Molecular cloning and transformation of cyclodiene resistance in Drosophila: an invertebrate gamma-aminobutyric acid subtype A receptor locus

A genomic approach to understanding Heliothis and Helicoverpa resistance to chemical and biological insecticides

Genomics is the comparative study of the structure and function of entire genomes. Although the complete sequencing of the genome of any insect pest is far in the future, a genomic approach can be useful in the study of mechanisms of insecticide resistance. We describe this strategy for Heliothis and Helicoverpa , two of the most destructive genera of pest moths (Lepidoptera) worldwide. Genome–wide linkage mapping provides the location of major and minor resistance genes. Positional cloning identifies novel resistance genes, even when the mechanisms are poorly understood, as with resistance to Bacillus thuringiensis toxins. Anchor loci provide the reference points for comparing the genomes and the genetic architecture of resistance mechanisms among related species. Collectively, these tools enable the description of the evolutionary response of related, but independent, genomes to the common selective pressure of insecticides in the environment. They also provide information that is useful for targeted management of specific resistance genes, and may even speed the search for families of novel insecticidal targets in Lepidoptera.

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.

Immunocytochemical Mapping of an RDL-Like GABA Receptor Subunit and of GABA in Brain Structures Related to Learning and Memory in the Cricket Acheta domesticus

The distribution of putative RDL-like GABA receptors and of γ-aminobutyric acid (GABA) in the brain of the adult house cricket Acheta domesticus was studied using specific antisera. Special attention was given to brain structures known to be related to learning and memory. The main immunostaining for the RDL-like GABA receptor was observed in mushroom bodies, in particular the upper part of mushroom body peduncle and the two arms of the posterior calyx. Weaker immunostaining was detected in the distal part of the peduncle and in the α and β lobes. The dorso- and ventrolateral protocerebrum neuropils appeared rich in RDL-like GABA receptors. Staining was also detected in the glomeruli of the antennal lobe, as well as in the ellipsoid body of the central complex. Many neurons clustered in groups exhibit GABA-like immunoreactivity. Tracts that were strongly immunostained innervated both the calyces and the lobes of mushroom bodies. The glomeruli of the antennal lobe, the ellipsoid body, as well as neuropils of the dorso- and ventrolateral protocerebrum were also rich in GABA-like immuno- reactivity. The data demonstrated a good correlation between the distribution of the GABA-like and of the RDL-like GABA receptor immunoreactivity. The prominent distribution of RDL-like GABA receptor subunits, in particular areas of mushroom bodies and antennal lobes, underlines the importance of inhibitory signals in information processing in these major integrative centers of the insect brain.

Immunocytochemical mapping of an RDL-like GABA receptor subunit and of GABA in brain structures related to learning and memory in the cricket Acheta domesticus

The distribution of putative RDL-like GABA receptors and of gamma-aminobutyric acid (GABA) in the brain of the adult house cricket Acheta domesticus was studied using specific antisera. Special attention was given to brain structures known to be related to learning and memory. The main immunostaining for the RDL-like GABA receptor was observed in mushroom bodies, in particular the upper part of mushroom body peduncle and the two arms of the posterior calyx. Weaker immunostaining was detected in the distal part of the peduncle and in the alpha and beta lobes. The dorso- and ventrolateral protocerebrum neuropils appeared rich in RDL-like GABA receptors. Staining was also detected in the glomeruli of the antennal lobe, as well as in the ellipsoid body of the central complex. Many neurons clustered in groups exhibit GABA-like immunoreactivity. Tracts that were strongly immunostained innervated both the calyces and the lobes of mushroom bodies. The glomeruli of the antennal lobe, the ellipsoid body, as well as neuropils of the dorso- and ventrolateral protocerebrum were also rich in GABA-like immunoreactivity. The data demonstrated a good correlation between the distribution of the GABA-like and of the RDL-like GABA receptor immunoreactivity. The prominent distribution of RDL-like GABA receptor subunits, in particular areas of mushroom bodies and antennal lobes, underlines the importance of inhibitory signals in information processing in these major integrative centers of the insect brain.

The effect of a transmembrane amino acid on etomidate sensitivity of an invertebrate GABA receptor

1. The gamma-aminobutyric acid (GABA)-modulatory and GABA-mimetic actions of etomidate at mammalian GABA(A) receptors are favoured by beta2- or beta3- versus beta1-subunit containing receptors, a selectivity which resides with a single transmembrane amino acid (beta2 N290, beta3 N289, beta1 S290). Here, we have utilized the Xenopus laevis oocyte expression system in conjunction with the two-point voltage clamp technique to determine the influence of the equivalent amino acid (M314) on the actions of this anaesthetic at an etomidate-insensitive invertebrate GABA receptor (Rdl) of Drosophila melanogaster. 2. Complementary RNA-injected oocytes expressing the wild type Rdl GABA receptor and voltage-clamped at -60 mV responded to bath applied GABA with a concentration-dependent inward current response and a calculated EC50 for GABA of 20+/-0.4 microM. Receptors in which the transmembrane methionine residue (M314) had been exchanged for an asparagine (RdlM314N) or a serine (RdlM314S) also exhibited a concentration-dependent inward current response to GABA, but in both cases with a reduced EC50 of 4.8+/-0.2 microM. 3. Utilizing the appropriate GABA EC10, etomidate (300 microM) had little effect on the agonist-evoked current of the wild type Rdl receptor. By contrast, at RdlM314N receptors, etomidate produced a clear concentration-dependent enhancement of GABA-evoked currents with a calculated EC50 of 64+/-3 microM and an Emax of 68+/-2% (of the maximum response to GABA). 4. The actions of etomidate at RdlM314N receptors exhibited an enantioselectivity common to that found for mammalian receptors, with 100 microM R-(+)-etomidate and S-(-)-etomidate enhancing the current induced by GABA (EC10) to 52+/-6% and 12+/-1% of the GABA maximum respectively. 5. The effects of this mutation were selective for etomidate as the GABA-modulatory actions of 1 mM pentobarbitone at wild type Rdl (49+/-4% of the GABA maximum) and RdlM314N receptors (53+/-2% of the GABA maximum) were similar. Additionally, the modest potentiation of GABA produced by the anaesthetic neurosteroid 5alpha-pregnan-3alpha-ol-20-one (Rdl = 25+/-4% of the GABA maximum) was not altered by this mutation (RdlM314N = 18+/-3% of the GABA maximum). 6. Etomidate acting at beta1 (S290)-containing mammalian GABA(A) receptors is known to produce only a modest GABA-modulatory effect. Similarly, etomidate acting at RdlM314S receptors produced an enhancement of GABA but the magnitude of the effect was reduced compared to RdlM314N receptors. 7. Etomidate acting at human alpha6beta3gamma2L receptors is known to produce a large enhancement of GABA-evoked currents and at higher concentrations this anaesthetic directly activates the GABA(A) receptor complex. Mutation of the human beta3 subunit asparagine to methionine (beta3 N289M found in the equivalent position in Rdl completely inhibited both the GABA-modulatory and GABA-mimetic action of etomidate (10-300 microM) acting at alpha6beta3 N289Mgamma2L receptors. 8. It was concluded that, although invertebrate and mammalian proteins exhibit limited sequence homology, allosteric modification of their function by etomidate can be influenced in a complementary manner by a single amino acid substitution. The results are discussed in relation to whether this amino acid contributes to the anaesthetic binding site, or is essential for transduction. Furthermore, this study provides a clear example of the specificity of anaesthetic action.

