Neurosteroids shift partial agonist activation of GABA(A) receptor channels from low- to high-efficacy gating patterns

Role of GABAA receptors in cognition

Complex brains have developed specialized mechanisms for the grouping of principal cells into temporal coalitions of local or distant networks: the inhibitory interneuron ‘clocking’ networks. They consist of GABAergic (where GABA is γ-aminobutyric acid) interneurons of a rich diversity. In cortical circuits, these neurons control spike timing of the principal cells, sculpt neuronal rhythms, select cell assemblies and implement brain states. On the basis of these considerations, the deficits in cognition, emotion and perception in psychiatric disorders such as anxiety, depression or schizophrenia are considered to manifest themselves through a dysregulation of the inhibitory interneuron ‘clocking’ network as a final common denominator, irrespective of the diverse underlying disease pathologies. The diversity of GABAergic interneurons is paralleled by a corresponding diversity of GABAA receptors in network regulation. The region-, cell- and domain-specific location of these receptor subtypes offers the possibility to gain functional insights into the role of behaviourally relevant neuronal circuits. Using genetic manipulation, the regulation of anxiety behaviour was attributed to neuronal circuits characterized by the expression of α2-GABAA receptors. Neurons expressing α3-GABAA receptors, located mainly in aminergic and basal forebrain cholinergic neurons, were related to a hyperdopaminergic phenotype, typical of schizophrenic symptoms. Temporal and spatial memory were selectively modulated by extrasynaptic α5-GABAA receptors. Chronic pathological pain was under the regulation of spinal and cortical α2- (and α3-) GABAA receptors. Thus the relevance of the diversity of inhibitory GABAA receptor subtypes for the regulation of cognition, emotion and memory is increasingly being recognized. The clinical proof-of-concept of a subtype-specific pharmacology is most advanced for the alleviation of cognitive dysfunctions in schizophrenia, based on the treatment of patients with an α2/α3-GABAA receptor ligand.

Influence of 17beta-estradiol and progesterone on GABAergic gene expression in the arcuate nucleus, amygdala and hippocampus of the rhesus macaque

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the brain, and the responsiveness of neurons to GABA can be modulated by sex steroids. To better understand how ovarian steroids influence the GABAergic system in the primate brain, we evaluated the expression of genes encoding GABA receptor subunits, glutamic acid decarboxylase (GAD) and a GABA transporter in the brains of female rhesus macaques. Ovariectomized adults were subjected to a hormone replacement paradigm involving either 17beta-estradiol (E), or E plus progesterone (E+P). Untreated animals served as controls. Using GeneChip microarray analysis and real-time RT-PCR (qPCR), we examined gene expression differences within and between the amygdala (AMD), hippocampus (HPC) and arcuate nuclei of the medial basal hypothalamus (MBH). The results from PCR corresponded with results from representative GeneChip probesets, and showed similar effects of sex steroids on GABA receptor subunit gene expression in the AMD and HPC, and a more pronounced expression than in the MBH. Exposure to E+P attenuated GAD1, GAD2 and SLC32A1 gene expression in the AMD and HPC, but not in the MBH. GABA receptor subunit gene expression was generally higher in the AMD and HPC than in the MBH, with the exception of receptor subunits epsilon and gamma 2. Taken together, the data demonstrate differential regulation of GABA receptor subunits and GABAergic system components in the MBH compared to the AMD and HPC of rhesus macaques. Elevated epsilon and reduced delta subunit expression in the MBH supports the hypothesis that the hypothalamic GABAergic system is resistant to the modulatory effects of sex steroids.

Structure of alpha6 beta3 delta GABA(A) receptors and their lack of ethanol sensitivity

Delta (delta) subunit containing GABA(A) receptors are expressed extra-synaptically and mediate tonic inhibition. In cerebellar granule cells, they often form a receptor together with alpha(6) subunits. We were interested to determine the architecture of these receptors. We predefined the subunit arrangement of 24 different GABA(A) receptor pentamers by subunit concatenation. These receptors (composed of alpha(6), beta(3) and delta subunits) were expressed in Xenopus oocytes and their electrophysiological properties analyzed. Currents elicited in response to GABA were determined in presence and absence of 3alpha, 21-dihydroxy-5alpha-pregnan-20-one and to 4,5,6,7-tetrahydroisoxazolo[5,4-c]-pyridin-3-ol. alpha(6)-beta(3)-alpha(6)/delta receptors showed a substantial response to GABA alone. Three receptors, beta(3)-alpha(6)-delta/alpha(6)-beta(3), alpha(6)-beta(3)-alpha(6)/beta(3)-delta and beta(3)-delta-beta(3)/alpha(6)-beta(3), were only uncovered in the combined presence of the neurosteroid 3alpha, 21-dihydroxy-5alpha-pregnan-20-one with GABA. All four receptors were activated by 4,5,6,7-tetrahydroisoxazolo[5,4-c]-pyridin-3-ol. None of the functional receptors was modulated by physiological concentrations (up to 30 mM) of ethanol. GABA concentration response curves indicated that the delta subunit can contribute to the formation of an agonist site. We conclude from the investigated receptors that the delta subunit can assume multiple positions in a receptor pentamer composed of alpha(6), beta(3) and delta subunits.

Inverse effects on gating and modulation caused by a mutation in the M2-M3 Linker of the GABA(A) receptor gamma subunit

M2-M3 linkers are receptor subunit domains known to be critical for the normal function of cysteine-loop ligand-gated ion channels. Previous studies of alpha and beta subunits of type "A" GABA receptors suggest that these linkers couple extracellular elements involved in GABA binding to the transmembrane segments that control the opening of the ion channel. To study the importance of the gamma subunit M2-M3 linker, we examined the macroscopic and single-channel effects of an engineered gamma2(L287A) mutation on GABA activation and propofol modulation. In the macroscopic analysis, we found that the gamma2(L287A) mutation decreased GABA potency but increased the ability of propofol to enhance both GABA potency and efficacy compared with wild-type receptors. Indeed, although propofol had significant effects on GABA potency in wild-type receptors, we found that propofol produced no corresponding increase in GABA efficacy. At the single-channel level, mutant receptors showed a loss in the longest of three open-time components compared with wild-type receptors under GABA activation. Furthermore, propofol reduced the duration of one closed-time component, increased the duration of two open-time components, and generated a third open component with a longer lifetime in mutant compared with wild-type receptors. Taken together, we conclude that although the gamma subunit is not required for the binding of GABA or propofol, the M2-M3 linker of this subunit plays a critical role in channel gating by GABA and allosteric modulation by propofol. Our results also suggest that in wild-type receptors, propofol exerts its enhancing effects by mechanisms extrinsic to channel gating.

