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

Structural insights into opposing actions of neurosteroids on GABAA receptors

γ-Aminobutyric acid type A (GABAA) receptors mediate fast inhibitory signaling in the brain and are targets of numerous drugs and endogenous neurosteroids. A subset of neurosteroids are GABAA receptor positive allosteric modulators; one of these, allopregnanolone, is the only drug approved specifically for treating postpartum depression. There is a consensus emerging from structural, physiological and photolabeling studies as to where positive modulators bind, but how they potentiate GABA activation remains unclear. Other neurosteroids are negative modulators of GABAA receptors, but their binding sites remain debated. Here we present structures of a synaptic GABAA receptor bound to allopregnanolone and two inhibitory sulfated neurosteroids. Allopregnanolone binds at the receptor-bilayer interface, in the consensus potentiator site. In contrast, inhibitory neurosteroids bind in the pore. MD simulations and electrophysiology support a mechanism by which allopregnanolone potentiates channel activity and suggest the dominant mechanism for sulfated neurosteroid inhibition is through pore block.

Synthetic neuroactive steroids as new sedatives and anaesthetics: Back to the future

Since the 1990s, there has been waning interest in researching general anaesthetics (anaesthetics). Although currently used anaesthetics are mostly safe and effective, they are not without fault. In paediatric populations and neonatal animal models, they are associated with learning impairments and neurotoxicity. In an effort to research safer anaesthetics, we have gone back to re-examine neuroactive steroids as anaesthetics. Neuroactive steroids are steroids that have direct, local effects in the central nervous system. Since the discovery of their anaesthetic effects, neuroactive steroids have been consistently used in human or veterinary clinics as preferred anaesthetic agents. Although briefly abandoned for clinical use due to unwanted vehicle side effects, there has since been renewed interest in their therapeutic value. Neuroactive steroids are safe sedative/hypnotic and anaesthetic agents across various animal species. Importantly, unlike traditional anaesthetics, they do not cause extensive neurotoxicity in the developing rodent brain. Similar to traditional anaesthetics, neuroactive steroids are modulators of synaptic and extrasynaptic γ-aminobutyric acid type A (GABA_A ) receptors and their interactions at the GABA_A receptor are stereo- and enantioselective. Recent work has also shown that these agents act on other ion channels, such as high- and low-voltage-activated calcium channels. Through these mechanisms of action, neuroactive steroids modulate neuronal excitability, which results in characteristic burst suppression of the electroencephalogram, and a surgical plane of anaesthesia. However, in addition to their interactions with voltage and ligand gated ions channels, neuroactive steroids interact with membrane bound metabotropic receptors and xenobiotic receptors to facilitate signaling of prosurvival, antiapoptotic pathways. These pathways play a role in their neuroprotective effects in neuronal injury and may also prevent extensive apoptosis in the developing brain during anaesthesia. The current review explores the history of neuroactive steroids as anaesthetics in humans and animal models, their diverse mechanisms of action, and their neuroprotective properties.

New perspectives on the role of the neurosteroid pregnenolone as an endogenous regulator of type-1 cannabinoid receptor (CB1R) activity and function

Pregnenolone is a steroid with specific characteristics, being the first steroid to be synthesised from cholesterol at all sites of steroidogenesis, including the brain. For many years, pregnenolone was defined as an inactive precursor of all steroids because no specific target had been discovered. However, over the last decade, it has become a steroid of interest because it has been recognised as being a biomarker for brain-related disorders through the development of metabolomic approaches and advanced analytical methods. In addition, physiological roles for pregnenolone emerged when specific targets were discovered. In this review, we highlight the discovery of the selective interaction of pregnenolone with the type-1 cannabinoid receptor (CB1R). After describing the specific characteristic of CB1Rs, we discuss the newly discovered mechanisms of their regulation by pregnenolone. In particular, we describe the action of pregnenolone as a negative allosteric modulator and a specific signalling inhibitor of the CB1R. These particular characteristics of pregnenolone provide a great strategic opportunity for therapeutic development in CB1-related disorders. Finally, we outline new perspectives using innovative genetic tools for the discovery of original regulatory mechanisms of pregnenolone on CB1-related functions.

Neurosteroid dehydroepiandrosterone sulphate enhances pain transmission in rat spinal cord dorsal horn

BACKGROUND

The neurosteroid dehydroepiandrosterone sulphate (DHEAS) activates the sigma-1 receptor, inhibits gamma-aminobutyric acid A (GABA_A) and glycine receptors, and induces hyperalgesic effects. Although its effects have been studied in various tissues of the nervous system, its synaptic mechanisms in nociceptive pathways remain to be elucidated.

METHODS

The threshold of mechanical hypersensitivity and spontaneous pain behaviour was assessed using the von Frey test in adult male Wistar rats after intrathecal administration of DHEAS. We also investigated the effects of DHEAS on synaptic transmission in the spinal dorsal horn using slice patch-clamp electrophysiology.

RESULTS

Intrathecally administered DHEAS elicited dose-dependent mechanical hyperalgesia and spontaneous pain behaviours (withdrawal threshold: saline; 51.0 [20.1] g, 3 μg DHEAS; 14.0 [7.8] g, P<0.01, 10 μg DHEAS; 6.9 [5.2] g, 15 min after administration, P<0.001). DHEAS at 100 μM increased the frequency of miniature postsynaptic currents in the rat dorsal spinal horn; this increase was extracellular Ca²⁺-dependent but not sigma-1 and N-methyl-d-aspartate receptor-dependent. DHEAS suppressed the frequency of miniature inhibitory postsynaptic currents in a GABA_A receptor- and sigma-1 receptor-dependent manner.

CONCLUSIONS

These results suggest that DHEAS participates in the pathophysiology of nociceptive synaptic transmission in the spinal cord by potentiation of glutamate release and inhibition of the GABA_A receptor.

Neurosteroid Actions in Memory and Neurologic/Neuropsychiatric Disorders

Memory dysfunction is a symptomatic feature of many neurologic and neuropsychiatric disorders; however, the basic underlying mechanisms of memory and altered states of circuitry function associated with disorders of memory remain a vast unexplored territory. The initial discovery of endogenous neurosteroids triggered a quest to elucidate their role as neuromodulators in normal and diseased brain function. In this review, based on the perspective of our own research, the advances leading to the discovery of positive and negative neurosteroid allosteric modulators of GABA type-A (GABA_A), NMDA, and non-NMDA type glutamate receptors are brought together in a historical and conceptual framework. We extend the analysis toward a state-of-the art view of how neurosteroid modulation of neural circuitry function may affect memory and memory deficits. By aggregating the results from multiple laboratories using both animal models for disease and human clinical research on neuropsychiatric and age-related neurodegenerative disorders, elements of a circuitry level view begins to emerge. Lastly, the effects of both endogenously active and exogenously administered neurosteroids on neural networks across the life span of women and men point to a possible underlying pharmacological connectome by which these neuromodulators might act to modulate memory across diverse altered states of mind.

