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The Journal of Neuroscience, May 1, 2002, 22(9):3366-3375

3beta -Hydroxypregnane Steroids Are Pregnenolone Sulfate-Like GABAA Receptor Antagonists

Mingde Wang1, Yejun He1, Lawrence N. Eisenman5, Christopher Fields1, Chun-Min Zeng3, Jose Mathews1, Ann Benz1, Tao Fu1, Erik Zorumski1, Joe Henry Steinbach2, Douglas F. Covey3, Charles F. Zorumski1, 4, and Steven Mennerick1, 4

Departments of 1 Psychiatry, 2 Anesthesiology, 3 Molecular Biology and Pharmacology, 4 Anatomy and Neurobiology, and 5 Neurology, Washington University School of Medicine, St. Louis, Missouri 63110


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Endogenous neurosteroids have rapid actions on ion channels, particularly GABAA 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 GABAA receptor antagonists. 3beta -Hydroxysteroids coapplied with GABA significantly inhibited responses to >= 15 µM GABA. The profile of block was similar to that exhibited by sulfated steroids, known blockers of GABAA 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 GABAA 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 GABAA receptors. Nevertheless, these steroids may be useful functional blockers of potentiating steroids when used at concentrations that do not affect baseline neurotransmission.

Key words: neurosteroids; inhibitory postsynaptic current; GABAA receptors; pregnenolone sulfate; anesthetic; hippocampal culture


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

GABAA receptors mediate most fast inhibitory neurotransmission in the CNS. Many important neuroactive compounds, including benzodiazepines, barbiturates, neuroactive steroids, and other general anesthetics, allosterically interact with GABAA receptors and thereby influence the balance between neuronal excitation and inhibition (Macdonald and Olsen, 1994). These drugs enhance the activity produced by low concentrations of GABA and/or directly gate GABAA receptor channels in the absence of GABA (Majewska, 1992).

Neuroactive steroids are of particular interest, because they are synthesized in the CNS and periphery and are present in the CNS at concentrations that may endogenously modulate GABAA receptor function (Robel and Baulieu, 1994). (3alpha ,5alpha )-3-Hydroxypregnan-20-one (3alpha 5alpha P), (3alpha ,5beta )-3-hydroxypregnan-20-one (3alpha 5beta P), (3alpha ,5beta )-3,21-dihydroxypregnan-20-one (3alpha 5beta THDOC), and (3alpha ,5alpha )-3,21-dihydroxypregnan-20-one (3alpha 5alpha THDOC) act as potent, efficacious positive modulators of GABAA receptors (Gasior et al., 1999; Lambert et al., 2001). Other endogenous neuroactive steroids, such as pregnenolone sulfate (PS) and (3beta ,5alpha )-3-hydroxypregnan-20-one sulfate (3beta 5alpha PS), inhibit GABA responses at high nanomolar to micromolar concentrations (for review, see Lambert et al., 2001). The role of endogenous neuroactive steroids in modulating GABAA receptor function remains unclear because of the lack of specific antagonists at the steroid-modulating sites. Precise sites of action of potentiating steroids on the GABAA receptor have also remained elusive (Lambert et al., 2001). Identification of antagonists may help to clarify interactions between potentiating steroids and the GABAA receptor.

Several previous studies explored potential steroid antagonists against potentiation by neuroactive steroids. In studies of [3H]flunitrazepam binding, which is a validated measure of GABAA receptor potentiation (Turner et al., 1989), both (3beta ,5beta )-3-hydroxypregnan-20-one (3beta 5beta P) and (3beta ,5alpha )-3-hydroxypregnan-20-one (3beta 5alpha P) produced insignificant changes in [3H]flunitrazepam binding when administered alone. However, both competitively antagonized the potentiation of [3H]flunitrazepam binding by 3alpha 5beta P and 3alpha 5alpha P (Prince and Simmonds, 1992, 1993). In electrophysiological studies, 3beta 5beta P antagonized the 3alpha 5beta P-induced enhancement of GABA current (Garrett and Gan, 1998; Maitra and Reynolds, 1998). Similarly, 3beta 5alpha P and 3beta 5beta P diminished the inhibitory effects of 3alpha 5alpha P and 3alpha 5beta P on population spikes evoked in rat hippocampal CA1 stratum pyramidale (Wang et al., 2000).

In the present study, we examined the effects of a series of endogenous and synthetic 3beta -hydroxypregnane and 3beta -hydroxyandrostane steroids on GABA-induced currents in Xenopus oocytes expressing recombinant GABAA receptors containing alpha 1, beta 2, and gamma 2L subunits and on GABA-mediated synaptic responses in cultured rat hippocampal neurons. We report here that 3beta -hydroxypregnane steroids exhibit block of GABAA receptor function that is dependent on GABA concentration, similar to block by sulfated steroids. The direct effect on GABAA receptors explains the apparent ability of 3beta -hydroxysteroids to antagonize the effects of positive modulators.


