Enantiomers of neuroactive steroids support a specific interaction with the GABA-C receptor as the mechanism of steroid action

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.

Conformations of Steroid Hormones: Infrared and Vibrational Circular Dichroism Spectroscopy

Steroid hormone molecules may exhibit very different functionalities based on the associated functional groups and their 3D arrangements in space, i.e., absolute configurations and conformations. Infrared (IR) and vibrational circular dichroism (VCD) spectra of four different steroid hormones, namely dehydroepiandrosterone (DHEA), 17α-methyltestosterone (MTTT), (16α,17)-epoxyprogesterone (Epoxy-P4), and dehydroepiandrosterone acetate (AcO-DHEA), were measured in deuterated dimethyl sulfoxide and some also in carbon tetrachloride. Extensive conformational searches were carried out using the recent developed conformer-rotamer ensemble sampling tool (CREST) which also accounts for solvent effects using an implicit solvation model. All the CREST conformational candidates were then reoptimized at the B3LYP-D3BJ/def2-TZVPD with the PCM of solvent. The good agreements between the experimental IR and VCD spectra and the theoretical simulations provide a conclusive information about their conformational distribution and absolute configurations. The experimental and theoretical IR and VCD spectra of AcO-DHEA in the carbonyl and alkene stretching region showed some discrepancies, and the possible causes related to solvent effects, large amplitude motions and levels of theory used in the modelling were explored in detail. As part of the investigation, additional calculations at the B3LYP-D3BJ/6-31++G (2d,p) and B3LYP-D3BJ/cc-pVTZ levels, as well as some 'mixed' calculations with the double-hybrid functional B2PLYP-D3 were also carried out. The results indicate that the double-hybrid functional is important for predicting the correct IR band pattern in the carbonyl and alkene stretching region.

Analysis of Modulation of the ρ1 GABAA Receptor by Combinations of Inhibitory and Potentiating Neurosteroids Reveals Shared and Distinct Binding Sites

The ρ1 GABA_A receptor is prominently expressed in the retina and is present at lower levels in several brain regions and other tissues. Although the ρ1 receptor is insensitive to many anesthetic drugs that modulate the heteromeric GABA_A receptor, it maintains a rich and multifaceted steroid pharmacology. The receptor is negatively modulated by 5β-reduced steroids, sulfated or carboxylated steroids, and β-estradiol, whereas many 5α-reduced steroids potentiate the receptor. In this study, we analyzed modulation of the human ρ1 GABA_A receptor by several neurosteroids, individually and in combination, in the framework of the coagonist concerted transition model. Experiments involving coapplication of two or more steroids revealed that the receptor contains at least three classes of distinct, nonoverlapping sites for steroids, one each for the inhibitory steroids pregnanolone (3α5βP), 3α5βP sulfate, and β-estradiol. The site for 3α5βP can accommodate the potentiating steroid 5αTHDOC. The findings are discussed with respect to receptor modulation by combinations of endogenous neurosteroids. SIGNIFICANCE STATEMENT: The study describes modulation of the ρ1 GABA_A receptor by neurosteroids. The coagonist concerted transition model was used to determine overlap of binding sites for several inhibitory and potentiating steroids.

A chimeric prokaryotic-eukaryotic pentameric ligand gated ion channel reveals interactions between the extracellular and transmembrane domains shape neurosteroid modulation

Pentameric ligand-gated ion channels (pLGICs) are the targets of several clinical and endogenous allosteric modulators including anesthetics and neurosteroids. Molecular mechanisms underlying allosteric drug modulation are poorly understood. Here, we constructed a chimeric pLGIC by fusing the extracellular domain (ECD) of the proton-activated, cation-selective bacterial channel GLIC to the transmembrane domain (TMD) of the human ρ1 chloride-selective GABAAR, and tested the hypothesis that drug actions are regulated locally in the domain that houses its binding site. The chimeric channels were proton-gated and chloride-selective demonstrating the GLIC ECD was functionally coupled to the GABAρ TMD. Channels were blocked by picrotoxin and inhibited by pentobarbital, etomidate and propofol. The point mutation, ρ TMD W328M, conferred positive modulation and direct gating by pentobarbital. The data suggest that the structural machinery mediating general anesthetic modulation resides in the TMD. Proton-activation and neurosteroid modulation of the GLIC-ρ chimeric channels, however, did not simply mimic their respective actions on GLIC and GABAρ revealing that across domain interactions between the ECD and TMD play important roles in determining their actions. Proton-induced current responses were biphasic suggesting that the chimeric channels contain an additional proton sensor. Neurosteroid modulation of the GLIC-ρ chimeric channels by the stereoisomers, 5α-THDOC and 5β-THDOC, were swapped compared to their actions on GABAρ indicating that positive versus negative neurosteroid modulation is not encoded solely in the TMD nor by neurosteroid isomer structure but is dependent on specific interdomain connections between the ECD and TMD. Our data reveal a new mechanism for shaping neurosteroid modulation.

