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

Protective effects of anthocyanins on the nervous system injury caused by fluoride-induced endoplasmic reticulum stress in rats

Long-term fluoride exposure can produce neurotoxicity. Anthocyanins, as antioxidants, have a certain protective effect in nerve damage. This study aimed to investigate the protective role of anthocyanins in fluoride-induced neurological damage due to endoplasmic reticulum stress (ERS). Using a fluoride-exposed Wistar rat model, we assessed learning memory capacity and pathologic and ultrastructural injury. The level of oxidative stress (OS) in vivo was detected by colorimetric method, the level of ERS was analyzed by immunohistochemistry, and the apoptosis of neuronal cells was observed by TUNEL staining. The results showed that fluoride exposure could decrease the learning and memory ability in rats, and led to histopathological and ultrastructural damage in the hippocampal CA1, CA3 and cortical regions. Fluoride exposure-induced OS in vivo, which further activates ERS, which was manifested by increased levels of ERS-related proteins GRP78, Caspase 12, and Caspase 3 in hippocampal CA1, CA3, and cortical regions, and eventually led to a significant increase in neuronal apoptosis rate. Notably, after anthocyanins treatment, pathological and ultrastructural damage was restored, the level of OS and ERS were significantly restored, and the apoptosis rate of neuronal cells was significantly reduced. In summary, as nutritional interventions, anthocyanins exert a protective role in fluoride-induced neurological injury.

Unraveling the role of abnormal AMPK and CRMP-2 phosphorylation in developmental fluoride neurotoxicity: Implications for synaptic damage and neurological disorders

Excessive fluoride exposure can be neurotoxic, although the exact mechanism remains unknown. This study aimed to investigate the neurotoxicity of continuous sodium fluoride exposure in offspring rats, focusing on the potential effects of fluoride exposure on hippocampal synaptic function and the role of AMPK and CRMP-2 in synaptic damage. We established an SD rat model of fluoride exposure (25, 50, and 100 mg/L NaF) and found that fluoride exposure damaged the learning and memory ability of F1 generation rats and caused ultrastructural changes in the hippocampus. Additionally, after the proteomic and phosphoproteomic analysis of rat hippocampal tissues, the Gene Ontology analysis revealed that sodium fluoride was involved in the enrichment of neuronal differentiation, synaptic signaling, and cytoskeleton-related biological processes. The Kyoto Encyclopedia of Genes and Genomes analysis showed that differential genes were enriched in synapse-related signaling pathways. Thus, we screened three differentially expressed proteins related to synaptic function for validation. The Western blotting analysis showed that AMPK and CRMP-2 were hyperphosphorylated in the hippocampus of fluoride-exposed rats. Our study found that abnormal AMPK and CRMP-2 phosphorylation leads to synaptic damage. This may be an important cause of memory impairment in fluorosis, offering new insights into the mechanism of fluoride-induced neurotoxicity.

Lithium attenuates ketamine-induced long-term neurotoxicity through DISC1-mediated GSK-3β/β-catenin and ERK/CREB pathways

Ketamine, an antagonist of N-methyl-D-aspartate receptor, is extensively employed in pediatric anesthesia. Multiple studies have shown that repeated ketamine exposure induces neuroapoptosis, synaptic changes and cognitive deficits during neurodevelopment. Therefore, it is essential to elucidate the mechanisms of ketamine-induced neurotoxicity and develop therapies to mitigate its harmful effects. Here, we investigated the role of disrupted in Schizophrenia 1 (DISC1) in ketamine-induced long-term neurotoxicity through a ketamine-exposed neuroapoptosis model. Neonatal rats received 2-5 intraperitoneal injections of ketamine (20 mg/kg b.w.) at 90 min intervals. Another cohort of pups received five intraperitoneal injections of ketamine (20 mg/kg×5 b.w.) with or without lithium (120 mg/kg×5 b.w.) at 90 min intervals over 6 h. Neuroapoptosis, DISC1-associated proteins expression in rats treated with ketamine, lithium, or a combination of both were detected, and the cognitive function of adolescent rats was evaluated by Morris water maze test. The length of dendrites and axons of primary neurons treated with lithium and ketamine were further measured. Results showed that ketamine time-dependently downregulated the levels of DISC1, pGSK-3β, β-catenin, pERK, pCREB and PSD95 in neonatal rats. Lithium could ameliorate neuroapoptosis, cognitive deficits and neurite growth inhibition triggered by ketamine. Mechanistically, lithium upregulated the levels of DISC1, PSD95 and GSK-3β/β-catenin and ERK/CREB signaling-related proteins. Consequently, lithium mitigated ketamine-induced long-term neurotoxicity by elevating DISC1 level and activating the GSK-3β/β-catenin and ERK/CREB signaling pathways.

Association between fluoride exposure and psychiatric disorders in adults

To explore the association between fluoride exposure and depression / anxiety in adults, the 1,169 participants were recruited. The demographic information of participants was obtained through questionnaire survey and physical measurements. Morning urine samples were collected, and urinary fluoride (UF) level was determined. Changes in depression and anxiety levels were evaluated using the Patient Health Questionnaire-2 and General Anxiety Disorder-2 scales. The association between psychiatric disorders and UF levels was analyzed. In the total population, the prevalence of depression and anxiety were 3.17% and 4.19%, respectively. These results showed no significant association between depression / anxiety scale scores and UF levels. Logistic regression suggested no significant association between depression / anxiety levels, and UF levels, but there was an interaction between UF and income on depression. Our findings highlighted the interaction between fluoride exposure and monthly income, which may affect depression in adults.

The role of GSK3β signaling mediated lysosomal biosynthesis dysregulation in fluoride-induced neurological impairment

Neurological dysfunction induced by fluoride is still one of major concern worldwide, yet the underlying mechanisms remain elusive. To explore whether fluoride disrupts lysosomal biosynthesis via the GSK3β signaling, leading to neurological damage, both in vivo rat models and in vitro PC12 cell models were conducted. Subsequent findings revealed reduced spatial learning and memory abilities, decreased hippocampal neurons, and disrupted neuronal arrangement in NaF-treated rats. In vitro, PC12 cells exhibited decreased cell viability and increased apoptosis rates after NaF treatment for 24 h. Moreover, immunofluorescence assays demonstrated that there is a reduction in the number of mature lysosomes and an increase in immature lysosomes in NaF-treated PC12 cells, evident by decreased co-localization of LAMP1 with Arl8b, and increased co-localization of LAMP1 with Rab7. Furthermore, both in vivo and in vitro, the protein expression of cleaved caspase-3 was upregulated, whereas the protein expressions of TFEB and CTSB were downregulated. The GSK3β signaling activation was detected, and this was confirmed by silencing GSK3β with siRNA in vitro. Collectively, these results indicate that NaF can impair lysosomal biosynthesis via GSK3β signaling, promoting neuronal apoptosis, and consequently impairing neurological function in rats.

Effects of Quercetin against fluoride-induced neurotoxicity in the medial prefrontal cortex of rats: A stereological, histochemical and behavioral study

BACKGROUND

Exposure to high levels of fluoride leads to brain developmental and functional damage. Motor performance deficits, learning and memory dysfunctions are related to fluoride neurotoxicity in human and rodent studies.

MATERIALS AND METHODS

Here, we evaluated the effects of Quercetin treatment (25 mg/kg) against sodium fluoride-induced neurotoxicity (NaF, 200 ppm) in the medial prefrontal cortex (mPFC) of male adult rats based on oxidative markers, behavioral performances, mRNA expressions, and stereological parameters. After a 4-week experimental period, the brains of rats were collected and used for molecular and histological analysis.

RESULTS

We found that 4 weeks of NaF exposure decreased body weight, working memory, Brain-derived neurotrophic factor (BDNF) mRNA expression, total volume of mPFC, number of neurons and non-neuronal cells in the mPFC, and anti-oxidative markers (CAT, SOD, and GSH-Px), while increased lipid peroxidation, P53 mRNA expression and anxiety. Quercetin treatment could significantly reverse the neurotoxic effect of NaF in the mPFC.

CONCLUSIONS

In summary, Quercetin could decrease the detrimental effects of NaF in the mPFC of adult rats by improving antioxidant potency and consequently decreasing neuronal and non-neuronal apoptosis.

Naringin alleviates fluoride-induced neurological impairment: A focus on the regulation of energy metabolism mediated by mitochondrial permeability transition pore

The neurological impairment induced by fluoride is associated with mitochondrial dysfunction. Normal mitochondrial permeability transition pore (mPTP) opening plays a pivotal role in mitochondrial function. However, it remains unclear whether p53-dependent mPTP-related mitochondrial apoptosis is associated with fluoride-induced neurological impairment, and the alleviation of naringin on those. In vivo, NaF-treated rats had impaired learning and memory abilities, damaged hippocampal structure, and higher respiratory exchange rates (RER). In vitro, the increased apoptosis rates, excessive opening of mPTP, and decreased mitochondrial membrane potential (MMP) were observed in PC12 cells treated with NaF. The protein expressions of p53, CytoC, and cleaved caspase 3 were significantly increased in hippocampi of rats treated with 50 mg/L and 100 mg/L NaF and in 40 mg/L and 80 mg/L NaF-treated PC12 cells, while the protein expression of CypD remains stable. And the changes of p53 and CypD were also confirmed by the immunofluorescence staining in vivo. After inhibiting the expression of p53 with pifithrin-α and p53-siRNA, the decreased apoptosis rates and mPTP opening, increased MMP, and decreased protein expressions of p53, CytoC, and cleaved caspase 3 were observed in NaF-treated PC12 cells. Rats, treated with NaF and naringin, had alleviated impaired neurological function, and had lower RER than rats treated with NaF alone. And compared with those in the NaF group, the decreased apoptosis rates and mPTP opening, and increased MMP were also found in PC12 cells treated with NaF and naringin. Furthermore, hippocampi of rats and PC12 cells treated with NaF and naringin had decreased protein expressions of p53, CytoC, and cleaved caspase 3. Our results indicate that fluoride activates the p53-dependent mPTP-related mitochondrial apoptosis, which then affects energy metabolism, resulting in neurological impairment. Additionally, naringin can alleviate this damage, and further studies on the potential health benefits of naringin are needed.