    MATERIALS AND METHODS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Chemicals. (3beta ,5beta )-3,21-dihydroxypregnan-20-one, (3alpha ,5beta )-3-hydroxypregnan-20-one sulfate (3alpha 5beta PS), (3alpha ,5alpha )-3-hydroxypregnan-20-one sulfate (3alpha 5alpha PS), and 3beta 5alpha PS were obtained from Steraloids Inc. (Newport, RI), and flumazenil was from Roche (Basel, Switzerland). (3beta ,5beta )-3-Hydroxypregnan-20-one sulfate (3beta 5beta PS) was synthesized as described previously (Park-Chung et al., 1997) and was a generous gift of Dr. Robert H. Purdy (Scripps Research Institute, La Jolla, CA). (3beta ,5beta ,17beta )-3-hydroxyandrostane-17-carbonitrile (3beta 5beta ACN) was synthesized as described previously (Han et al., 1996). Synthesis of (3beta ,5beta ,7alpha ,17beta )-3-hydroxy-7-methylpregnan-20-one [(7alpha Me)3beta 5beta P] was also described previously (Zeng et al., 2001). All of the remaining chemicals were from RBI/Sigma (St. Louis, MO). The steroids were dissolved in DMSO. Pentobarbital was dissolved in 0.1% NaOH. All chemicals were then diluted in saline for experiments. The concentration of DMSO in experimental solutions was <= 0.1%.

Hippocampal microcultures. Primary microisland cultures of hippocampal cells were prepared from 1- to 3-d-old postnatal Sprague Dawley rats using established methods (Mennerick et al., 1995). Under halothane anesthesia, rats were decapitated, and the hippocampi were dissected and cut into 500-µm-thick transverse slices. The slices were dissociated with 1 mg/ml papain in oxygenated Leibovitz L-15 medium and mechanical trituration in modified Eagle's medium containing 5% horse serum, 5% fetal calf serum, 17 mM D-glucose, 400 µM glutamine, 50 U/ml penicillin, and 50 µg/ml streptomycin. Isolated cells were plated onto 35 mm plastic culture dishes at a density of 75 cells/mm2. Before plating, culture dishes were coated with a layer of 0.15% agarose, dried overnight, and sprayed with small droplets of rat tail collagen using a microatomizer (Thomas Scientific, Swedeboro, NJ). The agarose layer serves as a nonpermissive background for cell adhesion. Cultures were treated with cytosine arabinoside (5-10 µM) after 3 d in vitro to halt glial proliferation. Electrophysiological recordings were performed 8-15 d after plating.

Culture electrophysiology. Whole-cell recordings were performed on solitary, inhibitory, hippocampal microculture neurons, using an Axopatch 1D amplifier (Axon Instruments, Foster City, CA) interfaced to a Pentium III-based computer via a Digidata 1200 acquisition board (Axon Instruments). Recordings were at room temperature. Electrodes had resistances of 1.5-4 MOmega for whole-cell recordings. Access resistance was electronically compensated 90-100%. Autaptic release of neurotransmitter was stimulated in voltage-clamped solitary neurons with a 2 msec voltage pulse to 0 mV from a holding potential of -70 mV. This stimulation protocol elicits an escaped action potential in the partially clamped axons that triggers transmitter release (Bekkers and Stevens, 1991; Mennerick et al., 1995). Whenever possible, at least three traces in each experimental condition were acquired for analysis. For all experiments, the interval between data sweeps was 25 sec for synaptic responses. Control conditions were interleaved with experimental conditions to counterbalance any time-dependent changes. Data sampling frequency was 5-10 kHz.

At the time of experiments, culture medium was replaced with an extracellular recording solution consisting of (in mM): 138 NaCl, 4 KCl, 2 CaCl2, 1 MgCl2, 10 glucose, 10 HEPES, and 0.025 D-APV, pH 7.25. Solutions were exchanged via a local multibarrel perfusion pipette, with a common delivery port placed 0.5 mm from the cell under study. The standard pipette solution for autaptic responses contained (in mM): 140 KCl, 4 NaCl, 0.5 CaCl2, 5 EGTA, and 10 HEPES, pH 7.25. IPSCs were easily distinguished from AMPA receptor-mediated EPSCs under these conditions by the >10-fold slower 10-90% decay times of IPSCs. There was no effect of either 3alpha -hydroxy (500 nM 3alpha 5alpha P) or 3beta -hydroxy (10 µM 3beta 5beta THDOC) neurosteroids on the amplitude or decay of AMPA receptor-mediated EPSCs (n = 4). IPSCs exhibited decays that were fitted by two or three exponential components, and potentiating steroid had most prominent effects on the slow components of decay (Zorumski et al., 1998). For ease of comparing effects of several drugs among cells with heterogeneous kinetics, we used the model-independent 10-90% decay time of IPSCs as our primary measure of IPSC duration (see Fig. 10).

Expression in Xenopus oocytes. Stage V-VI oocytes were harvested from sexually mature female Xenopus laevis (Xenopus One, Northland, MI) under 0.1% tricaine (3-aminobenzoic acid ethyl ester) anesthesia. Oocytes were defolliculated by shaking for 20 min at 37°C in collagenase (2 mg/ml) dissolved in calcium-free solution containing (in mM): 96 NaCl, 2 KCl, 1 MgCl2, and 5 HEPES, pH 7.4. Capped mRNA, encoding rat GABAA receptor alpha 1, beta 2, and gamma 2L subunits, was transcribed in vitro using the mMESSAGE mMachine Kit (Ambion, Austin, TX) from linearized pBluescript vectors containing receptor coding regions. Subunit transcripts were injected in equal parts (20-40 ng of total RNA) 8-24 hr after defolliculation. Oocytes were incubated up to 5 d at 18°C in ND96 medium containing 96 mM NaCl, 1 mM KCl, 1 mM MgCl2, 2 mM CaCl2 and 5 mM HEPES at pH 7.4, supplemented with 5 mM pyruvate, 100 U/ml penicillin, 100 µg/ml streptomycin, and 50 µg/ml gentamycin. The cDNAs for the rat GABAA receptor subunits were provided by A. Tobin (alpha 1) (University of California, Los Angels, CA), P. Malherbe (beta 2) (Hoffman-La Roche, Basal, Switzerland), and C. Fraser (gamma 2L) (National Institute on Alcohol Abuse and Alcoholism, Bethesda, MD). The V256S mutation, a point mutation in the alpha 1 subunit at the second residue N terminal to the beginning of transmembrane domain 2 (position 2'), was characterized previously and shown to dramatically reduce pregnenolone sulfate block of GABAA receptors (Akk et al., 2001).

