Differential antagonism by epipregnanolone of alphaxalone and pregnanolone potentiation of [3H]flunitrazepam binding suggests more than one class of binding site for steroids at GABAA receptors

Neuroactive steroids and depression in early pregnancy

Progesterone (P4) can be metabolized to two general classes of neuroactive steroids (NAS) -those like allopregnanolone (ALLO) and pregnanolone (PA) which are positive allosteric modulators of the Gamma Aminobutyric Acid type A (GABAA) receptor and those like isoallopregnanolone (ISOALLO) and epipregnanolone (EPI) which are negative allosteric modulators of the GABAA receptor. While exogenous administration of ALLO is effective in treating postpartum depression, knowledge gaps remain in the dynamic interplay of NAS across the perinatal period. In particular little is known about ALLO and PA in relation to depression earlier in pregnancy, and the role of ISOALLO and EPI in relation to depression at any point in the perinatal period. In a prospective, nested case/control study in low-income women of color, we compared the metabolism of P4 to four NAS (i.e., ratios ALLO:P4, PA:P4, ISOALLO:P4, EPI:P4) in pregnant women with depression at either or both of the first and second trimesters (cases) and women without depression at either time point (controls). Fifty women (36% depressed, 56% Black, 28% Latina) completed depression screening using a computerized adaptive test of mental health (CAT-MH™) and provided blood serum samples in both trimesters. In longitudinal mixed effects models of both trimesters, PND cases showed higher ratios of ALLO:P4 (p = .002) and PA:P4 (p = .03) compared to controls. In regression models of only first trimester data, there was no significant difference in NAS ratios between cases and controls (p > .05). Conversely, in models of the second trimester, ratios of PA:P4 (p = .002) and ISOALLO:P4 (p = .01) were significantly higher in cases compared to controls, and ratios of ALLO:P4 (p = .08) and EPI:P4 (p = .1) also trended higher in cases. The most severe cases, those with depression at both trimesters, showed an increase in ALLO:P4 (p = .06) and EPI:P4 (p < .001) ratios from the first to the second trimester, whereas controls showed a decrease in these ratios. Secondary analyses confirmed higher levels of ALLO (p = .04) and PA (p = .07) overall in cases compared to controls, along with higher levels of PA (p = .005) and ISOALLO (p = .02) in the second trimester alone. This work suggests a dynamic relationship between NAS and PND; whereas low ALLO levels have been previously associated with postpartum depression, earlier in pregnancy a higher metabolism of P4 to ALLO (and higher ALLO levels) is associated with depression. Some women may show a hormone-sensitive depressive response to acute increases in NAS metabolism in early pregnancy.

Epipregnanolone as a Positive Modulator of GABAA Receptor in Rat Cerebellar and Hippocampus Neurons

Epipregnanolone (3β-hydroxy-5β-pregnan-20-one, Epi) is an endogenous steroid with important physiological effects and high affinity for GABA_A receptors. The effect of Epi on GABA-induced chloride current (I_GABA) in native neurons has hardly been studied. In this work, we studied the influence of Epi on the I_GABA in the Purkinje cells of rat cerebellum and pyramidal neurons of rat hippocampus with the patch clamp technique. We showed that Epi is a positive modulator of the I_GABA with EC₅₀ of 5.7 µM in Purkinje cells and 9.3 µM in hippocampal neurons. Epi-induced potentiation of the I_GABA was more potent at low vs. high GABA concentrations. Isopregnanolone (3β-hydroxy-5α-pregnan-20-one, Iso) counteracted Epi, reducing its potentiating effect by 2–2.3 times. Flumazenil, a nonsteroidal GABA_A receptor antagonist, does not affect the Epi-induced potentiation. Comparison of the potentiating effects of Epi and allopregnanolone (3α-hydroxy-5α-pregnan-20-one, ALLO) showed that ALLO is, at least, a four times more potent positive modulator than Epi. The combined application of ALLO and Epi showed that the effects of these two steroids are not additive. We conclude that Epi has a dual effect on the I_GABA increasing the current in the control solution and decreasing the stimulatory effect of ALLO.

Photoaffinity labeling identifies an intersubunit steroid-binding site in heteromeric GABA type A (GABAA) receptors

Allopregnanolone (3α5α-P), pregnanolone, and their synthetic derivatives are potent positive allosteric modulators (PAMs) of GABA_A receptors (GABA_ARs) with in vivo anesthetic, anxiolytic, and anti-convulsant effects. Mutational analysis, photoaffinity labeling, and structural studies have provided evidence for intersubunit and intrasubunit steroid-binding sites in the GABA_AR transmembrane domain, but revealed only little definition of their binding properties. Here, we identified steroid-binding sites in purified human α1β3 and α1β3γ2 GABA_ARs by photoaffinity labeling with [³H]21-[4-(3-(trifluoromethyl)-3H-diazirine-3-yl)benzoxy]allopregnanolone ([³H]21-pTFDBzox-AP), a potent GABA_AR PAM. Protein microsequencing established 3α5α-P inhibitable photolabeling of amino acids near the cytoplasmic end of the β subunit M4 (β3Pro-415, β3Leu-417, and β3Thr-418) and M3 (β3Arg-309) helices located at the base of a pocket in the β⁺-α^- subunit interface that extends to the level of αGln-242, a steroid sensitivity determinant in the αM1 helix. Competition photolabeling established that this site binds with high affinity a structurally diverse group of 3α-OH steroids that act as anesthetics, anti-epileptics, and anti-depressants. The presence of a 3α-OH was crucial: 3-acetylated, 3-deoxy, and 3-oxo analogs of 3α5α-P, as well as 3β-OH analogs that are GABA_AR antagonists, bound with at least 1000-fold lower affinity than 3α5α-P. Similarly, for GABA_AR PAMs with the C-20 carbonyl of 3α5α-P or pregnanolone reduced to a hydroxyl, binding affinity is reduced by 1,000-fold, whereas binding is retained after deoxygenation at the C-20 position. These results provide a first insight into the structure-activity relationship at the GABA_AR β⁺-α^- subunit interface steroid-binding site and identify several steroid PAMs that act via other sites.

Allopregnanolone and pregnanolone analogues modified in the C ring: synthesis and activity

(25R)-3β-Hydroxy-5α-spirostan-12-one (hecogenin) and 11α-hydroxypregn-4-ene-3,20-dione (11α-hydroxyprogesterone) were used as starting materials for the synthesis of a series of 11- and 12-substituted derivatives of 5ξ-pregnanolone (3α-hydroxy-5α-pregnan-20-one and 3α-hydroxy-5β-pregnan-20-one), the principal neurosteroid acting via γ-aminobutyric acid (GABA). These analogues were designed to study the structural requirements of the corresponding GABAA receptor. Their biological activity was measured by in vitro test with [(3)H]flunitrazepam as radioligand in which allopregnanolone and its active analogues stimulated the binding to the GABAA receptor. Analysis of the SAR data suggests dependence of the flunitrazepam binding activity on the hydrophobic-hydrophilic balance of the groups at the C-ring edge rather than on specific interactions between them and the receptor.

Effects of pregnanolone and flunitrazepam on the retention of response sequences in rats

Neuroactive steroids produce effects similar to other GABA(A) modulators (e.g., benzodiazepines and barbiturates) and have a large therapeutic potential; however, a greater understanding of the effects of these substances on learning and memory is needed. To specifically assess the effects of a neurosteroid on memory, pregnanolone (1-18 mg/kg) was administered to male Long-Evans rats responding under a repeated acquisition and delayed-performance procedure in which different 4-response sequences were acquired and then retested after varying delays. Responding was maintained under a second-order fixed-ratio (FR) 2 schedule of food reinforcement, and incorrect responses (errors) produced a 5-sec timeout. For comparison purposes, both a high (flunitrazepam) and low efficacy agonist/antagonist (flumazenil) of the GABA(A) receptor complex were also administered both alone and in combination. Retention of each sequence was quantified as percent savings in errors-to-criterion and this dependent measure was shown to be sensitive to increases in delay. When administered 15 min prior to the end of either a 30- or 180-minute delay, pregnanolone produced both dose- and delay-dependent decreases in percent savings, response rate and accuracy; this effect was selective in that decreases in retention occurred at doses lower than those that disrupted response rate or accuracy. Flunitrazepam (0.056-1mg/kg) produced similar disruptions in retention and these disruptions were antagonized by 5.6 mg/kg of flumazenil. Both an ineffective (0.056 mg/kg) and an effective (0.18 mg/kg) dose of flunitrazepam also potentiated the dose- and delay-dependent disruptions in retention produced by pregnanolone. These data indicate that the neurosteroid pregnanolone disrupts retention in a manner similar to the benzodiazepine flunitrazepam, and suggests that the interaction of flunitrazepam and pregnanolone on retention may be mediated by the GABA(A) receptor complex.

