Co-existence in DUM neurones of two GluCl channels that differ in their picrotoxin sensitivity

Global, asynchronous partial sweeps at multiple insecticide resistance genes in Aedes mosquitoes

Aedes aegypti (yellow fever mosquito) and Ae. albopictus (Asian tiger mosquito) are globally invasive pests that confer the world's dengue burden. Insecticide-based management has led to the evolution of insecticide resistance in both species, though the genetic architecture and geographical spread of resistance remains incompletely understood. This study investigates partial selective sweeps at resistance genes on two chromosomes and characterises their spread across populations. Sweeps at the voltage-sensitive sodium channel (VSSC) gene on chromosome 3 correspond to one resistance-associated nucleotide substitution in Ae. albopictus and three in Ae. aegypti, including two substitutions at the same nucleotide position (F1534C) that have evolved and spread independently. In Ae. aegypti, we also identify partial sweeps at a second locus on chromosome 2. This locus contains 15 glutathione S-transferase (GST) epsilon class genes with significant copy number variation among populations and where three distinct genetic backgrounds have spread across the Indo-Pacific region, the Americas, and Australia. Local geographical patterns and linkage networks indicate VSSC and GST backgrounds probably spread at different times and interact locally with different genes to produce resistance phenotypes. These findings highlight the rapid global spread of resistance and are evidence for the critical importance of GST genes in resistance evolution.

Biochemical alterations in cotton leafworm, Spodoptera littoralis (Boisd.) related to emamectin benzoate and fipronil compared to their joint action

Cotton leafworm, Spodoptera littoralis (Boisduval), is one of the major destructive pests of ornamental, industrial, and vegetable crops. The efficacy of technical emamectin benzoate (EMB) and fipronil (FPR) was assessed against the 4th larval instar using leaf-dip bioassay method. EMB was more efficient than FPR based on 96 h LC50 values of 0.004 and 0.023 μg/ml, respectively. Joint toxic action of the dual exposure in sequence with time interval 24 h and in mix were evaluated at LC10:LC10, LC25:LC25 and LC50:LC50 after 96 h posttreatment, as well. Their impacts on detoxification enzymes, esterases (ESTs); alkaline phosphatase (ALP); and glutathione S-transferase (GST) as well as acetylcholine esterase (AChE) were also determined. The sequential exposure of EMB after FPR (S1) produced antagonism, potentiation, and potentiation effects, respectively while sequential exposure of FPR after EMB (S2) interacted as addition, potentiation, and potentiation respectively. The rest of binary mixtures (Mix) revealed antagonistic effect regardless of concentration. Orthogonal contrast analysis showed that the highest elevations of AChE, α-EST, β- EST and ALP enzymes were obtained from Mix at LC50:LC50 (181.6%, 288.4, 229.2 and 460.9%, respectively), LC25:LC25 (131.5%, 252.8, 205.60 and 252.0, respectively) and LC10:LC10 (106.6%, 215.6%, 201.8% and 170.0%, respectively). Differently, the greatest elevation of GST activity (157.7%) resulted from S1 at LC50:LC50, while it was significantly lower at LC25:LC25 and LC10:LC10 as well as Mix and S2 at all concentrations than corresponding concentrations of FPR. These findings shed some light on the role of GST in FPR toxicity and clarified the risk of these dual exposures in elevating detoxification enzymes dangerously compared to their individual insecticides. These dual exposures should be carefully handled. Although rotational exposure at low concentrations may enhance performance and mitigate resistance risk, rotational exposure at high concentrations and Mix may indirectly contribute to the evolution of cross-resistance to other insecticides.

Characterization of Glutamate-Gated Chloride Channel in Tribolium castaneum

The glutamate-gated chloride channels (GluCls) play essential roles in signal transduction by regulating fast inhibitory synaptic transmission in the nervous system of invertebrates. While there is only one GluCl subunit in the insect, the diversity of insect GluCls is broadened by alternative splicing. In the present study, three TcGluCl variant genes were cloned from the red flour beetle Tribolium castaneum. Analysis of the characteristics of TcGluCls including sequence features, genomic structures, and alternative splicing revealed that TcGluCls had the typical structural features of GluCls and showed high homologies with the GluCls from other insect orders. The TcGluCl-encoding gene consists of nine exons and three variants (TcGluCl-3a, TcGluCl-3b, and TcGluCl-3c) were generated by the alternative splicing of exon 3, which was a highly conserved alternative splicing site in insect GluCls. Homology modeling of TcGluCl-3a showed that the exon 3 coding protein located at the N-terminal extracellular domain, and there were no steric clashes encountered between the exon 3 coding region and ivermectin/glutamate binding pocket, which indicated that the alternative splicing of exon 3 might have no impact on the binding of GluCls to glutamate and insecticide. In addition to the head tissue, TcGluCl-3a and TcGluCl-3c also had high expressions in the ovary and testis of T. castaneum, whereas TcGluCl-3b showed high expression in the midgut, suggesting the diverse physiological functions of TcGluCl variants in T. castaneum. The total TcGluCl and three variants showed the highest expression levels in the early stage larvae. The expressions of TcGluCl, TcGluCl-3b, and TcGluCl-3c were significantly increased from the late-stage larvae to the early stage pupae and indicated that the TcGluCl might be involved in the growth and development of T. castaneum. These results are helpful to further understand the molecular characteristics of insect GluCls and provide foundations for studying the specific function of the GluCl variant.

