Neuronally restricted RNA splicing regulates the expression of a novel GABAA receptor subunit conferring atypical functional properties [corrected; erratum to be published]

Chemogenetic Modulation of Preoptic Gabre Neurons Decreases Body Temperature and Heart Rate

The preoptic area of the hypothalamus is critical for regulation of brain-body interaction, including circuits that control vital signs such as body temperature and heart rate. The preoptic area contains approximately 70 molecularly distinct cell types. The Gabre gene is expressed in a subset of preoptic area cell types. It encodes the GABA receptor ε-subunit, which is thought to confer resistance to anesthetics at the molecular level, but the function of Gabre cells in the brain remains largely unknown. We generated and have extensively characterized a Gabre-cre knock-in mouse line and used chemogenetic tools to interrogate the function of Gabre cells in the preoptic area. Comparison with macaque GABRE expression revealed the conserved character of Gabre cells in the preoptic area. In awake mice, we found that chemogenetic activation of Gabre neurons in the preoptic area reduced body temperature, whereas chemogenetic inhibition had no effect. Furthermore, chemogenetic inhibition of Gabre neurons in the preoptic area decreased the heart rate, whereas chemogenetic activation had no effect under isoflurane anesthesia. These findings suggest an important role of preoptic Gabre neurons in maintaining vital signs such as body temperature and heart rate during wakefulness and under anesthesia.

Activation of the Rat α1β2ε GABAA Receptor by Orthosteric and Allosteric Agonists

GABA_A receptors are a major contributor to fast inhibitory neurotransmission in the brain. The receptors are activated upon binding the transmitter GABA or allosteric agonists including a number of GABAergic anesthetics and neurosteroids. Functional receptors can be formed by various combinations of the nineteen GABA_A subunits cloned to date. GABA_A receptors containing the ε subunit exhibit a significant degree of constitutive activity and have been suggested to be unresponsive to allosteric agents. In this study, we have characterized the functional properties of the rat α1β2ε GABA_A receptor. We confirm that the α1β2ε receptor exhibits a higher level of constitutive activity than typical of GABA_A receptors and show that it is inefficaciously activated by the transmitter and the allosteric agonists propofol, pentobarbital, and allopregnanolone. Manipulations intended to alter ε subunit expression and receptor stoichiometry were largely without effect on receptor properties including sensitivity to GABA and allosteric agonists. Surprisingly, amino acid substitutions at the conserved 9' and 6' positions in the second transmembrane (TM2) domain in the ε subunit did not elicit the expected functional effects of increased constitutive activity and resistance to the channel blocker picrotoxin, respectively. We tested the accessibility of TM2 residues mutated to cysteine using the cysteine-modifying reagent 4-(hydroxymercuri)benzoic acid and found a unique pattern of water-accessible residues in the ε subunit.

Electrophysiology of ionotropic GABA receptors

GABA_A receptors are ligand-gated chloride channels and ionotropic receptors of GABA, the main inhibitory neurotransmitter in vertebrates. In this review, we discuss the major and diverse roles GABA_A receptors play in the regulation of neuronal communication and the functioning of the brain. GABA_A receptors have complex electrophysiological properties that enable them to mediate different types of currents such as phasic and tonic inhibitory currents. Their activity is finely regulated by membrane voltage, phosphorylation and several ions. GABA_A receptors are pentameric and are assembled from a diverse set of subunits. They are subdivided into numerous subtypes, which differ widely in expression patterns, distribution and electrical activity. Substantial variations in macroscopic neural behavior can emerge from minor differences in structure and molecular activity between subtypes. Therefore, the diversity of GABA_A receptors widens the neuronal repertoire of responses to external signals and contributes to shaping the electrical activity of neurons and other cell types.

Tonic GABAergic inhibition, via GABAA receptors containing αβƐ subunits, regulates excitability of ventral tegmental area dopamine neurons

The activity of midbrain dopamine neurons is strongly regulated by fast synaptic inhibitory γ‐Aminobutyric acid (GABA)ergic inputs. There is growing evidence in other brain regions that low concentrations of ambient GABA can persistently activate certain subtypes of GABA_A receptor to generate a tonic current. However, evidence for a tonic GABAergic current in midbrain dopamine neurons is limited. To address this, we conducted whole‐cell recordings from ventral tegmental area (VTA) dopamine neurons in brain slices from mice. We found that application of GABA_A receptor antagonists decreased the holding current, indicating the presence of a tonic GABAergic input. Global increases in GABA release, induced by either a nitric oxide donor or inhibition of GABA uptake, further increased this tonic current. Importantly, prolonged inhibition of the firing activity of local GABAergic neurons abolished the tonic current. A combination of pharmacology and immunohistochemistry experiments suggested that, unlike common examples of tonic inhibition, this current may be mediated by a relatively unusual combination of α4βƐ subunits. Lastly, we found that the tonic current reduced excitability in dopamine neurons suggesting a subtractive effect on firing activity.

Rational approaches for the design of various GABA modulators and their clinical progression

GABA (γ-amino butyric acid) is an important inhibitory neurotransmitter in the central nervous system. Attenuation of GABAergic neurotransmission plays an important role in the etiology of several neurological disorders including epilepsy, Alzheimer's disease, Huntington's chorea, migraine, Parkinson's disease, neuropathic pain, and depression. Increase in the GABAergic activity may be achieved through direct agonism at the GABAA receptors, inhibition of enzymatic breakdown of GABA, or by inhibition of the GABA transport proteins (GATs). These functionalities make GABA receptor modulators and GATs attractive drug targets in brain disorders associated with decreased GABA activity. There have been several reports of development of GABA modulators (GABA receptors, GABA transporters, and GABAergic enzyme inhibitors) in the past decade. Therefore, the focus of the present review is to provide an overview on various design strategies and synthetic approaches toward developing GABA modulators. Furthermore, mechanistic insights, structure-activity relationships, and molecular modeling inputs for the biologically active derivatives have also been discussed. Summary of the advances made over the past few years in the clinical translation and development of GABA receptor modulators is also provided. This compilation will be of great interest to the researchers working in the field of neuroscience. From the light of detailed literature, it can be concluded that numerous molecules have displayed significant results and their promising potential, clearly placing them ahead as potential future drug candidates.

