Pharmacological identification of cholinergic receptor subtypes: modulation of locomotion and neural circuit excitability in Drosophila larvae

Effects of Terbufos (Organophosphate) on Larval Behaviour of Two Forensically Important Diptera Species: Contributions for Entomotoxicology

Neurotoxicant compounds interfere with the behaviour and biology of insects, significantly altering their locomotion patterns. However, little is known about the effect of organophosphates, neurotoxicants for agricultural, domestic and industrial use, on the larval movement of necrophagous flies, although being responsible for frequent cases of poisoning and accidental or intentional deaths. Thus, we aimed to study the influence of Terbufos (organophosphate) on the activity and mobility patterns of Lucilia eximia (Wiedemann 1819) (Calliphoridae) and Peckia (Peckia) chrysostoma (Wiedemann 1830) (Sarcophagidae) immatures collected from rat carcasses intoxicated with 5, 10 or 20 mg/kg of Terbufos, to evaluate (i) peristaltic movements and body contractions, and (ii) distance and shape of the trajectory travelled by the larva. Behavioural parameters were analysed in loco and through videos. We observed that the presence of Terbufos altered poisoned larvae's activity and body mobility in both taxon and dose-dependent manner. Lucilia eximia larvae were more active, with greater frequency of body movements and lateral contractions when intoxicated with high and intermediate doses of Terbufos. On the other hand, P. (P.) chrysostoma immatures were less active, with fewer body and lateral contractions when intoxicated with the high dose of the compound. This work experimentally demonstrates that the presence of Terbufos can alters the mobility and movement of intoxicated necrophagous Diptera, essential components of the cadaveric fauna.

Perfluorooctanoic acid induces behavioral impairment and oxidative injury in Nauphoeta cinerea nymphs

Perfluorooctanoic acid (PFOA) is a persistent organic contaminant with potential health threats to both animals and humans. However, the impact of PFOA on insects, which play significant roles in ecosystems, is understudied. We evaluated the toxicological impact of ecologically relevant concentrations of PFOA (0, 25, 50, 100, and 200 µg L-1) on Nauphoeta cinerea nymphs following exposure for 42 consecutive days. We analyzed the behavior of the insects with automated video-tracking software and processed the head, midgut, and fat body for biochemical assays. PFOA-exposed insects exhibited significant reductions in locomotory abilities and an increase in freezing time. Furthermore, PFOA exposure reduced acetylcholinesterase activity in the insect head. PFOA exposure increased the activities of superoxide dismutase, glutathione peroxidase, and catalase in the head and midgut, but decreased them in the fat body. PFOA also significantly increased glutathione-S transferase activity, while decreasing glutathione levels in the head, midgut, and fat body. Additionally, PFOA exposure increased reactive oxygen and nitrogen species, nitric oxide, lipid peroxidation, and protein carbonyl contents in the head, midgut, and fat body of the insects. In conclusion, our findings indicate that PFOA exposure poses an ecological risk to Nauphoeta cinerea.

Persistent oxidative injury and neurobehavioral impairment in adult male and female Nauphoeta cinerea exposed to perfluorooctanoic acid

This study aimed to elucidate if the toxicity of perfluorooctanoic acid (PFOA), an emerging persistent organic contaminant, is reversible or not in adult male and female Nauphoeta cinerea. Both sexes of Nauphoeta cinerea were separately exposed to 0, 1 and 5 mg/L PFOA in drinking water for 21 consecutive days. PFOA-exposed Nauphoeta cinerea exhibited significant deficits in the locomotor and exploratory capabilities with concomitant increase in anxiogenic behaviors which persisted after cessation of PFOA exposure. Moreover, PFOA-induced decrease in acetylcholinesterase activity persisted after cessation of PFOA exposure in both insects' sexes. Catalase and superoxide dismutase activities were increased in the midgut but restored to control following cessation of PFOA exposure. The increased reactive oxygen and nitrogen species, nitric oxide and hydrogen peroxide levels persisted in the head whereas they were abated in the midgut after cessation of PFOA exposure. However, PFOA-induced persistent increase in lipid peroxidation and protein carbonyl levels in the head and midgut of insects. Collectively, PFOA exposure elicited persistent neurobehavioral and oxidative injury similarly in both sexes of adult Nauphoeta cinerea during this investigation.

