Activation and modulation of recombinant glycine and GABAA receptors by 4-halogenated analogues of propofol.

BACKGROUND AND PURPOSE: Glycine receptors are important players in pain perception and movement disorders and therefore important therapeutic targets. Glycine receptors can be modulated by the intravenous anaesthetic propofol (2,6-diisopropylphenol). However, the drug is more potent, by at least one order of magnitude, on GABAA receptors. It has been proposed that halogenation of the propofol molecule generates compounds with selective enhancement of glycinergic modulatory properties. EXPERIMENTAL APPROACH: We synthesized 4-bromopropofol, 4-chloropropofol and 4-fluoropropofol. The direct activating and modulatory effects of these drugs and propofol were compared on recombinant rat glycine and human GABAA receptors expressed in oocytes. Behavioural effects of the compounds were compared in the tadpole loss-of-righting assay. KEY RESULTS: Concentration-response curves for potentiation of homomeric α1, α2 and α3 glycine receptors were shifted to lower drug concentrations, by 2-10-fold, for the halogenated compounds. Direct activation by all compounds was minimal with all subtypes of the glycine receptor. The four compounds were essentially equally potent modulators of the α1ß3γ2L GABAA receptor with EC50 between 4 and 7 µM. The EC50 for loss-of-righting in Xenopus tadpoles, a proxy for loss of consciousness and considered to be mediated by actions on GABAA receptors, ranged from 0.35 to 0.87 µM. CONCLUSIONS AND IMPLICATIONS: We confirm that halogenation of propofol more strongly affects modulation of homomeric glycine receptors than α1ß3γ2L GABAA receptors. However, the effective concentrations of all tested halogenated compounds remained lower for GABAA receptors. We infer that 4-bromopropofol, 4-chloropropofol and 4-fluoropropofol are not selective homomeric glycine receptor modulators.

11-trifluoromethyl-phenyldiazirinyl neurosteroid analogues: potent general anesthetics and photolabeling reagents for GABAA receptors.

RATIONALE: While neurosteroids are well-described positive allosteric modulators of gamma-aminobutyric acid type A (GABAA) receptors, the binding sites that mediate these actions have not been definitively identified. OBJECTIVES: This study was conducted to synthesize neurosteroid analogue photolabeling reagents that closely mimic the biological effects of endogenous neurosteroids and have photochemical properties that will facilitate their use as tools for identifying the binding sites for neurosteroids on GABAA receptors. RESULTS: Two neurosteroid analogues containing a trifluromethyl-phenyldiazirine group linked to the steroid C11 position were synthesized. These reagents, CW12 and CW14, are analogues of allopregnanolone (5α-reduced steroid) and pregnanolone (5ß-reduced steroid), respectively. Both reagents were shown to have favorable photochemical properties with efficient insertion into the C-H bonds of cyclohexane. They also effectively replicated the actions of allopregnanolone and pregnanolone on GABAA receptor functions: they potentiated GABA-induced currents in Xenopus laevis oocytes transfected with α1ß2γ2L subunits, modulated [(35)S]t-butylbicyclophosphorothionate binding in rat brain membranes, and were effective anesthetics in Xenopus tadpoles. Studies using [(3)H]CW12 and [(3)H]CW14 showed that these reagents covalently label GABAA receptors in both rat brain membranes and in a transformed human embryonal kidney (TSA) cells expressing either α1 and ß2 subunits or ß3 subunits of the GABAA receptor. Photolabeling of rat brain GABAA receptors was shown to be both concentration-dependent and stereospecific. CONCLUSIONS: CW12 and CW14 have the appropriate photochemical and pharmacological properties for use as photolabeling reagents to identify specific neurosteroid-binding sites on GABAA receptors.

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

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

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

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

Neurosteroid analogues. 16. A new explanation for the lack of anesthetic effects of δ(16)-alphaxalone and identification of a δ(17(20)) analogue with potent anesthetic activity.