Influence of epipregnanolone on the modulation of rapid tolerance to ethanol by neurosteroids

OBJECTIVE: The objective of the present study was to investigate the effect of epipregnanolone on the influence of neurosteroids on the development of rapid tolerance to the motor impairing and hypothermic effects of ethanol. METHOD: Experiment 1: on Day 1 groups of mice were pretreated with saline or with epipregnanolone. After 30 min each group was further divided in subgroups that received ethanol or saline. Thirty, 60 and 90 min after the injections the animals were tested on the rota-rod or the body temperature was measured. On Day 2 all groups received ethanol and a similar procedure was followed to evaluate rapid tolerance. Experiment 2 and 3: On Day 1 groups of mice were treated with epipregnanolone and after 15 min each group was divided into three groups in order to receive pregnenolone sulfate, dehydroepiandrosterone sulfate or saline. Thirty minutes later, each group was further divided into two subgroups in order to receive ethanol or saline, respectively, and 30, 60 and 90 min later the animals were tested as in the experiment 1. On Day 2 all groups received ethanol and a similar procedure was followed to evaluate rapid tolerance. RESULTS: Pretreatment with epipregnanolone (0.10-0.30 mg/kg) significantly blocked the development of tolerance to the motor impairing and hypothermic effects induced by ethanol in mice. Considering tolerance to ethanol-induced motor impairment, epipregnanolone (0.15 mg/kg) reversed the stimulatory action of dehydroepiandrosterone sulfate (0.15 mg/kg), but did not affect the actions of pregnenolone sulfate (0.08 mg/kg). Moreover, epipregnanolone prevented the inhibitory action of allotetrahydrodeoxycorticosterone (0.10 mg/kg). In relation to ethanol-induced hypothermia, the results showed that pretreatment with epipregnanolone (0.30 mg/kg) significantly prevented the stimulatory action of dehydroepiandrosterone sulfate and pregnenolone sulfate, as well as the inhibitory action of allotetrahydrodeoxicorticosterone (0.20 mg/kg), on tolerance to this effect. CONCLUSIONS: The results suggest a differential interaction between neurosteroids that might modulate the development of rapid tolerance to ethanol.

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.

Neuroactive steroid effects on cognitive functions with a focus on the serotonin and GABA systems

This article will review neuroactive steroid effects on serotonin and GABA systems, along with the subsequent effects on cognitive functions. Neurosteroids (such as estrogen, progesterone, and allopregnanolone) are synthesized in the central and peripheral nervous system, in addition to other tissues. They are involved in the regulation of mood and memory, in premenstrual syndrome, and mood changes related to hormone replacement therapy, as well as postnatal and major depression, anxiety disorders, and Alzheimer's disease. Estrogen and progesterone have their respective hormone receptors, whereas allopregnanolone acts via the GABA(A) receptor. The action of estrogen and progesterone can be direct genomic, indirect genomic, or non-genomic, also influencing several neurotransmitter systems, such as the serotonin and GABA systems. Estrogen alone, or in combination with antidepressant drugs affecting the serotonin system, has been related to improved mood and well being. In contrast, progesterone can have negative effects on mood and memory. Estrogen alone, or in combination with progesterone, affects the brain serotonin system differently in different parts of the brain, which can at least partly explain the opposite effects on mood of those hormones. Many of the progesterone effects in the brain are mediated by its metabolite allopregnanolone. Allopregnanolone, by changing GABA(A) receptor expression or sensitivity, is involved in premenstrual mood changes; and it also induces cognitive deficits, such as spatial-learning impairment. We have shown that the 3beta-hydroxypregnane steroid UC1011 can inhibit allopregnanolone-induced learning impairment and chloride uptake potentiation in vitro and in vivo. It would be important to find a substance that antagonizes allopregnanolone-induced adverse effects.

Syntheses of 19-[O-(carboxymethyl)oxime] haptens of epipregnanolone and pregnanolone

O-(Carboxymethyl)oximes 1 and 2 derived from two epimeric 5beta-pregnanolones (3beta-hydroxy-5beta-pregnan-20-one and 3alpha-hydroxy-5beta-pregnan-20-one) in position 19 were prepared. Two synthetic routes were employed, both using protection of the 20-keto group after reduction into the (20R)-alcohol in the form of acetate. In the first route, (20R)-19-hydroxy-5beta-pregnan-3beta,20-diyl diacetate (3) was transformed into the corresponding 19-[O-(carboxymethyl)oxime] methyl ester 6, then deacetylated by acid and partially silylated with tert-butyldimethylsilyl chloride. The desired 3-O-silylated derivative 8 was separated, oxidized to the 20-ketone and protecting groups were sequentially removed to give the first title hapten 1. The second route started from (20R)-19-hydroxy-3-oxopregn-4-en-20-yl acetate (11), which was hydrogenated in the presence of base to the 5beta-pregnan-3-one derivative 12, protected in position 19 with tert-butyldimethylsilyl group and reduced with borohydride. The prevailing 3alpha-alcohol 15 was separated, protected in position 3 with a methoxymethyl group, deprotected in position 19 and transformed into the 19-[O-(carboxymethyl)oxime] 19. After deacetylation, esterification with diazomethane and oxidation in position 20, the pregnanolone skeleton was regenerated. Final deprotection steps gave the second title hapten 2. Both haptens, i.e., (19E)-3beta- and -3alpha-hydroxy-20-oxo-5beta-pregnan-19-al 19-[O-(carboxymethyl)oxime], were designed for the development of immunoassays of the corresponding parent neuroactive steroids.

Alphaxalone and epiallopregnanolone in rats trained to discriminate ethanol

BACKGROUND

Neurosteroids with a 3 alpha-hydroxy orientation share pharmacological effects with ethanol, increase in brain after ethanol administration, and may mediate ethanol effects. 3beta-hydroxy neurosteroids antagonize in vitro and some, but not all in vivo effects of ethanol and 3 alpha-hydroxy neurosteroids.

METHODS

We assessed the discriminative stimulus and rate altering effects of alphaxalone, a 3 alpha-hydroxy neurosteroid, and epiallopregnanolone, a 3beta-hydroxy neurosteroid, in rats trained to discriminate either 0.8 g/kg or 1.2 g/kg ethanol. The ability of epiallopregnanolone to antagonize the discriminative stimulus or rate-altering effects of ethanol or alphaxalone was also assessed.

RESULTS

Ethanol had similar discriminative ED50s (0.5 g/kg) in both groups; however rats trained with the lower ethanol dose were more sensitive to rate-decreasing effects of ethanol. Alphaxalone occasioned ethanol-appropriate responding in both training groups, although less effectively in rats trained on the lower ethanol dose (maximum 65% versus 80% ethanol-appropriate responding). No difference in sensitivity to the rate-decreasing effects of alphaxalone was present between groups. Epiallopregnanolone did not reliably occasion ethanol-appropriate responding in either training group, and rats trained on the lower ethanol dose were slightly more sensitive to epiallopregnanolone rate decreasing effects. Epiallopregnanolone did not alter any effects of ethanol or alphaxalone.

CONCLUSIONS

Our results agree with previous reports that 3 alpha-hydroxy neurosteroids occasion ethanol-appropriate responding, while 3beta-hydroxy neurosteroids do not; as well as reports showing no antagonism of the discriminative stimulus or rate-suppressant effects of ethanol or 3 alpha-hydroxy neurosteroids by 3beta-hydroxy neurosteroids. Results of the present study demonstrate that ethanol and 3 alpha-hydroxy neurosteroids share discriminative stimulus effects. However, these results are inconsistent with the hypothesis that such neurosteroids mediate the discriminative stimulus of ethanol.

Neuroactive steroids in schizophrenia

Schizophrenia is a psychiatric disorder with a complicated pathophysiology, involving many biochemical abnormalities in the brain. Because neuroactive steroids (NASs) modulate neurotransmitter systems that are implicated in the pathology of schizophrenia, recent research has focused on examining the role that NASs play in the illness. Although research in this area is relatively new, it appears that NASs may potentially be implicated in the pathophysiology of the illness. This paper reviews the current understanding of NASs, the research literature on NASs in schizophrenia and in animal models of the illness (including the effects of antipsychotic medication on NASs) and on the potential antipsychotic role of NASs themselves and, finally, discusses future directions for this area of schizophrenia research.

