Actions of the insecticide 2(nitromethylene)tetrahydro-1,3-thiazine on insect and vertebrate nicotinic acetylcholine receptors

Effects of cofactors RIC-3, TMX3 and UNC-50, together with distinct subunit ratios on the agonist actions of imidacloprid on Drosophila melanogaster Dα1/Dβ1 nicotinic acetylcholine receptors expressed in Xenopus laevis oocytes

Insect nicotinic acetylcholine receptors (nAChRs) require cofactors for functional heterologous expression. A previous study revealed that TMX3 was crucial for the functional expression of Drosophila melanogaster Dα1/Dβ1 nAChRs in Xenopus laevis oocytes, while UNC-50 and RIC-3 enhanced the acetylcholine (ACh)-induced responses of the nAChRs. However, it is unclear whether the coexpression of UNC-50 and RIC-3 with TMX3 and the subunit stoichiometry affect pharmacology of Dα1/Dβ1 nAChRs when expressed in X. laevis oocytes. We have investigated the effects of coexpressing UNC-50 and RIC-3 with TMX3 as well as changing the subunit stoichiometry on the agonist activity of ACh and imidacloprid on the Dα1/Dβ1 nAChRs. UNC-50 and RIC-3 hardly affected the agonist affinity of ACh and imidacloprid for the Dα1/Dβ1 nAChRs formed by injecting into X. laevis oocytes with an equal amount mixture of the subunit cRNAs, but enhanced current amplitude of the ACh-induced response. Imidacloprid showed higher affinity for the Dβ1 subunit-excess Dα1/Dβ1 (Dα1/Dβ1 = 1/5) nAChRs than the Dα1 subunit-excess Dα1/Dβ1 (Dα1/Dβ1 = 5/1) nAChRs, suggesting that imidacloprid prefers the Dα1-Dβ1 orthosteric site over the Dα1-Dα1 orthosteric site.

Robust functional expression of insect nicotinic acetylcholine receptors provides new insights into neonicotinoid actions and new opportunities for pest and vector control

Neonicotinoids are selective modulators of insect nicotinic acetylcholine receptors (nAChRs). These widely deployed insecticides interact with the orthosteric sites of nAChRs, not only to activate nAChRs on their own, but also to block the desensitizing component of nAChR responses. To date recombinant vertebrate or insect/vertebrate hybrid nAChRs have been deployed to understand the mechanism of selectivity and diversity of neonicotinoid actions as well as to show that both α/α and α/non-α interfaces are involved in the interactions with neonicotinoids. However, many of the fine details of insecticide interactions with sites on nAChRs remain to be resolved. The breakthrough of functional expression of insect nAChRs allows such questions to be addressed, not only for neonicotinoids but for other insecticides targeting insect nAChRs. © 2020 Society of Chemical Industry.

Neonicotinoid Insecticides: Molecular Targets, Resistance, and Toxicity

Neonicotinoids have been used to protect crops and animals from insect pests since the 1990s, but there are concerns regarding their adverse effects on nontarget organisms, notably on bees. Enhanced resistance to neonicotinoids in pests is becoming well documented. We address the current understanding of neonicotinoid target site interactions, selectivity, and metabolism not only in pests but also in beneficial insects such as bees. The findings are relevant to the management of both neonicotinoids and the new generation of pesticides targeting insect nicotinic acetylcholine receptors.

Nicotinic Receptors of Different Species Exhibit Differential Sensitivities to Nitromethylene and Organophosphate Insecticides

The effects of nitromethylene heterocycle (NMH) and organophosphate (OP) insecticides were studied on nicotinic acetylcholine receptors (nAChR) in cultured cells of different species origin, in order to examine the selectivity of these compounds at the level of the target sites. In mouse muscle BC3H1, mouse NIE-115 and human SH-SY5Y neuroblastoma, and locust thoracic ganglion cells, the neurotransmitter, acetylcholine (ACh), induces a similar transient inward current. Dependent on the cell type, the six NMHs acted as agonists and/or antagonists on nAChR. Distinct agonistic effects of NMHs on nAChR are observed on insect neurons only. Further, NMHs potently block nicotinic responses in insects, while mammalian cells are only moderately affected. In all cases, the neuronal type nAChR was more sensitive to blocking than the endplate type nAChR in mammalian cells.

