Behavioral effects of prenatal methoxy-ethyl-mercury chloride exposure in rat pups

A number of neurotoxic drugs, when administered prenatally, induce neurobehavioral impairments and cause delay of the development of central nervous functions, without morphological malformations. Experiments were undertaken to clarify the behavioral teratogenicity of the fungicide methoxy-ethyl-mercury chloride (MEMC). CFY rat dams were treated with different doses of MEMC during 7th-15th days of gestation (2.0, 0.62 and 0.02 mg/kg daily), perorally. Development of gait, motor coordination, behavior patterns in an open field test, swimming, and conditioned avoidance learning were tested at different ages of rat pups. MEMC did not cause any mortality of dams, but there was a mild ataxia at the 2.0 mg/kg treatment. While birthweight, number of offspring, ear-eye opening and gait were normal, unexpectedly high mortality occurred perinatally. After weaning, open field behavior was nearly normal, there was a mild decrease of rearing, grooming and ambulation and an initial preference for the periphery of the open field decreased. Ambulation increased significantly in 90-day-old pups. Motor coordination on a rotorod decreased in 23- and 36-day-old pups, but increased in 90-day-old pups at the 2.0 mg/kg dose. There was no difference among groups in amphetamine sensitivity tested in a swim stress test. During avoidance conditioning, pups treated with the two higher doses performed poorly when compared to controls and the latency of the positive conditioned response was lengthened significantly. Our results show a dose-dependent behavioral teratogenicity of this organomercurial fungicide. The so called no effect level--as far as the neurobehavioral impairments due to prenatal exposure are concerned--is 0.02 mg/kg daily.