Structural requirements of steroidal agonists of transient receptor potential melastatin 3 (TRPM3) cation channels

Background and Purpose

Transient receptor potential melastatin 3 (TRPM3) proteins form non-selective but calcium-permeable membrane channels, rapidly activated by extracellular application of the steroid pregnenolone sulphate and the dihydropyridine nifedipine. Our aim was to characterize the steroid binding site by analysing the structural chemical requirements for TRPM3 activation.

Experimental Approach

Whole-cell patch-clamp recordings and measurements of intracellular calcium concentrations were performed on HEK293 cells transfected with TRPM3 (or untransfected controls) during superfusion with pharmacological substances.

Key Results

Pregnenolone sulphate and nifedipine activated TRPM3 channels supra-additively over a wide concentration range. Other dihydropyridines inhibited TRPM3 channels. The natural enantiomer of pregnenolone sulphate was more efficient in activating TRPM3 channels than its synthetic mirror image. However, both enantiomers exerted very similar inhibitory effects on proton-activated outwardly rectifying anion channels. Epiallopregnanolone sulphate activated TRPM3 almost equally as well as pregnenolone sulphate. Exchanging the sulphate for other chemical moieties showed that a negative charge at this position is required for activating TRPM3 channels.

Conclusions and Implications

Our data demonstrate that nifedipine and pregnenolone sulphate act at different binding sites when activating TRPM3. The latter activates TRPM3 by binding to a chiral and thus proteinaceous binding site, as inferred from the differential effects of the enantiomers. The double bond between position C5 and C6 of pregnenolone sulphate is not strictly necessary for the activation of TRPM3 channels, but a negative charge at position C3 of the steroid is highly important. These results provide a solid basis for understanding mechanistically the rapid chemical activation of TRPM3 channels.

Cyclodextrins sequester neuroactive steroids and differentiate mechanisms that rate limit steroid actions

BACKGROUND AND PURPOSE

Neuroactive steroids are potent modulators of GABA(A) receptors and are thus of interest for their sedative, anxiolytic, anticonvulsant and anaesthetic properties. Cyclodextrins may be useful tools to manipulate neuroactive effects of steroids on GABA(A) receptors because cyclodextrins form inclusion complexes with at least some steroids that are active at the GABA(A) receptor, such as (3alpha,5alpha)-3-hydroxypregnan-20-one (3alpha5alphaP, allopregnanolone).

EXPERIMENTAL APPROACH

To assess the versatility of cyclodextrins as steroid modulators, we investigated interactions between gamma-cyclodextrin and neuroactive steroids of different structural classes.

KEY RESULTS

Both a bioassay based on electrophysiological assessment of GABA(A) receptor function and optical measurements of cellular accumulation of a fluorescent steroid analogue suggest that gamma-cyclodextrin sequesters steroids rather than directly influencing GABA(A) receptor function. Neither a 5beta-reduced A/B ring fusion nor a sulphate group at carbon 3 affected the presumed inclusion complex formation between steroid and gamma-cyclodextrin. Apparent dissociation constants for interactions between natural steroids and gamma-cyclodexrin ranged from 10-60 microM. Although gamma-cyclodextrin accommodates a range of natural and synthetic steroids, C(11) substitutions reduced inclusion complex formation. Using gamma-cyclodextrin to remove steroid not directly bound to GABA(A) receptors, we found that cellular retention of receptor-unbound steroid rate limits potentiation by 3alpha- hydroxysteroids but not inhibition by sulphated steroids.

CONCLUSIONS AND IMPLICATIONS

We conclude that gamma-cyclodextrins can be useful, albeit non-specific, tools for terminating the actions of multiple classes of naturally occurring neuroactive steroids.

5alpha-reduced neuroactive steroids alleviate thermal and mechanical hyperalgesia in rats with neuropathic pain

5alpha-reduced neuroactive steroids with selective modulatory action in vitro on T or combined modulatory action on T and GABA(A) currents present in peripheral sensory neurons have been shown to induce potent peripheral analgesia in vivo in intact animals. Although the role of T and GABA(A) currents in pathophysiology of neuropathic pain (NPP) is not established, it appears that blockade of T currents and/or potentiation of GABA(A) currents could be beneficial in the management of NPP. To study the potential usefulness of 5alpha-reduced neuroactive steroids in alleviating NPP, we selected two newly synthesized steroids-ECN and CDNC24-with a selective blocking effect on T currents and a selective potentiating effect on GABA(A) currents, respectively, and commercial analogs-alphaxalone and 3alpha5alphaP-with the effects on both ion channels. We used a sciatic nerve ligation model to induce thermal and mechanical hyperalgesia in adult rats and tested peripheral thermal and mechanical nociception following local injection of neuroactive steroids into the peripheral receptive fields of a ligated hind paw. We found that 5alpha-reduced neuroactive steroids alleviate thermal and mechanical hyperalgesia in NPP rats. ECN and CDNC24 were more selective in alleviating thermal nociception in NPP than in sham animals when compared to 3alpha5alphaP and alphaxalone although the anti-nociceptive effect induced by 3alpha5alphaP and alphaxalone was more profound. CDNC24 was most selective since it had very minimal anti-nociceptive effect in sham animals but a very profound anti-nociceptive effect in NPP animals suggesting that, under pathological conditions, peripheral GABA(A) receptors might be an attractive therapeutic target.

Epipregnanolone and a novel synthetic neuroactive steroid reduce alcohol self-administration in rats

This study was designed to compare the effects of several neuroactive steroids with varying patterns of modulation of gamma-aminobutyric acid (GABA)(A) and NMDA receptors on operant self-administration of ethanol or water. Once stable responding for 10% (w/v) ethanol was achieved, separate test sessions were conducted in which male Wistar rats were allowed to self-administer ethanol or water following pre-treatment with vehicle or one of the following neuroactive steroids: (3beta,5beta)-3-hydroxypregnan-20-one (epipregnanolone; 5, 10, 20 mg/kg; n=12), (3alpha,5beta)-20-oxo-pregnane-3-carboxylic acid (PCA; 10, 20, 30 mg/kg n=10), (3alpha,5beta)-3-hydroxypregnan-20-one hemisuccinate (pregnanolone hemisuccinate; 5, 10, 20 mg/kg; n=12) and (3alpha,5alpha)-3-hydroxyandrostan-17-one hemisuccinate (androsterone hemisuccinate; 5, 10, 20 mg/kg; n=11). The effect of the 3beta-epimer of PCA, (3beta,5beta)-20-oxo-pregnane-3-carboxylic acid (10, 20, 30 mg/kg; n=9), on ethanol self-administration was also examined. The compounds were administered using a Latin-square design 45 min prior to the weekly test sessions. The effects of the 30 mg/kg dose of the steroidal hemisuccinates on ethanol intake were also examined 5 min after administration of these drugs. Both epipregnanolone and PCA attenuated ethanol self-administration. However, neither of the hemisuccinate compounds significantly altered this behavior. The steroidal hemisuccinates (30 mg/kg; n=7) were also tested 5 min before behavior testing and had no effect on ethanol intake 5 min after administration. The 3beta-epimer of PCA also failed to alter ethanol intake. None of the test compounds altered water intake. In electrophysiological studies, the effects of PCA and androsterone hemisuccinate on evoked GABA(A) receptor-mediated inhibitory postsynaptic currents (GABA(A)-IPSCs) was examined in brain slices of the amygdala. PCA had a stimulatory effect at concentrations of 5 and 25 muM. Androsterone hemisuccinate had no agonist activity. The ability of epipregnanolone and PCA to alter ethanol intake appears to be related to different inhibitory actions of these compounds on either GABA(A) or NMDA receptors, respectively. Thus, dual modulation of these systems by selected neuroactive steroids may offer potential for modifying the reinforcing effects of alcohol.

The Enantiomer of Cholesterol

Cholesterol plays a variety of significant roles in biological systems. However, the mechanisms by which cholesterol functions remain largely unclear. The enantiomer of cholesterol (ent-cholesterol)--which has identical physical properties, but opposite three-dimensional configuration compared to cholesterol--is a unique tool that can be used to better understand the mechanisms of cholesterol function. We review the literature pertaining to ent-cholesterol, focusing in particular on its use in biological studies.

