Sequence of a Drosophila ligand-gated ion-channel polypeptide with an unusual amino-terminal extracellular domain

Identification and Spatiotemporal Expression of a Putative New GABA Receptor Subunit in the Human Body Louse Pediculus humanus humanus

The human louse (Pediculus humanus) is an obligatory blood feeding ectoparasite with two ecotypes: the human body louse (Pediculus humanus humanus), a competent vector of several bacterial pathogens, and the human head louse (Pediculus humanus capitis), responsible for pediculosis and affecting millions of people around the globe. GABA (γ-aminobutyric acid) receptors, members of the cys-loop ligand gated ion channel superfamily, are among the main pharmacological targets for insecticides. In insects, there are four subunits of GABA receptors: resistant-to-dieldrin (RDL), glycin-like receptor of drosophila (GRD), ligand-gated chloride channel homologue3 (LCCH3), and 8916 are well described and form distinct phylogenetic clades revealing orthologous relationships. Our previous studies in the human body louse confirmed that subunits Phh-RDL, Phh-GRD, and Phh-LCCH3 are well clustered in their corresponding clades. In the present work, we cloned and characterized a putative new GABA receptor subunit in the human body louse that we named HoCas, for Homologous to Cys-loop α like subunit. Extending our analysis to arthropods, HoCas was found to be conserved and clustered in a new (fifth) phylogenetic clade. Interestingly, the gene encoding this subunit is ancestral and has been lost in some insect orders. Compared to the other studied GABA receptor subunits, HoCas exhibited a relatively higher expression level in all development stages and in different tissues of human body louse. These findings improved our understanding of the complex nature of GABA receptors in Pediculus humanus and more generally in arthropods.

Transgenic line for characterizing GABA-receptor expression to study the neural basis of olfaction in the yellow-fever mosquito

The mosquito Aedes aegypti is an important vector of diseases including dengue, Zika, chikungunya, and yellow fever. Olfaction is a critical modality for mosquitoes enabling them to locate hosts, sources of nectar, and sites for oviposition. GABA is an essential neurotransmitter in olfactory processing in the insect brain, including the primary olfactory center, the antennal lobe. Previous work with Ae. aegypti has suggested that antennal lobe inhibition via GABA may be involved in the processing of odors. However, little is known about GABA receptor expression in the mosquito brain, or how they may be involved in odor attraction. In this context, generating mutants that target the mosquito's olfactory responses, and particularly the GABAergic system, is essential to achieve a better understanding of these diverse processes and olfactory coding in these disease vectors. Here we demonstrate the potential of a transgenic line using the QF2 transcription factor, GABA-B1QF2-ECFP, as a new neurogenetic tool to investigate the neural basis of olfaction in Ae. aegypti. Our results show that the gene insertion has a moderate impact on mosquito fitness. Moreover, the line presented here was crossed with a QUAS reporter line expressing the green fluorescent protein and used to determine the location of the metabotropic GABA-B1 receptor expression. We find high receptor expression in the antennal lobes, especially the cell bodies surrounding the antennal lobes. In the mushroom bodies, receptor expression was high in the Kenyon cells, but had low expression in the mushroom body lobes. Behavioral experiments testing the fruit odor attractants showed that the mutants lost their behavioral attraction. Together, these results show that the GABA-B1QF2-ECFP line provides a new tool to characterize GABAergic systems in the mosquito nervous system.

Physiological Function of RDL1 and RDL2 Subunits of the Ionotropic GABA Receptor in the Spodoptera litura with the CRISPR/Cas9 System In Vivo

In insect ionotropic γ-aminobutyric acid receptor (iGABAR) subunits, only resistance to dieldrin (RDL) can be individually and functionally expressed in vitro. In lepidopteran, two to three RDL subtypes are identified; however, their physiological roles have not been distinguished in vivo. In this study, SlRdl1 and SlRdl2 of S. litura were individually knocked out using CRISPR/Cas9, respectively. The mortality and larval and pupal duration of KOSlRdl1 and KOSlRdl2 were increased. The flight time and distance were increased by 43.30%-80.66% and 58.96%-198.22%, respectively, in KOSlRdl1. The GABA-induced current was significantly decreased by 53.57%-74.28% and 46.91%-63.34% in the ventral nerve cord, and the GABA titer was significantly reduced by 17.65%-28.05% and 19.85%-42.46% in KOSlRdl1 and KOSlRdl2, respectively. In conclusion, SlRdl1 and SlRdl2 are necessary for the transmission of GABA-induced neural signals; however, only SlRdl1 could regulate the flight capability of S. litura. Our results provided a new avenue to study lepidopteran iGABARs.

Shisa reduces the sensitivity of homomeric RDL channel to GABA in the two-spotted spider mite, Tetranychus urticae Koch

The γ-aminobutyric acid receptors (GABARs) mediate fast inhibitory transmission in central nervous system of insects and are important targets of insecticides. An auxiliary subunit, Shisa7, was identified in mammals as a single-passing transmembrane protein. However, the homology gene(s) of Shisa in invertebrates has not been reported to date. In the present study, a homolog Shisa gene was identified from the two-spotted spider mite, Tetranychus urticae Koch. Its open reading frame had 927 base pairs and encoded 308 amino acid residues, which has a typical Shisa domain at 13th-181st amino acid residues. According to the phylogenetic tree, the invertebrate Shisa was categorized apart with those of vertebrate, and TuShisa showed closest relationship with the Shisa9 of velvet mite, Dinothrombium tinctorium (L.). In the electrophysiological assay with two-electrode voltage clamp, the GABA-activated TuRDL channel was functionally formed in the Africa clawed frog Xenopus laevis (Daudin) oocytes (EC50 = 53.34 μM). No GABA-activated current could be observed in TuShisa-expressed oocytes, whereas TuShisa could reduce the sensitivity of TuRDL/TuShisa (mass ratio of 1: 4) channel to GABA. The homology structural models of TuRDL and TuShisa were built by the SWISS-MODEL server, their interaction was predicted using Z-DOCK and three predicted hydrogen bonds and interface residues were analysed by PyMOL. Meanwhile, the key interface residues of TuShisa affected the stability of complex were calculated by Discovery Studio 2019. In conclusion, the TuShisa, as the first reported invertebrate Shisa, was explored and functionally examined as the GABARs auxiliary subunit. Our findings provide a basis for research of invertebrate Shisa.

Alkaline taste sensation through the alkaliphile chloride channel in Drosophila

The sense of taste is an important sentinel governing what should or should not be ingested by an animal, with high pH sensation playing a critical role in food selection. Here we explore the molecular identities of taste receptors detecting the basic pH of food using Drosophila melanogaster as a model. We identify a chloride channel named alkaliphile (Alka), which is both necessary and sufficient for aversive taste responses to basic food. Alka forms a high-pH-gated chloride channel and is specifically expressed in a subset of gustatory receptor neurons (GRNs). Optogenetic activation of alka-expressing GRNs is sufficient to suppress attractive feeding responses to sucrose. Conversely, inactivation of these GRNs causes severe impairments in the aversion to high pH. Altogether, our discovery of Alka as an alkaline taste receptor lays the groundwork for future research on alkaline taste sensation in other animals.

