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NEUROPHARMACOLOGY

The Pheromone Androstenol (5-Androst-16-en-3-ol) Is a Neurosteroid Positive Modulator of GABA_A Receptors

Epilepsy Research Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland (R.M.K., H.M., M.A.R.); and Department of Physiology and Biophysics, Georgetown University School of Medicine, Washington, District of Columbia (P.I.O., S.V.)

Received November 8, 2005; accepted January 11, 2006.

	Abstract

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Androstenol is a steroidal compound belonging to the group of odorous 16-androstenes, first isolated from boar testes and also found in humans. Androstenol has pheromone-like properties in both animals and humans, but the molecular targets of its pheromonal activity are unknown. Androstenol is structurally similar to endogenous A-ring reduced neurosteroids that act as positive modulators of GABA_A receptors. Here we show that androstenol has neurosteroid-like activity as a GABA_A receptor modulator. In whole-cell recordings from cerebellar granule cells, androstenol (but not its 3-epimer) caused a concentration-dependent enhancement of GABA-activated currents (EC₅₀, 0.4 µM in cultures; 1.4 µM in slices) and prolonged the duration of spontaneous and miniature inhibitory postsynaptic currents. Androstenol (0.1–1 µM) also potentiated the amplitude of GABA-activated currents in human embryonic kidney 293 cells transfected with recombinant 122 and 222 GABA_A receptors and, at high concentrations (10–300 µM), directly activated currents in these cells. Systemic administration of androstenol (30–50 mg/kg) caused anxiolytic-like effects in mice in the open-field test and elevated zero-maze and antidepressant-like effects in the forced swim test (5–10 mg/kg). Androstenol, but not its 3-epimer, conferred seizure protection in the 6-Hz electroshock and pentylenetetrazol models (ED₅₀ values, 21.9 and 48.9 mg/kg, respectively). The various actions of androstenol in the whole-animal models are consistent with its activity as a GABA_A receptor modulator. GABA_A receptors could represent a target for androstenol as a pheromone, for which it is well suited because of high volatility and lipophilicity, or as a conventional hormonal neurosteroid.

View larger version (19K): [in this window] [in a new window]   Fig. 1. Structural similarity between the androstenol epimers (16–17 unsaturated) and A-ring-reduced (16–17 saturated) GABAA receptor-modulating neurosteroids, including the testosterone metabolites andro-stanediol and androsterone, the progesterone metabolite allopregnanolone, and the deoxycorticosterone metabolite allotetetrahydrodeoxycorticosterone.

Several studies have indicated that androstenol is able to modulate behavioral and social responses in humans (Cowley and Brooksbank, 1991; Gower and Ruparelia, 1993; Grammer et al., 2005), presumably reflecting the effects of the steroid on the nervous system. In examining the structure of androstenol, we recognized a similarity to the C₁₉ 5,3-reduced testosterone metabolites, androstanediol (5-andro-stan-3-ol-17-ol) and androsterone (5-androstan-3-ol-17-one) (Fig. 1). Androstanediol is a positive modulator of GABA_A receptors (Frye et al., 1996) with potent anxiolytic and anticonvulsant activity (Frye and Reed, 1998; Reddy, 2004a,2004b); recently, we confirmed that androsterone has similar systemic activities (Kaminski et al., 2005). A structural homology also exists between androstenol and the C₂₁ 5,3-reduced progesterone and deoxycorticosterone metabolites, allopregnanolone (5-pregnan-3-ol-20-one) and allotetrahy-drodeoxycorticosterone (5-pregnane-3,21-diol-20-one). These latter compounds are prototypic neurosteroids, which also act as positive modulators of GABA_A receptors (Puia et al., 1990) and have diverse behavioral properties characteristic of such GABA modulators, including anticonvulsant (Rogawski and Reddy, 2004) and anxiolytic (Bitran et al., 1991; Rodgers and Johnson, 1998; Finn et al., 2003) actions. Androstenol has been shown previously to inhibit [³⁵S]t-bu-tylbicyclophosphorothionate binding to rat cortical membranes, indicating that it interacts with GABA_A receptors (Bolger et al., 1996).

In the present study, we sought to demonstrate that androstenol, like the aforementioned 5,3-hydroxy A-ring-reduced congeners, has activity as a positive modulator of GABA_A receptors. In addition, we wished to determine whether the steroid has central nervous system actions typical of GABA_A receptor modulators when administered systemically, providing support for the hypothesis that endogenously produced androstenol or environmental exposure to the steroid could influence brain function.

	Materials and Methods

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In Vitro Studies
Cell Culture and Tissue Preparation. Primary cultures of mouse cerebellar granule cells were prepared from postnatal day 5 to 7 mice uncontrolled for sex. Mouse pups were sacrificed by decapitation in accordance with the guidelines of the Georgetown University Animal Care and Use Committee. The cerebella were dissociated with trypsin (0.25 mg/ml; Sigma-Aldrich, St. Louis, MO) and plated in 35-mm Nunc dishes at a density of 1.1 x 10⁶ cells/ml on 12-mm glass coverslips (Fisher Scientific, Pittsburgh, PA) coated with poly-L-lysine (10 µg/ml; Sigma-Aldrich). For the first 5 days, the cells were cultured in high (25 mM) K⁺ basal Eagle's medium supplemented with 10% bovine calf serum, 2 mM glutamine, and 100 µg/ml gentamycin (all from Invitrogen, Carlsbad, CA) and maintained at 37°C in 5% CO₂. At culture day 5, the medium was replaced with minimal essential medium (containing 5 mM K⁺) supplemented with 5 mg/ml glucose, 0.1 mg/ml transferrin, 0.025 mg/ml insulin, 2 mM glutamine, 20 µg/ml gentamicin (Invitrogen), and 10 µM cytosine arabinofuranoside (Sigma-Aldrich) as described previously (Chen et al., 2000; Losi et al., 2002). The cultures were generally used for recording on the 7th day in vitro (DIV 7).

