiota and increased the relative abundance of Lactobacillus. D-lactate content and diamine oxidase (DAO) activity, two biomarkers of gut permeability were reduced in the serum of probiotic-inoculated mice. ZO-1, an intestinal tight junction protein was reduced by fluoride in mRNA, and its protein levels were increased by the probiotic treatment. Moreover, the hippocampus which is essential to learning and memory, down-regulated mRNA level of both the myelin-associated glycoprotein (MAG), and protein levels of brain-derived neurotrophic factor (BDNF), including the improvement of cAMP response element-binding protein (CREB) by BS15 in fluoride-exposed mice after WAS. Via spearman correlation analysis, Lactobacillus displayed significantly positive associations with the behavioral tests, levels of nerve development related factors, and intestinal tight junction proteins ZO-1, and negative association with TNF-α of the hippocampus, highlighting regulatory effects of gut bacteria on memory potential and gut barrier. These results suggested the psychoactive effects of BS15 on fluoride-induced memory dysfunction after psychological stress. In addition, there may be some correlations between fluoride-induced memory dysfunction and reconstruction of gut microbiota. AVAILABILITY OF DATA AND MATERIALS: 16S rRNA sequencing reads have uploaded to NCBI. The accession code of 16S rRNA sequencing reads in the National Center for Biotechnology Information (NCBI) BioProject database: PRJNA660154.

A comprehensive review on the neuropathophysiology of selenium

As an essential micronutrient, selenium (Se) exerts its biological function as a catalytic entity in a variety of enzymes. From a toxicological perspective, however, Se can become extremely toxic at concentrations slightly above its nutritional levels. Over the last few decades, there has been a growing level of concern worldwide regarding the adverse effects of both inorganic and organic Se compounds on a broad spectrum of neurological functions. A wealth of evidence has shown that exposure to excess Se may compromise the normal functioning of various key proteins, neurotransmitter systems (the glutamatergic, dopaminergic, serotonergic, and cholinergic systems), and signaling molecules involved in the control and regulation of cognitive, behavioral, and neuroendocrine functions. Elevated Se exposure has also been suspected to be a risk factor for the development of several neurodegenerative and neuropsychiatric diseases. Nonetheless, despite the various deleterious effects of excess Se on the central nervous system (CNS), Se neurotoxicity and negative behavioral outcomes are still disregarded at the expense of its beneficial health effects. This review focuses on the current state of knowledge regarding the neurobehavioral effects of Se and discusses its potential mode of action on different aspects of the central and peripheral nervous systems. This review also provides a brief history of Se discovery and uses, its physicochemical properties, biological roles in the CNS, environmental occurrence, and toxicity. We also review potential links between exposure to different forms of Se compounds and aberrant neurobehavioral functions in humans and animals, and identify key knowledge gaps and hypotheses for future research.

Genotoxicity of fluoride subacute exposure in rats and selenium intervention

The aims of this study were to: (i) examine the toxic effects of sodium fluoride (NaF) in blood, liver, spleen, and brain cells of Wistar rats after the subacute exposure; (ii) explore the potential protective properties of selenium (Se) against fluoride toxicity after the simultaneous administration. Twenty male Wistar rats, eight weeks old, weighing approximately 140-190 g, were divided into four experimental groups (n = 5) as follows: I control-tap water; II NaF 150 ppm; III NaF 150 ppm and Se 1.5 mg/L; IV Se 1.5 mg/L, and had available water with solutions ad libitum for 28 days. DNA damage detected by comet assay was confirmed in the liver, spleen, and brain cells, but not in blood. Selenium supplementation together with NaF decreased DNA damage in liver and spleen cells. According to the histological findings, no changes were observed in spleen and brain tissues after NaF administration. Unlike the observed Se protective effect on the DNA level, no significant reduction of liver tissue injury was observed after the NaF and Se treatment, resulting in mild inflammation. Data of this study suggest that DNA damage after NaF subacute exposure at moderately high concentration was reduced in liver and spleen cells due to Se supplementation, but a similar change was not seen in the brain.