Competitive chrodrimanin B interactions with rat brain GABAA receptors revealed by radioligand binding assays

Meroterpenoid compounds chrodrimanins produced by Talaromyces sp. YO-2 have been shown to act as competitive antagonists of silkworm larval GABA_A receptors using electrophysiology, yet no further evidence has been provided to support such an action. We have investigated the actions of chrodrimanin B on rat brain GABA_A receptors by binding assays with non-competitive ligand of GABA_A receptors [³H]EBOB and competitive ligands [³H]gabazine and [³H]muscimol. Chrodrimanin B did not significantly affect the binding of [³H]EBOB while reducing the binding of [³H]gabazine and [³H]muscimol to the rat membrane preparations. Chrodrimanin B increased the dissociation constant K_d of [³H]gabazine and [³H]muscimol without significantly affecting the maximum binding, pointing to competitive interactions of chrodrimanin B with rat GABA_A receptors in support of our previous observation that the compound acts as a competitive antagonist on the silkworm larval GABA receptor.

Functional Characteristics of the Lepidopteran Ionotropic GABA Receptor 8916 Subunit Interacting with the LCCH3 or the RDL Subunit

The ionotropic γ-aminobutyric acid (iGABA) receptor is commonly considered as a fast inhibitory channel and is an important insecticide target. Since 1990, RDL, LCCH3, and GRD have been successively isolated and found to be potential subunits of the insect iGABA receptor. More recently, one orphan gene named 8916 was found and considered to be another potential iGABA receptor subunit according to its amino acid sequence. However, little information about 8916 has been reported. Here, the 8916 subunit from Chilo suppressalis was studied to determine whether it can form part of a functional iGABA receptor by co-expressing this subunit with CsRDL1 or CsLCCH3 in the Xenopus oocyte system. Cs8916 or CsLCCH3 did not form functional ion channels when expressed alone. However, Cs8916 was able to form heteromeric ion channels when expressed with either CsLCCH3 or CsRDL1. The recombinant heteromeric Cs8916/LCCH3 channel was a cation-selective channel, which was sensitive to GABA or β-alanine. The current of the Cs8916/LCCH3 channel was inhibited by dieldrin, endosulfan, fipronil, or ethiprole. In contrast, fluralaner, broflanilide, and avermectin showed little effect on the Cs8916/LCCH3 channel (IC₅₀s > 10 000 nM). The Cs8916/RDL1 channel was sensitive to GABA, but was significantly different in EC₅₀ and I_max for GABA to those of homomeric CsRDL1. Fluralaner, fipronil, or dieldrin showed antagonistic actions on Cs8916/RDL1. In conclusion, Cs8916 is a potential iGABA receptor subunit, which can interact with CsLCCH3 to generate a cation-selective channel that is sensitive to several insecticides. Also, as Cs8916/RDL1 has a higher EC₅₀ than homomeric CsRDL1, Cs8916 may affect the physiological functions of CsRDL1 and therefore play a role in fine-tuning GABAergic signaling.

TDP-43 regulates GAD1 mRNA splicing and GABA signaling in Drosophila CNS

Alterations in the function of the RNA-binding protein TDP-43 are largely associated with the pathogenesis of amyotrophic lateral sclerosis (ALS), a devastating disease of the human motor system that leads to motoneurons degeneration and reduced life expectancy by molecular mechanisms not well known. In our previous work, we found that the expression levels of the glutamic acid decarboxylase enzyme (GAD1), responsible for converting glutamate to γ-aminobutyric acid (GABA), were downregulated in TBPH-null flies and motoneurons derived from ALS patients carrying mutations in TDP-43, suggesting that defects in the regulation of GAD1 may lead to neurodegeneration by affecting neurotransmitter balance. In this study, we observed that TBPH was required for the regulation of GAD1 pre-mRNA splicing and the levels of GABA in the Drosophila central nervous system (CNS). Interestingly, we discovered that pharmacological treatments aimed to potentiate GABA neurotransmission were able to revert locomotion deficiencies in TBPH-minus flies, revealing novel mechanisms and therapeutic strategies in ALS.

Molecular and Functional Characterization of GABA Receptor Subunits GRD and LCCH3 from Human Louse Pediculus Humanus Humanus

Human louse Pediculus humanus is a cosmopolitan obligatory blood-feeding ectoparasite causing pediculosis and transmitting many bacterial pathogens. Control of infestation is difficult due to the developed resistance to insecticides that mainly target GABA (γ-aminobutyric acid) receptors. Previous work showed that Pediculus humanus humanus (Phh) GABA receptor subunit resistance to dieldrin (RDL) is the target of lotilaner, a synthetic molecule of the isoxazoline chemical class. To enhance our understanding of how insecticides act on GABA receptors, two other GABA receptor subunits were cloned and characterized: three variants of Phh-grd (glycine-like receptor of Drosophila) and one variant of Phh-lcch3 (ligand-gated chloride channel homolog 3). Relative mRNA expression levels of Phh-rdl, Phh-grd, and Phh-lcch3 revealed that they were expressed throughout the developmental stages (eggs, larvae, adults) and in the different parts of adult lice (head, thorax, and abdomen). When expressed individually in the Xenopus oocyte heterologous expression system, Phh-GRD1, Phh-GRD2, Phh-GRD3, and Phh-LCCH3 were unable to reconstitute functional channels, whereas the subunit combinations Phh-GRD1/Phh-LCCH3, Phh-GRD1/Phh-RDL, and Phh-LCCH3/Phh-RDL responded to GABA in a concentration-dependent manner. The three heteromeric receptors were similarly sensitive to the antagonistic effect of picrotoxin and fipronil, whereas Phh-GRD1/Phh-RDL and Phh-LCCH3/Phh-RDL were respectively about 2.5-fold and 5-fold more sensitive to ivermectin than Phh-GRD1/Phh-LCCH3. Moreover, the heteropentameric receptor constituted by Phh-GRD1/Phh-LCCH3 was found to be permeable and highly sensitive to the extracellular sodium concentration. These findings provided valuable additions to our knowledge of the complex nature of GABA receptors in human louse that could help in understanding the resistance pattern to commonly used pediculicides. SIGNIFICANCE STATEMENT: Human louse is an ectoparasite that causes pediculosis and transmits several bacterial pathogens. Emerging strains developed resistance to the commonly used insecticides, especially those targeting GABA receptors. To understand the molecular mechanisms underlying this resistance, two subunits of GABA receptors were cloned and described: Phh-grd and Phh-lcch3. The heteromeric receptor reconstituted with the two subunits was functional in Xenopus oocytes and sensitive to commercially available insecticides. Moreover, both subunits were transcribed throughout the parasite lifecycle.

Change the channel: CysLoop receptor antagonists from nature

Vertebrate and invertebrate ligand-gated ion channels (LGICs) exhibit significant structural homology and often share ligands. As a result, ligands with activity against one class can be brought to bear against another, including for development as insecticides. Receptor selectivity, metabolism and distribution must then be optimized using chemical synthesis. Here we review natural products (NPs) that ligate and inhibit the Cys-loop family of LGICs, which benefit from the unique physicochemical properties of natural product space but often present a high synthetic burden. Recent advances in chemical synthesis, however, have opened practical entries into these complex structures, several of which are highlighted. © 2020 Society of Chemical Industry.

