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.

A role for Caenorhabditis elegans in understanding the function and interactions of human disease genes

A growing number of medical research teams have begun to explore the experimental advantages of using a genetic animal model, the nematode worm Caenorhabditis elegans, with a view to enhancing our understanding of genes underlying human congenital disorders. In this study, we have compared sequences of positionally cloned human disease genes with the C.elegans database of predicted genes. Drawing on examples from spinal muscular atrophy, polycystic kidney disease, muscular dystrophy and Alzheimer's disease, we illustrate how data from C.elegans can yield new insights into the function and interactions of human disease genes.

Molecular biology of insect neuronal GABA receptors

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

Novel putative nicotinic acetylcholine receptor subunit genes, Dalpha5, Dalpha6 and Dalpha7, in Drosophila melanogaster identify a new and highly conserved target of adenosine deaminase acting on RNA-mediated A-to-I pre-mRNA editing

Genome analysis of the fruit fly Drosophila melanogaster reveals three new ligand-gated ion channel subunits with the characteristic YXCC motif found only in alpha-type nicotinic acetylcholine receptor subunits. The subunits are designated Dalpha5, Dalpha6, and Dalpha7. Cloning of the Dalpha5 embryonic cDNAs reveals an atypically large N terminus, part of which is without identifiable sequence motifs and is specified by two polymorphic alleles. Embryonic clones from Dalpha6 contain multiple variant transcripts arising from alternative splicing as well as A-to-I pre-mRNA editing. Alternative splicing in Dalpha6 involves exons encoding nAChR functional domains. The Dalpha6 transcript is a target of the Drosophila adenosine deaminase acting on RNA (dADAR). This is the first case for any organism where a nAChR gene is the target of mRNA editing. Seven adenosines could be modified in the extracellular ligand-binding region of Dalpha6, four of which are also edited in the Dalpha6 ortholog in the tobacco budworm Heliothis virescens. The conservation of an editing site between the insect orders Diptera and Lepidoptera makes nAChR editing the most evolutionarily conserved invertebrate RNA editing site so far described. These findings add to our understanding of nAChR subunit diversity, which is increased and regulated by mechanisms acting at the genomic and mRNA levels.

Edit, cut and paste in the nicotinic acetylcholine receptor gene family ofDrosophila melanogaster

Nicotinic acetylcholine receptors (nAChRs) are important for fast synaptic cholinergic transmission. They are targets of drugs/chemicals for human and animal health as well as for pest control. With the advent of genome sequencing, entire nAChR gene families have now been described for vertebrates and invertebrates. Mostly, these are extensive with a large number of distinct subunits, making possible many nAChR subtypes differing in transmitter affinity, channel conductance, ion selectivity, desensitization, modulation and pharmacology. The smallest nAChR gene family to date is that of the fruit fly, Drosophila melanogaster, with only 10 members. This apparently compact family belies its true diversity as 4 of the 10 subunits show alternative splicing. Also, using Drosophila, A-to-I pre-mRNA editing has been demonstrated for the first time in nAChRs. Such is the extent of this variation, that one subunit alone (Dalpha6) can potentially generate far more isoforms than seen in entire gene families from other species. We present here three-dimensional models constructed for insect nAChRs, which show that many variations introduced by alternative splicing and RNA editing may influence receptor function.

Sequence and functional expression of a single alpha subunit of an insect nicotinic acetylcholine receptor

