Neurosteroids: a new brain function?

The biosynthesis of neurosteroids proceeds through cholesterol side-chain cleavage, and gives rise to pregnenolone (P) and dehydroepiandrosterone (D). These steroids accumulate in the rat brain independently of the supply by peripheral endocrine glands. This led to the discovery of a steroid biosynthesis pathway in rat brain oligodendrocytes based on enzyme immunocytochemistry and conversion of radioactive precursors to C-21 steroids. Several biological functions have been proposed for P and D. They may serve as precursors of other steroids (such as progesterone and testosterone and their metabolites). They are implicated in the control of some behavioural activities. They have excitatory effects on neurons, and they modulate the function of GABAA-receptors. These observations may apply to all mammalian species including the human, and the physiological significance of neurosteroid synthesis needs further investigation. The relationship between steroids and cerebral function may be reconsidered in the light of a new fact: the existence of a biosynthetic pathway of these compounds from cholesterol, assured in the brain by the oligodendrocytes, glial cells which synthesize myelin.     

Quantification of neurosteroids in rat plasma and brain following swim stress and allopregnanolone administration using negative chemical ionization gas chromatography/mass spectrometry

A simplified method for the quantitative analysis of neurosteroids in rat plasma and brain is described. The method uses negative chemical ionization gas chromatography/mass spectrometry and involves the synthesis of pentafluorobenzyloxime/trimethylsilyl ether derivatives with excellent chromatographic and electron-capturing properties. Deuterium-labeled analogs of the steroids of interest were synthesized and used as internal standards. The steroids (allopregnanolone, epiallopregnanolone, pregnenolone, testosterone, and dehydroepiandrosterone) were isolated from the plasma or brain matrix by a rapid and straightforward solid-phase extraction procedure. The mass spectrometer was operated in a selective ion monitoring mode, allowing for picograms of neurosteroids to be quantified from biological extracts. The method was linear (typical R(2) = 0.999) over the concentration range (100 to 8000 pg from 0.3 ml plasma and 250 to 8000 pg from 100 mg brain tissue) with good precision and accuracy. In experimental protocols, the procedure was suitable for measuring concentrations of endogenous neurosteroids in rat plasma and brain. Significant elevations (P < 0.001) were observed in the frontal cortex for allopregnanolone and pregnenolone following a swim stress and for allopregnanolone and epiallopregnanolone following allopregnanolone injection (8 mg/kg, sc). The present method allows accurate determination of neurosteroids and will be helpful in elucidating the role of neurosteroids in health and disease.     

Immunocytochemical localization of cytochrome P-450scc in cultured rat oligodendrocytes

Primary cultures of glial cells from newborn rat forebrain were tested after 3 to 4 weeks. Oligodendrocytes and astrocytes were characterized by immunofluorescence with monoclonal antibodies to galactocerebroside and glial fibrillary acidic protein, respectively. The cytoplasm of oligodendrocytes was specifically and intensely immunostained with monospecific polyclonal antibodies to the cytochrome P-450scc involved in the synthesis of pregnenolone from cholesterol. This observation brings additional support to the concept of "neurosteroids".     

Steroid synthesis and metabolism in the nervous system: Trophic and protective effects

Steroids influence the activity and plasticity of neurons and glial cells during early development, and they continue to exert trophic and protective effects in the adult nervous system. Steroids are produced by the gonads and adrenal glands and reach the brain, the spinal cord and the peripheral nerves via the bloodstream. However, some of them, named “neurosteroids”, can also be synthesized within the nervous system. They include pregnenolone, progesterone, dehydroepiandrosterone and their reduced metabolites and sulfate esters. Little is known concerning the regulation of steroid synthesis in the nervous system, which involves interactions between different cell types. For example, the synthesis of progesterone by Schwann cells in peripheral nerves is regulated by a diffusible neuronal signal. Neurotrophic and neuroprotective effects of steroids have been documented both in cell culture and in vivo. PROG plays an important role in the neurological recovery from traumatic injury of the brain and spinal cord by mechanisms involving protection from excitotoxic cell death, lipid peroxydation and the induction of specific enzymes. After transection of the rat spinal cord, PROG increases the number of nitric oxide synthase expressing astrocytes immediately above and below the lesion. PROG also plays an important role in the formation of new myelin sheaths. This has been shown in the regenerating mouse sciatic nerve after lesion and in cocultures of sensory neurons and Schwann cells. PROG promotes myelination by activating the expression of genes coding for myelin proteins. The modulation of neurostransmitter receptors, in particular the type A γ-aminobutyric acid, the N-methyl-D-aspartate and the sigma 1 receptors, is involved in the psychopharmacological effects of steroids and allows to explain their anticonvulsant, anxiolytic, antidepressive and sedative effects as well as their influence on memory. Pregnenolone sulfate has been shown to reverse age-related deficits in spatial memory performance and to have protective effects on memory in different models of amnesia.     