Selenium Exerts Protective Effects Against Fluoride-Induced Apoptosis and Oxidative Stress and Altered the Expression of Bcl-2/Caspase Family

Fluoride is widely distributed in nature, and at high concentrations, it targets the kidney and especially proximal tubule epithelial cells. Selenium is a typical trace element beneficial to humans, and the role of selenium in the prevention and treatment of fluoride-induced organ damage is an important research topic. The purpose of this study was to investigate the possible protective effects of selenium against fluoride-induced oxidative stress and apoptosis in rat renal tubular epithelial cells. We showed that the activity of antioxidant enzymes (superoxide dismutase and glutathione peroxidase) and total antioxidant capacity were significantly reduced in NaF-treated normal rat kidney cells (NRK-52E), whereas the levels of nitrogen monoxide (NO) and malondialdehyde (MDA) were significantly increased. Moreover, the number of apoptotic cells, mRNA expression of Bax, Bad, caspase-3, caspase-8, and caspase-9, and protein expression of Bax were elevated, while mitochondrial membrane potential and the protein expression of Bcl-2 were reduced. Compared with the NaF group, pretreatment with selenium enhanced the activity of antioxidant enzymes, mitochondrial membrane potential, and protein expression of Bcl-2, while the levels of NO and MDA, number of apoptotic cells, mRNA expression of Bax, Bad, caspase-3, caspase-8, and caspase-9, and protein expression of Bax were decreased. In conclusion, selenium exerted remarkable protective effect against NaF-induced oxidative stress and apoptosis and altered the expression of Bcl-2/caspase family.

Fluoride exposure during pregnancy and lactation triggers oxidative stress and molecular changes in hippocampus of offspring rats

Long-term exposure to high concentrations of fluoride (F) can damage mineralized and soft tissues such as bones, liver, kidney, intestine, and nervous system of adult rats. The high permeability of the blood-brain barrier and placenta to F during pregnancy and lactation may be critical to neurological development. Therefore, this study aimed to investigate the effects of F exposure during pregnancy and lactation on molecular processes and oxidative biochemistry of offspring rats' hippocampus. Pregnant Wistar rats were randomly assigned into 3 groups in accordance with the drinking water received: G1 - deionized water (control); G2 - 10 mg/L of F and G3 - 50 mg/L of F. The exposure to fluoridated water began on the first day of pregnancy and lasted until the 21^st day of breastfeeding (when the offspring rats were weaned). Blood plasma samples of the offspring rats were collected to determine F levels. Hippocampi samples were collected for oxidative biochemistry analyses through antioxidant capacity against peroxyl (ACAP), lipid peroxidation (LPO), and nitrite (NO₂^-) levels. Also, brain-derived neurotrophic factor (BDNF) gene expression (RT-qPCR) and proteomic profile analyses were performed. The results showed that exposure to both F concentrations during pregnancy and lactation increased the F bioavailability, triggered redox imbalance featured by a decrease of ACAP, increase of LPO and NO₂^- levels, BDNF overexpression and changes in the hippocampus proteome. These findings raise novel questions regarding potential repercussions on the hippocampus structure and functioning in the different cognitive domains.

Lactobacillus johnsonii BS15 improves intestinal environment against fluoride-induced memory impairment in mice-a study based on the gut-brain axis hypothesis

Background

Excessive fluoride can lead to chronic neurodegeneration characterized by neuron and myelin loss and memory dysfunction. The gut–brain axis hypothesis suggests that gut microbiota plays a crucial role in regulating brain function. Thus, using probiotics to adjust the gut microenvironment may be a potential therapy for mental diseases.

Methods

Mice in the prob group were administrated with Lactobacillus johnsonii BS15 for 28 days prior to and throughout a 70-day exposure to sodium fluoride. The drinking water of all groups (F and prob groups) except the control group were replaced by high-fluoride water (100 mg NaF/L) on day 28. Animals in each group were divided into two subsets: one underwent behavioral test, and the other was sacrificed for sampling. The mRNA expression level and protein content related to inflammatory reaction in the ileum and hippocampus were respectively detected by reverse transcription quantitative polymerase chain reaction (RT-qPCR) and enzyme-linked immunosorbent assay (ELISA). The mRNA expression levels of proteins related to myelin structure, apoptosis, and memory in the hippocampus and tight junction proteins in the ileum were determined by RT-qPCR and/or immunohistochemistry. Gut permeability markers (D-lactate and diamine oxidase (DAO)) in the serum were also examined by ELISA.

