Allosteric modulators of the δ GABAA receptor subtype demonstrate a therapeutic effect in morphine-antinociceptive tolerance and withdrawal in mice

The present study evaluated the effects of compounds targeting extrasynaptic δ subunit-containing γ-aminobutyric acid type A receptors (δ*-GABAARs) to interrogate the role of tonic inhibition in the development of antinociceptive tolerance caused by repeated morphine administration. We investigated the effect of subchronic or acute treatment with non-steroidal positive allosteric modulators (PAMs) of δ*-GABAARs, such as 2-261, on the morphine-antinociceptive tolerance. Mice were treated twice daily with morphine for 9 days and antinociception was measured using the hot water tail immersion test. Co-treatment with 2-261 and morphine prevented morphine-antinociceptive tolerance and acute administration of 2-261 on day 9 was sufficient to reverse the tolerance. Other compounds with activity at δ*-GABAARs also reversed morphine tolerance, whereas an enaminone that lacked activity at δ*-GABAARs did not. Acute administration of 2-261 did not cause an additive or synergistic antinociceptive effect when combined with an acute submaximal dose of morphine. We then used Cre/LoxP recombination to generate GABAA δ-subunit knockout mice to corroborate the pharmacological results. Observations of male δ-knockout mice demonstrated that the δ*-GABAARs was necessary for 2-261 modulation of both analgesic tolerance and somatic withdrawal symptoms produced by subchronic morphine. While female mice still benefited from the positive effects of 2-261, the δ-subunit was not necessary for these effects, highlighting a distinction of the different pathways that could have implications for some of the sex-related differences seen in human opioid-induced outcomes. Consequently, subtype-specific allosteric modulators of GABAARs may warrant further investigation as pharmacological targets to manage tolerance and withdrawal from opioids.

Steroids in relation to epilepsy and anaesthesia

Increasing numbers of reports indicate direct effects of ovarian steroids on the central nervous system. Effects of progesterone and its metabolites on brain excitability in humans and in experimental animals have been studied. Anti-epileptic effects have been shown in cats and in women with partial epilepsy and well-defined epileptic foci. The reduced progesterone metabolite 5 alpha-pregnan-3 alpha-ol-20-one and its 5 beta analogue also decreased the epileptic activity resulting from a penicillin-induced cortical focus in cats. 5 alpha-Pregnan-3 alpha-ol-20-one protected mice against metrazol-, bicuculline- and picrotoxin-induced seizures but not against electroshock-and strychnine-induced seizures. Progesterone, 5 alpha-pregnan-3 alpha-ol-20-one and 5 beta-pregnan-3 alpha-ol-20-one also induce anaesthesia in humans and animals; in a rat model of anaesthesia 5 alpha-pregnan-3 alpha-ol-20-one was eight times more potent than methohexitone (the most potent anaesthetic barbiturate). Anaesthesia with loss of the eyelash reflex was observed in humans 75-90 seconds after the intravenous injection of 5 beta-pregnan-3 alpha-ol-20-one in lipid emulsion. The in vivo production and brain distribution of centrally active steroids has also been studied in relation to the phases of the ovarian and menstrual cycle. A subset of women with epilepsy show changes in seizure frequency in relation to hormonal variations during the menstrual cycle. In the luteal phase when progesterone levels are high the number of generalized seizures is low. It is possible that progesterone and its metabolites play a role in epileptic seizures and also in the premenstrual syndrome.

Behavioral characterization of Co 134444 (3alpha-hydroxy-21-(1'-imidazolyl)-3beta-methoxymethyl-5alpha- pregnan-20-one), a novel sedative-hypnotic neuroactive steroid

RATIONALE

Neuroactive steroids have been shown to exhibit a wide range of behavioral activities that are similar but not identical to those of benzodiazepines. These activities include anticonvulsant, anxiolytic and sedative-hypnotic effects.

OBJECTIVE

The purpose of the present study was to characterize Co 134444 (3alpha-hydroxy-21-(1'-imidazolyl)-3 -methoxymethyl-5alpha-pregnan-20-one), a novel sedative-hypnotic neuroactive steroid, in a variety of behavioral procedures.

METHODS

Anticonvulsant effects were determined by the ability to protect against pentylenetetrazol- and maximal electroshock-induced seizures in mice and rats. Anxiolytic-like effects were determined using a punished drinking procedure in rats. Ataxic effects were determined using a horizontal wire procedure in mice and a rotorod procedure in mice and rats. The discriminative stimulus effects were evaluated in rats trained to discriminate pregnanolone from vehicle.

RESULTS

Co 134444 exhibited oral anticonvulsant activity against pentylenetetrazol and maximal electroshock with ED50s of 9.8 and 20.6 mg/kg, respectively, in mice and 23.6 and 25.3 mg/kg, respectively, in rats. Anxiolytic-like efficacy was observed at a dose as low as 3.0 mg/kg, PO, in rats. Ataxic effects were observed with rapid onset and short duration. TD50s were 17.4 and 21.2 mg/kg orally in mice in the horizontal wire and rotorod procedures, respectively, and 39.0 mg/kg in rats using the rotorod. Co 134444 completely substituted for pregnanolone as a discriminative stimulus with little effect on response rate.

CONCLUSIONS

Co 134444 exhibits a wide variety of behavioral effects; however, its rapid onset and short duration are consistent with its potential use as a sedative-hypnotic drug.

Response-rate suppression in operant paradigm as predictor of soporific potency in rats and identification of three novel sedative-hypnotic neuroactive steroids

Novel neuroactive steroids were evaluated for their effects on operant responding, rotorod motor performance, and electroencephalogram recording in rats. Co 134444, Co 177843, and Co 127501 were compared with the prototypical gamma-aminobutyric acid(A)-positive allosteric modulators triazolam, zolpidem, pentobarbital, pregnanolone, and CCD 3693. Each of the compounds produced a dose-related decrease in response rates under a variable-interval 2-min schedule of positive reinforcement in an operant paradigm. In addition, all compounds produced a dose-related increase in ataxia and significant increases in nonrapid eye movement sleep in this experiment or have been previously reported to do so. Co 134444, Co 177843, and Co 127501 increased nonrapid eye movement sleep at doses that had no effect on rapid eye movement sleep. All of the compounds were more potent at decreasing operant responding than they were at increasing ataxia. Furthermore, the potency of compounds to produce response-rate suppression in an operant paradigm appeared to be a better predictor of soporific potency than did potency in the rotorod assay. The screening for sedative-hypnotic activity resulted in the identification of the novel orally active neuroactive steroids Co 134444, Co 177843, and Co 127501.

Positive allosteric modulators of the GABA(A) receptor: differential interaction of benzodiazepines and neuroactive steroids with ethanol

Endogenous pregnane steroids, such as allopregnanolone (3alpha-hydroxy-5alpha-pregnan-20-one; 3alpha, 5alpha-P) and pregnanolone (3alpha-hydroxy-5beta-pregnan-20-one; 3alpha,5beta-P), allosterically modulate GABA(A) receptor function and exhibit behavioral effects similar to benzodiazepines, though acting at a distinct recognition site. Inasmuch as some positive allosteric modulators of GABA(A) receptor function exhibit profound interactions with ethanol, the effects of 3alpha,5alpha-P and 3alpha,5beta-P were compared to those of two benzodiazepines, triazolam and diazepam, on the motor function of mice and rats when administered either alone or in combination with ethanol. All four test compounds exhibited dose-related impairment of motor function in the horizontal wire task in mice and the rotorod task in rats. Ethanol caused a marked enhancement of triazolamand diazepam-induced motor impairment. In contrast, ethanol enhanced to a lesser extent the motor impairment induced by both neurosteroids in mice and not at all in rats. All four compounds increased ethanol-induced behavioral sleep time in mice, although the benzodiazepines did so at a much smaller fraction of their ataxic doses as compared to the neurosteroids. As one of the undesired side-effects of therapeutic use of benzodiazepines is their interaction with ethanol, development of neuroactive steroids as drugs may offer therapeutic advantages.

Anxiolytic and anticonvulsant activity of a synthetic neuroactive steroid Co 3-0593

Endogeneously occurring neuroactive steroids, metabolites of progesterone and deoxycorticosterone, have been shown previously to interact with the GABAA receptor with great specificity in vitro and to have anticonvulsant, anxiolytic and sedative activity in vivo. However, these endogenously occurring steroids are not useful as therapeutic agents due to their potential metabolism to hormonally active steroids and their poor oral bioavailability. In an attempt to develop therapeutic agents which would maintain the pharmacological profiles of endogeneous neuroactive steroids but with increased oral bioavailability and reduced metabolic liability, we explored simple substitutions at the 3 beta-position of the endogenous neuroactive steroid, 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha, 5 alpha-P). This report describes part of the in vitro and in vivo pharmacological profile of a 3 beta-substituted analog, 3 beta-ethenyl-3 alpha-hydroxy-5 alpha-pregnan-20-one (Co 3-0593). The compound exhibited anticonvulsant activity against pentylenetrazol-induced seizures in mice and rats (ED50 = 5.6 and 11.5 mg/kg, i.p., respectively). Co 3-0593 showed robust anxiolytic effects, comparable to benzodiazepines in the Geller-Seifter test after both SC and oral administration. Furthermore, the anxiolytic activity was maintained after chronic administration suggesting an absence of tolerance. The compound did not affect the acquisition of a learned response at both anticonvulsant and anxiolytic doses. However, at higher doses the compound showed rotorod deficit which was further enhanced by ethanol. In summary, 3 beta-ethenyl-substituted 3 alpha, 5 alpha-P appeared to maintain the pharmacological activities of the endogenous neuroactive steroid with apparent oral activity.

