GABAA receptors involved in sleep and anaesthesia: β1- versus β3-containing assemblies

Comparative analysis of adenosine 1 receptor expression and function in hippocampal and hypothalamic neurons

BACKGROUND

Adenosine, an ATP degradation product, is a sleep pressure factor. The adenosine 1 receptor (A1R) reports sleep need. Histaminergic neurons (HN) of the tuberomamillary nucleus (TMN) fire exclusively during wakefulness and promote arousal. All of them express GABAA receptors and are inhibited by GABA. Does adenosine contribute to their silencing?

SUBJECTS AND TREATMENT

Responses to adenosine were studied in mouse brain slices and primary dissociated cultures. For HN identification single-cell (sc)RT-PCR, reporter protein and pharmacology were used. Hippocampal Dentate Gyrus granular layer cells (DGgc) were studied in parallel.

METHODS

Firing frequency was recorded in patch-clamp configuration or by microelectrode arrays. A1R-expression was studied by scRT-PCR and semiquantitative PCR.

RESULTS

Most DGgc were inhibited through A1R, detected with scRT-PCR in 7 out of 10 PDZd2-positive DGgc; all HN were A1R negative. One HN out of 25 was inhibited by adenosine. The A1R mRNA level in the hippocampus was 6 times higher than in the caudal (posterior) hypothalamus. Response to adenosine was weaker in hypothalamic compared to hippocampal cultures.

CONCLUSIONS

Most HN are not inhibited by adenosine.

γ-Aminobutyric acid type A receptor β1 subunit gene polymorphisms are associated with the sedative and amnesic effects of midazolam

Midazolam is widely used for intravenous sedation. However, wide interindividual variability is seen in the sensitivity to midazolam. The association between genetic factors and interindividual differences in midazolam sensitivity remains unclear. The present study explored the association between common genetic variants and sedative and amnesic effects of midazolam. This prospective study included patients who were scheduled to undergo dental procedures under intravenous sedation. The sedative effect was evaluated using the Ramsay sedation scale 5 min after midazolam (0.05 mg/kg) administration. We employed two parallel approaches in this study: genome-wide approach and candidate gene approach. The γ-aminobutyric acid type A receptor subunit genes were selected as candidate genes. Multivariate linear regression analyses were performed to investigate the association between the Ramsay sedation scale and genetic variants. We also analyzed the association between the presence of anterograde amnesia and genetic variants using multivariate binominal logistic regression analyses. The analyses were adjusted for potential confounding factors. A total of 191 patients were included in the analyses. In the genome-wide association analyses, no significant association was found between the genetic variants and Ramsay scores. In the candidate gene analyses, the rs73247636 (dominant model: β = 0.72 [95% confidence interval, 0.34 to 1.10], P < 0.001) and rs56278524 (dominant model: β = 0.73 [0.37 to 1.10], P < 0.001) polymorphisms of the GABRB1 gene were significantly associated with Ramsay scores. Additionally, the rs73247636 (dominant model: odds ratio [OR] = 8.39 [2.36 to 29.85], P = 0.001) and rs56278524 (dominant model: OR = 15.26 [3.42 to 68.07], P < 0.001) polymorphisms were also significantly associated with the presence of anterograde amnesia. The rs73247636 and rs56278524 single-nucleotide polymorphisms of GABRB1 were associated with the sedative and amnesic effects of midazolam.

Human ARHGEF9 intellectual disability syndrome is phenocopied by a mutation that disrupts collybistin binding to the GABAA receptor α2 subunit

Intellectual disability (ID) is a common neurodevelopmental disorder that can arise from genetic mutations ranging from trisomy to single nucleotide polymorphism. Mutations in a growing number of single genes have been identified as causative in ID, including ARHGEF9. Evaluation of 41 ARHGEF9 patient reports shows ubiquitous inclusion of ID, along with other frequently reported symptoms of epilepsy, abnormal baseline EEG activity, behavioral symptoms, and sleep disturbances. ARHGEF9 codes for the Cdc42 Guanine Nucleotide Exchange Factor 9 collybistin (Cb), a known regulator of inhibitory synapse function via direct interaction with the adhesion molecule neuroligin-2 and the α2 subunit of GABAA receptors. We mutate the Cb binding motif within the large intracellular loop of α2 replacing it with the binding motif for gephyrin from the α1 subunit (Gabra2-1). The Gabra2-1 mutation causes a strong downregulation of Cb expression, particularly at cholecystokinin basket cell inhibitory synapses. Gabra2-1 mice have deficits in working and recognition memory, as well as hyperactivity, anxiety, and reduced social preference, recapitulating the frequently reported features of ARHGEF9 patients. Gabra2-1 mice also have spontaneous seizures during postnatal development which can lead to mortality, and baseline abnormalities in low-frequency wavelengths of the EEG. EEG abnormalities are vigilance state-specific and manifest as sleep disturbance including increased time in wake and a loss of free-running rhythmicity in the absence of light as zeitgeber. Gabra2-1 mice phenocopy multiple features of human ARHGEF9 mutation, and reveal α2 subunit-containing GABAA receptors as a druggable target for treatment of this complex ID syndrome.