Genomic and Non-genomic Action of Neurosteroids in the Peripheral Nervous System

Since the former evidence of biologic actions of neurosteroids in the central nervous system, also the peripheral nervous system (PNS) was reported as a structure affected by these substances. Indeed, neurosteroids are synthesized and active in the PNS, exerting many important actions on the different cell types of this system. PNS is a target for neurosteroids, in their native form or as metabolites. In particular, old and recent evidence indicates that the progesterone metabolite allopregnanolone possesses important functions in the PNS, thus contributing to its physiologic processes. In this review, we will survey the more recent findings on the genomic and non-genomic actions of neurosteroids in nerves, ganglia, and cells forming the PNS, focusing on the mechanisms regulating the peripheral neuron-glial crosstalk. Then, we will refer to the physiopathological significance of the neurosteroid signaling disturbances in the PNS, in to identify new molecular targets for promising pharmacotherapeutic approaches.

Sex differences in depolarizing actions of GABAA receptor activation in rat embryonic hypothalamic neurons

GABA_A receptor activation exerts trophic actions in immature neurons through depolarization of resting membrane potential. The switch to its classical hyperpolarizing role is developmentally regulated. Previous results suggest that a hormonally biased sex difference exists at the onset of the switch in hypothalamic neurons. The aim of this work was to evaluate sex differences in GABA_A receptor function of hypothalamic neurons before brain masculinization by gonadal hormones. Hypothalamic cells were obtained from embryonic day 16 male and female rat foetuses, 2 days before the peak of testosterone production by the foetal testis, and grown in vitro for 9 days. Whole-cell and perforated patch-clamp recordings were carried out in order to measure several electrophysiological parameters. Our results show that there are more male than female neurons responding with depolarization to muscimol. Additionally, among cells with depolarizing responses, males have higher and longer lasting responses than females. These results highlight the relevance of differences in neural cell sex irrespective of exposure to sex hormones.

Characteristic expressions of GABA receptors and GABA producing/transporting molecules in rat kidney

Gamma-aminobutyric acid (GABA) is an important neurotransmitter, but recent reports have revealed the expression of GABAergic components in peripheral, non-neural tissues. GABA administration induces natriuresis and lowers blood pressure, suggesting renal GABA targets. However, systematic evaluation of renal GABAergic components has not been reported. In this study, kidney cortices of Wistar-Kyoto rats (WKY) were used to assay for messenger RNAs of GABA-related molecules using RT-PCR. In WKY kidney cortex, GABA_A receptor subunits, α1, β3, δ, ε and π, in addition to both types of GABA_B receptors, R1 and R2, and GABA_C receptor ρ1 and ρ2 subunit mRNAs were detected. Kidney cortex also expressed mRNAs of glutamate decarboxylase (GAD) 65, GAD67, 4-aminobutyrate aminotransferase and GABA transporter, GAT2. Western blot and/or immunohistochemistry were performed for those molecules detected by RT-PCR. By immunofluorescent observation, co-staining of α1, β3, and π subunits was observed mainly on the apical side of cortical tubules, and immunoblot of kidney protein precipitated with π subunit antibody revealed α1 and β3 subunit co-assembly. This is the first report of GABA_A receptor π subunit in the kidney. In summary, unique set of GABA receptor subunits and subtypes were found in rat kidney cortex. As GABA producing enzymes, transporters and degrading enzyme were also detected, a possible existence of local renal GABAergic system with an autocrine/paracrine mechanism is suggested.

Regulation of the neuronal transcription factor NPAS4 by REST and microRNAs

NPAS4 is a brain restricted, activity-induced transcription factor which regulates the expression of inhibitory synapse genes to control homeostatic excitatory/inhibitory balance in neurons. NPAS4 is required for normal social interaction and contextual memory formation in mice. Protein and mRNA expression of NPAS4 is tightly coupled to neuronal depolarization and most prevalent in the cortical and hippocampal regions in the brain, however the precise mechanisms by which the NPAS4 gene is controlled remain unexplored. Here we show that expression of NPAS4 mRNA is actively repressed by RE-1 silencing transcription factor/neuron-restrictive silencer factor (REST/NRSF) in embryonic stem cells and non-neuronal cells by binding multiple sites within the promoter and Intron I of NPAS4. Repression by REST also appears to correlate with the binding of the zinc finger DNA binding protein CTCF within Intron I of NPAS4. In addition, we show that the 3' untranslated region (3'UTR) of NPAS4 can be targeted by two microRNAs, miR-203 and miR-224 to further regulate its expression. miR-224 is a midbrain/hypothalamus enriched microRNA which is expressed from an intron within the GABAA receptor epsilon (GABRE) gene and may further regionalize NPAS4 expression. Our results reveal REST and microRNA dependent mechanisms that restrict NPAS4 expression to the brain.

Time-dependent modulation of GABA(A)-ergic synaptic transmission by allopregnanolone in locus coeruleus neurons of Mecp2-null mice

Rett syndrome (RTT) is a neurodevelopmental disorder with symptoms starting 6-18 mo after birth, while what underlies the delayed onset is unclear. Allopregnanolone (Allop) is a metabolite of progesterone and a potent modulator of GABAA-ergic currents whose defects are seen in RTT. Allop changes its concentration during the perinatal period, which may affect central neurons via the GABAA-ergic synaptic transmission, contributing to the onset of the disease. To determine whether Mecp2 disruption affects Allop modulation, we performed studies in brain slices obtained from wild-type (WT) and Mecp2(-/Y) mice. Allop dose dependently suppressed locus coeruleus (LC) neuronal excitability in WT mice, while Mecp2-null neurons showed significant defects. Using optogenetic approaches, channelrhodopsin was specifically expressed in GABA-ergic neurons in which optical stimulation evoked action potentials. In LC neurons of WT mice, Allop exposure increased the amplitude of GABAA-ergic inhibitory postsynaptic currents (IPSCs) evoked by optical stimulation and prolonged the IPSC decay time. Consistently, Allop augmented both frequency and amplitude of GABAA-ergic spontaneous IPSCs (sIPSCs) and extended the decay time of sIPSCs. The Allop-induced potentiation of sIPSCs was deficient in Mecp2(-/Y) mice. Surprisingly, the impairment occurred at 3 wk postnatal age, while no significant difference in Allop modulation was observed in 1-2 wk between WT and Mecp2(-/Y) mice. These results indicate that the modulation of GABAA-ergic synaptic transmission by Allop is impaired in LC neurons of Mecp2-null mice at a time when RTT-like symptoms manifest, suggesting a potential mechanism for the delayed onset of the disease.

