Automated phenotyping of mosquito larvae enables high-throughput screening for novel larvicides and offers potential for smartphone-based detection of larval insecticide resistance

Pyrethroid-impregnated nets have contributed significantly to halving the burden of malaria but resistance threatens their future efficacy and the pipeline of new insecticides is short. Here we report that an invertebrate automated phenotyping platform (INVAPP), combined with the algorithm Paragon, provides a robust system for measuring larval motility in Anopheles gambiae (and An. coluzzi) as well as Aedes aegypti with the capacity for high-throughput screening for new larvicides. By this means, we reliably quantified both time- and concentration-dependent actions of chemical insecticides faster than using the WHO standard larval assay. We illustrate the effectiveness of the system using an established larvicide (temephos) and demonstrate its capacity for library-scale chemical screening using the Medicines for Malaria Venture (MMV) Pathogen Box library. As a proof-of-principle, this library screen identified a compound, subsequently confirmed to be tolfenpyrad, as an effective larvicide. We have also used the INVAPP / Paragon system to compare responses in larvae derived from WHO classified deltamethrin resistant and sensitive mosquitoes. We show how this approach to monitoring larval response to insecticides can be adapted for use with a smartphone camera application and therefore has potential for further development as a simple portable field-assay with associated real-time, geo-located information to identify hotspots.

Turning a Drug Target into a Drug Candidate: A New Paradigm for Neurological Drug Discovery?

The conventional paradigm for developing new treatments for disease mainly involves either the discovery of new drug targets, or finding new, improved drugs for old targets. However, an ion channel found only in invertebrates offers the potential of a completely new paradigm in which an established drug target can be re-engineered to serve as a new candidate therapeutic agent. The L-glutamate-gated chloride channels (GluCls) of invertebrates are absent from vertebrate genomes, offering the opportunity to introduce this exogenous, inhibitory, L-glutamate receptor into vertebrate neuronal circuits either as a tool with which to study neural networks, or a candidate therapy. Epileptic seizures can involve L-glutamate-induced hyper-excitation and toxicity. Variant GluCls, with their inhibitory responses to L-glutamate, when engineered into human neurons, might counter the excitotoxic effects of excess L-glutamate. In reviewing recent studies on model organisms, it appears that this approach might offer a new paradigm for the development of candidate therapeutics for epilepsy.

Insect toxins - selective pharmacological tools and drug/chemical leads

Insect toxins comprise a diverse array of chemicals ranging from small molecules, polyamines and peptide toxins. Many target nervous system and neuromuscular ion channels and so rapidly affect the behaviour of animals to which the toxin is applied or injected. Other modes of action have also been identified. Wasps, bees, flies, beetles and ants generate a rich arsenal of channel-active toxins, some of which offer selective pharmacological probes that target particular ion channels, while others act on more than one type of channel. Philanthotoxins from the digger wasp have been fruitful in adding to our understanding of ligand-gated ion channels both in the nervous system and at neuromuscular junctions. Fire ants produce the toxic alkaloid solenopsin, a molecule which has stimulated attempts to generate synthetic compounds with insecticidal activity. Apamin from bee venom targets calcium-activated potassium channels, which can in turn influence the release of neuropeptides. Melittin, another bee venom component, is a membrane-acting peptide. The saliva of the assassin bug contains toxins that target the voltage-gated calcium channels of their insect prey. Certain beetles produce diamphotoxin, a haemolytic peptide toxin with traditional use as an arrow poison and others generate leptinotarsin with similar properties. Mastoparan is a powerful peptide toxin present in the venom of wasps. Its toxic actions can be engineered out leaving a potent antimicrobial molecule of interest. In this short review we describe the actions of selected insect toxins and evaluate their potential as neuroactive pharmacological tools, candidate lead molecules for insect control and therapeutic candidates with potential antimicrobial, antiviral and anti-cancer applications.

Improved reference genome of Aedes aegypti informs arbovirus vector control

Female Aedes aegypti mosquitoes infect more than 400 million people each year with dangerous viral pathogens including dengue, yellow fever, Zika and chikungunya. Progress in understanding the biology of mosquitoes and developing the tools to fight them has been slowed by the lack of a high-quality genome assembly. Here we combine diverse technologies to produce the markedly improved, fully re-annotated AaegL5 genome assembly, and demonstrate how it accelerates mosquito science. We anchored physical and cytogenetic maps, doubled the number of known chemosensory ionotropic receptors that guide mosquitoes to human hosts and egg-laying sites, provided further insight into the size and composition of the sex-determining M locus, and revealed copy-number variation among glutathione S-transferase genes that are important for insecticide resistance. Using high-resolution quantitative trait locus and population genomic analyses, we mapped new candidates for dengue vector competence and insecticide resistance. AaegL5 will catalyse new biological insights and intervention strategies to fight this deadly disease vector.

An improved, fully re-annotated Aedes aegypti genome assembly (AaegL5) provides insights into the sex-determining M locus, chemosensory systems that help mosquitoes to hunt humans and loci involved in insecticide resistance and will help to generate intervention strategies to fight this deadly disease vector.

The fungal alkaloid Okaramine-B activates an L-glutamate-gated chloride channel from Ixodes scapularis, a tick vector of Lyme disease

A novel L-glutamate-gated anion channel (IscaGluCl1) has been cloned from the black-legged tick, Ixodes scapularis, which transmits multiple pathogens including the agents of Lyme disease and human granulocytic anaplasmosis. When mRNA encoding IscaGluCl1 was expressed in Xenopus laevis oocytes, we detected robust 50-400 nA currents in response to 100 μM L-glutamate. Responses to L-glutamate were concentration-dependent (pEC₅₀ 3.64 ± 0.11). Ibotenate was a partial agonist on IscaGluCl1. We detected no response to 100 μM aspartate, quisqualate, kainate, AMPA or NMDA. Ivermectin at 1 μM activated IscaGluCl1, whereas picrotoxinin (pIC₅₀ 6.20 ± 0.04) and the phenylpyrazole fipronil (pIC₅₀ 6.90 ± 0.04) showed concentration-dependent block of the L-glutamate response. The indole alkaloid okaramine B, isolated from fermentation products of Penicillium simplicissimum (strain AK40) grown on okara pulp, activated IscaGluCl1 in a concentration-dependent manner (pEC₅₀ 5.43 ± 0.43) and may serve as a candidate lead compound for the development of new acaricides.