Chemogenetic Isolation Reveals Synaptic Contribution of δ GABAA Receptors in Mouse Dentate Granule Neurons

Two major GABA_A receptor classes mediate ionotropic GABA signaling, those containing a δ subunit and those with a γ2 subunit. The classical viewpoint equates γ2-containing receptors with IPSCs and δ-containing receptors with tonic inhibition because of differences in receptor localization, but significant questions remain because the populations cannot be pharmacologically separated. We removed this barrier using gene editing to confer a point mutation on the δ subunit in mice, rendering receptors containing the subunit picrotoxin resistant. By pharmacologically isolating δ-containing receptors, our results demonstrate their contribution to IPSCs in dentate granule neurons and weaker contributions to thalamocortical IPSCs. Despite documented extrasynaptic localization, we found that receptor localization does not preclude participation in isolated IPSCs, including mIPSCs. Further, phasic inhibition from δ subunit-containing receptors strongly inhibited summation of EPSPs, whereas tonic activity had little impact. In addition to any role that δ-containing receptors may play in canonical tonic inhibition, our results highlight a previously underestimated contribution of δ-containing receptors to phasic inhibition.SIGNIFICANCE STATEMENT GABA_A receptors play key roles in transient and tonic inhibition. The prevailing view suggests that synaptic γ2-containing GABA_A receptors drive phasic inhibition, whereas extrasynaptic δ-containing receptors mediate tonic inhibition. To re-evaluate the impact of δ receptors, we took a chemogenetic approach that offers a sensitive method to probe the synaptic contribution of δ-containing receptors. Our results reveal that localization does not strongly limit the contribution of δ receptors to IPSCs and that δ receptors make an unanticipated robust contribution to phasic inhibition.

Neurosteroid biosynthesis down-regulation and changes in GABAA receptor subunit composition: a biomarker axis in stress-induced cognitive and emotional impairment

By rapidly modulating neuronal excitability, neurosteroids regulate physiological processes, such as responses to stress and development. Excessive stress affects their biosynthesis and causes an imbalance in cognition and emotions. The progesterone derivative, allopregnanolone (Allo) enhances extrasynaptic and postsynaptic inhibition by directly binding at GABA_A receptors, and thus, positively and allosterically modulates the function of GABA. Allo levels are decreased in stress-induced psychiatric disorders, including depression and post-traumatic stress disorder (PTSD), and elevating Allo levels may be a valid therapeutic approach to counteract behavioural dysfunction. While benzodiazepines are inefficient, selective serotonin reuptake inhibitors (SSRIs) represent the first choice treatment for depression and PTSD. Their mechanisms to improve behaviour in preclinical studies include neurosteroidogenic effects at low non-serotonergic doses. Unfortunately, half of PTSD and depressed patients are resistant to current prescribed 'high' dosage of these drugs that engage serotonergic mechanisms. Unveiling novel biomarkers to develop more efficient treatment strategies is in high demand. Stress-induced down-regulation of neurosteroid biosynthesis and changes in GABA_A receptor subunit expression offer a putative biomarker axis to develop new PTSD treatments. The advantage of stimulating Allo biosynthesis relies on the variety of neurosteroidogenic receptors to be targeted, including TSPO and endocannabinoid receptors. Furthermore, stress favours a GABA_A receptor subunit composition with higher sensitivity for Allo. The use of synthetic analogues of Allo is a valuable alternative. Pregnenolone or drugs that stimulate its levels increase Allo but also sulphated steroids, including pregnanolone sulphate which, by inhibiting NMDA tonic neurotransmission, provides neuroprotection and cognitive benefits. In this review, we describe current knowledge on the effects of stress on neurosteroid biosynthesis and GABA_A receptor neurotransmission and summarize available pharmacological strategies that by enhancing neurosteroidogenesis are relevant for the treatment of SSRI-resistant patients. Linked Articles This article is part of a themed section on Pharmacology of Cognition: a Panacea for Neuropsychiatric Disease? To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.19/issuetoc.

Effects of acute and chronic administration of neurosteroid dehydroepiandrosterone sulfate on neuronal excitability in mice

Background

Neurosteroid dehydroepiandrosterone sulfate (DHEAS) has been associated with important brain functions, including neuronal survival, memory, and behavior, showing therapeutic potential in various neuropsychiatric and cognitive disorders. However, the antagonistic effects of DHEAS on γ-amino-butyric acid_A receptors and its facilitatory action on glutamatergic neurotransmission might lead to enhanced brain excitability and seizures and thus limit DHEAS therapeutic applications. The aim of this study was to investigate possible age and sex differences in the neuronal excitability of the mice following acute and chronic DHEAS administration.

Methods

DHEAS was administered intraperitoneally in male and female adult and old mice either acutely or repeatedly once daily for 4 weeks in a 10 mg/kg dose. To investigate the potential proconvulsant properties of DHEAS, we studied the effects of acute and chronic DHEAS treatment on picrotoxin-, pentylentetrazole-, and N-methyl-D-aspartate-induced seizures in mice. The effects of acute and chronic DHEAS administration on the locomotor activity, motor coordination, and body weight of the mice were also studied. We also investigated the effects of DHEAS treatment on [³H]flunitrazepam binding to the mouse brain membranes.

Results

DHEAS did not modify the locomotor activity, motor coordination, body weight, and brain [³H]flunitrazepam binding of male and female mice. The results failed to demonstrate significant effects of single- and long-term DHEAS treatment on the convulsive susceptibility in both adult and aged mice of both sexes. However, small but significant changes regarding sex differences in the susceptibility to seizures were observed following DHEAS administration to mice.

Conclusion

Although our findings suggest that DHEAS treatment might be safe for various potential therapeutic applications in adult as well as in old age, they also support subtle interaction of DHEAS with male and female hormonal status, which may underline observed sex differences in the relationship between DHEAS and various health outcomes.

Interactions of L-3,5,3'-Triiodothyronine [corrected], Allopregnanolone, and Ivermectin with the GABAA Receptor: Evidence for Overlapping Intersubunit Binding Modes

Structural mechanisms of modulation of γ-aminobutyric acid (GABA) type A receptors by neurosteroids and hormones remain unclear. The thyroid hormone L-3,5,3’-triiodothyronine (T3) inhibits GABA_A receptors at micromolar concentrations and has common features with neurosteroids such as allopregnanolone (ALLOP). Here we use functional experiments on α₂β₁γ₂ GABA_A receptors expressed in Xenopus oocytes to detect competitive interactions between T3 and an agonist (ivermectin, IVM) with a crystallographically determined binding site at subunit interfaces in the transmembrane domain of a homologous receptor (glutamate-gated chloride channel, GluCl). T3 and ALLOP also show competitive effects, supporting the presence of both a T3 and ALLOP binding site at one or more subunit interfaces. Molecular dynamics (MD) simulations over 200 ns are used to investigate the dynamics and energetics of T3 in the identified intersubunit sites. In these simulations, T3 molecules occupying all intersubunit sites (with the exception of the α-β interface) display numerous energetically favorable conformations with multiple hydrogen bonding partners, including previously implicated polar/acidic sidechains and a structurally conserved deformation in the M1 backbone.