Propofol differentially induces unconsciousness and respiratory depression through distinct interactions between GABAA receptor and GABAergic neuron in corresponding nuclei

Propofol is the most commonly used intravenous anesthetic worldwide. It can induce loss of consciousness prior to the occurrence of severe respiratory suppression, which is also a pharmacodynamic feature of all general anesthetics. However, the neural mechanisms underlying this natural phenomenon are controversial and highly related to patient safety. In the present study, we demonstrated that the pharmacodynamic effects of propofol (50 and 100 μM) on suppression of consciousness-related excitatory postsynaptic currents in the medial prefrontal cortex (mPFC) and centromedian nucleus of the thalamus (CMT) were lower than those in the kernel respiratory rhythmogenesis nucleus pre-Bötzinger complex (PrBo). Furthermore, we unexpectedly found that the GABAA receptor β3 subunit is the key target for propofol's action and that it is mutually and exclusively expressed in GABAergic neurons. It is also more abundant in the mPFC and CMT, but mainly co-localized with GABAergic neurons in the PrBo. As a result, the differentiated expression pattern should mediate more neuron suppression through the activation of GABAergic neurons in the mPFC and CMT at low doses of propofol (50 μM). However, PrBo GABAergic neurons were only activated by propofol at a high dose (100 μM). These results highlight the detailed pharmacodynamic effects of propofol on consciousness-related and respiration-related nuclei and provide the distinct interaction mechanism between the β3 subunit and GABAergic neurons in mediating the suppression of consciousness compared to the inhibition of respiration.

GABAA receptor β1 -subunit knock-out mice show increased delta power in NREM sleep and decreased theta power in REM sleep

γ-Aminobutyric acid (GABA) acting through heteropentameric GABA_A receptors plays a pivotal role in the sleep-promoting circuitry. Whereas the role of the different GABA_A receptor α-subunits in sleep regulation and in mediating the effect of benzodiazepines for treatment of insomnia is well-described, the β-subunits are less studied. Here we report the first study characterizing sleep in mice lacking the GABA_A receptor β₁ -subunit (β₁ ^-/- mice). We show that β₁ ^-/- mice have a distinct and abnormal sleep phenotype characterized by increased delta power in non-rapid eye movement (NREM) sleep and decreased theta activity in rapid eye movement (REM) sleep compared to β₁ ^+/+ mice, without any change in the overall sleep-wake architecture. From GABA_A receptor-specific autoradiography, it is further demonstrated that functional β₁ -subunit-containing GABA_A receptors display the highest binding levels in the hippocampus and frontal cortex. In conclusion, this study suggests that the GABA_A receptor β₁ -subunit does not play an important role in sleep initiation or maintenance but instead regulates the power spectrum and especially the expression of theta rhythm. This provides new knowledge on the complex role of GABA_A receptor subunits in sleep regulation. In addition, β₁ ^-/- mice could provide a useful mouse model for future studies of the physiological role of delta and theta rhythms during sleep.

The (Poly)Pharmacology of Cannabidiol in Neurological and Neuropsychiatric Disorders: Molecular Mechanisms and Targets

Cannabidiol (CBD), the major nonpsychoactive Cannabis constituent, has been proposed for the treatment of a wide panel of neurological and neuropsychiatric disorders, including anxiety, schizophrenia, epilepsy and drug addiction due to the ability of its versatile scaffold to interact with diverse molecular targets that are not restricted to the endocannabinoid system. Albeit the molecular mechanisms responsible for the therapeutic effects of CBD have yet to be fully elucidated, many efforts have been devoted in the last decades to shed light on its complex pharmacological profile. In particular, an ever-increasing number of molecular targets linked to those disorders have been identified for this phytocannabinoid, along with the modulatory effects of CBD on their cascade signaling. In this view, here we will try to provide a comprehensive and up-to-date overview of the molecular basis underlying the therapeutic effects of CBD involved in the treatment of neurological and neuropsychiatric disorders.