Inhibition of type A GABA receptors by L-type calcium channel blockers

Modulation of type A GABA receptors (GABAA) by L-type Ca++ channel blockers was investigated. The dihydropyridines nifedipine and nitrendipine, and the phenylalkylamine verapamil inhibited recombinant rat alpha1beta2gamma2 receptors recorded from human embryonic kidney (HEK) 293 cells; nifedipine at low concentrations also elicited modest stimulatory effects on GABA-gated current. The IC50 for GABA current inhibition was lowest for nitrendipine (17.3 +/- 1.3 microM), so subsequent studies were focused on further exploring its mechanism and possible site of action. When co-applied with GABA, nitrendipine had minimal effects on initial current amplitude, but significantly enhanced current decay rate. Nitrendipine-mediated inhibition was subunit-selective, as its IC50 was 10-fold lower in alpha1beta2 receptors. Nitrendipine's effect in recombinant human alpha1beta2gamma2 receptors was similar (IC50=23.0 +/- 1.3 microM) to that observed in rat receptors of the same configuration, indicating the site of action is conserved in the two species. The inhibitory effects were dependent on channel gating, were independent of transmembrane voltage, and were also observed in GABAA receptors recorded from hypothalamic brain slices. The pharmacologic mechanism of inhibition by nitrendipine was non-competitive, indicating it does not act at the GABA binding site. Nitrendipine block was retained in the presence of the benzodiazepine antagonist flumazenil, indicating it does not interact at the benzodiazepine site. The actions of nitrendipine were not affected by a mutation (beta2T246F) that confers resistance to the channel blocker picrotoxin, and they were not altered in the presence of the picrotoxin site antagonist alpha-isopropyl-alpha-methyl-gamma-butyrolactone, demonstrating nitrendipine does not act at the picrotoxin site of the GABAA receptor. Possible interaction of nitrendipine with the Zn++ site was also eliminated, as mutation of beta2 H267 to A, which confers resistance to Zn++, had no effect on nitrendipine-mediated inhibition. Our data suggest some of the central effects of dihydropyridines may be due to actions at GABAA receptors. Moreover, the effects may be mediated through interaction with a novel modulatory site on the GABAA receptor.

Neuroprotective agent riluzole potentiates postsynaptic GABA(A) receptor function

The antiepileptic drug riluzole is a use-dependent blocker of voltage-gated Na(+) channels and selectively depresses action potential-driven glutamate over gamma-aminobutyric acid (GABA) release. Here we report that in addition to its presynaptic effect, riluzole at higher concentrations also strongly potentiates postsynaptic GABA(A) responses both in cultured hippocampal neurons and in Xenopus oocytes expressing recombinant receptors. Although peak inhibitory postsynaptic currents (IPSCs) of autaptic hippocampal neurons were inhibited, 20-100 microM riluzole significantly prolonged the decay of IPSCs, resulting in little change in total charge transfer. The effect was dose-dependent and reversible. Riluzole selectively increased miniature IPSC fast and slow decay time constants, without affecting their relative proportions. Miniature IPSC peak amplitude, rise time and frequency were unaffected, indicating a postsynaptic mechanism. In the Xenopus oocyte expression system, riluzole potentiated GABA responses by lowering the EC(50) for GABA activation. Riluzole directly gated a GABA(A) current that was partially blocked by bicuculline and gabazine. Pharmacological experiments suggest that the action of riluzole did not involve a benzodiazepine, barbiturate, or neurosteroid site. Instead, riluzole-induced potentiation was inhibited by the lactone antagonist alpha-isopropyl-alpha-methyl-gamma-butyrolatone (alpha-IMGBL). While most anticonvulsants either block voltage-gated Na(+) channels or potentiate GABA(A) receptors, our results suggest that riluzole may define an advantageous class of anticonvulsants with both effects.

Steroid structure and pharmacological properties determine the anti-amnesic effects of pregnenolone sulphate in the passive avoidance task in rats

Pregnenolone sulphate (PREGS) has generated interest as one of the most potent memory-enhancing neurosteroids to be examined in rodent learning studies, with particular importance in the ageing process. The mechanism by which this endogenous steroid enhances memory formation is hypothesized to involve actions on glutamatergic and GABAergic systems. This hypothesis stems from findings that PREGS is a potent positive modulator of N-methyl-d-aspartate receptors (NMDARs) and a negative modulator of gamma-aminobutyric acid(A) receptors (GABA(A)Rs). Moreover, PREGS is able to reverse the amnesic-like effects of NMDAR and GABA(A)R ligands. To investigate this hypothesis, the present study in rats examined the memory-altering abilities of structural analogs of PREGS, which differ in their modulation of NMDAR and/or GABA(A)R function. The analogs tested were: 11-ketopregnenolone sulphate (an agent that is inactive at GABA(A)Rs and NMDARs), epipregnanolone ([3beta-hydroxy-5beta-pregnan-20-one] sulphate, an inhibitor of both GABA(A)Rs and NMDARs), and a newly synthesized (-) PREGS enantiomer (which is identical to PREGS in effects on GABA(A)Rs and NMDARs). The memory-enhancing effects of PREGS and its analogs were tested in the passive avoidance task using the model of scopolamine-induced amnesia. Both PREGS and its (-) enantiomer blocked the effects of scopolamine. The results show that, unlike PREGS, 11-ketopregnenolone sulphate and epipregnanolone sulphate failed to block the effect of scopolamine, suggesting that altering the modulation of NMDA receptors diminishes the memory-enhancing effects of PREGS. Moreover, enantioselectivity was demonstrated by the ability of natural PREGS to be an order of magnitude more effective than its synthetic enantiomer in reversing scopolamine-induced amnesia. These results identify a novel neuropharmacological site for the modulation of memory processes by neuroactive steroids.

Enantiospecificity of cholesterol function in vivo

The importance of the absolute configuration of cholesterol for its function in vivo is unknown. To directly test this question in vivo, we synthesized the enantiomer of cholesterol (ent-cholesterol) and tested its ability to substitute for natural cholesterol (nat-cholesterol) in the growth, viability, and behavior of Caenorhabditis elegans, a cholesterol auxotroph. First-generation animals grown on ent-cholesterol were viable with only mild behavioral defects. However, ent-cholesterol produced 100% lethality/arrest of their second generation progeny. Isotopically labeled ent-cholesterol incorporated into animals, indicating that its lethality was not secondary to cholesterol starvation. When mixed with nat-cholesterol, ent-cholesterol was not inert; rather, it antagonized the activity of nat-cholesterol. These results demonstrate for the first time that the absolute configuration of cholesterol, not just its physical properties, is essential for its functions in vivo.

The synthetic enantiomer of pregnenolone sulfate is very active on memory in rats and mice, even more so than its physiological neurosteroid counterpart: distinct mechanisms?

The demonstration that the neurosteroid pregnenolone sulfate (PREGS) is active on memory function at both the physiological and pharmacological levels led to us examining in detail the effects of the steroid on spatial working memory by using a two-trial recognition task in a Y-maze, a paradigm based on the natural drive in rodents to explore a novel environment. Dose-response studies in young male adult Sprague-Dawley rats and Swiss mice, after the postacquisition intracerebroventricular injection of steroid, showed an U-inverted curve for memory performance and indicated a greater responsiveness in rats compared with mice. Remarkably, the synthetic (-) enantiomer of PREGS not only also displayed promnesiant activity, but its potency was 10 times higher than that of the natural steroid. Intracerebroventricular coadministration experiments with DL-2-amino-5-phosphonovaleric acid, a competitive selective antagonist of the N-methyl-D-aspartate receptor, abolished the memory-enhancing effect of PREGS, but not that of the PREGS enantiomer, evoking enantiomeric selectivity at the N-methyl-d-aspartate receptor and/or different mechanisms for the promnestic function of the two enantiomers.

