Neurosteroid analogues. 6. The synthesis and GABAA receptor pharmacology of enantiomers of dehydroepiandrosterone sulfate, pregnenolone sulfate, and (3alpha,5beta)-3-hydroxypregnan-20-one sulfate

Structural insights into opposing actions of neurosteroids on GABAA receptors

γ-Aminobutyric acid type A (GABAA) receptors mediate fast inhibitory signaling in the brain and are targets of numerous drugs and endogenous neurosteroids. A subset of neurosteroids are GABAA receptor positive allosteric modulators; one of these, allopregnanolone, is the only drug approved specifically for treating postpartum depression. There is a consensus emerging from structural, physiological and photolabeling studies as to where positive modulators bind, but how they potentiate GABA activation remains unclear. Other neurosteroids are negative modulators of GABAA receptors, but their binding sites remain debated. Here we present structures of a synaptic GABAA receptor bound to allopregnanolone and two inhibitory sulfated neurosteroids. Allopregnanolone binds at the receptor-bilayer interface, in the consensus potentiator site. In contrast, inhibitory neurosteroids bind in the pore. MD simulations and electrophysiology support a mechanism by which allopregnanolone potentiates channel activity and suggest the dominant mechanism for sulfated neurosteroid inhibition is through pore block.

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.

A neuroactive steroid with a therapeutically interesting constellation of actions at GABAA and NMDA receptors

Neuroactive steroids are an ascendant class of treatment for neuropsychiatric illness. Effects on ligand-gated neurotransmitter receptors appear to be a major mechanism of action. Here we describe a neuroactive steroid with a unique constellation of receptor actions. MQ-221 is a sulfated, 3β-hydroxy neurosteroid analogue that inhibits NMDAR function but also potentiates GABAAR function, thereby exhibiting unusual but potentially clinically desirable effects. Although the compound also exhibited features of other sulfated steroids, namely activation-dependent inhibition of GABAAR function, net potentiation dominated under physiological conditions. Potentiation of GABAAR function was distinct from the mechanism governing potentiation by anesthetic neurosteroids. Inhibition of NMDAR function showed weaker channel activation dependence than pregnanolone sulfate (3α5βPS). MQ-221 was unique among four stereoisomers explored in the pattern of effects at GABAA and NMDARs. Taken together, MQ-221 may represent a new class of compound with unique psychoactive effects and beneficial prospects for treating neuropsychiatric disorders.

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.

Analysis of GABAA Receptor Activation by Combinations of Agonists Acting at the Same or Distinct Binding Sites

Under both physiologic and clinical conditions GABA_A receptors are exposed to multiple agonists, including the transmitter GABA, endogenous or exogenous neuroactive steroids, and various GABAergic anesthetic and sedative drugs. The functional output of the receptor reflects the interplay among all active agents. We have investigated the activation of the concatemeric α1β2γ2L GABA_A receptor by combinations of agonists. Simulations of receptor activity using the coagonist concerted transition model demonstrate that the response amplitude in the presence of agonist combinations is highly dependent on whether the paired agonists interact with the same or distinct sites. The experimental data for receptor activation by agonist combinations were in agreement with the established views of the overlap of binding sites for several pairs of orthosteric (GABA, β-alanine, and piperidine-4-sulfonic acid) and/or allosteric agents (propofol, pentobarbital, and several neuroactive steroids). Conversely, the degree of potentiation when two GABAergic agents are coapplied can be used to determine whether the compounds act by binding to the same or distinct sites. We show that common interaction sites mediate the actions of 5α- and 5β-reduced neuroactive steroids, and natural and enantiomeric steroids. Furthermore, the results indicate that the anesthetics propofol and pentobarbital interact with partially shared binding sites. We propose that the findings may be used to predict the efficacy of drug mixtures in combination therapy and thus have potential clinical relevance.

Visualizing pregnenolone sulfate-like modulators of NMDA receptor function reveals intracellular and plasma-membrane localization

Positive modulators of NMDA receptors are important candidates for therapeutic development to treat psychiatric disorders including autism and schizophrenia. Sulfated neurosteroids have been studied as positive allosteric modulators of NMDA receptors for years, but we understand little about the cellular fate of these compounds, an important consideration for drug development. Here we focus on a visualizable sulfated neurosteroid analogue, KK-169. As expected of a pregnenolone sulfate analogue, the compound strongly potentiates NMDA receptor function, is an antagonist of GABA_A receptors, exhibits occlusion with pregnenolone sulfate potentiation, and requires receptor domains important for pregnenolone sulfate potentiation. KK-169 exhibits somewhat higher potency than the natural parent, pregnenolone sulfate. The analogue contains a side-chain alkyne group, which we exploited for retrospective click labeling of neurons. Although the anionic sulfate group is expected to hinder cell entry, we detected significant accumulation of KK-169 in neurons with even brief incubations. Adding a photolabile diazirine group revealed that the expected plasma membrane localization of KK-169 is likely lost during fixation. Overall, our studies reveal new facets of the structure-activity relationship of neurosteroids at NMDA receptors, and their intracellular distribution suggests that sulfated neurosteroids could have unappreciated targets in addition to plasma membrane receptors.