Pathways of dehydroepiandrosterone formation in rat brain glia

In peripheral steroidogenic tissues, dehydroepiandrosterone (D) is formed from pregnenolone (P) by the microsomal cytochrome P450c17 enzyme. Although some steroidogenic P450s have been found in brain tissue, no enzyme has been shown to possess P450c17 activity. We recently demonstrated the presence of an alternative, Fe(2+)-dependent pathway responsible for D formation from alternative precursors in rat glioma cells. We and others could not find P450c17 mRNA and protein in rat brain, but demonstrate herein the presence of Fe(2+)-dependent alternative pathway for D formation in rat brain cortex microsomes. Using primary cultures of differentiating rat glial cells, we observed that P450c17 mRNA and protein were present in O-2A oligodendrocyte precursors and mature oligodendrocytes. In the presence of P, O-2A and mature oligodendrocytes formed D. Addition of Fe(2+) together with submaximal concentrations of P increased D formation by these cells. Treatment of oligodendrocytes with the P450c17 inhibitor SU 10603 in the presence or absence of P failed to inhibit D production. These data suggest that D formation in oligodendrocytes occurs independently of the P450c17 protein present in the cells. In isolated type I astrocytes we did not find neither P450c17 mRNA nor protein. These cells responded to Fe(2+) by producing D and addition of P together with Fe(2+) further increased D synthesis. SU 10603 failed to inhibit D formation by astrocytes. Taken together these results suggest that in differentiating rat brain oligodendrocytes and astrocytes D is formed via a P450c17-independent and oxidative stress-dependent alternative pathway.     

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.     

Enzyme-assisted synthesis and characterization of glucuronide conjugates of neuroactive steroids

Synthesis of reference standards is needed to determine the presence and function of steroid glucuronides in the brain or other tissues, because commercial sources of steroid glucuronide standards are limited or unavailable. In the present study porcine, rat, and bovine liver microsomes were tested to evaluate their ability to glucuronidate eight neurosteroids and neuroactive steroids of various types: dehydroepiandrosterone, pregnenolone, isopregnanolone, 5alpha-tetrahydrodeoxycorticosterone, corticosterone, cortisol, beta-estradiol, and testosterone. In general, the glucuronidation efficiency of rat liver was rather poor compared with that of bovine and porcine liver microsomes. Since porcine liver apparently has a relatively large amount of dehydrogenase, its microsomes also produced dehydrogenated steroids and their glucuronides, as well as various regioisomers in which the site of glucuronidation varied. In contrast, bovine liver microsomes produced mainly a single major glucuronidation product and few dehydrogenation products and gave the best overall yield for two-third of the steroids tested. The enzymatic synthesis of five glucuronides of four steroids was carried out and the conditions, purification, and analytical methods for the glucuronidation products were optimized. The steroid glucuronides synthesized were characterized by nuclear magnetic resonance spectroscopy (NMR) and liquid chromatography-mass spectrometry (LC-MS). The stereochemically pure steroid glucuronide conjugates were recovered in milligram amounts (yield 10-78%) and good purity (>85-90%), which is sufficient for LC-MS/MS method development and analyses of steroid glucuronides in biological matrices such as brain, urine, or plasma.     

Modulation of Neurosteroid Synthesis/Accumulation by l-Ascorbic Acid in Rat Brain Tissue: Inhibition by Selected Serotonin Antagonists

