STYRENE IMPAIRS SERIAL SPATIAL REVERSAL-LEARNING IN RATS

To evaluate the effects of styrene exposure on learning, adult male Lo ng-Evans rats learned repeated reversals of a spatial discrimination t ask. Styrene monomer (50% vol/vol in corn oil) was administered by gav age to groups of eight rats at 500 mg/kg/day, 5 days/week, for 8 weeks in Experiments (Exps) I and II (total dose = 20.0 g/kg) or for 1, 3, 5, or 8 weeks in Exp III (total dose = 2.5, 7.5, 12.5, or 20.0 g/kg). Control rats received corn oil vehicle for 8 weeks. Reversal training began 8 (Exp I), 10 (Exp II), or 32 (Exp III) weeks after termination of dosing. In Exp I, an instrumental (IN) schedule was used, under whi ch rats received food after each presentation of a ''positive'' respon se lever (S+) only if they had made at least one response during that presentation of S+. In Exps II and III, an automaintenance (AU) schedu le was used, under which rats received food after every presentation o f S+, regardless of responding. In all experiments, a second manipulan dum (S degrees) was presented randomly in time with respect to S+ and food delivery. A discrimination ratio (DR) was calculated as the propo rtion of total responses on S+ in each block of 10 trials. A reversal involved switching the reward values of S+ and S degrees. Serial rever sal learning was quantified in terms of trials to criterion. Reversal learning improved similarly in control and treated rats trained under the IN schedule, whereas treated rats trained under the AU schedule fa iled to improve as much as controls. Reversal learning of some styrene -treated AU rats in Exp III continued to be impaired for >1 year after treatment. Increased responding on S degrees featured prominently in the behavioral effect of styrene. An IN schedule requiring suppression of S degrees responses for food in Exp III revealed a clear deficit i n rats exposed to styrene. Not all treated rats were affected by styre ne; nevertheless, changes in the affected individuals were as large as those previously observed after trimethyltin-induced lesions of the C NS. The incidence of impairment was not related to the total dose of s tyrene given, suggesting the action of other, undetermined factors aff ecting individual sensitivity to styrene.