Results

The results showed that fluoride exposure induced a lower spontaneous exploration (P < 0.05) in T-maze test, which indicated an impairment of memory. Spontaneous exploration of BS15-treated mice was significantly higher (P < 0.05) than that in F group. Fluoride reduced (P < 0.05) levels of myelin structural protein (proteolipid protein) and neurogenesis-associated proteins (brain-derived neurotrophic factor and cAMP/Ca²⁺ responsive element-binding protein), induced disordered inflammatory cytokines (TNF-α, IFN-γ, and IL-6; P < 0.05), increased pro-apoptotic genes (caspase-3; P < 0.05), and decreased anti-apoptotic genes (Bcl-2; P < 0.05) in the hippocampus, of which the influences were reversed by BS15. BS15 treatment exerted significant preventive effects on reversing the gut inflammation induced by excessive fluoride intake by reducing (P < 0.05) the levels of pro-inflammatory cytokines (tumor necrosis factor-alpha (TNF-α) and interferon-gamma (IFN-γ)) and remarkably increasing (P < 0.05) the level of anti-inflammatory cytokines (IL-10). Moreover, the serum DAO activity and D-lactate concentration significantly increased by fluoride were also reduced (P < 0.05) by BS15. This result indicated the profitable effect of BS15 on gut permeability.

Conclusion

L. johnsonii BS15 intake could benefit the neuroinflammation and demyelination in the hippocampus by improving the gut environment and ameliorating fluorine-induced memory dysfunction.

Inorganic fluoride and functions of brain

Although actively disputed and questioned, it has been proposed that chronic exposure to inorganic fluoride (F^-) is toxic for brain. The major question for this review was whether an excessive F^- intake is causally related to adverse neurological and cognitive health conditions in human beings and animals. The paper systematically and critically summarizes the findings of the studies showing positive associations between F^- intoxication and various intellectual defects, as well as of those which attempted to clarify the nature of F^- neurotoxicity. Many works provide support for a link between pre- and postnatal F^- exposure and structural and functional changes in the central nervous system responsible for neurological and cognitive disorders. The mechanisms suggested to underlie F^- neurotoxicity include the disturbances in synaptic transmission and synaptic plasticity, premature death of neurons, altered activities of components of intracellular signaling cascades, impaired protein synthesis, deficit of neurotrophic and transcriptional factors, oxidative stress, metabolic changes, inflammatory processes. However, the majority of works have been performed on laboratory rodents using such F^- doses which are never exist in the nature even in the regions of endemic fluorosis. Thus, this kind of treatment is hardly comparable with human exposure even taking into account the higher rate of F^- clearance in animals. Of special importance are the data collected on humans chronically consuming excessive F^- doses in the regions of endemic fluorosis or contacting with toxic F^- compounds at industrial sites, but those works are scarce and often criticized due to low quality. New, expertly performed studies with repeated exposure assessment in independent populations are needed to prove an ability of F^- to impair neurological and intellectual development of human beings and to understand the molecular mechanisms implicated in F^--induced neurotoxicity.

Clofibrate, a PPAR-α agonist, abrogates sodium fluoride-induced neuroinflammation, oxidative stress, and motor incoordination via modulation of GFAP/Iba-1/anti-calbindin signaling pathways

Fluoride is an environmental contaminant that is ubiquitously present in air, water, and soil. It is commonly added in minute quantity to drinking water, toothpaste, and mouth rinses to prevent tooth decay. Epidemiological findings have demonstrated that exposure to fluoride induced neurodevelopmental toxicity, developmental neurotoxicity, and motor disorders. The neuroprotective effect of clofibrate, a peroxisome proliferator-activated receptor alpha agonist, was investigated in the present study. Forty male Wistar rats were used for this study and randomly grouped into 10 rats per group as control, sodium fluoride (NaF) alone (300 ppm), NaF plus clofibrate (250 mg/kg), and NaF plus lisinopril (10 mg/kg), respectively, for 7 days. NaF was administered in drinking water while clofibrate and lisinopril were administered by oral gavage. Markers of neuronal inflammation and oxidative stress, acetylcholinesterase activity, and neurobehavioral (hanging wire and open field) tests were performed. Immunohistochemistry was performed on brain tissues, and they were probed with glial fibrillary acidic protein, ionized calcium-binding adaptor molecule 1, and cerebellar Ca²⁺ -binding protein calbindin-D28k. The results showed that NaF significantly increased of oxidative stress and neuroinflammation and inhibited AChE activity. Immunostaining showed reactive astrocytes, microgliosis, loss of dendritic spines, and arborization in Purkinje cells in rats administered only NaF. Neurobehavioral results showed that cotreatment of NaF with clofibrate improved muscular strength and locomotion, reduced anxiety, and significantly reduced astrocytic count. Overall, cotreatment of NaF with either clofibrate or lisinopril showed neuroprotective effects by mitigating neuronal inflammation and oxidative and motor incoordination. Hence, clofibrate could be seen as a novel drug candidate against neurodegeneration and motor disorders.

