Subunit dependent modulation of GABAA receptor function by neuroactive steroids

Local effect of allopregnanolone in rat ovarian steroidogenesis, follicular and corpora lutea development

Allopregnanolone (ALLO) is a known neurosteroid and a progesterone metabolite synthesized in the ovary, CNS, PNS, adrenals and placenta. Its role in the neuroendocrine control of ovarian physiology has been studied, but its in situ ovarian effects are still largely unknown. The aims of this work were to characterize the effects of intrabursal ALLO administration on different ovarian parameters, and the probable mechanism of action. ALLO administration increased serum progesterone concentration and ovarian 3β-HSD2 while decreasing 20α-HSD mRNA expression. ALLO increased the number of atretic follicles and the number of positive TUNEL granulosa and theca cells, while decreasing positive PCNA immunostaining. On the other hand, there was an increase in corpora lutea diameter and PCNA immunostaining, whereas the count of TUNEL-positive luteal cells decreased. Ovarian angiogenesis and the immunohistochemical expression of GABAA receptor increased after ALLO treatment. To evaluate if the ovarian GABAA receptor was involved in these effects, we conducted a functional experiment with a specific antagonist, bicuculline. The administration of bicuculline restored the number of atretic follicles and the diameter of corpora lutea to normal values. These results show the actions of ALLO on the ovarian physiology of the female rat during the follicular phase, some of them through the GABAA receptor. Intrabursal ALLO administration alters several processes of the ovarian morpho-physiology of the female rat, related to fertility and oocyte quality.

Cryo-EM structures reveal native GABAA receptor assemblies and pharmacology

Type A γ-aminobutyric acid receptors (GABAARs) are the principal inhibitory receptors in the brain and the target of a wide range of clinical agents, including anaesthetics, sedatives, hypnotics and antidepressants1-3. However, our understanding of GABAAR pharmacology has been hindered by the vast number of pentameric assemblies that can be derived from 19 different subunits4 and the lack of structural knowledge of clinically relevant receptors. Here, we isolate native murine GABAAR assemblies containing the widely expressed α1 subunit and elucidate their structures in complex with drugs used to treat insomnia (zolpidem (ZOL) and flurazepam) and postpartum depression (the neurosteroid allopregnanolone (APG)). Using cryo-electron microscopy (cryo-EM) analysis and single-molecule photobleaching experiments, we uncover three major structural populations in the brain: the canonical α1β2γ2 receptor containing two α1 subunits, and two assemblies containing one α1 and either an α2 or α3 subunit, in which the single α1-containing receptors feature a more compact arrangement between the transmembrane and extracellular domains. Interestingly, APG is bound at the transmembrane α/β subunit interface, even when not added to the sample, revealing an important role for endogenous neurosteroids in modulating native GABAARs. Together with structurally engaged lipids, neurosteroids produce global conformational changes throughout the receptor that modify the ion channel pore and the binding sites for GABA and insomnia medications. Our data reveal the major α1-containing GABAAR assemblies, bound with endogenous neurosteroid, thus defining a structural landscape from which subtype-specific drugs can be developed.

Regulated assembly and neurosteroid modulation constrain GABA A receptor pharmacology in vivo

Type A GABA receptors (GABA _A Rs) are the principal inhibitory receptors in the brain and the target of a wide range of clinical agents, including anesthetics, sedatives, hypnotics, and antidepressants. However, our understanding of GABA _A R pharmacology has been hindered by the vast number of pentameric assemblies that can be derived from a total 19 different subunits and the lack of structural knowledge of clinically relevant receptors. Here, we isolate native murine GABA _A R assemblies containing the widely expressed α ₁ subunit, and elucidate their structures in complex with drugs used to treat insomnia (zolpidem and flurazepam) and postpartum depression (the neurosteroid allopregnanolone). Using cryo-EM analysis and single-molecule photobleaching experiments, we uncover only three structural populations in the brain: the canonical α ₁ β2γ ₂ receptor containing two α ₁ subunits and two unanticipated assemblies containing one α ₁ and either an α ₂ , α ₃ or α ₅ subunit. Both of the noncanonical assemblies feature a more compact arrangement between the transmembrane and extracellular domains. Interestingly, allopregnanolone is bound at the transmembrane α/β subunit interface, even when not added to the sample, revealing an important role for endogenous neurosteroids in modulating native GABA _A Rs. Together with structurally engaged lipids, neurosteroids produce global conformational changes throughout the receptor that modify both the pore diameter and binding environments for GABA and insomnia medications. Together, our data reveal that GABA _A R assembly is a strictly regulated process that yields a small number of structurally distinct complexes, defining a structural landscape from which subtype-specific drugs can be developed.

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.

Possible influence of neurosteroids in the anxiolytic effects of alpha-casozepine

Alpha-casozepine (α CZP), a tryptic hydrolysate of milk casein is a decapeptide shown to promote sleep and produce anxiolytic or anticonvulsant activity. Intriguingly, studies indicate structural similarities to benzodiazepine (BZD)-like molecules (e.g., diazepam), resulting in positive modulation of γ-aminobutyric acid A type (GABA_A) receptors. However, some unexplained anomalous behaviour of α-CZP includes 1) 1000 times less affinity for BZD site on GABA_A receptor in vitro conditions, whereas in vivo it showed 10-fold increased affinity when compared to diazepam; 2) anxiolytic effects were observed only in stressed conditions and 3) unlike diazepam, it failed to exhibit dependence or habituation. Interestingly, neurosteroids like allopregnanolone or its analogues that are synthesized de novo have both genomic and non-genomic actions. The rapid nongenomic neuronal inhibition of these compounds is mediated by GABA_A receptors through autocrine and paracrine actions. Studies have shown that changes in the levels of neurosteroids during acute (rise) and chronic stress (decreased), consequently, altering the senetivity of GABA_A receptor subunits. Neurosteroids even at low concentration (nanomolar range) potentiate the response of GABA indirectly, while at higher concentrations they directly activate the receptor-channel complex. Interestingly, coadministration of neurosteroids and BZPs has shown not only to prevent the development of tolerance of BZP and augmented recovery from BZP withdrawal anxiety and hyperactivity in mice. The combination also produced synergetic anxiolytic effects. Taken together, the evidence suggests possible implications of neurosteroids in the actions of CZP via BZD receptors. The present hypothesis brings out the possible role of neurosteroids and the various factors that might participate in CZP-induce anxiolytic effects.

From Cannabinoids and Neurosteroids to Statins and the Ketogenic Diet: New Therapeutic Avenues in Rett Syndrome?

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder caused mainly by mutations in the MECP2 gene, being one of the leading causes of mental disability in females. Mutations in the MECP2 gene are responsible for 95% of the diagnosed RTT cases and the mechanisms through which these mutations relate with symptomatology are still elusive. Children with RTT present a period of apparent normal development followed by a rapid regression in speech and behavior and a progressive deterioration of motor abilities. Epilepsy is one of the most common symptoms in RTT, occurring in 60 to 80% of RTT cases, being associated with worsening of other symptoms. At this point, no cure for RTT is available and there is a pressing need for the discovery of new drug candidates to treat its severe symptoms. However, despite being a rare disease, in the last decade research in RTT has grown exponentially. New and exciting evidence has been gathered and the etiopathogenesis of this complex, severe and untreatable disease is slowly being unfolded. Advances in gene editing techniques have prompted cure-oriented research in RTT. Nonetheless, at this point, finding a cure is a distant reality, highlighting the importance of further investigating the basic pathological mechanisms of this disease. In this review, we focus our attention in some of the newest evidence on RTT clinical and preclinical research, evaluating their impact in RTT symptomatology control, and pinpointing possible directions for future research.

Neurosteroids in Schizophrenia: Pathogenic and Therapeutic Implications

Neurosteroids are a group of important endogenous molecules affecting many neural functions in the brain. Increasing evidence suggests a possible role of these neurosteroids in the pathology and symptomatology of schizophrenia (SZ) and other mental disorders. The aim of this review is to summarize the current knowledge about the neural functions of neurosteroids in the brain, and to evaluate the role of the key neurosteroids as candidate modulators in the etiology and therapeutics of SZ. The present paper provides a brief introduction of neurosteroid metabolism and distribution, followed by a discussion of the mechanisms underlying neurosteroid actions in the brain. The content regarding the modulation of the GABAA receptor is elaborated, given the considerable knowledge of its interactions with other neurotransmitter and neuroprotective systems, as well as its ameliorating effects on stress that may play a role in the SZ pathophysiology. In addition, several preclinical and clinical studies suggested a therapeutic benefit of neurosteroids in SZ patients, even though the presence of altered neurosteroid pathways in the circulating blood and/or brain remains debatable. Following treatment of antipsychotic drugs in SZ, therapeutic benefits have also been linked to the regulation of neurosteroid signaling. Specifically, the neurosteroids such as pregnenolone and dehydroepiandrosterone affect a broad spectrum of behavioral functions through their unique molecular characteristics and may represent innovative therapeutic targets for SZ. Future investigations in larger cohorts with long-term follow-ups will be required to ascertain the neuropsychopharmacological role of this yet unexploited class of neurosteroid agents.