Heterogeneous expression of GABA receptor-like subunits LCCH3 and GRD reveals functional diversity of GABA receptors in the honeybee Apis mellifera

BACKGROUND AND PURPOSE

Despite a growing awareness, annual losses of honeybee colonies worldwide continue to reach threatening levels for food safety and global biodiversity. Among the biotic and abiotic stresses probably responsible for these losses, pesticides, including those targeting ionotropic GABA receptors, are one of the major drivers. Most insect genomes include the ionotropic GABA receptor subunit gene, Rdl, and two GABA-like receptor subunit genes, Lcch3 and Grd. Most studies have focused on Rdl which forms homomeric GABA-gated chloride channels, and a complete analysis of all possible molecular combinations of GABA receptors is still lacking.

EXPERIMENTAL APPROACH

We cloned the Rdl, Grd, and Lcch3 genes of Apis mellifera and systematically characterized the resulting GABA receptors expressed in Xenopus oocytes, using electrophysiological assays, fluorescence microscopy and co-immunoprecipitation techniques.

KEY RESULTS

The cloned subunits interacted with each other, forming GABA-gated heteromeric channels with particular properties. Strikingly, these heteromers were always more sensitive than AmRDL homomer to all the pharmacological agents tested. In particular, when expressed together, Grd and Lcch3 form a non-selective cationic channel that opens at low concentrations of GABA and with sensitivity to insecticides similar to that of homomeric Rdl channels.

CONCLUSION AND IMPLICATIONS

For off-target species like the honeybee, chronic sublethal exposure to insecticides constitutes a major threat. At these concentration ranges, homomeric RDL receptors may not be the most pertinent target to study and other ionotropic GABA receptor subtypes should be considered in order to understand more fully the molecular mechanisms of sublethal toxicity to insecticides.

Potential of Competitive Antagonists of Insect Ionotropic γ-Aminobutyric Acid Receptors as Insecticides

Ionotropic γ-aminobutyric acid (GABA) receptors (GABARs) represent an important insecticide target. Currently used GABAR-targeting insecticides are non-competitive antagonists (NCAs) of these receptors. Recent studies have demonstrated that competitive antagonists (CAs) of GABARs have functions of inhibiting insect GABARs similar to NCAs and that they also exhibit insecticidal activity. CAs have different binding sites and different mechanisms of action compared to those of NCAs. Therefore, GABAR CAs should have the potential to be developed into novel insecticides, which could be used to overcome the developed resistance of insect pests to conventional NCA insecticides. Although research on insect GABAR CAs has lagged behind that on mammalian GABAR CAs, research on the CAs of insect ionotropic GABARs has made great progress in recent years, and several series of heterocyclic compounds, such as 3-isoxazolols and 6-iminopyridazines, have been identified as insect GABAR CAs. In this review, we briefly summarize the design strategies, structures, and biological activities of the novel GABAR CAs that have been found in the past decade. Updated information about GABAR CAs may benefit the design and development of novel GABAR-targeting insecticides.

Functional integrity of honeybee (Apis mellifera L.) resistant to dieldrin γ-aminobutyric acid receptor channels conjugated with three fluorescent proteins

To generate an efficient tool used in Xenopus oocyte expression for in situ investigation of channel receptor expression, distribution and function, the C-terminus of the honeybee (Apis mellifera L.) resistant to dieldrin (RDL) subunit was fused with *FP, including monomeric red, enhanced yellow or enhanced green fluorescent protein (referred to as mRFP, EYFP and EGFP, respectively). In the present study, all fused *FP-AmRDLs could be visualized using fluorescence and laser confocal microscopy in cRNA-injected oocytes. Fluorescence was distributed isotropically in the cellular membrane. The potencies of the agonist γ-aminobutyric acid (GABA), but not β-alanine, and the test antagonists (fipronil, flufiprole, dieldrin, α-endosulfan, bifenazate and avermectin B1a) in the *FP-AmRDL receptor did not significantly differ from that of the untagged receptor with two-electrode voltage clamp detection. The half maximal effective concentrations (EC50 s) of GABA in AmRDL, EGFP-AmRDL, EYFP-AmRDL and mRFP-AmRDL receptors were 11.98, 12.61, 18.92 and 22.11 μM, respectively, and those of β-alanine were 651.6, 629.6, 1643.0 and 2146.0 μM, respectively. Inhibition percentages of test antagonists against *FP-AmRDL and AmRDL were not significantly different from each other. Overall, the consistency in functional properties between *FP-AmRDL and AmRDL receptors makes pGH19-*FP a promising tool for further in situ investigation of GABA receptors.

Identification of the ionotropic GABA receptor-like subunits from the striped stem borer, Chilo suppressalis Walker (Lepidoptera: Pyralidae)

Ionotropic γ-aminobutyric acid (GABA) receptors (GABARs) mediate rapid inhibitory neurotransmission in both vertebrates and invertebrates, and are important molecular targets of insecticides. However, components of insect GABARs remain elusive. In addition to CsRDL1 and CsRDL2, the complementary DNAs (cDNAs) of another two GABA receptor-like subunits, CsLCCH3 and Cs8916, were identified from the rice striped stem borer, Chilo suppressalis Walker in the present study. Both CsLCCH3 and Cs8916 subunits shared common structural features, such as a highly-conserved Cys-loop structure, six distinct regions involved in ligand binding (loops A-F), and four transmembrane domains (TM 1-4). Transcript analysis demonstrated that the relative mRNA expression levels of both CsLCCH3 and Cs8916 subunits were the highest in the ventral nerve cord. Regarding developmental stage, transcript levels of both subunits were highest in eggs. Injections of double-stranded RNAs (dsRNAs), including dsRDL1, dsRDL2, dsLCCH3, or ds8916, significantly reduced mRNA abundance after 24 and 48 h. However, no observable effects on the development of C. suppressalis were observed. Injection of dsRDL1 or dsRDL2 did significantly reduce the mortality of C. suppressalis treated with fluralaner. Our results indicated that CsRDLs mediated the susceptibility of C. suppressalis to fluralaner, whereas CsLCCH3 and CsL8916 did not. The current investigation enhances our knowledge of Lepidopteran GABARs and offers a molecular basis for the development of novel insecticides to control C. suppressalis.

Molecular cloning, spatiotemporal and functional expression of GABA receptor subunits RDL1 and RDL2 of the rice stem borer Chilo suppressalis

Insect γ-aminobutyric acid (GABA) receptor (GABAR) is one of the major targets of insecticides. In the present study, cDNAs (CsRDL1A and CsRDL2S) encoding the two isoforms of RDL subunits were cloned from the rice stem borer Chilo suppressalis. Transcripts of both genes demonstrated similar expression patterns in different tissues and developmental stages, although CsRDL2S was ∼2-fold more abundant than CsRDL1A throughout all development stages. To investigate the function of channels formed by CsRDL subunits, both genes were expressed in Xenopus laevis oocytes singly or in combination in different ratios. Electrophysiological results using a two-electrode voltage clamp demonstrated that GABA activated currents in oocytes injected with both cRNAs. The EC₅₀ value of GABA in activating currents was smaller in oocytes co-injected with CsRDL1A and CsRDL2S than in oocytes injected singly. The IC₅₀ value of the insecticide fluralaner in inhibiting GABA responses was smaller in oocytes co-injected with different cRNAs than in oocytes injected singly. Co-injection also changed the potency of the insecticide dieldrin in oocytes injected singly. These findings suggested that heteromeric GABARs were formed by the co-injections of CsRDL1A and CsRDL2S in oocytes. Although the presence of Ser at the 2'-position in the second transmembrane segment was responsible for the insensitivity of GABARs to dieldrin, this amino acid did not affect the potencies of the insecticides fipronil and fluralaner. These results lead us to hypothesize that C. suppressalis may adapt to insecticide pressure by regulating the expression levels of CsRDL1A and CsRDL2S and the composition of both subunits in GABARs.