Inhibition of the GABAA Receptor by Sulfated Neurosteroids: A Mechanistic Comparison Study between Pregnenolone Sulfate and Dehydroepiandrosterone Sulfate

The γ-aminobutyric acid type A receptor (GABAAR) is negatively modulated by two structurally similar neurosteroids, pregnenolone sulfate (PS) and dehydroepiandrosterone sulfate (DHEAS). This study attempted to ascertain the molecular mechanisms of inhibition of the GABA-ergic current by neurosteroids. We demonstrated that the presence of the γ subunit in GABAAR enhances the efficacy of DHEAS without altering its binding affinity. A saturating concentration of DHEAS blocked approximately 75 % of currents mediated by GABAAR, which is composed of human α1, β1, and γ2S subunits, whereas the inhibition was only 35 % in GABAAR containing only α1 and β1 subunits. The IC50 values of DHEAS with and without the γ subunit were almost identical. In contrast to DHEAS, neither the affinity nor the efficacy of PS was altered by the γ subunit. When Val256 of α1 subunit was mutated to Ser, the mutant channel became resistant to inhibition by both DHEAS and PS. PS exerted its inhibitory effect by enhancing the desensitization kinetics of GABAAR possibly through promoting the interaction between the M2-M3 linker and extracellular loop 7/loop 2. Mutant α1, containing double Cys in loop 2/loop 7 and the M2-M3 linker, formed disulfide bonds three times as much fast, when treated with saturating GABA+PS, compared with GABA alone or with GABA+DHEAS. We demonstrated that PS, but not DHEAS, mediates GABA-ergic inhibition by promoting collisions between the structural elements involved in receptor desensitization, i.e., loop 2, loop 7, and the M2-M3 linker, thus following different inhibitory mechanisms.

Effect of neurosteroids on a model lipid bilayer including cholesterol: An Atomic Force Microscopy study

Amphiphilic molecules which have a biological effect on specific membrane proteins, could also affect lipid bilayer properties possibly resulting in a modulation of the overall membrane behavior. In light of this consideration, it is important to study the possible effects of amphiphilic molecule of pharmacological interest on model systems which recapitulate some of the main properties of the biological plasma membranes. In this work we studied the effect of a neurosteroid, Allopregnanolone (3α,5α-tetrahydroprogesterone or Allo), on a model bilayer composed by the ternary lipid mixture DOPC/bSM/chol. We chose ternary mixtures which present, at room temperature, a phase coexistence of liquid ordered (Lo) and liquid disordered (Ld) domains and which reside near to a critical point. We found that Allo, which is able to strongly partition in the lipid bilayer, induces a marked increase in the bilayer area and modifies the relative proportion of the two phases favoring the Ld phase. We also found that the neurosteroid shifts the miscibility temperature to higher values in a way similarly to what happens when the cholesterol concentration is decreased. Interestingly, an isoform of Allo, isoAllopregnanolone (3β,5α-tetrahydroprogesterone or isoAllo), known to inhibit the effects of Allo on GABAA receptors, has an opposite effect on the bilayer properties.

Structural requirements of steroidal agonists of transient receptor potential melastatin 3 (TRPM3) cation channels

Background and Purpose

Transient receptor potential melastatin 3 (TRPM3) proteins form non-selective but calcium-permeable membrane channels, rapidly activated by extracellular application of the steroid pregnenolone sulphate and the dihydropyridine nifedipine. Our aim was to characterize the steroid binding site by analysing the structural chemical requirements for TRPM3 activation.

Experimental Approach

Whole-cell patch-clamp recordings and measurements of intracellular calcium concentrations were performed on HEK293 cells transfected with TRPM3 (or untransfected controls) during superfusion with pharmacological substances.

Key Results

Pregnenolone sulphate and nifedipine activated TRPM3 channels supra-additively over a wide concentration range. Other dihydropyridines inhibited TRPM3 channels. The natural enantiomer of pregnenolone sulphate was more efficient in activating TRPM3 channels than its synthetic mirror image. However, both enantiomers exerted very similar inhibitory effects on proton-activated outwardly rectifying anion channels. Epiallopregnanolone sulphate activated TRPM3 almost equally as well as pregnenolone sulphate. Exchanging the sulphate for other chemical moieties showed that a negative charge at this position is required for activating TRPM3 channels.

Conclusions and Implications

Our data demonstrate that nifedipine and pregnenolone sulphate act at different binding sites when activating TRPM3. The latter activates TRPM3 by binding to a chiral and thus proteinaceous binding site, as inferred from the differential effects of the enantiomers. The double bond between position C5 and C6 of pregnenolone sulphate is not strictly necessary for the activation of TRPM3 channels, but a negative charge at position C3 of the steroid is highly important. These results provide a solid basis for understanding mechanistically the rapid chemical activation of TRPM3 channels.