Parathion and paraoxon at micromolar concentrations inhibit ACh-induced nicotinic inward currents. The insecticide, parathion, is a 50-fold more potent inhibitor than its acetyl-cholinesterase-inhibiting metabolite, paraoxon. Moderate differences in sensitivity to the blocking action of the OPs appear to exist among cells of different species. The results demonstrate that the experimental approach of fundamental electrophysiology and the use of cell lines are relevant tools for investigating the species specificity of neurotoxic compounds.

Neonicotinoids: molecular mechanisms of action, insights into resistance and impact on pollinators

Neonicotinoids are insecticides that target insect nicotinic acetylcholine receptors (nAChRs), exhibiting high selective toxicity to insects over vertebrates and good systemic activity in crop plants. For these reasons, neonicotinoids currently make up ∼30% of insecticide sales worldwide. However, due to their adverse impact on pollinators such as honey bees and bumble bees, neonicotinoids are being banned from the EU, and other countries may follow. It is therefore crucial to understand the mechanism underlying neonicotinoid actions on pollinators as well as on the nAChRs of pests, with a view to understanding their selectivity. Here we review the molecular mechanisms of neonicotinoid actions at an atomic level, through structural and resistance mechanism studies and propose relevant research topics for further studies on the future of pest management.

Probing new components (loop G and the α-α interface) of neonicotinoid binding sites on nicotinic acetylcholine receptors

Neonicotinoid insecticides interact with the orthosteric site on the extracellular ligand binding domain (LBD) of nicotinic acetylcholine receptors (nAChRs), typically activating the cation permeable ion channels. In nAChRs consisting of two α and three non-α subunits, LBDs contain six loops (loops A, B and C on the α subunit and loops D, E and F on the non-α subunit) which make up the orthosteric binding site at the α/non-α subunit interfaces. Recently, an additional site (loop G) on the β1 strand has been identified. Also, when the α/non-α subunit ratio is 3/2, another binding site is generated at the interface of two adjacent α subunits. Roles for loop G and the α-α interface in the interactions with neonicotinoids are discussed with reference to recent structural and physiological data.

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.

Neuroactive insecticides: targets, selectivity, resistance, and secondary effects

Neuroactive insecticides are the principal means of protecting crops, people, livestock, and pets from pest insect attack and disease transmission. Currently, the four major nerve targets are acetylcholinesterase for organophosphates and methylcarbamates, the nicotinic acetylcholine receptor for neonicotinoids, the γ-aminobutyric acid receptor/chloride channel for polychlorocyclohexanes and fiproles, and the voltage-gated sodium channel for pyrethroids and dichlorodiphenyltrichloroethane. Species selectivity and acquired resistance are attributable in part to structural differences in binding subsites, receptor subunit interfaces, or transmembrane regions. Additional targets are sites in the sodium channel (indoxacarb and metaflumizone), the glutamate-gated chloride channel (avermectins), the octopamine receptor (amitraz metabolite), and the calcium-activated calcium channel (diamides). Secondary toxic effects in mammals from off-target serine hydrolase inhibition include organophosphate-induced delayed neuropathy and disruption of the cannabinoid system. Possible associations between pesticides and Parkinson's and Alzheimer's diseases are proposed but not established based on epidemiological observations and mechanistic considerations.

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.

Neonicotinoids-from zero to hero in insecticide chemistry

In recent years, neonicotinoids have been the fastest-growing class of insecticides in modern crop protection, with widespread use against a broad spectrum of sucking and certain chewing pests. As potent agonists, they act selectively on insect nicotinic acetylcholine receptors, their molecular target site. The discovery of neonicotinoids can be considered as a milestone in insecticide research and facilitates greatly the understanding of the functional properties of insect nicotinic acetylcholine receptors. Because of the relatively low risk for non-target organisms and environment, the high target specificity of neonicotinoid insecticides and their versatility in application methods, this important class has to be maintained globally for integrated pest management strategies and insect resistance management programmes. This review comprehensively describes particularly the origin, structure and bonding as well as associated properties of neonicotinoid insecticides.