Discriminative stimulus effects of 5.6 mg/kg pregnanolone in DBA/2J and C57BL/6J inbred mice

Neurosteroids represent a class of endogenous compounds that exert rapid, nongenomic effects through neurotransmitter receptor systems such as gamma-aminobutyric acid(A) (GABA(A)). Two neurosteroids, allopregnanolone (3alpha-hydroxy-5alpha-pregnan-20-one) and pregnanolone (3alpha-hydroxy-5beta-pregnan-20-one), possess anxiolytic and sedative properties and show substitution for ethanol, benzodiazepines, and barbiturates in drug discrimination assays. A previous study examining the discriminative stimulus effects of 10 mg/kg pregnanolone in DBA/2J and C57BL/6J mice showed pregnanolone's discriminative stimulus to be mediated primarily through GABA(A) positive modulation. This study examined the discriminative stimulus effects of a lower training dose (5.6 mg/kg) of pregnanolone in DBA/2J and C57BL/5J mice. Twelve male DBA/2J mice and 12 male C57BL/6J mice were trained to discriminate 5.6 mg/kg pregnanolone. GABA(A)-receptor positive modulators, neuroactive steroids, NMDA receptor antagonists, and 5-HT(3) receptor agonists were tested for pregnanolone substitution. In DBA/2J and C57BL/6J mice benzodiazepine, barbiturate, and GABAergic neuroactive steroids all substituted for pregnanolone. In the DBA/2J mice, NMDA receptor antagonists showed generalization to the discriminative stimulus cues of pregnanolone, an effect not seen in the C57BL/6J mice. 5-HT(3) receptor agonists and zolpidem failed to substitute for pregnanolone's discriminative stimulus in either strain. AlloTHDOC and midazolam were more potent in producing pregnanolone-like discriminative stimulus effects in DBA/2J mice. These results provide a comprehensive look at pregnanolone's discriminative stimulus effects in two commonly used strains of mice. The present data suggest GABA(A)-receptor positive modulation as the predominant receptor mechanism mediating the discriminative stimulus effects of pregnanolone. NMDA receptor antagonism was suggested in the DBA/2J mice and may represent a heterogenous cue produced by the lower training dose of pregnanolone.

Characterization of the discriminative stimulus effects of the neuroactive steroid pregnanolone in DBA/2J and C57BL/6J inbred mice

Neurosteroids represent a class of endogenous compounds that exert rapid, nongenomic effects through neurotransmitter receptor systems such as GABA(A). Two neurosteroids, allopregnanolone (3alpha-hydroxy-5alpha-pregnan-20-one) and pregnanolone (3alpha-hydroxy-5beta-pregnan-20-one), possess anxiolytic and sedative properties and show substitution for ethanol, benzodiazepines, and barbiturates in drug discrimination assays. This study aimed to examine the effects of strain and sex on the discriminative stimulus effects of pregnanolone. Twelve male and female DBA/2J mice and 12 male and female C57BL/6J mice were trained to discriminate 10 mg/kg pregnanolone from saline. The male C57BL/6J mice had to be removed from the study due to increased seizures apparently associated with the chronic intermittent pregnanolone administration used in drug discrimination. GABA(A)-positive modulators, neuroactive steroids, N-methyl-d-aspartate (NMDA) antagonists, and 5-hydroxytryptamine (5-HT)(3) agonists were tested for pregnanolone substitution. In DBA/2J and C57BL/6J mice, a benzodiazepine, barbiturate, and GABAergic neuroactive steroids all substituted for the stimulus effects of pregnanolone. NMDA antagonists, 5-HT(3) agonists, and zolpidem failed to substitute for pregnanolone's discriminative stimulus in either sex or strain. Pentobarbital and midazolam were more potent in producing pregnanolone-like discriminative stimulus effects in DBA/2J mice. Differences in sensitivities to neurosteroids between the two strains were not evident. These results provide a comprehensive look at pregnanolone's discriminative stimulus effects in two commonly used strains of mice. The present data suggest that many of the previously documented neurosteroid-induced behavioral differences between the DBA/2J and C57BL/6J are acute effects and are not apparent in a drug discrimination procedure.

Social isolation stress-induced aggression in mice: a model to study the pharmacology of neurosteroidogenesis

Long-term social isolation of laboratory animals is a model to study the behavioral and neurochemical consequences of the absence of social interaction in rodents. Many of the symptoms induced by isolation resemble depression and anxiety disorder symptomatology. Our studies have revealed that male mice socially isolated for more than 4 weeks, exhibit increased aggressiveness, a reduced responsiveness to GABA(A) receptor acting drugs, and a downregulation of brain levels of 3alpha,5alpha-tetrahydroprogesterone (allopregnanolone: 3alpha,5alpha-THP), a neurosteroid endowed with potent positive allosteric modulatory activity of the action of GABA at various GABA(A) receptor subtypes. This downregulation of 3alpha,5alpha-THP appeared to be associated with the reduction of brain type I 5alpha-reductase mRNA and protein expression. Systemic administration of the selective serotonin reuptake inhibitor fluoxetine and its metabolite norfluoxetine normalized brain 3alpha,5alpha-THP content and reduced responsiveness to GABA(A) mimetic drugs in a stereospecific manner. These drugs in nanomolar doses also reduced social isolation-induced aggressiveness with the same stereospecificity as detected in their action on 3alpha,5alpha-THP brain content, while their ex vivo inhibition of serotonin reuptake occurred at high micromolar doses and lacked stereospecificity. From these results we infer that the brain 3alpha,5alpha-THP content physiologically upregulates GABA(A) receptor responsiveness to GABA and that social isolation induces a reduction of brain 3alpha,5alpha-THP content that is probably causally related to the onset of aggression.

Isoallopregnanolone; an antagonist to the anaesthetic effect of allopregnanolone in male rats

The interaction of isoallopregnanolone (3 beta-OH-5 alpha-pregnan-20-one) on allopregnanolone (3 alpha-OH-5 alpha-pregnan-20-one) induced anaesthesia was studied in male rats using burst suppression of 1 s ("silent second") with an electroencephalographic-threshold method. The i.v. administration of isoallopregnanolone was varied in relation to induction of "silent second". Pre-treatment with isoallopregnanolone (12.5-50 mg/kg iv) 2 min prior to the threshold test gave an increase in the threshold dose of allopregnanolone (ANOVA df(3;36), F=13.61, P<0.001), which was dose dependent (r=0.73, b [slope]=0.08, df=38, P<0.001). After isoallopregnanolone pre-treatment, but not in the controls, anaesthesia time was positively related to the dose of allopregnanolone (r=0.52, b=1.72, df=28, P<0.01). Anaesthesia times were not influenced by a corresponding administration of isoallopregnanolone immediately after induction of "silent second". When allopregnanolone and isoallopregnanolone were infused together at molar ratios of 1:1, 1:1.23, 1:1.43, a linear increase of the threshold doses of allopregnanolone was seen in relation to the dose of isoallopregnanolone (r=0.86, b=0.40, df=8, P<0.01). Thus isoallopregnanolone can antagonise the anaesthetic action of allopregnanolone.

Dipole potential and head-group spacing are determinants for the membrane partitioning of pregnanolone

The membrane interactions of pregnanolone, an intravenous general anesthetic steroid, were characterized using fluorescence spectroscopy and monolayer technique. di-8-ANEPPS [4-[2-[6-(dioctylamino)-2-naphthalenyl]ethenyl]-1-(3-sulfopropyl)-pyridinium], a membrane dipole potential (Psi)-sensitive probe, revealed pregnanolone to decrease Psi similarly as reported previously for other anesthetics. The decrement in Psi was approximately 16 and 10 mV in dipalmitoylphosphatidylcholine (DPPC) and DPPC/cholesterol (90:10, mol/mol) vesicles, respectively. Diphenylhexatriene anisotropy indicated pregnanolone to have a negligible effect on the acyl chain order. In contrast, substantial changes were observed for the fluorescent dye Prodan, thus suggesting pregnanolone to reside in the interfacial region of lipid bilayers. Langmuir balance studies indicated increased association of pregnanolone to DPPC monolayers containing cholesterol or 6-ketocholestanol at surface pressures pi > 20 mN/m as well as to monolayers of the unsaturated 1-palmitoyl-2-oleoylphosphatidylcholine. In the same surface pressure range, the addition of phloretin, which decreases Psi, reduced the penetration of pregnanolone into the monolayers. These results suggest that membrane partitioning of pregnanolone is influenced by the spacing of the phosphocholine head groups as well as by membrane dipole potential. The latter can be explained in terms of electrostatic dipole-dipole interactions between pregnanolone and the membrane lipids with their associated water molecules. Considering the universal nature of these interactions, they are likely to affect membrane partitioning of most, if not all, weakly amphiphilic drugs.