Oocyte electrophysiology. Two-electrode voltage-clamp experiments were performed with an Axoclamp 2B amplifier or Warner Instruments (Hamden, CT) OC725 amplifier 2-5 d after RNA injection. The extracellular recording solution was ND96 medium with no supplements. Intracellular recording pipettes were filled with 3 M KCl and had open tip resistances of ~1 MOmega . Drugs were applied from a common tip via a gravity-driven multibarrel drug-delivery system. Drugs were always coapplied with GABA and were not preapplied in the absence of GABA. Cells were clamped at -70 mV for all experiments, and current at the end of 20-30 sec drug applications was measured for quantification of current amplitudes. In three oocytes, we pretreated cells with a low concentration of potentiating steroid (3alpha 5alpha P, 100 nM for 40 sec) before coapplying GABA. This protocol resulted in little difference in response size compared with the standard (coapplication only) protocol used for all figures (101 ± 11% potentiation with pretreatment, followed by coapplication; 85 ± 12% with coapplication alone). Likewise, pretreatment with a moderate concentration of blocking steroid (3beta 5beta THDOC, 6 µM for 40 sec) yielded similar block at the end of 20 µM GABA coapplication as with coapplication alone (36 ± 4% depression with pretreatment vs 36 ± 2% depression with coapplication alone) (see Fig. 6).

Data analysis. Data acquisition and analysis were performed with pClamp software (Axon Instruments). Data plotting and curve fitting were done with Sigma Plot software (SPSS, Chicago, IL). Data are presented in the text and figures as mean ± SE. Statistical differences were determined using a two-tailed Student's t test or a one-way ANOVA. The percentage of modulation of GABA-activated current was calculated as (IM - IN)/IN, where IN (normalizing current) and IM (measured current) are the amplitudes of the GABA-activated current in the absence and presence of the test substance, respectively. Fitting of the dose-response relationships were performed using the Hill equation as follows: I = Imax × Cn/(EC50n + Cn), where C is the concentration of steroid (or GABA) (see Fig. 6), Imax is the maximum current amplitude, EC50 is the concentration of steroid (or GABA) that produces 50% of Imax, and n is the Hill coefficient.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Noncompetitive antagonism of steroid and barbiturate potentiation by 3beta -hydroxypregnane steroids

To characterize the action of 3beta -hydroxypregnane steroids at GABAA receptors, we examined their putative antagonist profile against GABA-potentiating neurosteroids. Figure 1 shows the pregnane-androstane steroid ring system, and the chiral centers at carbon 3 (C3) and C5 are highlighted. The steroid shown is a 3alpha ,5alpha steroid. The steroids used in the present study varied in the side chain group at C17 (Fig. 1), but these side groups all conformed to the general rule that the side chain contained a hydrogen bond acceptor (Covey et al., 2001). This side chain group at C17 in the beta  configuration is necessary (but not sufficient) for GABAA receptor potentiation (Lambert et al., 2001). Potentiation by neuroactive steroids is also critically dependent on a hydrogen bond donor in the alpha  configuration at C3 (Lambert et al., 2001). For naturally occurring steroids, such as 3alpha 5alpha P, 3alpha 5beta P, and 3alpha 5beta THDOC, this hydrogen bond donor is a hydroxyl group. We tested the hypothesis in the current work that the 3beta diastereomers of these potentiating neurosteroids are competitive antagonists of the potentiators (Prince and Simmonds, 1992, 1993).



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Figure 1.   Steroid structure, with emphasis (arrows) on the chiral centers at C3 and C5. The structure shown is 3alpha ,5alpha ,17beta . 3beta -Hydroxypregnane steroids have been suggested to antagonize the potentiating actions of steroids with a 3alpha -hydroxy configuration. For the steroids tested in this work, the R group was CN, COCH3, or COCH2OH.

Figure 2A shows raw traces from a Xenopus oocyte expressing a combination of alpha 1beta 2gamma 2L GABAA receptor subunits. We applied a low concentration of GABA (2 µM) alone, or we coapplied (with GABA) varied concentrations of 3alpha 5alpha P plus or minus 10 µM 3beta 5alpha P. Note that 3beta 5alpha P significantly inhibited only the response to the high concentration of potentiating steroid. Figure 2B shows a similar experiment using another 3beta -hydroxypregnane steroid, 3beta 5beta THDOC, which exhibited a similar profile of apparent antagonism. Figure 2C-E shows summaries of the effect of three different 3beta -hydroxypregnane steroids (3beta 5alpha P, 3beta 5beta THDOC, and 3beta 5beta P) on potentiation by 3alpha 5alpha P. Table 1 gives values from fits to the Hill equation of the concentration-response curves shown in Figure 2C-E. Note that the EC50 values for potentiation were not substantially affected by the presence of the 3beta -hydroxysteroid, suggesting a noncompetitive mechanism of reduced potentiation. The similarity of the effects of the three different steroids (Fig. 2C-E) suggests that neither the configuration of the hydrogen atom at C5 nor the structure of the hydrogen bond acceptor group at C17 is critical to the blocking effect.



