Messenger RNA of steroid 21-hydroxylase (CYP21) is expressed in the human hippocampus

Are Mutations in the DHRS9 Gene Causally Linked to Epilepsy? A Case Report

The DHRS9 gene is involved in several pathways including the synthesis of allopregnanolone from progesterone. Allopregnanolone is a positive modulator of gamma aminobutyric acid (GABA) action and plays a role in the control of neuronal excitability and seizures. Whole-exome sequencing performed on a girl with an early onset epilepsy revealed that she was a compound heterozygote for two novel missense mutations of the DHRS9 gene likely to disrupt protein function. No previous studies have reported the implication of this gene in epilepsy. We discuss a new potential pathogenic mechanism underlying epilepsy in a child, due to a defective progesterone pathway.

An alternative theory for hormone effects on sex differences in PTSD: The role of heightened sex hormones during trauma

Women are at least twice as susceptible to developing post-traumatic stress disorder (PTSD) compared to men. Although most research seeking to explain this discrepancy has focussed on the role of oestradiol during fear extinction learning, the role of progesterone has been overlooked, despite relatively consistent findings being reported concerning the role of progesterone during consolidation of emotional and intrusive memories. In this review article, we outline literature supporting the role of progesterone on memory formation, with particular emphasis on potential memory-enhancing properties of progesterone when subjects are placed under stress. It is possible that progesterone directly and indirectly exerts memory-enhancing effects at the time of trauma, which is an effect that may not be necessarily captured during non-stressful paradigms. We propose a model whereby progesterone's steroidogenic relationship to cortisol and brain-derived neurotrophic factor in combination with elevated oestradiol may enhance emotional memory consolidation during trauma and therefore present a specific vulnerability to PTSD formation in women, particularly during the mid-luteal phase of the menstrual cycle.

Expression and activity profiling of the steroidogenic enzymes of glucocorticoid biosynthesis and the fdx1 co-factors in zebrafish

The spatial and temporal expression of steroidogenic genes in zebrafish has not been fully characterised. Because zebrafish are increasingly employed in endocrine and stress research, a better characterisation of steroidogenic pathways is required to target specific steps in the biosynthetic pathways. In the present study, we have systematically defined the temporal and spatial expression of steroidogenic enzymes involved in glucocorticoid biosynthesis (cyp21a2, cyp11c1, cyp11a1, cyp11a2, cyp17a1, cyp17a2, hsd3b1, hsd3b2), as well as the mitochondrial electron-providing ferredoxin co-factors (fdx1, fdx1b), during zebrafish development. Our studies showed an early expression of all these genes during embryogenesis. In larvae, expression of cyp11a2, cyp11c1, cyp17a2, cyp21a2, hsd3b1 and fdx1b can be detected in the interrenal gland, which is the zebrafish counterpart of the mammalian adrenal gland, whereas the fdx1 transcript is mainly found in the digestive system. Gene expression studies using quantitative reverse transcriptase-PCR and whole-mount in situ hybridisation in the adult zebrafish brain revealed a wide expression of these genes throughout the encephalon, including neurogenic regions. Using ultra-high-performance liquid chromatography tandem mass spectrometry, we were able to demonstrate the presence of the glucocorticoid cortisol in the adult zebrafish brain. Moreover, we demonstrate de novo biosynthesis of cortisol and the neurosteroid tetrahydrodeoxycorticosterone in the adult zebrafish brain from radiolabelled pregnenolone. Taken together, the present study comprises a comprehensive characterisation of the steroidogenic genes and the fdx co-factors facilitating glucocorticoid biosynthesis in zebrafish. Furthermore, we provide additional evidence of de novo neurosteroid biosynthesising in the brain of adult zebrafish facilitated by enzymes involved in glucocorticoid biosynthesis. Our study provides a valuable source for establishing the zebrafish as a translational model with respect to understanding the roles of the genes for glucocorticoid biosynthesis and fdx co-factors during embryonic development and stress, as well as in brain homeostasis and function.