A hypothesis to account for the selective and diverse actions of neonicotinoid insecticides at their molecular targets, nicotinic acetylcholine receptors: catch and release in hydrogen bond networks

The low mammalian toxicity of neonicotinoid insecticides has been shown to be attributable, at least in part, to their selective actions on insect nicotinic acetylcholine receptors (nAChRs). There are multiple nAChRs in insects and a wealth of neonicotinoid chemicals. Studies to date have discribed a wide range of effects on nAChRs, notably partial agonist, super agonist and antagonist actions. Both the diversity of the neonicotinoid actions and their selectivity for insect over vertebrate nAChRs are the result of physicochemical and steric interactions at their molecular targets (nAChRs). In such interactions, the formation and breakage of hydrogen bond (HB) networks plays a key role. Therefore the loss or gain of even a single HB resulting from either structural changes in neonicotinoids, or the amino acid sequence of a particular nAChR subunit, could result in a drastic modification of neonicotinoid actions. In addition to the amino acid residues, the backbone carbonyl of nAChRs may also be involved in the formation of HB networks with neonicotinoids.

Neonicotinoid insecticide toxicology: mechanisms of selective action

The neonicotinoids, the newest major class of insecticides, have outstanding potency and systemic action for crop protection against piercing-sucking pests, and they are highly effective for flea control on cats and dogs. Their common names are acetamiprid, clothianidin, dinotefuran, imidacloprid, nitenpyram, thiacloprid, and thiamethoxam. They generally have low toxicity to mammals (acute and chronic), birds, and fish. Biotransformations involve some activation reactions but largely detoxification mechanisms. In contrast to nicotine, epibatidine, and other ammonium or iminium nicotinoids, which are mostly protonated at physiological pH, the neonicotinoids are not protonated and have an electronegative nitro or cyano pharmacophore. Agonist recognition by the nicotinic receptor involves cation-pi interaction for nicotinoids in mammals and possibly a cationic subsite for interaction with the nitro or cyano substituent of neonicotinoids in insects. The low affinity of neonicotinoids for vertebrate relative to insect nicotinic receptors is a major factor in their favorable toxicological profile.

Suppression by phthalates of the calcium signaling of human nicotinic acetylcholine receptors in human neuroblastoma SH-SY5Y cells

Phthalates are widely used in industry and cause public concern since they have genomic estrogenic-like effects via estrogen receptors. We previously found that some phthalates have nongenomic effects, exerting inhibitory effects on the functional activities of nicotinic acetylcholine receptors (nAChRs) in bovine chromaffin cells. In this study, we investigated the effects of eight phthalates on the calcium signaling of human nAChR by using human neuroblastoma SH-SY5Y cells. All eight phthalates, with different potency, have inhibitory roles on the calcium signaling coupled with human nAChR, but not muscarinic acetylcholine receptors (mAChRs). For inhibition of human nAChR, the strongest to weakest potencies were observed as di-n-pentyl phthalate (DPP) --> butyl benzyl phthalate (BBP) --> di-n-butyl phthalate (DBP) --> dicyclohexyl phthalate (DCHP) --> di-n-hexyl phthalate (DHP) --> di-(2-ethyl hexyl) phthalate (DEHP) --> di-n-propyl phthalate (DPrP) --> diethyl phthalate (DEP). The potencies of phthalates were associated with their structures such that the most effective ones had dialkyl group carbon numbers of C4 or C5, with shorter or longer numbers resulting in decreased potency. At as low as 0.1 microM, DPP, DBP, BBP, DCHP and DHP significantly inhibited the calcium signaling of human nAChR. The IC50 of phthalates on human nAChR, ranging from 0.32 to 7.96 microM, were 10-50 lower than those for bovine nAChR. We suggest that some phthalates effectively inhibit the calcium signaling of human nAChR, and these nongenomic effects are cause for concern.

Selective toxicity of neonicotinoids attributable to specificity of insect and mammalian nicotinic receptors

Neonicotinoids, the most important new class of synthetic insecticides of the past three decades, are used to control sucking insects both on plants and on companion animals. Imidacloprid (the principal example), nitenpyram, acetamiprid, thiacloprid, thiamethoxam, and others act as agonists at the insect nicotinic acetylcholine receptor (nAChR). The botanical insecticide nicotine acts at the same target without the neonicotinoid level of effectiveness or safety. Fundamental differences between the nAChRs of insects and mammals confer remarkable selectivity for the neonicotinoids. Whereas ionized nicotine binds at an anionic subsite in the mammalian nAChR, the negatively tipped ("magic" nitro or cyano) neonicotinoids interact with a proposed unique subsite consisting of cationic amino acid residue(s) in the insect nAChR. Knowledge reviewed here of the functional architecture and molecular aspects of the insect and mammalian nAChRs and their neonicotinoid-binding site lays the foundation for continued development and use of this new class of safe and effective insecticides.