Synthesis and chemical reactions of the steroidal hormone 17α-methyltestosterone

Structural modifications of natural products with complex structures like steroids require great synthetic effort. A review of literature is presented on the chemistry of the steroidal hormone 17α-methyltestosterone that is approved by Food and Drug Administration (FDA) in the United States as an androgen for estrogen-androgen hormone replacement therapy treatment. The analog also offers special possibilities for the prevention/treatment of hormone-sensitive cancers. The testosterone skeleton has important functionalities in the molecule that can act as a carbonyl component, an active methylene compound, α,β-unsaturated enone and tertiary hydroxyl group in various chemical reactions to access stereoisomeric steroidal compounds with potent activity. In addition, microbiological methods of synthesis and transformation of this hormone are presented.

Chirality and anaesthetic drugs: A review and an update

Many molecules can exist as right-handed and left-handed forms that are non-superimposable mirror images of each other. They are known as enantiomers or substances of opposite shape. Such compounds are also said to be chiral (Greek chiros meaning ‘hand’). Such chiral molecules are of great relevance to anaesthetic theory and practice. This review summarizes the basic concepts, pharmacokinetic and pharmacodynamic aspects of chirality, and some specific examples of their application in anaesthesia, along with recent advances to elucidate the anaesthetic mechanisms. Chirality is relevant to anaesthesia, simply because more than half of the synthetic agents used in anaesthesia practice are chiral drugs. Almost all these synthetic chiral drugs are administered as racemic mixture, rather than as single pure enantiomers. These mixtures are not drug formulations containing two or more therapeutic substances, but combination of isomeric substances, with the therapeutic activity residing mainly in one of the enantiomer. The other enantiomer can have undesirable properties, have different therapeutic activities or be pharmacologically inert. Specific examples of application of chirality in anaesthetic drugs include inhalational general anaesthetics (e.g. isoflurane), intravenous anaesthetics (e.g. etomidate, thiopentone), neuromuscular blocking agents (e.g. cisatracurium), local anaesthetics (e.g. ropivacaine and levobupivacaine) and other agents (e.g. levosimendan, dexmedetomidine, L-cysteine). In the recent advances, chirality study has not only helped new drug development as mentioned above, but has also contributed in a more profound way to the understanding of the mechanism of anaesthesia and anaesthetic drugs.

Hydrophobic anions potently and uncompetitively antagonize GABA(A) receptor function in the absence of a conventional binding site

BACKGROUND AND PURPOSE

A 'lock-and-key' binding site typically accounts for the effect of receptor antagonists. However, sulphated neurosteroids are potent non-competitive antagonists of GABA(A) receptors without a clear structure-activity relationship. To gain new insights, we tested two structurally unrelated hydrophobic anions with superficially similar properties to sulphated neurosteroids.

EXPERIMENTAL APPROACH

We used voltage-clamp techniques in Xenopus oocytes and hippocampal neurons to characterize dipicrylamine (DPA) and tetraphenylborate (TPB), compounds previously used to probe membrane structure and voltage-gated ion channel function.

KEY RESULTS

Both DPA and TPB potently antagonized GABA(A) receptors. DPA exhibited an IC₅₀ near 60 nM at half-maximal GABA concentration and antagonism with features indistinguishable from pregnenolone sulphate antagonism, including sensitivity to a point mutation in transmembrane domain 2 of the α1 subunit. Bovine serum albumin, which scavenges free membrane-associated DPA, accelerated both capacitance offset and antagonism washout. Membrane interactions and antagonism were explored using the voltage-dependent movement of DPA between membrane leaflets. Washout of DPA antagonism was strongly voltage-dependent, paralleling DPA membrane loss, although steady-state antagonism lacked voltage dependence. At antagonist concentrations, DPA failed to affect inhibitory post-synaptic current (IPSC) amplitude or decay, but DPA accelerated pharmacologically prolonged IPSCs.

CONCLUSIONS AND IMPLICATIONS

Neurosteroid-like GABA(A) receptor antagonism appears to lacks a conventional binding site. These features highlight key roles of membrane interactions in antagonism. Because its membrane mobility can be controlled, DPA may be a useful probe of GABA(A) receptors, but its effects on excitability via GABA(A) receptors raise caveats for its use in monitoring neuronal activity.