Peptides from the Sea Anemone Metridium senile with Modified Inhibitor Cystine Knot (ICK) Fold Inhibit Nicotinic Acetylcholine Receptors

Nicotinic acetylcholine receptors (nAChRs) play an important role in the functioning of the central and peripheral nervous systems, and other organs of living creatures. There are several subtypes of nAChRs, and almost all of them are considered as pharmacological targets in different pathological states. The crude venom of the sea anemone Metridium senile showed the ability to interact with nAChRs. Four novel peptides (Ms11a-1-Ms11a-4) with nAChR binding activity were isolated. These peptides stabilized by three disulfide bridges have no noticeable homology with any known peptides. Ms11a-1-Ms11a-4 showed different binding activity towards the muscle-type nAChR from the Torpedo californica ray. The study of functional activity and selectivity for the most potent peptide (Ms11a-3) revealed the highest antagonism towards the heterologous rat α9α10 nAChR compared to the muscle and α7 receptors. Structural NMR analysis of two toxins (Ms11a-2 and Ms11a-3) showed that they belong to a new variant of the inhibitor cystine knot (ICK) fold but have a prolonged loop between the fifth and sixth cysteine residues. Peptides Ms11a-1-Ms11a-4 could represent new pharmacological tools since they have structures different from other known nAChRs inhibitors.

Decabromodiphenyl ethane induced hyperactivity in developing zebrafish at environmentally relevant concentrations

Decabromodiphenyl ethane (DBDPE), a widely used novel brominated flame retardant, is gaining concerns due to rapidly increased contents in various environmental and biota samples. In the present study, zebrafish (Danio rerio) embryos were exposed to 2.91, 9.71, 29.14 and 97.12 μg/L of DBDPE until 120 h post-fertilization (hpf) to investigate the potential developmental neurotoxicity and underlying mechanisms. Chemical analysis revealed concentration-dependently increased body burdens of DBDPE in zebrafish larvae, with bioaccumulation factors (BCFs) ranging from 414 to 726. Embryonic exposure to DBDPE caused hyperactivity without affecting the development of secondary motoneuron axons and muscle fibers. However, further results implicated that DBDPE may affect the locomotor regulatory network via different mechanisms at lower and higher concentrations. On the one hand, embryonic exposure to 2.91 μg/L DBDPE transiently promoted spontaneous coiling contractions, but showed no effects on touch-response and swimming activity in zebrafish larvae. The whole-body contents of neurotransmitters were significantly decreased. Significant decreased protein abundances of α1-TUBULIN and SYN2a and molecular docking results pointed out possible interactions of DBDPE with these two proteins. However, these changes may be unconcerned with the transient hyperactivity, and the exact molecular mechanisms need further investigation. On the other hand, 29.14 and 97.12 μg/L DBDPE exposure caused longer-lasting effects in promoting spontaneous coiling contractions, and also touch-response and swimming activity. At the same time, increased ACh contents (without changes of other neurotransmitters) and ChAT activity and inhibited transcription of nAChRs were observed at higher concentrations. Molecular docking indicated direct interaction of DBDPE with ChAT. The results suggested that DBDPE induced hyperactivity at higher concentrations was probably involved with disrupted cholinergic system, with ChAT as a potential target. Given that the body burden of DBDPE in lower concentration group was comparable with those detected in wild fish, the current results may provide useful information for ecological risk assessment.