Nuclear deterrents: Intrinsic regulators of IL-1β-induced effects on hippocampal neurogenesis

Hippocampal neurogenesis, the process by which new neurons are born and develop into the host circuitry, begins during embryonic development and persists throughout adulthood. Over the last decade considerable insights have been made into the role of hippocampal neurogenesis in cognitive function and the cellular mechanisms behind this process. Additionally, an increasing amount of evidence exists on the impact of environmental factors, such as stress and neuroinflammation on hippocampal neurogenesis and subsequent impairments in cognition. Elevated expression of the pro-inflammatory cytokine interleukin-1β (IL-1β) in the hippocampus is established as a significant contributor to the neuronal demise evident in many neurological and psychiatric disorders and is now known to negatively regulate hippocampal neurogenesis. In order to prevent the deleterious effects of IL-1β on neurogenesis it is necessary to identify signalling pathways and regulators of neurogenesis within neural progenitor cells that can interact with IL-1β. Nuclear receptors are ligand regulated transcription factors that are involved in modulating a large number of cellular processes including neurogenesis. In this review we focus on the signalling mechanisms of specific nuclear receptors involved in regulating neurogenesis (glucocorticoid receptors, peroxisome proliferator activated receptors, estrogen receptors, and nuclear receptor subfamily 2 group E member 1 (NR2E1 or TLX)). We propose that these nuclear receptors could be targeted to inhibit neuroinflammatory signalling pathways associated with IL-1β. We discuss their potential to be therapeutic targets for neuroinflammatory disorders affecting hippocampal neurogenesis and associated cognitive function.

Effects of neuroactive steroids on cochlear hair cell death induced by gentamicin

As neuroactive steroids, sex steroid hormones have non-reproductive effects. We previously reported that 17β-estradiol (βE2) had protective effects against gentamicin (GM) ototoxicity in the cochlea. In the present study, we examined whether the protective action of βE2 on GM ototoxicity is mediated by the estrogen receptor (ER) and whether other estrogens (17α-estradiol (αE2), estrone (E1), and estriol (E3)) and other neuroactive steroids, dehydroepiandrosterone (DHEA) and progesterone (P), have similar protective effects. The basal turn of the organ of Corti was dissected from Sprague-Dawley rats and cultured in a medium containing 100 μM GM for 48h. The effects of βE2 and ICI 182,780, a selective ER antagonist, were examined. In addition, the effects of other estrogens, DHEA and P were tested using this culture system. Loss of outer hair cells induced by GM exposure was compared among groups. βE2 exhibited a protective effect against GM ototoxicity, but its protective effect was antagonized by ICI 182,780. αE2, E1, and E3 also protected hair cells against gentamicin ototoxicity. DHEA showed a protective effect; however, the addition of ICI 182,780 did not affect hair cell loss. P did not have any effect on GM-induced outer hair cell death. The present findings suggest that estrogens and DHEA are protective agents against GM ototoxicity. The results of the ER antagonist study also suggest that the protective action of βE2 is mediated via ER but that of DHEA is not related to its conversion to estrogen and binding to ER. Further studies on neuroactive steroids may lead to new insights regarding cochlear protection.

Endogenous synthesis of corticosteroids in the hippocampus

BACKGROUND

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).

METHODOLOGY/PRINCIPAL FINDINGS

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 (3)H-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.

CONCLUSIONS/SIGNIFICANCE

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.

Extra-adrenal glucocorticoids and mineralocorticoids: evidence for local synthesis, regulation, and function

Glucocorticoids and mineralocorticoids are steroid hormones classically thought to be secreted exclusively by the adrenal glands. However, recent evidence has shown that corticosteroids can also be locally synthesized in various other tissues, including primary lymphoid organs, intestine, skin, brain, and possibly heart. Evidence for local synthesis includes detection of steroidogenic enzymes and high local corticosteroid levels, even after adrenalectomy. Local synthesis creates high corticosteroid concentrations in extra-adrenal organs, sometimes much higher than circulating concentrations. Interestingly, local corticosteroid synthesis can be regulated via locally expressed mediators of the hypothalamic-pituitary-adrenal (HPA) axis or renin-angiotensin system (RAS). In some tissues (e.g., skin), these local control pathways might form miniature analogs of the pathways that regulate adrenal corticosteroid production. Locally synthesized glucocorticoids regulate activation of immune cells, while locally synthesized mineralocorticoids regulate blood volume and pressure. The physiological importance of extra-adrenal glucocorticoids and mineralocorticoids has been shown, because inhibition of local synthesis has major effects even in adrenal-intact subjects. In sum, while adrenal secretion of glucocorticoids and mineralocorticoids into the blood coordinates multiple organ systems, local synthesis of corticosteroids results in high spatial specificity of steroid action. Taken together, studies of these five major organ systems challenge the conventional understanding of corticosteroid biosynthesis and function.