We report the isolation and sequence of a cDNA clone that encodes a locust (Schistocerca gregaria) nervous system nicotinic acetylcholine receptor (AChR) subunit (alpha L1). The calculated molecular weight of the unglycosylated polypeptide, which contains in the proposed extracellular domain two adjacent cysteine residues which are characteristic of alpha (ligand binding) subunits, is 60,641 daltons. Injection into Xenopus oocytes, of RNA synthesized from this clone in vitro, results in expression of functional nicotinic receptors in the oocyte membrane. In these, nicotine opens a cation channel; the receptors are blocked by both alpha-bungarotoxin (alpha-Bgt) and kappa-bungarotoxin (kappa-Bgt). Reversible block of the expressed insect AChR by mecamylamine, d-tubocurarine, tetraethylammonium, bicuculline and strychnine has also been observed. These data are entirely consistent with previously reported electrophysiological studies on in vivo insect nicotinic receptors and also with biochemical studies on an alpha-Bgt affinity purified locust AChR. Thus, a functional receptor exhibiting the characteristic pharmacology of an in vivo insect nicotinic AChR can be expressed in Xenopus oocytes by injection with a single subunit RNA.

Actions of the insecticide fipronil, on dieldrin-sensitive and- resistant GABA receptors of Drosophila melanogaster

1. Blocking actions of the novel insecticide, fipronil, were examined on GABA responses recorded from Xenopus oocytes expressing either wild type (dieldrin-sensitive) or mutant (dieldrin-resistant) forms of the Drosophila melanogaster GABA-gated chloride channel homo-oligomer, RDL (the product of the resistance to dieldrin locus: Rdl). 2. In the case of the wild type receptor, fipronil blocked GABA-induced currents inducing both a shift to the right in the GABA dose-response curve and depressing the maximum amplitude of responses to GABA. The potency of fipronil was dependent on the GABA concentration but was unaffected by membrane potential. 3. Mutant RDL GABA-receptors, which have a naturally occurring amino acid substitution (A302-->S) in the putative ion-channel lining region, conferring resistance to dieldrin and picrotoxinin, were markedly less sensitive to fipronil than the wild-type receptors. 4. Fipronil antagonism is qualitatively similar to that produced by the structurally distinct compound, picrotoxinin. As the mutation A302-->S reduces the potency of both fipronil and picrotoxinin, homooligomeric RDL receptors should facilitate detailed studies of the molecular basis of convulsant/insecticide antagonist actions on GABA receptors.

The Caenorhabditis elegans orthologue of the human gene responsible for spinal muscular atrophy is a maternal product critical for germline maturation and embryonic viability

Spinal muscular atrophy (SMA) is a common disorder characterized by loss of lower motor neurones of the spinal cord. The disease is caused by mutations in the survival motor neurone ( SMN ) gene. SMN is ubiquitously expressed and evolutionarily conserved, and its role in RNA processing has been well established. However, these properties do not explain the observed specificity of motor neurone death. To gain further insight into the function of SMN, we have isolated and characterized the Caenorhabditis elegans orthologue of the SMN gene ( CeSMN ). Here we show that CeSMN is transmitted maternally as a predominantly nuclear factor, which remains present in all the blastomeres throughout embryonic development and onwards into adulthood. In adult nematodes, a CeSMN-green fluorescent protein fusion protein is expressed in a number of cell types including the germline. Both disruption of the endogenous CeSMN function and overexpression of the gene result in a severe decrease in the number of progeny and in locomotive defects. In addition, its transient knockdown leads to sterility caused by a defect in germ cell maturation. The expression pattern and functional properties so far observed for CeSMN, together with its unusual behaviour in the germline, indicate that SMN may be involved in specific gene expression events at these very early developmental stages. We have also identified a deletion in the CeSMN promoter region in egl-32. This mutant may become a useful genetic tool with which to explore regulation of CeSMN and hence provide possible clues for novel therapeutic strategies for SMA.

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.

