Lack of enantiomeric specificity in the effects of anesthetic steroids on lipid bilayers

Neurosteroid enantiomers as potentially novel neurotherapeutics

Endogenous neurosteroids and synthetic neuroactive steroids (NAS) are important targets for therapeutic development in neuropsychiatric disorders. These steroids modulate major signaling systems in the brain and intracellular processes including inflammation, cellular stress and autophagy. In this review, we describe studies performed using unnatural enantiomers of key neurosteroids, which are physiochemically identical to their natural counterparts except for rotation of polarized light. These studies led to insights in how NAS interact with receptors, ion channels and intracellular sites of action. Certain effects of NAS show high enantioselectivity, consistent with actions in chiral environments and likely direct interactions with signaling proteins. Other effects show no enantioselectivity and even reverse enantioselectivity. The spectrum of effects of NAS enantiomers raises the possibility that these agents, once considered only as tools for preclinical studies, have therapeutic potential that complements and in some cases may exceed their natural counterparts. Here we review studies of NAS enantiomers from the perspective of their potential development as novel neurotherapeutics.

The Mechanism of Enantioselective Neurosteroid Actions on GABAA Receptors

The neurosteroid allopregnanolone (ALLO) and pregnanolone (PREG), are equally effective positive allosteric modulators (PAMs) of GABAA receptors. Interestingly, the PAM effects of ALLO are strongly enantioselective, whereas those of PREG are not. This study was aimed at determining the basis for this difference in enantioselectivity. The oocyte electrophysiology studies showed that ent-ALLO potentiates GABA-elicited currents in α1β3 GABAA receptors with lower potency and efficacy than ALLO, PREG or ent-PREG. The small PAM effect of ent-ALLO was prevented by the α1(Q242L) mutation in the intersubunit neurosteroid binding site between the β3 and α1 subunits. Consistent with this result, neurosteroid analogue photolabeling with mass spectrometric readout, showed that ent-ALLO binds weakly to the β3-α1 intersubunit binding site in comparison to ALLO, PREG and ent-PREG. Rigid body docking predicted that ent-ALLO binds in the intersubunit site with a preferred orientation 180° different than ALLO, PREG or ent-PREG, potentially explaining its weak binding and effect. Photolabeling studies did not identify differences between ALLO and ent-ALLO binding to the α1 or β3 intrasubunit binding sites that also mediate neurosteroid modulation of GABAA receptors. The results demonstrate that differential binding of ent-ALLO and ent-PREG to the β3-α1 intersubunit site accounts for the difference in enantioselectivity between ALLO and PREG.

The Enantiomer of Allopregnanolone Prevents Pressure-Mediated Retinal Degeneration Via Autophagy

In an ex vivo rat ocular hypertension (OHT) model, the neurosteroid allopregnanolone (AlloP) exerts neuroprotective effects via enhancement of both GABA_A receptors and autophagy. We now examine whether its enantiomer (ent-AlloP), which is largely inactive at GABA receptors, offers similar neuroprotection in ex vivo and in vivo rat OHT models. Ex vivo rat retinal preparations were incubated in a hyperbaric condition (10 and 75 mmHg) for 24 h. An in vivo ocular hypertension (OHT) model was induced by intracameral injection of polystyrene microbeads. We examined pharmacological effects of AlloP, ent-AlloP, picrotoxin (a GABA_A receptor antagonist), and 3-MA (an autophagy inhibitor) histologically and biochemically. We found that both AlloP and ent-AlloP have marked neuroprotective effects in the retina, but effects of the unnatural enantiomer are independent of GABA_A receptors. Electron microscopic analyses show that pressure elevation significantly increased autophagosomes (APs) in the nerve fiber layer and addition of AlloP also increased APs and degenerative autophagic vacuoles (AVds). ent-AlloP markedly increased APs and AVds compared to AlloP. Examination of LC3B-II and SQSTM1 protein levels using immunoblotting revealed that AlloP increased LC3B-II, and ent-AlloP further enhanced LC3B-II and suppressed SQSTM1, indicating that autophagy is a major mechanism underlying neuroprotection by ent-AlloP. In an rat in vivo OHT model, single intravitreal ent-AlloP injection prevented apoptotic cell death of retinal ganglion cells similar to AlloP. However, even in this model, ent-AlloP was more effective in activating autophagy than AlloP. We conclude that ent-AlloP may be a prototype of potential therapeutic for treatment of glaucoma as an autophagy enhancer without affecting GABA receptors.