An in vivo and in silico probing of the protective potential of betaine against sodium fluoride-induced neurotoxicity

Excessive fluoride exposure beyond the tolerable limit may adversely impacts brain functionality. Betaine (BET), a trimethyl glycine, possesses antioxidant, anti-inflammatory and anti-apoptotic functions, although the underlying mechanisms of the role of BET on fluoride-induced neurotoxicity remain unelucidated. To assess the mechanism involved in the neuro-restorative role of BET on behavioural, neurochemical, and histological changes, we employed a rat model of sodium fluoride (NaF) exposure. Animals were treated with NaF (9 mg/kg) body weight (bw) only or co-treated with BET (50 and 100 mg/kg bw) orally uninterrupted for 28 days. We obtained behavioural phenotypes in an open field, performed negative geotaxis, and a forelimb grip test, followed by oxido-inflammatory, apoptotic, and histological assessment. Behavioural endpoints indicated lessened locomotive and motor and heightened anxiety-like performance and upregulated oxidative, inflammatory, and apoptotic biomarkers in NaF-exposed rats. Co-treatment with BET significantly enhanced locomotive, motor, and anxiolytic performance, increased the antioxidant signalling mechanisms and demurred oxidative, inflammatory, and apoptotic biomarkers and histoarchitectural damage in the cerebrum and cerebellum cortices mediated by NaF. The in-silico analysis suggests that multiple hydrogen bonds and hydrophobic interactions of BET with critical amino acid residues, including arginine (ARG380 and ARG415) in the Keap1 Kelch domain, which may disrupt Keap1-Nrf2 complex and activate Nrf2. This may account for the observed increased in the Nrf2 levels, elevated antioxidant response and enhanced anti-inflammatory response. The BET-Keap1 complex was also observed to exhibit structural stability and conformational flexibility in solvated biomolecular systems, as indicated by the thermodynamic parameters computed from the trajectories obtained from a 100 ns full atomistic molecular dynamics simulation. Therefore, BET mediates neuroprotection against NaF-induced cerebro-cerebellar damage through rats' antioxidant, anti-inflammatory, and anti-apoptotic activity, which molecular interactions with Keap1-Nrf2 may drive.

Effect of Voluntary Wheel Running on Anxiety- and Depression-Like Behaviors in Fluoride-Exposed Mice

Fluoride, an environmental toxicant, could induce endoplasmic reticulum stress (ERS) in neuronal cells ultimately leading to apoptosis and emotional dysfunction. Meanwhile, voluntary wheel running contributes to mitigate anxiety and depression. Our investigation aimed to study the effect of voluntary wheel running on anxiety- and depression-like behaviors in fluoride-exposure mice. The results showed that exposure to 100 mg/L sodium fluoride (NaF) for 6 months can induce anxiety- and depression-like behavior in mice. Fluorosis mice subjected to voluntary wheel running have less anxiety- and depression-like behaviors. Nissl and TUNEL staining demonstrated that fluoride led to a reduced proportion of Nissl body area in the cerebral cortex and an increased apoptotic ratio of nerve cells in the cerebral cortex. In contrast, these pathologic damages were improved in voluntary wheel running mice exposed to NaF. Moreover, the expressions of mRNA in the cerebral cortex GABA, GAD65, GAD67, DR, vGLU, 5-HT1A, BDNF, NMDAR1, and Bcl2 were downregulated and the levels of c-fos, GRP78, PERK, eIF2α, CHOP, Caspase-12, and Caspase-3 mRNA were upregulated in mice exposed to fluoride. NaF treatment had increased the PERK, ATF6, IRE1, p-eIF2α, and Caspase-3 protein levels and reduced the expressions of proteins, including GAD67, VGAT, BDNF, NMDAR1, PSD95, and SYN. By contrast, fluorosis mice subjected to voluntary wheel running enhanced the expression of GAD65, GAD67, VGAT, and neuroplasticity-related proteins in mice and inhibited the PERK-CHOP pathway. It is worth noting that the correlation between the amount of exercise and the behavioral indicators as well as neurotransmitter levels was found. In conclusion, voluntary wheel running inhibits the fluoride-induced ERS and GRP78 expression through the PERK-CHOP pathway and plays an anti-apoptotic role, ultimately ameliorating emotional dysfunction in NaF-exposed mice.

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.

NTP monograph on the state of the science concerning fluoride exposure and neurodevelopment and cognition: a systematic review

BACKGROUND

Fluoride is a common exposure in our environment that comes from a variety of sources and is widely promoted for its dental and overall oral health benefits. Contributions to an individual's total exposure come primarily from fluoride in drinking water, food, beverages and dental products. A 2006 evaluation by the National Research Council (NRC) found support for an association between consumption of high levels of naturally occurring fluoride in drinking water and adverse neurological effects in humans and recommended further investigation. The evidence reviewed at that time was from dental and skeletal fluorosis-endemic regions of China. Since the NRC evaluation, the number and location of studies examining cognitive and neurobehavioral effects of fluoride in humans have grown considerably, including several recent North American prospective cohort studies evaluating prenatal fluoride exposure. In 2016, the National Toxicology Program (NTP) published a systematic review of the evidence from experimental animal studies on the effects of fluoride on learning and memory. That systematic review found a low-to-moderate level of evidence that deficits in learning and memory occur in non-human mammals exposed to fluoride.

OBJECTIVE

To conduct a systematic review of the human, experimental animal, and mechanistic literature to evaluate the extent and quality of the evidence linking fluoride exposure to neurodevelopmental and cognitive effects in humans.

METHOD

A systematic review protocol was developed and utilized following the standardized OHAT systematic review approach for conducting literature-based health assessments. This monograph presents the current state of evidence associating fluoride exposure with cognitive or neurodevelopmental health effects and incorporated predefined assessments of study quality and confidence levels. Benefits of fluoride with respect to oral health are not addressed in this monograph.

RESULTS

The bodies of experimental animal studies and human mechanistic evidence do not provide clarity on the association between fluoride exposure and cognitive or neurodevelopmental human health effects. Human mechanistic studies were too heterogenous and limited in number to make any determination on biological plausibility. This systematic review identified studies that assessed the association between estimated fluoride exposure and cognitive or neurodevelopmental effects in both adults and children, which were evaluated separately. The most common exposure assessment measures were drinking water concentrations and estimates of total fluoride exposure, as reflected in biomarkers such as urinary fluoride. In adults, only two high-quality cross-sectional studies examining cognitive effects were available. The literature in children was more extensive and was separated into studies assessing intelligence quotient (IQ) and studies assessing other cognitive or neurodevelopmental outcomes. Eight of nine high-quality studies examining other cognitive or neurodevelopmental outcomes reported associations with estimated fluoride exposure. Seventy-two studies assessed the association between fluoride exposure and IQ in children. Nineteen of those studies were considered to be high quality; of these, 18 reported an inverse association between estimated fluoride exposure and IQ in children. The 18 studies, which include 3 prospective cohort studies and 15 cross-sectional studies, were conducted in 5 different countries. Forty-six of the 53 low-quality studies in children also found evidence of an inverse association between estimated fluoride exposure and IQ in children.

DISCUSSION

Existing animal studies provide little insight into the question of whether fluoride exposure affects IQ. In addition, studies that evaluated fluoride exposure and mechanistic data in humans were too heterogenous and limited in number to make any determination on biological plausibility. The body of evidence from studies in adults is also limited and provides low confidence that fluoride exposure is associated with adverse effects on adult cognition. There is, however, a large body of evidence on associations between fluoride exposure and IQ in children. There is also some evidence that fluoride exposure is associated with other neurodevelopmental and cognitive effects in children; although, because of the heterogeneity of the outcomes, there is low confidence in the literature for these other effects. This review finds, with moderate confidence, that higher estimated fluoride exposures (e.g., as in approximations of exposure such as drinking water fluoride concentrations that exceed the World Health Organization Guidelines for Drinking-water Quality of 1.5 mg/L of fluoride) are consistently associated with lower IQ in children. More studies are needed to fully understand the potential for lower fluoride exposure to affect children's IQ.