Characterization and comparative pharmacological studies of a functional gamma-aminobutyric acid (GABA) receptor cloned from the tobacco budworm, Heliothis virescens (Noctuidae:Lepidoptera)

This paper reports the functional expression and pharmacological characterization of a full length complementary deoxyribonucleic acid (cDNA) (pIVY12) cloned from a Heliothis virescens fertilized egg cDNA library that encodes for a gamma-aminobutyric acid (GABA) receptor subunit (HVRDL-Ser 285). Two electrode voltage clamp recordings of Xenopus oocytes expressing the HVRDL GABA-gated chloride channel revealed robust chloride ion conductance in response to GABA and the GABAA receptor agonist, muscimol. Baclofen, a GABAB agonist had no effect. Phenobarbital showed a positive dose-dependent allosteric modulatory effect, whereas the benzodiazepine, flunitrazepam, had no effect. Chloride conductance was depressed by the novel insecticide, fipronil ((+/-)-5-amino-1-(2,6 dichloro-alpha, alpha, alpha-trifluoro-p-tolyl)-4-trifluoromethyl-sulfinylpyrazole-3-carb onitrile) and the GABAA antagonist, picrotoxinin. The HVRDL GABA receptor was insensitive to blockage by dieldrin and the GABAA antagonist, bicuculline. The comparative actions of fipronil, picrotoxinin and dieldrin were examined on oocytes expressing the H. virescens wild-type (HVRDL-Ser 285), the site-directed mutant (HVRDL-Ala 285), the Drosophila melanogaster Rdl wild-type (DMRDL-Ala 302) and the Rdl dieldrin resistant (DMRDL-Ser 302) homo-oligomeric GABA receptors. HVRDL-Ala 285 was 15-fold more sensitive to blockage by fipronil than HVRDL-Ser 285. DMRDL-Ala 302 and DMRDL-Ser-302 showed a similar level of sensitivity to blockage by fipronil. HVRDL-Ser 285 and DMRDL-Ser 302 exhibited a similar level of insensitivity to picrotoxinin. HVRDL-Ala 285 and DMRDL-Ala 302 showed a similar range of picrotoxinin sensitivity. DMRDL-Ala 302 and HVRDL-Ala 285 showed some sensitivity to blockage by dieldrin. Fipronil sensitivity was significantly altered by the serine to alanine mutation at position 285 in the M2 region of the HVRDL subunit, whereas no difference was observed between the DMRDL-Ser 302 and DMRDL-Ala 302 receptors.

Immunocytochemistry of a novel GABA receptor subunit Rdl in Drosophila melanogaster

Following our recent cloning of a novel gamma-aminobutyric acid (GABA) receptor subunit gene Resistance to dieldrin or Rdl form cyclodiene resistance locus in Drosophila melanogaster, we were interested in defining its pattern of expression during development. Here we report the raising of an anti-Rdl polyclonal antibody that recognizes a single protein of the expected 65 kDa size in immunoblots of Drosophila head homogenates. In situ hybridization using Rdl cDNA probes and the anti-Rdl antibody shows that Rdl message and protein are highly expressed in the developing central nervous system (CNS) of 15-17 h embryos. Interestingly, despite the use of GABA in both the peripheral and CNS of insects, Rdl GABA receptor subunits appear to be confined to the CNS. Detailed immunocytochemistry of Drosophila brain sections showed particularly strong anti-Rdl antibody staining in the optic lobes, ellipsoid body, fan shaped body, ventrolateral protocerebrum and the glomeruli of the antennal lobes. Results are compared with the distribution of staining observed in the insect CNS with antibodies against GABA itself and synaptotagmin, a synaptic vesicle protein.

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.

Effects of [3H]-BIDN, a novel bicyclic dinitrile radioligand for GABA-gated chloride channels of insects and vertebrates

1. The radiolabelled bicyclic dinitrile, [3H]-3,3-bis-trifluoromethyl-bicyclo[2.2.1]heptane-2,2-dicarbonitrile ([3H]-BIDN), exhibited, specific binding of high affinity to membranes of the southern corn rootworm (Diabrotica undecimpunctata howardi) and other insects. A variety of gamma-aminobutyric acid (GABA) receptor convulsants, including the insecticides heptachlor (IC50, 35 +/- 3 nM) and dieldrin (IC50, 93 +/- 7 nM), displaced [3H]-BIDN from rootworm membranes. When tested at 100 microM, 1-(4-ethynylphenyl)-4-n-propyl-2,6,7-trioxabicyclo[2.2.2]oct ane(EBOB), 4-t-butyl-2,6,7-trioxa-1-phosphabicy-clo[2.2.2]octane-1-thio ne (TBPS), 1-phenyl-4-t-butyl-2,6,7-trioxabicyclo[2.2.2]octane (TBOB) and picrotoxin failed to displace 50% of [3H]-BIDN binding to rootworm membranes indicating that the bicyclic dinitrile radioligand probes a site distinct from those identified by other convulsant radioligands. 2. Dissociation studies showed that dieldrin, ketoendrin, toxaphene, heptachlor epoxide and alpha and beta endosulphan displace bound [3H]-BIDN from rootworm membranes by a competitive mechanism. 3. Rat brain membranes were also shown to possess a population of saturable, specific [3H]-BIDN binding sites, though of lower affinity than in rootworm and with a different pharmacological profile. Of the insecticidal GABAergic convulsants that displaced [3H]-BIDN from rootworm, cockroach (Periplaneta americana) and rat brain membranes, many were more effective in rootworm. 4. Functional GABA-gated chloride channels of rootworm nervous system and of cockroach nerve and muscle were blocked by BIDN, whereas cockroach neuronal GABA(B) receptors were unaffected. 5. Expression in Xenopus oocytes of either rat brain mRNA, or cDNA-derived RNA encoding a GABA receptor subunit (Rdl) that is expressed widely in the nervous system of Drosophila melanogaster resulted in functional, homo-oligomeric GABA receptors that were blocked by BIDN. Thus, BIDN probes a novel site on GABA-gated Cl- channels to which a number of insecticidally-active molecules bind.

gamma-Aminobutyric acid receptor distribution in the mushroom bodies of a fly (Calliphora erythrocephala): a functional subdivision of Kenyon cells?