CCD-3693: an orally bioavailable analog of the endogenous neuroactive steroid, pregnanolone, demonstrates potent sedative hypnotic actions in the rat

An endogenous neuroactive steroid, pregnanolone, and an orally available synthetic analog, CCD-3693, were administered to rats at the middle of their circadian activity phase (6 hr after lights off). Electroencephalogram-defined sleep-wake states, locomotor activity and body temperature were concurrently measured 30 hr before and after treatment. Identical procedures were used to test triazolam and zolpidem. Triazolam (0.1-1.6 mg/kg), zolpidem (2.5-10 mg/kg) and the neuroactive steroids (10-30 mg/kg) produced dose-dependent increases in non-rapid eye movement (NREM) sleep. At this dose and time of day (in which the rats were predominantly awake during the 6 hr before treatment) the neuroactive steroids appeared more intrinsically efficacious in promoting NREM sleep than the benzodiazepine ligands. The neurosteroids did not, however, significantly interfere with rapid eye movement sleep and were more selective in reducing (EEG) wakefulness, with relatively less locomotor activity impairment during waking than triazolam and zolpidem. In addition, the benzodiazepine receptor ligands showed distinct "rebound" wakefulness after the NREM sleep-promoting effect subsided, although the neuroactive steroids did not. In addition, in vitro binding studies and in vivo pharmacological data confirmed that CCD-3693 was orally active in standard tests of anxiety, anticonvulsant, loss-of-righting and passive avoidance.

Partial agonism by 3alpha,21-dihydroxy-5beta-pregnan-20-one at the gamma-aminobutyric acidA receptor neurosteroid site

3alpha,21-Dihydroxy-5alpha-pregnan-20-one (5alpha-THDOC) and 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha,5alpha-P) have full efficacy as allosteric modulators of [35S]t-butylbicyclophosphorothionate ([35S]TBPS) binding to sites on the gamma-aminobutyric acid (GABA) type A receptor complex (GRC). Relative to 3alpha,5alpha-P and 5alpha-THDOC, 3alpha,21-dihydroxy-5beta-pregnan-20-one (5beta-THDOC) has limited efficacy as an allosteric modulator of [35S]TBPS binding. Interactions between 3alpha,5alpha-P, 5alpha-THDOC and 5beta-THDOC were examined to determine whether these neuroactive steroids share a common site for modulation of the GRC. The concentration-response curves for both 3alpha,5alpha-P and 5alpha-THDOC modulation of [35S]TBPS binding to brain and recombinantly derived GRCs are shifted rightward in the presence of various concentrations of 5beta-THDOC. Similarly, 5beta-THDOC modulates GABA-evoked Cl- currents with low efficacy and inhibits the potentiation of GABA-evoked Cl- currents by 3alpha,5alpha-P. Furthermore, behavioral studies reveal that 5beta-THDOC antagonizes 3alpha,5alpha-P-induced loss of the righting reflex in mice at a dose that has no effect alone. These results represent the first demonstration of antagonist-like actions of a neuroactive steroid on the GRCs at levels ranging from the receptor to animal behavior and suggest the existence of partial agonist neurosteroids.

Characterization of the anticonvulsant properties of ganaxolone (CCD 1042; 3alpha-hydroxy-3beta-methyl-5alpha-pregnan-20-one), a selective, high-affinity, steroid modulator of the gamma-aminobutyric acid(A) receptor

Ganaxolone (CCD 1042) is a 3beta-methyl-substituted analog of the endogenous neuroactive steroid 3alpha-hydroxy-5alpha-pregnan-20-one. Ganaxolone inhibited binding of the gamma-aminobutyric acid (GABA)A receptor-chloride channel ligand t-[35S]butylbicyclophosphorothionate (IC50 of 80 nM) and enhanced binding of the benzodiazepine site ligand [3H]flunitrazepam (EC50 of 125 nM) and the GABA site ligand [3H]muscimol (EC50 of 86 nM), consistent with activity as a positive allosteric modulator of the GABA(A) receptor. Electrophysiological recordings showed that, whereas nanomolar concentrations of ganaxolone potentiated GABA-evoked chloride currents in Xenopus oocytes expressing the human GABA(A) receptor subunits alpha1beta1gamma2L, alpha2beta1gamma2L or alpha3beta1gamma2L, direct activation of chloride flux occurred to a limited extent only at micromolar concentrations. Ganaxolone was effective in nontoxic doses against clonic convulsions induced by s.c. pentylenetetrazol administration in mice and rats (ED50 values of 4.3 and 7.8 mg/kg i.p., respectively). Ganaxolone also exhibited potent anticonvulsant activity against seizures induced by s.c. bicuculline (ED50 of 4.6 mg/kg i.p.), i.p. TBPS (ED50 of 11.7 mg/kg i.p.) and i.p. aminophylline (ED50 of 11.5 mg/kg i.p.) in mice. Although ganaxolone effectively blocked tonic seizures induced by maximal electroshock in mice (ED50 of 29.7 mg/kg i.p.), it did so only at doses that produced ataxia on the Rotorod (TD50 of 33.4 mg/kg i.p.). Conversely, ganaxolone was a potent anticonvulsant against fully kindled stage 5 seizures induced by corneal kindling in rats (ED50 of 4.5 mg/kg i.p.), producing these effects at doses well below those that resulted in ataxia (TD50 of 14.2 mg/kg i.p.). The seizure threshold, as determined by an increase in the dose of i.v. infused pentylenetetrazol required to induce clonus, was also significantly elevated by nontoxic doses of ganaxolone in mice. In summary, these data indicate that ganaxolone is a high-affinity, stereoselective, positive allosteric modulator of the GABA(A) receptor complex that exhibits potent anticonvulsant activity across a range of animal procedures. The profile of anticonvulsant activity obtained for ganaxolone supports clinical evaluation of this drug as an antiepileptic therapy with potential utility in the treatment of generalized absence seizures as well as simple and complex partial seizures.

Synthesis and in vitro activity of 3 beta-substituted-3 alpha-hydroxypregnan-20-ones: allosteric modulators of the GABAA receptor

Two naturally occurring metabolites of progesterone, 3 alpha-hydroxy-5 alpha- and 5 beta-pregnan-20-one (1 and 2), are potent allosteric modulators of the GABAA receptor. Their therapeutic potential as anxiolytics, anticonvulsants, and sedative/hypnotics is limited by rapid metabolism. To avoid these shortcomings, a series of 3 beta-substituted derivatives of 1 and 2 was prepared. Small lipophilic groups generally maintain potency in both the 5 alpha- and 5 beta-series as determined by inhibition of [35S]TBPS binding. In the 5 alpha-series, 3 beta-ethyl, -propyl, -trifluoromethyl and -(benzyloxy)methyl, as well as substituents of the form 3 beta-XCH2, where X is Cl, Br, or I or contains unsaturation, show limited efficacy in inhibiting [35S]TBPS binding. In the 5 beta-series, the unsubstituted parent 2 is a two-component inhibitor, whereas all of the 3 beta-substituted derivatives of 2 inhibit TBPS via a single class of binding sites. In addition, all of the 3-substituted 5 beta-sterols tested are full inhibitors of [35S]TBPS binding. Electrophysiological measurements using alpha 1 beta 2 gamma 2L receptors expressed in oocytes show that 3 beta-methyl- and 3 beta-(azidomethyl)-3 alpha-hydroxy-5 alpha-pregnan-20-one (6 and 22, respectively) are potent full efficacy modulators and that 3 alpha-hydroxy-3 beta-(trifluoromethyl)-5 alpha-pregnan -20-one (24) is a low-efficacy modulator, confirming the results obtained from [35S]TBPS binding. These results indicate that modification of the 3 beta-position in 1 and 2 maintains activity at the neuroactive steroid site on the GABAA receptor. In animal studies, compound 6 (CCD 1042) is an orally active anticonvulsant, while the naturally occurring progesterone metabolites 1 and 2 are inactive when administered orally, suggesting that 3 beta-substitution slows metabolism of the 3-hydroxyl, resulting in orally bioavailable steroid modulators of the GABAA receptor.