Impaired neurogenesis induced by fluoride via the Notch1 signaling and effects of carvacrol intervention

The negative regulation on neurogenesis has been implicated in fluoride neurotoxicity, while the evidence is limited. To explore whether fluoride interferes with neurogenesis via the Notch1 signaling and the potential alleviation effects of carvacrol (CAR), we conducted in vivo and in vitro experiments, as well as epidemiological analyses in this study. The results showed that urinary fluoride levels and circulating Notch1 levels were associated with IQ levels in boys. NaF-treated rats had fewer neurons, lower densities of dendritic spines, depressed neurogenesis, and impaired learning and memory abilities. In vitro experiments using undifferentiated PC12 cells mimicking neurogenesis revealed that NaF suppressed differentiation and neurite outgrowth. Besides, Notch1 signaling activation was detected in vivo and in vitro. The latter was confirmed using an in vitro model supplemented with DAPT, a potent Notch1 inhibitor. Furthermore, CAR supplementation negatively regulated NICD1 and Hes1 expressions and promoted hippocampal neurogenesis, thereby improving neurological functions in NaF-treated rats. These findings indicated that the inhibition of neurogenesis in hippocampi induced by fluoride via Notch1 signaling activation may be one of the underlying mechanisms of its neurotoxicity, and that CAR significantly alleviated the neurotoxicity of NaF via the Notch1 signaling.

GPX4 degradation contributes to fluoride-induced neuronal ferroptosis and cognitive impairment via mtROS-chaperone-mediated autophagy

Ferroptosis is a newly recognized type of programmed cell death that is implicated in the pathophysiological process of neurological disorders. Our previous studies have revealed that exposure to high concentrations of fluoride for long periods of time induces hippocampal neural injury and cognitive deficits. However, whether ferroptosis is involved in fluoride-induced neuronal death and the underlying mechanism remain unknown. In this study, the results indicated that exposure to high fluoride triggered ferroptosis in SH-SY5Y cells and in the hippocampus of mice. Fluoride exposure accelerated the lysosomal degradation of GPX4 and led to neuronal ferroptosis, while GPX4 overexpression protected SH-SY5Y cells against fluoride-induced neurotoxicity. Intriguingly, the enhanced chaperone-mediated autophagy (CMA) induced by fluoride stimulation was responsible for GPX4 degradation because the inhibition of CMA activity by LAMP2A knockdown effectively prevented fluoride-induced GPX4 loss. Furthermore, mitochondrial ROS (mtROS) accumulation caused by fluoride contributed to CMA activation-mediated GPX4 degradation and subsequent neuronal ferroptosis. Notably, the ferroptosis-specific inhibitor ferrostatin-1 (Fer-1) or the ROS scavenger N-acetyl-L-cysteine (NAC) alleviated fluoride-evoked hippocampal neuronal death and synaptic injury as well as cognitive deficits in mice. The present studies indicates that ferroptosis is a novel mechanism of fluoride-induced neurotoxicity and that chronic fluoride exposure facilitates GPX4 degradation via mtROS chaperone-mediated autophagy, leading to neuronal ferroptosis and cognitive impairment.

Ketamine impairs growth cone and synaptogenesis in human GABAergic projection neurons via GSK-3β and HDAC6 signaling

The growth cone guides the axon or dendrite of striatal GABAergic projection neurons that protrude into the midbrain and cortex and form complex neuronal circuits and synaptic networks in a developing brain, aberrant projections and synaptic connections in the striatum related to multiple brain disorders. Previously, we showed that ketamine, an anesthetic, reduced dendritic growth, dendritic branches, and spine density in human striatal GABAergic neurons. However, whether ketamine affects the growth cone, the synaptic connection of growing striatal GABAergic neurons has not been tested. Using human GABAergic projection neurons derived from human inducible pluripotent stem cells (hiPSCs) and embryonic stem cells (ES) in vitro, we tested ketamine effects on the growth cones and synapses in developing GABAergic neurons by assessing the morphometry and the glycogen synthase kinase-3 (GSK-3) and histone deacetylase 6 (HDAC6) pathway. Ketamine exposure impairs growth cone formation, synaptogenesis, dendritic development, and maturation via ketamine-mediated activation of GSK-3 pathways and inhibiting HDAC6, an essential stabilizing protein for dendritic morphogenesis and synapse maturation. Our findings identified a novel ketamine neurotoxic pathway that depends on GSK-3β and HDAC6 signaling, suggesting that microtubule acetylation is a potential target for reducing ketamine's toxic effect on GABAergic projection neuronal development.

Triaging between post-translational modification of cell cycle regulators and their therapeutics in neurodegenerative diseases

Neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, and Huntington's disease, present challenges in healthcare because of their complicated etiologies and absence of healing remedies. Lately, the emerging role of post-translational modifications (PTMs), in the context of cell cycle regulators, has garnered big interest as a potential avenue for therapeutic intervention. The review explores the problematic panorama of PTMs on cell cycle regulators and their implications in neurodegenerative diseases. We delve into the dynamic phosphorylation, acetylation, ubiquitination, SUMOylation, Glycation, and Neddylation that modulate the key cell cycle regulators, consisting of cyclins, cyclin-dependent kinases (CDKs), and their inhibitors. The dysregulation of these PTMs is related to aberrant cell cycle in neurons, which is one of the factors involved in neurodegenerative pathologies. Moreover, the effect of exogenous activation of CDKs and CDK inhibitors through PTMs on the signaling cascade was studied in postmitotic conditions of NDDs. Furthermore, the therapeutic implications of CDK inhibitors and associated alteration in PTMs were discussed. Lastly, we explored the putative mechanism of PTMs to restore normal neuronal function that might reverse NDDs.

Sodium Butyrate Ameliorates Fluorosis-Induced Neurotoxicity by Regulating Hippocampal Glycolysis In Vivo

Fluorosis can induce neurotoxicity. Sodium butyrate (SB), a histone deacetylase inhibitor, has important research potential in correcting glucose metabolism disorders and is widely used in a variety of neurological diseases and metabolic diseases, but it is not yet known whether it plays a role in combating fluoride-induced neurotoxicity. This study aims to evaluate the effect of SB on fluoride neurotoxicity and the possible associated mechanisms. The results of HE staining and Morris water maze showed that, in mice exposed to 100 mg/L fluoride for 3 months, the hippocampal cells arranged in loosely with large cell gaps and diminished in number. One thousand milligram per kilogram per day SB treatment improved fluoride-induced neuronal cell damage and spatial learning memory impairment. Western blot results showed that the abundance of malate dehydrogenase 2 (MDH2) and pyruvate dehydrogenase (PDH) in the hippocampus of fluorosis mice was increased, the abundance of pyruvate kinase M (PKM), lactate dehydrogenase (LDH), hexokinase (HK), phosphatidylinositol 3-kinase (PI3K), phosphorylated Akt (P-AKT), and hypoxia-inducible factor 1α (HIF-1α) was inhibited, and the content of lactate and ATP was decreased. SB treatment reversed the decreased glycolysis in the hippocampus of fluorosis mice. These results suggested that SB could ameliorate fluorosis-induced neurotoxicity, which might be linked with its function in regulating glycolysis as well as inhibition of the PI3K/AKT/HIF-1α pathway. Sodium butyrate ameliorates fluorosis-induced neurotoxicity by regulating hippocampal glycolysis in vivo (created with MedPeer (www.medpeer.cn)).

Fecal microbiota transplantation and short-chain fatty acids improve learning and memory in fluorosis mice by BDNF-PI3K/AKT pathway

Fluoride, an environmental toxicant, not only arouses intestinal microbiota dysbiosis, but also causes neuronal apoptosis and a decline in learning and memory ability. The purpose of this study was to explore whether fecal microbiota transplantation (FMT) from healthy mice and bacteria-derived metabolites short-chain fatty acids (SCFAs) supplement protect against fluoride-induced learning and memory impairment. Results showed that FMT reversed the elevated percentage of working memory errors (WME) and reference memory errors (RME) in fluorosis mice during the eight-arm maze test. Nissl and TUNEL staining presented that fluoride led to a decreased proportion of Nissl bodies area in the hippocampal CA3 region and an increased apoptotic ratio of nerve cells in CA1, CA3 and DG areas, whereas FMT alleviated those pathological damages. Moreover, the expressions of mRNA in hippocampal BDNF, PDK1, AKT, Bcl-2, and Bcl-xL were downregulated in mice exposed to fluoride, but the levels of PI3K, Bax, Bak, and Caspase-7 mRNA were upregulated. NaF treatment had an increase in PI3K and Caspase-3 protein levels and reduced the expressions of these four proteins, including BDNF, p-PI3K, AKT and p-AKT. By contrast, FMT enhanced the expression of BDNF and thus activated the PI3K/AKT pathway. Besides, the 16S rRNA sequencing revealed that fluoride caused a reduction in certain SCFA producers in the colon as evidenced by a decline in Erysipelatoclostridiaceae, and a downward trend in Akkermansia, Blautia and Alistipes. However, the disordered gut microbiome was restored via frequent FMT. Of note, SCFAs administration also increased BDNF levels and regulated its downstream pathways, which contributed to cell survival and learning and memory function recovery. In conclusion, FMT and SCFAs may activate the BDNF-PI3K/AKT pathway to play an anti-apoptotic role and ultimately improve learning and memory deficits in fluorosis mice.