Neurochemical and behavioral features in genetic absence epilepsy and in acutely induced absence seizures

The absence epilepsy typical electroencephalographic pattern of sharp spikes and slow waves (SWDs) is considered to be due to an interaction of an initiation site in the cortex and a resonant circuit in the thalamus. The hyperpolarization-activated cyclic nucleotide-gated cationic I h pacemaker channels (HCN) play an important role in the enhanced cortical excitability. The role of thalamic HCN in SWD occurrence is less clear. Absence epilepsy in the WAG/Rij strain is accompanied by deficiency of the activity of dopaminergic system, which weakens the formation of an emotional positive state, causes depression-like symptoms, and counteracts learning and memory processes. It also enhances GABAA receptor activity in the striatum, globus pallidus, and reticular thalamic nucleus, causing a rise of SWD activity in the cortico-thalamo-cortical networks. One of the reasons for the occurrence of absences is that several genes coding of GABAA receptors are mutated. The question arises: what the role of DA receptors is. Two mechanisms that cause an infringement of the function of DA receptors in this genetic absence epilepsy model are proposed.

Bicuculline- and neurosteroid-sensitive tonic chloride current in rat hypoglossal motoneurons and atypical dual effect of SR95531

Hypoglossal motoneurons (HMs) are known to be under 'permanent' bicuculline-sensitive inhibition and to show 'transient' synaptic γ-aminobutyric acid (GABA)(A) and glycine inhibitory responses. The present paper describes a permanent bicuculline-sensitive current that should contribute to their tonic inhibition. This current was recorded in brainstem slices superfused without any exogenous agonist and remained detectable with tetrodotoxin. It could also be blocked by the other GABA(A) antagonists picrotoxin (PTX) and 2-(3-carboxypropyl)-3-amino-6-(4 methoxyphenyl)pyridazinium bromide) (SR95531; gabazine), but persisted in the presence of a specific blocker of α5-containing GABA(A) receptors. Addition of 2 μm 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol hydrochloride (THIP), known to preferentially activate GABA(A) receptors devoid of a γ-subunit, induced a sustained anionic current that could be further enhanced by neurosteroids such as allopregnanolone (100 nm). Thus, HMs show a tonic inhibitory current carried by extrasynaptic γ-free GABA(A) receptors, highly sensitive to neurosteroids. A second result was obtained by using SR95531 at concentrations sufficiently high to rapidly block the tonic current above the chloride equilibrium potential (E(C) (l)). Surprisingly, below E(C) (l) , SR95531 (10-40 μm) activated a sustained inward current, associated with a conductance increase, and resistant to bicuculline or PTX (100 μm). Similarly, after blockade of the bicuculline-sensitive current, SR95531 activated an outward current above E(C) (l). The bicuculline-resistant anionic current activated by SR95531 could be blocked by a GABA(C) receptor antagonist. Thus, two types of inhibitory GABA receptors, belonging to the GABA(A) and GABA(C) families, are able to show a sustained activity in HMs and provide promising targets for neuroprotection under overexcitatory situations known to easily damage these particularly fragile neurons.

Enhancement of GABAergic activity: neuropharmacological effects of benzodiazepines and therapeutic use in anesthesiology

GABA is the major inhibitory neurotransmitter in the central nervous system (CNS). The type A GABA receptor (GABA(A)R) system is the primary pharmacological target for many drugs used in clinical anesthesia. The α1, β2, and γ2 subunit-containing GABA(A)Rs located in the various parts of CNS are thought to be involved in versatile effects caused by inhaled anesthetics and classic benzodiazepines (BZD), both of which are widely used in clinical anesthesiology. During the past decade, the emergence of tonic inhibitory conductance in extrasynaptic GABA(A)Rs has coincided with evidence showing that these receptors are highly sensitive to the sedatives and hypnotics used in anesthesia. Anesthetic enhancement of tonic GABAergic inhibition seems to be preferentially increased in regions shown to be important in controlling memory, awareness, and sleep. This review focuses on the physiology of the GABA(A)Rs and the pharmacological properties of clinically used BZDs. Although classic BZDs are widely used in anesthesiological practice, there is a constant need for new drugs with more favorable pharmacokinetic and pharmacodynamic effects and fewer side effects. New hypnotics are currently developed, and promising results for one of these, the GABA(A)R agonist remimazolam, have recently been published.

Human locus coeruleus neurons express the GABA(A) receptor gamma2 subunit gene and produce benzodiazepine binding

Noradrenergic neurons of the locus coeruleus project throughout the cerebral cortex and multiple subcortical structures. Alterations in the locus coeruleus firing are associated with vigilance states and with fear and anxiety disorders. Brain ionotropic type A receptors for gamma-aminobutyric acid (GABA) serve as targets for anxiolytic and sedative drugs, and play an essential regulatory role in the locus coeruleus. GABA(A) receptors are composed of a variable array of subunits forming heteropentameric chloride channels with different pharmacological properties. The gamma2 subunit is essential for the formation of the binding site for benzodiazepines, allosteric modulators of GABA(A) receptors that are clinically often used as sedatives/hypnotics and anxiolytics. There are contradictory reports in regard to the gamma2 subunit's expression and participation in the functional GABA(A) receptors in the mammalian locus coeruleus. We report here that the gamma2 subunit is transcribed and participates in the assembly of functional GABA(A) receptors in the tyrosine hydroxylase-positive neuromelanin-containing neurons within postmortem human locus coeruleus as demonstrated by in situ hybridization with specific gamma2 subunit oligonucleotides and autoradiographic assay for flumazenil-sensitive [(3)H]Ro 15-4513 binding to benzodiazepine sites. These sites were also sensitive to the alpha1 subunit-preferring agonist zolpidem. Our data suggest a species difference in the expression profiles of the alpha1 and gamma2 subunits in the locus coeruleus, with the sedation-related benzodiazepine sites being more important in man than rodents. This may explain the repeated failures in the transition of novel drugs with a promising neuropharmacological profile in rodents to human clinical usage, due to intolerable sedative effects.

The role of Loop F in the activation of the GABA receptor

Functional studies of the ligand gated ion channel family (nicotinic acetylcholine, serotonin Type 3, glycine and GABA receptors) along with the crystal structure of the acetylcholine binding protein (AChBP) and molecular dynamics simulations of the nAChR structure have resulted in a structural model in which the agonist-binding pocket comprises six loops (A-F) contributed by adjacent subunits. It is presumed that the binding of agonist results in a local structural rearrangement that is then transduced to the gate, causing the pore to open. Efforts are underway to better define the specific roles of the six binding loops. Several studies have suggested Loop F may play a direct role in linking the structural rearrangement within the binding pocket to the gate, although other investigations have indicated Loop F may be crucial for locking the agonist molecule into the binding site. This review will focus on the controversy surrounding the role of Loop F during GABA receptor activation.

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.