Antibodies against the Drosophila gamma-aminobutyric acid (GABA) receptor subunit RDL were used to investigate the significance of inhibitory inputs to the mushroom bodies in the blowfly (Calliphora erythrocephala) brain. The pedunculus and the lobes of the mushroom body, which mainly consist of Kenyon cell fibers, revealed strong immunoreactivity against RDL. Pedunculi, alpha- and beta-lobe show characteristic unstained core structures with concentric labeling along the neuropile axis. The gamma-lobes in contrast exhibit a compartmentalized RDL-immunoreactive pattern. These data suggest an important role of GABAergic inhibition in the pedunculus and the lobes of insect mushroom bodies. It is most likely that the RDL-immunoreactivity in the mushroom bodies is closely related to Kenyon cell fibers suggesting that Kenyon cells are an inhomogeneous class of neurons, only part of which receive inhibitory GABAergic input from extrinsic elements. GABAergic inhibition, therefore, may play a substantial role in the process of learning and memory formation in the insect mushroom bodies.

The N-terminal domain of human GABA receptor rho1 subunits contains signals for homooligomeric and heterooligomeric interaction

gamma-Aminobutyric acid type C (GABAC) receptors identified in retina appear to be composed of GABA rho subunits. The purpose of this study was to localize signals for homooligomeric assembly of rho1 subunits and to investigate whether the same region contained signals for heterooligomeric interaction with rho2 subunits. In vitro translated human rho1 was shown to be membrane-associated, and proteinase K susceptibility studies indicated that the N terminus was oriented in the lumen of ER-derived microsomal vesicles. This orientation suggested the involvement of the N terminus of rho1 in the initial steps of subunit assembly. To test this hypothesis, mutants were created containing only N-terminal sequences (N-rho1) or C-terminal sequences (C-rho1) of rho1. Co-immunoprecipitation studies revealed that N-rho1, but not C-rho1, interacted with rho1 in vitro. When coexpressed in Xenopus oocytes, N-rho1 interfered with rho1 receptor formation. Together, these data suggested that signals for rho1 homooligomeric assembly reside in the N-terminal half of the subunit. Sequential immunoprecipitations were then performed upon cotranslated rho1 and rho2 subunits which demonstrated that rho1 and rho2 interacted in vitro. Co-immunoprecipitation indicated that N-rho1 specifically associated with rho2. Therefore, the N-terminal regions of rho subunits contain the initial signals for both homooligomeric and heterooligomeric assembly into receptors with GABAC properties.

Fatal association between dieldrin-resistant and susceptible Australian sheep blowflies,Lucilia cuprina

A novel phenomenon of interactions between genotypes of the dieldrin-resistance (Rdl) locus of the Australian sheep blowfly (Lucilia cuprina) is described. Susceptible adult flies exposed to dieldrin-resistant (Rdl/Rdl or Rdl/S) adults, raised from larvae grown on media containing sublethal concentrations of dieldrin, display mortality related to the concentration on which the resistant flies developed. The resistant flies excrete quantities of dieldrin that are toxic to susceptible flies. These observations provide an additional mechanism to those previously identified for the rapid evolution of resistance to dieldrin by L. cuprina.

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.

Actions of picrodendrin antagonists on dieldrin-sensitive and -resistant Drosophila GABA receptors

1. A series of terpenoid compounds, recently isolated from Picrodendron baccatum, share a picrotoxane skeleton with picrotoxinin, an antagonist of ionotropic GABA receptors. Referred to as picrodendrins, they inhibit the binding of [35S]-tert-butylbicyclophosphorothionate (TBPS) to rat GABAA receptors. Hitherto, their effects on GABA receptors have not been investigated electrophysiologically. Under two-electrode voltage-clamp, the actions of picrodendrins and related terpenoids have been assayed on homooligomeric GABA receptors formed by the expression of a Drosophila GABA receptor subunit (RDLac) in Xenopus oocytes. 2. All the terpenoids tested, dose-dependently antagonized currents induced by 30 microM (EC50) GABA. 3. Tutin and its analogues (dihydrotutin and isohyenanchin) differ in the structure of their axial C4 substituents. Of these compounds, tutin, which bears an isopropenyl group at this carbon atom, was the most potent antagonist of RDLac homo-oligomers, whereas isohyenanchin, which bears a hydroxyisopropyl group, was the least potent antagonist tested. 4. Picrodendrins differ mainly in the structure of their C9 substituents. The IC50s of picrodendrins ranged from 17 +/- 1.3 nM (picrodendrin-Q) to 1006 +/- 1.3 nM (picrodendrin-O). As such, the most potent picrodendrins (Q, A and B) were approximately equipotent with picrotoxinin as antagonists of RDLac homo-oligomers. 5. Certain picrodendrin compounds effected a use-dependent blockade of RDLac homo-oligomers. Such a biphasic block was not observed with tutin analogues. 6. Picrotoxin-resistant RDLacA3025 homo-oligomers, which have a single amino acid substitution (A302S) in the 2nd transmembrane region, were markedly less sensitive to picrodendrin-O than the wild-type, dieldrin-sensitive, homo-oligomers. 7. The relative potency of tutin analogues demonstrates that the structure-activity relationship of the C4 substituent of picrotoxane-based compounds is conserved in vertebrates and insects. However, the relative order of potency of picrodendrins on RDLac homo-oligomers is distinctly different from that observed in previous radioligand binding studies performed on vertebrate GABAA receptors. As picrodendrin compounds differ in the structure of their C9 substituents, these data suggest that the optimal convulsant pharmacophores of vertebrate GABAA receptors and RDLac homo-oligomers differ with respect to this substituent.

pH-dependent actions of THIP and ZAPA on an ionotropic Drosophila melanogaster GABA receptor

The actions of THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) and ZAPA (Z-3-[(aminoiminomethyl)thio]prop-2-enoic acid) were tested on an ionotropic homo-oligomeric GABA receptor of Drosophila melanogaster. The amplitude of currents activated by THIP and ZAPA declined rapidly during agonist application and a rebound response was observed on washout. By correcting the pH shift induced by these acid salts, responses more typical of GABA agonists were seen. Less striking pH-dependence was observed in the case of GABA responses.

