The Evolution of GABAA Receptor–Like Genes

Evaluation of diazepam as a drug treatment for water hemlock (Cicuta species) poisoning in Spanish goats

Water hemlocks (Cicuta spp.) are toxic members of the Apiaceae plant family. The best drug treatment for the convulsions associated with acute water hemlock poisoning in livestock and humans has not been determined experimentally. This work compared the therapeutic actions of benzodiazepines (diazepam) and barbiturates (phenobarbital) on water hemlock poisoning in a goat model. C. maculata tubers were orally dosed to goats. Experimental groups consisted of; control saline; 20 mg/kg phenobarbital; 1.0 mg/kg diazepam; 10 mg/kg diazepam; and 1.0 mg/kg diazepam administered as needed to moderate convulsions by intravenous (i.v.) infusion. Diazepam provided nearly instant control of convulsions. Clinical signs of poisoning were completely controlled for the duration of the experiment in the goats that received the 10 mg/kg diazepam dose. These results suggest that diazepam is effective at managing the clinical signs of water hemlock poisoning in goats. We speculate that diazepam can be used as a potential treatment for water hemlock poisoning in other livestock species and humans.

The role of ligand-gated chloride channels in behavioural alterations at elevated CO2 in a cephalopod

Projected future carbon dioxide (CO2) levels in the ocean can alter marine animal behaviours. Disrupted functioning of γ-aminobutyric acid type A (GABAA) receptors (ligand-gated chloride channels) is suggested to underlie CO2-induced behavioural changes in fish. However, the mechanisms underlying behavioural changes in marine invertebrates are poorly understood. We pharmacologically tested the role of GABA-, glutamate-, acetylcholine- and dopamine-gated chloride channels in CO2-induced behavioural changes in a cephalopod, the two-toned pygmy squid (Idiosepius pygmaeus). We exposed squid to ambient (∼450 µatm) or elevated (∼1000 µatm) CO2 for 7 days. Squid were treated with sham, the GABAA receptor antagonist gabazine or the non-specific GABAA receptor antagonist picrotoxin, before measurement of conspecific-directed behaviours and activity levels upon mirror exposure. Elevated CO2 increased conspecific-directed attraction and aggression, as well as activity levels. For some CO2-affected behaviours, both gabazine and picrotoxin had a different effect at elevated compared with ambient CO2, providing robust support for the GABA hypothesis within cephalopods. In another behavioural trait, picrotoxin but not gabazine had a different effect in elevated compared with ambient CO2, providing the first pharmacological evidence, in fish and marine invertebrates, for altered functioning of ligand-gated chloride channels, other than the GABAAR, underlying CO2-induced behavioural changes. For some other behaviours, both gabazine and picrotoxin had a similar effect in elevated and ambient CO2, suggesting altered function of ligand-gated chloride channels was not responsible for these CO2-induced changes. Multiple mechanisms may be involved, which could explain the variability in the CO2 and drug treatment effects across behaviours.

Predator prey interactions between predatory gastropod Reishia clavigera, barnacle Amphibalanusamphitriteamphitrite and mussel Brachidontesvariabilis under ocean acidification

Since the response to ocean acidification is species specific, differences in responses between predator and prey will alter their interactions, hence affect the population dynamics of both species. Changes in predator prey interactions between a predatory muricid gastropod Reishia clavigera and its prey, the barnacle Amphibalanus amphitrite amphitrite and mussel Brachidontes variabilis under three pCO₂ levels (380, 950, and 1250 μatm) were investigated. The searching time for barnacles increased and the ability to locate them decreased at higher pCO₂ levels. The movement speed and the prey consumption rate, however, were independent of pCO₂. There was no preference towards either B. variabilis or A. amphitrite amphitrite regardless of pCO₂. Exposure experiments involving multiple generations are suggested to assess transgenerational effects of ocean acidification and the potential compensation responses before any realistic predictions on the long term changes of population dynamics of the interacting species can be made.

Molecular Architecture of Genetically-Tractable GABA Synapses in C. elegans

Inhibitory synapses represent a minority of the total chemical synapses in the mammalian brain, yet proper tuning of inhibition is fundamental to shape neuronal network properties. The neurotransmitter γ-aminobutyric acid (GABA) mediates rapid synaptic inhibition by the activation of the type A GABA receptor (GABA_AR), a pentameric chloride channel that governs major inhibitory neuronal transduction in the nervous system. Impaired GABA transmission leads to a variety of neuropsychiatric diseases, including schizophrenia, autism, epilepsy or anxiety. From an evolutionary perspective, GABA_AR shows remarkable conservations, and are found in all eukaryotic clades and even in bacteria and archaea. Specifically, bona fide GABA_ARs are found in the nematode Caenorhabditis elegans. Because of the anatomical simplicity of the nervous system and its amenability to genetic manipulations, C. elegans provide a powerful system to investigate the molecular and cellular biology of GABA synapses. In this mini review article, we will introduce the structure of the C. elegans GABAergic system and describe recent advances that have identified novel proteins controlling the localization of GABA_ARs at synapses. In particular, Ce-Punctin/MADD-4 is an evolutionarily-conserved extracellular matrix protein that behaves as an anterograde synaptic organizer to instruct the excitatory or inhibitory identity of postsynaptic domains.