Clock gene-dependent glutamate dynamics in the bean bug brain regulate photoperiodic reproduction

Animals adequately modulate their physiological status and behavior according to the season. Many animals sense photoperiod for seasonal adaptation, and the circadian clock is suggested to play an essential role in photoperiodic time measurement. However, circadian clock-driven neural signals in the brain that convey photoperiodic information remain unclear. Here, we focused on brain extracellular dynamics of a classical neurotransmitter glutamate, which is widely used for brain neurotransmission, and analyzed its involvement in photoperiodic responses using the bean bug Riptortus pedestris that shows clear photoperiodism in reproduction. Extracellular glutamate levels in the whole brain were significantly higher under short-day conditions, which cause a reproductive diapause, than those under long-day conditions. The photoperiodic change in glutamate levels was clearly abolished by knockdown of the clock gene period. We also demonstrated that genetic modulation of glutamate dynamics by knockdown of glutamate-metabolizing enzyme genes, glutamate oxaloacetate transaminase (got) and glutamine synthetase (gs), attenuated photoperiodic responses in reproduction. Further, we investigated glutamate-mediated photoperiodic modulations at a cellular level, focusing on the pars intercerebralis (PI) neurons that photoperiodically change their neural activity and promote oviposition. Electrophysiological analyses showed that L-Glutamate acts as an inhibitory signal to PI neurons via glutamate-gated chloride channel (GluCl). Additionally, combination of electrophysiology and genetics revealed that knockdown of got, gs, and glucl disrupted cellular photoperiodic responses of the PI neurons, in addition to reproductive phenotypes. Our results reveal that the extracellular glutamate dynamics are photoperiodically regulated depending on the clock gene and play an essential role in the photoperiodic control of reproduction via inhibitory pathways.

The cys-loop ligand-gated ion channel gene superfamilies of the cockroaches Blattella germanica and Periplaneta americana

BACKGROUND

Cockroaches are serious urban pests that can transfer disease-causing microorganisms as well as trigger allergic reactions and asthma. They are commonly managed by pesticides that act on cys-loop ligand-gated ion channels (cysLGIC). To provide further information that will enhance our understanding of how insecticides act on their molecular targets in cockroaches, we used genome and reverse transcriptase polymerase chain reaction (RT-PCR) data to characterize the cysLGIC gene superfamilies from Blattella germanica and Periplaneta americana.

RESULTS

The B. germanica and P. americana cysLGIC superfamilies consist of 30 and 32 subunit-encoding genes, respectively, which are the largest insect cysLGIC superfamilies characterized to date. As with other insects, the cockroaches possess ion channels predicted to be gated by acetylcholine, γ-aminobutyric acid, glutamate and histamine, as well as orthologues of the drosophila pH-sensitive chloride channel (pHCl), CG8916 and CG12344. The large cysLGIC superfamilies of cockroaches are a result of an expanded number of divergent nicotinic acetylcholine receptor subunits, with B. germanica and P. americana, respectively, possessing eight and ten subunit genes. Diversity of the cockroach cysLGICs is also broadened by alternative splicing and RNA A-to-I editing. Unusually, both cockroach species possess a second glutamate-gated chloride channel as well as another CG8916 subunit.

CONCLUSION

These findings on B. germanica and P. americana enhance our understanding of the evolution of the insect cysLGIC superfamily and provide a useful basis for the study of their function, the detection and management of insecticide resistance, and for the development of improved pesticides with greater specificity towards these major pests. © 2020 Society of Chemical Industry.

Tribolium castaneum: A Model for Investigating the Mode of Action of Insecticides and Mechanisms of Resistance

The red flour beetle, Tribolium castaneum, is a worldwide insect pest of stored products, particularly food grains, and a powerful model organism for developmental, physiological and applied entomological research on coleopteran species. Among coleopterans, T. castaneum has the most fully sequenced and annotated genome and consequently provides the most advanced genetic model of a coleopteran pest. The beetle is also easy to culture and has a short generation time. Research on this beetle is further assisted by the availability of expressed sequence tags and transcriptomic data. Most importantly, it exhibits a very robust response to systemic RNA interference (RNAi), and a database of RNAi phenotypes (iBeetle) is available. Finally, classical transposonbased techniques together with CRISPR/Cas-mediated gene knockout and genome editing allow the creation of transgenic lines. As T. castaneum develops resistance rapidly to many classes of insecticides including organophosphates, methyl carbamates, pyrethroids, neonicotinoids and insect growth regulators such as chitin synthesis inhibitors, it is further a suitable test system for studying resistance mechanisms. In this review, we will summarize recent advances in research focusing on the mode of action of insecticides and mechanisms of resistance identified using T. castaneum as a pest model.