Sleep-inducing effect of Passiflora incarnata L. extract by single and repeated oral administration in rodent animals

Social cost of insomnia in modern society is gradually increasing. Due to various social phenomena and lifestyles that take away the opportunity of good quality of sleep, problems of insomnia cannot be easily figured out. Prescription of sleeping pills for insomnia patients can cause other inconveniences due to their side effects beyond their intended purposes. On the other hand, Passiflora incarnata L. (PI) has been widely used in South America for several centuries, showing effectiveness for sleep, sedation, anxiety, and so on in the civilian population. However, reports on the treatment efficacy of this herbal medicinal plant for insomnia patients through standardization as a sleeping agent have been very rare. Therefore, we obtained leaves and fruits of PI (8:2 by weight) as powder to prepare an extract. It was then applied to C6 rat glioma cells to quantitate mRNA expression levels of GABA receptors. Its sleep-inducing effect was investigated using experimental animals. PI extract (6 μg/ml) significantly decreased GABA receptors at 6 hr after treatment. Immobility time and palpebral closing time were significantly increased after single (500 mg/kg) or repeated (250 mg/kg) oral administration. In addition, blood melatonin levels were significantly increased in PI extract-treated animals after both single and repeated administrations. These results were confirmed through several repeated experiments. Taken together, these results confirmed that PI extract had significant sleep-inducing effects in cells and animals, suggesting that PI extract might have potential for treating human insomnia.

GABAa receptor subunits expression in silver catfish (Rhamdia quelen) brain and its modulation by Nectandra grandiflora Nees essential oil and isolated compounds

Studies using silver catfish (Rhamdia quelen) as experimental models are often applied to screen essential oils (EO) with GABAergic-mediated effects. However, the expression of GABAa receptors in the silver catfish brain remains unknown. Thus, we assessed whether silver catfish express GABAa receptor subunits associated with sedation/anesthetic process and/or neurological diseases. Additionally, we evaluated the brain expression of GABAa receptor subunits in fish sedated with Nectandra grandiflora EO and its isolated compounds, the fish anesthetic (+)-dehydrofukinone (DHF), and dehydrofukinone epoxide (DFX), eremophil-11-en-10-ol (ERM) and selin-11-en-4-α-ol (SEL), which have GABAa-mediated anxiolytic-like effects in mice. The expression of the subunits gabra1, gabra2, gabra3, gabrb1, gabrd and gabrg2 in the silver catfish brain were assessed after a 24h-sedation bath by real time PCR. Since qPCR data rarely describes mechanisms of action, which are usually found through interactions with receptors, we also performed an antagonist-driven experiment using flumazenil (FMZ). Real-time PCR detected the mRNA expression of all targeted genes in R. quelen brain. The expression of gabra1 was decreased in fish sedated with ERM; EO increased gabra2, gabra3, gabrb1 and gabrg2 expression; SEL increased gabrb1, gabrd and gabrg2 expression. EO and compounds DFX, SEL and ERM induced sustained sedation in fish and FMZ-bath prompted the recovery from ERM- and DFX-induced sedation. Our results suggest that the EO, SEL, ERM and DFX sedative effects involve interaction with the GABAergic system. Our findings support the use of the silver catfish as robust and reliable experimental model to evaluate the efficacy of drugs with putative GABAergic-mediated effects.

International Union of Basic and Clinical Pharmacology. CVI: GABAA Receptor Subtype- and Function-selective Ligands: Key Issues in Translation to Humans

GABAA receptors are the major inhibitory transmitter receptors in the brain. They are ligand-gated chloride channels and the site of action of benzodiazepines, barbiturates, neuroactive steroids, anesthetics, and convulsants. GABAA receptors are composed of five subunits that can belong to different subunit classes. The existence of 19 homologous subunits and their distinct regional, cellular, and subcellular distribution gives rise to a large number of GABAA receptor subtypes with distinct pharmacology, which modulate different functions of the brain. A variety of compounds have been identified that were claimed to modulate selectively individual GABAA receptor subtypes. However, many of these compounds have only incompletely been investigated or, in addition to a preferential modulation of a receptor subtype, also modulate other subtypes at similar concentrations. Although their differential efficacy at distinct receptor subtypes reduced side effects in behavioral experiments in rodents, the exact receptor subtypes mediating their behavioral effects cannot be unequivocally delineated. In addition, the discrepant in vivo effects of some of these compounds in rodents and man raised doubts on the applicability of the concept of receptor subtype selectivity as a guide for the development of clinically useful drugs. Here, we provide an up-to-date review on the currently available GABAA receptor subtype-selective ligands. We present data on their actual activity at GABAA receptor subtypes, discuss the translational aspect of subtype-selective drugs, and make proposals for the future development of ligands with better anxioselectivity in humans. Finally, we discuss possible ways to strengthen the conclusions of behavioral studies with the currently available drugs.