Anxiolytic properties of endogenously occurring pregnanediols in two rodent models of anxiety

Certain endogenously occurring 3 alpha-hydroxylated, 5-reduced pregnane steroids act at a specific site on the GABAA receptor complex (GRC) to modulate the effects of GABA at its receptor. Modulators that potentiate GABA at the GABAA receptor often possess anxiolytic properties. The anxiolytic potential of four 5-reduced, 3 alpha, 20-pregnanediols, differing only in the stereochemical orientation of the steroid A-ring and the 20-hydroxyl group, were tested in the Vogel test following intracerebroventricular (ICV) administration. The effects of these pregnanediols were compared to those of their 20-ketone analogues, 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha, 5 alpha-P) and 3 alpha-hydroxy-5 beta-pregnan-20-one (3 alpha, 5 beta-P). All four pregnanediols tested significantly enhanced punished drinking at doses ranging from 10 to 60 micrograms. The rank order of potency based on the minimum effective dose (MED) observed was 5 alpha-pregnan-3 alpha,20 alpha-diol = 5 beta-pregnan-3 alpha,20 alpha-diol > 5 beta-pregnan-3 alpha,20 beta-diol > 5 alpha-pregnan-3 alpha, 20 beta-diol. 3 alpha,5 beta-P and 3 alpha,5 alpha-P enhanced punished responding when administered at 2.5 and 5 micrograms, respectively. 3 beta,5 alpha-P which is inactive at the GRC was also inactive (up to 100 micrograms) in the Vogel test. The benzodiazepine control diazepam was efficacious when administered at 2.5 micrograms. 5 alpha-Pregnan-3 alpha,20 alpha-diol was further tested in the mouse elevated plus-maze model following systemic administration where it was found to be active in a dose range of 10-40 mg/kg IP. These results raise the possibility that in addition to 3 alpha,5 alpha-P and 3 alpha,5 beta-P, some of their endogenously occurring pregnanediol metabolites may also influence physiological processes related to anxiety via the GRC.

3 alpha-Hydroxy-3 beta-trifluoromethyl-5 alpha-pregnan-20-one (Co 2-1970): a partial agonist at the neuroactive steroid site of the gamma-aminobutyric acidA receptor

Neuroactive steroids bind to a unique site on the gamma-aminobutyric acidA (GABAA) receptor complex and allosterically modulate the binding of convulsant ([35S]t-butylbicyclophosphorothionate, [35S]TBPS), GABA ([3H]muscimol), and benzodiazepine ([3H]flunitrazepam) site ligands. In rat cortical membranes, 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha, 5 alpha-P) is a full agonist at the steroid site, inhibiting 96% of specific [35S]TBPS binding and enhancing [3H]flunitrazepam and [3H]muscimol binding 95% and 69% above control levels, respectively. In contrast, the synthetic steroid 3 alpha-hydroxy-3 beta-trifluoromethyl-5 alpha-pregnan-20-one (Co 2-1970) has limited efficacy for modulating the binding of [35S]TBPS (44% inhibition), [3H]flunitrazepam (41% enhancement), and [3H]muscimol (< 10% enhancement). In competition experiments, Co 2-1970 (10 microM) reduced the apparent potency of 3 alpha, 5 alpha-P by 7-17-fold for modulating the binding of these radioligands in rat cortical membranes, suggesting that it has partial agonist properties. Because cortical membranes contain a heterogeneous population of receptors, Co 2-1970 was examined in recombinant GABAA receptors stably expressed in human embryonic kidney 293 cells. Co 2-1970 inhibited [35S]TBPS binding with limited efficacy (39-65% inhibition) in the five receptor combinations examined and, at 10 microM, reduced the apparent potency of 3 alpha, 5 alpha-P 57-fold for inhibiting [35S]TBPS binding to alpha 1 beta 1 gamma 2L receptors. To verify these findings functionally, the effects of 3 alpha, 5 alpha-P and Co 2-1970 were examined electrophysiologically in Xenopus oo-cytes expressing alpha 1 beta 1 gamma 2L receptors. Co 2-1970 showed limited efficacy potentiation of GABA-evoked chloride currents relative to 3 alpha, 5 alpha-P (28% and 86% of the GABA maximum current, respectively). Moreover, Co 2-1970 produced a concentration-dependent antagonism of the 3 alpha, 5 alpha-P-induced potentiation that was associated with a reduction in the apparent affinity of 3 alpha, 5 alpha-P (11-fold at 10 microM Co 2-1970). Taken together, these data indicate that Co 2-1970 is a partial agonist at the neuroactive steroid site associated with GABAA receptors.

Loreclezole modulates [35S]t-butylbicyclophosphorothionate and [3H]flunitrazepam binding via a distinct site on the GABAA receptor complex

The allosteric modulation of [35S]t-butylbicyclophosphorothionate ([35S]TBPS) and [3H]flunitrazepam binding was utilized to evaluate the actions of loreclezole at the GABAA receptor complex in the rat brain. Loreclezole was observed to allosterically inhibit the binding of [35S]TBPS in a dose-dependent manner with micromolar potency (IC50 = 1 microM). Loreclezole was found to have an additive effect on neuroactive steroid modulation of [35S]TBPS binding, but merely potentiated the effect of Ro5-4864 (4"-chlorodiazepam) modulation of [35S]TBPS binding. These observations suggest that loreclezole modulates [35S]TBPS binding through a site independent of the neuroactive steroid and Ro5-4864 sites on the GABAA receptor complex. The enhancement of [3H]flunitrazepam binding to the benzodiazepine receptor by loreclezole as well as the effect of loreclezole on CL218872/[3H]flunitrazepam dose-response curves suggest that loreclezole does not act through the benzodiazepine site on the GABAA receptor complex, nor does it selectively modulate benzodiazepine receptor subtypes. The potency of loreclezole as and inhibitor of [35S]TBPS binding in rat brain was regionally dependent and GABA-sensitive. Loreclezole modulation of [35S]TBPS binding showed greater potency and GABA sensitivity in the cerebellum and thalamus when compared to other brain regions such as the cortex, hippocampus and striatum. This finding is consistent with previous reports of the selectivity of loreclezole for GABAA receptor complex's containing beta 2 and beta 3 subunits. These beta subunit isoforms predominate in the cerebellum and thalamus. Collectively the evidence suggests that loreclezole modulates [35S]TBPS and [3H]flunitrazepam binding through a site distinct from benzodiazepine, neuroactive steroid, Ro5-4864 and GABA sites on the GABAA receptor complex.

Soporific effect of the neurosteroid pregnanolone in relation to the substance's plasma level: a pilot study

The soporific effect of the neuroactive steroid pregnanolone, a metabolite of progesterone, and its relationship with plasma concentrations was assessed in 18 young, healthy, male volunteers for 2 h after administration of a single dose of pregnanolone prepared in two different formulations. Sedation was measured as sleep propensity and power increase in the low frequency delta band of the quantified EEG, based on 5-min polygraphic (EEG, EOG, EMG) recordings under resting conditions, which were performed immediately before, and 30, 60, 90, and 120 min after intake of the study drug. With both formulations, there was a time-dependent increase in plasma concentration of pregnanolone with highest values 1-2 h postdosing. The model of short polygraphic recordings under resting conditions demonstrated soporific effects of pregnanolone. Compared to predosing baseline the number of sleep attempts and the time asleep increased after treatment with a peak 60 min postdosing. Quantitative EEG analysis revealed an increase of absolute amplitude in the delta frequency range with a comparable temporal pattern. Correlations between the soporific effect and plasma concentrations of pregnanolone suggest that the effects were drug-related, although this has to be replicated with placebo control.

Characterization of peripheral benzodiazepine type sites in a cultured murine BV-2 microglial cell line

It is known that the density of peripheral benzodiazepine receptors (PBR) increases after brain damage. Astrocytes are among the cell types where PBR ligand binding has been detected and may be involved in the response to neuronal injury and regeneration. Consistent with the hypothesis, the apparent density of PBR sites in astrocytes is increased by both cytokines and neurotoxins. However, microglia, the resident macrophages which represent 5-15% of glial cell populations have not been evaluated for the presence of the PBR. In the present study, we report the presence of [3H]Ro5-4864 binding in microglial cells. In particular, we used BV-2 cells, an immortalized cell line of murine microglial cells. High affinity binding of [3H]Ro5-4864 to a single site was detected in membranes prepared from BV-2 cells (KD = 4.4 nM, Bmax = 3,800 fmoles/mg protein). Various ligands for the PBR displaced [3H]Ro5-4864 binding with the following rank order of potencies: PK11195 = Ro5-4864 > FGIN-1-27 > triazolam = diazepam > beta-pro-pyl-beta-carboline-3-carboxylate = clonazepam > lorazepam = flurazepam >> chlordiazepoxide = clorazepate. Subcellular fractionationstudies indicate that the majority of the Ro5-4864 binding sites is in the mitochondrial fraction. The remainder is found in nonmitochondrial cell fractions. The [3H]Ro5-4864 binding observed on intact cells had characteristics similar to those found on membranes. The presence of a high density of PBRs in these cells establish the basis for additional investigations into their possible functional role, if any, in the microglial response to neuronal injury.

