The behavioral effects of perinatal methimazole administration in Swiss Webster mice

Methimazole was tested for use as a positive control agent in behavioral studies of mice. Continuous administration of the antithyroid agent via drinking water (0.1 mg/ml) from Day 16 of pregnancy through Day 10 postpartum produced developmental delays in mice offspring. Ten methimazole and 12 untreated litters were studied. Developmental milestones were unaltered; i.e., time of pinna detachment, incisor eruption, eye opening, vaginal patency, and testicular descent were not different between groups. Mean body weights of methimazole offspring were consistently reduced, but significant differences were isolated to a few days in the preweaning period and a few weeks during the postweaning period. There was no enduring effect. All preweaning tests showed some significant treatment-related changes; methimazole pups were developmentally delayed. Surface righting time was increased while time pivoting and the number of quadrants traveled were decreased in methimazole pups. Negative geotaxis showed significant treatment-related increases in the time to orient 180 degrees uphill, the percentage of pups orienting 180 degrees uphill, and the percentage of pups orienting less than 180 degrees. Ontogeny of swimming ability also showed significant delays. The only postweaning test evaluated, time on a rotating rod, showed no treatment-related effects. Brain weights Postnatal Day (PND) 120 were not different between groups. In this study, methimazole produced developmental delays in mice that were detectable by behavioral tests. Thus, methimazole has potential as a positive control agent for mice, not only to validate preweaning test sensitivity, but also to validate a laboratory's ability to perform preweaning behavioral studies.