Research Article: Endogenous Synthesis of Corticosteroids in the Hippocampus

Date Published: July 28, 2011

Publisher: Public Library of Science

Author(s): Shimpei Higo, Yasushi Hojo, Hirotaka Ishii, Yoshimasa Komatsuzaki, Yuuki Ooishi, Gen Murakami, Hideo Mukai, Takeshi Yamazaki, Daiichiro Nakahara, Anna Barron, Tetsuya Kimoto, Suguru Kawato, Maria A. Deli.

Abstract: Brain synthesis of steroids including sex-steroids is attracting much attention. The endogenous synthesis of corticosteroids in the hippocampus, however, has been doubted because of the inability to detect deoxycorticosterone (DOC) synthase, cytochrome P450(c21).

The expression of P450(c21) was demonstrated using mRNA analysis and immmunogold electron microscopic analysis in the adult male rat hippocampus. DOC production from progesterone (PROG) was demonstrated by metabolism analysis of 3H-steroids. All the enzymes required for corticosteroid synthesis including P450(c21), P450(2D4), P450(11β1) and 3β-hydroxysteroid dehydrogenase (3β-HSD) were localized in the hippocampal principal neurons as shown via in situ hybridization and immunoelectron microscopic analysis. Accurate corticosteroid concentrations in rat hippocampus were determined by liquid chromatography-tandem mass spectrometry. In adrenalectomized rats, net hippocampus-synthesized corticosterone (CORT) and DOC were determined to 6.9 and 5.8 nM, respectively. Enhanced spinogenesis was observed in the hippocampus following application of low nanomolar (10 nM) doses of CORT for 1 h.

These results imply the complete pathway of corticosteroid synthesis of ‘pregnenolone →PROG→DOC→CORT’ in the hippocampal neurons. Both P450(c21) and P450(2D4) can catalyze conversion of PROG to DOC. The low nanomolar level of CORT synthesized in hippocampal neurons may play a role in modulation of synaptic plasticity, in contrast to the stress effects by micromolar CORT from adrenal glands.

Partial Text: The hippocampus is a target of corticosterone (CORT) modulatory actions, with high levels of glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) expression [1], [2]. Traditionally, CORT had been thought to be synthesized exclusively in the adrenal cortex, reaching the brain via blood circulation. De novo synthesis of CORT from PROG in the brain has been doubted partly because brain CORT disappears after adrenalectomy (ADX) in rats [3]. However, recent evidence shows that the hippocampus expresses steroidogenic enzymes required for corticosteroid synthesis, including cytochromes P450scc, P450(11β1), P450(11β2), and 3β-hydroxysteroid dehydrogenase (3β-HSD) [4]-[10]. Some of these enzymes are also necessary for sex-steroid synthesis [11]-[14]. Yet, the complete corticosteroid synthesis of ‘pregnenolone (PREG) → progesterone (PROG)→ deoxycorticosterone (DOC)→CORT or aldosterone (ALDO)’ in the hippocampus has not been proven (see Fig. S1). Although previous studies have shown parts of the corticosteroid synthesis pathway in brain, including, PREG→PROG and DOC→CORT and DOC→ALDO[5], [6], [10], the conversion of PROG→DOC to be demonstrated. Cytochrome P450(c21) (DOC synthase), a key enzyme catalyzing the conversion of PROG to DOC, has not been detected in the hippocampus, although mRNA expression has been demonstrated in other brain regions including the hypothalamus, cortex, cerebellum and striatum [15], [16].

In the current study, we clarified the complete pathway for corticosteroid synthesis ‘PREG→PROG→DOC→CORT’ in hippocampal neurons. We demonstrated for the first time the expression, neuronal localization and activity of P450(c21) in the hippocampus. In addition, the localization of P450(2D4), another enzyme participating in DOC synthesis, was demonstrated.