Neurosteroid and GABA-A receptor alterations in Alzheimer's disease, Parkinson's disease and multiple sclerosis

Steroid hormones (e.g. estrogens, androgens, progestagens) which are synthesized de novo or metabolized within the CNS are called neurosteroids. There is substantial evidence from animal studies suggesting that these steroids can affect brain function by modulating neurotransmission, and influence neuronal survival, neuronal and glial differentiation and myelination in the CNS by regulating gene expression of neurotrophic factors and anti-inflammatory molecules. Indeed, evidence is emerging that expression of the enzymes responsible for the synthesis of neurosteroids changes in neurodegenerative diseases. Some of these changes may contribute to the pathology, while others, conversely, may represent an attempted rescue program in the diseased brain. Here we review the data on changes in neurosteroid levels and neurosteroid synthesis pathways in the human brain in three neurodegenerative conditions, Alzheimers's (AD) and Parkinson's (PD) diseases and Multiple Sclerosis (MS) and the extent to which these findings may implicate protective or pathological roles for neurosteroids in the course of these diseases.Some neurosteroids can modulate neurotransmitter activity, for example, the pregnane steroids allopregnanolone and 3α5α-tetrahydro-deoxycorticosterone which are potent positive allosteric modulators of ionotropic GABA-A receptors. Therefore, neurosteroid-modulated GABA-A receptor subunit alterations found in AD and PD will also be discussed. These data imply an involvement of neurosteroid changes in the neurodegenerative and neuroinflammatory processes and suggest that they may deserve further investigation as potential therapeutic agents in AD, PD and MS. Finally, suggestions for therapeutic strategies will be included. This article is part of a Special Issue entitled: Neuroactive Steroids: Focus on Human Brain.

Hippocampal synthesis of sex steroids and corticosteroids: essential for modulation of synaptic plasticity

Sex steroids play essential roles in the modulation of synaptic plasticity and neuroprotection in the hippocampus. Accumulating evidence shows that hippocampal neurons synthesize both estrogen and androgen. Recently, we also revealed the hippocampal synthesis of corticosteroids. The accurate concentrations of these hippocampus-synthesized steroids are determined by liquid chromatography-tandem mass-spectrometry in combination with novel derivatization. The hippocampal levels of 17β-estradiol (E2), testosterone (T), dihydrotestosterone (DHT), and corticosterone (CORT), are 5-15 nM, and these levels are sufficient to modulate synaptic plasticity. Hippocampal E2 modulates memory-related synaptic plasticity not only slowly/genomically but also rapidly/non-genomically. Slow actions of E2 occur via classical nuclear receptors (ERα or ERβ), while rapid E2 actions occur via synapse-localized or extranuclear ERα or ERβ. Nanomolar concentrations of E2 change rapidly the density and morphology of spines in hippocampal neurons. ERα, but not ERβ, drives this enhancement/suppression of spinogenesis in adult animals. Nanomolar concentrations of androgens (T and DHT) and CORT also increase the spine density. Kinase networks are involved downstream of ERα and androgen receptor. Newly developed Spiso-3D mathematical analysis is useful to distinguish these complex effects by sex steroids and kinases. Significant advance has been achieved in investigations of rapid modulation by E2 of the long-term depression or the long-term potentiation.