Neuroprotection by the synthetic neurosteroid enantiomers ent-PREGS and ent-DHEAS against Aβ₂₅₋₃₅ peptide-induced toxicity in vitro and in vivo in mice

RATIONALE

Pregnenolone sulfate (PREGS) and dehydroepiandrosterone sulphate (DHEAS) are pro-amnesic, anti-amnesic and neuroprotective steroids in rodents. In Alzheimer's disease (AD) patient's brains, their low concentrations are correlated with high levels of Aβ and tau proteins. The unnatural enantiomer ent-PREGS enhanced memory in rodents. We investigated here whether ent-PREGS and ent-DHEAS could be neuroprotective in AD models.

OBJECTIVE

The effects of PREGS, ent-PREGS, DHEAS and ent-DHEAS against Aβ25-35 peptide-induced toxicity were examined in vitro on B104 neuroblastoma cells and in vivo in mice.

METHODS

B104 cells pretreated with the steroids before Aβ25-35 were analysed by flow cytometry measuring cell viability and death processes. Mice injected intracerebroventricularly with Aβ25-35 and the steroids were analysed for their memory abilities. Additionally, lipid peroxidation levels in the hippocampus were measured.

RESULTS

ent-PREGS and PREGS significantly attenuated the Aβ25-35-induced decrease in cell viability. Both steroids prevented the Aβ25-35-induced increase in late apoptotic cells. PREGS further attenuated the ratio of necrotic cells. ent-DHEAS and DHEAS significantly reduced the Aβ25-35-induced toxicity and prevented the cells from entering late apoptosis and necrosis. All steroids stimulated neurite outgrowth per se and prevented the Aβ25-35-induced decrease. In vivo, ent-PREGS and ent-DHEAS significantly attenuated the Aβ25-35-induced decrease in memory (spontaneous alternation and passive avoidance) and an increase in lipid peroxidation levels. In contrast to the natural steroids, both enantiomers prevented amnesia when injected 6 h before Aβ25-35 in contrast to the natural steroids.

CONCLUSION

The unnatural steroids ent-PREGS and ent-DHEAS are potent neuroprotective agents and could be effective therapeutical tools in AD.

Structure-activity relationship studies on neuroactive steroids in memory, alcohol and stress-related functions: a crucial benefit from endogenous level analysis

RATIONALE

New research findings in the field of neuroactive steroids strongly suggest that to understand their role in physiopathology, it is essential to accurately measure their tissue levels. Through his broad chemical expertise and extensive knowledge of steroids, Dr. Robert H. Purdy pioneered structure-activity relationship studies on these compounds and developed innovative detection assays that are essential to assess their function in biological tissues.

OBJECTIVE

The goal of the present paper is to point out the specific contributions of Dr. Purdy and his collaborators to the current knowledge on the role of neuroactive steroids in the modulation of memory and alcohol- and stress-related effects with particular emphasis on the detection assays he developed to assess their endogenous levels. Reviewed here are the major results as well as the original and valuable methodological strategies issued by the long-term collaboration between Dr Purdy and many scientists worldwide on the investigation of the structure-activity relationship of neuroactive steroids.

RESULTS

Altogether, the data presented herein put forward the original notion that knowledge of the chemical structure of steroids is essential for their detection and the understanding of their role in physiological and pathological conditions, including the stress response.

CONCLUSIONS

The current challenge is to identify and quantify using appropriate methods neuroactive steroids in the context of both animal and clinical studies in order to reveal how their levels change under physiological and disease states. Dr. Purdy passed away in September 2012, but scientists all over the world will always be grateful for his pioneering work on steroid chemistry and for his great enthusiasm in research.

Neurosteroid analogues. 18. Structure-activity studies of ent-steroid potentiators of γ-aminobutyric acid type A receptors and comparison of their activities with those of alphaxalone and allopregnanolone

A model of the alignment of neurosteroids and ent-neurosteroids at the same binding site on γ-aminobutyric acid type A (GABAA) receptors was evaluated for its ability to identify the structural features in ent-neurosteroids that enhance their activity as positive allosteric modulators of this receptor. Structural features that were identified included: (1) a ketone group at position C-16, (2) an axial 4α-OMe group, and (3) a C-18 methyl group. Two ent-steroids were identified that were more potent than the anesthetic steroid alphaxalone in their threshold for and duration of loss of the righting reflex in mice. In tadpoles, loss of righting reflex for these two ent-steroids occurs with EC50 values similar to those found for allopregnanolone. The results indicate that ent-steroids have considerable potential to be developed as anesthetic agents and as drugs to treat brain disorders that are ameliorated by positive allosteric modulators of GABAA receptor function.

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.

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.