An automated high-throughput system for phenotypic screening of chemical libraries on C. elegans and parasitic nematodes

Parasitic nematodes infect hundreds of millions of people and farmed livestock. Further, plant parasitic nematodes result in major crop damage. The pipeline of therapeutic compounds is limited and parasite resistance to the existing anthelmintic compounds is a global threat. We have developed an INVertebrate Automated Phenotyping Platform (INVAPP) for high-throughput, plate-based chemical screening, and an algorithm (Paragon) which allows screening for compounds that have an effect on motility and development of parasitic worms. We have validated its utility by determining the efficacy of a panel of known anthelmintics against model and parasitic nematodes: Caenorhabditis elegans, Haemonchus contortus, Teladorsagia circumcincta, and Trichuris muris. We then applied the system to screen the Pathogen Box chemical library in a blinded fashion and identified compounds already known to have anthelmintic or anti-parasitic activity, including tolfenpyrad, auranofin, and mebendazole; and 14 compounds previously undescribed as anthelmintics, including benzoxaborole and isoxazole chemotypes. This system offers an effective, high-throughput system for the discovery of novel anthelmintics.

Genomic insights into the Ixodes scapularis tick vector of Lyme disease

Ticks transmit more pathogens to humans and animals than any other arthropod. We describe the 2.1 Gbp nuclear genome of the tick, Ixodes scapularis (Say), which vectors pathogens that cause Lyme disease, human granulocytic anaplasmosis, babesiosis and other diseases. The large genome reflects accumulation of repetitive DNA, new lineages of retro-transposons, and gene architecture patterns resembling ancient metazoans rather than pancrustaceans. Annotation of scaffolds representing ∼57% of the genome, reveals 20,486 protein-coding genes and expansions of gene families associated with tick-host interactions. We report insights from genome analyses into parasitic processes unique to ticks, including host 'questing', prolonged feeding, cuticle synthesis, blood meal concentration, novel methods of haemoglobin digestion, haem detoxification, vitellogenesis and prolonged off-host survival. We identify proteins associated with the agent of human granulocytic anaplasmosis, an emerging disease, and the encephalitis-causing Langat virus, and a population structure correlated to life-history traits and transmission of the Lyme disease agent.

Automated, high-throughput, motility analysis in Caenorhabditis elegans and parasitic nematodes: Applications in the search for new anthelmintics

The scale of the damage worldwide to human health, animal health and agricultural crops resulting from parasitic nematodes, together with the paucity of treatments and the threat of developing resistance to the limited set of widely-deployed chemical tools, underlines the urgent need to develop novel drugs and chemicals to control nematode parasites. Robust chemical screens which can be automated are a key part of that discovery process. Hitherto, the successful automation of nematode behaviours has been a bottleneck in the chemical discovery process. As the measurement of nematode motility can provide a direct scalar readout of the activity of the neuromuscular system and an indirect measure of the health of the animal, this omission is acute. Motility offers a useful assay for high-throughput, phenotypic drug/chemical screening and several recent developments have helped realise, at least in part, the potential of nematode-based drug screening. Here we review the challenges encountered in automating nematode motility and some important developments in the application of machine vision, statistical imaging and tracking approaches which enable the automated characterisation of nematode movement. Such developments facilitate automated screening for new drugs and chemicals aimed at controlling human and animal nematode parasites (anthelmintics) and plant nematode parasites (nematicides).

Glutamate-gated chloride channels of Haemonchus contortus restore drug sensitivity to ivermectin resistant Caenorhabditis elegans

Anthelmintic resistance is a major problem in livestock farming, especially of small ruminants, but our understanding of it has been limited by the difficulty in carrying out functional genetic studies on parasitic nematodes. An important nematode infecting sheep and goats is Haemonchus contortus; in many parts of the world this species is resistant to almost all the currently available drugs, including ivermectin. It is extremely polymorphic and to date it has proved impossible to relate any sequence polymorphisms to its ivermectin resistance status. Expression of candidate drug-resistance genes in Caenorhabditis elegans could provide a convenient means to study the effects of polymorphisms found in resistant parasites, but may be complicated by differences between the gene families of target and model organisms. We tested this using the glutamate-gated chloride channel (GluCl) gene family, which forms the ivermectin drug target and are candidate resistance genes. We expressed GluCl subunits from C. elegans and H. contortus in a highly resistant triple mutant C. elegans strain (DA1316) under the control of the avr-14 promoter; expression of GFP behind this promoter recapitulated the pattern previously reported for avr-14. Expression of ivermectin-sensitive subunits from both species restored drug sensitivity to transgenic worms, though some quantitative differences were noted between lines. Expression of an ivermectin-insensitive subunit, Hco-GLC-2, had no effect on drug sensitivity. Expression of a previously uncharacterised parasite-specific subunit, Hco-GLC-6, caused the transgenic worms to become ivermectin sensitive, suggesting that this subunit also encodes a GluCl that responds to the drug. These results demonstrate that both orthologous and paralogous subunits from C. elegans and H. contortus are able to rescue the ivermectin sensitivity of mutant C. elegans, though some quantitative differences were observed between transgenic lines in some assays. C. elegans is a suitable system for studying parasitic nematode genes that may be involved in drug resistance.

A Cys-loop mutation in the Caenorhabditis elegans nicotinic receptor subunit UNC-63 impairs but does not abolish channel function

The nematode Caenorhabditis elegans is an established model organism for studying neurobiology. UNC-63 is a C. elegans nicotinic acetylcholine receptor (nAChR) α-subunit. It is an essential component of the levamisole-sensitive muscle nAChR (L-nAChR) and therefore plays an important role in cholinergic transmission at the nematode neuromuscular junction. Here, we show that worms with the unc-63(x26) allele, with its αC151Y mutation disrupting the Cys-loop, have deficient muscle function reflected by impaired swimming (thrashing). Single-channel recordings from cultured muscle cells from the mutant strain showed a 100-fold reduced frequency of opening events and shorter channel openings of L-nAChRs compared with those of wild-type worms. Anti-UNC-63 antibody staining in both cultured adult muscle and embryonic cells showed that L-nAChRs were expressed at similar levels in the mutant and wild-type cells, suggesting that the functional changes in the receptor, rather than changes in expression, are the predominant effect of the mutation. The kinetic changes mimic those reported in patients with fast-channel congenital myasthenic syndromes. We show that pyridostigmine bromide and 3,4-diaminopyridine, which are drugs used to treat fast-channel congenital myasthenic syndromes, partially rescued the motility defect seen in unc-63(x26). The C. elegans unc-63(x26) mutant may therefore offer a useful model to assist in the development of therapies for syndromes produced by altered function of human nAChRs.

