Early Developmental Low-Dose Methylmercury Exposure Alters Learning and Memory in Periadolescent but Not Young Adult Rats

Abstract

Few studies have assessed the effects of developmental methylmercury (MeHg) exposure on learning and memory at different ages. The possibility of the amelioration or worsening of the effects has not been sufficiently investigated. This study aimed to assess whether low-dose MeHg exposure in utero and during suckling induces differential disturbances in learning and memory of periadolescent and young adult rats. Four experimental groups of pregnant Sprague-Dawley rats were orally exposed to MeHg or vehicle from gestational day 5 to weaning: (1) control (vehicle), (2) 250 μg/kg/day MeHg, (3) 500 μg/kg/day MeHg, and (4) vehicle, and treated on the test day with MK-801 (0.15 mg/kg i.p.), an antagonist of the N-methyl D-aspartate receptor. The effects were evaluated in male offspring through the open field test, object recognition test, Morris water maze, and conditioned taste aversion. For each test and stage assessed, different groups of animals were used. MeHg exposure, in a dose-dependent manner, disrupted exploratory behaviour, recognition memory, spatial learning, and acquisition of aversive memories in periadolescent rats, but alterations were not observed in littermates tested in young adulthood. These results suggest that developmental low-dose exposure to MeHg induces age-dependent detrimental effects. The relevance of decreasing exposure to MeHg in humans remains to be determined.

Figure 1

Experimental design. Pregnant rats were orally exposed to MeHg (250 or 500 μg/kg/day) from gestational day (GD) 5 to weaning (postnatal day- (PND-) 21). All exposure to MeHg was stopped after weaning. The learning and memory tasks were performed at PND-40 (periadolescence) and PND-90 (young adulthood), using a different cohort of rats for each task and evaluation time. ^∗ n = 10 per experimental group; ^∗∗ n = 8 per experimental group.

Figure 2

The effects of methylmercury on locomotor and exploratory behaviour. OFT was performed at PND-40 (a-b) and PND-90 (c-d). Graphs (a) and (c) show the total distance travelled (cm), and graphs (b) and (d) show the number of rearings (mean ± SEM) at PND-40 and PND-90, respectively. The data were analysed using one-way ANOVA followed by Bonferroni post hoc tests. Significant differences compared with the control group (^∗ p < 0.05; ^∗∗∗ p < 0.001) or with the group exposed to 250 μg/kg/day MeHg (^# p < 0.05) are indicated.

Figure 3

Methylmercury exposure disrupted recognition memory. ORT was performed at PND-40 (a) and PND-90 (b). The graphs show the recognition index (RI: time exploring new object/total exploration time) as the mean ± SEM; an RI below 0.50 indicates that the animal spent more time exploring the familiar object, and an RI above 0.50 indicates that the animal spent more time exploring the new object. The results were analysed using one-way ANOVA and Bonferroni post hoc tests. Significant differences compared with the control group (^∗ p < 0.05; ^∗∗ p < 0.01; ^∗∗∗ p < 0.001) or with the group exposed to 250 μg/kg/day MeHg (^# p < 0.05) are indicated.

Figure 4

Methylmercury exposure altered spatial learning. Graphs show the learning curve normalized as a percentage of the latency of the group on the first day of training (mean ± SEM) at PND-40 (a) and PND-90 (b). Escape latency results were analysed using two-way ANOVA, followed by Bonferroni post hoc tests; statistically significant differences compared with the control group are indicated (^∗∗ p < 0.01; ^∗∗∗ p < 0.001).

Figure 5

Methylmercury exposure disturbed the acquisition of aversive memories. The aversion index (AI) (mean ± SEM) of rats at PND-40 (a) and PND-90 (b). The data were analysed using two-way ANOVA followed by Bonferroni post hoc tests; significant differences compared with the control group are indicated (^∗∗ p < 0.01; ^∗∗∗ p < 0.001).