Actions of agonists and convulsant antagonists on a Drosophila melanogaster GABA receptor (Rdl) homo-oligomer expressed in Xenopus oocytes

Cytoplasmic injection of Xenopus oocytes with cDNA-derived RNA (cRNA) encoding a wild-type Drosophila melanogaster GABA receptor subunit (Rdl) resulted in functional expression of a GABA receptor homo-oligomer. Membrane currents recorded in response to muscimol, trans (TACA)- and cis (CACA)-4-aminocrotonic acid reversed at membrane potentials close to ECl- and were insensitive to 1.0 x 10(-4) M bicuculline methiodide. An order of potency of GABA approximately muscimol approximately TACA > CACA > glycine was observed. Response of the expressed Drosophila Rdl receptor to GABA was reduced by t-butylbicyclophosphorothionate (TBPS), 4-n-propyl-4'-ethynylbicycloorthobenzoate (EBOB), picrotoxinin and the novel insecticide, fipronil (all at 1.0 x 10(-5) M).

Indoxacarb, an oxadiazine insecticide, blocks insect neuronal sodium channels

1. Decarbomethoxyllated JW062 (DCJW), the active component of a new oxadiazine insecticide DPX-JW062 (Indoxacarb), was tested on action potentials and the inward sodium current recorded from short-term cultured dorsal unpaired median neurones of the cockroach Periplaneta americana. 2. Under whole-cell current-clamp conditions, 100 nM DCJW reduced the amplitude of action potentials and induced a large hyperpolarization of the resting membrane potential associated with a 41% increase in input resistance. 3. In voltage-clamp, DCJW resulted in a dose-dependent inhibition (IC(50) 28 nM) of the peak sodium current. Based on IC(50) values, the effect of DCJW was about 10 fold less potent than tetrodotoxin (TTX) but 1000 fold more potent than the local anaesthetic lidocaine. DCJW (100 nM) was without effect on activation properties of the sodium current, reversal potential, voltage dependence of sodium conductance and on both fast and slow steady-state inactivations. 4. TTX (2 nM) resulted in 48% inhibition of the peak inward sodium current. Co-application of TTX (2 nM) with various concentrations of DCJW produced an additional inhibition of the peak inward current, indicating that the blocking actions of DCJW and TTX were distinct. Co-application of lidocaine (IC(50) 30 microM) with various concentrations of DCJW produced a reduction of the apparent potency of DCJW, suggesting that DCJW and lidocaine acted at the same site. 5. DCJW (100 nM) did not affect inward calcium or outward potassium currents. 6. This study describes, for the first time, the action on insect neuronal voltage-dependent sodium channels of Indoxacarb, a new class of insecticides.

Inositol 1,4,5-trisphosphate receptors are strongly expressed in the nervous system, pharynx, intestine, gonad and excretory cell of Caenorhabditis elegans and are encoded by a single gene (itr-1)

Inositol 1,4,5-trisphosphate (InsP3) activates receptors (InsP3Rs) that mediate intracellular Ca(2+ )release, thereby modulating intracellular calcium signals and regulating important aspects of cellular physiology and gene expression. To further our understanding of InsP3Rs we have characterised InsP3Rs and the InsP3R gene, itr-1, from the model organism Caenorhabditis elegans. cDNAs encoding InsP3Rs were cloned enabling us to: (a) identify three putative transcription start sites that result in alternative mRNA 5' ends: (b) detect alternative splicing at three sites and: (c) determine the full genomic organisation of the itr-1 gene. The InsP3R protein (ITR-1) is approximately 42 % identical with known InsP3Rs and possesses conserved structural features. When the putative InsP3 binding domain was expressed in Escherichia coli, specific binding of InsP3 was detected. Using antibodies against ITR-1 we detected a protein of 220 kDa in C. elegans membranes. These antibodies and itr-1::GFP (green fluorescent protein) reporter constructs were used to determine the expression pattern of itr-1 in C. elegans. Strong expression was observed in the intestine, pharynx, nerve ring, excretory cell and gonad. These results demonstrate the high degree of structural and functional conservation of InsP3Rs from nematodes to mammals and the utility of C. elegans as a system for studies on InsP3R mediated signalling.