Insight into the mechanism of action of neuroactive steroids

Rho(1) receptor-channels (rho(1)Rs) are GABA-gated chloride channels that exhibit slow kinetics, little desensitization, and inert pharmacology to most anesthetics, except for neuroactive steroids (NSs). NSs differentially modulate rho(1)Rs dependent on the steric arrangement of the hydrogen atom at the fifth carbon position. In particular, the NS allotetrahydrodeoxycorticosterone (5alpha-THDOC) potentiates, whereas 5beta-pregnane-3alpha-ol-20-one (pregnanolone) and 5beta-dihydroprogesterone (5beta-DHP) inhibit rho(1) GABA currents. Here, we used Xenopus laevis oocytes expressing rho(1)Rs as a model system to study the mechanism of NS modulation. The second transmembrane residue, Ile307, was mutated to 16 amino acids. Subsequent testing of these mutants with 5alpha- and 5beta-NSs, at equivalent GABA activity, showed the following paradigm. For 5beta-DHP, Ile307 mutation either altered the degree of inhibition or entirely reversed the direction of modulation, rendering 5beta-DHP a potentiator. Dependent on the mutation, pregnanolone remained an inhibitor, transformed into a potentiator, or converted to inhibitor and potentiator based on concentration. The extent of mode reversal for both 5beta compounds showed a correlation with the side-chain hydrophilicity of the 307 residue. In contrast, Ile307 substitutions did not alter the direction of modulation for 5alpha-THDOC but caused a significant increase in the level of potentiation. Paradoxical to their impact on the mode and/or the degree of modulation, none of the mutations altered the concentration range producing the response significantly for any of the above NSs. Moreover, preincubation of Ile307 mutants with 5alpha or 5beta alone produced an equivalent effect on the activation time course. Based on the above data, a universal model is presented wherein anesthetic compounds like NSs can potentiate or inhibit the activity of ligand-gated ion channels distinct from interaction with alternative binding sites.

Selective antagonism of 5alpha-reduced neurosteroid effects at GABA(A) receptors

Although neurosteroids have rapid effects on GABA(A) receptors, study of steroid actions at GABA receptors has been hampered by a lack of pharmacological antagonists. In this study, we report the synthesis and characterization of a steroid analog, (3alpha,5alpha)-17-phenylandrost-16-en-3-ol (17PA), that selectively antagonized neurosteroid potentiation of GABA responses. We examined 17PA using the alpha1beta2gamma2 subunit combination expressed in Xenopus laevis oocytes. 17PA had little or no effect on baseline GABA responses but antagonized both the response augmentation and the direct gating of GABA receptors by 5alpha-reduced potentiating steroids. The effect was selective for 5alpha-reduced potentiating steroids; 5beta-reduced potentiators were only weakly affected. Likewise, 17PA did not affect barbiturate and benzodiazepine potentiation. 17PA acted primarily by shifting the concentration response for steroid potentiation to the right, suggesting the possibility of a competitive component to the antagonism. 17PA also antagonized 5alpha-reduced steroid potentiation and gating in hippocampal neurons and inhibited anesthetic actions in X. laevis tadpoles. Analogous to benzodiazepine site antagonists, the development of neurosteroid antagonists may help clarify the role of GABA-potentiating neurosteroids in health and disease.

Chronic prenatal ethanol exposure alters hippocampal GABAA receptors and impairs spatial learning in the guinea pig

Chronic prenatal ethanol exposure (CPEE) can injure the developing brain, and may lead to the fetal alcohol syndrome (FAS). Previous studies have demonstrated that CPEE upregulates gamma-aminobutyric acid type A (GABA(A)) receptor expression in the cerebral cortex, and decreases functional synaptic plasticity in the hippocampus, in the adult guinea pig. This study tested the hypothesis that CPEE increases GABA(A) receptor expression in the hippocampus of guinea pig offspring that exhibit cognitive deficits in a hippocampal-dependent spatial learning task. Timed, pregnant guinea pigs were treated with ethanol (4 g/kg maternal body weight per day), isocaloric-sucrose/pair-feeding, or water throughout gestation. GABA(A) receptor subunit protein expression in the hippocampus was measured at two development ages: near-term fetus and young adult. In young adult guinea pig offspring, CPEE increased spontaneous locomotor activity in the open-field and impaired task acquisition in the Morris water maze. CPEE did not change GABA(A) receptor subunit protein expression in the near-term fetal hippocampus, but increased expression of the beta2/3-subunit of the GABA(A) receptor in the hippocampus of young adult offspring. CPEE did not change either [(3)H]flunitrazepam binding or GABA potentiation of [(3)H]flunitrazepam binding, but decreased the efficacy of allopregnanolone potentiation of [(3)H]flunitrazepam binding, to hippocampal GABA(A) receptors in adult offspring. Correlational analysis revealed a relationship between increased spontaneous locomotor activity and growth restriction in the hippocampus induced by CPEE. Similarly, an inverse relationship was found between performance in the water maze and the efficacy of allopregnanolone potentiation of [(3)H]flunitrazepam binding in the hippocampus. These data suggest that alterations in hippocampal GABA(A) receptor expression and pharmacological properties contribute to hippocampal-related behavioral and cognitive deficits associated with CPEE.

Allopregnanolone-stimulated GABA-mediated chloride ion flux is inhibited by 3beta-hydroxy-5alpha-pregnan-20-one (isoallopregnanolone)

Altered gamma-aminobutyric acid (GABA)-ergic function is associated with neurological and psychiatric disorders. Certain progesterone metabolite 3alpha-hydroxy-5alpha-pregnan-20-one, or allopregnanolone (ALLO), increases the GABA-mediated chloride ion (Cl(-)) flux through GABA(A) receptors in a similar fashion as benzodiazepines and barbiturates. We have studied the effect of its 3beta diastereomer, 3beta-hydroxy-5alpha-pregnan-20-one, or isoallopregnanolone (ISO), on the Cl(-) flux and investigated the interaction between ISO and ALLO on GABA-mediated Cl(-) uptake in cortical homogenates from adult male Wistar rats. We found that ISO from 1 microM to 1 mM does not affect baseline Cl(-) uptake in rat cortical homogenates. Neither does ISO at dose range of 100 nM to 100 microM interact with 10 microM GABA in the Cl(-) uptake assay. In addition, ISO at the dose range of 1-30 microM does not affect flunitrazepam and pentobarbital-induced increase of Cl(-) uptake. We conclude that ISO selectively inhibits the ALLO-induced Cl(-) uptake with respect to ALLO concentrations. The IC(50) of ISO inhibition on 1 microM ALLO-induced Cl(-) uptake was calculated to be 12.25 microM. On the other hand, we have studied the effect of 30 microM ISO on ALLO (0.01 nM to 1 microM) induced displacement of tert-butylbicyclophosphorothionate (TBPS) binding. We did not note any interaction between ALLO and ISO on TBPS binding assay. These results indicate that ISO may be useful functional blocker of GABA(A) receptor potentiating steroid ALLO when used at concentrations that do not affect baseline GABAergic neurotransmission.

3beta -hydroxypregnane steroids are pregnenolone sulfate-like GABA(A) receptor antagonists

Endogenous neurosteroids have rapid actions on ion channels, particularly GABA(A) receptors, which are potentiated by nanomolar concentrations of 3alpha-hydroxypregnane neurosteroids. Previous evidence suggests that 3beta-hydroxypregnane steroids may competitively antagonize potentiation induced by their 3alpha diastereomers. Because of the potential importance of antagonists as experimental and clinical tools, we characterized the functional effect of 3beta-hydroxysteroids. Although 3beta-hydroxysteroids reduced the potentiation induced by 3alpha-hydroxysteroids, 3beta-hydroxysteroids acted noncompetitively with respect to potentiating steroids and inhibited the largest degrees of potentiation most effectively. Potentiation by high concentrations of barbiturates was also reduced by 3beta-hydroxysteroids. 3beta-Hydroxysteroids are also direct, noncompetitive GABA(A) receptor antagonists. 3beta-Hydroxysteroids coapplied with GABA significantly inhibited responses to > or =15 microm GABA. The profile of block was similar to that exhibited by sulfated steroids, known blockers of GABA(A) receptors. This direct, noncompetitive effect of 3beta-hydroxysteroids was sufficient to account for the apparent antagonism of potentiating steroids. Mutated receptors exhibiting decreased sensitivity to sulfated steroid block were insensitive to both the direct effects of 3beta-hydroxysteroids on GABA(A) responses and the reduction of potentiating steroid effects. At concentrations that had little effect on GABAergic synaptic currents, 3beta-hydroxysteroids and low concentrations of sulfated steroids significantly reversed the potentiation of synaptic currents induced by 3alpha-hydroxysteroids. We conclude that 3beta-hydroxypregnane steroids are not direct antagonists of potentiating steroids but rather are noncompetitive, likely state-dependent, blockers of GABA(A) receptors. Nevertheless, these steroids may be useful functional blockers of potentiating steroids when used at concentrations that do not affect baseline neurotransmission.

Pregnenolone sulfate, dehydroepiandrosterone sulfate and allotetrahydrodeoxycorticosterone affect rapid tolerance to the hypothermic effect of ethanol

Our previous study showed that the neurosteroids pregnenolone sulfate (PS) and epipregnanolone stimulated and blocked, respectively, the demonstration of chronic tolerance to the incoordinating effect of ethanol. The aim of the present study was to investigate the influence of three neurosteroids on the demonstration of tolerance to ethanol-induced hypothermia in mice using the rapid tolerance paradigm. The first experiment defined the doses of ethanol that did or did not induce rapid tolerance to ethanol-induced hypothermia. In the second, the influence of pretreatment of mice with PS (0.08 or 0.15 mg/kg, i.p.) or dehydroepiandrosterone sulfate (DHEAS; 0.15 or 0.20 mg/kg, i.p.) before ethanol (4.0 g/kg, i.p.) on rapid tolerance was studied. The third experiment examined the effect of allotetrahydrodeoxicorticosterone (ALLOT; 0.10 or 0.20 mg/kg, i.p.) before ethanol (4.0 g/kg, i.p.) on rapid tolerance. Results showed that pretreatment with PS or with DHEAS significantly facilitated the demonstration of rapid tolerance, whereas pretreatment with ALLOT interfered with the demonstration of tolerance to the hypothermic effect.