This study addresses the hypothesis that the lack of anesthetic activity for (3α,5α)-3-hydroxypregn-16-ene-11,20-dione (Δ(16)-alphaxalone) is explained by the steroid Δ(16) double bond constraining the steroid 20-carbonyl group to a position that prevents it from favorably interacting with γ-aminobutyric acid type A (GABA(A)) receptors. A series of Δ(16) and Δ(17(20)) analogues of Δ(16)-alphaxalone was prepared to evaluate this hypothesis in binding, electrophysiological, and tadpole anesthesia experiments. The results obtained failed to support the hypothesis. Instead, the results indicate that it is the presence of the C-21 methyl group in Δ(16)-alphaxalone, not the location of the constrained C-20 carbonyl group, that prevents Δ(16)-alphaxalone from interacting strongly with the GABA(A) receptor and having anesthetic activity. Consistent with this conclusion, a Δ(17(20)) analogue of Δ(16)-alphaxalone without a C-21 methyl group was found to be very similar to the anesthetic steroid (3α,5α)-3-hydroxypregnane-11,20-dione (alphaxalone) with regard to time of onset and rate of recovery from anesthesia when administered to mice by tail vein injection.

Neurosteroid analogues. 15. A comparative study of the anesthetic and GABAergic actions of alphaxalone, Δ16-alphaxalone and their corresponding 17-carbonitrile analogues.

Alphaxalone, a neuroactive steroid containing a 17ß-acetyl group, has potent anesthetic activity in humans. This pharmacological activity is attributed to this steroid's enhancement of γ-amino butyric acid-mediated chloride currents at γ-amino butyric acid type A receptors. The conversion of alphaxalone into Δ(16)-alphaxalone produces an analogue that lacks anesthetic activity in humans and that has greatly diminished receptor actions. By contrast, the corresponding 17ß-carbonitrile analogue of alphaxalone and the Δ(16)-17-carbonitrile analogue both have potent anesthetic and receptor actions. The differential effect of the Δ(16)-double bond on the actions of alphaxalone and the 17ß-carbonitrile analogue is accounted for by a differential effect on the orientation of the 17-acetyl and 17-carbonitrile substituents.

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

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

Neurosteroid analogues. 14. Alternative ring system scaffolds: GABA modulatory and anesthetic actions of cyclopenta[b]phenanthrenes and cyclopenta[b]anthracenes.

Although the structural features of binding sites for neuroactive steroids on gamma-aminobutryic acid type A (GABA A) receptors are still largely unknown, structure-activity studies have established a pharmacophore for potent enhancement of GABA A receptor function by neuroactive steroids. This pharmacophore emphasizes the importance of the position and stereochemistry of hydrogen-bonding groups on the steroid. However, the importance of the steroid ring system in mediating hydrophobic interactions with the GABA A receptor is unclear. We have taken the cyclopenta[ b]phenanthrene (tetracyclic compounds with a nonlinear ring system different from that of steroids) and cyclopenta[ b]anthracene (tetracyclic molecules with a linear 6-6-6-5 carbocyclic ring system) ring systems and properly substituted them to satisfy the pharmacophore requirements of the critical hydrogen-bond donor and acceptor groups found in neuroactive steroids. We have found these cyclopenta[ b]phenanthrene and cyclopenta[ b]anthracene analogues to have potent activity at the GABA A receptor, rivaling that of the most potent steroid modulators. Single-channel analysis of electrophysiological data indicates that similarly substituted analogues in the different ring systems affect the kinetic components of macroscopic currents in different ways. Mutations to the hydrogen bonding amino acids at the putative steroid binding site (alpha1Q241L mutation and alpha1N407A/Y410F double mutation) produce similar effects on macroscopic current amplitude by the different ring system analogues suggesting that the different kinetic effects are explained by the precise interactions of each analogue with the same binding site(s).

Neurosteroid analogues. 12. Potent enhancement of GABA-mediated chloride currents at GABAA receptors by ent-androgens.

Allopregnanolone (1) and pregnanolone (2), steroids containing a 17beta-acetyl group, are potent enhancers of GABA (gamma-aminobutyric acid) action at GABAA receptors. Their effects are enantioselective with the non-naturally occurring enantiomers (ent-1 and ent-2) being less potent. Androsterone (3) and etiocholanolone (4), steroids with a C-17 carbonyl group, are weak enhancers of GABA action at GABAA receptors. Unexpectedly, their enantiomers (ent-3 and ent-4) have been found to have enhanced, not diminished, activity at GABAA receptors. Furthermore, the C-17 spiro-epoxide analogues (ent-5 and ent-6) of ent-3 and ent-4, respectively, have activities comparable to those of steroids 1 and 2. The results indicate that some ent-steroids are potent modulators of GABAA receptors and might have clinical potential as GABAergic drugs of the future.