Effect of Fluoride on the Expression of 8-Hydroxy-2'-Deoxyguanosine in the Blood, Kidney, Liver, and Brain of Rats

Excessive exposure of fluoride not only leads to damage on bone, but also has an adverse effect on soft tissues. Oxidative DNA damage induced by fluoride is thought to be one of the toxic mechanisms of fluoride effect. However, the dose-response of fluoride on oxidative DNA damage is barely studied in organisms. This study investigated the concentration of fluoride in rat blood, kidney, liver, and brain as well as the dose-time effect of fluoride on the expression of 8-hydroxy-2'-deoxyguanosine (8-OHdG) in the above tissues. Rats were exposed to 0 mg/L, 25 mg/L, 50 mg/L, and 100 mg/L of fluorine ion and treated for one and three months. The results showed that the accumulation of fluoride in soft tissues was very different. At the first month, blood fluoride was increased, liver and brain fluoride showed a U-shaped change, and kidney fluoride was not significant. At the third month, blood fluoride was altered with an inverted U-shaped change, kidney and brain fluoride increased, but liver fluoride decreased. Both the exposure concentration and the time of exposure had a significant effect on the expression of 8-OHdG in the above tissues. However, the effect patterns of fluoride on these tissues were notably different at different times. At the first month of fluoride treatment, blood, kidney, and liver 8-OHdG decreased with the increasing fluoride concentration. At the third month, blood 8-OHdG showed a U-shaped change, but kidney 8-OHdG altered with an inverted U-shaped change. Liver 8-OHdG increased, while brain 8-OHdG decreased at the third month. Correlation analysis showed that only blood 8-OHdG was significantly inversely correlated with blood fluoride and dental fluorosis grade in both the first and third months. Liver 8-OHdG was negatively and significantly correlated with liver fluoride. There was a weak but nonsignificant correlation between kidney and brain 8-OHdG and fluoride in both tissues. Additionally, blood 8-OHdG was positively correlated with kidney and liver 8-OHdG at the first month and positively correlated with brain 8-OHdG at the third month. Taken together, our data suggests that concentration and time of fluoride exposure had a significant effect on 8-OHdG, but the effect patterns of fluoride on 8-OHdG were different in the tissues, which suggests that the impact of fluoride on 8-OHdG may be a tissue-specific, as well as a non-monotonic positive correlation.

Fluoride immobilization and release in cemented PG backfill and its influence on the environment

Waste recycling must consider secondary pollution, which is affected by recycling methods. Cemented phosphogypsum (PG) backfill is a cost-effective method for PG recycling. However, due to dynamic environmental conditions, the impurity fluoride is challenging to stabilize. In this study, we investigated the immobilization and release of fluoride and its influence on backfill strength. The results showed that the fluoride was temporarily immobilized by PG. However, when the binder was mixed with PG to make the backfill, immobilized fluoride was re-released into the backfill slurry due to the increased pH caused by binder hydration. Therefore, simply converting fluoride into CaF precipitation cannot avoid the risk of fluoride exceeding the Chinese standard (GB8978-1996) (10 mg/L). Furthermore, fluoride deteriorated strength development by inhibiting binder hydration and weakening the backfill structure. The fluoride content in the slurry, rather than in PG, directly affected the backfill strength. Considering the recycling of PG as aggregate for backfill, fluoride should be removed in advance or immobilized in other low-solubility forms instead of CaF precipitation. These results were of great significance for the large-scale resource recycling and safety management of PG.

Influence of interflow carbonate-clay association for groundwater fluoride contamination in eastern Deccan, central India

In central India, fluoride contamination in deeper basaltic aquifer is geogenic. This study demonstrates the source of fluorine enrichment in aquifer matrix and its release mechanism into groundwater. Magmatic-hydrothermal residual melt, i.e., albitic-calcic-amphibole-apatite-rich intrusive rock is the main source of fluorine enrichment. The association of this rock with interflow carbonate-clay assemblage played a significant role for fluoride contamination. Fluorine-enriched residual melt interacted with interflow carbonate-clay association, and this interaction metasomatized the carbonates and enhanced fluorine concentration in sediments. Bulk fluorine concentration of 988 ppm is measured in the soil developed over the association of intrusive rock and carbonate-clay assemblage. X-ray diffraction and electron-probe micro analysis confirmed the presence of fluorine-bearing and/or containing minerals, i.e., fluorite, fluorapatite, and palygorskite. The presence of bicarbonate and Na⁺ (from albitic feldspar) in alkaline water enhanced desorption of fluoride from clays, and dissociation from fluorapatite and fluorite from carbonate-clay assemblage, which released fluoride from aquifer matrix to groundwater. Clay horizon acts as an impervious cap on the deeper aquifer and increases the residence time of groundwater. In such favorable physico-chemical condition, fluoride released from aquifer matrix to groundwater and gradually increasing the degree of fluoride contamination.

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.

Fluoride exposure causes behavioral, molecular and physiological changes in adult zebrafish (Danio rerio) and their offspring

Fluoride exposure through drinking water, foods, cosmetics, and drugs causes genotoxic effects, oxidative damage, and impaired cognitive abilities. In our study, the effects of fluoride on anxiety caused by the circadian clock and circadian clock changes in a zebrafish model were investigated at the molecular level on parents and the next generations. For this purpose, adult zebrafish were exposed to 1.5 ppm, 5 ppm, and 100 ppm fluoride for 6 weeks. At the end of exposure, anxiety-like behaviors and sleep/wake behaviors of the parent fish were evaluated with the circadian rhythm test and the novel tank test. In addition, antioxidant enzyme activities and melatonin levels in brain tissues were measured. In addition, morphological, physiological, molecular and behavioral analyzes of offspring taken from zebrafish exposed to fluoride were performed. In addition, histopathological analyzes were made in the brain tissues of both adult zebrafish and offspring, and the damage caused by fluoride was determined. The levels of BMAL1, CLOCK, PER2, GNAT2, BDNF and CRH proteins were measured by immunohistochemical analysis and significant changes in their levels were determined in the F- treated groups. The data obtained as a result of behavioral and molecular analyzes showed that parental fluoride exposure disrupts the circadian rhythm, causes anxiety-like behaviors, and decreases the levels of brain antioxidant enzymes and melatonin in parents. In addition, delay in hatching, increase in death and body malformations, and decrease in blood flow velocity, and locomotor activity was observed in parallel with dose increase in offspring. On the other hand, an increase in offspring apoptosis rate, ROS level, and lipid accumulation was detected. As a result, negative effects of fluoride exposure on both parents and next generations have been identified.

Cdk5 and aberrant cell cycle activation at the core of neurodegeneration

Neurodegenerative diseases are caused by the progressive loss of specific neurons. The exact mechanisms of action of these diseases are unknown, and many studies have focused on pathways related to abnormal accumulation and processing of proteins, mitochondrial dysfunction, and oxidative stress leading to apoptotic death. However, a growing body of evidence indicates that aberrant cell cycle re-entry plays a major role in the pathogenesis of neurodegeneration. The activation of the cell cycle in mature neurons could be promoted by several signaling mechanisms, including c-Jun N-terminal kinases, p38 mitogen-activated protein kinases, and mitogen-activated protein kinase/extracellular signal-regulated kinase cascades; post-translational modifications such as Tau-phosphorylation; and DNA damage response. In all these events, implicated Cdk5, a proline-directed serine/threonine protein kinase, seems to be responsible for several cellular processes in neurons including axon growth, neurotransmission, synaptic plasticity, neuronal migration, and maintenance of neuronal survival. However, under pathological conditions, Cdk5 dysregulation may lead to cell cycle re-entry in post-mitotic neurons. Thus, Cdk5 hyperactivation, by its physiologic activator p25, hyper-phosphorylates downstream substrates related to neurodegenerative diseases. This review summarizes factors such as oxidative stress, DNA damage response, signaling pathway disturbance, and Ubiquitin proteasome malfunction contributing to cell cycle re-entry in post-mitotic neurons. It also describes how all these factors are linked to a greater or lesser extent with Cdk5. Thus, it offers a global vision of the function of cell cycle-related proteins in mature neurons with a focus on Cdk5 and how this protein contributes to the development of Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease by cell cycle activation.

Brain structure and synaptic protein expression alterations after antidepressant treatment in a Wistar-Kyoto rat model of depression

BACKGROUND

Structural MRI has demonstrated brain alterations in depression pathology and antidepressants treatment. While synaptic plasticity has been previously proposed as the potential underlying mechanism of MRI findings at a cellular and molecular scale, there is still insufficient evidence to link the MRI findings and synaptic plasticity mechanisms in depression pathology.

METHODS

In this study, a Wistar-Kyoto (WKY) depression rat model was treated with antidepressants (citalopram or Jie-Yu Pills) and tested in a series of behavioral tests and a 7.0 MRI scanner. We then measured dendritic spine density within altered brain regions. We also examined expression of synaptic marker proteins (PSD-95 and SYP).

RESULTS

WKY rats exhibited depression-like behaviors in the sucrose preference test (SPT) and forced swim test (FST), and anxiety-like behaviors in the open field test (OFT). Both antidepressants reversed behavioral changes in SPT and OFT but not in FST. We found a correlation between SPT performance and brain volumes as detected by MRI. All structural changes were consistent with alterations of the corpus callosum (white matter), dendritic spine density, as well as PSD95 and SYP expression at different levels. Two antidepressants similarly reversed these macro- and micro-changes.

LIMITATIONS

The single dose of antidepressants was the major limitation of this study. Further studies should focus on the white matter microstructure changes and myelin-related protein alterations, in addition to comparing the effects of ketamine.

CONCLUSION

Translational evidence links structural MRI changes and synaptic plasticity alterations, which promote our understanding of SPT mechanisms and antidepressant response in WKY rats.

