Exploring the pharmacological properties of insect nicotinic acetylcholine receptors

Inhibition of cholinergic pathways in Drosophila melanogaster by α-conotoxins

Nicotinic acetylcholine receptors (nAChRs) play a pivotal role in synaptic transmission of neuronal signaling pathways and are fundamentally involved in neuronal disorders, including Alzheimer's disease, Parkinson's disease, and schizophrenia. In vertebrates, cholinergic pathways can be selectively inhibited by α-conotoxins; we show that in the model organism Drosophila, the cholinergic component of the giant fiber system is inhibited by α-conotoxins MII, AuIB, BuIA, EI, PeIA, and ImI. The injection of 45 pmol/fly of each toxin dramatically decreases the response of the giant fiber to dorsal longitudinal muscle (GF-DLM) connection to 20 ± 13.9% for MII; 26 ± 13.7% for AuIB, 12 ± 9.9% for BuIA, 30 ± 11.3% for EI, 1 ± 1% for PeIA, and 34 ± 15.4% for ImI. Through bioassay-guided fractionation of the venom of Conus brunneus, we found BruIB, an α-conotoxin that inhibits Drosophila nicotinic receptors but not its vertebrate counterparts. GF-DLM responses decreased to 43.7 ± 8.02% on injection of 45 pmol/fly of BruIB. We manipulated the Dα7 nAChR to mimic the selectivity of its vertebrate counterpart by placing structurally guided point mutations in the conotoxin-binding site. This manipulation rendered vertebrate-like behavior in the Drosophila system, enhancing the suitability of Drosophila as an in vivo tool to carry out studies related to human neuronal diseases. .

Synthesis and biological activity of fluorescent neonicotinoid insecticide thiamethoxam

Here, we describe the synthesis of two new fluorescent derivatives of thiamethoxam and compared their toxicity on aphid Acyrthosiphon pisum and their mode of action on insect nicotinic acetylcholine receptors expressed on the sixth abdominal ganglion. The compound 3 with two 2-chlorothiazole moieties was found to be more toxic using toxicological bioassays 24 h and 48 h after exposure while compound 4 appeared more active using cockroach ganglionic depolarization. Interestingly, thiamethoxam appeared more effective than component 3 and 4, respectively. Our results demonstrated that component 3 and 4 act as agonists of insect nicotinic acetylcholine receptors.

Optogenetic and pharmacologic dissection of feedforward inhibition in Drosophila motion vision

Visual systems extract directional motion information from spatiotemporal luminance changes on the retina. An algorithmic model, the Reichardt detector, accounts for this by multiplying adjacent inputs after asymmetric temporal filtering. The outputs of two mirror-symmetrical units tuned to opposite directions are thought to be subtracted on the dendrites of wide-field motion-sensitive lobula plate tangential cells by antagonistic transmitter systems. In Drosophila, small-field T4/T5 cells carry visual motion information to the tangential cells that are depolarized during preferred and hyperpolarized during null direction motion. While preferred direction input is likely provided by excitation from T4/T5 terminals, the origin of null direction inhibition is unclear. Probing the connectivity between T4/T5 and tangential cells in Drosophila using a combination of optogenetics, electrophysiology, and pharmacology, we found a direct excitatory as well as an indirect inhibitory component. This suggests that the null direction response is caused by feedforward inhibition via yet unidentified neurons.

Comparative analysis of response to selection with three insecticides in the dengue mosquito Aedes aegypti using mRNA sequencing

Background

Mosquito control programmes using chemical insecticides are increasingly threatened by the development of resistance. Such resistance can be the consequence of changes in proteins targeted by insecticides (target site mediated resistance), increased insecticide biodegradation (metabolic resistance), altered transport, sequestration or other mechanisms. As opposed to target site resistance, other mechanisms are far from being fully understood. Indeed, insecticide selection often affects a large number of genes and various biological processes can hypothetically confer resistance. In this context, the aim of the present study was to use RNA sequencing (RNA-seq) for comparing transcription level and polymorphism variations associated with adaptation to chemical insecticides in the mosquito Aedes aegypti. Biological materials consisted of a parental susceptible strain together with three child strains selected across multiple generations with three insecticides from different classes: the pyrethroid permethrin, the neonicotinoid imidacloprid and the carbamate propoxur.

Results

After ten generations, insecticide-selected strains showed elevated resistance levels to the insecticides used for selection. RNA-seq data allowed detecting over 13,000 transcripts, of which 413 were differentially transcribed in insecticide-selected strains as compared to the susceptible strain. Among them, a significant enrichment of transcripts encoding cuticle proteins, transporters and enzymes was observed. Polymorphism analysis revealed over 2500 SNPs showing > 50% allele frequency variations in insecticide-selected strains as compared to the susceptible strain, affecting over 1000 transcripts. Comparing gene transcription and polymorphism patterns revealed marked differences among strains. While imidacloprid selection was linked to the over transcription of many genes, permethrin selection was rather linked to polymorphism variations. Focusing on detoxification enzymes revealed that permethrin selection strongly affected the polymorphism of several transcripts encoding cytochrome P450 monooxygenases likely involved in insecticide biodegradation.

Conclusions

The present study confirmed the power of RNA-seq for identifying concomitantly quantitative and qualitative transcriptome changes associated with insecticide resistance in mosquitoes. Our results suggest that transcriptome modifications can be selected rapidly by insecticides and affect multiple biological functions. Previously neglected by molecular screenings, polymorphism variations of detoxification enzymes may play an important role in the adaptive response of mosquitoes to insecticides.

A novel method using Autographa californica multiple nucleopolyhedrovirus for increasing the sensitivity of insecticide through calcium influx in insect cell line

Due to an intensive use of chemical insecticides, resistance mechanisms to insecticides together with adverse effects on non-target organisms have been largely reported. Improvement in pest control strategy represents an urgent need to optimize efficiency in the control of pest insects. In this context, a novel method based on the use of insect specific virus applied in combination with chemical insecticide, which could lead to sensitization of the insect target to insecticides is described. Insect virus, the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV), applied onto Sf9 cells induces an increase of intracellular calcium concentration via extracellular calcium influx. Co-application of AcMNPV with chlorpyrifos-ethyl onto Sf9 cells expressing the key enzyme acetylcholinesterase (AChE), known to be targeted by organophosphate insecticides, increases 1.5-fold the sensitivity of AChE to the insecticide. This effect is correlated with intracellular calcium concentration rise since AcMNPV-induced potentiating insecticide effect is counteracted by pretreatment with the calcium channel blocker, cadmium chloride. Increasing insecticide target sensitivity through intracellular calcium modulation by using insect virus co-applied with a chemical insecticide is a very promising strategy allowing optimization of insecticide treatment while reducing the concentration of insecticides used.