Recent developments in structure-activity relationships for steroid modulators of GABA(A) receptors

GABAergic neurotransmission can be both positively and negatively modulated by steroids. The steroid effects are thought to be mediated by binding of steroids to specific sites on GABA(A) receptors. It appears that the receptor sites for positive and negative modulatory steroids are different. Thus far, the location and number of binding sites for steroids on these receptors have not been established. In this brief review, we concentrate largely on results from our own structure-activity studies. Novel analogues have been studied to further delineate the structural features required for compounds to modulate receptor function via steroid binding sites. Non-naturally occurring enantiomers of both positive and negative modulators have been studied to provide further evidence for the existence of specific steroid binding sites on the receptors.

Effects on gamma-aminobutyric acid (GABA)(A) receptors of a neuroactive steroid that negatively modulates glutamate neurotransmission and augments GABA neurotransmission

Neurosteroids positively and negatively modulate gamma-aminobutyric acid (GABA)(A) receptors and glutamate receptors, which underlie most fast inhibition and excitation in the central nervous system. We report the identification of a neuroactive steroid, (3 alpha,5 beta)-20-oxo-pregnane-3-carboxylic acid (3 alpha 5 beta PC), with unique cellular actions. 3 alpha 5 beta PC positively modulates GABA(A) receptor function and negatively modulates N-methyl-D-aspartate (NMDA) receptor function, a combination that may be of particular clinical benefit. 3 alpha 5 beta PC promotes net GABA(A) potentiation at low steroid concentrations (<10 microM) and at negative membrane potentials. At higher concentrations, the steroid also blocks GABA receptors. Because this block would presumably counteract the NMDA receptor blocking actions of 3 alpha 5 beta PC, we characterize the GABA receptor block in some detail. Agonist concentration, depolarization, and high extracellular pH increase the block. The apparent pK for both potentiation and block was 6.4 to 6.9, substantially higher than expected from carboxylated steroid in an aqueous environment. Block is not dependent on the stereochemistry of the carboxylic acid at carbon 3 and is relatively insensitive to placement of the carboxylic acid at the opposite end of the steroid (carbon 24). Potentiation is critically dependent on the stereochemistry of the carboxylic acid group at carbon 3. Consistent with the pH dependence of potentiation, effects of the amide derivative (3 alpha,5 beta)-20-oxo-pregnane-3-carboxamide, suggest that the un-ionized form of 3 alpha 5 beta PC is important for potentiation, whereas the ionized form is probably responsible for block. Further refinement of the neuroactive steroid to promote GABA potentiation and NMDA receptor block and diminish GABA receptor block may lead to a clinically useful neuroactive steroid.

Pentylenetetrazole-induced inhibition of recombinant gamma-aminobutyric acid type A (GABA(A)) receptors: mechanism and site of action

Pentylenetetrazole (PTZ) is a central nervous system convulsant that is thought, based on binding studies, to act at the picrotoxin (PTX) site of the gamma-aminobutyric acid type A (GABA(A)) receptor. In the present study, we have investigated the mechanism and site of action of PTZ in recombinant GABA(A) receptors. In rat alpha 1 beta 2 gamma 2 receptors, PTZ inhibited GABA-activated Cl(-) current in a concentration-dependent, voltage-independent manner, with an IC(50) of 0.62 +/- 0.13 mM. The mechanism of inhibition appeared competitive with respect to GABA in both rat and human alpha 1 beta 2 gamma 2 receptors. Varying subunit configuration (change or lack of alpha subunit isoform or lack of gamma 2 subunit) had modest effects on PTZ-induced inhibition, as evidenced by comparable IC(50) values (0.6-2.2 mM) in all receptor configurations tested. This contrasts with PTX and other PTX-site ligands, which have greater affinity in receptors lacking an alpha subunit. Using a one-site model for PTZ interaction with alpha 1 beta 2 gamma 2 receptors, the association rate (k(+1)) was found to be 1.14 x 10(3) M(-1) s(-1) and the dissociation rate (k(-1)) was 0.476 s(-1), producing a functional k(d) of 0.418 mM. PTZ could only gain access to its binding site extracellularly. Single-channel recordings demonstrated that PTZ decreased open probability by increasing the duration of closed states but had no effect on single-channel conductance or open state duration. alpha-Isopropyl-alpha-methyl-gamma-butyrolactone, a compound known to antagonize effects of PTX, also diminished the effects of PTZ. Taken together, our results indicate that pentylenetetrazole and picrotoxin interact with overlapping but distinct domains of the GABA(A) receptor.

Synthesis of (5alpha)-17-azaandrostan-3-ols and (5alpha)-17-aza-D-homoandrostan-3-ols and their N-acylated derivatives

Two groups of N-acylated D-azasteroids (4 and 5) were synthesized to explore structure-activity relationships for steroid modulation of GABA(A) receptor function. The target compounds were prepared conveniently from (5alpha)-3-hydroxyandrostan-17-ones (6 and 7) via the intermediate (5alpha)-17-aza-D-homoandrostan-3-ols (14 and 15) or (5alpha)-17-azaandrostan-3-ols (18 and 19) precursors in high overall yields. A Beckmann rearrangement and a Hofmann rearrangement were employed as two key steps in the synthetic sequences.

The nonfeminizing enantiomer of 17beta-estradiol exerts protective effects in neuronal cultures and a rat model of cerebral ischemia

Estrogens are potent neuroprotective compounds in a variety of animal and cell culture models, and data indicate that estrogen receptor (ER)-mediated gene transcription is not required for some of these effects. To further address the requirement for an ER in estrogen enhancement of neuronal survival, we assessed the enantiomer of 17beta-estradiol (ENT-E(2)), which has identical chemical properties but interacts only weakly with known ERs, for neuroprotective efficacy. ENT-E(2) was both as potent and efficacious as 17beta-estradiol in attenuating oxidative stress-induced death in HT-22 cells, a murine hippocampal cell line. Further, ENT-E(2) completely attenuated H(2)O(2) toxicity in human SK-N-SH neuroblastoma cells at a 10 nM concentration. In a rodent model of focal ischemia, 17beta-estradiol (100 microgram/kg) or ENT-E(2) (100 microgram/kg), injected 2 h before middle cerebral artery occlusion, resulted in a 60 and 61% reduction in lesion volume, respectively. ENT-E(2), at the doses effective in this study, did not stimulate uterine growth or vaginal opening in juvenile female rats when administered daily for 3 days. These data indicate that the neuroprotective effects of estrogens, both in vitro and in vivo, can be disassociated from the peripheral estrogenic actions.

Correlation of the apparent affinities and efficacies of gamma-aminobutyric acid(C) receptor agonists

gamma-Aminobutyric acid (GABA), trans-4-aminocrotonic acid (TACA), muscimol, imidazole-4-acetic acid (I4AA), cis-4-aminocrotonic acid (CACA), and isoguvacine are all GABA(C) receptor agonists. These compounds have different apparent sensitivities (EC(50)) and efficacies (I(max)) on exogenously expressed human rho1 homomeric GABA(C) receptors. It is not clear if these differences are due to distinct binding affinities and/or distinct gating kinetics. In this study, using a recently developed single oocyte binding technique, we determined the apparent dissociation constants (K(i) values) of these compounds from their IC(50) values for [(3)H]GABA displacement. The apparent K(i) values fell into two distinct groups. The high affinity group was comprised of agonists with longer distances between the nitrogen atom of the amino or imidazole group and the carbon atom of the carboxyl or isoxazole group. The single oocyte binding technique, in conjunction with two-electrode voltage clamp, has allowed a direct correlation of the apparent affinity, efficacy, and potency of agonists on intact functional GABA(C) receptors. The correlation and coupling of these parameters are discussed in terms of a simple proposed activation mechanism.

Potential clinical uses of neuroactive steroids

Neuroactive steroids rapidly modulate gamma-aminobutyric acid (GABA) and glutamate receptors. GABA-enhancing steroids have potential clinical utility as anesthetics, hypnotics, anticonvulsants and anxiolytics. Furthermore, GABAergic neurosteroids may participate in regulating mood and the effects of alcohol on the nervous system, suggesting a potential role in major psychiatric disorders. Neuroactive steroids that alter the function of glutamate receptors could be useful for treating neurodegenerative disorders, and as cognitive enhancers. Recent progress in developing water-soluble steroids and steroids with enhanced oral efficacy foster optimism that certain neuroactive steroids will be developed for clinical use.