Neurosteroid interactions with synaptic and extrasynaptic GABA(A) receptors: regulation of subunit plasticity, phasic and tonic inhibition, and neuronal network excitability

RATIONALE

Neurosteroids are steroids synthesized within the brain with rapid effects on neuronal excitability. Allopregnanolone, allotetrahydrodeoxycorticosterone, and androstanediol are three widely explored prototype endogenous neurosteroids. They have very different targets and functions compared to conventional steroid hormones. Neuronal γ-aminobutyric acid (GABA) type A (GABA(A)) receptors are one of the prime molecular targets of neurosteroids.

OBJECTIVE

This review provides a critical appraisal of recent advances in the pharmacology of endogenous neurosteroids that interact with GABA(A) receptors in the brain. Neurosteroids possess distinct, characteristic effects on the membrane potential and current conductance of the neuron, mainly via potentiation of GABA(A) receptors at low concentrations and direct activation of receptor chloride channel at higher concentrations. The GABA(A) receptor mediates two types of inhibition, now characterized as synaptic (phasic) and extrasynaptic (tonic) inhibition. Synaptic release of GABA results in the activation of low-affinity γ2-containing synaptic receptors, while high-affinity δ-containing extrasynaptic receptors are persistently activated by the ambient GABA present in the extracellular fluid. Neurosteroids are potent positive allosteric modulators of synaptic and extrasynaptic GABA(A) receptors and therefore enhance both phasic and tonic inhibition. Tonic inhibition is specifically more sensitive to neurosteroids. The resulting tonic conductance generates a form of shunting inhibition that controls neuronal network excitability, seizure susceptibility, and behavior.

CONCLUSION

The growing understanding of the mechanisms of neurosteroid regulation of the structure and function of the synaptic and extrasynaptic GABA(A) receptors provides many opportunities to create improved therapies for sleep, anxiety, stress, epilepsy, and other neuropsychiatric conditions.

Translocator protein (18 kDa) as a target for novel anxiolytics with a favourable side-effect profile

Anxiety disorders are frequent and highly disabling diseases with considerable socio-economic impact. In the treatment of anxiety disorders, benzodiazepines (BZDs) as direct modulators of the GABA(A) receptor are used as emergency medication because of their rapid onset of action. However, BZDs act also as sedatives and rather quickly induce tolerance and abuse liability associated with withdrawal symptoms. Antidepressants with anxiolytic properties are also applied as first line long-term treatment of anxiety disorders. However, the onset of action of antidepressants takes several weeks. Obviously, novel pharmacological approaches are needed that combine a rapid anxiolytic efficacy with the lack of tolerance induction, abuse liability and withdrawal symptoms. Neurosteroids are potent allosteric modulators of GABA(A) receptor function. The translocator protein (18 kDa) (TSPO) plays an important role for the synthesis of neurosteroids by promoting the transport of cholesterol from the outer to the inner mitochondrial membrane, which is the rate-limiting step in neurosteroidogenesis. Etifoxine not only exerts anxiolytic effects as a TSPO ligand by enhancing neurosteroidogenesis, but also acts as a weak direct GABA(A) receptor enhancer. The TSPO ligand XBD173 enhances GABAergic neurotransmission via the promotion of neurosteroidogenesis without direct effects at the GABA(A) receptor. XBD173 counteracts pharmacologically-induced panic in rodents in the absence of sedation and tolerance development. Also in humans, XBD173 displays antipanic activity and does not cause sedation and withdrawal symptoms after 7 days of treatment. XBD173 therefore appears to be a promising candidate for fast-acting anxiolytic drugs with less severe side-effects than BZDs. In this review, we focus on the pathophysiology of anxiety disorders and TSPO ligands as a novel pharmacological approach in the treatment of these disorders.

Translocator protein (18 kDa) (TSPO) as a therapeutic target for neurological and psychiatric disorders

The translocator protein (18 kDa) (TSPO) is localized primarily in the outer mitochondrial membrane of steroid-synthesizing cells, including those in the central and peripheral nervous system. One of its main functions is the transport of the substrate cholesterol into mitochondria, a prerequisite for steroid synthesis. TSPO expression may constitute a biomarker of brain inflammation and reactive gliosis that could be monitored by using TSPO ligands as neuroimaging agents. Moreover, initial clinical trials have indicated that TSPO ligands might be valuable in the treatment of neurological and psychiatric disorders. This Review focuses on the biology and pathophysiology of TSPO and the potential of currently available TSPO ligands for the diagnosis and treatment of neurological and psychiatric disorders.

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.

Brain pathways mediating the pro-aggressive effect of the steroid sulfatase (Sts) gene

STS is the single enzyme that converts all steroid sulfates into their free steroid forms. Initiation of attack behavior against conspecific male mice appeared to be linked to Sts. Here we have confirmed the role of Sts through an association study with attack behavior. Previous studies indicated a positive correlation between the initiation of attack behavior and liver STS concentration levels in male mice, but this finding was not compatible with established knowledge of STS mechanisms. High STS concentrations induce low concentrations of sulfated steroids. Sulfated and un-sulfated steroids are GABA(A) receptor agonists and NMDA receptor positive allosteric modulators. This synaptic pattern of functioning can generate attack behavior and we have confirmed here that an injection of the sulfated steroid dehydroepiandrosterone sulfate (DHEA-S) increases attack behavior. To solve the paradox, we measured the transcription activity of the genes underlying the pathways involved in the hydrolysis of sulfated steroids and leading to the formation of un-conjugated steroids in the mouse brain. We observed that the genes monitoring the steroid biosynthesis pathways exhibited a transcription pattern resulting in an increased sulfotransferase activity in the attacking males that could counterbalance the de-sulfating activity of Sts in the attacking mice.

Allopregnanolone-induced rise in intracellular calcium in embryonic hippocampal neurons parallels their proliferative potential

Background

Factors that regulate intracellular calcium concentration are known to play a critical role in brain function and neural development, including neural plasticity and neurogenesis. We previously demonstrated that the neurosteroid allopregnanolone (APα; 5α-pregnan-3α-ol-20-one) promotes neural progenitor proliferation in vitro in cultures of rodent hippocampal and human cortical neural progenitors, and in vivo in triple transgenic Alzheimer's disease mice dentate gyrus. We also found that APα-induced proliferation of neural progenitors is abolished by a calcium channel blocker, nifedipine, indicating a calcium dependent mechanism for the proliferation.

Methods

In the present study, we investigated the effect of APα on the regulation of intracellular calcium concentration in E18 rat hippocampal neurons using ratiometric Fura2-AM imaging.

Results

Results indicate that APα rapidly increased intracellular calcium concentration in a dose-dependent and developmentally regulated manner, with an EC₅₀ of 110 ± 15 nM and a maximal response occurring at three days in vitro. The stereoisomers 3β-hydroxy-5α-hydroxy-pregnan-20-one, and 3β-hydroxy-5β-hydroxy-pregnan-20-one, as well as progesterone, were without significant effect. APα-induced intracellular calcium concentration increase was not observed in calcium depleted medium and was blocked in the presence of the broad spectrum calcium channel blocker La³⁺, or the L-type calcium channel blocker nifedipine. Furthermore, the GABA_A receptor blockers bicuculline and picrotoxin abolished APα-induced intracellular calcium concentration rise.

Conclusion

Collectively, these data indicate that APα promotes a rapid, dose-dependent, stereo-specific, and developmentally regulated increase of intracellular calcium concentration in rat embryonic hippocampal neurons via a mechanism that requires both the GABA_A receptor and L-type calcium channel. These data suggest that APα-induced intracellular calcium concentration increase serves as the initiation mechanism whereby APα promotes neurogenesis.