Neural effects of elevated CO2 in fish may be amplified by a vicious cycle

Maladaptive behavioural disturbances have been reported in some fishes and aquatic invertebrates exposed to projected future CO₂ levels. These disturbances have been linked to altered ion gradients and neurotransmitter function in the brain. Still, it seems surprising that the relatively small ionic changes induced by near-future CO₂ levels can have such profound neural effects. Based on recent transcriptomics data, we propose that a vicious cycle can be triggered that amplifies the initial disturbance, explaining how small pH regulatory adjustments in response to ocean acidification can lead to major behavioural alterations in fish and other water-breathing animals. The proposed cycle is initiated by a reversal of the function of some inhibitory GABA_A receptors in the direction of neural excitation and then amplified by adjustments in gene expression aimed at suppressing the excitation but in reality increasing it. In addition, the increased metabolic production of CO₂ by overexcited neurons will feed into the cycle by elevating intracellular bicarbonate levels that will lead to increased excitatory ion fluxes through GABA_A receptors. We also discuss the possibility that an initiation of a vicious cycle could be one of the several factors underlying the differences in neural sensitivity to elevated CO₂ displayed by fishes.

Origin and Evolution of the Neuroendocrine Control of Reproduction in Vertebrates, With Special Focus on Genome and Gene Duplications

In humans, as in the other mammals, the neuroendocrine control of reproduction is ensured by the brain-pituitary gonadotropic axis. Multiple internal and environmental cues are integrated via brain neuronal networks, ultimately leading to the modulation of the activity of gonadotropin-releasing hormone (GnRH) neurons. The decapeptide GnRH is released into the hypothalamic-hypophysial portal blood system and stimulates the production of pituitary glycoprotein hormones, the two gonadotropins luteinizing hormone and follicle-stimulating hormone. A novel actor, the neuropeptide kisspeptin, acting upstream of GnRH, has attracted increasing attention in recent years. Other neuropeptides, such as gonadotropin-inhibiting hormone/RF-amide related peptide, and other members of the RF-amide peptide superfamily, as well as various nonpeptidic neuromediators such as dopamine and serotonin also provide a large panel of stimulatory or inhibitory regulators. This paper addresses the origin and evolution of the vertebrate gonadotropic axis. Brain-pituitary neuroendocrine axes are typical of vertebrates, the pituitary gland, mediator and amplifier of brain control on peripheral organs, being a vertebrate innovation. The paper reviews, from molecular and functional perspectives, the evolution across vertebrate radiation of some key actors of the vertebrate neuroendocrine control of reproduction and traces back their origin along the vertebrate lineage and in other metazoa before the emergence of vertebrates. A focus is given on how gene duplications, resulting from either local events or from whole genome duplication events, and followed by paralogous gene loss or conservation, might have shaped the evolutionary scenarios of current families of key actors of the gonadotropic axis.

Positive allosteric modulators of nonbenzodiazepine γ-aminobutyric acidA receptor subtypes for the treatment of chronic pain

Chronic neuropathic pain may be caused, in part, by loss of inhibition in spinal pain processing pathways due to attenuation of local GABAergic tone. Nociception and nocifensive behaviors are reduced after enhancement of tonically activated extrasynaptic GABAAR-mediated currents by agonist ligands for δ subunit-containing GABAARs. However, typical ligands that target δ subunit-containing GABAARs are limited due to sedative effects at higher doses. We used the spinal nerve ligation (SNL) and gp120 models of experimental neuropathic pain to evaluate compound 2-261, a nonbenzodiazepine site positive allosteric modulator of α4β3δ GABAARs optimized to be nonsedative by selective activation of β2/3-subunit-containing GABAARs over receptor subtypes incorporating β1 subunits. Similar levels of 2-261 were detected in the brain and plasma after intraperitoneal administration. Although systemic 2-261 did not alter sensory thresholds in sham-operated animals, it significantly reversed SNL-induced thermal and tactile hypersensitivity in a GABAAR-dependent fashion. Intrathecal 2-261 produced conditioned place preference and elevated dopamine levels in the nucleus accumbens of nerve-injured, but not sham-operated, rats. In addition, systemic pretreatment with 2-261 blocked conditioned place preference from spinal clonidine in SNL rats. Moreover, 2-261 reversed thermal hyperalgesia and partially reversed tactile allodynia in the gp120 model of HIV-related neuropathic pain. The effects of 2-261 likely required interaction with the α4β3δ GABAAR because 2-301, a close structural analog of 2-261 with limited extrasynaptic receptor efficacy, was not active. Thus, 2-261 may produce pain relief with diminished side effects through selective modulation of β2/3-subunit-containing extrasynaptic GABAARs.