Inhibitory transmission in locus coeruleus neurons expressing GABAA receptor epsilon subunit has a number of unique properties

Fast inhibitory synaptic transmission in the brain relies on ionotropic GABA(A) receptors (GABA(A)R). Eighteen genes code for GABA(A)R subunits, but little is known about the epsilon subunit. Our aim was to identify the synaptic transmission properties displayed by native receptors incorporating epsilon. Immunogold localization detected epsilon at synaptic sites on locus coeruleus (LC) neurons. In situ hybridization revealed prominent signals from epsilon, and mRNAs, some low beta1 and beta3 signals, and no gamma signal. Using in vivo extracellular and in vitro patch-clamp recordings in LC, we established that neuron firing rates, GABA-activated currents, and mIPSC charge were insensitive to the benzodiazepine flunitrazepam (FLU), in agreement with the characteristics of recombinant receptors including an epsilon subunit. Surprisingly, LC provided binding sites for benzodiazepines, and GABA-induced currents were potentiated by diazepam (DZP) in the micromolar range. A number of GABA(A)R ligands significantly potentiated GABA-induced currents, and zinc ions were only active at concentrations above 1 muM, further indicating that receptors were not composed of only alpha and beta subunits, but included an epsilon subunit. In contrast to recombinant receptors including an epsilon subunit, GABA(A)R in LC showed no agonist-independent opening. Finally, we determined that mIPSCs, as well as ensemble currents induced by ultra-fast GABA application, exhibited surprisingly slow rise times. Our work thus defines the signature of native GABA(A)R with a subunit composition including epsilon: differential sensitivity to FLU and DZP and slow rise time of currents. We further propose that alpha(3,) beta(1/3,) and epsilon subunits compose GABA(A)R in LC.

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.

Pentobarbital produces activation and block of {alpha}1{beta}2{gamma}2S GABAA receptors in rapidly perfused whole cells and membrane patches: divergent results can be explained by pharmacokinetics

Millimolar concentrations of the barbiturate pentobarbital (PB) activate γ-aminobutyric acid (GABA) type A receptors (GABARs) and cause blockade reported by a paradoxical current increase or “tail” upon washout. To explore the mechanism of blockade, we investigated PB-triggered currents of recombinant α₁β₂γ_2S GABARs in whole cells and outside-out membrane patches using rapid perfusion. Whole cell currents showed characteristic bell-shaped concentration dependence where high concentrations triggered tail currents with peak amplitudes similar to those during PB application. Tail current time courses could not be described by multi-exponential functions at high concentrations (≥3,000 μM). Deactivation time course decayed over seconds and was slowed by increasing PB concentration and application time. In contrast, macropatch tail currents manifested eightfold greater relative amplitude, were described by multi-exponential functions, and had millisecond rise times; deactivation occurred over fractions of seconds and was insensitive to PB concentration and application time. A parsimonious gating model was constructed that accounts for macropatch results (“patch” model). Lipophilic drug molecules migrate slowly through cells due to avid partitioning into lipophilic subcellular compartments. Inclusion of such a pharmacokinetic compartment into the patch model introduced a slow kinetic component in the extracellular exchange time course, thereby providing recapitulation of divergent whole cell results. GABA co-application potentiated PB blockade. Overall, the results indicate that block is produced by PB concentrations sixfold lower than for activation involving at least three inhibitory PB binding sites, suggest a role of blocked channels in GABA-triggered activity at therapeutic PB concentrations, and raise an important technical question regarding the effective rate of exchange during rapid perfusion of whole cells with PB.

Expression of GABA(A) receptor alpha3-, theta-, and epsilon-subunit mRNAs during rat CNS development and immunolocalization of the epsilon subunit in developing postnatal spinal cord

Ionotropic GABA(A) receptors are heteromeric structures composed of a combination of five from at least 16 different subunits. Subunit genes are expressed in distinct cell types at specific times during development. The most abundant native GABA(A) receptors consist of alpha1-, beta2-, and gamma2-subunits that are co-expressed in numerous brain areas. alpha3-, theta-, And epsilon-subunits are clustered on the X chromosome and show striking overlapping expression patterns throughout the adult rat brain. To establish whether these subunits are temporally and spatially co-expressed, we used in situ hybridization to analyze their expression throughout rat development from embryonic stage E14 to postnatal stage P12. Each transcript exhibited a unique or a shared regional and temporal developmental expression profile. The thalamic expression pattern evolved from a restricted expression of epsilon and theta transcripts before birth, to a theta and alpha3 expression at birth, and finally to a grouped epsilon, theta and alpha3 expression postpartum. However, strong similarities occurred, such as a grouped expression of the three subunits within the hypothalamus, tegmentum and pontine nuclei throughout the developmental process. At early stages of development (E17), epsilon and theta appeared to have a greater spatial distribution before the dominance of the alpha3 subunit transcript around birth. We also revealed expression of alpha3, theta, and epsilon in the developing spinal cord and identified neurons that express epsilon in the postnatal dorsal horn, intermediolateral column and motoneurons. Our findings suggest that various combinations of alpha3-, theta- and epsilon-subunits may be assembled at a regional and developmental level in the brain.

Chapter 2: hypothalamic neural systems controlling the female reproductive life cycle gonadotropin-releasing hormone, glutamate, and GABA

The hypothalamic-pituitary-gonadal (HPG) axis undergoes a number of changes throughout the reproductive life cycle that are responsible for the development, puberty, adulthood, and senescence of reproductive systems. This natural progression is dictated by the neural network controlling the hypothalamus including the cells that synthesize and release gonadotropin-releasing hormone (GnRH) and their regulatory neurotransmitters. Glutamate and GABA are the primary excitatory and inhibitory neurotransmitters in the central nervous system, and as such contribute a great deal to modulating this axis throughout the lifetime via their actions on receptors in the hypothalamus, both directly on GnRH neurons as well as indirectly through other hypothalamic neural networks. Interactions among GnRH neurons, glutamate, and GABA, including the regulation of GnRH gene and protein expression, hormone release, and modulation by estrogen, are critical to age-appropriate changes in reproductive function. Here, we present evidence for the modulation of GnRH neurosecretory cells by the balance of glutamate and GABA in the hypothalamus, and the functional consequences of these interactions on reproductive physiology across the life cycle.

Investigation of gamma-aminobutyric acid (GABA) A receptors genes and migraine susceptibility

BACKGROUND

Migraine is a neurological disorder characterized by recurrent attacks of severe headache, affecting around 12% of Caucasian populations. It is well known that migraine has a strong genetic component, although the number and type of genes involved is still unclear. Prior linkage studies have reported mapping of a migraine gene to chromosome Xq 24-28, a region containing a cluster of genes for GABA A receptors (GABRE, GABRA3, GABRQ), which are potential candidate genes for migraine. The GABA neurotransmitter has been implicated in migraine pathophysiology previously; however its exact role has not yet been established, although GABA receptors agonists have been the target of therapeutic developments. The aim of the present research is to investigate the role of the potential candidate genes reported on chromosome Xq 24-28 region in migraine susceptibility. In this study, we have focused on the subunit GABA A receptors type epsilon (GABRE) and type theta (GABRQ) genes and their involvement in migraine.

