Neurosteroid analogues: structure-activity studies of benz[e]indene modulators of GABAA receptor function. 1. The effect of 6-methyl substitution on the electrophysiological activity of 7-substituted benz[e]indene-3-carbonitriles

Comparison of Behavioral Effects of GABAergic Low- and High-Efficacy Neuroactive Steroids in the Zebrafish Larvae Assay

Activation of the GABAA receptor is associated with numerous behavioral end points ranging from anxiolysis to deep anesthesia. The specific behavioral effect of a GABAergic compound is considered to correlate with the degree of its functional effect on the receptor. Here, we tested the hypothesis that a low-efficacy allosteric potentiator of the GABAA receptor may act, due to a ceiling effect, as a sedative with reduced and limited action. We synthesized a derivative, named (3α,5β)-20-methyl-pregnane-3,20-diol (KK-235), of the GABAergic neurosteroid 5β-pregnane-3α,20α-diol. Using electrophysiology, we showed that KK-235 is a low-efficacy potentiator of the synaptic-type α1β2γ2L GABAA receptor. In the zebrafish larvae behavioral assay, KK-235 was found to only partially block the inverted photomotor response (PMR) and to weakly reduce swimming behavior, whereas the high-efficacy GABAergic steroid (3α,5α,17β)-3-hydroxyandrostane-17-carbonitrile (ACN) fully blocked PMR and spontaneous swimming. Coapplication of KK-235 reduced the potentiating effect of ACN in an electrophysiological assay and dampened its sedative effect in behavioral experiments. We propose that low-efficacy GABAergic potentiators may be useful as sedatives with limited action.

Comparison of Steroid Modulation of Spontaneous Inhibitory Postsynaptic Currents in Cultured Hippocampal Neurons and Steady-State Single-Channel Currents from Heterologously Expressed α1β2γ2L GABA(A) Receptors

Neuroactive steroids are efficacious modulators of γ-aminobutyric acid type A receptor (GABA(A)) receptor function. The effects of steroids on the GABA(A) receptor are typically determined by comparing steady-state single-channel open probability or macroscopic peak responses elicited by GABA in the absence and presence of a steroid. Due to differences in activation conditions (exposure duration, concentration of agonist), it is not obvious whether modulation measured using typical experimental protocols can be used to accurately predict the effect of a modulator on native receptors under physiologic conditions. In the present study, we examined the effects of 14 neuroactive steroids and analogs on the properties of spontaneous inhibitory postsynaptic currents (sIPSCs) in cultured rat hippocampal neurons. The goal was to determine whether the magnitude of modulation of the decay time course of sIPSCs correlates with the extent of modulation and kinetic properties of potentiation as determined in previous single-channel studies. The steroids were selected to cover a wide range of efficacy on heterologously expressed rat α1β2γ2L GABA(A) receptors, ranging from essentially inert to highly efficacious (strong potentiators of single-channel and macroscopic peak responses). The data indicate a strong correlation between prolongation of the decay time course of sIPSCs and potentiation of single-channel open probability. Furthermore, changes in intracluster closed time distributions were the single best predictor of prolongation of sIPSCs. We infer that the information obtained in steady-state single-channel recordings can be used to forecast modulation of synaptic currents.

Interactions of L-3,5,3'-Triiodothyronine [corrected], Allopregnanolone, and Ivermectin with the GABAA Receptor: Evidence for Overlapping Intersubunit Binding Modes

