Neurosteroid synthesis-mediated regulation of GABA(A) receptors: relevance to the ovarian cycle and stress

Stoichiometry of expressed alpha(4)beta(2)delta gamma-aminobutyric acid type A receptors depends on the ratio of subunit cDNA transfected

The gamma-aminobutyric acid type A receptor (GABA(A)R) is the target of many depressants, including benzodiazepines, anesthetics, and alcohol. Although the highly prevalent alphabetagamma GABA(A)R subtype mediates the majority of fast synaptic inhibition in the brain, receptors containing delta subunits also play a key role, mediating tonic inhibition and the actions of endogenous neurosteroids and alcohol. However, the fundamental properties of delta-containing GABA(A)Rs, such as subunit stoichiometry, are not well established. To determine subunit stoichiometry of expressed delta-containing GABA(A)Rs, we inserted the alpha-bungarotoxin binding site tag in the alpha(4), beta(2), and delta subunit N termini. An enhanced green fluorescent protein tag was also inserted into the beta(2) subunit to shift its molecular weight, allowing us to separate subunits using SDS-PAGE. Tagged alpha(4)beta(2)delta GABA(A)Rs were expressed in HEK293T cells using various ratios of subunit cDNA, and receptor subunit stoichiometry was determined by quantitating fluorescent alpha-bungarotoxin bound to each subunit on Western blots of surface immunopurified tagged GABA(A)Rs. The results demonstrate that the subunit stoichiometry of alpha(4)beta(2)delta GABA(A)Rs is regulated by the ratio of subunit cDNAs transfected. Increasing the ratio of delta subunit cDNA transfected increased delta subunit incorporation into surface receptors with a concomitant decrease in beta(2) subunit incorporation. Because receptor subunit stoichiometry can directly influence GABA(A)R pharmacological and functional properties, considering how the transfection protocols used affect subunit stoichiometry is essential when studying heterologously expressed alpha(4)beta(2)delta GABA(A)Rs. Successful bungarotoxin binding site tagging of GABA(A)R subunits is a novel tool with which to accurately quantitate subunit stoichiometry and will be useful for monitoring GABA(A)R trafficking in live cells.

Regulation of GABA(A) receptor subunit expression by pharmacological agents

The gamma-aminobutyric acid (GABA) type A receptor system, the main fast-acting inhibitory neurotransmitter system in the brain, is the pharmacological target for many drugs used clinically to treat, for example, anxiety disorders and epilepsy, and to induce and maintain sedation, sleep, and anesthesia. These drugs facilitate the function of pentameric GABA(A) receptors that exhibit widespread expression in all brain regions and large structural and pharmacological heterogeneity as a result of composition from a repertoire of 19 subunit variants. One of the main problems in clinical use of GABA(A) receptor agonists is the development of tolerance. Most drugs, in long-term use and during withdrawal, have been associated with important modulations of the receptor subunit expression in brain-region-specific manner, participating in the mechanisms of tolerance and dependence. In most cases, the molecular mechanisms of regulation of subunit expression are poorly known, partly as a result of neurobiological adaptation to altered neuronal function. More knowledge has been obtained on the mechanisms of GABA(A) receptor trafficking and cell surface expression and the processes that may contribute to tolerance, although their possible pharmacological regulation is not known. Drug development for neuropsychiatric disorders, including epilepsy, alcoholism, schizophrenia, and anxiety, has been ongoing for several years. One key step to extend drug development related to GABA(A) receptors is likely to require deeper understanding of the adaptational mechanisms of neurons, receptors themselves with interacting proteins, and finally receptor subunits during drug action and in neuropsychiatric disease processes.

Plasticity of the alpha4betadelta GABAA receptor

The GABAR [GABA(A) (gamma-aminobutyric acid type A) receptor], which mediates most inhibition in the brain, is regulated homoeostatically to maintain an optimal level of neuronal excitability. In particular, the alpha(4)betadelta subtype of the GABAR plays a pivotal role in this regulation. This receptor, which is expressed extrasynaptically on the dendrites, normally has low expression in the brain, but displays a remarkable degree of plasticity. It can also be a sensitive target for endogenous neurosteroids such as THP (3alpha-hydroxy-5[alpha]beta-pregnan-20-one (allo-pregnanolone); a neurosteroid and positive modulator of the GABAR), which is released during stress, although the effect of the steroid is polarity-dependent, such that it increases inward current, but decreases outward current, at alpha(4)beta(2)delta GABAR. Expression of alpha(4)beta(2)delta GABAR in CA1 hippocampus is also tightly regulated by fluctuating levels of neurosteroids, as seen at the onset of puberty. Declining levels of inhibition resulting from the decrease in THP at puberty are compensated for by an increase in alpha(4)betadelta GABAR along the apical dendrites of CA1 hippocampal pyramidal cells, which reduces neuronal excitability by decreasing the input resistance. However, excessive decrease of neuronal function is averted when THP levels rise, as would occur during stress, because this steroid decreases the outward GABAergic tonic current via inhibition of alpha(4)beta(2)delta GABAR, thereby restoring measures of neuronal excitability to pre-pubertal levels. Thus the homoeostatic regulation of alpha(4)betadelta GABAR expression plays an important role in maintaining ambient levels of neuronal excitability at puberty.

Trace and contextual fear conditioning is enhanced in mice lacking the alpha4 subunit of the GABA(A) receptor

The GABA(A)R alpha4 subunit is highly expressed in the dentate gyrus region of the hippocampus at predominantly extra synaptic locations where, along with the GABA(A)R delta subunit, it forms GABA(A) receptors that mediate a tonic inhibitory current. The present study was designed to test hippocampus-dependent and hippocampus-independent learning and memory in GABA(A)R alpha4 subunit-deficient mice using trace and delay fear conditioning, respectively. Mice were of a mixed C57Bl/6J X 129S1/X1 genetic background from alpha4 heterozygous breeding pairs. The alpha4-knockout mice showed enhanced trace and contextual fear conditioning consistent with an enhancement of hippocampus-dependent learning and memory. These enhancements were sex-dependent, similar to previous studies in GABA(A)R delta knockout mice, but differences were present in both males and females. The convergent findings between alpha4 and delta knockout mice suggests that tonic inhibition mediated by alpha4betadelta GABA(A) receptors negatively modulates learning and memory processes and provides further evidence that tonic inhibition makes important functional contributions to learning and behavior.

Stress and GABA receptors

GABA(A) receptors are sensitive to subtle changes in the environment in both early-life and adulthood. These neurochemical responses to stress in adulthood are sex-dependent. Acute stress induces rapid changes in GABA(A) receptors in experimental animals, with the direction of the changes varying according to the sex of the animals and the stress-paradigm studied. These rapid alterations are of particular interest as they provide an example of fast neurotransmitter system plasticity that may be mediated by stress-induced increases in neurosteroids, perhaps via effects on phosphorylation and/or receptor trafficking. Interestingly, some studies have also provided evidence for long-lasting changes in GABA(A) receptors as a result of exposure to stressors in early-life. The short- and long-term stress sensitivity of the GABAergic system implicates GABA(A) receptors in the non-genetic etiology of psychiatric illnesses such as depression and schizophrenia in which stress may be an important factor.

Are neuroactive steroids promising therapeutic agents in the management of acute and chronic pain?