Immunocytochemical mapping of a C-terminus anti-peptide antibody to the GABA receptor subunit, RDL in the nervous system in Drosophila melanogaster

An antibody raised against a peptide based on the C-terminal derived amino acid sequence from a cloned Drosophila melanogaster (fruit fly) gene, Rdl (resistant to dieldrin), was used to investigate localization of a GABA receptor subunit in adult male D. melanogaster. Many regions in the brain and thoracic ganglia were stained with this antibody. For example, staining was detected in the medulla, lobula and lobular plate optic neurpiles. Also stained were the antennal lobe glomeruli, the ellipsoid body of the central complex and the mushroom bodies. These results suggest possible roles for an RDL-like GABA receptor subunit in the processing of olfactory, visual and mechanosensory information in the nervous system of D. melanogaster.

Pharmacological and biochemical properties of insect GABA receptors

The first evidence for the existence of GABA receptors in any tissue was provided by studies on an invertebrate preparation but, until recently, characterization of GABA receptors from such lower organisms has advanced slowly. The identification of GABA receptors as putative target sites for a variety of insecticidal agents has contributed to the resurgence of interest in amino acid receptors of insects and other invertebrates. In this review, James Rauh and colleagues describe the properties of GABA receptors of insects and detail some striking pharmacological differences between the well-characterized GABA receptors of vertebrates and those of insects and other invertebrate organisms. A detailed understanding of invertebrate receptor pharmacology will be increasingly important for defining the mode of action of numerous modern pesticides.

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).

Temperature-sensitive expression of Drosophila neuronal nicotinic acetylcholine receptors

Heterologous expression of cloned Drosophila nicotinic acetylcholine receptor (nAChR) subunits indicates that these proteins misfold when expressed in mammalian cell lines at 37 degrees C. This misfolding can, however, be overcome either by growing transfected mammalian cells at lower temperatures or by the expression of Drosophila nAChR subunits in a Drosophila cell line. Whereas the Drosophila nAChR beta subunit (SBD) cDNA, reported previously, lacked part of the SBD coding sequence, here we report the construction and expression of a full-length SBD cDNA. We have examined whether problems in expressing functional Drosophila nAChRs in either Xenopus oocytes or mammalian cell lines can be attributed to an inability of these expression systems to assemble correctly Drosophila nAChRs. Despite expression in what might be considered a more native cellular environment, we have been unable to detect functional nAChRs in a Drosophila cell line unless Drosophila nAChR subunit cDNAs are coexpressed with vertebrate nAChR subunits. Our results indicate that the folding of Drosophila nAChR subunits is temperature-sensitive and strongly suggest that the inability of these Drosophila nAChR subunits to generate functional channels in the absence of vertebrate subunits is due to a requirement for coassembly with as yet unidentified Drosophila nAChR subunits.

Stable expression of a functional homo-oligomeric Drosophila GABA receptor in a Drosophila cell line

A cloned Drosophila gamma-aminobutyric acid GABA receptor subunit (Rdl) has been stably expressed as a functional homo-oligomeric ion channel in a Drosophila cell line. Stably-transfected clonal cell lines which expressed high levels of GABA receptor were identified by specific [3H]-muscimol binding. Expression of functional GABA-gated ion channels in these cell lines was demonstrated by electrophysiological recording. Rapid and pronounced rundown of responses to GABA during whole-cell patch clamp recordings was overcome by the inclusion of EGTA in the pipette solution, indicating a possible role for calcium-dependent processes in the functional regulation of this GABA receptor. Relative agonist potencies of the expressed receptor were found to be in the order GABA = TACA > CACA. We have observed a reversible block of the receptor by the convulsant antagonists, picrotoxinin and EBOB, and by the insecticide fipronil. Potentiation of GABA responses was seen with the anaesthetic steroid 5 alpha-pregnan-3 alpha-ol-20-one. No significant effects (either agonist, antagonist or modulatory) were observed with bicuculline (a vertebrate GABAAR antagonist), benzodiazepines or barbiturates (vertebrate GABAAR modulators), or with glycine agonist of the closely related vertebrate glycine receptors). The suitability of this Drosophila stable expression system for the characterization of receptors and ion channels is discussed.