A novel Caenorhabditis elegans allele, smn-1(cb131), mimicking a mild form of spinal muscular atrophy, provides a convenient drug screening platform highlighting new and pre-approved compounds

Spinal muscular atrophy (SMA), an autosomal recessive genetic disorder, is characterized by the selective degeneration of lower motor neurons, leading to muscle atrophy and, in the most severe cases, paralysis and death. Deletions and point mutations cause reduced levels of the widely expressed survival motor neuron (SMN) protein, which has been implicated in a range of cellular processes. The mechanisms underlying disease pathogenesis are unclear, and there is no effective treatment. Several animal models have been developed to study SMN function including the nematode, Caenorhabditis elegans, in which a large deletion in the gene homologous to SMN, smn-1, results in neuromuscular dysfunction and larval lethality. Although useful, this null mutant, smn-1(ok355), is not well suited to drug screening. We report the isolation and characterization of smn-1(cb131), a novel allele encoding a substitution in a highly conserved residue of exon 2, resembling a point mutation found in a patient with type IIIb SMA. The smn-1(cb131) animals display milder yet similar defects when compared with the smn-1 null mutant. Using an automated phenotyping system, mutants were shown to swim slower than wild-type animals. This phenotype was used to screen a library of 1040 chemical compounds for drugs that ameliorate the defect, highlighting six for subsequent testing. 4-aminopyridine, gaboxadol hydrochloride and N-acetylneuraminic acid all rescued at least one aspect of smn-1 phenotypic dysfunction. These findings may assist in accelerating the development of drugs for the treatment of SMA.

Allosteric modulation by benzodiazepines of GABA-gated chloride channels of an identified insect motor neurone

The actions of benzodiazepines were studied on the responses to GABA of the fast coxal depressor (D(f)) motor neurone of the cockroach, Periplaneta americana. Ro5-4864, diazepam and clonazepam were investigated. Responses to GABA receptors were enhanced by both Ro5-4864 and diazepam, whereas clonazepam, a potent-positive allosteric modulator of human GABA(A) receptors, was ineffective on the native insect GABA receptors of the D(f) motor neurone. Thus, clear pharmacological differences exist between insect and mammalian native GABA-gated chloride channels with respect to the actions of benzodiazepines. The results enhance our understanding of invertebrate GABA-gated chloride channels which have recently proved important in (a) comparative studies aimed at identifying human allosteric drug-binding sites and (b) understanding the actions of compounds used to control ectoparasites and insect crop pests.

Fast, automated measurement of nematode swimming (thrashing) without morphometry

Background

The "thrashing assay", in which nematodes are placed in liquid and the frequency of lateral swimming ("thrashing") movements estimated, is a well-established method for measuring motility in the genetic model organism Caenorhabditis elegans as well as in parasitic nematodes. It is used as an index of the effects of drugs, chemicals or mutations on motility and has proved useful in identifying mutants affecting behaviour. However, the method is laborious, subject to experimenter error, and therefore does not permit high-throughput applications. Existing automation methods usually involve analysis of worm shape, but this is computationally demanding and error-prone. Here we present a novel, robust and rapid method of automatically counting the thrashing frequency of worms that avoids morphometry but nonetheless gives a direct measure of thrashing frequency. Our method uses principal components analysis to remove the background, followed by computation of a covariance matrix of the remaining image frames from which the interval between statistically-similar frames is estimated.

Results

We tested the performance of our covariance method in measuring thrashing rates of worms using mutations that affect motility and found that it accurately substituted for laborious, manual measurements over a wide range of thrashing rates. The algorithm used also enabled us to determine a dose-dependent inhibition of thrashing frequency by the anthelmintic drug, levamisole, illustrating the suitability of the system for assaying the effects of drugs and chemicals on motility. Furthermore, the algorithm successfully measured the actions of levamisole on a parasitic nematode, Haemonchus contortus, which undergoes complex contorted shapes whilst swimming, without alterations in the code or of any parameters, indicating that it is applicable to different nematode species, including parasitic nematodes. Our method is capable of analyzing a 30 s movie in less than 30 s and can therefore be deployed in rapid screens.

Conclusion

We demonstrate that a covariance-based method yields a fast, reliable, automated measurement of C. elegans motility which can replace the far more time-consuming, manual method. The absence of a morphometry step means that the method can be applied to any nematode that swims in liquid and, together with its speed, this simplicity lends itself to deployment in large-scale chemical and genetic screens.

Comparative pharmacology and computational modelling yield insights into allosteric modulation of human alpha7 nicotinic acetylcholine receptors

The human alpha7 nicotinic acetylcholine receptor (nAChR) subunit and its Caenorhabditis elegans homolog, ACR-16, can generate functional recombinant homomeric receptors when expressed in Xenopus laevis oocytes. Both nAChRs express robustly in the presence of the co-injected chaperone, RIC-3, and show striking differences in the actions of a type I positive allosteric modulator (PAM), ivermectin (IVM). Type I PAMs are characterised by an increase in amplitude only of the response to acetylcholine (ACh), whereas type II PAMs exhibit, in addition, changes in time-course/desensitization of the ACh response. The type I PAMs, ivermectin, 5-hydroxyindole (5-HI), NS-1738 and genistein and the type II PAM, PNU-120596, are all active on human alpha7 but are without PAM activity on ACR-16, where they attenuate the amplitude of the ACh response. We used the published structure of avermectin B1a to generate a model of IVM, which was then docked into the candidate transmembrane allosteric binding site on alpha7 and ACR-16 in an attempt to gain insights into the observed differences in IVM actions. The new pharmacological findings and computational approaches being developed may inform the design of novel PAM drugs targeting major neurological disorders.