Formation of stable vesicles from N- or 3-alkylindoles: possible evidence for tryptophan as a membrane anchor in proteins

Twelve indole derivatives have been prepared and studied. Five were 1-substituted: 1, methyl; 2, n-hexyl; 3, n-octyl; 4, n-octadecyl; and 5, cholestanyloxycarbonylmethyl. Four were 3-substituted: 6, methyl; 7, n-hexyl; 8, n-octyl; and 9, n-octadecyl. Three were disubstituted as follows: 10, 1-n-decyl-3- n-decyl; 11, 1-methyl-3-n-decyl; and 12, 1,3-bis(n-octadecyl)indole. Sonication of aqueous suspensions afforded stable aggregates from 3-5 and 8-12. Laser light scattering, dye entrapment, and electron microscopy were used to characterize the aggregates. Aggregates formed from N-substituted indoles proved to be more robust than those formed from 3-alkylindoles. A stable monolayer formed from 3-n-octadecylindole but not from N- or 1,3-disubstituted analogues by using a Langmuir-Blodgett trough. The formation of aggregates was explained in terms of stacking by the relatively polar indole headgroup. In the monolayer experiment, this force was apparently overwhelmed by H-bonding interactions with the aqueous phase.

Potential clinical uses of neuroactive steroids

Neuroactive steroids rapidly modulate gamma-aminobutyric acid (GABA) and glutamate receptors. GABA-enhancing steroids have potential clinical utility as anesthetics, hypnotics, anticonvulsants and anxiolytics. Furthermore, GABAergic neurosteroids may participate in regulating mood and the effects of alcohol on the nervous system, suggesting a potential role in major psychiatric disorders. Neuroactive steroids that alter the function of glutamate receptors could be useful for treating neurodegenerative disorders, and as cognitive enhancers. Recent progress in developing water-soluble steroids and steroids with enhanced oral efficacy foster optimism that certain neuroactive steroids will be developed for clinical use.

Steroid inhibition of rat neuronal nicotinic alpha4beta2 receptors expressed in HEK 293 cells

Steroids, in addition to regulating gene expression, directly affect a variety of ion channels. We examined the action of steroids on human embryonic kidney 293 cells stably transfected to express rat alpha4beta2 neuronal nicotinic receptors. Each steroid that was tested inhibited acetylcholine responses from these receptors, with slow kinetics requiring seconds for block to develop and recover. The action of one steroid [3alpha,5alpha, 17beta-3-hydroxyandrostane-17-carbonitrile (ACN)] was studied in detail. Block showed enantioselectivity, with an IC(50) value of 1.5 microM for ACN and 4.5 microM for the enantiomer. Inhibition curves had Hill slopes larger than 1, indicating more than one binding site per receptor. Block did not require intracellular compounds containing high-energy phosphate bonds and was not affected by analogs of GTP, suggesting that the mechanism does not require the activation of second messengers. Block did not appear to be strongly selective between open and closed channel states or to involve changes in desensitization. A comparison of different steroids showed that a beta-orientation of groups at the 17 position produced more block than alpha-orientated diastereomers. The stereochemistry at the 3 and 5 positions was less influential for block of alpha4beta2 nicotinic receptors, despite its importance for potentiation of gamma-aminobutyric acid(A) receptors. The ability of steroids to block neuronal nicotinic receptors correlated with their ability to produce anesthesia in Xenopus tadpoles, but the concentrations required for inhibition are generally greater. Similarly, the concentrations of endogenous neurosteroids required to inhibit receptors are larger than estimates of brain concentrations.

Effects of cholesterol and enantiomeric cholesterol on P-glycoprotein localization and function in low-density membrane domains

Multidrug resistance P-glycoprotein (Pgp) has been reported to localize in low-density, cholesterol-enriched membranes. However, effects of low-density membrane domains on function of Pgp remain unexplored in whole cell systems. In cells that express modest levels of the protein endogenously or through drug selection, Pgp predominantly localized to low-density membranes following separation on a sucrose gradient. When highly overexpressed in NIH 3T3 cells, a prominent amount of Pgp also was detected in high-density membranes. Removing cholesterol from cells with beta-methylcyclodextrin (CD), a sterol acceptor molecule, shifted fractions that contained Pgp from low toward high density, and this effect was reversed to a similar extent by restoring sterols with either cholesterol or enantiomeric cholesterol. However, function of human MDR1 Pgp as probed with Tc-Sestamibi, a transport substrate for Pgp, was not dependent on localization of Pgp in cholesterol-enriched membranes. Specific inhibition of MDR1 Pgp with GF120918 or LY335979 also was independent of cholesterol. Cell-type-specific effects of cholesterol content on function of human Pgp were detected by use of daunomycin, another substrate for Pgp, although efficacy of inhibitors remained independent of cholesterol. Conversely, both function and inhibition of hamster Pgp as measured with Tc-Sestamibi and daunomycin were in part dependent on normal cell content of cholesterol. These data show that Pgp preferentially localizes to low-density, cholesterol-enriched membrane domains, but acute depletion of cholesterol impacts Pgp-mediated drug transport in a substrate- and cell-type-specific manner.

Subunit-specific action of an anticonvulsant thiobutyrolactone on recombinant glycine receptors involves a residue in the M2 membrane-spanning region

The anticonvulsant alpha-ethyl, alpha-methyl-gamma-thiobutyrolactone (alphaEMTBL) potentiates the response to a submaximal concentration of glycine produced by receptors composed of human glycine alpha1-subunits but reduces the response of receptors composed of rat glycine alpha3-subunits. Both the potentiating and blocking actions of alphaEMTBL are reduced by higher concentrations of glycine. The subunit specificity of alphaEMTBL block is conferred by a residue in the second membrane-spanning region (M2), which is alanine in the alpha3-subunit (A254) and glycine in the alpha1-subunit. The mutation A254G in the alpha3-subunit removes blocking by alphaEMTBL and reveals potentiation. Picrotin, a picrotoxinin analog, blocks responses of receptors composed of either alpha1 or alpha3-subunits. Blocking of alpha3 receptors by picrotin is reduced in the presence of alphaEMTBL, indicating that the mechanisms interact at some point, but the mutation alpha3 A254G does not remove block by picrotin. However, mutation of a nearby residue alpha3 T258F does remove block by picrotin, picrotoxinin and alphaEMTBL. These observations suggest that alphaEMTBL and picrotin act on glycine alpha3 receptors to produce block by an allosteric mechanism that involves overlapping sets of residues in the M2 region. Coexpression of the alpha3-subunit with the beta-subunit of the glycine receptor also removes block by alphaEMTBL and reveals potentiation, suggesting that receptors containing either alpha3 or alpha1 glycine receptor subunits are potentiated in the adult brain.

Enantioselectivity of pregnanolone-induced gamma-aminobutyric acid(A) receptor modulation and anesthesia

This study reports the actions of enantiomer pairs of anesthetic steroids 3alpha5alphaP/ent-3alpha5alphaP and 3alpha5betaP/ent-3alpha5betaP as modulators of gamma-aminobutyric acid (GABA)(A) receptors and as anesthetics. The enantiomers of structurally related 17-carbonitrile analogs also are examined. These studies were aimed at 1) determining whether the steroid recognition site could distinguish between molecules differing in shape, but not other physical properties (enantioselectivity); 2) providing further insight into the structure-activity relationships of anesthetic steroids; and 3) determining whether modulation of GABA(A) receptor function correlates with anesthetic potency for anesthetic steroid enantiomers. Stereoselective actions of the compounds were evaluated in four different bioassays: 1) noncompetitive displacement of [(35)S]t-butylbicyclophosphorothionate from the picrotoxin site of GABA(A) receptors present in rat brain membrane preparations; 2) modulation of GABA currents in cultured rat hippocampal neurons; 3) loss of righting reflex in tadpoles; and 4) loss of righting reflex in mice. The data indicate that 5alpha-reduced steroids, but not 5beta-reduced steroids, show a high degree of enantioselectivity/enantiospecificity in their actions as modulators of GABA(A) receptors and as anesthetics. For all compounds studied, the effects on GABA(A) receptor function closely tracked with anesthetic effects. These data show that the anesthetic steroid recognition site is capable of distinguishing enantiomers, suggesting a protein-binding site of specific dimensions and shape. The results are compatible either with a structural model of the binding site that can accommodate 3alpha5alphaP, 3alpha5betaP, and ent-3alpha5betaP, but not ent-3alpha5alphaP, or with two different binding sites for steroid anesthetics.