Human embryonic kidney 293 (HEK 293) cells (CRL1573; American Type Culture Collection, Manassas, VA) were grown in minimal essential medium, supplemented with 10% fetal bovine serum, 100 units/ml penicillin, and 100 units/ml streptomycin (all from Invitrogen) in a 5% CO₂ incubator. Growing cells were dispersed with trypsin and seeded at approximately 2 x 10⁵ cells/35-mm dish in 2 ml of culture medium on 12-mm glass coverslips coated with poly-Llysine. The cells were transfected with mouse GABA_A receptor subunit cDNAs (kind gifts of Dr. Peter H. Seeburg, Max-Planck-Institute for Medical Research, Heidelberg, Germany and Dr. Hartmut Lüddens, University of Mainz, Mainz, Germany) and green fluorescent protein using calcium phosphate precipitation. Mixed plasmids (5 µg total) were added to the dish containing 2 ml of culture medium for 8 to 12 h. The cells were used for electrophysiological recording 2 to 3 days after transfection.

Sagittal cerebellar slices (200 µm thick) were prepared from postnatal day 11 to 12 hybrid mice of mixed genetic background (C57BL/6J x 129/Sv/SvJ). Slices were cut with a Vibratome (Technical Products International, St. Louis, MO). The ice-cold slicing solution contained 85 mM NaCl, 2.5 mM KCl, 1 mM CaCl₂, 4 mM MgCl₂, 1 mM NaH₂PO₄, 25 mM NaHCO₃, 25 mM glucose, and 75 mM sucrose and was brought to pH 7.4 by continuous bubbling with carbogen gas (95% O₂ and 5% CO₂).

Recording of GABA_A Receptor Currents. Cultured granule cells and HEK 293 cells transfected with GABA_A receptors subunits (on 12-mm glass coverslips), and mouse cerebellar slices were placed on the stage of an Nikon E600FN upright microscope equipped with Nomarski optics and an electrically insulated 60x water immersion objective. Cultured granule cells and HEK 293 cells were washed with phosphate-buffered saline and then continuously perfused with extracellular solution containing 145 mM NaCl, 5 mM KCl, 1 mM MgCl₂, 1 mM CaCl₂, 5 mM HEPES, 5 mM glucose, 15 mM sucrose, 0.25 mg/l phenol red, and 0.01 mM D-serine adjusted to pH 7.4 with NaOH. For recordings of miniature IPSCs, 0.5 µM tetrodotoxin (Sigma-Aldrich) was added to the extracellular solution. Transfected HEK 293 cells were identified by the fluorescent label. The extracellular solution for cerebellar slices contained 120 mM NaCl, 3.1 mM KCl, 2 mM CaCl₂, 1 mM MgCl₂, 1 mM KH₂PO₄, 25 mM NaHCO₃, 2.5 mM glucose, and 30 mM sucrose [pH 7.4 when continuously bubbled with carbogen gas). All recordings were performed at room temperature (24–26°C)]. Electrodes were pulled in two stages on a vertical pipette puller from borosilicate glass capillaries (Wiretrol II; Drummond, Broomall, PA) and filled with recording solution containing 145 mM KCl, 10 mM HEPES, 5 mM ATP·Mg, 0.2 mM GTP·Na, and 10 mM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, adjusted to pH 7.2 with KOH. Pipette resistance was 5 to 8 M. Whole-cell voltage-clamp recordings were made at –60 mV with an Axopatch-1D amplifier (Axon Instruments, Union City, CA). Access resistance was monitored throughout the recordings. Cell capacitance was determined from the transient current response to a 10-mV hyperpolarizing voltage step. Currents were filtered at 1 kHz with an 8-pole low-pass Bessel filter (Frequency Devices, Haverhill, MA), digitized at 5 to 10 kHz using an IBM-compatible microcomputer equipped with a Digidata 1322A data acquisition board and pCLAMP 9 software (both from Axon Instruments). Off-line data analysis was performed as described previously (Ortinski et al., 2004). In the analysis of sIPSCs and mIPSCs, infrequent AMPA receptor-mediated excitatory postsynaptic currents were easily identified by their rapid decay (<2 ms) and were excluded from the analysis. The decay phase of sIPSCs and mIPSCs was fitted by the least-squares method using a simplex algorithm according to the triple exponential equation I(t) = I₁ exp(–t/₁) + I₂ exp(–t/₂) + I₃ exp(–t/₃), where I_k=1.3 is a peak amplitude of a decay component and _k=1.3 is the corresponding decay time constant. To allow for easier comparison of decay kinetics between experimental conditions, weighted time constants are reported that were calculated from the individual component time constant values according to the formula _w = ₁ [I₁/(I₁ + I₂ + I₃)] + ₂ [I₂/(I₁ + I₂ + I₃)] + ₃ [I₃/(I₁ + I₂ + I₃)]. For display, 30 or more traces were averaged.