Differential involvement of glutamate-gated chloride channel splice variants in the olfactory memory processes of the honeybee Apis mellifera

Glutamate-gated chloride channels (GluCl) belong to the cys-loop ligand-gated ion channel superfamily and their expression had been described in several invertebrate nervous systems. In the honeybee, a unique gene amel_glucl encodes two alternatively spliced subunits, Amel_GluCl A and Amel_GluCl B. The expression and differential localization of those variants in the honeybee brain had been previously reported. Here we characterized the involvement of each variant in olfactory learning and memory processes, using specific small-interfering RNA (siRNA) targeting each variant. Firstly, the efficacy of the two siRNAs to decrease their targets' expression was tested, both at mRNA and protein levels. The two proteins showed a decrease of their respective expression 24h after injection. Secondly, each siRNA was injected into the brain to test whether or not it affected olfactory memory by using a classical paradigm of conditioning the proboscis extension reflex (PER). Amel_GluCl A was found to be involved only in retrieval of 1-nonanol, whereas Amel_GluCl B was involved in the PER response to 2-hexanol used as a conditioned stimulus or as new odorant. Here for the first time, a differential behavioral involvement of two highly similar GluCl subunits has been characterized in an invertebrate species.

Expression pattern and function of alternative splice variants of glutamate-gated chloride channel in the housefly Musca domestica

Glutamate-gated chloride channels (GluCls) mediate fast inhibitory neurotransmission in invertebrate nervous systems. cDNAs encoding two alternative splice variants (MdGluClB and C) of the GluCl subunit were cloned from the housefly Musca domestica. The expression patterns of three variants, including the previously reported MdGluClA, differed among the body parts (head, thorax, abdomen, and leg) of the adult housefly and among developmental stages (embryo, larva, pupa, and adult). The MdGluClA and B transcripts were abundant in the central nervous system of the adult, whereas the MdGluClC transcript was expressed in the central nervous system and as the predominant variant in the peripheral tissues. The sensitivities to the agonist glutamate and the allosteric activator ivermectin B1a did not differ between channels containing MdGluCl variants when they were singly or co-expressed in Xenopus oocytes. By contrast, MdGluClA and B channels were more sensitive to the channel blockers fipronil and picrotoxinin than was MdGluClC channels. Heteromeric channels containing different subunit variants were more sensitive to picrotoxinin than were homomeric channels. Heteromeric channels were more sensitive to fipronil than were homomeric MdGluClC channels but not than homomeric MdGluClA and B channels. These results suggest that functionally indistinguishable but pharmacologically distinct GluCls are expressed in a spatially and temporally distinct manner in the housefly.

Glutamate is an inhibitory neurotransmitter in the Drosophila olfactory system

Glutamatergic neurons are abundant in the Drosophila central nervous system, but their physiological effects are largely unknown. In this study, we investigated the effects of glutamate in the Drosophila antennal lobe, the first relay in the olfactory system and a model circuit for understanding olfactory processing. In the antennal lobe, one-third of local neurons are glutamatergic. Using in vivo whole-cell patch clamp recordings, we found that many glutamatergic local neurons are broadly tuned to odors. Iontophoresed glutamate hyperpolarizes all major cell types in the antennal lobe, and this effect is blocked by picrotoxin or by transgenic RNAi-mediated knockdown of the GluClα gene, which encodes a glutamate-gated chloride channel. Moreover, antennal lobe neurons are inhibited by selective activation of glutamatergic local neurons using a nonnative genetically encoded cation channel. Finally, transgenic knockdown of GluClα in principal neurons disinhibits the odor responses of these neurons. Thus, glutamate acts as an inhibitory neurotransmitter in the antennal lobe, broadly similar to the role of GABA in this circuit. However, because glutamate release is concentrated between glomeruli, whereas GABA release is concentrated within glomeruli, these neurotransmitters may act on different spatial and temporal scales. Thus, the existence of two parallel inhibitory transmitter systems may increase the range and flexibility of synaptic inhibition.

Expression and localization of glutamate-gated chloride channel variants in honeybee brain (Apis mellifera)

Due to its specificity to invertebrate species, glutamate-gated chloride channels (GluCls) are the target sites of antiparasitic agents and insecticides, e.g. ivermectin and fipronil, respectively. In nematodes and insects, the GluCls diversity is broadened by alternative splicing. GluCl subunits have been characterized according to their sensitivity to drugs, and to their anatomical localization. In the honeybee, the GluCl gene can encode different alpha subunits due to alternative splicing of exon 3. We examined mRNA expression in brain parts and we confirmed the existence of two GluCl variants with RT-PCR, Amel_GluCl A and Amel_GluCl B. Surprisingly, a mixed isoform not yet described in insect was obtained, we called it Amel_GluCl C. We determined precise immunolocalization of peptide sequence corresponding to Amel_GluCl A and Amel_GluCl B in the honeybee brain. Amel_GluCl A is mainly located in neuropils, whereas Amel_GluCl B is mostly expressed in cell bodies. Both proteins can also be co-localized. According to their anatomical localization, different GluCl variants might be involved in olfactory and visual modalities and in learning and memory.