Neurosteroids, stress and depression: potential therapeutic opportunities

Neurosteroids are potent and effective neuromodulators that are synthesized from cholesterol in the brain. These agents and their synthetic derivatives influence the function of multiple signaling pathways including receptors for γ-aminobutyric acid (GABA) and glutamate, the major inhibitory and excitatory neurotransmitters in the central nervous system (CNS). Increasing evidence indicates that dysregulation of neurosteroid production plays a role in the pathophysiology of stress and stress-related psychiatric disorders, including mood and anxiety disorders. In this paper, we review the mechanisms of neurosteroid action in brain with an emphasis on those neurosteroids that potently modulate the function of GABA(A) receptors. We then discuss evidence indicating a role for GABA and neurosteroids in stress and depression, and focus on potential strategies that can be used to manipulate CNS neurosteroid synthesis and function for therapeutic purposes.

Neurosteroids and GABA-A Receptor Function

Neurosteroids represent a class of endogenous steroids that are synthesized in the brain, the adrenals, and the gonads and have potent and selective effects on the GABAA-receptor. 3α-hydroxy A-ring reduced metabolites of progesterone, deoxycorticosterone, and testosterone are positive modulators of GABA(A)-receptor in a non-genomic manner. Allopregnanolone (3α-OH-5α-pregnan-20-one), 5α-androstane-3α, 17α-diol (Adiol), and 3α5α-tetrahydrodeoxycorticosterone (3α5α-THDOC) enhance the GABA-mediated Cl(-) currents acting on a site (or sites) distinct from the GABA, benzodiazepine, barbiturate, and picrotoxin binding sites. 3α5α-P and 3α5α-THDOC potentiate synaptic GABA(A)-receptor function and activate δ-subunit containing extrasynaptic receptors that mediate tonic currents. On the contrary, 3β-OH pregnane steroids and pregnenolone sulfate (PS) are GABA(A)-receptor antagonists and induce activation-dependent inhibition of the receptor. The activities of neurosteroid are dependent on brain regions and types of neurons. In addition to the slow genomic action of the parent steroids, the non-genomic, and rapid actions of neurosteroids play a significant role in the GABA(A)-receptor function and shift in mood and memory function. This review describes molecular mechanisms underlying neurosteroid action on the GABA(A)-receptor, mood changes, and cognitive functions.

Effects of mirtazapine on dehydroepiandrosterone-sulfate and cortisol plasma concentrations in depressed patients

BACKGROUND

Among the neuroactive steroids, dehydroepiandrosterone sulfate (DHEA-S) is at least in part produced in the adrenal gland and is therefore under the control of the hypothalamic-pituitary-adrenocortical (HPA)-system. In the present study, the impact of mirtazapine on DHEA-S and cortisol (COR) levels was investigated in relation to clinical response in depressed patients.

METHODS

A total of 23 inpatients suffering from a major depressive episode (DSM-IV criteria) underwent 5-week treatment with mirtazapine (45 mg/day). Plasma samples were taken weekly at 0800 h and quantified for COR and DHEA-S levels.

RESULTS

Mirtazapine significantly reduced both COR and DHEA-S concentrations, but had no impact on the COR/DHEA-S ratio. The percentage decrease of DHEA-S, but not that of COR was significantly and positively correlated with the percentage reduction in the sum score of the Hamilton Depression Rating Scale at week 5, suggesting a relationship between DHEA-S reduction and clinical efficacy of mirtazapine. There was a significant positive correlation between the decline in COR and DHEA-S levels.

CONCLUSIONS

Apparently, the decrease in COR and DHEA-S concentrations conjointly reflects an attenuating impact of mirtazapine on HPA axis activity, thereby decreasing the adrenal secretion of COR and DHEA-S.

Modulation of neurotransmitter systems by dehydroepiandrosterone and dehydroepiandrosterone sulfate: mechanism of action and relevance to psychiatric disorders

Dehydroepiandrosterone (DHEA) is synthesized in the brain and several studies have shown that this steroid is a modulator of synaptic transmission. The effect of DHEA, and its sulfate ester DHEAS, on glutamate and GABA neurotransmission has been extensively studied but some effects on other neurotransmitter systems, such as dopamine, serotonin and nitric oxide, have also been reported. This review summarizes studies showing the effect of DHEA and DHEAS on neurotransmitter systems at different levels (metabolism, release, reuptake, receptor activation), as well as the activation of voltage-gated ion channels and calcium homeostasis, showing the variety of effects that these steroids exert on those systems, allowing the discussion of its mechanisms of action and its relevance to psychiatric disorders.

Neurosteroid binding sites on GABA(A) receptors

Controlling neuronal excitability is vitally important for maintaining a healthy central nervous system (CNS) and this relies on the activity of type A gamma-aminobutyric acid (GABA(A)) neurotransmitter receptors. Given this role, it is therefore important to understand how these receptors are regulated by endogenous modulators in the brain and determine where they bind to the receptor. One of the most potent groups of modulators is the neurosteroids which regulate the activity of synaptic and extrasynaptic GABA(A) receptors. This level of regulation is thought to be physiologically important and its dysfunction may be relevant to numerous neurological conditions. The aim of this review is to summarise those studies that over the last 20 years have focussed upon finding the binding sites for neurosteroids on GABA(A) receptors. We consider the nature of steroid binding sites in other proteins where this has been determined at atomic resolution and how their generic features were mapped onto GABA(A) receptors to help locate 2 putative steroid binding sites. Altogether, the findings strongly suggest that neurosteroids do bind to discrete sites on the GABA(A) receptor and that these are located within the transmembrane domains of alpha and beta receptor subunits. The implications for neurosteroid binding to other inhibitory receptors such as glycine and GABA(C) receptors are also considered. Identifying neurosteroid binding sites may enable the precise pathophysiological role(s) of neurosteroids in the CNS to be established for the first time, as well as providing opportunities for the design of novel drug entities.

Pregnenolone sulphate and Zn2+ inhibit recombinant rat GABA(A) receptor through different channel property

AIMS

We compared the antagonistic effects of state-dependent gamma-aminobutyric acid A (GABA(A)) receptor blockers picrotoxin, Zn(2+) and pregnenolone sulphate (PS) on GABA- and pentobarbital-activated currents in recombinant rat GABA(A) receptors in Xenopus oocytes.

METHODS

Experiments were performed with wild type rat alpha1 beta2 gamma2L and alpha1beta2 receptors, mutants alpha1V256S beta2 gamma2L and alpha1beta2A252Sgamma2L receptors by the two-electrode voltage-clamp technique.

RESULTS

In contrast to respective 3840- and 56-fold increases in Zn(2+) potencies to inhibit GABA- and pentobarbital-activated currents in the alpha1beta2 receptor, the corresponding potencies of PS remained unchanged in comparison with the alpha1 beta2 gamma2L receptor. A homologous mutation of the residue at 2' position closest to the cytoplasmic end of the M(2) helix to serine on both alpha1 and beta2 subunit, alpha1V256S and beta2A252S, abolished the inhibition of GABA(A) receptor by PS. In comparison with the wild type alpha1beta2gamma2L receptor, mutants alpha1V256S beta2 gamma2L and alpha1beta2 A252S gamma2L receptors did not affect the Zn(2+) inhibition. Furthermore, a significant increase in GABA potency was observed in the mutant alpha1V256S beta2 gamma2L receptor (P < 0.05), but not the mutant alpha1beta2 A252S gamma2L receptor compared with the wild type receptor.