Molecular characterization and expression pattern of three GABA receptor-like subunits in the small brown planthopper Laodelphax striatellus (Hemiptera: Delphacidae)

Ionotropic γ-aminobutyric acid (GABA)-gated chloride channel receptors mediate rapid inhibitory neurotransmission in vertebrates and invertebrates. GABA receptors are well known to be the molecular targets of synthetic insecticides or parasiticides. Three GABA receptor-like subunits, LsLCCH3, LsGRD and LS8916, of the small brown planthopper, Laodelphax striatellus (Fallén), a major insect pest of crop systems in East Asia, had been identified and characterized in this study. All three genes were cloned using the reverse transcriptase polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). They shared common structural features with known Cys-loop ligand-gated ion channels (LGICs): the well-conserved dicysteine-loop structures, an extracellular N-terminal domain containing six distinct regions (loops A-F) that form the ligand binding sites and four transmembrane regions (TM1-4). Additionally, temporal and spatial transcriptional profiling analysis indicated that Lslcch3 was significantly higher than the other two genes. All of them were expressed at higher levels in fifth-instar nymph and adults than in eggs and from first- to fourth-instar nymph. They were predominantly expressed in the heads of 2-d old female adults. These findings enhanced our understanding of cys-loop LGIC functional characterization in Hemiptera and provided a useful basis for the development of improved insecticides that targeting this important agricultural pest.

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.

Global and local missions of cAMP signaling in neural plasticity, learning, and memory

The fruit fly Drosophila melanogaster has been a popular model to study cAMP signaling and resultant behaviors due to its powerful genetic approaches. All molecular components (AC, PDE, PKA, CREB, etc) essential for cAMP signaling have been identified in the fly. Among them, adenylyl cyclase (AC) gene rutabaga and phosphodiesterase (PDE) gene dunce have been intensively studied to understand the role of cAMP signaling. Interestingly, these two mutant genes were originally identified on the basis of associative learning deficits. This commentary summarizes findings on the role of cAMP in Drosophila neuronal excitability, synaptic plasticity and memory. It mainly focuses on two distinct mechanisms (global versus local) regulating excitatory and inhibitory synaptic plasticity related to cAMP homeostasis. This dual regulatory role of cAMP is to increase the strength of excitatory neural circuits on one hand, but to act locally on postsynaptic GABA receptors to decrease inhibitory synaptic plasticity on the other. Thus the action of cAMP could result in a global increase in the neural circuit excitability and memory. Implications of this cAMP signaling related to drug discovery for neural diseases are also described.

Suppression of inhibitory GABAergic transmission by cAMP signaling pathway: alterations in learning and memory mutants

The cAMP signaling pathway mediates synaptic plasticity and is essential for memory formation in both vertebrates and invertebrates. In the fruit fly Drosophila melanogaster, mutations in the cAMP pathway lead to impaired olfactory learning. These mutant genes are preferentially expressed in the mushroom body (MB), an anatomical structure essential for learning. While cAMP-mediated synaptic plasticity is known to be involved in facilitation at the excitatory synapses, little is known about its function in GABAergic synaptic plasticity and learning. In this study, using whole-cell patch-clamp techniques on Drosophila primary neuronal cultures, we demonstrate that focal application of an adenylate cyclase activator forskolin (FSK) suppressed inhibitory GABAergic postsynaptic currents (IPSCs). We observed a dual regulatory role of FSK on GABAergic transmission, where it increases overall excitability at GABAergic synapses, while simultaneously acting on postsynaptic GABA receptors to suppress GABAergic IPSCs. Further, we show that cAMP decreased GABAergic IPSCs in a PKA-dependent manner through a postsynaptic mechanism. PKA acts through the modulation of ionotropic GABA receptor sensitivity to the neurotransmitter GABA. This regulation of GABAergic IPSCs is altered in the cAMP pathway and short-term memory mutants dunce and rutabaga, with both showing altered GABA receptor sensitivity. Interestingly, this effect is also conserved in the MB neurons of both these mutants. Thus, our study suggests that alterations in cAMP-mediated GABAergic plasticity, particularly in the MB neurons of cAMP mutants, account for their defects in olfactory learning.

Molecular characterization of agonists that bind to an insect GABA receptor

Ionotropic GABA receptors are widely distributed throughout the vertebrate and invertebrate central nervous system (CNS) where they mediate inhibitory neurotransmission. One of the most widely studied insect GABA receptors is constructed from RDL (resistance to dieldrin) subunits from Drosophila melanogaster. The aim of this study was to determine critical features of agonists binding to RDL receptors using in silico and experimental data. Partial atomic charges and dipole separation distances of a range of GABA analogues were calculated, and the potency of the analogues was determined using RDL receptors expressed in Xenopus oocytes. These data revealed functional agonists require an ammonium group and an acidic group with an optimum separation distance of ∼5 Å. To determine how the agonists bind to the receptor, a homology model of the extracellular domain was generated and agonists were docked into the binding site. The docking studies support the requirements for functional agonists and also revealed a range of potential interactions with binding site residues, including hydrogen bonds and cation−π interactions. We conclude that the model and docking procedures yield a good model of the insect GABA receptor binding site and the location of agonists within it.

RDL receptors

RDL receptors are invertebrate members of the Cys-loop family of ligand-gated ion channels. They are GABA (gamma-aminobutyric acid)-activated chloride-selective receptors that are closely related to their vertebrate orthologues, the GABA(A) receptors, as well as other Cys-loop receptors such as the ionotropic glycine, nicotinic acetylcholine and 5-HT(3) receptors. RDL receptors are widely expressed throughout the insect CNS (central nervous system) and are important in inhibitory neurotransmission. They are therefore a major insecticidal target site.

RNA editing regulates insect gamma-aminobutyric acid receptor function and insecticide sensitivity

A-to-I pre-mRNA editing by adenosine deaminase enzymes has been reported to enhance protein diversity in the nervous system. In Drosophila, the resistance to dieldrin (RDL) gamma-aminobutyric acid (GABA) receptor subunit displays an editing site (R122) that is close to the putative GABA-binding site. We assessed the functional effects of editing at this site by expressing homomeric RDL receptors in Xenopus oocytes. After replacement of arginine 122 with a glycine, both agonist and fipronil potencies were shifted to the right in either fipronil-sensitive receptors or mutated resistant receptors (A301G/T350M). These data provide the first insight on the influence of RNA editing on GABA receptor function.

The cys-loop ligand-gated ion channel gene superfamily of the red flour beetle, Tribolium castaneum

BACKGROUND

Members of the cys-loop ligand-gated ion channel (cys-loop LGIC) superfamily mediate chemical neurotransmission and are studied extensively as potential targets of drugs used to treat neurological disorders such as Alzheimer's disease. Insect cys-loop LGICs are also of interest as they are targets of highly successful insecticides. The red flour beetle, Tribolium castaneum, is a major pest of stored agricultural products and is also an important model organism for studying development.