Neurosteroid modulation of recombinant and synaptic GABAA receptors

Certain pregnane steroids are now established as potent, positive allosteric modulators of the gamma-aminobutyric acid type A (GABAA) receptor. These compounds are known to be synthesized in the periphery by endocrine glands, such as the ovaries and the adrenal glands, and can rapidly cross the blood-brain barrier. Therefore, such steroids could act as endogeneous modulators of the major inhibitory receptor in the mammalian central nervous system. However, the demonstration that certain neurons and glia can synthesize the pregnane steroids (i.e., neurosteroids) additionally suggests that they may serve a paracrine role by influencing GABAA-receptor function through their local release in the brain itself. Here, we demonstrate that these neurosteroids are highly selective and extremely potent modulators of the GABAA receptor. The subunit composition of the GABAA receptor may influence the actions of the neurosteroids, particularly when considering concentrations of these agents thought to occur physiologically, which may underlie their reported differential effects at certain inhibitory synapses. However, recent work suggests that the phosphorylation status of either the synaptic GABAA receptor or its associated proteins may also influence neurosteroid sensitivity; these findings are discussed. Upon administration, the neurosteroids exhibit clear behavioral effects, including sedation, anticonvulsant actions, and behaviors predictive of anxiolysis; when given at high doses, they induce general anesthesia. Numerous synthetic steroids have been synthesized in an attempt to therapeutically exploit these properties, and these data are reviewed in this chapter. However, targeting the brain enzymes that synthesize and metabolize the neurosteroids may offer a new approach to exploit this novel endocrine-paracrine neurotransmitter interaction.

Influence of neurosteroids on the development of rapid tolerance to ethanol in mice

Our recent study demonstrated that neurosteroids might either facilitate or block chronic tolerance to the incoordinating effects of ethanol. The present study investigated the effects of neurosteroids on the development of rapid tolerance to ethanol-induced motor impairment using the N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine [(+)-MK-801] or the gamma-aminobutyric acid (GABA) type A (GABA(A)) receptor agonist muscimol. Male Swiss mice were pretreated with pregnenolone sulfate (0.03 to 0.15 mg/kg) or dehydroepiandrosterone sulfate (0.05 to 0.20 mg/kg) before administration of ethanol (1.9 or 2.25 g/kg) and tested with the rota-rod apparatus. Twenty-four hours later, all animals were re-tested with the rota-rod after receiving the same dose of ethanol. Pretreatment with pregnenolone sulfate or with dehydroepiandrosterone sulfate significantly facilitated the acquisition of tolerance. However, the administration of (+)-MK-801 reversed the stimulatory action of pregnenolone sulfate but did not affect the actions of dehydroepiandrosterone sulfate on ethanol tolerance. Pretreatment with pregnenolone sulfate or dehydroepiandrosterone sulfate prevented the inhibitory action of muscimol on tolerance development. Taken together, our results suggest that neurosteroids may stimulate the development of rapid tolerance to ethanol and that GABA(A) and NMDA receptor systems may be involved in these actions.

Modulation of native and recombinant GABA(A) receptors by endogenous and synthetic neuroactive steroids

Upon administration, certain pregnane steroids produce clear behavioural effects including, anxiolysis, sedation, analgesia, anaesthesia and are anti-convulsant. This behavioural profile is characteristic of compounds that act to enhance the actions of GABA acting at the GABA(A) receptor. In agreement, numerous studies have now demonstrated these steroids to be potent, positive allosteric modulators of the GABA(A) receptor. The pregnane steroids are synthesized in the periphery by endocrine glands such as the adrenals and the ovaries, but are also made by neurons and glial cells in the central nervous system itself. Hence, these compounds could play both an endocrine and a paracrine role to influence neuronal excitability by promoting inhibition. Here we review evidence that the pregnane steroids are highly selective and extremely potent GABA(A) receptor modulators and that their effects at 'physiological' concentrations (low nanomolar) may be influenced by the subunit composition of the GABA(A) receptor. This feature may underlie recent findings demonstrating the effects of the neurosteroids on inhibitory synaptic transmission to be brain region dependent, although recent reports suggest that phosphorylation mechanisms may additionally influence neurosteroid sensitivity of the GABA(A) receptor. Numerous synthetic steroids have been synthesized in an attempt to therapeutically exploit the behavioural effects of the pregnane steroids and progress with this approach will be discussed. However, the demonstration that the steroids may be made within the central nervous system offers the alternative strategy of targeting the enzymes that synthesize/metabolise the neurosteroids to exploit this novel endocrine/paracrine interaction.

In ovo chronic neurosteroid treatment affects the function and allosteric interactions of GABAA receptor modulatory sites

We investigated the effects of in ovo chronic administration of the endogenous neurosteroid epipregnanolone (5beta-pregnan-3beta-ol-20-one) on the GABA(A) receptor complex present in chick optic lobe synaptic membranes. Chronic epipregnanolone treatment failed to exert any effect on the chick optic lobe total protein content and wet weight at the different doses tested. [3H]Flunitrazepam control binding remained unaltered after neurosteroid exposure, however, the positive allosteric modulation of this ligand by 4 microM allopregnanolone was reduced in a dose-dependent manner by neurosteroid treatment. Embryo exposure to 30 microM epipregnanolone decreased allopregnanolone EC(50) and E(max) values. Analyses of saturation binding isotherms disclosed that such administration had no effect on K(d) and B(max) values for [3H]flunitrazepam and [3H]GABA binding. [3H]GABA binding modulation disclosed an increase in allopregnanolone EC(50) value with a decrease in its E(max) value. Diazepam EC(50) and E(max) values were enhanced, while low affinity sodium pentobarbital EC(50) value was reduced by epipregnanolone treatment. The investigation of the GABA(A) receptor function revealed that administration of this neurosteroid reduces the efficacy of GABA to induce 36Cl(-) influx into microsacs prepared from chick optic lobe. These results indicate that endogenous neurosteroid epipregnanolone chronically administered in ovo produces homologous uncoupling between steroid modulatory sites, and those corresponding to benzodiazepine and GABA receptors. Thus epipregnanolone is able to induce heterologous changes in the allosteric linkage between benzodiazepine and barbiturate modulatory sites, and the GABA receptor site. Taken jointly with results on epipregnanolone enhancing effects on [3H]flunitrazepam and [3H]GABA binding, in the context of its endogenous synthesis, our present findings support this neurosteroid as the endogenous modulator of GABA(A) receptor sites and function during chick optic lobe development.

Additivities of compounds that increase the numbers of high affinity [3H]muscimol binding sites by different amounts define more than 9 GABA(A) receptor complexes in rat forebrain: implications for schizophrenia and clozapine research

BACKGROUND

The numbers of [3H]MUS binding sites were reported to be elevated in layers II and III, but not V or VI, in cingulate cortex of schizophrenic brains post mortem. These increases in [3H]MUS binding sites are probably due to compensatory up-regulation of GABA(A) receptors on pyramidal cells as a consequence of a selective loss of GABAergic interneurons in layer II of cingulate cortex. The number of [3H]flunitrazepam binding sites was reported to be reduced in schizophrenic cingulate cortex, and this may directly reflect the loss of GABAergic interneurons. Chronic administration of clozapine to rats was reported to significantly reduce the numbers of [3H]MUS binding sites in temporal cortex and hippocampus which may be due to selective blockade of GABA(A) receptors on GABAergic interneurons that make synaptic contact with pyramidal cells. Basket cells are GABAergic interneurons that make synaptic contact with pyramidal cells as well as other interneurons. Basket cells can also generate both theta and gamma oscillations. Clozapine increases the power of theta and gamma EEG. Schizophrenic patients show reduced EEG power at 40 Hz (gamma frequency) but not at lower frequencies during auditory stimulation. The GABA(A) receptor blocker bicuculline at 10 nM, but not 10 microM, was reported to increase the amplitude of slow oscillations (< or =1 Hz) in rat hippocampal slices. It therefore seems possible that clozapine, by selectively blocking another GABA(A) receptor, could increase the amplitude of gamma oscillations.