Distribution of two GABA receptor-like subunits in the Drosophila CNS

Previously we have described the distribution of the Rdl GABA receptor subunit in the Drosophila CNS. Knowing that Rdl can coassemble with LCCH3 (a Drosophila GABA receptor-like subunit showing sequence similarity to vertebrate beta subunit GABAA receptors) in baculovirus infected insect cells, we compared the localization of these two receptor subunits in order to identify any potential overlap in their spatial or temporal distribution. The two subunits show very different patterns of localization. Early in development LCCH3 is found in the majority of developing neuroblasts and later is localized to the cell bodies of the embryonic nerve cord and brain, and the neuronal cell bodies surrounding the adult brain. In contrast, Rdl receptor subunits appear confined to the neuropil in all developmental stages. These results have two important implications. Firstly, they suggest that although these two subunits can coassemble in heterologous expression systems, they may not be found in the same tissues in the nervous system. Secondly, production of LCCH3 before neuronal differentiation leads us to speculate on the role of that LCCH3 containing receptors in the developing nervous system.

Allosteric modulation of an expressed homo-oligomeric GABA-gated chloride channel of Drosophila melanogaster

1. Functional GABA-gated chloride channels are formed when cRNA encoding the Drosophila melanogaster GABA receptor subunit RDL is injected into the cytoplasm of Xenopus oocytes. Two-electrode voltage-clamp was used to investigate allosteric modulation of GABA-induced currents recorded from the expressed, bicuculline-insensitive, RDL homo-oligomers. 2. Flunitrazepam (0.1 microM to 100 microM) had no effect on the amplitude of responses to 10 microM GABA (approximately EC10), whereas 4'chlorodiazepam (100 microM) enhanced the amplitude of submaximal responses to GABA. 3-Hydroxymethyl-beta-carboline (1 microM) and ethyl-beta-carboline-3-carboxylate (both 1 and 100 microM) had no effect on currents induced by 30 microM (approximately EC50) GABA. However 100 microM 3-hydroxymethyl-beta-carboline reduced potentiation by 4'chlorodiazepam. 3. The sodium salts of pentobarbitone (10 microM to 1 mM) and phenobarbitone (50 microM to 1 mM) dose-dependently enhanced submaximal GABA responses. Neither barbiturate activated currents in the absence of GABA. 4. At 10 microM, the steroids 5 alpha-pregnan-3 alpha-ol-20-one and alphaxalone (5 alpha-pregnan-3 alpha-ol-11,20-dione), potentiated submaximal GABA responses. The stereoselectivity of steroid action seen on vertebrate GABAA receptors was observed on RDL homo-oligomers as 5 alpha-pregnan-3 beta-ol-20-one (10 microM) was without effect. None of the three steroids tested activated currents in the absence of GABA. 5. The novel anticonvulsant, loreclezole (100 microM), potentiated the response to 10 microM GABA, but not that of saturating concentrations of GABA. delta-Hexachlorocyclohexane (0.1 microM to 30 microM) was a potent enhancer of submaximal responses to GABA of RDL. 6. The potencies of barbiturates and steroids on RDL homo-oligomers resemble those observed for several in situ insect GABA receptors, whereas those of benzodiazepine binding-site ligands are considerably reduced. The differences in the benzodiazepine pharmacology of RDL homo-oligomers and native GABA receptors, may reflect roles of other subunits in native insect receptors.

Single-stranded DNA conformation polymorphism at the Rdl locus in Hypothenemus hampei (Coleoptera: Scolytidae)

The homologue of the resistance to dieldrin gene (Rdl) in Drosophila melanogaster was cloned and sequenced in the scolytid beetle Hypothenemus hampei, a coffee pest resistant to cyclodiene insecticides in New Caledonia. The amino acid sequence of the Rdl exon no. 7 protein product in H. hampei was identical to that in D. melanogaster and showed the same amino acid change as that characterizing susceptible vs. resistant D. melanogaster. Samples from natural H. hampei populations (from Asia, the Pacific Islands, Africa and Central America), from reference susceptible (S) and resistant (R) laboratory strains, and from their hybrid progenies, were analysed at the Rdl locus using single-stranded DNA conformation polymorphism on polymerase chain reaction products. The susceptible allele was the only allele present in all samples from natural populations except in the only resistant population known to date (Ponerihouen, New Caledonia). Females and some males obtained at F1 from R x S crosses were heterozygous at the Rdl locus, confirming that this local mate competing species is diplo-diploid.

Wild-type and insecticide-resistant homo-oligomeric GABA receptors of Drosophila melanogaster stably expressed in a Drosophila cell line

RDL is an ionotropic GABA receptor subunit, a product of the Rdl gene, originally identified in the Maryland strain of Drosophila melanogaster. Here, we report the generation of a Drosophila melanogaster cell line (S2-RDLA302S) stably expressing a mutated, dieldrin-resistant (A302S) form of RDL. The properties of this dieldrin-resistant, homo-oligomeric receptor have been compared with those of the stably expressed, wild-type form (S2-RDL). Using these stable lines, a striking reduction in sensitivity to both picrotoxinin and dieldrin was observed for responses to GABA of S2-RDLA302S compared to S2-RDL. To determine if these stable insect cell lines generate results similar to those obtained by transient expression in Xenopus laevis oocytes, we have examined the actions of two widely used convulsants, EBOB and TBPS, and a recently developed convulsant BIDN, on RDL-mediated GABA responses in the two expression systems. In both oocytes and S2 cells, the three convulsants suppressed the amplitude of responses to GABA. Thus, in accord with earlier work on agonist and allosteric sites, the S2-RDL cell line is found to yield similar pharmacological results to those obtained in transient expression studies. Stable cell lines are now available expressing susceptible and resistant forms of an ionotropic receptor by GABAergic insecticides.