The insecticide imidacloprid is a partial agonist of the nicotinic receptor of honeybee Kenyon cells

The main targets of the insecticide imidacloprid are neuronal nicotinic acetylcholine receptors (nAChRs) within the insect brain. We tested the effects of imidacloprid on ligand-gated ion channels of cultured Kenyon cells of the honeybee, Apis mellifera. Kenyon cells build up the mushroom body neuropils, which are involved in higher order neuronal processes such as olfactory learning. We measured whole-cell currents through nicotinic and gamma-aminobutyric acid (GABA) receptors using patch-clamp techniques. Pressure applications of imidacloprid elicited inward currents, which were irreversibly blocked by alpha-bungarotoxin. Imidacloprid was a partial nicotinic agonist, since it elicited only 36% of ACh-induced currents and competitively blocked 64% of the peak ACh-induced currents. GABA-induced currents were partially blocked when imidacloprid was coapplied and this block was independent upon activation of nAChRs. Our results identify the honeybee nAChR as a target of imidacloprid and an imidacloprid-induced inhibition of the insect GABA receptor.

Laboratory evaluation of fipronil and imidacloprid topical insecticides for control of the plague vector Oropsylla montana (Siphonaptera: Ceratophyllidae) on california ground squirrels (Rodentia: Sciuridae)

Two insecticides, fipronil and imidacloprid, were evaluated for efficacy and longevity against Oropsylla montana (Baker), the most important vector of plague in California. Wild-caught California ground squirrels, Spermophilus beecheyi (Richardson), were individually housed in the laboratory to serve as natural hosts to O. montana and for on-animal insecticide trials. Several concentrations oftechnical grade fipronil and imidacloprid in acetone were applied to samples of clean rodent bedding to determine residual activity and longevity against fleas. Immature and adult cat fleas, Ctenocephalides felis (Bouche), were used as representative fleas for periodic assays in place of less fecund O. montana. Toxicity of treated bedding did not decrease significantly for 1 yr at all applied concentrations. Fipronil provided 100% kill for at least 1 yr at > or = 100 ppm, whereas imidacloprid required 10,000 ppm for similar performance. Laboratory squirrels were treated with topical formulations of fipronil (Frontline Top Spot) and imidacloprid (Advantage Flea Adulticide) at a dosage rate of 15 mg/kg and evaluated for residual activity every 2 wk against adult O. montana. Residual activity was determined by percent recovery of O. montana adults released on treated and untreated animals after 48 h. Frontline provided 100% kill of adult fleas for at least 10 wk, and up to 26 wk on one animal. Advantage failed to provide 100% kill of adult fleas at 2 wk, with complete loss of efficacy by week 6. Concurrent assays with bedding samples from squirrel nest boxes showed negligible toxicity transfer from treated animals to nest bedding.

Neonicotinoids: insecticides acting on insect nicotinic acetylcholine receptors

Imidacloprid is increasingly used worldwide as an insecticide. It is an agonist at nicotinic acetylcholine receptors (nAChRs) and shows selective toxicity for insects over vertebrates. Recent studies using binding assays, molecular biology and electrophysiology suggest that both alpha- and non-alpha-subunits of nAChRs contribute to interactions of these receptors with imidacloprid. Electrostatic interactions of the nitroimine group and bridgehead nitrogen in imidacloprid with particular nAChR amino acid residues are likely to have key roles in determining the selective toxicity of imidacloprid. Chemical calculation of atomic charges of the insecticide molecule and a site-directed mutagenesis study support this hypothesis.