Evodiamine Reduces Caffeine-Induced Sleep Disturbances and Excitation in Mice

Worldwide, caffeine is among the most commonly used stimulatory substances. Unfortunately, significant caffeine consumption is associated with several adverse effects, ranging from sleep disturbances (including insomnia) to cardiovascular problems. This study investigates whether treatment with the Evodia rutaecarpa aqueous extract (ERAE) from berries and its major molecular component, evodiamine, can reduce the adverse caffeine-induced sleep-related and excitation effects. We combined measurements from the pentobarbital-induced sleep test, the open field test, and the locomotor activity test in mice that had been dosed with caffeine. We found that ERAE and evodiamine administration reduced the degree of caffeine-induced sleep disruption during the sleep test. Additionally, we found that evodiamine significantly inhibits caffeine-induced excitation during the open field test, as well as decreasing hyperlocomotion in the locomotor activity test. Additional in vitro experiments showed that caffeine administration decreased the expression of γ-aminobutyric acid (GABA)_A receptor subunits in the mouse hypothalamus. However, evodiamine treatment significantly reversed this expression reduction. Taken together, our results demonstrate that ERAE and its major compound, evodiamine, provide an excellent candidate for the treatment or prevention of caffeine-induced sleep disturbances and excitatory states, and that the mechanism of these beneficial effects acts, at least in part, through the GABA_A-ergic system.

Nuclear Respiratory Factor 1 (NRF-1) Controls the Activity Dependent Transcription of the GABA-A Receptor Beta 1 Subunit Gene in Neurons

While the exact role of β1 subunit-containing GABA-A receptors (GABARs) in brain function is not well understood, altered expression of the β1 subunit gene (GABRB1) is associated with neurological and neuropsychiatric disorders. In particular, down-regulation of β1 subunit levels is observed in brains of patients with epilepsy, autism, bipolar disorder and schizophrenia. A pathophysiological feature of these disease states is imbalance in energy metabolism and mitochondrial dysfunction. The transcription factor, nuclear respiratory factor 1 (NRF-1), has been shown to be a key mediator of genes involved in oxidative phosphorylation and mitochondrial biogenesis. Using a variety of molecular approaches (including mobility shift, promoter/reporter assays, and overexpression of dominant negative NRF-1), we now report that NRF-1 regulates transcription of GABRB1 and that its core promoter contains a conserved canonical NRF-1 element responsible for sequence specific binding and transcriptional activation. Our identification of GABRB1 as a new target for NRF-1 in neurons suggests that genes coding for inhibitory neurotransmission may be coupled to cellular metabolism. This is especially meaningful as binding of NRF-1 to its element is sensitive to the kind of epigenetic changes that occur in multiple disorders associated with altered brain inhibition.

Neonatal Propofol and Etomidate Exposure Enhance Inhibitory Synaptic Transmission in Hippocampal Cornus Ammonis 1 Pyramidal Neurons

Background:

Propofol and etomidate are the most important intravenous general anesthetics in the current clinical use and that mediate gamma-aminobutyric acid's (GABAergic) synaptic transmission. However, their long-term effects on GABAergic synaptic transmission induced by neonatal propofol or etomidate exposure remain unclear. We investigated the long-term GABAergic neurotransmission alterations, following neonatal propofol and etomidate administration.

Methods:

Sprague-Dawley rat pups at postnatal days 4–6 were underwent 6-h-long propofol-induced or 5-h-long etomidate-induced anesthesia. We performed whole-cell patch-clamp recording from pyramidal cells in the cornus ammonis 1 area of acute hippocampal slices of postnatal 80–90 days. Spontaneous and miniature inhibitory GABAergic currents (spontaneous inhibitory postsynaptic currents [sIPSCs] and miniature inhibitory postsynaptic currents [mIPSCs]) and their kinetic characters were measured. The glutamatergic tonic effect on inhibitory transmission and the effect of bumetanide on neonatal propofol exposure were also examined.