RESULTS

As part of the T. castaneum genome sequencing effort, we have characterized the beetle cys-loop LGIC superfamily which is the third insect superfamily to be described after those of Drosophila melanogaster and Apis mellifera, and also the largest consisting of 24 genes. As with Drosophila and Apis, Tribolium possesses ion channels gated by acetylcholine, gamma-amino butyric acid (GABA), glutamate and histamine as well as orthologs of the Drosophila pH-sensitive chloride channel subunit (pHCl), CG8916 and CG12344. Similar to Drosophila and Apis, Tribolium cys-loop LGIC diversity is broadened by alternative splicing although the beetle orthologs of RDL and GluCl possess more variants of exon 3. Also, RNA A-to-I editing was observed in two Tribolium nicotinic acetylcholine receptor subunits, Tcasalpha6 and Tcasbeta1. Editing in Tcasalpha6 is evolutionarily conserved with D. melanogaster, A. mellifera and Heliothis virescens, whereas Tcasbeta1 is edited at a site so far only observed in the beetle.

CONCLUSION

Our findings reveal that in diverse insect species the cys-loop LGIC superfamily has remained compact with only minor changes in gene numbers. However, alternative splicing, RNA editing and the presence of divergent subunits broadens the cys-loop LGIC proteome and generates species-specific receptor isoforms. These findings on Tribolium castaneum enhance our understanding of cys-loop LGIC functional genomics and provide a useful basis for the development of improved insecticides that target an important agricultural pest.

Synaptic organization of lobula plate tangential cells inDrosophila: γ-Aminobutyric acid receptors and chemical release sites

In flies, the large tangential cells of the lobula plate represent an important processing center for visual navigation based on optic flow. Although the visual response properties of these cells have been well studied in blowflies, information on their synaptic organization is mostly lacking. Here we study the distribution of presynaptic release and postsynaptic inhibitory sites in the same set of cells in Drosophila melanogaster. By making use of transgenic tools and immunohistochemistry, our results suggest that HS and VS cells of Drosophila express gamma-aminobutyric acid (GABA) receptors in their dendritic region within the lobula plate, thus being postsynaptic to inhibitory input there. At their axon terminals in the protocerebrum, both cell types express synaptobrevin, suggesting the presence of presynaptic specializations there. HS- and VS-cell terminals additionally show evidence for postsynaptic GABAergic input, superimposed on this synaptic polarity. Our findings are in line with the general circuit for visual motion detection and receptive field properties as postulated from electrophysiological and optical recordings in blowflies, suggesting a similar functional organization of lobula plate tangential cells in the two species.

Decreased expression of the GABAA receptor in fragile X syndrome

After our initial discovery of under expression of the GABA(A) receptor delta subunit in a genome wide screening for differentially expressed mRNAs in brain of fragile X mice, a validated model for fragile X mental retardation syndrome, we analyzed expression of the 17 remaining subunits of the GABA(A) receptor using real-time PCR. We confirmed nearly 50% under expression of the delta subunit and found a significant 35%-50% reduction in expression of 7 additional subunit mRNAs, namely alpha(1), alpha(3), and alpha(4), beta(1) and beta(2) and gamma(1) and gamma(2), in fragile X mice compared to wild-type littermates. In concordance with previous results, under expression was found in cortex, but not in cerebellum. Moreover, decreased expression of specific GABA(A) receptor subunits in fragile X syndrome seems to be an evolutionary conserved hallmark since in the fragile X fly (Drosophila melanogaster) model we also found almost 50% under expression of all 3 subunits which make up the invertebrate GABA receptor, namely Grd, Rdl and Lcch3. In addition, we demonstrated a direct correlation between the amount of dFmrp and the expression of the GABA receptor subunits Rdl and Grd. Our results add evidence to previous observations of an altered GABAergic system in fragile X syndrome. Because GABA(A) receptors are the major inhibitory receptors in brain, involved in anxiety, depression, insomnia, learning and memory and epilepsy, processes also disturbed in fragile X patients, the well described GABA(A) receptor pharmacology might open new powerful opportunities for treatment of the behavioral and epileptic phenotype associated with fragile X syndrome.

Role of GABAergic inhibition in shaping odor-evoked spatiotemporal patterns in the Drosophila antennal lobe

Drosophila olfactory receptor neurons project to the antennal lobe, the insect analog of the mammalian olfactory bulb. GABAergic synaptic inhibition is thought to play a critical role in olfactory processing in the antennal lobe and olfactory bulb. However, the properties of GABAergic neurons and the cellular effects of GABA have not been described in Drosophila, an important model organism for olfaction research. We have used whole-cell patch-clamp recording, pharmacology, immunohistochemistry, and genetic markers to investigate how GABAergic inhibition affects olfactory processing in the Drosophila antennal lobe. We show that many axonless local neurons (LNs) in the adult antennal lobe are GABAergic. GABA hyperpolarizes antennal lobe projection neurons (PNs) via two distinct conductances, blocked by a GABAA- and GABAB-type antagonist, respectively. Whereas GABAA receptors shape PN odor responses during the early phase of odor responses, GABAB receptors mediate odor-evoked inhibition on longer time scales. The patterns of odor-evoked GABAB-mediated inhibition differ across glomeruli and across odors. Finally, we show that LNs display broad but diverse morphologies and odor preferences, suggesting a cellular basis for odor- and glomerulus-dependent patterns of inhibition. Together, these results are consistent with a model in which odors elicit stimulus-specific spatial patterns of GABA release, and as a result, GABAergic inhibition increases the degree of difference between the neural representations of different odors.

Acetylcholine, GABA and glutamate induce ionic currents in cultured antennal lobe neurons of the honeybee, Apis mellifera

The honeybee, Apis mellifera, is a valuable model system for the study of olfactory coding and its learning and memory capabilities. In order to understand the synaptic organisation of olfactory information processing, the transmitter receptors of the antennal lobe need to be characterized. Using whole-cell patch-clamp recordings, we analysed the ligand-gated ionic currents of antennal lobe neurons in primary cell culture. Pressure applications of acetylcholine (ACh), gamma-amino butyric acid (GABA) or glutamate induced rapidly activating ionic currents. The ACh-induced current flows through a cation-selective ionotropic receptor with a nicotinic profile. The ACh-induced current is partially blocked by alpha-bungarotoxin. Epibatidine and imidacloprid are partial agonists. Our data indicate the existence of an ionotropic GABA receptor which is permeable to chloride ions and sensitive to picrotoxin (PTX) and the insecticide fipronil. We also identified the existence of a chloride current activated by pressure applications of glutamate. The glutamate-induced current is sensitive to PTX. Thus, within the honeybee antennal lobe, an excitatory cholinergic transmitter system and two inhibitory networks that use GABA or glutamate as their neurotransmitter were identified.