Cortisol and corticosterone in immune organs and brain of European starlings: developmental changes, effects of restraint stress, comparison with zebra finches

Glucocorticoids (GCs) are produced in the adrenal glands and also in extra-adrenal sites, including immune organs and brain. Here, we examined regulation of systemic GC levels in plasma and local GC levels in immune organs and brain during development. We conducted two studies and examined a total of 462 samples from 70 subjects. In study 1, we determined corticosterone and cortisol levels in the plasma, immune organs, and brain of wild European starlings on posthatch day 0 (P0) and P10 (at baseline and after 45 min of restraint). Baseline corticosterone and cortisol levels were low in the immune organs and brain at P0 and P10, providing little evidence for local GC synthesis in starlings. At P0, restraint had no significant effects on corticosterone or cortisol levels in the plasma or tissues; however, there was a trend for restraint to increase both corticosterone and cortisol in the immune organs. At P10, restraint increased corticosterone levels in the plasma and all tissues, but restraint increased cortisol levels in the plasma, thymus, and diencephalon only. In study 2, we directly compared GC levels in European starlings and zebra finches at P4. In zebra finches but not starlings, cortisol levels were higher in the immune organs than in plasma. This difference in immune GC levels might be due to evolutionary lineage, life history strategy, or experiential factors, such as parasite exposure. This is the first study to measure immune GC levels in wild animals and one of the first studies to measure local GC levels after restraint stress.

Neurosteroid biosynthesis in the human brain and its clinical implications

This paper summarizes the current knowledge concerning the biosynthesis of neurosteroids in the human brain, the enzymes mediating these reactions, their localization, and the putative effects of neurosteroids. The presence of the steroidogenic enzymes cytochrome P450(SCC), aromatase, 5alpha-reductase, 3alpha-hydroxysteroid dehydrogenase, and 17beta-hydroxysteroid dehydrogenase in the human brain has now been firmly established by molecular biological and biochemical studies. Their presence in the cerebral cortex and in the subcortical white matter indicates that various cell types, either neurons or glial cells, are involved in the biosynthesis of neuroactive steroids in the brain. The following functions are attributed to specific neurosteroids: modulation of GABA(A), N-methyl-d-aspartate (NMDA), nicotinic, muscarinic, serotonin (5-HT(3)), kainate, glycine and sigma receptors, neuroprotection and induction of neurite outgrowth, dendritic spines, and synaptogenesis. We still do not know whether and how the steroidogenic enzymes are involved in the pathophysiology of the nervous system. The first clinical investigations in humans produced evidence for an involvement of neuroactive steroids in conditions such as fatigue during pregnancy, premenstrual syndrome, postpartum depression, catamenial epilepsy, and depressive disorders. Further and improved knowledge of the biochemical pathways of neurosteroidogenesis and their actions on the brain may enable new perspectives in the understanding of the physiology of the human brain as well as in the pharmacological treatment of its disturbances.

Cytochrome P450 2D catalyze steroid 21-hydroxylation in the brain

mRNA of cytochrome P450 21-hydroxylase (P450c21) is expressed in the brain, but little is known about the enzymatic properties of P450c21 in the brain. In the present study, we showed, by using various recombinant cytochrome P450 (CYP)2D enzymes and anti-CYP2D4- or P450c21-specific antibodies, that rat brain microsomal steroid 21-hydroxylation is catalyzed not by P450c21, but by CYP2D isoforms. Rat CYP2D4 and human CYP2D6, which are the predominant CYP2D isoforms in the brain, possess 21-hydroxylation activity for both progesterone and 17alpha-hydroxyprogesterone. In rat brain microsomes, these activities were not inhibited by anti-P450c21 antibodies, but they were effectively inhibited by the CYP2D-specific chemical inhibitor quinidine and by anti-CYP2D4 antibodies. mRNA and protein of CYP2D4 were expressed throughout the brain, especially in cerebellum, striatum, pons, and medulla oblongata, whereas the mRNA and protein levels of P450c21 were extremely low or undetectable. These results support the idea that CYP2D4, not P450c21, works as steroid 21-hydroxylase in the brain. Allopregnanolone, a representative gamma-aminobutyric acid receptor modulator, was also hydroxylated at the C-21 position by recombinant CYP2D4 and CYP2D6. Rat brain microsomal allopregnanolone 21-hydroxylation was inhibited by fluoxetine with an IC(50) value of 2 microm, suggesting the possibility that the brain CYP2D isoforms regulate levels of neurosteroids such as allopregnanolone, and that this regulation is modified by central nervous system-active drugs such as fluoxetine.