Role of loop D of the alpha7 nicotinic acetylcholine receptor in its interaction with the insecticide imidacloprid and related neonicotinoids

1. The nitroguanidine insecticide imidacloprid along with a second generation of related compounds including nitenpyram, all nicotinic acetylcholine (ACh) receptor ligands, are used increasingly in many countries. Site-directed mutagenesis and heterologous expression in Xenopus laevis oocytes have been deployed to investigate mutants (G189D and G189E) of the chicken alpha7 homomer-forming nicotinic receptor subunit which are predicted to enhance the negative charge at the negative subsite (loop D) of the ACh binding site. 2. Xenopus oocytes expressing wild-type alpha7 nicotinic receptors respond to imidacloprid with rapid inward currents. Imidacloprid and nitenpyram are partial agonists, whereas ACh, (-)-nicotine and (+)-epibatidine are full agonists. 3. Compared to wild-type alpha7, the mutant G189D and G189E receptors are much less sensitive to the insecticides, whereas their sensitivity to (-)-nicotine, ACh and (+)-epibatidine is only slightly reduced. In contrast, G189N and G189Q mutants are sensitive not only to ACh, (-)-nicotine and (+)-epibatidine, but also to the two insecticides. Thus reduction of the insecticide-sensitivity by the mutations G189D and G189E are attributed to an increase in negativity of loop D. Desnitro-imidacloprid (DN-IMI), an imidacloprid derivative lacking the nitro group is a potent agonist on the G189D and G189E mutants suggesting an important role of loop D in nicotinic receptor interactions with the nitro group of nitroguanidine insecticides.

A Simple Technique for Monitoring the Synaptic Actions of Pharmacological Agents

1. A simple electrophysiological method for the pharmacological investigation of synaptic transmission in insects is described.

2. By means of this technique synaptic potentials and ganglionic polarization can be recorded from the sixth abdominal ganglion of Periplaneta americana L. for long periods.

3. In the absence of stimulation, spontaneous excitatory (EPSP) and inhibitory (IPSP) postsynaptic potentials can be recorded. The mechanical stimulation of cercal receptors demonstrates the summation of EPSPs leading to spike generation.

4. Electrical stimulation of the cercal nerves (X, XI) enables the recording of a monosynaptic EPSP (XI) and a bisynaptic IPSP (X).

5. The action of acetylcholine on synaptic transmission, including a dose-response curve, is described.

6. The potential of this technique as a tool in pharmacological research is discussed.

Conduction Processes in the Nerve Cord of the Moth Manduca Sexta In Relation to its Ultrastructure and Haemolymph Ionic Composition

1. Connectives of the ventral nerve cord of Manduca sexta consist of glia-ensheathed axons surrounded by a perineuriurn and an acellular neural lamella, which is greatly expanded on the dorsal surface. The glial cells are linked to one another by desmosomea and tight junctions; the latter also occur between adjacent perineurial cells. There no continuous circum-neural fat-body sheath.

2. A ten-fold change in the external potassium concentration results in a 43 mV change in the resting potential of de-sheathed connectives. Action potentials of such exposed axons are rapidly blocked in low-sodium or sodium-free saline and under these conditions neither calcium nor magnesium is able to maintain conduction. Spikes from de-sheathed preparations are rapidly abolished on exposure to 10-6M tetrodotoxin. These iindmgs indicate a conventional ionic basis of excitation for the axonal membrane of this insect.

3. Analyses of the haemolymph reveal a mean sodium concentration of 25.4 (s.E. ± 0.98) mM/1 and a mean potassium concentration of 25.1 (s.E. ± 1.74) mM/.1

4. Action potentials recorded from sheathed connectives are maintained for extended periods in sodium-free saline.

5. Exposure of most sheathed connectives to elevated potassium concentrations results in a two-stage depolarization. A rapid, single-stage, apparently extraneuronal potential change is, however, observed in some preparations.