Enhanced tonic GABA current in normotopic and hilar ectopic dentate granule cells after pilocarpine-induced status epilepticus

In temporal lobe epilepsy, loss of inhibitory neurons and circuit changes in the dentate gyrus promote hyperexcitability. This hyperexcitability is compensated to the point that dentate granule cells exhibit normal or even subnormal excitability under some conditions. This study explored the possibility that compensation involves enhanced tonic GABA inhibition. Whole cell patch-clamp recordings were made from normotopic granule cells in hippocampal slices from control rats and from both normotopic and hilar ectopic granule cells in slices from rats subjected to pilocarpine-induced status epilepticus. After status epilepticus, tonic GABA current was an order of magnitude greater than control in normotopic granule cells and was significantly greater in hilar ectopic than in normotopic granule cells. These differences could be observed whether or not the extracellular GABA concentration was increased by adding GABA to the superfusion medium or blocking plasma membrane transport. The enhanced tonic GABA current had both action potential-dependent and action potential-independent components. Pharmacological studies suggested that the small tonic GABA current of granule cells in control rats was mediated largely by high-affinity alpha(4)beta(x)delta GABA(A) receptors but that the much larger current recorded after status epilepticus was mediated largely by the lower-affinity alpha(5)beta(x)gamma(2) GABA(A) receptors. A large alpha(5)beta(x)gamma(2)-mediated tonic current could be recorded from controls only when the extracellular GABA concentration was increased. Status epilepticus seemed not to impair the control of extracellular GABA concentration by plasma membrane transport substantially. Upregulated tonic GABA inhibition may account for the unexpectedly modest excitability of the dentate gyrus in epileptic brain.

Differential drug responses on native GABA(A) receptors revealing heterogeneity in extrasynaptic populations in cultured hippocampal neurons

Hippocampal pyramidal neurons potentially express multiple subtypes of GABA(A) receptors at extrasynaptic locations that could therefore respond to different drugs. We activated extrasynaptic GABA(A) receptors in cultured rat hippocampal pyramidal neurons and measured single-channel currents in order to compare the actions of two drugs that potentially target different GABA(A) receptor subtypes. Despite the possible difference in receptor targets of etomidate and diazepam, the two drugs were similar in their actions on native extrasynaptic GABA(A) receptors. Each drug produced three distinct responses that differed significantly in current magnitude, implying heterogeneous GABA(A) receptor populations. In the majority of patches, drug application increased both the single-channel conductance (>40 pS) and the open probability of the channels. By contrast, in the minority of patches, drug application caused an increase in open probability only. In the third group high-conductance channels were observed upon GABA activation and drug application increased their open probability only. The currents potentiated by etomidate or diazepam were substantially larger in patches displaying high-conductance GABA channels compared to those displaying only low-conductance channels. Factors contributing to the large magnitude of these currents were the long mean open time of high-conductance channels and the presence of multiple channels in these patches. In conclusion, we suggest that the local density of extrasynaptic GABA(A) receptors may influence their single-channel properties and may be an additional regulating factor for tonic inhibition and, importantly, differential drug modulation.

Unanticipated structural and functional properties of delta-subunit-containing GABAA receptors

GABA(A) receptors mediate inhibitory neurotransmission in the mammalian brain via synaptic and extrasynaptic receptors. The delta (delta)-subunit-containing receptors are expressed exclusively extra-synaptically and mediate tonic inhibition. In the present study, we were interested in determining the architecture of receptors containing the delta-subunit. To investigate this, we predefined the subunit arrangement by concatenation. We prepared five dual and three triple concatenated subunit constructs. These concatenated dual and triple constructs were used to predefine nine different GABA(A) receptor pentamers. These pentamers composed of alpha(1)-, beta(3)-, and delta-subunits were expressed in Xenopus oocytes and maximal currents elicited in response to 1 mm GABA were determined in the presence and absence of THDOC (3alpha, 21-dihydroxy-5alpha-pregnane-20-one). beta(3)-alpha(1)-delta/alpha(1)-beta(3) and beta(3)-alpha(1)-delta/beta(3)-alpha(1) resulted in the expression of large currents in response to GABA. Interestingly, the presence of the neurosteroid THDOC uncovered alpha(1)-beta(3)-alpha(1)/beta(3)-delta receptors, additionally. The functional receptors were characterized in detail using the agonist GABA, THDOC, Zn(2+), and ethanol and their properties were compared with those of non-concatenated alpha(1)beta(3) and alpha(1)beta(3)delta receptors. Each concatenated receptor isoform displayed a specific set of properties, but none of them responded to 30 mm ethanol. We conclude from the investigated receptors that delta can assume multiple positions in the receptor pentamer. The GABA dose-response properties of alpha(1)-beta(3)-alpha(1)/beta(3)-delta and beta(3)-alpha(1)-delta/alpha(1)-beta(3) match most closely the properties of non-concatenated alpha(1)beta(3)delta receptors. Furthermore, we show that the delta-subunit can contribute to the formation of an agonist site in alpha(1)-beta(3)-alpha(1)/beta(3)-delta receptors.

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.

Tryptophan mutations at azi-etomidate photo-incorporation sites on alpha1 or beta2 subunits enhance GABAA receptor gating and reduce etomidate modulation

The potent general anesthetic etomidate produces its effects by enhancing GABA(A) receptor activation. Its photolabel analog [(3)H]azi-etomidate labels residues within transmembrane domains on alpha and beta subunits: alphaMet236 and betaMet286. We hypothesized that these methionines contribute to etomidate sites formed at alpha-beta subunit interfaces and that increasing side-chain bulk and hydrophobicity at either locus would mimic etomidate binding and block etomidate effects. Channel activity was electrophysiologically quantified in alpha(1)beta(2)gamma(2L) receptors with alpha(1)M236W or beta(2)M286W mutations, in both the absence and the presence of etomidate. Measurements included spontaneous activation, GABA EC(50), etomidate agonist potentiation, etomidate direct activation, and rapid macrocurrent kinetics. Both alpha(1)M236W and beta(2)M286W mutations induced spontaneous channel opening, lowered GABA EC(50), increased maximal GABA efficacy, and slowed current deactivation, mimicking effects of etomidate on alpha(1)beta(2)gamma(2L) channels. These changes were larger with alpha(1)M236W than with beta(2)M286W. Etomidate (3.2 muM) reduced GABA EC(50) much less in alpha(1)M236Wbeta(2)gamma(2L) receptors (2-fold) than in wild type (23-fold). However, etomidate was more potent and efficacious in directly activating alpha(1)M236Wbeta(2)gamma(2L) compared with wild type. In alpha(1)beta(2)M286Wgamma(2L) receptors, etomidate induced neither agonist-potentiation nor direct channel activation. These results support the hypothesis that alpha(1)Met236 and beta(2)Met286 are within etomidate sites that allosterically link to channel gating. Although alpha(1)M236W produced the larger impact on channel gating, beta(2)M286W produced more profound changes in etomidate sensitivity, suggesting a dominant role in drug binding. Furthermore, quantitative mechanistic analysis demonstrated that wild-type and mutant results are consistent with the presence of only one class of etomidate sites mediating both agonist potentiation and direct activation.

Molecular mechanisms supporting a paracrine role of GABA in rat adrenal medullary cells

GABA is known to produce membrane depolarization and secretion in adrenal medullary (AM) cells in various species. However, whether the GABAergic system is intrinsic or extrinsic or both in the adrenal medulla and the role that GABA plays are controversial. Therefore, these issues were addressed by combining a biochemical and functional analysis. Glutamic acid decarboxylase (GAD), a GABA synthesizing enzyme, and vesicular GABA transporter (VGAT) were expressed in rat AM cells at the mRNA and protein levels, and the adrenal medulla had no nerve fibre-like structures immunoreactive to an anti-GAD Ab. The double staining for VGAT and chromogranin A indicates that GABA was stored in chromaffin granules. The alpha1, alpha3, beta2/3, gamma2 and delta subunits of GABA(A) receptors were identified in AM cells at the mRNA and protein levels. Pharmacological properties of GABA-induced Cl(-) currents, immunoprecipitation experiments and immunocytochemistry indicated the expression of not only gamma2-, but also delta-containing GABA(A) receptors, which have higher affinities for GABA and neurosteroids. Expression of GATs, which are involved in the clearance of GABA at GABAergic synapses, were conspicuously suppressed in the adrenal medulla, compared with expression levels of GABA(A) receptors. Increases in Ca(2+) signal in AM cells evoked trans-synaptically by nerve stimulation were suppressed during the response to GABA, and this suppression was attributed to the shunt effect of the GABA-induced increase in conductance. Overall Ca(2+) responses to electrical stimulation and GABA in AM cells were larger or smaller than those to electrical stimulation alone, depending on the frequency of stimulation. The results indicate that GABA functions as a paracrine in rat AM cells and this function may be supported by the suppression of GAT expression and the expression of not only gamma2-, but also delta-GABA(A) receptors.

