6-Methyl-1,3,8-trichlorodibenzofuran (MCDF), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and TCDD plus MCDF were administered to C57BL/6 mice and their effects on several aryl hydrocarbon (Ah) receptor-mediated responses including hepatic microsomal aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin O-deethylase (EROD) induction, immunotoxicity and teratogenicity were determined. MCDF did not induce hepatic microsomal AHH and EROD at doses up to 500 μmol/kg, however, co-administration of MCDF (50 μmol/kg) with a dose of TCDD which elicited a submaximal induction response (i.e. ED80-100, 15 nmol/kg) resulted in some small but significant inhibition of the induction of hepatic microsomal AHH and EROD (14 and 17%, respectively) compared to that observed with TCDD alone. Co-administration of TCDD and other doses of MCDF (10, 100, 200 or 500 μmol/kg) did not effect the induction response. These results were in contrast to the effectiveness of MCDF as an antagonist of the induction of AHH and EROD by TCDD in the rat (up to 50% inhibition of monooxygenase induction). Administration of MCDF (4, 20 and 40 μmol/kg) to C57BL/6 mice caused some inhibition of the splenic plaque-forming cell response to sheep erythrocytes only at the highest dose (26% decrease); the interaction of MCDF (4, 20 and 40 μmol/ kg) and an immunotoxic dose of TCDD (3.7 nmol/kg) resulted in significant protection from the immunotoxic effects of TCDD at the 2 higher dose levels of MCDF. Similarly, MCDF (400 μmol/kg) did not cause cleft palate in mice but at this dose level MCDF afforded some protection from TCDD (20 μg/kg)-mediated cleft palate in mice. However, studies utilizing [3H]TCDD suggested that the protective effects may be due to modulation of TCDD reaching the palate in the co-treated animals (MCDF plus TCDD). Although both MCDF and Aroclor 1254 were both weak Ah receptor agonists in C57BL/6 mice, the former compound was much less effective as a TCDD antagonist. The observed species-specific effects for these 2 TCDD antagonists may be related species-dependent differences in receptor structure and receptor-ligand (i.e. agonist or antagonist) interactions/.
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