6. These results on sheathed connectives indicate the presence of some peripheral barrier to the movements of sodium and potassium; the tight junctions between adjacent perineurial cells are considered to be possible sites of this restriction.

Pharmacology of insect GABA receptors

A GABA-operated Cl- channel that is bicuculline-insensitive is abundant in the nervous tissue of cockroach, in housefly head preparations and thorax/abdomen preparations, and in similar preparations from several insect species. Bicuculline-insensitive GABA-operated Cl- channels, which are rare in vertebrates, possess sites of action of benzodiazepines, steroids and insecticides that are pharmacologically-distinct from corresponding sites on vertebrate GABAA receptors. The pharmacological profile of the benzodiazepine-binding site linked to an insect CNS GABA-operated Cl- channel resembles more closely that of vertebrate peripheral benzodiazepine-binding sites. Six pregnane steroids and certain polychlorocycloalkane insecticides, which are active at t-butylbicyclophosphorothionate (TBPS)-binding sites, also differ in their effectiveness on vertebrate and insect GABA receptors. Radioligand binding and physiological studies indicate that in insects there may be subtypes of the GABA receptor. Molecular biology offers experimental approaches to understanding the basis of this diversity.

Agonist pharmacology of two Drosophila GABA receptor splice variants

1. The Drosophila melanogaster gamma-aminobutyric acid (GABA) receptor subunits, RDLac and DRC 17-1-2, form functional homo-oligomeric receptors when heterologously expressed in Xenopus laevis oocytes. The subunits differ in only 17 amino acids, principally in regions of the N-terminal domain which determine agonist pharmacology in vertebrate ionotropic neurotransmitter receptors. A range of conformationally restricted GABA analogues were tested on the two homo-oligomers and their agonists pharmacology compared with that of insect and vertebrate iontropic GABA receptors. 2. The actions of GABA, isoguvacine and isonipecotic acid on RDLac and DRC 17-1-2 homo-oligomers were compared, by use of two-electrode voltage-clamp. All three compounds were full agonists of both receptors, but were 4-6 fold less potent agonists of DRC 17-1-2 homo-oligomers than of RDLac. However, the relative potencies of these agonists on each receptor were very similar. 3. A more complete agonist profile was established for RDLac homo-oligomers. The most potent agonists of these receptors were GABA, muscimol and trans-aminocrotonic acid (TACA), which were approximately equipotent. RDLac homo-oligomers were fully activated by a range of GABA analogues, with the order of potency: GABA > ZAPA ((Z)-3-[(aminoiminomethyl)thio]prop-2-enoic acid) > isoguvacine > imidazole-4-acetic acid > or = isonipecotic acid > or = cis-aminocrotonic acid (CACA) > beta-alanine. 3-Aminopropane sulphonic acid (3-APS), a partial agonist of RDLac homo-oligomers, was the weakest agonist tested and 100 fold less potent than GABA. 4. SR95531, an antagonist of vertebrate GABAA receptors, competitively inhibited the GABA responses of RDLac homo-oligomers, which have previously been found to insensitive to bicuculline. However, its potency (IC50 500 microM) was much reduced when compared to GABAA receptors. 5. The agonist pharmacology of Drosophila RDLac homo-oligomers exhibits aspects of the characteristic pharmacology of certain native insect GABA receptors which distinguish them from vertebrate GABA receptors. The high potency and efficacy of isoguvacine and ZAPA distinguishes RDLac homo-oligomers from bicuculline-insensitive vertebrate GABAC receptors, while the low potency of SR95531 and 3-APS distinguishes them from GABAA receptors. The differences in the potency of agonists on RDLac and DRC 17-1-2 homo-oligomers observed in the present study may assist in identification of further molecular determinants of GABA receptor function.