Results:

Neonatal propofol exposure significantly increased the frequency of mIPSCs (from 1.87 ± 0.35 Hz to 3.43 ± 0.51 Hz, P < 0.05) and did not affect the amplitude of mIPSCs and sIPSCs. Both propofol and etomidate slowed the decay time of mIPSCs kinetics (168.39 ± 27.91 ms and 267.02 ± 100.08 ms vs. 68.18 ± 12.43 ms; P < 0.05). Bumetanide significantly blocked the frequency increase and reversed the kinetic alteration of mIPSCs induced by neonatal propofol exposure (3.01 ± 0.45 Hz and 94.30 ± 32.56 ms).

Conclusions:

Neonatal propofol and etomidate exposure has long-term effects on inhibitory GABAergic transmission. Propofol might act at pre- and post-synaptic GABA receptor A (GABA_A) receptors within GABAergic synapses and impairs the glutamatergic tonic input to GABAergic synapses; etomidate might act at the postsynaptic site.

Histaminergic H1 and H2 Receptors Mediate the Effects of Propofol on the Noradrenalin-Inhibited Neurons in Rat Ventrolateral Preoptic Nucleus

The ventrolateral preoptic nucleus is a sleep-promoting nucleus located in the basal forebrain. A commonly used intravenous anesthetic, propofol, had been reported to induce sleep spindles and augment the firing rate of neurons in ventrolateral preoptic nucleus, but the underlining mechanism is yet to be known. By using patch clamp recording on neuron in acute brain slice, present study tested if histaminergic H1 and H2 receptors play a role in the effect of propofol on the noradrenalin-inhibited neurons in ventrolateral preoptic nucleus. We found that the firing rate of noradrenalin-inhibited neurons were significantly augmented by propofol; the frequency of inhibitory postsynaptic currents of noradrenalin-inhibited neuron were evidently attenuated by propofol; such inhibition effect was suppressed by histamine; and both triprolidine (antagonist for H1 histamine receptor) and ranitidine (antagonist for H2 histamine receptor) were able to increase the inhibition rate of propofol in presence of histamine. Present study demonstrated that propofol-induced inhibition of inhibitory postsynaptic currents on noradrenalin-inhibited neurons were mediated by histaminergic H1 and H2 receptors.

Pharmacological characterisation of murine α4β1δ GABAA receptors expressed in Xenopus oocytes

Background

GABA_A receptor subunit composition has a profound effect on the receptor’s physiological and pharmacological properties. The receptor β subunit is widely recognised for its importance in receptor assembly, trafficking and post-translational modifications, but its influence on extrasynaptic GABA_A receptor function is less well understood. Here, we examine the pharmacological properties of a potentially native extrasynaptic GABA_A receptor that incorporates the β1 subunit, specifically composed of α4β1δ and α4β1 subunits.

Results

GABA activated concentration-dependent responses at α4β1δ and α4β1 receptors with EC₅₀ values in the nanomolar to micromolar range, respectively. The divalent cations Zn²⁺ and Cu²⁺, and the β1-selective inhibitor salicylidine salicylhydrazide (SCS), inhibited GABA-activated currents at α4β1δ receptors. Surprisingly the α4β1 receptor demonstrated biphasic sensitivity to Zn²⁺ inhibition that may reflect variable subunit stoichiometries with differing sensitivity to Zn²⁺. The neurosteroid tetrahydro-deoxycorticosterone (THDOC) significantly increased GABA-initiated responses in concentrations above 30 nM for α4β1δ receptors.

Conclusions

With this study we report the first pharmacological characterisation of various GABA_A receptor ligands acting at murine α4β1δ GABA_A receptors, thereby improving our understanding of the molecular pharmacology of this receptor isoform. This study highlights some notable differences in the pharmacology of murine and human α4β1δ receptors. We consider the likelihood that the α4β1δ receptor may play a role as an extrasynaptic GABA_A receptor in the nervous system.

The role of K₂p channels in anaesthesia and sleep

Tandem two-pore potassium channels (K_2Ps) have widespread expression in the central nervous system and periphery where they contribute to background membrane conductance. Some general anaesthetics promote the opening of some of these channels, enhancing potassium currents and thus producing a reduction in neuronal excitability that contributes to the transition to unconsciousness. Similarly, these channels may be recruited during the normal sleep-wake cycle as downstream effectors of wake-promoting neurotransmitters such as noradrenaline, histamine and acetylcholine. These transmitters promote K_2P channel closure and thus an increase in neuronal excitability. Our understanding of the roles of these channels in sleep and anaesthesia has been largely informed by the study of mouse K_2P knockout lines and what is currently predicted by in vitro electrophysiology and channel structure and gating.