GABA and glutamate receptors have different effects on excitability and are differentially regulated by calcium in spider mechanosensory neurons

GABA and glutamate receptors belonging to the ligand-gated chloride-channel family are primary targets of insecticides and antiparasitics, so their molecular structure, pharmacology and biophysical properties have attracted significant attention. However, little is known about the physiological roles of these channels or how they regulate neuronal excitability and animal behavior. Mechanosensory neurons of VS-3 slit sensilla in the patella of the tropical wandering spider, Cupiennius salei, react to the GABA(A)-receptor agonists, GABA and muscimol, with depolarization and an increase in intracellular [Ca(2+)] and, during random noise stimulation, with a mixed inhibitory-excitatory response. We established that the GABA(A)-receptors in all VS-3 neurons are identical, but there are at least two types of glutamate receptors and some neurons do not respond to glutamate at all. Immunohistochemistry with antibodies against Drosophila inhibitory glutamate receptor (GluCls) α-subunit suggests that in addition to VS-3 neurons, these receptors may also be present in the efferent neurons surrounding the sensory neurons. Most VS-3 neurons were inhibited but not depolarized by glutamate during random stimulation, but some depolarized and had a similar excitatory-inhibitory response to glutamate as to muscimol. The membrane-permeable Ca(2+)-chelator BAPTA-AM abolished muscimol effects but potentiated glutamate effects, indicating that GABA and glutamate receptors are differentially modulated by Ca(2+), leading to diverse regulation of neuronal excitability. We hypothesize that this could be achieved by different Ca(2+)-triggered phosphorylation processes at each receptor type. These findings are important for understanding the significance of Ca(2+)-mediated regulation of transmitter receptor molecules and its role in controlling excitability.

Spider-venom peptides as bioinsecticides

Over 10,000 arthropod species are currently considered to be pest organisms. They are estimated to contribute to the destruction of ~14% of the world's annual crop production and transmit many pathogens. Presently, arthropod pests of agricultural and health significance are controlled predominantly through the use of chemical insecticides. Unfortunately, the widespread use of these agrochemicals has resulted in genetic selection pressure that has led to the development of insecticide-resistant arthropods, as well as concerns over human health and the environment. Bioinsecticides represent a new generation of insecticides that utilise organisms or their derivatives (e.g., transgenic plants, recombinant baculoviruses, toxin-fusion proteins and peptidomimetics) and show promise as environmentally-friendly alternatives to conventional agrochemicals. Spider-venom peptides are now being investigated as potential sources of bioinsecticides. With an estimated 100,000 species, spiders are one of the most successful arthropod predators. Their venom has proven to be a rich source of hyperstable insecticidal mini-proteins that cause insect paralysis or lethality through the modulation of ion channels, receptors and enzymes. Many newly characterized insecticidal spider toxins target novel sites in insects. Here we review the structure and pharmacology of these toxins and discuss the potential of this vast peptide library for the discovery of novel bioinsecticides.

Glutamate-activated chloride channels: Unique fipronil targets present in insects but not in mammals

Selectivity to insects over mammals is one of the important characteristics for a chemical to become a useful insecticide. Fipronil was found to block cockroach GABA receptors more potently than rat GABA(A) receptors. Furthermore, glutamate-activated chloride channels (GluCls), which are present in cockroaches but not in mammals, were very sensitive to the blocking action of fipronil. The IC(50)s of fipronil block were 30 nM in cockroach GABA receptors and 1600 nM in rat GABA(A) receptors. Moreover, GluCls of cockroach neurons had low IC(50)s for fipronil. Two types of glutamate-induced chloride current were obswerved: desensitizing and non-desensitizing, with fipronil IC(50)s of 800 and 10 nM, respectively. We have developed methods to separately record these two types of GluCls. The non-desensitizing and desensitizing currents were selectively inhibited by trypsin and polyvinylpyrrolidone, respectively. In conclusion, in addition to GABA receptors, GluCls play a crucial role in selectivity of fipronil to insects over mammals. GluCls form the basis for development of selective and safe insecticides.

Cys-Loop Ligand-Gated Chloride Channels in Dorsal Unpaired Median Neurons of Locusta migratoria

In insects, inhibitory neurotransmission is generally associated with members of the cys-loop ligand-gated anion channels, such as the glutamate-gated chloride channel (GluCl), the GABA-gated chloride channels (GABACl), and the histamine-gated chloride channels (HisCl). These ionotropic receptors are considered established target sites for the development of insecticides, and therefore it is necessary to obtain a better insight in their distribution, structure, and functional properties. Here, by combining electrophysiology and molecular biology techniques, we identified and characterized GluCl, GABACl, and HisCl in dorsal unpaired median (DUM) neurons of Locust migratoria. In whole cell patch-clamp recordings, application of glutamate, GABA, or histamine induced rapidly activating ionic currents. GluCls were sensitive to ibotenic acid and blocked by picrotoxin and fipronil. The pharmacological profile of the L. migratoria GABACl fitted neither the vertebrate GABAA nor GABAC receptor and was similar to the properties of the cloned Drosophila melanogaster GABA receptor subunit (Rdl). The expression of Rdl-like subunit-containing GABA receptors was shown at the molecular level using RT-PCR. Sequencing analysis indicated that the orthologous GABACl of D. melanogaster CG10357-A is expressed in DUM neurons of L. migratoria. Histamine-induced currents exhibited a fast onset and desensitized completely on continuous application of histamine. In conclusion, within the DUM neurons of L. migratoria, we identified three different cys-loop ligand-gated anion channels that use GABA, glutamate, or histamine as their neurotransmitter.