Profiling transcript levels for steroidogenic enzymes in fetal tissues

Cytochrome P450 (CYP) and hydroxysteroid dehydrogenase enzymes are involved in the conversion of cholesterol to steroid hormones. These enzymes are primarily expressed in the placenta, adrenal and gonads. Interestingly, some of these enzyme activities have been demonstrated in non-endocrine tissues, where they may be involved in important paracrine and autocrine actions. This is particularly the case in the human fetus where steroid precursors circulate at high levels and could be metabolized within tissues to produce active steroid hormones. Herein, we tested the hypothesis that transcripts for steroidogenic enzymes are expressed in fetal tissues other than the classical steroidogenic organs. To test this hypothesis, real-time reverse transcription polymerase chain reaction (RT-RTPCR) assays were developed that quantify mRNA levels for steroidogenic acute regulatory protein (StAR), cholesterol side-chain cleavage (CYP11A), 3beta-hydroxysteroid dehydrogenase types 1 and 2 (HSD3B1 and HSD3B2), 17alpha-hydroxylase (CYP17), 21-hydroxylase (CYP21), 11beta-hydroxylase (CYP11B1), aldosterone synthase (CYP11B2) and aromatase (CYP19). The use of RT-RTPCR allows the specific detection of these transcripts at levels that would not be detectable using northern analysis. In addition, this method can detect levels of transcript that would not lead to sufficient protein for detection of enzymatic activity of protein by western analysis. Thus, this methodology can detect low levels of expression that could play a role in regulating intra-tissue concentrations of steroid hormone. Total RNAs used for RT-RTPCR analysis were isolated from several human fetal tissues, including adrenal, testis, ovary, placenta, aorta, brain, liver, kidney, heart, lung, pancreas, prostate, stomach, and thymus. Our findings suggest that RT-RTPCR is a powerful tool for the examination of steroidogenic enzyme mRNA expressions. Using this approach, we have identified and quantified transcript levels of StAR and steroidogenic enzymes in several endocrine and non-endocrine fetal tissues. Even though some of the mRNA levels measured in these peripheral tissues are extremely lower in respect to the steroidogenic tissues, they could be sufficient to produce local (i.e. autocrine and paracrine) effects because produced steroids are not diluted into the entire circulation. These findings open new perspectives on the role of steroid hormones synthesized locally as probable regulatory factors of the development of several organ systems.

Steroid 21-hydroxylase expression in cultured rat astrocytes

Over the past decade or so it has become widely recognised that the brain is a significant steroidogenic organ. Many publications have highlighted the ability of the brain to synthesise and interconvert a large number of steroid products including cholesterol, progesterone and testosterone. In this study, in vitro experiments were performed to determine if 21-hydroxylation of steroids is undertaken by rat brain astrocytes in culture. This is a common reaction that occurs in the adrenal gland and other organs in mammals, catalysing the conversion of pregn-4-ene-3,20-dione (progesterone) to 21-hydroxypregn-4-ene-3,20-dione (deoxycorticosterone). Previous reports have indicated that 21-hydroxylation occurs within the rat brain, however, the precise identity of the cells expressing 21-hydroxylase has not yet been determined. Several metabolites, such as 5alpha-pregnan-3alpha-ol-20-one (tetrahydroprogesterone) and 3alpha,21-dihydroxy-5-pregnan-20-one (tetrahydrodeoxycorticosterone) were of particular interest because of their modulatory role in neuronal function, such as their agonist activity at gamma-aminobutyric acid (GABA(A)) receptors. Evidence was obtained for the expression of peripheral 21-hydroxylase enzyme (P450c21) in cultured rat brain astrocytes by a combination of mass spectroscopy and molecular biology techniques. This is a significant finding as expression of 21-hydroxylase within astrocytes may be indicative of a wider role for these cells in modulating neuronal behaviour.