METHODS

We have performed an association analysis in a large population of case-controls (275 unrelated Caucasian migraineurs versus 275 controls) examining a set of 3 single nucleotide polymorphisms (SNPs) in the coding region (exons 3, 5 and 9) of the GABRE gene and also the I478F coding variant of the GABRQ gene.

RESULTS

Our study did not show any association between the examined SNPs in our test population (P>0.05).

CONCLUSION

Although these particular GABA receptor genes did not show positive association, further studies are necessary to consider the role of other GABA receptor genes in migraine susceptibility.

International Union of Pharmacology. LXX. Subtypes of gamma-aminobutyric acid(A) receptors: classification on the basis of subunit composition, pharmacology, and function. Update

In this review we attempt to summarize experimental evidence on the existence of defined native GABA(A) receptor subtypes and to produce a list of receptors that actually seem to exist according to current knowledge. This will serve to update the most recent classification of GABA(A) receptors (Pharmacol Rev 50:291-313, 1998) approved by the Nomenclature Committee of the International Union of Pharmacology. GABA(A) receptors are chloride channels that mediate the major form of fast inhibitory neurotransmission in the central nervous system. They are members of the Cys-loop pentameric ligand-gated ion channel (LGIC) superfamily and share structural and functional homology with other members of that family. GABA(A) receptors are assembled from a family of 19 homologous subunit gene products and form numerous, mostly hetero-oligomeric, pentamers. Such receptor subtypes with properties that depend on subunit composition vary in topography and ontogeny, in cellular and subcellular localization, in their role in brain circuits and behaviors, in their mechanisms of regulation, and in their pharmacology. We propose several criteria, which can be applied to all the members of the LGIC superfamily, for including a receptor subtype on a list of native hetero-oligomeric subtypes. With these criteria, we develop a working GABA(A) receptor list, which currently includes 26 members, but will undoubtedly be modified and grow as information expands. The list is divided into three categories of native receptor subtypes: "identified," "existence with high probability," and "tentative."

Etazolate, a neuroprotective drug linking GABA(A) receptor pharmacology to amyloid precursor protein processing

Pharmacological modulation of the GABA(A) receptor has gained increasing attention as a potential treatment for central processes affected in Alzheimer disease (AD), including neuronal survival and cognition. The proteolytic cleavage of the amyloid precursor protein (APP) through the alpha-secretase pathway decreases in AD, concurrent with cognitive impairment. This APP cleavage occurs within the beta-amyloid peptide (Abeta) sequence, precluding formation of amyloidogenic peptides and leading to the release of the soluble N-terminal APP fragment (sAPPalpha) which is neurotrophic and procognitive. In this study, we show that at nanomolar-low micromolar concentrations, etazolate, a selective GABA(A) receptor modulator, stimulates sAPPalpha production in rat cortical neurons and in guinea pig brains. Etazolate (20 nM-2 microM) dose-dependently protected rat cortical neurons against Abeta-induced toxicity. The neuroprotective effects of etazolate were fully blocked by GABA(A) receptor antagonists indicating that this neuroprotection was due to GABA(A) receptor signalling. Baclofen, a GABA(B) receptor agonist failed to inhibit the Abeta-induced neuronal death. Furthermore, both pharmacological alpha-secretase pathway inhibition and sAPPalpha immunoneutralization approaches prevented etazolate neuroprotection against Abeta, indicating that etazolate exerts its neuroprotective effect via sAPPalpha induction. Our findings therefore indicate a relationship between GABA(A) receptor signalling, the alpha-secretase pathway and neuroprotection, documenting a new therapeutic approach for AD treatment.

Differential expression of gamma-aminobutyric acid receptor A (GABA(A)) and effects of homocysteine

BACKGROUND

gamma-Aminobutyric acid (GABA) is a known inhibitory neurotransmitter in the mammalian central nervous system, and homocysteine (Hcy) behaves as an antagonist for GABA(A) receptor. Although the properties and functions of GABA(A) receptors are well studied in mouse neural tissue, its presence and significance in non-neural tissue remains obscure. The aim of the present study was to examine the expression of GABA(A) receptor and its subunits in non-neural tissue.

METHODS

The mice were analyzed. The presence of GABA(A) receptor and its subunits was evaluated using Western blot and reverse transcription polymerase chain reaction.

RESULTS

We report that GABA(A) receptor protein is abundant in the renal medulla, cortex, heart, left ventricle, aorta and pancreas. Low levels of GABA(A) receptor protein were detected in the atria of the heart, right ventricle, lung and stomach. The mRNA protein expression of GABA(A) receptor subunit shows that alpha1, beta1, beta3 and gamma1 subunits are present only in brain. The mRNA protein expression levels of GABA(A) receptor alpha2, alpha6, beta2 and gamma3 subunits were highly expressed in brain compared to other tested tissue, while GABA(A) receptor gamma2 subunit was expressed only in brain and kidney. Treatment of microvascular endothelial cells with Hcy decreased GABA(A) receptor protein level, which was restored to its baseline level in the presence of GABA(A) receptor agonist, muscimol. The distribution of GABA(A) and GABA(B) receptors in wild type mice was determined and tissue-specific expression patterns were found showing that several receptor subtypes were also expressed in the central nervous system.

CONCLUSIONS

Hcy, a GABA(A) agonist, was found to decrease GABA(A) expression levels. These data enlarge knowledge on distribution of GABA receptors and give novel ideas of the effects of Hcy on different organs.

Identification, coassembly, and activity of gamma-aminobutyric acid receptor subunits in renal proximal tubular cells

Although the properties and functions of GABA(A) receptors in the mammalian central nervous system have been well studied, the presence and significance of GABA(A) receptors in non-neural tissues are less clear. The goal of this study was to examine the expression of GABA(A) receptor alpha(1), alpha(2), alpha(4), alpha(5), beta(1), gamma(1), gamma(2), and delta subunits in the kidney and to determine whether these subunits coassemble to form an active renal epithelial cell GABA(A) receptor. Using reverse transcriptase products from RNA isolated from rat and rabbit kidney cortex and brain or cerebellum through polymerase chain reaction (PCR) and sequencing of the PCR products, we revealed that rat kidney cortex contained the alpha(1), alpha(5), beta(1), gamma(1), and gamma(2) subunits and that they were similar to the neuronal subunits. Sequencing of the PCR products revealed that the rabbit kidney cortex contained the alpha(1) and gamma(2) subunits and that they were similar to their neuronal counterparts. Immunoprecipitation and immunoblot studies using GABA(A) receptor subunit-specific antibodies and detergent-solubilized rat kidney cortex membranes identified a GABA(A) receptor complex containing alpha(5), beta(1), and gamma(1). Isolated rat renal proximal tubular cells exhibited GABA-mediated, picrotoxin-sensitive (36)Cl(-) uptake. These studies demonstrate the presence of numerous GABA(A) receptor subunits in the kidneys of two species, the assembly of the subunits into at least one novel receptor complex, and an active GABA(A) receptor in renal proximal tubular cells.