Glutamatergic and GABAergic effects of fipronil on olfactory learning and memory in the honeybee

We investigated here the role of transmissions mediated by GABA and glutamate-gated chloride channels (GluCls) in olfactory learning and memory in honeybees, both of these channels being a target for fipronil. To do so, we combined olfactory conditioning with injections of either the GABA- and glutamate-interfering fipronil alone, or in combination with the blocker of glutamate transporter L-trans-Pyrrolidine-2,4-Dicarboxylicacid (L-trans-PDC), or the GABA analog Trans-4-Aminocrotonic Acid (TACA). Our results show that a low dose of fipronil (0.1 ng/bee) impaired olfactory memory, while a higher dose (0.5 ng/bee) had no effect. The detrimental effect induced by the low dose of fipronil was rescued by the coinjection of L-trans-PDC but was rather increased by the coinjection of TACA. Moreover, using whole-cell patch-clamp recordings, we observed that L-trans-PDC reduced glutamate-induced chloride currents in antennal lobe cells. We interpret these results as reflecting the involvement of both GluCl and GABA receptors in the impairment of olfactory memory induced by fipronil.

Role of a serine residue (S278) in the pore-facing region of the housefly L-glutamate-gated chloride channel in determining sensitivity to noncompetitive antagonists

Gamma-hexachlorocyclohexane (gamma-HCH), fipronil and picrotoxinin are noncompetitive antagonists (NCAs) of L-glutamate-gated chloride channels (GluCls), yet their potencies are weaker than those on gamma-aminobutyric acid receptors (GABARs). The A302S mutation of Drosophila RDL (resistant to dieldrin) GABAR confers NCA resistance, and housefly GluCls (MdGluCls) possess S278 as the residue corresponding to the A302. Thus, the effects of S278A mutation on the NCA actions on MdGluCls were investigated. The S278A mutation resulted in enhanced blocking by NCAs of the MdGluCl response to 30 microM L-glutamate. However, such actions of gamma-HCH and picrotoxinin, but not of fipronil, on the S278A mutant were reduced with 200 microM L-glutamate. Further increases in the L-glutamate concentration led to potentiation by NCAs of the mutant response to L-glutamate.

Dorsal Unpaired Median Neurons ofLocusta migratoriaExpress Ivermectin- and Fipronil-Sensitive Glutamate-Gated Chloride Channels

Together with type A GABA and strychnine-sensitive glycine receptors, glutamate-gated chloride channels (GluCl) are members of the Cys-loop family of ionotropic receptors, which mediate fast inhibitory neurotransmission. To date, GluCls are found in invertebrates only and therefore represent potential specific targets for insecticides, such as ivermectin and fipronil. In this study, we identified the functional expression of GluCls in dorsal unpaired median (DUM) neurons of the metathoracic ganglion of Locusta migratoria using electrophysiological and molecular biological techniques. In whole cell patch-clamped DUM neurons, glutamate-induced changes in both their membrane potentials (current-clamp) and currents (voltage-clamp) were dependent on the chloride equilibrium potential. On continuous application of glutamate, the glutamate-elicited current response became rapidly and completely desensitized. Application of glutamate in the presence of 10 μM fipronil or 100 μM picrotoxin reversibly decreased GluCl-mediated currents by 87 and 39%, respectively. Furthermore, 1 μM ivermectin induced a persistent chloride current, suggesting the expression of ivermectin-sensitive GluCl α subunits. A degenerate PCR/RACE strategy was used to clone the full-length L. migratoria LmGlClα subunit. Finally, RT-PCR experiments demonstrated the presence of LmGluClα transcripts in locust DUM neurons. Our results provide the first direct evidence of a functional ivermectin-sensitive GluCl channel on the cell surface of DUM neurons of L. migratoria.