Neurosteroid analogues. 17. Inverted binding orientations of androsterone enantiomers at the steroid potentiation site on γ-aminobutyric acid type A receptors

The enantiomer pair androsterone and ent-androsterone are positive allosteric modulators of γ-aminobutyric acid (GABA) type A receptors. Each enantiomer was shown to bind at the same receptor site. Binding orientations of the enantiomers at this site were deduced using enantiomer pairs containing OBn substituents at either C-7 or C-11. 11β-OBn-substituted steroids and 7α-OBn-substituted ent-steroids potently displace [(35)S]-tert-butylbicyclophosphorothionate, augment GABA currents, and anesthetize tadpoles. In contrast, 7β-OBn-substituted steroids and 11α-OBn-substituted ent-steroids have diminished actions. The results suggest that the binding orientations of the active analogues are inverted relative to each other with the 7α- and 11β-substituents similarly located on the edges of the molecules not in contact with the receptor surface. Analogue potentiation of the GABA current was abrogated by an α(1) subunit Q241L mutation, indicating that the active analogues act at the same sites in α(1)β(2)γ(2L) receptors previously associated with positive neurosteroid modulation.

Pregnenolone sulfate and its enantiomer: differential modulation of memory in a spatial discrimination task using forebrain NMDA receptor deficient mice

This study examined the role of forebrain N-methyl-D-aspartate receptors (NMDA-Rs) in the promnesiant effects of natural (+) pregnenolone sulfate (PREGS) and its synthetic (-) enantiomer ent-PREGS in young adult mice. Using the two-trial arm discrimination task in a Y-maze, PREGS and ent-PREGS administration to control mice increased memory performances. In mice with a knock-out of the NR1 subunit of NMDA-Rs in the forebrain, the promnesiant effect of ent-PREGS was maintained whereas the activity of PREGS was lost. Memory enhancement by PREGS involves the NMDA-R activity in the hippocampal CA1 area and possibly in some locations of the cortical layers, whereas ent-PREGS acts independently of NMDA-R function.

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 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.

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.

Emergence of NMDAR-independent long-term potentiation at hippocampal CA1 synapses following early adolescent exposure to chronic intermittent ethanol: role for sigma-receptors

Adolescent humans who abuse alcohol are more vulnerable than adults to the development of memory impairments. Memory impairments often involve modifications in the ability of hippocampal neurons to establish long-term potentiation (LTP) of excitatory neurotransmission; however, few studies have examined how chronic ethanol exposure during adolescence affects LTP mechanisms in hippocampus. We investigated changes in LTP mechanisms in hippocamal slices from rats exposed to intoxicating concentrations of chronic intermittent ethanol (CIE) vapors in their period of early-adolescent (i.e., prepubescent) or late-adolescent (i.e., postpubescent) development. LTP was evaluated at excitatory CA1 synapses in hippocampal slices at 24 h after the cessation of air (control) or CIE vapor treatments. CA1 synapses in control slices showed steady LTP following induction by high-frequency stimulation, which was fully dependent on NMDAR function. By contrast, slices from early-adolescent CIE exposed animals showed a compound form of LTP consisting of an NMDAR-dependent component and a slow-developing component independent of NMDARs. These components summated to yield LTP of robust magnitude above LTP levels in age-matched control slices. Bath-application of the sigma-receptor antagonist BD1047 and the neuroactive steroid pregnenolone sulfate, but not acute ethanol application, blocked NMDAR-independent LTP, while leaving NMDAR-dependent LTP intact. Analysis of presynaptic function during NMDAR-independent LTP induction demonstrated increased presynaptic function via a sigma-receptor-dependent mechanism in slices from early-adolescent CIE-exposed animals. By contrast, CIE exposure after puberty onset in late-adolescent animals produced decrements in LTP levels. The identification of a role for sigma-receptors and neuroactive steroids in the development of NMDAR-independent LTP suggests an important pathway by which hippocampal synaptic plasticity, and perhaps memory, may be uniquely altered by chronic ethanol exposure during the prepubescent phase of adolescent development.

Pregnenolone sulfate in the brain: a controversial neurosteroid

Pregnenolone sulfate (PREGS) has been shown, either at high nanomolar or at micromolar concentrations, to increase neuronal activity by inhibiting GABAergic and by stimulating glutamatergic neurotransmission. PREGS is also a potent modulator of sigma type 1 (sigma1) receptors. It has been proposed that these actions of PREGS underlie its neuropharmacological effects, and in particular its influence on memory processes. On the other hand, the PREGS-mediated increase in neuronal excitability may become dangerous under particular conditions, for example in the case of excitotoxic stress or convulsions. However, the physiopathological significance of these observations has recently been put into question by the failure to detect significant levels of PREGS within the brain and plasma of rats and mice, either by direct analytical methods based on liquid chromatography/mass spectrometry (LC/MS) or enzyme linked immunosorbent assay (ELISA) with specific antibodies against PREGS, or by indirect gas chromatography/mass spectrometry (GC/MS) analysis with improved sample workup. These recent results have not come to the attention of a large number of neurobiologists interested in steroid sulfates. However, although available direct analytical methods have failed to detect levels of PREGS above 0.1-0.3 ng/g in brain tissue, it may be premature to completely exclude the local formation of biologically active PREGS within specific and limited compartments of the nervous system. In contrast to the situation in rodents, significant levels of sulfated 3beta-hydroxysteroids have been measured in human plasma and brain. Previous indirect measures of steroid sulfates by radioimmunoassays (RIA) or GC/MS had detected elevated levels of PREGS in rodent brain. The discrepancies between the results of different assay procedures have revealed the danger of indirect analysis of steroid sulfates. Indeed, PREGS must be solvolyzed/hydrolyzed prior to RIA or GC/MS analysis, and it is the released, unconjugated PREG which is then quantified. Extreme caution needs to be exercised during the preparation of samples for RIA or GC/MS analysis, because the fraction presumed to contain only steroid sulfates can be contaminated by nonpolar components from which PREG is generated by the solvolysis/hydrolysis/derivatization reactions.