Systematic impacts of fluoride exposure on the metabolomics of rats

Fluoride is widely present in the environment. Excessive fluoride exposure leads to fluorosis, which has become a global public health problem and will cause damage to various organs and tissues. Only a few studies focus on serum metabolomics, and there is still a lack of systematic metabolomics associated with fluorosis within the main organs. Therefore, in the current study, a non-targeted metabolomics method using gas chromatography-mass spectrometry (GC-MS) was used to research the effects of fluoride exposure on metabolites in different organs, to uncover potential biomarkers and study whether the affected metabolic pathways are related to the mechanism of fluorosis. Male Sprague-Dawley rats were randomly divided into two groups: a control group and a fluoride exposure group. GC-MS technology was used to identify metabolites. Multivariate statistical analysis identified 16, 24, 20, 20, 24, 13, 7, and 13 differential metabolites in the serum, liver, kidney, heart, hippocampus, cortex, kidney fat, and brown fat, respectively, in the two groups of rats. Fifteen metabolic pathways were affected, involving toxic mechanisms such as oxidative stress, mitochondrial damage, inflammation, and fatty acid, amino acid and energy metabolism disorders. This study provides a new perspective on the understanding of the mechanism of toxicity associated with sodium fluoride, contributing to the prevention and treatment of fluorosis.

Impaired V-ATPase leads to increased lysosomal pH, results in disrupted lysosomal degradation and autophagic flux blockage, contributes to fluoride-induced developmental neurotoxicity

Fluoride is capable of inducing developmental neurotoxicity, yet its mechanisms remain elusive. We aimed to explore the possible role and mechanism of autophagic flux blockage caused by abnormal lysosomal pH in fluoride-induced developmental neurotoxicity, focusing on the role of V-ATPase in regulating the neuronal lysosomal pH. Using Sprague-Dawley rats exposed to sodium fluoride (NaF) from gestation through delivery until the neonatal offspring reached six months of age as an in vivo model. The results showed that NaF impaired the cognitive abilities of the offspring rats. In addition, NaF reduced V-ATPase expression, diminished lysosomal degradation capacity and blocked autophagic flux, and increased apoptosis in the hippocampus of offspring. Consistently, these results were validated in SH-SY5Y cells incubated with NaF. Moreover, NaF increased the SH-SY5Y lysosomal pH. Mechanistically, V-ATPase B2 overexpression and ATP effectively restored V-ATPase expression, reducing NaF-induced lysosomal alkalinization while increasing lysosomal degradation capacity. Notably, those above pharmacological and molecular interventions diminished NaF-induced apoptosis by restoring autophagic flux. Collectively, the present findings suggested that NaF impairs the lysosomal pH raised by V-ATPase. This leads to reduced lysosomal degradation capacity and triggers autophagic flux blockage and apoptosis, thus contributing to neuronal death. Therefore, V-ATPase might be a promising indicator of developmental fluoride neurotoxicity.

Toxicity studies of select ionic liquids (1-ethyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium chloride, 1-butyl-1-methylpyrrolidinium chloride, and n-butylpyridinium chloride) administered in drinking water to Sprague Dawley (Hsd:Sprague Dawley SD) rats and B6C3F1/N mice

Ionic liquids (ILs) are synthetic solvents with applications in a variety of industrial and chemical industries. Human exposure to this diverse chemical class is primarily through dermal or oral routes. Research suggests toxicity may be associated with IL structural characteristics, including the type of cation base or alkyl chain substitutions associated with the cation. To further investigate this hypothesis, the National Toxicology Program (NTP) conducted 3-month toxicity studies in male and female Sprague Dawley (Hsd:Sprague Dawley SD) rats and B6C3F1/N mice (n = 10/sex/exposure group; 3 exposure concentrations per IL) to compare the relative toxicities of four ILs administered via drinking water-1-ethyl-3-methylimidazolium chloride (Emim-Cl), 1-butyl-3-methylimidazolium chloride (Bmim-Cl), 1-butyl-1-methylpyrrolidinium chloride (Bmpy-Cl), and n-butylpyridinium chloride (NBuPy-Cl). (Abstract Abridged).

Characterization of drinking groundwater quality in rural areas of Inner Mongolia and assessment of human health risks

Groundwater is an important natural resource of drinking water in rural areas in Inner Mongolia, China. In this study, 4438 drinking groundwater samples were collected from the rural areas of 81 counties in Inner Mongolia, and were analyzed for 16 parameters, including pH, total hardness (TH), chemical oxygen demand (COD), total dissolved solids (TDS), sulfate (SO42-), chloride (Cl-), fluoride (F-), iron (Fe), manganese (Mn), arsenic (As), cadmium (Cd), hexavalent chromium (Cr), lead (Pb), aluminum (Al), cuprum (Cu), zinc (Zn). The groundwater quality was evaluated with water quality index (WQI) and human health risk assessment (HRA). Monte Carlo simulation were applied for the uncertainty and sensitivity analysis in the health risk assessment. The spatial map was employed based on the inverse distance weighted (IDW) interpolation technique. The results reveal that while the hazard quotient (HQ) suggests that the risk of single element contamination is feeble, the hazard index (HI) indicates a potential health risk for the local population. The observed cumulative carcinogenic risk (CCR) indicates a probable risks of carcinogenic health hazards in the study area. The sensitivity analysis revealed that daily ingestion rate (IR), exposure frequency (EF), and the concentrations of As, Mn, F-, and Cr are the most influential parameters for health hazards. The highly polluted areas are mainly distributed in the central and western regions of Inner Mongolia, including Xianghuangqi, New Barag Zuoqi, and Togtoh. It is observed that the groundwater may cause a potential health risk after long-term ingestion. The results of this study will contribute to groundwater management and protection in Inner Mongolia.

Potential Protective Effect of Riboflavin Against Pathological Changes in the Main Organs of Male Mice Induced by Fluoride Exposure

Long-term exposure to excessive fluorine could cause damage to various tissues and organs in human and animals. However, there is no effective antidote to prevent and cure fluorosis except for avoiding fluoride intake. As an essential nutrient, riboflavin (VB₂) has been identified to relieve oxidative stress and inflammation in animal tissues caused by other toxic substances, whether it can alleviate the damage caused by fluoride is unknown. For this, 32 ICR male mice were allocated to four groups of eight each. They were treated with 0 (distilled water), 100 mg/L sodium fluoride (NaF), 40 mg/L VB₂, and their combination (100 mg/L NaF plus 40 mg/L VB₂) via the drinking water for 90 consecutive days, respectively. The content of bone fluoride and the histomorphology of the main organs including liver, kidney, cerebral cortex, epididymis, small intestine, and colon were evaluated and pathologically scored. The results found that fluoride caused the pathological changes in liver, kidney, cerebral cortex, epididymis, small intestine, and colon at varying degrees, while riboflavin supplementation reduced significantly the accumulation of fluoride in bone, alleviated the morphological damage to cerebral cortex, epididymis, ileum, and colon. This study provides new clues for deeply exploring the mechanism of riboflavin intervention in fluorosis.

Impacts of Fluoride Neurotoxicity and Mitochondrial Dysfunction on Cognition and Mental Health: A Literature Review

This review focuses on the synthesis of current experimental and observational data regarding the effect of fluoride exposure on childhood mental health and the role of mitochondrial function as a mechanism of action. We aggregated data on the relationships between fluoride neurotoxicity, mitochondrial function, and cognitive and mental health using PubMed. Current animal and human research suggest that prenatal and perinatal fluoride exposure might have neurotoxic effects. These studies observed physical changes (fur loss and delayed reflex development in animals), intelligence loss, increased hyperactivity, and irregular moods associated with fluoride exposure. Two gaps in the literature were identified: (1) there is limited research on the mental and emotional impacts of fluoride exposure compared to research on cognitive outcomes, and (2) human studies primarily focus on prenatal and perinatal exposure, with little research conducted at other time points (e.g., adolescence). Furthermore, there is no agreed-upon mechanism for the neurotoxic effects of fluoride; however, fluoride can induce mitochondrial damage, including decreasing circulating mitochondrial DNA content, dysregulating biogenesis, and circular structure loss. Additionally, many neurodevelopmental conditions have mitochondrial underpinnings. More work is needed to elucidate the impact and timing of fluoride exposure on mental health and the role of mitochondrial function as a biological mechanism

The GSK-3β/β-Catenin Signaling-Mediated Brain-Derived Neurotrophic Factor Pathway Is Involved in Aluminum-Induced Impairment of Hippocampal LTP In Vivo

The neurotoxic effects of aluminum (Al) are associated with the impairment of synaptic plasticity, the biological basis of learning and memory, the major form of which is long-term potentiation (LTP). The canonical glycogen synthase kinase-3β (GSK-3β)/β-catenin signaling-mediated brain-derived neurotrophic factor (BDNF) pathway has been suggested to play important roles in memory. Thus, Al may affect LTP through this pathway. In this study, a Sprague-Dawley rat model of neurotoxicity was established through intracerebroventricular (i.c.v.) injection of aluminum maltol (Al(mal)3), which was achieved by preimplantation of a cannula into the lateral ventricle. The rats in the control and Al-treated groups received a daily injection of SB216763, an inhibitor of GSK-3β. Electrophysiology and western blot analysis were used to investigate the regulatory effect of the GSK-3β/β-catenin signaling-mediated BDNF pathway on LTP impairment induced by Al(mal)3. The results confirmed that i.c.v. injection of Al(mal)3 significantly suppressed the field excitatory postsynaptic potential (fEPSP) amplitude, as indicated by a decrease in BDNF protein expression, which was accompanied by dose-dependent decreases in β-catenin protein expression and the phosphorylation of GSK-3β at Ser9. Rats that received SB216763, a GSK-3β inhibitor, exhibited higher fEPSP amplitudes than control rats. Furthermore, SB216763 treatment upregulated the hippocampal protein expression of BDNF and β-catenin while increasing the ratio of p-GSK-3β/GSK-3β. From the perspective of the identified β-catenin-BDNF axis, Al impairs hippocampal LTP, possibly through the GSK-3β/β-catenin signaling-mediated BDNF pathway.