Functional characterization of Musca glutamate- and GABA-gated chloride channels expressed independently and coexpressed in Xenopus oocytes

Ligand-gated chloride channels (LGICs) are important targets for insecticides and parasiticides. Genes encoding subunits of two LGICs, a glutamate-gated chloride channel (MdGluCl-alpha) and a gamma-aminobutyric acid (GABA)-gated chloride channel (MdRdl), were cloned from house-flies (Musca domestica L.). These genes were first expressed independently in Xenopus laevis oocytes by cRNA injection in order to investigate the pharmacology of these ligand-gated channels using two-electrode voltage-clamp electrophysiology. It was found that L-glutamate and GABA activated the MdGluCl-alpha homo-oligomers with an EC(50) value of 30 microM and the MdRdl homo-oligomers with an EC(50) value of 101 microM, respectively. Both channels were chloride ion-permeable, and the MdRdl channel was more sensitive to chloride channel blockers, such as gamma-hexachlorocyclohexane (gamma-HCH), fipronil and picrotoxinin, than the MdGluCl-alpha channel. MdGluCl-alpha required only 1-2 days of incubation after cRNA injection to be expressed in oocytes, whereas 4-7 days of incubation was necessary to achieve MdRdl expression. However, when the cRNA of MdGluCl-alpha was injected at a dose of 1% (w/w) 1 day after the injection of the cRNA of MdRdl, a significant increase in the current amplitude of responses to GABA was observed, and the incubation period necessary for MdRdl expression became shorter. These results suggest that MdGluCl-alpha assists in the expression of MdRdl when the two are coexpressed.

Differential blocking actions of 4′-ethynyl-4-n-propylbicycloorthobenzoate (EBOB) and γ-hexachlorocyclohexane (γ-HCH) on γ-aminobutyric acid- and glutamate-induced responses of American cockroach neurons

4'-Ethynyl-4-n-propylbicycloorthobenzoate (EBOB) has been employed extensively as a radioligand in binding assays to evaluate the pharmacology of gamma-aminobutyric acid (GABA)-gated Cl- channels (GABARs) of insects and mammals, and gamma-hexachlorocyclohexane (gamma-HCH) was used as an insecticide targeting insect GABARs. Since recent studies have shown that not only GABARs but also glutamate-gated chloride channels (GluCls) are blocked by picrotoxinin, dieldrin and fipronil, the actions of EBOB and gamma-HCH on native GABARs and GluCls of terminal abdominal ganglion neurons in American cockroach (Periplaneta americana) were tested using patch-clamp electrophysiology. A marked run-down of the GABA- and glutamate-induced responses of the cockroach neurons occurred, when a standard pipette solution was employed, but addition of pyruvate to the solution permitted stable recordings of these responses. With this solution, EBOB and gamma-HCH were found to block not only the GABA- but also glutamate-gated responses, with the actions augmented by repeated co-application with the agonists. It was also found that prolonged pre-application of EBOB and gamma-HCH prior to co-application with GABA and glutamate resulted in enhanced blocking actions, indicating resting-state actions of the blockers. The blocking actions of EBOB and gamma-HCH on the GABA- and glutamate-induced responses were compared by determining IC50 values under steady state condition. The IC50 values for the actions of EBOB on GABAR and GluCls differed less than those of gamma-HCH.

Effects of sublethal doses of fipronil on the behavior of the honeybee (Apis mellifera)

Fipronil is a phenylpyrazole insecticide introduced for pest control, but it can also affect non-target insects such as honeybees. In insects, fipronil is known to block GABA receptors and to inhibit ionotropic glutamate-gated chloride channels, but the behavioral effects of low doses are not yet fully understood. We have studied the effect of sublethal doses of fipronil on the behavior of the honeybee (Apis mellifera) under controlled laboratory conditions. The drug was either administered orally or applied topically on the thorax. A significant reduction of sucrose sensitivity was observed for the dose of 1 ng/bee 1 h after a thoracic application. No significant effect on sucrose sensitivity was obtained with acute oral treatment. A lower dose of fipronil (0.5 ng/bee applied topically) impaired the olfactory learning of the honeybees. By contrast, locomotor activity was not affected. Our results suggest a particular vulnerability of the olfactory memory processes and sucrose perception to sublethal doses of fipronil in the honeybee.

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.

Resistance to fipronil in Drosophila simulans: influence of two point mutations in the RDL GABA receptor subunit

The Eyguieres 42 strain of Drosophila simulans, obtained by laboratory selection, displayed approximately 20,000-fold resistance to the insecticide fipronil. Molecular cloning of the cDNA encoding the RDL GABA receptor subunit of this strain revealed the presence of two mutations: the Rdl mutation (A301G) and an additional mutation in the third transmembrane domain (T350M). In order to assess the individual and combined roles of the two mutations in fipronil resistance, the functional properties of wild-type, A301G, T350M and A301G/T350M homomultimeric RDL receptors were compared by expression in Xenopus oocytes. In wild-type receptors, the inhibition of GABA (EC(30))-induced currents by fipronil and picrotoxin was enhanced by repeated GABA applications. The A301G mutation nearly abolished this effect, decreased the sensitivity to fipronil and picrotoxin and increased the reversibility of inhibition. The T350M mutation also reduced the sensitivity to both antagonists. Of the four receptor variants tested, the double mutant showed the highest resistance to fipronil, following repeated GABA applications. In conclusion, the present study emphasizes new aspects of the pharmacological alterations induced by the Rdl mutation and shows that resistance to GABA receptor-directed insecticides may implicate a mutation distinct from Rdl.

