Deficit in learning and memory of rats with chronic fluorosis correlates with the decreased expressions of M1 and M3 muscarinic acetylcholine receptors

Abstract

To reveal the molecular mechanism of deficit in learning and memory induced by chronic fluorosis, the expression of muscarinic acetylcholine receptors (mAChRs) and oxidative stress were investigated. Sixty Sprague-Dawley (SD) rats were divided randomly into two groups (30 cases in each), i.e., the control group (<0.5 ppm fluoride in drinking water) and the fluoride group (50 ppm fluoride) for 10 months of treatment. The pups born from SD mothers with or without chronic fluorosis were selected at postnatal days 1, 7, 14, 21 and 28 for experiments (10 for each age). Spatial learning and memory were evaluated by Morris water maze test. The expressions of M1 and M3 mAChRs at the protein and mRNA levels were determined by Western blotting and real-time PCR, respectively. In addition, the contents of (·)OH, H2O2, O2(·-) and malondialdehyde (MDA), and activities of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) in brains were quantitated by biochemical methods. Our results showed that as compared to controls, the abilities of learning and memory were declined in the adult rats and the offspring rats of postnatal day 28 in the fluoride groups; the expressions of both M1 and M3 mAChRs were significantly reduced at protein and mRNA levels; and the levels of (·)OH, H2O2, O2(·-) and MDA were significantly increased, while the activities of SOD and GSH-Px decreased. Interestingly, the decreased protein levels of M1 and M3 mAChRs were significantly correlated with the deficits of learning and memory and high level of oxidative stress induced by chronic fluorosis. Our results suggest that the mechanism for the deficits in learning and memory of rats with chronic fluorosis may be associated with the decreased expressions of M1 and M3 in mAChRs, in which the changes in the receptors might be the result of the high level of oxidative stress occurring in the disease.

Exposure to fluoride aggravates the impairment in learning and memory and neuropathological lesions in mice carrying the APP/PS1 double-transgenic mutation

“…Therefore, the doses of fluoride exposed to mice were equivalent to 1.5 ppm (close to the drinking water standard set by WHO) and 15 ppm, respectively, in drinking water for humans. In previous studies by our group, we observed that the rats with chronic fluorosis exhibited a significantly declined ability of learning and memory [24,25]. In the behavioral test, both the WT and APP/PS1 double-transgenic mice exposed to a high level of fluoride performed more poorly than the controls, indicating that impairment of spatial learning and memory.…”

“…Oxidative stress is often defined as an imbalance between free radical formation and their removal by cellular anti-oxidative systems, which leads to accumulation of toxic oxy-radicals, a process that plays a key role in both fluorosis and AD [55][56][57][58]. In recent years, it has been recognized that higher concentrations of fluoride result in neuronal dysfunction by elevating levels of free radicals while reducing the activities of antioxidant enzymes and contents of antioxidant compounds [24,25]. Oxidative stress damages all classes of organic molecules in the cells, causing excessive lipid peroxidation and protein oxidation in the brains of patient with AD [59,60].…”

“…Appropriate fluoride exposure and usage is beneficial to tooth integrity and, as such, has an important, positive impact on health throughout life [65], while water fluoridation also brings about negative impact such as the chronic toxicity of fluoride [66]. Chronic fluorosis has been found to cause damage to the central nervous system and cognitive impairment [20,21,[24][25][26][27]67], but the relationship between fluoride and AD is still elusive. In present study here, we found that long-term exposure to fluoride (especially exposure to low fluoride) could enhanced the deficit in Table 3 The MDA content and activities of SOD and GSH-Px in the brains of mice following 12 weeks of exposure to fluoride learning and memory and raised the numbers of senile plaque in mice carrying APP/PS1 double-transgenic mutation, which suggesting that long-term exposure to fluoride may be considered as risk factor of in development of AD.…”

“…Fluoride injures the central nervous system (CNS) by several mechanisms [18][19][20], in particular by elevating the level of oxidative stress [21][22][23][24]. Earlier studies in our own laboratory have documented direct toxic effects on the brains of experimental animals exposed to high levels of fluoride, including enhanced oxidative stress, reduction in the levels of nicotinic and muscarinic acetylcholine receptors, and mitochondrial abnormalities, along with impaired learning and memory [21,[24][25][26][27]. Moreover, fluoride increases lipid peroxidation and decreases the activity of antioxidant enzymes in rats, causing neurotoxicity, even in the second and third generations following exposure [28].…”

Original Research: Influence of okadaic acid on hyperphosphorylation of tau and nicotinic acetylcholine receptors in primary neurons

“…24 In brief, 200 ml of the supernatant prepared as described above were placed into a micro-centrifuge tube and 600 ml of the TBARS solution then added. This mixture was incubated at 95 C for 60 min and cooled to room temperature in an ice bath for 10 min.…”