Structural mechanisms of modulation of γ-aminobutyric acid (GABA) type A receptors by neurosteroids and hormones remain unclear. The thyroid hormone L-3,5,3’-triiodothyronine (T3) inhibits GABA_A receptors at micromolar concentrations and has common features with neurosteroids such as allopregnanolone (ALLOP). Here we use functional experiments on α₂β₁γ₂ GABA_A receptors expressed in Xenopus oocytes to detect competitive interactions between T3 and an agonist (ivermectin, IVM) with a crystallographically determined binding site at subunit interfaces in the transmembrane domain of a homologous receptor (glutamate-gated chloride channel, GluCl). T3 and ALLOP also show competitive effects, supporting the presence of both a T3 and ALLOP binding site at one or more subunit interfaces. Molecular dynamics (MD) simulations over 200 ns are used to investigate the dynamics and energetics of T3 in the identified intersubunit sites. In these simulations, T3 molecules occupying all intersubunit sites (with the exception of the α-β interface) display numerous energetically favorable conformations with multiple hydrogen bonding partners, including previously implicated polar/acidic sidechains and a structurally conserved deformation in the M1 backbone.

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.

The use of symmetry in enantioselective synthesis: four pairs of chrysene enantiomers prepared from 19-nortestosterone

Expansion of the D-ring of 19-norsteroids with incorporation of the steroid C-18 methyl group into a newly formed six-membered ring provides easy access to the chrysene ring system. By taking advantage of the symmetry of the chrysene ring system and avoiding meso chrysene intermediates, four optically pure 2,8-difunctionalized (C-2 hydroxyl group and C-8 oxo group) hexadecahydrochrysene diastereomers, and their corresponding optically pure enantiomers were prepared from 19-nortestosterone. The eight chrysene stereoisomers are of interest as starting materials for preparing chrysene analogues of physiologically important neurosteroids.

A synthetic 18-norsteroid distinguishes between two neuroactive steroid binding sites on GABAA receptors

In the absence of GABA, neuroactive steroids that enhance GABA-mediated currents modulate binding of [35S]t-butylbicyclophosphorothionate in a biphasic manner, with enhancement of binding at low concentrations (site NS1) and inhibition at higher concentrations (site NS2). In the current study, compound (3alpha,5beta,17beta)-3-hydroxy-18-norandrostane-17-carbonitrile (3alpha5beta-18-norACN), an 18-norsteroid, is shown to be a full agonist at site NS1 and a weak partial agonist at site NS2 in both rat brain membranes and heterologously expressed GABAA receptors. 3alpha5beta-18-norACN also inhibits the action of a full neurosteroid agonist, (3alpha,5alpha,17beta)-3-hydroxy-17-carbonitrile (3alpha5alphaACN), at site NS2. Structure-activity studies demonstrate that absence of the C18 methyl group and the 5beta-reduced configuration both contribute to the weak agonist effect at the NS2 site. Electrophysiological studies using heterologously expressed GABAA receptors show that 3alpha5beta-18-norACN potently and efficaciously potentiates the GABA currents elicited by low concentrations of GABA but that it has low efficacy as a direct activator of GABAA receptors. 3alpha5beta-18-norACN also inhibits direct activation of GABAA receptors by 3alpha5alphaACN. 3alpha5beta-18-norACN also produces loss of righting reflex in tadpoles and mice, indicating that action at NS1 is sufficient to mediate the sedative effects of neurosteroids. These data provide insight into the pharmacophore required for neurosteroid efficacy at the NS2 site and may prove useful in the development of selective agonists and antagonists for neurosteroid sites on the GABAA receptor.

Synthesis and analytics of 2,2,3,4,4-d5-19-nor-5alpha-androsterone--an internal standard in doping analysis

A short and efficient synthesis of pentadeuterated 2,2,3,4,4-d5-19-nor-5alpha-androsterone 7 starting from 19-norandrost-4-ene-3,17-dione 1 by a d1-L-Selectride mediated stereo- and regioselective reduction of the 3-keto group is presented. The use of compound 7 as internal standard for the detection of anabolic steroids via mass spectrometric techniques such as gas chromatography-mass spectrometry (GC-MS) is discussed.