Abstract: We have investigated the possibility that the synthesis/accumulation of neurosteroids, i.e., brain-produced steroids putatively endowed with modulatory actions in the CNS, is regulated by monoaminergic receptor-mediated mechanisms. In minces of rat brain cortex, l -ascorbic acid concentration-dependently (0.07–1.0 m M ) increases the levels of pregnenolone, allotetrahydrodeoxycorticosterone, and dehydroepiandrosterone. This effect of l -ascorbic acid is region-dependent: in hippocampus, progesterone and allopregnanolone are also increased, whereas dehydroepiandrosterone is unchanged, and in corpus striatum only progesterone is increased significantly. 5-Hydroxytryptamine (10 µ M ), 1-(3-chlorophenyl)piperazine (1.0 µ M ), and 5-methoxytryptamine (0.4 µ M ) mimic the effect of l -ascorbic acid, whereas a pretreatment with p -chlorophenylalanine (400 mg/kg i.p., 2 days) reduces the amplitude of the l -ascorbic acid effect on brain cortical neurosteroids. The effect of l -ascorbic acid is blocked by the nonselective serotonin antagonists methiothepin, clozapine, methysergide, and pizotifen, but not mesulergine, spiperone, MDL 72222, and dl -propranolol, nor by the catecholaminergic receptor antagonists prazosin and S (−)-sulpiride. l -Ascorbic acid is not additive with dibutyryl-cyclic AMP and, furthermore, the inhibition of adenylate cyclase by MDL 12330A, but not of phospholipase C by U-73122, markedly attenuates the l -ascorbic acid-induced increase of pregnenolone in rat brain cortical minces. Together these data suggest that l -ascorbic acid plays a role in the modulation of neurosteroidogenesis, presumably by favoring the activation of the purported serotonin type 6 receptor by endogenous serotonin.     

Simultaneous radioimmunoassay of progesterone, androst-4-enedione, pregnenolone, dehydroepiandrosterone and 17-hydroxyprogesterone in specific regions of human brain

Simultaneous determination of progesterone, androst-4-enedione, pregnenolone, dehydroepiandrosterone (DHEA) and 17-hydroxyprogesterone has been developed for human cerebral tissue. Before immunoassay, steroids were separated on a Celite column with propylene glycol as stationary phase with hexane containing increasing proportions of dichloromethane as mobile phase. This system allowed separation of steroids of similar polarity, especially of pregnenolone and progesterone. The brain regions studied cortex (prefrontal, parietal and temporal), cerebellum and corpus callosum, were obtained after autopsy from 9 women and 1 man between 76 and 93 years of age. Steroids were found in all regions. The overall concentrations expressed in nmol/kg of tissue were: 10.1, 7.6, 120.7, 19.6 and 10.4 respectively, for progesterone, androst-4-enedione, pregnenolone, dehydroepiandrosterone and 17-hydroxyprogesterone, corresponding to 7.3, 4.9, 74, 6.5 and 9.2 times the plasma levels. These very high concentrations, not previously described in human brain tissue, pose the question of the existence of local biosynthetic pathways independent of the peripheral endocrine gland system as well as that of progressive accumulation of steroids over a lifetime. Concentrations of each steroid in each subject varied little among the various brain regions studied, but there was much variation among the subjects with respect to the concentrations of a given steroid.     

Fetal alcohol exposure alters neurosteroid levels in the developing rat brain

Neurosteroids are modulators of neuronal function that may play important roles in brain maturation. We determined whether chronic prenatal ethanol exposure altered neurosteroid levels in the developing brain. Rat dams were exposed to: (i) a 5% ethanol-containing liquid diet that produces peak maternal blood alcohol levels near the legal intoxication limit (approximately 0.08 g/dL); (ii) an isocaloric liquid diet containing maltose-dextrin instead of ethanol with pair-feeding; (iii) rat chow ad libitum. Neurosteroid levels were assessed in offspring brains using radioimmunoassay or gas chromatography-mass spectrometry techniques. A prenatal ethanol exposure-induced increase in pregnenolone sulfate levels, but not dehydroepiandrosterone sulfate levels, was evident at the earliest time point studied (embryonic day 14). This effect lasted until post-natal day 5. Levels of other neurosteroids were assessed at embryonic day 20; pregnenolone levels, but not allopregnanolone levels, were elevated. Pregnenolone sulfate levels were not altered in the maternal brain. Neither pregnenolone nor pregnenolone sulfate levels were significantly altered in the fetal liver, placenta and maternal blood, indicating that the effect of ethanol is not secondary to accumulation of peripherally-produced steroids. Fetal ethanol exposure has been shown to decrease both cellular and behavioral responsiveness to neurosteroids, and our findings provide a plausible explanation for this effect.     