Nicotinic acetylcholine receptor signalling: roles in Alzheimer's disease and amyloid neuroprotection

Alzheimer's disease (AD), the major contributor to dementia in the elderly, involves accumulation in the brain of extracellular plaques containing the beta-amyloid protein (Abeta) and intracellular neurofibrillary tangles of hyperphosphorylated tau protein. AD is also characterized by a loss of neurons, particularly those expressing nicotinic acetylcholine receptors (nAChRs), thereby leading to a reduction in nAChR numbers. The Abeta(1-42) protein, which is toxic to neurons, is critical to the onset and progression of AD. The discovery of new drug therapies for AD is likely to be accelerated by an improved understanding of the mechanisms whereby Abeta causes neuronal death. We examine the evidence for a role in Abeta(1-42) toxicity of nAChRs; paradoxically, nAChRs can also protect neurons when activated by nicotinic ligands. Abeta peptides and nicotine differentially activate several intracellular signaling pathways, including the phosphatidylinositol 3-kinase/v-akt murine thymoma viral oncogene homolog pathway, the extracellular signal-regulated kinase/mitogen-activated protein kinase, and JAK-2/STAT-3 pathways. These pathways control cell death or survival and the secretion of Abeta peptides. We propose that understanding the differential activation of these pathways by nicotine and/or Abeta(1-42) may offer the prospect of new routes to therapy for AD.

Strategies for automated analysis of C. elegans locomotion

Automated analysis of C. elegans behaviour is a rapidly developing field, offering the possibility of behaviour-based, high-throughput drug screens and systematic phenotyping. Standard methods for parameterizing worm shapes and movements are emerging, and progress has been made towards overcoming the difficulties introduced by interactions between worms, as well as worm coiling and omega turning. Current methods have facilitated the identification of subtle phenotypes and the characterisation of roles of neurones in forward locomotion and chemotaxis, as well as the quantitative characterisation of behaviour choice and circadian patterns of activity. Given the speed with which C. elegans has been deployed in genetic screens and chemical screens, it is to be hoped that wormtrackers may eventually provide similar rapidity in assaying behavioural phenotypes. However, considerable progress must be made before this can be accomplished. In the case of genome-wide RNAi screens, for example, the presence in the worm genome of some 19,000 genes means that even the minimal user intervention in an automatic phenotyping system will be very costly. Nonetheless, recent advances have shown that drug actions on large numbers of worms can be tracked, raising hopes that high-throughput behavioural screens may soon be available.

The Abeta1-42M35C mutated amyloid peptide Abeta1-42 and the 25-35 fragment fail to mimic the subtype-specificity of actions on recombinant human nicotinic acetylcholine receptors (alpha7, alpha4beta2, alpha3beta4)

Alzheimer's disease (AD) is a neurodegenerative condition involving accumulation of the beta-amyloid peptide, Abeta1-42. Previously we have shown that amyloid peptides (Abeta1-42, Abeta1-40) have different actions on the three major brain nicotinic acetylcholine receptor (nAChR) subtypes (alpha7, alpha4beta2 and alpha3beta4). The methionine in position 35 of Abeta (M35) has been shown to be important in the toxicity of Abeta and the 25-35 fragment can mimic some of the actions of the Abeta1-42 peptide. However, the extent to which this mutant and the fragment mimic subtype selectivity is unknown. Two-electrode voltage-clamp electrophysiology has been used to study the actions on alpha7, alpha4beta2 and alpha3beta4 recombinant nAChRs expressed in Xenopus laevis oocytes of full length Abeta1-42, and Abeta peptide fragments, scrambled peptides, and the Abeta1-42 peptide containing mutations of the methionine in position 35. The Abeta25-35 fragment did not display subunit specificity. Abeta1-42 with an M35C mutation showed similar subtype-specificity to wild-type Abeta1-42. However, Abeta1-42 with an M35V substitution reduced the peak amplitude of ACh-induced currents recorded from alpha4beta2 nAChRs, but did not affect those recorded from alpha7 or alpha3beta4. These results indicate that the amino acid in position 35 of Abeta1-42 is an important determinant of the subtype-specificity of this peptide on human recombinant alpha7, alpha4beta2 and alpha3beta4 nAChRs and that the 25-35 fragment fails to mimic all of the actions of the full-length peptide.

Actions of snake neurotoxins on an insect nicotinic cholinergic synapse

Here we examine the actions of six snake neurotoxins (alpha-cobratoxin from Naja naja siamensis, erabutoxin-a and b from Laticauda semifasciata; CM12 from N. haje annulifera, toxin III 4 from Notechis scutatus and a long toxin from N. haje) on nicotinic acetylcholine receptors in the cercal afferent, giant interneuron 2 synapse of the cockroach, Periplaneta americana. All toxins tested reduced responses to directly-applied ACh as well as EPSPs evoked by electrical stimulation of nerve XI with similar time courses, suggesting that their action is postsynaptic. Thus, these nicotinic receptors in a well-characterized insect synapse are sensitive to both long and short chain neurotoxins. This considerably expands the range of snake toxins that block insect nicotinic acetylcholine receptors and may enable further pharmacological distinctions between nAChR subtypes.

Functional genomics: applying calcium imaging and RNA interference to Drosophila cell lines to identify new roles for gene products

The selective knockdown of genes using RNA interference, and the analysis of the resultant phenotype using calcium imaging, is proving to be a powerful combination for discovering new genes involved in calcium signalling. This technical note describes the application of this approach to the Drosophila S2 cell line and its derivatives.

Neonicotinoid insecticides display partial and super agonist actions on native insect nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are present in high density in insect nervous tissue and are targeted by neonicotinoid insecticides. Improved understanding of the actions of these insecticides will assist in the development of new compounds. Here, we have used whole-cell patch-clamp recording of cholinergic neurons cultured from the central nervous system of 3rd instar Drosophila larvae to examine the actions of acetylcholine (ACh) and nicotine, as well as the neonicotinoids imidacloprid, clothianidin and P-CH-clothianidin on native nAChRs of these neurons. Dose-response data yield an EC(50) value for ACh of 19 microm. Both nicotine and imidacloprid act as low efficacy agonists at native nAChRs, evoking maximal current amplitudes 10-14% of those observed for ACh. Conversely, clothianidin and P-CH-clothianidin evoke maximal current amplitudes up to 56% greater than those evoked by 100 microm ACh in the same neurons. This is the first demonstration of 'super' agonist actions of an insecticide on native insect nAChRs. Cell-attached recordings indicate that super agonism results from more frequent openings at the largest (63.5 pS) conductance state observed.