Conserved site for neurosteroid modulation of GABA A receptors

This study addresses whether the potentiation site for neurosteroids on GABA(A) receptors is conserved amongst different GABA(A) receptor isoforms. The neurosteroid potentiation site was previously identified in the alpha1beta2gamma2S receptor by mutation of Q241 to methionine or leucine, which reduced the potentiation of GABA currents by the naturally occurring neurosteroids, allopregnanolone or tetrahydrodeoxycorticosterone (THDOC). By using heterologous expression of GABA(A) receptors in HEK cells, in combination with whole-cell patch clamp recording methods, a relatively consistent potentiation by allopregnanolone of GABA-activated currents was evident for receptors composed of one alpha subunit isoform (alpha2-5) assembled with beta3 and gamma2S subunits. Using mutant alphabetagamma receptors, the neurosteroid potentiation was universally dependent on the conserved glutamine residue in M1 of the respective alpha subunit. Studying wild-type and mutant receptors composed of alpha4beta3delta subunits revealed that the delta subunit is unlikely to contribute to the neurosteroid potentiation binding site and probably affects the efficacy of potentiation. Thus, in keeping with the ability of neurosteroids to potentiate GABA currents via a broad variety of GABA(A) receptor isoforms in neurons, the potentiation site is structurally highly conserved on this important neurotransmitter receptor family.

Neuroactive steroids and GABAA receptor plasticity in the brain of the WAG/Rij rat, a model of absence epilepsy

The role of neuroactive steroids and GABA(A) receptors in the generation of spontaneous spike-and-wave discharges (SWDs) was investigated in the WAG/Rij rat model of absence epilepsy. The plasma, cerebrocortical, and thalamic concentrations of the progesterone metabolite 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha,5alpha-TH PROG) were increased in the WAG/Rij rat at 2 months of age compared with those in control (Wistar) rats. In contrast, the brain and peripheral levels of 3alpha,5alpha-tetrahydrodeoxycorticosterone (3alpha,5alpha-TH DOC) did not differ between the two rat strains at this age. At 6 months of age, when absence epilepsy worsens in WAG/Rij rats, the plasma concentration of 3alpha,5alpha-TH PROG remained high whereas that of 3alpha,5alpha-TH DOC had increased, the cerebrocortical levels of both 3alpha,5alpha-TH PROG and 3alpha,5alpha-TH DOC had increased, and the thalamic concentrations of these metabolites had decreased. At 6 months of age the expression of the alpha(4) and delta subunits of the GABA(A) receptor in relay nuclei was increased. Finally, chronic stress induced by social isolation elicited a reduction in the amount of 3alpha,5alpha-TH PROG in the thalamus of 2-month-old WAG/Rij rats that was associated with a reduction in the number and overall duration of SWDs at 6 months of age. Absence epilepsy in the WAG/Rij rat is thus associated with changes in the abundance of neuroactive steroids and in the expression of specific GABA(A) receptor subunits in the thalamus, a brain area key to the pathophysiology of this condition.

The relevance of neuroactive steroids in schizophrenia, depression, and anxiety disorders

1. Neuroactive steroids are steroid hormones that exert rapid, nongenomic effects at ligand-gated ion channels. There is increasing awareness of the possible role of these steroids in the pathology and manifestation of symptoms of psychiatric disorders. The aim of this paper is to review the current knowledge of neuroactive steroid functioning in the central nervous system, and to assess the role of neuroactive steroids in the pathophysiology and treatment of symptoms of schizophrenia, depression, and anxiety disorders. Particular emphasis will be placed on GABAA receptor modulation, given the extensive knowledge of the interactions between this receptor complex, neuroactive steroids, and psychiatric illness. 2. A brief description of neuroactive steroid metabolism is followed by a discussion of the interactions of neuroactive steroids with acute and chronic stress and the HPA axis. Preclinical and clinical studies related to psychiatric disorders that have been conducted on neuroactive steroids are also described. 3. Plasma concentrations of some neuroactive steroids are altered in individuals suffering from schizophrenia, depression, or anxiety disorders compared to values in healthy controls. Some drugs used to treat these disorders have been reported to alter plasma and brain concentrations in clinical and preclinical studies, respectively. 4. Further research is warranted into the role of neuroactive steroids in the pathophysiology of psychiatric illnesses and the possible role of these steroids in the successful treatment of these disorders.

The neuroactive steroids alphaxalone and pregnanolone increase the conductance of single GABAA channels in newborn rat hippocampal neurons

The effects of the neuroactive steroids alphaxalone and pregnanolone on single GABA(A) receptor channels were tested in cell-attached and inside-out patches from cultured newborn rat hippocampal neurons. The conductance of these single channels ranged between 10 and 80 pS when exposed to low (0.5-3 microM) GABA concentrations. These GABA concentrations activated low-conducting channels (<40 pS) in 78% of the patches, 22% of patches had channels with a maximum conductance above 40 pS. Alphaxalone at concentrations above 1 microM, and pregnanolone at concentrations above 0.1 microM, significantly increased the conductance of initially low-conducting single channels activated by GABA up to seven-fold and at all concentrations tested, both drugs increased open probability and mean open time and decreased closed probability and mean closed time of channels. Both steroids at higher concentrations could directly activate high conductance (>40 pS) chloride channels. Both the directly activated channels and those channels that had been previously affected by alphaxalone were modulated by diazepam, a benzodiazepine drug that is known to specifically modulate GABA(A) channels. The present study is the first one to show that neurosteroids can significantly increase single GABA(A) channel conductance, thus enlarging our current knowledge on the molecular mechanism of action of these compounds.

Potentiation of inhibitory amino acid receptors-mediated responses by lanthanum in rat sacral dorsal commissural neurons

Lanthanum is one of rare earth cations with extremely active chemical property and has been reported to influence neuronal transmitter systems. To date, little attention has been directed towards the sacral dorsal commissural nucleus (SDCN), which serves as a relay of sensory information from the pelvic viscera in the spinal cord. Therefore, the effect of lanthanum on the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) and glycine (Gly) responses in neurons acutely dissociated from the rat SDCN was investigated using the nystatin-perforated patch-recording configuration under voltage-clamp conditions. At a holding potential of -40 mV, La(3+) reversibly potentiated GABA (3 microM)-activated currents (I(GABA)) in a concentration-dependent manner over the concentration range of 10 microM to 30 mM, with the EC(50) value of 67.3+/-16.4 microM. Similarly, La(3+) reversibly potentiated glycine (10 microM)-activated currents (I(Gly)) in a concentration-dependent manner over the concentration range of 1 microM to 1 mM, with the EC(50) value of 52.3+/-10.9 microM. The effects of La(3+) on I(GABA) and I(Gly) were voltage-independent. Moreover, both of the potentiations were not use-dependent and were overcome by increasing the concentration of agonist. Our results indicate that La(3+) potentiates the inhibitory amino acid receptors-mediated responses in SDCN, which may reduce the transmission of the pelvic visceral information. The information provided by this work may help to elucidate the mechanisms and effects of lanthanum on brain functions.

Inhibition by alpha-tetrahydrodeoxycorticosterone (THDOC) of pre-sympathetic parvocellular neurones in the paraventricular nucleus of rat hypothalamus

BACKGROUND AND PURPOSE

alpha-tetrahydrodeoxycorticosterone (THDOC) is an endogenous neuroactive steroid which increases in plasma and brain concentration during stress. It has both positive and negative modulatory effects on GABA activated GABAA currents, dependent upon the dose. We investigated the effects of THDOC on spinally-projecting "pre-sympathetic" neurones in the parvocellular subnucleus of the hypothalamic paraventricular nucleus (PVN), to determine whether it activates or inhibits these neurones, and by what mechanism.

EXPERIMENTAL APPROACH

Rat spinally-projecting (parvocellular) PVN neurones were identified by retrograde labelling and the action of THDOC investigated with three modes of patch-clamp: cell-attached action current, whole-cell voltage-clamp and cell-attached single-channel recording.

KEY RESULTS

In cell-attached patch mode, parvocellular neurones fired action potentials spontaneously with an average frequency of 3.6 +/- 1.1 Hz. Bath application of THDOC reduced this with an EC50 of 67 nM (95% confidence limits: 54 to 84 nM), Hill coefficient 0.8 +/- 0.04, n = 5. In whole-cell patch-clamp mode, pressure ejection of GABA evoked inward currents. These were clearly GABAA currents, since they were inhibited by the GABAA receptor antagonist bicuculline, and reversed near the chloride equilibrium potential. THDOC significantly potentiated GABAA currents (1 microM THDOC: 148 +/- 15% of control, n = 5, p < or = 0.05, ANOVA). Single-channel analysis showed no differences in conductance or corrected mean open times in the presence of 1 microM THDOC.

CONCLUSIONS AND IMPLICATIONS

THDOC inhibited parvocellular neuronal activity without showing any evidence of the bidirectional activity demonstrated previously with cultured hypothalamic neurones. Our data are consistent with the hypothesis that THDOC acts by potentiating the post-synaptic activity of endogenously released GABA.