Progesterone as a neuroactive neurosteroid, with special reference to the effect of progesterone on myelination

Some steroids are synthesized within the central and peripheral nervous system, mostly by glial cells. These are known as neurosteroids. In the brain, certain neurosteroids have been shown to act directly on membrane receptors for neurotransmitters. For example, progesterone inhibits the neuronal nicotinic acetylcholine receptor, whereas its 3alpha,5alpha-reduced metabolite 3alpha, 5alpha-tetrahydroprogesterone (allopregnanolone) activates the type A gamma-aminobutyric acid receptor complex. Besides these effects, neurosteroids also regulate important glial functions, such as the synthesis of myelin proteins. Thus, in cultures of glial cells prepared from neonatal rat brain, progesterone increases the number of oligodendrocytes expressing the myelin basic protein (MBP) and the 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNPase). An important role for neurosteroids in myelin repair has been demonstrated in the rodent sciatic nerve, where progesterone and its direct precursor pregnenolone are synthesized by Schwann cells. After cryolesion of the male mouse sciatic nerve, blocking the local synthesis or action of progesterone impairs remyelination of the regenerating axons, whereas administration of progesterone to the lesion site promotes the formation of new myelin sheaths.     

Strain differences of neurosteroid levels in mouse brain

Neurosteroids, pregnenolone (Preg), dehydroepiandrosterone (DHEA) and their sulfates (PregS and DHEAS) are reported to exert their modulatory effects of neuronal excitability and synaptic plasticity via amino acid receptors, which affect and regulate the learning and memory process, mood, and depression. Although the brain levels of these steroids have been reported in rodents, the strain differences of the levels of these steroids have not been demonstrated. We examined the concentrations of Preg, 17-OH-Preg, DHEA, androstenediol (ADIOL) and their sulfates in whole brains from DBA/2, C57BL/6, BALB/c, ddY and ICR mice, the genetic backgrounds of which are different. No differences in the brain levels of Preg and DHEA were found among the strains. In contrast, PregS levels in DBA/2 were significantly lower than in the others, while DHEAS concentrations in DBA/2 were significantly higher than those in other strains. Strain differences were found in 17-OH-Preg, ADIOL and 17-OH-PregS but not in ADIOLS levels. The ranges of Preg and PregS levels were the highest among the steroids studied. Further, we measured serum these steroid levels. Although strain differences were also found in serum steroids, correlation study between brain and serum levels revealed that brain neurosteroids studied may not come from peripheral circulation. In conclusion, this is the first report of demonstrating mammalian brain levels of 17-OH-Preg, ADIOL, 17-OH-PregS and ADIOLS and the strain differences in neurosteroid levels in mice brains. The differences in levels may involve the strain differences in their behavior, e.g. aggression, adaptation to stress or learning, in mice.     

Neurosteroid biosynthesis in the quail brain: a review

The brain traditionally has been considered to be a target site of peripheral steroid hormones. In contrast to this classical concept, new findings over the past decade have shown that the brain itself also has the capability of forming steroids de novo, the so-called "neurosteroids". De novo neurosteroidogenesis in the brain from cholesterol is a conserved property of vertebrates. Our studies using the quail, as an excellent animal model, have demonstrated that the avian brain possesses cytochrome P450 side-chain cleavage enzyme (P450scc), 3beta-hydroxysteroid dehydrogenase/Delta(5)-Delta(4)-isomerase (3beta-HSD), cytochrome P450 17alpha-hydroxylase/c17,20-lyase (P450(17alpha,lyase)), 17beta-HSD, etc., and produces pregnenolone, progesterone, 3beta, 5beta-tetrahydroprogesterone, androstenedione, testosterone and estradiol from cholesterol. However, the biosynthetic pathway of neurosteroids in the avian brain from cholesterol may be still incomplete, because we recently found that the quail brain actively produces 7alpha-hydroxypregnenolone, a previously undescribed avian neurosteroid. This paper summarize the advances made in our understanding of biosynthesis of neurosteroids in the avian brain.     

Neurosteroidogenesis in Rat Retinas

Abstract: Neurosteroids (steroids synthesized in the CNS) function by modulating neurotransmission. To establish an experimental model for investigation of neurosteroid synthesis and regulation, independent of blood-borne steroids, we examined the steroidogenic activity of isolated rat retinas. We identified progesterone, pregnenolone, dehydroepiandrosterone, desoxycorticosterone, 3α,5α-tetrahydrodesoxycorticosterone, 3α-hydroxy-5α-dihydroprogesterone, 17-hydroxyprogesterone, and 17-hydroxypregnenolone together with their esterified forms. As pregnenolone is the precursor of all steroids, its formation was studied in detail as an index of a steroid-synthesizing tissue. Pregnenolone was identified further by gas chromatography coupled to mass spectrometry, and its in vitro synthesis was inhibited by lovastatin, an inhibitor of mevalonolactone and cholesterol biosynthesis. We then examined pregnenolone synthesis in the presence of mevalonolactone as a precursor of sterol formation together with lovastatin, which reduces endogenous mevalonolactone synthesis, as well as with inhibitors of pregnenolone metabolism. The incorporation of mevalonolactone into pregnenolone and its sulfate ester was time- and concentration-dependent and blocked by aminoglutethimide, a competitive inhibitor of cytochrome P450 side-chain cleavage (P450_scc) enzyme. Immunocytochemical studies with a specific antibody to P450_scc revealed a primary localization of the enzyme at the retinal ganglion cell layer. A less pronounced immunostaining was also seen at cells of the inner nuclear layer. Compounds known to stimulate cyclic AMP content also stimulated pregnenolone formation by rat retinas. These results demonstrate that rat retinas synthesize steroids and, for the first time, they reveal the steroidogenic ability of neuronal cells. We propose rat retinas as an in vitro model system to study neurosteroidogenesis in the CNS.     