Drosophila melanogaster GRD and LCCH3 subunits form heteromultimeric GABA-gated cation channels

In addition to its action as a fast inhibitory neurotransmitter, gamma-aminobutyric acid (GABA) is thought to mediate excitatory action by activating cation currents in some cell types in invertebrates. However, to date no GABA receptor capable of mediating such action has been identified at the molecular level in insects. Using a systematic expression screening approach, we found that the Drosophila ligand-gated ion channel subunits GRD and LCCH3 combine to form cation-selective GABA-gated ion channels when coexpressed in Xenopus laevis oocytes. The heteromultimeric receptor is activated by GABA (EC50=4.5 microm), muscimol (EC50=4.8 microm) and trans-4-aminocrotonic acid (EC50=104.5 microm), and partially by cis-4-aminocrotonic acid (EC50=106.3 microm). Picrotoxin effectively blocked the GABA-gated channel (IC50=0.25 microm), but bicuculline, TPMTA, dieldrin and lindane did not. The benzodiazepines flunitrazepam and diazepam did not potentiate the GABA-evoked current. Our data suggest that heteromultimeric channels composed of GRD and LCCH3 subunits form GABA-gated cation channels in insects.

GABA receptors containing Rdl subunits mediate fast inhibitory synaptic transmission in Drosophila neurons

GABAergic inhibition in Drosophila, as in other insects and mammals, is important for regulation of activity in the CNS. However, the functional properties of synaptic GABA receptors in Drosophila have not been described. Here, we report that spontaneous GABAergic postsynaptic currents (sPSCs) in cultured embryonic Drosophila neurons are mediated by picrotoxin-sensitive chloride-conducting receptors. A rapid increase in spontaneous firing in response to bath application of picrotoxin demonstrates that these GABA receptors mediate inhibition in the neuronal networks formed in culture. Many of the spontaneous GABAergic synaptic currents are sodium action potential independent [miniature IPSCs (mIPSCs)] but are regulated by external calcium levels. The large variation in mIPSC frequency, amplitude, and kinetics properties between neurons suggests heterogeneity in GABA receptor number, location, and/or subtype. A decrease in the mean mIPSC decay time constant between 2 and 5 d, in the absence of a correlated change in rise time, demonstrates that the functional properties of the synaptic GABA receptors are regulated during maturation in vitro. Finally, neurons from the GABA receptor subunit mutant Rdl exhibit reduced sensitivity to picrotoxin blockade of the mIPSCs and resistance to picrotoxin-induced increases in spontaneous firing frequency. This demonstrates that Rdl containing GABA receptors play a direct role in mediating synaptic inhibition in Drosophila neural circuits formed in culture.

Ca2+/calmodulin-dependent protein kinase regulates GABA-activated Cl- current in cockroach dorsal unpaired median neurons

We studied gamma-aminobutyric acid (GABA)-mediated currents in short-term cultured dorsal unpaired median (DUM) neurons of cockroach Periplaneta americana using the whole cell patch-clamp technique in symmetrical chloride solutions. All DUM neurons voltage-clamped at -50 mV displayed inward currents (I(GABA)) when 10(-4) M of GABA was applied by pneumatic pressure-ejection pulses. The semi-logarithmic curve of I(GABA) amplitude versus the ejection time yielded a Hill coefficient of 4.0. I(GABA) was chloride (Cl-) because the reversal potential given by the current-voltage (I-V) curve varied according to the value predicted by the Nernst equation for Cl- dependence. In addition, I(GABA) was almost completely blocked by bath application of the chloride channel blockers picrotoxin (PTX) or 3,3-bis(trifluoromethyl)bicyclo-[2,2,1]heptane-2,2-diacarbonitrile (BIDN). The I-V curve for I(GABA) displayed a unexpected biphasic aspect and was best fitted by two linear regressions giving two slope conductances of 35.6 +/- 2.1 and 80.9 +/- 4.1 nS for potentials ranging from 0 to -30 and -30 to -70 mV, respectively. At -50 mV, the current amplitude was decreased by cadmium chloride (CdCl2, 10(-3) M) and calcium-free solution. The semi-logarithmic curve for CdCl2-resistant I(GABA) gave a Hill coefficient of 2.4. Hyperpolarizing voltage step from -50 to -80 mV was known to increase calcium influx through calcium-resting channels. According to this protocol, a significant increase of I(GABA) amplitude was observed. However, this effect was never obtained when the same protocol was applied on cell body pretreated with CdCl2. When the calmodulin blocker N-(6-aminohexyl)-5-chloro-1-naphtalene-sulfonamide or the calcium-calmodulin-dependent protein kinase blocker 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine (KN-62) was added in the pipette solution, I(GABA) amplitude was decreased. Pressure ejection application of the cis-4-aminocrotonic acid (CACA) on DUM neuron cell body held at -50 mV, evoked a Cl- inward current which was insensitive to CdCl2. The Hill plot yielded a Hill coefficient of 2.3, and the I-V curve was always linear in the negative potential range with a slope conductance of 32.4 +/- 1.1 nS. These results, similar to those obtained with GABA in the presence of CdCl2 and KN-62, indicated that CACA activated one subtype of GABA receptor. Our study demonstrated that at least two distinct subtypes of Cl--dependent GABA receptors were expressed in DUM neurons, one of which is regulated by an intracellular Ca2+-dependent mechanism via a calcium-dependent protein kinase. The consequences of the modulatory action of Ca2+ in GABA receptors function and their sensitivity to insecticide are discussed.

Maintenance of GABA receptor function of small-diameter cockroach neurons by adenine nucleotides

Small diameter (<20 microm) neurons from the sixth abdominal ganglion of the American cockroach, Periplaneta americana, were enzymatically isolated and responses to exogenously applied gamma-aminobutyric acid (GABA) were recorded using the whole-cell patch clamp technique. With a minimal intracellular medium, responses to repeated applications of GABA decreased to zero within a few minutes. The rate of rundown of GABA responses was decreased by the intracellular inclusion of the phosphatase inhibitors microcystin and okadaic acid, suggesting that phosphorylation is necessary for the maintenance of cockroach GABA receptor function. ATP (5 mM) prevented GABA response rundown. ADP (5 mM) also slowed GABA response rundown, but responses stabilized at a level about half that seen with ATP. In the presence of protein kinase A inhibitory peptide (PKI), ATP was only as efficacious as ADP in slowing rundown. PKI had no effect on the ability of ADP to slow rundown, suggesting that the beta-phosphate of ADP is not involved in PKA-dependent phosphorylation of the GABA receptor. These results suggest that in cockroach neurons, GABA receptor function is maintained intracellularly by adenine nucleotides, not only by phosphorylation, but also possibly by an interaction with a nucleotide recognition site unrelated to PKA-dependent phosphorylation.

Interaction of non-competitive blockers within the gamma-aminobutyric acid type A chloride channel using chemically reactive probes as chemical sensors for cysteine mutants

Selected channel-lining cysteine mutants from the M2 segment of rat alpha1 gamma-aminobutyric acid (GABA) type A receptor subunit, at positions 257, 261, 264, and 272 were co-expressed with beta1 and gamma2 subunits in Xenopus oocytes. They generated functional receptors displaying conductance and response to both GABA and picrotoxinin similar to the wild type alpha1beta1gamma2 receptor. Three chemically reactive affinity probes derived from non-competitive blockers were synthesized to react with the engineered cysteines: 1) dithiane bis-sulfone derivative modified by an isothiocyanate function (probe A); 2) fiprole derivatives modified by an alpha-chloroketone (probe B) and alpha-bromoketone (probe C) moiety. These probes blocked the GABA-induced currents on all receptors. This blockade could be fully reversed by a washing procedure on the wild type, the alpha1T261Cbeta1gamma2 and alpha1L264Cbeta1gamma2 mutant receptors. In contrast, an irreversible effect was observed for all three probes on both alpha1V257Cbeta1gamma2 and alpha1S272Cbeta1gamma2 mutant receptors. This effect was probe concentration-dependent and could be abolished by picrotoxinin and/or t-butyl bicyclophosphorothionate. These data indicate a major interaction of non-competitive blockers at position 257 of the presumed M2 segment of rat alpha1 subunit but also suggest an interaction at the more extracellular position 272.