The neurosteroids dehydroepiandrosterone sulfate and pregnenolone sulfate inhibit the UNC-49 GABA receptor through a common set of residues

Neurosteroids are endogenous neuromodulators that bind and allosterically regulate GABA(A) receptors. Residues were recently identified in the first transmembrane domain (M1) of GABA(A) receptor subunits that are important for neurosteroid modulation. We are studying the inhibition of GABA(A) receptors by sulfated neurosteroids. One of these neurosteroid, pregnenolone sulfate (PS), depends on six identified M1 residues to inhibit the UNC-49 GABA receptor, a homomeric GABA receptor from Caenorhabditis elegans that is homologous to the mammalian GABA(A) receptor. Here, we investigate the inhibition of the UNC-49 GABA receptor by another sulfated neurosteroid, dehydroepiandrosterone sulfate (DHEAS). DHEAS is identical to PS except that it contains a carbonyl oxygen instead of an acetyl group at C17 on the steroid D ring. UNC-49 mutations that affect PS inhibition had broadly parallel effects on DHEAS, suggesting the two neurosteroids act through similar mechanisms. However, certain M1 mutations affected DHEAS differently than PS. Considering that first, the D ring contains the only structural difference between PS and DHEAS, and second, the strongest chemical and steric effects of a mutation are likely to be felt in the local environment of the altered residues, this result implies that the steroid D ring may contact M1 near the mutated residues. This possibility is interesting because current models of neurosteroid interactions with GABA(A) receptors, based on pregnane steroids, suggest that the steroid A ring binds M1, whereas the D ring binds M4. Our findings suggest that there may be considerable diversity in the way different classes of neurosteroids interact with GABA(A) receptors.

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.

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.

Residues in the first transmembrane domain of the Caenorhabditis elegans GABA(A) receptor confer sensitivity to the neurosteroid pregnenolone sulfate

The GABA(A) receptor is a target of endogenous and synthetic neurosteroids. Little is known about the residues required for neurosteroid action on GABA(A) receptors. We have investigated pregnenolone sulfate (PS) inhibition of the Caenorhabditis elegans UNC-49 GABA receptor, a close homolog of the mammalian GABA(A) receptor. The UNC-49 locus encodes two GABA receptor subunits, UNC-49B and UNC-49C. UNC-49C is sensitive to PS but UNC-49B is not sensitive. By analyzing chimeric receptors and receptors containing site-directed mutations, we identified two regions required for PS inhibition. Four residues in the first transmembrane domain are required for the majority of the sensitivity to PS, but a charged extracellular residue at the end of the M2 helix also plays a role. Strikingly, mutation of one additional M1 residue reverses the effect of PS from an inhibitor to an enhancer of receptor function. Mutating the M1 domain had little effect on sensitivity to the inhibitor picrotoxin, suggesting that these residues may mediate neurosteroid action specifically, and not allosteric regulation in general.

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

Neuroactive steroids can either potentiate or inhibit a variety of membrane channels. Most studies have suggested that the effects are mediated by specific association of the steroid with the affected channel. However, a recent study of the rho1 (GABA-C) receptor (Mol Pharmacol 66:56-69, 2004) concluded that the actions were consistent with an action of the steroid in the lipid bilayer to alter the lateral pressure profile in the membrane. The enantiomers of an optically active compound are expected to have identical physical properties, including interactions with hydrophobic portions of the cell membrane. We have used two pairs of enantiomers (pregnanolone and ent-pregnanolone, allopregnanolone and ent-allopregnanolone) and show that the ability to potentiate (allopregnanolone) or inhibit (pregnanolone) the rho1 receptor is enantioselective. Therefore, these results strongly suggest that the actions of these neuroactive steroids are mediated by interactions with chiral regions of the target protein, rather than by a change in membrane properties (including lateral pressure).