Pregnenolone sulfate modulates inhibitory synaptic transmission by enhancing GABA(A) receptor desensitization

We examined the effects of the neurosteroid pregnenolone sulfate (PS) on GABA(A) receptor-mediated synaptic currents and currents elicited by rapid applications of GABA onto nucleated outside-out patches in cultured postnatal rat hippocampal neurons. At 10 microm, PS significantly depressed peak responses and accelerated the decay of evoked inhibitory synaptic currents. In nucleated outside-out patches, PS depressed peak currents and speeded deactivation after 5 msec applications of a saturating concentration of GABA. PS also increased the rate and degree of macroscopic GABA receptor desensitization during prolonged GABA applications. In a paired GABA application paradigm, PS slowed the rate of recovery from desensitization. In contrast to its prominent effects on currents produced by saturating GABA concentrations, PS had only small effects on peak currents and failed to alter deactivation after brief applications of the weakly desensitizing GABA(A) receptor agonists taurine and beta-alanine. However, when beta-alanine was applied for a sufficient duration to promote receptor desensitization, PS augmented macroscopic desensitization and slowed deactivation. These results suggest that PS inhibits GABA-gated chloride currents by enhancing receptor desensitization and stabilizing desensitized states. This contention is supported by kinetic modeling studies in which increases in the rate of entry into doubly liganded desensitized states mimic most effects of PS.

Slow death of postnatal hippocampal neurons by GABA(A) receptor overactivation

Neurotransmitters can have both toxic and trophic functions in addition to their role in neural signaling. Surprisingly, chronic blockade of GABA(A) receptor activity for 5-8 d in vitro enhanced survival of hippocampal neurons, suggesting that GABA(A) receptor overactivation may be neurotoxic. Potentiating GABA(A) receptor activity by chronic treatment with the endogenous neurosteroid (3alpha,5alpha)-3-hydroxypregnan-20-one caused massive cell loss over 1 week in culture. Other potentiators of GABA(A) receptors, including benzodiazepines, mimicked the cell loss, suggesting that potentiating endogenous GABA activity is sufficient to produce neuronal death. Neurosteroid-treated neurons had lower resting intracellular calcium levels than control cells and produced smaller calcium rises in response to depolarizing challenges. Manipulating intracellular calcium levels with chronic elevated extracellular potassium or with the calcium channel agonist Bay K 8644 protected neurons. The results may have implications for the mechanisms of programmed cell death in the developing CNS as well as implications for the long-term consequences of chronic GABAmimetic drug use during development.

Effects of anticonvulsant lactams on in vitro seizures in the hippocampal slice preparation

Some 3,3-disubstituted 2-pyrrolidinones and 2-piperidinones (five- and six-membered ring lactams, respectively) possess potent in vivo anticonvulsant activity. In vitro these lactams potentiate GABA(A) receptor-mediated chloride currents, which is thought to be the mechanism by which they exert their therapeutic effects. However, the apparent affinity for these GABA(A) interactions is low: EC50s range from hundreds of micromolar to low millimolar values. In order to more completely characterize the activities of these compounds, it was necessary to know the concentrations required to curtail epileptiform activity in an intact neural network, and the mechanism by which this occurs. To address these questions, we used two methods of inducing ictal activity in hippocampal-entorhinal cortical slices: 4-aminopyridine (4-AP) and low Mg2+. We found that 3,3-diethyl-2-pyrrolidinone (diethyl-lactam) prevents seizure-like discharges with IC50s of 1.1 and 2.1 mM in the two models, respectively. These values are nearly identical to the EC50 value obtained in whole-cell studies of diethyl-lactam's GABA(A) receptor modulation. The addition of the GABA(A) antagonist picrotoxin to the low Mg2+ ACSF produced seizures which persisted during diethyl-lactam application. Neither 3-benzyl-3-ethyl-2-piperidinone (3-BEP) nor alpha-ethyl-alpha-methyl-gamma-thiobutyrolactone (alpha-EMTBL), two compounds which are similar to diethyl-lactam, but demonstrate picrotoxin-insensitive inhibition of voltage-dependent currents, diminished low Mg2+/picrotoxin seizure activity. Our results support the hypothesis that diethyllactam and related compounds exert their anticonvulsant activity primarily, if not exclusively, by modulating the GABA(A) receptor.

Blockade of Ba2+ current through human alpha1E channels by two steroid analogs, (+)-ACN and (+)-ECN

Previous work suggests that different neuroactive steroids may exhibit some selectivity in their blocking effects on different high-voltage activated (HVA) Ca2+ currents. At least some of these effects appear to involve direct blocking actions on Ca2+ channels. Thus, direct investigation of the effects of various steroids on cloned Ca2+ channel variants may lead to the development of potent and selective small-molecular weight Ca2+ channel blockers. Here we examine the effects of two steroids on a cloned human alpha1E Ca2+ channel both with and without a beta3 subunit, when expressed in HEK293 cells. One compound, (+)-ACN, has been previously shown to block N-, Q-, and R-subtypes of HVA current without affecting L- and P-type current. The second compound, (+)-ECN, weakly blocks total HVA current in hippocampal neurons. (+)-ECN differs from (+)-ACN in lacking effects on GABA receptors, but shares with (+)-ACN an ability to partially inhibit T current in DRG neurons (Todorovic, S.M., Prakriya, M., Nakashima, Y.M. et al., 1998. Enantioselective blockade of T-type Ca2+ current in adult rat sensory neurons by a steroid lacking GABA-mimetic activity. Mol. Pharmacol. 54, 918-927). (+)-ACN can block 100% of Ba2+ current in HEK cells arising either from the alpha1E subunit (IC50 approximate to 10 microM) or the alpha1Ebeta3 combination (IC50 approximate to 5 microM), while (+)-ECN maximally blocks only about 80% of the alpha1E (10 microM) or alpha1Ebeta3 (16 microM) current. Blockade by (+)-ACN exhibits several differences from blockade by (+)-ECN. (+)-ACN increases the apparent rate of onset of inactivation, particularly for the alpha1E variant, slows recovery from inactivation, and more profoundly shifts the voltage-dependence of current availability for both alpha1E and alpha1Ebeta3 variants than does (+)-ECN. Although the complexity of the normal inactivation kinetics of alpha1E variants makes interpretation of the (+)-ACN-induced kinetic alterations difficult, the results suggest that the two steroids are to some extent acting by distinct mechanisms, and perhaps at different sites.

Multidrug resistance (MDR1) P-glycoprotein enhances esterification of plasma membrane cholesterol

Class I P-glycoproteins (Pgp) confer multidrug resistance in tumors, but the physiologic function of Pgp in normal tissues remains uncertain. In cells derived from tissues that normally express Pgp, recent data suggest a possible role for Pgp in cholesterol trafficking from the plasma membrane to the endoplasmic reticulum. We investigated the esterification of plasma membrane cholesterol under basal conditions and in response to sphingomyelinase treatment in transfected and drug-selected cell lines expressing differing amounts of functional class I Pgp. Compared with parental NIH 3T3 fibroblasts, cells transfected with human multidrug resistance (MDR1) Pgp esterified more cholesterol both without and with sphingomyelinase. Esterification also was greater in drug-selected Dox 6 myeloma cells than parental 8226 cells, which express low and non-immunodetectable amounts of Pgp, respectively. However, no differences in total plasma membrane cholesterol were detected. Transfection of fibroblasts with the multidrug resistance-associated protein (MRP) did not alter esterification, showing that cholesterol trafficking was not generally affected by ATP-binding cassette transporters. Steroidal (progesterone, dehydroepiandrosterone) and non-steroidal antagonists (verapamil, PSC 833, LY335979, and GF120918) were evaluated for effects on both cholesterol trafficking and the net content of 99mTc-Sestamibi, a reporter of drug transport activity mediated by Pgp. In Pgp-expressing cells treated with nonselective and selective inhibitors, both the kinetics and efficacy of inhibition of cholesterol esterification differed from the antagonism of drug transport mediated by Pgp. Thus, although the data show that greater expression of class I Pgp within a given cell type is associated with enhanced esterification of plasma membrane cholesterol in support of a physiologic function for Pgp in facilitating cholesterol trafficking, the molecular mechanism is dissociated from the conventional drug transport activity of Pgp.