Human pharmacology of positive GABA-A subtype-selective receptor modulators for the treatment of anxiety

Anxiety disorders arise from disruptions among the highly interconnected circuits that normally serve to process the streams of potentially threatening stimuli. The resulting imbalance among these circuits can cause a fundamental misinterpretation of neural sensory information as threatening and can lead to the inappropriate emotional and behavioral responses observed in anxiety disorders. There is considerable preclinical evidence that the GABAergic system, in general, and its α2- and/or α5-subunit-containing GABA(A) receptor subtypes, in particular, are involved in the pathophysiology of anxiety disorders. However, the clinical efficacy of GABA-A α2-selective agonists for the treatment of anxiety disorders has not been unequivocally demonstrated. In this review, we present several human pharmacological studies that have been performed with the aim of identifying the pharmacologically active doses/exposure levels of several GABA-A subtype-selective novel compounds with potential anxiolytic effects. The pharmacological selectivity of novel α2-subtype-selective GABA(A) receptor partial agonists has been demonstrated by their distinct effect profiles on the neurophysiological and neuropsychological measurements that reflect the functions of multiple CNS domains compared with those of benzodiazepines, which are nonselective, full GABA(A) agonists. Normalizing the undesired pharmacodynamic side effects against the desired on-target effects on the saccadic peak velocity is a useful approach for presenting the pharmacological features of GABA(A)-ergic modulators. Moreover, combining the anxiogenic symptom provocation paradigm with validated neurophysiological and neuropsychological biomarkers may provide further construct validity for the clinical effects of novel anxiolytic agents. In addition, the observed drug effects on serum prolactin levels support the use of serum prolactin levels as a complementary neuroendocrine biomarker to further validate the pharmacodynamic measurements used during the clinical pharmacological study of novel anxiolytic agents.

Expression of the eight GABAA receptor α subunits in the developing zebrafish central nervous system

GABA is a robust regulator of both developing and mature neural networks. It exerts many of its effects through GABA_A receptors, which are heteropentamers assembled from a large array of subunits encoded by distinct genes. In mammals, there are 19 different GABA_A subunit types, which are divided into the α, β, γ, δ, ε, π, θ and ρ subfamilies. The immense diversity of GABA_A receptors is not fully understood. However, it is known that specific isoforms, with their distinct biophysical properties and expression profiles, tune responses to GABA. Although larval zebrafish are well-established as a model system for neural circuit analysis, little is known about GABA_A receptors diversity and expression in this system. Here, using database analysis, we show that the zebrafish genome contains at least 23 subunits. All but the mammalian θ and ε subunits have at least one zebrafish ortholog, while five mammalian GABA_A receptor subunits have two zebrafish orthologs. Zebrafish contain one subunit, β4, which does not have a clear mammalian ortholog. Similar to mammalian GABA_A receptors, the zebrafish α subfamily is the largest and most diverse of the subfamilies. In zebrafish there are eight α subunits, and RNA in situ hybridization across early zebrafish development revealed that they demonstrate distinct patterns of expression in the brain, spinal cord, and retina. Some subunits were very broadly distributed, whereas others were restricted to small populations of cells. Subunit-specific expression patterns in zebrafish resembled were those found in frogs and rodents, which suggests that the roles of different GABA_A receptor isoforms are largely conserved among vertebrates. This study provides a platform to examine isoform specific roles of GABA_A receptors within zebrafish neural circuits and it highlights the potential of this system to better understand the remarkable heterogeneity of GABA_A receptors.

Predator-prey interaction between muricid gastropods and mussels under ocean acidification

Predation of the muricid gastropod Thais clavigera on two-sized groups of the mussel Brachidontes variabilis was studied under three pCO₂ levels, 380, 950, and 1250μatm. At 950μatm pCO₂ level, the prey handling time decreased significantly and large-sized B. variabilis were preferred by T. clavigera. However, the prey consumption rate was independent of pCO₂ levels, although the prey searching time increased significantly at elevated pCO₂. These findings indicated that the predator-prey interaction between T. clavigera and B. variabilis was altered under ocean acidification, which will have a long-term impact on the population dynamics of the interacting species.

