Previous studies have shown that As has the property of accumulating in tissues. In animal experiments with arsenate or arsenite, the concentration of As in different tissues and organs of animals increases, such as kidney, liver, lung and spleen[33-35]. As is neurotoxic and causes CNS dysfunction, such as cognitive dysfunction, behavioral deficits and mood disorders[36]. However, there are few reports on As accumulation in the hippocampus and cortex following chronic As exposure and whether As exposure disrupts changes in other trace elements in the hippocampus and cortex.
In the present study, we found that As levels in hippocampal and cortical tissues of rats in the arsenic-exposed group were significantly higher than those in the control group, and that As levels in the cortex were higher than those in the hippocampus. This suggests that As can accumulate in hippocampal and cortical tissues after exposure, and is distributed differently in the hippocampus and cortex. As exposure can cause a variety of neurotoxic effects through various molecular mechanisms, including oxidative stress, energy depletion, mitochondrial dysfunction, epigenetic alterations, neurotransmitter homeostasis and synaptic transmission, cell death pathways and inflammation, resulting in significant changes in hippocampal and cortical function, morphology and signaling, with severe effects on learning, memory and cognitive behavior in humans and animals[32, 37-40]. Although the pattern of As exposure and concentrations vary widely from study to study, the overall results are consistent. On the other hand, the literature reports that As can accumulate in different parts of the rodent brain, such as the hippocampus, cerebral cortex, striatum and midbrain, and that the distribution of As levels varies between locations[41]. Similar to this study, a previous study has observed higher levels of total As in the rat cortex than in the hippocampus following As exposure, and different mechanisms of neurotoxicity resulting from As exposure in the hippocampus and cortex[42]. Not only that, another study observed tissue specificity of As and its metabolites of methylation in the hippocampus and cortex of mice following a single oral dose of arsenite, with dimethylarsenate (DMA) being the only metabolite of As methylation in the cortex. In contrast, very high concentrations and percentages of monomethylarsenic (MMA) were found in the hippocampus[43]. These results indicate that the different distribution of As and even its metabolites in different brain regions after exposure, as well as the resulting tissue specificity or neurotoxic differences, deserve further study.
In this study, As exposure led to changes in some other trace elements, such as Se and Cd. Se is an essential trace element for mammals. Through selenoproteins, this mineral is involved in various biological processes such as antioxidant defense, thyroid hormone production and immune responses[44]. Se has been found to prevent oxidative damage to cellular components through the corresponding selenoproteins with antioxidant activity, such as glutathione peroxidase (GSH-Px) and thioredoxin reductase (TrxR)[45, 46]. It may have a protective effect against oxidative damage in the CNS[47], and improve mood and enhance energy and cognitive function[48]. Therefore, reduced brain Se levels may lead to disturbances in certain functions of the CNS. The effects of As on human Se levels have been extensively studied. A previous study reported that the Se level of patients with chronic As poisoning was lower than that of the unaffected controls[49]. Another study examined blood and urine samples from adults in areas with arsenic-contaminated groundwater and found a negative correlation between plasma Se concentrations and whole blood and urine As concentrations[50]. A previous study has also found that the concentration of Se in the brain of mice exposed to As2O3 was significantly lower than that of the control group[51]. Similarly, in the present study, Se concentrations in the hippocampus and cortex of arsenic-exposed rats were significantly lower than those in the control group, suggesting that As exposure may reduce Se levels in the brain.
Many studies have shown interactions between As and Se that may explain the mechanisms by which Se is reduced during As exposure. Arsenite and selenite form selenite bis (S-glutathione) arsenic ions through in vivo antagonism as the selenium compound [(GS)2AsSe], which is subsequently excreted in the bile[52]. Arsenic and selenium complexes in the form of selenotoxic (S-glutathione) arsenic ions have been identified in the bile of rabbits and rats[53, 54]. In addition, since both elements have similar physical and chemical properties, e.g., Se4+ and As3+ both have the same electronic structure (neither of their 4p and 4d orbitals has an electron), leading to a significant inhibition of selenite (Se4+) uptake by arsenite (As3+) in tissue or organelle membranes[55]. Moreover, both elements are methylated in vivo via one-carbon metabolism to facilitate excretion via urine[56, 57], suggesting potential pathways of interaction.