Etomidate, propofol and the neurosteroid THDOC increase the GABA efficacy of recombinant alpha4beta3delta and alpha4beta3 GABA A receptors expressed in HEK cells

General anesthetics, once thought to exert their effects through non-specific membrane effects, have highly specific ion channel targets that can silence neuronal populations in the nervous system, thereby causing unconsciousness and immobility, characteristic of general anesthesia. Inhibitory GABA(A) receptors (GABA(A)Rs), particularly highly GABA-sensitive extrasynaptic receptor subtypes that give rise to sustained inhibitory currents, are uniquely sensitive to GABA(A)R-active anesthetics. A prominent population of extrasynaptic GABA(A)Rs is made up of alpha4, beta2 or beta3, and delta subunits. Considering the demonstrated importance of GABA receptor beta3 subunits for in vivo anesthetic effects of etomidate and propofol, we decided to investigate the effects of GABA anesthetics on "extrasynaptic" alpha4beta3delta and also binary alpha4beta3 receptors expressed in human embryonic kidney (HEK) cells. Consistent with previous work on similar receptor subtypes we show that maximal GABA currents through "extrasynaptic" alpha4beta3delta receptors, receptors defined by sensitivity to EtOH (30mM) and the beta-carboline beta-CCE (1microM), are enhanced by the GABA(A)R-active anesthetics etomidate, propofol, and the neurosteroid anesthetic THDOC. Furthermore, we show that receptors formed by alpha4beta3 subunits alone also show high GABA sensitivity and that saturating GABA responses of alpha4beta3 receptors are increased to the same extent by etomidate, propofol, and THDOC as are alpha4beta3delta receptors. Therefore, both alpha4beta3 and alpha4beta3delta receptors show low GABA efficacy, and GABA is also a partial agonist on certain binary alphabeta receptor subtypes. Increasing GABA efficacy on alpha4/6beta3delta and alpha4beta3 receptors is likely to make an important contribution to the anesthetic effects of etomidate, propofol and the neurosteroid THDOC.

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.

Physiology and pharmacology of alcohol: the imidazobenzodiazepine alcohol antagonist site on subtypes of GABAA receptors as an opportunity for drug development?

Alcohol (ethanol, EtOH) has pleiotropic actions and induces a number of acute and long-term effects due to direct actions on alcohol targets, and effects of alcohol metabolites and metabolism. Many detrimental health consequences are due to EtOH metabolism and metabolites, in particular acetaldehyde, whose high reactivity leads to nonspecific chemical modifications of proteins and nucleic acids. Like acetaldehyde, alcohol has been widely considered a nonspecific drug, despite rather persuasive evidence implicating inhibitory GABA(A) receptors (GABA(A)Rs) in acute alcohol actions, for example, a GABA(A)R ligand, the imidazobenzodiazepine Ro15-4513 antagonizes many low-to-moderate dose alcohol actions in mammals. It was therefore rather surprising that abundant types of synaptic GABA(A)Rs are generally not responsive to relevant low concentrations of EtOH. In contrast, delta-subunit-containing GABA(A)Rs and extrasynaptic tonic GABA currents mediated by these receptors are sensitive to alcohol concentrations that are reached in blood and tissues during low-to-moderate alcohol consumption. We recently showed that low-dose alcohol enhancement on highly alcohol-sensitive GABA(A)R subtypes is antagonized by Ro15-4513 in an apparently competitive manner, providing a molecular explanation for behavioural Ro15-4513 alcohol antagonism. The identification of a Ro15-4513/EtOH binding site on unique GABA(A)R subtypes opens the possibility to characterize this alcohol site(s) and screen for compounds that modulate the function of EtOH/Ro15-4513-sensitive GABA(A)Rs. The utility of such drugs might range from novel alcohol antagonists that might be useful in the emergency room, to drugs for the treatment of alcoholism, as well as alcohol-mimetic drugs to harness acute positive effects of alcohol.

alpha1beta2delta, a silent GABAA receptor: recruitment by tracazolate and neurosteroids

BACKGROUND AND PURPOSE

This study investigated the alpha(1)beta(2)delta isoform of the GABA(A) receptor that is presumably expressed in the forebrain. The functional and pharmacological properties of this receptor combination are largely unknown.

EXPERIMENTAL APPROACH

We expressed alpha(1)beta(2)delta GABA(A) receptors in Xenopus laevis oocytes. GABA-activated currents, in the presence and absence of modulators, were recorded using the two-electrode voltage clamp technique.

KEY RESULTS

The alpha(1)beta(2)delta isoform of the GABA(A) receptor exhibited an extremely small GABA-mediated current. Tracazolate increased the current amplitude evoked by a half-maximal concentration (EC(50)) of GABA by 59-fold. The maximum current was increased 23-fold in the presence of a saturating GABA concentration. Concomitant with the increase in the maximum, was a 4-fold decrease in the EC(50). Finally, a mutation in the second transmembrane domain of the delta subunit that increases receptor efficacy (L286S), eliminated the increase in the maximum GABA-activated current. The endogenous neurosteroid, tetrahydrodeoxycorticosterone (THDOC), also decreased the EC(50) and increased the maximum current amplitude, although to a lesser degree than that of tracazolate.

CONCLUSIONS AND IMPLICATIONS

Taken all together, these findings indicate that the small GABA-mediated currents in the absence of the modulator are due to a low efficacy for activation. In the absence of modulators, alpha(1)beta(2)delta GABA receptors would be effectively silent and therefore contribute little to inhibition in the CNS. In the presence of tracazolate or endogenous neurosteroids however, this particular receptor isoform could exert a profound inhibitory influence on neuronal activity.

Enhanced behavioral sensitivity to the competitive GABA agonist, gaboxadol, in transgenic mice over-expressing hippocampal extrasynaptic alpha6beta GABA(A) receptors

The behavioral and functional significance of the extrasynaptic inhibitory GABA(A) receptors in the brain is still poorly known. We used a transgenic mouse line expressing the GABA(A) receptor alpha6 subunit gene in the forebrain under the Thy-1.2 promoter (Thy1alpha6) mice ectopically expressing alpha6 subunits especially in the hippocampus to study how extrasynaptically enriched alphabeta(gamma2)-type receptors alter animal behavior and receptor responses. In these mice extrasynaptic alpha6beta receptors make up about 10% of the hippocampal GABA(A) receptors resulting in imbalance between synaptic and extrasynaptic inhibition. The synthetic GABA-site competitive agonist gaboxadol (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol; 3 mg/kg) induced remarkable anxiolytic-like response in the light : dark exploration and elevated plus-maze tests in Thy1alpha6 mice, while being almost inactive in wild-type mice. The transgenic mice also lost quicker and for longer time their righting reflex after 25 mg/kg gaboxadol than wild-type mice. In hippocampal sections of Thy1alpha6 mice, the alpha6beta receptors could be visualized autoradiographically by interactions between gaboxadol and GABA via [(35)S]TBPS binding to the GABA(A) receptor ionophore. Gaboxadol inhibition of the binding could be partially prevented by GABA. Electrophysiology of recombinant GABA(A) receptors revealed that GABA was a partial agonist at alpha6beta3 and alpha6beta3delta receptors, but a full agonist at alpha6beta3gamma2 receptors when compared with gaboxadol. The results suggest strong behavioral effects via selective pharmacological activation of enriched extrasynaptic alphabeta GABA(A) receptors, and the mouse model represents an example of the functional consequences of altered balance between extrasynaptic and synaptic inhibition.

