Cys-Loop Ligand-Gated Chloride Channels in Dorsal Unpaired Median Neurons of Locusta migratoria

Ivermectin: A Multifaceted Drug With a Potential Beyond Anti-parasitic Therapy

Ivermectin was first discovered in the 1970s by Japanese microbiologist Satoshi Omura and Irish parasitologist William C. Campbell. Ivermectin has become a versatile pharmaceutical over the past 50 years. Ivermectin is a derivative of avermectin originally used to treat parasitic infections. Emerging literature has suggested that its role goes beyond this and may help treat inflammatory conditions, viral infections, and cancers. Ivermectin's anti-parasitic, anti-inflammatory, anti-viral, and anticancer effects were explored. Its traditional mechanism of action in parasitic diseases, such as scabies and malaria, rests on its ability to interfere with the glutamate-gated chloride channels in invertebrates and the lack of P-glycoprotein in many parasites. More recently, it has been discovered that the ability of ivermectin to block the nuclear factor kappa-light-chain enhancer of the activated B (NF-κB) pathway that modulates the expression and production of proinflammatory cytokines is implicated in its role as an anti-inflammatory agent to treat rosacea. Ivermectin has also been evaluated for treating infections caused by viruses, such as SARS-CoV-2 and adenoviruses, through inhibition of viral protein transportation and acting on the importin α/β1 interface. It has also been suggested that ivermectin can inhibit the proliferation of tumorigenic cells through various pathways that lead to the management of certain cancers. The review aimed to evaluate its multifaceted effects and potential clinical applications beyond its traditional use as an anthelmintic agent.

GABA signaling affects motor function in the honey bee

GABA is the most common inhibitory neurotransmitter in both vertebrate and invertebrate nervous systems. In insects, inhibition plays important roles at the neuromuscular junction, in the regulation of central pattern generators, and in the modulation of information in higher brain processing centers. Additionally, increasing our understanding of the functions of GABA is important since GABA_A receptors are the targets of several classes of pesticides. To investigate the role of GABA in motor function, honey bee foragers were injected with GABA or with agonists or antagonists specific for either GABA_A or GABA_B receptors. Compounds that activated either type of GABA receptor decreased activity levels. Bees injected with the GABA_A receptor antagonist picrotoxin lost the ability to right themselves, whereas blockade of GABA_B receptors led to increases in grooming. Injection with antagonists of either GABA_A or GABA_B receptors resulted in an increase in extended wing behavior, during which bees kept their wings out at right angles to their body rather than folded along their back. These data suggest that the GABA receptor types play distinct roles in behavior and that GABA may affect behavior at several different levels.

Acidification and γ-aminobutyric acid independently alter kairomone-induced behaviour

Exposure to high pCO₂ or low pH alters sensation and behaviour in many marine animals. We show that crab larvae lose their ability to detect and/or process predator kairomones after exposure to low pH over a time scale relevant to diel pH cycles in coastal environments. Previous work suggests that acidification affects sensation and behaviour through altered neural function, specifically the action of γ-aminobutyric acid (GABA), because a GABA antagonist, gabazine, restores the original behaviour. Here, however, gabazine resulted in a loss of kairomone detection/processing, regardless of pH. Our results also suggest that GABAergic signalling is necessary for kairomone identification in these larvae. Hence, the mechanism for the observed pH effect varies from the original GABA hypothesis. Furthermore, we suggest that this pH effect is adaptive under diel-cycling pH.

Mosquitocidal properties of IgG targeting the glutamate-gated chloride channel in three mosquito disease vectors (Diptera: Culicidae)