Neuroactive steroids with potentiating effects on GABA(A) channels and inhibitory effects on T-type Ca2+ channels which are located in peripheral sensory neurons are potent modulators of pain perception. The focus of this review is on peripheral anti-nociceptive properties of 5alpha- and 5beta-reduced neuroactive steroids with either selective or combined modulatory action on GABA(A) and T-type Ca2+ channel-mediated neurotransmission. We report that these neuroactive steroids are very effective in alleviating peripheral nociception in both acute and chronic pain conditions in animal models of pain. We believe that promising animal data warrant the exploration of their usefulness in clinical settings especially considering the fact that chronic pain sufferers are often young and otherwise healthy people.

Puberty, steroids and GABA(A) receptor plasticity

GABA(A) receptors (GABAR) mediate most inhibition in the CNS and are also a target for neuroactive steroids such as 3alpha,5[alpha]beta-THP (3alphaOH-5[alpha]beta-OH-pregnan-20-one or [allo]pregnanolone). Although these steroids robustly enhance current gated by alpha1beta2delta GABAR, we have shown that 3alpha,5[alpha]beta-THP effects at recombinant alpha4beta2delta GABAR depend on the direction of Cl(-) flux, where the steroid increases outward flux, but decreases inward flux through the receptor. This polarity-dependent inhibition of alpha4beta2delta GABAR resulted from an increase in the rate and extent of rapid desensitization of the receptor, recorded from recombinant receptors expressed in HEK-293 cells with whole cell voltage clamp techniques. This inhibitory effect of 3alpha,5[alpha]beta-THP was not observed at other receptor subtypes, suggesting it was selective for alpha4beta2delta GABAR. Furthermore, it was prevented by a selective mutation of basic residue arginine 353 in the intracellular loop of the receptor, suggesting that this might be a putative chloride modulatory site. Expression of alpha4betadelta GABAR increases markedly at extrasynaptic sites at the onset of puberty in female mice. At this time, 3alpha,5[alpha]beta-THP decreased the inhibitory tonic current, recorded with perforated patch techniques to maintain the physiological Cl(-) gradient. By decreasing this shunting inhibition, 3alpha,5[alpha]beta-THP increased the excitability of CA1 hippocampal pyramidal cells at puberty. These effects of the steroid were opposite to those observed before puberty when 3alpha,5[alpha]beta-THP reduced neuronal excitability as a pre-synaptic effect. Behaviorally, the excitatory effect of 3alpha,5[alpha]beta-THP was reflected as an increase in anxiety at the onset of puberty in female mice. Taken together, these findings suggest that the emergence of alpha4beta2delta GABAR at the onset of puberty reverses the effect of a stress steroid. These findings may be relevant for the mood swings and increased response to stressful events reported in adolescence.

Neurosteroids: endogenous allosteric modulators of GABA(A) receptors

In the mammalian central nervous system activation of the ionotropic GABA(A) receptor by the neurotransmitter GABA plays a crucial role in controlling neuronal excitability. This essential form of neuronal regulation may be subject to "fine tuning" by particular metabolites of progesterone and deoxycorticosterone, which bind directly to the GABA(A) receptor to enhance the actions of GABA. Originally such steroids were considered to act as endocrine messengers, being synthesised in peripheral glands such as the adrenals and ovaries and crossing the blood brain barrier to influence neuronal signalling. However, it is now evident that certain neurons and glia may produce such "neurosteroids" and that these locally synthesised modulators may act in a paracrine, or indeed an autocrine manner to influence neuronal activity. Neurosteroid synthesis may change dynamically in a variety of physiological situations (e.g. stress, pregnancy) and perturbations in their levels are implicated in a variety of neurological and psychiatric disorders. Here we will consider (1) evidence supporting the concept that neurosteroids act as local regulators of neuronal inhibition, (2) that extrasynaptic GABA(A) receptors appear to be a particularly important neurosteroid target and (3) recent advances in defining the neurosteroid binding site(s) on the GABA(A) receptor.

Steroid hormone fluctuations and GABA(A)R plasticity

Conditions of changing steroid hormone levels are a particularly vulnerable time for the manifestation of neurological disorders, including catamenial epilepsy, premenstrual syndrome (PMS), and postpartum depression. The pathophysiology of these disorders may be related to changes in neurosteroid levels, which can dramatically impact neuronal excitability. Robust changes in neurosteroid levels, such as those that occur following stress, over the ovarian cycle, and throughout pregnancy, profoundly alter GABAA receptor (GABAAR) structure and function and underlie the associated changes in neuronal excitability. A moderate and brief exposure to elevated neurosteroids, such as those that occur over the ovarian cycle and following an acute stressful episode, result in a decrease in GABAAR gamma2 subunit expression and an increase in GABAAR delta subunit expression. These changes are accompanied by a decrease in seizure susceptibility and decreased anxiety-like behavior in mice, demonstrating altered neuronal excitability associated with changes in the receptor composition. More robust changes in steroid hormone levels, such as those that occur throughout pregnancy, result in a decrease in both GABAAR gamma2 and delta subunit expression and are associated with an increase in neuronal excitability evident from the shift in the input-output relationship. Alterations in GABAAR subunit composition may represent a homeostatic mechanism to maintain an ideal level of inhibition in the face of fluctuating neurosteroid levels. Neurosteroids potentiate the effects of GABA on GABAARs, particularly those containing the delta subunit, and reorganization of these receptors may be necessary to prevent sedation and/or anaesthesia in the face of high levels of neurosteroids or to prevent hyperexcitability in the absence of these compounds. Alterations in GABAARs under conditions of altered steroid hormone levels result in measurable changes in neuronal excitability and dysregulation of GABAARs may play a role in steroid hormone-associated neurological disorders.

Chronic exposure to anabolic androgenic steroids alters neuronal function in the mammalian forebrain via androgen receptor- and estrogen receptor-mediated mechanisms

Anabolic androgenic steroids (AAS) can promote detrimental effects on social behaviors for which GABA type A (GABA(A)) receptor-mediated circuits in the forebrain play a critical role. While all AAS bind to androgen receptors (AR), they may also be aromatized to estrogens and thus potentially impart effects via estrogen receptors (ER). Chronic exposure of wild-type male mice to a combination of chemically distinct AAS increased action potential (AP) frequency, selective GABA(A) receptor subunit mRNAs, and GABAergic synaptic current decay in the medial preoptic area (mPOA). Experiments performed with pharmacological agents and in AR-deficient Tfm mutant mice suggest that the AAS-dependent enhancement of GABAergic transmission in wild-type mice is AR-mediated. In AR-deficient mice, the AAS elicited dramatically different effects, decreasing AP frequency, spontaneous IPSC amplitude and frequency and the expression of selective GABA(A) receptor subunit mRNAs. Surprisingly, in the absence of AR signaling, the data indicate that the AAS do not act as ER agonists, but rather suggest a novel in vivo action in which the AAS inhibit aromatase and impair endogenous ER signaling. These results show that the AAS have the capacity to alter neuronal function in the forebrain via multiple steroid signaling mechanisms and suggest that effects of these steroids in the brain will depend not only on the balance of AR- versus ER-mediated regulation for different target genes, but also on the ability of these drugs to alter steroid metabolism and thus the endogenous steroid milieu.