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.

Extrasynaptic GABAA receptors in the crosshairs of hormones and ethanol

Gamma-aminobutyric acid (GABA) is the main chemical inhibitory neurotransmitter in the brain. In the central nervous system (CNS) it acts on two distinct types of receptor: an ion channel, i.e., an "ionotropic" receptor permeable to Cl- and HCO3- (GABAA receptors) and a G-protein coupled "metabotropic" receptor that is linked to various effector mechanisms (GABAB receptors). This review will summarize novel developments in the physiology and pharmacology of GABAA receptors (GABAARs), specifically those found outside synapses. The focus will be on a particular combination of GABAAR subunits sensitive to ovarian and adrenal cortical steroid hormone metabolites that are synthesized in the brain (neurosteroids) and to sobriety impairing concentrations of ethanol. These receptors may be the final common pathway for interactions between ethanol and ovarian and stress-related 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.

Ethanol acts directly on extrasynaptic subtypes of GABAA receptors to increase tonic inhibition

Based on the similarity of ethanol intoxication to the behavioral effects of drugs known to target gamma-aminobutyric acid type A (GABAA) receptors (GABARs), it has been suspected for decades that ethanol facilitates the activity of GABA. Even so, it has been surprisingly difficult to identify molecular targets of ethanol. Research conducted over the past several years suggests that a subclass of GABARs (those containing delta subunits) responds in a relevant concentration range to ethanol. Although delta subunit-containing GABARs are not ubiquitously expressed at inhibitory synapses like their gamma subunit-containing, synaptic counterparts, they are found in many neurons in extrasynaptic locations. Here, they give rise to a tonic form of inhibition that can potently suppress neuronal excitability. Studies have shown that both recombinant and native delta subunit-containing GABARs (1) are modulated by behaviorally relevant (i.e., low millimolar) concentrations of ethanol, (2) directly bind ethanol over the same concentration range, (3) show altered function upon single amino substitutions linked to changes in behavioral responsiveness to ethanol, and (4) are a site of action of Ro15-4513, a competitive antagonist of ethanol binding and a drug which prevents many of the behavioral aspects of ethanol intoxication. Despite such comprehensive evidence, however, the field is not free from controversy. This review evaluates published data for and against a central role of delta subunit-containing GABARs in ethanol actions and suggests future directions that might help settle points of controversy.

GABAA receptor subtypes: the "one glass of wine" receptors

This review discusses evidence for and apparent controversy about, gamma-aminobutyric acid type A (GABAA) receptor (GABAAR) subtypes that mediate alcohol effects experienced during social drinking. GABAARs that contain the beta3 and delta subunits were shown to be enhanced by alcohol concentrations that mirror the concentration dependence of alcohol responses in humans. A mutation (alpha6R100Q) previously found in alcohol nontolerant rats in the cerebellar GABAAR alpha6 subunit is sufficient for increased alcohol-induced ataxia in rats homozygous for this mutation (alpha6-100QQ) and further increases alcohol sensitivity of tonic GABA currents (mediated by alpha6betadelta receptors) in cerebellar granule cells of alpha6-100QQ rats and in recombinant alpha6R100Qbeta3delta receptors. This provided the first direct evidence that these types of receptors mediate behavioral effects of ethanol. Furthermore, the behavioral alcohol antagonist Ro15-4513 specifically reverses ethanol enhancement on alpha4/6beta3delta receptors. Unexpectedly, native and recombinant alpha4/6beta3delta receptors bind the behavioral alcohol antagonist Ro15-4513 with high affinity and this binding is competitive with EtOH, suggesting a specific and mutually exclusive (competitive) ethanol/Ro15-4513 site, which explains the puzzling activity of Ro15-4513 as a behavioral alcohol antagonist. Our conclusion from these findings is that alcohol/Ro15-4513-sensitive GABAAR subtypes are important alcohol targets and that alcohol at relevant concentrations is more specific than previously thought. In this review, we discuss technical difficulties in expressing recombinant delta subunit-containing receptors in oocytes and mammalian cells that may have contributed to negative results and confusion. Not only because we have reproduced detailed positive results numerous times, and we and many others have built extensively on basic findings, but also because we explain and combine many previously puzzling results into a coherent and highly plausible paradigm on how alcohol exerts an important part of its action in the brain, we are confident about our findings and conclusions. However, many important open questions remain to be answered.

A new meaning for "Gin & Tonic": tonic inhibition as the target for ethanol action in the brain

Gamma-aminobutyric acid (GABA) is the main chemical inhibitory neurotransmitter in the brain. In the central nervous system, it acts on two distinct types of receptor: an ion channel, that is, an "ionotropic" receptor permeable to Cl- and HCO3- (GABAA receptors [GABAARs]) and a G-protein coupled "metabotropic" receptor that is linked to various effector mechanisms (GABAB receptors). This review will summarize novel developments in the physiology and pharmacology of GABAARs, specifically those found outside synapses. The focus will be on a particular combination of GABAAR subunits responsible for mediating tonic inhibition and sensitive to concentrations of ethanol legally considered to be sobriety impairing. Since the same receptors are also a preferred target for the metabolites of steroid hormones synthesized in the brain (neurosteroids), the ethanol-sensitive tonic inhibition may be a common pathway for interactions between the effects of alcohol and those of ovarian and stress-related neurosteroids.

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.

Neurosteroid binding sites on GABA(A) receptors

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

Mechanisms of neurosteroid interactions with GABA(A) receptors

Neuroactive steroids have some of their most potent actions by augmenting the function of GABA(A) receptors. Endogenous steroid actions on GABA(A) receptors may underlie important effects on mood and behavior. Exogenous neuroactive steroids have potential as anesthetics, anticonvulsants, and neuroprotectants. We have taken multiple approaches to understand more completely the interaction of neuroactive steroids with GABA(A) receptors. We have developed many novel steroid analogues in this effort. Recent work has resulted in synthesis of new enantiomer analogue pairs, novel ligands that probe various properties of the steroid pharmacophore, fluorescent neuroactive steroid analogues, and photoaffinity labels. Using these tools, combined with receptor binding and electrophysiological assays, we have begun to untangle the complexity of steroid actions at this important class of ligand-gated ion channel.

Molecular regulation of cognitive functions and developmental plasticity: impact of GABAA receptors

By controlling spike timing and sculpting neuronal rhythms, inhibitory interneurons play a key role in regulating neuronal circuits and behavior. The pronounced diversity of GABAergic (gamma-aminobutyric acid) interneurons is paralleled by an extensive diversity of GABAA receptor subtypes. The region- and domain-specific location of these receptor subtypes offers the opportunity to gain functional insights into the role of defined neuronal circuits. These developments are reviewed with regard to the regulation of sleep, anxiety, memory, sensorimotor processing and post-natal developmental plasticity.