The glutamate-gated chloride channel (GluCl) is a highly sensitive insecticide target of the avermectin class of insecticides. As an alternative to using chemical insecticides to kill mosquitoes, we tested the effects of purified immunoglobulin G (IgG) targeting the extracellular domain of GluCl from Anopheles gambiae (AgGluCl) on the survivorship of three key mosquito disease vectors: Anopheles gambiae s.s., Aedes aegypti and Culex tarsalis. When administered through a single blood meal, anti-AgGluCl IgG reduced the survivorship of A. gambiae in a dose-dependent manner (LC50: 2.82 mg ml(-1), range 2.68-2.96 mg ml(-1)) but not A. aegypti or C. tarsalis. We previously demonstrated that AgGluCl is only located in tissues of the head and thorax of A. gambiae. To verify that AgGluCl IgG is affecting target antigens found outside the midgut, we injected it directly into the hemocoel via intrathoracic injection. A single, physiologically relevant concentration of anti-AgGluCl IgG injected into the hemocoel equally reduced mosquito survivorship of all three species. To test whether anti-AgGluCl IgG was entering the hemocoel of each of these mosquitoes, we fed mosquitoes a blood meal containing anti-AgGluCl IgG and subsequently extracted their hemolymph. We only detected IgG in the hemolymph of A. gambiae, suggesting that resistance of A. aegypti and C. tarsalis to anti-AgGluCl IgG found in blood meals is due to deficient IgG translocation across the midgut. We predicted that anti-AgGluCl IgG's mode of action is by antagonizing GluCl activity. To test this hypothesis, we fed A. gambiae blood meals containing anti-AgGluCl IgG and the GluCl agonist ivermectin (IVM). Anti-AgGluCl IgG attenuated the mosquitocidal effects of IVM, suggesting that anti-AgGluCl IgG antagonizes IVM-induced activation of GluCl. Lastly, we stained adult, female A. aegypti and C. tarsalis for GluCl expression. Neuronal GluCl expression in these mosquitoes was similar to previously reported A. gambiae GluCl expression; however, we also discovered GluCl staining on the basolateral surface of their midgut epithelial cells, suggesting important physiological differences in Culicine and Anopheline mosquitoes.

Characterization of the target of ivermectin, the glutamate-gated chloride channel, from Anopheles gambiae

The use of insecticide-treated nets and indoor residual insecticides targeting adult mosquito vectors is a key element in malaria control programs. However, mosquito resistance to the insecticides used in these applications threatens malaria control efforts. Recently, the mass drug administration of ivermectin (IVM) has been shown to kill Anopheles gambiae mosquitoes and disrupt Plasmodium falciparum transmission in the field. We cloned the molecular target of IVM from A. gambiae, the glutamate-gated chloride channel (AgGluCl), and characterized its transcriptional patterns, protein expression and functional responses to glutamate and IVM. AgGluCl cloning revealed an unpredicted fourth splice isoform as well as a novel exon and splice site. The predicted gene products contained heterogeneity in the N-terminal extracellular domain and the intracellular loop region. Responses to glutamate and IVM were measured using two-electrode voltage clamp on Xenopus laevis oocytes expressing AgGluCl. IVM induced non-persistent currents in AgGluCl-a1 and did not potentiate glutamate responses. In contrast, AgGluCl-b was insensitive to IVM, suggesting that the AgGluCl gene could produce IVM-sensitive and -insensitive homomultimers from alternative splicing. AgGluCl isoform-specific transcripts were measured across tissues, ages, blood feeding status and sex, and were found to be differentially transcribed across these physiological variables. Lastly, we stained adult, female A. gambiae for GluCl expression. The channel was expressed in the antenna, Johnston's organ, supraesophageal ganglion and thoracic ganglia. In summary, we have characterized the first GluCl from a mosquito, A. gambiae, and described its unique activity and expression with respect to it as the target of the insecticide IVM.

Mining gene expression data for pollutants (dioxin, toluene, formaldehyde) and low dose of gamma-irradiation

General and specific effects of molecular genetic responses to adverse environmental factors are not well understood. This study examines genome-wide gene expression profiles of Drosophila melanogaster in response to ionizing radiation, formaldehyde, toluene, and 2,3,7,8-tetrachlorodibenzo-p-dioxin. We performed RNA-seq analysis on 25,415 transcripts to measure the change in gene expression in males and females separately. An analysis of the genes unique to each treatment yielded a list of genes as a gene expression signature. In the case of radiation exposure, both sexes exhibited a reproducible increase in their expression of the transcription factors sugarbabe and tramtrack. The influence of dioxin up-regulated metabolic genes, such as anachronism, CG16727, and several genes with unknown function. Toluene activated a gene involved in the response to the toxins, Cyp12d1-p; the transcription factor Fer3's gene; the metabolic genes CG2065, CG30427, and CG34447; and the genes Spn28Da and Spn3, which are responsible for reproduction and immunity. All significantly differentially expressed genes, including those shared among the stressors, can be divided into gene groups using Gene Ontology Biological Process identifiers. These gene groups are related to defense response, biological regulation, the cell cycle, metabolic process, and circadian rhythms. KEGG molecular pathway analysis revealed alteration of the Notch signaling pathway, TGF-beta signaling pathway, proteasome, basal transcription factors, nucleotide excision repair, Jak-STAT signaling pathway, circadian rhythm, Hippo signaling pathway, mTOR signaling pathway, ribosome, mismatch repair, RNA polymerase, mRNA surveillance pathway, Hedgehog signaling pathway, and DNA replication genes. Females and, to a lesser extent, males actively metabolize xenobiotics by the action of cytochrome P450 when under the influence of dioxin and toluene. Finally, in this work we obtained gene expression signatures pollutants (dioxin, toluene), low dose of gamma-irradiation and common molecular pathways for different kind of stressors.