Kindling-induced alterations in GABAAreceptor-mediated inhibition and neurosteroid activity in the rat piriform cortex

The piriform cortex makes strong interconnections with limbic structures (amygdala, entorhinal cortex and hippocampus) that are involved in memory processing. These connections have also been implicated in the development of temporal lobe epilepsy. However, little is known about how neurones in this region may change during seizure genesis. Here we tested the hypothesis that in the kindling model of temporal lobe epilepsy GABAA receptor-mediated inhibition is altered in the piriform cortex. To do this we performed whole-cell patch-clamp recordings in piriform cortex brain slices obtained from non-kindled and amygdala-kindled adult rats. We found that kindling coincided with an increase in the amplitude and duration of miniature inhibitory post-synaptic currents (mIPSCs) recorded from non-pyramidal neurones, whereas the mIPSCs occurring on pyramidal (excitatory) cells did not change. Non-stationary noise analysis of mIPSCs occurring on the non-pyramidal neurones showed that inferred unitary conductance of synaptic channels were the same before and after kindling, implying that the channel number increased significantly. Immunocytochemical analysis of the inhibitory innervation showed that it was also unaltered by seizure induction. We also found that the effect of the positive modulator tetrahydrodeoxycorticosterone was reduced on the pyramidal neurones after kindling. In contrast, the potentiating effects of tetrahydrodeoxycorticosterone on non-pyramidal cells were about the same after kindling as in control (sham) rats. These data indicate that amygdala kindling causes a shift in the inhibition 'balance' between the pyramidal and non-pyramidal cells, perhaps leading to the disinhibition of pyramidal cells.

Activity of B-nor analogues of neurosteroids on the GABA(A) receptor in primary neuronal cultures

A GABA(A) receptor study of several B-nor analogues of allopregnanolone and pregnanolone has been carried out. B-norallopregnanolone (i.e., 3alpha-hydroxy-7-nor-5alpha-pregnan-20-one) was found comparable to allopregnanolone when measured with labeled TBPS. Analogous results were obtained from their effect on neurons in culture: this time, both 3alpha-hydroxy-7-nor-5xi-pregnan-20-ones (5 and 6) were found to stimulate [3H]flunitrazepam binding and GABA-induced 36Cl- influx. These effects were inhibited by GABA(A) receptor antagonists. Other analogues carrying electronegative substituents (epoxides 9 and 10 and ketone 12) in the B ring were inactive. Similarly, B-normal ketones 17, and 18 and 6-azasteroids 20 and 21 were also inactive. B-Nor analogues 5 and 6 did not induce neurotoxicity at relevant concentrations. A computational analysis of active and inactive neurosteroid analogues allowed the proposal of a 3D pharmacophoric hypothesis of their interaction with the GABA(A) receptor.

Plasticity of GABAA receptor subunit expression during the oestrous cycle of the rat: implications for premenstrual syndrome in women

Many women experience psychological changes during the luteal phase of their menstrual cycle. The late luteal (premenstrual) phase, when symptoms become most severe, is characterized by declining levels of ovarian progesterone. In female rats, withdrawal from prolonged dosing with progesterone leads to upregulation of alpha4 and delta subunits of the GABAA receptor in several brain regions. During the oestrous cycle of the rat, the natural fall in progesterone that occurs in late dioestrus is associated with a parallel increase in expression of alpha4, beta1 and delta GABAA receptor subunits in neurones in the periaqueductal grey matter (PAG), suggesting that new receptors of the alpha4beta1delta composition have been formed. Recombinant alpha4beta1delta receptors display a low EC50 for GABA, which is consistent with activation by extracellular levels of GABA. They are also likely to be located extrasynaptically and to carry tonic currents. In the PAG, a region involved in mediating panic-like anxiety, alpha4, beta1 and delta GABAA receptor subunits are located principally on GABAergic interneurones. On-going GABAergic neuronal activity normally limits and controls the excitability of the panic circuitry. During late dioestrus, when expression of alpha4, beta1 and delta subunits on GABAergic interneurones is upregulated, the increase in tonic current would be expected to lead to a reduction in the activity of the GABAergic population. Thus the panic circuitry would become intrinsically more excitable. It is suggested that during the menstrual cycle in women, plasticity of GABAA receptor subunit expression in brain regions such as the PAG, which are involved in mediating anxiety behaviour, may underlie some of the changes in mood that occur during the premenstrual period.

Cardiovascular and respiratory responses to a panicogenic agent in anaesthetised female Wistar rats at different stages of the oestrous cycle

In urethane-anaesthetised female Wistar rats, intravenous injection of the panicogenic CCK(B) receptor agonist pentagastrin (0.002-80 microg/kg) evoked a dose-related increase in blood pressure, heart rate and ventilation. The response was blocked in the presence of the selective CCK(B) receptor antagonist CR2945 (1 mg/kg i.v.). The same pattern of cardiovascular and respiratory changes was evoked by microinjection of pentagastrin (0.3 nmol in 250 nL) into the dorsal half of the periaqueductal grey matter (PAG). The effect of intra-PAG administration of pentagastrin was also abolished following injection of CR2945 (1 mg/kg, i.v.). Responsiveness to systemically administered pentagastrin was enhanced in rats in late dioestrus. At the highest dose tested (80 microg/kg), the pressor response, tachycardia and tachypnoea evoked in rats in late dioestrus was significantly higher than rats in proestrus. For rats in oestrus, the pressor response and tachycardia but not tachypnoea were also significantly larger than the response evoked in rats in early dioestrus. The results suggest that the dorsal half of the PAG (dPAG) plays a key role in mediating the cardiovascular and respiratory responses evoked by systemically administered CCK(B) agonists. The enhanced responsiveness to panicogenic agents during late dioestrus may be related to changes in the functional responsiveness of gamma-aminobutyric acid (GABA)ergic circuitry in the dPAG due to plasticity of GABA(A) receptor subunit expression as a consequence of falling progesterone levels.

Negative GABA(A) modulators attenuate the discriminative stimulus effects of benzodiazepines and the neuroactive steroid pregnanolone in rhesus monkeys

RATIONALE

Negative GABA(A) modulators (i.e., inverse agonists) might be useful for identifying mechanisms at the GABA(A) receptor complex that mediate the effects of positive GABA(A) modulators, especially those for which there are no available competitive antagonists.

OBJECTIVE

Drug discrimination was used to examine antagonism of a 5-beta neuroactive steroid (pregnanolone) and a benzodiazepine (midazolam) by several negative GABA(A) modulators in rhesus monkeys.

METHODS

One group of monkeys (n=5) received 5.6 mg kg(-1) day(-1) of diazepam (p.o.) and discriminated the benzodiazepine antagonist flumazenil (0.1 or 0.32 mg/kg s.c.); another group of monkeys (n=5) discriminated the benzodiazepine midazolam (0.32 mg/kg s.c.).

RESULTS

In diazepam-treated monkeys, negative GABA(A) modulators with increasing efficacy, including Ro 15-4513, ethyl beta-carboline-3-carboxylate (beta-CCE), methyl beta-carboline-3-carboxylate (beta-CCM) and methyl-6,7-dimethoxyl-4-ethyl-beta-carboline-3-carboxylate (DMCM), substituted for flumazenil. In monkeys discriminating midazolam, pregnanolone occasioned high levels of midazolam-lever responding, and these effects were attenuated by beta-CCE and beta-CCM, but not by flumazenil, Ro 15-4513, or DMCM. The midazolam discriminative stimulus also was attenuated by beta-CCM and DMCM; Schild analysis was consistent with a simple competitive interaction between midazolam and beta-CCM but not between midazolam and DMCM.

CONCLUSIONS

Negative modulators are qualitatively similar to neutral modulators in diazepam-treated animals; however, interactions between negative modulators and midazolam suggest that different receptors mediate the effects of some (DMCM) and not other (beta-CCM) negative modulators. Negative modulators at benzodiazepine sites exert efficacy-dependent antagonism of positive modulators at neuroactive steroid sites. Without competitive antagonists at neuroactive steroid or barbiturate sites, negative modulators could prove useful for examining the mechanism of action of different classes of positive GABA(A) modulator.