Neuronal nicotinic receptors in the locust Locusta migratoria. Cloning and expression

We have identified five cDNA clones that encode nicotinic acetylcholine receptor (nAChR) subunits expressed in the nervous system of the locust Locusta migratoria. Four of the subunits are ligand-binding alpha subunits, and the other is a structural beta subunit. The existence of at least one more nAChR gene, probably encoding a beta subunit, is indicated. Based on Northern analysis and in situ hybridization, the five subunit genes are expressed. localpha1, localpha3, and locbeta1 are the most abundant subunits and are expressed in similar areas of the head ganglia and retina of the adult locust. Because Loc<alpha3 binds alpha-bungarotoxin with high affinity, it may form a homomeric nAChR subtype such as the mammalian alpha7 nAChR. Localpha1 and Locbeta1 may then form the predominant heteromeric nAChR in the locust brain. localpha4 is mainly expressed in optic lobe ganglionic cells and localpha2 in peripherally located somata of mushroom body neurons. localpha3 mRNA was additionally detected in cells interspersed in the somatogastric epithelium of the locust embryo, suggesting that this isoform may also be involved in functions other than neuronal excitability. Transcription of all nAChR subunit genes begins approximately 3 days before hatching and continues throughout adult life. Electrophysiological recordings from head ganglionic neurons also indicate the existence of more than one functionally distinct nAChR subtype. Our results suggest the existence of several nAChR subtypes, at least some of them heteromeric, in this insect species.

Effects of the alpha subunit on imidacloprid sensitivity of recombinant nicotinic acetylcholine receptors

1. Imidacloprid is a new insecticide with selective toxicity for insects over vertebrates. Recombinant (alpha4beta2) chicken neuronal nicotinic acetylcholine receptors (AChRs) and a hybrid nicotinic AChR formed by co-expression of a Drosophila melanogaster neuronal alpha subunit (SAD) with the chicken beta2 subunit were heterologously expressed in Xenopus oocytes by nuclear injection of cDNAs. The agonist actions of imidacloprid and other nicotinic AChR ligands ((+)-epibatidine, (-)-nicotine and acetylcholine) were compared on both recombinant nicotinic AChRs by use of two-electrode, voltage-clamp electrophysiology. 2. Imidacloprid alone of the 4 agonists behaved as a partial agonist on the alpha4beta2 receptor; (+)-epibatidine, (-)-nicotine and acetylcholine were all full, or near full, agonists. Imidacloprid was also a partial agonist of the hybrid Drosophila SAD chicken beta2 receptor, as was (-)-nicotine, whereas (+)-epibatidine and acetylcholine were full agonists. 3. The EC50 of imidacloprid was decreased by replacing the chicken alpha4 subunit with the Drosophila SAD alpha subunit. This alpha subunit substitution also resulted in an increase in the EC50 for (+)-epibatidine, (-)-nicotine and acetylcholine. Thus, the Drosophila (SAD) alpha subunit contributes to the greater apparent affinity of imidacloprid for recombinant insect/vertebrate nicotinic AChRs. 4. Imidacloprid acted as a weak antagonist of ACh-mediated responses mediated by SADbeta2 hybrid receptors and as a weak potentiator of ACh responses mediated by alpha4beta2 receptors. This suggests that imidacloprid has complex effects upon these recombinant receptors, determined at least in part by the alpha subunit.

Structural factors contributing to insecticidal and selective actions of neonicotinoids

Nicotinoids and neonicotinoids are characterized by the presence of the 3-pyridylmethylamine moiety in their structure. In the former, the amino nitrogen atom is ionized, while in the latter the corresponding nitrogen atom is not ionized but bears a partial positive charge. Both types of insecticides interact with nicotinic acetylcholine receptor (nAChR) of insect origin. The poor interaction of neonicotinoids with vertebrate nAChR was shown by its poor binding affinity to the nAChR from Torpedo electric organ and rat brain and poor activation with nAChR expressed in Xenopus oocytes. The full positive charge was essential to interact with the vertebrate nAChR, while the 3-pyridylmethylamine moiety with a partial positive charge was enough to interact with the insect nAChR. For penetration into the insect central nervous system, hydrophobicity seemed to play an important role, as indicated by the binding of the injected compounds to the housefly head nAChR. The ionization reduced hydrophobicity and limited the penetration of nicotinoids, resulting in less insecticidal activity. Among neonicotinoids, nitromethylene type compounds, though far higher in binding affinity, were less hydrophobic than the corresponding nitroimine type, and the net result was better or inferior insecticidal activity. A chlorine atom at the 6 position of the 3-pyridyl group found in commercialized neonicotinoids contributes to increased binding affinity and more importantly hydrophobicity, thus increasing insecticidal activity. N-Me-imidacloprid was found to be a propesticide of imidacloprid.

