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Age-related behavioral effects of methomyI in Brown Norway rats.
Citation:
MACPHAIL, R. C., K. JAREMA, AND P. M. Phillips. Age-related behavioral effects of methomyI in Brown Norway rats. Presented at Society of Toxicology (SOT) Annual Meeting, Washington, DC, March 06 - 10, 2011.
Impact/Purpose:
This experiment, part of a larger program on life-stage susceptibility, compared methomyl effects on the behavior of adolescent, young-adult and middle-age male Brown Norway rats (Charles River, N=24/age) that were initially 1, 4 or 12 months of age, respectively. Another group of 12-month rats (NIA) was included to evaluate vendor differences.
Description:
Methomyl is a cholinesterase-inhibiting carbamate pesticide that is used in the field on cotton and a variety of fruits and vegetables. Concerns have been raised generally about age-related differences in susceptibility to cholinesterase-inhibiting pesticides, especially for children. This experiment, part of a larger program on life-stage susceptibility, compared methomyl effects on the behavior of adolescent, young-adult and middle-age male Brown Norway rats (Charles River, N=24/age) that were initially 1, 4 or 12 months of age, respectively. Another group of 12-month rats (NIA) was included to evaluate vendor differences. Testing occurred at weekly intervals in photocell devices that recorded horizontal and vertical locomotor activity during 30-min sessions. Methomyl (ChemService) was dissolved in deionized water and given orally in doses of 0 (i.e.,vehicle), 0.5, 1.25, 2.0 and 2.75mg/kg, I5-min before a test session. Order of dosing was counter balanced so that each rat received all treatments. Following vehicle, horizontal activity was highest in young adults and roughly equal in adolescent and middle-age rats; vertical activity was highest in young adults, slightly lower in adolescents, and considerably lower in middle-age rats. Methomyl produced dose-related decreases in both horizontal and vertical activity. Adolescent rats were least affected. ED50s for horizontal activity were ca. 5.72, 2.43 and 2.19 mg/kg for the adolescent, young-adult and middle-age rats, respectively. Corresponding ED50s for vertical activity were 4. 41, 2.20 and 1.85 mg/kg. Middle-age rats from NIA were similarly affected (horizontal ED50=1.96 and vertical ED50=1.61 mg/kg). These results indicate aging increases susceptibility to methomyl in Brown Norway rats; they also emphasize the importance of evaluating pesticide effects at multiple life stages in order to make informed inferences regarding susceptibility and risk. (This is an abstract of a proposed presentation and does not reflect US EPA policy.)