Fluoride exposure decreased learning ability and the expressions of the insulin receptor in male mouse hippocampus and olfactory bulb

Fluoride is one of the common environmental pollutants. Internal exposure to fluoride is related to the lowered cognitive function and intelligence, particularly for children. Determination of protein content in brain tissue is a means to reflect the functional development of the central nervous system. Insulin and insulin receptor (IR) signaling systems are associated with cognitive ability. The present research focused on the assessment of the expressions of IR protein and mRNA in hippocampus and olfactory bulb (OB), as well as learning and memory ability of male Kunming mice. Mice were exposed to 50, 100, and 150 mg/L NaF for 90 continuous days. The results showed that learning and memory abilities as well as protein content of male mice brain was significantly decreased by fluoride. Fluoride could inhibit the protein and mRNA expressions of the IR in the hippocampus and OB of mice. IRs mainly distributed in the olfactory nerve layer of the outermost layer of the OB, and most distributed in the hippocampal cornu ammon 3 (CA3) region, followed by the dentate gyrus (DG) and cornu ammon 1 (CA1) regions. These findings suggested that inhibition of the IR protein and mRNA expressions in the hippocampus and OB by fluoride might in part affect learning and memory ability in male mice.

Chronic fluoride exposure induces neuronal apoptosis and impairs neurogenesis and synaptic plasticity: Role of GSK-3β/β-catenin pathway

Fluoride is becoming an ineluctable environmental pollutant and its longterm exposure would cause fluorosis and irreversible brain damage, but the molecular mechanisms remain far from fully understood. In the present study, we firstly evaluated the glycogen synthase kinase 3β (GSK-3β)/β-catenin pathway in the hippocampus of rats exposed to fluoride, given the well-established role of GSK-3β/β-catenin pathway in neuronal death and survival. Our data showed that sustained exposure to 50 mg/L and 100 mg/L NaF in drinking water dose-dependently induced neuronal loss and apoptosis in rat hippocampus. Neurogenesis was also weakened by fluoride administration in the hippocampal dentate gyrus region. Additionally, the synaptic markers, synaptophysin (SYP) and post-synaptic density 95 (PSD95) protein levels, were decreased by 100 mg/L NaF treatment, whereas 50 mg/L NaF only reduced SYP expression, indicating a compromised synaptic function. We further demonstrated that NaF, especially the higher dose, induced GSK-3β activity, with decreased inactive phosphorylated GSK-3β levels and increased GSK-3β, the active form of the kinase. Correspondingly, downstream β-catenin signaling was undermined by NaF treatment as evidenced by the fact that both two doses of NaF decreased nucleus β-catenin status and the higher dose of NaF also reduced cytoplasmic β-catenin protein expression. Taken together, the present study firstly showed the aberrant changes of GSK-3β/β-catenin signaling in the fluoride-exposed brain, highlighting the involvement of GSK-3β/β-catenin signaling in the fluoride-induced neurotoxicity.

Fluoride activates microglia, secretes inflammatory factors and influences synaptic neuron plasticity in the hippocampus of rats