Fluoride Stimulates Anxiety- and Depression-like Behaviors Associated with SIK2-CRTC1 Signaling Dysfunction

Using Sprague-Dawley rats and rat PC12 cells treated with sodium fluoride (NaF), we investigated the effects of SIK2-CRTC1 signaling on the neurobehavioral toxicity induced by fluoride. The in vivo results demonstrated that NaF treatment induced anxiety- and depression-like behaviors in juvenile rats, resulting in histological and ultrastructural abnormalities in the rat hippocampus and medial prefrontal cortex. Moreover, NaF exposure induced neuronal loss and excessive apoptosis. We also found that NaF elevated the expression of SIK2 and reduced the expression of CRTC1, brain-derived neurotrophic factor (BDNF), and VGF. The in vitro results showed that NaF suppressed cell viability, induced SIK2-CRTC1 signaling dysfunction, and caused excessive apoptosis in PC12 cells. Notably, targeted knockout of SIK2 with SIK2-siRNA or blocking of SIK2-CRTC1 signaling with 7,8-dihydroxyflavone (7,8-DHF) (as well as venlafaxine) can reduce apoptosis and increase cell viability in vitro. These findings suggest that neuronal death resulting from abnormal SIK2-CRTC1 signaling contributes to neurobehavioral toxicity induced by fluoride.

Regulatory mechanism of cyclins and cyclin-dependent kinases in post-mitotic neuronal cell division

Neurodegenerative diseases (NDDs) are the most common life-threatening disease of the central nervous system and it cause the progressive loss of neuronal cells. The exact mechanism of the disease's progression is not clear and thus line of treatment for NDDs is a baffling issue. During the progression of NDDs, oxidative stress and DNA damage play an important regulatory function, and ultimately induces neurodegeneration. Recently, aberrant cell cycle events have been demonstrated in the progression of different NDDs. However, the pertinent role of signaling mechanism, for instance, post-translational modifications, oxidative stress, DNA damage response pathway, JNK/p38 MAPK, MEK/ERK cascade, actively participated in the aberrant cell cycle reentry induced neuronal cell death. Mounting evidence has demonstrated that aberrant cell cycle re-entry is a major contributing factor in the pathogenesis of NDDs rather than a secondary phenomenon. In the brain of AD patients with mild cognitive impairment, post miotic cell division can be seen in the early stage of the disease. However, in the brain of PD patients, response to various neurotoxic signals, the cell cycle re-entry has been observed that causes neuronal apoptosis. On contrary, the contributing factors that leads to the induction of cell cycle events in mature neurons in HD and ALS brain pathology is remain unclear. Various pharmacological drugs have been developed to reduce the pathogenesis of NDDs, but they are still not helpful in eliminating the cause of these NDDs.

Sirt3-mediated mitochondrial dysfunction is involved in fluoride-induced cognitive deficits

Excessive fluoride is capable of inducing cognitive deficits, but the mechanisms remain elusive. This study aimed to investigate the effects and underlying mechanisms of fluoride on mitochondrial dysfunction and neurobiological alterations, as well as cognitive impairment. C57BL/6 mice were orally administered 25, 50, and 100 mg/L NaF for 90 days. Cultured human neuroblastoma SH-SY5Y cells were exposed to NaF (110 mg/L) for 24 h in the presence or absence of Sirt3 overexpression. The results demonstrated that chronic exposure to high fluoride induced cognitive deficits and neural/synaptic injury in mice. Fluoride reduced mitochondrial antioxidant enzyme activities and elevated SOD2 acetylation by downregulating Sirt3 expression in the brains of mice and NaF-treated SH-SY5Y cells. Moreover, fluoride lowered mtDNA transcription and induced mitochondrial dysfunction along with increased FoxO3A acetylation in the brains of mice and NaF-treated SH-SY5Y cells. Subsequent experiments revealed that overexpression of Sirt3 significantly attenuated the adverse effects of fluoride on radical scavenging capabilities and mtDNA transcription, as well as mitochondrial function in SH-SY5Y cells. These results suggest that chronic long-term fluoride exposure evokes neural/synaptic injury and cognitive impairment through mitochondrial dysfunction and its associated oxidative stress, which is, at least partly, mediated by Sirt3 inhibition in the mouse brain.

Sensitive assessment of groundwater-associated, multi-exposure health hazards in a fluoride-enriched region of West Bengal, India

Ninety groundwater samples were collected from Khayrasole and Rajnagar blocks of Birbhum district, West Bengal, India, during pre-monsoon and post-monsoon in 2016 to assess the hazards of fluoride in groundwater. Fluoride concentration fluctuated from 0.3 to 17.6 mg/L, with 70% of samples reported beyond the modified regional optimal fluoride level (0.7 mg/L) with a statistically significant level of p < 1.7E-24. The average cation and anion concentrations exhibited a descending order of Ca²⁺ > Mg²⁺ > Na⁺ > K⁺ and HCO₃^- > Cl^- > SO₄^2- > NO₃^- > F^-, respectively. Notably, groundwater quality in 50% of the places ranged from poor to unfit for drinking purposes in terms of water quality index. The mean total hazard index (THI) was 1.1 for adults and 1.9 for children, signifying a greater chance of non-carcinogenic threats to both age groups. In calculating the THI, ingestion and dermal pathways accounted for approximately 96% and 3% health hazards, respectively. The Monte Carlo simulation and sensitivity analysis identified that the diurnal water ingestion rate, exposure duration, and fluoride concentration were the significant sensitive variables that triggered most groundwater-associated non-carcinogenic health issues, signifying more risks among children. Further, dental health surveys (N = 746), following Dean's norms for classification based on regional optimal fluoride level, designated the borderline grade of the community dental hazard. The subsequent hydrogeochemical characterization directed that dissolution from fluoride-bearing minerals and water-rock interaction, such as halite dissolution and calcite-dolomite precipitation, were the governing factors for F^- enrichment in groundwater. This study will serve as baseline data for delineating fluoride-induced dental and other health hazards through sensitivity and spatial analysis in the GIS platform for hazard zonation and effective groundwater quality management.

Hesperidin protects liver and kidney against sodium fluoride-induced toxicity through anti-apoptotic and anti-autophagic mechanisms

AIM

High dose of fluoride intake is associated with toxic effects on liver and kidney tissues. One approach to tackle these toxicities is using natural antioxidants as supplements. This study evaluated the ameliorative effects of hesperidin (HSP) against sodium fluoride (NaF)-induced hepatotoxicity and nephrotoxicity in wistar albino rats.

MATERIALS AND METHODS

In the present study, the rats were randomly allocated into five groups of seven male rats each group: control, NaF (600 ppm), HSP-200, NaF + HSP-100 and NaF + HSP 200.

KEY FINDINGS

Hepatic and renal injuries induced by NaF were confirmed by the alteration in kidney function parameters in the serum (urea and creatinine), levels of liver enzymes (ALT, ALP and AST), activities of the antioxidant enzymes (SOD, CAT and GPx) and levels of inflammatory markers (NF-κB, IL-1β and TNF-α). NaF also inhibited PI3K/Akt/mTOR pathway, increased levels of autophagic markers (Beclin-1, LC3A and LC3B) and expression levels of apoptotic and anti-apoptotic proteins (Bax, Bcl-2, cytochrome c, p53 and procaspase-3) in the liver and kidney tissues. Administration of HSP concurrently with NaF significantly ameliorated the deviation in the above-studied parameters.

SIGNIFICANCE

The results of the current study revealed that HSP could be used as a beneficial adjuvant that confers protection against NaF-induced liver and kidney damage through antioxidant, anti-inflammatory, anti-apoptotic and anti-autophagic mechanisms.

Efficacy of bisphosphonates in detection of early enamel caries using NIR fluorescence imaging and inhibition of caries progression

NIR fluorescence imaging using bisphosphonate-Indocyanine green has been indicated for early interproximal caries detection. This study assessed diagnostic accuracy of caries detection by NIR fluorescence imaging with OsteoSense 750^® (OS750) in vitro and ex vivo, and to analyze the therapeutic efficacy of a bisphosphonate (Etidronate) in inhibiting enamel caries progression in vitro. Methods: Four experiments were conducted using extracted human teeth; 1) to calculate the infiltration rate of OS750 into interproximal white spot lesions using fluorescence microscope, 2) to assess diagnostic accuracy of interproximal natural white spot lesions using desktop NIR fluorescence imaging device in vitro setting, 3) to assess diagnostic accuracy of artificially created deeper enamel carious lesion (0.5 mm~1.0 mm) using NIR fluorescence image through the head-mount display in ex vivo setting, 4) to compare the progression on the enamel caries lesions treated by Etidronate, NaF and distilled-water. Diagnostic accuracy was analyzed using sensitivity, specificity and receiver operating curves (ROC). The caries progression was calculated with micro-CT and was statistically analyzed using a two-way ANOVA and the Tukey HDS post-hoc test. Results: 1) The infiltration rate of OS750 was 101.83% ± 8.66 (Min: 90.10%, Max: 133.94%). 2) The average of sensitivity and specificity in vitro setting experiments were 86.7% ± 4.4% and 70% ± 11%, respectively. The average of area under the ROC curves (AUC) was 0.883 ± 0.059 indicating excellent performance. 3) The mean sensitivity and specificity in ex vivo setting was 82.97% ± 15% and 76.78% ± 13.27% respectively. 4) The carious lesion volume treated by Etidronate was significantly smaller at post treatment-1 (p<0.05) and treatment-2 (p<0.01) than the control. There was no significant difference in lesion volume in the Etidronate and NaF group at the time point of post treatment-1. Conclusion: This study suggests that bisphosphonates contribute to both early diagnosis of enamel caries and inhibition of caries progression.

