Suppression of TH mRNA expression by si-AhR (siRNA against AhR). N2a-Rβ cells were transiently transfected with si-AhR or scrambled siRNA (Scramble) and then exposed to 10 nM TCDD. Twenty-four hours after TCDD exposure, total RNA extracted from transfected cells was subjected to real-time quantitative PCR. Expression levels of AhR mRNA (left panel) and TH mRNA (right panel) were significantly suppressed by si-AhR. Results are normalized to the expression level of cyclophilin mRNA and are expressed as the ratio of si-AhR to scramble. Data are presented as mean ± SD of four replicate determinations.**P < 0.01 and *P < 0.05.