Effects of melatonin on learning and memory abilities of fluoride-exposed offspring rats / 中华地方病学杂志

ABSTRACT

Objective:To investigate the effect of melatonin (MEL) on learning and memory abilities of fluoride-exposed offspring rats and the role of gut microbiota.Methods:Twelve 8-week-old Sprague-Dawley (SD) rats (8 females and 4 males) with a body weight ranging from 180 to 220 g were selected and divided into control group 1 and fluoride-exposed group 1 using a random number table method, with 6 rats in each group (female ∶ male = 2 ∶ 1). They were free to drink purified water or purified water containing 100 mg/L sodium fluoride, respectively. After 2 months, male and female rats were raised together in cages, and the first postnatal day (PND) of the offspring rats was recorded as PND0. In PND21, the offspring rats of fluoride-exposed group 1 were divided into fluoride-exposed group (Group F, n = 6) and fluoride + MEL group (Group FM, n = 6) using a group design, and continued to be exposed to fluoride through drinking water. The offspring rats of control group 1 were divided into control group (Group C, n = 6) and MEL group ( n = 6). The groups FM and MEL were given 20 mg/kg MEL by gavage, while the groups C and F were given the same dose of normal saline by gavage. In PND60, novel object recognition and Morris water maze tests were used to observe the learning and memory abilities of the offspring rats. Western blotting (WB) was used to detect the expression level of brain derived neurotrophic factor (BDNF) in the hippocampus of the offspring rats. And 16S rDNA sequencing technology was used to detect the changes in the structure and composition of gut microbiota in fecal samples. Results:The results of novel object recognition test showed that there was a statistically significant difference in the discrimination index (DI) among the four groups of offspring rats ( F = 3.95, P = 0.024). The DI in groups C and FM was higher than that of Group F ( P < 0.05). The results of Morris water maze test showed that compared with Group C, the platform-crossing time of the offspring rats of Group F were less and they had a longer time to reach the platform for the first time ( P < 0.05). Compared with Group F, the platform-crossing time of the offspring rats of Group FM were increased and they had a shorter time to reach the platform for the first time ( P < 0.05). The WB results showed that compared with Group C (1.00 ± 0.07), the expression level of BDNF protein in Group F (0.68 ± 0.26) was lower ( P < 0.05). Compared with Group F, the expression level of BDNF protein in Group FM (0.99 ± 0.14) was higher ( P < 0.05). Anosim similarity analysis showed significant differences in the structure and composition of gut microbiota in the four groups of offspring rats ( R = 0.395 062, P = 0.002). The distribution characteristics of gut microbiota species showed that at the phylum level, compared with Group C, the relative abundance of Bacteroidetes in Group F increased from 14.26% to 37.00%, and the relative abundance of Firmicutes decreased from 68.78% to 45.95%. Compared with Group F, the relative abundance of Firmicutes in Group FM increased from 45.95% to 65.26%, and the relative abundance of Bacteroidetes decreased from 37.00% to 23.00%. At the genus level, compared with Group C, the relative abundance of Lactobacillus, Dubosiella, HT002 and UCG-005 in Group F was lower, while the relative abundance of unclassified Muribaculaceae was higher. Compared with Group F, the relative abundance of Lactobacillus, Dubosiella, HT002 and UCG-005 in Group FM was higher, while the relative abundance of unclassified Muribaculaceae was lower. The results of linear discriminant analysis revealed that the Candidatus-Saccharimonas and Incertae-Sedis were significantly enriched in Group C, unclassified Muribaculaceae and Muribaculum were significantly enriched in Group F, and Allorhizobium- Neorhizobium- Pararhizobium- Rhizobium were significantly enriched in Group FM. Conclusion:MEL can improve the learning and memory impairment of offspring rats induced by fluoride exposure by changing the structure and composition of gut microbiota.