Steroid modulation of GABAA receptor-mediated transmission in the hypothalamus: effects on reproductive function

The hypothalamus, the seat of neuroendocrine control, is exquisitely sensitive to gonadal steroids. For decades it has been known that androgens, estrogens and progestins, acting through nuclear hormone receptors, elicit both organizational and activational effects in the hypothalamus and basal forebrain that are essential for reproductive function. While changes in gene expression mediated by these classical hormone pathways are paramount in governing both sexual differentiation and the neural control of reproduction, it is also clear that steroids impart critical control of neuroendocrine functions through non-genomic mechanisms. Specifically, endogenous neurosteroid derivatives of deoxycorticosterone, progesterone and testosterone, as well and synthetic anabolic androgenic steroids that are self-administered as drugs of abuse, elicit acute effects via allosteric modulation of gamma-aminobutyric acid type A receptors. GABAergic transmission within the hypothalamus and basal forebrain is a key regulator of pubertal onset, the expression of sexual behaviors, pregnancy and parturition. Summarized here are the known actions of steroid modulators on GABAergic transmission within the hypothalamus/basal forebrain, with a focus on the medial preoptic area and the supraoptic/paraventricular nuclei that are known to be central players in the control of reproduction.

Ethanol Modulates the Interaction of the Endogenous Neurosteroid Allopregnanolone with the α1β2γ2L GABAA Receptor

We have examined alpha1beta2gamma2L GABAA receptor modulation by the endogenous steroids allopregnanolone (3alpha5alphaP), pregnenolone sulfate, and beta-estradiol in the absence and presence of ethanol. Coapplication of 0.1 to 1% (17-170 mM) ethanol influenced receptor modulation by 3alpha5alphaP but not that by pregnenolone sulfate or beta-estradiol. One of the three kinetic effects evident in channel potentiation by 3alpha5alphaP, prolongation of the longest-lived open time component (OT3), was affected by ethanol with the midpoint of its dose-response curve moved to lower steroid concentrations by 2 orders of magnitude without significantly affecting the maximal effect. Manipulations designed to affect the ability of 3alpha5alphaP to prolong OT3 also affected OT3 prolongation in the presence of ethanol. A mutation to the gamma2 subunit, which reduces the ability of 3alpha5alphaP to prolong OT3, also reduces the interaction between ethanol and 3alpha5alphaP. And the presence of the competitive steroid antagonist (3alpha,5alpha)-17-phenylandrost-16-en-3-ol (17-PA) diminishes the positive interaction between ethanol and 3alpha5alphaP on the GABAA receptor. Together, the findings suggest that steroid interactions with the classic steroid binding site underlie the effect seen in the presence of ethanol, and that ethanol acts by increasing the affinity of 3alpha5alphaP for the site. Tadpole behavioral assays showed that the presence of 3alpha5alphaP at a concentration ineffective at causing changes in tadpole behavior shifted the ethanol dose-response curve for loss of righting reflex to lower concentrations and that this effect was neutralized by coapplication of 17-PA with 3alpha5alphaP.

Auto-modulation of neuroactive steroids on GABA A receptors: a novel pharmacological effect

GABA(A) receptor function is modulated by various important drugs including neuroactive steroids that act on allosteric modulatory sites and can directly activate GABA(A) receptor channels at high concentrations. We used whole cell patch-clamp recordings and rapid applications of the neuroactive steroid alphaxalone to investigate repetitive steroid effects. Alphaxalone potentiation of submaximal GABA-evoked currents was enhanced significantly by repetitive coapplications at all investigated recombinant isoforms (alpha1beta3delta, alpha1beta3gamma2L, alpha6beta3delta, alpha6beta3gamma2L) and at GABA(A) receptors of differentiated human NT2 neurons. A similar increase of current amplitudes was induced by repetitive applications of a high steroid concentration without GABA. We refer to these reversible effects as auto-modulation because repeated interactions of steroids enhanced their own pharmacological impact at the receptor sites in a time and concentration dependent manner without affecting GABA controls. Pronounced auto-modulatory actions were also measured using the neurosteroid 5alpha-THDOC in contrast to indiplon, THIP, and pentobarbital indicating a steroid specificity. Protein kinase A inhibition significantly reduced alphaxalone auto-modulation at alpha1beta3gamma2L, alpha6beta3gamma2L, and alpha6beta3delta subtypes while it enhanced potentiation at alpha1beta3delta isoforms suggesting a crucial influence of receptor subunit composition and phosphorylation for steroid actions. Especially at extrasynaptic GABA(A) receptor sites containing the delta subunit steroid auto-modulation may have a critical role in enhancing potentiation of GABA-induced currents.

GABA(A) receptor diversity and pharmacology

Because of its control of spike-timing and oscillatory network activity, gamma-aminobutyric acid (GABA)-ergic inhibition is a key element in the central regulation of somatic and mental functions. The recognition of GABA(A) receptor diversity has provided molecular tags for the analysis of distinct neuronal networks in the control of specific pharmacological and physiological brain functions. Neurons expressing alpha(1)GABA(A) receptors have been found to mediate sedation, whereas those expressing alpha(2)GABA(A) receptors mediate anxiolysis. Furthermore, associative temporal and spatial memory can be regulated by modulating the activity of hippocampal pyramidal cells via extrasynaptic alpha(5)GABA(A) receptors. In addition, neurons expressing alpha(3)GABA(A) receptors are instrumental in the processing of sensory motor information related to a schizophrenia endophenotype. Finally, during the postnatal development of the brain, the maturation of GABAergic interneurons seems to provide the trigger for the experience-dependent plasticity of neurons in the visual cortex, with alpha(1)GABA(A) receptors setting the time of onset of a critical period of plasticity. Thus, particular neuronal networks defined by respective GABA(A) receptor subtypes can now be linked to the regulation of various clearly defined behavioural patterns. These achievements are of obvious relevance for the pharmacotherapy of certain brain disorders, in particular sleep dysfunctions, anxiety disorders, schizophrenia and diseases associated with memory deficits.

Low dose acute alcohol effects on GABA A receptor subtypes

GABA(A) receptors (GABA(A)Rs) are the main inhibitory neurotransmitter receptors and have long been implicated in mediating at least part of the acute actions of ethanol. For example, ethanol and GABAergic drugs including barbiturates and benzodiazepines share many pharmacological properties. Besides the prototypical synaptic GABA(A)R subtypes, nonsynaptic GABA(A)Rs have recently emerged as important regulators of neuronal excitability. While high doses (> or =100 mM) of ethanol have been reported to enhance activity of most GABA(A)R subtypes, most abundant synaptic GABA(A)Rs are essentially insensitive to ethanol concentrations that occur during social ethanol consumption (< 30 mM). However, extrasynaptic delta and beta3 subunit-containing GABA(A)Rs, associated in the brain with alpha4 or alpha6 subunits, are sensitive to low millimolar ethanol concentrations, as produced by drinking half a glass of wine. Additionally, we found that a mutation in the cerebellar alpha6 subunit (alpha6R100Q), initially reported in rats selectively bred for increased alcohol sensitivity, is sufficient to produce increased alcohol-induced motor impairment and further increases of alcohol sensitivity in recombinant alpha6beta3delta receptors. Furthermore, the behavioral alcohol antagonist Ro15-4513 blocks the low dose alcohol enhancement on alpha4/6/beta3delta receptors, without reducing GABA-induced currents. In binding assays alpha4beta3delta GABA(A)Rs bind [(3)H]Ro15-4513 with high affinity, and this binding is inhibited, in an apparently competitive fashion, by low ethanol concentrations, as well as analogs of Ro15-4513 that are active to antagonize ethanol or Ro15-4513's block of ethanol. We conclude that most low to moderate dose alcohol effects are mediated by alcohol actions on alcohol/Ro15-4513 binding sites on GABA(A)R subtypes.