Localization of muscarinic acetylcholine receptor-dependent rhythm-generating modules in the Drosophila larval locomotor network

Mechanisms of rhythm generation have been extensively studied in motor systems that control locomotion over terrain in limbed animals; however, much less is known about rhythm generation in soft-bodied terrestrial animals. Here we explored how muscarinic acetylcholine receptor (mAChR)-modulated rhythm-generating networks are distributed in the central nervous system (CNS) of soft-bodied Drosophila larvae. We measured fictive motor patterns in isolated CNS preparations, using a combination of Ca2+ imaging and electrophysiology while manipulating mAChR signaling pharmacologically. Bath application of the mAChR agonist oxotremorine potentiated bilaterally asymmetric activity in anterior thoracic regions and promoted bursting in posterior abdominal regions. Application of the mAChR antagonist scopolamine suppressed rhythm generation in these regions and blocked the effects of oxotremorine. Oxotremorine triggered fictive forward crawling in preparations without brain lobes. Oxotremorine also potentiated rhythmic activity in isolated posterior abdominal CNS segments as well as isolated anterior brain and thoracic regions, but it did not induce rhythmic activity in isolated anterior abdominal segments. Bath application of scopolamine to reduced preparations lowered baseline Ca2+ levels and abolished rhythmic activity. Overall, these results suggest that mAChR signaling plays a role in enabling rhythm generation at multiple sites in the larval CNS. This work furthers our understanding of motor control in soft-bodied locomotion and provides a foundation for study of rhythm-generating networks in an emerging genetically tractable locomotor system.NEW & NOTEWORTHY Using a combination of pharmacology, electrophysiology, and Ca2+ imaging, we find that signaling through mACh receptors plays a critical role in rhythmogenesis in different regions of the Drosophila larval CNS. mAChR-dependent rhythm generators reside in distal regions of the larval CNS and provide functional substrates for central pattern-generating networks (CPGs) underlying headsweep behavior and forward locomotion. This provides new insights into locomotor CPG operation in soft-bodied animals that navigate over terrain.

Identification of putative muscarinic acetylcholine receptor genes in Bactrocera dorsalis and functional analysis of Bdor-mAChR-B

Muscarinic acetylcholine receptors (mAChRs) play important roles in the insect nervous system. These receptors are G protein-coupled receptors, which are potential targets for insecticide development. While the investigation of pharmacological properties of insect mAChRs is growing, the physiological roles of the receptor subtype remain largely indeterminate. Here, we identified three mAChR genes in an important agricultural pest Bactrocera dorsalis. Phylogenetic analysis defined these genes as mAChR-A, -B, and -C. Transcripts of the three mAChRs are most prevalent in 1-d-old larvae and are more abundant in the brain than other body parts in adults. Functional assay of Bdor-mAChR-B transiently expressed in Chinese hamster ovary cells showed that it was activated by acetylcholine (EC₅₀, 205.11 nM) and the mAChR agonist oxotremorine M (EC₅₀, 2.39 μM) in a dose-dependent manner. Using the CRISPR/Cas9 technique, we successfully obtained a Bdor-mAChR-B knockout strain based on wild-type (WT) strain. When compared with WT, the hatching and eclosion rate of Bdor-mAChR-B mutants are significantly lower. Moreover, the crawl speed of Bdor-mAChR-B knockout larvae was lower than that of WT, while climbing performance was enhanced in the mutant adults. Adults with loss of function of Bdor-mAChR-B showed declined copulation rates and egg numbers (by mated females). Our results indicate that Bdor-mAChR-B plays a key role in the development, locomotion, and mating behavior of B. dorsalis.

Loss of the Dβ1 nicotinic acetylcholine receptor subunit disrupts bursicon-driven wing expansion and diminishes adult viability in Drosophila melanogaster

Cholinergic signaling dominates the insect central nervous system, contributing to numerous fundamental pathways and behavioral circuits. However, we are only just beginning to uncover the diverse roles different cholinergic receptors may play. Historically, insect nicotinic acetylcholine receptors have received attention due to several subunits being key insecticide targets. More recently, there has been a focus on teasing apart the roles of these receptors, and their constituent subunits, in native signaling pathways. In this study, we use CRISPR-Cas9 genome editing to generate germline and somatic deletions of the Dβ1 nicotinic acetylcholine receptor subunit and investigate the consequences of loss of function in Drosophila melanogaster. Severe impacts on movement, male courtship, longevity, and wing expansion were found. Loss of Dβ1 was also associated with a reduction in transcript levels for the wing expansion hormone bursicon. Neuron-specific somatic deletion of Dβ1 in bursicon-producing neurons (CCAP-GAL4) was sufficient to disrupt wing expansion. Furthermore, CCAP-GAL4-specific expression of Dβ1 in a germline deletion background was sufficient to rescue the wing phenotype, pinpointing CCAP neurons as the neuronal subset requiring Dβ1 for the wing expansion pathway. Dβ1 is a known target of multiple commercially important insecticides, and the fitness costs exposed here explain why field-isolated target-site resistance has only been reported for amino acid replacements and not loss of function. This work reveals the importance of Dβ1-containing nicotinic acetylcholine receptors in CCAP neurons for robust bursicon-driven wing expansion.