Test Substance Application in GABA_A Receptor-Current Recordings. GABA (Sigma-Aldrich) was dissolved in water and androstenol (Sigma-Aldrich), its 3-epimer 5-androst-16-en-3-ol (Steraloids, Newport, RI) and allopregnanolone (Sigma-Aldrich) in hydroxypropyl--cyclodextrin (Trappsol; CTD, High Springs, FL). Stock solutions were diluted in the extracellular medium to the concentration desired. The concentration of hydroxypropyl--cyclodextrin in the extracellular medium never exceeded 0.04%. To investigate the potentiation of whole-cell GABA responses in cerebellar granule cell cultures, slices, and transfected HEK 293 cells, a fixed concentration of GABA was coapplied locally with varying concentrations of androstenol by means of a Y-tube (Murase et al., 1989). The steroid perfusion began 15 to 20 s before the GABA application. The GABA concentration differed among the various experimental preparations and was taken to be the EC₂₀ (concentration eliciting 20% of maximal current amplitude). Under our experimental conditions, the EC₂₀ value for cerebellar granule cell cultures (DIV 7) and cerebellar slices (P11–12) was 0.25 µM and varied from 1 to 3 µM for 122and 222-transfected HEK cells. In experiments examining the direct effect of androstenol where high (1–300 µM) concentrations were used, responses to 5 µM GABA (EC₃₀ concentration; Huang et al., 2004) were determined in several 122-transfected cells in each culture before the androstenol applications. The average GABA current amplitude obtained in these naive cells was then used to normalize whole-cell responses evoked by application of androstenol. Because high concentrations of androstenol tended to accumulate in the cultures, this procedure allowed a reliable baseline GABA response to be determined.

In Vivo Studies
Animals. Male NIH Swiss mice (25–30 g) and Sprague-Dawley rats (120–180 g) (Taconic Farms, Germantown, NY) were housed two per cage. Animals were kept in a vivarium under controlled laboratory conditions (temperature 22–26°C; humidity 40–50%) with an artificial 12-h light/dark cycle and free access to food and water. Animals were allowed to acclimate to the vivarium for at least 5 days. The experimental groups consisted of six to eight animals. The experiments were performed during the light phase of the light/dark cycle (between 9:30 AM and 3:30 PM), after at least a 30-min period of acclimation to the experimental room. Animals were maintained in facilities fully accredited by the Association for Assessment and Accreditation of Laboratory Animal Care, and experiments were performed under protocols approved by the Animal Care and Use Committee of the National Institute of Neurological Disorders and Stroke in strict compliance with the Guide for the Care and Use of Laboratory Animals of the National Research Council (National Academy Press, Washington, DC; http://www.nap.edu/readingroom/books/labrats/).

Test Substance and Convulsant Administration. Solutions of androstenol and its 3-epimer were made fresh daily in 40% hydroxypropyl--cyclodextrin in sterile 0.9% saline. Further dilutions were made using sterile saline. The steroids were injected intraperitoneally; control animals received injections of 40% hydroxypropyl-cyclodextrin. The convulsant agent pentylenetetrazol (PTZ; SigmaAldrich) was dissolved in saline immediately before use. All drug solutions were administered in a volume equaling 0.01 ml/g of the animal's body weight.

Open-Field Test and Spontaneous Locomotor Activity. Spontaneous exploratory activity was assessed with a VersaMax automated locomotor activity monitoring system (Accuscan Instruments, Columbus, OH). The test area was a square arena (40 x 40 x 30 cm) with clear Plexiglas walls and floor, evenly illuminated by overhead fluorescent white room lighting. Two sets of 16 infrared beams and photocell detectors spaced 2.5 cm apart were arrayed on each side of the arena 2.2 cm above the floor left-to-right and frontto-back on the walls, forming a grid of 256 equally sized squares. To detect vertical movement, a third set of 16 photocell beams was located above the square grid capable of sensing movement at a height of up to 8.4 cm off the floor. Mice were injected with the test compound and placed in the center of the open field and allowed to freely explore for 60 min. The number of horizontal and vertical beam breaks was taken as a measure of horizontal and vertical activity. Time spent in the central square of the open field (2.5 cm away from the walls) was recorded by the VersaMax system as center time. Data were collected in 10-min epochs during the 60-min observation time. Center time is expressed as a percentage of total time of the epoch. Stereotypy counts represent the number of beam breaks detected by one sensor when the same beam was broken three or more times in succession without another beam being broken. The test chamber was cleaned with 70% ethanol solution after each subject to prevent subsequent test subjects from being influenced by deposited odors. For each behavioral measure, the values in 10-min epochs were analyzed with a two-way ANOVA with repeated measures. Specific comparisons between the treatments were made by a post hoc multiple comparisons procedure (Bonferroni's t test). The area under the curve for the total time spent in the center of the arena was determined by the trapezoidal method, and the values for various androstenol doses were compared with vehicle by Student's t test.

Horizontal Screen Test. Androstenol was evaluated for motor toxicity by using a modification of the horizontal screen test as described previously (Kokate et al., 1994). Mice were placed on a horizontally oriented grid (consisting of parallel 1.5-mm diameter rods situated 1 cm apart), and the grid was inverted. Animals that fell from the grid within 10 s were scored as impaired. The doseproducing impairment in 50% of mice (TD₅₀) and the 95% confidence limits were estimated by log-probit analysis using the Litchfield and Wilcoxon method (PHARM/PCS, version 4.2; MicroComputer Specialists, Philadelphia, PA).

Elevated Zero-Maze. Testing was performed as originally described by Shepherd et al. (1994). The elevated zero-maze test is a behavioral test of anxiety based on the naturalistic tendency of rodents to avoid open and elevated areas. It is similar to the more widely used elevated plus maze, with the exception that the open and closed arms are arranged circularly, thus eliminating the center area, which removes ambiguity in interpretation of time spent in the central square of the traditional design. The elevated zero-maze (Hamilton-Kinder, Poway, CA) was positioned in the center of a room under dim lighting. Each mouse was individually removed from its home cage and placed just inside a closed arm. Five-min test sessions were video-recorded with a tripod-mounted camcorder. Several anxiety-related behaviors were subsequently scored from the taped records using Observer 3.0 software (Noldus, Wageningen, The Netherlands). The measures included 1) latency to first open area entry, 2) number of open-area entries, and 3) percentage of time spent in open areas. An animal was considered to have entered the open arm if all four paws had left the closed arm. Open-arm time was considered terminated once a single paw was placed back into the closed arm.

