Objective To study the effects of postnatal exposure to methylmercury on learning, memory and the expression of N-methyl-D-aspartate (NMDA) receptor mRNA in rats.
Methods The rats treated with 5 mg/kg methylmercury chloride (MMC) were taken as MMC-treated group and those with 5 mL/kg sodium chloride solution as control group, by oral for 7 consecutive days. The MMC-treated group was subdivided by different postnatal periods (PND7, PND14, PND28 and PND60) and tested for learning and memory with Morris water maze. The concentration of mercury was checked by atomic fluorescence spectrometry (AFS). Expression of NMDA receptor 2A, 2B and 2C subunits were measured by semi-quantitative reverse transcript polymerasechain reaction (RT-PCR).
Results Mercury concentrations in both hippocampus and cerebral cortex were elevated significantly in the MMC-treated group (P < 0.01); while the level of mercury in cortex of PND28 and both cortex and hippocampus of PND60 was significantly higher in comparison with that of the control (P < 0.01). The latency of PND7 and PND14 MMC-treated rats in water maze was much longer than that of the control (P < 0.01). The levels of NR2A mRNA were significantly decreased in both hippocampus and cortex of PND14 subgroup (P < 0.05); the levels of NR2B mRNA were significantly reduced in hippocampus of PND14 subgroup (P < 0.01); and an increment of NR2C mRNA was observed in hippocampus of PND14 and in cortex of PND14 and PND28 subgroups (P < 0.01).
Conclusion The first two weeks after birth is critical to methylmercury exposure, which would result in the impairment of learning and memory ability for life span. The altering expression of NR2 subunits is an influencing factor.
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