Pregnenolone sulfate and dehydroepiandrosterone sulfate inhibit GABA-gated chloride currents in Xenopus oocytes expressing picrotoxin-insensitive GABA(A) receptors

We examined the effects of picrotoxinin, pregnenolone sulfate (PS) and dehydroepiandrosterone sulfate (DHEAS) on gamma-aminobutyric acid (GABA) responses in Xenopus oocytes injected with wild type alpha1, beta2 and gamma2 GABA(A) receptor subunits and in oocytes injected with wild type alpha1 and beta2 subunits and a mutated gamma2 subunit that eliminates picrotoxin sensitivity. All three agents inhibited GABA currents in oocytes injected with wild type subunits. Oocytes injected with the mutated gamma2 subunit showed no inhibition of GABA responses by picrotoxinin at concentrations up to 100 microM. PS and DHEAS inhibited GABA currents at similar concentrations in both sets of oocytes. These results indicate that PS and DHEAS do not require a functional picrotoxin site for inhibition of GABA responses.

Enantioselective blockade of T-type Ca2+ current in adult rat sensory neurons by a steroid that lacks gamma-aminobutyric acid-modulatory activity

A number of steroids seem to have anesthetic effects resulting primarily from their ability to potentiate currents gated by gamma-aminobutyric acidA (GABAA) receptor activation. One such compound is (3alpha,5alpha, 17beta)-3-hydroxyandrostane-17-carbonitrile [(+)-ACN]. We were interested in whether carbonitrile substitution at other ring positions might result in other pharmacological consequences. Here we examine effects of (3beta,5alpha, 17beta)-17-hydroxyestrane-3-carbonitrile [(+)-ECN] on GABAA receptors and Ca2+ channels. In contrast to (+)-ACN, (+)-ECN does not potentiate GABAA-receptor activated currents, nor does it directly gate GABAA-receptor mediated currents. However, both steroids produce an enantioselective reduction of T-type current. (+)-ECN blocked T current with an IC50 value of 0.3 microM with a maximal block of 41%. (+)-ACN produced a partial block of T current (44% maximal block) with an IC50 value of 0.4 microM. Block of T current showed mild use- and voltage-dependence. The (-)-ECN enantiomer was about 33 times less potent than (+)-ECN, with an IC50 value of 10 microM and an amount of maximal block comparable to (+)-ECN. (+)-ECN was less effective at blocking high-voltage-activated Ca2+ current in DRG neurons (IC50 value of 9. 3 microM with maximal block of about 27%) and hippocampal neurons. (+)-ECN (10 microM) had minimal effects on voltage-gated sodium and potassium currents in rat chromaffin cells. The results identify a steroid with no effects on GABAA receptors that produces a partial inhibition of T-type Ca2+ current with reasonably high affinity and selectivity. Further study of steroid actions on T currents may lead to even more selective and potent agents.

The anesthetic steroid (+)-3alpha-hydroxy-5alpha-androstane-17beta-carbonitrile blocks N-, Q-, and R-type, but not L- and P-type, high voltage-activated Ca2+ current in hippocampal and dorsal root ganglion neurons of the rat

High voltage-activated (HVA) Ca2+ current (ICa) was recorded from neonatal rat hippocampal and adult rat dorsal root ganglion neurons. In both cell types, (+)-3alpha-hydroxy-5alpha-androstane-17beta-carbonitrile [(+)-ACN], a neuroactive steroid, had no effect on nifedipine- (L-type) or omega-agatoxin IVA- (P-type) sensitive ICa. Selective blockade of N-type current with omega-conotoxin GVIA and of Q-type current with omega-conotoxin MVIIC indicated that (+)-ACN inhibits both N- and Q-type current components in both cell types. Current persisting after blockade of all other current components (R-type) was also sensitive to (+)-ACN. Half-blockade of (+)-ACN-sensitive HVA current occurred in the range of 3-25 microM, with N-type current somewhat more sensitive than Q- or R-type. The (+)-ACN enantiomer, (-)-ACN, and pregnanolone were somewhat less effective at inhibiting total HVA current than (+)-ACN, whereas several steroid analogs, including alfaxalone, were relatively ineffective at inhibiting total HVA current. Neither guanosine-5'-O-(2-thio)diphosphate nor guanosine-5'-O-(3-thio)triphosphate altered the ability of (+)-ACN to inhibit HVA current in dorsal root ganglion neurons, indicating that (+)-ACN acts directly on Ca2+ channels. The partial selectivity exhibited by (+)-ACN among different HVA current components suggests that manipulations of steroid analogues may be a useful strategy in the generation of more selective, more potent, small-molecular-weight HVA channel blockers.

The diazoxide derivative 7-chloro-3-methyl-3,4-dihydro-2H-1,2,4-benzothiadiazine-S,S-dioxide augments AMPA- and GABA-mediated synaptic responses in cultured hippocampal neurons

The diazoxide derivative 7-chloro-3-methyl-3,4-dihydro-2H-1,2,4-benzothiadiazine-S,S-dioxide (IDRA21) enhances memory and learning in rodents, most likely by potentiating AMPAergic synaptic activity. We examined IDRA21's effect upon AMPAergic synaptic currents and whole-cell glutamate currents in cultured rat hippocampal neurons to determine whether IDRA21 was a partial modulator of AMPA receptor desensitization and deactivation. Comparable to cyclothiazide, IDRA21 prolonged AMPAergic autaptic currents (5.6 times control, EC50 150 microM) and slowed the rate of AMPA deactivation (3 times control) following 1-ms applications of 1 mM glutamate to excised, outside-out membrane patches. IDRA21 also augmented autaptic GABA currents by 27 +/- 8.1%, although it had two opposing effects, reducing the peak amplitude versus prolonging autaptic GABA currents. IDRA21 (200 microM) inhibited whole-cell GABA currents elicited by exogenously applied 1 mM GABA by 41 +/- 11%. At sufficient concentrations, IDRA21 reduced AMPA receptor desensitization and slowed the rate of deactivation, most consistent with full agonist activity with lower potency compared to cyclothiazide. IDRA21 slightly augments GABAergic synaptic currents.

Contribution of subsaturating GABA concentrations to IPSCs in cultured hippocampal neurons

The time course of EPSCs and IPSCs is at least partly determined by the concentration profile of neurotransmitter acting on postsynaptic receptors. Several recent reports have suggested that the peak synaptic cleft concentration of the inhibitory neurotransmitter GABA likely reaches at least 500 microM, a level that saturates the GABAA receptor. In the course of investigating the experimental anticonvulsant 3,3-diethyl-2-pyrrolidinone (diethyl-lactam), we have observed an important contribution to IPSC decay by subsaturating concentrations of GABA. Diethyl-lactam augments currents elicited by the exogenous application of subsaturating concentrations of GABA in voltage-clamped, cultured hippocampal neurons and significantly prolongs the decay of autaptic IPSCs and miniature IPSCs in our cultures. In addition, diethyl-lactam potentiates currents in excised outside-out membrane patches elicited by the prolonged application of low concentrations of GABA. However, when patches are exposed to 1-2 msec pulses of 1 mM GABA, diethyl-lactam does not alter current decay. Tiagabine, which blocks GABA reuptake, does not prolong IPSCs, so it is unlikely that uptake inhibition accounts for the enhancement of IPSCs. EPSCs and miniature IPSC frequency are unaffected by diethyl-lactam, again consistent with a postsynaptic site of action. We propose that during an IPSC, a substantial number of postsynaptic receptors must be exposed to subsaturating concentrations of GABA. A simplified model of GABAA receptor kinetics can account for the effects of diethyl-lactam on exogenous GABA and IPSCs if diethyl-lactam has its main effect on the monoliganded states of the GABAA receptor.