Targeting the Multiple Physiologic Roles of VDAC With Steroids and Hydrophobic Drugs

There is accumulating evidence that endogenous steroids and non-polar drugs are involved in the regulation of mitochondrial physiology. Many of these hydrophobic compounds interact with the Voltage Dependent Anion Channel (VDAC). This major metabolite channel in the mitochondrial outer membrane (MOM) regulates the exchange of ions and water-soluble metabolites, such as ATP and ADP, across the MOM, thus governing mitochondrial respiration. Proteomics and biochemical approaches together with molecular dynamics simulations have identified an impressively large number of non-polar compounds, including endogenous, able to bind to VDAC. These findings have sparked speculation that both natural steroids and synthetic hydrophobic drugs regulate mitochondrial physiology by directly affecting VDAC ion channel properties and modulating its metabolite permeability. Here we evaluate recent studies investigating the effect of identified VDAC-binding natural steroids and non-polar drugs on VDAC channel functioning. We argue that while many compounds are found to bind to the VDAC protein, they do not necessarily affect its channel functions in vitro. However, they may modify other aspects of VDAC physiology such as interaction with its cytosolic partner proteins or complex formation with other mitochondrial membrane proteins, thus altering mitochondrial function.

Neurosteroids as novel antidepressants and anxiolytics: GABA-A receptors and beyond

The recent FDA approval of the neurosteroid, brexanolone (allopregnanolone), as a treatment for women with postpartum depression, and successful trials of a related neuroactive steroid, SGE-217, for men and women with major depressive disorder offer the hope of a new era in treating mood and anxiety disorders based on the potential of neurosteroids as modulators of brain function. This review considers potential mechanisms contributing to antidepressant and anxiolytic effects of allopregnanolone and other GABAergic neurosteroids focusing on their actions as positive allosteric modulators of GABAA receptors. We also consider their roles as endogenous "stress" modulators and possible additional mechanisms contributing to their therapeutic effects. We argue that further understanding of the molecular, cellular, network and psychiatric effects of neurosteroids offers the hope of further advances in the treatment of mood and anxiety disorders.

A clickable neurosteroid photolabel reveals selective Golgi compartmentalization with preferential impact on proximal inhibition

Anesthetic, GABA-active neurosteroids potently augment GABAA receptor function, leading to important behavioral consequences. Neurosteroids and their synthetic analogues are also models for a wide variety of cell-permeant neuroactive compounds. Cell permeation and compartmentalization raise the possibility that these compounds' actions are influenced by their cellular partitioning, but these contributions are not typically considered experimentally or therapeutically. To examine the interplay between cellular accumulation and pharmacodynamics of neurosteroids, we synthesized a novel chemical biology analogue (bio-active, clickable photolabel) of GABA-active neurosteroids. We discovered that the analogue selectively photo-labels neuronal Golgi in rat hippocampal neurons. The active analogue's selective distribution was distinct from endogenous cholesterol and not completely shared by some non-GABA active, neurosteroid-like analogues. On the other hand, the distribution was not enantioselective and did not require energy, in contrast to other recent precedents from the literature. We demonstrate that the soma-selective accumulation can act as a sink or source for steroid actions at plasma-membrane GABA receptors, altering steady-state and time course of effects at somatic GABAA receptors relative to dendritic receptors. Our results suggest a novel mechanism for compartment-selective drug actions at plasma-membrane receptors.

Anticancer steroids: linking natural and semi-synthetic compounds

Steroids, a widespread class of natural organic compounds occurring in animals, plants and fungi, have shown great therapeutic value for a broad array of pathologies. The present overview is focused on the anticancer activity of steroids, which is very representative of a rich structural molecular diversity and ability to interact with various biological targets and pathways. This review encompasses the most relevant discoveries on steroid anticancer drugs and leads through the last decade and comprises 668 references.

