Behavioral effects on the progeny of mice treated with methylmercury

Abstract

The effects of prenatal methylmercury (MeHg) exposure on the behavior of postnatal offspring were determined in 129 SvSl mice. A single dose of 0, 6, 8, or 12 mg/kg of methylmercury hydroxide (MeHgOH) was injected sc to pregnant mice on Day 10 of gestation. The following three types of behavioral tests were conducted on offspring from these mothers at various ages: (1) exploratory behavior by open-field test at age 23 days old, (2) spontaneous locomotor activity at ages 24, 44, and 64 days old, and (3) convulsive behavior to a convulsant, flurothyl at age 70 days old. Another group of 33-day-old offspring whose mothers had received three multiple doses of MeHgOH or daily doses of 0 or 4 mg/kg from Day 10 to Day 12 of gestation were also used for the open-field behavioral study. Results from the openfield test indicated that prenatal MeHg exposure decreases exploratory behavior as indicated by longer center square latency, lower number of peripheral squares traversed, lower frequency of rearing and grooming, lower frequency of urination, and higher frequency of backing, in treated groups in comparison to control groups. Depression in spontaneous locomotor activity was also observed in offspring prenatally exposed to MeHg; however, the effect diminished gradually as the animals grew older. In the flurothyl convulsion behavior test, it was found that animals prenatally treated with MeHg demonstrated significantly lower threshold to preliminary clonic convulsion as well as maximal clonic-tonic convulsion; thus, indicating an increase in susceptibility to flurothyl-induced convulsions. The observed decrease in exploratory behavior and spontaneous locomotor activity with an increase in convulsive susceptibility are not necessarily inconsistent findings and may be due to MeHg affecting two independent mechanisms. Therefore, it has been demonstrated that behavioral effects can be induced in offspring prenatally exposed to MeHg. Furthermore, behavioral effects of MeHg can be detected in offspring having no detectable gross congenital malformations.