Mechanisms of neurosteroid interactions with GABA(A) receptors

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

Neurosteroid access to the GABAA receptor

GABAA receptors are a pivotal inhibitory influence in the nervous system, and modulators of the GABAA receptor are important anesthetics, sedatives, anticonvulsants, and anxiolytics. Current views of receptor modulation suggest that many exogenous drugs access and bind to an extracellular receptor domain. Using novel synthetic steroid analogs, we examined the access route for neuroactive steroids, potent GABAA receptor modulators also produced endogenously. Tight-seal recordings, in which direct aqueous drug access to receptor was prevented, demonstrated that steroids can reach the receptor either through plasma membrane lateral diffusion or through intracellular routes. A fluorescent neuroactive steroid accumulated intracellularly, but recordings from excised patches indicated that the intracellular reservoir is not necessary for receptor modulation, although it can apparently equilibrate with the plasma membrane within seconds. A membrane impermeant neuroactive steroid modulated receptor activity only when applied to the inner membrane leaflet, demonstrating that the steroid does not access an extracellular modulatory site. Thus, neuroactive steroids do not require direct aqueous access to the receptor, and membrane accumulation is required for receptor modulation.

Dual potentiating and inhibitory actions of a benz[e]indene neurosteroid analog on recombinant alpha1beta2gamma2 GABAA receptors

Benz[e]indenes are tricyclic analogs of neuroactive steroids and can be modulators of GABA(A) receptor activity. We have examined the mechanisms of action of the benz[e]indene compound [3S-(3alpha,3aalpha,5abeta,7beta,9aalpha,9bbeta)]-dodecahydro-7-(2-hydroxyethyl)-3a-methyl-1H-benz[e]indene-3-carbonitrile (BI-2) using single-channel patch-clamp and whole-cell recordings from human embryonic kidney cells transfected with rat GABA(A) receptor alpha1, beta2, and gamma2L subunits. The data demonstrate that BI-2 is a positive modulator of GABA(A) receptor activity with a peak effect at 2 microM. The mechanism of modulation is similar but not identical to that of neuroactive steroids. Similar to steroids, BI-2 acts by prolonging the mean open time duration through an effect on the duration and prevalence of the longest open time component. However, in contrast to many steroids, BI-2 does not selectively reduce the channel closing rate. The potentiating action of BI-2 seems to be mediated through interactions with the classic neuroactive steroid binding site. Mutation to the membrane-spanning region in the alpha1 subunit Q242W and the double mutation alpha1N408A/Y411F, previously shown to abolish potentiation by neurosteroids, also diminish potentiation by BI-2. At higher concentrations (>5 microM), BI-2 inhibits receptor function by enhancing the apparent rate of desensitization. From single-channel recordings, we estimate that the entry rate into the inhibited or blocked state, k(+B), is 0.50 microM(-1) s(-1). Based on the kinetic mechanism of action, and the finding that this effect is blocked by the alpha1V256S mutation, we propose that BI-2 acts through an inhibitory site first postulated for the inhibitory neurosteroid pregnenolone sulfate.

New evidence that both T-type calcium channels and GABAA channels are responsible for the potent peripheral analgesic effects of 5α-reduced neuroactive steroids