Forced Swim Test. This test was performed as originally described by Porsolt et al. (1977). In brief, mice were pretreated with either androstenol or vehicle and forced to swim in a glass cylinder (21 x 12 cm) containing fresh water (25 ± 2°C) at a depth of 9 cm for a period of 6 min. The last 4 min of the test session was videorecorded and scored for immobility time by two observers blind to the treatment. Mice were judged to be immobile if they ceased struggling and remained motionless, making only those movements necessary to keep their head above water. Reduction in the duration of immobility by a drug was considered an antidepressant-like effect. Mean immobility time values for androstenol-treated animals were compared with vehicle values by Student's t test.

Pentylenetetrazol Seizure Test. Testing was carried out as described previously (Kokate et al., 1994). In brief, mice were injected subcutaneously with PTZ (80 mg/kg) and were observed for a 30-min period. Mice failing to show clonic seizures lasting longer than 5 s were scored as protected. To construct dose-response curves, steroids were tested at several doses spanning the dose producing 50% protection (ED₅₀) or if no protection was obtained at a dose of 100 mg/kg. At least six to eight mice were tested at each dose. ED₅₀ values and corresponding 95% confidence limit were determined by log-probit analysis as for the horizontal screen test.

Six-Hertz Seizure Test. Testing was carried out as described previously (Kaminski et al., 2004). In brief, 3-s corneal stimulation (200 µs-duration, 32-mA monopolar rectangular pulses at 6 Hz) was delivered by a constant current device (ECT Unit 5780; Ugo Basile, Comerio, Italy). Ocular anesthetic (0.5% tetracaine) was applied to the corneas 15 min before stimulation. Immediately before stimulation, the corneal electrodes were wetted with saline to provide good electrical contact. After the stimulation, the animals exhibited a "stunned" posture associated with rearing and automatic movements that lasted from 60 to 120 s in untreated animals. Animals resumed their normal exploratory behavior after the seizure. The experimental endpoint was protection against the seizure. An animal was considered to be protected if it resumed its normal exploratory behavior within 10 s of stimulation. Statistical comparisons were as for the PTZ test.

Physiochemical Properties of Steroids. Vapor-pressure values were estimated using the Clausius-Clapeyron equation from boiling point and enthalpy of vaporization values calculated using Advanced Chemistry Development (ACD/Labs, Toronto, ON, Canada) software version 4.67. Octanol-water partition coefficient values (log P) were also calculated using the ACD/Labs software.

	Results

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Androstenol Potentiates GABA Responses in Cerebellar Granule Cells. To investigate the physiological actions of androstenol on GABA_A receptor responses, we first characterized the effects of the steroid on currents induced by exogenously applied GABA in mouse-cultured cerebellar granule cells. Application of 0.25 µM GABA for 2 to 5 s evoked brief inward current responses that were enhanced in a concentration-dependent fashion by increasing concentrations of androstenol, preapplied for 15 to 20 s and then coapplied during the GABA application (Figs. 2 and 3A). The effect of androstenol was fully reversible but required longwash times at higher concentrations (800 s following 1000 nM androstenol; Fig. 2). At a concentration of 100 nM, the mean percentage of control value for androstenol was 109.9 ± 12.7, and the mean value in a separate series of experiments for allopregnanolone was 150.1 ± 44.6 (n = 7). Androstenol produced a similar concentration-dependent enhancement of GABA-evoked currents in recordings from granule cells in cerebellar slices obtained from 11to 12-dayold mice (Fig. 3B). To determine whether the potentiating effect is structurally specific, we compared the response to androstenol with that of its 3-epimer in cultured cerebellar granule cells. As shown in Fig. 4, only the natural 3-isomer was active at concentrations of 1000 and 3000 nM.

View larger version (24K): [in this window] [in a new window]   Fig. 2. Androstenol reversibly enhances GABA-activated whole-cell currents in DIV 7-cultured mouse cerebellar granule cells. Black bars indicate period of application of 0.25 µM GABA; gray bars indicate preapplication/coapplication of androstenol at the concentrations noted. Recovery was obtained at 100 s with 300 nM androstenol and at 800 s with 1000 nM androstenol. Top and bottom represent different neurons. The holding potential was –60 mV.

View larger version (23K): [in this window] [in a new window]   Fig. 3. Effects of androstenol on GABA-activated currents in cultured mouse cerebellar granule cells at DIV 7 (A) and granule cells in cerebellar slices from 11- to 12-day-old mice (B). A and B, top, whole-cell currents elicited by 0.25 µM GABA (black bars) during the coapplication of androstenol at the indicated concentration (in nanomolars, gray bars). Androstenol was preapplied for 15 to 20 s before the combined androstenol and GABA applications. Full duration of preapplication is not shown. Each trace is from a separate cell. Bottom, concentration-response curves from experiments similar to those illustrated. Percentage of control values were calculated from the peak current values with respect to the peak response to GABA alone in the same cell. Each data point represents the mean ± S.E.M. of data from 4 to 18 cultured granule cells and four to five granule cells in slice recordings. The curves represent logistic fits to the data points. The EC50 values for cultured granule cells and granule cells in slices were 382 and 1400 nM, respectively. The mean ± S.E.M. peak amplitude values of GABA responses in the absence of androstenol for granule cells and granule cells in slices were 48.0 ± 6.5 pA (n = 18) and 38.4 ± 3.7 pA (n = 11), respectively. Small closed and open squares are mean values from experiments of Fig. 5 for HEK 293 cells transfected with 122 and 222 subunit combinations, respectively.