Modulation of human recombinant GABAA receptors by pregnanediols

Utilising two point voltage-clamp techniques on Xenopus laevis oocytes expressing human (alpha 1 beta 1 gamma 2L) recombinant GABAA receptors, the GABA modulatory actions of six naturally occurring neurosteroids have been determined and compared with those of known positive allosteric modulators. The anaesthetic steroids 5 alpha- and 5 beta-pregnan-3 alpha-ol-20-one produced a concentration-dependent enhancement of the GABA-evoked current. The maximal enhancement of the agonist-induced response produced by these steroids was intermediate between that of pentobarbitone and diazepam, but much greater than that caused by bretazenil. For both the 5 alpha and 5 beta steroid a reduction of the 20 ketone group to form either the corresponding 20 alpha or 20 beta hydroxy steroid produced, in all cases, a reduction in potency and a decrease in the maximal effect. The relationship of steroid structure to these two parameters is considered. The influence of the alpha subtype (alpha x beta 1 gamma 2L, where x = 1, 2 or 3) for the behaviourally active 5 alpha-pregnan-3 alpha,20 alpha-diol is also determined. Although the maximal effect of the steroid is not influenced by the alpha subtype, the alpha 2-containing receptor exhibits a modest decrease (approximately 6-fold) in potency compared to alpha 1- and alpha 3-containing receptors. The results described here are discussed in relation to the distinct behavioural actions of the neurosteroids.

Influence of the estrus cycle on the discrimination of apparent neuroactive steroid site subtypes on the gamma-aminobutyric acidA receptor complex in the rat

Estrus cycle-related changes in gamma-aminobutyric acid (GABA)A receptor complex (GRC) sensitivity to modulation by reduced progesterone metabolites is suggestive of a possible mechanism for maintaining brain homeostasis in the presence of fluctuating levels of these neuroactive metabolites. In addition, certain endogenously occurring pregnanediols are selective for apparent neuroactive steroid site subtypes discriminated by the progesterone metabolite 3 alpha-hydroxy-5 beta-pregnan-20-one (3 alpha,5 beta-P) on the GRC. Thus, it was of interest to evaluate the influence of gender and the estrus cycle on the ability of 3 alpha,5 beta-P and its 20-reduced analog 5 beta-pregnan-3 alpha,20 beta-diol to differentiate neuroactive steroid site subtypes. Neuroactive steroid modulation of [35S]t-butylbicyclophosphorothionate ([35S]TBPS) binding to the GRC in rats during estrus, diestrus and after ovariectomy (OVX) was measured in washed cortical P2 homogenates in the presence or absence of exogenous GABA. During diestrus, the inability of 5 beta-pregnan-3 alpha,20 beta-diol to allosterically modulate [35S]TBPS binding in the absence of GABA coincides with the inability of 3 alpha,5 beta-P to modulate [35S]TBPS binding with high potency. In contrast, the addition of GABA to the assay produced high potency inhibition of [35S]TBPS binding by each steroid. Remarkably, although findings in diestrus and OVX homogenates were no different from those observed in males, the proportions and IC50 values of the two sites discriminated by 3 alpha,5 beta-P in [35S]TBPS binding assays during the estrus phase were significantly different from male, OVX and diestrus rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Potency of lipid and protein formulation of 5 alpha-pregnanolone at induction of anaesthesia and the corresponding regional brain distribution

We have studied the anaesthetic potencies of 5 alpha-pregnanolone albumin solution (PAS) and 5 alpha-pregnanolone Intralipid emulsion (PLE) at equivalent concentrations in male rats using an EEG threshold method. The criterion of anaesthesia was burst suppression of the EEG of 1 s or more (the "silent second" (SS)) as a sign of deep anaesthesia. The potency of the two formulations was assessed by comparing the threshold doses of 5 alpha-pregnanolone at three dose rates (1.0, 2.0 and 3.0 mg kg-1 min-1). We found that SS was initiated in all rats after infusions of PAS, while no SS could be induced in rats after infusion of PLE at a larger dose. A higher concentration of 5 alpha-pregnanolone was found in all brain and peripheral tissues of PAS-treated rats than in those treated with PLE. In rats with PAS-induced anaesthesia (3.0 mg kg-1 min-1), the highest concentrations were detected in striatum (mean 19.40 (SD 1.21) ng mg-1). Although there was a small insignificant reduction in threshold doses with dose rates at 2.0-3.0 mg kg-1 min-1, the tissue concentrations in striatum, frontal cortex and occipital cortex were found to be significantly increased. We conclude that PAS was more potent than PLE in inducing anaesthesia. Brain distribution of 5 alpha-pregnanolone varied regionally in a manner similar to the variation in GABAA receptor sensitivity to this neuroactive steroid.

Benzodiazepines and peptides stimulate pregnenolone synthesis in brain mitochondria

Mitochondria isolated from rat brain were found to cleave cholesterol to produce pregnenolone, the precursor for hormonal steroids, at a mean rate of 21.0 pmol pregnenolone.mg protein-1.min-1. This rate-limiting step in steroidogenesis was significantly stimulated by PK 11195 (1-(2-chlorophenyl)-N-methyl-(1-methylpropyl)-3-isoquinoline carboxamide) and Ro5 4864 (4'-chlorodiazepam), ligands which bind to peripheral benzodiazepine receptors with high affinity. Low-affinity ligands for the peripheral benzodiazepine receptor such as Ro15 1788 (ethyl-8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5 alpha][1,4] benzo-3-carboxylate) and clonazepam had no significant effect on the rate of pregnenolone synthesis. Furthermore, the rank order of potency of these compounds as inhibitors of [3H]Ro5 4864 binding was identical to the rank order for steroid production. Since the 86-amino acid peptide diazepam binding inhibitor is also thought to bind to the peripheral benzodiazepine receptor, four fragments of this peptide, a random sequence and steroidogenesis activator peptide were also evaluated for their ability to interact with peripheral benzodiazepine receptors and to stimulate steroidogenesis in rat brain mitochondria. Steroidogenesis activator peptide and two fragments of diazepam binding inhibitor significantly stimulated pregnenolone biosynthesis. In contrast to the peripheral benzodiazepine receptor ligands, no correlation between peptide potency in displacing [3H]Ro5 4864 binding and steroidogenesis was observed.

Differential effects of 4'-chlorodiazepam on expressed human GABAA receptors

The interactions of the atypical benzodiazepine 4'-chlorodiazepam (Ro 5-4864) with functionally expressed human GABAA receptor cDNAs were determined. Cotransfection of human alpha 2, beta 1, and gamma 2 subunits was capable of reconstituting a 4'-chlorodiazepam recognition site as revealed by a dose-dependent potentiation of t-[35S]butylbicyclophosphorothionate ([35S]-TBPS) binding to the GABA-activated chloride channel. This site is found on GABAA receptor complexes containing sites for GABA agonist-like benzodiazepines and neuroactive steroids. The importance of the alpha subunit was further demonstrated as substitution of either alpha 1 or alpha 3 for the alpha 2 subunit did not reconstitute a 4'-chlorodiazepam recognition site that was capable of modulating [35S]TBPS binding under the same experimental conditions. The 4'-chlorodiazepam modulatory site was shown to be distinct from the benzodiazepine site, but the phenylquinolines PK 8165 and PK 9084 produced effects similar to 4'-chlorodiazepam, consistent with the previous analysis of the 4'-chlorodiazepam site in brain homogenates. Further analysis of the subunit requirements revealed that coexpression of alpha 2 and beta 1 alone reconstituted a 4'-chlorodiazepam recognition site. It is interesting, however, that the 4'-chlorodiazepam site was found to inhibit [35S]TBPS binding to the GABA-activated chloride channel. Thus, the 4'-chlorodiazepam site may be reconstituted with only the alpha and beta polypeptides.