Global Proteome Profiling of a Marine Copepod and the Mitigating Effect of Ocean Acidification on Mercury Toxicity after Multigenerational Exposure

Previously, we found that ocean acidification (OA) mitigates mercury (Hg) toxicity to marine copepod Tigriopus japonicus under multigenerational exposure (four generations, F0-F3). To determine the response mechanisms of T. japonicus against long-term exposure to OA and Hg pollution, we investigated the proteome of F3 copepods after multigenerational exposure to four conditions: pCO₂ 400 μatm + control; pCO₂ 1000 μatm + control; pCO₂ 400 μatm +1.0 μg/L Hg; and pCO₂ 1000 μatm +1.0 μg/L Hg. Functional enrichment analysis indicated that OA enhanced the copepod's energy production mainly by increasing protein assimilation and proteolysis as a compensatory strategy, which explained its physiological resilience to reduced pH. Conversely, Hg treatment decreased many critical processes, including ferric iron binding, antioxidant activity, cellular homeostasis, and glutathione metabolism, and these toxic events could translate into higher-level responses, i.e., restrained reproduction in copepods. Importantly, the mediation of Hg toxicity in T. japonicus by OA could be explained by the enhanced lysosome-autophagy pathway proteomes that are responsible for repairing and removing damaged proteins and enzymes under stress. Overall, this study provided molecular insights into the response of T. japonicus to long-term exposure of OA and Hg, with a particular emphasis on the mitigating impact of the CO₂-driven acidification on Hg toxicity.

Home alone-The effects of isolation on uptake of a pharmaceutical contaminant in a social fish

A wide range of biologically active pharmaceutical residues is present in aquatic systems worldwide. As uptake potential and the risk of effects in aquatic wildlife are directly coupled, the aim of this study was to investigate the relationships between stress by isolation, uptake and effects of the psychiatric pharmaceutical oxazepam in fish. To do this, we measured cortisol levels, behavioral stress responses, and oxazepam uptake under different stress and social conditions, in juvenile perch (Perca fluviatilis) that were either exposed (1.03μgl^-1) or not exposed to oxazepam. We found single exposed individuals to take up more oxazepam than individuals exposed in groups, likely as a result of stress caused by isolation. Furthermore, the bioconcentration factor (BCF) was significantly negatively correlated with fish weight in both social treatments. We found no effect of oxazepam exposure on body cortisol concentration or behavioral stress response. Most laboratory experiments, including standardized bioconcentration assays, are designed to minimize stress for the test organisms, however wild animals experience stress naturally. Hence, differences in stress levels between laboratory and natural environments can be one of the reasons why predictions from artificial laboratory experiments largely underestimate uptake of oxazepam, and other pharmaceuticals, in the wild.

A cellular and regulatory map of the GABAergic nervous system of C. elegans

Neurotransmitter maps are important complements to anatomical maps and represent an invaluable resource to understand nervous system function and development. We report here a comprehensive map of neurons in the C. elegans nervous system that contain the neurotransmitter GABA, revealing twice as many GABA-positive neuron classes as previously reported. We define previously unknown glia-like cells that take up GABA, as well as 'GABA uptake neurons' which do not synthesize GABA but take it up from the extracellular environment, and we map the expression of previously uncharacterized ionotropic GABA receptors. We use the map of GABA-positive neurons for a comprehensive analysis of transcriptional regulators that define the GABA phenotype. We synthesize our findings of specification of GABAergic neurons with previous reports on the specification of glutamatergic and cholinergic neurons into a nervous system-wide regulatory map which defines neurotransmitter specification mechanisms for more than half of all neuron classes in C. elegans.

DOI:http://dx.doi.org/10.7554/eLife.17686.001

Immunochemical Localization of GABAA Receptor Subunits in the Freshwater Polyp Hydra vulgaris (Cnidaria, Hydrozoa)

γ-aminobutyric acid (GABA) receptors, responding to GABA positive allosteric modulators, are present in the freshwater polyp Hydra vulgaris (Cnidaria, Hydrozoa), one of the most primitive metazoans to develop a nervous system. We examined the occurrence and distribution of GABA_A receptor subunits in Hydra tissues by western blot and immunohistochemistry. Antibodies against different GABA_A receptor subunits were used in Hydra membrane preparations. Unique protein bands, inhibited by the specific peptide, appeared at 35, 60, ∼50 and ∼52 kDa in membranes incubated with α3, β1, γ3 or δ antibodies, respectively. Immunohistochemical screening of whole mount Hydra preparations revealed diffuse immunoreactivity to α3, β1 or γ3 antibodies in tentacles, hypostome, and upper part of the gastric region; immunoreactive fibers were also present in the lower peduncle. By contrast, δ antibodies revealed a strong labeling in the lower gastric region and peduncle, as well as in tentacles. Double labeling showed colocalization of α3/β1, α3/γ3 and α3/δ immunoreactivity in granules or cells in tentacles and gastric region. In the peduncle, colocalization of both α3/β1 and α3/γ3 immunoreactivity was found in fibers running horizontally above the foot. These data indicate that specific GABA_A receptor subunits are present and differentially distributed in Hydra body regions. Subunit colocalization suggests that Hydra GABA receptors are heterologous multimers, possibly sub-serving different physiological activities.