Differential distribution of glutamate- and GABA-gated chloride channels in the housefly Musca domestica

l-Glutamic acid (glutamate) mediates fast inhibitory neurotransmission by affecting glutamate-gated chloride channels (GluCls) in invertebrates. The molecular function and pharmacological properties of GluCls have been well studied, but not much is known about their physiological role and localization in the insect body. The distribution of GluCls in the housefly (Musca domestica L.) was thus compared with the distribution of γ-aminobutyric acid (GABA)-gated chloride channels (GABACls). Quantitative PCR and ligand-binding experiments indicate that the GluCl and GABACl transcripts and proteins are predominantly expressed in the adult head. Intense GluCl immunostaining was detected in the lamina, leg motor neurons, and legs of adult houseflies. The GABACl (Rdl) immunostaining was more widely distributed, and was found in the medulla, lobula, lobula plate, mushroom body, antennal lobe, and ellipsoid body. The present findings suggest that GluCls have physiological roles in different tissues than GABACls.

Glutamate-gated chloride channels

Glutamate-gated chloride channels (GluCls) are found only in protostome invertebrate phyla but are closely related to mammalian glycine receptors. They have a number of roles in these animals, controlling locomotion and feeding and mediating sensory inputs into behavior. In nematodes and arthropods, they are targeted by the macrocyclic lactone family of anthelmintics and pesticides, making the GluCls of considerable medical and economic importance. Recently, the three-dimensional structure of a GluCl was solved, the first for any eukaryotic ligand-gated anion channel, revealing a macrocyclic lactone-binding site between the channel domains of adjacent subunits. This minireview will highlight some unique features of the GluCls and illustrate their contribution to our knowledge of the entire Cys loop ligand-gated ion channel superfamily.

GABA and glutamate receptors have different effects on excitability and are differentially regulated by calcium in spider mechanosensory neurons

GABA and glutamate receptors belonging to the ligand-gated chloride-channel family are primary targets of insecticides and antiparasitics, so their molecular structure, pharmacology and biophysical properties have attracted significant attention. However, little is known about the physiological roles of these channels or how they regulate neuronal excitability and animal behavior. Mechanosensory neurons of VS-3 slit sensilla in the patella of the tropical wandering spider, Cupiennius salei, react to the GABA(A)-receptor agonists, GABA and muscimol, with depolarization and an increase in intracellular [Ca(2+)] and, during random noise stimulation, with a mixed inhibitory-excitatory response. We established that the GABA(A)-receptors in all VS-3 neurons are identical, but there are at least two types of glutamate receptors and some neurons do not respond to glutamate at all. Immunohistochemistry with antibodies against Drosophila inhibitory glutamate receptor (GluCls) α-subunit suggests that in addition to VS-3 neurons, these receptors may also be present in the efferent neurons surrounding the sensory neurons. Most VS-3 neurons were inhibited but not depolarized by glutamate during random stimulation, but some depolarized and had a similar excitatory-inhibitory response to glutamate as to muscimol. The membrane-permeable Ca(2+)-chelator BAPTA-AM abolished muscimol effects but potentiated glutamate effects, indicating that GABA and glutamate receptors are differentially modulated by Ca(2+), leading to diverse regulation of neuronal excitability. We hypothesize that this could be achieved by different Ca(2+)-triggered phosphorylation processes at each receptor type. These findings are important for understanding the significance of Ca(2+)-mediated regulation of transmitter receptor molecules and its role in controlling excitability.