Anticonvulsant and anesthetic effects of a fluorescent neurosteroid analog activated by visible light.

Most photoactivatable compounds suffer from the limitations of the ultraviolet wavelengths that are required for activation. We synthesized a neuroactive steroid analog with a fluorescent (7-nitro-2,1,3-benzoxadiazol-4-yl) amino (NBD) group in the beta configuration at the C2 position of (3alpha,5alpha)-3-hydroxypregnan-20-one (allopregnanolone, 3alpha5alphaP). Light wavelengths (480 nm) that excite compound fluorescence strongly potentiate GABAA receptor function. Potentiation is limited by photodepletion of the receptor-active species. Photopotentiation is long-lived and stereoselective and shows single-channel hallmarks similar to steroid potentiation. Other NBD-conjugated compounds also generate photopotentiation, albeit with lower potency. Thus, photopotentiation does not require a known ligand for neurosteroid potentiating sites on the GABAA receptor. Photoactivation of a membrane-impermeant, fluorescent steroid analog demonstrates that membrane localization is critical for activity. The photoactivatable steroid silences pathological spiking in cultured rat hippocampal neurons and anesthetizes tadpoles. Fluorescent steroids photoactivated by visible light may be useful for modulating GABAA receptor function in a spatiotemporally defined manner.

Ethanol modulates the interaction of the endogenous neurosteroid allopregnanolone with the alpha1beta2gamma2L GABAA receptor.

We have examined alpha1beta2gamma2L GABAA receptor modulation by the endogenous steroids allopregnanolone (3alpha5alphaP), pregnenolone sulfate, and beta-estradiol in the absence and presence of ethanol. Coapplication of 0.1 to 1% (17-170 mM) ethanol influenced receptor modulation by 3alpha5alphaP but not that by pregnenolone sulfate or beta-estradiol. One of the three kinetic effects evident in channel potentiation by 3alpha5alphaP, prolongation of the longest-lived open time component (OT3), was affected by ethanol with the midpoint of its dose-response curve moved to lower steroid concentrations by 2 orders of magnitude without significantly affecting the maximal effect. Manipulations designed to affect the ability of 3alpha5alphaP to prolong OT3 also affected OT3 prolongation in the presence of ethanol. A mutation to the gamma2 subunit, which reduces the ability of 3alpha5alphaP to prolong OT3, also reduces the interaction between ethanol and 3alpha5alphaP. And the presence of the competitive steroid antagonist (3alpha,5alpha)-17-phenylandrost-16-en-3-ol (17-PA) diminishes the positive interaction between ethanol and 3alpha5alphaP on the GABAA receptor. Together, the findings suggest that steroid interactions with the classic steroid binding site underlie the effect seen in the presence of ethanol, and that ethanol acts by increasing the affinity of 3alpha5alphaP for the site. Tadpole behavioral assays showed that the presence of 3alpha5alphaP at a concentration ineffective at causing changes in tadpole behavior shifted the ethanol dose-response curve for loss of righting reflex to lower concentrations and that this effect was neutralized by coapplication of 17-PA with 3alpha5alphaP.

Neurosteroid analogues. 11. Alternative ring system scaffolds: gamma-aminobutyric acid receptor modulation and anesthetic actions of benz[f]indenes.

Benz[f]indenes are tricyclic compounds with a linear 6-6-5 fused carbocyclic ring system. When properly substituted, benz[f]indenes can satisfy the pharmacophore requirements of the critical hydrogen-bond donor and acceptor groups found in neuroactive steroids that modulate gamma-aminobutyric acidA (GABAA) receptor function. Thus, the benz[f]indene ring system provides an opportunity to extend the previously well-studied GABAA receptor structure-activity relationships (SAR) of neuroactive steroids to a different ring system. Depending on whether the stereochemistry of the 6-6-5 ring fusions are trans-trans or cis-trans, either planar or nonplanar benz[f]indenes are obtained. We found that the planar trans-trans benz[f]indenes are active, but less active than the steroids they were designed to mimic, whereas the nonplanar cis-trans compounds have little, if any, activity. The results provide new insight into the importance of the steroid framework for the actions of neuroactive steroids at GABAA receptors.