Invertebrate studies and their ongoing contributions to neuroscience

Invertebrates have been deployed very successfully in experimental studies of the nervous system and neuromuscular junctions. Many important discoveries on axonal conduction, synaptic transmission, integrative neurobiology and behaviour have been made by investigations of these remarkable animals. Their advantages as model organisms for investigations of nervous systems include (a) the large diameter of neurons, glia and muscle cells of some invertebrates, thereby facilitating microelectrode recordings; (b) simple nervous systems with few neurons, enhancing the tractability of neuronal circuitry; and (c) well-defined behaviours, which lend themselves to physiological and genetic dissection. Genetic model organisms such as Drosophila melanogaster and Caenorhabditis elegans have provided powerful genetic approaches to central questions concerning nervous system development, learning and memory and the cellular and molecular basis of behaviour. The process of attributing function to particular gene products has been greatly accelerated in recent years with access to entire genome sequences and the application of reverse genetic (e.g. RNA interference, RNAi) and other post-genome technologies (e.g. microarrays). Studies of many other invertebrates, notably the honeybee (Apis mellifera), a nudibranch mollusc (Aplysia californica), locusts, lobsters, crabs, annelids and jellyfish have all assisted in the development of major concepts in neuroscience. The future is equally bright with ease of access to genome-wide reverse genetic technologies, and the development of optical recordings using voltage and intracellular calcium sensors genetically targeted to selected individual and groups of neurons.

Atypical Phenotypes From Flatworm Kv3 Channels

Divergence of the Shaker superfamily of voltage-gated (Kv) ion channels early in metazoan evolution created numerous electrical phenotypes that were presumably selected to produce a wide range of excitability characteristics in neurons, myocytes, and other cells. A comparative approach that emphasizes this early radiation provides a comprehensive sampling of sequence space that is necessary to develop generally applicable models of the structure–function relationship in the Kv potassium channel family. We have cloned and characterized two Shaw-type potassium channels from a flatworm ( Notoplana atomata) that is arguably a representative of early diverging bilaterians. When expressed in Xenopus oocytes, one of these cloned channels, N.at-Kv3.1, exhibits a noninactivating, outward current with slow opening kinetics that are dependent on both the holding potential and the activating potential. A second Shaw-type channel, N.at-Kv3.2, has very different properties, showing weak inward rectification. These results demonstrate that broad phylogenetic sampling of proteins of a single family will reveal unexpected properties that lead to new interpretations of structure–function relationships.

Replacement of asparagine with arginine at the extracellular end of the second transmembrane (M2) region of insect GABA receptors increases sensitivity to penicillin G

The actions of penicillin-G (PCG) on wild-type and mutant Drosophila GABA receptor (RDL) subunits expressed in Xenopus oocytes were studied under two-electrode voltage-clamp. PCG was found to be a non-competitive antagonist of homomeric Drosophila RDL receptors with an IC(50) of 20.41 +/- 1.66 mM at EC(50) GABA. Substitution of a single amino acid (N318R) at the extracellular end of the channel lining region of the RDL subunit increased the potency of GABA approximately four fold, and increased the IC(50) of PCG to 5.09 +/- 0.38 mM. Although the antagonism by PCG on wild-type RDL receptors was independent of membrane potential, PCG action on the N318R mutant showed pronounced voltage-dependency, being much more effective at positive membrane potentials. Thus, in RDL homomers, the replacement of N318 by R318, a residue present at the equivalent position in vertebrate GABA(A) receptors, confers a vertebrate-like PCG pharmacology to the N318R mutant receptor. The A301S mutation that confers resistance to dieldrin did not significantly affect the antagonism by PCG.

Contributions from Caenorhabditis elegans functional genetics to antiparasitic drug target identification and validation: nicotinic acetylcholine receptors, a case study

Following the complete sequencing of the genome of the free-living nematode, Caenorhabditis elegans, in 1998, rapid advances have been made in assigning functions to many genes. Forward and reverse genetics have been used to identify novel components of synaptic transmission as well as determine the key components of antiparasitic drug targets. The nicotinic acetylcholine receptors (nAChRs) are prototypical ligand-gated ion channels. The functions of these transmembrane proteins and the roles of the different members of their extensive subunit families are increasingly well characterised. The simple nervous system of C. elegans possesses one of the largest nicotinic acetylcholine receptor gene families known for any organism and a combination of genetic, microarray, physiological and reporter gene expression studies have added greatly to our understanding of the components of nematode muscle and neuronal nAChR subtypes. Chemistry-to-gene screens have identified five subunits that are components of nAChRs sensitive to the antiparasitic drug, levamisole. A novel, validated target acting downstream of the levamisole-sensitive nAChR has also been identified in such screens. Physiology and molecular biology studies on nAChRs of parasitic nematodes have also identified levamisole-sensitive and insensitive subtypes and further subdivisions are under investigation.

Data mining for protein-protein interactions in invertebrate model organisms

Well-annotated genome databases are available for many invertebrate species, notably the fruitfly, Drosophila melanogaster, and the nematode, Caenorhabditis elegans. An adequate interpretation of this information at the biological level requires the exploration of the interactions between the gene products. Knowledge of protein interactions and the components of cell signalling pathways in the fly and worm are particularly valuable as hypotheses can be rapidly tested using the powerful genetic toolkits available. Invertebrates offer additional experimental advantages when attempting to characterise protein-protein interactions (PPIs). Their relatively small genome size compared to mammals helps to reduce missed interactions due to redundancy, and their function can be addressed using forward (mutants) and reverse (RNA interference) genetics. However, the researcher looking for evidence of PPIs for a protein of interest is faced with the challenge of extracting interaction data from sources that are highly varied, such as the results of microarray experiments in the unstructured text of research papers. This challenge is greatly reduced by a range of public databases of curated information, as well as publicly available, enhanced search engines, which can provide either direct experimental evidence for a PPI, or valuable clues for generating new hypotheses.

Sgbeta1, a novel locust (Schistocerca gregaria) non-alpha nicotinic acetylcholine receptor-like subunit with homology to the Drosophila melanogaster Dbeta1 subunit

The cloning, sequencing and functional expression of Sgbeta1, a novel locust (Schistocerca gregaria) non-alpha nicotinic acetylcholine receptor (nAChR) subunit is described. This subunit shows 80% identity with the Drosophila melanogaster Dbeta1 and 92% identity with the Locusta migratoria beta1, non-alpha subunits but only 38% identity to Sgalpha1 (also referred to as alphaL1), a previously cloned S. gregaria nAChR alpha-subunit. When expressed in Xenopus laevis oocytes, Sgbeta1 does not respond to nicotine. Responses to nicotine are observed, however, in oocytes co-expressing Sgalpha1 and Sgbeta1, but the pharmacology is indistinguishable from that of currents produced by expressing Sgalpha1 alone. We conclude that either Sgbeta1 does not co-assemble with Sgalpha1, or that it is unable to contribute to the functional properties of the receptor, in the Xenopus oocyte expression system.