FINDINGS

Twenty-six compounds that inhibit [35S]TBPS binding in ways that are reversible by 10 nM R-5135 were found to increase [3H]MUS binding to membranes prepared from rat whole forebrain. In almost all cases the increases in binding were due to increases in the number of binding sites with little effect on affinity (Kd) for [3H]MUS. Concentration-response curves for the compounds revealed maximum increases in [3H]MUS (Esat) binding ranging from 140% (for meclizine) to 313% of control for honokiol. Additivity experiments showed that propofol (44% above control) and diflunisal (50% above control) were almost entirely additive, but there was also a small, but significant overlap, suggesting the existence of three groups of [3H]MUS binding sites defined by propofol and diflunisal. Meclizine was entirely additive with both propofol and diflunisal, indicating the existence of a fourth [3H]MUS binding site. Alphaxalone is also completely additive with meclizine, and has an Esat value significantly larger than that for propofol + diflunisal suggesting a fifth [3H]MUS binding site. The Esat for mefenamate is significantly greater than the Esat for alphaxalone, and mefenamate is also completely additive with meclizine, suggesting the existence of a sixth [3H]MUS binding site. The Esat for magnolol is significantly greater then the Esat, for mefenamate, and the Esat for honokiol is greater than that for magnolol, suggesting, but not proving, the existence of a seventh and an eighth group of [3H]MUS binding sites. The binding of [3H]MUS alone, without enhancers may represent a ninth group of binding sites which is probably heterogeneous as indicated by the very low pseudo Hill coefficients for bicuculline and strychnine in displacing [3H]MUS without enhancer. Altogether, our results suggest the existence of more than 9 different [3H]MUS binding sites. Clozapine was a very weak overall displacer of [3H]MUS (IC50 = 280 microM). However, 5 microM clozapine reduced [3H]MUS binding 6% (P < 0.0001, n = 10) and significantly reduced [3H]MUS binding enhanced by propofol (approximately 14%) or clotrimazole (approximately 17%) but not 17 other compounds tested.

TENTATIVE CONCLUSIONS

In the absence of enhancers [3H]MUS may bind preferentially to GABA(A) receptors on pyramidal cells and less to interneurons in cerebral cortex. Conversely, [3H]flunitrazepam may bind preferentially to GABA(A) receptors (allosterically) on interneurons and less to pyramidal cells. Clozapine appears to selectively block a small fraction (10-20%) of [3H]MUS binding sites with an IC50 value in the low micromolar range. This fraction may be preferentially located on certain GABAergic interneurons (basket cells?) that make synaptic contact with pyramidal cells. The blockade of these GABA(A) receptors by clozapine would be expected to increase the firing rate of the interneurons and the release of GABA onto pyramidal cells. Such blockade would also increase the generation of gamma oscillations by the basket cells. Some of these interneurons appear to be destroyed selectively, probably during the second trimester of gestation by a non-paralytic polio virus, in individuals who wil

Effect of epipregnanolone and pregnenolone sulfate on chronic tolerance to ethanol

The aim of the present study was to investigate the influence of neurosteroids on the development of tolerance to ethanol. Male Swiss mice were injected daily with the positive allosteric modulator of the gamma amino butyric acid-A (GABA(A)) receptor epipregnanolone (5beta-pregnan-3beta-ol-20-one; 0.15 mg/kg i.p.) or pregnenolone sulfate (5-pregnen-3beta-ol-20-one sulfate sodium; 0.08 mg/kg i.p.) - considered a negative allosteric modulator of this receptor and/or positive allosteric modulator of the N-methyl-D-aspartate (NMDA) receptor - 30 min before ethanol (2.5 g/kg i.p.). They were tested on the rota-rod apparatus, under continuous acceleration (1rpm/s), at 30, 60 and 90 min after ethanol injections for 5 days. The results showed that tolerance to the motor incoordinating effect of ethanol occurred on the fifth day of treatment when this effect was blocked by pretreatment with epipregnanolone. On the other hand, ethanol tolerance was enhanced by pretreatment with pregnenolone sulfate from the second to the fifth days of treatment. Taken together, our results suggest that neurosteroids can either stimulate or block the development of chronic tolerance to ethanol. Moreover, since neurosteroids can interact with GABA(A) or NMDA receptor systems, our results suggest the involvement of these systems in the actions of neurosteroids upon ethanol tolerance.

The inhibitory effects of allopregnanolone and pregnanolone on the population spike, evoked in the rat hippocampal CA1 stratum pyramidale in vitro, can be blocked selectively by epiallopregnanolone

The progesterone metabolites allopregnanolone (Allo, 3alpha-hydroxy-5alpha-pregnan-20-one) and pregnanolone (Preg, 3alpha-hydroxy-5beta-pregnan-20-one) enhance the gamma-aminobutyric acid (GABA) action through a distinct site on the GABAA-receptor. Their 3beta-isomers epiallopregnanolone (Epiallo, 3beta-hydroxy-5alpha-pregnan-20-one) and epipregnanolone (Epipreg, 3beta-hydroxy-5beta-pregnan-20-one), do not have these effects on GABAA-receptors. We have studied the interaction between Allo/Preg and their 3beta-isomers on action potentials in rat hippocampal slices in vitro. The Schaffer collaterals were stimulated electrically in CA1 striatum radiatum and the population spike (POPSP) was recorded in stratum pyramidale. A 0.5-nL droplet of drug was applied locally onto stratum oriens-pyramidale via a pressure pipett. Muscimol (Mus) (12.5 fmol), Allo and Preg (6.25 fmol) caused a reversible inhibition of POPSP. On the other hand, 6.25 fmol Epiallo had no significant effect on POPSP compared with the vehicle control. Combined Epiallo and Allo application caused a dose-dependent reduction of the Allo inhibition of POPSP. A full blockage was seen at a molar ratio of 1:1. Epiallo also blocked the Preg inhibition of POPSP, when the two drugs were combined in a molar ratio of 1:1. Epiallo did not block the Mus inhibition of POPSP, when the two drugs were combined at a molar ratio of 1:2. Bath perfusion of 12.5 microM Epiallo blocked the inhibition of 6.25 fmol Allo on POPSP, but not the inhibition caused by 12.5 fmol Mus. Epipreg did not block the inhibition of Allo and Preg on POPSP, when it was combined with the two latter drugs at a molar ratio of 1:1. Our data suggest that the steroid modulation of the GABAA transmitted inhibition of the CA1 pyramidal neurones is selectively and dose dependently blocked by Epiallo, the 3beta-hydroxy-isomer of Allo, but not by Epipreg, the 3beta-hydroxy-isomer of Preg.

Subunit dependent modulation of GABAA receptor function by neuroactive steroids

Neurosteroids are potent, endogenous modulators of GABAA receptor function in the central nervous system. The endogenous progesterone metabolite allopregnanolone (ALP) and the synthetic steroid compound alphaxalone (AFX) have been shown to both directly activate and potentiate GABAA receptor-activated membrane current (IGABA). The role of different alpha and gamma subunit subtypes in modulation of IGABA by ALP and AFX was investigated using recombinant GABAA receptor isoforms expressed in Xenopus oocytes. Changing or removal of the alpha subunit subtype altered the efficacy of both ALP and AFX (alpha2beta1gamma2L>alpha1beta1gamma2L>>beta1gamma2L) to potentiate IGABA, but did not alter the potency of the neuroactive steroids at these receptor isoforms. The efficacy of ALP to enhance IGABA was also dependent on the gamma subunit subtype (alpha1beta1gamma3>alpha1beta1gamma2L = alpha1beta1gamma1). AFX also had higher efficacy in the alpha1beta1gamma3 receptor isoform compared to alpha1beta1gamma1. In contrast to ALP, the potency of AFX was greater in the alpha1beta1gamma3 and alpha1beta1gamma1 receptor isoforms compared to alpha1beta1gamma2L. This study provides evidence that the alpha subunit subtype determines the efficacy, but not the potency, of these neuroactive steroids to potentiate IGABA. The gamma3 subunit subtype increases the maximal efficacy of neuroactive steroids compared to other gamma subunit subtypes. These results suggest that the heteromeric assembly of different GABAA receptor isoforms containing different subunit subtypes results in multiple steroid recognition sites on GABAA receptors that in turn produce distinctly different modulatory interactions between neuroactive steroids acting at the GABAA receptor.