Neurosteroid analogues. 10. The effect of methyl group substitution at the C-6 and C-7 positions on the GABA modulatory and anesthetic actions of (3alpha,5alpha)- and (3alpha,5beta)-3-hydroxypregnan-20-one.

The planar 5alpha-reduced steroid (3alpha,5alpha)-3-hydroxypregnan-20-one and the nonplanar 5beta-reduced steroid (3alpha,5beta)-3-hydroxypregnan-20-one act at GABA(A) receptors to induce general anesthesia. The structural features of the binding sites for these anesthetic steroids on GABA(A) receptors have not been determined. To determine how structural modifications at the steroid C-6 and C-7 positions effect the actions of these anesthetic steroids, an axial or equatorial methyl group was introduced at these positions. The analogues were evaluated (1) in [(35)S]-tert-butylbicyclophosphorothionate binding experiments, (2) in electrophysiological experiments using rat alpha(1)beta(2)gamma(2L) GABA(A) receptors expressed in Xenopus laevis oocytes, and (3) as tadpole anesthetics. The effects of methyl group substitution in the 5alpha- and 5beta-reduced series of compounds were strikingly similar. In both series, a 6beta-Me group gave compounds with actions similar to or greater than those of the parent steroids. A 6alpha-, 7beta- or 7alpha-Me substituent resulted in reduced potency for inhibition of radioligand binding, GABA(A) receptor modulation and tadpole anesthesia. Because of the similar effects of methyl group substitution in the two series of compounds and previous results from other studies showing that structural modifications in the steroid D ring/side chain region produce similar effects regardless of the stereochemistry of the A,B-ring fusion, we propose that either the 3alpha-hydroxyl groups of planar and nonplanar anesthetic steroids hydrogen bond to different amino acids on GABA(A) receptors or that this critical hydrogen bonding group interacts with membrane lipids instead of the receptor.

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.

Neuroactive steroid interactions with voltage-dependent anion channels: lack of relationship to GABA(A) receptor modulation and anesthesia.

Neuroactive steroids modulate the function of gamma-aminobutyric acid type A (GABA(A)) receptors in brain; this is the presumed basis of their action as anesthetics. In a previous study using the neuroactive steroid analog, (3alpha,5beta)-6-azi-3-hydroxypregnan-20-one (6-AziP), as a photoaffinity-labeling reagent, we showed that voltage-dependent anion channel-1 (VDAC-1) was the predominant protein labeled in brain. Antisera to VDAC-1 were shown to coimmunoprecipitate GABA(A) receptors, suggesting a functional relationship between steroid binding to VDAC-1 and modulation of GABA(A) receptor function. This study examines the contribution of steroid binding to VDAC proteins to modulation of GABA(A) receptor function and anesthesia. Photolabeling of 35-kDa protein with [(3)H]6-AziP was reduced 85% in brain membranes prepared from VDAC-1-deficient mice but was unaffected by deficiency of VDAC-3. The photolabeled 35-kDa protein in membranes from VDAC-1-deficient mice was identified by two-dimensional electrophoresis and electrospray ionization-tandem mass spectrometry as VDAC-2. The absence of VDAC-1 or VDAC-3 had no effect on the ability of neuroactive steroids to modulate GABA(A) receptor function as evidenced by radioligand ([(35)S] t-butylbicyclophosphorothionate) binding or by electrophysiological studies. Electrophysiological studies also showed that neuroactive steroids modulate GABA(A) receptor function normally in VDAC-2-deficient fibroblasts transfected with alpha(1)beta(2)gamma(2) GABA(A) receptor subunits. Finally, the neuroactive steroid pregnanolone [(3alpha,5beta)-3-hydroxypregnan-20-one] produced anesthesia (loss of righting reflex) in VDAC-1- and VDAC-3-deficient mice, and there was no difference in the recovery time between the VDAC-deficient mice and wild-type controls. These data indicate that neuroactive steroid binding to VDAC-1, -2, or -3 is unlikely to mediate GABA(A) receptor modulation or anesthesia.