CONCLUSIONS

Pregnenolone sulphate was a gamma2-subunit independent inhibitor in the GABA(A) receptor, whereas the Zn(2+) antagonism was profoundly influenced by the gamma2-subunit. It is likely that the 2' residue closest to the N-terminus of the protein at M(2) helix on both alpha1 and beta2 subunit are critical to the inhibitory actions of PS and the function of Cl(-) channels. These results are consistent with the hypothesis that PS behaves as a Cl(-) channel blocker that does not share with Zn(2+), the coincident channel property in the GABA(A) receptors.

Sulfated steroids as endogenous neuromodulators

Central nervous system function is critically dependent upon an exquisitely tuned balance between excitatory synaptic transmission, mediated primarily by glutamate, and inhibitory synaptic transmission, mediated primarily by GABA. Modulation of either excitation or inhibition would be expected to result in altered functionality of finely tuned synaptic pathways and global neural systems, leading to altered nervous system function. Administration of positive or negative modulators of ligand-gated ion channels has been used extensively and successfully in CNS therapeutics, particularly for the induction of sedation and treatment of anxiety, seizures, insomnia, and pain. Excessive activation of excitatory glutamate receptors, such as in cerebral ischemia, can result in neuronal damage via excitotoxic mechanisms. The discovery that neuroactive steroids exert rapid, direct effects upon the function of both excitatory and inhibitory neurotransmitter receptors has raised the possibility that endogenous neurosteroids may play a regulatory role in synaptic transmission by modulating the balance between excitatory and inhibitory neurotransmission. The sites to which neuroactive steroids bind may also serve as targets for the discovery of therapeutic neuromodulators.

Residues in the first transmembrane domain of the Caenorhabditis elegans GABA(A) receptor confer sensitivity to the neurosteroid pregnenolone sulfate

The GABA(A) receptor is a target of endogenous and synthetic neurosteroids. Little is known about the residues required for neurosteroid action on GABA(A) receptors. We have investigated pregnenolone sulfate (PS) inhibition of the Caenorhabditis elegans UNC-49 GABA receptor, a close homolog of the mammalian GABA(A) receptor. The UNC-49 locus encodes two GABA receptor subunits, UNC-49B and UNC-49C. UNC-49C is sensitive to PS but UNC-49B is not sensitive. By analyzing chimeric receptors and receptors containing site-directed mutations, we identified two regions required for PS inhibition. Four residues in the first transmembrane domain are required for the majority of the sensitivity to PS, but a charged extracellular residue at the end of the M2 helix also plays a role. Strikingly, mutation of one additional M1 residue reverses the effect of PS from an inhibitor to an enhancer of receptor function. Mutating the M1 domain had little effect on sensitivity to the inhibitor picrotoxin, suggesting that these residues may mediate neurosteroid action specifically, and not allosteric regulation in general.

Interaction between allopregnanolone and pregnenolone sulfate in modulating GABA-mediated synaptic currents in neurons from the rat medial preoptic nucleus

The two neurosteroids 3alpha-hydroxy-5alpha-pregnane-20-one (allopregnanolone; AlloP) and pregnenolone sulfate (PregS) affect neuronal GABA(A) receptors differently. While AlloP mainly potentiates the currents through GABA(A) receptors, PregS reduces such currents. The present study aimed at clarifying the interaction of AlloP and PregS at GABA(A) receptors in neurons from the medial preoptic nucleus of male rat. AlloP has previously been shown to dramatically prolong GABA-mediated spontaneous inhibitory postsynaptic currents (sIPSCs) in these neurons. Here, by recording sIPSCs under voltage-clamp conditions with the perforated-patch technique, it was shown that PregS by itself did not significantly affect the amplitude or time course of such currents. However, PregS, in a concentration-dependent manner, reduced the AlloP-evoked prolongation of sIPSC decay when the two neurosteroids were applied together. In contrast to sIPSC amplitude and time course, sIPSC frequency was significantly reduced by 10 microM PregS alone. Further, although 1.0 microM AlloP alone induced a clear increase in sIPSC frequency, the frequency was not significantly different from control when 1.0 microM AlloP was applied in combination with 10 microM PregS. In addition to the effects on sIPSC parameters, PregS reduced the baseline current evoked by 1.0 microM AlloP in the absence of GABA application or synaptic activity. PregS by itself did not significantly affect the baseline current. The main effects of AlloP and PregS on the sIPSC time course were mimicked by a simplified model with AlloP assumed to reduce the rate of GABA unbinding from the receptor and PregS assumed to increase the rate of desensitization.

Modeling spontaneous activity in the developing spinal cord using activity-dependent variations of intracellular chloride

We investigated how spontaneous activity is generated in developing, hyperexcitable networks. We focused our study on the embryonic chick spinal cord, a preparation that exhibits rhythmic discharge on multiple timescales: slow episodes (lasting minutes) and faster intraepisode cycling (approximately 1 Hz frequency). For this purpose, we developed a mean field model of a recurrent network with slow chloride dynamics and a fast depression variable. We showed that the model, in addition to providing a biophysical mechanism for the slow dynamics, was able to account for the experimentally observed activity. The model made predictions on how interval and duration of episodes are affected when changing chloride-mediated synaptic transmission or chloride flux across cell membrane. These predictions guided experiments, and the model results were compared with experimental data obtained with electrophysiological recordings. We found agreement when transmission was affected through changes in synaptic conductance and good qualitative agreement when chloride flux was varied through changes in external chloride concentration or in the rate of the Na+-K+-2Cl- cotransporter. Furthermore, the model made predictions about the time course of intracellular chloride concentration and chloride reversal potential and how these are affected by changes in synaptic conductance. Based on the comparison between modeling and experimental results, we propose that chloride dynamics could be an important mechanism in rhythm generation in the developing chick spinal cord.

Inhibition of the activity of the native γ-aminobutyric acidA receptor by metabolites of thyroid hormones: correlations with molecular modeling studies

To characterize the direct effects of thyroid hormones on native gamma-aminobutyric acid(A) (GABA(A)) receptors, rapid (5 s) actions of a series of iodothyronines on muscimol-stimulated uptake of (36)Cl(-) were investigated in synaptoneurosomes prepared from rat brain. The results were correlated with molecular modeling of the active compounds. Dose-response curves for muscimol in the presence of 3,3', 5-L-triiodothyronine (L-T3) indicated a noncompetitive inhibition of muscimol-stimulated (36)Cl(-) uptake by the thyroid hormone. Synaptoneurosomes prepared from cerebellum were less sensitive to L-T3 than those from cerebral cortex, in terms of the potency of the hormone. The overall efficacy approached complete inhibition for both brain regions. Muscimol-stimulated (36)Cl(-) uptake was inhibited differentially by iodothyronine derivatives. One group of compounds with IC(50) values of 18-30 microM included L-thyroxine (L-T4), D-thyroxine (D-T4), 3,3', 5,5'-tetraiodothyroacetic acid (Tetrac), and 3,3', 5-triiodothyroacetic acid (Triac). A second group with values of 75-100 microM included 3,3', 5'-l-triiodothyronine (reverse T3; r-T3), 3,3'-diiodo-L-thyronine (3,3'-l-T2) and 3,5-diiodo-L-thyronine (3,5-D-T2). A final group of inactive compounds with IC(50) values greater than 100 microM included 3',5'-diiodo-L-thyronine (3',5'-l-T2), 3-iodo-L-thyronine (L-T1), 3'-iodo-L-thyronine (3'-L-T1), and L-thyronine (L-T0). Molecular modeling of the active iodothyronines using the Gaussian03 series of programs indicated close correspondences with models of the GABA-inhibitory neurosteroid pregnenolone sulfate (PREGS), suggesting common mechanisms of action at the GABA(A) receptor.