Effect of bioconcentration and trophic transfer on realized exposure to oxazepam in 2 predators, the dragonfly larvae (Aeshna grandis) and the Eurasian perch (Perca fluviatilis)

Psychoactive substances are used worldwide and constitute one of the most common groups of pharmaceutical contaminants in surface waters. Although these pharmaceuticals are designed to be efficiently eliminated from the human body, very little is known about their trophic-transfer potential in aquatic wildlife. Therefore, the goal of the present study was to quantify and compare uptake of an anxiolytic (oxazepam) from water (bioconcentration) and via the consumption of contaminated diet (trophic transfer) in 2 common freshwater predators: Eurasian perch (Perca fluviatilis) and the dragonfly larvae Aeshna grandis. Bioconcentration and trophic transfer of oxazepam were found in both predator species. However, higher bioconcentrations were observed for perch (bioconcentration factor [BCF], 3.7) than for dragonfly larvae (BCF, 0.5). Perch also retained more oxazepam from consumed prey (41%) than dragonfly larvae (10%), whereas the relative contribution via prey consumption was 14% and 42% for perch and dragonflies, respectively. In addition, bioconcentration was negatively correlated with perch weight, indicating that exposure levels in natural contaminated environments differ between individuals of different size or between different developmental stages. Hence, trophic transfer of pharmaceuticals may indeed occur, and estimates of environmental exposures that do not consider intake via food or size-dependent bioconcentration may therefore lead to wrongful estimations of realized exposure levels in natural contaminated ecosystems.

Ocean acidification impairs crab foraging behaviour

Anthropogenic elevation of atmospheric CO2 is driving global-scale ocean acidification, which consequently influences calcification rates of many marine invertebrates and potentially alters their susceptibility to predation. Ocean acidification may also impair an organism's ability to process environmental and biological cues. These counteracting impacts make it challenging to predict how acidification will alter species interactions and community structure. To examine effects of acidification on consumptive and behavioural interactions between mud crabs (Panopeus herbstii) and oysters (Crassostrea virginica), oysters were reared with and without caged crabs for 71 days at three pCO2 levels. During subsequent predation trials, acidification reduced prey consumption, handling time and duration of unsuccessful predation attempt. These negative effects of ocean acidification on crab foraging behaviour more than offset any benefit to crabs resulting from a reduction in the net rate of oyster calcification. These findings reveal that efforts to evaluate how acidification will alter marine food webs should include quantifying impacts on both calcification rates and animal behaviour.

Deep mRNA sequencing of the Tritonia diomedea brain transcriptome provides access to gene homologues for neuronal excitability, synaptic transmission and peptidergic signalling

BACKGROUND

The sea slug Tritonia diomedea (Mollusca, Gastropoda, Nudibranchia), has a simple and highly accessible nervous system, making it useful for studying neuronal and synaptic mechanisms underlying behavior. Although many important contributions have been made using Tritonia, until now, a lack of genetic information has impeded exploration at the molecular level.

RESULTS

We performed Illumina sequencing of central nervous system mRNAs from Tritonia, generating 133.1 million 100 base pair, paired-end reads. De novo reconstruction of the RNA-Seq data yielded a total of 185,546 contigs, which partitioned into 123,154 non-redundant gene clusters (unigenes). BLAST comparison with RefSeq and Swiss-Prot protein databases, as well as mRNA data from other invertebrates (gastropod molluscs: Aplysia californica, Lymnaea stagnalis and Biomphalaria glabrata; cnidarian: Nematostella vectensis) revealed that up to 76,292 unigenes in the Tritonia transcriptome have putative homologues in other databases, 18,246 of which are below a more stringent E-value cut-off of 1x10-6. In silico prediction of secreted proteins from the Tritonia transcriptome shotgun assembly (TSA) produced a database of 579 unique sequences of secreted proteins, which also exhibited markedly higher expression levels compared to other genes in the TSA.

CONCLUSIONS

Our efforts greatly expand the availability of gene sequences available for Tritonia diomedea. We were able to extract full length protein sequences for most queried genes, including those involved in electrical excitability, synaptic vesicle release and neurotransmission, thus confirming that the transcriptome will serve as a useful tool for probing the molecular correlates of behavior in this species. We also generated a neurosecretome database that will serve as a useful tool for probing peptidergic signalling systems in the Tritonia brain.