Extrasynaptic GABAA receptors: form, pharmacology, and function

GABA is the principal inhibitory neurotransmitter in the CNS and acts via GABA(A) and GABA(B) receptors. Recently, a novel form of GABA(A) receptor-mediated inhibition, termed "tonic" inhibition, has been described. Whereas synaptic GABA(A) receptors underlie classical "phasic" GABA(A) receptor-mediated inhibition (inhibitory postsynaptic currents), tonic GABA(A) receptor-mediated inhibition results from the activation of extrasynaptic receptors by low concentrations of ambient GABA. Extrasynaptic GABA(A) receptors are composed of receptor subunits that convey biophysical properties ideally suited to the generation of persistent inhibition and are pharmacologically and functionally distinct from their synaptic counterparts. This mini-symposium review highlights ongoing work examining the properties of recombinant and native extrasynaptic GABA(A) receptors and their preferential targeting by endogenous and clinically relevant agents. In addition, it emphasizes the important role of extrasynaptic GABA(A) receptors in GABAergic inhibition throughout the CNS and identifies them as a major player in both physiological and pathophysiological processes.

Early life stress as an influence on limbic epilepsy: an hypothesis whose time has come?

The pathogenesis of mesial temporal lobe epilepsy (MTLE), the most prevalent form of refractory focal epilepsy in adults, is thought to begin in early life, even though seizures may not commence until adolescence or adulthood. Amongst the range of early life factors implicated in MTLE causation (febrile seizures, traumatic brain injury, etc.), stress may be one important contributor. Early life stress is an a priori agent deserving study because of the large amount of neuroscientific data showing enduring effects on structure and function in hippocampus and amygdala, the key structures involved in MTLE. An emerging body of evidence directly tests hypotheses concerning early life stress and limbic epilepsy: early life stressors, such as maternal separation, have been shown to aggravate epileptogenesis in both status epilepticus and kindling models of limbic epilepsy. In addition to elucidating its influence on limbic epileptogenesis itself, the study of early life stress has the potential to shed light on the psychiatric disorder that accompanies MTLE. For many years, psychiatric comorbidity was viewed as an effect of epilepsy, mediated psychologically and/or neurobiologically. An alternative – or complementary – perspective is that of shared causation. Early life stress, implicated in the pathogenesis of several psychiatric disorders, may be one such causal factor. This paper aims to critically review the body of experimental evidence linking early life stress and epilepsy; to discuss the direct studies examining early life stress effects in current models of limbic seizures/epilepsy; and to suggest priorities for future research.

Excitability changes related to GABAA receptor plasticity during pregnancy

Alterations in GABA(A) receptor (GABA(A)R) expression and function, similar to those we described previously during pregnancy in the mouse dentate gyrus, may also occur in other brain regions. Here we show, using immunohistochemical techniques, a decreased delta subunit-containing GABA(A)R (deltaGABA(A)R) expression in the dentate gyrus, hippocampal CA1 region, thalamus, and striatum but not in the cerebral cortex. In the face of the highly elevated neurosteroid levels during pregnancy, which can act on deltaGABA(A)Rs, it may be beneficial to decrease the number of neurosteroid-sensitive receptors to maintain a steady-state level of neuronal excitability throughout pregnancy. Consistent with this hypothesis, the synaptic input/output (I/O) relationship in the dentate gyrus molecular layer in response to lateral perforant path stimulation was shifted to the left in hippocampal slices from pregnant compared with virgin mice. The addition of allopregnanolone, at levels comparable with those found during pregnancy (100 nM), shifted the I/O curves in pregnant mice back to virgin levels. There was a decreased threshold to induce epileptiform local field potentials in slices from pregnant mice compared with virgin, but allopregnanolone reverted the threshold for inducing epileptiform activity to virgin levels. According to these data, neuronal excitability is increased in pregnant mice in the absence of allopregnanolone attributable to brain region-specific downregulation of deltaGABA(A)R expression. In brain regions, such as the cortex, that do not exhibit alterations in deltaGABA(A)R expression, there were no changes in the I/O relationship during pregnancy. Similarly, no changes in network excitability were detected in pregnant Gabrd(-/-) mice that lack deltaGABA(A)Rs, suggesting that changes in neuronal excitability during pregnancy are attributable to alterations in the expression of these receptors. Our findings indicate that alterations in deltaGABA(A)R expression during pregnancy result in brain region-specific increases in neuronal excitability that are restored by the high levels of allopregnanolone under normal conditions but under pathological conditions may result in neurological and psychiatric disorders associated with pregnancy and postpartum.

The role of neurosteroids in the pathophysiology and treatment of catamenial epilepsy

Catamenial epilepsy is a multifaceted neuroendocrine condition in which seizures are clustered around specific points in the menstrual cycle, most often around perimenstrual or periovulatory period. Generally, a twofold or greater increase in seizure frequency during a particular phase of the menstrual cycle could be considered as catamenial epilepsy. Based on this criteria, recent clinical studies indicate that catamenial epilepsy affects 31-60% of the women with epilepsy. Three types of catamenial seizures (perimenstrual, periovulatory and inadequate luteal) have been identified. However, there is no specific drug available today for catamenial epilepsy, which has not been successfully treated with conventional antiepileptic drugs. Elucidation of the pathophysiology of catamenial epilepsy is a prerequisite to develop specific targeted approaches for treatment or prevention of the disorder. Cyclical changes in the circulating levels of estrogens and progesterone play a central role in the development of catamenial epilepsy. There is emerging evidence that endogenous neurosteroids with anticonvulsant or proconvulsant effects could play a critical role in catamenial epilepsy. It is thought that perimenstrual catamenial epilepsy is associated with the withdrawal of anticonvulsant neurosteroids. Progesterone and other hormonal agents have been shown in limited trials to be moderately effective in catamenial epilepsy, but may cause endocrine side effects. Synthetic neurosteroids, which enhance the tonic GABA-A receptor function, might provide an effective approach for the catamenial epilepsy therapy without producing hormonal side effects.

Age- and gender-related differences in GABAA receptor-mediated postsynaptic currents in GABAergic neurons of the substantia nigra reticulata in the rat

The responsiveness of the rat anterior substantia nigra pars reticulata (SNR) GABAergic neurons to GABA(A)ergic drugs changes with age and gender, altering its role in seizure control. To determine whether maturational and gender-specific differences in the properties of spontaneous GABA(A)Rs-mediated inhibitory postsynaptic currents (sIPSCs) underlie these events, we studied sIPSCs at baseline and after application of the alpha1 GABA(A)Rs subunit selective agonist zolpidem, at postnatal days (PN) 5-9, PN12-15, and PN28-32. Results were correlated with the alpha1 and alpha 3 GABA(A)Rs subunit immunoreactivity (-ir) at PN5, PN15, and PN30, using immunochemistry. The mean frequency, amplitude and charge transfer increased whereas the 10-90% rise time and decay time accelerated with age in both genders. The faster sIPSC kinetics in older rats were paralleled by increased alpha1-ir and decreased alpha 3-ir. At PN5-9, males had more robust sIPSCs (frequency, amplitude, charge carried per event and charge transfer) than females. At PN28-32, males exhibited higher amplitudes and faster kinetics than females. The zolpidem-induced increase of decay times, amplitude and charge transfer and alpha1-ir expression were the lowest in PN5-9 males but increased with age, in both genders. Our findings demonstrate that alterations in GABA(A)Rs subunit expression partially underlie age- and gender-specific sIPSC changes in SNR neurons. However, the observation of gender differences in sIPSC kinetics that cannot be attributed to changes in perisomatic alpha1 expression suggests the existence of additional gender-specific factors that control the sIPSC kinetics in rat SNR.