Neurosteroids, stress and depression: potential therapeutic opportunities

Neurosteroids are potent and effective neuromodulators that are synthesized from cholesterol in the brain. These agents and their synthetic derivatives influence the function of multiple signaling pathways including receptors for γ-aminobutyric acid (GABA) and glutamate, the major inhibitory and excitatory neurotransmitters in the central nervous system (CNS). Increasing evidence indicates that dysregulation of neurosteroid production plays a role in the pathophysiology of stress and stress-related psychiatric disorders, including mood and anxiety disorders. In this paper, we review the mechanisms of neurosteroid action in brain with an emphasis on those neurosteroids that potently modulate the function of GABA(A) receptors. We then discuss evidence indicating a role for GABA and neurosteroids in stress and depression, and focus on potential strategies that can be used to manipulate CNS neurosteroid synthesis and function for therapeutic purposes.

Modulating the activity of short arginine-tryptophan containing antibacterial peptides with N-terminal metallocenoyl groups

A series of small synthetic arginine and tryptophan containing peptides was prepared and analyzed for their antibacterial activity. The effect of N-terminal substitution with metallocenoyl groups such as ferrocene (FcCO) and ruthenocene (RcCO) was investigated. Antibacterial activity in different media, growth inhibition, and killing kinetics of the most active peptides were determined. The toxicity of selected derivatives was determined against erythrocytes and three human cancer cell lines. It was shown that the replacement of an N-terminal arginine residue with a metallocenoyl moiety modulates the activity of WRWRW-peptides against Gram-positive and Gram-negative bacteria. MIC values of 2–6 µM for RcCO-W(RW)₂ and 1–11 µM for (RW)₃ were determined. Interestingly, W(RW)₂-peptides derivatized with ferrocene were significantly less active than those derivatized with ruthenocene which have similar structural but different electronic properties, suggesting a major influence of the latter. The high activities observed for the RcCO-W(RW)₂- and (RW)₃-peptides led to an investigation of the origin of activity of these peptides using several important activity-related parameters. Firstly, killing kinetics of the RcCO-W(RW)₂-peptide versus killing kinetics of the (RW)₃ derivative showed faster reduction of the colony forming units for the RcCO-W(RW)₂-peptide, although MIC values indicated higher activity for the (RW)₃-peptide. This was confirmed by growth inhibition studies. Secondly, hemolysis studies revealed that both peptides did not lead to significant destruction of erythrocytes, even up to 500 µg/mL for (RW)₃ and 250 µg/mL for RcCO-W(RW)₂. In addition, toxicity against three human cancer cell lines (HepG2, HT29, MCF7) showed that the (RW)₃-peptide had an IC₅₀ value of ~140 µM and the RcW(RW)₂ one of ~90 µM, indicating a potentially interesting therapeutic window. Both the killing kinetics and growth inhibition studies presented in this work point to a membrane-based mode of action for these two peptides, each having different kinetic parameters.

The sticky issue of neurosteroids and GABA(A) receptors

Endogenous neurosteroids and their synthetic analogs (neuroactive steroids) are potent modulators of GABA(A) receptors. Thus, they are of physiological and clinical relevance for their ability to modulate inhibitory function in the CNS. Despite their importance, fundamental issues of neurosteroid actions remain unresolved. Recent evidence suggests that glutamatergic principal neurons, rather than glia, are the major sources of neurosteroid synthesis. Other recent studies have identified putative neurosteroid binding sites on GABA(A) receptors. In this Opinion, we argue that neurosteroids require a membranous route of access to transmembrane-domain binding sites within GABA(A) receptors. This has implications for the design of future neuroactive steroids because the lipid solubility and related accessibility properties of the ligand are likely to be key determinants of receptor modulation.