Nicotinic acetylcholine receptor agonists: a milestone for modern crop protection

The destruction of crops by invertebrate pests is a major threat against a background of a continuously rising demand in food supply for a growing world population. Therefore, efficient crop protection measures in a vast range of agricultural settings are of utmost importance to guarantee sustainable yields. The discovery of synthetic agonists selectively addressing the nicotinic acetylcholine receptors (nAChRs), located in the central nervous system of insects, for use as insecticides was a major milestone in applied crop protection research. These compounds, as a result of their high target specificity and versatility in application methods, opened a new innovative era in the control of some of the world's most devastating insect pests. These insecticides also contributed massively to extending our knowledge of the biochemistry of insect nicotinic acetylcholine receptors. The global economic success of synthetic nAChR agonists as insecticides renders the nicotinic acetylcholine receptor still one of the most attractive target sites for exploration in insecticide discovery.

Ca²⁺/calmodulin-dependent protein kinase II in the cockroach Periplaneta americana: identification of five isoforms and their tissues distribution

Calcium/calmodulin-dependent protein kinase II (CaMKII) is a key kinase that transduces Ca²⁺ signals into downstream effects acting on a range of cellular processes in nervous system and muscular tissues. In insects, different CaMKII isoforms have been reported in Drosophila melanogaster, Apis florae, Bombus terrestris, and Bombus impatiens but little is known on the organization and tissue-specific expression of these isoforms with the exception of Drosophila. The present study reports the cloning of five CaMKII splice variants issued from a single gene and their tissue-specific expression in the cockroach Periplaneta americana. Each CaMKII isoform shared 82-90% identity with Drosophila CaMKII isoforms and accordingly were named PaCaMKII-A, PaCaMKII-B,PaCaMKII-C,PaCaMKII-D, and PaCaMKII-E. PaCaMKII-A and PaCaMKII-D isoforms are ubiquitously expressed in all tissues, but some such as PaCaMKII-B andPaCaMKII-C are preferentially expressed in the nerve cord and muscle. In addition, using single-cell reverse transcriptase-polymerase chain reaction (RT-PCR), we found a tissue-specific expression of PaCaMKII-E in the dorsal unpaired median neurons. Alternative splicing of PaCaMKII transcripts is likely a common mechanism in insects to control the pattern of isoform expression in the different tissues.

Calcium pathways such as cAMP modulate clothianidin action through activation of α-bungarotoxin-sensitive and -insensitive nicotinic acetylcholine receptors

Clothianidin is a neonicotinoid insecticide developed in the early 2000s. We have recently demonstrated that it was a full agonist of α-bungarotoxin-sensitive and -insensitive nicotinic acetylcholine receptors expressed in the cockroach dorsal unpaired median neurons. Clothianidin was able to act as an agonist of imidacloprid-insensitive nAChR2 receptor and internal regulation of cAMP concentration modulated nAChR2 sensitivity to clothianidin. In the present study, we demonstrated that cAMP modulated the agonist action of clothianidin via α-bungarotoxin-sensitive and insensitive receptors. Clothianidin-induced current-voltage curves were dependent to clothianidin concentrations. At 10 μM clothianidin, increasing cAMP concentration induced a linear current-voltage curve. Clothianidin effects were blocked by 0.5 μM α-bungarotoxin suggesting that cAMP modulation occurred through α-bungarotoxin-sensitive receptors. At 1 mM clothianidin, cAMP effects were associated to α-bungarotoxin-insensitive receptors because clothianidin-induced currents were blocked by 5 μM mecamylamine and 20 μM d-tubocurarine. In addition, we found that application of 1mM clothianidin induced a strong increase of intracellular calcium concentration. These data reinforced the finding that calcium pathways including cAMP modulated clothianidin action on insect nicotinic acetylcholine receptors. We proposed that intracellular calcium pathways such as cAMP could be a target to modulate the mode of action of neonicotinoid insecticides.

Structure-activity relationships of acetylcholine derivatives with Lucilia cuprina nicotinic acetylcholine receptor α1 and α2 subunits in chicken β2 subunit hybrid receptors in comparison with chicken nicotinic acetylcholine receptor α4/β2

Insect nicotinic acetylcholine (ACh) receptors (nAChRs) are the targets of several insecticide classes. In the present study, we report the gene identification and cloning of nAChR α1 and α2 subunits (Lcα1 and Lcα2) from the sheep blowfly Lucilia cuprina. Xenopus oocytes voltage clamp experiments as hybrids with the chicken β2 nAChR (Ggβ2) subunit resulted in ACh-gated ion channels with distinct dose-response curves for Lcα1/Ggβ2 (effective concentration 50% [EC50 ] = 80 nM; nH  = 1.05), and Lcα2/Ggβ2 (EC50  = 5.37 μM, nH  = 1.46). The neonicotinoid imidacloprid was a potent agonist for the α-bungarotoxin-sensitive Lcα1/Ggβ2 (EC50 ∼ 20 nM), while the α-bungarotoxin-resistant Lcα2/Ggβ2 showed a 30-fold lower sensitivity to this insecticide (EC50  = 0.62 μM). Thirteen close derivatives of ACh were analysed in EC50 , Hill coefficient and maximum current (relative to ACh) determinations for Lcα1/Ggβ2 and Lcα2/Ggβ2 and the chicken Ggα4/Ggβ2 nAChRs, and comparisons relative to ACh allowed the definition of novel structure-activity and structure-selectivity relationships. In the case of N-ethyl-acetylcholine, the EC50 of the chicken Ggα4/Ggβ2 rose by a factor of 1000, while for both Lcα1/Ggβ2 and Lcα2/Ggβ2, potency remained unchanged. Further derivatives with insect nAChR selectivity potential were acetyl-α-methylcholine and trimethyl-(3-methoxy-3-oxopropyl)ammonium, followed by acetylhomocholine and trimethyl-(4-oxopentyl) ammonium. Our results may provide guidance for the identification or design of insect-specific nAChR agonists using structure-based or in silico methods.