Steroid hormones and neurosteroids in normal and pathological aging of the nervous system

Without medical progress, dementing diseases such as Alzheimer's disease will become one of the main causes of disability. Preventing or delaying them has thus become a real challenge for biomedical research. Steroids offer interesting therapeutical opportunities for promoting successful aging because of their pleiotropic effects in the nervous system: they regulate main neurotransmitter systems, promote the viability of neurons, play an important role in myelination and influence cognitive processes, in particular learning and memory. Preclinical research has provided evidence that the normally aging nervous system maintains some capacity for regeneration and that age-dependent changes in the nervous system and cognitive dysfunctions can be reversed to some extent by the administration of steroids. The aging nervous system also remains sensitive to the neuroprotective effects of steroids. In contrast to the large number of studies documenting beneficial effects of steroids on the nervous system in young and aged animals, the results from hormone replacement studies in the elderly are so far not conclusive. There is also little information concerning changes of steroid levels in the aging human brain. As steroids present in nervous tissues originate from the endocrine glands (steroid hormones) and from local synthesis (neurosteroids), changes in blood levels of steroids with age do not necessarily reflect changes in their brain levels. There is indeed strong evidence that neurosteroids are also synthesized in human brain and peripheral nerves. The development of a very sensitive and precise method for the analysis of steroids by gas chromatography/mass spectrometry (GC/MS) offers new possibilities for the study of neurosteroids. The concentrations of a range of neurosteroids have recently been measured in various brain regions of aged Alzheimer's disease patients and aged non-demented controls by GC/MS, providing reference values. In Alzheimer's patients, there was a general trend toward lower levels of neurosteroids in different brain regions, and neurosteroid levels were negatively correlated with two biochemical markers of Alzheimer's disease, the phosphorylated tau protein and the beta-amyloid peptides. The metabolism of dehydroepiandrosterone has also been analyzed for the first time in the aging brain from Alzheimer patients and non-demented controls. The conversion of dehydroepiandrosterone to Delta5-androstene-3beta,17beta-diol and to 7alpha-OH-dehydroepiandrosterone occurred in frontal cortex, hippocampus, amygdala, cerebellum and striatum of both Alzheimer's patients and controls. The formation of these metabolites within distinct brain regions negatively correlated with the density of beta-amyloid deposits.

Pharmacokinetics of clozapine and its metabolites in hippocampal HT22 cells

Up to now, it is not yet clear whether and how clozapine and its metabolites are metabolized in neuronal cells. The interconversion of clozapine and its metabolites, clozapine-N-oxide and norclozapine, was studied in the hippocampal neuronal in vitro system of HT22 cells. Clinically relevant concentrations of clozapine (200+400 ng/ml) and its metabolites (100+200 ng/ml) were used for the examination of the metabolizing effects after short- (4 h) and long- (24 h) term incubation. Two-way analysis of variance revealed a significant decrease of clozapine (P<0.01) and norclozapine (P<0.01) levels in the supernatants of HT22 cells after the treatment procedures. Student-Newman-Keuls tests showed a significant decrease of clozapine 400 after 24 h of incubation (P=0.01) as well as of all concentrations of norclozapine. No significant treatment effects were found for the clozapine-N-oxide degradation. Using semi-quantification by reverse transcriptase-polymerase chain reaction methods, we could show a significant increase of cytochrome P450 (CYP) 1A2 mRNA levels (P<0.05) after clozapine treatment with 200 ng/ml. The results of the present study strongly suggest that clozapine and norclozapine are metabolized in hippocampal neuronal HT22 cells by CYP1A2, whereas the levels of clozapine-N-oxide were not affected. Moreover, CYP1A2 mRNA levels were significantly changed by incubation with clozapine 200.

Extra-adrenal production of corticosteroids

1. The major corticosteroids aldosterone and cortisol (corticosterone in rodents) are secreted from the adrenal cortex under the regulation of the renin-angiotensin system and the hypothalamic-pituitary-adrenal axis. 2. In addition to their accepted roles in such processes as blood pressure regulation, glycogenesis, hepatic glyconeogenesis and immunosuppression, the corticosteroids have been implicated in the development of cardiac fibrosis, modulation of hippocampal neuron excitability, memory formation and neurodegeneration. 3. The advent of sensitive molecular biological techniques has produced a wealth of evidence to support the existence of extra-adrenal corticosteroidogenic systems. Most attention has been paid to the cardiovascular system and the central nervous system, where the full array of enzymes required for the de novo synthesis of corticosteroids from cholesterol has been identified. 4. Although the evidence for local corticosteroid production is strong, the quantities of steroid would be small compared with adrenal production. Therefore, it is still a matter of debate as to whether extra-adrenal corticosteroids are of any physiological significance. This will depend on factors such as local concentration, proximity to target cells and, possibly, to tissue-specific control mechanisms.