Benzodiazepine-site pharmacology on GABAA receptors in histaminergic neurons

BACKGROUND AND PURPOSE

The histaminergic tuberomamillary nucleus (TMN) of the posterior hypothalamus controls the cognitive aspects of vigilance which is reduced by common sedatives and anxiolytics. The receptors targeted by these drugs in histaminergic neurons are unknown. TMN neurons express nine different subunits of the GABAA receptor (GABAA R) with three α- (α1, α2 and α5) and two γ- (γ1, γ 2) subunits, which confer different pharmacologies of the benzodiazepine-binding site.

EXPERIMENTAL APPROACH

We investigated the actions of zolpidem, midazolam, diazepam, chlordiazepoxide, flumazenil (Ro15-1788) and methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate (DMCM) in TMN neurons using mouse genetics, electrophysiological and molecular biological methods.

KEY RESULTS

We find the sensitivity of GABAA R to zolpidem, midazolam and DMCM significantly reduced in TMN neurons from γ2F77I mice, but modulatory activities of diazepam, chlordiazepoxide and flumazenil not affected. Potencies and efficacies of these compounds are in line with the dominance of α2- and α1-subunit containing receptors associated with γ2- or γ1-subunits. Functional expression of the γ1-subunit is supported by siRNA-based knock-down experiments in γ2F77I mice.

CONCLUSIONS AND IMPLICATIONS

GABAA R of TMN neurons respond to a variety of common sedatives with a high affinity binding site (γ2F77I) involved. The γ1-subunit likely contributes to the action of common sedatives in TMN neurons. This study is relevant for understanding the role of neuronal histamine and benzodiazepines in disorders of sleep and metabolism.

Sergeeva O. Partial agonism of taurine at gamma-containing native and recombinant GABAA receptors

Taurine is a semi-essential sulfonic acid found at high concentrations in plasma and mammalian tissues which regulates osmolarity, ion channel activity and glucose homeostasis. The structural requirements of GABA_A-receptors (GABA_AR) gated by taurine are not yet known. We determined taurine potency and efficacy relative to GABA at different types of recombinant GABA_AR occurring in central histaminergic neurons of the mouse hypothalamic tuberomamillary nucleus (TMN) which controls arousal. At binary α_1/2β_1/3 receptors taurine was as efficient as GABA, whereas incorporation of the γ_1/2 subunit reduced taurine efficacy to 60–90% of GABA. The mutation γ_2F77I, which abolishes zolpidem potentiation, significantly reduced taurine efficacy at recombinant and native receptors compared to the wild type controls. As taurine was a full- or super- agonist at recombinant α_xβ₁δ-GABA_AR, we generated a chimeric γ₂ subunit carrying the δ subunit motif around F77 (MTVFLH). At α_1/2β₁γ_2(MTVFLH) receptors taurine became a super-agonist, similar to δ-containing ternary receptors, but remained a partial agonist at β₃-containing receptors. In conclusion, using site-directed mutagenesis we found structural determinants of taurine’s partial agonism at γ-containing GABA_A receptors. Our study sheds new light on the β₁ subunit conferring the widest range of taurine-efficacies modifying GABA_AR function under (patho)physiological conditions.

GABAergic transmission in hepatic encephalopathy

Hepatic encephalopathy (HE)(1) is a neuropsychiatric disorder caused by chronic or acute liver failure. Nearly thirty years ago a hypothesis was formulated explaining the neuropathology of HE by increased GABAergic tone. Recent progress in the GABAA-receptor (GABAAR) molecular pharmacology and biochemistry as well as the physiology of GABAergic transmission provided better understanding of GABA's role in health and disease. A detailed analysis of neuronal populations and their GABAergic afferents affected in HE is still missing. The slow progress in understanding the pathology of GABAergic transmission in HE is due to the high complexity of brain circuitries controlled by multiple types of GABAergic interneurons and the large variety of GABAAR, which are differently affected by pathological conditions and not yet fully identified. The mechanisms of action of the GABAAR agonist taurine, allosteric positive modulators (inhibitory neurosteroids, anaesthetics, benzodiazepines and histamine) and inhibitors of the GABAAR (excitatory neurosteroids, Ro15-4513) are discussed with respect to HE pathophysiology. Perspectives for GABAergic drugs in the symptomatic treatment of HE are suggested.

