Previous studies showed learning and
memory deficit in adult rats that were prenatally exposed to
methylmercury chloride (MMC) in an advanced stage of pregnancy (15 days). Under these conditions, the cognitive deficits found at 60 days of age paralleled particularly changes in the
N-methyl-D-aspartate (
NMDA) receptor characteristics. In the present study, we report the behavioral effects of a single oral dose of MMC (8 mg/kg) administered earlier at gestational day 8. The use of different learning and memory tests (passive avoidance, object recognition, water maze) showed a general
cognitive impairment in the in utero-exposed rats tested at 60 days of age compared with matched controls. Considering the importance of the glutamatergic receptor system and its endogenous
ligands in learning and memory process regulation, we surmised that MMC could affect the gene expression of
NMDA receptor subtypes. The use of a sensitive
RNase protection assay allowed the evaluation of gene expression of two families of
NMDA receptors (NR-1 and NR-2 subtypes). The result obtained in 60-day-old rats prenatally exposed to MMC, showed increased
mRNA levels of the NR-2B subunit in the hippocampus but not in the frontal cortex. The data suggest that the behavioral abnormalities of MMC-exposed rats might be ascribed to a neurotoxic effect of the
metal that alters the gene expression of a specific
NMDA receptor subunit in the hippocampus.