Role of GABAA receptors in the physiology and pharmacology of sleep

Most sedative-hypnotics used in insomnia treatment target the gamma-aminobutyric acid (GABA)(A) receptors. A vast repertoire of GABA(A) receptor subtypes has been identified and displays specific electrophysiological and functional properties. GABA(A)-mediated inhibition traditionally refers to 'phasic' inhibition, arising from synaptic GABA(A) receptors which transiently inhibit neurons. However, there is growing evidence that peri- or extra-synaptic GABA(A) receptors are continuously activated by low GABA concentrations and mediate a 'tonic' conductance. This slower type of signaling appears to play a key role in controlling cell excitability. This review aims at summarizing recent knowledge on GABA transmission, including the emergence of tonic conductance, and highlighting the importance of GABA(A) receptor heterogeneity. The mechanism of action of sedative-hypnotic drugs and their effects on sleep and the electroencephalogram will be reported. Furthermore, studies using genetically engineered mice will be emphasized, providing insights into the role of GABA(A) receptors in mechanisms underlying physiological and pharmacological sleep. Finally, we will address the potential of GABA(A) receptor pharmacology for the treatment of insomnia.

Sex differences and reproductive hormone influences on human odor perception

The question of whether men and women differ in their ability to smell has been the topic of scientific investigation for over a hundred years. Although conflicting findings abound, most studies suggest that, for at least some odorants, women outperform men on tests of odor detection, identification, discrimination, and memory. Most functional imaging and electrophysiological studies similarly imply that, when sex differences are present, they favor women. In this review we examine what is known about sex-related alterations in human smell function, including influences of the menstrual cycle, pregnancy, gonadectomy, and hormone replacement therapy on a range of olfactory measures. We conclude that the relationship between reproductive hormones and human olfactory function is complex and that simple associations between circulating levels of gonadal hormones and measures of olfactory function are rarely present.

Neuroactive steroids inhibit spinal reflex potentiation by selectively enhancing specific spinal GABA(A) receptor subtypes

Recently, we demonstrated a spinal GABA(A) receptor (GABA(A)R)-dependent inhibition on the induction of repetitive stimulation-induced spinal reflex potentiation. However, it remains unclear whether steroid hormones modulate such an inhibition. Here, we show that progesterone is capable of producing GABA(A)Rs-dependent inhibition of the induction of spinal reflex potentiation by actions through neurosteroid metabolites. Progesterone (5mg/kg, twice daily for 4 days) up-regulates the expression of GABA(A)R alpha2, alpha3, alpha4 and delta subunits, and is associated with attenuated repetitive stimulation-induced spinal reflex activity in ovariectomized rats. These changes were blocked by finasteride (50mg/kg, twice daily), an antagonist of neurosteroid synthesis from progesterone, but not by the progesterone receptor antagonist, RU486 (100mg/kg, twice daily). The induction of spinal reflex potentiation was attenuated after a short (30 min) intrathecal treatment with the neurosteroids, allopregnanolone (ALLOP, 10 microM, 10 microL) and 3 alpha,5 alpha-tetrahydrodeoxycorticosterone (THDOC, 10 microM, 10 microL). Acute intrathecal administration of the GABA(A)R antagonist, bicuculline (10 microM, 10 microL) reversed the inhibition produced by progesterone, THDOC and allopregnanolone. These results imply that progesterone-mediated effects on GABA(A)R expression and neural inhibition are regulated by neurosteroids synthesis rather than progesterone receptor activation.

Changes in expression and function of extrasynaptic GABAA receptors in the rat hippocampus during pregnancy and after delivery

Pregnancy is associated with changes in mood and anxiety level as well as with marked hormonal fluctuations. Increases in the brain concentrations of neuroactive steroids during pregnancy in rats are accompanied by changes in expression of subunits of the GABA type A receptor (GABA(A)-R) in the brain. Granule cells of the dentate gyrus (DGGCs) exhibit two components of inhibitory GABAergic transmission: a phasic component mediated by synaptic GABA(A)-Rs, and a tonic component mediated by extrasynaptic GABA(A)-Rs. Recordings of GABAergic currents were obtained from hippocampal slices prepared from rats in estrus, at pregnancy day 15 (P15) or P19, or at 2 d after delivery. Exogenous GABA or 3alpha,5alpha-THP induced an increase in tonic current in DGGCs that was significantly greater at P19 than in estrus. Neither tonic nor phasic currents were affected by pregnancy in CA1 pyramidal cells. Immunohistochemical analysis revealed a marked increase in the abundance of the delta subunit of the GABA(A)-R and a concomitant decrease in that of the gamma(2) subunit in the hippocampus at P19. Expression of the alpha(4) subunit did not change during pregnancy but was increased 2 d after delivery. Treatment of rats from P12 to P18 with the 5alpha-reductase inhibitor finasteride prevented the changes in tonic current and in delta and gamma(2) subunit expression normally apparent at P19. These data suggest that the number of extrasynaptic GABA(A)-Rs is increased in DGGCs during late pregnancy as a consequence of the associated marked fluctuations in the brain levels of neuroactive steroids.

Novel compounds selectively enhance delta subunit containing GABA A receptors and increase tonic currents in thalamus

Inhibition in the brain is dominated by the neurotransmitter gamma-aminobutyric acid (GABA); operating through GABA(A) receptors. This form of neural inhibition was presumed to be mediated by synaptic receptors, however recent evidence has highlighted a previously unappreciated role for extrasynaptic GABA(A) receptors in controlling neuronal activity. Synaptic and extrasynaptic GABA(A) receptors exhibit distinct pharmacological and biophysical properties that differentially influence brain physiology and behavior. Here we used a fluorescence-based assay and cell lines expressing recombinant GABA(A) receptors to identify a novel series of benzamide compounds that selectively enhance, or activate alpha4beta3delta GABA(A) receptors (cf. alpha4beta3gamma2 and alpha1beta3gamma2). Utilising electrophysiological methods, we illustrate that one of these compounds, 4-chloro-N-[6,8-dibromo-2-(2-thienyl)imidazo[1,2-a]pyridine-3-yl benzamide (DS1) potently (low nM) enhances GABA-evoked currents mediated by alpha4beta3delta receptors. At similar concentrations DS1 directly activates this receptor and is the most potent known agonist of alpha4beta3delta receptors. 4-chloro-N-[2-(2-thienyl)imidazo[1,2-a]pyridine-3-yl benzamide (DS2) selectively potentiated GABA responses mediated by alpha4beta3delta receptors, but was not an agonist. Recent studies have revealed a tonic form of inhibition in thalamus mediated by the alpha4beta2delta extrasynaptic GABA(A) receptors that may contribute to the regulation of thalamocortical rhythmic activity associated with sleep, wakefulness, vigilance and seizure disorders. In mouse thalamic relay cells DS2 enhanced the tonic current mediated by alpha4beta2delta receptors with no effect on their synaptic GABA(A) receptors. Similarly, in mouse cerebellar granule cells DS2 potentiated the tonic current mediated by alpha6betadelta receptors. DS2 is the first selective positive allosteric modulator of delta-GABA(A) receptors and such compounds potentially offer novel therapeutic opportunities as analgesics and in the treatment of sleep disorders. Furthermore, these drugs may be valuable in elucidating the physiological and pathophysiological roles played by these extrasynaptic GABA(A) receptors.

