The effects of methylmercury on motor activity are sex- and age-dependent, and modulated by genetic deletion of adenosine receptors and caffeine administration

Abstract

Adenosine and its receptors are, as part of the brain stress response, potential targets for neuroprotective drugs. We have investigated if the adenosine receptor system affects the developmental neurotoxicity caused by the fish pollutant methylmercury (MeHg). Behavioral outcomes of low dose perinatal MeHg exposure were studied in mice where the A 1 and A 2A adenosine receptors were either partially blocked by caffeine treatment or eliminated by genetic modification (A 1R and A 2AR knock-out mice). From gestational day 7 to day 7 of lactation dams were administered doses that mimic human intake via normal diet, i.e. 1 μM MeHg and/or 0.3 g/l caffeine in the drinking water. This exposure to MeHg resulted in a doubling of brain Hg levels in wild type females and males at postnatal day 21 (PND21). Open field analysis was performed at PND21 and 2 months of age. MeHg caused time-dependent behavioral alterations preferentially in male mice. A decreased response to amphetamine in 2-month-old males pointed to disturbances in dopaminergic functions. Maternal caffeine intake induced long-lasting changes in the offspring evidenced by an increased motor activity and a modified response to psychostimulants in adult age, irrespectively of sex. Similar alterations were observed in A 1R knock-out mice, suggesting that adenosine A 1 receptors are involved in the alterations triggered by caffeine exposure during development. Perinatal caffeine treatment and, to some extent, genetic elimination of adenosine A 1 receptors, attenuated the behavioral consequences of MeHg in males. Importantly, also deletion of the A 2A adenosine receptor reduced the vulnerability to MeHg, consistent with the neuroprotective effects of adenosine A 2A receptor inactivation observed in hypoxia and Parkinson's disease. Thus, the consequences of MeHg toxicity during gestation and lactation can be reduced by adenosine A 1 and A 2A receptor inactivation, either via their genetic deletion or by treatment with their antagonist caffeine.