The influence of the membrane on neurosteroid actions at GABA(A) receptors

Modern views of anesthetic neurosteroid interaction with the GABA(A) receptor conceptualize steroid ligands interacting with a protein binding site on the receptor. It has generally been assumed that the steroid interaction/binding site is contained in an extracellular domain of the receptor, and that steroid interactions are of high potency, evidenced by the low aqueous ligand concentrations required to achieve potentiation of channel function. We have been considering implications of the observations that steroids are quite lipophilic and that recently identified putative steroid binding sites are in transmembrane domains of the receptor. Accordingly, we expect that both the effective plasma membrane steroid concentration and steroid pharmacophore properties will contribute to steady-state potency and to the lifetime of steroid actions following removal of free aqueous steroid. Here we review our recent studies that address the evidence that membrane partitioning and intracellular accumulation are non-specific contributors to the effects of anesthetic steroids at GABA(A) receptors. We compare and contrast the profile of anesthetic steroids with that of sulfated steroids that negatively regulate GABA(A) receptor function. These studies give rise to the view that the inherent affinity of anesthetic steroid for GABA(A) receptors is very low; low effective aqueous concentrations are accounted for by lipid partitioning. This yields a very different picture of the interaction of neurosteroids with the GABA(A) receptor than that of steroid interactions with classical intracellular steroid receptors, which exhibit inherently high affinity. These considerations have practical implications for actions of endogenous neurosteroids. Lipophilicity will tend to promote autocrine actions of neurosteroids at GABA(A) receptors within cells that synthesize neurosteroids, and lipophilic retention will limit intercellular diffusion from the source of steroid synthesis. Lipophilicity and steroid access to the receptor binding sites also must be considerations in drug design if drugs are to effectively reach the target GABA(A) receptor site.

ent-Steroids: novel tools for studies of signaling pathways

Membrane receptors are often modulated by steroids and it is necessary to distinguish the effects of steroids at these receptors from effects occurring at nuclear receptors. Additionally, it may also be mechanistically important to distinguish between direct effects caused by binding of steroids to membrane receptors and indirect effects on membrane receptor function caused by steroid perturbation of the membrane containing the receptor. In this regard, ent-steroids, the mirror images of naturally occurring steroids, are novel tools for distinguishing between these various actions of steroids. The review provides a background for understanding the different actions that can be expected of steroids and ent-steroids in biological systems, references for the preparation of ent-steroids, a short discussion about relevant forms of stereoisomerism and the requirements that need to be fulfilled for the interaction between two molecules to be enantioselective. The review then summarizes results of biophysical, biochemical and pharmacological studies published since 1992 in which ent-steroids have been used to investigate the actions of steroids in membranes and/or receptor-mediated signaling pathways.

The influence of neuroactive steroid lipophilicity on GABAA receptor modulation: evidence for a low-affinity interaction

Anesthetic steroids with actions at gamma-aminobutyric acid type A receptors (GABA(A)Rs) may access transmembrane domain binding site(s) directly from the plasma cell membrane. Accordingly, the effective concentration in lipid phase and the ability of the steroid to meet pharmacophore requirements for activity will both contribute to observed steady-state potency. Furthermore, onset and offset of receptor effects may be rate limited by lipid partitioning. Here we show that several GABA-active steroids, including naturally occurring neurosteroids, of different lipophilicity differ in kinetics and potency at GABA(A)Rs. The hydrophobicity ranking predicted relative potency of GABA(A)R potentiation and predicted current offset kinetics. Kinetic offset differences among steroids were largely eliminated by gamma-cyclodextrin, a scavenger of unbound steroid, suggesting that affinity differences among the analogues are dwarfed by the contributions of nonspecific accumulation. A 7-nitrobenz-2-oxa-1,3-diazole (NBD)-tagged fluorescent analogue of the low-lipophilicity alphaxalone (C17-NBD-alphaxalone) exhibited faster nonspecific accumulation and departitioning than those of a fluorescent analogue of the high-lipophilicity (3alpha,5alpha)-3-hydroxypregnan-20-one (C17-NBD-3alpha5alphaA). These differences were paralleled by differences in potentiation of GABA(A)R function. The enantiomer of C17-NBD-3alpha5alphaA, which does not satisfy pharmacophore requirements for steroid potentiation, exhibited identical fluorescence kinetics and distribution to C17-NBD-3alpha5alphaA, but was inactive at GABA(A)Rs. Simple simulations supported our major findings, which suggest that neurosteroid binding affinity is low. Therefore both specific (e.g., fulfilling pharmacophore requirements) and nonspecific (e.g., lipid solubility) properties contribute to the potency and longevity of anesthetic steroid action.