Pretreatment of the cockroach cercal afferent/giant interneuron synapses with nicotinoids and neonicotinoids differently affects acetylcholine and nicotine-induced ganglionic depolarizations

We have recently demonstrated that neonicotinoid insecticides were able to act as agonists of postsynaptic nicotinic acetylcholine receptors (nAChRs) expressed at the synapse between the cercal nerve XI and the giant interneurons, in the sixth abdominal ganglion. In this work, we demonstrated that nicotinoids such as nornicotine acted as an agonist of nicotinic acetylcholine receptors expressed at cercal afferent/giant interneurons while cotinine was a poor agonist. Indeed, nornicotine induced a ganglionic depolarization which was blocked by the nicotinic antagonist mecamylamine. In addition, we found that pretreatment of the sixth abdominal ganglion with 1 and 10 μM nornicotine and cotinine had no significant effect on acetylcholine and nicotine-induced depolarization. But pretreatment with 1 and 10 μM acetamiprid and imidacloprid had a strong effect. 1 and 10 μM acetamiprid completely blocked acetylcholine-induced depolarization, whereas imidacloprid had a partial effect. The present work therefore suggests, in agreement with previous studies, that nornicotine and cotinine bind to distinct cockroach postsynaptic nAChRs, whereas acetamiprid and imidacloprid have competitive effects with acetylcholine and nicotine on ganglionic depolarization.

A fluorinated quinuclidine benzamide named LMA 10203 acts as an agonist of insect nicotinic acetylcholine receptors

In the present study, we take advantage of the fact that cockroach dorsal unpaired median neurons express different nicotinic acetylcholine receptor subtypes to demonstrate that simple quinuclidine benzamides such as the 2-fluorinated benzamide LMA 10203, could act as an agonist of cockroach α-bungarotoxin-insensitive nicotinic acetylcholine receptor subtype, called nAChR2. Indeed, 1 mM LMA 10203 induced ionic currents which were partially blocked by 0.5 μM α-bungarotoxin and methyllycaconitine and completely blocked by 5 μM mecamylamine. Moreover, the current-voltage curve revealed that the ionic current induced by LMA 10203 increased from -30 mV to +20 mV confirming that it acted as an agonist of α-bungarotoxin-insensitive nAChR2. In addition, 1 mM LMA 10203 induced a depolarization of the sixth abdominal ganglion and this neuroexcitatory activity was completely blocked by 5 μM mecamylamine. These data suggest that nAChR2 was also expressed at the postsynaptic level on the synapse between the cercal afferent nerve and the giant interneurons. Interestingly, despite LMA 10203 being an agonist of cockroach nicotinic receptors, it had a poor insecticidal activity. We conclude that LMA 10203 could be used as an interesting compound to identify specific insect nAChR subtypes.

Insights from honeybee (Apis mellifera) and fly (Drosophila melanogaster) nicotinic acetylcholine receptors: from genes to behavioral functions

Nicotinic acetylcholine receptors (nAChRs) are widely expressed throughout the central nervous system of insects where they supply fast synaptic excitatory transmission and represent a major target for several insecticides. The unbalance is striking between the abundant literature on nAChR sensitivity to insecticides and the rarity of information regarding their molecular properties and cognitive functions. The recent advent of genome sequencing disclosed that nAChR gene families of insects are rather small-sized compared to vertebrates. Behavioral experiments performed in the honeybee demonstrated that a subpopulation of nAChRs sensitive to the venom α-bungarotoxin and permeant to calcium is necessary for the formation of long-term memory. Concomitant data in Drosophila reported that repetitive exposure to nicotine results in a calcium-dependent plasticity of the nAChR-mediated response involving cAMP signaling cascades and indicated that ACh-induced Ca++ currents are modulated by monoamines involved in aversive and appetitive learning. As in vertebrates, in which glutamate and NMDA-type glutamate receptors are involved in experience-associated synaptic plasticity and memory formation, insects could display a comparable system based on ACh and α-Bgt-sensitive nAChRs.

Transmembrane potential polarization, calcium influx, and receptor conformational state modulate the sensitivity of the imidacloprid-insensitive neuronal insect nicotinic acetylcholine receptor to neonicotinoid insecticides

Neonicotinoid insecticides act selectively on insect nicotinic acetylcholine receptors (nAChRs). Recent studies revealed that their efficiency was altered by the phosphorylation/dephosphorylation process and the intracellular signaling pathway involved in the regulation of nAChRs. Using whole-cell patch-clamp electrophysiology adapted for dissociated cockroach dorsal unpaired median (DUM) neurons, we demonstrated that intracellular factors involved in the regulation of nAChR function modulated neonicotinoid sensitivity. DUM neurons were known to express two α-bungarotoxin-insensitive nAChR subtypes: nAChR1 and nAChR2. Whereas nAChR1 was sensitive to imidacloprid, nAChR2 was insensitive to this insecticide. Here, we demonstrated that, like nicotine, acetamiprid and clothianidin, other types of neonicotinoid insecticides, acted as agonists on the nAChR2 subtype. Using acetamiprid, we revealed that both steady-state depolarization and hyperpolarization affected nAChR2 sensitivity. The measurement of the input membrane resistance indicated that change in the acetamiprid-induced agonist activity was related to the receptor conformational state. Using cadmium chloride, ω-conotoxin GVIA, and (R,S)-(3,4-dihydro-6,7-dimethoxy-isoquinoline-1-yl)-2-phenyl-N,N-di-acetamide (LOE 908), we found that inhibition of calcium influx through high voltage-activated calcium channels and transient receptor potential γ (TRPγ) activated by both depolarization and hyperpolarization increased nAChR2 sensitivity to acetamiprid. Finally, using N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W7), forskolin, and cAMP, we demonstrated that adenylyl cyclase sensitive to the calcium/calmodulin complex regulated internal cAMP concentration, which in turn modulated TRPγ function and nAChR2 sensitivity to acetamiprid. Similar TRPγ-induced modulatory effects were also obtained when clothianidin was tested. These findings bring insights into the signaling pathway modulating neonicotinoid efficiency and open novel strategies for optimizing insect pest control.