Sex and estrous cycle-dependent changes in neurosteroid and benzodiazepine effects on food consumption and plus-maze learning behaviors in rats

Experiments were designed to investigate the influence of estrous cycle and gender of the rat on the effects of a gamma-aminobutyric acid type A (GABA(A)) receptor active neurosteroid, 3alpha-hydroxy-5alpha-pregnan-20-one (allopregnanolone), the benzodiazepine, triazolam, and a GABA(A) receptor antagonistic neurosteroid, delta5-androsten-3beta-ol-17-one sulfate (dehydroepiandrosterone sulfate), on food intake and elevated plus-maze learning behaviors. Allopregnanolone (0.25 mg/kg, s.c.) and triazolam (0.25 mg/kg, i.p.) produced a hyperphagic effect, while dehydroepiandrosterone sulfate (5 mg/kg, s.c.) elicited an anorectic effect. However, allopregnanolone was more potent in diestrous females, whereas triazolam exhibited significantly higher hyperphagic potency in estrus females. The extent of anorexia following dehydroepiandrosterone sulfate was alike in male and female rats. The triazolam- and allopregnanolone-induced hyperphagic effect was blocked by bicuculline (1 mg/kg, i.p.), a selective GABA(A) receptor antagonist. In contrast to triazolam, the hyperphagic effect of allopregnanolone was insensitive to flumazenil (5 mg/kg, i.p.), a benzodiazepine antagonist. Vehicle-treated diestrous rats displayed moderately higher latencies in the elevated plus-maze learning task than estrus or proestrus females. Although allopregnanolone and triazolam elicited equipotent learning deficits in plus-maze learning in male and female rats, the magnitude of impairment-induced by triazolam was significantly higher in diestrous females than proestrus females. Dehydroepiandrosterone sulfate enhanced memory performance only in male rats. Although the use of the elevated plus-maze as a learning paradigm with benzodiazepines and neurosteroids may be sensitive to changes in anxiety, the differential data suggest that neurosteroid-induced effects are at least partly specific to learning behavior. These results confirm the role of estrous cycle and sex of rats in modifying the potency of neurosteroids and benzodiazepines on food consumption and learning and memory processes.

Modulation of gamma-aminobutyric acid (GABA) receptors and the feeding response by neurosteroids in Hydra vulgaris

Gamma-Aminobutyric acid (GABA) receptors are present in membrane preparations from Hydra vulgaris, one of the most primitive organisms with a nervous system. These receptors are sensitive to muscimol and benzodiazepines and appear to be important in the regulation of the feeding response. The effects of neurosteroids, general anaesthetics, and GABA antagonists on GABA(A) receptors in membranes prepared from Hydra and on the feeding response have now been investigated. The neurosteroids tetrahydroprogesterone and tetrahydrodeoxycorticosterone increased [3H]GABA binding to hydra membranes with nanomolar potency (EC50, 141+/-11 and 623+/-36 nM, respectively) and high efficacy (maximal increase 79+/-6.5 and 62+/-4%, respectively), whereas the 3beta-hydroxy epimer of tetrahydroprogesterone was ineffective. The benzodiazepine receptor ligands diazepam (100 microM), clonazepam (100 microM) and abecarnil (30 microM) enhanced [3H]GABA binding to Hydra membranes by 22, 20 and 24%, respectively; effects abolished by the specific benzodiazepine antagonist flumazenil (100 microM). On the contrary, the peripheral benzodiazepine receptor ligand 4'chlorodiazepam failed to affect [3H]GABA binding to Hydra membranes. The general anaesthetics propofol and alphaxalone similarly increased (+38% and +30% respectively) [3H]GABA binding. Moreover, [3H]GABA binding to Hydra membranes was completely inhibited by the GABA(A) receptor antagonist SR 95531, whereas bicuculline was without effect. The modulation of GABA(A) receptors in vitro by these various drugs correlated with their effects on the glutathione-induced feeding response in the living animals. Tetrahydroprogesterone and tetrahydrodeoxy-corticosterone (1 to 10 microM) prolonged, in a dose-dependent manner, the duration of mouth opening induced by 10 microM glutathione, with maximal effects of +33 and +29%, respectively, apparent at 10 microM neurosteroid. Alphaxalone (10 microM) similarly increased (+33%) the effect of glutathione. The effects of steroids on the feeding response were inhibited by SR 95531 in a dose-dependent manner; t-butylbyclophosphorothyonate (1 microM), a specific Cl- channel blocker, which per se, like picrotoxin but not bicuculline, shortened the duration of the response, also counteracted the steroids effects at 1 microM. These results suggest that the modulation of GABA(A) receptors by steroids is an ancient characteristic of the animal kingdom and that the pharmacological properties of these receptors have been highly conserved through evolution.

Prenatal experience and postnatal stress modulate the adult neurosteroid and catecholaminergic stress responses

Allopregnanolone (3 alpha-hydroxy-5 alpha-regnan-20-one) is a neuroactive steroid recently shown to be involved in the neurochemical stress response via its positive modulation of the GABAA receptor complex. This experiment investigated the effects of postnatal stress (daily maternal separation during the first week of life) on the subsequent adult response to a stressor (10 min forced swim) in Long-Evans rats from one of three prenatal treatment groups (alcohol, pair-fed and control). Indices of stress response were allopregnanolone concentrations in plasma, cortex and hippocampus, and dopamine and norepinephrine concentrations in prefrontal cortex, nucleus accumbens and striatum. Females had higher levels of allopregnanolone than males in both plasma and brain. Prenatal alcohol exposure combined with early maternal separation stress resulted in an increase in the endogenous levels of allopregnanolone in the prefrontal cortex and hippocampus of adult offspring in response to a stressor compared to subjects without a prior history of postnatal stress; this effect was greater in females. This increased allopregnanolone was also associated with decreased dopamine and norepinephrine levels in the prefrontal cortex. In the prenatal alcohol-exposed offspring, postnatal maternal separation blunted the increase in dopamine levels in the striatum seen in both control groups. Postnatal maternal separation increased norepinephrine levels in the nucleus accumbens regardless of prenatal experience, while in the prefrontal cortex only prenatal diet condition (pair-feeding and alcohol) resulted in lower norepinephrine levels. The results of this experiment suggest that experience, both pre- and postnatal, can have long-term consequences for the developing neurochemical responses to stressors.

Neurosteroids act on the GABA(A) receptor at sites on the N-terminal side of the middle of TM2

Two sets of chimeras between alphaxalone-sensitive GABA(A) receptor alpha2 or beta1 subunits and the alphaxalone-insensitive glycine receptor alpha1 subunit were constructed to determine the structural domains important for the modulatory actions of neuroactive steroids. These data suggest that the site of action for neurosteroids on GABA(A) receptors is not the same as that for volatile anesthetics and ethanol, but is on the N-terminal side of the middle of TM2.

Comparative modulation by 3 alpha,5 alpha and 3 beta,5 beta neurosteroids of GABA binding sites during avian central nervous system development

Neurosteroids are endogenous Central Nervous System (CNS) compounds which act mainly by allosteric modulation of the GABAA receptor complex. The presence of a 3 alpha-hydroxyl group and a 5 alpha-hydrogen atom have been found to be essential structural requirements for biological activity in mammals. In the present work we report the enhancing activity on [3H]GABA binding to its receptor sites in chick optic lobe produced by progesterone metabolites 3 alpha-hydroxy,5 alpha-pregnan-20-one (3 alpha,5 alpha-P) and 3 beta-hydroxy,5 beta-pregnan-20-one (3 beta,5 beta-P). Both steroids were found able to enhance [3H]GABA binding along ontogeny, displaying a similar profile at early developmental stages, while in adulthood 3 alpha,5 alpha-P had greater potency (EC50 0.22 microM) and enhancing effect (Emax: 122%). In adult synaptic membranes, the two compounds displayed a complex interaction with the GABAA receptor, disclosed by a Schild plot with slope below one and an incomplete displacement of 3 alpha,5 alpha-P by its 3 beta,5 beta isomer. Such complexity could be related to the steroidogenic profile in avian CNS, with 5 alpha-reduced progesterone metabolites present since early development, while 3 alpha,5 alpha-P is found only in adulthood. Bearing in mind differences between avian and mammalian steroidogenic profiles and the relevance of 5 beta-steroids in early avian development, we propose that 3 beta,5 beta-P, instead of the classical potent 3 alpha,5 alpha-steroids, may be the endogenous modulator of GABAergic activity in developing avian brain.

Neurosteroid modulation of [3H]flunitrazepam binding in the medulla: an autoradiographic study

Neurosteroids bind to unique sites on the GABA(A) receptor complex and modulate receptor function. The effects of neurosteroids on GABA(A) receptors have been well characterized in forebrain regions. However, little is known about their effects on GABA(A) receptors in the medulla, especially those areas involved in autonomic reflex pathways. Stimulation of [3H]flunitrazepam binding to the GABA(A) receptor by two progesterone metabolites, 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha-OH-DHP) and 3beta-hydroxy-5alpha-pregnan-20-one (3beta-OH-DHP), was studied using autoradiographic methods in the medulla and cerebellum of female rats at estrus. [3H]Flunitrazepam binding was enhanced by 3alpha-OH-DHP in every nucleus examined in the medulla and cerebellum. This effect was stereoselective since 3beta-OH-DHP had no effect on binding in any region. No differences were observed in the degree of stimulation of [3H]flunitrazepam binding by 3alpha-OH-DHP among medullary brain regions. However, in the cerebellum, the stimulation of binding was significantly greater in the granular layer than in the molecular layer. Stimulation of [3H]flunitrazepam binding by 3alpha-OH-DHP in nuclei involved in the baroreflex pathways supports previous studies which report that neurosteroids modulate autonomic regulation of blood pressure. These actions may also underlie alterations in autonomic function during pregnancy.

