Varied duration of congenital hypothyroidism potentiates perseveration in a response alternation discrimination task

The behavior of five groups of rats (seven rats per group) made hypothyroid for varying lengths of time and one group of seven normal control rats was assessed under forced alternation fixed-ratio (FR1, FR3, FR5 and FR10), al ternating lever cyclic-ratio (ALCR) and progressive-ratio (PR3) schedules o f reinforcement. Hypothyroidism was produced by adding methimazole (MMI) to the drinking water of pregnant dams from embryonic day E16 to postnatal da y P25. Four groups were given replacement thyroxine (T4) injections beginni ng at specific time points (P1, P7, P13, and P19). There were no difference s in behavioral performance between control and experimental groups under t he FR schedule, which indicates that the animals' sensorimotor abilities we re intact. Under the forced ALCR schedule. all groups reached criteria simi larly. However, under the choice lever ALCR schedule. control animals and t hose which received T4 replacement from early on (P1, P7, P13 groups) perfo rmed well and all had leached criteria by 11 sessions. In contrast, animals which did not receive any T4 replacement or received it late (P19 group) t ook longer to reach criteria and 5/14 animals had not reached criteria at a ll by 20 sessions. This deterioration in performance was paralleled by an i ncrease in perseverative behavior as evidenced by an increased frequency of pressing the wrong lever when alternation of lever was required. This sugg ests that congenital hypothyroidism results in increased perseveration lead ing to a decrease in learning when a discrimination between comet and incor rect operanda is made available. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.