Learning behavior in rat offspring after in utero and lactational exposure to either TCDD or PCB126

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I studies and compared the possible effects of in utero and lactational exposure to 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD) or 3, 3', 4, 4', 5-pentachlorobiphenyl (PCB 126) on learning behaviour in offspring. Pregnant Long-Evans Hooded rats were administered either TCDD (50, 200 or 800 ng/kg) or PCB126 (500, 2000 or 8000 ng/kg) on gestational day 15. A procedure of schedule-controlled operant behavior was applied to examine learning behavior in the male and female offspring at 11 weeks of age for 30 days. Three indices, namely, response rates in a fixed ratio (FR) and in a differential reinforcement of low rates (DRL), and reward rate in DRL component in a multiple FR20 DRL2Os (mult-FR20 DRL20s) test sessions, were used for the evaluation of learning behavior. Toxic effects on learning behaviour in male and female pups following in utero and lactational exposure to TCDD or PCB 126 were observed mainly in the FR learning component. However, effects of either of the two components were observed for the above three indices. The response rates of animals in the low-dose TCDD and PCB126 groups decreased and those in medium-dose TCDD and PCB 126 groups appeared to induce hyperactive behavior. The high dose of PCB 126 appeared to have a distinct toxicity from that of TCDD in terms of the acquisition of learning behavior. Toxicities of TCDD and PCB 126 in learning behaviour might be similar to each other and the current toxic equivalency factor (TEF) of 0.1 for PCB 126 can be considered to be appropriate for this endpoint.