Stress, the hippocampus, and epilepsy

Stress is among the most frequently self-reported precipitants of seizures in patients with epilepsy. This review considers how important stress mediators like corticotropin-releasing hormone, corticosteroids, and neurosteroids could contribute to this phenomenon. Cellular effects of stress mediators in the rodent hippocampus are highlighted. Overall, corticosterone--with other stress hormones--rapidly enhances CA1/CA3 hippocampal activity shortly after stress. At the same time, corticosterone starts gene-mediated events, which enhance calcium influx several hours later. This later effect serves to normalize activity but also imposes a risk for neuronal injury if and when neurons are concurrently strongly depolarized, for example, during epileptic activity. In the dentate gyrus, stress-induced elevations in corticosteroid level are less effective in changing membrane properties such as calcium influx; here, enhanced inhibitory tone mediated through neurosteroid effects on gamma-aminobutyric acid (GABA) receptors might dominate. Under conditions of repetitive stress (e.g., caused from experiencing repetitive and unpredictable seizures) and/or early life stress, hormonal influences on the inhibitory tone, however, are diminished; instead, enhanced calcium influx and increased excitation become more important. In agreement, perinatal stress and elevated steroid levels accelerate epileptogenesis and lower seizure threshold in various animal models for epilepsy. It will be interesting to examine how curtailing the effects of stress in adults, for example, by brief treatment with antiglucocorticoids, may be beneficial to the treatment of epilepsy.

GABA(A)R plasticity during pregnancy: relevance to postpartum depression

Fluctuating neurosteroid levels over the ovarian cycle modulate neuronal excitability through effects on GABA(A) receptors (GABA(A)Rs). The large increase in progesterone-derived neurosteroids during pregnancy and their precipitous decline at parturition may have considerable effects on GABA(A)Rs during pregnancy and postpartum. Here we show a significant decrease in tonic and phasic inhibitions in pregnant mice, mediated by a downregulation of GABA(A)R delta and gamma2 subunits, respectively, which rebounds immediately postpartum. Mice which do not exhibit GABA(A)R delta subunit regulation throughout pregnancy (Gabrd(+/-) and Gabrd(-/-)) exhibit depression-like and abnormal maternal behaviors, resulting in reduced pup survival. These abnormal postpartum behaviors were ameliorated in Gabrd(+/-) mice by a GABA(A)R delta-subunit-selective agonist, THIP. We suggest that Gabrd(+/-) and Gabrd(-/-) mice constitute a mouse model of postpartum depression that may be useful for evaluating potential therapeutic interventions.

Reduced serum level of THDOC, an anticonvulsant steroid, in women with perimenstrual catamenial epilepsy

PURPOSE

Seizure exacerbation in catamenial epilepsy (CE) is associated with the decrease in progesterone secretion and increase in estradiol secretion during the premenstrual period. Moreover, experimental evidence suggests that tetrahydrodeoxycorticosterone (THDOC), a positive modulator of the type A receptor for gamma-aminobutyric acid (GABA), and dehydroepiandrosterone sulfate (DHEAS), a negative modulator of this receptor, might play a crucial role in modulating seizure frequency during the menstrual cycle. Following these studies it seems of interest to investigate possible variations, among other hormonal parameters, of THDOC and DHEAS in CE patients.

METHODS

The serum concentrations of progesterone (P4), pregnenolone, allopregnanolone (AP), THDOC, DHEAS, cortisol, and DHEAS/cortisol ratio were measured throughout the menstrual cycle at the 7th, 11th, 15th, 19th, 23rd, and 27th day from the onset of spontaneous menstrual blood loss in young premenopausal women with CE (n = 17) and age-matched controls (n = 13).

RESULTS

At each time of the study, the serum concentration of THDOC and the DHEAS/cortisol ratio were lower (p < 0.05) in women with CE than in control women. The concentrations of P4, pregnenolone, and AP did not differ between the two groups of subjects.

CONCLUSIONS

The reduced serum concentration of THDOC and the reduced DHEAS/cortisol ratio detected throughout the menstrual cycle in women with CE might play a role in CE. Moreover, the peculiar pattern of CE seizure exacerbation might suggest that these neuroendocrine variations are worth investigating in other epileptic syndromes, particularly in those characterized by relevant and uncontrolled variations in seizure frequency.

Estradiol regulation of progesterone synthesis in the brain

Steroidogenesis is now recognized as a global phenomenon in the brain, but how it is regulated and its relationship to circulating steroids of peripheral origin have remained more elusive issues. Neurosteroids, steroids synthesized de novo in nervous tissue, have a large range of actions in the brain, but it is only recently that the role of neuroprogesterone in the regulation of arguably the quintessential steroid-dependent neural activity, regulation of the reproduction has been appreciated. Circuits involved in controlling the LH surge and sexual behaviors were thought to be influenced by estradiol and progesterone synthesized in the ovary and perhaps the adrenal. It is now apparent that estradiol of ovarian origin regulates the synthesis of neuroprogesterone, and it is the locally produced neuroprogesterone that is involved in the initiation of the LH surge and subsequent ovulation. In this model, estradiol induces the transcription of progesterone receptors while stimulating synthesis of neuroprogesterone. Although the complete signaling cascade has not been elucidated, many of the features have been characterized. The synthesis of neuroprogesterone occurs primarily in astrocytes and requires the interaction of membrane-associated estrogen receptor-alpha with metabotropic glutamate receptor-1a. This G protein-coupled receptor activates a phospholipase C that in turn increases inositol trisphosphate (IP3) levels mediating the release of intracellular stores of Ca2+ via an IP3 receptor gated Ca2+ channel. The large increase in free cytoplasmic Ca2+ ([Ca2+]i) stimulates the synthesis of progesterone, which can then diffuse out of the astrocyte and activate estradiol-induced progesterone receptors in local neurons to trigger the neural cascade to produce the LH surge. Thus, it is a cooperative action of astrocytes and neurons that is needed for estrogen positive feedback and stimulation of the LH surge.

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.

GABA(A) receptor properties in catastrophic infantile epilepsy

Catastrophic epilepsy due to cortical dysplasia is often intractable to anticonvulsant treatment. Many of the medications used unsuccessfully in treating this disorder are thought to exert at least a portion of their action through enhancement of inhibitory GABA(A) neurotransmission. In the present study, GABA(A) receptor properties in resected brain tissue from four infants with infantile spasms and intractable epilepsy due to cortical dysplasia were measured to determine if this clinical resistance to pharmacologic treatment correlates with alterations in receptor function. Results from epileptic cortex were compared with those from autopsy control samples. To perform these studies, we utilized the technique of injection of brain cellular membrane preparations into the Xenopus oocyte, which results in the incorporation of human GABA(A) receptors in their native configuration into the oocyte plasma membrane. Two-electrode voltage-clamp electrophysiology analysis was then performed to assess GABA(A) receptor pharmacologic properties. The intrinsic properties of affinity, reversal potential, current decay, and current rundown were unchanged in the epileptic infants. Current enhancement by benzodiazepines was also unaltered, as was the response to barbiturates. However, a significant decrease was found in the degree of GABA(A) current enhancement by neurosteroids in the epileptic infants, along with an increase in current inhibition by zinc. These findings may contribute to the mechanisms of intractability in catastrophic infantile epilepsy due to cortical dysplasia, and suggest alternative therapeutic approaches.