Emerging Pharmacological Properties of Cholinergic Synaptic Transmission: Comparison between Mammalian and Insect Synaptic and Extrasynaptic Nicotinic Receptors

Acetylcholine (ACh) is probably the oldest signalling neurotransmitter which appeared in evolution before the nervous system. It is present in bacteria, algae, protozoa and plants. In insects and mammals it is involved in cell-to-cell communications in various neuronal and non-neuronal tissues. The discovery of nicotinic acetylcholine receptors (nAChRs) as the main receptors involved in rapid cholinergic neurotransmission has helped to understand the role of ACh at synaptic level. Recently, several lines of evidence have indicated that extrasynaptically expressed nAChRs display distinct pharmacological properties from the ones expressed at synaptic level. The role of both nAChRs at insect extrasynaptic and/or synaptic levels has been underestimated due to the lack of pharmacological tools to identify different nicotinic receptor subtypes. In the present review, we summarize recent electrophysiological and pharmacological studies on the extrasynaptic and synaptic differences between insect and mammalian nAChR subtypes and we discuss on the pharmacological impact of several drugs such as neonicotinoid insecticides targeting these receptors. In fact, nAChRs are involved in a wide range of pathophysiological processes such as epilepsy, pain and a wide range of neurodegenerative and psychiatric disorders. In addition, they are the target sites of neonicotinoid insecticides which are known to act as nicotinic agonists causing severe poisoning in insects and mammals.

Natural Products Genomics: A novel approach for the discovery of anti-cancer therapeutics

Plants continue to retain some advantages over combinatorial chemistry as sources of novel compounds, for example, they can generate metabolites with a complexity beyond synthetic chemistry. However, this comes with its own problems in production and synthetic modification of these compounds. Natural Products Genomics (NPG) aims to access the plants own genomic capacity to increase yields, and modify complex bioactive metabolites, to alleviate these limitations. NPG uses a combination of gain of function mutagenesis and selection to a) mimic the evolution of novel compounds in plants, and b) to increase yields of known bioactive metabolites. This process is performed rapidly at the cell culture level in large populations of mutants. Two examples demonstrating proof of concept in Nicotiana tabacum (tobacco) and proof of application in the medicinal plant species Catharanthus roseus, are included to illustrate the feasibility of this approach. This biotechnology platform may alter the way in which plant drug discovery is perceived by the pharmaceutical industry, and provides an alternative to combinatorial chemistry for the discovery, modification and production of highly complex bioactive molecules.

New insights on the molecular features and electrophysiological properties of dinotefuran, imidacloprid and acetamiprid neonicotinoid insecticides

Structural features and hydrogen-bond interactions of dinotefuran (DIN), imidacoloprid (IMI) and acetamiprid (ACE) have been investigated experimentally through analyses of new crystal structures and observations in structural databases, as well as by Density Functional Theory quantum chemical calculations. Several conformations are observed experimentally in the solid state, highlighting the large flexibility of these compounds. This feature is confirmed by the theoretical calculations in the gas phase, the numerous and different energetic minima of the three neonicotinoids being located within a 10kJ/mol range. Comparisons of the observed and simulated data sheds light on the hydrogen-bond (HB) strength of the functional group at the tip of the electronegative fragment of each pharmacophore (NO(2) for DIN and IMI and CN for ACE). This effect originates in the 'push-pull' nature of these fragments and the related extensive electron delocalization. Molecular electrostatic potential calculations provide a ranking of the two fragments of the three neonicotinoid in terms of HB strength. Thus, the NO(2) group of DIN is the strongest HB acceptor of the electronegative fragment, closely followed by the cyano group of ACE. These two groups are significantly more potent than the NO(2) group of IMI. With respect to the other fragments of the three neonicotinoids, the nitrogen atom of the pyridine of IMI and ACE are stronger HB acceptors than the oxygen atom of the furanyl moiety of DIN. Finally, compared to electrophysiological studies obtained from cockroach synaptic and extrasynaptic receptors, DIN appears more effective than IMI and ACE because it strongly increases dose-dependently the ganglionic depolarisation and the currents amplitudes. These data suggest that DIN, IMI and ACE belong to two subgroups which act differently as agonists of insect nicotinic receptors.

Design, synthesis, crystal structure analysis, and insecticidal evaluation of phenylazoneonicotinoids

On the basis of research of the proposed modes of action between neonicotinoids and insect nicotinic acetylcholine receptor (nAChR), a series of phenylazoneonicotinoids were designed and synthesized to further promote the π-π interaction between molecule and amino acid residues. The target compounds have been identified on the basis of satisfactory analytical and spectral ((1)H NMR, (13)C NMR, HRMS, and X-ray) data. The preliminary results revealed that tiny differences in substitutes resulted in different configurations and great bioactivity variations. Some compounds with electron-donating groups on positions 2 and 6 of the phenyl ring presented higher insecticidal activity than imidacloprid against cowpea aphids ( Aphis craccivora ). The impressive crystal structure of the excellent insecticidal activity compound 9q clearly proved that the functional electronegative pharmacophore was approximately vertical to the methyleneimidazolidine plane. The differences in the mode of interaction on nAChR of typical compounds 9h and 9q remain unclear.

Expression patterns of nicotinic subunits α2, α7, α8, and β1 affect the kinetics and pharmacology of ACh-induced currents in adult bee olfactory neuropiles

Acetylcholine (ACh) is the main excitatory neurotransmitter of the insect brain, where nicotinic acetylcholine receptors (nAChRs) mediate fast cholinergic synaptic transmission. In the honeybee Apis mellifera, nAChRs are expressed in diverse structures including the primary olfactory centers of the brain, the antennal lobes (ALs) and the mushroom bodies (MBs), where they participate in olfactory information processing. To understand the nature and properties of the nAChRs involved in these processes, we performed a pharmacological and molecular characterization of nAChRs on cultured Kenyon cells of the MBs, using whole cell patch-clamp recordings combined with single-cell RT-PCR. In all cells, applications of ACh as well as nicotinic agonists such as nicotine and imidacloprid induced inward currents with fast desensitization. These currents were fully blocked by saturating doses of the antagonists α-bungarotoxin (α-BGT), dihydroxy-β-erythroidine (DHE), and methyllycaconitine (MLA) (MLA ≥ α-BGT ≥ DHE). Molecular analysis of ACh-responding cells revealed that of the 11 nicotinic receptor subunits encoded within the honeybee genome, α2, α8, and β1 subunits were expressed in adult Kenyon cells. Comparison with the expression pattern of adult AL cells revealed the supplementary presence of subunit α7, which could be responsible for the kinetic and pharmacological differences observed when comparing ACh-induced currents from AL and Kenyon cells. Together, our data demonstrate the existence of functional nAChRs on adult MB Kenyon cells that differ from nAChRs on AL cells in both their molecular composition and pharmacological properties, suggesting that changing receptor subsets could mediate different processing functions depending on the brain structure within the olfactory pathway.