The impact of genetic factors on response to anaesthetics

In recent years, exceptional progress has been observed in pharmacogenetics, i.e. investigations of inherited conditioning of the organism's response to drugs or xenobiotics. On the other hand, modern molecular biology techniques have been implemented, making it possible to perform studies determining the involvement of genetic factors in differing responses to agents employed in general anaesthesia. Unexpected and incorrect response of the organism to the administration of specific anaesthetics is most commonly associated with a genetic defect of the metabolic pathway of a given agent or its receptor. The majority of agents used in anaesthesia are metabolised in the liver by the cytochrome P450 superfamily enzymes (CYPs) and phase II drug-metabolising enzymes: glutathione S-transferases (GSTs), sulphotransferases (SULTs), UDP-glucuronosyltransferases (UGTs) and NAD(P)H:quinone oxidoreductase (NQO1). Propofol is presently widely used for gastrointestinal (GI) and several other procedures. Among genes associated with metabolism of the most commonly applied anaesthetics such as propofol and sevoflurane, the following ones can be mentioned: CYP2E1, CYP2B6, CYP2C9, GSTP1, UGT1A9, SULT1A1 and NQO1. Moreover, the basic mechanism of propofol action involves its interaction with an ionotropic receptor GABAA inhibiting transfer of nerve impulses. Molecular studies have shown that polymorphic changes in GABRG2 receptor gene turn out to be important in the propofol anaesthesia. Planning of optimal anaesthesia can be considerably assisted by the determination of genetic factors of prognostic value taking advantage of genotyping and making it possible to select anaesthetics and reduce risk of side effects as well as undesirable actions.

Evolution of neurotransmitter gamma-aminobutyric acid, glutamate and their receptors

Gamma-aminobutyric acid (GABA) and glutamate are two important amino acid neurotransmitters widely present in the nervous systems of mammals, insects, round worm, and platyhelminths, while their receptors are quite diversified across different animal phyla. However, the evolutionary mechanisms between the two conserved neurotransmitters and their diversified receptors remain elusive, and antagonistic interactions between GABA and glutamate signal transduction systems, in particular, have begun to attract significant attention. In this review, we summarize the extant results on the origin and evolution of GABA and glutamate, as well as their receptors, and analyze possible evolutionary processes and phylogenetic relationships of various GABAs and glutamate receptors. We further discuss the evolutionary history of Excitatory/Neutral Amino Acid Transporter (EAAT), a transport protein, which plays an important role in the GABA-glutamate "yin and yang" balanced regulation. Finally, based on current advances, we propose several potential directions of future research.

A factor analysis of global GABAergic gene expression in human brain identifies specificity in response to chronic alcohol and cocaine exposure

Although expression patterns of GABAergic genes in rodent brain have largely been elucidated, no comprehensive studies have been performed in human brain. The purpose of this study was to identify global patterns of GABAergic gene expression in healthy adults, including trans and cis effects in the GABA_A gene clusters, before determining the effects of chronic alcohol and cocaine exposure on gene expression in the hippocampus. RNA-Seq data from ‘BrainSpan’ was obtained across 16 brain regions from postmortem samples from nine adults. A factor analysis was performed on global expression of 21 GABAergic pathway genes. Factor specificity for response to chronic alcohol/cocaine exposure was subsequently determined from the analysis of RNA-Seq data from postmortem hippocampus of eight alcoholics, eight cocaine addicts and eight controls. Six gene expression factors were identified. Most genes loaded (≥0.5) onto one factor; six genes loaded onto two. The largest factor (0.30 variance) included the chromosome 5 gene cluster that encodes the most common GABA_A receptor, α₁β₂γ₂, and genes encoding the α₃β₃γ₂ receptor. Genes within this factor were largely unresponsive to chronic alcohol/cocaine exposure. In contrast, the chromosome 4 gene cluster factor (0.14 variance) encoding the α₂β₁γ₁ receptor was influenced by chronic alcohol/cocaine exposure. Two other factors (0.17 and 0.06 variance) showed expression changes in alcoholics/cocaine addicts; these factors included genes involved in GABA synthesis and synaptic transport. Finally there were two factors that included genes with exceptionally low (0.10 variance) and high (0.09 variance) expression in the cerebellum; the former factor was unaffected by alcohol/cocaine exposure. This study has shown that there appears to be specificity of GABAergic gene groups, defined by covariation in expression, for response to chronic alcohol/cocaine exposure. These findings might have implications for combating stress-related craving and relapse.