Trafficking and potential assembly patterns of epsilon-containing GABAA receptors

Incorporation of the epsilon subunit into the GABAA receptor has been suggested to confer unusual, but variable, biophysical and pharmacological characteristics to both recombinant and native receptors. Due to their structural similarity with the gamma subunits, epsilon subunits have been assumed to substitute at the single position of the gamma subunit in assembled receptors. However, prior work suggests that functional variability in epsilon-containing receptors may reflect alternative sites of incorporation and of not just one, but possibly multiple epsilon subunits in the pentameric receptor complex. Here we present data indicating that increased expression of epsilon, in conjunction with alpha2 and beta3 subunits, results in expression of GABAA receptors with correspondingly altered rectification, deactivation and levels of spontaneous openings, but not increased total current density. We also provide data that the epsilon subunit, like the beta3 subunit, can self-export and data from chimeric receptors suggesting that similarities between the assembly domains of the beta3 and the epsilon subunits may allow the epsilon subunit to replace the beta, as well as the gamma, subunit. The substitution of an epsilon for a beta, as well as the gamma subunit and formation of receptors with alternative patterns of assembly with respect to epsilon incorporation may underlie the observed variability in both biophysical and pharmacological properties noted not only in recombinant, but also in native receptors.

Autoradiographic analysis of GABAA receptors in mu-opioid receptor knockout mice

Anatomical evidence indicates that gamma-aminobutyric acid (GABA)-ergic and opioidergic systems are closely linked and act on the same neurons. However, the regulatory mechanisms between GABAergic and opioidergic system have not been well characterized. In the present study, we investigated whether there are changes in GABA(A) receptors in mice lacking mu-opioid receptor gene. The GABA(A) receptor binding was carried out by autoradiography using [(3)H]-muscimol (GABA(A)), [(3)H]-flunitrazepam (FNZ, native type 1 benzodiazepine) and [(35)S]-t-butylbicyclophosphorothionate (TBPS, binding to GABA(A)-gated chloride channels) in brain slices of wild type and mu-opioid receptor knockout mice. The binding of [(3)H]-FNZ in mu-opioid receptor knockout mice was significantly higher than that of the wild type controls in most of the cortex and hippocampal CA1 and CA2 formations. mu-Opioid receptor knockout mice show significantly lower binding of [(35)S]-TBPS than that of the wild type mice in few of the cortical areas including ectorhinal cortex layers I, III, and V, but not in the hippocampus. There was no significant difference in binding of [(3)H]-muscimol between mu-opioid receptor knockout and wild type mice in the cortex and hippocampus. These data indicate that there are specific regional changes in GABA(A) receptor binding sites in mu-opioid receptor knockout mice. These data also suggest that there are compensatory up-regulation of benzodiazepine binding site of GABA(A) receptors in the cortex and hippocampus and down-regulation of GABA-gated chloride channel binding site of GABA(A) receptors in the cortex of the mu-opioid receptor knockout mice.

Emerging roles for ubiquitin and protein degradation in neuronal function

Alterations in cellular structure and synapse composition are central to proper nervous system function. Recent work has identified the ubiquitin-proteasome system (UPS) as a key regulator of neuronal biology. The UPS is essential for the growth and development of immature neurons and is a critical mediator of synaptic adaptability in mature neurons. Furthermore, proteinaceous deposits that accumulate in diverse neurodegenerative disorders are enriched in components of the UPS, suggesting that UPS dysfunction may be pivotal for pathogenesis. Here, we summarize existing knowledge about the role of the UPS in brain function, highlighting recent work delineating its importance in neuronal development, plasticity, and degeneration.

Studies of ethanol actions on recombinant delta-containing gamma-aminobutyric acid type A receptors yield contradictory results

The gamma-aminobutyric acid type A receptors (GABAA-Rs) display a wide variety of subunit combinations. Drugs such as benzodiazepines have shown differential effects based on GABAA-R subunit composition. Actions of alcohols and volatile anesthetics generally do not vary markedly with subunit composition, with low concentrations of ethanol being poor modulators of these receptors. Recent studies showed alpha(4/6)- and delta-containing GABAA-Rs (located extrasynaptically and responsible for tonic currents in selective brain regions) presenting high sensitivity to low concentrations of ethanol, but these results have not been obtained in other laboratories. We carried out additional experiments varying the receptor level of expression, and GABA and ethanol concentration, but no sensitivity to low concentrations of ethanol was detected. We will discuss these results and attempt an analysis of the possible causes for the discrepancies.

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.

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.

Two types of GABAergic miniature inhibitory postsynaptic currents in mouse substantia gelatinosa neurons

The physiological and pharmacological properties of gamma-aminobutyric acid (GABA)ergic miniature inhibitory postsynaptic currents (mIPSCs) were investigated in substantia gelatinosa neurons of mouse spinal cord using whole-cell patch clamp recordings. Two cell populations were pharmacologically identified based on the effect of propofol (10 muM) on the mIPSC decay kinetics: those exhibiting propofol-sensitive mIPSCs, with a slow decay kinetic (mIPSC(SLOW)), and those exhibiting propofol-resistant mIPSCs, with a fast decay kinetic (mIPSC(FAST)) (decay time constants of 14.2+/-0.7 and 7.4+/-0.8 ms, respectively). The frequency and amplitude of both types of mIPSCs were not affected by propofol. Miniature IPSC(FAST) showed midazolam insensitivity, while midazolam prolonged the decay phase of mIPSC(SLOW) without modulation of the frequency and amplitude. Exogenous GABA-evoked responses in the neurons with mIPSC(SLOW) were potentiated by propofol, while those in neurons with mIPSC(FAST) were unaffected by propofol. Furthermore, non-stationary noise analysis of the two kinetically and pharmacologically distinct mIPSCs revealed different conductance of GABA(A) receptor channels underlying the synaptic events. Pharmacological responses to propofol and midazolam suggested that mIPSC(FAST) and mIPSC(SLOW) in substantia gelatinosa neurons can be mediated by GABA(A) receptors with different subunit compositions.