Molecular and Cellular Mechanisms Associated with Effects of Molecular Hydrogen in Cardiovascular and Central Nervous Systems

The increased production of reactive oxygen species and oxidative stress are important factors contributing to the development of diseases of the cardiovascular and central nervous systems. Molecular hydrogen is recognized as an emerging therapeutic, and its positive effects in the treatment of pathologies have been documented in both experimental and clinical studies. The therapeutic potential of hydrogen is attributed to several major molecular mechanisms. This review focuses on the effects of hydrogen on the cardiovascular and central nervous systems, and summarizes current knowledge about its actions, including the regulation of redox and intracellular signaling, alterations in gene expressions, and modulation of cellular responses (e.g., autophagy, apoptosis, and tissue remodeling). We summarize the functions of hydrogen as a regulator of nuclear factor erythroid 2-related factor 2 (Nrf2)-mediated redox signaling and the association of hydrogen with mitochondria as an important target of its therapeutic action. The antioxidant functions of hydrogen are closely associated with protein kinase signaling pathways, and we discuss possible roles of the phosphoinositide 3-kinase/protein kinase B (PI3K/Akt) and Wnt/β-catenin pathways, which are mediated through glycogen synthase kinase 3β and its involvement in the regulation of cellular apoptosis. Additionally, current knowledge about the role of molecular hydrogen in the modulation of autophagy and matrix metalloproteinases-mediated tissue remodeling, which are other responses to cellular stress, is summarized in this review.

Effects of Fluoride Exposure on Primary Human Melanocytes from Dark and Light Skin

Fluoride exposure has adverse effects on human health that have been studied in vitro in cell culture systems. Melanocytes are the melanin pigment-producing cells that have a significant role in the regulation of the process of melanogenesis, which provides several health benefits. Melanocytes are present in the oral cavity, skin, brain, lungs, hair, and eyes. However, to date, there has been no study on the effects of fluoride exposure on melanocytes. Hence, in the current study, we have studied the effects of sodium fluoride (NaF) exposure on neonatal human epidermal melanocytes (HEMn) derived from two different skin phototypes, lightly pigmented (LP) and darkly pigmented (DP). We have assessed the impact of a 24 h and 72 h NaF exposure on metabolic activity and membrane integrity of these cells. In addition, we have evaluated whether NaF exposure might have any impact on the physiological functions of melanocytes associated with the production of melanin, which is regulated by activity of the enzyme tyrosinase. We have also assessed if NaF exposure might induce any oxidative stress in LP and DP melanocytes, by evaluation of production of reactive oxygen species (ROS) and measurement of mitochondrial membrane potential (MMP) levels. Our results showed that HEMn-LP cells showed a higher sensitivity to NaF cytotoxicity than HEMn-DP cells, with significant cytotoxicity at concentrations >1 mM, while concentration range 0.25–1 mM were nontoxic and did not lead to oxidative stress, and also did not alter the levels of intracellular melanin or cellular tyrosinase activity, indicating that treatment up to 1 mM NaF is generally safe to melanocytes from both pigmentation phototypes.

Upregulation of miR-200c-3p induced by NaF promotes endothelial apoptosis by activating Fas pathway

Fluoride has been considered as a risk factor of cardiovascular disease due to its endothelial toxicology. However, the mechanism underlying the endothelial toxicity of fluoride has not been clearly illustrated. MiR-200c-3p was strongly linked with endothelial function and its level is increased in serum of fluorosis patients, but it is unclear the role of miR-200c-3p in the fluoride induced endothelial dysfunction. In this study, we confirmed that fluoride exposure induced the apoptosis of endothelial cells both in established rats model and cultured human umbilical vein endothelial cells (HUVECs). And miR-200c-3p was found to be upregulated in NaF treated HUVECs. Fluoride stimulation increased caspase-dependent apoptosis through miR-200c-3p upregulation, with repressing expression of its target gene Fas-associated phosphatase 1 (Fap-1), which functioned as Fas inhibitor. This resulted in activation of Fas-associated extrinsic apoptosis via interaction with increased Fas, Fadd, Cleaved Caspase-8 and Cleaved Caspase-3. The activation of Fas-associated extrinsic apoptosis was abrogated by miR-200c-3p inhibitor. Furthermore, the antiapoptotic effect of downregulated miR-200c-3p was restored by Fap-1 siRNA. These results suggested a determinant role of the miR-200c-3p/Fap-1 axis in fluoride induced endothelial apoptosis.

Akt inhibition-dependent downregulation of the Wnt/β-Catenin Signaling pathway contributes to antimony-induced neurotoxicity

Antimony (Sb), as a newly identified nerve poison, can lead to neuronal apoptosis. However, its neurotoxicological mechanisms remain largely unclear. Here, we evaluated the role and regulation of Wnt/β-catenin pathway in Sb-mediated neurotoxicity. Under Sb treatment, β-catenin was dramatically downregulated in vivo and in vitro. Moreover, overexpression of β-catenin effectively attenuated Sb-induced survivin gene expression suppression and subsequent apoptosis in PC12 cells. In addition, Sb stimualted glycogen synthase kinase-3β (GSK-3β) activation, shown as decreased phosphorylation levels at Ser 9 both in PC12 cells and mice brain. Paramacological inhibition of GSK-3β using lithium chloride (LiCl) significantly rescued β-catenin expression. For upstream pathway analysis, we found Sb treatment decreased protein kinase B (Akt) phosphorylation, and Akt activator protected PC12 cells from GSK-3β activation and subsequent β-catenin suppression. In summary, our data provided a novel molecular mechanism of Sb-associated neurotoxicity, namely that Sb induces Wnt/β-catenin pathway suppression through Akt inhibition, thus resulted in neuronal apoptosis.

Histone acetyltransferase promotes fluoride toxicity in LS8 cells

Previously we demonstrated that fluoride increased acetylated-p53 (Ac-p53) in LS8 cells that are derived from mouse enamel organ epithelia and in rodent ameloblasts. However, how p53 is acetylated by fluoride and how the p53 upstream molecular pathway responds to fluoride is not well characterized. Here we demonstrate that fluoride activates histone acetyltransferases (HATs) including CBP, p300, PCAF and Tip60 to acetylate p53. HAT activity is regulated by post-translational modifications such as acetylation and phosphorylation. HAT proteins and their post-translational modifications (p300, Acetyl-p300, CBP, Acetyl-CBP, Tip60 and phospho-Tip60) were analyzed by Western blots. p53-HAT binding was detected by co-immunoprecipitation (co-IP). Cell growth inhibition was analyzed by MTT assays. LS8 cells were treated with NaF with/without HAT inhibitors MG149 (Tip60 inhibitor) and Anacardic Acid (AA; inhibits p300/CBP and PCAF). MG149 or AA was added 1 h prior to NaF treatment. Co-IP results showed that NaF increased p53-CBP binding and p53-PCAF binding. NaF increased active Acetyl-p300, Acetyl-CBP and phospho-Tip60 levels, suggesting that fluoride activates these HATs. Fluoride-induced phospho-Tip60 was decreased by MG149. MG149 or AA treatment reversed fluoride-induced cell growth inhibition at 24 h. MG149 or AA treatment decreased fluoride-induced p53 acetylation to inhibit caspase-3 cleavage, DNA damage marker γH2AX expression and cytochrome-c release into the cytosol. These results suggest that acetylation of p53 by HATs contributes, at least in part, to fluoride-induced toxicity in LS8 cells via cell growth inhibition, apoptosis, DNA damage and mitochondrial damage. Modulation of HAT activity may, therefore, be a potential therapeutic target to mitigate fluoride toxicity in ameloblasts.

Fisetin Prevents HT22 Cells From High Glucose-Induced Neurotoxicity via PI3K/Akt/CREB Signaling Pathway

Hyperglycemia has been widely considered as a key risk factor for diabetic encephalopathy which can cause neuronal apoptosis and cognitive deficits. The flavonoid compound, fisetin, possesses potential neuroprotective effects and also enhances learning and memory. However, the role of fisetin in hyperglycemia-induced neuronal cytotoxicity has not been fully elucidated. In the present study, HT22 murine hippocampal neuronal cell line was used to establish the injured cell model. Cell proliferation and cytotoxicity assay, Hoechst 33258 staining, qRT-PCR, western blot analysis, and specific inhibitor were used to investigate the effect and molecular mechanisms of fisetin on high glucose (HG)-induced neurotoxicity in HT22 cells. Our results showed that 125 μM and 48 h of treatment was identified as optimal damage parameter of HG. Fisetin significantly improved HG-inhibited cell viability. The levels of LDH, malondialdehyde (MDA), and superoxide dismutase (SOD) were noticeably modulated by fisetin, which alleviated HG-induced HT22 cell oxidative damage. Besides, the apoptosis of HT22 cells was rescued by fisetin pretreatment. In addition, fisetin also prevented HG-induced downregulation of the mRNA expression of Bdnf, Gdnf, synaptophysin (Syp), and glutamate ionotropic receptor AMPA type subunit 1 (Gria1) in cells. More importantly, the decreased phosphorylation of phosphoinositide 3 kinase (PI3K), Akt, and cAMP-response element binding protein (CREB) was rescued by fisetin treatment and that neuroprotective effect of fisetin was partially blocked by PI3K inhibitor, LY294002. These findings indicate that fisetin has potent neuroprotective effect and prevents HG-induced neurotoxicity by activation of PI3K/Akt/CREB pathway.

