GABA receptor molecules of insects

Methemoglobinemia Following Indoxacarb Ingestion: A Unique Toxicological Presentation

Indoxacarb, an oxadiazine insecticide, is known for its selective lethality by blocking neuronal voltage-dependent sodium channels. While primarily developed to target insect populations resistant to other pesticides, its toxicity in humans remains poorly understood. We present a case of methemoglobinemia resulting from indoxacarb ingestion, a rare manifestation of its toxic effects. A 38-year-old farmer attempted suicide by ingesting the insecticide, leading to cyanosis, hypoxemia, and characteristic arterial blood gas findings indicative of methemoglobinemia. Prompt diagnosis was challenging due to the absence of specific tests, necessitating clinical suspicion. Treatment with methylene blue and supportive therapy resulted in significant clinical improvement, highlighting the importance of early intervention in managing indoxacarb poisoning. This case underscores the need for increased awareness among healthcare providers regarding the potential toxic effects of indoxacarb. It emphasizes the importance of prompt recognition and treatment of methemoglobinemia in pesticide-related poisonings. Further research is warranted to elucidate the mechanisms underlying indoxacarb toxicity in humans and to optimize treatment strategies for affected individuals.

Methyl Eugenol Binds Recombinant Gamma-Aminobutyric Acid Receptor-Associated Protein from the Western Flower Thrips Frankliniella occidentalis

The western flower thrips (Frankliniella occidentalis) is a major pest insect in agriculture. However, few insecticides are effective for their control. The recombinant gamma-aminobutyric acid receptor-associated protein (rGABARAP) was examined as a potential target of the monoterpenoids responsible for their insecticidal activities. The insecticidal activity of anethole, linalool, and methyl eugenol (ME) was evaluated in the laboratory. The half-maximum lethal concentration (LC₅₀) of ME against second-instar nymphs of F. occidentalis was 5.5 mg/L using membrane and leaf immersion methods, while that of spinosyn A was 1.0 mg/L. The dissociation constants of ME binding to rGABARAP were 1.30 and 4.22 μmol/L, respectively, according to microscale thermophoresis (MST) and isothermal titration calorimetry (ITC) measurements. A molecular docking study showed interactions between ME and Tyr174 via π-π stacking. The MST and ITC experiments showed loss of specific binding between ME and the rGABARAP^Y174A mutant. Therefore, Tyr174 is a key amino acid residue of rGABARAP involving ME binding. The results revealed GABARAP as a potential target for the development of monoterpenoid insecticides.

Groundnut Bud Necrosis Virus Modulates the Expression of Innate Immune, Endocytosis, and Cuticle Development-Associated Genes to Circulate and Propagate in Its Vector, Thrips palmi

Thrips palmi (Thysanoptera: Thripidae) is the predominant tospovirus vector in Asia-Pacific region. It transmits economically damaging groundnut bud necrosis virus (GBNV, family Tospoviridae) in a persistent propagative manner. Thrips serve as the alternate host, and virus reservoirs making tospovirus management very challenging. Insecticides and host plant resistance remain ineffective in managing thrips–tospoviruses. Recent genomic approaches have led to understanding the molecular interactions of thrips–tospoviruses and identifying novel genetic targets. However, most of the studies are limited to Frankliniella species and tomato spotted wilt virus (TSWV). Amidst the limited information available on T. palmi–tospovirus relationships, the present study is the first report of the transcriptome-wide response of T. palmi associated with GBNV infection. The differential expression analyses of the triplicate transcriptome of viruliferous vs. nonviruliferous adult T. palmi identified a total of 2,363 (1,383 upregulated and 980 downregulated) significant transcripts. The Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses showed the abundance of differentially expressed genes (DEGs) involved in innate immune response, endocytosis, cuticle development, and receptor binding and signaling that mediate the virus invasion and multiplication in the vector system. Also, the gene regulatory network (GRN) of most significant DEGs showed the genes like ABC transporter, cytochrome P450, endocuticle structural glycoprotein, gamma-aminobutyric acid (GABA) receptor, heat shock protein 70, larval and pupal cuticle proteins, nephrin, proline-rich protein, sperm-associated antigen, UHRF1-binding protein, serpin, tyrosine–protein kinase receptor, etc., were enriched with higher degrees of interactions. Further, the expression of the candidate genes in response to GBNV infection was validated in reverse transcriptase-quantitative real-time PCR (RT-qPCR). This study leads to an understanding of molecular interactions between T. palmi and GBNV and suggests potential genetic targets for generic pest control.

