Progesterone metabolism in the pineal, brain stem, thalamus and corpus callosum of the female rat

Specific brain regions, namely, thalamus, tectum, tegmentum, cerebellum, medulla and pineal, from five proestrous rats were incubated for 30 min with [³H]progesterone. After reverse isotopic dilution analysis, the following metabolites were identified in all incubations by purification to constant specific activity, derivative formation and/or gas liquid chromatography trapping: [³H]5α-pregnane-3, 20-dione (10-20% of the starting substrate except pineal - 0.7%), [³H]3α-hydroxy-5α-pregnan-20-one (1.6-3.8% except for pineal - 0.5%) and [³H]20α-hydroxy-4-pregnen-3-one (0.05-0.11%). Preliminary results from the corpus collosum incubation indicated the presence of the same metabolites. Although some apparent constant specific activities were obtained for 20α-hydroxy-5α-pregnan-3-one and 5β-pregnane-3, 20-dione, the low levels of ³H associated with these steroids did not permit a definitive identification. The results indicate the presence of at least Δ⁴-steroid 5α-reductase, 3α-hydroxysteroid dehydrogenase and 20α-hydroxysteroid dehydrogenase activities with progesterone as substrate in the brain regions examined.