Fluoride Affects Dopamine Metabolism and Causes Changes in the Expression of Dopamine Receptors (D1R and D2R) in Chosen Brain Structures of Morphine-Dependent Rats

Disturbances caused by excess or shortages of certain elements can affect the cerebral reward system and may therefore modulate the processes associated with the development of dependence as was confirmed by behavioural studies on animals addicted to morphine. Earlier publications demonstrated and proved the neurodegenerative properties of both low and high doses of fluoride ions in animal experiments and in epidemiological and clinical studies. The aim of the experiments conducted in the course of the present study was to analyse the effect of pre- and postnatal exposure to 50 ppm F⁻ on the initiation/development of morphine dependence. For this purpose, the following were conducted: behavioural studies, the analysis of concentrations of dopamine and its metabolites, and the analyses of mRNA expression and dopamine receptor proteins D1 and D2 in the prefrontal cortex, striatum, hippocampus, and cerebellum of rats. In this study, it was observed for the first time that pre- and postnatal exposure to fluoride ions influenced the phenomenon of morphine dependence in a model expressing withdrawal symptoms. Behavioural, molecular, and neurochemical studies demonstrated that the degenerative changes caused by toxic activity of fluoride ions during the developmental period of the nervous system may impair the functioning of the dopaminergic pathway due to changes in dopamine concentration and in dopamine receptors. Moreover, the dopaminergic disturbances within the striatum and the cerebellum played a predominant role as both alterations of dopamine metabolism and profound alterations in striatal D1 and D2 receptors were discovered in these structures. The present study provides a new insight into a global problem showing direct associations between environmental factors and addictive disorders.

Glycogen synthase kinase-3 as a key regulator of cognitive function

Glycogen synthase kinase-3 (GSK-3) is a highly conserved and multifunctional serine/threonine protein kinase widely distributed in eukaryotic cells. GSK-3 is originally thought to be an enzyme that regulates glycogen synthesis. It was subsequently found that GSK-3 influences many critical cellular functions, such as cell structure, neural plasticity, gene expression, and neuronal survival. Recently, GSK-3 has been found to be associated with cognition, and its dysregulation leads to cognitive impairments in many diseases, including Alzheimer's disease, diabetes, depression, Parkinson's disease, and others. In this review, we summarized the current knowledge about the structure of GSK-3, the regulation of GSK-3 activity, and its role in cognitive function and cognitive-related disease.

Hydrogeochemical characterisation and health hazards of fluoride enriched groundwater in diverse aquifer types

High concentration of fluoride (up to 20.9 mg/L) in groundwater with significant variation (p = 5.9E-128) among samples was reported from Birbhum district, an acknowledged fluoride endemic region in India. The groundwater samples (N = 368) were grouped based on their hydrochemical properties and aquifer geology for hydro-geochemical characterization. Friedman's test showed p < 0.0001 confidence level which indicates that fluoride concentration among geological groups and water groups are independent. Bland-Altman plot was used to study the inter-relationships among the groups through bias value (∂) and limit of agreement (LoA). Among the geological groups, laterites and granite-gneiss groups exhibited statistically significantly difference in fluoride geochemistry; whereas the younger and older alluvium groups displayed similar characteristics. The fluoride concentration was found to be in the order Lateritic > Granite-gneiss > Older alluvium ≥ Younger alluvium. Dissolution of minerals (such as fluorite, biotite) in laterite sheeted basalt, and granite-gneiss is the main source of groundwater fluoride in the region. Fluoride concentration is also influenced by depth of water table. Hydrochemical study indicated that fluoride concentration was higher in Na-HCO₃ than in Ca-SO₄ and Ca-HCO₃ type of groundwater. The fluoride concentration were positively correlated with Na⁺ and pH and negatively correlated with the Ca²⁺ and Mg²⁺ signifying linkage with halite dissolution and calcite, dolomite precipitation. Geostatistical mapping of WQI through empirical bayesian kriging (EBK) with respect to regional optimal guideline value (0.73 mg/L) classified that groundwater in some parts of the district are unfit for drinking purpose. Health survey (N = 1767) based on Dean's criteria for dental fluorosis indicated presence of slight to moderate dental hazard. Besides, providing baseline data for management of groundwater quality in the study area, the study demonstrated the applicability of Bland-Altman analysis and empirical bayesian kriging (EBK) in delineation and interpolation of fluoride contaminated region.

Hydrogen exerts neuroprotection by activation of the miR-21/PI3K/AKT/GSK-3β pathway in an in vitro model of traumatic brain injury

Few studies have explored the effect of hydrogen on neuronal apoptosis or impaired nerve regeneration after traumatic brain injury, and the mechanisms involved in these processes are unclear. In this study, we explored neuroprotection of hydrogen‐rich medium through activation of the miR‐21/PI3K/AKT/GSK‐3β pathway in an in vitro model of traumatic brain injury. Such model adopted PC12 cells with manual scratching. Then, injured cells were cultured in hydrogen‐rich medium for 48 hours. Expression of miR‐21, p‐PI3K, p‐Akt, p‐GSK‐3β, Bax and Bcl‐2 was measured using RT‐qPCR, Western blot analysis and immunofluorescence staining. Rate of apoptosis was determined using TUNEL staining. Neuronal regeneration was assessed using immunofluorescence staining. The results showed that hydrogen‐rich medium improved neurite regeneration and inhibited apoptosis in the injured cells. Scratch injury was accompanied by up‐regulation of miR‐21, p‐PI3K, p‐Akt and p‐GSK‐3β. A miR‐21 antagomir inhibited the expression of these four molecules, while a PI3K blocker only affected the three proteins and not miR‐21. Both the miR‐21 antagomir and PI3K blocker reversed the protective effect of hydrogen. In conclusion, hydrogen exerted a neuroprotective effect against neuronal apoptosis and impaired nerve regeneration through activation of miR‐21/PI3K/AKT/GSK‐3β signalling in this in vitro model of traumatic brain injury.

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.

Fluoride contamination in groundwater and associated health risks in Karbi Anglong District, Assam, Northeast India

Health hazards due to excess intake of fluoride via groundwater contamination are a major concern worldwide. This study provides a comprehensive report on the human health risks associated with the consumption of groundwater contaminated by fluoride. Several groundwater samples were collected across 8 blocks of Karbi Anglong district of Assam, India. The concentration of fluoride was observed in the range of 0.15-17.13 mg/L. In 4 out of 8 studied blocks, the mean fluoride level exceeded the permissible limit (1.5 mg/L) as prescribed by the World Health Organization. Elevated fluoride levels in some parts of the district may be attributed mainly to dissolution from fluoride-containing minerals in the granitic rocks and regional geological settings. The health risk of fluoride was assessed in terms of hazard quotient (HQ). The HQ was observed in the ranges of 0.06-10.7 (adult) and 0.2-35 (children). Mean HQ values exceeded the safe level (HQ > 1) for children in all blocks, except B-6 and B-8. For adult population, the HQ value was above the safe limits in 13-40% of the sampled locations in different blocks and HQ values were within safe limits in B-6 and B-8. These findings suggest that some sites in the district need serious attention in order to ensure the health safety of local residents.

Exploring a multi-exposure-pathway approach to assess human health risk associated with groundwater fluoride exposure in the semi-arid region of east India

Human health risks associated with groundwater fluoride have been assessed using USEPA method in few parts of India, but those assessments were conducted based on a single value for each parameter, which may lead to certain inaccuracy and uncertainties in results. In this study, a higher degree of accuracy in health risk assessment was achieved through Monte Carlo simulations, sensitivity analysis and uncertainty analysis. As fluoride hazards to human health are chronic, a total of 4560 water samples (N = 4560) were collected during consecutive four seasons (2 pre-monsoon and 2 post-monsoon seasons; 1140 samples/season) from the entire Birbhum district, covering all the blocks and geological settings to obtain the spatiotemporal variation of fluoride level. The Empirical Bayesian Kriging geostatistical model was employed to determine fluoride endemic areas. Amongst all blocks, Nalhati-1 had exhibited the highest fluoride level (18.25 mg/L). The study revealed that most of the blocks are vulnerable to groundwater fluoride due to its occurrences in excess level. The average and 95th percentile values of total hazard index indicate that the infants and children populations of the district are more susceptible than the adults and teens. The sensitivity analysis revealed that water ingestion rate and fluoride concentration are the most influential parameters for higher risk of fluoride-related health hazards. Health risks were evaluated through ingestion and dermal exposure routes for infants, children, teens, and adult residents. The exposure was much higher through ingestion than dermal contacts. Ingestion of defluoridated water will reduce the health risks associated with groundwater fluoride exposure.