Cd is a non-essential metal element in the human body and is a class of carcinogen. Studies have shown that Cd can cross the blood-brain barrier in rats[58] and accumulate in the adult rat brain, causing neurological damage[59]. Our results showed that Cd levels in hippocampus and cortex of arsenic-exposed group increased significantly. Cd induces cellular oxidative stress, increases the production of prooxidants, decreases the body's antioxidant capacity and leads to lipid peroxidation, DNA damage and sulfhydryl (SH) depletion[60, 61]. It has been reported that Cd-induced ROS production mediates mitogenesis in the mouse brain as an upstream signal, leading to cadmium neurotoxicity[62]. Cd exposure leads to increased lipid peroxidation and antioxidant enzyme activity in the rat hippocampus, resulting in altered neuronal morphology in hippocampal cells and affecting cognitive tasks[58]. GSH concentrations and SOD (superoxide dismutase) activity in the cortex of cadmium-treated rats were significantly reduced, suggesting that Cd weakens the antioxidant defense system in the brain, leading to the production of ROS[63]. In addition, Cd binds to metallothionein (MT-III) and brain-derived neurotrophic factor (BDNF) in the brain, thereby reducing their activity and also leading to increased levels of ROS[64]. Cd can also cause apoptosis of neurons in the cerebral cortex[65]. In vitro experiments showed that Cd induced PC12 and SH-SY5Y cells to produce ROS in a time-and concentration-dependent manner, leading to neuronal apoptosis[66, 67]. There is growing evidence that cadmium-induced neuronal toxicity is due to the induction of ROS, leading to oxidative stress, which is the main mechanism of cadmium neurotoxicity.
In addition to Se and Cd mentioned above, in the hippocampus, increased concentrations of Rb and Ho were also observed, as well as decreased concentrations of Au, Ba, Ce, Cs, Pd, Sr and Th. In the cortex, the levels of Rb were increased, and Au were decreased. The effects of As exposure on these elements have not been reported. However, oxidative stress is the main mechanism by which As poisoning causes damage to the body, and some of these elements have been reported to be associated with oxidative stress, suggesting that they have influence on neurotoxicity caused by As. For example, Ce and Ba have been reported to be associated with oxidative stress. CeO2 nanoparticles reduce the production of ROS, inhibit inflammation and maintain the levels of antioxidant enzymes and glutathione in the biological system[68]. BaCl2 and BaCO3 significantly reduce the activity of antioxidant enzymes, resulting in the inability of the antioxidant defense system to reduce ROS[69, 70]. These results suggest that As exposure may lead to alterations in the levels or distribution of some trace elements in the hippocampus and cortex of rats.
Subsequently, we analyzed elemental interactions to further elucidate the relationship between elemental disorders associated with As exposure and As toxicity. In neurotoxicity studies, it has been found that there are interactions among various metals[71]. As and cadmium may have synergistic effects on several key neurotoxic events, including oxidative stress[72, 73] and apoptosis[18, 74]. Synergistic effects of some learning disability-related genes (BDNF, CASP3, CAT, HMOX1, and MAPK1/MAPK3) have also been identified, and combined As and Cd exposure may lead to synergistic activation or inactivation of pathways associated with the expected neurotoxicity[75]. In addition, there are interactions between arsenic and other elements such as lead, selenium, manganese, and methylmercury[76, 77] that have synergistic or antagonistic effects in the neurotoxic damaging effects induced by arsenic exposure. Combined exposure to high levels of Se and As can significantly reduce the risk of disease caused by exposure to As alone. Oxidative stress is one of the key mechanisms in the pathogenesis of As, and Se is thought to act as an antidote to As toxicity primarily through its antioxidant effects[26], and Se supplementation has been reported in the treatment or adjunctive therapy of patients with As toxicity[78, 79]. Lead counteracts the toxicity of As, which has been found to have a greater toxic effect on neurobehavioral and biochemical changes than lead, and there may be an antagonistic effect between the action and accumulation of these two toxicants[27]. In the present study, a significant positive correlation between As and Cd was observed in the arsenic-exposed group in the hippocampus, while a significant positive correlation between As and Rb was observed in both the hippocampus and cortex. Together, these results suggest that As exposure disrupts a variety of trace elements in the hippocampus and cortex. However, the neurotoxicity or mechanism of action associated with their interaction needs further investigation.
There is growing evidence of gender differences in the health effects of As. In this study, there were no sex differences in the distribution of As in the hippocampus or cortex of rats. However, there were sex differences in the effects of As on rubidium, gold, barium, cerium, palladium, strontium and thallium concentrations in the rat hippocampus, as well as rubidium and molybdenum concentrations in the cortex. Similarly, a previous study found sex differences in the distribution of elements (Cu, Se) in the rat brain and cerebellum following subchronic As exposure[28]. Studies have shown that males are more susceptible to arsenic-related skin effects than females[80],and women have a higher risk of arsenic-induced bladder and kidney cancers than men[81]. Gender differences in As biotransformation through methylation have also been reported, with females being more effective than males in As methylation[82, 83]. Animal studies have shown lower levels of As methylation in male rats than in female rats, and higher levels of the As metabolite MMA in the same group of males than in females[84]. Compared to female rats, arsenic-treated males exhibited reduced hippocampal neurons and increased and altered apoptosis of the BMP2/Smad and BDNF/TrkB pathways at high doses of As, whereas endogenous E2 modulated hippocampal BMP and BDNF signaling and inhibited arsenic-induced neuronal dysfunction in female rats[85]. However, there are few reports on sex differences in As distribution and their effects on trace element levels in the brain. Our findings suggest that As exposure may lead to sex differences in trace element disorder levels in the hippocampus and cortex of rat.