GABAergic tonic inhibition of substantia gelatinosa neurons in mouse spinal cord

We used whole-cell recording to identify, for the first time, GABAergic tonic current in subpopulations of substantia gelatinosa neurons in mouse spinal cord slices. Application of the gamma-aminobutyric acid type A receptor antagonist bicuculline revealed tonic currents in 49 of 73 substantia gelatinosa neurons. These tonic currents were dissected into three groups according to their diverse pharmacological properties. In the first group, furosemide failed to block the tonic current while midazolam and propofol potentiated it. In the second group, furosemide blocked the tonic current but midazolam and propofol failed to enhance the current. In the third group, furosemide blocked and 3alpha, 21-dihydroxy-5alpha-pregnan-20-one enhanced the tonic current. The presence of mRNAs for alpha4, alpha5, delta and epsilon subunits in the substantia gelatinosa (reverse-transcriptase polymerase chain reaction) suggest that gamma-aminobutyric acid type A receptors comprising alpha5betaxgamma2, alphaxbetaxepsilon and alpha4betaxdelta subunits are responsible for tonic currents.

2,3,7-Trisubstituted pyrazolo[1,5-d][1,2,4]triazines: Functionally selective GABAA α3-subtype agonists

Novel synthetic routes have been devised for the preparation of previously inaccessible 2,3,7-trisubstituted pyrazolo[1,5-d][1,2,4]triazines 2. These compounds are high affinity ligands for the GABA(A) benzodiazepine binding site and some analogues show functional selectivity for agonism at alpha3-containing receptors over alpha1-containing receptors with the lead compound being 32.

Expression profiling of genes encoding glutamate and GABA receptor subunits in three immortalized GnRH cell lines

Gonadotropin-releasing hormone (GnRH) plays a central role in regulating development and function of the reproductive axis, and its secretion is known to be influenced by glutamate and GABA. In the present study, we used gene microarrays and RT-PCR to compare the expression profiles of glutamate and GABA receptor subunits in three immortalized GnRH cell lines: GT1-1, GT1-7, and Gn10. All of these cell lines expressed the AMPA glutamate receptor subunit genes GluR2 and GluR4, but only the GT1-1 and GT1-7 cells expressed the kainate glutamate receptor subunit gene KA2. Additionally, GluRdelta2, a subunit that can form heteromeric receptors with kainate and AMPA subunits, was present in GT1-1 and Gn10 cells but not in GT1-7 cells. Genes encoding the GABA(A) receptor alpha3, beta2, beta3, epsilon, and pi subunits, as well as the GABA(B) receptor 1 subunit, were evident in all three cell lines. However, the gene encoding the expression of GABA(A) receptor gamma subunit was noticeably absent. Taken together, these data demonstrate comprehensive screening of neurotransmitter receptor genes in a controlled neuronal culture system, and reveal novel features.

Mechanisms of anabolic androgenic steroid inhibition of mammalian epsilon-subunit-containing GABAA receptors

GABAergic transmission regulates the activity of gonadotrophin-releasing hormone (GnRH) neurons in the preoptic area/hypothalamus that control the onset of puberty and the expression of reproductive behaviours. One of the hallmarks of illicit use of anabolic androgenic steroids (AAS) is disruption of behaviours under neuroendocrine control. GnRH neurons are among a limited population of cells that express high levels of the epsilon-subunit of the GABAA receptor. To better understand the actions of AAS on neuroendocrine mechanisms, we have characterized modulation of GABAA receptor-mediated currents in mouse native GnRH neurons and in heterologous cells expressing recombinant alpha2beta3epsilon-receptors. GnRH neurons exhibited robust currents in response to millimolar concentrations of GABA and a picrotoxin (PTX)-sensitive, bicuculline-insensitive current that probably arises from spontaneous openings of GABAA receptors. The AAS 17alpha-methyltestosterone (17alpha-MeT) inhibited spontaneous and GABA-evoked currents in GnRH neurons. For recombinant alpha2beta3epsilon-receptors, 17alpha-MeT inhibited phasic and tonic GABA-elicited responses, accelerated desensitization and slowed paired pulse response recovery. Single channel analysis indicated that GABA-evoked events could be described by three open dwell components and that 17alpha-MeT enhanced residence in the intermediate dwell state. This AAS also inhibited a PTX-sensitive, spontaneous current (open probability, approximately 0.15-0.2) in a concentration-dependent fashion (IC50 approximately 9 microm). Kinetic modelling indicated that the inhibition induced by 17alpha-MeT occurs by an allosteric block in which the AAS interacts preferentially with a closed state and promotes accumulation in that state. Finally, studies with a G302S mutant epsilon-subunit suggest that this residue within the transmembrane domain TM2 plays a role in mediating AAS binding and modulation. In sum, our results indicate that inclusion of the epsilon-subunit significantly alters the profile of AAS modulation and that this allosteric inhibition of native GnRH neurons should be considered with regard to AAS disruption of neuroendocrine control.

Imidazo[1,2-a]pyrimidines as functionally selective and orally bioavailable GABA(A)alpha2/alpha3 binding site agonists for the treatment of anxiety disorders

A series of high-affinity GABA(A) agonists with good oral bioavailability in rat and dog and functional selectivity for the GABA(A)alpha2 and -alpha3 subtypes is reported. The 7-trifluoromethylimidazopyrimidine 14g and the 7-propan-2-olimidazopyrimidine 14k are anxiolytic in both conditioned and unconditioned animal models of anxiety with minimal sedation observed at full BZ binding site occupancy.

Impact of epsilon and theta subunits on pharmacological properties of alpha3beta1 GABAA receptors expressed in Xenopus oocytes

Background

γ-Aminobutyric acid type A (GABA_A) receptors provide the main inhibitory control in the brain. Their heterogeneity may make it possible to precisely target drug effects to selected neuronal populations. In situ hybridization using rat brain sections has revealed a unique expression of GABA_A receptor ε and θ subunit transcripts in the locus coeruleus, where they are accompanied at least by α3, α2, β1 and β3 subunits. Here, we studied the pharmacology of the human α3β1, α3β1ε, α3β1θ and α3β1εθ receptor subtypes expressed in Xenopus oocytes and compared them with the γ2 subunit-containing receptors.

Results

The GABA sensitivites and effects of several positive modulators of GABA_A receptors were studied in the absence and the presence of EC₂₅ GABA using the two-electrode voltage-clamp method. We found 100-fold differences in GABA sensitivity between the receptors, α3β1ε subtype being the most sensitive and α3β1γ2 the least sensitive. Also gaboxadol dose-response curves followed the same sensitivity rank order, with EC₅₀ values being 72 and 411 μM for α3β1ε and α3β1γ2 subtypes, respectively. In the presence of EC₂₅ GABA, introduction of the ε subunit to the receptor complex resulted in diminished modulatory effects by etomidate, propofol, pregnanolone and flurazepam, but not by pentobarbital. Furthermore, the α3β1ε subtype displayed picrotoxin-sensitive spontaneous activity. The θ subunit-containing receptors were efficiently potentiated by the anesthetic etomidate, suggesting that θ subunit could bring the properties of β2 or β3 subunits to the receptor complex.