View larger version (21K): [in this window] [in a new window]   Fig. 4. Stereoselective effects of the 3-hydroxy epimers of androstenol on GABA-activated currents. Recordings were obtained from DIV 7 cultured cerebellar granule cells. Representative current traces shown at the top compare the effects of 1000 nM 3-androstenol (5,3-a) with the natural 3-isomer (5,3-a) at the same concentration. The bar graph below shows percentage of control values calculated from peak current values obtained in a series of similar experiments. The percentage of control values were calculated from the peak response to 0.25 µM GABA in the presence of an androstenol epimer at a concentration of either 1000 or 3000 nM, with respect to the peak response to GABA alone in the same cell. Each bar represents the mean ± S.E.M of values from 4 to 16 granule cells. 3-Androstenol produced a significant potentiation at both concentrations (p < 0.05; two-tailed t test), whereas 3-androstenol did not.

View larger version (22K): [in this window] [in a new window]   Fig. 5. Androstenol potentiates GABA currents in HEK 293 cells transfected with 122 and 222 GABAA receptor subunit combinations. Representative current traces shown at the top illustrate the effects of 100, 300, and 1000 nM androstenol (gray bars) on currents evoked by 1 or 3 µM GABA (black bars). Bar chart below summarizes results from six to nine cells transfected with 122 subunits and four to six cells with 222 subunits. Bars represent mean ± S.E.M. percentage peak amplitude values with respect to the amplitude of GABA alone in the same cell. The mean ± S.E.M. peak amplitude values for GABA in the absence of androstenol in 122- and 222-transfected cells were 60.0 ± 11.7 (n = 11) and 59.0 ± 11.6 pA (n = 9), respectively.

Direct Activation of Inward Current in HEK 293 Cells Transfected with 122 GABA_A Receptor Subunits. Perfusion of HEK 293 cells expressing 122 GABA_A receptor subunits with 1 to 300 µM androstenol (in the absence of GABA) resulted in activation of long-lasting inward current responses (Fig. 6). Because of the concern that these high concentrations of androstenol would modify the response to subsequent GABA applications, the responsiveness to 5 µM GABA was assayed in two to three cells in the culture dish prior to application of androstenol. The androstenol response amplitudes were normalized to the mean amplitude of the GABA response in the corresponding dish. As shown in the graph of Fig. 6, increasing concentrations of androstenol produced a concentration-dependent increase in the peak current elicited. The maximal amplitudes of direct androstenol responses were approximately 30% of the 5 µM GABA response amplitude obtained from a sample of cells in the same cultures, indicating that androstenol is less efficacious in activating GABA_A receptors than is GABA (Fig. 6).

View larger version (19K): [in this window] [in a new window]   Fig. 6. Direct activation of inward current in HEK 293 cells expressing 122 GABAA receptors subunits. Traces (left) compare responses to 5 µM GABA and 100 µM androstenol in the same cell. The interval between the GABA and androstenol perfusions was 30 s. Graph (right) summarizes results from a series of similar experiments. In each culture dish, the response to 5 µM GABA was determined in two to three cells before application of androstenol. The data points indicate the peak direct androstenol current expressed as a fraction of the mean current amplitude evoked by 5 µM GABA in the dish. The average response to 5 µM GABA in two dishes was 381 ± 53 pA (n = 5).

Androstenol Prolongs Spontaneous and Miniature IPSCs. Spontaneous IPSCs were recorded from DIV 7-cultured cerebellar granule cells. As shown in Fig. 7, application of 100 nM androstenol caused a prolongation of the decay of the IPSCs as reflected by the increase in –_w. There was only a small increase in the IPSC amplitude that did not reach statistical significance. mIPSCs were recorded from cultured cerebellar granule cells as in the experiments studying spontaneous IPSCs, with the exception that 0.5 µM tetrodotoxin was included in the bathing solution. As for spontaneous IPSCs, androstenol did not influence the mIPSC peak amplitude but did cause a concentration-dependent increase in mIPSC duration (Fig. 8).

View larger version (18K): [in this window] [in a new window]   Fig. 7. Effects of 100 nM androstenol on sIPSCs in cultured cerebellar granule cells. Top left, sample recordings before (control) and after application of androstenol. Currents are presumed to be mediated by GABAA receptors; no fast AMPA receptor-mediated currents are present in the traces shown. Bottom left, superimposed traces show representative averaged sIPSCs from control (black) and androstenol (gray) recordings scaled to the same peak amplitude and displayed on a faster time scale. The weighted time constant (w) values for each trace are indicated. The chart on right plots mean ± S.E.M. values of peak amplitude (left two bars) and w (right two bars) values from 24 control cells and nine cells exposed to androstenol. For each cell, a minimum of 30 sIPSCs were analyzed, and the peak amplitude and weighted time constant values were averaged. *, p < 0.01.

View larger version (21K): [in this window] [in a new window]   Fig. 8. Effects of androstenol on mIPSCs in cultured cerebellar granule cells. Recordings were carried out in the presence of 0.5 µM tetrodotoxin. Top left, sample recordings before (control) and after application of 300 nM androstenol. Bottom left, superimposed traces show representative averaged mIPSCs from control (black) and androstenol (gray) recordings scaled to the same peak amplitude and displayed on a faster time scale. Mean peak amplitude and mean weighted time constant values during application of androstenol at various concentrations are plotted in the graph as percentage of control (prior to androstenol application). The control mean ± S.E.M. peak amplitude and weighted time constant values are 45.5 ± 7.0 pA and 46.8 ± 3.3 ms (n = 9). Each data point represents data from recordings from four to eight cells. In each recording, a minimum of 30 mIPSCs were analyzed, and the peak amplitude and weighted time constant values were averaged.

