In adult mammalian brain, occurrence of the synthesis of estradiol from endogenous cholesterol has been doubted because of the inability to detect dehydroepiandrosterone synthase, P45017␣. In adult male rat hippocampal formation, significant localization was demonstrated for both cytochromes P45017␣ and P450 aromatase, in pyramidal neurons in the CA1-CA3 regions, as well as in the granule cells in the dentate gyrus, by means of immunohistochemical staining of slices. Only a weak immunoreaction of these P450s was observed in astrocytes and oligodendrocytes. ImmunoGold electron microscopy revealed that P45017␣ and P450 aromatase were localized in pre-and postsynaptic compartments as well as in the endoplasmic reticulum in principal neurons. The expression of these cytochromes was further verified by using Western blot analysis and RT-PCR. Stimulation of hippocampal neurons with N-methyl-D-aspartate induced a significant net production of estradiol. Analysis of radioactive metabolites demonstrated the conversion from [ 3 H]pregnenolone to [ 3 H]estradiol through dehydroepiandrosterone and testosterone.This activity was abolished by the application of specific inhibitors of cytochrome P450s. Interestingly, estradiol was not significantly converted to other steroid metabolites. Taken together with our previous finding of a P450scc-containing neuronal system for pregnenolone synthesis, these results imply that 17␤-estradiol is synthesized by P45017␣ and P450 aromatase localized in hippocampal neurons from endogenous cholesterol. This synthesis may be regulated by a glutamate-mediated synaptic communication that evokes Ca 2؉ signals.

SUMMARY Male behaviors require both testosterone and estrogen. Circulating testosterone activates the androgen receptor (AR) and is also converted into estrogen in the brain via aromatase. This conversion is the primary source of estrogen to the male brain. It is unclear whether testosterone and estrogen signaling interact to masculinize neural circuits. Using a genetic approach, we show extensive sexual dimorphism in the number and projections of aromatase expressing neurons. The masculinization of these cells is independent of AR but can be induced by either testosterone or estrogen, indicating a role for aromatase in sexual differentiation of these neurons. We provide evidence suggesting that aromatase is also important in activating male aggression and urine marking as these behaviors can be elicited by testosterone in males mutant for AR. Taken together with additional findings, our results suggest that aromatization of testosterone into estrogen is important for the development and activation of neural circuits that control male territorial behaviors.

Much evidence indicates that women have a higher risk of developing Alzheimer's disease (AD) than do men. The reason for this gender difference is unclear. We hypothesize that estrogen deficiency in the brains of women with AD may be a key risk factor. In rapidly acquired postmortem brains from women with AD, we found greatly reduced estrogen levels compared with those from age-and gender-matched normal control subjects; AD and control subjects had comparably low levels of serum estrogen. We examined the onset and severity of AD pathology associated with estrogen depletion by using a gene-based approach, by crossing the estrogen-synthesizing enzyme aromatase gene knockout mice with APP23 transgenic mice, a mouse model of AD, to produce estrogen-deficient APP23 mice. Compared with APP23 transgenic control mice, estrogen-deficient APP23 mice exhibited greatly reduced brain estrogen and early-onset and increased ␤ amyloid peptide (A␤) deposition. These mice also exhibited increased A␤ production, and microglia cultures prepared from the brains of these mice were impaired in A␤ clearance͞degradation. In contrast, ovariectomized APP23 mice exhibited plaque pathology similar to that observed in the APP23 transgenic control mice. Our results indicate that estrogen depletion in the brain may be a significant risk factor for developing AD neuropathology.amyloid deposition ͉ aromatase ͉ transgenic animal N europathological hallmarks of Alzheimer's disease (AD) include significant deposition of extracellular ␤ amyloid peptide (A␤) and presence of neurofibrillary tangles in the brain (1). A␤ is derived from the two-step enzymatic processing of amyloid precursor protein (APP) in which ␤-secretase (BACE) cleaves the ␤-site of APP to release the N terminus of A␤, and the ␥-secretase protein complex cleaves the ␥-site of APP to release the C terminus of A␤ (2, 3). Overproduction and progressive deposition of A␤ are known to underlie, in part, A␤ plaque formation, a key pathologic feature of AD. The initial cleavage of APP by BACE is critical for A␤ associated with AD neuropathology (4). Recent studies have shown that BACE activity increases with age and is elevated in AD brains (5, 6).Impaired A␤ clearance and͞or degradation may also contribute to A␤ plaque formation. Our previous findings support this hypothesis: Microglia isolated from AD brains have impaired phagocytic activity, leading to reduced A␤ clearance (7). Other groups have found that cytoplasmic A␤ granules in the plaque-associated glia and microglia participate in the clearance of A␤ in A␤-immunized AD patients and APP transgenic mice (8, 9).Two enzymes are involved in A␤ degradation and clearance: insulin-degrading enzyme (IDE) and neprilysin (NEP). IDE is expressed in high concentrations in the brain. Besides degrading insulin and several regulatory peptides, IDE also degrades the intracellular domain of APP and is responsible for degrading and clearing A␤ from the brain (10, 11). Indeed, genetic linkage studies have shown that late-onset AD loci on chromosome 1...