Selective actions of certain neuroactive pregnanediols at the gamma-aminobutyric acid type A receptor complex in rat brain

Certain endogenous pregnanediols (5 alpha-pregnan-3 alpha,20 alpha-diol and 5 beta-pregnan-3 alpha,20 beta-diol) were observed to have limited efficacy as allosteric modulators of t-[35S]butylbicyclophosphorothionate ([35S]TBPS) and [3H]flunitrazepam binding to sites on the gamma-aminobutyric acid (GABA)A receptor complex in rat brain. In contrast, 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha,5 alpha-P) and 3 alpha-hydroxy-5 beta-pregnan-20-one (3 alpha,5 beta-P) have full efficacy. Moreover, 3 alpha,5 beta-P but not 3 alpha,5 alpha-P recognizes high (nanomolar) and low (micromolar) affinity neuroactive steroid sites in these allosteric modulatory assays. The concentration-response curve for 3 alpha,5 alpha-P modulation of [35S]TBPS binding was shifted rightward in the presence of these pregnanediols and GABA. The maximum shift produced by these pregnanediols never exceeded the concentration-response curve obtained with 3 alpha,5 alpha-P alone in the absence of GABA. Additionally, neither 5 alpha-pregnan-3 alpha,20 alpha-diol nor 5 beta-pregnan-3 alpha,20 beta-diol had any effect on the site recognized by 3 alpha,5 alpha-P in the absence of GABA. The difference in the affinities of the two apparent sites (29 nM versus 152 nM in the presence and absence of GABA, respectively) recognized by 3 alpha,5 alpha-P is only approximately 5-fold. In contrast, the difference between the high (30 nM) and low (7 microM) affinity sites discriminated by 3 alpha,5 beta-P is > 200-fold. Thus, the selective interaction between the high affinity site recognized by 3 alpha,5 beta-P and these pregnanediols can be clearly observed. A saturating concentration of 5 beta-pregnan-3 alpha,20 beta-diol selectively eliminated the high affinity component recognized by 3 alpha,5 beta-P, whereas 5 alpha-pregnan-3 alpha,20 alpha-diol did not completely abolish the high affinity site. 5 alpha-Pregnan-3 alpha,20 alpha-diol recognized only a portion of the high affinity sites discriminated by 3 alpha,5 beta-P, relative to 5 beta-pregnan-3 alpha,20 beta-diol, whereas the two pregnanediols recognized a similar population of sites mediating 3 alpha,5 alpha-P inhibition of [35S]TBPS binding. Collectively, these studies provide evidence that the limited efficacy of certain pregnanediols as allosteric modulators of [35S]TBPS binding may be explained in part by selectivity for the high affinity site recognized by 3 alpha,5 beta-P.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuroactive steroid actions at the GABAA receptor

Neuroactive steroids are a new class of steroids that do not interact with any of the classical cytosolic hormonal steroid receptors. The most well-documented examples are those that interact with the gamma-aminobutyric acidA (GABAA) receptor/chloride channel complex in the central nervous system. The GABAA receptors are known to contain allosteric modulatory sites for therapeutically useful drugs such as benzodiazepines (BZs) and barbiturates. The interaction of neuroactive steroids with the GABAA receptor is specific to a site on the receptor complex distinct from the benzodiazepine and barbiturate modulatory sites. Neuroactive steroids exist endogenously; the examples are metabolites of progesterone and deoxycorticosterone, 3 alpha-hydroxy-5 alpha-pregnane-20-one, and 5 alpha-pregnane-3 alpha, 21 alpha-dihydroxy-20-one, respectively, and their 5 beta stereoisomers. The GABAA receptor agonist-like effects that these neuroactive steroids produce in vivo are similar, but not identical, to those of BZs and barbiturates. Representatives of all three classes of modulators are active as sedative-hypnotics, anticonvulsants, and anxiolytics in animal models. Because of the heterogeneity of GABAA receptors and their differential distribution in the brain, dissimilar in vivo pharmacological profiles displayed by BZs, barbiturates and neuroactive steroids are not surprising. Studies of neuroactive steroid interactions with the GABAA receptor revealed a unique subset of these steroids that modulate the receptor with limited efficacy. Another endogenously occurring progesterone metabolite, 5 alpha-pregnane-3 alpha,20 alpha-diol, is an example of this subset of neuroactive steroids. At present, it is not clear whether the observed limited efficacy is due to receptor subtype selectivity, partial agonist activity or both.(ABSTRACT TRUNCATED AT 250 WORDS)

The neuroactive steroid 3 alpha-hydroxy-5 beta-pregnan-20-one is a two-component modulator of ligand binding to the GABAA receptor

Neuroactive steroids allosterically inhibit [35S]t-butylbicyclophosphorothionate ([35S]TBPS) and enhance [3H]flunitrazepam binding to the GABAA receptor complex. In the presence of 5 microM GABA, 3 alpha-hydroxy-5 beta-pregnan-20-one (3 alpha, 5 beta-P) inhibits [35S]TBPS binding with high- (IC50 21-32 nM) and low- (IC50 24-63 microM) affinity components in bovine cortical, cerebellar, and hippocampal membranes. The percentage of high-affinity sites ranges from 53% in cortex to 65% in cerebellum and hippocampus. However, 3 alpha, 5 beta-P is a single-site inhibitor in thalamus (IC50 43 nM). In the absence of GABA, similar affinities for the high- and low-affinity components were detected, although the percentages of high-affinity sites were reduced. Similarly, 3 alpha, 5 beta-P enhances [3H]flunitrazepam binding with high- (EC50 44-58 nM) and low- (EC50 2-13 microM) affinity components which account for 71-77% and 23-29% of the sites, respectively, in cortex, cerebellum and hippocampus. 3 alpha, 5 beta-P is a single-site enhancer in thalamus (EC50 80 nM). In contrast to 3 alpha,5 beta-P, 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha,5 alpha-P) is a single site modulator of [35S]TBPS and [3H]flunitrazepam binding in all regions examined. These data provide pharmacological evidence consistent with receptor heterogeneity for neuroactive steroids.

The estrus cycle, sensitivity to convulsants and the anticonvulsant effect of a neuroactive steroid

Recent in vitro work in our laboratory suggests that functional sensitivity of the gamma-aminobutyric acidA receptor complex to the neuroactive progesterone metabolite 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha,5 alpha-P) changes during the estrus cycle. Therefore, the current in vivo studies were conducted to evaluate estrus cycle-related differences in sensitivity to convulsants and the anticonvulsant effect of 3 alpha, 5 alpha-P. The threshold dose for onset to myoclonic twitch, running bouncing clonus and tonic hind-limb extension was measured by constant i.v. infusion of (+)-bicuculline, picrotoxin, pentylenetetrazol, strychnine and methyl-6,7-dimethoxy-4-ethyl-beta- carboline-3-carboxylate. Females in estrus were more sensitive than females in diestrus 1 or males to (+)-bicuculline and methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3- carboxylate. Administration of 3 alpha, 5 alpha-P (15 mg/kg i.p. in beta-cyclodextrin) 15 min before infusion of pentylenetetrazol significantly increased the threshold dose for onset to all three convulsions and provided equal protection against tonic convulsions. The dose for onset to myoclonic twitch was significantly higher in females in diestrus 1 than females in estrus or males. Plasma 3 alpha, 5 alpha-P did not differ between groups injected with 3 alpha, 5 alpha-P, suggesting that the difference in sensitivity to the anticonvulsant effect of 3 alpha, 5 alpha-P was not pharmacokinetic.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison of Ro 16-6028 with benzodiazepine receptor 'full agonists' on GABAA receptor function

Ro 16-6028 (bretazenil) has a pharmacological profile characteristic of a partial agonist at the gamma-aminobutyric acidA (GABAA) receptor-linked benzodiazepine site. The present study utilized modulation of [35S]t-butylbicyclophosphorothionate ([35S]TBPS) binding and enhancement of GABA-stimulated 36Cl- uptake to further assess Ro 16-6028's partial agonist profile in vitro. Ro 16-6028 was the most potent benzodiazepine examined, exhibiting an IC50 (concentration at which half-maximal inhibition of specific [35S]TBPS binding occurs) of 6.1 nM, compared to clonazepam (7.9 nM), flunitrazepam (13.6 nM) and diazepam (91.1 nM). The rank order of potency for inhibition of [35S]TBPS binding was identical to that for inhibition of [3H]flunitrazepam binding. However, Ro 16-6028 was less efficacious in that it produced 27% inhibition of specific [35S]TBPS binding, compared to clonazepam (34%), flunitrazepam (41%) or diazepam (49%). Ro 16-6028 antagonized the inhibition of [35S]TBPS binding produced by 10 microM diazepam. Ro 16-6028 was also more potent and less efficacious than diazepam in potentiating GABA-stimulated 36Cl- uptake. These results provide further evidence that Ro 16-6028 is acting as a partial agonist at the benzodiazepine receptor in modulating function of the GABAA receptor complex.

The influence of estrus cycle on neurosteroid potency at the gamma-aminobutyric acidA receptor complex

The reduced metabolites of progesterone (3 alpha-hydroxy-5 alpha-pregnan-20-one or 3 alpha,5 alpha-P) and deoxycorticosterone (3 alpha,21-dihydroxy-5 alpha-pregnan-20-one or 5 alpha-THDOC) exhibit a gamma-aminobutyric acid (GABA) agonist pharmacological profile and bind with high affinity to a unique steroid recognition site on the GABAA receptor complex (GRC). In female rats, peak levels of 3 alpha,5 alpha-P occur between proestrus and estrus and at concentrations which appear sufficient to affect GRC-mediated inhibitory events. Therefore, steroid modulation of the binding of [35S]t-butylbicyclophosphorothionate was conducted to examine sensitivity of the GRC during the estrus cycle. The results in unwashed tissue (cortex, cerebellum, hippocampus and striatum) indicated that 3 alpha, 5 alpha-P was most potent in estrus. Subsequent studies in washed tissue and in washed tissue plus 3 microM (+)bicuculline indicated that 3 alpha,5 alpha-P was more potent in diestrus 1 than in estrus. Brain regional differences in potency of 3 alpha,5 alpha-P were also observed. Overall, the results in washed tissue suggest that an inherent change in the sensitivity of the GRC occurs during the estrus cycle, whereas the results in unwashed tissue may be influenced by the presence of endogenous GRC-active neurosteroids and GABA. The change in sensitivity of the GRC during the estrus cycle may occur to help maintain homeostatic regulation of brain excitability.