Neurosteroids alter gamma-aminobutyric acid postsynaptic currents in gonadotropin-releasing hormone neurons: a possible mechanism for direct steroidal control

Pulsatile GnRH release is required for fertility and is regulated by steroid feedback. Whether or not steroids or their metabolites act directly on GnRH neurons is not well established. In some neurons, steroid metabolites known as neurosteroids modulate the function of the GABAA receptor. Specifically, the progesterone derivative allopregnanolone is an allosteric agonist at this receptor, whereas the androgen dehydroepiandrosterone sulfate (DHEAS) is an allosteric antagonist. We hypothesized these metabolites act similarly on GnRH neurons to modify the response to GABA. Whole-cell voltage-clamp recordings of GABAergic miniature postsynaptic currents (mPSCs) were made from green fluorescent protein-identified GnRH neurons in brain slices from diestrous mice. Glutamatergic currents were blocked with antagonists and action potentials blocked with tetrodotoxin, minimizing presynaptic effects of treatments. Allopregnanolone (5 microm) increased mPSC rate of rise, amplitude and decay time by 15.9 +/- 6.1%, 16.5 +/- 6.3%, and 58.3 +/- 18.6%, respectively (n = 7 cells). DHEAS (5 microm) reduced mPSC rate of rise (32.1 +/- 5.7%) and amplitude (27.6 +/- 4.3%) but did not alter decay time (n = 8). Effects of both neurosteroids were dose dependent between 0.1 and 10 microm. In addition to independent actions, DHEAS also reversed effects of allopregnanolone on rate of rise and amplitude so that these parameters were returned to pretreatment baseline values (n = 6). These data indicate allopregnanolone increases and DHEAS decreases responsiveness of GnRH neurons to activation of GABAA receptors by differentially modulating current flow through GABAA receptor chloride channels. This provides one mechanism for direct steroid feedback to GnRH neurons.

A presynaptic action of the neurosteroid pregnenolone sulfate on GABAergic synaptic transmission

The endogenous neurosteroid pregnenolone sulfate (PS) is known to enhance memory and cognitive function at nanomolar concentrations. However, the effect of these low concentrations on synaptic transmission has not been previously studied. The effects of PS on GABAA receptor-mediated inhibitory postsynaptic currents were studied in cultured hippocampal pyramidal neurons. Concentrations of PS similar to those endogenous in the hippocampus (10-30 nM) reduced the frequency of both action potential-dependent (spontaneous inhibitory postsynaptic current) and -independent (miniature inhibitory postsynaptic current; mIPSC) inhibitory postsynaptic currents. This effect of PS was mimicked by the selective sigma1 receptor agonist [2S-(2alpha,6alpha,11R]-1,2,3,4,5,6-hexahydro-6,11-dimethyl-3-(2-propenyl)-2,6-methano-3-benzazocin-8-ol hydrochloride [(+)-SKF 10047] and blocked the specific sigma1 receptor antagonists 1-[2-(3,4-dichlorophenyl)ethyl]-4-methylpiperazine dihydrochloride (BD-1063) and haloperidol and by pertussis toxin. The GABAB antagonist baclofen and the metabotropic glutamate receptor antagonist (R,S)-a-cyclopropyl-4-phosphonophenylglycine had no effect on the PS-mediated inhibition of mIPSC frequency. The postsynaptic effects of PS occurred at micromolar concentrations but not at nanomolar concentrations. A comparison of the pre- and postsynaptic effects of PS demonstrated that it was 100-fold more potent in inhibiting presynaptic GABAergic synaptic mechanisms than GABAA receptors. These studies demonstrate that concentrations of PS, similar to those endogenous in the hippocampus, inhibit GABAergic synaptic transmission by a presynaptic effect. PS causes specific activation of G protein-coupled sigma1 receptors, resulting in modulation of both action potential-dependent and -independent IPSCs. These findings improve our understanding of the physiological function of PS.

Activation-dependent properties of pregnenolone sulfate inhibition of GABAA receptor-mediated current

Sulfated steroids like pregnenolone sulfate (PS) are found endogenously in the central nervous system where they may modulate GABAA receptors. Understanding the mechanism of steroid inhibition is important for understanding the conditions under which endogenous steroids modulate GABAA receptor function, assessing their potential clinical utility, and for evaluating sulfated steroids as probes of receptor behaviour. Some previous studies suggest that sulfated steroid inhibition exhibits activation dependence, whilst other studies suggest only slow, time-dependent inhibition, perhaps reflecting slow PS association with receptors. We tested activation dependence in several ways. Steroid potency increased 2- to 3-fold with approximately 10-fold change in GABA concentration. PS inhibition of saturating partial agonist responses suggested that the level of channel activation, rather than receptor occupancy by agonist, is important for PS inhibition. Inhibition by sulfated steroids exhibited weak or no voltage dependence. Responses to rapid applications of exogenous GABA differed little whether PS was pre-applied or simply co-applied with GABA, consistent with the hypothesis that the actions of PS are facilitated by receptor activation. PS applied during steady-state GABA responses exhibited slow onset and offset rate constants. The offset, rather than onset, was significantly slowed by elevated GABA concentration. At hippocampal synapses, large, multiquantal IPSCs were inhibited more effectively by a fixed concentration of PS than small quantal content IPSCs, consistent with known 'pooling' of transmitter following multiquantal release. Picrotoxinin, although superficially similar to PS in its activation dependence, was dissimilar from PS in a number of details. In summary, PS inhibition exhibits activation dependence that may be explained by activation-dependent binding and altered desensitization.

Nongenomic steroid action: controversies, questions, and answers

Steroids may exert their action in living cells by several ways: 1). the well-known genomic pathway, involving hormone binding to cytosolic (classic) receptors and subsequent modulation of gene expression followed by protein synthesis. 2). Alternatively, pathways are operating that do not act on the genome, therefore indicating nongenomic action. Although it is comparatively easy to confirm the nongenomic nature of a particular phenomenon observed, e.g., by using inhibitors of transcription or translation, considerable controversy exists about the identity of receptors that mediate these responses. Many different approaches have been employed to answer this question, including pharmacology, knock-out animals, and numerous biochemical studies. Evidence is presented for and against both the participation of classic receptors, or proteins closely related to them, as well as for the involvement of yet poorly understood, novel membrane steroid receptors. In addition, clinical implications for a wide array of nongenomic steroid actions are outlined.

