Effects of in Utero Methylmercury Exposure on a Spatial Delayed Alternation Task in Monkeys

Abstract

Adult female monkeys ( Macaca fascicularis) were exposed to 0, 50, 70, or 90 μg/kg/day of methylmercury prior to and throughout pregnancy and produced 11, 9, 2, and 2 infants, respectively. At birth, blood mercury levels of treated infants ranged from 1.04 to 2.46 ppm. At approximately 7 to 9 years of age, the monkeys were trained by successive approximation to respond on a lit button for a small amount of apple juice. The monkeys were then trained on a 0.1-sec spatial delayed alternation task to a specified criterion of performance. This was followed by 10 sessions each of fixed delay times of 0.5, 1, 3, 5, and 10 sec, followed by 20 sessions containing variable delay times of 0.1 to 15 sec. Data from all treated monkeys were combined. There were no differences between treated and control monkeys in initial button training or number of sessions to reach criterion on 0.1-sec delay procedure. On the fixed delay sessions, the treated monkeys had significantly more correct trials, and fewer incorrect responses, perseverative responses, and delay responses than controls. There were no differences between the treated and control monkeys on performance on the variable delay schedule. Results from this study indicate that in utero methylmercury exposure did not adversely affect the spatial memory of adult monkeys when tested on a delayed alternation task and may have facilitated performance on this task.