Complex interactions between the steroid derivative RU 5135 and the GABAA-receptor complex

The modulation of [35S]t-butylbicyclophosporothionate ([35S]TBPS) binding was used to evaluate the actions of the steroid derivative RU 5135 at the gamma-aminobutyric acidA (GABAA) receptor complex. The inhibition of [35S]TBPS binding by GABA in the presence of various concentrations of RU 5135 was consistent with the hypothesis that RU 5135 is a competitive antagonist at the GABAA receptor. Despite common structural features (i.e., 3 alpha-hydroxylated, 5 beta-reduced A ring) with GABAA receptor-active neurosteroids, RU 5135 did not appear to be competitive at the putative steroid site on the GABAA receptor-active, as demonstrated by Schild analysis of 5 alpha-pregnane-3 alpha-ol-20-one (3 alpha,5 alpha-P) modulation of [35S]TBPS binding in the presence of different concentrations of RU 5135. On the other hand, the reduced potency of 3 alpha,5 alpha-P as an inhibitor of [35S]TBPS binding in the presence of RU 5135, as well as blockade of 5 alpha-pregnane-3 alpha-20 alpha-diol (5 alpha-pregnanediol) inhibition of [35S]TBPS binding by RU 5135 provide further support for the GABAA receptor antagonist properties of RU 5135. Moreover, this amidine steroid was able to partially inhibit [35S]TBPS binding independent of GABA with nanomolar potency; yet the mechanism by which this occurs remains to be determined.

Gamma-aminobutyric acidA receptor complexes in rat frontal cortex and spinal cord show differential responses to steroid modulation

Regional differences in neuroactive steroid modulation of the gamma-aminobutyric acidA receptor-chloride ionophore complex (GBRC), as measured by t-butylbicyclophosphoro[35S]thionate ([35S] TBPS) binding and 36Cl- uptake, were demonstrated in rat spinal cord versus frontal cortex. The rank order of potencies of a series of 5 alpha- and 5 beta-reduced isomers of 3 alpha-hydroxylated steroids against [35S]TBPS binding were different between regions. The differences in rank order of potencies imply the possible existence of heterogeneous populations of GBRC-coupled steroid recognition sites. The relative potencies of selected 5 alpha- and 5 beta-reduced isomers as potentiators of 36Cl- uptake paralleled their potencies as inhibitors of [35S]TBPS binding. Differential sensitivity of the steroid recognition site to the allosteric influence of gamma-aminobutyric acid was also demonstrated. It appears that regionally specific responses to GBRC-active steroids do occur, although the functional consequences of these effects await evaluation in appropriate in vivo models.

Anxiolytic activity of the progesterone metabolite 5 alpha-pregnan-3 alpha-o1-20-one

3 alpha-hydroxylated pregnane steroids have been shown to possess anesthetic, hypnotic, anticonvulsant and anxiolytic properties. In this study, metabolites of progesterone and deoxycorticosterone, 5 alpha-pregnan-3 alpha-o1-20-one (3 alpha-OH-DHP) and 5 alpha-pregnan-3 alpha,21-diol-20-one (5 alpha-THDOC), respectively, were tested for anxiolytic effects in N.I.H. Swiss-Webster mice using the light/dark transition, open-field and lick-suppression tests. Similar to the benzodiazepine (BZ) diazepam, 3 alpha-OH-DHP (5-40 mg/kg) and 5 alpha-THDOC (5-40 mg/kg) significantly increased the number of light/dark transitions. 3 alpha-OH-DHP's effects were stereospecific as its diasteriomer, 3 beta-OH-DHP was devoid of activity. The benzodiazepine antagonist CGS-8216 (10 mg/kg) blocked diazepam's (1.0 mg/kg) anxiolytic effects, but did not have any effect against 3 alpha-OH-DHP (20 mg/kg). The data indicate that the pregnane steroids produce their anxiolytic effects through a separate mechanism than the BZs. 3 alpha-OH-DHP (20 mg/kg), 5 alpha-THDOC (20 mg/kg) and diazepam (1.0 mg/kg) increased activity in a open-field test. 3 beta-OH-DHP had no effect in the open-field test. Furthermore, 3 alpha-OH-DHP produced a 235% increase in punished responding in a lick-suppression test. These results demonstrate that the endogenous pregnane steroids possess anxiolytic effects that may be clinically relevant.

Differential responses of expressed recombinant human gamma-aminobutyric acidA receptors to neurosteroids

Neuroactive steroids, in particular 3 alpha-hydroxypregnanes, are allosteric modulators of the gamma-aminobutyric acidA (GABAA) receptor. Regionally selective expression of receptor subunit subtypes may account for differential responsiveness of tissues to GABAergic inhibition and neurosteroid modulatory effects. The effect of 5 alpha-pregnan-3 alpha-ol-20-one (epiallopregnanolone) on heterotropic cooperativity on the GABAA receptor complex has been studied in three subtypes of expressed recombinant human receptors and in rat brain and spinal cord. Steroid potentiation of [3H]flunitrazepam binding was greatest for the alpha 3 beta 1 gamma 2 receptor complex, whereas alpha 1 beta 1 gamma 2 and alpha 2 beta 1 gamma 2 complexes showed less than 100% enhancement in binding. Previous studies suggest that the spinal cord is devoid of alpha 1, whereas cerebellum is rich in alpha 1 subunits. Correspondingly, a differential enhancement of [3H]flunitrazepam binding in spinal cord (51%) versus cerebellum (28%) was also observed. The structure of neuroactive steroids is important in determinikng the extent of neuromodulatory activity. The 5 beta-pregnanes,5 beta-pregnan-3 alpha-ol-20-one (epipregnanolone) and 5 beta-pregnan-3 alpha,21-diol-20-one (5 beta-tetrahydrodeoxycorticosterone), were both less potent than their corresponding 5 alpha derivatives. A 3 alpha hydroxyl group is essential for neuromodulatory activity in the expressed receptors, as demonstrated by the observation that 5 alpha-pregnan-3 beta-ol-20-one (allopregnanolone) and 4-pregnen-3, 20-dione (progesterone) were both inactive.(ABSTRACT TRUNCATED AT 250 WORDS)

Anticonvulsant steroids and the GABA/benzodiazepine receptor-chloride ionophore complex

The ability of steroids to influence brain excitability is well documented. Certain 3 alpha-hydroxylated pregnanes are known to possess anticonvulsant and sedative-hypnotic/anesthetic properties. It has been observed that the seizure susceptibility in menstruating women with catamenial epilepsy appears to be correlated with changes in ovarian steroid levels. However, the underlying mechanism of these steroid influences on brain activity has only been recently revealed by pharmacological studies. These studies have provided compelling evidence for the presence of a novel steroid recognition site on the GABAA-benzodiazepine receptor complex (GBRC). Steroids may interact with this site with high affinity and stereospecificity to enhance chloride channel conductance in a manner similar to that produced by benzodiazepines (BZs) or barbiturates. The existence of such a steroid site on the GBRC is further supported by recent experiments involving the transfection of GABAA receptor cDNAs into a human embryonic kidney cell line. Based on the knowledge of the structure-activity requirements for the interaction of steroids with this novel recognition site, it is conceivable that the development of new anticonvulsant steroids with high therapeutic indices can be achieved.

Heterotropic cooperativity between putative recognition sites for progesterone metabolites and the atypical benzodiazepine Ro 5-4864

The binding of the cage convulsant t-butylbicyclophosphorothionate (TBPS) and 36Cl- uptake by synaptoneurosomes were used to test the ability of progesterone metabolites to modulate allosterically the Ro 5-4864 (4'-chlorodiazepam) binding site that is functionally coupled to the gamma-aminobutyric acid (GABA)/benzodiazepine receptor complex (GBRC) in rat brain. Dose-dependent enhancement of [35S]TBPS binding by Ro 5-4864 occurs in rat cerebral cortex in the presence of the progesterone metabolites 5 alpha-pregnan-3 alpha-ol-20-one (3 alpha-OH-DHP) and 5 alpha-pregnan-3 alpha, 20 alpha-diol (pregnanediol). The pregnanediol effect is completely GABA dependent, whereas that of 3 alpha-OH-DHP is not. Conversely, Ro 5-4864 opposed the action of 3 alpha-OH-DHP by increasing the IC50 for 3 alpha-OH-DHP inhibition of [35S]TBPS binding. In cortical synaptoneurosomes, Ro 5-4864 antagonized both 3 alpha-OH-DHP and pregnanediol enhancement of GABA-stimulated 36Cl- uptake. In both binding and functional studies, pregnanediol showed limited efficacy relative to 3 alpha-OH-DHP, as previously reported. These findings provide the initial evidence that the GBRC-linked Ro 5-4864 binding site is allosterically coupled to the putative progesterone metabolite recognition site and confirm the GABA-mimetic properties of 3 alpha-OH-DHP and pregnanediol.