Effects of neurosteroids on epileptiform activity induced by picrotoxin and 4-aminopyridine in the rat hippocampal slice

The neurosteroids allopregnanolone (5alpha-pregnan-3alpha-ol-20-one; 5alpha,3alpha-P) and its 5beta-epimer pregnanolone (5beta,3alpha-P), and pregnenolone sulfate (PS) were examined for effects on spontaneous epileptiform discharges induced by 100 microM picrotoxin (PTX) and 55 microM 4-aminopyridine (4-AP) in the CA3 region of the rat hippocampal slice. At a concentration of 10 microM, 5alpha,3alpha-P partially reduced PTX-induced bursting and at 30 and 90 microM completely suppressed bursting. In contrast, 100 microM 5beta,3alpha-P failed to alter the discharge frequency. 5alpha,3alpha-P depressed 4-AP-induced bursting with similar potency as in the PTX model; 100 microM 5beta,3alpha-P was also partially effective. In the 4-AP model, 5alpha,3alpha-P inhibited both the more frequent predominantly positive-going potentials as well as the less frequent negative-going potentials that may be generated by synchronous GABAergic interneuron firing. PS enhanced the PTX bursting frequency and, in the 4-AP model, increased the frequency of negative potentials but did not alter the frequency of positive potentials. By itself, PS did not induce bursting. The effects of the steroids in the in vitro seizure models largely correspond with their activities on GABA(A) receptors; suppression of discharges may occur as a result of direct activation of these receptors rather than modulation of GABA-mediated synaptic responses. PTX and 4-AP-induced bursting in the hippocampal slice are useful models for directly assessing neurosteroid effects on seizure susceptibility under conditions that eliminate the factor of brain bioavailability.

Increased neurosteroid sensitivity of hippocampal GABAA receptors during postnatal development

To investigate developmental changes in neurosteroid modulation of GABA(A) receptors, whole-cell currents were elicited by applying GABA with allopregnanolone or pregnenolone sulfate (PS) to dentate granule cells (DGCs), acutely isolated from 7-14-day-old and adult rats. GABA evoked larger currents from dentate granule cells acutely isolated from adult rats (adult DGCs) than from neonatal DGCs, due to increased efficacy (1662+/-267 pA in adult DGCs versus 1094+/-198 pA in neonatal DGCs, P=0.004), and current density (0.072+/-0.01 pA/microm(2) in neonatal rat DGCs to 0.178+/-0.02 pA/microm(2) in adult DGCs), but unchanged potency (EC(50) was 18.5+/-2 microm in adult DGCs, and 26.6+/-7.9 microm in neonatal DGCs, P=0.21). Allopregnanolone sensitivity of GABA(A) receptor currents increased during development due to an increased potency (21.1+/-4.7 nM in adult DGCs versus 94.6+/-9 nM in neonatal DGCs, P=0.0002). The potency and efficacy of PS inhibition of GABA(A) receptor currents were remained unchanged during development (13+/-6 microm and 13.2+/-5.9 microm, P=0.71 and 85.5%+/-3.5% and 83.6%+/-0.8%, P=0.29, respectively). To investigate possible mechanism of developmental changes in GABA(A) receptor properties, in situ hybridization for alpha1, alpha4 and gamma2 subunit mRNAs was performed in dentate gyrus of the two age groups. Qualitatively, alpha1 subunit mRNA was expressed at low levels in neonatal rats while it was well expressed in adult rats. The alpha4 and gamma2 subunits were well expressed in the dentate gyrus of adult and neonatal rats. Immunohistochemical staining for alpha1 subunit in hippocampal slices from neonatal and adult rats was examined under confocal laser scanning microscope. This demonstrated that cell bodies and dendrites of granule cells are moderately positive for the alpha1 staining in adult rats but weakly so in neonatal rats. Higher-magnification images demonstrate large number of clusters of alpha1-subunit in the cell bodies of dentate granule cells of adult rat but rare clusters in granule cells of neonatal rats. Maturation of GABA(A) receptors in DGCs is characterized by increased number of GABA(A) receptors that are more sensitive to endogenous neurosteroid allopregnanolone, which might be related to increased expression of alpha1 subunit.

Inhibition by pregnenolone sulfate of nicotinic acetylcholine response in adrenal chromaffin cells

To evaluate whether pregnenolone sulfate, an abundant neurosteroid in the brain, modulates nicotinic receptor-mediated responses, the effect of pregnenolone sulfate on acetylcholine-induced catecholamine secretion was investigated in cultured bovine adrenal chromaffin cells. Pregnenolone sulfate inhibited acetylcholine-induced catecholamine secretion (IC(50): 27 microM). In addition, pregnenolone sulfate inhibited acetylcholine-induced Na(+) (IC(50): 12 microM) and Ca(2+) (IC(50): 20 microM) influxes. However, pregnenolone sulfate did not inhibit either catecholamine secretion or Ca(2+) influx stimulated by high K(+). Binding of [3H]nicotine to nicotinic receptors was not altered by pregnenolone sulfate. The inhibitory effect on the acetylcholine-induced secretion was insurmountable by increasing acetylcholine concentrations, but was enhanced by decreasing external Na(+) concentrations. These results suggest strongly that pregnenolone sulfate noncompetitively inhibits nicotinic receptor-operated ion channels, thereby suppressing Na(+) influx through the channels and, consequently, attenuates both Ca(2+) influx and catecholamine secretion. Our results further indicate that pregnenolone sulfate may modulate nicotinic receptor-mediated responses in the brain.

Neuroactive steroids, their precursors and polar conjugates during parturition and postpartum in maternal and umbilical blood: 3.3beta-hydroxy-5-ene steroids

Five 3beta-hydroxy-5-ene steroids involved in the metabolic route from pregnenolone sulfate to dehydroepiandrosterone and its sulfate, of which three are known allosteric modulators of neurotransmitter receptors, were monitored in the serum of 20 women around parturition. In addition, their levels in maternal and umbilical serum were compared at delivery. On the basis of these data, a scheme of steroid biosynthesis in maternal organism during the critical stages around parturition is proposed. In maternal serum, all the steroids except dehydroepiandrosterone sulfate decreased during labor and even first day after delivery, although their changes were less distinct the more distant from pregnenolone sulfate (PregS) in the metabolic pathway. Calculation of product/immediate precursor ratios in maternal serum over all stages around parturition enabled identification of the respective changes in the activities of the relevant enzymes. The ratio of 17-hydroxypregnenolone/pregnenolone did not change significantly, while that of dehydroepiandrosterone/17-hydroxypregnenolone grew, indicating increased C17,20 side chain cleavage on the account of C17-hydroxylation both catalyzed by C17-hydroxylase-C17,20-lyase. As was shown by factor analysis, the changes in the maternal steroids were associated with a single common factor, which strongly correlated with all the steroids except dehydroepiandrosterone sulfate. The lack of change in the pregnenolone sulfate/pregnenolone ratio and a marked increase of the ratio dehydroepiandrosterone sulfate to unconjugated dehydroepiandrosterone indicate a different means of formation of both steroid sulfates. On the basis of these data, a scheme of steroid biosynthesis in maternal organism during the critical stages around parturition is proposed.

