Prenatal exposure to methylmercury changes dopamine-modulated motor activity during early ontogeny: age and gender-dependent effects

We have shown previously that prenatal exposure of rats to 0.5 mg/kg/day of methylmercury (MeHg) produces gender-dependent changes in motor activity i n adulthood. In the present study we have investigated whether changes in m otor activity could also be found during early ontogeny of the offspring. P regnant rats were treated with MeHg from day 7 of pregnancy to day 7 of lac tation. The habituation to a novel environment (spontaneous activity) and t he response to stimulation of the dopaminergic system were studied on postn atal day 14 and 21. Measures of spontaneous activity showed a slight increa se in MeHg-prenatal exposed male and female rats at 14 days, but not at 21 days. Following administration of U91356A, a selective dopamine D-2 recepto r agonist, a significantly lower dopamine-mediated locomotor activity was o bserved in the 21 day old MeHg-treated males, but not in females. These res ults show that prenatal exposure to MeHg alters postjunctional dopaminergic activity during the period of maturation of the dopamine system in the bra in. Moreover, the gender-dependent susceptibility previously found in adult hood is already evident at the prepubertal stage. (C) 2001 Elsevier Science B.V. All rights reserved.