Neurosteroids exhibit differential effects on mIPSCs recorded from normal and seizure prone rats

In the perirhinal cortex of seizure prone (SP) rats, GABA(A)-mediated miniature inhibitory postsynaptic currents (mIPSCs) are smaller in amplitude but have longer deactivation phases than mIPSCs recorded in normal control (NC; outbred) rats. These differences in mIPSCs are correlated to the relatively higher alpha1 subunit expression in the NC rat strains and the higher alpha2, alpha3, and alpha5 subunit expression in the SP strain. Using patch-clamp recording, we investigated how the neurosteroids tetrahydrodeoxcorticosterone (THDOC) and allopregnanolone at physiological and pharmacological concentrations may differentially affect the mIPSCs in the perirhinal cortex of brain slices isolated from SP and NC rats. We found that 100 nM THDOC prolonged the time course and increased the amplitude of both the mono- and biphasic mIPSCs in the SP rats, but these effects were smaller in the NC rats. By comparison, allopregnanolone (100 nM) had small effects in both the NC and SP rats. At 1.0 microM, THDOC enhanced mIPSCs in both strains, but this effect was not greater in the SP rat than it was at 100 nM. By contrast, allopregnanolone (500 nM) enhanced the time course of the mIPSCs in both strains but it reduced mIPSC amplitudes as well. THDOC (100 nM) was much more effective than 100 nM allopregnanolone in inducing a tonic current in SP and NC rats. These data show that neurosteroids modulate synaptic activity at synapses having different biophysical behaviors. As differing GABA(A) receptors are targeted by subsets of interneurons, these data suggest these neurosteroids may selectively modulate one inhibitory input over another.

Potentiation, activation and blockade of GABAA receptors by etomidate in the rat sacral dorsal commissural neurons

Etomidate (ET), an imidazole general anesthetic, has been medically widely used. Recent evidence suggests that the inhibitory neurotransmitter GABA receptor may be one of the important molecular target(s) of general anesthetics. Up to date, little attention has been directed toward the sacral dorsal commissural nucleus (SDCN), which serves as a relay of sensory information from the pelvic viscera in the spinal cord. Therefore, the effect of ET on GABA(A) receptor function in neurons acutely dissociated from the SDCN was investigated using the nystatin-perforated patch-recording configuration under voltage-clamp conditions. At a holding potential of -40 mV, ET (above 10 microM) induced an inward ET-activated current (I(ET)) with the EC(50) value of 33 +/- 3 microM, which was reversibly blocked by bicuculline and picrotoxin. The reversal potential of I(ET) was close to the Cl(-) equilibrium potential. ET also displayed a biphasic modulatory effect on GABA responses. At lower concentrations (0.1-100 microM), ET reversibly potentiated GABA (1 microM)-activated Cl(-) currents in a bell-shaped manner, with the maximal facilitative effect at 10 microM, whereas at concentrations >100 microM, the peak of the ET-induced current was suppressed in the absence or presence of GABA (1 microM). These results suggest that in SDCN, in addition to the potentiation of GABA(A) receptor-mediated responses at low concentrations and the direct activation of GABA(A) receptors at moderate concentrations as expected, ET produced a fast blocking action at high concentrations. The general anesthetic-induced effects in SDCN, at least the potentiation of GABA responses, may significantly contribute to anesthesia of pelvic viscera during the general anesthesia.

Sites of positive allosteric modulation by neurosteroids on ionotropic gamma-aminobutyric acid receptor subunits

Neurosteroids are known as allosteric modulators of ionotropic gamma-aminobutyric acid (GABA) receptors. Here, we investigated sites of positive allosteric modulation by allotetrahydrodeoxycorticosterone (5alpha-THDOC) at GABA receptors using the technique of chimeragenesis and the Xenopus oocyte expression system. Our findings have demonstrated that the region from transmembrane segment (TM) 4 to the C-terminus of the GABA(A) receptor alpha1 subunit is crucial for the action of 5alpha-THDOC, but insufficient for the action of another neurosteroid allopregnanolone, suggesting that a specific region critical for neurosteroid action at GABA receptors exists in the domain between TM4 and the C-terminus of GABA receptor subunits.

Chronic prenatal ethanol exposure alters hippocampal GABAA receptors and impairs spatial learning in the guinea pig

Chronic prenatal ethanol exposure (CPEE) can injure the developing brain, and may lead to the fetal alcohol syndrome (FAS). Previous studies have demonstrated that CPEE upregulates gamma-aminobutyric acid type A (GABA(A)) receptor expression in the cerebral cortex, and decreases functional synaptic plasticity in the hippocampus, in the adult guinea pig. This study tested the hypothesis that CPEE increases GABA(A) receptor expression in the hippocampus of guinea pig offspring that exhibit cognitive deficits in a hippocampal-dependent spatial learning task. Timed, pregnant guinea pigs were treated with ethanol (4 g/kg maternal body weight per day), isocaloric-sucrose/pair-feeding, or water throughout gestation. GABA(A) receptor subunit protein expression in the hippocampus was measured at two development ages: near-term fetus and young adult. In young adult guinea pig offspring, CPEE increased spontaneous locomotor activity in the open-field and impaired task acquisition in the Morris water maze. CPEE did not change GABA(A) receptor subunit protein expression in the near-term fetal hippocampus, but increased expression of the beta2/3-subunit of the GABA(A) receptor in the hippocampus of young adult offspring. CPEE did not change either [(3)H]flunitrazepam binding or GABA potentiation of [(3)H]flunitrazepam binding, but decreased the efficacy of allopregnanolone potentiation of [(3)H]flunitrazepam binding, to hippocampal GABA(A) receptors in adult offspring. Correlational analysis revealed a relationship between increased spontaneous locomotor activity and growth restriction in the hippocampus induced by CPEE. Similarly, an inverse relationship was found between performance in the water maze and the efficacy of allopregnanolone potentiation of [(3)H]flunitrazepam binding in the hippocampus. These data suggest that alterations in hippocampal GABA(A) receptor expression and pharmacological properties contribute to hippocampal-related behavioral and cognitive deficits associated with CPEE.

Neurosteroids, GABAA receptors, and escalated aggressive behavior

Aggressive behavior can serve important adaptive functions in social species. However, if it exceeds the species-typical pattern, it may become maladaptive. Very high or escalated levels of aggressive behavior can be induced in laboratory rodents by pharmacological (alcohol-heightened aggression), environmental (social instigation), or behavioral (frustration-induced aggression) means. These various forms of escalated aggressive behavior may be useful in further elucidating the neurochemical control over aggression and violence. One neurochemical system most consistently linked with escalated aggression is the GABAergic system, in conjunction with other amines and peptides. Although direct stimulation of GABA receptors generally suppresses aggression, a number of studies have found that positive allosteric modulators of GABAA receptors can cause increases in aggressive behavior. For example, alcohol, benzodiazepines, and many neurosteroids are all positive modulators of the GABAA receptor and all can cause increased levels of aggressive behavior. These effects are dose-dependent and higher doses of these compounds generally shift from heightening aggressive behavior to being sedative and anti-aggressive. In addition, these modulators interact with each other and can have additive effects on the GABAA receptor and on behavior, including aggression. The GABAA receptor is a heteropentameric protein that can be constituted from various subunits. It has been shown that subunit composition can affect sensitivity of the receptor to some modulators and that subunit composition differentially affects the sedative vs anxiolytic actions of benzodiazepines. Initial studies targeting alpha subunits of the GABAA receptor point to their significant role in the aggression-heightening effects of alcohol, benzodiazepines, and neurosteroids.

The modulatory effects of the anxiolytic etifoxine on GABA(A) receptors are mediated by the beta subunit

The anxiolytic compound etifoxine (2-ethylamino-6-chloro-4-methyl-4-phenyl-4H-3,1-benzoxazine hydrochloride) potentiates GABA(A) receptor function in cultured neurons (Neuropharmacology 39 (2000) 1523). However, the molecular mechanisms underlying these effects are not known. In this study, we have determined the influence of GABA(A) receptor subunit composition on the effects of etifoxine, using recombinant murine GABA(A) receptors expressed in Xenopus oocytes. Basal chloride currents mediated by homomeric beta receptors were reduced by micromolar concentrations of etifoxine, showing that beta subunits possess a binding site for this modulator. In oocytes expressing alpha(1)beta(x) GABA(A) receptors (x=1, 2 or 3), etifoxine evoked a chloride current in the absence of GABA and enhanced GABA (EC10)-activated currents, in a dose-dependent manner. Potentiating effects were also observed with alpha(2)beta(x), beta(x)gamma(2s) or alpha(1)beta(x)gamma(2s) combinations. The extent of potentiation was clearly beta-subunit-dependent, being more pronounced at receptors containing a beta(2) or a beta(3) subunit than at receptors incorporating a beta(1) subunit. The mutation of Asn 289 in the channel domain of beta(2) to a serine (the homologous residue in beta(1)) did not significantly depress the effects of etifoxine at alpha(1)beta(2) receptors. This specific pattern of inhibition/potentiation was compared with that of other known modulators of GABA(A) receptor function like benzodiazepines, neurosteroids, barbiturates or loreclezole.

