Methylmercury, attention, and memory: baseline-dependent effects of adult d-amphetamine and marginal effects of adolescent methylmercury.

Methylmercury (MeHg) is an environmental neurotoxicant known to disrupt behavior related to dopamine neurotransmission in experimental models. Such disruptions are sensitive to dopamine agonists when administered acutely after exposure to MeHg has ended or when administered concurrently with MeHg exposure. Sustained attention and short-term remembering, components of attention-deficit/hyperactivity disorder (ADHD), are partially mediated by dopamine neurotransmission. In order to observe MeHg-related alterations in sustained attention and short-term memory, as well as determine sensitivity of MeHg exposed animals to dopamine agonists commonly used in the treatment of ADHD symptoms, rats were exposed to 0, 0.5, or 5 ppm MeHg throughout adolescence and trained in a hybrid sustained attention/short term memory visual signal detection task in adulthood. Behavior was then probed with acute i.p. injections of the dopamine agonist, d-amphetamine, which improves impaired attention and inhibits short-term memory in clinical syndromes like ADHD. Acute d-amphetamine dose-dependently decreased short-term memory as well as sustained attention. While MeHg alone did not impair accuracy or memory, it did interact with d-amphetamine to produce baseline-dependent inhibition of behavior. These findings further show that changes in behavior following low-level exposure to MeHg during adolescence are augmented by dopamine agonists. Observed impairments in memory following acute d-amphetamine are consistent with previous findings.