Evolution and overview of classical transmitter molecules and their receptors

All the classical transmitter ligand molecules evolved at least 1000 million years ago. With the possible exception of the Porifera and coelenterates (Cnidaria), they occur in all the remaining phyla. All transmitters have evolved the ability to activate a range of ion channels, resulting in excitation, inhibition and biphasic or multiphasic responses. All transmitters can be synthesised in all three basic types of neurones, i.e. sensory, interneurone and motoneurone. However their relative importance as sensory, interneurone or motor transmitters varies widely between the phyla. It is likely that all neurones contain more than one type of releasable molecule, often a combination of a classical transmitter and a neuroactive peptide. Second messengers, i.e. G proteins and phospholipase C systems, appeared early in evolution and occur in all phyla that have been investigated. Although the evidence is incomplete, it is likely that all the classical transmitter receptor subtypes identified in mammals, also occur throughout the phyla. The invertebrate receptors so far cloned show some interesting homologies both between those from different invertebrate phyla and with mammalian receptors. This indicates that many of the basic receptor subtypes, including benzodiazepine subunits, evolved at an early period, probably at least 800 million years ago. Overall, the evidence stresses the similarity between the major phyla rather than their differences, supporting a common origin from primitive helminth stock.

A single amino acid in gamma-aminobutyric acid rho 1 receptors affects competitive and noncompetitive components of picrotoxin inhibition

A class of bicuculline-insensitive gamma-aminobutyric acid (GABA) receptors, GABAC, has been identified in retina. Several lines of evidence indicate that GABAC receptors are formed partially or wholly of GABA rho subunits. These receptors generate a Cl- current in response to GABA but differ from GABAA receptors in a number of ways. Picrotoxin, widely accepted as a noncompetitive antagonist of GABAA receptors, displays competitive and noncompetitive antagonism of GABAC receptors in perch and bovine retina and GABA rho 1 receptors expressed in Xenopus oocytes. The aim of this study was to identify the molecular basis of the two components of picrotoxin inhibition of GABA rho 1 receptors. By using a domain-swapping and mutagenesis strategy, a difference in picrotoxin sensitivity between rho 1 and rho 2 receptors was localized to a single amino acid in the putative second transmembrane domain. Substitution of this amino acid with residues found in the analogous position in highly picrotoxin-sensitive glycine alpha and GABAA subunits increased the sensitivity of rho 1 mutants 10- to 500-fold. Importantly, the competitive component of picrotoxin inhibition of the rho 1 mutant receptors was almost eliminated. These findings demonstrate that an amino acid in the putative channel domain of GABA rho 1 receptors influences picrotoxin sensitivity and mediates agonist binding by an allosteric mechanism.

Channel gating in the absence of agonist by a homo-oligomeric molluscan GABA receptor expressed in Xenopus oocytes from a cloned cDNA

We have previously described the isolation of a complementary DNA (cDNA) from the freshwater mollusc Lymnaea stagnalis encoding a polypeptide that exhibits approximately 50% identity to the beta-subunits of vertebrate gamma-aminobutyric acid (GABA) type A (GABAA) receptor. When expressed in Xenopus laevis oocytes from in vitro-transcribed RNA, the snail subunit forms functional homo-oligomeric receptors possessing chloride-selective ion channels. In recordings from voltage-clamped oocytes held at -60 mV, GABA induced an inward current, whereas application of the chloride-channel blocker picrotoxin (in the absence of agonist) elicited an apparent outward current. Single channel recordings obtained from cell-attached patches have revealed a single population of approximately 20 pS channels, with an open probability greater than 90% (at a pipette potential of -100 mV) in the absence of GABA. The relationship between single channel current and pipette potential was linear over the studied range (-100 mV to +60 mV), but the open probability was less for hyperpolarizations than for depolarizations. The spontaneous channel openings were blocked by micromolar concentrations of picrotoxin. Functional hetero-oligomeric receptors were formed when the molluscan subunit was co-expressed in oocytes with the bovine GABAA receptor alpha 1-subunit, but the channels gated by these receptors did not open spontaneously.

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.

The GABAA receptors

Images

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.

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.

Functional expression of insecticide-resistant GABA receptors from the mosquito Aedes aegypti

We are interested in cloning insecticide resistance genes from vector mosquitos for use as selectable markers in their genetic transformation. As a first step towards this goal, we here report the functional homomultimeric expression of a gamma-aminobutyric acid (GABA) receptor subunit gene, Resistance to dieldrin (Rdl), from the yellow fever mosquito Aedes aegypti in baculovirus-infected insect cell lines. Replacement of alanine296 with a serine leads to approximately 100-fold insensitivity to picrotoxin as previously observed in Drosophila. This shows not only that the mosquito GABA receptor cDNA is functional but also that it can be simply mutated to resistance. Strategies for incorporation of this cDNA into a minigene for the genetic transformation of mosquitoes are discussed.

Tissue-specific expression of the diazepam-binding inhibitor in Drosophila melanogaster: cloning, structure, and localization of the gene

The diazepam-binding inhibitor (DBI; also called acyl coenzyme A-binding protein or endozepine) is a 10-kDa polypeptide found in organisms ranging from yeasts to mammals. It has been shown that DBI and its processing products are involved in various specific biological processes such as GABAA/benzodiazepine receptor modulation, acyl coenzyme A metabolism, steroidogenesis, and insulin secretion. We have cloned and sequenced the Drosophila melanogaster gene and cDNA encoding DBI. The Drosophila DBI gene encodes a protein of 86 amino acids that shows 51 to 56% identity with previously known DBI proteins. The gene is composed of one noncoding 5' and two coding exons and is localized on the chromosomal map at position 65E. Several transcription initiation sites were detected by RNase protection and primer extension experiments. Computer analysis of the promoter region revealed features typical of housekeeping genes, such as the lack of TATA and CCAAT elements. However, in its low GC content and lack of a CpG island, the region resembles promoters of tissue-specific genes. Northern (RNA) analysis revealed that the expression of the DBI gene occurred from the larval stage onwards throughout the adult stage. In adult flies, DBI mRNA and immunoreactivity were detected in the cardia, part of the Malpighian tubules, the fat body, and gametes of both sexes. Developmentally regulated expression, disappearing during metamorphosis, was detected in the larval and pupal brains. No expression was detected in the adult nervous system. On the basis of the expression of DBI in some but not all tissues with high energy consumption, we propose that in D. melanogaster, DBI is involved in energy metabolism in a manner that depends on the substrate used for energy production.