Intrinsic and Network Mechanisms Constrain Neural Synchrony in the Moth Antennal Lobe

Projection-neurons (PNs) within the antennal lobe (AL) of the hawkmoth respond vigorously to odor stimulation, with each vigorous response followed by a ~1 s period of suppression—dubbed the “afterhyperpolarization-phase,” or AHP-phase. Prior evidence indicates that this AHP-phase is important for the processing of odors, but the mechanisms underlying this phase and its function remain unknown. We investigate this issue. Beginning with several physiological experiments, we find that pharmacological manipulation of the AL yields surprising results. Specifically, (a) the application of picrotoxin (PTX) lengthens the AHP-phase and reduces PN activity, whereas (b) the application of Bicuculline-methiodide (BIC) reduces the AHP-phase and increases PN activity. These results are curious, as both PTX and BIC are inhibitory-receptor antagonists. To resolve this conundrum, we speculate that perhaps (a) PTX reduces PN activity through a disinhibitory circuit involving a heterogeneous population of local-neurons, and (b) BIC acts to hamper certain intrinsic currents within the PNs that contribute to the AHP-phase. To probe these hypotheses further we build a computational model of the AL and benchmark our model against our experimental observations. We find that, for parameters which satisfy these benchmarks, our model exhibits a particular kind of synchronous activity: namely, “multiple-firing-events” (MFEs). These MFEs are causally-linked sequences of spikes which emerge stochastically, and turn out to have important dynamical consequences for all the experimentally observed phenomena we used as benchmarks. Taking a step back, we extract a few predictions from our computational model pertaining to the real AL: Some predictions deal with the MFEs we expect to see in the real AL, whereas other predictions involve the runaway synchronization that we expect when BIC-application hampers the AHP-phase. By examining the literature we see support for the former, and we perform some additional experiments to confirm the latter. The confirmation of these predictions validates, at least partially, our initial speculation above. We conclude that the AL is poised in a state of high-gain; ready to respond vigorously to even faint stimuli. After each response the AHP-phase functions to prevent runaway synchronization and to “reset” the AL for another odor-specific response.

Cloning and characterization of a GABA receptor from Plutella xylostella (Lepidoptera: Plutellidae)

A full-length cDNA, with an open reading frame (ORF) of 1,449 bp, encoding a subunit of the gamma-aminobutyric acid (GABA)-activated chloride channel was isolated from Plutella xylostella (L.) (Lepidoptera: Plutellidae) (GenBank accession no. EF156251). The subunit gene encoded a 483-amino acid polypeptide that showed 84% sequence identity with DmRdl subunit (U02042) (Drosophila melanogaster resistant to dieldrin). When expressed in Xenopus laevis oocytes, the subunit assembled as a functional homomeric complex activated by GABA and abamectin in a dose-dependent manner. The EC50 value of GABA was 0.49 mM (0.41-0.58) (n = 5). However, the responses to abamectin were very robust, with an EC50 of 4.85 microM (4.02-5.89) (n = 6), indicating that abamectin was > 100-fold more potent in activating chloride currents than GABA. The results suggest that this subunit is vital to the formation of a functional channel and contains the binding site of abamectin.

Ca2+/calmodulin-dependent protein kinase regulates GABA-activated Cl- current in cockroach dorsal unpaired median neurons

We studied gamma-aminobutyric acid (GABA)-mediated currents in short-term cultured dorsal unpaired median (DUM) neurons of cockroach Periplaneta americana using the whole cell patch-clamp technique in symmetrical chloride solutions. All DUM neurons voltage-clamped at -50 mV displayed inward currents (I(GABA)) when 10(-4) M of GABA was applied by pneumatic pressure-ejection pulses. The semi-logarithmic curve of I(GABA) amplitude versus the ejection time yielded a Hill coefficient of 4.0. I(GABA) was chloride (Cl-) because the reversal potential given by the current-voltage (I-V) curve varied according to the value predicted by the Nernst equation for Cl- dependence. In addition, I(GABA) was almost completely blocked by bath application of the chloride channel blockers picrotoxin (PTX) or 3,3-bis(trifluoromethyl)bicyclo-[2,2,1]heptane-2,2-diacarbonitrile (BIDN). The I-V curve for I(GABA) displayed a unexpected biphasic aspect and was best fitted by two linear regressions giving two slope conductances of 35.6 +/- 2.1 and 80.9 +/- 4.1 nS for potentials ranging from 0 to -30 and -30 to -70 mV, respectively. At -50 mV, the current amplitude was decreased by cadmium chloride (CdCl2, 10(-3) M) and calcium-free solution. The semi-logarithmic curve for CdCl2-resistant I(GABA) gave a Hill coefficient of 2.4. Hyperpolarizing voltage step from -50 to -80 mV was known to increase calcium influx through calcium-resting channels. According to this protocol, a significant increase of I(GABA) amplitude was observed. However, this effect was never obtained when the same protocol was applied on cell body pretreated with CdCl2. When the calmodulin blocker N-(6-aminohexyl)-5-chloro-1-naphtalene-sulfonamide or the calcium-calmodulin-dependent protein kinase blocker 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine (KN-62) was added in the pipette solution, I(GABA) amplitude was decreased. Pressure ejection application of the cis-4-aminocrotonic acid (CACA) on DUM neuron cell body held at -50 mV, evoked a Cl- inward current which was insensitive to CdCl2. The Hill plot yielded a Hill coefficient of 2.3, and the I-V curve was always linear in the negative potential range with a slope conductance of 32.4 +/- 1.1 nS. These results, similar to those obtained with GABA in the presence of CdCl2 and KN-62, indicated that CACA activated one subtype of GABA receptor. Our study demonstrated that at least two distinct subtypes of Cl--dependent GABA receptors were expressed in DUM neurons, one of which is regulated by an intracellular Ca2+-dependent mechanism via a calcium-dependent protein kinase. The consequences of the modulatory action of Ca2+ in GABA receptors function and their sensitivity to insecticide are discussed.

