Putative long-term learning and memory effects of low-dose exposure to the cholinesterase inhibitor organophosphate methamidophos (Tamaron®) early in life were studied in two parallel studies in middle-aged rats. Methamidophos was administered via the drinking water to female and male Wistar rats using nominal concentrations of 0 (control), 0.5, 1.5 and 4.5 ppm active ingredient for 16 weeks. Animals were then maintained for a recovery period of about 14 months without treatment. They were tested in the standard and repeated acquisition version of the Moms water escape task in two series of tests starting 33 and 55 weeks after termination of the methamidophos treatment. Functional observations and motor activity measurements preceded each series of testing. Exposure to methamidophos was confirmed by measurement of brain cholinesterase (ChE-B) at the end of the 16 weeks of treatment in satellite animals. At 4.5 ppm a biologically relevant reduction in ChE-B activity was observed without clinical signs of intoxication (males: 66%, females: 64% of control activity). Mid- and low-dose exposure to methamidophos revealed ChE-B activity of 90% and 100% in males and 88% and 97% in females, respectively. General examinations of the animals during treatment revealed no clinical signs suggesting cholinergic stimulation. Functional observations and motor activity measurements exhibited no relevant differences between treatment groups and controls. Neither the performance in the standard Morris water escape task that predominantly measures spatial reference memory, nor in the repeated acquisition task in the Morris tank, which predominantly measures spatial working memory, was affected by treatment with methamidophos. A small number of statistically statistically significant differences were noted in the mean performance level between treatment groups, or between treatment by sex groups in both versions of the Morris task. However, these findings appeared to be idiosyncratic for a particular experiment and were not supported by findings from the other. They were consequently not considered as reflecting a consistent effect of methamidophos on learning and memory. In conclusion, administration of low doses of methamidophos to female and male Wistar rats for 16 weeks during early adulthood did not impair spatial working and reference memory in the Morris water escape task 33 and 55 weeks after cessation of treatment.
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