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Figure 2.   3beta 5alpha P and 3beta 5beta THDOC reversed the effect of high concentrations of GABA potentiating steroids. A, Sample traces showing that 10 µM 3beta 5alpha P inhibited potentiation by high but not low concentrations of 3alpha 5alpha P. B, Similar traces for 10 µM 3beta 5beta THDOC. C-E, Summary of effect of 3beta 5alpha P (C), 3beta 5beta THDOC (D), and 3beta 5beta P (E) from oocytes tested with a range of 3alpha 5alpha P concentrations. C, Summary of the effect of 10 µM 3beta 5alpha P against increasing concentrations of 3alpha 5alpha P. Normalized responses in this and subsequent figures were calculated as follows: (IM - IN)/IN, where IM is the amplitude of the measured current in a given experimental condition, and IN is the normalizing current. For these data and data in D and E, IN was the response to 2 µM GABA in the absence of modulator. The solid lines represent least-squares fits of the data to the Hill equation as follows: I = Imax × Cn/(EC50n + Cn), where C is the concentration of potentiator, EC50 is the concentration of potentiator that produced half-maximum potentiation, and n is the Hill coefficient. Parameters of the fit for potentiators in the absence (filled circles) and presence (open squares) of 3beta 5alpha P are given in Table 1. D, Summary of the effect of 10 µM 3beta 5beta THDOC under similar experimental conditions to those in C. E, Concentration-response relationship for 3alpha 5alpha P in the presence and absence of 3beta 5beta P (10 µM). F, Concentration-response relationship of the benzodiazepine agonist lorazepam in the presence and absence of the benzodiazepine antagonist flumazenil. The solid line through lorazepam concentration-response values (circles) is the best fit of the Hill equation, with an EC50 of 76.7 nM and a Hill coefficient of 1.2 (n = 7). The solid line through lorazepam-flumazenil interaction values (squares) is the best fit of the Hill equation, with an EC50 of 348.5 nM and a Hill coefficient of 1.1 (n = 7).


                              
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Table 1.   Fit parameters for potentiation in the presence and absence of 3beta -hydroxysteroids

We were surprised by the apparently noncompetitive nature of the interaction between the 3beta -hydroxysteroids and 3alpha 5alpha P (Fig. 2C-E). To be sure that our protocol was capable of detecting a competitive interaction between potentiator and antagonist, we examined the effect of the benzodiazepine antagonist flumazenil on the effect of a potentiating benzodiazepine lorazepam. Flumazenil (100 nM) competitively inhibited the GABA-enhancing effect of lorazepam by shifting lorazepam concentration-response curves to the right (n = 7) (Fig. 2F). The EC50 of lorazepam increased from 76.7 to 348.5 nM in the presence of 100 nM flumazenil (Fig. 2F). The Hill coefficients (Fig. 2F, see legend) and maximal response were quite close between two lorazepam concentration-response curves. This control experiment suggests that our experimental protocol should have detected a competitive interaction between a steroid potentiator and antagonist if present.

We explored the actions of several other 3beta -hydroxysteroids at 10 µM, and, consistent with the results of Figure 2, we found little structural specificity in the action against the potentiating neurosteroid 3alpha 5alpha P (Fig. 3A). Addition of an alpha  methyl group at C7 did not affect block of potentiation, nor did substitution of a carbonitrile group for the acetyl side group at C17 (Fig. 3A).



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Figure 3.   Effects of a series 3beta -hydroxysteroids on 2 µM GABA responses in the presence and absence of 3alpha 5alpha P. A, Effects of several 3beta -hydroxysteroids (10 µM) against potentiation induced by 3alpha 5alpha P (3 µM) in the presence of 2 µM GABA. For these data, the normalizing response (IN, representing 0 on the y-axis) was the current in the combined presence of 2 µM GABA plus 3 µM 3alpha 5alpha P (n = 6). B, Effects of several 3beta -hydroxysteroids on responses to 2 µM GABA alone. Most compounds were effectively inert at 10 µM, except for 3beta 5beta P, which slightly potentiated responses to 2 µM GABA. IN represented the response to 2 µM GABA alone (n = 5).

We also examined this panel of 3beta -hydroxysteroids (using a fixed concentration of 10 µM) against the response to 2 µM GABA alone (Fig. 3B). We found that most 3beta steroids were inert, but, as reported previously (Puia et al., 1990; Woodward et al., 1992; Kokate et al., 1994), 10 µM 3beta 5beta P mildly potentiated GABA responses. For many of the remaining experiments, we used 3beta 5beta THDOC as our standard 3beta -hydroxysteroid because of its relative inertness against GABA alone (Fig. 3B), the slightly better blocking activity than some other 3beta -hydroxysteroids (Fig. 3A), and its commercial availability.