A steroid recognition site is functionally coupled to an expressed GABA(A)-benzodiazepine receptor

Pregnane steroids, particularly 3 alpha-hydroxylated metabolites of progesterone, are known to have rapid and profound effects on brain excitability. Recent evidence suggests that the gamma-aminobutyric acid (GABA(A))-benzodiazepine receptor-Cl- ionophore complex may mediate these actions. The data further suggest that these steroids modulate the complex through a novel site independent of other known sites on the complex. The hypothesis that this site is on the GABA(A)-benzodiazepine receptor-Cl- ionophore complex is tested in the present study by determining its presence on transiently expressed GABAA-benzodiazepine receptors.

5 alpha-pregnan-3 alpha,20 alpha-diol behaves like a partial agonist in the modulation of GABA-stimulated chloride ion uptake by synaptoneurosomes

In rat cortical synaptoneurosomes, the maximum potentiation of GABA-stimulated 36Cl uptake produced by 5 alpha-pregnan-3 alpha,20 alpha-diol (5 alpha-pregnanediol) is significantly less than that elicited by 5 alpha-pregnan-3 alpha-ol-20-one (3 alpha-OH-DHP). This observation suggests that 5 alpha-pregnanediol may be a partial agonist whereas 3 alpha-OH-DHP acts as a full agonist at a common site on or near the GABAA/benzodiazepine receptor-chloride ionophore complex (GBRC). This hypothesis is supported by the finding that 5 alpha-pregnanediol will antagonize in a dose-dependent manner the enhancement of GABA-stimulated 36Cl uptake produced by 3 alpha-OH-DHP under certain conditions. Collectively, these findings support the hypothesis that GBRC-active progesterone metabolites with varying degrees of efficacy exist as reflected by their differential ability to potentiate 36Cl uptake in brain synaptoneurosomes.

Anticonvulsant profile of the progesterone metabolite 5 alpha-pregnan-3 alpha-ol-20-one

5 alpha-Pregnan-3 alpha-ol-20-one (3 alpha-OH-DHP) is a naturally occurring metabolite of progesterone that can modulate brain excitability through a specific steroid recognition site on the GABA/benzodiazepine receptor-chloride ionophore complex. The anticonvulsant properties of 3 alpha-OH-DHP were determined using standardized anticonvulsant screening tests in mice. This steroid was found to be effective against metrazol-, (+)-bicuculline- and picrotoxin-induced seizures. The steroid has maximum potency against (+)-bicuculline-induced convulsions and no activity against maximal electroshock and strychnine-induced seizures. These findings support the hypothesis that therapeutically useful anticonvulsant steroids active at the putative steroid recognition site associated with the GABA/benzodiazepine receptor-chloride ionophore complex can be identified.

Complex interactions between pregnenolone sulfate and the t-butylbicyclophosphorothionate-labeled chloride ionophore in rat brain

Detailed studies of the interactions between the 'neuroactive' steroid, pregnenolone sulfate (PS), and the gamma-aminobutyric acid (GABA)/benzodiazepine receptor-linked chloride ionophore (GBRC) labeled by [35S]t-butylbicyclophosphorothionate ([35S]TBPS) in the rat brain reveal a site of action allosteric to the TBPS-labeled site as demonstrated by the ability of PS to accelerate TBPS-initiated dissociation of [35S]TBPS. In contrast to previous findings, PS modulates [35S]TBPS binding in a GABA 'agonist'-like fashion with micromolar potencies. The role of PS in the regulation of neuronal excitability through the GBRC is questioned, based upon the observation that brain concentrations of PS are 2-3 orders of magnitude less than that necessary for the modulation of chloride conductance in vitro.

Steroid modulation of the GABA/benzodiazepine receptor-linked chloride ionophore

Recent findings suggest that steroids with sedative-hypnotic properties interact specifically with the gamma-aminobutyric acidA/benzodiazepine receptor-chloride ionophore complex (GBRC). They show positive heterotropic cooperativity by allosterically enhancing the binding of GABA agonists and the clinically useful benzodiazepines (BZs) to their respective recognition sites. These steroids have stringent structural requirements for activity at the GBRC, with the essential requirements for high potency being a 3 alpha-hydroxyl group and a 5 alpha-reduced A-ring. Some of these steroids are naturally occurring metabolites of progesterone and deoxycorticosterone and have nanomolar potencies as potentiators of chloride channel conductance. These 3 alpha-hydroxylated, 5 alpha-reduced steroids do not act through any known sites on the GBRC. Thus, the exact site and mechanism of action remain to be determined. Together with the observation that physiological levels of these metabolites are sufficient to influence the function of the GBRC, the evidence clearly suggests a role for these steroids in the normal regulation of brain excitability by potentiating the postsynaptic effects of gamma-aminobutyric acid (GABA). Pharmacological studies of the GBRC-active steroids show that they possess anxiolytic and anticonvulsant activities. The potential therapeutic application of these steroids in the treatment of mood disorders and catamenial exacerbation of seizures associated with the menstrual cycle is discussed. Collectively, the evidence from the studies of these steroids imply that another mechanism by which the endocrine system influences brain function has been identified. Its characterization will provide important insight into how steroids modulate brain excitability under normal and pathophysiological states.

Steroid modulation of the chloride ionophore in rat brain: structure-activity requirements, regional dependence and mechanism of action

Further in vitro studies of steroids active at the gamma-aminobutyric acidA (GABAA) receptor regulated Cl- channel labeled by [35S]-t-butylbicyclophosphorothionate ([35S]TBPS) reveal additional structural requirements necessary for activity. Evaluation of selected steroids for activity against TBPS-induced convulsions show similar requirements for activity. Interestingly, steroids (e.g., 5 alpha-pregnan-3 alpha, 20 alpha-diol) were identified that have high potency but limited efficacy as modulators of [35S]TBPS binding. These characteristics are reminiscent of the clinically useful benzodiazepines (BZs) such as clonazepam. However, interactions between the prototypical anesthetic-barbiturate, sodium pentobarbital, and steroids active at the Cl- channel suggest that they do not share a common site of action as allosteric modulators of [35S]TBPS and BZ receptor binding. The most potent steroid evaluated, 5 alpha-pregnan-3 alpha-ol-20-one, modulates [35S]TBPS binding at low concentrations (IC50 approximately 17 nM) in a regionally dependent manner. All [35S]TBPS binding sites appear to be functionally coupled to a steroid "modulatory site." Because several of the active steroids are metabolites of progesterone, their ability to inhibit the binding of [3H]promegestrone to the cytosolic progestin receptor in rat uterus was evaluated. Those steroids showing potent activity at the GABAA receptor-Cl- ionophore were inactive at the intracellular progestin receptor. Such specificity coupled with their high potency provide additional support for the hypothesis that some of these steroids may be involved in the homeostatic regulation of brain excitability via the GABAA-BZ receptor complex.

Regional distribution of a Ro5 4864 binding site that is functionally coupled to the gamma-aminobutyric acid/benzodiazepine receptor complex in rat brain

The hypothesis that a novel drug binding site linked to a gamma-aminobutyric acid (GABA)-regulated chloride ionophore mediates the excitatory effects of the atypical benzodiazepine (BZ) Ro5 4864 is further evaluated in the present study. Dose-dependent inhibition of [3H]flunitrazepam to the central BZ receptor in rat cerebral cortex by the cage convulsant t-butylbicyclophosphorotionate (TBPS) is modulated by Ro5 4864 and the isoquinoline PK 11195 in a manner consistent with their reported pro/anticonvulsant effects. The ability of Ro5 4864 to enhance the binding of [35S]TBPS to a GABA-regulated chloride ionophore in rat cortex is unchanged after the irreversible labeling of the central BZ receptor by the photoaffinity label Ro15 4513. Together, these observations further suggest that 1) the effect of Ro5 4864 on [35S]TBPS is not mediated by the central BZ receptor and 2) the Ro5 4864 binding site is allosterically coupled to the GABA/BZ receptor-chloride ionophore complex in rat cerebral cortex. Anatomical localization of Ro5 4864-stimulated [35S]TBPS binding in rat brain by autoradiography reveals a distribution of chloride ionophore-coupled Ro5 4864 sites which is in many instances similar to that of the GABA/BZ receptor-chloride ionophore complex. These studies lend additional support to the postulate that this drug binding site represents an additional locus for the regulation of GABAergic neurotransmission in the central nervous system.