Characterization of enantiomeric bile acid-induced apoptosis in colon cancer cell lines

Bile acids are steroid detergents that are toxic to mammalian cells at high concentrations; increased exposure to these steroids is pertinent in the pathogenesis of cholestatic disease and colon cancer. Understanding the mechanisms of bile acid toxicity and apoptosis, which could include nonspecific detergent effects and/or specific receptor activation, has potential therapeutic significance. In this report we investigate the ability of synthetic enantiomers of lithocholic acid (ent-LCA), chenodeoxycholic acid (ent-CDCA), and deoxycholic acid (ent-DCA) to induce toxicity and apoptosis in HT-29 and HCT-116 cells. Natural bile acids were found to induce more apoptotic nuclear morphology, cause increased cellular detachment, and lead to greater capase-3 and -9 cleavage compared with enantiomeric bile acids in both cell lines. In contrast, natural and enantiomeric bile acids showed similar effects on cellular proliferation. These data show that bile acid-induced apoptosis in HT-29 and HCT-116 cells is enantiospecific, hence correlated with the absolute configuration of the bile steroid rather than its detergent properties. The mechanism of LCA- and ent-LCA-induced apoptosis was also investigated in HT-29 and HCT-116 cells. These bile acids differentially activate initiator caspases-2 and -8 and induce cleavage of full-length Bid. LCA and ent-LCA mediated apoptosis was inhibited by both pan-caspase and selective caspase-8 inhibitors, whereas a selective caspase-2 inhibitor provided no protection. LCA also induced increased CD95 localization to the plasma membrane and generated increased reactive oxygen species compared with ent-LCA. This suggests that LCA/ent-LCA induce apoptosis enantioselectively through CD95 activation, likely because of increased reactive oxygen species generation, with resulting procaspase-8 cleavage.

Oxidative stress in NPC1 deficient cells: protective effect of allopregnanolone

Niemann-Pick C disease (NPC) is an autosomal recessive neurodegenerative disorder caused by the abnormal function of NPC1 or NPC2 proteins, leading to an accumulation of unesterified cholesterol and glycosphingolipids (GSLs) in the lysosomes. The mechanisms underlying the pathophysiology in NPC disease are not clear. Oxidative damage is implicated in the pathophysiology of different neurological disorders and the effect of GSL accumulation on the intracellular redox state has been documented. Therefore, we determined whether the intracellular redox state might contribute to the NPC disease pathophysiology. Because the treatment of NPC mice with allopregnanolone (ALLO) increases their lifespan and delays the onset of neurological impairment, we analysed the effect of ALLO on the oxidative damage in human NPC fibroblasts. Concentrations of reactive oxygen species (ROS) and lipid peroxidation were higher in fibroblasts from NPC patients than in fibroblasts from normal subjects. Fibroblasts from NPC patients were more susceptible to cell death through apoptosis after an acute oxidative insult. This process is mediated by activation of the NF-κB signalling pathway. Knockdown of NPC1 mRNA both in normal fibroblasts and in human SH-SY5Y neuroblastoma cells caused increased ROS concentrations. ALLO treatment of fibroblasts from NPC patients or NPC1 knockdown cells reduced the levels of ROS and lipid peroxidation and prevented peroxide-induced apoptosis and NF-kB activation. Thus, these findings suggest that oxidative stress might contribute to the NPC disease and ALLO might be beneficial in the treatment of the disease, at least in part, due to its ability to restore the intracellular redox state.

Conserved site for neurosteroid modulation of GABA A receptors

This study addresses whether the potentiation site for neurosteroids on GABA(A) receptors is conserved amongst different GABA(A) receptor isoforms. The neurosteroid potentiation site was previously identified in the alpha1beta2gamma2S receptor by mutation of Q241 to methionine or leucine, which reduced the potentiation of GABA currents by the naturally occurring neurosteroids, allopregnanolone or tetrahydrodeoxycorticosterone (THDOC). By using heterologous expression of GABA(A) receptors in HEK cells, in combination with whole-cell patch clamp recording methods, a relatively consistent potentiation by allopregnanolone of GABA-activated currents was evident for receptors composed of one alpha subunit isoform (alpha2-5) assembled with beta3 and gamma2S subunits. Using mutant alphabetagamma receptors, the neurosteroid potentiation was universally dependent on the conserved glutamine residue in M1 of the respective alpha subunit. Studying wild-type and mutant receptors composed of alpha4beta3delta subunits revealed that the delta subunit is unlikely to contribute to the neurosteroid potentiation binding site and probably affects the efficacy of potentiation. Thus, in keeping with the ability of neurosteroids to potentiate GABA currents via a broad variety of GABA(A) receptor isoforms in neurons, the potentiation site is structurally highly conserved on this important neurotransmitter receptor family.