Effects of amino acid neurotransmitter candidates on an identified insect motoneurone

The cell body of the fast coxal depressor motoneurone (Df) was used to examine the effects of a group of putative amino acid neurotransmitters. gamma-Aminobutyric acid (GABA) elicited a biphasic hyperpolarizing response, each component having a separate reversal potential. Both components were affected by the GABA antagonist picrotoxinin. L-Glutamate produced a hyperpolarizing response with a reversal potential of between -80 and -85 mV, and L-aspartate gave a similar but less potent effect. Kainic acid and quisqualic acid elicited depolarizing responses of up to 10 mV in amplitude. beta-Alanine and taurine produced a single hyperpolarization with a reversal potential similar to glutamate. Glycine was ineffective on motoneurone Df.

GABA receptors on the cell-body membrane of an identified insect motor neuron

The pharmacology of a gamma-aminobutyric acid (GABA) receptor on the cell body of an identified motor neuron of the cockroach (Periplaneta americana) was investigated by current-clamp and voltage-clamp methods. Iontophoretic application of GABA increased membrane conductance to chloride ions, and prolonged application resulted in desensitization. Hill coefficients, determined from dose-response data, indicated that binding of at least two GABA molecules was required to activate the chloride channel. Differences between vertebrate GABAA receptors and insect neuronal GABA receptors were detected. For the GABA receptor of motor neuron Df, the following rank order of potency was observed: isoguvacine greater than muscimol greater than or equal to GABA greater than 3-aminopropanesulphonic acid. The GABAB receptor agonist baclofen was inactive. Of the potent vertebrate GABA receptor antagonists (bicuculline, pitrazepin, RU5135 and picrotoxin), only picrotoxin (10(-7) M) produced a potent, reversible block of the response to GABA of motor neuron Df. Both picrotoxinin and picrotin also blocked GABA-induced currents. Bicuculline hydrochloride (10(-4) M) and bicuculline methiodide (10(-4) M) were both ineffective when applied at resting membrane potential (-65 mV), although at hyperpolarized levels partial block of GABA-induced current was sometimes observed. Pitrazepin (10(-4) M) caused a partial, voltage-independent block of GABA-induced current. The steroid derivative RU5135 was inactive at 10(-5) M. In contrast to the potent competitive blockade of vertebrate GABAA receptors by bicuculline, pitrazepin and RU5135, none of the weak antagonism caused by these drugs on the insect GABA receptor was competitive. Flunitrazepam (10(-6) M) potentiated GABA responses, providing evidence for a benzodiazepine site on an insect GABA-receptor-chloride-channel complex.

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

The nitromethylene heterocyclic compound 2(nitromethylene)tetrahydro)1,3-thiazine (NMTHT) inhibits the binding of [125I]alpha-bungarotoxin to membranes prepared from cockroach (Periplaneta americana) nerve cord and fish (Torpedo californica) electric organ. Electrophysiological studies on the cockroach fast coxal depressor motorneuron (Df) reveal a dose-dependent depolarization in response to bath-applied NMTHT. Responses to ionophoretic application of NMTHT onto the cell-body membrane of motorneuron Df are suppressed by bath-applied mecamylamine (1.0 x 10(-4) M) and alpha-bungarotoxin (1.0 x 10(-7) M). These findings, together with the detection of a reversal potential close to that estimated for acetylcholine, provide evidence for an agonist action of this nitromethylene on an insect neuronal nicotinic acetylcholine receptor. The binding of [3H]H12-histrionicotoxin to Torpedo membranes was enhanced in the presence of NMTHT indicating an agonist action at this vertebrate peripheral nicotinic acetylcholine receptor. NMTHT is ineffective in radioligand binding assays for rat brain GABAA receptors, rat brain L-glutamate receptors and insect (Musca domestica) L-glutamate receptors. Partial block of rat brain muscarinic acetylcholine receptors is detected at millimolar concentrations of NMTHT. Thus nitromethylenes appear to exhibit selectivity for acetylcholine receptors and exhibit an agonist action at nicotinic acetylcholine receptors.