RNA interference: from model organisms towards therapy for neural and neuromuscular disorders

Experimental RNA interference (RNAi) leading to the selective knockdown of gene function is induced by introducing into cells either double stranded RNA (dsRNA), or short interfering RNA (siRNA) fragments into which dsRNA is cut. The siRNA triggers degradation of homologous messenger RNA (mRNA). Widely used as a research tool in the genetic model organisms Caenorhabditis elegans, Drosophila melanogaster and mouse to investigate the function of individual genes, RNAi has also been deployed in genome-wide, specific gene-knockdown screens. Recent rapid progress in the application of RNAi to mammalian cells, including neurons and muscle cells, offers new approaches to drug target identification and validation. Advances in targeted delivery of RNAi-inducing molecules has raised the possibility of using RNAi directly as a therapy for a variety of human genetic and other neural and neuromuscular disorders. Here, we review examples of the application of RNAi to worm, fly and mouse models of such diseases aimed at understanding their pathophysiology and we address problems to be solved in developing RNAi-based therapies.

Structure and function of two-pore-domain K+ channels: contributions from genetic model organisms

K+ channels that possess two pore domains in each channel subunit are common in many animal tissues. Such channels are generated from large families of subunits and are implicated in several functions, including temperature sensation, responses to ischaemia, K+ homeostasis and setting the resting potential of the cell. Their activity can be modulated by polyunsaturated fatty acids, pH and oxygen, and some are candidate targets of volatile anaesthetics. However, despite their potential as targets for novel drugs for human health, comparatively little is known about the molecular basis of their diverse physiological and pharmacological properties. Genetic model organisms have considerable potential for improving our understanding of these channels. In this article, we review the contributions of some of these genetic model organisms to recent advances in our knowledge of two-pore-domain K+ channels.

Computer simulations of high-pass filtering in zebrafish larval muscle fibres

Larval somatic muscle of the zebrafish, Danio rerio, like that of some other organisms, responds to a sustained depolarization with one, and only one, action potential. Here, we report computer simulations, using the NEURON simulation programme, of sodium and potassium currents of somatic muscle of larval Danio rerio to investigate their possible contribution to once-only firing. Our computer model incorporated simulated sodium and potassium ion channels based on steady-state and kinetic parameters derived from a recent electrophysiological study. The model responded to sustained depolarizations with a single action potential at all levels of depolarization above threshold. By varying several parameters of the sodium and potassium currents systematically, the minimum changes necessary to produce repetitive firing were found to be a positive shift in the half-inactivation and a negative shift in the half-activation potentials for the sodium current, accompanied by a slowing of the rate of inactivation to half of the experimentally observed values. This suggests that once-only spiking can be attributed to the steady-state values of activation and inactivation of the sodium current, along with a slower rate of inactivation. Mapping of the resultant firing properties against steady-state and kinetic ion channel parameters revealed a high safety factor for once-only firing and showed that the time constant of inactivation of the sodium current was the key determinant of once-only or repetitive firing. The rapidly inactivating potassium current does not influence once-only firing or the maximum rate of firing in response to periodic excitation in these simulations. Although a contribution of other currents to produce once-only firing has not been excluded, this model suggests that the properties of the sodium current are sufficient to account for once-only firing.

Chemistry-to-gene screens in Caenorhabditis elegans

The nematode worm Caenorhabditis elegans is a genetic model organism linked to an impressive portfolio of fundamental discoveries in biology. This free-living nematode, which can be easily and inexpensively grown in the laboratory, is also a natural vehicle for screening for drugs that are active against nematode parasites. Here, we show that chemistry-to-gene screens using this animal model can define targets of antiparasitic drugs, identify novel candidate drug targets and contribute to the discovery of new drugs for treating human diseases.

A7DB: a relational database for mutational, physiological and pharmacological data related to the alpha7 nicotinic acetylcholine receptor

BACKGROUND

Nicotinic acetylcholine receptors (nAChRs) are pentameric proteins that are important drug targets for a variety of diseases including Alzheimer's, schizophrenia and various forms of epilepsy. One of the most intensively studied nAChR subunits in recent years has been alpha7. This subunit can form functional homomeric pentamers (alpha7)5, which can make interpretation of physiological and structural data much simpler. The growing amount of structural, pharmacological and physiological data for these receptors indicates the need for a dedicated and accurate database to provide a means to access this information in a coherent manner.

DESCRIPTION

A7DB http://www.lgics.org/a7db/ is a new relational database of manually curated experimental physiological data associated with the alpha7 nAChR. It aims to store as much of the pharmacology, physiology and structural data pertaining to the alpha7 nAChR. The data is accessed via web interface that allows a user to search the data in multiple ways: 1) a simple text query 2) an incremental query builder 3) an interactive query builder and 4) a file-based uploadable query. It currently holds more than 460 separately reported experiments on over 85 mutations.

CONCLUSIONS

A7DB will be a useful tool to molecular biologists and bioinformaticians not only working on the alpha7 receptor family of proteins but also in the more general context of nicotinic receptor modelling. Furthermore it sets a precedent for expansion with the inclusion of all nicotinic receptor families and eventually all cys-loop receptor families.

Sodium and potassium currents of larval zebrafish muscle fibres

The steady-state and kinetic properties of Na(+) and K(+) currents of inner (white) and outer (red) muscles of zebrafish larvae 4-6 days post-fertilization (d.p.f.) are described. In inner muscle, the outward currents were half-activated at -1.0 mV and half-inactivated at -30.4 mV, and completely inactivated within 100 ms of depolarization. The inward currents of inner fibres were half-activated at -7.3 mV and half-inactivated at -74.5 mV and completely inactivated within 5 ms of depolarization. Inner muscle fibres were found to support action potentials, while no action potentials could be evoked in outer muscles. In inner muscle fibres, all tested levels of depolarizing current above a threshold value evoked only one action potential. However, spiking at frequencies of up to 200 cycles s(-1) was evoked by the injection of depolarizing pulses separated by short hyperpolarizing currents. We suggest that the properties of the inward sodium and outward potassium currents permit high frequency firing in response to a pulsatile depolarizing input of the kind expected in fast swimming, whilst safeguarding against tetany during a strong depolarization.

