Genetic Evidence for Androgen-Dependent and Independent Control of Aromatase Activity in the Rat Brain*

Abstract

To investigate the role of androgen receptors in the regulation of brain aromatase activity (AA) in adult rats, the levels of AA in discrete brain areas of androgen-insensitive testicular feminized (Tfm) rats were compared with those in their normal male littermates (NL). AA was measured in homogenates of brain tissue by using a radiometric assay that quantifies the production of 3H2O from [1 beta-3H]androstenedione as an index of estrogen formation. Initially, we assessed the capability of block-dissected tissues to aromatize androgens. We found that the AA in the amygdala and hypothalamus-preoptic area of Tfm rats was significantly lower (P less than 0.001) than the AA in NL despite the fact that circulating androgen concentrations in the Tfm were significantly higher. Kinetics studies demonstrated that the apparent Michaelis constant was equivalent for both groups (0.02-0.03 microM). Administration of testosterone propionate to castrated males produced 3 to 4-fold elevations of AA in NL, but did not affect brain AA in Tfm rats. To pinpoint specific sites where AA is affected in Tfm rats, we measured AA in 10 hypothalamic and limbic nuclei that were dissected from 300-micron frozen brain sections. Compared to NL, Tfm rats exhibited significantly lower levels of AA in all micro-dissected brain regions studied, except for the medial and cortical amygdala. These data provide genetic evidence for both androgen-dependent and independent regulation of AA in the rat brain.

The role of androgen receptors in the masculinization of brain and behavior: What we've learned from the testicular feminization mutation

“…However, Tfm male rats show only a partial masculinization of the SDN-POA: volume is fully masculine but neuronal soma size is not, with Tfm males having smaller neurons in the SDN-POA than wt males . Again, as in several other brain regions, aromatase activity is decreased in the medial preoptic area of Tfm male rats (Roselli et al, 1987), so ARs may not be directly necessary for the masculinization of neuronal soma size in the SDN-POA, if sufficient estrogen is provided. Nevertheless, ARs appear to contribute normally to the full masculinization of the rat SDN-POA.…”

“…Even if administration of DHT did act solely upon ARs, it might not reveal a contribution of the AR in cases where T acts synergistically upon both ERs and ARs. For example, T acts upon ARs to increase aromatase activity in many brain regions (Roselli et al, 1987;Rosenfeld et al, 1977), which would provide more estrogens to stimulate ERs. Therefore DHT treatment alone would not reveal such a normal role for ARs because while it might boost aromatase activity, there would be no aromatizable androgenic precursor to be converted by the enzyme to provide estrogens for ERs.…”

“…In this regard, the Tfm rat resembles untreated humans with CAIS, who also have circulating T levels in the high-male range (Vague, 1983). In both rat and mouse Tfm models, indirect evidence suggests that T levels are near the normal male range in the perinatal period (Olsen, 1979;Goldstein and Wilson, 1972), but no one has actually measured circulating androgens in perinatal Tfm rodents.Aromatase activity is also decreased in several areas of the adult Tfm brain compared to wt males (Roselli et al, 1987;Rosenfeld et al, 1977), so differences between wt males and Tfm males in brain morphology and behaviors associated with these regions may be caused by reduced ER activation. Therefore in the survey that follows, as we find evidence that the Tfm allele of the AR gene affects brain and behavior, we must keep in mind that such findings do not disprove a role for ER, since AR may be acting in part by affecting aromatase.…”

“…Morris et al (2005) analyzed the SCN in Tfm male rats and found that volume and neuronal soma size was decreased compared to wt male rats, indicating that functional ARs are essential for the full masculinization of this brain area. However, given that aromatase activity is decreased in the SCN of Tfm male rats (Roselli et al, 1987), it is possible that the main role the AR plays in masculinization of the SCN is to regulate the level of aromatase and that ERs mediate downstream changes in SCN structure. This scenario would still indicate an important role for ARs in normal masculinization of the SCN, but it would also suggest that sufficient stimulation of ERs, such as with exogenous estrogen, can fully masculinize the nucleus.…”

Androgen receptors are required for full masculinization of the ventromedial hypothalamus (VMH) in rats

“…Like the VMH, both the volume of the MePD and the size of MePD neuronal somata are larger in males than in females, with TFM males having intermediate volumes and neuronal sizes (Morris et al, 2005). It does not seem likely that the partial masculinization of the MePD in TFM males reflects reduced activation of estrogen receptors (MacLusky et al, 1997), since neither testosterone levels nor aromatase activity in the medial amygdala of TFM males are reduced compared to WT males (Roselli, et al, 1987). However, in the VMH aromatase activity levels are lower in TFM males compared to WT males (Roselli, et al 1987), so a reduction in aromatase activity may underlie demasculinization of the VMH in TFM males.…”