Epidemiological studies have reported that highly fluoridated drinking water may significantly decrease the Intelligence Quotient (IQ) of exposed children. It is thought that synaptic plasticity is the basis of learning and memory skills in developing children. However, the effect on synaptic plasticity by activated microglia induced via fluoride treatment is less clear. Our previous research showed that fluoride ions activated microglia which then released pro-inflammatory cytokines. In this study, hippocampal-dependent memory status was evaluated in rat models sub-chronically exposed to fluoride in their drinking water. Microglial activation in the hippocampus was examined using immunofluorescence staining and the expression of synaptophysin (SYP) and postsynaptic density protein 95 (PSD-95), Long-term potentiation (LTP) and the expression of Amino-3-hydroxy-5-methy-4-isoxazole propionate (AMPA) receptor subunit GluR2 as well as N-methyl-d-aspartate (NMDA) receptor subunit NMDAR2β of exposed rats. We found that fluoride exposure activated microglia and increased the expression of DAP12 and TREM2, as well as promoted pro-inflammatory cytokines secretion via ERK/MAPK and P38/MAPK signal pathways. Furthermore fluoride depressed LTP and decreased PSD-95 protein levels as well as expression of ionotropic glutamate receptors GluR2 and NMDAR2β. We concluded that the role of fluoride on synaptic plasticity may be associated with neuroinflammation induced by microglia.

ERK1/2-mediated disruption of BDNF-TrkB signaling causes synaptic impairment contributing to fluoride-induced developmental neurotoxicity

Excessive exposure to fluoride has adverse effects on neurodevelopment, but the mechanisms remain unclear. This study aimed to investigate the effects of fluoride exposure on synaptogenesis, and focused on the role of brain-derived neurotrophic factor (BDNF)-tyrosine kinase B (TrkB) signaling in these effects. Using Sprague-Dawley rats developmentally exposed to sodium fluoride (NaF) from pregnancy until 6 months of delivery as in vivo model, we showed that fluoride impaired the cognitive abilities of offspring rats, decreased the density of dendritic spines and the expression of synapse proteins synaptophysin (SYN) and postsynaptic density protein-95 (PSD-95) in hippocampus, suggesting fluoride-induced cognitive deficit associates with synaptic impairment. Consistently, NaF treatment reduced dendritic outgrowth and expression of SYN and PSD-95 in human neuroblastoma SH-SY5Y cells. Further studies demonstrated that the BDNF-TrkB axis was disrupted in vivo and in vitro, as manifested by BDNF accumulation and TrkB reduction. Importantly, fluoride treatment increased phospho-extracellular signal-regulated kinases 1 and 2 (p-ERK1/2) expression, while inhibition of p-ERK1/2 significantly attenuated the effects of NaF, indicating a regulating role of p-ERK1/2 in BDNF-TrkB signaling disruption. Collectively, these data suggest that the developmental neurotoxicity of fluoride is associated with the impairment of synaptogenesis, which is caused by ERK1/2-mediated BDNF-TrkB signaling disruption.

Fluoride-induced alterations of synapse-related proteins in the cerebral cortex of ICR offspring mouse brain

Fluoride (F) exposure causes cognitive dysfunction in humans and animals. However, the precise molecular mechanisms by which fluoride exerts its neurotoxic effects are poorly understood. In this study, an animal model of fluoride exposure was created by providing ICR mice were treated with vehicle F at a dose of 0 (control group), 50 (low-fluoride group) or 100 mg/L (high-fluoride group) in water for one month. After the mice mated, parents and offspring were treated and maintained under these conditions. The cognitive abilities of the mice were examined using a Morris water maze test. Results indicated that fluoride exposure significantly prolonged the escape latency period and decreased the number of crossings in a particular zone. Histopathologic analysis revealed the shrinkage and fragmentation of glial cells in the fluoride-treated groups. Pyramidal cells in the cerebral cortices of fluoride-treated groups were fewer than those of the control group. The expression of microtubule-associated protein 2 (MAP2) and synaptic proteins of the cerebral cortex in mouse offspring was assayed using RT-PCR and Western blot. Fluoride exposure possibly induced a significantly decreased expression of MAP2, synaptophysin (SYP) and developmentally regulated brain protein (Dbn) at protein and mRNA levels. Glutamate receptor (N-methyl-d-aspartate receptor, NMDAR) was also expressed, and this finding was consistent with the reduced MAP2, SYP and Dbn expression. Therefore, fluoride-mediated reduction in cognitive dysfunction is likely caused by the disruption of the expression of these synapse-associated proteins, resulting in attenuated neuronal functioning.