Sulfone Metabolite of Fipronil Blocks γ-Aminobutyric Acid- and Glutamate-Activated Chloride Channels in Mammalian and Insect Neurons

Fipronil sulfone, a major metabolite of fipronil in both insects and mammals, binds strongly to GABA receptors and is thought to play a significant role in poisoning by fipronil. To better understand the mechanism of selective insecticidal action of fipronil, we examined the effects of its sulfone metabolite on GABA- and glutamate-activated chloride channels (GluCls) in cockroach thoracic ganglion neurons and on GABA(A) receptors in rat dorsal root ganglion neurons using the whole-cell patch-clamp technique. Fipronil sulfone blocked both desensitizing and nondesensitizing GluCls in the cockroach. Activation was required for block and unblock of desensitizing GluCls. In contrast, activation was not prerequisite for block and unblock of nondesensitizing channels. After repetitive activation of the receptors, the IC50 of fipronil sulfone to block the desensitizing GluCls was reduced from 350 to 25 nM and that for blocking nondesensitizing GluCls was reduced from 31.2 to 8.8 nM. This use-dependent block may be explained by its slow unbinding rate. In cockroach and rat neurons, fipronil sulfone blocked GABA receptors in both activated and resting states, with IC50 values ranging from 20 to 70 nM. In conclusion, although fipronil sulfone is a potent inhibitor of cockroach GABA receptors, desensitizing and nondesensitizing GluCls, and rat GABA(A) receptors, its selective toxicity in insects over mammals appears to be associated with its potent blocking action on both desensitizing and nondesensitizing GluCls, which are lacking in mammals.

Kinetic and Pharmacological Characterization of Desensitizing and Non-desensitizing Glutamate-gated Chloride Channels in Cockroach Neurons

Glutamate-gated chloride channels (GluCls) are found only in invertebrate nerve and muscle, where they mediate inhibitory synaptic transmission, and are important target sites of insecticides. Two GluCl subtypes have previously been distinguished in isolated cockroach CNS neurons based on differential pharmacology. The present study characterizes the kinetics and pharmacological properties of desensitizing and non-desensitizing GluCls. Both types of GluCls were sensitive to glutamate and ibotenic acid. The non-desensitizing GluCl subtype was elicited by glutamate with an EC(50) of 115.8 microM and a Hill coefficient of 2.6 and was also sensitive to the agonist ibotenic acid with an EC(50) of 42 microM and a Hill coefficient of 1.7. The desensitizing and non-desensitizing currents were carried by chloride ions, and occurred either separately or in combination in individual neurons. The GluCls were also found to coexist with and function independently of the GABA-activated chloride channels. The desensitizing and non-desensitizing GluCls exhibited different sensitivities to the ligand-gated channel blocker picrotoxinin. The desensitizing GluCls were blocked only 8% by 30 microM picrotoxinin, whereas the non-desensitizing GluCls were potently blocked by picrotoxinin with an IC(50) of 4.1 microM. The insecticides fipronil and dieldrin at 1 microM inhibited the desensitizing currents by 56 and 13%, respectively, and the non-desensitizing currents by 98 and 43%, respectively. It is concluded that the two types of GluCls found in cockroach neurons exhibit significantly different electrophysiological and pharmacological characteristics.

Fipronil Is a Potent Open Channel Blocker of Glutamate-Activated Chloride Channels in Cockroach Neurons

Fipronil, a phenylpyrazole insecticide, displays high insecticidal activity and reduced mammalian toxicity. To better elucidate the mechanism of its selective toxicity between insects and mammals and activity against dieldrin-resistant insects, we studied fipronil action on glutamate-gated chloride channels (GluCls), unique invertebrate ligand-gated chloride channels, in cockroach thoracic ganglion neurons, using the whole-cell patch clamp technique. Glutamate evoked two types of chloride currents, a desensitizing current and a nondesensitizing current. Fipronil differentially inhibited these two types of currents with different potencies and with different rates of reversibility. Fipronil inhibited the desensitizing and nondesensitizing GluCls with IC50 values of 801 and 10 nM, respectively. Kinetic analysis revealed that fipronil blocks required channel opening. Recovery of the desensitizing current from fipronil block required channel opening, whereas recovery of nondesensitizing current from block was independent of channel opening. The high potency of fipronil against the nondesensitizing current was due to a slow unblocking rate constant. In addition, when the nondesensitizing GluCls were occupied by picrotoxinin, the receptors became less sensitive to fipronil block. It is concluded that GluCls are a critical target for fipronil, especially for the selective toxicity between mammals and insects, and that fipronil block of GluCls may play a role in the lack of the cross-resistance with dieldrin.