Insect nicotinic acetylcholine receptor agonists as flea adulticides in small animals

Fleas are significant ectoparasites of small animals. They can be a severe irritant to animals and serve as a vector for a number of infectious diseases. In this article, we discuss the pharmacological characteristics of four insect nicotinic acetylcholine receptor (nAChR) agonists used as flea adulticides in dogs and cats, which include three neonicotinoids (imidacloprid, nitenpyram, and dinotefuran) and a macrocyclic lactone (spinosad). Insect nAChR agonists are one of the most important classes of insecticides, which are used to control sucking insects on both plants and animals. These novel compounds provide a new approach for practitioners to safely and effectively eliminate adult fleas.

Unique neonicotinoid binding conformations conferring selective receptor interactions

Neonicotinoid agonists selectively act on the insect nicotinic acetylcholine receptor (nAChR). The molecular basis for this specificity is deciphered by comparisons of two acetylcholine binding proteins (AChBPs) with distinct pharmacological profiles that serve as structural homologues for the nAChR subtypes. Aplysia AChBP has high neonicotinoid sensitivity, whereas Lymnaea AChBP has low neonicotinoid sensitivity, pharmacologies reminiscent of insect and vertebrate nAChR subtypes, respectively. The ligand-receptor interactions for these AChBPs were established by chemical and structural neurobiology approaches. Neonicotinoids and nicotinoids bind in a single conformation with Aplysia AChBP, wherein the electronegative nitro or cyano pharmacophore of the neonicotinoid faces in a reversed orientation relative to the cationic nicotinoid functionality. For Lymnaea AChBP, the neonicotinoids have two binding conformations in this vertebrate receptor model, which are completely inverted relative to each other, whereas nicotinoids are nestled in only one conserved conformation. Therefore, the unique binding conformations of nicotinic agonists determine the selective receptor interactions.

Differential sensitivity of Ctenocephalides felis and Drosophila melanogaster nicotinic acetylcholine receptor α1 and α2 subunits in recombinant hybrid receptors to nicotinoids and neonicotinoid insecticides

Nicotinic acetylcholine receptors (nAChRs) are the binding sites for nicotinoid drugs, such as nicotine and epibatidine, and are the molecular targets of the selectively insecticidal neonicotinoids. In this study we report the full length cDNA cloning of the three Ctenocephalides (C.) felis (cat flea) nAChR α subunits Cfα1, Cfα2, and Cfα3. When expressed in Xenopus oocytes as hybrid receptors with the Gallus gallus (chicken) β2 (Ggβ2) subunit, these cat flea α subunits formed acetylcholine-responsive ion channels. Acetylcholine-evoked currents of Cfα2/Ggβ2 were resistant to α-bungarotoxin, while those of Cfα1/Ggβ2 were sensitive to this snake toxin. The pharmacological profiles of Cfα1/Ggβ2, Cfα2/Ggβ2 and the chicken neuronal receptor Ggα4/Ggβ2 for acetylcholine, two nicotinoids and 6 insecticidal neonicotinoids were determined and compared. Particularly remarkable was the finding that Cfα1/Ggβ2 was far more sensitive to acetylcholine, nicotine and neonicotinoid agonists than either Cfα2/Ggβ2 or Ggα4/Ggβ2: for the anti flea neonicotinoid market compound imidacloprid the respective EC₅₀s were 0.02 μM, 1.31 μM and 10 μM. These results were confirmed for another insect species, Drosophila melanogaster, where the pharmacological profile of the Dmα1 and Dmα2 subunits as hybrid receptors with Ggβ2 in Xenopus oocyte expressions resulted in a similar sensitivity pattern as those identified for the C. felis orthologs. Our results show that at least in a Ggβ2 hybrid receptor setting, insect α1 subunits confer higher sensitivity to neonicotinoids than α2 subunits, which may contribute in vivo to the insect-selective action of this pesticide class.

Thiamethoxam, a poor agonist of nicotinic acetylcholine receptors expressed on isolated cell bodies, acts as a full agonist at cockroach cercal afferent/giant interneuron synapses

Thiamethoxam (TMX) is a second-generation neonicotinoid which is known to induce toxic effects on insects and mammalians. Recently, it has been proposed that TMX is a poor agonist of insect nicotinic acetylcholine receptors (nAChRs) on isolated cell bodies. Here, we have studied its effect on synaptic transmission. Our results demonstrate that TMX acts as an agonist of nAChRs expressed on cockroach cercal afferent giant/interneuron synapses as bath applications of TMX induce a strong reversible depolarization of the sixth abdominal ganglion. This response was reduced by the nicotinic antagonists mecamylamine and methyllicaconitine, but was insensitive to d-tubocurarine. Interestingly, TMX-induced depolarization was partially reduced by the muscarinic antagonist atropine, suggesting that TMX could bind to a 'mixed nicotinic/muscarinic' receptor. Compared to previous studies, we proposed that TMX is able to act as agonist of insect nAChRs expressed at cercal afferent/giant interneuron synapses. Moreover, our results suggest that nAChRs expressed on synaptic ganglion are distinct to nAChRs expressed on isolated cell bodies and that synaptic receptors have higher affinity to TMX resulting to a depolarization of postsynaptic nicotinic receptors.

Effect of thiamethoxam on cockroach locomotor activity is associated with its metabolite clothianidin

BACKGROUND

In the present study, the effect of thiamethoxam and clothianidin on the locomotor activity of American cockroach, Periplaneta americana (L.), was evaluated. Because it has been proposed that thiamethoxam is metabolised to clothianidin, high-performance liquid chromatography coupled with mass spectrometry was used to evaluate the amount of clothianidin on thiamethoxam-treated cockroaches.