Tissue-specific expression of the diazepam-binding inhibitor in Drosophila melanogaster: cloning, structure, and localization of the gene

The diazepam-binding inhibitor (DBI; also called acyl coenzyme A-binding protein or endozepine) is a 10-kDa polypeptide found in organisms ranging from yeasts to mammals. It has been shown that DBI and its processing products are involved in various specific biological processes such as GABAA/benzodiazepine receptor modulation, acyl coenzyme A metabolism, steroidogenesis, and insulin secretion. We have cloned and sequenced the Drosophila melanogaster gene and cDNA encoding DBI. The Drosophila DBI gene encodes a protein of 86 amino acids that shows 51 to 56% identity with previously known DBI proteins. The gene is composed of one noncoding 5' and two coding exons and is localized on the chromosomal map at position 65E. Several transcription initiation sites were detected by RNase protection and primer extension experiments. Computer analysis of the promoter region revealed features typical of housekeeping genes, such as the lack of TATA and CCAAT elements. However, in its low GC content and lack of a CpG island, the region resembles promoters of tissue-specific genes. Northern (RNA) analysis revealed that the expression of the DBI gene occurred from the larval stage onwards throughout the adult stage. In adult flies, DBI mRNA and immunoreactivity were detected in the cardia, part of the Malpighian tubules, the fat body, and gametes of both sexes. Developmentally regulated expression, disappearing during metamorphosis, was detected in the larval and pupal brains. No expression was detected in the adult nervous system. On the basis of the expression of DBI in some but not all tissues with high energy consumption, we propose that in D. melanogaster, DBI is involved in energy metabolism in a manner that depends on the substrate used for energy production.

Immunoaffinity purification of avermectin-binding proteins from the free-living nematode Caenorhabditis elegans and the fruitfly Drosophila melanogaster

Avermectin-binding proteins from the free-living nematode worm Caenorhabditis elegans and from the fruitfly Drosophila melanogaster were purified to homogeneity via a three-step procedure. The binding proteins were covalently labelled using a radioactive photoaffinity probe and then partially purified on a Sephacryl S-300 gel-filtration column. The radiolabelled binding proteins were then purified by immunoaffinity chromatography using a monoclonal antibody to avermectin covalently attached to Protein A-Sepharose beads. Three affinity-labelled Drosophila proteins with molecular masses between 45 and 50 kDa were isolated in this way and then separated from each other by electroelution. This three-step protocol provides a rapid technique for receptor purification which may be of use in the purification of other binding proteins.

A unique amino acid of the Drosophila GABA receptor with influence on drug sensitivity by two mechanisms

1. The Drosophila gene Rdl (resistance to dieldrin) encodes a GABA receptor. An alanine-to-serine mutation in this gene at residue 302 confers resistance to cyclodiene insecticides and picrotoxin. Patch clamp analysis of GABA receptors in cultured neurons from wild type and mutant Drosophila was undertaken to investigate the biophysical basis of resistance. 2. In cultured neurons from both wild type and mutant strains, GABA activated a channel that reversed near 0 mV in symmetrical chloride. GABA dose-response characteristics of wild type and mutant receptors were very similar. 3. GABA responses in neurons from the mutant strains showed reduced sensitivity to the GABA antagonists picrotoxin, lindane and t-butyl-bicyclophosphorothionate. Resistance ratios were 116, 970 and 9 for the three blockers, respectively. Inhibition increased with blocker concentration in a manner consistent with saturation of a single binding site. 4. The mutation reduced the single channel conductance by 5% for inward current and 17% for outward current. The single channel current was approximately 60% lower for outward current than for inward current in both wild type and mutant. 5. Open and closed times were both well fitted by the sum of two exponentials. Resistance was associated with longer open times and shorter closed times, reflecting a net stabilization of the channel open state by a factor of approximately five. 6. The mutation was associated with a marked reduction in the rate of GABA-induced desensitization, and a net destabilization of the desensitized conformation by a factor of 29. 7. The Rdl mutation manifests resistance through two different mechanisms. (a) The mutation weakens drug binding to the antagonist-favoured (desensitized) conformation by a structural change at the drug binding site. (b) The mutation destabilizes the antagonist-favoured conformation in an allosteric sense. The global association of a single amino acid replacement with cyclodiene resistance suggests that the resistance phenotype depends on changes in both of these properties, and that insecticides have selected residue 302 of Rdl for replacement because of its unique ability to influence both of these functions. 8. The location of alanine 302 in the sequence of the Rdl gene product supports a mechanism of action in which convulsants such as picrotoxin bind within the channel lumen, where they induce a rapid conformational change to the desensitized state.

Cloning and functional expression of a Drosophila gamma-aminobutyric acid receptor

A cDNA encoding a functional gamma-aminobutyric (GABA)-activated Cl- channel has been isolated from an adult Drosophila head cDNA library. When expressed in Xenopus laevis oocytes, the subunit functions efficiently, presumably as a homooligomeric complex and is activated by GABA or muscimol. GABA-evoked currents are highly sensitive to antagonism by picrotoxin but are insensitive to bicuculline, RU 5135, or zinc. Pentobarbitone greatly enhances GABA-evoked currents, whereas the neurosteroid 5 alpha-pregnan-3 alpha-ol-20-one demonstrates a large reduction in both the potency and maximal effect when compared with its actions upon vertebrate GABA type A receptors. Although zinc-insensitive, the subunit is also insensitive to flunitrazepam. Hence, the GABA receptors formed by this subunit exhibit a unique pharmacology when compared with vertebrate GABA type A receptors or those composed of rho subunits. Because the receptor-channel complex functions as a homooligomer, this subunit may be of value in mutagenesis studies aiming to define drug-binding sites.

Sequence of a Drosophila ligand-gated ion-channel polypeptide with an unusual amino-terminal extracellular domain

We report the isolation of a full-length clone from a Drosophila melanogaster head cDNA library that encodes a 614-residue polypeptide that exhibits all of the features of a ligand-gated chloride-channel/receptor subunit. This polypeptide, which has been named GRD (denoting that the polypeptide is a GABAA and glycine receptor-like subunit of Drosophila), displays between 33 and 44% identity to vertebrate GABAA and glycine receptor subunits and 32-37% identity to the GABAA receptor-like polypeptides from Drosophila and Lymnaea. It is interesting that the large amino-terminal, presumed extracellular domain of the GRD protein contains an insertion, between the dicysteine loop and the first putative membrane-spanning domain, of 75 amino acids that is not found in any other ligand-gated chloride-channel subunit. Analysis of cDNA and genomic DNA reveals that these residues are encoded by an extension of an exon that is equivalent to exon 6 of vertebrate GABAA and glycine receptor genes. The gene (named Grd) that encodes the Drosophila polypeptide has been mapped, by in situ hybridization, to position 75A on the left arm of chromosome 3.