Neurotransmitters and neuropeptides in the brain of the locust

As part of continuous research on the neurobiology of the locust, the distribution and functions of neurotransmitter candidates in the nervous system have been analyzed particularly well. In the locust brain, acetylcholine, glutamate, gamma-aminobutyric acid (GABA), and the biogenic amines serotonin, dopamine, octopamine, and histamine most likely serve a transmitter function. Increasing evidence, furthermore, supports a signalling function for the gaseous molecule nitric oxide, but a role for neuroptides is so far suggested only by immunocytochemistry. Acetylcholine, glutamate, and GABA appear to be present in large numbers of interneurons. As in other insects, antennal sensory afferents might be cholinergic, while glutamate is the transmitter candidate of antennal motoneurons. GABA is regarded as the principle inhibitory transmitter of the brain, which is supported by physiological studies in the antennal lobe. The cellular distribution of biogenic amines has been analyzed particularly well, in some cases down to physiologically characterized neurons. Amines are present in small numbers of interneurons, often with large branching patterns, suggesting neuromodulatory roles. Histamine, furthermore, is the transmitter of photoreceptor neurons. In addition to these "classical transmitter substances," more than 60 neuropeptides were identified in the locust. Many antisera against locust neuropeptides label characteristic patterns of neurosecretory neurons and interneurons, suggesting that these peptides have neuroactive functions in addition to hormonal roles. Physiological studies supporting a neuroactive role, however, are still lacking. Nitric oxide, the latest addition to the list of neurotransmitter candidates, appears to be involved in early stages of sensory processing in the visual and olfactory systems.

The pharmacological profile of GABA receptors on cultured insect neurones

Neuronal cultures of the cockroach, Periplaneta americana, were used to study the pharmacological profile of GABA receptors using the whole-cell-voltage clamp technique. The results indicated that insect GABA receptors are linked to a chloride channel that can be activated by both GABA(A) and GABA(C) receptor agonists. The receptors are blocked by GABA(A) chloride channel blockers and some insecticides but not by competitive GABA(A) receptor antagonists. The GABA(C) receptor competitive antagonists were either full or partial agonists of the cockroach GABA receptors. The receptors were modulated by the enantiomers of lindane. In conclusion, insect GABA receptors appear to have a distinct pharmacological profile that does not conform to either vertebrate GABA(A) or GABA(C) receptors.

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.

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.

Drosophila GABAergic systems. II. Mutational analysis of chromosomal segment 64AB, a region containing the glutamic acid decarboxylase gene

The Drosophila melanogaster Gad gene maps to region 64A3-5 of chromosome 3L and encodes glutamic acid decarboxylase (GAD), the rate-limiting enzyme for the synthesis of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Because this neurotransmitter has been implicated in developmental functions, we have begun to study the role of GABA synthesis during Drosophila embryogenesis. We show that Gad mRNA is expressed in a widespread pattern within the embryonic nervous system. Similarly, GAD-immunoreactive protein is present during embryogenesis. These results prompted us to screen for embryonic lethal mutations that affect GAD activity. The chromosomal region to which Gad maps, however, has not been subjected to an extensive mutational analysis, even though it contains several genes encoding important neurobiological, developmental, or cellular functions. Therefore, we have initially generated both chromosomal rearrangements and point mutations that map to the Drosophila 64AB interval. Altogether, a total of 33 rearrangements and putative point mutations were identified within region 64A3-5 to 64B12. Genetic complementation analysis suggests that this cytogenetic interval contains a minimum of 19 essential genes. Within our collection of lethal mutations are several chromosomal rearrangements, two of which are in the vicinity of the Gad locus. One of these rearrangements, Df(3L)C175, is a small deletion that removes the Gad locus and at least two essential genes; the second, T(2;3)F10, is a reciprocal translocation involving the second and third chromosomes with a break within region 64A3-5. Both of these rearrangements are associated with embryonic lethality and decreased GAD enzymatic activity.