Fast non-genomic effects of progesterone-derived neurosteroids on nociceptive thresholds and pain symptoms

Fast Inhibitory controls mediated by glycine (GlyRs) and GABAA receptors (GABAARs) play an important role to prevent the apparition of pathological pain symptoms of allodynia and hyperalgesia. The use of positive allosteric modulators of these receptors, specifically expressed in the spinal cord, may represent an interesting strategy to limit or block pain expression. In this study, we have used stereoisomers of progesterone metabolites, acting only via non-genomic effects, in order to evaluate the contribution of GlyRs and GABAARs for the reduction of mechanical and thermal heat hypernociception. We show that 3alpha neurosteroids were particularly efficient to elevate nociceptive thresholds in naive animal. It also reduced mechanical allodynia and thermal heat hyperalgesia in the carrageenan model of inflammatory pain. This effect is likely to be mediated by GABAA receptors since 3beta isomer was inefficient. More interestingly, 3alpha5beta neurosteroid was only efficient on mechanical allodynia while having no effect on thermal heat hyperalgesia. We characterized these paradoxical effects of 3alpha5beta neurosteroid using the strychnine and bicuculline models of allodynia. We clearly show that 3alpha5beta neurosteroid exerts an antinociceptive effect via a positive allosteric modulation of GABAARs but, at the same time, is pronociceptive by reducing GlyR function. This illustrates the importance of the inhibitory amino acid receptor channels and their allosteric modulators in spinal pain processing. Moreover, our results indicate that neurosteroids, which are synthesized in the dorsal horn of the spinal cord and have limited side effects, may be of significant interest in order to treat pathological pain symptoms.

GABAA receptor-mediated tonic currents in substantia gelatinosa neurons of rat spinal trigeminal nucleus pars caudalis

In the present study, we describe GABAA receptor-mediated tonic inhibitory currents in the substantia gelatinosa (SG) region of rat spinal trigeminal nucleus pars caudalis (Vc). The GABA(A) receptor-mediated tonic currents were identified by bath-application of the GABAA receptor antagonists, picrotoxin (1mM), SR95531 (100microM) and bicuculline (100microM). All three antagonists completely blocked outward spontaneous (phasic) inhibitory postsynaptic currents, but only picrotoxin and bicuculline induced a significant (>5pA) inward shift of holding currents at a holding potential (Vh) of 0mV in 60-70% of SG neurons, revealing the existence of tonic outward currents. The tonic currents were resistant to further the blockades of glycine receptors or those in addition to glutamate receptors and voltage-dependent sodium channels. An acute bath-application of THDOC (0.1microM), the stress-related neurosteroid, did enhance tonic currents, but only in a small population of SG neurons. In addition, slices incubated with THDOC for 30min increased the probability of neurons with significant tonic currents. The GABAergic tonic inhibition demonstrated in this study may play a significant role in the sensory processing system of the Vc.

Synthesis and function of hypothalamic neuroprogesterone in reproduction

The physiology and regulation of steroid synthesis in the brain have emerged as important for understanding brain function. Neurosteroids, those steroids synthesized de novo in nervous tissue, have been associated with numerous central nervous system functions, including myelination, mental retardation, and epilepsy. Central regulation of reproduction was thought to depend on steroids of peripheral origin. Only recently has the role of neurosteroids in reproduction been appreciated. This minireview describes our work trying to understand how circulating estradiol modulates the synthesis of neuroprogesterone. The synthesis of neuroprogesterone occurs primarily in astrocytes, and requires the interaction of membrane-associated estrogen receptor with metabotropic glutamate receptor and the release of intracellular calcium stores. The newly synthesized neuroprogesterone acts on estradiol-induced progesterone receptors in nearby neurons to initiate the LH surge.

Cell type-specific GABA A receptor-mediated tonic inhibition in mouse neocortex

Activity of extrasynaptic GABA A receptors mediating tonic inhibition is thought to play an important role for the excitability of the mammalian cerebral cortex. However, little is known about the cell type-specific expression of tonic inhibition in particular types of cortical interneurons. Here, we used transgenic mice expressing green fluorescent protein (GFP) in somatostatin-positive (SOM) interneurons and investigated tonic inhibition in SOM interneurons versus pyramidal cells in neocortical layers 2/3. In brain slices, pyramidal cells showed a tonic current of 66 +/- 19 pA in response to the delta-subunit selective GABA A agonist THIP (1 microM). On the other hand, tonic inhibition was absent in SOM interneurons (8 +/- 1 pA) in response to THIP. As opposed to pyramidal cells, SOM interneurons were also insensitive to the delta-subunit preferring neurosteroid allotetrahydrodeoxycorticosterone (THDOC) (100 nM) and to elevated endogenous GABA levels in the slice. Finally, SOM interneurons received only 45% of the phasic charge transfer during GABA A receptor-mediated synaptic activity compared with pyramidal cells. Altogether, our study indicates that SOM interneurons receive relatively weak inhibitory input and cannot be brought under the influence of tonic inhibition.

Regional differences in the decay kinetics of GABA(A) receptor-mediated miniature IPSCs in the dorsal horn of the rat spinal cord are determined by mitochondrial transport of cholesterol

We examined the possibility of a differential spatial control in the endogenous production of 3alpha5alpha-reduced steroids and its consequences on GABA(A) receptor-mediated miniature IPSCs (mIPSCs) in laminas II and III-IV of the rat spinal cord dorsal horn (DH). Early in postnatal development [younger than postnatal day 8 (P8)], mIPSCs displayed slow decay kinetics in laminas II and III-IV resulting from a continuous local production of 3alpha5alpha-reduced steroids. This was mediated by the tonic activity of the translocator protein of 18 kDa (TSPO), which controls neurosteroid synthesis by regulating the transport of cholesterol across the mitochondrial membrane system. TSPO activity disappeared in laminas III-IV after P8 and was functionally downregulated in lamina II after P15, resulting in a marked reduction of mIPSC duration in these laminas. TSPO-mediated synthesis of 3alpha5alpha-reduced steroids was spatially restricted, because, at P9-P15, when their production was maximal in lamina II, no sign of spillover to laminas III-IV was apparent. Interestingly, after P8, the enzymes necessary for the synthesis of 3alpha5alpha-reduced steroids remained functional in laminas III-IV and could produce such steroids from various precursors or after a single subcutaneous injection of progesterone. Moreover, induction of an acute peripheral inflammation by intraplantar injection of carrageenan, restored a maximal TSPO-mediated neurosteroidogenesis in laminas III-IV. Our results indicate that the decay kinetics of GABA(A) receptor-mediated mIPSCs in the DH of the spinal cord are primarily controlled by 3alpha5alpha-reduced steroids, which can be produced from circulating steroid precursors and/or in a spatially restricted manner by the modulation of the activity of TSPO.