Neurosteroid analogues. 6. The synthesis and GABAA receptor pharmacology of enantiomers of dehydroepiandrosterone sulfate, pregnenolone sulfate, and (3alpha,5beta)-3-hydroxypregnan-20-one sulfate

The unnatural enantiomers of dehydroepiandrosterone sulfate (1), pregnenolone sulfate (2), and (3alpha,5beta)-3-hydroxypregnan-20-one sulfate (3), compounds 4-6, respectively, were prepared by total steroid synthesis. The enantioselectivity of the compounds as negative modulators of the GABAA receptors present in cultured rat hippocampal neurons was examined using electrophysiological methods. Enantioselectivity was found for the inhibitory actions of the dehydroepiandrosterone enantiomers. The IC50s for compounds 1 and 4 were 11 +/- 1 and 80 +/- 14 microM, respectively. Little, if any, enantioselectivity was found for the other two pairs of steroid sulfate inhibitors. The IC50s for compounds 2 and 5 were 82 +/- 12 and 76 +/- 27 microM, respectively. The IC50s for compounds 3 and 6 were 39 +/- 7 and 46 +/- 2 microM, respectively. The results suggest that the sites of action for the androstane and pregnane series of steroid sulfate blockers of GABA-mediated current are different. The observed enantioselectivity for the actions of dehydroepiandrosterone sulfate indicates that its inhibitory actions are mediated via a chiral recognition site and provides new evidence in support of the earlier hypothesis that there is a binding site for this compound on GABAA receptors. Conversely, the failure to observe enantioselectivity for the actions of pregnenolone sulfate and steroid sulfate 3 indicates that a chiral recognition site for these steroids does not exist on GABAA receptors and suggests that the effects of these compounds on this receptor's function may arise indirectly as a consequence of steroid-induced membrane perturbation.

Inhibition of voltage-dependent sodium channels by the anticonvulsant gamma-aminobutyric acid type A receptor modulator, 3-benzyl-3-ethyl-2-piperidinone

3-Benzyl-3-ethyl-2-piperidinone (3-BEP) belongs to a family of compounds that includes alpha- substituted gamma-butyrolactones, gamma-thiobutyrolactones, 2-pyrrolidinones and hexahydro-2H-azepin-2-ones. Many of these drugs exhibit potent in vivo anticonvulsant activity in mice. Previous electrophysiological studies demonstrated that they potentiate gamma-aminobutyric acid- (GABA) mediated chloride currents. This GABAA receptor modulation was thought to be the main mechanism of anticonvulsant activity. We report that 3-BEP also modulates sodium channels. It decreased sodium currents in cultured rat hippocampal neurons in a voltage- and concentration-dependent manner. The drug's apparent affinity increased as neurons were depolarized. At a holding potential of -60 mV, the apparent IC50 was 487 microM. This concentration is comparable to its EC50 for GABAA modulation (575 microM). Current blockade occurred over all activation voltages tested. The steady state inactivation curve was shifted by 600 microM 3-BEP from V50 = -65.3 mV to -72.0 mV, and recovery from inactivation was slowed from tau = 4.9 to 12.8 msec. Sodium current inhibition was not observed for three related compounds, suggesting a degree of chemical specificity for this activity. We conclude that in addition to its known effects on GABAA receptors, 3-BEP modulates sodium channels. Therefore this compound may prevent seizures by both enhancing inhibition and diminishing neuronal excitability.

Enantioselective modulation of GABAergic synaptic transmission by steroids and benz[e]indenes in hippocampal microcultures

The effects of enantiomers of the neurosteroid analogues, 3alpha-hydroxy-5alpha-pregnan-20-one (DHP) and 3alpha-hydroxy-5alpha-androstane-17beta-carbonitrile (ACN), and the benz[e]indene, BI-1, on synaptic currents were examined in microcultures of rat hippocampal neurons. Over the range of 0.1-10 microM, the (+)-enantiomers were more potent and effective than their (-)-enantiomeric counterparts in enhancing gamma-aminobutyric acid (GABA)A receptor-mediated evoked synaptic currents. The (+)-enantiomers had small effects on peak currents, but slowed the decay of inhibitory synaptic currents, resulting in 2-3-fold increases in charge transfer during inhibitory synaptic events at 10 microM. Similar prolongations of spontaneous miniature inhibitory postsynaptic currents (IPSCs) and responses to brief GABA pulses to outside-out patches suggest that the prolongations of evoked synaptic currents result primarily from postsynaptic effects. In contrast, the (-)-enantiomers had little effect on evoked IPSCs at concentrations < or = 1 microM, but enhanced inhibitory transmission at 10 microM. At concentrations < or = 1 microM, neither the (+)- nor (-)-enantiomers altered glutamate-mediated excitatory synaptic currents. At 10 microM, (+)-DHP and (+)-ACN depressed excitatory responses in a bicuculline-sensitive fashion, suggesting that direct chloride channel gating by the steroids contributed to the depression. These data indicate that certain steroids and benz[e]indenes augment inhibitory synaptic transmission enantioselectively and provide strong support for the hypothesis that steroids act at specific sites on synaptic GABA(A) receptors rather than via alteration of membrane lipids.

The diazoxide derivative IDRA 21 enhances ischemic hippocampal neuron injury

The diazoxide derivative IDRA 21 and other positive modulators of (AMPA)-type glutamate receptors are considered potential memory-enhancing agents. However, AMPA receptor activation contributes to CA1 hippocampal neuron damage from global ischemia in rodents, raising the possibility that 7-chloro-3-methyl-3-4-dihydro-2H-1,2,4 benzothiadiazine S,S-dioxide (IDRA 21) or drugs with similar actions may worsen ischemic neuronal injury. Here we demonstrate that glutamate plus IDRA 21 kills cultured rat hippocampal neurons by AMPA receptor activation, and, in vivo, 12 and 24 mg/kg of IDRA 21 given orally increases CA1 neuron loss produced by 10 minutes of global ischemia. Treating patients with drugs that potentiate AMPA receptor activation will have to consider these potential effects, particularly when coexistent with conditions in which excessive activation of AMPA receptors may occur (eg, stroke, seizures).

Structure-activity studies of fluoroalkyl-substituted gamma-butyrolactone and gamma-thiobutyrolactone modulators of GABA(A) receptor function

Dihydro-2(3H)-furanones (gamma-butyrolactones) and dihydro-2(3H)-thiophenones (gamma-thiobutyrolactones) containing fluoroalkyl groups at positions C-3, C-4, and C-5 of the heterocyclic rings were prepared. The anticonvulsant/convulsant activities of the compounds were evaluated in mice. Brain concentrations of the compounds were determined and the effects of the compounds on [35S]-tert-butylbicyclophosphorothionate ([35S]TBPS) binding to the picrotoxin site on GABAA receptors were investigated. The effects of the compounds on GABAA receptor function were studied using electrophysiological methods and cultured rat hippocampal neurons. Fluorination at C-3 results in either subtle or pronounced effects on the pharmacological activity of the compounds. When hydrogens are replaced with fluorines at the methylene carbon of an ethyl group, as in 3-(1,1-difluoroethyl)dihydro-3-methyl-2(3H)-furanone (1), the anticonvulsant actions of the compound are not much changed from those found for the corresponding alkyl-substituted analogue. In marked contrast, fluorination at the methyl carbon of the ethyl group, as in dihydro-3-methyl-3-(2,2,2-trifluoroethyl)-2(3H)-furanone (3), produces a compound having convulsant activity. This convulsant activity seems to be due to an increased affinity of the compound for the picrotoxin site on GABAA receptors caused by an interaction that involves the trifluoromethyl group. Results obtained with gamma-butyrolactones containing either a 3-(1-trifluoromethyl)ethyl or a 3-(1-methyl-1-trifluoromethyl)ethyl substituent indicate that the interactions of the trifluoromethyl group with the picrotoxin binding site are subject to both stereochemical and steric constraints. Sulfur for oxygen heteroatom substitution, as in the corresponding gamma-thiobutyrolactones, affects the type (competitive, non-competitive, etc.) of binding interactions that these compounds have with the picrotoxin site in a complex manner. Fluorination of alkyl groups at the C-4 and C-5 positions of gamma-butyrolactones having convulsant activity increases convulsant potency.