Immunocytochemical Mapping of an RDL-Like GABA Receptor Subunit and of GABA in Brain Structures Related to Learning and Memory in the Cricket Acheta domesticus

The distribution of putative RDL-like GABA receptors and of γ-aminobutyric acid (GABA) in the brain of the adult house cricket Acheta domesticus was studied using specific antisera. Special attention was given to brain structures known to be related to learning and memory. The main immunostaining for the RDL-like GABA receptor was observed in mushroom bodies, in particular the upper part of mushroom body peduncle and the two arms of the posterior calyx. Weaker immunostaining was detected in the distal part of the peduncle and in the α and β lobes. The dorso- and ventrolateral protocerebrum neuropils appeared rich in RDL-like GABA receptors. Staining was also detected in the glomeruli of the antennal lobe, as well as in the ellipsoid body of the central complex. Many neurons clustered in groups exhibit GABA-like immunoreactivity. Tracts that were strongly immunostained innervated both the calyces and the lobes of mushroom bodies. The glomeruli of the antennal lobe, the ellipsoid body, as well as neuropils of the dorso- and ventrolateral protocerebrum were also rich in GABA-like immuno- reactivity. The data demonstrated a good correlation between the distribution of the GABA-like and of the RDL-like GABA receptor immunoreactivity. The prominent distribution of RDL-like GABA receptor subunits, in particular areas of mushroom bodies and antennal lobes, underlines the importance of inhibitory signals in information processing in these major integrative centers of the insect brain.

Picrodendrin and related terpenoid antagonists reveal structural differences between ionotropic GABA receptors of mammals and insects

Twenty-eight picrotoxane terpenoids, including picrodendrins isolated from the Euphorbiaceae plant, Picrodendron baccatum (L.) Krug and Urban, have been evaluated for their ability to inhibit the specific binding of [3H]EBOB, the noncompetitive antagonist of ionotropic GABA receptors, to rat-brain and housefly (Musca domestica L.)-head membranes. Picrodendrin Q was the most potent competitive inhibitor of [3H]EBOB binding, with IC50 values of 16 nM (rat) and 22 nM (Musca). We find that the spiro gamma-butyrolactone moiety at the 13-position, which contains a carbonyl group conjugated with an unsaturated bond, and the substituents at the 4-position play important roles in the interaction of picrodendrins with their binding site in rat receptors. In contrast, such structural features are not strictly required in the case of the interaction with Musca receptors; the spiro saturated gamma-butyrolactone moiety at the 13-position, which bears the 16-sp3 carbon atom, and the hydroxyl groups at various positions are somewhat tolerated. Quantitative structure-activity studies have clearly shown that the electronegativity of the 16-carbon atom and the presence or absence of the 4- and 8-hydroxyl groups are important determinants of the potency of nor-diterpenes in Musca receptors, while the negative charge on the 17-carbonyl oxygen atom is likely important in the case of rat receptors. These findings indicate that there are significant differences between the structures of the complementary binding sites in rat GABA receptors and Musca GABA receptors. We also infer differences between native Musca GABA receptors and the Drosophila Rdl subunit-containing homo-oligomeric GABA receptors in the structures of their binding sites.

Characterization and comparative pharmacological studies of a functional gamma-aminobutyric acid (GABA) receptor cloned from the tobacco budworm, Heliothis virescens (Noctuidae:Lepidoptera)

This paper reports the functional expression and pharmacological characterization of a full length complementary deoxyribonucleic acid (cDNA) (pIVY12) cloned from a Heliothis virescens fertilized egg cDNA library that encodes for a gamma-aminobutyric acid (GABA) receptor subunit (HVRDL-Ser 285). Two electrode voltage clamp recordings of Xenopus oocytes expressing the HVRDL GABA-gated chloride channel revealed robust chloride ion conductance in response to GABA and the GABAA receptor agonist, muscimol. Baclofen, a GABAB agonist had no effect. Phenobarbital showed a positive dose-dependent allosteric modulatory effect, whereas the benzodiazepine, flunitrazepam, had no effect. Chloride conductance was depressed by the novel insecticide, fipronil ((+/-)-5-amino-1-(2,6 dichloro-alpha, alpha, alpha-trifluoro-p-tolyl)-4-trifluoromethyl-sulfinylpyrazole-3-carb onitrile) and the GABAA antagonist, picrotoxinin. The HVRDL GABA receptor was insensitive to blockage by dieldrin and the GABAA antagonist, bicuculline. The comparative actions of fipronil, picrotoxinin and dieldrin were examined on oocytes expressing the H. virescens wild-type (HVRDL-Ser 285), the site-directed mutant (HVRDL-Ala 285), the Drosophila melanogaster Rdl wild-type (DMRDL-Ala 302) and the Rdl dieldrin resistant (DMRDL-Ser 302) homo-oligomeric GABA receptors. HVRDL-Ala 285 was 15-fold more sensitive to blockage by fipronil than HVRDL-Ser 285. DMRDL-Ala 302 and DMRDL-Ser-302 showed a similar level of sensitivity to blockage by fipronil. HVRDL-Ser 285 and DMRDL-Ser 302 exhibited a similar level of insensitivity to picrotoxinin. HVRDL-Ala 285 and DMRDL-Ala 302 showed a similar range of picrotoxinin sensitivity. DMRDL-Ala 302 and HVRDL-Ala 285 showed some sensitivity to blockage by dieldrin. Fipronil sensitivity was significantly altered by the serine to alanine mutation at position 285 in the M2 region of the HVRDL subunit, whereas no difference was observed between the DMRDL-Ser 302 and DMRDL-Ala 302 receptors.

Molecular biology of insect neuronal GABA receptors

Ionotropic gamma-aminobutyric acid (GABA) receptors are distributed throughout the nervous systems of many insect species. As with their vertebrate counterparts, GABAA receptors and GABAC receptors, the binding of GABA to ionotropic insect receptors elicits a rapid, transient opening of anion-selective ion channels which is generally inhibitory. Although insect and vertebrate GABA receptors share a number of structural and functional similarities, their pharmacology differs in several aspects. Recent studies of cloned Drosophila melanogaster GABA receptors have clarified the contribution of particular subunits to these differences. Insect ionotropic GABA receptors are also the target of numerous insecticides and an insecticide-resistant form of a Drosophila GABA-receptor subunit has enhanced our understanding of the structure-function relationship of one aspect of pharmacology common to both insect and vertebrate GABA receptors, namely antagonism by the plant-derived toxin picrotoxinin.