Characterization of an acetylcholine receptor gene of Haemonchus contortus in relation to levamisole resistance

The anthelminitic drug levamisole is thought to bind to nicotinic acetylcholine receptors of nematodes. It is possible that resistance to this drug is associated with either a change in binding characteristics or a reduction in the number of nicotinic acetylcholine receptors. Therefore, the molecular mechanism of levamisole resistance in the parasitic nematode Haemonchus contortus was studied by isolating and characterising cDNA clones encoding a putative ligand binding nicotinic acetylcholine receptor subunit, HCAl, of two susceptible and one levamisole resistant population. Hcal is related to unc-38, a nicotinic acetylcholine receptor subunit gene associated with levamisole resistance in Caenorhabditis elegans. Although extensive sequence analyses of hcal sequences revealed polymorphism at amino acid level, no association with levamisole resistance could be detected. Restriction fragment length polymorphism analyses confirmed that, although polymorphism was detected, no selection of a specific allele of hcal has taken place during selection for levamisole resistance in various levamisole resistant populations.

Genetic analysis of cholinergic nerve terminal function in invertebrates

Genetic analysis of nerve terminal function is proving fruitful and studies on invertebrates are making a substantial impact. In this survey, particular emphasis has been placed on cholinergic chemical synaptic transmission. The advanced genetics of Drosophila melanogaster and Caenorhabditis elegans with their rich diversity of behavioural and biochemical mutants is providing new insights into the functions of key molecular components of synapses. A 'space-invader' mutant of Periplaneta americana permits investigations of competition between neurons during synaptogenesis and its impact on neurotransmitter release. The growing importance of the C. elegans genome as a major research resource is emphasized in this survey.

Neuronal ion channels as the target sites of insecticides

Certain types of neuronal ions channels have been demonstrated to be the major target sites of insecticides. The insecticide-channel interactions that have been studied most extensively are pyrethroid actions on the voltage-gated sodium channel and cyclodiene/lindane actions on the GABAA receptor chloride channel complex. With the exception of organophosphate and carbamate insecticides which inhibit acetylcholinesterases, most insecticide commercially developed act on the sodium channel and the GABA system. Pyrethroids show the kinetics of both activation and inactivation gates of sodium channels resulting in prolonged openings of individual channels. This causes membrane depolarization, repetitive discharges and synaptic disturbances leading to hyperexcitatory symptoms of poisoning in animals. Only a very small fraction (approximately 1%) of sodium channel population is required to be modified by pyrethroids to produce severe hyperexcitatory symptoms. This toxicity amplification theory applies to pharmacological and toxicological action of other drugs that go through a threshold phenomenon. Selective toxicity of pyrethroids between invertebrates and mammals can be explained based largely on the responses of sodium channels and partly on metabolic degradation. The pyrethroid-sodium channel interaction is also supported by Na+ uptake and batrachotoxin binding experiments. Cyclodienes and lindane exert a dual action on the GABAA system, the initial transient stimulation being followed by a suppression. The stimulation requires the presence of the gamma 2 subunit. The suppression of the GABA system is also documented by Cl- flux and ligand binding experiments. It appears that the sodium channel and the GABA system merit continuing efforts for development of newer and better insecticides. Nitromethylene heterocycles including imidacloprid act on nicotinic acetylcholine receptors. Insect receptors are more sensitive to these compounds than mammalian receptors. Single-channel analyses of the nicotinic acetylcholine receptor of PC12 cells have shown that imidacloprid increases the activity of subconductance state currents and decreases that of main conductance state currents. This may explain the imidacloprid suppression of acetylcholine responses.

Species-specific Effects of Nitromethylene Heterocycle Insecticides on Nicotinic Receptors in Locust Neurons and Mouse N1E-115 and BC3H1 Cells