Ro15-4513 differentially affects ethanol-induced hypnosis and hypothermia

Recent reports of in vivo antagonism of the acute anti-conflict, ataxic, and lethal effects of ethanol with the partial inverse agonist imidazobenzodiazepine Ro15-4513 implicate the GABAa/benzodiazepine receptor in mediating several acute effects of ethanol. These data raise the question as to whether all of the acute effects of ethanol can be antagonized by Ro15-4513. The present study addressed this issue by investigating the effect of Ro15-4513 on ethanol-induced loss of righting reflex and hypothermia in mice. We found that Ro15-4513 antagonized the hypnotic, but not the hypothermic, effects of ethanol when tested under identical conditions. These results suggest that the GABAa/benzodiazepine receptor may not be involved in mediating all pharmacological effects of ethanol.

GABA-dependent modulation of the Cl- ionophore by steroids in rat brain

Steroids inhibit the binding of [35S]t-butylbicyclophosphorothionate ([ 35S]TBPS) to the GABAA-benzodiazepine receptor (GBR) linked Cl- ionophore in a GABA dependent manner but not through the GABAA receptor. The most potent steroid evaluated is a naturally occurring metabolite of progesterone, 3 alpha-hydroxy,5 alpha-dihydroprogesterone with an IC50 of approximately 17 nM. Structural requirements necessary for inhibitory activity coincide with those reported for anticonvulsant and anesthetic actions. Coupled with earlier evidence that these steroids do not act directly at the benzodiazepine receptor nor the [35S]TBPS labeled site to modulate the Cl- ionophore, the possibility is proposed that a distinct membrane-bound 'steroid site' coupled to the GBR-Cl- ionophore complex exists.

Phenylquinolines PK 8165 and PK 9084 allosterically modulate [35S]t-butylbicyclophosphorothionate binding to a chloride ionophore in rat brain via a novel Ro5 4864 binding site

The binding of the cage convulsant, [35S]t-butylbicyclophosphorothionate ([35S]TBPS), to a picrotoxin-sensitive site in rat cerebral cortical homogenates was used to identify and characterize the site of action of the phenylquinolines PK 8165 and PK 9084, the isoquinoline PK 11195 and the atypical benzodiazepine (BZ) Ro5 4864. These agents were found to allosterically modulate the binding of 2 nM [35S]TBPS in a pharmacologically relevant fashion. Evidence is presented to suggest that these compounds share a common site of action as modulators of [35S]TBPS binding. The relative potencies of these compounds in vitro are in the submicromolar to micromolar concentration range and correlate well with the concentrations reported to elicit specific responses in behavioral and electrophysiologic studies. Modulation of [35S]TBPS binding in vitro is affected by micromolar quantities of gamma-aminobutyric acid in a (+)-bicuculline-sensitive fashion and is unaffected by the central BZ receptor "antagonist" Ro15 1788. Collectively, the evidence presented suggests the existence of a novel drug binding site that is functionally coupled to a gamma-aminobutyric acid-A receptor and a [35S]TBPS-labeled chloride ionophore. Moreover, this site is distinct from the central BZ receptor recognized by clonazepam and the high-affinity peripheral BZ binding site labeled by [3H] Ro5 4864. The hypothesis is proposed that the novel "Ro5 4864 site" identified in the present study is a functionally relevant binding site that mediates some of the pharmacologic effects of Ro5 4864, PK 8165, PK 9084 and PK 11195 in the mammalian central nervous system.

Modulation of the chloride ionophore by benzodiazepine receptor ligands: influence of gamma-aminobutyric acid and ligand efficacy

t-Butylbicyclophosphorothionate (TBPS) produces dose-dependent enhancement of [3H]propyl beta-carboline-3-carboxylate ([3H]PCC, 40 pM) binding to the benzodiazepine1 (BZ1) receptor subtype in hippocampus. Furthermore, TBPS enhancement of [3H]PCC binding was antagonized by micromolar concentrations of gamma-aminobutyric acid (GABA) in a way reversible by bicuculline. BZ receptor ligands that are "GABA positive" (i.e., enhance GABA neurotransmission) allosterically inhibited [35S]TBPS binding, whereas "GABA-negative" ligands (i.e., inhibit GABA neurotransmission) produced the opposite effect. The efficacy of the ligands as modulators of [35S]TBPS binding was consistent with their reported in vivo pharmacology. The effects of positive and negative ligands on [35S]TBPS binding were modulated by micromolar concentrations of GABA. Examination of the kinetics of [35S]TBPS binding suggested the presence of slowly and rapidly dissociating components. The GABA-positive clonazepam stabilized the rapidly dissociating component of [35S]TBPS binding, whereas methyl beta-carboline-3-carboxylate had a similar effect on the slowly dissociating component. It is speculated that the slowly dissociating component of [35S]TBPS binding is associated with a closed chloride channel, whereas the opposite is proposed for the rapidly dissociating component. The differential effects of GABA-positive versus GABA-negative ligands on [35S]TBPS binding and the modulatory effect of GABA provide further evidence to suggest that [35S]TBPS labels a site near the chloride ionophore linked to the GABA-BZ receptor complex.

t-[3H]butylbicycloorthobenzoate: new radioligand probe for the gamma-aminobutyric acid-regulated chloride ionophore

t-[3H]Butylbicycloorthobenzoate [( 3H]TBOB; 22 Ci/mmol) was prepared by reductive dechlorination of its 4-chlorophenyl analog with tritium gas. This new radioligand binds reversibly to fresh washed rat brain P2 membranes in 500 mM NaCl plus 50 mM sodium-potassium phosphate buffer (pH 7.4) at 25 degrees C, with 80-90% specific relative to total binding, a KD of 61 +/- 15 nM, and a Bmax of 1.6 +/- 0.5 pmol/mg of protein. [3H]TBOB association with its binding site(s) is monophasic, but its dissociation is biphasic. The binding characteristics of [3H]TBOB are essentially identical to those of t-[35S]butylbicyclophosphorothionate [( 35S]TBPS) with respect to pH dependence, stimulation by anions, regional distribution in the brain, and pharmacological profile. Saturation analyses and dissociation studies further indicate that TBOB and TBPS have a common binding site. However, binding of the two radioligands differs in respect to temperature effects. In contrast to [35S]TBPS, which exhibits negligible binding at 0 degrees C, [3H]TBOB binds to rat brain membranes at 0, 25, and 37 degrees C with similar KD values. [3H]TBOB with its long radioactive half-life and temperature-independent KD is a valuable supplement to [35S]TBPS in further biochemical and pharmacological characterization of the gamma-aminobutyric acid receptor-ionophore complex.

Regional distribution of putative vasopressin receptors in rat brain and pituitary by quantitative autoradiography

Quantitative light microscopic autoradiography was used to map and characterize the distribution of [3H]arginine vasopressin [( 3H]AVP) binding sites in the rat brain. HPLC analysis for possible degradation of AVP during binding indicated that addition of specific peptidase inhibitors prevented metabolism of AVP. Binding sites for [3H]AVP were observed in the hypothalamus and pituitary as well as in brain regions where AVP may act as a neuroregulator. Within the hypothalamus, dense AVP binding sites were seen in the suprachiasmatic, supraoptic, and paraventricular nuclei. High specific binding was also apparent in the median eminence tubero-infundibular region and in the posterior lobe of the pituitary. [3H]AVP labeling at possible neuroregulatory sites was observed in the hippocampus, lateral septum, superficial cortex, cerebellum, nucleus tractus solitarious, adenohypophysis, and spinal cord.

Benzodiazepine receptor heterogeneity: possible molecular basis and functional significance

The pharmacological actions of the benzodiazepines (BZs) are thought to be mediated through specific receptor sites in the mammalian central nervous system. Characterization of these receptor sites in the brain has yielded evidence for heterogeneity of BZ receptor sites. Current theories on the molecular basis of the apparent BZ receptor heterogeneity and the possible functional significance of BZ receptor subtypes are presented. Studies of BZ receptor heterogeneity have provided insights into the molecular events that may be responsible for BZ modulation of gamma-aminobutyric-ergic function.

Benzodiazepine anticonvulsant action: gamma-aminobutyric acid-dependent modulation of the chloride ionophore

Brain-specific benzodiazepine receptors are allosterically coupled to chloride ionophore-associated binding sites for sulfur-35-labeled t-butylbicyclophosphorothionate. The specific binding of t-butylbicyclophosphorothionate to fresh unwashed rat cortical membranes is inhibited by nanomolar concentrations of five benzodiazepine agonists but not by the antagonist Ro 15-1788. Their inhibitory potencies in this assay are closely related to their antimetrazol activities. Studies with solubilized receptor-complex preparations establish an absolute requirement for gamma-aminobutyric acid (3 to 10 microM), strongly suggesting that the antagonism of metrazol-induced seizures by the benzodiazepines involves an action on the chloride ionophore mediated through the low affinity gamma-aminobutyric acid receptor.