Structurally diverse amphiphiles exhibit biphasic modulation of GABAA receptors: similarities and differences with neurosteroid actions

BACKGROUND AND PURPOSE

Some neurosteroids, notably 3alpha-hydroxysteroids, positively modulate GABA(A) receptors, but sulphated steroids negatively modulate these receptors. Recently, other lipophilic amphiphiles have been suggested to positively modulate GABA receptors. We examined whether there was similarity among the actions of these agents and the mechanisms of neurosteroids. Significant similarity would affect theories about the specificity of steroid actions.

EXPERIMENTAL APPROACH

Xenopus laevis oocytes were challenged with Triton X-100, octyl-beta-glucoside, capsaicin, docosahexaenoic acid and sodium dodecyl sulphate (SDS), along with different GABA concentrations.

KEY RESULTS

These compounds have both positive and negative effects on GABA currents, which can be accentuated according to the degree of receptor activation. A low GABA concentration (1 microM) promoted potentiation and a high concentration (20 microM) promoted inhibition of current, except for SDS that inhibited function even at low GABA concentrations. Amphiphile inhibition was characterized by enhanced apparent desensitization and by weak voltage dependence, similar to pregnenolone sulphate antagonism. We then tested amphiphile effects on mutated receptor subunits that are insensitive to negative (alpha1V256S) and positive (alpha1Q241L or alpha1N407A/Y410F) steroid modulation. Negative regulation by amphiphiles was nearly abolished in alpha1V256S-mutated receptors, but potentiation was unaffected. In alpha1Q241L- or alpha1N407A/Y410F-mutated receptors, potentiation by amphiphiles remained intact.

CONCLUSIONS AND IMPLICATIONS

Structurally diverse amphiphiles have antagonist actions at GABA(A) receptors very similar to those of sulphated neurosteroids, while the potentiating mechanisms of these amphiphiles are distinct from those of neurosteroid-positive modulators. Thus, such antagonism at GABA(A) receptors does not have a clear pharmacophore requirement.

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.

Neurosteroids differentially modulate P2X ATP-gated channels through non-genomic interactions

As neuroactive steroids modulate several ionotropic receptors, we assessed whether the ATP-gated currents elicited by P2X(4) receptors are modulated by these compounds. We transfected HEK293 cells or injected Xenopus laevis oocytes with the cDNA coding for rat P2X(4) receptor. Application of 0.1-10 microM alfaxolone potentiated within 60-s the 1 microM ATP-evoked currents with a maximal potentiation of 1.8 and 2.6-fold in HEK293 or oocytes cells respectively. Allopregnalolone or 3alpha, 21-dihydroxy-5alpha-pregnan-20-one (THDOC) also potentiated the ATP-gated currents but with a maximal effect only averaging 1.25 and 1.35-fold respectively. In contrast, 0.3-10 microM pregnanolone, but not its sulfated derivative, inhibited the ATP-gated currents; the maximal inhibition reached 40% in both cell types. THDOC, but not other neurosteroids increased significantly the tau(off) of the ATP-evoked currents, revealing another mode of neurosteroid modulation. Sexual steroids such as 17beta-estradiol or progesterone were inactive revealing explicit structural requirements. Alfaxolone or THDOC at concentrations 30- to 100-fold larger than required to modulate the receptor, gated the P2X(4) receptor eliciting ATP-like currents that were reduced with suramin or brilliant blue G, but potentiated the P2X(4) receptor more than 10-fold by 10 microM zinc. In conclusion, neurosteroids rapidly modulate via non-genomic mechanisms and with nanomolar potencies, the P2X4 receptor interacting likely at distinct modulator sites.

Expanding proteomics into the analysis of chiral drugs

The chiralities of chiral drugs have been investigated extensively with the purpose of enlightening the role of chirality in drug action. Proteomics, though in its infancy, has recently emerged as the foremost technology in drug development research, possessing the advantage of providing more useful information about an organism than genomics, as it directly addresses the level of genome products and their interactions. In this review, we will discuss the background of chiral drug investigation from which contemporary drug chirality research has emerged, the techniques involved in proteomics technology, the application of proteomics in this exciting area, and the perspectives in future applications of this field.