Neurosteroid analogues. 9. Conformationally constrained pregnanes: structure-activity studies of 13,24-cyclo-18,21-dinorcholane analogues of the GABA modulatory and anesthetic steroids (3alpha,5alpha)- and (3alpha,5beta)-3-hydroxypregnan-20-one.

The hydrogen-bond-acceptor properties of the carbonyl moiety in the 17beta-acetyl group on the D-ring of the anesthetic steroids (3alpha,5alpha)- and (3alpha,5beta)-3-hydroxypregan-20-one form an important part of the anesthetic steroid pharmacophore. 13,24-Cyclo-18,21-dinorcholanes containing a ketone or conjugated ketone group at C-20, C-22, C-23, or C-24 were prepared as conformationally constrained analogues of these anesthetic steroids and were used to probe for alternate locations for the D-ring hydrogen-bond-accepting carbonyl group. The analogues were evaluated (1). in [(35)S]-tert-butylbicyclophosphorothionate binding experiments, (2). in electrophysiological experiments using rat alpha(1)beta(2)gamma(2L) GABA(A) receptors expressed in Xenopus laevis oocytes, and (3). as tadpole anesthetics. In the binding assay, the relative order of potencies for the analogues in the 5alpha- and 5beta-series is identical. For the ketones, the order is 24-one >or= 23-one > 20-one > 22-one. Likewise, for the enones, the order is delta(22)-24-one > delta(20(22))-23-one > delta(22)-20-one > delta(23)-22-one. Similar relative orders of potencies are also found in the other two bioassays. The activities of the 24-one and delta(22)-24-one compounds were expected to be very low, because the carbonyl group in these compounds is located over the steroid C-ring and oriented toward C-8. Instead, these compounds have the highest activities in their respective series, with the delta(22)-24-one compounds having activities comparable to those of the reference anesthetic steroids. The electrophysiology results obtained with the 24-oxo-cyclosteroids suggest that rat alpha(1)beta(2)gamma(2L) GABA(A) receptors contain more than one donor for the hydrogen-bond-acceptor group of anesthetic steroids. The family of cyclosteroids should be useful for future structure-activity relationship studies of steroid modulation of other GABA(A) receptor subtypes.

Photoaffinity labeling with a neuroactive steroid analogue. 6-azi-pregnanolone labels voltage-dependent anion channel-1 in rat brain.

Neuroactive steroids modulate the function of gamma-aminobutyric acid, type A (GABA(A)) receptors in the central nervous system by an unknown mechanism. In this study we have used a novel neuroactive steroid analogue, 3 alpha,5 beta-6-azi-3-hydroxypregnan-20-one (6-AziP), as a photoaffinity labeling reagent to identify neuroactive steroid binding sites in rat brain. 6-AziP is an effective modulator of GABA(A) receptors as evidenced by its ability to inhibit binding of [(35)S]t-butylbicyclophosphorothionate to rat brain membranes and to potentiate GABA-elicited currents in Xenopus oocytes and human endothelial kidney 293 cells expressing GABA(A) receptor subunits (alpha(1)beta(2)gamma(2)). [(3)H]6-AziP produced time- and concentration-dependent photolabeling of protein bands of approximately 35 and 60 kDa in rat brain membranes. The 35-kDa band was half-maximally labeled at a [(3)H]6-AziP concentration of 1.9 microM, whereas the 60-kDa band was labeled at higher concentrations. The photolabeled 35-kDa protein was isolated from rat brain by two-dimensional PAGE and identified as voltage-dependent anion channel-1 (VDAC-1) by both matrix-assisted laser desorption ionization time-of-flight and ESI-tandem mass spectrometry. Monoclonal antibody directed against the N terminus of VDAC-1 immunoprecipitated labeled 35-kDa protein from a lysate of rat brain membranes, confirming that VDAC-1 is the species labeled by [(3)H]6-AziP. The beta(2) and beta(3) subunits of the GABA(A) receptor were co-immunoprecipitated by the VDAC-1 antibody suggesting a physical association between VDAC-1 and GABA(A) receptors in rat brain membranes. These data suggest that neuroactive steroid effects on the GABA(A) receptor may be mediated by binding to an accessory protein, VDAC-1.