Preclinical development of neurosteroids as neuroprotective agents for the treatment of neurodegenerative diseases

Recent literature has emphasized the unique role that the neurosteroid subclass of steroids, which includes dehydroepiandrosterone (DHEA) and dehydroepiandrosterone sulfate (DHEAS), play in the developing and adult central nervous system (CNS). Both DHEA and DHEAS are found in abundance in the CNS (Majewska, 1995), and both can be synthesized and metabolized in the brain of many species (Baulieu, 1981, 1998; Corpéchot et al., 1981, 1983; Zwain and Yen, 1999). DHEA and DHEAS have been implicated as potential signaling molecules for neocortical organization during neuronal development, suggesting that they have trophic factor-like activity (neurotrophic or neurotropic) or can interact with various neurotransmitter systems to promote neuronal remodeling (Compagnone and Mellon, 1998; Mao and Barger, 1998). Consistent with a neurotrophic role for these steroids, studies have shown that DHEAS protects certain neuronal populations against neurotoxic insults inflicted by the excitatory amino acid glutamate (Kimonides et al., 1998; Mao and Barger, 1998). This finding suggests that DHEAS may be useful in treating neurodegenerative diseases in which excitotoxicity is believed to be the underlying cause or a major contributor to cell death. Moreover, because DHEA and DHEAS are multifunctional and exhibit a variety of properties in the CNS, including memory consolidation, neuroprotection, and reduction of neurodegeneration (Majewska, 1992, 1995; Lapchak et al., 2000), their potential therapeutic benefits may be extended to include the treatment of other neurodegenerative diseases not directly linked to excitotoxicity.

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.

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.

Influence of membrane cholesterol on modulation of the GABA(A) receptor by neuroactive steroids and other potentiators

1. The influence of membrane cholesterol on some pharmacological properties of the GABA(A) receptor was investigated in acutely dissociated rat hippocampal neurones with whole cell patch clamp recording. The cholesterol levels were varied between 56% and 235% control using methyl-beta-cyclodextrin as the cholesterol carrier. 2. Enrichment of neurones with cholesterol increased the effects of the non-steroidal GABA potentiators propofol, flunitrazepam and pentobarbitone. A similar result was obtained after pre-incubation of neurones with epicholesterol, the 3alpha-hydroxy isomer of cholesterol. 3. In contrast, the effects of the steroidal GABA potentiators pregnanolone and alfaxalone were reduced by cholesterol enrichment, but not by epicholesterol. Depletion of membrane cholesterol increased the potentiation of GABA by pregnanolone and alfaxalone but did not affect the non-steroidal potentiators. 4. The steroidal antagonist of GABA, pregnenolone sulphate, reduced the maximum response to GABA. This effect, also, was diminished in cholesterol-enriched neurones and enhanced in cholesterol-depleted neurones. 5. The effects of the cholesterol manipulations that were selective for the steroidal modulators of GABA are suggested to arise from direct interactions between membrane cholesterol and the GABA(A) receptor. The separate effects on the non-steroidal potentiators of GABA of cholesterol-enrichment or addition of epicholesterol to the neurones are suggested to be due to changes in membrane fluidity. 6. In view of the likely physiological modulation of GABA(A) receptors by endogenous neuroactive steroids and evidence of the in vivo lability of membrane cholesterol, the present observations may have physiological as well as pharmacological relevance.

Analysis of GABA(A)- and GABA(B)-receptor mediated effects on intracellular Ca(2+) in DRG hybrid neurones

1. Using pharmacological analysis and fura-2 spectrofluorimetry, we examined the effects of gamma-aminobutyric acid (GABA) and related substances on intracellular Ca(2+) concentration ([Ca(2+)]i) of hybrid neurones, called MD3 cells. The cell line was produced by fusion between a mouse neuroblastoma cell and a mouse dorsal root ganglion (DRG) neurone. 2. MD3 cells exhibited DRG neurone-like properties, such as immunoreactivity to microtubule-associated protein-2 and neurofilament proteins. Bath applications of capsaicin and alpha, beta-methylene adenosine triphosphate reversibly increased [Ca(2+)]i. However, repeated applications of capsaicin were much less effective. 3. Pressure applications of GABA (100 microM), (Z)-3-[(aminoiminomethyl) thio] prop-2-enoic acid sulphate (ZAPA; 100 microM), an agonist at low affinity GABA(A)-receptors, or KCl (25 mM), transiently increased [Ca(2+)]i. 4. Bath application of bicuculline (100 nM - 100 microM), but not picrotoxinin (10 - 25 microM), antagonized GABA-induced increases in [Ca(2+)]i in a concentration-dependent manner (IC(50)=9.3 microM). 5. Ca(2+)-free perfusion reversibly abolished GABA-evoked increases in [Ca(2+)]i. Nifedipine and nimodipine eliminated GABA-evoked increases in [Ca(2+)]i. These results imply GABA response dependence on extracellular Ca(2+). 6. Baclofen (500 nM - 100 microM) activation of GABA(B)-receptors reversibly attenuated KCl-induced increases in [Ca(2+)]i in a concentration-dependent manner (EC(50)=1.8 microM). 2-hydroxy-saclofen (1 - 20 microM) antagonized the baclofen-depression of the KCl-induced increase in [Ca(2+)]i. 7. In conclusion, GABA(A)-receptor activation had effects similar to depolarization by high external K(+), initiating Ca(2+) influx through high voltage-activated channels, thereby transiently elevating [Ca(2+)]i. GABA(B)-receptor activation reduced Ca(2+) influx evoked by depolarization, possibly at Ca(2+)-channel sites in MD3 cells.

Effect of treatment of hypothyroidism on the plasma concentrations of neuroactive steroids and homocysteine

Autoimmune thyroiditis with hypothyroidism is frequently accompanied by symptoms of psychiatric disorders and atherogenic changes in lipid metabolism. Recent studies suggest that some neuroactive steroids and homocysteine are involved in the pathophysiology of both disorders. Homocysteine metabolism may be affected by some steroids. We were interested if the treatment of hypothyroidism would affect the above factors. We studied plasma concentrations of allopregnanolone, pregnenolone sulfate, dehydroepiandosterone and its sulfate, progesterone, estradiol and homocysteine in 14 patients (12 women, 2 men) during the 3-month treatment with levothyroxine. Steroids and thyroid function were monitored by measuring thyrotropin, free triiodothyronine, free thyroxine and levels of thyroid antimicrosomal antibodies and antibodies to thyroglobulin. We have found that with the restoration of the thyrotropin level, free triiodothyronine, free thyroxine and homocysteine levels decreased, but the levels of steroids were not significantly altered. Steroid concentrations correlated negatively with the level of thyroid antimicrosomal antibodies.