Effects of Androstenol on Spontaneous Motor Behavior. The effects of androstenol were examined on several measures of spontaneous motor behavior, including spontaneous locomotor activity (horizontal behavior); repetitive head, limb, or body movements (stereotyped behavior); and open field activity, which is considered a measure of anxiety level. Figure 9 summarizes the results of experiments comparing a single 10-, 30-, 50-, or 100-mg/kg injection of androstenol with vehicle. As shown in Fig. 9, top, animals exhibited a reduction in horizontal and stereotyped behaviors over the 60-min observation period, reflecting normal habituation. Two-way repeated measures ANOVA demonstrated a significant effect of post-treatment time on these two parameters (respectively, F_5,191 = 96.8; p < 0.001 and F_5,191 = 100.9; p < 0.001). Androstenol at doses of 10 to 50 mg/kg did not affect either of these measures of activity. Thus, for horizontal activity, the treatment effect (F_3,191 = 0.36 p > 0.05) and treatment versus time interaction (F_15,191 = 1.4; p > 0.05) were not statistically significant. However, when the data from the 100 mg/kg dose was included in the analysis, the treatment effect (F_4,221 = 5.87) and treatment versus time interaction (F_20,221 = 5.26) were significant (p < 0.001), indicating that at the highest dose there is an effect on horizontal activity. For stereotyped behavior, the treatment effect (F_3,191 = 0.527; p > 0.05) and treatment versus time interaction (F_15,191 = 1.2; p > 0.05) for doses of 10 to 50 mg/kg were not statistically significant. Including the data from the 100 mg/kg dose in the analysis did not alter the outcome of the statistical analysis (treatment effect: F_4,221 = 1.49; treatment versus time interaction: F_20,221 = 1.12; p > 0.05). Thus, at doses of 50 mg/kg and below, androstenol did not affect these spontaneous motor behaviors and their habituation. However, at the 100 mg/kg dose, there was an effect on horizontal activity but not sterotypy.

View larger version (32K): [in this window] [in a new window]   Fig. 9. Effects of androstenol on spontaneous motor behavior in mice. Graphs (top) show horizontal activity counts and stereotypy counts for the preceding 10-min period after injection of vehicle or the indicated androstenol doses at zero time. The time in the center of the recording area (expressed as a percentage of total time) is plotted in the bottom left graph. The data points represent the mean ± S.E.M. of values from 11 vehicle-treated control mice or six to seven androstenol-treated mice. The bar chart in lower right shows area-under-curve values from the time in center of the recording area plots. Androstenol was injected i.p. just before the placement of the animals in the openfield arena. *, p < 0.05; **, p < 0.01 versus vehicle; Bonferroni's t test, except AUC where Student's t test was used.

In contrast to the lack of effect of androstenol on gross motor activity at doses of 50 mg/kg and below, the steroid at these doses did increase, in a dose-dependent fashion, the fraction of time spent in the center of the arena (Fig. 9, bottom left), indicating an anxiolytic-like action. Two-way repeated measures ANOVA revealed a significant effect of time spent in center of the arena for both treatment (F_3,191 =3.2; p < 0.05) and time (F_5,191 = 8.2; p < 0.001) but no treatment versus time effect (F_15,191 = 0.8; p > 0.05). We assume that the significant time effect is due to the rise and fall in plasma levels of the steroid during the 60-min observation period. The various steroid doses were compared by determining the mean area-under-the-curve values for percentage time in center at each dose. As shown in Fig. 9, bottom right, androstenol doses of 30 and 50 mg/kg were associated with significant increases in the mean areaunder-the-curve values. Because the 100 mg/kg dose of androstenol was associated with an overall reduction in motor behavior, percentage of time in center values are not meaningful and are therefore not presented.

Effects of Androstenol in the Horizontal Screen Test. The horizontal screen test was used to further characterize the sedative/motor impairing action of high doses of androstenol. The steroid was administered 15 min before testing to groups of six mice at doses of 100 to 300 mg/kg. None of the six animals receiving the 100 mg/kg dose of androstenol scored positive in the test, whereas at 200 and 300 mg/kg, four and five animals, respectively, showed impairment. The TD₅₀ (95% confidence limit) value was 181 (108–303) mg/kg.

Effects of Androstenol in Elevated Zero-Maze. We used the elevated zero-maze as an alternative to the openfield test for assessing the anxiolytic activity of androstenol. As shown in Fig. 10, androstenol caused a dose-dependent reduction in the latency to first open-arm entry, a dosedependent increase in the number of open-arm entries and a dose-dependent increase in the percentage time in open arms; significant effects were observed at doses of 10 to 50 mg/kg, depending upon the measure. Similar reductions in latency to first open-arm entry and increases in percentage time in open arm have been observed for anxiolytic drugs (Mombereau et al., 2004).

View larger version (15K): [in this window] [in a new window]   Fig. 10. Effects of androstenol on anxiety-related behaviors in the elevated zero-maze. Androstenol or vehicle was administered i.p. 15 min before testing. Top, latency to first open-area entry; middle, number of open area entries; bottom, time spent in open areas. The bars represent the mean ± S.E.M. of values from eight vehicle-treated control mice or six to seven androstenol-treated mice. *, p < 0.05; **, p < 0.01 versus vehicle; Student's t test.

View larger version (20K): [in this window] [in a new window]   Fig. 11. Immobility times in the forced swim test following treatment with vehicle or different doses of androstenol. Androstenol was administered i.p. 15 min before the beginning of the test. *, p < 0.05, Student's t test.

Anticonvulsant Activity of Androstenol in 6-Hz and PTZ Seizure Models. As shown in Fig. 12, androstenol (10–100 mg/kg) conferred protection against seizures in the 6-Hz and PTZ models. The ED₅₀ (95% confidence limit) values in the two models were 21.9 (11.4–42.2) and 48.9 (33.0–72.6) mg/kg. The 3-epimer at a dose of 100 mg/kg failed to protect any animals in the PTZ model.