Given that the interaction between 3beta -hydroxysteroids and 3alpha 5alpha P was noncompetitive, we examined whether this noncompetitive interaction held true for other potentiating neurosteroids. Indeed, Figure 4A-C shows that 3beta 5beta THDOC similarly inhibited potentiation by 5alpha - and 5beta -reduced neuroactive steroids. A noncompetitive profile was evident for all steroid potentiators. When we tested other classes of GABA potentiators, we were surprised to find that barbiturate (Fig. 5B,C) but not benzodiazepine (Fig. 5A,C) potentiation was inhibited by 3beta -hydroxysteroids. The potentiation induced by 150 µM pentobarbital was significantly reduced by 10 µM 3beta 5beta THDOC (Fig. 5B,C). As with steroid potentiation, the effect on pentobarbital potentiation was dependent on potentiator concentration. The potentiation by 50 µM pentobarbital (121 ± 25%) was not affected by 3beta 5beta THDOC (130 ± 28%; n = 13) (data not shown). Also, we found that maximum lorazepam potentiation (1 µM; 142 ± 8% potentiation) was unaffected by 3beta 5beta THDOC (138 ± 14% potentiation) (Fig. 5C).



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Figure 4.   Effect of 3beta -hydroxypregnane steroids on other 5alpha -reduced and 5beta -reduced steroid potentiators of GABAA responses. A, Concentration dependence of 3alpha 5alpha THDOC potentiation in the absence (filled circles) and presence (open squares) of 10 µM 3beta 5beta THDOC. GABA (2 µM) was coapplied with steroids, and the response to 2 µM GABA was used as the normalizing response (IN). B, C, Similar analyses using the 5beta -reduced steroids 3alpha 5beta P and 3alpha 5beta THDOC as potentiators. In all panels, the solid line through the concentration-response values in the absence and presence of 3beta 5beta THDOC is the best fit of the Hill equation. Parameters of the fits and n values are given in Table 1.



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Figure 5.   Effect of 3beta -hydroxysteroids on two other classes of GABAA receptor potentiators. A, Effect of 3beta 5beta THDOC on potentiation by the benzodiazepine lorazepam in an oocyte. B, Effect of 3beta 5beta THDOC on potentiation by the barbiturate pentobarbital in another oocyte. C, Summary of the effects of 10 µM 3beta 5beta THDOC on pentobarbital (150 µM; n = 13) and lorazepam (1 µM; n = 5) potentiation. Note that the more robust potentiation by the barbiturate was more effectively antagonized. *p < 0.01 indicates significant inhibition.

At high concentrations, 3alpha -hydroxysteroids can weakly gate the GABAA receptor (Barker et al., 1987). We investigated whether this direct gating was antagonized by 3beta -hydroxy steroids. We found that 3alpha 5alpha P (30 µM) in the absence of GABA gated a current with an amplitude 34 ± 4% of response to 2 µM GABA in the same cell. Consistent with a lack of direct antagonism between 3beta -hydroxysteroids and effects of 3alpha -hydroxysteroids, we observed no significant reduction in this steroid-gated current when 3beta 5beta THDOC was coapplied with 30 µM 3alpha 5alpha P (30 ± 6% of the response to 2 µM GABA; n = 4).

Direct 3beta -hydroxysteroid inhibition of GABA responses mimics the action of sulfated steroids and explains steroid antagonism

The pattern of noncompetitive inhibition of potentiation, the lack of structural specificity to the blocking effects, and the promiscuity of the effects of 3beta -hydroxysteroids against at least two classes of potentiators suggest that the mechanism of 3beta -hydroxysteroids does not result from a direct interaction with potentiating steroids sites on the GABAA receptor. Among all three classes of potentiators, a pattern emerged in which only strong potentiation was inhibited by 3beta -hydroxysteroids (effects on only high concentrations of steroids and barbiturates, no effect on the modest benzodiazepine potentiation). This pattern suggested the hypothesis that block by 3beta -hydroxysteroids may correlate with opening of GABAA receptors.

To test this hypothesis, we examined the effect of 3beta -hydroxysteroids on responses to GABA alone over a range of GABA concentrations. We found that responses to low concentrations (<15 µM) of GABA were unaffected by 10 µM 3beta 5beta THDOC (Fig. 6A,B). However, responses to >= 15 µM GABA were significantly depressed by 10 µM 3beta 5beta THDOC. The profile of 3beta 5beta THDOC effects against GABA was similar to that observed against potentiation by 3alpha -hydroxysteroids (Fig. 6B) (compare Figs. 2D, 4). The effect of 3beta 5beta THDOC was concentration dependent, with an IC50 against 20 µM GABA responses of 6.8 µM (n = 6) (Fig. 6C). We again tested the panel of 3beta -hydroxysteroids shown in Figure 3 and found that, at a fixed concentration of 10 µM, all similarly blocked currents in response to 20 µM GABA (n = 6) (data not shown). IC50 values for other 3beta -hydroxysteroids against 20 µM GABA were as follows: 3beta 5beta P, 12.8 µM (n = 5); 3beta 5alpha P, 14.4 µM (n = 5); and 3beta 5beta ACN, 9.5 µM (n = 6).