Dehydroepiandrosterone sulfate and estrone sulfate reduce GABA-recurrent inhibition in the hippocampus via muscarinic acetylcholine receptors

Several recent studies have established a role for estrogens in ameliorating specific neurodegenerative disorders, mainly those associated with the cholinergic neurons of the basal forebrain and their targets in the cortex and hippocampus. We have previously demonstrated that endogenous and exogenous application of the neurosteroid dehydroepiandrosterone sulfate (DHEAS) markedly reduces GABA-mediated recurrent inhibition and synchronizes hippocampal unit activity to theta rhythm (Steffensen (1995) Hippocampus 5:320-328). In this study, we evaluated the role of muscarinic receptors in mediating the effects of DHEAS and estrone sulfate (ES), the principal circulating estrogen in humans, on short-latency-evoked potential responses, paired-pulse inhibition (PPI), paired-pulse facilitation, and GABA interneuron activity in the dentate gyrus and CA1 subfields of the rat hippocampus. In situ microelectrophoretic application of the muscarinic M2 subtype cholinergic receptor agonist cis-dioxolane, DHEAS, and ES markedly reduced PPI in the dentate and CA1 that was blocked by the M2 receptor antagonist gallamine. Similar to DHEAS, microelectrophoretic administration of ES increased population spike amplitudes, without increasing excitatory transmission, but this effect was not blocked by gallamine. Microelectrophoretic application of cis-dioxolane and ES markedly increased the firing rate of dentate hilar interneurons and CA1 oriens/alveus interneurons and enhanced their synchrony to hippocampal theta rhythm. These findings suggest that select GABA-modulating neurosteroids and neuroactive estrogen sulfates alter septohippocampal cholinergic modulation of hippocampal GABAergic interneurons mediating recurrent, but not feedforward, inhibition of hippocampal principal cell activity.

Novel 17β-Substituted Conformationally Constrained Neurosteroids that Modulate GABAA Receptors

The goal of this study was to develop a series of allopregnanolone analogues substituted by conformationally constrained 17beta side chains to obtain additional information about the structure-activity relationship of 5alpha-reduced steroids to modulate GABA(A) receptors. Specifically, we introduced alkynyl-substituted 17beta side chains in which the triple bond is either directly attached to the 17beta-position or to the 21-position of the steroid skeleton. Furthermore, we investigated the effects of C22 and C20 modification. The in vitro binding affinity for the GABA(A) receptor of the new analogues was measured by allosteric displacement of the specific binding of [(3)H]4'-ethynyl-4-n-propyl-bicycloorthobenzoate (EBOB) to GABA(A) receptors on synaptosomal membranes of rat cerebellum. An allosteric binding model that has been successfully applied to ionotropic glycine receptors was employed. The most active derivative is (20R)-17beta-(1-hydroxy-2,3-butadienyl)-5alpha-androstane-3-ol (20), which possesses low nanomolar potency to modulate cerebellar GABA(A) receptors and is 71 times more active than the control compound allopregnanolone. Theoretical conformational analysis was employed in an attempt to correlate the in vitro results with the active conformations of the most potent of the new analogues.

New insights into the role of neuroactive steroids in cognitive aging

The aim of this article is to describe neuroactive steroid research that has been focused on their physiological role in cognitive aging, an attractive new field in experimental gerontology. Neuroactive steroids have been recently proposed as biomarkers of cognitive aging, however, their specific functions have not yet been fully established. For instance, data emerging from human and animal studies suggest a complex relationship between neuroactive steroids and/or metabolites and cognitive processes during aging. Thus, a better knowledge of neuroactive steroid brain distribution and function could broaden our understanding of their physiological roles and lead to novel and more effective treatments for the management of age-related brain disorders. To this end, newly developed sensitive, specific, and accurate mass spectrometry assays may allow the quantification of neuroactive steroids in discrete brain regions and greatly contribute to unravel their role in age-related cognitive deficits.

Inhibition of the activity of the native γ-aminobutyric acidA receptor by metabolites of thyroid hormones: correlations with molecular modeling studies

To characterize the direct effects of thyroid hormones on native gamma-aminobutyric acid(A) (GABA(A)) receptors, rapid (5 s) actions of a series of iodothyronines on muscimol-stimulated uptake of (36)Cl(-) were investigated in synaptoneurosomes prepared from rat brain. The results were correlated with molecular modeling of the active compounds. Dose-response curves for muscimol in the presence of 3,3', 5-L-triiodothyronine (L-T3) indicated a noncompetitive inhibition of muscimol-stimulated (36)Cl(-) uptake by the thyroid hormone. Synaptoneurosomes prepared from cerebellum were less sensitive to L-T3 than those from cerebral cortex, in terms of the potency of the hormone. The overall efficacy approached complete inhibition for both brain regions. Muscimol-stimulated (36)Cl(-) uptake was inhibited differentially by iodothyronine derivatives. One group of compounds with IC(50) values of 18-30 microM included L-thyroxine (L-T4), D-thyroxine (D-T4), 3,3', 5,5'-tetraiodothyroacetic acid (Tetrac), and 3,3', 5-triiodothyroacetic acid (Triac). A second group with values of 75-100 microM included 3,3', 5'-l-triiodothyronine (reverse T3; r-T3), 3,3'-diiodo-L-thyronine (3,3'-l-T2) and 3,5-diiodo-L-thyronine (3,5-D-T2). A final group of inactive compounds with IC(50) values greater than 100 microM included 3',5'-diiodo-L-thyronine (3',5'-l-T2), 3-iodo-L-thyronine (L-T1), 3'-iodo-L-thyronine (3'-L-T1), and L-thyronine (L-T0). Molecular modeling of the active iodothyronines using the Gaussian03 series of programs indicated close correspondences with models of the GABA-inhibitory neurosteroid pregnenolone sulfate (PREGS), suggesting common mechanisms of action at the GABA(A) receptor.