GABAergic inhibition of histaminergic neurons regulates active waking but not the sleep-wake switch or propofol-induced loss of consciousness

The activity of histaminergic neurons in the tuberomammillary nucleus (TMN) of the hypothalamus correlates with an animal's behavioral state and maintains arousal. We examined how GABAergic inputs onto histaminergic neurons regulate this behavior. A prominent hypothesis, the "flip-flop" model, predicts that increased and sustained GABAergic drive onto these cells promotes sleep. Similarly, because of the histaminergic neurons' key hub-like place in the arousal circuitry, it has also been suggested that anesthetics such as propofol induce loss of consciousness by acting primarily at histaminergic neurons. We tested both these hypotheses in mice by genetically removing ionotropic GABA(A) or metabotropic GABA(B) receptors from histidine decarboxylase-expressing neurons. At the cellular level, histaminergic neurons deficient in synaptic GABA(A) receptors were significantly more excitable and were insensitive to the anesthetic propofol. At the behavioral level, EEG profiles were recorded in nontethered mice over 24 h. Surprisingly, GABAergic transmission onto histaminergic neurons had no effect in regulating the natural sleep-wake cycle and, in the case of GABA(A) receptors, for propofol-induced loss of righting reflex. The latter finding makes it unlikely that the histaminergic TMN has a central role in anesthesia. GABA(B) receptors on histaminergic neurons were dispensable for all behaviors examined. Synaptic inhibition of histaminergic cells by GABA(A) receptors, however, was essential for habituation to a novel environment.

A novel GABA(A) receptor pharmacology: drugs interacting with the α(+) β(-) interface

GABA(A) receptors are ligand-gated chloride channels composed of five subunits that can belong to different subunit classes. The existence of 19 different subunits gives rise to a multiplicity of GABA(A) receptor subtypes with distinct subunit composition; regional, cellular and subcellular distribution; and pharmacology. Most of these receptors are composed of two α, two β and one γ2 subunits. GABA(A) receptors are the site of action of a variety of pharmacologically and clinically important drugs, such as benzodiazepines, barbiturates, neuroactive steroids, anaesthetics and convulsants. Whereas GABA acts at the two extracellular β(+) α(-) interfaces of GABA(A) receptors, the allosteric modulatory benzodiazepines interact with the extracellular α(+) γ2(-) interface. In contrast, barbiturates, neuroactive steroids and anaesthetics seem to interact with solvent accessible pockets in the transmembrane domain. Several benzodiazepine site ligands have been identified that selectively interact with GABA(A) receptor subtypes containing α2βγ2, α3βγ2 or α5βγ2 subunits. This indicates that the different α subunit types present in these receptors convey sufficient structural differences to the benzodiazepine binding site to allow specific interaction with certain benzodiazepine site ligands. Recently, a novel drug binding site was identified at the α(+) β(-) interface. This binding site is homologous to the benzodiazepine binding site at the α(+) γ2(-) interface and is thus also strongly influenced by the type of α subunit present in the receptor. Drugs interacting with this binding site cannot directly activate but only allosterically modulate GABA(A) receptors. The possible importance of such drugs addressing a spectrum of receptor subtypes completely different from that of benzodiazepines is discussed.

α4βδ GABA(A) receptors are high-affinity targets for γ-hydroxybutyric acid (GHB)

γ-Hydroxybutyric acid (GHB) binding to brain-specific high-affinity sites is well-established and proposed to explain both physiological and pharmacological actions. However, the mechanistic links between these lines of data are unknown. To identify molecular targets for specific GHB high-affinity binding, we undertook photolinking studies combined with proteomic analyses and identified several GABA(A) receptor subunits as possible candidates. A subsequent functional screening of various recombinant GABA(A) receptors in Xenopus laevis oocytes using the two-electrode voltage clamp technique showed GHB to be a partial agonist at αβδ- but not αβγ-receptors, proving that the δ-subunit is essential for potency and efficacy. GHB showed preference for α4 over α(1,2,6)-subunits and preferably activated α4β1δ (EC(50) = 140 nM) over α4β(2/3)δ (EC(50) = 8.41/1.03 mM). Introduction of a mutation, α4F71L, in α4β1(δ)-receptors completely abolished GHB but not GABA function, indicating nonidentical binding sites. Radioligand binding studies using the specific GHB radioligand [(3)H](E,RS)-(6,7,8,9-tetrahydro-5-hydroxy-5H-benzocyclohept-6-ylidene)acetic acid showed a 39% reduction (P = 0.0056) in the number of binding sites in α4 KO brain tissue compared with WT controls, corroborating the direct involvement of the α4-subunit in high-affinity GHB binding. Our data link specific GHB forebrain binding sites with α4-containing GABA(A) receptors and postulate a role for extrasynaptic α4δ-containing GABA(A) receptors in GHB pharmacology and physiology. This finding will aid in elucidating the molecular mechanisms behind the proposed function of GHB as a neurotransmitter and its unique therapeutic effects in narcolepsy and alcoholism.

