Prenatal methylmercury exposure results in dendritic spine dysgenesis in rats.

To test the prenatal neurotoxic effect of low doses of methylmercury chloride (MM), the toxic agent was given by gavage to pregnant Wistar rats on days 6 to 9 after conception in doses of 0.025, 0.05, 0.5 and 5.0 mg/kg/day. The offspring of these animals were subjected to a routine developmental and behavioral testing battery. In essence, functional changes including impaired swimming behavior, increased auditory startle amplitude, increased passiveness and increased locomotor stereotypy compared to controls were the result of prenatal MM exposure, tested from the second to the seventh month postnatally. Further, in each group, ten prenatally treated rats were investigated histopathologically at the age of twelve months. The most striking effect was a distinct difference in the morphology of the dendritic spines of the pyramidal neurons in the somatosensory cortex demonstrated by Golgi impregnation. The spine abnormalities in the experimental animals consisted of a reduction of stubby and mushroom-shaped spines and a predominance of long and tortuous spines. Dendritic spine dysgenesis implies defective development and might be the pathological feature of the impaired behavior and learning of the MM-exposed animals.