The Effect of Chronic Fluorosis on Calcium Ions and CaMKIIα, and c-fos Expression in the Rat Hippocampus

This study investigated neurotoxicity of chronic fluorosis in the rat hippocampus. Newly weaning, male, Sprague-Dawley (SD) rats were administered 15, 30, and 60 mg/L sodium fluoride (NaF) solution (fluorine ion concentration 8.25, 16.50, and 33.00 mg/L, respectively), and tap water, for 18 months. The neurotoxicological mechanism was examined with a focus on intracellular calcium overload. Results showed that as the fluoride concentration increased, calcium ion concentration [Ca²⁺], the expression of calcium/calmodulin-dependent protein kinase II α (CaMKIIα), and the expression of catus proto-oncogene protein c-fos (c-fos) all tend to increase. Compared to the control group, Ca²⁺, CaMKIIα, and c-fos significantly increased (P < 0.05) in the moderate-fluoride and the high-fluoride groups. These results indicate that Ca²⁺/CaMKIIα/c-fos channel signal may be the molecular mechanism of central nervous system damage caused by chronic fluoride intoxication. Moreover, elevated Ca²⁺ concentration in the hippocampus may be the initiating factor of neuronal apoptosis induced by fluoride.

Effects of perinatal fluoride exposure on the expressions of miR-124 and miR-132 in hippocampus of mouse pups

To investigate the effects of perinatal fluoride exposure on learning and memory ability of mouse offspring, ICR female mice were received different doses of sodium fluoride (0, 25, 50, 100 mg/L NaF) from pregnant day 7 to lactational day 21. Pups were exposed to fluoride through the cord blood and breast milk. Open field test showed that compared to the control group, perinatal fluoride exposure significantly decreased the number of entries into the center zone in 100 mg/L NaF group. In the eight-arm maze test, the number of working memory errors, reference memory errors, and the total arm entries were significantly increased in fluoride treatment groups, compared to the control group. Additionally, 100 mg/L NaF significantly elevated the expression levels of miR-124, miR-132, and DiGeorge syndrome chromosomal region 8 (DGCR8) in hippocampus of mouse pups at postnatal day (PND) 21. Contrarily, methyl CpG binding protein 2 (MeCP2) were dramatically reduced in 50 and 100 mg/L NaF groups, while cAMP-response element binding protein (CREB) mRNA level was significantly decreased in all fluoride groups. These findings suggested that the impairment of learning and memory in mouse offspring induced by perinatal fluoride exposure may partly result from the enhanced miR-124 and miR-132 and the alterations of their target genes.

Effects of fluoride on synapse morphology and myelin damage in mouse hippocampus

To investigate the fluoride-induced neurotoxicity on mice hippocampus, healthy adult mice were exposed to 25, 50, and 100 mg NaF/L for 60 days. The results showed that medium and high fluoride administration induced ultrastructural alterations in the structure of neuron synapse, including indistinct and short synaptic cleft, and thickened postsynaptic density (PSD). The significant reduced mRNA expressions of proteolipid protein (PLP) in medium and high fluoride groups suggested that myelin damage occurred in hippocampus. The myelin damage in turn was determined by the increased myelin-associated glycoprotein (MAG) level, which is naturally released by injured myelin, in high fluoride group, compared to the medium fluoride group. In addition, high fluoride exposure also reduced the mRNA and protein levels of cAMP response element-binding protein (CREB), brain-derived neurotrophic factor (BDNF), and neural cell adhesion molecule (NCAM). These findings suggested that the alteration in synaptic structure and myelin damage may partly be due to adverse effects of fluoride on the neurotrophy and neuron adhesion in mice hippocampus.

Effects of Calcium on Drinking Fluorosis-induced Hippocampal Synaptic Plasticity Impairment in the Offspring of Rats

OBJECTIVE

This study investigated the effects of calcium on fluorosis-induced impairment in learning and memory of offspring rats. Methods Seventy-five newly weaned female Sprague-Dawley (SD) rats were randomly divided into five groups as follows: Control group (Control) drank tap water, and ate the normal diet (calcium content of 0.79%); fluoride group (F) drank 100 mg/L NaF solution, and ate the normal diet; low calcium group (LCa) drank tap water, and ate the low calcium diet (calcium content of 0.063%); low calcium fluoride group (F+LCa) drank 100 mg/L NaF solution, and ate the low calcium diet; high calcium fluoride group (F+HCa) drank 100 mg/L NaF solution, and ate the high calcium diet (calcium content of 7%). After exposing rats to fluoride for three months, male and female rats were mated and 14 and 28 days old offspring were obtained as experimental subjects. Examinations determined the submicroscopic parameters of the synaptic interface and expression levels of specific proteins: doublecortin (DCX) and synaptophysin (p38).