Neurosteroid modulation of GABAA receptors

Certain metabolites of progesterone and deoxycorticosterone are established as potent and selective positive allosteric modulators of the gamma-aminobutyric acid type A (GABA(A)) receptor. Upon administration these steroids exhibit clear behavioural effects that include anxiolysis, sedation and analgesia, they are anticonvulsant and at high doses induce a state of general anaesthesia, a profile consistent with an action to enhance neuronal inhibition. Physiologically, peripherally synthesised pregnane steroids derived from endocrine glands such as the adrenals and ovaries function as hormones by crossing the blood brain barrier to influence neuronal signalling. However, the demonstration that certain neurons and glial cells within the central nervous system (CNS) can synthesize these steroids either de novo, or from peripherally derived progesterone, has led to the proposal that these steroids (neurosteroids) can additionally function in a paracrine manner, to locally influence GABAergic transmission. Steroid levels are known to change dynamically, for example in stress and during pregnancy. Given that GABA(A) receptors are ubiquitously expressed throughout the central nervous system, such changes in steroid levels would be predicted to cause a global enhancement of inhibitory neurotransmission throughout the brain, a scenario that would seem incompatible with a physiological role as a selective neuromodulator. Here, we will review emerging evidence that the GABA-modulatory actions of the pregnane steroids are highly selective, with their actions being brain region and indeed neuron dependent. Furthermore, the sensitivity of GABA(A) receptors is not static but can dynamically change. The molecular mechanisms underpinning this neuronal specificity will be discussed with particular emphasis being given to the role of GABA(A) receptor isoforms, protein phosphorylation and local steroid metabolism and synthesis.

Neuroactive steroids: mechanisms of action and neuropsychopharmacological properties

Steroids influence neuronal function through binding to cognate intracellular receptors which may act as transcription factors in the regulation of gene expression. In addition, certain so-called neuroactive steroids modulate ligand-gated ion channels via non-genomic mechanisms. Especially distinct 3alpha-reduced metabolites of progesterone and deoxycorticosterone are potent positive allosteric modulators of gamma-aminobutyric acid type A (GABA(A)) receptors. However, also classical steroid hormones such as 17beta-estradiol, testosterone and progesterone are neuroactive steroids because they may act as functional antagonists at the 5-hydroxytryptamine type 3 (5-HT(3)) receptor, a ligand-gated ion channel or distinct glutamate receptors. A structure-activity relationship for the actions of a variety of steroids at the 5-HT(3) receptor was elaborated that differed considerably from that known for GABA(A) receptors. Although a bindings site for steroids at GABA(A) receptors is still a matter of debate, meanwhile there is also evidence that steroids interact allosterically with ligand-gated ion channels at the receptor membrane interface. On the other hand, also 3alpha-reduced neuroactive steroids may regulate gene expression via the progesterone receptor after intracellular oxidation into 5alpha-pregnane steroids. Animal studies showed that progesterone is converted rapidly into GABAergic neuroactive steroids in vivo. Progesterone reduces locomotor activity in a dose-dependent fashion in male Wistar rats. Moreover, progesterone and 3alpha-reduced neuroactive steroids produce a benzodiazepine-like sleep EEG profile in rats and humans. During major depression, there is a disequilibrium of such 3alpha-reduced neuroactive steroids which is corrected by successful treatment with antidepressant drugs. Neuroactive steroids may further be involved in the treatment of depression and anxiety with antidepressants in patients during ethanol withdrawal. Studies in patients with panic disorder suggest that neuroactive steroids may also play a role in modulating human anxiety. Both the genomic and non-genomic effects of steroids in the brain may contribute to the pathophysiology of psychiatric disorders and the mechanisms of action of antidepressants. Neuroactive steroids affect a broad spectrum of behavioral functions through their unique molecular properties and may represent a new treatment strategy for neuropsychiatric disorders.

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.

Allopregnanolone modulates spontaneous GABA release via presynaptic Cl- permeability in rat preoptic nerve terminals

The endogenous neurosteroid 3alpha-hydroxy-5alpha-pregnane-20-one (allopregnanolone) affects presynaptic nerve terminals and thereby increases the frequency of spontaneous GABA release. The present study aimed at clarifying the mechanisms underlying this presynaptic neurosteroid action, by recording the frequency of spontaneous GABA-mediated inhibitory postsynaptic currents (sIPSCs) in neurons from the medial preoptic nucleus (MPN) of rat. Acutely dissociated neurons with functional adhering nerve terminals were studied by perforated-patch recording under voltage-clamp conditions. It was shown that the sIPSC frequency increased with the external K(+) concentration ([K(+)](o)). Further, the effect of allopregnanolone on the sIPSC frequency was strongly dependent on [K(+)](o). In a [K(+)](o) of 5 mM, 2.0 microM allopregnanolone caused a clear increase in sIPSC frequency. However, the effect declined rapidly with increased [K(+)](o) and at high [K(+)](o) allopregnanolone reduced the sIPSC frequency. The effect of allopregnanolone was also strongly dependent on the external Cl(-) concentration ([Cl(-)](o)). In a reduced [Cl(-)](o) (40 mM, but with a standard [K(+)](o) of 5 mM), the effect on sIPSC frequency was larger than that in the standard [Cl(-)](o) of 146 mM. The dependence of the effect of allopregnanolone on [K(+)](o) and on estimated presynaptic membrane potential was also altered by the reduction in [Cl(-)](o). As in standard [Cl(-)](o), the effect in low [Cl(-)](o) declined when [K(+)](o) was raised, but reversed at a higher [K(+)](o). The GABA(A) receptor agonist muscimol also potentiated the sIPSC frequency. Altogether, the results suggest that allopregnanolone exerts its presynaptic effect by increasing the presynaptic Cl(-) permeability, most likely via GABA(A) receptors.

Neuroactive steroid mechanisms and GABA type A receptor subunit assembly in hypothalamic and extrahypothalamic regions

Gonadal- and neuronal-derived steroids are capable of altering brain functions through two basic mechanisms: slow (genomic) and rapid (novel nongenomic membrane) types of activities. The genomic activities that are circumscribed to the numerous neuronal and glial expressed receptor actions involve transcriptional processes regulated largely by classical steroids. On the other hand, rapid nongenomic activities are linked to the stereoselective interactions of potent neuroactive steroids. It appears that both of these steroid mechanisms can be successfully evoked at the ligand-gated heteroligomeric GABA type A receptor. However, the precise structural prerequisites and type of molecular steroid interactions implicated in this neuronal target have not been fully investigated. This article reviews the most common subunits (alpha, beta, and gamma) of the native GABA type A receptor involved in signaling pathways of slow and rapid steroidal mechanisms. Different beta-containing compositions (alpha1beta1-3gamma2) are necessary for the slow type of mechanism, whereas different alpha-containing constructs (alpha2-6beta 1/2 gamma2/2L) are linked to the rapid type. Because of the major role played by neuroactive steroids in GABA-dependent neuroendocrine and sociosexual events, distinction of the specific subunit combination is essential not only for elucidating neuronal communicative expressions during such events but also for elucidating their potential neuroprotective role in neurodegenerative disorders.

Functional characterization of GABA(A) receptors in neonatal hypothalamic brain slice

The hypothalamus influences a number of autonomic functions. The activity of hypothalamic neurons is modulated in part by release of the inhibitory neurotransmitter GABA onto these neurons. GABA(A) receptors are formed from a number of distinct subunits, designated alpha, beta, gamma, delta, epsilon, and theta, many of which have multiple isoforms. Little data exist, however, on the functional characteristics of the GABA(A) receptors present on hypothalamic neurons. To gain insight into which GABA(A) receptor subunits are functionally expressed in the hypothalamus, we used an array of pharmacologic assessments. Whole cell recordings were made from thin hypothalamic slices obtained from 1- to 14-day-old rats. GABA(A) receptor-mediated currents were detected in all neurons tested and had an average EC(50) of 20 +/- 1.6 microM. Hypothalamic GABA(A) receptors were modulated by diazepam (EC(50) = 0.060 microM), zolpidem (EC(50) = 0.19 microM), loreclezole (EC(50) = 4.4 microM), methyl-6,7-dimethoxy-4-ethyl-beta-carboline (EC(50) = 7.7 microM), and 5alpha-pregnan-3alpha-hydroxy-20-one (3alpha-OH-DHP). Conversely, these receptors were inhibited by Zn(2+) (IC(50) = 70.5 microM), dehydroepiandrosterone sulfate (IC(50) = 16.7 microM), and picrotoxin (IC(50) = 2.6 microM). The alpha4/6-selective antagonist furosemide (10-1,000 microM) was ineffective in all hypothalamic neurons tested. The results of our pharmacological analysis suggest that hypothalamic neurons express functional GABA(A) receptor subtypes that incorporate alpha1 and/or alpha2 subunits, beta2 and/or beta3 subunits, and the gamma2 subunit. Our results suggest receptors expressing alpha3-alpha6, beta1, gamma1, and delta, if present, represent a minor component of functional hypothalamic GABA(A) receptors.