RESULTS

One hour after neonicotinoid treatment, the time spent in the open-field-like apparatus significantly increased, suggesting a decrease in locomotor activity. The percentage of cockroaches displaying locomotor activity was significantly reduced 1 h after haemolymph application of 1 nmol g(-1) neonicotinoid, while no significant effect was found after topical and oral administration. However, at 24 and 48 h, all neonicotinoids were able to reduce locomotor activity, depending on their concentrations and the way they were applied. Interestingly, it was found that thiamethoxam was converted to clothianidin 1 h after application, but the amount of clothianidin did not rise proportionately to thiamethoxam, especially after oral administration.

CONCLUSION

The data suggest that the effect of thiamethoxam on cockroach locomotor activity is due in part to clothianidin action because (1) thiamethoxam levels remained persistent 48 h after application and (2) the amount of clothianidin in cockroach tissues was consistent with the toxicity of thiamethoxam.

Furan-2,5-dimethylene-tethered bis-imidacloprid insecticide conferring high potency

Bis-imidacloprid (bis-IMI) analogues with suitable alkylene spacers have plant-systemic insecticidal properties. The alkylene-tethered bis-IMI binds in a unique mode to the insect nicotinic acetylcholine receptor (nAChR) wherein the chloropyridine moieties are embraced by two distinct and distant domains. The heptamethylene spacer optimally bridges these two subsites, yet the linker itself binds in a relatively nonspecific manner. This investigation examines the hypothesis that a bis-IMI analogue with a heteroaromatic tether, which undergoes specific interaction(s) with the newly recognized receptor cavity, may enhance the potency relative to those of the alkylene-tethered derivatives. Remarkably, a novel bis-IMI with a furan-2,5-dimethylene fulcrum showed highest receptor potency and insecticidal activity among the analogues with various chemotype spacers. The nAChR structural model, simulating the binding site interactions of the furan-2,5-dimethylene-tethered bis-IMI, reveals that the furan ring is nestled in a hydrophobic pocket, consisting of three aromatic amino acids, and is stabilized via hydrogen bonding.

Critical roles of mecamylamine-sensitive mushroom body neurons in insect olfactory learning

In insects, cholinergic neurons are thought to transmit olfactory conditioned stimulus (CS) to the sites for associating the CS with unconditioned stimulus (US), but the types of acetylcholine (ACh) receptor used by neurons participating in the association have not been determined. In cockroaches, a type of nicotinic ACh receptor specifically antagonized by mecamylamine (MEC) has been characterized. Here we investigated the roles of neurons possessing MEC-sensitive ACh receptors (MEC-sensitive neurons) in olfactory conditioning of salivation, monitored by changes in activities of salivary neurons, in cockroaches. Local and bilateral microinjection of MEC into each of the three olfactory centers, antennal lobes, calyces of the mushroom bodies and lateral protocerebra, impaired olfactory responses of salivary neurons, indicating that MEC-sensitive neurons in all olfactory centers participate in pathways mediating olfactory responses of salivary neurons. Conditioning of olfactory CS with sucrose US was impaired by injection of MEC into the antennal lobes or calyces, i.e., conditioned responses were absent even after recovery from MEC injection, suggesting that the CS-US association occurs in MEC-sensitive neurons in calyces (most probably Kenyon cells) or in neurons in downstream pathways. In contrast, conditioned responses appeared after recovery from MEC injection into the lateral protocerebra, suggesting that MEC-sensitive neurons in the lateral protocerebra are downstream of the association sites. Since lateral protocerebra are major termination areas of mushroom body efferent neurons, we suggest that input synapses of MEC-sensitive Kenyon cells, or their output synapses upon mushroom body efferent neurons, are the sites for CS-US association for conditioning of salivation.

Transcripts of the nicotinic acetylcholine receptor subunit gene Pxylα6 with premature stop codons are associated with spinosad resistance in diamondback moth, Plutella xylostella

The cDNA sequence of the α6 nicotinic acetylcholine receptor subunit of diamondback moth (Plutella xylostella) was cloned and sequenced. Transcripts were similar between the spinosad-susceptible G88 and Wapio strains. All transcripts from the spinosad-resistant Pearl-Sel strain contained premature stop codons, and most transcripts have not been previously reported. None of these truncated transcripts were seen in the spinosad-susceptible strains. Proteins made from these transcripts would likely have no, or greatly altered, receptor function. An F(2) backcross and spinosad bioassay showed that all spinosad bioassay survivors produced truncated α6 transcripts. Thus, it appears that spinosad resistance in diamondback moth is due to a mutation(s) that results in no functional Pxylα6 being produced.

Identification of cholinergic synaptic transmission in the insect nervous system

A major criteria initially used to localize cholinergic neuronal elements in nervous systems tissues that involve acetylcholine (ACh) as neurotransmitter is mainly based on immunochemical studies using choline acetyltransferase (ChAT), an enzyme which catalyzes ACh biosynthesis and the ACh degradative enzyme named acetylcholinesterase (AChE). Immunochemical studies using anti-ChAT monoclonal antibody have allowed the identification of neuronal processes and few types of cell somata that contain ChAT protein. In situ hybridization using cRNA probes to ChAT or AChE messenger RNA have brought new approaches to further identify cell bodies transcribing the ChAT or AChE genes. Combined application of all these techniques reveals a widespread expression of ChAT and AChE activities in the insect central nervous system and peripheral sensory neurons which implicates ACh as a key neurotransmitter. The discovery of the snake toxin alpha-bungatoxin has helped to identify nicotinic acetylcholine receptors (nAChRs). In fact, nicotine when applied to insect neurons, resulted in the generation of an inward current through the activation of nicotinic receptors which were blocked by alpha-bungarotoxin. Thus, insect nAChRs have been divided into two categories, sensitive and insensitive to this snake toxin. Up to now, the recent characterization and distribution pattern of insect nAChR subunits and the biochemical evidence that the insect central nervous system contains different classes of cholinergic receptors indicated that ACh is involved in several sensory pathways.