Neurosteroids are potent blockers of neuronal low-voltage activated (T-type) Ca(2+) channels and potentiators of GABA(A) ligand-gated channels, but their effects in peripheral pain pathways have not been studied previously. To investigate potential analgesic effects and the ion channels involved, we tested the ability of locally injected 5alpha-reduced neurosteroids to modulate peripheral thermal nociception to radiant heat in adult rats in vivo and to modulate GABA(A) and T-type Ca(2+) channels in vitro. The steroid anesthetic alphaxalone (ALPX), the endogenous neurosteroid allopregnanolone (3alpha5alphaP), and a related compound ((3alpha,5alpha,17beta)-3-hydroxyandrostane-17-carbonitrile, (ACN)), induced potent, dose-dependent, enantioselective anti-nociception in vivo and modulation of both T-type Ca(2+) currents and GABA(A)-mediated currents in vitro. Analgesic effects of ALPX were incompletely antagonized by co-injections of the GABA(A) receptor antagonist bicuculline. The neurosteroid analogue ((3alpha,5alpha)-3-hydroxy-13,24-cyclo-18,21-dinorchol-22-en-24-ol (CDNC24), a compound with GABAergic but not T-type activity, was not analgesic. However, (3beta,5alpha,17beta)-17-hydroxyestrane-3-carbonitrile (ECN)), which has effects on T-type channels but not on GABA(A) receptors, also induced potent enantioselective peripheral anti-nociception. ECN increased pain thresholds less than ALPX, 3alpha5alphaP and ACN. However, when an ineffective dose of CDNC24 was combined with ECN, anti-nociceptive activity was greatly enhanced, and this effect was bicuculline-sensitive. These results strongly suggest that GABA(A) channels do not contribute to baseline pain transmission, but they can enhance anti-nociception mediated by blockade of T-type Ca(2+) channels. In conclusion, we demonstrate that potent peripheral analgesia induced by 5alpha-reduced neurosteroid is mediated in part by effects on T-type Ca(2+) channels. Our results also reveal a role of GABA-gated ion channels in peripheral nociceptive signaling.

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.

Antinociceptive properties of neurosteroids III: experiments with alphadolone given intravenously, intraperitoneally, and intragastrically

The veterinary neurosteroid anaesthetic Saffan has the same formulation as Althesin now withdrawn from human use and is a mixture of two neurosteroids, alphadolone, and alphaxalone. The molecular structures of these two pregnanes and their properties as i.v. anaesthetics were reported to be similar. Preliminary experiments showed that alphadolone caused powerful antinociceptive effects without sedation when given i.p. In this study, alphadolone was given to rats (weight 100-200 g) i.v., i.p., and intragastrically. I.v. injections of alphadolone (25 mg kg(-1)) caused anaesthesia and sedation, whereas i.p. (0.1-100 mg kg(-1)) and intragastric administration (750 mg kg(-1)) produced no such effects. Intragastric alphadolone caused antinociceptive effects assessed with the electrical current threshold test (response 2.2 x pre-drug control values) without sedation. These effects were reversed at the level of the spinal cord by intrathecally-administered bicuculline (10 pmol). We conclude that a metabolite of alphadolone acetate produced in the liver leads to antinociceptive effects after i.p. and intragastric administration of the parent compound. This antinociception involves spinal cord GABA(A) receptors, even though the drug was administered via a non-spinal route.

Antinociceptive properties of neurosteroids II. Experiments with Saffan and its components alphaxalone and alphadolone to reveal separation of anaesthetic and antinociceptive effects and the involvement of spinal cord GABA(A) receptors

Studies have shown that the steroid anaesthetic alphaxalone positively modulates gamma-aminobutyric acid (GABA) receptors in vitro. It has also been reported that positive modulation of GABA(A) receptors in the rat spinal cord can produce antinociception in vivo. This present study looks at the interaction of an intraperitoneal injection (i.p.) of the steroid anaesthetic combination Saffan (alphaxalone 9 mg/ml, alphadolone acetate 3 mg/ml) with GABA(A) receptors in the spinal cord. Full recovery from anaesthesia induced by Saffan 2 ml/kg i.p., as assessed by the rotarod test, occurred after 28.78 +/- 0.86 min. Residual antinociceptive effects were assessed by application of electrical current at two skin sites (neck and tail) and also tail withdrawal from noxious heat. Residual antinociception was observed at both skin sites assessed by the electrical test but not when assessed by noxious heat. The antinociceptive effects in the tail but not the neck were suppressed by intrathecal administration of GABA(A) antagonists (bicuculline and SR-95531). In a separate group of experiments alphaxalone and alphadolone were given i.p. individually at the same doses that were given when formulated in Saffan. Alphaxalone produced sedative and anaesthetic effects with no antinociception. Alphadolone caused no sedation but it did cause antinociceptive effects equal in magnitude to those produced by Saffan. We conclude that Saffan produces antinociception in rats when given i.p. by an interaction with spinal GABA(A) receptors. Furthermore, this antinociception is due to the alphadolone content of the neurosteroid anaesthetic and not the alphaxalone.