Effects of etifoxine on ligand binding to GABA(A) receptors in rodents

The GABA(A) receptor/chloride ionophore is allosterically modulated by several classes of anxiolytic and anticonvulsant agents, including benzodiazepines, barbiturates and neurosteroids. Etifoxine, an anxiolytic and anticonvulsant compound competitively inhibited the binding of [(35)S]t-butylbicyclophosphoro-thionate (TBPS), a specific ligand of the GABA(A) receptor chloride channel site. To investigate the etifoxine modulatory effects on the different binding sites of the GABA(A) receptor complex, we have examined the effects of etifoxine on binding of the receptor agonist [(3)H]muscimol and the benzodiazepine modulator [(3)H]flunitrazepam in rat brain membrane preparations. The anticonvulsant properties of etifoxine combined with muscimol and flunitrazepam were performed in mice with picrotoxin-induced clonic seizures. Etifoxine modestly enhanced binding of [(3)H]muscimol and of [(3)H]flunitrazepam by increasing the number of binding sites without changing the binding affinity of [(3)H]flunitrazepam. In contrast, the compound decreased the affinity of muscimol for its binding site. In vivo, the combination of subactive doses of etifoxine with muscimol or flunitrazepam produced an anticonvulsant additive effect against the picrotoxin-induced clonic seizures in mice. These results suggest that the interaction of etifoxine on the GABA(A) receptor complex would allosterically modify different binding sites due to conformational changes. Functionally, the resulting facilitation of GABA transmission underlies the pharmacological properties of etifoxine.

Allopregnanolone activates GABA(A) receptor/Cl(-) channels in a multiphasic manner in embryonic rat hippocampal neurons

Although 3alpha-substituted metabolites of progesterone are well established to interact with GABA(A) receptor/Cl(-) channels, the nature of the interaction(s) remains uncertain. We used patch-clamp recording to study the interaction with GABA(A) receptor/Cl(-) channels expressed by embryonic hippocampal neurons differentiating in culture and nonneuronal cells transfected with GABA(A) receptor subunits. Allopregnanolone primarily induced multiphasic current responses in neurons, which were eliminated by bicuculline, an antagonist of GABA at GABA(A) receptor/Cl(-) channels. Similar multiphasic responses blocked by bicuculline were induced by allopregnanollone in nonneuronal cells transfected with alpha(1) and gamma(2) subunits, indicating that the steroid activation of GABA(A) receptor/Cl(-) channels occurred independently of GABA. Fluctuation analyses of current responses to allopregnanolone and GABA revealed underlying channel activities with similar estimated unitary properties. However, although both agonists activated Cl(-) channels with similar estimated short and long burst-length durations, most of those stimulated by the steroid were short, while most of those opened by GABA were long. Allopregnanolone potentiated GABA-evoked Cl(-) currents in nonneuronal cells transfected with alpha(1) and beta(2) or beta(3) subunits, which did not exhibit multiphasic responses to the steroid, indicating another, independent action of the steroid at activated receptors. Pertussis toxin treatment eliminated the low-amplitude current and attenuated the high-amplitude current induced by allopregnanolone in a reversible manner. Mastoparan, which activates G proteins directly, triggered a high-amplitude current after a delay, which was blocked by bicuculline. The results indicate that allopregnanolone interacts with GABA(A) receptor/Cl(-) channels expressed by embryonic hippocampal neurons in multiple ways, some of which are mediated by G proteins.

Rapid neuromodulation by cortisol in the rat paraventricular nucleus: an in vitro study

1. We have used a range of in vitro electrophysiological techniques to investigate the mechanism of rapid cortisol neuromodulation of parvocellular neurones in the rat paraventricular nucleus. 2. In our study, we found that cortisol (10 microM) increased spontaneous action-current firing frequency to 193%. This effect was insensitive to the glucocorticoid intracellular-receptor antagonist mifepristone. 3. Cortisol (0.1-10 microM) had no detectable effects on whole-cell GABA current amplitudes, or GABA(A) single-channel kinetics. 4. Cortisol (10 microM) inhibited whole-cell potassium currents in parvocellular neurones by shifting the steady-state activation curve by 14 mV to the right. 5. Additionally, in a cell line expressing both the glucocorticoid intracellular receptor and recombinant, fast inactivating potassium channels (hKv1.3), cortisol (1 and 10 microM) inhibited potassium currents by shifting their steady-state activation curves to the right by 12 mV (10 microM cortisol). This effect was also insensitive to the cortisol antagonist, mifepristone. 6. These data suggest that inhibition of voltage-gated potassium channels may contribute to the rapid neuromodulatory effects of cortisol, possibly by direct interaction with the ion channel itself.

The influence of subunit composition on the interaction of neurosteroids with GABA(A) receptors

The influence of the subunit composition of human GABA(A) receptors upon the GABA-modulatory properties of 5alpha-pregnan-3alpha-ol-20-one (5alpha,3alpha) has been examined using the Xenopus laevis oocyte expression system and the two electrode voltage-clamp technique. Steroid potency (EC(50)) is modestly influenced by the alpha-isoform (alpha(x)beta(1)gamma(2L); x=1-6). alpha(2)-, alpha(4)- and alpha(5)-containing receptors are significantly less sensitive to the action of low concentrations of 5alpha,3alpha (10-100 nM) when compared to alpha(1,3,6)beta(1)gamma(2L) receptors. Additionally, the maximal effect of the steroid is favoured at alpha(6)-containing receptors. The beta-isoform (alpha(1)beta(y)gamma(2L); y=1-3) has little influence on the GABA-modulatory effect of the neurosteroid. The EC(50) of 5alpha,3alpha is only modestly influenced by the omission of the gamma(2) subunit (alpha(1)beta(1)gamma(2L) vs alpha(1)beta(1)): while the maximal effect is favoured by the binary complex. However, the identity of the gamma subunit influences the GABA(A)-modulatory potency of 5alpha,3alpha with gamma(2)- and gamma(1)-containing receptors being the most and the least sensitive to 5alpha,3alpha, respectively. Finally, incorporation of the epsilon, or delta subunit dramatically reduces and augments the GABA-enhancing actions of the steroid, respectively. These findings provide evidence that 5alpha,3alpha discriminates amongst recombinant receptors of varied subunit composition. Furthermore, this selectivity may contribute to their neuronal specificity and behavioural profile.

Tracazolate reveals a novel type of allosteric interaction with recombinant gamma-aminobutyric acid(A) receptors

Tracazolate, a pyrazolopyridine, is an anxiolytic known to interact with gamma-aminobutyric acid (GABA)(A) receptors, adenosine receptors, and phosphodiesterases. Its anxiolytic effect is thought to be via its interaction with GABA(A) receptors. We now report the first detailed pharmacological study examining the effects of tracazolate on a range of recombinant GABA(A) receptors expressed in Xenopus laevis oocytes. Replacement of the gamma2s subunit within the alpha1beta3gamma2s receptor with the epsilon subunit caused a dramatic change in the functional response to tracazolate from potentiation to inhibition. The gamma2s subunit was not critical for potentiation because alpha1beta3 receptors were also potentiated by tracazolate. gamma2/epsilon chimeras revealed a critical N-terminal domain between amino acids 206 and 230 of gamma2, governing the nature of this response. Replacement of the beta3 subunit with the beta1 subunit within alpha1beta3gamma2s and alpha1beta3epsilon receptors also revealed selectivity of tracazolate for beta3-containing receptors, determined by asparagine at position 265 within transmembrane 2. Replacement of gamma2s with gamma1 or gamma3 revealed a profile intermediate to that of alpha1beta1epsilon and alpha1beta1gamma2s. alpha1beta1delta receptors were also potentiated by tracazolate; however, the maximum potentiation of the EC(20) was much greater than on alpha1beta1gamma2. Concentration-response curves to GABA in the presence of tracazolate for alpha1beta1epsilon and alpha1beta1gamma2s revealed a concentration-related decrease in maximum current amplitude, but a leftward shift in the EC(50) only on alpha1beta1gamma2. Like alpha1beta1gamma2s, GABA concentration-response curves on alpha1beta1delta receptors were shifted to the left with increased maximum responses. Tracazolate has a unique pharmacological profile on recombinant GABA(A) receptors: its potency (EC(50)) is influenced by the nature of the beta subunit; but more importantly, its intrinsic efficacy, potentiation, or inhibition is determined by the nature of the third subunit (gamma1-3, delta, or epsilon) within the receptor complex.

