Effects of fluoride on the endocannabinoid system, stress and hypothalamic-pituitary-adrenal axis

Fluoride compounds are commonly used today for the treatment of osteoporosis and to prevent dental caries. To this end, drinking water with low fluoride concentrations (<0.5 ppm) is fluoridated, or topical or systemic fluoride applications are employed. This practice can lead to exceeding the daily optimal dose and may result in various systemic effects in the body at high doses. The endocannabinoid System (ECS), considered an important neuromodulatory system in maintaining brain homeostasis, is critical in controlling motor coordination, learning and memory, emotion and motivation, addiction, and pain modulation and coordinating the HPA axis during stress responses. Our study used behavioral and biochemical methods to investigate the effects of fluoride doses, which are considered optimal and those leading to fluorosis, on the ECS, HPA axis, and stress response. Our study used 40 Balb-c mice, randomly assigned into four groups: Control Group, Stress Group, Low Fluoride+Stress Group, and High Fluoride+Stress Group, with 10 animals in each group. Throughout the 45-day experiment, the control and stress groups were given deionized drinking water, the Low Fluoride+Stress Group was given deionized drinking water containing 10 mg/L NaF, and the High Fluoride+Stress Group was given deionized drinking water containing 50 mg/L NaF. At the end of the 45th day, a predator-based stress model was applied to all experimental groups except the control group. The Elevated Plus Maze (EPM) test was conducted on the animals, blood samples were collected, and the animals were sacrificed. Brain tissue was extracted, and the hippocampus and amygdala regions were isolated. Statistical analysis was performed using the Kruskal-Wallis Variance Analysis for differences between groups and the Mann-Whitney U Test for pairwise comparisons. Our study showed that serum corticosterone levels in the stress group animals were significantly higher than in the control group (p<0.05). Moreover, serum corticosterone levels were significantly lower in the low fluoride+stress group (p<0.05) and the high fluoride+stress group (p<0.01) compared to the stress group. According to the Elevated Plus Maze test results, the time spent in the open arm by the stress group was significantly reduced, and the time spent in the closed arm was significantly increased compared to the control group. Regarding the components of the endocannabinoid system, it was found that arashidoniletanolamin (AEA) levels significantly decreased in both the hippocampus and amygdala of the high fluoride+stress group compared to the low flour+stress group, 2- arashidonoilglyserol (2-AG) levels decreased in the hippocampus of the high fluoride+stress group. At the same time, no change was observed in the amygdala. There was no significant change in fat acid amin hydrolase (FAAH) levels between the hippocampus and amygdala groups. This study is the first to investigate the effects of fluoride on the ECS, HPA axis, and stress response. Keywords: Fluoride, endocannabinoid system, HPA axis, stress response