General anaesthetic actions on ligand-gated ion channels

The molecular mechanisms of general anaesthetics have remained largely obscure since their introduction into clinical practice just over 150 years ago. This review describes the actions of general anaesthetics on mammalian neurotransmitter-gated ion channels. As a result of research during the last several decades, ligand-gated ion channels have emerged as promising molecular targets for the central nervous system effects of general anaesthetics. The last 10 years have witnessed an explosion of studies of anaesthetic modulation of recombinant ligand-gated ion channels, including recent studies which utilize chimeric and mutated receptors to identify regions of ligand-gated ion channels important for the actions of general anaesthetics. Exciting future directions include structural biology and gene-targeting approaches to further the understanding of general anaesthetic molecular mechanisms.

Blockade of Ba2+ current through human alpha1E channels by two steroid analogs, (+)-ACN and (+)-ECN

Previous work suggests that different neuroactive steroids may exhibit some selectivity in their blocking effects on different high-voltage activated (HVA) Ca2+ currents. At least some of these effects appear to involve direct blocking actions on Ca2+ channels. Thus, direct investigation of the effects of various steroids on cloned Ca2+ channel variants may lead to the development of potent and selective small-molecular weight Ca2+ channel blockers. Here we examine the effects of two steroids on a cloned human alpha1E Ca2+ channel both with and without a beta3 subunit, when expressed in HEK293 cells. One compound, (+)-ACN, has been previously shown to block N-, Q-, and R-subtypes of HVA current without affecting L- and P-type current. The second compound, (+)-ECN, weakly blocks total HVA current in hippocampal neurons. (+)-ECN differs from (+)-ACN in lacking effects on GABA receptors, but shares with (+)-ACN an ability to partially inhibit T current in DRG neurons (Todorovic, S.M., Prakriya, M., Nakashima, Y.M. et al., 1998. Enantioselective blockade of T-type Ca2+ current in adult rat sensory neurons by a steroid lacking GABA-mimetic activity. Mol. Pharmacol. 54, 918-927). (+)-ACN can block 100% of Ba2+ current in HEK cells arising either from the alpha1E subunit (IC50 approximate to 10 microM) or the alpha1Ebeta3 combination (IC50 approximate to 5 microM), while (+)-ECN maximally blocks only about 80% of the alpha1E (10 microM) or alpha1Ebeta3 (16 microM) current. Blockade by (+)-ACN exhibits several differences from blockade by (+)-ECN. (+)-ACN increases the apparent rate of onset of inactivation, particularly for the alpha1E variant, slows recovery from inactivation, and more profoundly shifts the voltage-dependence of current availability for both alpha1E and alpha1Ebeta3 variants than does (+)-ECN. Although the complexity of the normal inactivation kinetics of alpha1E variants makes interpretation of the (+)-ACN-induced kinetic alterations difficult, the results suggest that the two steroids are to some extent acting by distinct mechanisms, and perhaps at different sites.