Distribution of two GABA receptor-like subunits in the Drosophila CNS

Previously we have described the distribution of the Rdl GABA receptor subunit in the Drosophila CNS. Knowing that Rdl can coassemble with LCCH3 (a Drosophila GABA receptor-like subunit showing sequence similarity to vertebrate beta subunit GABAA receptors) in baculovirus infected insect cells, we compared the localization of these two receptor subunits in order to identify any potential overlap in their spatial or temporal distribution. The two subunits show very different patterns of localization. Early in development LCCH3 is found in the majority of developing neuroblasts and later is localized to the cell bodies of the embryonic nerve cord and brain, and the neuronal cell bodies surrounding the adult brain. In contrast, Rdl receptor subunits appear confined to the neuropil in all developmental stages. These results have two important implications. Firstly, they suggest that although these two subunits can coassemble in heterologous expression systems, they may not be found in the same tissues in the nervous system. Secondly, production of LCCH3 before neuronal differentiation leads us to speculate on the role of that LCCH3 containing receptors in the developing nervous system.

Structure and pharmacological properties of a molluscan glutamate-gated cation channel and its likely role in feeding behavior

We describe the isolation of a molluscan (Lymnaea stagnalis) full-length complementary DNA that encodes a mature polypeptide (which we have named Lym-eGluR2) with a predicted molecular weight of 105 kDa that exhibits 44-48% identity to the mammalian kainate-selective glutamate receptor GluR5, GluR6, and GluR7 subunits. Injection of in vitro-transcribed RNA from this clone into Xenopus laevis oocytes results in the robust expression of homo-oligomeric cation channels that can be gated by L-glutamate (EC50 = 1.2 +/- 0.3 micron) and several other glutamate receptor agonists; rank order of potency: glutamate >> kainate > ibotenate > AMPA. These currents can be blocked by the mammalian non-NMDA receptor antagonists 6,7-dinitroquinoxaline-2,3-dione, 6-cyano-7-nitroquinoxaline-2,3-dione, and 1-(4-chlorobenzoyl)piperazine-2,3-dicarboxylic acid. Ionic-replacement experiments have shown that the agonist-induced current is carried entirely by sodium and potassium ions. In situ hybridization has revealed that the Lym-eGluR2 transcript is present in all 11 ganglia of the Lymnaea CNS, including the 4-cluster motorneurons within the paired buccal ganglia. The pharmacological properties and deduced location of Lym-eGluR2 are entirely consistent with it being (a component of) the receptor, which has been identified previously on buccal motorneurons, that mediates the excitatory effects of glutamate released from neurons within the feeding central pattern generator.

Allosteric modulation of an expressed homo-oligomeric GABA-gated chloride channel of Drosophila melanogaster

1. Functional GABA-gated chloride channels are formed when cRNA encoding the Drosophila melanogaster GABA receptor subunit RDL is injected into the cytoplasm of Xenopus oocytes. Two-electrode voltage-clamp was used to investigate allosteric modulation of GABA-induced currents recorded from the expressed, bicuculline-insensitive, RDL homo-oligomers. 2. Flunitrazepam (0.1 microM to 100 microM) had no effect on the amplitude of responses to 10 microM GABA (approximately EC10), whereas 4'chlorodiazepam (100 microM) enhanced the amplitude of submaximal responses to GABA. 3-Hydroxymethyl-beta-carboline (1 microM) and ethyl-beta-carboline-3-carboxylate (both 1 and 100 microM) had no effect on currents induced by 30 microM (approximately EC50) GABA. However 100 microM 3-hydroxymethyl-beta-carboline reduced potentiation by 4'chlorodiazepam. 3. The sodium salts of pentobarbitone (10 microM to 1 mM) and phenobarbitone (50 microM to 1 mM) dose-dependently enhanced submaximal GABA responses. Neither barbiturate activated currents in the absence of GABA. 4. At 10 microM, the steroids 5 alpha-pregnan-3 alpha-ol-20-one and alphaxalone (5 alpha-pregnan-3 alpha-ol-11,20-dione), potentiated submaximal GABA responses. The stereoselectivity of steroid action seen on vertebrate GABAA receptors was observed on RDL homo-oligomers as 5 alpha-pregnan-3 beta-ol-20-one (10 microM) was without effect. None of the three steroids tested activated currents in the absence of GABA. 5. The novel anticonvulsant, loreclezole (100 microM), potentiated the response to 10 microM GABA, but not that of saturating concentrations of GABA. delta-Hexachlorocyclohexane (0.1 microM to 30 microM) was a potent enhancer of submaximal responses to GABA of RDL. 6. The potencies of barbiturates and steroids on RDL homo-oligomers resemble those observed for several in situ insect GABA receptors, whereas those of benzodiazepine binding-site ligands are considerably reduced. The differences in the benzodiazepine pharmacology of RDL homo-oligomers and native GABA receptors, may reflect roles of other subunits in native insect receptors.

Evolution and overview of classical transmitter molecules and their receptors

All the classical transmitter ligand molecules evolved at least 1000 million years ago. With the possible exception of the Porifera and coelenterates (Cnidaria), they occur in all the remaining phyla. All transmitters have evolved the ability to activate a range of ion channels, resulting in excitation, inhibition and biphasic or multiphasic responses. All transmitters can be synthesised in all three basic types of neurones, i.e. sensory, interneurone and motoneurone. However their relative importance as sensory, interneurone or motor transmitters varies widely between the phyla. It is likely that all neurones contain more than one type of releasable molecule, often a combination of a classical transmitter and a neuroactive peptide. Second messengers, i.e. G proteins and phospholipase C systems, appeared early in evolution and occur in all phyla that have been investigated. Although the evidence is incomplete, it is likely that all the classical transmitter receptor subtypes identified in mammals, also occur throughout the phyla. The invertebrate receptors so far cloned show some interesting homologies both between those from different invertebrate phyla and with mammalian receptors. This indicates that many of the basic receptor subtypes, including benzodiazepine subunits, evolved at an early period, probably at least 800 million years ago. Overall, the evidence stresses the similarity between the major phyla rather than their differences, supporting a common origin from primitive helminth stock.

Channel gating in the absence of agonist by a homo-oligomeric molluscan GABA receptor expressed in Xenopus oocytes from a cloned cDNA

We have previously described the isolation of a complementary DNA (cDNA) from the freshwater mollusc Lymnaea stagnalis encoding a polypeptide that exhibits approximately 50% identity to the beta-subunits of vertebrate gamma-aminobutyric acid (GABA) type A (GABAA) receptor. When expressed in Xenopus laevis oocytes from in vitro-transcribed RNA, the snail subunit forms functional homo-oligomeric receptors possessing chloride-selective ion channels. In recordings from voltage-clamped oocytes held at -60 mV, GABA induced an inward current, whereas application of the chloride-channel blocker picrotoxin (in the absence of agonist) elicited an apparent outward current. Single channel recordings obtained from cell-attached patches have revealed a single population of approximately 20 pS channels, with an open probability greater than 90% (at a pipette potential of -100 mV) in the absence of GABA. The relationship between single channel current and pipette potential was linear over the studied range (-100 mV to +60 mV), but the open probability was less for hyperpolarizations than for depolarizations. The spontaneous channel openings were blocked by micromolar concentrations of picrotoxin. Functional hetero-oligomeric receptors were formed when the molluscan subunit was co-expressed in oocytes with the bovine GABAA receptor alpha 1-subunit, but the channels gated by these receptors did not open spontaneously.

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.