Conclusion

The ε and θ subunits bring additional features to α3β1 GABA_A receptors. These receptor subtypes may constitute as novel drug targets in selected brain regions, e.g., in the brainstem locus coeruleus nuclei.

Anabolic androgenic steroids and forebrain GABAergic transmission

Anabolic androgenic steroids are synthetic derivatives of testosterone designed for therapeutic purposes, but now taken predominantly as drugs of abuse. The most common behavioral effects associated with anabolic androgenic steroid use are changes in anxiety, aggression and reproductive behaviors, including the onset of puberty and sexual receptivity. GABAergic circuits in the forebrain underlie these behaviors and are regulated by gonadal steroids. Work from our laboratories has shown that the expression and function of GABA(A) receptors in the rat and mouse forebrain varies between the sexes and across the estrous cycle. We have also shown that there are significant changes in GABA(A) receptor expression that occur with the progression through puberty to adulthood. Because GABAergic systems are both steroid-sensitive and critical for the expression of behaviors altered with anabolic androgenic steroid use, forebrain GABA(A) receptors are an attractive candidate to assess how molecular actions of anabolic androgenic steroids may be translated to known behavioral outcomes. Our studies demonstrate that anabolic androgenic steroids elicit both acute modulation of GABA(A) receptor-mediated currents, as well as chronic regulation of GABA(A) receptor expression and forebrain GABAergic transmission. Because anabolic androgenic steroid use has now become prevalent not only among adolescent boys, but in an increasing number of adolescent girls, we have also been particularly interested in determining age- and sex-specific effects of anabolic androgenic steroids. Our data show that the effects of chronic anabolic androgenic steroid exposure can be greater for adolescent than adult animals and are more marked in females than in males. These data have particularly important implications with respect to studies we have done demonstrating that chronic anabolic androgenic steroid exposure alters the onset of puberty, estrous cyclicity and sexual receptivity.

7-(1,1-Dimethylethyl)-6-(2-ethyl-2H-1,2,4- triazol-3-ylmethoxy)-3-(2-fluorophenyl)- 1,2,4-triazolo[4,3-b]pyridazine: A Functionally Selective γ-Aminobutyric AcidA (GABAA) α2/α3-Subtype Selective Agonist That Exhibits Potent Anxiolytic Activity but Is Not Sedating in Animal Models

There is increasing evidence that compounds with selectivity for gamma-aminobutyric acid(A) (GABA(A)) alpha2- and/or alpha3-subtypes may retain the desirable anxiolytic activity of nonselective benzodiazepines but possess an improved side effect profile. Herein we describe a novel series of GABA(A) alpha2/alpha3 subtype-selective agonists leading to the identification of the development candidate 17, a nonsedating anxiolytic in preclinical animal assays.

Glycinergic and GABAergic synaptic transmission are differentially affected by gephyrin in spinal neurons

In the present study, we have examined the physiological properties of synaptic currents mediated by GlyRs and GABAARs after culturing spinal neurons with a gephyrin antisense oligonucleotide. Application of gephyrin antisense, but not the sense, reduced the glycinergic mIPSC amplitude ( approximately 50%) and frequency ( approximately 85%), indicating the importance of gephyrin for GlyR anchoring at postsynaptic sites. On the other hand, the glycine-evoked current amplitude was unchanged indicating that functional GlyRs were still located in the extrasynaptic membrane. The analysis of the GABAergic transmission in the same neurons revealed approximately 70% reduction in the frequency of the GABAergic mIPSCs, without changes in the amplitude. Interestingly, the modulation of remaining GABAAR-mediated synaptic events by zinc and diazepam was significantly altered by the antisense. These results indicate that gephyrin is required for the membrane insertion/stabilization of the GABAAR gamma2 subunit as well as for its subsequent localization in the postsynaptic membrane.

Distinct patterns of expression and regulation of GABA receptors containing the delta subunit in cerebellar granule and hippocampal neurons

Neuronal plasticity is achieved by regulation of the expression of genes for neurotransmitter receptors such as the type A receptor (GABA(A)R) for gamma-aminobutyric acid. We now show that two different rat neuronal populations in culture manifest distinct patterns of GABA(A)R plasticity in response to identical stimuli. Whereas prolonged exposure to ethanol had no effect on expression of the delta subunit of GABA(A)Rs at the mRNA or protein level in cerebellar granule neurons, it increased the abundance of delta subunit mRNA and protein in hippocampal neurons. Subsequent ethanol withdrawal transiently down-regulated delta subunit expression in cerebellar granule neurons and gradually normalized that in hippocampal neurons. These effects of ethanol exposure and withdrawal were accompanied by corresponding functional changes in GABA(A)Rs. GABA(A)Rs containing the delta subunit were also distributed differentially in the cerebellar and hippocampal neurons. These findings reveal complex and distinct mechanisms of regulation of the expression of GABA(A)Rs that contain the delta subunit in different neuronal types.

Differential expression of GABAA receptor subunits in the distinct nuclei of the rat amygdala

Detailed knowledge of the anatomical distribution of different GABA(A) receptor subunits is crucial for understanding the physiological actions of GABA in individual brain areas and for developing drugs acting through the individual GABA receptor subtypes. Since the amygdala is a key brain structure in the processing of emotional information with distinct functions in each nucleus, GABA(A) receptors in the amygdala are an important target of treatment for emotional disorders. In this study, we analyzed by quantitative RT-PCR the expression levels of all GABA(A) receptor subunits in distinct nuclei of the amygdala, the central (Ce) and the lateral/basolateral (LA/BLA) amygdala. We found the strongest expression of the gamma(2) subunit mRNA in both the Ce and LA/BLA, modest expressions of alpha(1), alpha(2) and alpha(3) mRNAs in the LA/BLA and alpha(2) and gamma(1) mRNAs in the Ce, and weak expressions of alpha(6), rho(2) and rho(3) mRNAs in both regions. We further revealed the significantly different expressions of alpha(1), alpha(3), alpha(5), gamma(1), gamma(2), delta, epsilon and theta subunit mRNAs in the Ce and LA/BLA. Differences in the expression levels of GABA(A) receptor subunits suggest different sensitivity to a variety of drugs including benzodiazepines and anesthetics in amygdala nuclei with distinct functions.

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.