Neuroactive steroids: new biomarkers of cognitive aging

Intensive studies in animals established that neuroactive steroids display neuronal actions and influence behavioral functions. We describe here investigations on the role of neuroactive steroids in learning and memory processes during aging and suggest their role as biomarkers of cognitive aging. Our work demonstrated the role of the steroid pregnenolone (PREG) sulfate as a factor underlying an individual's age-related cognitive decline in animals. As new perspectives of research we argue that knowing whether neuroactive steroids exist as endogenous neuromodulators and modulate physiologically behavioral functions is essential. To this end, a new approach using the sensitive, specific, and accurate quantitative determination of neuroactive steroids by mass spectrometry seems to have potential for examining the role of each steroid in discrete brain areas in learning and memory alterations, as observed during aging.

First synthesis of ent-desmosterol and its conversion to ent-deuterocholesterol

We report the first synthesis of the unnatural enantiomer of desmosterol (ent-desmosterol). The sterol nucleus was constructed enantiospecifically, followed by stepwise addition of the side chain. Beginning with ent-androst-4-ene-3,17-dione, ent-desmosterol was synthesized in 13 steps and 20% yield. Protected ent-desmosterol was subjected to catalytic deuteration to afford ent-deuterocholesterol. Ent-desmosterol and ent-deuterocholesterol will be used to study the importance of sterol absolute configuration for sterol-lipid interactions in biophysical studies and in biological systems.

Neurosteroids enhance spontaneous glutamate release in hippocampal neurons. Possible role of metabotropic sigma1-like receptors

Pregnenolone sulfate (PREGS), one of the most abundantly produced neurosteroids in the mammalian brain, improves cognitive performance in rodents. The mechanism of this effect has been attributed to its allosteric modulatory actions on glutamate- and gamma-aminobutyric acid-gated ion channels. Here we report a novel effect of PREGS that could also mediate some of its actions in the nervous system. We found that PREGS induces a robust potentiation of the frequency but not the amplitude of miniature excitatory postsynaptic currents (mEPSCs) mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors in cultured hippocampal neurons. PREGS also decreased paired pulse facilitation of autaptic EPSCs evoked by depolarization, indicating that it modulates glutamate release probability presynaptically. PREGS potentiation of mEPSCs was mimicked by dehydroepiandrosterone sulfate and (+)-pentazocine but not by (-)-pentazocine, the synthetic (-)-enantiomer of PREGS or the inactive steroid isopregnanolone. The sigma receptor antagonists, haloperidol and BD-1063, blocked the effect of PREGS on mEPSCs, as did pertussis toxin and the membrane-permeable Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (acetoxymethyl) ester. These results suggest that PREGS increases spontaneous glutamate release via activation of a presynaptic G(i/o)-coupled sigma receptor and an elevation in intracellular Ca2+ levels. We postulate that presynaptic actions of neurosteroids have a role in the maturation and/or maintenance of synaptic networks and the processing of information in the central nervous system.

Neurosteroids in learning and memory processes

The discovery that neurosteroids could be synthesized de novo in the brain independent from the periphery and display neuronal actions led to great enthusiasm for the study of their physiological role. Pharmacological studies suggest that neurosteroids may be involved in several physiological processes, such as learning and memory. This chapter summarizes the effects of the administration of neurosteroids on learning and memory capabilities in rodents and in models of amnesia. We address the central mechanisms involved in mediating the modulation of learning and memory processes by neurosteroids. In this regard, the neurosteroid-modulated neurotransmitter systems, such as gamma-aminobutyric acid type A, N-methyl-D-aspartate, and cholinergic and sigma opioid systems, appear to be potential targets for the rapid memory alteration actions of neurosteroids. Moreover, given that some neurosteroids affect neuronal plasticity, this neuronal change could be involved in the long-term modulation of learning and memory processes. To understand the role of endogeneous neurosteroids in learning and memory processes, we present some physiological studies in rodents and humans. However, the latter do not successfully prove a role of endogenous neurosteroids in age-related memory impairments. Finally, we discuss the relative implication of a given neurosteroid vs its metabolites. For this question, a new approach using the quantitative determination of traces of neurosteroids by mass spectrometry seems to have potential for examining the role of each neurosteroid in discrete brain areas in learning and memory alterations, as observed during aging.