Steroid withdrawal in the mouse results in anxiogenic effects of 3alpha,5beta-THP: a possible model of premenstrual dysphoric disorder

RATIONALE

3alpha-OH-5alpha[beta]-pregnan-20-one (THP) is a positive modulator of the GABAA receptor (GABAR), which underlies its reported anxiolytic effect. However, there are conditions such as premenstrual dysphoric disorder (PMDD) where increases in THP levels can be associated with adverse mood.

OBJECTIVES

In order to test for conditions where THP might be anxiogenic, we developed a mouse model of THP withdrawal. Because delta-containing GABAR are highly sensitive to THP modulation, results were compared in wild-type and delta knockout mice.

METHODS

Finasteride, a 5alpha-reductase blocker, was administered for 3 days to female wild-type or delta knockout mice. Then, animals were tested in the elevated plus maze, following acute administration of THP, lorazepam, flumazenil, or 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), and results compared to vehicle-injected controls. CA1 hippocampal GABAR alpha4 subunit levels were assessed by Western blot.

RESULTS

After THP withdrawal, THP produced anxiogenic effects, decreasing open arm entries on the elevated plus maze, following a brief shock, in contrast to its expected anxiolytic effects. As we have shown in rats, THP withdrawal also resulted in increased expression of the alpha4 subunit in mouse CA1 hippocampus. As expected for increases in alpha4-containing GABAR, THP withdrawn mice were relatively insensitive to the benzodiazepine (BDZ) lorazepam and had atypical responses to the BDZ antagonist flumazenil when tested on the plus maze. In contrast, they showed a greater anxiolytic response to THIP, which has greater efficacy at alpha4betadelta than other GABAR. Although THP withdrawal in delta knockout mice also increased the alpha4 GABAR subunit, the anxiogenic effects of THP and the anxiolytic effects of THIP were not observed, implicating alpha4betadelta GABAR in these effects.

CONCLUSIONS

Based on these behavioral and pharmacological findings, we suggest that THP withdrawal in the mouse may serve as a rodent model of PMDD.

Identification of structures within GABAA receptor alpha subunits that regulate the agonist action of pentobarbital

Barbiturates act on GABA(A) receptors (GABARs) through three distinct mechanisms, resulting in positive allosteric modulation, direct activation, and inhibition. These effects are observed at different concentrations and are differentially affected by some mutations and by the receptor's subunit composition. Mammalian GABARs can be formed from a combination of 16 different subunit subtypes. Although the effect of barbiturates depends largely on the beta subunit, their agonist activity is substantially influenced by the alpha subunit subtype. Pentobarbital is a more effective agonist than GABA only when receptors contain an alpha6 subunit. Results from chimeric alpha1/alpha6 subunits suggested that structural differences within the extracellular N-terminal domain were responsible for this characteristic. Within this domain, we examined 15 amino acid residues unique to the alpha6 subtype. Each of these sites was individually mutated in the alpha6 subunit to the corresponding residue of the alpha1 subunit. The effect of the mutation on direct activation by pentobarbital was determined with whole-cell electrophysiological recordings. Our results indicate that only one of these mutations, alpha6(T69K), altered pentobarbital efficacy. This single mutation reduced the response to pentobarbital to a level intermediate to the wild-type alpha1beta1gamma2L and alpha6beta1gamma2L isoforms. The mutation did not affect the sensitivity of the receptor to GABA but did reduce the efficacy of etomidate, another i.v. anesthetic with activity similar to pentobarbital. The reverse mutation in the alpha1 subunit (K70T) did not alter the response to pentobarbital. This is the first identification of a structural difference in GABAR alpha subtypes that regulates direct activation by barbiturates.

Basis of the gabamimetic profile of ethanol

This article summarizes the proceedings of a symposium held at the 2005 Research Society on Alcoholism meeting. The initial presentation by Dr. Wallner provided evidence that selected GABA(A) receptors containing the delta subunit display sensitivity to low intoxicating ethanol concentrations and this sensitivity is further increased by a mutation in the cerebellar alpha6 subunit, found in alcohol-hypersensitive rats. Dr. Mameli reported that ethanol affects gamma-aminobutyric acid (GABA) function by affecting neural circuits that influence GABA release. Dr. Parsons presented data from electrophysiological and microdialysis investigations that ethanol is capable of releasing GABA from presynaptic terminals. Dr. Morrow demonstrated that systemic ethanol increases neuroactive steroids in brain, the absence of which alters various functional responses to ethanol. Dr. Criswell presented evidence that the ability of ethanol to increase GABA was apparent in some, but not all, brain regions indicative of regional specificity. Further, Dr. Criswell demonstrated that neurosteroids alone and when synthesized locally by ethanol act postsynaptically to enhance the effect of GABA released by ethanol in a region specific manner. Collectively, this series of reports support the GABAmimetic profile of acutely administered ethanol being dependent on several specific mechanisms distinct from a direct effect on the major synaptic isoforms of GABA(A) receptors.

Modes and models of GABA(A) receptor gating

Upon activation by agonist, the type A gamma-aminobutyric acid receptor (GABAR) 'gates', allowing chloride ions to permeate membranes and produce fast inhibition of neurons. There is no consensus kinetic model for the GABAR gating mechanism. We expressed human alpha(1)beta(1)gamma(2S) GABARs in HEK 293 cells and recorded single channel currents in the cell-attached configuration using various GABA concentrations (50-5000 microm). Closed and open events occurred individually and in clusters that had at least three different modes that were distinguishable by open probability (P(O)): High (P(O)= 0.73), Mid (P(O)= 0.50), and Low (P(O)= 0.21). We used a critical time to isolate shorter bursts of openings and to thus eliminate long-lived, desensitized events. Bursts from all three modes contained three closed and three open components. We employed maximum likelihood fitting, autocorrelation analysis and macroscopic current simulation to distinguish kinetic schemes. The 'core' gating scheme for most models contained two closed states that preceded an open state (C(1) C(2) O(1)). The two best-fitting models had a third closed state connected to C(1) and a second open state (O(2)) connected to C(2). The third open state, whose occupancy varied greatly between modes, could be connected either to O(2) or C(2). We estimated rate constants for two identical, independent GABA binding steps by globally fitting data across GABA concentrations ranging from 50 to 1000 microm. For the most highly ranked model the binding rate constants were: k(+)= 3 microm(-1) s(-1) and k(-)= 272 s(-1) (K(D)= 91 microm).