Bicuculline-insensitive GABA-gated Cl? channels in the larval nervous system of the moth Manduca sexta

GABA-gated Cl(-) channels were studied in the nervous system of the larval tobacco hawk moth, Manduca sexta, using electrophysiology, (36)Cl(-) uptake into membrane microsacs and immunocytochemistry. A GABA-induced increase in Cl(-) conductance was recorded from a visually identifiable neurone ( fg1) in the desheathed frontal ganglion. The response was insensitive to the vertebrate GABA(A) receptor antagonist, bicuculline, but was blocked by picrotoxinin. Bicuculline-insensitive, picrotoxinin-sensitive, GABA-stimulated (36)Cl(-) uptake was also detected in membrane microsacs prepared from the isolated larval M. sexta nervous system. Such receptors appear to be the major type of GABA receptor in larval nervous system membrane microsac preparations. An antibody raised against a 17 amino acid peptide, based on the predicted C-terminus of the Drosophila GABA receptor subunit (RDL), stained not only cell bodies, including that of fg1, but also the neuropile in the frontal ganglion, indicating the existence of RDL-like GABA receptor subunits in neurones of this ganglion. Thus, bicuculline-insensitive GABA-gated Cl(-) channels are present in the larval nervous system of M. sexta.

Role of high-voltage activated potassium currents in high-frequency neuronal firing: evidence from a basal metazoan

Certain neurons of vertebrates are specialized for high-frequency firing. Interestingly, high-frequency firing is also seen in central neurons in basal bilateral metazoans. Recently, the role of potassium currents with rightward-shifted activation curves in producing high-frequency firing has come under scrutiny. We apply intracellular recording, patch-clamp techniques, and compartmental modeling to examine the roles of rightward-shifted potassium currents in repetitive firing and shaping of action potentials in central neurons of the flatworm, Notoplana atomata (Phylum Platyhelminthes). The kinetic properties of potassium and sodium currents were determined from patch-clamp experiments on dissociated brain cells. To predict the effects of changing the steady-state and kinetic properties of these potassium currents, these data were incorporated into a computer model of a 30-microm spherical cell with the levels of current adjusted to approximate the values recorded in voltage-clamp experiments. The model was able to support regenerative spikes at high frequencies in response to injected current. Current-clamp recordings of cultured cells and of neurons in situ also showed evidence of very-high-frequency firing. Adjusting the ratio of inactivating to non-inactivating potassium currents had little effect upon the firing pattern of the cell or its ability to fire at high frequencies, whereas the presence of the non-inactivating current was necessary for repetitive firing. Computer simulations suggested that the rightward shift in voltage sensitivity confers a raised firing threshold, while rapid channel kinetics underlie high frequency firing, and the large activation range enhances the coding range of the cell.

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.

Alternative splicing of a Drosophila GABA receptor subunit gene identifies determinants of agonist potency

Alternative splicing of the Drosophila melanogaster Rdl gene yields four ionotropic GABA receptor subunits. The two Rdl splice variants cloned to date, RDL(ac) and RDL(bd) (DRC17-1-2), differ in their apparent agonist affinity. Here, we report the cloning of a third splice variant of Rdl, RDL(ad). Two-electrode voltage clamp electrophysiology was used to investigate agonist pharmacology of this expressed subunit following cRNA injection into Xenopus laevis oocytes. The EC(so) values for GABA and its analogues isoguvacine, muscimol, isonipecotic acid and 3-amino sulphonic acid on the RDL(ad) homomeric receptor differed from those previously described for RDL(ac) and DRC17-1-2 receptors. In addition to providing a possible physiological role for the alternative splicing of Rdl, these data delineate a hitherto functionally unassigned region of the N-terminal domain of GABA receptor subunits, which affects agonist potency and aligns closely with known determinants of potency in nicotinic acetylcholine receptors. Thus, using expression in Xenopus oocytes, we have demonstrated differences in agonist potency for the neurotransmitter GABA (and four analogues) between splice variant products of the Drosophila melanogaster Rdl gene encoding homomer-forming GABA receptor subunits.

K(+) currents in cultured neurones from a polyclad flatworm

Cells from the brain of the polyclad flatworm Notoplana atomata were dispersed and maintained in primary culture for up to 3 weeks. Whole-cell patch-clamp of presumed neurones revealed outwardly directed K(+) currents that comprised, in varying proportions, a rapidly activating (time constant tau =0.94+/−0.79 ms; N=15) and inactivating (tau =26.1+/−1.9 ms; N=22) current and a second current that also activated rapidly (tau =1.1+/−0.2 ms; N=9) (means +/− s.e.m.) but did not inactivate within 100 ms. Both current types activated over similar voltage ranges. Activation and steady-state inactivation overlap and are markedly rightward-shifted compared with most Shaker-like currents (half-activation of 16.9+/−1. 9 mV, N=7, half-inactivation of −35.4+/−3.0 mV, N=5). Recovery from inactivation was rapid (50+/−2.5 ms at −90 mV). Both currents were unaffected by tetraethylammonium (25 mmol l(−1)), whereas 4-aminopyridine (10 mmol l(−1)) selectively blocked the inactivating current. The rapidly inactivating current, like cloned K(+) channels from cnidarians and certain cloned K(+) channels from molluscs and the Kv3 family of vertebrate channels, differed from most A-type K(+) currents reported to date. These findings suggest that K(+) currents in Notoplana atomata play novel roles in shaping excitability properties.