RESULTS

(1) High fluorosis significantly reduced synapse density, length of synaptic active zone, thickness of postsynaptic density, and led to abnormal changes in the structural parameter of synaptic gap width, which was significantly reduced or increased. High dietary calcium significantly reversed the abnormal changes in structural parameters, and low calcium aggravated these variations. (2) Dietary calcium resulted in nonsignificant effect on expression levels of DCX and p38.

CONCLUSION

The results suggested that dietary calcium significantly affected hippocampal synaptic plasticity of offspring of mothers exposed to water fluorosis, but its molecular mechanism may not be related to the expression of DCX and p38 in the brain. The findings also demonstrate the important effects of maternal exposure to water fluorosis on offspring brain functions before water improvement.

The effects of fluoride on neuronal function occurs via cytoskeleton damage and decreased signal transmission

It has been reported that fluoride exposure may cause serious public health problems, particularly neurotoxicity. However, the underlying mechanisms remain unclear. This study used Neuro-2A cells to investigate the effects of fluoride on the cytoskeleton. The Neuro-2A cells were exposed to 0, 1, 2, 4 and 6 mM sodium fluoride (NaF) for 24 h. Cell viability and lactate dehydrogenase (LDH) release were examined. It was observed that exposure to NaF reduced cell viability, disrupted cellular membrane integrity, and high levels of LDH were released. The observed changes occurred in a dose response manner. Morphologic observations showed that cell became rounded and were loosely adherent following exposure to NaF. Axon spines and normal features disappeared with high dose NaF treatment. The expression of MAP2 and synaptophysin decreased, particularly at 4 mM and 6 mM (P < 0.05) for MAP2. These results corroborate the morphologic observations. The content of glutamate and NMDAR (glutamate receptor) protein were assessed to help understand the relationship between synapses and neurotransmitter release using ELISA and Western-blot. Compared with the control, glutamate and NMDAR expression declined significantly at 4 mM and 6 mM (P < 0.05) group. Finally, the ultrastructural changes observed with increasing doses of NaF were: disappearance of synapses, mitochondrial agglutination, vacuole formation, and cellular edema. Taken together, NaF exposure disrupted cellular integrity and suppressed the release of neurotransmitters, thus effecting neuronal function. These findings provide deeper insights into roles of NaF in neuron damage, which could contribute to a better understanding of fluoride-induced neurotoxicity.

The History, Status, Gaps, and Future Directions of Neurotoxicology in China

BACKGROUND

Rapid economic development in China has produced serious ecological, environmental, and health problems. Neurotoxicity has been recognized as a major public health problem. The Chinese government, research institutes, and scientists conducted extensive studies concerning the source, characteristics, and mechanisms of neurotoxicants.

OBJECTIVES

This paper presents, for the first time, a comprehensive history and review of major sources of neurotoxicants, national bodies/legislation engaged, and major neurotoxicology research in China.

METHODS

Peer-reviewed research and pollution studies by Chinese scientists from 1991 to 2015 were examined. PubMed, Web of Science and Chinese National Knowledge Infrastructure (CNKI) were the major search tools.

RESULTS

The central problem is an increased exposure to neurotoxicants from air and water, food contamination, e-waste recycling, and manufacturing of household products. China formulated an institutional framework and standards system for management of major neurotoxicants. Basic and applied research was initiated, and international cooperation was achieved. The annual number of peer-reviewed neurotoxicology papers from Chinese authors increased almost 30-fold since 2001.

CONCLUSIONS

Despite extensive efforts, neurotoxicity remains a significant public health problem. This provides great challenges and opportunities. We identified 10 significant areas that require major educational, environmental, governmental, and research efforts, as well as attention to public awareness. For example, there is a need to increase efforts to utilize new in vivo and in vitro models, determine the potential neurotoxicity and mechanisms involved in newly emerging pollutants, and examine the effects and mechanisms of mixtures. In the future, we anticipate working with scientists worldwide to accomplish these goals and eliminate, prevent and treat neurotoxicity.

CITATION

Cai T, Luo W, Ruan D, Wu YJ, Fox DA, Chen J. 2016. The history, status, gaps, and future directions of neurotoxicology in China. Environ Health Perspect 124:722-732; http://dx.doi.org/10.1289/ehp.1409566.