Lead-induced neurodegenerative events and abnormal behaviors occur via ORXRergic/GABA(A)Rergic mechanisms in a marine teleost

The hindering effects of metals and in particular lead (Pb) are representing a growing threat to aquatic organisms such as fish. This observation derives from toxic concentrations of Pb accounting for altered neurophysiological activities of some interesting teleost models like Thalassoma pavo, a fish species highly known for its host-cleaning symbiosis. In this study, the nominal PbNO(3) concentration of 1.6 mg/L was capable of reducing feeding and resting bouts as early as 24 h of exposure while hyperactive swimming episodes were also detected. Such abnormal behaviors were tightly correlated to up-regulated orexin receptor (ORXR) mRNA expression levels in some brain areas such as the lateral thalamic nucleus (+213%) and the optic tectum (+90%) with respect to controls. Interestingly, these transcriptional effects seemed to be attenuated when Pb-exposed fish received either 100 ng/g of ORX-A (-70%) or 0.1 μg/g of γ-aminobutyric acid(A) receptor (GABA(A)R) agonist muscimol (MUS; -97%) compared to fish exposed to Pb alone. Moreover, a net neurodegenerative process of the different brain areas was reported after Pb exposure as displayed by their marked amino cupric silver stained cells while these cells were devoid of any staining reaction after treatment with MUS only. Conversely, addition of the GABA(A)R antagonist bicuculline (BIC; 1 μg/g) moderately (p<0.05) enhanced Pb-dependent behavioral and neurodegenerative actions. Overall, these first indications strongly point to altered ORXR/GABA(A)R interactions during neurotoxic events of a metal that by evoking harmful neurobiological dysfunctions may endanger the survival of commercially valuable fish with eventual repercussions on human health.

Identification of the minimal promoter for specific expression of the GABAρ1 receptor in retinal bipolar cells

γ-aminobutyric acid (GABA)ρ receptors regulate rapid synaptic ion currents in the axon end of retinal ON bipolar neurons, acting as a point of control along the visual pathway. In the GABAρ1 subunit knock out mouse, inhibition mediated by this receptor is totally eliminated, showing its role in neural transmission in retina. GABAρ1 mRNA is expressed in mouse retina after post-natal day 7, but little is known about its transcriptional regulation. To identify the GABAρ1 promoter, in silico analyses were performed and indicated that a 0.290-kb fragment, flanking the 5'-end of the GABAρ1 gene, includes putative transcription factor-binding sites, two Inr elements, and lacks a TATA-box. A rapid amplification of cDNA ends (RACE) assay showed three transcription start sites (TSS) clustered in the first exon. Luciferase reporter assays indicated that a 0.232-kb fragment upstream from the ATG is the minimal promoter in transfected cell lines and in vitro electroporated retinae. The second Inr and AP1 site are important to activate transcription in secretin tumor cells (STC-1) and retina. Finally, the 0.232-kb fragment drives green fluorescent protein (GFP) expression to the inner nuclear layer, where bipolar cells are present. This first work paves the way for further studies of molecular elements that control GABAρ1 transcription and regulate its expression during retinal development.

Conserved toxic responses across divergent phylogenetic lineages: a meta-analysis of the neurotoxic effects of RDX among multiple species using toxicogenomics

At military training sites, a variety of pollutants such as hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), may contaminate the area originating from used munitions. Studies investigating the mechanism of toxicity of RDX have shown that it affects the central nervous system causing seizures in humans and animals. Environmental pollutants such as RDX have the potential to affect many different species, therefore it is important to establish how phylogenetically distant species may respond to these types of emerging pollutants. In this paper, we have used a transcriptional network approach to compare and contrast the neurotoxic effects of RDX among five phylogenetically disparate species: rat (Sprague-Dawley), Northern bobwhite quail (Colinus virginianus), fathead minnow (Pimephales promelas), earthworm (Eisenia fetida), and coral (Acropora formosa). Pathway enrichment analysis indicated a conservation of RDX impacts on pathways related to neuronal function in rat, Northern bobwhite quail, fathead minnows and earthworm, but not in coral. As evolutionary distance increased common responses decreased with impacts on energy and metabolism dominating effects in coral. A neurotransmission related transcriptional network based on whole rat brain responses to RDX exposure was used to identify functionally related modules of genes, components of which were conserved across species depending upon evolutionary distance. Overall, the meta-analysis using genomic data of the effects of RDX on several species suggested a common and conserved mode of action of the chemical throughout phylogenetically remote organisms.