View larger version (15K): [in this window] [in a new window]   Fig. 12. Dose-response relationships for protection by androstenol and its 3-isomer () against 6-Hz and PTZ-induced seizures in mice. Steroids were administered i.p. 15 min before electrical stimulation or injection of PTZ. Data points indicate percentage of animals protected against seizures. Each point represents six to eight mice. The smooth curves represent logistic fits to the data.

	Discussion

Top Abstract Materials and Methods Results Discussion References

 
The pheromone androstenol is a 16 to 17-unsaturated steroid that is structurally similar to GABA_A receptor-modulating neurosteroids. We now show that androstenol has GABA_A receptor-modulating activity comparable to such neurosteroids. Because androstenol is synthesized endogenously, it too can be considered to be a neurosteroid. Neurosteroids are conventionally viewed as hormones; i.e., they are produced endogenously and convey a biological signal between separate cells of the same organism, either by bloodborne or local transmission. Androstenol has not previously been considered to be a hormone, although it is now apparent that it could act in this fashion. Rather, androstenol is well recognized to be a pheromone, which serves as a signaling molecule between individuals of the same species. Our results raise the possibility that an interaction with GABA_A receptors could play a role in the signaling function of this steroid.

A critical structural characteristic of endogenous GABA_A receptor-modulating neurosteroids is that they are reduced at the 5and 3-positions of the A-ring (Rogawski and Reddy, 2004) (Fig. 1). Among such structures, maximal potency is obtained when the substituents at the 3and 5-positions are in the -configuration. Altering the stereochemistry at these critical positions dramatically influences activity. In particular, the 3-epimers all largely lack effects on GABA_A receptors (Gee et al., 1988; Peters et al., 1988; Kokate et al., 1994). Androstenol shares an A-ring structure with these other endogenous neurosteroids. Thus, our first objective was to demonstrate that androstenol exhibits functional actions on GABA_A receptors, similar to that of A-ring-reduced neurosteroids. We found that the steroid markedly potentiates responses to GABA in granule cell cultures in a concentration-dependent manner (EC₅₀, 0.4 µM), with up to a four-fold increase in current amplitude at the highest concentration tested (3 µM). A similar action was observed in granule cell slices at modestly higher concentrations (EC₅₀, 1.4 µM), presumably because access to GABA_A receptors is more restricted in the slices. Comparison with the prototypic 5,3reduced neurosteroid allopregnanolone in granule cell cultures suggested that androstenol is less potent (see also Bolger et al., 1996; Fodor et al., 2005), although it produces comparable maximal potentiation to that reported for allopregnanolone and allotetrahydrodeoxycorticosterone in various in vitro preparations and thus has similar efficacy (Peters et al., 1988; Puia et al., 1990; Wohlfarth et al., 2002). In addition, we found that androstenol produces a concentration-dependent prolongation of the decay time of spontaneous inhibitory postsynaptic currents without a significant alteration in the amplitude of the currents. These actions are similar to those of other GABA_A receptor-modulating neurosteroids that have generally been found to enhance the amplitude of currents induced by exogenously applied GABA but mainly influence the decay of synaptically generated currents (Zhu and Vicini, 1997; Cooper et al., 1999). Androstenol is produced in the body and is able to access the brain. Thus, similar to these other steroids, it could have a hormonal role in the modulation of central nervous system neuronal excitability. In contrast to androstenol, the 3-epimer failed to potentiate responses to exogenous GABA, even when applied at high (1 and 3 µM) concentrations, as expected from the aforementioned structure-activity relationship for neurosteroid stereoisomers.

Our results with native GABA_A receptors in granule cells were corroborated in studies with recombinant GABA_A receptors in a mammalian cell expression system. We found that GABA-evoked responses of GABA_A receptors composed of 122 and 222 subunits, which are abundant subunit combinations in many brain regions (Sieghart, 1995; Mehta and Ticku, 1999), including the olfactory bulb (Laurie et al., 1992; Pirker et al., 2000), were potentiated by the steroid to a similar extent as native receptors. In addition, androstenol at higher concentrations (1–300 µM) directly gated the recombinant GABA_A receptors, as has previously been observed for other neurosteroids (Kokate et al., 1994; Reddy and Rogawski, 2002; Wohlfarth et al., 2002).

Having found androstenol to be active in vitro, we sought to determine whether systemic administration of the steroid has behavioral actions expected of GABA_A receptor-modulating neurosteroids, including anxiolytic-like, antidepressantlike, and anticonvulsant activities. In the open-field test, we observed that androstenol increased the time spent by mice in the center of the arena at doses that did not alter spontaneous locomotor activity, which suggests that it has anxiolytic properties. There is a large body of evidence indicating that neurosteroids, like other GABA_A receptor modulators, have such anxiolytic actions (Bitran et al., 1991; Rodgers and Johnson, 1998; Finn et al., 2003). At higher doses (100 mg/kg), androstenol inhibited locomotor activity and caused gross impairment of motor function, as is also typical of neurosteroids (Kokate et al., 1994).

There is also evidence that neurosteroids have antidepressant-like activity in the forced swim test (Khisti et al., 2000). In this regard, neurosteroids are similar to GABA_A receptor agonists or drugs that elevate brain GABA levels (Borsini et al., 1986), although it is not clear that such drugs have true antidepressant actions. We examined androstenol in the forced swim test and found that low doses (5–10 mg/kg) cause a significant reduction in immobility time, an effect that is produced by some drugs with antidepressant properties (Porsolt et al., 1977, 2001). A higher dose of androstenol (30 mg/kg) was inactive. Anxiolytic drugs that lack intrinsic antidepressant activity, such as benzodiazepines, can reverse the reduction in immobility produced by antidepressants (Flugy et al., 1992). Thus, it seems plausible that, at higher doses, the anxiolytic activity of androstenol could counteract the antidepressant-like reduction in immobility times.

