Adult male rat hippocampus synthesizes estradiol from pregnenolone by cytochromes P45017α and P450 aromatase localized in neurons

Abstract

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.

Functional significance of the rapid regulation of brain estrogen action: Where do the estrogens come from?

“…This scenario would be in agreement with the concept that "estrogen synthesized in extragonadal compartments acts predominantly at the local tissue level in a paracrine or intracrine fashion" and consequently "the total amount of estrogen synthesized by these extragonadal sites may be small, but the local tissue concentrations achieved are probably high and exert biological influence locally" [239,241]. This model is supported by reports of higher concentrations of estrogen in certain brain areas, such as the hypothalamus, preoptic area and the hippocampus, than in the plasma [40,114]. It must also be noticed that these measures of tissue concentrations are likely to be underestimations of the actual concentrations present at discrete locations close to the site of synthesis and action.…”

“…Interestingly, this effect resulted in a 2-fold increase in E 2 concentration in the preparation. The authors of this study hypothesized that this effect depends on NMDAinduced Ca 2+ influx and thereby depends on synaptic communication [114]. Why application of NMDA tended to decrease estrogen formation in the quail preoptic/ hypothalamic explants and increase it in the rat hippocampus remains unclear at present.…”

“…Aromatase activity is also enriched in synaptosomes prepared from quail or rat brain [230,250] and dense aromatase immunoreactivity is observed in presynaptic boutons at the surface of synaptic vesicles [114,178,194]. Together, these data suggest that aromatase activity can be rapidly (with latencies in the minute or possibly the second range) modulated at the pre-synaptic level by afferent inputs potentially providing a mechanism able to quickly change the local production of estrogens.…”

Can endocrine disruptors influence neuroplasticity in the aging brain?

“…Elements of steroid synthesis in adult male hippocampus (based on Hojo et al, 2004;. Relationship of free testosterone to neuropsychological test scores in older men (based on Yaffe et al, 2002).…”

Estrogen Enhances Neurogenesis and Behavioral Recovery after Stroke

“…Rodent models have also provided definitive evidence for a role of E2 in ameliorating acute neuronal damage , but much less is known regarding the potential of E2 to enhance neurogenesis after an injury. In the central nervous system, E2 can be directly synthesized by hippocampal neurons in adult rats (Hojo et al, 2004) by the actions of P450 aromatase, which catalyzes the final step in E2 biosynthesis from C19 steroids such as testosterone (Garcia-Segura, 2008). Aromatase is rapidly upregulated after injury, and via the local production of E2, may be an important contributor to its proliferative and reparative effects (Garcia-Segura, 2008;Roselli et al, 2009).…”