Sex steroids and 5-en-3 beta-hydroxysteroids in specific regions of the human brain and cranial nerves

Sex steroids and 5-en-3 beta-hydroxysteroids were determined by radioimmunoassay in specific regions of the human brain, in the anterior and posterior pituitary, in one sensory organ, the retina and in the cranial nerves. Progesterone, androstenedione, testosterone and estrone were found in all areas of the brain and in all the cranial nerves but not in all cases. There was no sex difference except in the case of androstenedione where values were higher in women in some brain areas. Estrone values were always higher than those of estradiol in both men and women. No 5 alpha-dihydrotestosterone was detected in any of the samples studied. The values for pregnenolone, dehydroepiandrosterone and their sulfates were much higher than those of the sex steroids in all areas of the brain and in all the cranial nerves. Values for pregnenolone were greater than those of its sulfate while those of dehydroepiandrosterone were in general equal to or higher than those of its sulfate. The values for pregnenolone were greater than those of dehydroepiandrosterone. There were no obvious regional differences in the concentrations of the 5-en-3 beta-hydroxysteroids either in specific areas of the brain or in the cranial nerves. But there was a definite trend for the free dehydroepiandrosterone values to be higher in women. The possible significance of these observations is discussed.     

Neurosteroids: trophic effects in the nervous system]

Some steroids, named "neurostero?ds", can be synthesized from cholesterol within both the central and peripheral nervous systems. Thus, pregnenolone and progesterone persist in the brain and in peripheral nerves long after removal of the steroidogenic endocrine glands by castration and adrenalectomy. The role of neurosteroids during the development of the nervous system is not well known, although they are synthesized by glial cells and some populations of neurons already during embryonic life. Cell culture experiments suggest that neurosteroids may influence the survival and differentiation of neurons and glial cells. In the adult nervous system, neurosteroids play an important role during regeneration. Progesterone is indeed synthesized by Schwann cells in peripheral nerves, where it plays an important role in the formation of new myelin sheaths after lesion. This is the first demonstration of a vital role for a neurosteroid in the nervous system.     

Models for the study of memory and neurosteroids]

The steroids dehydroepiandrosterone sulfate (DHEA-S) and pregnenolone sulfate (Preg-S) are naturally synthetized in the brain. They improve short term and long term memory performances in a variety of learning tasks and models of amnesia in rodents. DHEA-S and Preg-S modulate GABAergic and glutamatergic synaptic transmission through direct interactions with GABA-A, NMDA and/or sigma 1 membrane receptors. In addition, these two neurosteroids facilitate the release of acetylcholine and modulate synaptic plasticity phenomena in cerebral structures, such as the hippocampus, known to play a role in learning and memory processes. The possible links between these actions and the promnestic effects of DHEA-S and Preg-S are discussed in the present review.     

The implications of 7-dehydrosterol-7-reductase deficiency (Smith-Lemli-Opitz syndrome) to neurosteroid production

Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive, multiple malformation/mental retardation syndrome with an estimated incidence among individuals of European ancestry of 1 in 20000 to 1 in 30000. It is caused by inactivity of the enzyme 7-dehydrosterol-delta(7)-reductase, which catalyses the terminal transformation in cholesterol synthesis. Patients show not only an increased level of 7-dehydrocholesterol in blood and tissues, but also increased 8-dehydrocholesterol because of the presence of an active delta(8)-delta(7) isomerase. A major consequence of these biochemical abnormalities is the alteration of normal embryonic and fetal somatic development causing postnatal abnormalities of growth, learning, language and behavior. While deficient cholesterol during early development is the primary cause of central nervous system (CNS) abnormalities and retardation, we questioned whether neurosteroids could also be involved since they can have a profound influence on behavioral characteristics. Disordered neurosteroidogenesis would be expected in SLOS and could be caused by a deficiency in classical neurosteroid synthesis secondary to cholesterol deficiency, or by synthesis from 7- and 8-dehydrocholesterol of novel neurosteroids with delta(7) or delta(8) unsaturation which may have altered activity compared with conventional neurosteroids. In particular we sought analogues of dehydroepiandrosterone sulfate, pregnenolone sulfate and the pregnanolone epimers. We targeted urine from post-pubertal females, as this type of sample would be most likely to yield identifiable amounts of the pregnanolone metabolites of progesterone. Analysis by GC/MS of urinary steroids excreted by post-pubertal females confirmed the presence of neurosteroid-like compounds in SLOS patient's urine. Even though the new neuroactive steroids identified were unlikely to have been formed in the brain, it is likely that mechanisms for their synthesis are operable in this organ.     

Neuroactive steroids.

Neuroactive steroids are natural or synthetic steroids that rapidly alter the excitability of neurons by binding to membrane-bound receptors such as those for inhibitory and (or) excitatory neurotransmitters. The best-studied neuroactive steroids are a series of sedative-hypnotic 3 alpha-hydroxy ring A-reduced pregnane steroids that include the major metabolites of progesterone and deoxycorticosterone, 3 alpha-hydroxy-5 alpha-pregnan-20-one (allopregnanolone) and 3 alpha,21-dihydroxy-5 alpha-pregnan-20-one (allotetrahydroDOC), respectively. These 3 alpha-hydroxysteroids do not interact with classical intracellular steroid receptors but bind stereoselectively and with high affinity to receptors for the major inhibitory neurotransmitter in brain, gamma-amino-butyric acid (GABA). Biochemical and electrophysiological studies have shown that these steroids markedly augment GABA-activated chloride ion currents in a manner similar (but not identical) to that of anesthetic barbiturates. Several steroids have also been observed to have convulsant or proconvulsant properties, including the synthetic amidine 3 alpha-hydroxy-16-imino-5 beta-17-azaandrostan-11-one (RU5135) and the natural sulfate esters of pregnenolone and dehydroepiandrosterone. Several of these have been shown to be bicuculline or picrotoxin-like GABAA receptor antagonists. Examples of steroids that alter neuronal excitability rapidly by augmenting or inhibiting excitatory amino acid receptor-mediated responses have also been reported. Recently, allopregnanolone and allotetrahydroDOC have also been measured in brain and plasma where their levels have been shown to fluctuate in response to stress and during the estrous and menstrual cycles of rats and humans, respectively. Although the major fraction of allopregnanolone in tissue, including brain, is of adrenal and/or ovarian origin, appreciable levels of allopregnanolone can still be measured in the brains of adrenalectomized and/or oophorectomized animals. Receptor-active neurosteroids may represent an important class of neuromodulators that can rapidly alter central nervous system excitability via novel nongenomic mechanisms.     

Neuropsychopharmacological properties of neuroactive steroids.

In addition to the well-known genomic effects of steroid molecules via intracellular steroid receptors, certain steroids rapidly alter neuronal excitability through interaction with neurotransmitter-gated ion channels. Several of these steroids accumulate in the brain after local synthesis or after metabolism of adrenal steroids. The 3alpha-hydroxy ring A-reduced pregnane steroids allopregnanolone and tetrahydrodeoxycorticosterone have been thought not to interact with intracellular receptors, but enhance gamma-aminobutyric acid (GABA)-mediated chloride currents, whereas pregnenolone sulfate and dehydroepiandrosterone (DHEA) sulfate display functional antagonistic properties at GABA(A) receptors. We demonstrated that these neuroactive steroids can regulate also gene expression via the progesterone receptor after intracellular oxidation. Thus, in physiological concentrations these neuroactive steroids regulate neuronal function through their concurrent influence on transmitter-gated ion channels and gene expression. When administered in animal studies, memory-enhancing effects have been shown for pregnenolone sulfate and DHEA. The 3alpha-hydroxy ring A-reduced neuroactive steroids predominantly display anxiolytic, anticonvulsant, and hypnotic activities. Sleep studies evaluating the effects of progesterone as a precursor molecule for these neuroactive steroids revealed a sleep electroencephalogram pattern similar to that obtained by the administration of benzodiazepines. These findings extend the concept of a "cross-talk" between membrane and nuclear hormone effects and provide a new role for the therapeutic application of these steroids in neurology and psychiatry.