Waking action of ursodeoxycholic acid (UDCA) involves histamine and GABAA receptor block

Since ancient times ursodeoxycholic acid (UDCA), a constituent of bile, is used against gallstone formation and cholestasis. A neuroprotective action of UDCA was demonstrated recently in models of Alzheimer's disease and retinal degeneration. The mechanisms of UDCA action in the nervous system are poorly understood. We show now that UDCA promotes wakefulness during the active period of the day, lacking this activity in histamine-deficient mice. In cultured hypothalamic neurons UDCA did not affect firing rate but synchronized the firing, an effect abolished by the GABA_AR antagonist gabazine. In histaminergic neurons recorded in slices UDCA reduced amplitude and duration of spontaneous and evoked IPSCs. In acutely isolated histaminergic neurons UDCA inhibited GABA-evoked currents and sIPSCs starting at 10 µM (IC₅₀ = 70 µM) and did not affect NMDA- and AMPA-receptor mediated currents at 100 µM. Recombinant GABA_A receptors composed of α1, β1–3 and γ2L subunits expressed in HEK293 cells displayed a sensitivity to UDCA similar to that of native GABA_A receptors. The mutation α1V256S, known to reduce the inhibitory action of pregnenolone sulphate, reduced the potency of UDCA. The mutation α1Q241L, which abolishes GABA_AR potentiation by several neurosteroids, had no effect on GABA_AR inhibition by UDCA. In conclusion, UDCA enhances alertness through disinhibition, at least partially of the histaminergic system via GABA_A receptors.

Proton- and ammonium-sensing by histaminergic neurons controlling wakefulness

The histaminergic neurons in the tuberomamillary nucleus (TMN) of the posterior hypothalamus are involved in the control of arousal. These neurons are sensitive to hypercapnia as has been shown in experiments examining c-Fos expression, a marker for increased neuronal activity. We investigated the mechanisms through which TMN neurons respond to changes in extracellular levels of acid/CO₂. Recordings in rat brain slices revealed that acidification within the physiological range (pH from 7.4 to 7.0), as well as ammonium chloride (5 mM), excite histaminergic neurons. This excitation is significantly reduced by antagonists of type I metabotropic glutamate receptors and abolished by benzamil, an antagonist of acid-sensing ion channels (ASICs) and Na⁺/Ca²⁺ exchanger, or by ouabain which blocks Na⁺/K⁺ ATPase. We detected variable combinations of 4 known types of ASICs in single TMN neurons, and observed activation of ASICs in single dissociated TMN neurons only at pH lower than 7.0. Thus, glutamate, which is known to be released by glial cells and orexinergic neurons, amplifies the acid/CO₂-induced activation of TMN neurons. This amplification demands the coordinated function of metabotropic glutamate receptors, Na⁺/Ca²⁺ exchanger and Na⁺/K⁺ ATPase. We also developed a novel HDC-Cre transgenic reporter mouse line in which histaminergic TMN neurons can be visualized. In contrast to the rat, the mouse histaminergic neurons lacked the pH 7.0-induced excitation and displayed only a minimal response to the mGluR I agonist DHPG (0.5 μM). On the other hand, ammonium-induced excitation was similar in mouse and rat. These results are relevant for the understanding of the neuronal mechanisms controlling acid/CO₂-induced arousal in hepatic encephalopathy and obstructive sleep apnoea. Moreover, the new HDC-Cre mouse model will be a useful tool for studying the physiological and pathophysiological roles of the histaminergic system.

Receptor and channel heteromers as pain targets

Recent discoveries indicate that many G-protein coupled receptors (GPCRs) and channels involved in pain modulation are able to form receptor heteromers. Receptor and channel heteromers often display distinct signaling characteristics, pharmacological properties and physiological function in comparison to monomer/homomer receptor or ion channel counterparts. It may be possible to capitalize on such unique properties to augment therapeutic efficacy while minimizing side effects. For example, drugs specifically targeting heteromers may have greater tissue specificity and analgesic efficacy. This review will focus on current progress in our understanding of roles of heteromeric GPCRs and channels in pain pathways as well as strategies for controlling pain pathways via targeting heteromeric receptors and channels. This approach may be instrumental in the discovery of novel classes of drugs and expand our repertoire of targets for pain pharmacotherapy.