Lactone modulation of the gamma-aminobutyric acid A receptor: evidence for a positive modulatory site

The gamma-aminobutyric acid-A (GABA(A)) receptor complex is allosterically modulated by a variety of substances, some of clinical importance. Barbiturates and neurosteroids augment GABA-currents and also directly gate the channel. A variety of gamma-butyrolactone analogues also modulate GABA-induced currents, with some potentiating and others inhibiting. Because several gamma-thiobutyrolactone analogues have biphasic effects on GABA currents, experiments with wild-type and picrotoxinin-insensitive GABA(A) receptors were performed to analyze whether some gamma-thiobutyrolactones interact with two distinguishable sites on the GABA(A) receptor. beta-Ethyl-beta-methyl-gamma-thiobutyrolactone inhibited GABA-induced currents at low concentrations (0.001-1 mM), but potentiated GABA-induced currents at higher concentrations (3-10 mM) in wild-type alpha1beta2gamma2-subunit containing ionophores. The related alpha-ethyl-alpha-methyl-gamma-thiobutyrolactone potentiated submaximal GABA currents in wild-type receptors at both low and high concentrations (0.1-10 mM). Mutations in the second transmembrane domain of alpha1, beta2, or gamma2 conferred picrotoxinin-insensitivity onto GABA(A) receptor complexes. When these mutated alpha1, beta2, or gamma2 subunits were incorporated into the receptor complex, beta-ethyl-beta-methyl-gamma-thiobutyrolactone potentiated GABA currents over the entire concentration range (0.1-10 mM). Neither the potentiating activity nor the EC50 of alpha-ethyl-alpha-methyl-gamma-thiobutyrolactone changed in the mutant receptors. Further studies demonstrated that the mutations did not affect the EC50 of chlordiazepoxide or phenobarbital. These and our earlier results identify a modulatory site on the GABA(A) receptor distinct from that interacting with barbiturates, benzodiazepines, and steroids. Additionally, they show that the gamma-butyrolactones probably interact at two different sites on the ionophore to produce opposite effects on GABA-mediated current.

Voltage-dependent calcium channels as targets for convulsant and anticonvulsant alkyl-substituted thiobutyrolactones

Alkyl-substituted thiobutyrolactones increase or decrease gamma-aminobutyric acidA responses at or near the picrotoxin site, but they are structurally similar to ethosuximide, which prompted us to determine the actions of thiobutyrolactones on voltage-dependent Ca++ currents. We measured Ca++ currents in cultured neonatal rat dorsal root ganglion neurons in the absence and presence of the anticonvulsant alpha-ethyl,alpha-methyl-gamma-thiobutyrolactone (alpha-EMTBL) and the convulsant beta-ethyl,beta-methyl-gamma-thiobutyrolactone (beta-EMTBL). Low-voltage-activated (T-type) currents were reduced in a concentration-dependent manner, with a maximal reduction of 26% and 30% by alpha-EMTBL and beta-EMTBL, respectively. alpha-EMTBL reduced high-voltage-activated currents in a concentration- and voltage-dependent manner: maximal responses were 7% when evoked from -80 mV, with more rapid current inactivation; 29% when evoked from -40 mV, with little effect on current inactivation. beta-EMTBL increased high-voltage-activated currents < or = 20% at 10 to 300 microM, but reduced currents at higher concentrations; the latter action was similar to that of alpha-EMTBL in its magnitude and voltage dependence. Block of N-type channels with omega-conotoxin GVIA (10 microM) reduced the effect of alpha-EMTBL and eliminated its voltage dependence. The L-type current component was also reduced by alpha-EMTBL, with little effect on P- or Q-type current components. The related compound, alpha-ethyl,alpha-methyl-gamma-butyrolactone, had no effect on Ca++ currents. We conclude that thiobutyrolactones affect voltage-dependent Ca++ currents in a concentration- and voltage-dependent manner, with greater potency on low-voltage. activated channels. Both the ring structure and the position of its alkyl substitutions determine the identity of the targeted Ca++ channel subtypes and the manner of regulation.

Synthesis and anticonvulsant activities of 3,3-dialkyl- and 3-alkyl-3-benzyl-2-piperidinones (delta-valerolactams) and hexahydro-2H-azepin-2-ones (epsilon-caprolactams)

A series of 3-substituted 2-piperidinone (delta-valerolactam) and hexahydro-2H-azepin-2-one (epsilon-caprolactam) derivatives were prepared and evaluated as anticonvulsants in mice. In the 2-piperidinone series, 3,3-diethyl compound 7b is the most effective anticonvulsant against pentylenetetrazole-induced seizures (ED50, 37 mg/kg; PI (TD50/ED50), 4.46), and 3-benzyl compound 4c (ED50, 41 mg/kg; PI, 7.05) is the most effective anticonvulsant against seizures induced by maximal electroshock. By contrast, none of the epsilon-caprolactams tested had anticonvulsant effects below doses causing rotorod toxicity. log P values were correlated with neurotoxicity and [35S]TBPS displacement, but not with anticonvulsant activity. Electrophysiological evaluations of selected compounds from each series indicated that both the delta-valero-lactams and epsilon-caprolactams potentiated GABA-mediated chloride currents in rat hippocampal neurons.

Enantioselectivity of steroid-induced gamma-aminobutyric acidA receptor modulation and anesthesia

Neuroactive steroids have been postulated to cause anesthesia by binding to unique steroid recognition sites on gamma-aminobutyric acid (GABA) receptors and modulating GABA receptor function. Steroids interact with these sites diastereoselectively, but it is unknown whether steroid sites show enantioselectivity. To address this issue, we synthesized enantiomers to (+)-3alpha-hydroxy-5alpha-androstane-17beta-carbonitrile and (+)-3alpha-hydroxy-5alpha-pregnan-20-one. In this study, we show that potentiation of GABA-mediated currents and gating of the GABA(A) channel by steroids, as well as steroid-induced anesthesia in tadpoles and mice, is enantioselective, with the (+)-enantiomers exhibiting significantly greater potency in all assays. The correlation between the effects of steroid enantiomers on channel behavior and their effects as anesthetics provides strong evidence that GABA(A) receptors play a predominant role in steroid-induced anesthesia. The enantiomers also provide a tool to probe the relative contributions of direct chloride channel activation versus potentiation of GABA-elicited currents to the induction of anesthesia. Studies examining the effects of combinations of (+)- and (-)-3alpha-hydroxy-5alpha-androstane-17beta-carbonitrile were consistent with the hypothesis that potentiation of GABA-activated currents contributes to steroid-induced anesthesia but indicated that direct steroid activation of GABA(A) receptors is not mechanistically important in producing anesthesia.

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

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

3,3-Dialkyl- and 3-alkyl-3-benzyl-substituted 2-pyrrolidinones: a new class of anticonvulsant agents

A series of 3,3-dialkyl- and 3-alkyl-3-benzyl-substituted 2-pyrrolidinones (lactams) have been prepared and evaluated for their anticonvulsant activities. In the pentylenetetrazole mouse seizure model, 3,3-diethyl lactam 7c and 3-benzyl-3-ethyl lactam 7j are the most effective anticonvulsants (ED50 = 46 and 42 mg/kg, respectively) and have protective index (PI = TD50/ED50) values of 5.65 and 3.00, respectively. These protective index values compare favorably to those of the clinically used antiepileptic drugs ethosuximide (ED50 = 161 mg/kg), phenobarbital (ED50 = 22 mg/kg), and valproic acid (ED50 = 133 mg/kg), which have PI values of 2.35, 4.00, and 2.12, respectively. The benzyl compounds [3-substituents are Bn, H (7h); Bn, Me (7i); and Bn, Et (7j)] are also very effective anticonvulsants against seizures induced by maximal electroshock (ED50 = 41, 55, and 74 mg/kg, respectively) and have PI values of 3.51, 3.04, and 1.70, respectively. The corresponding PI values for phenobarbital and valproic acid are 1.37 and 5.18, respectively. As a class of anticonvulsants, the 3,3-disubstituted 2-pyrrolidinones have a broad spectrum of action and may be useful for the treatment of human epilepsies.