Suppression of the febrile response in late gestation: evidence, mechanisms and outcomes

Fever is a beneficial host defence response. However, fever caused by the immune stimulant, lipopolysaccharide (LPS), are attenuated in many species during pregnancy, particularly near term. A number of parallel mechanisms may be responsible, and these vary in magnitude according to the time of gestation, type of inflammatory stimulus and species of animal. Some studies report a reduction in the plasma levels of circulating pro-inflammatory cytokines such as tumour necrosis factor-alpha, interleukin-1beta and interleukin-6 along with increased levels of anti-inflammatory cytokines such as interleukin-1 receptor antagonist. Associated with the attenuated febrile response to LPS is a reduction in the activation of the prostaglandin synthesising enzyme, cyclo-oxygenase 2, resulting in reduced levels of the obligatory prostaglandin mediators of the febrile response in the brain. There is also a reduction in the sensitivity of the brain to the pyrogenic action of prostaglandins, which does not appear to be due to a change in the levels of hypothalamic EP3 prostaglandin receptors. The suppression of fever at term may be important for the health of the neonate because fever in pregnant mothers may be harmful to the late-term foetus and neonate.

Neurosteroid modulation of GABAA receptors: molecular determinants and significance in health and disease

Over the past 20 years it has become apparent that certain steroids, synthesised de novo in the brain, hence named neurosteroids, produce immediate changes (within seconds) in neuronal excitability, a time scale that precludes a genomic locus of action. Identified molecular targets underlying modulation of brain excitability include both the inhibitory GABA(A) and the excitatory NMDA receptor. Of particular interest is the interaction of certain neurosteroids with the GABA(A) receptor, the major inhibitory receptor in mammalian brain. During the last decade, compelling evidence has accrued to reveal that locally produced neurosteroids may selectively "fine tune" neuronal inhibition. A range of molecular mechanisms including the subunit composition of the receptor(s), phosphorylation and local steroid metabolism, underpin the region- and neuronal selectivity of action of neurosteroids at synaptic and extrasynaptic GABA(A) receptors. The relative contribution played by each of these mechanisms in a variety of physiological and pathophysiological scenarios is currently being scrutinised at a cellular and molecular level. However, it is not known how such mechanisms may act in concert to influence behavioural profiles in health and disease. An important question concerns the identification of the anatomical substrates mediating the repertoire of behaviours produced by neurosteroids. "Knock-in" mice expressing mutant GABA(A) subunits engineered to be insensitive to benzodiazepines or general anaesthetics have proved invaluable in evaluating the role of GABA(A) receptor subtypes in complex behaviours such as sedation, cognition and anxiety [Rudolph, U., Mohler, H., 2006. GABA-based therapeutic approaches: GABA(A) receptor subtype functions. Curr. Opin. Pharmacol. 6, 18-23]. However, the development of a similar approach for neurosteroids has been hampered by the limited knowledge that, until recently, has surrounded the identity of the amino acid residues contributing to the neurosteroid binding pocket. Here, we will review recent progress in identifying the neurosteroid binding site on the GABA(A) receptor, and discuss how these discoveries will impact on our understanding of the role of neurosteroids in health and disease.

Neurosteroid regulation of GABA(A) receptors: Focus on the alpha4 and delta subunits

Neurosteroids, such as the progesterone metabolite 3alpha-OH-5alpha[beta]-pregnan-20-one (THP or [allo]pregnanolone), function as potent positive modulators of the GABA(A) receptor (GABAR) when acutely administered. However, fluctuations in the circulating levels of this steroid at puberty, across endogenous ovarian cycles, during pregnancy or following chronic stress produce periods of prolonged exposure and withdrawal, where changes in GABAR subunit composition may occur as compensatory responses to sustained levels of inhibition. A number of laboratories have demonstrated that both chronic administration of THP as well as its withdrawal transiently increase expression of the alpha4 subunit of the GABAR in several areas of the central nervous system (CNS) as well as in in vitro neuronal systems. Receptors containing this subunit are insensitive to benzodiazepine (BDZ) modulation and display faster deactivation kinetics, which studies suggest underlie hyperexcitability states. Similar increases in alpha4 expression are triggered by withdrawal from other GABA-modulatory compounds, such as ethanol and BDZ, suggesting a common mechanism. Other studies have reported puberty or estrous cycle-associated increases in delta-GABAR, the most sensitive target of these steroids which underlies a tonic inhibitory current. In the studies reported here, the effect of steroids on inhibition, which influence anxiety state and seizure susceptibility, depend not only on the subunit composition of the receptor but also on the direction of Cl(-) current generated by these target receptors. The effect of neurosteroids on GABAR function thus results in behavioral outcomes relevant for pubertal mood swings, premenstrual dysphoric disorder and catamenial epilepsy, which are due to fluctuations in endogenous steroids.

Changes in neuroactive steroid content during social isolation stress modulate GABAA receptor plasticity and function

Rats deprived of social contact with other rats at a young age experience a form of prolonged stress that leads to long-lasting alteration in their behavior profile. This chronic stress paradigm is thus thought to be anxiogenic for these normally gregarious animals and their abnormal reactivity to environmental stimuli, when reared under this condition, is thought to be a product of prolonged stress. Neurochemical, molecular, and electrophysiological evidences demonstrate that social isolation is associated with alteration in the structure and function of GABA(A) receptors and suggest that endogenous content of the progesterone metabolite 3alpha,5alpha-TH PROG may be an important determinant in regulating brain excitability and sensitivity to stimuli and point out its possible role in psychiatric and neurological disorder.

Exposure to novelty and forced swimming evoke stressor-dependent changes in extracellular GABA in the rat hippocampus

In the hippocampus, a brain structure critically important in the stress response, GABA controls neuronal activity not only via synaptic inhibition, but also via tonic inhibition through stimulation of extrasynaptic GABA receptors. The extracellular level of GABA may represent a major determinant for tonic inhibition and, therefore, it is surprising that its responsiveness to stress has hardly been investigated. To clarify whether hippocampal extracellular GABA levels change in response to acute stress, we conducted an in vivo microdialysis study in rats. We found that dialysate GABA levels respond to various neuropharmacological manipulations such as reuptake inhibition, elevated concentrations of K(+), tetrodotoxin and baclofen, indicating that a large proportion of hippocampal extracellular GABA depends on neuronal release and that GABA re-uptake plays a role in determining the extracellular levels of this neurotransmitter. Next, rats were exposed to a novel cage or to forced swimming in 25 degrees C water. Interestingly, these two stressors resulted in opposite effects. Novelty caused a fast increase in GABA (120% of baseline), whereas forced swimming resulted in a profound decrease (70% of baseline). To discriminate between the psychological and physical aspects (i.e. the effects on body temperature) of forced swimming, another group of animals was forced to swim at 35 degrees C. This stressor, like novelty, caused an increase in hippocampal GABA, suggesting a stimulatory effect of psychological stress. The effects of novelty could not be blocked by the corticotropin-releasing factor receptor antagonist D-Phe-CRF(12-41). These results are the first to demonstrate stressor-dependent changes in hippocampal extracellular GABA; an observation which may be of particular significance for GABAergic tonic inhibition of hippocampal neurons.