Neuroactive steroids and human recombinant rho1 GABAC receptors

The gamma-aminobutyric acid type C (GABAC) receptor is structurally related to the GABAA receptors, yet quite distinct physiologically and pharmacologically. Neuroactive steroids are known to be potent and efficacious modulators of the GABAA receptor, but they are less well characterized in their actions on the GABAC receptor. We have examined the actions of neuroactive steroids and analogs on rho1 (GABAC) receptors expressed in Xenopus laevis oocytes, with two goals in mind. First, we tested a larger number of endogenous steroids, to determine whether particularly potent steroids could be found. Second, we examined the structure-activity relationship for steroid actions, and some mechanistic features, to determine the possible numbers of steroid binding sites and mechanisms of action. In total, 41 compounds were examined. Estradiols are inhibitors, essentially equipotent with picrotoxinin. No endogenous steroid tested was highly efficacious at potentiation. The results of the structure-activity studies and the effects of two mutations to the second transmembrane region of the rho1 GABAC receptor indicate that there are several mechanisms by which steroids can inhibit the receptor. Surprisingly, estradiol shares some features with picrotoxin. Inhibition by negatively charged compounds was not sensitive to membrane potential, and inhibition by all compounds tested was reduced at higher concentrations of GABA. The data indicate that the binding sites mediating potentiation and inhibition differ from each other and that there are several (three or more) mechanisms for producing inhibition.

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.

Natural and enantiomeric etiocholanolone interact with distinct sites on the rat alpha1beta2gamma2L GABAA receptor

We have studied the ability of the androgen etiocholanolone and its enantiomer (ent-etiocholanolone) to modulate rat alpha1beta2gamma2L GABA(A) receptor function transiently expressed in human embryonic kidney cells. Studies on steroid enantiomer pairs can yield powerful new information on the pharmacology of steroid interactions with the GABA(A) receptor. Both steroids enhance currents elicited by GABA, but ent-etiocholanolone is much more powerful than etiocholanolone at producing potentiation. At a low GABA concentration (0.5 microM, <EC(5)), the presence of 10 microM ent-etiocholanolone potentiates whole-cell currents by almost 30-fold, whereas 10 microM etiocholanolone merely doubles the peak response. At higher GABA concentration (5 microM, approximately EC(25)), the potentiation curve for ent-etiocholanolone is positioned at lower concentrations than that for etiocholanolone. Single-channel kinetic analysis shows that exposure to etiocholanolone has a single effect on currents: the relative frequency of long openings is increased in the presence of steroid. But exposure to ent-etiocholanolone produces two kinetic effects: an increase in the relative frequency of long openings and a decrease in the frequency of long closed times. The presence of etiocholanolone does not inhibit potentiation by ent-etiocholanolone, suggesting that etiocholanolone is unable to interact with the sites through which ent-etiocholanolone modifies receptor function. The double mutation alpha1(N407A/Y410F) prevents potentiation by etiocholanolone but not by ent-etiocholanolone, and the alpha1(Q241A) and alpha1(I238N) point mutations fully abolish potentiation by etiocholanolone but not by ent-etiocholanolone. We conclude that etiocholanolone and its enantiomer interact with distinct sites on the alpha1beta2gamma2L GABA(A) receptor.

Biophysical properties of lipids and dynamic membranes

The lipid bilayer is a 3D assembly with a rich variety of physical features that modulate cell signaling and protein function. Lateral and transverse forces within the membrane are significant and change rapidly as the membrane is bent or stretched and as new constituents are added, removed or chemically modified. Recent studies have revealed how differences in structure between the two leaflets of the bilayer and between different areas of the bilayer can interact together with membrane deformation to alter the activities of transmembrane channels and peripheral membrane binding proteins. Here, we highlight some recent reports that the physical properties of the membrane can help control the function of transmembrane proteins and the motor-dependent elongation of internal organelles, such as the endoplasmic reticulum.

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

The most important target protein for many anesthetics, including volatile and steroid anesthetics, appears to be the type A gamma-amino butyric acid receptor (GABA(A)R), yet direct binding remains to be demonstrated. Hypotheses of lipid-mediated anesthesia suggest that lipid bilayer properties are changed by anesthetics and that this in turn affects the functions of proteins. While other data could equally well support direct or lipid-mediated action, enantiomeric specificity displayed by some anesthetics is not reflected in their interactions with lipids. In the present study, we studied the effects of two pairs of anesthetic steroid enantiomers on bilayers of several compositions, measuring potentially relevant physical properties. For one of the pairs, allopregnanolone and ent-allopregnanolone, the natural enantiomer is 300% more efficacious as an anesthetic, while for the other, pregnanolone and ent-pregnanolone, there is little difference in anesthetic potency. For each enantiomer pair, we could find no differences. This strongly favors the view that the effects of these anesthetics on lipid bilayers are not relevant for the main features of anesthesia. These steroids also provide tools to distinguish in general the direct binding of steroids to proteins from lipid-mediated effects.