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Figure 6.   3beta -Hydroxypregnane steroids block responses to high concentrations of GABA. A, Sample traces showing lack of effect of 10 µM 3beta 5beta THDOC on responses to 2 µM GABA but inhibition of responses to 20 µM GABA. Drugs were coapplied for 20 sec. Note the change in vertical calibration bars between the left and right panels. B, GABA concentration-response curves in the absence (filled circles) and presence (open squares) of 10 µM 3beta 5beta THDOC. Each point is calculated relative to the normalizing response (IN) activated by 2 µM GABA. The solid line through GABA concentration-response values (circles) is the best fit of the Hill equation, with an EC50 of 12.6 µM and a Hill coefficient of 2.6 (n = 8). The solid line through GABA plus 3beta 5beta THDOC interaction values (squares) is the best fit of the Hill equation, with an EC50 of 9.4 µM and a Hill coefficient of 2.1 (n = 8). C. The graph shows the concentration-response curve of 3beta 5beta THDOC against 20 µM GABA. The normalizing current (IN) was the response to 20 µM GABA. The solid lines are the fit of the Hill equation, with an IC50 of 6.8 µM and a Hill coefficient of 0.8, with maximum inhibition of -1.0 (n = 6).

It is well known that another class of endogenous neurosteroids, steroids sulfated at C3, block GABAA receptor-mediated responses (Majewska et al., 1988). Block is not dependent on the stereochemistry of the sulfate group at C3, with 3beta -sulfated steroids blocking GABA responses with similar potency as 3alpha -sulfated steroids (Park-Chung et al., 1999). Block is not dependent on the presence of a sulfate group, because steroids containing other anionic groups also block GABA currents (Park-Chung et al., 1999). This class of blockers exhibits additional structural nonspecificity, because the charged group can be placed at C24 rather than C3 (Mennerick et al., 2001). Previous work has also shown that charge is not essential for blocking activity, because dehydroepiandrosterone sulfate and dehydroepiandrosterone both block GABAA receptors (Demirgoren et al., 1991; Le Foll et al., 1997). We addressed the possibility that 3beta -hydroxysteroids are GABAA receptor blockers, acting similarly to sulfated steroids. As a first test of this hypothesis, we repeated the experiments shown in Figure 4, substituting 300 nM 3beta 5alpha PS for 10 µM 3beta 5beta THDOC. The 3beta -sulfated steroid produced inhibition of steroid potentiation that was essentially indistinguishable from the inhibition produced by the 3beta -hydroxysteroids (Fig. 7A1,A2). In addition, 3beta 5alpha PS clearly inhibited responses to high GABA concentrations more effectively than it inhibited responses to low concentrations of GABA (Fig. 7B). When examined against a fixed GABA concentration of 20 µM, the IC50 for 3beta 5alpha PS block was ~190 nM (n = 6) (Fig. 7C), much lower than the IC50 for 3beta 5beta THDOC under the same conditions (Fig. 6C). Pregnenolone sulfate was also quite potent, with an IC50 against 20 µM GABA responses of 385 nM (data not shown).



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Figure 7.   The action of 3beta -hydroxysteroids is similar to that of sulfated steroid block. A, Inhibition of potentiation by a sulfated steroid. A1, Sample traces showing the very small effect of 300 nM 3beta 5alpha PS against potentiation by 1 µM 3alpha 5alpha P but stronger inhibition against GABA responses potentiated by 10 µM 3alpha 5alpha P. A2, Summary of experiments like that shown in A1, in which multiple concentrations of potentiator were examined. GABA (2 µM) was coapplied with varied concentrations of 3alpha 5alpha P either without (filled circles) or with (open squares) 300 nM 3beta 5alpha PS. Parameters of fits to the Hill equation and experimental n are given in Table 1. B, Sulfated steroid block, like 3beta -hydroxysteroid block, is dependent on GABA concentration. Shown are concentration- response curves for GABA in the absence (filled circles) and presence (open squares) of 300 nM 3beta 5alpha PS. In the absence of steroid, the EC50 for GABA was 12.5 µM, with a Hill coefficient of 2.8 (n = 7). In the presence of steroid, the GABA EC50 was 11.4 µM, with a Hill coefficient of 3.0 (n = 7). The normalized maximum response was decreased from 13.9 to 7.8 in the presence of steroid. C, Against a fixed GABA concentration of 20 µM, 3beta 5alpha PS produced a concentration-dependent inhibition of responses. The solid line is a fit of the Hill equation, with an IC50 of 189 nM and a Hill coefficient of 0.7 (n = 6).

To further explore whether the action of 3beta -hydroxypregnane steroids is similar to that of sulfated steroids, we used a recently characterized point mutation causing resistance to sulfated steroid block (Akk et al., 2001). The mutant carries a valine to serine substitution at position 256 of the alpha 1 subunit, located on the cytoplasmic side of transmembrane domain 2. Because we observed that the degree of block by both 3beta -hydroxysteroids and 3beta -sulfated steroids was dependent on GABA concentration, we examined the GABA concentration-response profile for wild-type and mutated receptors. We found that the mutation caused a large (~20-fold) leftward shift of the GABA concentration-response relationship. To ensure that we examined the effect of 3beta -hydroxysteroids and sulfated steroids under conditions that should promote block, we examined the effect of the steroids against 5 µM GABA in mutated receptors, a concentration that produced nearly maximum responses (Fig. 8A, arrow). Comparison was made with wild-type responses at 20 µM, a concentration somewhat higher than the EC50 (Fig. 8A, arrow).