Neurosteroids that act as positive GABA_A receptor modulators have anticonvulsant activity in various animal seizure models, including the 6-Hz and PTZ models (Kokate et al., 1994, 1996; Reddy and Rogawski, 2002; Kaminski et al., 2004, 2005). In the present study, we found that androstenol similarly is protective in these two seizure models. The potency in the 6-Hz model was greater than in the PTZ model, as observed previously for other neurosteroids. In accordance with the stereospecificity for potentiation of GABA_A receptor currents, the 3-epimer of androstenol was devoid of anticonvulsant activity. Thus, it is likely that modulation of GABA_A receptors accounts for the in vivo anticonvulsant activity, and indeed, there is extensive evidence that the target site for anticonvulsant effects of neurosteroids in the 6-Hz and PTZ models is the GABA_A receptor (Kaminski et al., 2004; Rogawski and Reddy, 2004). The potency of androstenol in these models was similar to that of its 16-keto analog androsterone (Kaminski et al., 2005). However, androstenol was relatively less potent in the two seizure models than other neurosteroids, such as allopregnanolone.

The key question raised by our studies is whether the GABA_A-potentiating activity of androstenol contributes to its pheromonal properties. We did not discern any differences between androstenol and other endogenous neurosteroids with respect to their effects on GABA_A receptors. However, androstenol has certain physiochemical properties that make it better suited to serve as a pheromone than other neurosteroids. The calculated vapor pressure of androstenol at 37°C is 254 µtorr, which is markedly greater than other neurosteroids (values for allopregnanolone, allotetrahydrodeoxycorticosterone, androstanediol, and androsterone are 26, 4, 55, and 60 µtorr, respectively). Thus, androstenol is more volatile than other neurosteroids and is better able to serve as an airborne vapor-phase chemical messenger. By the universal gas law, the concentration of androstenol in air at 37°C is estimated to be 13 nM. Interestingly, this value is comparable to the basal serum concentrations of allopregnanolone (3–10 nM) (Paul and Purdy, 1992), indicating that whether neurosteroids are delivered as blood-borne hormones or as pheromones carried in a vapor, the concentrations in the delivery medium are remarkably similar. The estimated vapor-phase concentration of androstenol is below the concentration active on GABA_A receptors (threshold concentration, 100 nM). However, it is likely that the steroid would reach the required concentration in mucus within seconds or less (T. H. Morton, personal communication). A second characteristic of androstenol that makes it well suited as a pheromone is its hydrophobicity. Androstenol has a calculated log P (octanol-water partition coefficient) value of 5.9. Thus, it has significantly greater hydrophobicity than other neurosteroids (comparable values, 3.8–4.9), which would promote accumulation in mucus and transcellular delivery following exposure to a vapor.

The receptive organ for the pheromonal activity of androstenol is not known, although the olfactory system is an obvious possibility, particularly because androstenol is detected as an odorant by 70% of humans (Gower and Ruparelia, 1993) and possibly all nonhuman primates (Laska et al., 2005). Although functional GABA neurons are present transiently in the nose during embryonic development (Tobet et al., 1996; Wray et al., 1996), as of yet, GABA_A receptors have not been identified in olfactory receptor cells situated in the adult olfactory epithelium. However, virtually every type of neuron in the olfactory bulb receives GABAergic input. The glomerular layer in the most superficial zone of the bulb has among the highest densities of GABA_A receptors of any region in the brain (Palacios et al., 1981; Bowery et al., 1987; Laurie et al., 1992). Within the glomerular layer, periglomerular cells inhibit mitral cells through GABA-mediated dendrodendritic inhibition (Shepherd and Greer, 1998). There is extensive evidence that hydrophobic, small molecular weight substances can be transported from the nasal cavity into the olfactory bulb and other brain structures (Illum, 2004). Thus, inhaled androstenol could access GABA_A receptors, which are present in high abundance in superficially located mitral cell dendrites as well as GABA_A receptors in deeper structures in the bulb and elsewhere in the brain. In addition to environmental and blood-borne delivery, the olfactory system may also be exposed to locally synthesized androstenol. In this regard, it is noteworthy that the nasal mucosa contains many of the enzymes required for androstenol synthesis, including 3-hydroxysteroid oxidoreductase (Brittebo and Rafter, 1984), which can synthesize androstenol from androstenone, a companion steroidal pheromone to androstenol. Androstenol and androstenone are among the best-accepted mammalian pheromones (Grammer et al., 2005). The present results raise the possibility that effects on GABA_A receptors could contribute to their pheromonal activity.

	Acknowledgements

 
We thank Dr. Thomas H. Morton for advice.

	Footnotes

 
This research was supported in part by the Intramural Research Program of the National Institute of Neurological Disorders and Stroke, National Institutes of Health and by National Institute of Mental Health Grant MH-64797 (to S.V.).

doi:10.1124/jpet.105.098319.

ABBREVIATIONS: DIV 7, 7th day in vitro; HEK, human embryonic kidney; IPSC, inhibitory postsynaptic current; sIPSC, spontaneous IPSC; mIPSC, miniature IPSC; PTZ, pentylenetetrazol.

¹ Current affiliation: Faculty of Medicine and Surgery, Azienda Ospedaliera Universitaria G. Martino, University of Messina, Messina, Italy.

Address correspondence to: Dr. Michael A. Rogawski, Epilepsy Research Section, Porter Neuroscience Research Center, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Building 35, Room 1C-1002, MSC 3702, 35 Convent Drive, Bethesda, MD 20892-3702. E-mail: michael.rogawski{at}nih.gov

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