Enantioselective blockade of T-type Ca2+ current in adult rat sensory neurons by a steroid that lacks gamma-aminobutyric acid-modulatory activity

A number of steroids seem to have anesthetic effects resulting primarily from their ability to potentiate currents gated by gamma-aminobutyric acidA (GABAA) receptor activation. One such compound is (3alpha,5alpha, 17beta)-3-hydroxyandrostane-17-carbonitrile [(+)-ACN]. We were interested in whether carbonitrile substitution at other ring positions might result in other pharmacological consequences. Here we examine effects of (3beta,5alpha, 17beta)-17-hydroxyestrane-3-carbonitrile [(+)-ECN] on GABAA receptors and Ca2+ channels. In contrast to (+)-ACN, (+)-ECN does not potentiate GABAA-receptor activated currents, nor does it directly gate GABAA-receptor mediated currents. However, both steroids produce an enantioselective reduction of T-type current. (+)-ECN blocked T current with an IC50 value of 0.3 microM with a maximal block of 41%. (+)-ACN produced a partial block of T current (44% maximal block) with an IC50 value of 0.4 microM. Block of T current showed mild use- and voltage-dependence. The (-)-ECN enantiomer was about 33 times less potent than (+)-ECN, with an IC50 value of 10 microM and an amount of maximal block comparable to (+)-ECN. (+)-ECN was less effective at blocking high-voltage-activated Ca2+ current in DRG neurons (IC50 value of 9. 3 microM with maximal block of about 27%) and hippocampal neurons. (+)-ECN (10 microM) had minimal effects on voltage-gated sodium and potassium currents in rat chromaffin cells. The results identify a steroid with no effects on GABAA receptors that produces a partial inhibition of T-type Ca2+ current with reasonably high affinity and selectivity. Further study of steroid actions on T currents may lead to even more selective and potent agents.

Bicuculline and gabazine are allosteric inhibitors of channel opening of the GABAA receptor

Anesthetic drugs are known to interact with GABAA receptors, both to potentiate the effects of low concentrations of GABA and to directly gate open the ion channel in the absence of GABA; however, the site(s) involved in direct gating by these drugs is not known. We have studied the ability of alphaxalone (an anesthetic steroid) and pentobarbital (an anesthetic barbiturate) to directly activate recombinant GABAA receptors containing the alpha 1, beta 2, and gamma 2L subunits. Steroid gating was not affected when either of two mutated beta 2 subunits [beta 2 (Y157S) and beta 2 (Y205S)] are incorporated into the receptors, although these subunits greatly reduce the affinity of GABA binding. These observations indicate that steroid binding and subsequent channel gating do not require these particular residues, as already shown for barbiturates. Bicuculline or gabazine (two competitive antagonists of GABA binding) reduced the currents elicited by alphaxalone and pentobarbital from wild-type GABAA receptors; however, gabazine produced only a partial block of response pentobarbital or alphaxalone, and bicuculline only partially blocked responses to pentobarbital. These observations indicate that the blockers do not compete with alphaxalone or pentobarbital for a single class of sites on the GABAA receptor. Finally, at receptors containing alpha 1 beta 2 (Y157S) gamma 2L subunits, both bicuculline and gabazine showed weak agonist activity and actually potentiated responses to alphaxalone. These observations indicate that the blocking drugs can produce allosteric changes in GABAA receptors, at least those containing this mutated beta 2 subunit. We conclude that the sites for binding steroids and barbiturates do not overlap with the GABA-binding site. Furthermore, neither gabazine nor bicuculline competes for binding at the steroid or barbiturate sites. The data are consistent with a model in which both gabazine and bicuculline act as allosteric inhibitors of channel opening for the GABAA receptor after binding to the GABA-binding site.

GABAA receptor pharmacology

gamma-Aminobutyric acid (GABA)A receptors for the inhibitory neurotransmitter GABA are likely to be found on most, if not all, neurons in the brain and spinal cord. They appear to be the most complicated of the superfamily of ligand-gated ion channels in terms of the large number of receptor subtypes and also the variety of ligands that interact with specific sites on the receptors. There appear to be at least 11 distinct sites on GABAA receptors for these ligands.