Expression of a Drosophila GABA receptor in a baculovirus insect cell system. Functional expression of insecticide susceptible and resistant GABA receptors from the cyclodiene resistance gene Rdl

Recombinant baculoviruses containing two alternative splice forms of the Drosophila Rdl GABA receptor gene were constructed. Spodoptera frugiperda (Sf21) cells infected with either splice form expressed a transcript of expected size (2.5 kb). Western blotting of cell membrane extracts and immunoprecipitation experiments with an anti-Rdl antiserum recognized a protein of the expected size of approximately 65 kDa. Whole cell patch clamp analysis of cells infected with either splice form revealed functional expression of GABA gated chloride ion channels which were blocked by application of 1 microM picrotoxinin. Following replacement of alanine 302 with a serine, a mutation associated with resistance to picrotoxinin and cyclodiene insecticides, mutant channels showed similar levels of insensitivity to picrotoxinin (approximately 100-fold) as those observed in recordings from cultured Drosophila neurons. The significance of the expression of an insect GABA receptor in an insect cell line and the similarity of the results from these functional expression studies to recordings from cultured neurons is discussed.

Genes required for GABA function in Caenorhabditis elegans

gamma-Aminobutyric acid (GABA) neurotransmission is widespread in vertebrate and invertebrate nervous systems. Here we use a genetic approach to identify molecules specific to GABA function. On the basis of the known in vivo roles of GABAergic neurons in controlling behaviour of the nematode Caenorhabditis elegans, we identified mutants defective in GABA-mediated behaviours. Five genes are necessary either for GABAergic neuronal differentiation or for pre- or postsynaptic GABAergic function. The gene unc-30 is required for the differentiation of a specific type of GABAergic neuron, the type-D inhibitory motor neuron. The gene unc-25 is necessary for GABA expression and probably encodes the GABA biosynthetic enzyme glutamic acid decarboxylase. The genes unc-46 and unc-47 seem to be required for normal GABA release. Finally, the gene unc-49 is apparently necessary postsynaptically for the inhibitory effect of GABA on the body muscles and might encode a protein needed for the function of a GABAA-like receptor. Some of these genes are likely to encode previously unidentified proteins required for GABA function.

A novel invertebrate GABAA receptor-like polypeptide. Sequence and pattern of gene expression

A full-length complementary DNA has been isolated, from the fresh-water mollusc Lymnaea stagnalis, that encodes a polypeptide (which we have named zeta) that exhibits between 30% and 40% identity to vertebrate GABAA and glycine receptor subunit sequences. The locations of seven introns have been determined in the corresponding gene and six of these occur at similar relative positions as those found in vertebrate GABAA receptor genes. RNase protection studies have revealed that the transcript for this Lymnaean polypeptide is present at highest levels in the adult nervous system but that it can also be detected in peripheral tissues.

Cloning and sequencing of the cyclodiene insecticide resistance gene from the yellow fever mosquito Aedes aegypti. Conservation of the gene and resistance associated mutation with Drosophila

In order to examine the conservation of the mechanism of cyclodiene insecticide resistance between species we cloned a cDNA from the yellow fever mosquito Aedes aegypti homologous to the resistance gene Rdl in Drosophila. In D. melanogaster, resistance to cyclodienes and picrotoxinin is caused by a single amino acid substitution (alanine to serine) in the putative channel lining of a gamma-aminobutyic acid gated chloride ion channel. We report that the mosquito gene not only shows high homology to that of Drosophila but also that resistant strains display substitution of the same amino acid. The significance of this result in relation to the evolution of pesticide resistance, the use of Drosophila as a model insect for resistance studies and the potential use of this gene as a selectable marker in the genetic transformation of non-Drosophilids is discussed.

Glutamate receptors of Drosophila melanogaster. Primary structure of a putative NMDA receptor protein expressed in the head of the adult fly

The NMDA subtype of ionotropic glutamate receptors has been implicated in the activity-dependent modification of synaptic efficacy in the mammalian brain. Here we describe a cDNA isolated from Drosophila melanogaster which encodes a putative invertebrate NMDA receptor protein (DNMDAR-I). The deduced amino acid sequence of DNMDAR-I displays 46% amino acid identity to the rat NMDAR1 polypeptide and shows significant homology (16-23%) to other vertebrate and invertebrate glutamate receptor proteins. The DNMDAR-I gene maps to position 83AB of chromosome 3R and is highly expressed in the head of adult flies. Our data indicate that the NMDA subtype of glutamate receptors evolved early during phylogeny and suggest the existence of activity-dependent synaptic plasticity in the insect brain.

A point mutation in a Drosophila GABA receptor confers insecticide resistance

Vertebrates and invertebrates both have GABA (gamma-aminobutyric acid) as a major inhibitory neurotransmitter. GABAA receptors in vertebrates assemble as heteromultimers to form an integral chloride ion channel. These receptors are targets for drugs and pesticides and are also implicated in seizure-related diseases. Picrotoxinin (PTX) and cyclodiene insecticides are GABAA receptor antagonists which competitively displace each other from the same binding site. Insects and vertebrates showing resistance to cyclodienes also show cross-resistance to PTX. Previously, we used a field-isolated Drosophila mutant Rdl (Resistant to dieldrin) insensitive to PTX and cyclodienes to clone a putative GABA receptor. Here we report the functional expression and novel pharmacology of this GABA receptor and examine the functionality of a resistance-associated point mutation (alanine to serine) within the second membrane-spanning domain, the region thought to line the chloride ion channel pore. This substitution is found globally in Drosophila populations. This mutation not only identifies a single amino acid conferring high levels of resistance to the important GABA receptor antagonist PTX but also, by conferring resistance to cyclodienes, may account for over 60% of reported cases of insecticide resistance.