Neurosteroid modulation of recombinant and synaptic GABAA receptors

Certain pregnane steroids are now established as potent, positive allosteric modulators of the gamma-aminobutyric acid type A (GABAA) receptor. These compounds are known to be synthesized in the periphery by endocrine glands, such as the ovaries and the adrenal glands, and can rapidly cross the blood-brain barrier. Therefore, such steroids could act as endogeneous modulators of the major inhibitory receptor in the mammalian central nervous system. However, the demonstration that certain neurons and glia can synthesize the pregnane steroids (i.e., neurosteroids) additionally suggests that they may serve a paracrine role by influencing GABAA-receptor function through their local release in the brain itself. Here, we demonstrate that these neurosteroids are highly selective and extremely potent modulators of the GABAA receptor. The subunit composition of the GABAA receptor may influence the actions of the neurosteroids, particularly when considering concentrations of these agents thought to occur physiologically, which may underlie their reported differential effects at certain inhibitory synapses. However, recent work suggests that the phosphorylation status of either the synaptic GABAA receptor or its associated proteins may also influence neurosteroid sensitivity; these findings are discussed. Upon administration, the neurosteroids exhibit clear behavioral effects, including sedation, anticonvulsant actions, and behaviors predictive of anxiolysis; when given at high doses, they induce general anesthesia. Numerous synthetic steroids have been synthesized in an attempt to therapeutically exploit these properties, and these data are reviewed in this chapter. However, targeting the brain enzymes that synthesize and metabolize the neurosteroids may offer a new approach to exploit this novel endocrine-paracrine neurotransmitter interaction.

Regulation of uterine 5 alpha-reductase type 1 in mice

A null mutation in the murine gene encoding steroid 5 alpha-reductase type 1 (5 alpha R1) leads to failure of normal parturition at term. This observation, together with the finding that mRNA levels of uterine 5 alpha R1 increase significantly at term in normal pregnant animals, indicates that 5 alpha R1 plays an important role in murine parturition. The current studies were conducted to elucidate the regulation of 5 alpha R1 in uterine tissues of nonpregnant and pregnant animals. Nonpregnant, ovariectomized ICR mice were treated with vehicle (control), 17 beta-estradiol (E(2)), progesterone (P(4) ), or E(2)+P(4) for 3 days. Thereafter, uterine tissues were obtained for histology, quantification of 5 alpha R1 specific activity, and Northern blot analysis of 5 alpha R1 mRNA expression. The 5 alpha R1 enzyme activity was significantly increased in animals treated with E(2)+P(4). However, activity was much less in uterine tissues from E(2)+P(4)-treated animals than in uterine tissues from pregnant animals near term. To evaluate further the regulation of 5 alpha R1 during gestation, mice underwent unilateral tubal ligation before timed matings. The 5 alpha R1 activity increased eightfold in uterine tissues from the fetal horn from Gestational Days 12 to 18. This temporal pattern in 5 alpha R1 activity paralleled marked increases in uterine diameter. Taken together, these studies indicate that expression of 5 alpha R1 is regulated by E(2)+P(4) in uterine tissues. Whereas E(2) alone is insufficient to induce enzyme activity, E(2) may be required to increase P(4) receptors and, thereby, mediate the effects of P(4) on 5 alpha R1 gene expression. Further increases in enzyme activity during late gestation are mediated by fetal occupancy, possibly through stretch-induced increases in endometrial growth. Thus, like other genes involved in parturition, expression of 5 alpha R1 is regulated by both hormonal and fetal-derived signaling pathways.

Modulation of native and recombinant GABA(A) receptors by endogenous and synthetic neuroactive steroids

Upon administration, certain pregnane steroids produce clear behavioural effects including, anxiolysis, sedation, analgesia, anaesthesia and are anti-convulsant. This behavioural profile is characteristic of compounds that act to enhance the actions of GABA acting at the GABA(A) receptor. In agreement, numerous studies have now demonstrated these steroids to be potent, positive allosteric modulators of the GABA(A) receptor. The pregnane steroids are synthesized in the periphery by endocrine glands such as the adrenals and the ovaries, but are also made by neurons and glial cells in the central nervous system itself. Hence, these compounds could play both an endocrine and a paracrine role to influence neuronal excitability by promoting inhibition. Here we review evidence that the pregnane steroids are highly selective and extremely potent GABA(A) receptor modulators and that their effects at 'physiological' concentrations (low nanomolar) may be influenced by the subunit composition of the GABA(A) receptor. This feature may underlie recent findings demonstrating the effects of the neurosteroids on inhibitory synaptic transmission to be brain region dependent, although recent reports suggest that phosphorylation mechanisms may additionally influence neurosteroid sensitivity of the GABA(A) receptor. Numerous synthetic steroids have been synthesized in an attempt to therapeutically exploit the behavioural effects of the pregnane steroids and progress with this approach will be discussed. However, the demonstration that the steroids may be made within the central nervous system offers the alternative strategy of targeting the enzymes that synthesize/metabolise the neurosteroids to exploit this novel endocrine/paracrine interaction.

Effect of alpha subunit on allosteric modulation of ion channel function in stably expressed human recombinant gamma-aminobutyric acid(A) receptors determined using (36)Cl ion flux

Inhibitory gamma-aminobutyric acid (GABA)(A) receptors are subject to modulation at a variety of allosteric sites, with pharmacology dependent on receptor subunit combination. The influence of different alpha subunits in combination with beta3gamma2s was examined in stably expressed human recombinant GABA(A) receptors by measuring (36)Cl influx through the ion channel pore. Muscimol and GABA exhibited similar maximal efficacy at each receptor subtype, although muscimol was more potent, with responses blocked by picrotoxin and bicuculline. Receptors containing the alpha3 subunit exhibited slightly lower potency. The comparative pharmacology of a range of benzodiazepine site ligands was examined, revealing a range of intrinsic efficacies at different receptor subtypes. Of the diazepam-sensitive GABA(A) receptors (alpha1, alpha2, alpha3, alpha5), alpha5 showed the most divergence, being discriminated by zolpidem in terms of very low affinity, and CL218,872 and CGS9895 with different efficacies. Benzodiazepine potentiation at alpha3beta3gamma2s with nonselective agonist chlordiazepoxide was greater than at alpha1, alpha2, or alpha5 (P < 0.001). The presence of an alpha4 subunit conferred a unique pharmacological profile. The partial agonist bretazenil was the most efficacious benzodiazepine, despite lower alpha4 affinity, and FG8205 displayed similar efficacy. Most striking were the lack of affinity/efficacy for classical benzodiazepines and the relatively high efficacy of Ro15-1788 (53 +/- 12%), CGS8216 (56 +/- 6%), CGS9895 (65 +/- 6%), and the weak partial inverse agonist Ro15-4513 (87 +/- 5%). Each receptor subtype was modulated by pentobarbital, loreclezole, and 5alpha-pregnan-3alpha-ol-20-one, but the type of alpha subunit influenced the level of potentiation. The maximal pentobarbital response was significantly greater at alpha4beta3gamma2s (226 +/- 10% increase in the EC(20) response to GABA) than any other modulator. The rank order of potentiation for pregnanolone was alpha5 > alpha2 > alpha3 = alpha4 > alpha1, for loreclezole alpha1 = alpha2 = alpha3 > alpha5 > alpha4, and for pentobarbital alpha4 = alpha5 = alpha2 > alpha1 = alpha3.

Neurosteroid modulation of GABA IPSCs is phosphorylation dependent

The neurosteroid 3alpha-hydroxy-5alpha-pregnan-20-one (allopregnanolone) facilitates GABA(A) receptor-mediated ionic currents via allosteric modulation of the GABA(A) receptor. Accordingly, allopregnanolone caused an increase in the slow decay time constant of spontaneous GABA-mediated IPSCs in magnocellular neurons recorded in hypothalamic slices. The allopregnanolone effect on IPSCs was inhibited by a G-protein antagonist as well as by blocking protein kinase C and, to a lesser extent, cAMP-dependent protein kinase activities. G-protein and protein kinase C activation in the absence of the neurosteroid had no effect on spontaneous IPSCs but enhanced the effect of subsequent allopregnanolone application. These findings together suggest that the neurosteroid modulation of GABA-mediated IPSCs requires G-protein and protein kinase activation, although not via a separate G-protein-coupled steroid receptor.