Aromatase activity in cultured brain cells: difference between neurons and glia

Abstract

At the level of the central nervous system (CNS) of several mammalian and non-mammalian species, estrogens may be intracellularly formed from circulating androgens through the action of the aromatase complex. Estrogenic steroids play a crucial role in organizing and directing certain behavioral and neuroendocrine responses both during the fetal/neonatal life and in adulthood. Biochemical and immunocytochemical studies have shown that the aromatase is particularly concentrated in CNS areas involved in the control of reproductive functions, such as the hypothalamus, the preoptic area and the limbic system; despite this large body of evidence, the exact cellular localization of this enzymatic complex within the different cell populations of the brain is still uncertain. In the experiments described here, the presence of the aromatase has been evaluated in the two main cellular components of the brain: the neurons and the glia. In these experiments, cultures of neurons obtained from the brains of 14–15-day-old rat embryos, mixed glial cells from 1-day-old rats and type 1 astrocytes derived from cultured glial cells, have been utilized. The aromatase has been also evaluated in oligodendrocytes isolated from adult male rat brain by density gradient ultracentrifugation. The aromatase activity has been assayed by an ‘in vitro’ radiometric method which quantifies the production of tritiated water from [1β- 3H]-androstenedione as an index of estrogen formation. The validity of the method has been verified both on the placental microsomes and on rat hypothalamic tissue, in which the actual formation of estrogens has also been measured. Among the different cells tested, only neurons possess the aromatase complex to a significant degree, while mixed glial cell and astrocyte preparations have an extremely low enzymatic activity: oligodendrocytes seem to be completely inactive in the respect. The calculation of the kinectic parameters of the neuronal aromatase has shown that this enzymatic activity is linear with respect to the protein content and the incubation time. The neuronal aromatase follows a Michaelis-Menten model with a K m of 52 nM: this value is analogous to that calculated for other central or peripheral structures in which the enzyme is known to be present.