Drosophila gamma-aminobutyric acid receptor gene Rdl shows extensive alternative splicing

The Drosophila gamma-aminobutyric acid (GABA) receptor subunit gene Rdl was isolated on the basis of a mutant phenotype showing high levels of insensitivity to picrotoxinin and cyclodiene insecticides. Following analysis of two dissimilar cDNAs isolated from the locus, we report that Rdl undergoes extensive alternative splicing at two locations in the putative extracellular domain. At each location a choice is made between exons of the same size: "a" or "b" (23 amino acids long with two substitutions) and "c" or "d" (46 residues long with 10 substitutions). The function of these alternative exons remains unclear; however, exon d contains a putative site for casein kinase II phosphorylation. All possible combinations of exons (a with c or d and b with c or d) were found in RNA isolated from early embryos. This is the first demonstration of alternative splicing in a GABA receptor gene from invertebrates.

A single-amino acid substitution in a gamma-aminobutyric acid subtype A receptor locus is associated with cyclodiene insecticide resistance in Drosophila populations

Resistance to cyclodiene insecticides, documented in at least 277 species, is perhaps the most common kind of resistance to any pesticide. By using cyclodiene resistance to localize the responsible gene, a gamma-aminobutyric acid type A receptor/chloride ion-channel gene was previously cloned and sequenced from an insecticide-susceptible Drosophila melanogaster strain. We now describe the molecular genetics of the resistance allele. A single-base-pair mutation, causing a single-amino acid substitution (Ala-->Ser) within the second membrane-spanning region of the channel, was found to be the only consistent difference between resistant and susceptible strains of D. melanogaster. Some resistant strains of Drosophila simulans show the same mutation, whereas others show an alternative single-base-pair mutation in the same codon, resulting in the substitution of a different amino acid (glycine). These constitute single-box-pair mutations in insects that confer high levels of resistance to insecticides. The presence of the resistance mutations was then tested in a much larger set of strains by the PCR and subsequent digestion with a diagnostic restriction endonuclease. Both resistance-associated mutations cause the loss of a Hae II site. This site was invariably present in 122 susceptible strains but absent in 58 resistant lines of the two species sampled from five continents. PCR/restriction endonuclease treatment was also used to examine linkage of an EcoRI polymorphism in a neighboring intron in D. melanogaster, which was found associated with resistance in all but 3 of 48 strains examined. These PCR-based techniques are widely applicable to examination of the uniqueness of different resistance alleles in widespread populations, the identification of resistance mechanisms in different species, and the determination of resistance frequencies in monitoring.

GABA receptor molecules of insects

Receptors for 4-aminobutyric acid (GABA) have been identified in both central and peripheral nervous systems of several invertebrate phyla. To date, much of the information derived from physiological and biochemical studies on insect GABA receptors relates to GABA-gated chloride channels that show some similarities with vertebrate GABAA receptors. Like their vertebrate central nervous system (CNS) counterparts, agonist activation of such insect GABA receptors leads to a rapid, picrotoxin-sensitive increase in chloride ion conductance across the cell membrane. In insects, responses to GABA can be modulated by certain benzodiazepines and barbiturates. However, recent studies have detected a number of striking pharmacological differences between GABA-gated chloride channels of insects and vertebrates. Receptor binding, electrophysiological and 36Cl- flux assays have indicated that many insect receptors of this type are insensitive to the vertebrate GABAA antagonists bicuculline and pitrazepin. Benzodiazepine binding sites coupled to insect GABA receptors display a pharmacological profile distinct from that of corresponding sites in vertebrate CNS. Receptor binding studies have also demonstrated differences between convulsant binding sites of insect and vertebrate receptors. Insect GABA receptor molecules are important target sites for several chemically-distinct classes of insecticidally-active molecules. By characterizing these pharmacological properties in detail, it may prove possible to exploit differences between vertebrate and insect GABA receptors in the rational design of novel, more selective pest control agents. The recent application of the powerful techniques of molecular biology has revealed a diversity of vertebrate GABAA receptor subunits and their respective isoforms that can assemble in vivo to form a multiplicity of receptor subtypes. Molecular cloning of insect GABA receptor subunits will not only enhance our understanding of invertebrate neurotransmitter receptor diversity but will also permit the precise identification of the sites of action of pest control agents.

Cloning of a putative GABAA receptor from cyclodiene-resistant Drosophila: a case study in the use of insecticide-resistant mutants to isolate neuroreceptors

This chapter uses the isolation and cloning of cyclodiene resistance from Drosophila melanogaster to illustrate how mutants resistant to a toxicant can be used to study neuroreceptors. Isolation of mutants from the field, mapping of the single gene responsible and its subsequent cloning are described. As confirmation of gene cloning a susceptible allele of the gene has been used to genetically transform resistant individuals to susceptibility. The gene product appears to code for a subunit of a receptor highly similar to vertebrate GABAA receptor/chloride ion channels, and functional expression studies are described which will elucidate its pharmacology. Cyclodiene resistance is extremely widespread, occurring in both invertebrates and vertebrates. Thus examination of resistance-associated mutations in this receptor in a range of species will enhance our understanding of both the binding sites of toxic ligands and the genetic basis of pesticide resistance.

Molluscan ligand-gated ion-channel receptors

In this chapter we introduce the reader to the structures of the different types of ligand-gated ion-channel receptor, and the numerous receptor subtypes that have recently been revealed to exist, in both invertebrate and vertebrate species, by the application of molecular biological methods. We then review some of the data in support of the existence, in molluscs, of such receptor/channel complexes for gamma-aminobutyric acid, glutamate and acetylcholine. Finally, recent results from our laboratory on the cloning and expression of complementary DNAs, from the pond-snail Lymnaea stagnalis, that encode GABA(A) and glutamate receptor subunits will be described.

Drosophila cyclodiene resistance gene shows conserved genomic organization with vertebrate gamma-aminobutyric acidA receptors

Genomic clones from the Rdl locus of Drosophila, whose mutant phenotype is resistant to cyclodiene insecticides and picrotoxin, were characterized by restriction mapping and partial sequencing to determine intron/exon structure. The coding region of the gene comprises nine identified exons and spans greater than 25 kb of genomic DNA. The structure of the Drosophila Rdl receptor subunit was compared with those of vertebrate gamma-aminobutyric acid subtype A (GABAA) receptors and nicotinic acetylcholine receptors (nAChRs). The first six introns in Rdl show positions similar to those in vertebrate GABAA receptors, whereas the last two differ. It is interesting that the last intron appears to be in a position similar to that in nAChRs. These results are examined in relation to the proposal, based on amino acid identities, that Rdl codes for a novel class of GABAA receptor subunit more closely related to glycine receptors, and the possible place of Rdl in the lineage of the receptor superfamily is discussed.