Apparent receptor-mediated activation of Ca2+-dependent conductive Cl−transport by shark-derived polyaminosterols

The shark liver antimicrobial polyaminosterol squalamine is an angiogenesis inhibitor under clinical investigation as an anti-cancer agent and as a treatment for the choroidal neovascularization associated with macular degeneration of the retina. The related polyaminosterol MSI-1436 is an appetite suppressant that decreases systemic insulin resistance. However, the mechanisms of action of these polyaminosterols are unknown. We report effects of MSI-1436 on Xenopus oocytes consistent with the existence of a receptor for polyaminosterols. MSI-1436 activates bidirectional, trans-chloride-independent Cl-flux in Xenopus oocytes. At least part of this DIDS-sensitive Cl−flux is conductive, as measured using two-electrode voltage-clamp and on-cell patch-clamp techniques. MSI-1436 also elevates cytosolic Ca2+concentration ([Ca2+]) and increases bidirectional45Ca2+flux. Activation of Cl−flux and elevation of cytosolic [Ca2+] by MSI-1436 both are accelerated by lowering bath Ca2+and are not acutely inhibited by extracellular EGTA. Elevation of cytosolic [Ca2+] by MSI-1436 requires heparin-sensitive intracellular Ca2+stores. Although injected EGTA abolishes the increased conductive Cl−flux, that Cl−flux is not dependent on heparin-sensitive stores. In low-bath Ca2+conditions, several structurally related polyaminosterols act as strong agonists or weak agonists of conductive Cl−flux in oocytes. Weak agonist polyaminosterols antagonize the strong agonist, MSI-1436, but upon addition of the conductive Cl−transport inhibitor DIDS, they are converted into strong agonists. Together, these properties operationally define a polyaminosterol receptor at or near the surface of the Xenopus oocyte, provide an initial description of receptor signaling, and suggest routes toward further understanding of a novel class of appetite suppressants and angiogenesis inhibitors.

Neuroactive steroids and inhibitory neurotransmission: mechanisms of action and physiological relevance

Dysfunction of GABA(A) receptor-mediated inhibition is implicated in a number of neurological and psychiatric conditions including epilepsy and affective disorders. Some of these conditions have been associated with abnormal levels of certain endogenously occurring neurosteroids, which potently and selectively enhance the function of the brain's major inhibitory receptor, the GABA(A) receptor. Consistent with their ability to enhance neuronal inhibition, such steroids exhibit in animals and humans anxiolytic, anticonvulsant and anesthetic actions. Neurosteroids, exemplified by the potent progesterone metabolite, 5alpha-pregnan-3alpha-ol-20-one can be synthesized de novo in the CNS both in neurones and glia in levels sufficient to modulate GABA(A) receptor function. Neurosteroid levels are not static, but are subject to dynamic fluctuations, for example during stress, or the later stages of pregnancy. These observations suggest that these endogenous modulators may refine the function of the brain's major inhibitory receptor and thus, play an important physiological and pathophysiological role. However, given the ubiquitous expression of GABA(A) receptors throughout the mammalian CNS, changes in neurosteroid levels should be widely experienced, causing a generalized enhancement of neuronal inhibition. Such a non-specific action would seem incompatible with a physiological role. However, neurosteroid action is both brain region and neurone selective. This specificity results from a variety of molecular mechanisms including receptor subunit composition, local steroid metabolism and phosphorylation. This paper will evaluate the relative contribution these mechanisms play in defining the interaction of neurosteroids with synaptic and extra-synaptic GABA(A) receptors.

A conceptualization of integrated actions of ethanol contributing to its GABAmimetic profile: a commentary

Early behavioral investigations supported the contention that systemic ethanol displays a GABAmimetic profile. Microinjection of GABA agonists into brain and in vivo electrophysiological studies implicated a regionally specific action of ethanol on GABA function. While selectivity of ethanol to enhance the effect of GABA was initially attributed an effect on type-I-benzodiazepine (BZD)-GABA(A) receptors, a lack of ethanol's effect on GABA responsiveness from isolated neurons with this receptor subtype discounted this contention. Nonetheless, subsequent work identified GABA(A) receptor subtypes, with limited distribution in brain, sensitive to enhancement of GABA at relevant ethanol concentrations. In view of these data, it is hypothesized that the GABAmimetic profile for ethanol is due to activation of mechanisms associated with GABA function, distinct from a direct action on the majority of postsynaptic GABA(A) receptors. The primary action proposed to account for ethanol's regional specificity on GABA transmission is its ability to release GABA from some, but not all, presynaptic GABAergic terminals. As systemic administration of ethanol increases neuroactive steroids, which can enhance GABA responsiveness, this elevated level of neurosteroids is proposed to magnify the effect of GABA released by ethanol. Additional factors contributing to the degree to which ethanol interacts with GABA function include an involvement of GABA(B) and other receptors that influence ethanol-induced GABA release, an effect of phosphorylation on GABA responsiveness, and a regional reduction of glutamatergic tone. Thus, an integration of these consequences induced by ethanol is proposed to provide a logical basis for its in vivo GABAmimetic profile.

Neurosteroids: endogenous regulators of the GABAA receptor

GABA(A) (gamma-aminobutyric acid type A) receptors mediate most of the 'fast' synaptic inhibition in the mammalian brain and are targeted by many clinically important drugs. Certain naturally occurring pregnane steroids can potently and specifically enhance GABA(A) receptor function in a nongenomic (direct) manner, and consequently have anxiolytic, analgesic, anticonvulsant, sedative, hypnotic and anaesthetic properties. These steroids not only act as remote endocrine messengers, but also can be synthesized in the brain, where they modify neuronal activity locally by modulating GABA(A) receptor function. Such 'neurosteroids' can influence mood and behaviour in various physiological and pathophysiological situations, and might contribute to the behavioural effects of psychoactive drugs.

Nanomolar allopregnanolone potentiates rat cerebellar GABAA receptors

The ionophore function of gamma-aminobutyric acid A (GABA(A)) receptors was studied by whole-cell patch clamp electrophysiology in primary cultures of rat cerebellar cortex. Chloride currents elicited by 1 microM GABA were potentiated by allopregnanolone with a plateau of high affinity (EC(50) = 14 nM) and a peak of potentiation around 1 microM allopregnanolone. Furosemide (0.1 mM) eliminated the high affinity phase and increased the EC(50) to 685 nM. GABA(A) receptors of rat cerebellar synaptosomal membranes were labelled with [(3)H]ethynylbicycloorthobenzoate (EBOB). Allopregnanolone displaced [(3)H]EBOB binding with IC(50) = 320 nM. The displacing potency of allopregnanolone was strongly enhanced (IC(50) = 39 nM) in the presence of 400 nM GABA and 60 nM SR 95531. Nanomolar potentiation by allopregnanolone can be associated with cerebellar GABA(A) receptors containing alpha(6), beta(2-3) and delta subunits. This might be suitable for physiological modulation of tonic inhibitory neurotransmission via extrasynaptic GABA(A) receptors in cerebellar granule cells by neurosteroids.

Variations on an inhibitory theme: phasic and tonic activation of GABA(A) receptors

The proper functioning of the adult mammalian brain relies on the orchestrated regulation of neural activity by a diverse population of GABA (gamma-aminobutyric acid)-releasing neurons. Until recently, our appreciation of GABA-mediated inhibition focused predominantly on the GABA(A) (GABA type A) receptors located at synaptic contacts, which are activated in a transient or 'phasic' manner by GABA that is released from synaptic vesicles. However, there is growing evidence that low concentrations of ambient GABA can persistently activate certain subtypes of GABA(A) receptor, which are often remote from synapses, to generate a 'tonic' conductance. In this review, we consider the distinct roles of synaptic and extrasynaptic GABA receptor subtypes in the control of neuronal excitability.