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Figure 8.   A point mutation in the alpha 1 subunit (V256S) reduces sulfate and 3beta -hydroxysteroid block of GABAA receptors. A, GABA concentration-response curves for wild-type (filled squares; n = 5) and mutated (open squares; n = 6) GABAA receptors. Responses at 1 µM were set to 1 for both wild-type and mutated receptors, and other responses in the same oocyte were normalized to this response. Note that the left y-axis corresponds to the wild-type normalized responses, and the right y-axis corresponds to the normalized responses from mutated receptors. Absolute amplitudes of maximum responses did not notably differ between wild-type and mutated receptors, but the mutation resulted in an ~20-fold leftward shift in the GABA EC50, from 9.6 to 0.53 µM, resulting in the apparently larger normalized maximum responses for wild-type receptors. Arrows indicate concentrations of GABA used in subsequent studies of steroid block of GABAA receptors (B, C). B, Sulfated steroid effects (1 µM) in wild-type (WT, filled bars; n = 5) and alpha 1V256S mutated (open bars; n = 5) GABAA receptors. C, 3beta -Hydroxysteroid effects (10 µM) in wild-type (WT, filled bars; n = 6) and alpha 1V256S (open bars; n = 9) mutated GABAA receptors.

Figure 8B (filled bars) shows a summary of the effect of various sulfated steroids at a fixed concentration of 1 µM against responses to 20 µM GABA in wild-type receptors. All sulfated steroids blocked more than half the current under these conditions. In contrast, none of the sulfated steroids significantly inhibited responses of mutated receptors, despite the use of a functionally higher GABA concentration than used for the wild-type receptors (Fig. 8B, open bars). We found a similar pattern when a panel of 3beta -hydroxysteroids were examined against wild-type and mutated receptors (Fig. 8C). In contrast, we found no difference in the ability of other GABAA receptor antagonists (gabazine, bicuculline, and picrotoxin) to inhibit mutated receptors (n = 8) (data not shown). These results are consistent with the idea that 3beta -hydroxysteroids and sulfated steroids block GABAA receptors similarly, albeit with different potencies.

To address whether the direct effects on GABAA receptors are sufficient to explain the apparent antagonism of potentiating steroids, in the same oocytes, we matched the size of GABA currents produced by a moderate concentration of GABA alone to that produced by 2 µM GABA plus potentiating steroid (Fig. 9A,C). We found that 20 µM GABA produced responses of similar amplitude to those produced by 3 µM 3alpha 5alpha P plus 2 µM GABA. Responses to 20 µM GABA alone or the combination of 2 µM GABA plus 3alpha 5alpha P were inhibited to a similar degree by 10 µM 3beta 5beta THDOC (Fig. 9A,C). These results are consistent with the idea that the direct effect on GABAA receptors can account for the apparent antagonism of potentiating steroid effects.



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Figure 9.   Direct GABAA receptor block explains 3beta -hydroxysteroid inhibition of potentiation. A, Comparison of responses "potentiated" by increasing GABA concentration or by adding 3 µM 3alpha 5alpha P. The traces show that 20 µM GABA and 3 µM 3alpha 5alpha P cause a similar increase in current relative to 2 µM GABA. 3beta 5beta THDOC (10 µM) inhibited both increased currents to a similar degree. B, Mutated receptors are potentiated by 3alpha -hydroxysteroids, but steroid potentiation is not inhibited by 3beta -hydroxysteroids. C, Summary of the blocking actions of 3beta 5beta THDOC on 20 µM GABA-activated current and potentiation of 2 µM GABA-activated current by 3 µM 3alpha 5alpha P (n = 7) in oocytes expressing wild-type receptors. The bar graph represents current amplitudes normalized to the 2 µM GABA response. D, The bar graph represents a summary of the lack of effect of 3beta 5beta THDOC on potentiation by 3alpha 5alpha P in mutated receptors. Normalizing current was the response activated by 0.1 µM GABA alone (n = 8).

As another test of whether direct block of GABAA receptor function can account for the apparent antagonism of potentiating steroids, we examined steroid potentiation of alpha 1 V256S mutated receptors and the effect of 3beta -hydroxypregnane steroids on potentiation. For studies of potentiation at wild-type receptors, we typically used 2 µM GABA, approximately fivefold below the GABA EC50. For studies of potentiation at mutated receptors, we used 100 nM GABA to functionally match the GABA concentration with that used on wild-type receptors (Fig. 8A). We found that 3 µM 3alpha 5alpha P robustly potentiated currents generated by mutated GABAA receptors. No additional potentiation was observed at 10 µM 3alpha 5alpha P (n = 8) (data not shown). In contrast to wild-type receptors, we observed no significant antagonism of this potentiation by 3beta 5beta THDOC (Fig. 9, compare B,D with C).