Chronic anabolic-androgenic steroid treatment affects brain GABA(A) receptor-gated chloride ion transport

Previous research in this laboratory has shown that chronic treatment of adult male rats with an anabolic-androgenic steroid (AAS) produced anxiolytic behavior and increased the functional response of cortical gamma-aminobutyric acid(A) (GABA(A)) receptors. The experiments reported here were aimed at further characterizing the effect of chronic AAS exposure on cerebral cortical GABA(A) receptors. Adult male rats were injected with dianabol (1,4-androstadien-17alpha-methyl-17beta-ol-3-one; 10 mg/kg/day, SC) for 4 weeks. A significant decrease in ventral prostate gland weight was found after 2 weeks of dianabol, and returned to control levels 3 and 10 days after steroid discontinuation. Testicular weights decreased throughout the treatment period but reached statistical significance only during the withdrawal period. Serum 3alpha-androstanediol level was marginally increased afer 2 weeks of dianabol injection, and was significantly decreased at 3 and 10 days after withdrawal. GABA-stimulated 36chloride (Cl-) influx in cortical synaptoneurosomes was increased in animals treated with dianabol for 2 and 4 weeks, and remained elevated 3 days after dianabol withdrawal, returning to control levels at withdrawal day 10. The increase in receptor efficacy was associated with a transient increase in receptor sensitivity (inverse of EC50), apparent after 2 weeks of AAS treatment and at withdrawal day 3. In a follow-up experiment, metabolites of dianabol were tested for the in vitro efficacy in potentiating GABA-stimulated Cl- transport. Only 3alpha-androstanedial and androsterone were found to have potent stimulatory effects. The 3beta-reduced metabolites were inactive, as were metabolites that contained a methyl group at the 17alpha position. These results point to significant facilitative effects of dianabol treatment on brain GABA(A) receptors via the metabolic formation of neuroactive steroids.

Effects of neurosteroid and benz[e]indene enantiomers on GABAA receptors in cultured hippocampal neurons and transfected HEK-293 cells

The effects of the enantiomers of the neurosteroid, 3 alpha-hydroxy-5 alpha-pregnan-20-one (DHP), and the benz[e]indene, BI-1, on gamma-aminobutyric acid (GABA) responses were studied using whole-cell recording techniques in cultured rat hippocampal neurons and human embryonic kidney cells (HEK-293) transfected with either alpha 1 beta 2 gamma 2 or alpha 6 beta 2 gamma 2 GABAA receptor subunits. At 10 microM, the (+)-enantiomers enhanced currents gated by 2 microM GABA in all cells, whereas the (-)-enantiomers were significantly less effective. The enhancement of 2 microM GABA responses in HEK-293 cells transfected with alpha 6 beta 2 gamma 2 subunits was about half that of hippocampal neurons or HEK-293 cells transfected with alpha 1 beta 2 gamma 2. The lower sensitivity of alpha 6 beta 2 gamma 2 receptors for (+)-DHP and (+)-BI-1 is accounted for by their greater apparent affinity for GABA. When the GABA concentration was decreased to 0.5 microM to take into account the four-fold higher apparent affinity of alpha 6 beta 2 gamma 2 receptors, these receptors exhibited enhancement similar to alpha 1 beta 2 gamma 2 receptors. These results indicate that both native and recombinant GABAA receptors have enantioselective sites at which neurosteroids and benz[e]indenes modulate GABA responses, and that differences in agonist affinity contribute to apparent differences in steroid sensitivity among GABAA receptors.

Potentiation of GABA-induced inhibition by 20-hydroxyecdysone, a neurosteroid, in cultured rat cortical neurons

Effects of 20-hydroxyecdysone (20-HE), a neurosteroid, on cultured rat cortical neurons were examined using the whole cell recording technique. Under the voltage and current clamp conditions, brief application (5 sec) of 20-HE alone did not produce current changes nor any changes in the membrane potential. However, the chemical dose-dependently potentiated the GABA-induced current and hyperpolarization, which were blocked by bicuculline. These results suggest that 20-HE acts on the modulatory site of the GABAA receptor and potentiates GABAergic inhibition in rat cortical neurons.