Molecular mechanism of brain impairment caused by drinking-acquired fluorosis and selenium intervention

This study investigated the molecular mechanism of brain impairment induced by drinking fluoridated water and selenium intervention. Results showed that the learning and memory of rats in NaF group significantly decreased. Moreover, the number of apoptotic cells, the expression levels of Cytc mRNA and protein, and the expression levels of Caspase-9 and Caspase-3 mRNA significantly increased; by contrast, Caspase-9 and Caspase-3 protein levels significantly decreased. Compared with the NaF group, the mRNA levels of Cytc and Caspase-9, as well as the protein levels of Cytc in NaF+Se group, significantly decreased. Conversely, the protein levels of Caspase-3 and Caspase-9, as well as the mRNA levels of Caspase-3, significantly increased. Thus, the mitochondrial CytC-Caspase-9-Caspase-3 apoptosis pathway in the hippocampus was one of the mechanisms leading to fluorosis-induced brain damage. Furthermore, the Cytc signaling molecules were possibly the key target molecules in fluorosis-induced apoptosis, and selenium could alleviate fluorosis-induced brain injury.

Selenium deficiency induced damages and altered expressions of metalloproteinases and their inhibitors (MMP1/3, TIMP1/3) in the kidneys of growing rats

Selenium is an essential trace element for the maintenance of structures and functions of kidney. To evaluate the effects of low selenium on the kidneys of growing rats, newborn rats were fed with selenium deficient and normal diets respectively for 109 days. As a result, rats fed with low selenium diets resulted in a decline in the body weight and the concentration of selenium in the kidney, especially the male rats from the low selenium groups. Moreover, the ultrastructure of glomerulus and tubules were damaged in low selenium group: the glomeruli were observed with hyperplasia of mesangial cells, fusion of podocyte foot processes and thickening of basement membrane; and the tubules were observed with vacuolar degenerated epithelial cells, increased edema fluid or protein solution between cells, microvilli edema, increased cell gaps and decreased cell links. Furthermore, the pathological changes in selenium deficient group included the increase of fibers around renal hilum aorta and in the renal collecting duct, and shed of cells in the proximal convoluted tubules. In addition, up-regulated expressions of matrix metalloproteinases (MMP1/3) and down-regulated expressions of their inhibitors (TIMP1/3) at the mRNA and protein levels were also appeared to be relevant to low selenium. The results suggested that low selenium in diet may cause low selenium concentration in the kidney of growing rat and lead to damages of the ultrastructure and extracellular matrix (ECM) of kidney.

Effects of chronic fluoride exposure on object recognition memory and mRNA expression of SNARE complex in hippocampus of male mice

This study aimed to investigate the effects of long-term fluoride exposure on object recognition memory and mRNA expression of soluble N-ethylmaleimidesensitive fusion protein attachment protein receptors (SNARE) complex (synaptosome-associated protein of 25 kDa (SNAP-25), vesicle-associated membrane protein 2 (VAMP-2), and syntaxin 1A) in the hippocampus of male mice. Sixty sexually matured male Kunming mice were randomly divided into four groups: control group (given distilled water), low F group (25 mg/L NaF, corresponding to 11 mg/L F(-)), medium F group (50 mg/L NaF, corresponding to 22 mg/L F(-)), and high F group (100 mg/L NaF, corresponding to 45 mg/L F(-)). After 180 days, the spontaneous locomotor behavior and object recognition memory were detected by open field test and novel object recognition (NOR) test. Results showed that compared with the control group, frequency in each zone, total distance, and line crosses were significantly increased in low F and medium F groups, suggesting fluoride enhanced excitement of mice, while there were no marked changes in high F group. Twenty-four hours after training, a deficit of long-term memory (LTM) occurred only in high F group (P < 0.05), but there was no significant change of short-term memory (STM) 1.5 h later. The mRNA expression levels of SNAP-25, VAMP-2, and syntaxin 1A were detected by real-time quantitative RT-PCR, which revealed that the mRNA expression of VAMP-2 was significantly increased in medium F and high F groups (P < 0.01). Taken together, these results indicated that long-term fluoride administration can enhance the excitement of male mice, impair recognition memory, and upregulate VAMP-2 mRNA expression, which are involved in the adverse effects of fluoride on the object recognition memory of nervous system.