Sex Differences and Steroid Control of Testosterone‐Metabolizing Enzyme Activity in the Quail Brain

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Abstract
The activity of three testosterone-metabolizing enzymes (aromatase, 5α-reductase and 5β-reductase) was determined in the quail brain using the Palkovits punch technique combined with a very sensitive radioenzyme assay. Sex differences and the effects of gonadectomy and testosterone treatment on the activity of the three enzymes were quantified in eight brain nuclei which are implicated in the control of various aspects of reproductive behavior and physiology. The aromatase was only present in a few brain areas in which its activity was strongly controlled by testosterone. In two brain regions (medial preoptic nucleus and preoptic area in general) the activity of the enzyme was higher in males than in females. These sex differences disappeared in gonadectomized birds and in gonadectomized birds treated with testosterone, suggesting that they might only result from different circulating steroids in both sexes. However, in the posterior part of the medial preoptic nucleus, there was a strong tendency for the induction of aromatase by testosterone to be larger in males than in females. This supports our earlier finding that in the preoptic area, the aromatase activity is sexually differentiated. This difference probably has a restricted neuroanatomical localization and could only be demonstrated by more precise anatomical methods such as immunocytochemistry. The two testosterone reductases (5α and 5β) showed a more homogeneous distribution in the brain. They were not affected by the hormonal treatments or the sex of the birds except for the 5β-reductase which was significantly more active in three brain nuclei of the females (ventromedial nucleus of the hypothalamus, area hypothalamica lateralis and tuber) by comparison with the males. These sex differences were maintained irrespective of the hormonal status of the birds suggesting that they might be organizational in nature. The relation of these enzymes and their regulation to the control of reproduction is discussed and the usefulness of this approach combining punch technique and radioenzyme assay is evaluated.

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