Precise subcellular input retinotopy and its computational consequences in an identified visual interneuron

The Lobula Giant Movement Detector (LGMD) is a higher-order visual interneuron of Orthopteran insects that responds preferentially to objects approaching on a collision course. It receives excitatory input from an entire visual hemifield that anatomical evidence suggests is retinotopic. We show that this excitatory projection activates calcium-permeable nicotinic acetylcholine receptors. In vivo calcium imaging reveals that the excitatory projection preserves retinotopy down to the level of a single ommatidium. Examining the impact of retinotopy on the LGMD's computational properties, we show that sublinear synaptic summation can explain orientation preference in this cell. Exploring retinotopy's impact on directional selectivity leads us to infer that the excitatory input to the LGMD is intrinsically directionally selective. Our results show that precise retinotopy has implications for the dendritic integration of visual information in a single neuron.

Molecular characterization of two nicotinic acetylcholine receptor subunits from Liposcelis bostrychophila Badonnel (Psocoptera: Liposcelididae)

Two nicotinic acetylcholine receptor (nAChR) subunit genes, Lbalpha1 and Lbalpha8, were isolated and characterized from psocid, Liposcelis bostrychophila Badonnel, using the rapid amplification of cDNA ends (RACE) technique. They are the first two nAChR family members isolated from the insect order of Psocoptera. The full-length cDNAs of Lbalpha1 (GenBank accession number: EU871527) and Lbalpha8 (EU871526) consist of 2,025 and 1,763 nucleotides, respectively, and an open reading frame of 1,644 and 1,608 bp encoding 547 and 535 amino acid proteins, respectively. Both genes have typical features of nAChR family members, though they share only 56% identity in amino acid sequence. The dendrogram generated by the MEGA 3.1 program shows that the protein deduced by Lbalpha1 had the closest phylogenetic relationship to Agamalpha1 from Anopheles gambiae and Amelalpha1 from Apis mellifera, and Lbalpha8 shares the highest identity with Agamalpha8 from An. gambiae and Amelalpha8 from A. mellifera. Quantitative real-time PCR analysis showed that Lbalpha1 was expressed 2.03-6.54-fold higher than Lbalpha8 at the different developmental stages of L. bostrychophila. The highest expression levels of Lbalpha1 and Lbalpha8 were both detected at adult stage and the lowest were at the third and fourth nymphal stages, respectively. There was a stable and relatively low expression level for Lbalpha1, whereas there was a descending expression pattern for Lbalpha8 in the 1st through the 4th nymphal stadia.

Diverse actions and target-site selectivity of neonicotinoids: structural insights

The nicotinic acetylcholine receptors (nAChRs) are targets for human and veterinary medicines as well as insecticides. Subtype-selectivity among the diverse nAChR family members is important for medicines targeting particular disorders, and pest-insect selectivity is essential for the development of safer, environmentally acceptable insecticides. Neonicotinoid insecticides selectively targeting insect nAChRs have important applications in crop protection and animal health. Members of this class exhibit strikingly diverse actions on their nAChR targets. Here we review the chemistry and diverse actions of neonicotinoids on insect and mammalian nAChRs. Electrophysiological studies on native nAChRs and on wild-type and mutagenized recombinant nAChRs have shown that basic residues particular to loop D of insect nAChRs are likely to interact electrostatically with the nitro group of neonicotinoids. In 2008, the crystal structures were published showing neonicotinoids docking into the acetylcholine binding site of molluscan acetylcholine binding proteins with homology to the ligand binding domain (LBD) of nAChRs. The crystal structures showed that 1) glutamine in loop D, corresponding to the basic residues of insect nAChRs, hydrogen bonds with the NO(2) group of imidacloprid and 2) neonicotinoid-unique stacking and CH-pi bonds at the LBD. A neonicotinoid-resistant strain obtained by laboratory-screening has been found to result from target site mutations, and possible reasons for this are also suggested by the crystal structures. The prospects of designing neonicotinoids that are safe not only for mammals but also for beneficial insects such as honey bees (Apis mellifera) are discussed in terms of interactions with non-alpha nAChR subunits.

Pharmacological analysis of tonic activity in motoneurons during stick insect walking

Stick insect middle leg (mesothoracic) motoneurons receive tonic excitatory input during front leg stepping on a treadmill. We studied the pharmacology of this excitatory input to the motoneurons during single-legged treadmill walking (in situ). During bath application of drugs restricted to the mesothoracic ganglion, activity in motoneurons contralateral to the stepping front leg was recorded from neuropilar processes. Application of the cholinergic antagonist atropine reduced the tonic depolarization amplitude. These results were compared with findings in acutely dissociated motoneuron cell bodies (in vitro) under whole cell voltage-clamp conditions. The presence of an acetylcholine-induced current in situ was supported by the finding of an acetylcholine evoked biphasic inward current with a sustained component that could be blocked by atropine. In situ the tonic depolarization was generally increased by application of the neuro-modulator octopamine and decreased by its antagonist mianserin. In vitro, however, octopamine reduced the inward current evoked by acetylcholine application to motoneurons. Intracellular application of bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) into motoneurons in situ revealed a dependence of the tonic depolarization on Ca(2+) and application of the membrane-permeable cAMP analogue 8-bromo-cAMP increased the tonic depolarization. In contrast, 8-bromo-cAMP reduced the inward current evoked by acetylcholine application to motoneurons in vitro. We conclude that during walking, acetylcholine contributes to mediating the tonic depolarization possibly by acting on atropine-sensitive receptors on motoneurons. Octopamine that is released during walking increases the tonic depolarization. This increase, however, is not based on modulation of cholinergic action on motoneurons but rather on effects on premotor neurons. Both, Ca(2+) and cAMP are likely second messengers involved in mediating the tonic depolarization, but whereas Ca(2+) acts in motoneurons, cAMP does not appear to mediate a cholinergic depolarization in motoneurons.

Transcriptional diversity and allelic variation in nicotinic acetylcholine receptor subunits of the red flour beetle, Tribolium castaneum

Sequence analysis of 168 cDNA clones encoding 12 nicotinic acetylcholine receptor subunits, Tcasalpha1-Tcasalpha11 and Tcasbeta1, from the red flour beetle, Tribolium castaneum, revealed extensive post-transcriptional modification and multiple alleles. The greatest diversity was found for Tcasa6, where 18 unique transcripts, as a result of alternative and optional exon usage, were seen. A novel alternative exon 8d was found in one Tcasalpha6 transcript. Tcasalpha5 transcripts did not contain previously reported exons 8-10. Six subunits had transcripts that contained unspliced introns, which introduced premature stop codons. Intron 3' splice site variants were seen at six intron boundaries across five subunits. A-to-I RNA editing was seen only in Tcasalpha6. Alleles were found for all subunit genes, except Tcasalpha1 and Tcasalpha10. Transcriptional and allelic diversity are discussed with respect to receptor function and potential interactions with insecticides.