Identification, localization and function of glutamate-gated chloride channel receptors in the honeybee brain

Glutamate-gated chloride channels (GluCls) are members of the cys-loop ligand-gated ion channel superfamily whose presence has been reported in a variety of invertebrate tissues. In the honeybee, a single gene, amel_glucl, encoding a GluClα subunit, was found in the genome but both the pattern of expression of this gene in the bee brain and its functional role remained unknown. Here we localised the expression sites of the honeybee GluClα subunit at the mRNA and protein levels. To characterise the functional role of GluCls in the honeybee brain, we studied their implication in olfactory learning and memory by means of RNA interference (RNAi) against the GluClα subunit. We found that the GluClα subunit is expressed in the muscles, the antennae and the brain of honeybees. Expression of the GluClα protein was necessary for the retrieval of olfactory memories; more specifically, injection of dsRNA or siRNA resulted in a decrease in retention performances ∼24 h after injection. Knockdown of GluClα subunits impaired neither olfaction nor sucrose sensitivity, and did not affect the capacity to associate odor and sucrose. Our data provide the first evidence for the involvement of glutamate-gated chloride channels in olfactory memory in an invertebrate.

Transmembrane potential polarization, calcium influx, and receptor conformational state modulate the sensitivity of the imidacloprid-insensitive neuronal insect nicotinic acetylcholine receptor to neonicotinoid insecticides

Neonicotinoid insecticides act selectively on insect nicotinic acetylcholine receptors (nAChRs). Recent studies revealed that their efficiency was altered by the phosphorylation/dephosphorylation process and the intracellular signaling pathway involved in the regulation of nAChRs. Using whole-cell patch-clamp electrophysiology adapted for dissociated cockroach dorsal unpaired median (DUM) neurons, we demonstrated that intracellular factors involved in the regulation of nAChR function modulated neonicotinoid sensitivity. DUM neurons were known to express two α-bungarotoxin-insensitive nAChR subtypes: nAChR1 and nAChR2. Whereas nAChR1 was sensitive to imidacloprid, nAChR2 was insensitive to this insecticide. Here, we demonstrated that, like nicotine, acetamiprid and clothianidin, other types of neonicotinoid insecticides, acted as agonists on the nAChR2 subtype. Using acetamiprid, we revealed that both steady-state depolarization and hyperpolarization affected nAChR2 sensitivity. The measurement of the input membrane resistance indicated that change in the acetamiprid-induced agonist activity was related to the receptor conformational state. Using cadmium chloride, ω-conotoxin GVIA, and (R,S)-(3,4-dihydro-6,7-dimethoxy-isoquinoline-1-yl)-2-phenyl-N,N-di-acetamide (LOE 908), we found that inhibition of calcium influx through high voltage-activated calcium channels and transient receptor potential γ (TRPγ) activated by both depolarization and hyperpolarization increased nAChR2 sensitivity to acetamiprid. Finally, using N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W7), forskolin, and cAMP, we demonstrated that adenylyl cyclase sensitive to the calcium/calmodulin complex regulated internal cAMP concentration, which in turn modulated TRPγ function and nAChR2 sensitivity to acetamiprid. Similar TRPγ-induced modulatory effects were also obtained when clothianidin was tested. These findings bring insights into the signaling pathway modulating neonicotinoid efficiency and open novel strategies for optimizing insect pest control.

Development of histamine-immunoreactivity in the Central nervous system of the two locust species Schistocerca gregaria and Locusta migratoria

Locusts are attractive model preparations for cellular investigations of neurodevelopment. In this study, we investigate the immunocytochemical localization of histamine in the developing ventral nerve cord of two locust species, Schistocerca gregaria and Locusta migratoria. Histamine is the fast neurotransmitter of photoreceptor neurons in the compound eye of insects, but it is also synthesized in interneurons of the central nervous system. In the locust ventral nerve cord, the pattern of histamine-immunoreactive neurons follows a relatively simple bauplan. The histaminergic system comprises a set of single, ascending projection neurons that are segmentally arranged in almost every neuromere. The neurons send out their axons anteriorly, forming branches and varicosities throughout the adjacent ganglia. In the suboesophageal ganglion, the cell bodies lie in a posteriolateral position. The prothoracic ganglion lacks histaminergic neurons. In the posterior ganglia of the ventral nerve cord, the somata of the histaminergic neurons are ventromedially positioned. Histamine-immunoreactivity starts around 50% of embryonic development in interneurons of the brain. Subsequently, the neurons of the more posterior ganglia of the ventral nerve cord become immunoreactive. From 60% embryonic development, the pattern of soma staining in the nerve cord appears mature. Around 65% of embryonic development, the photoreceptor cells show histamine-immunoreactivity. The histaminergic innervation of the neuropile develops from the central branches toward the periphery of the ganglia and is completed right before hatching.