REGIONAL DISTRIBUTION AND EXPRESSION MODULATION OF CYTOCHROME-P-450 AND EPOXIDE HYDROLASE MESSENGER-RNAS IN THE RAT-BRAIN

In the present study, we developed a very sensitive, semiquantitative assay based on the reverse transcriptase-coupled polymerase chain reac tion to measure, in a region-selective manner, mRNA expression pattern s within the brain for microsomal epoxide hydrolase and several cytoch rome P-450s (P-450s) known to be induced by prototypic agents in other tissues. The P-450s assessed included the polyaromatic hydrocarbon-in ducible CYP1A1 and CYP1A2 systems, together with the phenobarbital-ind ucible P-450s, CYP2B1, CYP2B2, CYP3A1, which were examined 1 8 hr afte r a single intraperitoneal dose of the respective inducing agents. Hig hly region-specific patterns of expression were evident for P-450 mRNA s within the rat brain. In the control, uninduced brain, CYP1A1 mRNAs were readily detected in the striatum and in the hypothalamus, and to a lesser extent in the other regions examined. The regional pattern of expression was similar for CYP1A2; however, a major difference was no ted in the olfactory bulbs, characterized by a relatively high level o f CYP1A2 mRNA but correspondingly low levels of CYP1A1. Within the bra in regions examined, the highest content of CYP2B1 and CYP2B2 mRNAs we re present in the striatum and in the cerebellum, whereas CYP3A1 level s varied only slightly across the respective regions. In contrast to t he P-450s, microsomal epoxide hydrolase mRNAs were expressed at relati ve homogeneous amounts throughout the brain. Beta-Naphthoflavone marke dly increased the CYP1A1 and CYP1A2 mRNA contents of each brain region investigated, although this agent did not affect levels of epoxide hy drolase. At 18 hr post-treatment with phenobarbital, an optimal time p eriod for hepatic induction, brain expression was characterized by a c omplex pattern of effects, with increased levels noted for CYP2B1 mRNA content in the medulla oblongata, midbrain, and cortex, but decreased contents measured in the cerebellum, the hypothalamus, and the striat um. In each of these respective regions, CYP2B2 content was profoundly decreased whereas epoxide hydrolase expression was slightly increased by the same treatment. These results establish that the central nervo us system actively expresses a number of different biotransformation g ene products in a regional specific and inducer-dependent manner, and suggest that for tissues exhibiting low regenerative capacity, like th e brain, such reactions are likely to be of critical toxicological sig nificance.