Effects on membrane capacitance of steroids with antagonist properties at GABAA receptors

We investigated the electrophysiological signature of neuroactive steroid interactions with the plasma membrane. We found that charged, sulfated neuroactive steroids, those that exhibit noncompetitive antagonism of GABA(A) receptors, altered capacitive charge movement in response to voltage pulses in cells lacking GABA receptors. Uncharged steroids, some of which are potent enhancers of GABA(A) receptor activity, produced no alteration in membrane capacitance. We hypothesized that the charge movements might result from physical translocation of the charged steroid through the transmembrane voltage, as has been observed previously with several hydrophobic anions. However, the charge movements and relaxation time constants of capacitive currents did not exhibit the Boltzmann-type voltage dependence predicted by a single barrier model. Further, a fluorescently tagged analog of a sulfated neurosteroid altered membrane capacitance similar to the parent compound but produced no voltage-dependent fluorescence change, a result inconsistent with a strong change in the polar environment of the fluorophore during depolarization. These findings suggest that negatively charged sulfated steroids alter the plasma membrane capacitance without physical movement of the molecule through the electric field.

Endogenous and synthetic neurosteroids in treatment of Niemann-Pick Type C disease

The functions for neurosteroids during development and in response to nervous system injury are beginning to be identified. We focused on a mouse model in which we believed neurosteroid production would be altered, and which had a neurodegenerative phenotype. Niemann-Pick Type-C (NP-C) is an autosomal recessive neurodegenerative disease caused by mutations in NPC1 (95%) or NPC2 (5%), resulting in lysosomal accumulation of unesterified cholesterol and glycolipids. The NIH mouse model of NP-C has a mutation in the NPC1 gene, and exhibits several pathological features of the most severe NP-C patients. How lysosomal storage and trafficking defects lead to neurodegeneration is unknown. We found that these mice had normal neurosteroidogenic enzyme activity during development, but lost this activity in the early neonatal period, prior to onset of neurological symptoms. Neurons that expressed P450scc, 3beta HSD, as well as those that expressed 3alpha HSD and 5alpha reductase were lost in adult NP-C brains, resulting in diminished concentrations of allopregnanolone. We treated NP-C mice with allopregnanolone and found that a single dose in the neonatal period resulted in a doubling of life span, substantial delay in onset of neurological symptoms, survival of cerebellar Purkinje and granule cell neurons, and reduction in cholesterol and ganglioside accumulation. The mechanism by which allopregnanolone elicited these effects is unknown. Our in vitro studies showed that Purkinje cell survival promoted by allopregnanolone was lost by treatment with bicuculline, suggesting GABA(A) receptors may play a role. We treated NP-C mice with a synthetic GABA(A) neurosteroid, ganaxolone (3alpha-hydroxy-3beta-methyl-5alpha-pregnan-20-one). Ganaxolone treatment of NP-C mice produced beneficial neurological effects, but these effects were not as robust as those obtained using allopregnanolone. Thus, allopregnanolone may elicit its effects through GABA(A) receptors and through other mechanisms. Additional studies also suggest that allopregnanolone may elicit its effects through pregnane-X-receptors (PXR). Our data suggest that mouse models of neurodegeneration may be beneficial in establishing both physiologic and pharmacologic actions of neurosteroids. These animal models further establish the wide range of functions of these compounds, which may ultimately be useful for treatment of human diseases.

Neurosteroid modulation of GABAA receptors: molecular determinants and significance in health and disease