Changes in ventral respiratory column GABAaR ε- and δ-subunits during hibernation mediate resistance to depression by EtOH and pentobarbital

During hibernation in the 13-lined ground squirrel, Ictidomys tridecemlineatus, the cerebral cortex is electrically silent, yet the brainstem continues to regulate cardiorespiratory function. Previous work showed that neurons in slices through the medullary ventral respiratory column (VRC) but not the cortex are insensitive to high doses of pentobarbital during hibernation, leading to the hypothesis that GABA(A) receptors (GABA(A)R) in the VRC undergo a seasonal modification in subunit composition. To test whether alteration of GABA(A)R subunits are responsible for hibernation-associated pentobarbital insensitivity, we examined an array of subunits using RT-PCR and Western blots and identified changes in ε- and δ-subunits in the medulla but not the cortex. Using immunohistochemistry, we confirmed that during hibernation, the expression of ε-subunit-containing GABA(A)Rs nearly doubles in the VRC. We also identified a population of δ-subunit-containing GABA(A)Rs adjacent to the VRC that were differentially expressed during hibernation. As δ-subunit-containing GABA(A)Rs are particularly sensitive to ethanol (EtOH), multichannel electrodes were inserted in slices of medulla and cortex from hibernating squirrels and EtOH was applied. EtOH, which normally inhibits neuronal activity, excited VRC but not cortical neurons during hibernation. This excitation was prevented by bicuculline pretreatment, indicating the involvement of GABA(A)Rs. We propose that neuronal activity in the VRC during hibernation is unaffected by pentobarbital due to upregulation of ε-subunit-containing GABA(A)Rs on VRC neurons. Synaptic input from adjacent inhibitory interneurons that express δ-subunit-containing GABA(A)Rs is responsible for the excitatory effects of EtOH on VRC neurons during hibernation.

The origin and evolution of synapses

Understanding the evolutionary origins of behaviour is a central aim in the study of biology and may lead to insights into human disorders. Synaptic transmission is observed in a wide range of invertebrate and vertebrate organisms and underlies their behaviour. Proteomic studies of the molecular components of the highly complex mammalian postsynaptic machinery point to an ancestral molecular machinery in unicellular organisms--the protosynapse--that existed before the evolution of metazoans and neurons, and hence challenges existing views on the origins of the brain. The phylogeny of the molecular components of the synapse provides a new model for studying synapse diversity and complexity, and their implications for brain evolution.

Effect of GABAergic substances on firing rate and thermal coefficient of hypothalamic neurons in the juvenile chicken

The goal of the study is to investigate the GABAergic action on firing rate (FR) and temperature coefficient (TC) on hypothalamic neurons in the juvenile chicken. Extracellular recordings were obtained from 37 warm-sensitive, 32 cold-sensitive and 56 temperature-insensitive neurons in brain slices to determine the effect of GABA(A)-receptor agonist muscimol, GABA(A)-receptor antagonist bicuculline, GABA(B)-receptor agonist baclofen and GABA(B)-receptor antagonist CGP 35348. Muscimol and baclofen in equimolar concentrations (1 microM) significantly inhibited FR of the neurons, regardless of their type of thermosensitivity. In contrast, bicuculline, as well as CGP 35348 (10 microM) increased FR of the majority of the neurons. The TC of most chick hypothalamic neurons could not be estimated during muscimol application because FR was completely inhibited. GABA(B)-receptor agonist specifically increased TC. This effect was restricted to cold-sensitive neurons, which were determined in a high number. The TC was significantly increased (p<0.05) by baclofen and significantly decreased (p<0.05) by CGP 35348. The effects of muscimol and baclofen on FR and TC were prevented by co-perfusion of the appropriate antagonists bicuculline and CGP 35348. The results suggest that the fundamental mechanisms of GABAergic influence on temperature sensitive and insensitive neurons in the chicken PO/AH are conserved during evolution of amniotes.

Early evolution of ionotropic GABA receptors and selective regimes acting on the mammalian-specific theta and epsilon subunits

Background

The amino acid neurotransmitter GABA is abundant in the central nervous system (CNS) of both invertebrates and vertebrates. Receptors of this neurotransmitter play a key role in important processes such as learning and memory. Yet, little is known about the mode and tempo of evolution of the receptors of this neurotransmitter. Here, we investigate the phylogenetic relationships of GABA receptor subunits across the chordates and detail their mode of evolution among mammals.

Principal Findings

Our analyses support two major monophyletic clades: one clade containing GABA_A receptor α, γ, and ε subunits, and another one containing GABA_A receptor ρ, β, δ, θ, and π subunits. The presence of GABA receptor subunits from each of the major clades in the Ciona intestinalis genome suggests that these ancestral duplication events occurred before the divergence of urochordates. However, while gene divergence proceeded at similar rates on most receptor subunits, we show that the mammalian-specific subunits θ and ε experienced an episode of positive selection and of relaxed constraints, respectively, after the duplication event. Sites putatively under positive selection are placed on a three-dimensional model obtained by homology-modeling.

Conclusions

Our results suggest an early divergence of the GABA receptor subunits, before the split from urochordates. We show that functional changes occurred in the lineages leading to the mammalian-specific subunit θ, and we identify the amino acid sites putatively responsible for the functional divergence. We discuss potential consequences for the evolution of mammals and of their CNS.