Steroid structure and pharmacological properties determine the anti-amnesic effects of pregnenolone sulphate in the passive avoidance task in rats

Pregnenolone sulphate (PREGS) has generated interest as one of the most potent memory-enhancing neurosteroids to be examined in rodent learning studies, with particular importance in the ageing process. The mechanism by which this endogenous steroid enhances memory formation is hypothesized to involve actions on glutamatergic and GABAergic systems. This hypothesis stems from findings that PREGS is a potent positive modulator of N-methyl-d-aspartate receptors (NMDARs) and a negative modulator of gamma-aminobutyric acid(A) receptors (GABA(A)Rs). Moreover, PREGS is able to reverse the amnesic-like effects of NMDAR and GABA(A)R ligands. To investigate this hypothesis, the present study in rats examined the memory-altering abilities of structural analogs of PREGS, which differ in their modulation of NMDAR and/or GABA(A)R function. The analogs tested were: 11-ketopregnenolone sulphate (an agent that is inactive at GABA(A)Rs and NMDARs), epipregnanolone ([3beta-hydroxy-5beta-pregnan-20-one] sulphate, an inhibitor of both GABA(A)Rs and NMDARs), and a newly synthesized (-) PREGS enantiomer (which is identical to PREGS in effects on GABA(A)Rs and NMDARs). The memory-enhancing effects of PREGS and its analogs were tested in the passive avoidance task using the model of scopolamine-induced amnesia. Both PREGS and its (-) enantiomer blocked the effects of scopolamine. The results show that, unlike PREGS, 11-ketopregnenolone sulphate and epipregnanolone sulphate failed to block the effect of scopolamine, suggesting that altering the modulation of NMDA receptors diminishes the memory-enhancing effects of PREGS. Moreover, enantioselectivity was demonstrated by the ability of natural PREGS to be an order of magnitude more effective than its synthetic enantiomer in reversing scopolamine-induced amnesia. These results identify a novel neuropharmacological site for the modulation of memory processes by neuroactive steroids.

The synthetic enantiomer of pregnenolone sulfate is very active on memory in rats and mice, even more so than its physiological neurosteroid counterpart: distinct mechanisms?

The demonstration that the neurosteroid pregnenolone sulfate (PREGS) is active on memory function at both the physiological and pharmacological levels led to us examining in detail the effects of the steroid on spatial working memory by using a two-trial recognition task in a Y-maze, a paradigm based on the natural drive in rodents to explore a novel environment. Dose-response studies in young male adult Sprague-Dawley rats and Swiss mice, after the postacquisition intracerebroventricular injection of steroid, showed an U-inverted curve for memory performance and indicated a greater responsiveness in rats compared with mice. Remarkably, the synthetic (-) enantiomer of PREGS not only also displayed promnesiant activity, but its potency was 10 times higher than that of the natural steroid. Intracerebroventricular coadministration experiments with DL-2-amino-5-phosphonovaleric acid, a competitive selective antagonist of the N-methyl-D-aspartate receptor, abolished the memory-enhancing effect of PREGS, but not that of the PREGS enantiomer, evoking enantiomeric selectivity at the N-methyl-d-aspartate receptor and/or different mechanisms for the promnestic function of the two enantiomers.

Recent developments in structure-activity relationships for steroid modulators of GABA(A) receptors

GABAergic neurotransmission can be both positively and negatively modulated by steroids. The steroid effects are thought to be mediated by binding of steroids to specific sites on GABA(A) receptors. It appears that the receptor sites for positive and negative modulatory steroids are different. Thus far, the location and number of binding sites for steroids on these receptors have not been established. In this brief review, we concentrate largely on results from our own structure-activity studies. Novel analogues have been studied to further delineate the structural features required for compounds to modulate receptor function via steroid binding sites. Non-naturally occurring enantiomers of both positive and negative modulators have been studied to provide further evidence for the existence of specific steroid binding sites on the receptors.

Role of pregnenolone, dehydroepiandrosterone and their sulfate esters on learning and memory in cognitive aging

Aging is a general process of functional decline which involves in particular a decline of cognitive abilities. However, the severity of this decline differs from one subject to another and inter-individual differences have been reported in humans and animals. These differences are of great interest especially as concerns investigation of the neurobiological factors involved in cognitive aging. Intensive pharmacological studies suggest that neurosteroids, which are steroids synthesized in the brain in an independent manner from peripheral steroid sources, could be involved in learning and memory processes. This review summarizes data in animals and humans in favor of a role of neurosteroids in cognitive aging. Studies in animals demonstrated that the neurosteroids pregnenolone (PREG) and dehydroepiandrosterone (DHEA), as sulfate derivatives (PREGS and DHEAS, respectively), display memory-enhancing properties in aged rodents. Moreover, it was recently shown that memory performance was correlated with PREGS levels in the hippocampus of 24-month-old rats. Human studies, however, have reported contradictory results. First, improvement of learning and memory dysfunction was found after DHEA administration to individuals with low DHEAS levels, but other studies failed to detect significant cognitive effects after DHEA administration. Second, cognitive dysfunctions have been associated with low DHEAS levels, high DHEAS levels, or high DHEA levels; while in other studies, no relationship was found. As future research perspectives, we propose the use of new methods of quantification of neurosteroids as a useful tool for understanding their respective role in improving learning and memory impairments associated with normal aging and/or with pathological aging, such as Alzheimer's disease.