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

Rieko Hojo,Junzo Yonemoto,Chiharu Tohyama,Yasunobu Aoki,Yoshika Kurokawa,Masaki Kakeyama

doi:10.1007/s12199-008-0026-0

Abstract

We studied 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 (PCB126) on learning behavior in offspring. Pregnant Long-Evans Hooded rats were administered either TCDD (50, 200, or 800 ng/kg) or PCB126 (500, 2,000 or 8,000 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 the DRL component in multiple FR 20 DRL 20 s (mult-FR 20 DRL 20-s) test sessions, were used for the evaluation of learning behavior. Toxic effects on learning behavior in male and female pups following in utero and lactational exposure to TCDD or PCB126 were observed mainly in the FR learning component. However, no linear dose-dependent effects of either of the two compounds 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 PCB126 groups appeared to induce hyperactive behavior. The high dose of PCB126 appeared to have a distinct toxicity from that of TCDD in terms of the acquisition of learning behavior. Toxicities of PCB126 and TCDD in learning behavior might be similar to each other and the current toxic equivalency factor (TEF) of 0.1 for PCB126 can be considered to be appropriate for this endpoint.

Highlights

Polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and polychlorinated biphenyls (PCBs) are a group of persistent environmental chemicals
Neither TCDD nor PCB126 exposure to dams affected the body weights of either the male and female pups, suggesting that no marked toxicity was relevant to the possible effects of either compound on the learning behavior of the offspring (Table 2)
Repeated measures analysis of variance (ANOVA) was used to evaluate the possible effects of dose, chemical (TCDD or PCB126), sex, phase and dose–chemical interaction in the response rate of male and female offspring in the fixed ratio (FR) component (Fig. 1)

Summary

Introduction

Polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and polychlorinated biphenyls (PCBs) are a group of persistent environmental chemicals. Among the more than 400 congeners belonging to this group, 29 are classified as dioxins and related compounds because they have been shown to exert several toxic responses similar to those of 2,3,7,8-tetrachloro-p-dibenzodioxin (TCDD), the prototype of these compounds [1]. For practical risk assessment purposes, the toxic equivalents (TEQ) is used to evaluate the total dioxin toxicity of various congeners to which humans are exposed through food and environmental media. A degree of toxicities of these congeners are expressed as toxic equivalent factors (TEF) and range from 1.0 to 0.00001, with the TEF value 1.0 given to the most toxic congener, TCDD, as the standard [1, 3]. The TEF value is a composite one that is derived, on the basis of a so-called expert assessment, from the relative potency (REP) obtained from various experiments, both in vivo and in vitro

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