Altered localization of GABA(A) receptor subunits on dentate granule cell dendrites influences tonic and phasic inhibition in a mouse model of epilepsy

Complex changes in GABA(A) receptors (GABA(A)Rs) in animal models of temporal lobe epilepsy during the chronic period include a decrease in the delta subunit and increases in the alpha4 and gamma2 subunits in the dentate gyrus. We used postembedding immunogold labeling to determine whether the subcellular locations of these subunits were also altered in pilocarpine-treated epileptic mice, and related functional changes were identified electrophysiologically. The ultrastructural studies confirmed a decrease in delta subunit labeling at perisynaptic locations in the molecular layer of the dentate gyrus where these subunits are critical for tonic inhibition. Unexpectedly, tonic inhibition in dentate granule cells was maintained in the epileptic mice, suggesting compensation by other GABA(A)Rs. An insensitivity of the tonic current to the neurosteroid tetrahydrodeoxy-corticosterone was consistent with decreased expression of the delta subunit. In the pilocarpine-treated mice, alpha4 subunit labeling remained at perisynaptic locations, but increased gamma2 subunit labeling was also found at many perisynaptic locations on granule cell dendrites, consistent with a shift of the gamma2 subunit from synaptic to perisynaptic locations and potential partnership of the alpha4 and gamma2 subunits in the epileptic animals. The decreased gamma2 labeling near the center of synaptic contacts was paralleled by a corresponding decrease in the dendritic phasic inhibition of granule cells in the pilocarpine-treated mice. These GABA(A)R subunit changes appear to impair both tonic and phasic inhibition, particularly at granule cell dendrites, and could reduce the adaptive responses of the GABA system in temporal lobe epilepsy.

GABAA and glycine receptor-mediated transmission in rat lamina II neurones: relevance to the analgesic actions of neuroactive steroids

Analgesic neurosteroids such as 5alpha-pregnan-3alpha-ol-20-one (5alpha3alpha) are potent selective endogenous modulators of the GABA(A) receptor (GABA(A)R) while certain synthetic derivatives (i.e. minaxolone) additionally enhance the function of recombinant glycine receptors (GlyR). Inhibitory transmission within the superficial dorsal horn has been implicated in mediating the analgesic actions of neurosteroids. However, the relative contribution played by synaptic and extrasynaptic receptors is unknown. In this study, we have compared the actions of 5alpha3alpha and minaxolone upon inhibitory transmission mediated by both GABA(A) and strychnine-sensitive GlyRs in lamina II neurones of juvenile (P15-21) rats. At the near physiological temperature of 35 degrees C and at a holding potential of -60 mV we recorded three kinetically distinct populations of miniature IPSCs (mIPSCs): GlyR-mediated, GABA(A)R-mediated and mixed GABA(A)R-GlyR mIPSCs, arising from the corelease of both inhibitory neurotransmitters. In addition, sequential application of strychnine and bicuculline revealed a small (5.2 +/- 1.0 pA) GlyR- but not a GABA(A)R-mediated tonic conductance. 5alpha3alpha (1-10 microm) prolonged GABA(A)R and mixed mIPSCs in a concentration-dependent manner but was without effect upon GlyR mIPSCs. In contrast, minaxolone (1-10 microm) prolonged the decay of GlyR mIPSCs and, additionally, was approximately 10-fold more potent than 5alpha3alpha upon GABA(A)R mIPSCs. However, 5alpha3alpha and minaxolone (1 microm) evoked a similar bicuculline-sensitive inhibitory conductance, indicating that the extrasynaptic GABA(A)Rs do not discriminate between these two steroids. Furthermore, approximately 92% of the effect of 1 microm 5alpha3alpha upon GABAergic inhibition could be accounted for by its action upon the extrasynaptic conductance. These findings are relevant to modulation of inhibitory circuits within spinally mediated pain pathways and suggest that extrasynaptic GABA(A)Rs may represent a relevant molecular target for the analgesic actions of neurosteroids.

A 17beta-derivative of allopregnanolone is a neurosteroid antagonist at a cerebellar subpopulation of GABA A receptors with nanomolar affinity

BACKGROUND AND PURPOSE

High-affinity, subtype-selective antagonists of the neurosteroid binding sites of GABA(A) receptors are not available. We have characterized an allopregnanolone derivative as an antagonist of cerebellar GABA(A) receptors with nanomolar affinity.

EXPERIMENTAL APPROACH

Receptor binding and electrophysiological methods were used for the allosteric modulation of cerebellar GABA(A) receptors by an allopregnanolone derivative, (20R)-17beta-(1-hydroxy-2,3-butadienyl)-5alpha-androstane-3alpha-ol (HBAO). GABA(A) receptors of rat cerebellar membranes were labelled with the chloride channel blocker [(3)H]ethynylbicycloorthobenzoate (EBOB). The ionophore function of GABA(A) receptors was studied by whole-cell patch clamp electrophysiology in cultured rat cerebellar granule and cortical cells.

KEY RESULTS

Partial displacement of cerebellar [(3)H]EBOB binding by nanomolar HBAO was attenuated by 0.1 mM furosemide, an antagonist of alpha(6) and beta(2-3) subunit-containing GABA(A) receptors. Displacement curves of HBAO were reshaped by 30 nM GABA and shifted to the right. However, the micromolar potency of full displacement by allopregnanolone was not affected by 0.1 mM furosemide or 30 nM GABA. The nanomolar, but not the micromolar phase of displacement of [(3)H]EBOB binding by GABA was attenuated by 100 nM HBAO. Submicromolar HBAO did not affect [(3)H]EBOB binding to cortical and hippocampal GABA(A) receptors. HBAO up to 1 microM did not affect chloride currents elicited by 0.3-10 microM GABA, while it abolished potentiation by 1 microM allopregnanolone with nanomolar potency in cerebellar but not in cortical cells. Furosemide attenuated cerebellar inhibition by 100 nM HBAO.

CONCLUSIONS AND IMPLICATIONS

HBAO is a selective antagonist of allopregnanolone, a major endogenous positive modulator via neurosteroid sites of cerebellar (probably alpha(6)beta(2-3)delta) GABA(A) receptors.

Neurosteroid modulation of synaptic and extrasynaptic GABA(A) receptors

Certain naturally occurring pregnane steroids act in a nongenomic manner to potently and selectively enhance the interaction of the inhibitory neurotransmitter GABA with the GABA(A) receptor. Consequently such steroids exhibit anxiolytic, anticonvulsant, analgesic, sedative, hypnotic, and anesthetic properties. In both physiological and pathophysiological scenarios, the pregnane steroids may function as endocrine messengers (e.g., produced in the periphery and cross the blood-brain barrier) to influence behaviour. However, additionally "neurosteroids" can be synthesised in the brain and spinal cord to act in a paracrine or autocrine manner and thereby locally influence neuronal activity. Given the ubiquitous expression of the GABA(A) receptor throughout the mammalian central nervous system (CNS), physiological, pathophysiological, or drug-induced pertubations of neurosteroid levels may be expected to produce widespread changes in brain excitability. However, the neurosteroid/GABA(A) receptor interaction is brain region and indeed neuron specific. The molecular basis of this specificity will be reviewed here, including (1) the importance of the subunit composition of the GABA(A) receptor; (2) how protein phosphorylation may dynamically influence the sensitivity of GABA(A) receptors to neurosteroids; (3) the impact of local steroid metabolism; and (4) the emergence of extrasynaptic GABA(A) receptors as a neurosteroid target.