Innovative applications for insect viruses: towards insecticide sensitization

The effective management of emerging insect-borne disease is dependent on the use of safe and efficacious chemical insecticides. Given the inherent ability of insects to develop resistance, it is essential to propose innovative strategies because insecticides remain the most important element of integrated approaches to vector control. Recently, intracellular phosphorylation and dephosphorylation of membrane receptors and ion channels targeted by insecticides have been described as new processes for increasing the sensitivity of insecticides. An efficient method might be to infect host insects with recombinant viruses overexpressing specific protein phosphatases/kinases known to regulate specific insecticide-sensitive targets. This attractive strategy could lead to sensitization of the insects, thus reducing the doses of insecticides and increasing the efficacy of treatments.

Molecular recognition of neonicotinoid insecticides: the determinants of life or death

Until the mid-20th century, pest insect control in agriculture relied on largely inorganic and botanical insecticides, which were inadequate. Then, the remarkable insecticidal properties of several organochlorines, organophosphates, methylcarbamates, and pyrethroids were discovered, leading to an arsenal of synthetic organics. The effectiveness of these insecticides, however, diminished over time due to the emergence of resistant insect strains with less sensitive molecular targets in their nervous systems. This created a critical need for a new type of neuroactive insecticide with a different yet highly sensitive target. Nicotine in tobacco extract was for centuries the best available agent to prevent sucking insects from damaging crops, although this alkaloid was hazardous to people and not very effective. The search for unusual structures and optimization revealed a new class of potent insecticides, known as neonicotinoids, which are similar to nicotine in their structure and action as agonists of the nicotinic acetylcholine receptor (nAChR). Fortunately, neonicotinoids are much more toxic to insects than mammals due in large part to differences in their binding site interactions at the corresponding nAChRs. This Account discusses the progress that has been made in defining the structural basis of neonicotinoid and nicotinoid potency and selectivity. The findings are based on comparisons of two acetylcholine binding proteins (AChBPs) with distinct pharmacological profiles that serve as structural surrogates for the extracellular ligand-binding domain of the nAChRs. Saltwater mollusk (Aplysia californica) AChBP has high neonicotinoid sensitivity, whereas freshwater snail (Lymnaea stagnalis) AChBP has low neonicotinoid and high nicotinoid sensitivities, pharmacologies reminiscent of insect and vertebrate nAChR subtypes, respectively. The ligand-receptor interactions for these AChBPs were established by photoaffinity labeling and X-ray crystallography. Both azidopyridinyl neonicotinoid and nicotinoid photoprobes bind in a single conformation with Aplysia AChBP; this is consistent with high-resolution crystal structures. Surprisingly, though, the electronegative nitro or cyano moiety of the neonicotinoid faced in a reversed orientation relative to the cationic nicotinoid functionality. For the Lymnaea AChBP, the azidoneonicotinoid probes modified two distinct and distant sites, while the azidonicotinoid probes, surprisingly, derivatized only one point. This meant that the neonicotinoids have two bound conformations in the vertebrate receptor model, which are completely inverted relative to each other, whereas nicotinoids appear buried in only one conserved conformation. Therefore, the unique binding conformations of nicotinic agonists in these insect and vertebrate receptor homologues define the basis for molecular recognition of neonicotinoid insecticides as the determinants of life or death.

Actions of quinolizidine alkaloids on Periplaneta americana nicotinic acetylcholine receptors

BACKGROUND

Botanical insecticides do not play a major role as crop protectants, but they are beneficial in some applications. The authors investigated the actions of naturally occurring alkaloids on insect nicotinic acetylcholine (ACh) receptors (nAChRs) by evaluating their abilities to inhibit specific binding of [(3)H]imidacloprid (IMI) to nerve-cord membranes from Periplaneta americana L. Two alkaloids were also tested for their actions on nAChRs expressed by cockroach neurons using patch-clamp electrophysiology.

RESULTS

Four natural quinolizidine alkaloids (matrine, sophocarpine, cytisine and aloperine) exhibited more than 50% inhibition of [(3)H]IMI binding at 10 microM, although other compounds were found to have no or low inhibitory activity. The rank order of potency based on concentration-inhibition curves was cytisine > sophocarpine >or= aloperine >or= matrine. Patch-clamp analysis indicated that sophocarpine and aloperine were not agonists of nAChRs expressed in P. americana neurons, yet, at 10 microM, aloperine, but not sophocarpine, suppressed ACh-induced inward currents significantly.

CONCLUSION

Three of the four natural alkaloids tested possess structural moieties that are necessary for interaction with P. americana nAChRs. Aloperine, which possesses a unique structure and showed a distinctive dose-response curve, was found to act as an antagonist. Appropriate modifications of these alkaloids might result in novel insecticidal nAChR ligands.

Involvement of NO-synthase and nicotinic receptors in learning in the honey bee

Restrained worker honey bees (Apis mellifera) are one of the main models for the comparative study of learning and memory processes. Bees easily learn to associate a sucrose reward to antennal tactile scanning of a small metal plate (associative learning). Their proboscis extension response can also be habituated through repeated sucrose stimulations (non-associative learning). We studied the role of nitric oxide synthase and nicotinic acetylcholine receptors in these two forms of learning. The nicotinic antagonist MLA or the nitric oxide synthase inhibitor l-NAME impaired the formation of tactile associative long-term memory that specifically occurs during multiple-trial training; however these drugs had no effect on single-trial training. None of the drugs affected retrieval processes. These pharmacological results are consistent with data previously obtained with olfactory conditioning and indicate that MLA-sensitive nicotinic receptors and NO-synthase are specifically involved in long-term memory. MLA and l-NAME both reduced the number of trials required for habituation to occur. This result suggests that a reduction of cholinergic nicotinic neurotransmission promotes PER habituation in the honey bee.