Fipronil Modulation of Glutamate-Induced Chloride Currents in Cockroach Thoracic Ganglion Neurons

Fipronil is the first phenylpyrazole insecticide introduced for pest control. It is effective against some insects that have become resistant to other insecticides, and exhibits low mammalian toxicity. Although fipronil is known to block GABA receptors, the mechanisms of its selective toxicity and efficacy against insects with dieldrin-resistant GABA receptors are not fully understood. We studied the effects of fipronil on the inhibitory glutamate receptor-chloride channel complex, which is found only in invertebrates. Glutamate-activated chloride currents were recorded from neurons isolated from cockroach thoracic ganglia using the whole-cell patch clamp technique. When glutamate was applied to a neuron, it evoked inward currents with an EC50 of 36.8 +/- 3.0 microM and a Hill coefficient of 1.56 +/- 0.17. The similarity between the reversal potential and the calculated chloride equilibrium potential indicated that glutamate-induced currents were carried by chloride ions. Fipronil suppressed the glutamate-induced peak currents in a dose-dependent manner with an IC50 of 0.73 +/- 0.27 microM and a Hill coefficient of 0.68 +/- 0.15. The current decay phases were greatly prolonged after fipronil application in a concentration-dependent manner. Picrotoxinin (PTX) at 100 microM slightly suppressed glutamate-induced currents to 87.8 +/- 3.7% of the control, and dieldrin at 100 microM had no effect (96.7 +/- 3.1%). AP5 and CNQX, mammalian glutamate receptor antagonists, were without effect on glutamate-induced Cl- currents. It is concluded that the potent blocking action of fipronil against glutamate-gated chloride channels may contribute to the higher toxicity against insects than mammals, as well as the efficacy against insects resistant to other insecticides.

Differential actions of fipronil and dieldrin insecticides on GABA-gated chloride channels in cockroach neurons

Fipronil and dieldrin are known to inhibit GABA receptors in both mammals and insects. However, the mechanism of selective toxicity of these insecticides between mammals and insects remains to be seen. One possible mechanism is that insect GABA receptors are more sensitive than mammalian GABAA receptors to fipronil and dieldrin. We examined differential actions of fipronil and dieldrin on GABA-gated chloride channels in insects and compared them with the data on mammalian GABAA receptors. Neurons were acutely dissociated from the American cockroach thoracic ganglia, and currents evoked by GABA were recorded by the whole-cell patch-clamp technique. GABA-evoked currents were carried by chloride ions, blocked by picrotoxinin, but not by bicuculline. Fipronil inhibited GABA currents with an IC50 value of 28 nM, whereas dieldrin exhibited a dual action potentiation with an EC50 value of 4 nM followed by inhibition with an IC50 value of 16 nM. Fipronil and dieldrin acted on the resting receptor at comparable rates, whereas fipronil blocked the activated receptor 10 times faster than dieldrin. Fipronil inhibition was partially reversible, whereas dieldrin inhibition was irreversible. Fipronil was 59 times more potent on cockroach GABA receptors than on rat GABAA receptors. However, the potentiating and inhibitory potencies of dieldrin in cockroach GABA receptors were comparable with those in rat GABAA receptors. It was concluded that the higher toxicity of fipronil in insects than in mammals is due partially to the higher sensitivity of GABA receptors. The mechanism of dieldrin's selective toxicity must lie in factors other than the sensitivity of GABA receptors.

Novel animal-health drug targets from ligand-gated chloride channels

The world's three best-selling veterinary antiparasitic drugs ('parasiticides') act on ligand-gated ion channels. The sequencing of the complete genomes of the invertebrate genetic model organisms Caenorhabditis elegans and Drosophila melanogaster has led to the recent cloning of new subunits of 5-hydroxytryptamine-gated and histamine-gated chloride channels. Together with L-glutamate-gated chloride channels, which are important targets of known parasiticides, and acetylcholine-gated chloride channels, these new classes of ligand-gated chloride channels, which are known only from invertebrates, add to our understanding of inhibitory neural signalling. They could offer the prospect of being targets for a new generation of selective drugs to control nematode and insect parasites.

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.

Taurine-, aspartate- and glutamate-like immunoreactivity identifies chemically distinct subdivisions of Kenyon cells in the cockroach mushroom body

The lobes of the mushroom bodies of the cockroach Periplaneta americana consist of longitudinal modules called laminae. These comprise repeating arrangements of Kenyon cell axons, which like their dendrites and perikarya have an affinity to one of three antisera: to taurine, aspartate, or glutamate. Taurine-immunopositive laminae alternate with immunonegative ones. Aspartate-immunopositive Kenyon cell axons are distributed across the lobes. However, smaller leaf-like ensembles of axons that reveal particularly high affinities to anti-aspartate are embedded within taurine-positive laminae and occur in the immunonegative laminae between them. Together, these arrangements reveal a complex architecture of repeating subunits whose different levels of immunoreactivity correspond to broader immunoreactive layers identified by sera against the neuromodulator FMRFamide. Throughout development and in the adult, the most posterior lamina is glutamate immunopositive. Its axons arise from the most recently born Kenyon cells that in the adult retain their juvenile character, sending a dense system of collaterals to the front of the lobes. Glutamate-positive processes intersect aspartate- and taurine-immunopositive laminae and are disposed such that they might play important roles in synaptogenesis or synapse modification. Glutamate immunoreactivity is not seen in older, mature axons, indicating that Kenyon cells show plasticity of neurotransmitter phenotype during development. Aspartate may be a universal transmitter substance throughout the lobes. High levels of taurine immunoreactivity occur in broad laminae containing the high concentrations of synaptic vesicles.