Alcohol effects on gamma-aminobutyric acid type A receptors: are extrasynaptic receptors the answer?

GABA(A) receptors have long been implicated in mediating at least part of the actions of ethanol in mammalian brain. However, until very recently, reports of the actions of EtOH on recombinant receptors have required very high doses of ethanol and animals lacking receptor subunits shown to be important for ethanol actions in vitro did not support the view that these subunits are crucial in ethanol actions. Recombinant alpha4beta3delta and alpha6beta3delta GABA(A) receptors are uniquely sensitive to ethanol, with a dose-response relationship mirroring the well known effects of alcohol consumption on the human brain. Receptors containing the delta subunit are thought to be located extrasynaptically and it will be important to determine if these extrasynaptic GABA(A) receptor subunit combinations mediate low dose alcohol effects in vivo.

The alpha 1 and alpha 6 subunit subtypes of the mammalian GABA(A) receptor confer distinct channel gating kinetics

The GABA(A) receptors show a large degree of structural heterogeneity, with seven different subunit families, and 16 different subtypes in mammalian species. The alpha family is the largest, with six different subtypes. The alpha1 and alpha6 subtypes are among the most diverse within this family and confer distinct pharmacological properties to recombinant and neuronal receptors. To determine whether different single channel and macroscopic kinetic properties were also associated with these subtypes, the alpha1 or alpha6 subunit was expressed in mammalian cells along with beta3 and gamma2L subunits and the kinetic properties examined with outside-out patch recordings. The alpha1 beta3 gamma2L receptors responded to GABA with long-duration openings organized into multi-opening bursts. In contrast, channel openings of the alpha6 beta3 gamma2L receptors were predominately short in duration and occurred as isolated, single openings. The subunit subtype also affected the deactivation rate of the receptor, which was almost 2-fold slower for alpha6 beta3 gamma2L, compared with the alpha1 beta3 gamma2L isoform. Onset of fast desensitization did not differ between the isoforms. To determine the structural domains responsible for these differences in kinetic properties, we constructed six chimeric subunits, combining different regions of the alpha1 and alpha6 subunits. The properties of the chimeric subunits indicated that structures within the third transmembrane domain (TM3) and the TM3-TM4 intracellular loop conferred differences in single channel gating kinetics that subsequently affected the deactivation rate and GABA EC50. The effect of agonist concentration on the rise time of the current showed that the extracellular N-terminal domain was largely responsible for binding characteristics, while the transmembrane domains determined the activation rate at saturating GABA concentrations. This suggests that subunit structures outside of the agonist binding and pore-lining domains are responsible for the kinetic differences conferred by the alpha1 and alpha6 subtypes. Structural heterogeneity within these transmembrane and intracellular regions can therefore influence the characteristics of the postsynaptic response of GABA(A) receptors with different subunit composition.

Fishing for allosteric sites on GABAA receptors

GABA(A) receptors have structural and functional homology with a super-family of cys-loop ligand-gated ion channel receptors including the nicotinic acetylcholine receptors. Amino acid residues involved in ligand-binding pockets are homologous among super-family members, leading to the multiple-loop model of binding sites situated at subunit interfaces, validated by structural studies on the nicotinic acetylcholine receptor and water-soluble snail acetylcholine binding protein. This article will briefly review the literature on the agonist binding sites on the receptor super-family, and then describe the current situation for attempts to identify sites for allosteric modulators on the GABA(A) receptors. A combination of mutagenesis and photoaffinity labeling with anesthetic ligands has given some leads in this endeavor. Current work by others and ourselves focuses on three putative sites for modulators: (1) within the ion channel domain TM2, near the extracellular end; (2) the agonist binding sites and homologous pockets at other subunit interfaces of the pentameric receptor; and (3) on the linker region stretching from the agonist site loop C to the top of the TM1 region. It is likely that concrete structural information will be forthcoming soon.

Multiple actions of propofol on alphabetagamma and alphabetadelta GABAA receptors

GABAA receptors are predominantly composed of alphabetagamma and alphabetadelta isoforms in the brain. It has been proposed that alphabetagamma receptors mediate phasic inhibition, whereas alphabetadelta receptors mediate tonic inhibition. Propofol (2,6-di-isopropylphenol), a widely used anesthetic drug, exerts its effect primarily by modulating GABAA receptors; however, the effects of propofol on the kinetic properties of alphabetagamma and alphabetadelta receptors are uncertain. We transfected human embryonic kidney (HEK293T) cells with cDNAs encoding rat alpha1, alpha6, beta3, gamma2L, or delta subunits and performed whole-cell patch-clamp recordings to explore this issue. Propofol (3 microM) increased GABA concentration-response curve maximal currents similarly for both alpha1beta3gamma2L and alpha6beta3gamma2L receptors, but propofol increased those for alpha1beta3delta and alpha6beta3delta receptors differently, the increase being greater for alpha1beta3delta than for alpha6beta3delta receptors. Propofol (10 microM) produced similar alterations in alpha1beta3gamma2L and alpha6beta3gamma2L receptor currents when using a preapplication protocol; peak currents were not altered, desensitization was reduced, and deactivation was prolonged. Propofol enhanced peak currents for both alpha1beta3delta and alpha6beta3delta receptors, but the enhancement was greater for alpha1beta3delta receptors. Desensitization of these two isoforms was not modified by propofol. Propofol did not alter the deactivation rate of alpha1beta3delta receptor currents but did slow deactivation of alpha6beta3delta receptor currents. The findings that propofol reduced desensitization and prolonged deactivation of gamma2L subunit-containing receptors and enhanced peak currents or prolonged deactivation of delta subunit-containing receptors suggest that propofol enhancement of both phasic and tonic inhibition may contribute to its anesthetic effect in the brain.

Pentobarbital Differentially Modulates α1β3δ and α1β3γ2L GABAA Receptor Currents

GABAA receptors are modulated by a variety of compounds, including the neurosteroids and barbiturates. Although the effects of barbiturates on alphabetagamma isoforms, thought to dominate phasic (synaptic) GABAergic inhibition, have been extensively studied, the effects of pentobarbital on kinetic properties of alphabetadelta GABAA receptors, thought to mediate tonic (extra- or perisynaptic) inhibition, are unknown. Using ultrafast drug delivery and single channel recording techniques, we demonstrate isoform-specific pentobarbital modulation of low-efficacy, minimally desensitizing alpha1beta3 currents and high-efficacy, rapidly desensitizing alpha1beta3gamma2L currents. Specifically, with saturating concentrations of GABA, pentobarbital substantially potentiated peak alpha1beta3delta receptor currents but failed to potentiate peak alpha1beta3gamma2L receptor currents. Also, pentobarbital had opposite effects on the desensitization of alpha1beta3delta (increased) and alpha1beta3gamma2L (decreased) receptor currents evoked by saturating GABA. Pentobarbital increased steady-state alpha1beta3delta receptor single channel open duration primarily by introducing a longer duration open state, whereas for alpha1beta3gamma2L receptor channels, pentobarbital increased mean open duration by increasing the proportion and duration of the longest open state. The data support previous suggestions that GABA may be a partial agonist at alphabetadelta isoforms, which may render them particularly sensitive to allosteric modulation. The remarkable increase in gating efficacy of alpha1beta3delta receptors suggests that alphabetadelta isoforms, and by inference tonic forms of inhibition, may be important targets for barbiturates.