Actions of 3beta -hydroxysteroids and sulfated steroids at GABA synapses

Although these experiments show that 3beta -hydroxypregnane steroids are not true antagonists of potentiating steroids and act through an action similar to sulfated steroids, it is possible that either class of steroid could be a useful functional antagonist against postsynaptic potentiation at synapses. It is thought that a high GABA concentration is present at the synapse but only briefly (~1 msec) (Maconochie et al., 1994; Jones and Westbrook, 1995). If binding and action of 3beta -hydroxysteroids or sulfated steroids is dependent on GABA concentration and time, it is possible that blocking steroids are relatively inert against normal GABA transmission but more effective against responses potentiated by 3alpha -hydroxysteroids or other postsynaptic potentiators. In pilot experiments on cultured hippocampal neurons, we found that 2 µM 3beta 5alpha PS produced inhibition of responses to exogenous GABA that was equivalent to 10 µM 3beta 5beta THDOC (data not shown). This suggests that, in hippocampal cells, there is only approximately fivefold difference in potency between the 3beta -hydroxysteroid and the sulfated steroid compared with the 35-fold difference observed in oocytes (Figs. 6C, 7C). We therefore examined the effect of 10 µM 3beta 5beta THDOC and 2 µM 3beta 5alpha PS on IPSCs generated by recurrent (autaptic) synapses formed in culture (Mennerick et al., 1995). We found that 3beta 5beta THDOC and 3beta 5alpha PS at these concentrations had little effect on IPSCs generated in the absence of postsynaptic modulator (13 ± 12% decrease in 10-90% decay time for 3beta 5beta THDOC; 20 ± 9% decrease for 3beta 5alpha PS; n = 6) (Fig. 10A,C). Despite the relative inertness of the steroids on baseline IPSCs, the same concentrations significantly inhibited the ability of a potentiating steroid (3alpha 5alpha P, 0.5 µM) to prolong IPSCs (Fig. 10B,C). Also, consistent with a direct effect on GABAA receptors rather than a competitive inhibition of the potentiating steroid, 30 µM 3beta 5beta THDOC alone significantly speeded the 10-90% decay time of baseline IPSCs (38 ± 14%; n = 15). Neither 10 nor 30 µM 3beta 5beta THDOC had any appreciable effect on the peak IPSC amplitude (4 ± 3 and 8 ± 4% increase, respectively). In summary, these data are consistent with the idea that 3beta -hydroxysteroids and sulfated steroids share a similar mechanism and suggest that these blockers might be useful under some circumstances as functional antagonists of postsynaptic potentiation at GABAergic synapses.



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Figure 10.   Similar synaptic effect of 3beta 5beta THDOC and 3beta 5alpha PS. A, Autaptic IPSCs were elicited from solitary GABAergic hippocampal neurons in microcultures with 2 msec voltage pulse to 0 mV from a holding potential of -70 mV. The traces represent responses obtained in the absence and presence of 10 µM 3beta 5beta THDOC. Note that the drug alone only slightly affected the IPSC. B, The traces represent responses obtained in the absence and presence of 0.5 µM 3alpha 5alpha P and 0.5 µM 3alpha 5alpha P plus 10 µM 3beta 5beta THDOC. Note that 3alpha 5alpha P significantly prolonged the decay time course of the IPSC. 3beta 5beta THDOC inhibited the prolongation induced by potentiating steroid. C, Summary of the effect of 10 µM 3beta 5beta THDOC and 2 µM 3beta 5alpha PS on IPSCs in the absence and presence of 0.5 µM 3alpha 5alpha P (n = 6 neurons). For this analysis, we used 10-90% decay times, which averaged 315 ± 71 msec in the control situation. We also fit IPSC decays with multiple exponential components (Zorumski et al., 1998). Weighted time constants (Sigma aitau I, where ai is the fractional amplitude and tau I is the time constant of each exponential component) were also used to quantify the data (Jones and Westbrook, 1997). Raw values from the weighted time constant analysis were 137 ± 30 msec (control), 119 ± 20 msec (3beta 5beta THDOC alone), 101 ± 18 msec (3beta 5alpha PS alone), 486 ± 63 msec (3alpha 5alpha P alone), 176 ± 20 msec (3alpha 5alpha P plus 3beta 5beta THDOC), and 266 ± 48 msec (3alpha 5alpha P plus 3beta 5alpha PS).


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

We characterized the actions of 3beta -hydroxypregnane steroids, which previous reports suggested are competitive antagonists against 3alpha -hydroxypregnane steroid potentiators. Although the 3beta -hydroxysteroids behave as functional inhibitors of GABAA receptor potentiation, we find that the antagonism is noncompetitive with respect to 3alpha -hydroxypregnane steroids. In fact, 3beta -hydroxysteroids are also noncompetitive antagonists of GABAA receptors themselves by actions similar to that of sulfated steroids. Both sulfated steroids and 3beta -hydroxysteroids block GABAA receptors more effectively under conditions that promote channel opening, suggesting that the direct antagonism of GABA responsiveness may represent ligand-dependent or state-dependent block. This direct action at GABAA receptors accounts for the apparent antagonism of potentiating steroid effects.

Our results contrast with those reported by others using different methods. Using [3H]flunitrazepam binding, it was shown previously that 3beta 5beta P competitively reduced the increase in binding produced by potentiating steroids (Prince and Simmonds, 1992, 1993). Other studies suggest that 3beta 5beta P acts as a partial agonist at the potentiating steroid site (Pignataro and Fiszer de Plazas, 1997). Our results do not support the competitive interaction, observed in binding studies, between 3beta -hydroxypregnane steroids and 3alpha -hydroxypregnane steroids.

Antagonism of potentiation by 3beta -hydroxysteroids has also been reported in previous electrophysiological studies (Garrett and Gan, 1998; Maitra and Reynolds, 1998), but the nature of the interaction between 3alpha - and 3beta -hydroxysteroids was not explored in any of these studies. In previous studies, 3beta 5beta P marginally potentiated or had no effect on responses to GABA alone at GABA concentrations less than EC50, similar to our r