3 beta-OH-5 beta-pregnan-20-one enhances [3H]GABA binding in developing chick optic lobe

The neurosteroids are synthesized in the CNS and act mainly through allosteric modulation of the GABAA receptor. Structure-activity relationship studies in mammalian CNS have shown that a 3 alpha-hydroxyl group and a 5 alpha-reduced A-ring are striking features for their biological activity, while the 3 beta,5 beta structures as in 3 beta,5 beta-P are completely inactive. In this work we report the enhancing activity of epipregnanolone on [3H]GABA binding to its receptor sites in the chick optic lobe. Concentration-effect curves for this neurosteroid showed a concentration-dependent activity with different potencies at the three developmental stages studied, the hatching stage being the most sensitive to the steroid stimulatory effect. The displacement of a potent 3 alpha,5 alpha steroid by epipregnanolone indicated that this steroid behaved as a partial agonist of the steroid recognition site. Considering the developmental profile for steroidogenesis in avian tissues and the biological relevance of 5 beta-reduced steroids in early development, we propose that 3 or its 3 alpha-epimer, pregnanolone, instead of the potent 3 alpha,5 alpha neurosteroids, modulates GABAA receptors in the chick optic lobe during development.

The anxiolytic-like effects of the neurosteroid allopregnanolone: interactions with GABA(A) receptors

The neurosteroid 3alpha-hydroxy-5alpha-pregnan-20-one (allopregnanolone) was administered systemically to rats which were tested in the Geller-Seifter conflict paradigm, an established animal model of anxiety. Allopregnanolone was found to produce significant anxiolytic-like effects at a dose of 8 mg/kg. When three ligands that function at different sites on the gamma-aminobutyric acid/benzodiazepine receptor-chloride ionophore complex (GABA(A) receptors) were examined in conjunction with allopregnanolone, the anti-conflict effects of allopregnanolone were effectively reversed only by the benzodiazepine receptor inverse agonist RO15-4513 (ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo [1,5-alpha]-[1,4]benzodiazepine-3-carboxylate). Since this inverse agonist has been reported to inhibit the GABA(A)-activated chloride flux in neuronal membranes, it is likely that the stimulation of the chloride channel in GABA(A) receptors is an important component of the effects of allopregnanolone. In contrast, the benzodiazepine receptor antagonist flumazenil (ethyl-8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo [1,5-alpha]-[1,4]benzodiazepine-3-carboxylate) did not block the anxiolytic-like actions of allopregnanolone, indicating that allopregnanolone does not bind at the benzodiazepine site directly. Isopropylbicyclophosphate, which binds at the picrotoxinin site on the GABA(A) receptors and blocks the behavioral actions of ethanol, also dose-dependently reversed the anti-conflict effect of this neurosteroid. The results suggest that allopregnanolone may be working either at a site specific for the benzodiazepine receptor inverse agonist RO15-4513 or at the picrotoxinin site to produce its potent anxiolytic-like behavioral effects.

Epipregnanolone acts as a partial agonist on a common neurosteroid modulatory site of the GABA(A) receptor complex in avian CNS

Neurosteroid modulatory sites present in the GABA(A) receptor complex in chick optic lobe were investigated, in order to evaluate whether allopregnanolone and alphaxalone act through a common site of action. Results showed that either allopregnanolone or alphaxalone present a single-component enhancement of [3H]flunitrazepam binding with EC50 of 1.18 +/- 0.12 and 6.56 +/- 0.86 microM and Emax of 82.18 +/- 5.80 and 62.98 +/- 3.73%, respectively. Epipregnanolone behaved as a partial agonist of these steroid modulatory sites with EC50 of 0.49 +/- 0.15 microM and Emax 12.34 +/- 1.03%. Moreover, the addition of 16 microM epipregnanolone to either allopregnanolone or alphaxalone decreased EC50 values to 0.54 +/- 0.09 and 1.24 +/- 0.25 microM respectively, while Emax values were not significantly affected. On the other hand, additivity experiments disclosed that a maximal concentration (16 microM) of alphaxalone in the presence of allopregnanolone failed to enhance [3H]flunitrazepam binding in excess of that produced by allopregnanolone alone. Results indicate that not only allopregnanolone and alphaxalone act through a common site of action, but such site is highly stereospecific with regard to the neurosteroid spatial configuration.

Prenatal alcohol exposure influences the effects of neuroactive steroids on separation-induced ultrasonic vocalizations in rat pups

Fetal alcohol exposure has been reported to be associated with hyper-responsiveness to stress. Using a maternal separation paradigm, this study examined whether prenatal alcohol exposure affected sensitivity to neurosteroid modulation of stress. We have shown that the neuroactive steroid allopregnanolone reduces ultrasonic vocalizations (USVs) after brief maternal separation in week-old rat pups. Prenatal alcohol exposure, however, resulted in reduced sensitivity to this neurosteroid. In this study's first experiment, the behavioral effects of pregnenolone sulfate, a neurosteroid with reportedly opposite modulatory effects on the GABAA receptor, were characterized. Pregnenolone sulfate had a triphasic effect on the production of ultrasonic vocalizations and on open field activity. Blockade of conversion of pregnenolone sulfate to allopregnanolone via the 5 alpha-reductase inhibitor 4-MA also blocked the drug-related reduction in USVs, but not the higher-dose augmentation. The enzyme inhibitor alone had no significant effects on USV production, nor did progesterone. These results suggest that the neuroactive steroid pregnenolone sulfate may play an independent role in the stress response after maternal separation as well as being a precursor for the anxiolytic neurosteroid allopregnanolone. In the second experiment, prenatal alcohol exposure was found to eliminate both the low dose USV-reducing effect and the higher dose USV-increasing effect. These results support previous results demonstrating that prenatal alcohol exposure may cause an altered sensitivity to the neuromodulatory effects of neurosteroids.

Neurosteroid analogues. 4. The effect of methyl substitution at the C-5 and C-10 positions of neurosteroids on electrophysiological activity at GABAA receptors

A series of analogues of the neuroactive steroids 3 alpha-hydroxy-5 alpha-pregnan-20-one and 3 alpha-hydroxy-5 beta-pregnan-20-one were studied to elucidate the mode of binding of 5 alpha-and 5 beta-reduced steroids to steroid binding sites on GABAA receptors. Analogues which were either 3 alpha-hydroxy-20-ketosteroids or 3 alpha-hydroxysteroid-17 beta-carbonitriles and which contained various methyl group substitution patterns at C-5 and C-10 were prepared. Evaluations utilized whole-cell patch clamp electrophysiological methods carried out on cultured rat hippocampal neurons, and the results obtained with the rigid 17 beta-carbonitrile analogs were analyzed using molecular modeling methods. The molecular modeling results provide a rationale for the observation that the configuration of the hydroxyl group at C-3 is a greater determinant of anesthetic potency than the configuration of the A,B ring fusion at C-5. The electrophysiological results identify steric restrictions for the space that can be occupied in 5 alpha- and 5 beta-reduced steroid modulators of GABAA receptors in the regions of space proximate to the steroid C-5, C-10, and possibly C-4 positions. This information is useful for the development of nonsteroidal analogues that can modulate GABAA receptors via interactions at steroid binding sites.

Neurosteroid modulation of native and recombinant GABAA receptors

1. The pioneering work of Hans Selye over 50 years ago demonstrated that certain steroid metabolites can produce a rapid depression of central nervous system activity. 2. Research during the last 10 years has established that such effects are mediated by a nongenomic and specific interaction of these steroids with the brain's major inhibitory receptor, the GABAA receptor. 3. Here we describe the molecular mechanism of action of such steroids and review attempts to define the steroid binding site on the receptor protein. The therapeutic potential of such neurosteroids is discussed.

The neurosteroid pregnenolone sulfate blocks deficits induced by a competitive NMDA antagonist in active avoidance and lever-press learning tasks in mice

The neurosteroid pregnenolone sulfate (PREG-S) has been shown to modulate positively NMDA receptor activity and to have memory enhancing properties in mice. The present study was designed to evaluate the effects of post-training administration of PREG-S, alone or in combination with D-2-amino-5-phosphonovalerate (D-AP5), a competitive NMDA receptor antagonist, in Y-maze avoidance and appetitively motivated lever-press learning tasks and in a traction reflex test in mice. Intracerebroventricular (i.c.v.) administration of PREG-S (0.01-0.1 nmol/mouse) blocked the selective retention deficits induced by 0.02 nmol D-AP5 in the Y-maze avoidance task. PREG-S (0.1 nmol, i.c.v.) also blocked the retention deficits induced by 0.02 nmol D-AP5 in the lever-press task. Post-training administration of PREG-S alone (0.001-0.1 nmol, i.c.v.) had no effect on retention performance in the Y-maze and the lever-press tasks. PREG-S (1-10 nmol, i.c.v.) significantly reduced the impairment of the traction reflex induced by 2 nmol D-AP5. The ability of PREG-S to block retention performance deficits as well as motor impairment induced by D-AP5 is in agreement with its positive modulatory action at NMDA receptors.

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.