Early change and adaptation in the social behaviour or rats given methyl mercury in the diet

Rats were fed on diets containing methyl mercury dicyandiamide (MMD) at concentrations of 1.5, 7.5 or 75 ppm, and observations made of their social and exploratory behaviour and of gross neurotoxicity (ataxia). Mercury concentration in the blood was monitored. MMD at 75 ppm (50 ppm Hg) for 24 days caused ataxia with a sudden onset at 21-27 days. Social behaviour was reduced at 16-17 days. In two experiments at 7.5 ppm MMD, activity was increased in observations of social behaviour after 2 to 17 days, and treated rats found water in an unfamiliar cage sooner than controls. No difference from controls was apparent from then until increased activity re-appeared after 9 mon of the diet (exp. 1) or after 7 days recovery from 31 or 45 days diet (exp. 2). Ataxia was not observed after 7.5 ppm MMD for up to 47 weeks or in 14 weeks recovery. No consistent effect was observed at 1.5 ppm MMD for 31-45 days. MMD, therefore, had a behavioural effect in rats, independent of gross neurotoxicity; its details were consistent with a hyperresponsiveness to stimuli. Tolerance occurred to this effect at a time when blood-mercury concentration was still rising, but the tolerance appeared to be subject to overload.