Cultured insect mushroom body neurons express functional receptors for acetylcholine, GABA, glutamate, octopamine, and dopamine

Fluorescence calcium imaging with fura-2 and whole cell, patch-clamp electrophysiology was applied to cultured Kenyon cells (interneurons) isolated from the mushroom bodies of adult crickets (Acheta domesticus) to demonstrate the presence of functional neurotransmitter receptors. In all cells investigated, 5 microM acetylcholine (ACh, n = 52) evoked an increase in intracellular free calcium ([Ca2+]i). Similar effects were observed in response to 10 microM nicotine. The ACh response was insensitive to atropine (50 microM) but was reduced by mecamylamine (50 microM) and alpha-bungarotoxin (alpha-bgt, 10 microM). ACh-induced inward ion currents (n = 28, EACh approximately 0 mV) were also blocked by 1 microM mecamylamine and by 1 microM alpha-bgt. Nicotine-induced inward currents desensitized more rapidly than ACh responses. Thus functional alpha-bgt-sensitive nicotinic ACh receptors are abundant on all Kenyon cells tested, and their activation leads to an increase in [Ca2+]i. gamma-Aminobutyric acid (GABA, 100 microM) triggered a sustained decrease in [Ca2+]i. Similar responses were seen with a GABAA agonist, muscimol (100 microM), and a GABAB agonist, 3-APPA (1 mM), suggesting that more than one type of GABA receptor can affect [Ca2+]i. This action of GABA was not observed when the extracellular KCl concentration was lowered. All cells tested (n = 26) with patch-clamp electrophysiology showed picrotoxinin (PTX)-sensitive, GABA-induced (30-100 microM) currents with a chloride-sensitive reversal potential. Thus, an ionotropic PTX-sensitive GABA receptor was found on all Kenyon cells tested. Most (61%) of the 54 cells studied responded to -glutamate (100 microM) application either with a biphasic increase in [Ca2+]i or with a single, delayed, sustained [Ca2+]i increase. Nearly all cells tested (95%, n = 19) responded to (100 microM) -glutamate with rapidly desensitizing, inward currents that reversed at approximately -30 mV. Dopamine (100 microM) elicited either a rapid or a delayed increase in [Ca2+]i in 63% of the 26 cells tested. The time course of these responses varied greatly among cells. Dopamine failed to elicit currents in patch-clamped cells (n = 4). A brief decrease in [Ca2+]i was induced by octopamine (100 microM) in approximately 54% of the cells tested (n = 35). However, when extracellular CaCl2 was lowered, octopamine triggered a substantial increase in [Ca2+]i in 35% of the cells tested (n = 26). No octopamine-elicited currents were detected in patched-clamped cells (n = 10).

Antagonist profile and molecular dynamic simulation of a Drosophila melanogaster muscarinic acetylcholine receptor

A stably-transfected, Drosophila cell line (S2-DMl-1) expressing the Drosophila DMl muscarinic acetylcholine receptor (mAChR) exhibits high-affinity, saturable, specific binding of the radiolabelled muscarinic antagonist [3H]-N-methyl scopolamine ([3H]-NMS) with an equilibrium dissociation constant (Kd) of 0.67 +/- 0.02 and a Bmax of 1.53 +/- 0.3 pmol/mg protein. Displacement of [3H]-NMS by mAChR antagonists results in the pharmacological profile: 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) > hexahydrosiladifenidol > p-fluorohexahydrosiladifenidol > nitrocaramiphen > pirenzepine > methoctramine > AFDX-116. This antagonist profile most closely resembles that of the vertebrate M3 mAChR subtype. In this study, however, we have demonstrated that the antagonist profile of DM1 is distinct from those of vertebrate mAChR subtypes. Molecular dynamic simulations of the Drosophila muscarinic receptor are presented in the free, carbamylcholine-bound and NMS-bound forms. Theoretical, quantitative structure-activity relationship models have been developed; a good correlation is observed between the interaction energies of the minimized ligand-receptor complexes and the pharmacological affinities of the antagonists tested.

Functional characterization of a mutated chicken alpha7 nicotinic acetylcholine receptor subunit with a leucine residue inserted in transmembrane domain 2

1. Site-directed mutagenesis was used to create an altered form of the chicken alpha7 nicotinic acetylcholine (ACh) receptor subunit (alpha7x61) in which a leucine residue was inserted between residues Leu9' and Ser10' in transmembrane domain 2. The properties of alpha7x61 receptors are distinct from those of the wild-type receptor. 2. Oocytes expressing wild-type alpha7 receptors responded to 10 microM nicotine with rapid inward currents that desensitized with a time-constant of 710+/-409 ms (mean+/-s.e.mean, n=5). However in alpha7x61 receptors 10 microM nicotine resulted in slower onset inward currents that desensitized with a time-constant of 5684+/-3403 ms (mean+/-s.e.mean, n = 4). No significant difference in the apparent affinity of nicotine or acetylcholine between mutant and wild-type receptors was observed. Dihydro-beta-erythroidine (DHbetaE) acted as an antagonist on both receptors. 3. Molecular modelling of the alpha7x61 receptor channel pore formed by a bundle of M2 alpha-helices suggested that three of the channel lining residues would be altered by the leucine insertion i.e.; Ser10 would be replaced by the leucine insertion, Val13' and Phe14' would be replaced, by Thr and Val, respectively. 4 When present in the LEV-1 nicotinic ACh receptor subunit from Caenorhabditis elegans the same alteration conferred resistance to levamisole anthelmintic drug. Levamisole blocked responses to nicotine of wild-type and alpha7x61 receptors. However, block was more dependent on membrane potential for the alpha7x61 receptors. 5. We conclude that the leucine insertion in transmembrane domain 2 has the unusual effect of slowing desensitization without altering apparent agonist affinity.

Adult insect mushroom body neurons in primary culture: cell morphology and characterization of potassium channels

The dissociation and maintenance in culture of cells derived from the mushroom bodies of adult crickets (Acheta domesticus) are described. This primary culture was developed in order to investigate maturation and differentiation of mushroom-body cells including Kenyon cells, the major intrinsic interneurons of mushroom bodies, which have been shown to be involved in learning and memory in insects. Three distinct cell types were observed, all identified as neural cells on the basis of their size, morphology and immunocytochemical staining with horseradish peroxidase. These cells appear to correspond to the three cell types observed in vivo: Kenyon cells, ganglion mother cells and neuroblasts. Some cells showed neurite growth, usually with long unipolar processes, occasionally with either bipolar or, more rarely, multipolar processes. Neuronal cell bodies readily formed seals with patch pipettes, allowing stable, whole-cell, patch-clamp electrophysiological recordings. Depolarization of the cell under voltage-clamp resulted in at least two types of outwardly directed potassium currents: a delayed rectifier-type of current that was sensitive to tetraethylammonium, and a cadmium-sensitive current with rapid inactivation. Neither type of current was affected by quinidine, a blocker of potassium currents recorded from pupal honeybee Kenyon cells. Other ionic currents, which have yet to be characterized, were also observed.

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.