PRENATAL EXPOSURE TO METHYLMERCURY DURING LATE-GESTATION AFFECTS CEREBRAL OPIATERGIC SYSTEM IN RAT OFFSPRING

Pregnant female rats were orally administered a single dose (a mg/kg) of methylmercury chloride (MMC) on Day 15 of gestation. The binding ch aracteristics of opioid receptors were studied in the brain of develop ing rats at different stages of age, An increased density of opioid re ceptors was found in whole brain of MMC-exposed rats at 21 days (delta receptors) and 60 days (mu and delta receptors) of age, in comparison with matched controls. An enhanced response to morphine administratio n was detected in MMC-exposed rat offspring at Day 60 of postnatal lif e, which, however, was not apparently due to an impaired liver metabol ization or renal excretion. Hence, it is reasonable to surmise a possi ble correlation between receptor up-regulation and increased response to pharmacological challenge. These data seem to indicate that neuroch emical alterations produced in the rat developing organism by prenatal exposure to methylmercury involve the opiatergic system which undergo es a supersensitivity phenomenon. This effect, which is not detectable in the first postnatal period, shows a delayed onset, being detectabl e only at the adult stage, These findings seem to indicate that pre-an d postnatal methylmercury exposure induces latent neurochemical and be havioral alterations which could last even after the clearance of the metal from the brain. (C) 1997 Academic Press.