Role of human GABA(A) receptor beta3 subunit in insecticide toxicity

The gamma-aminobutyric acid type A (GABA(A)) receptor is the target for the major insecticides alpha-endosulfan, lindane, and fipronil and for many analogs. Their action as chloride channel blockers is directly measured by binding studies with [(3)H]ethynylbicycloorthobenzoate ([(3)H]EBOB). This study tests the hypothesis that GABA(A) receptor subunit composition determines the sensitivity and selectivity of insecticide toxicity. Human receptor subtypes were expressed individually (alpha1, alpha6, beta1, beta3, and gamma2) and in combination in insect Sf9 cells. Binding parameters were similar for [(3)H]EBOB in the beta3 homooligomer, alpha1beta3gamma2 heterooligomer, and native brain membranes, but toxicological profiles were very different. Surprisingly, alpha-endosulfan, lindane, and fipronil were all remarkably potent on the recombinant beta3 homooligomeric receptor (IC50 values of 0.5-2.4 nM), whereas they were similar in potency on the alpha1beta3gamma2 subtype (IC50 values of 16-33 nM) and highly selective on the native receptor (IC50 values of 7.3, 306, and 2470 nM, respectively). The selectivity order for 29 insecticides and convulsants as IC50 ratios for native/beta3 or alpha1beta3gamma2/beta3 was as follows: fipronil > lindane > 19 other insecticides including alpha-endosulfan and picrotoxinin > 4 trioxabicyclooctanes and dithianes (almost nonselective) > tetramethylenedisulfotetramine, 4-chlorophenylsilatrane, or alpha-thujone. Specificity between mammals and insects at the target site (fipronil > lindane > alpha-endosulfan) paralleled that for toxicity. Potency at the native receptor is more predictive for inhibition of GABA-stimulated chloride uptake than that at the beta3 or alpha1beta3gamma2 receptors. Therefore, the beta3 subunit contains the insecticide target and other subunits differentially modulate the binding to confer compound-dependent specificity and selective toxicity.

Pregnenolone sulfate block of GABA(A) receptors: mechanism and involvement of a residue in the M2 region of the alpha subunit

Neurosteroids are produced in the brain, and can have rapid actions on membrane channels of neurons. Pregnenolone sulfate (PS) is a sulfated neurosteroid which reduces the responses of the [gamma]-aminobutyric acid A (GABA(A)) receptor. We analysed the actions of PS on single-channel currents from recombinant GABA(A) receptors formed from [alpha]1, [beta]2 and [gamma]2L subunits. Currents were elicited by a concentration of GABA eliciting a half-maximal response (50 microM) and a saturating concentration (1 mM). PS reduced the duration of clusters of single-channel activity at either concentration of GABA. PS had no discernable effect on rapid processes: no effects were apparent on channel opening and closing, nor on GABA affinity, and a rapidly recovering desensitised state was not affected. Instead, PS produced a slowly developing block which occurred at a similar rate for receptors with open or closed channels and with one or two bound GABA molecules. The rate of block was independent of membrane potential, implying that the charged sulfate moiety does not move through the membrane field. Change in a specific residue near the intracellular end of the channel lining portion of the [alpha]1 subunit had a major effect on the rate of block. Mutation of the residue [alpha]1 V256S reduced the rate of block by 30-fold. A mutation at the homologous position of the [beta]2 subunit ([beta]2 A252S) had no effect, nor did a complementary mutation in the [gamma]2L subunit ([gamma]2L S266A). It seems likely that this residue is involved in a conformational change underlying block by PS, instead of forming part of the binding site for PS.

Dehydroepiandrosterone sulfate is neuroprotective in a reversible spinal cord ischemia model: possible involvement of GABA(A) receptors

BACKGROUND AND PURPOSE

Dehydroepiandrosterone (DHEA) and DHEA sulfate (DHEAS) may function as neurotrophic or neuroprotective factors to protect central nervous system (CNS) neurons against a variety of insults, including excitotoxicity. The present study evaluated the pharmacological effects of DHEAS in a reversible spinal cord ischemia model.

METHODS

DHEAS was administered (50 mg/kg IV) 5 or 30 minutes after the start of occlusion to groups of rabbits exposed to ischemia induced by temporary (15 to 60 minutes) occlusion of the infrarenal aorta. The group P(50) represents the duration of ischemia (in minutes) associated with 50% probability of resultant permanent paraplegia.

RESULTS

The P(50) of the vehicle-treated control group, when behavioral analysis was assessed 18 hours after aortal occlusion, was 28.8+/-2.0 minutes. Neuroprotection was demonstrated if a drug significantly prolonged the P(50) compared with the vehicle-treated control group. Treatment with DHEAS at 5 minutes significantly (P<0.05) prolonged the P(50) of the group to 36.8+/-3.9 minutes. In addition, the DHEAS effect appeared durable, because a significant difference between the control and DHEAS-treated groups was still measurable at the 4-day time point. At 4 days, the P(50) of the control group was 26.1+/-2.2 minutes, whereas the P(50) for the DHEAS-treated group was 38.6+/-5. 9 minutes. DHEAS was not neuroprotective if administered 30 minutes after occlusion. In addition, the GABA(A) antagonist bicuculline abolished the neuroprotective effect of DHEAS.

CONCLUSIONS

The present study suggests that neurosteroids may have substantial therapeutic benefit for the treatment of ischemic stroke.

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.

Differential modulatory actions of the volatile convulsant flurothyl and its anesthetic isomer at inhibitory ligand-gated ion channels

A challenge for theories of general anesthesia is the existence of compounds predicted to be anesthetics but which, instead, do not produce anesthesia and often elicit other behavioral effects such as convulsions. This study focused on flurothyl (bis[2,2, 2-trifluoroethyl] ether), a potent volatile convulsant, and its anesthetic isomer, 'iso-flurothyl' (1,1,1,3,3, 3-hexafluoro-2-methoxypropane). The effects of flurothyl and iso-flurothyl were studied using the whole-cell patch-clamp technique on agonist-activated chloride currents in human GABA(A), glycine, and GABA(C) rho(1) receptors expressed in HEK 293 cells. GABA(A) and glycine receptors are promising molecular targets for the actions of inhaled ether general anesthetics. Flurothyl acted as a non-competitive antagonist at GABA(A) alpha(2)beta(1) and alpha(2)beta(1)gamma(2s) receptors, but had no effect at glycine alpha(1) receptors. Flurothyl had biphasic actions on GABA responses at GABA(C) rho(1) receptors. In contrast, iso-flurothyl enhanced ('potentiated') submaximal agonist responses at GABA(A) and glycine receptors, but had no effect on GABA responses at GABA(C) rho(1) receptors. Point mutations in GABA(A) and glycine receptor subunits, which have been previously shown to abolish potentiation of agonist responses by the ether anesthetics enflurane and isoflurane, also ablated potentiation of agonist responses by iso-flurothyl. These same mutations in the GABA(A) receptor had only modest effects on the inhibitory actions of flurothyl. GABA(A) receptors with mutations conferring insensitivity to antagonism by picrotoxin were still inhibited by flurothyl, suggesting that picrotoxin and flurothyl antagonize GABA responses by distinct sites or mechanisms of action. In summary, antagonism of GABA(A) receptors is likely to account for the convulsant effects of flurothyl, while the general anesthetic actions of iso-flurothyl, like those of other ether anesthetics, may be related to positive modulation of GABA(A) and/or glycine receptors.