Hippocampal cytochrome P450s synthesize brain neurosteroids which are paracrine neuromodulators of synaptic signal transduction

Hippocampal pyramidal neurons and granule neurons of adult male rats are equipped with a complete machinery for the synthesis of pregnenolone, dehydroepiandrosterone, 17beta-estradiol and testosterone as well as their sulfate esters. These brain neurosteroids are synthesized by cytochrome P450s (P450scc, P45017alpha and P450arom) from endogenous cholesterol. Synthesis is acutely dependent on the Ca(2+) influx attendant upon neuron-neuron communication via N-methyl-D-aspartate (NMDA) receptors. Pregnenolone sulfate, estradiol and corticosterone rapidly modulate neuronal signal transduction and the induction of long-term potentiation via NMDA receptors and putative membrane steroid receptors. Brain neurosteroids are therefore promising neuromodulators that may either activate or inactivate neuron-neuron communication, thereby mediating learning and memory in the hippocampus.

Selective and potent inhibitors of human 20alpha-hydroxysteroid dehydrogenase (AKR1C1) that metabolizes neurosteroids derived from progesterone

Neuroactive steroids, such as 3alpha,5alpha-tetrahydroprogesterone (3alpha,5alpha-THP) and 3alpha,5alpha-tetrahydrodeoxycorticosterone have been shown to be synthesized from progesterone in animal brains. Comparison of kinetic constants for the neuroactive steroids and their precursors among four human 3(20)alpha-hydroxysteroid dehydrogenases (AKR1C1-AKR1C4) suggests that AKR1C1 and AKR1C2 are involved in the catabolism and synthesis, respectively, of the neuroactive steroids in the human brain. In our efforts to identify agents that would specifically inhibit the two enzymes, benzbromarone and 3',3",5',5"-tetrabromophenolphthalein were found to be relatively selective and potent inhibitors of AKR1C1. Kinetic analyses in the oxidoreduction catalyzed by AKR1C1 in the presence of the inhibitors suggest that the inhibitors bind to the enzyme-NADP(H) complex (K(i)=0.7 nM) in the ordered bi-bi pathway, including an isomerization step. The inhibitors effectively also decreased the reduction of 3alpha,5alpha-THP to its 20alpha-hydroxy metabolite in HepG2 cells treated with ethacrynic acid.

Local renin-angiotensin systems and their interactions with extra-adrenal corticosteroid production

Adrenal aldosterone production is regulated by the renin-angiotensin system (RAS). It is now known that several other tissues are capable of extra-adrenal aldosterone biosynthesis and that these tissues can also generate angiotensin II through local RAS. Therefore, the regulation of local aldosterone production by the local RAS is a distinct possibility. In this review, we present evidence for the existence of such systems in the vascular system, heart and brain. We then discuss the possibility of interactions between the RAS and aldosterone synthesis at the local level and speculate on the possible physiological effects of such systems in these tissues.

Steroidogenic enzyme gene expression in the human brain

mRNA, protein and activity for the enzymes required for the synthesis of adrenal corticosteroids have been demonstrated in rat brains by several laboratories. In this study real-time RT-PCR was used to determine whether mRNA for these enzymes are expressed in the human amygdala, caudate nucleus, cerebellum, corpus callosum, hippocampus, spinal cord, and thalamus. Published sequences for the human adrenal enzymes were used to construct primers.

RESULTS

mRNAs encoding cholesterol side-chain cleavage enzyme (CYP11A gene), 17beta-hydroxylase (CYP17), 3beta-hydroxysteroid dehydrogenase (3beta-HSD), 21-hydroxylase (CYP21), 11beta-hydroxysteroid dehydrogenase (11beta-HSD2) and glucocorticoid and mineralocorticoid receptors were detectable in all anatomical regions evaluated. The 11beta-hydroxylase mRNA was detected in all except cerebellum and hippocampus. The aldosterone synthase mRNA was not found in amygdala, cerebellum or hippocampus. Levels of transcripts were 10(-1)-10(-7)-fold lower than those in the adrenal, with corpus callosum and spinal cord having the highest concentrations. Enzyme activity or relevance is yet unknown.