The effects of nitromethylene heterocycle (NMH) insecticides on subtypes of nicotinic acetylcholine (nACh) receptors were investigated in locust thoracic ganglion neurons, mouse N1E-115 neuroblastoma cells, and mouse BC3H1 muscle cells by using electrophysiological techniques. In locust neurons, all of the six NMH insecticides tested induced transient inward currents resembling nicotinic ACh-induced inward currents, while, in the continued presence of the NMH compounds, the ACh-induced inward current was blocked. The amplitude of the inward current and the blocking effects of the NMH insecticides were enhanced by concentrations between 0.1 and 10μM. Cross-desensitisation with the ACh-induced inward current confirmed that the NMH-induced inward current was governed by the activation of nACh receptors. Mammalian endplate type nACh receptors in BC3H1 cells and mammalian neuronal type nACh receptors in N1E-115 cells were much less sensitive to the NMH insecticides than the locust neuronal nACh receptors. At a concentration of 10μM, which blocked 80–100% of the ACh-induced inward current in locust neurons, NMH insecticides only partially blocked the ACh-induced inward currents mediated by the two subtypes of mammalian nACh receptors. NMH insecticides also failed to induce significant agonist effects in the mammalian cells at this concentration.

The results provide a possible explanation for the selectively greater toxicity of NMH insecticides to insects than to vertebrates, at the level of nACh receptor subtypes and, hence, demonstrate that this in vitro approach is valuable for the investigation of species-specific interactions of compounds at their target site.

Actions of acromelic acid on nervous system L-glutamate receptors

Acromelic acid, a naturally occurring kainoid, isolated from the mushroom Clitocybe acromelalga, is a weak displacer of [3H]L-glutamate binding to cockroach (Periplaneta americana) nerve cord membranes. Acromelic acid (1 mM) displaces approximately 60% of specifically bound [3H]L-glutamate. When applied by bath perfusion to the cell body membrane of the cockroach fast coxal depressor motor neurone, acromelic acid generated slow, prolonged, dose-dependent depolarizations at concentrations of 0.3 microM and above. Thus acromelic acid is among the most potent of the excitatory amino acids tested to date on insect neurones.

Acetylcholine receptor/channel molecules of insects

Acetylcholine-gated ion channels of the nicotinic type are abundant in the nervous system of insects. The channels are permeable to Na+, K+ and probably Ca(2+), and unlike most vertebrate neuronal nicotinic acetylcholine receptors the receptor/channel molecule is blocked by alpha-bungarotoxin (alpha-Bgt). Such alpha-Bgt-sensitive receptors are present at synapses and on cell bodies of insect neurones. Single channel recordings have shown the existence of multiple conductances of nAChRs. Studies on several different insect preparations have provided evidence for more than one open state and several closed states of insect nAChRs. Functional insect nAChR channels have now been investigated in situ, following reconstitution of a purified protein in bilayers, and as a result of expressing in Xenopus oocytes messenger RNA encoding receptor subunits.

Acetylcholine receptor molecules of the nematode Caenorhabditis elegans

Receptors for acetylcholine are present in nematodes. Studies using physiological and biochemical methods have revealed the existence of nicotinic acetylcholine receptors with a novel pharmacology. Caenorhabditis elegans provides a particularly suitable organism with which to investigate such receptors using molecular genetic approaches. Mutants resistant to the cholinergic agonist (and anthelmintic drug) levamisole have permitted the isolation of a number of genes, including structural subunits of the nicotinic acetylcholine receptor. The only known viable mutants of nicotinic receptors are those of Caenorhabditis elegans. This organism offers the prospect of studying the developmental and regulatory effects of the loss of a single component of the receptor. Using Caenorhabditis elegans it is possible to select interesting phenotypic mutations by in vivo mutagenesis before determining the causative lesion. Resistance genes other than those encoding structural subunits are of particular interest, as they will encode additional polypeptides closely associated with nicotinic receptor function. Such proteins are often difficult or impossible to identify using conventional biochemical approaches, whereas genetic selection should permit their identification.

Nitromethylene actions on in situ and expressed insect nicotinic acetylcholine receptors

Single channel recordings from dissociated housefly (Musca domestica) neurons show that a novel type of nitromethylene insecticide, 2(nitro-methylene)tetrahydro-1,3-thiazine (NMTHT) gates a channel, the conductance and open time histogram of which resemble those obtained when acetylcholine is the agonist. Injection into Xenopus oocytes of a locust (Schistocerca gregaria) alpha-subunit mRNA results in the expression of functional nicotinic receptors sensitive to NMTHT. Control oocytes injected with distilled water are insensitive to the same concentration of this compound. Thus NMTHT exhibits agonist actions at both in situ and expressed insect nicotinic receptors, and one site of action of this compound is on an insect nicotinic receptor alpha-subunit.