Over the past 20 years it has become apparent that certain steroids, synthesised de novo in the brain, hence named neurosteroids, produce immediate changes (within seconds) in neuronal excitability, a time scale that precludes a genomic locus of action. Identified molecular targets underlying modulation of brain excitability include both the inhibitory GABA(A) and the excitatory NMDA receptor. Of particular interest is the interaction of certain neurosteroids with the GABA(A) receptor, the major inhibitory receptor in mammalian brain. During the last decade, compelling evidence has accrued to reveal that locally produced neurosteroids may selectively "fine tune" neuronal inhibition. A range of molecular mechanisms including the subunit composition of the receptor(s), phosphorylation and local steroid metabolism, underpin the region- and neuronal selectivity of action of neurosteroids at synaptic and extrasynaptic GABA(A) receptors. The relative contribution played by each of these mechanisms in a variety of physiological and pathophysiological scenarios is currently being scrutinised at a cellular and molecular level. However, it is not known how such mechanisms may act in concert to influence behavioural profiles in health and disease. An important question concerns the identification of the anatomical substrates mediating the repertoire of behaviours produced by neurosteroids. "Knock-in" mice expressing mutant GABA(A) subunits engineered to be insensitive to benzodiazepines or general anaesthetics have proved invaluable in evaluating the role of GABA(A) receptor subtypes in complex behaviours such as sedation, cognition and anxiety [Rudolph, U., Mohler, H., 2006. GABA-based therapeutic approaches: GABA(A) receptor subtype functions. Curr. Opin. Pharmacol. 6, 18-23]. However, the development of a similar approach for neurosteroids has been hampered by the limited knowledge that, until recently, has surrounded the identity of the amino acid residues contributing to the neurosteroid binding pocket. Here, we will review recent progress in identifying the neurosteroid binding site on the GABA(A) receptor, and discuss how these discoveries will impact on our understanding of the role of neurosteroids in health and disease.

Neurosteroid binding sites on GABA(A) receptors

Controlling neuronal excitability is vitally important for maintaining a healthy central nervous system (CNS) and this relies on the activity of type A gamma-aminobutyric acid (GABA(A)) neurotransmitter receptors. Given this role, it is therefore important to understand how these receptors are regulated by endogenous modulators in the brain and determine where they bind to the receptor. One of the most potent groups of modulators is the neurosteroids which regulate the activity of synaptic and extrasynaptic GABA(A) receptors. This level of regulation is thought to be physiologically important and its dysfunction may be relevant to numerous neurological conditions. The aim of this review is to summarise those studies that over the last 20 years have focussed upon finding the binding sites for neurosteroids on GABA(A) receptors. We consider the nature of steroid binding sites in other proteins where this has been determined at atomic resolution and how their generic features were mapped onto GABA(A) receptors to help locate 2 putative steroid binding sites. Altogether, the findings strongly suggest that neurosteroids do bind to discrete sites on the GABA(A) receptor and that these are located within the transmembrane domains of alpha and beta receptor subunits. The implications for neurosteroid binding to other inhibitory receptors such as glycine and GABA(C) receptors are also considered. Identifying neurosteroid binding sites may enable the precise pathophysiological role(s) of neurosteroids in the CNS to be established for the first time, as well as providing opportunities for the design of novel drug entities.

Mechanisms of neurosteroid interactions with GABA(A) receptors

Neuroactive steroids have some of their most potent actions by augmenting the function of GABA(A) receptors. Endogenous steroid actions on GABA(A) receptors may underlie important effects on mood and behavior. Exogenous neuroactive steroids have potential as anesthetics, anticonvulsants, and neuroprotectants. We have taken multiple approaches to understand more completely the interaction of neuroactive steroids with GABA(A) receptors. We have developed many novel steroid analogues in this effort. Recent work has resulted in synthesis of new enantiomer analogue pairs, novel ligands that probe various properties of the steroid pharmacophore, fluorescent neuroactive steroid analogues, and photoaffinity labels. Using these tools, combined with receptor binding and electrophysiological assays, we have begun to untangle the complexity of steroid actions at this important class of ligand-gated ion channel.

Neurosteroid analogues. 12. Potent enhancement of GABA-mediated chloride currents at GABAA receptors by ent-androgens

Allopregnanolone (1) and pregnanolone (2), steroids containing a 17beta-acetyl group, are potent enhancers of GABA (gamma-aminobutyric acid) action at GABAA receptors. Their effects are enantioselective with the non-naturally occurring enantiomers (ent-1 and ent-2) being less potent. Androsterone (3) and etiocholanolone (4), steroids with a C-17 carbonyl group, are weak enhancers of GABA action at GABAA receptors. Unexpectedly, their enantiomers (ent-3 and ent-4) have been found to have enhanced, not diminished, activity at GABAA receptors. Furthermore, the C-17 spiro-epoxide analogues (ent-5 and ent-6) of ent-3 and ent-4, respectively, have activities comparable to those of steroids 1 and 2. The results indicate that some ent-steroids are potent modulators of GABAA receptors and might have clinical potential as GABAergic drugs of the future.