GABAergic regulation of locomotion before and during an ethanol exposure in Drosophila melanogaster

Ethanol at low doses induces a locomotor stimulant response across a range of phylogenetically diverse species. In rodents, this response is commonly used as an index of ethanol's disinhibitory, anxiolytic, or reinforcing effects, and its expression is regulated by signaling through a number of conserved neurotransmitter systems. In the current experiments, we asked whether ethanol-induced locomotor stimulation in the fruit fly Drosophila melanogaster might be mediated by ionotropic GABA receptors. We measured basal and ethanol-stimulated locomotion in flies expressing RNAi directed against three known subunits of ionotropic GABA receptors, and also examined the effects of picrotoxin feeding on these behaviors. We found that RNAi-mediated knockdown of a subunit of fly ionotropic GABA receptors, RDL, in all neurons resulted in an increased ethanol-induced locomotor stimulant response, while knockdown of two other subunits, LCCH3 and GRD, did not affect the responses. The effect of pan neuronal RDL knockdown was recapitulated with selective RDL knockdown in cholinergic neurons, and increased ethanol-induced locomotor stimulation was also seen by feeding the GABA_A antagonist picrotoxin to flies prior to behavioral testing. However, the increase in ethanol-stimulated locomotion in each of these experiments was largely accounted for by decreased baseline activity. Our results indicate that ionotropic GABA receptors might be a conserved mediator of the locomotor stimulant effects of ethanol, but that alternative experimental approaches will be necessary to disentangle effects of GABAergic manipulations on baseline and ethanol-stimulated locomotion in flies.

Chronic ciprofloxacin and atrazine co-exposure aggravates locomotor and exploratory deficits in non-target detritivore speckled cockroach (Nauphoeta cinerea)

The global detection of ciprofloxacin and atrazine in soil is linked to intensive anthropogenic activities in agriculture and inadvertent discharge of industrial wastes to the environment. Nauphoeta cinerea is a terrestrial insect with cosmopolitan distribution and great environmental function. The current study probed the neurobehavioral and cellular responses of N. cinerea singly and jointly exposed to atrazine (1.0 and 0.5 μg g^-1 feed) and ciprofloxacin (0.5 and 0.25 μg g^-1 feed) for 63 days. Results demonstrated that the reductions in the body rotation, maximum speed, turn angle, path efficiency, distance traveled, episodes, and time of mobility induced by atrazine or ciprofloxacin per se was exacerbated in the co-exposure group. The altered exploratory and locomotor in insects singly and jointly exposed to ciprofloxacin and atrazine were verified by track plots and heat maps. Furthermore, we observed a decrease in acetylcholinesterase and anti-oxidative enzyme activities with concomitant elevation in the levels of lipid peroxidation, nitric oxide, and reactive oxygen and nitrogen species were significantly intensified in the midgut, hemolymph, and head of insects co-exposed to ciprofloxacin and atrazine. In conclusion, exposure to binary mixtures of ciprofloxacin and atrazine elicited greater locomotor and exploratory deficits than upon exposure to the individual compound by inhibiting acetylcholinesterase activity and induction of oxido-inflammatory stress responses in the insects. N. cinerea may be a usable model insect for checking contaminants of ecological risks.

Smoking flies: testing the effect of tobacco cigarettes on heart function of Drosophila melanogaster

Studies about the relationship between substances consumed by humans and their impact on health, in animal models, have been a challenge due to differences between species in the animal kingdom. However, the homology of certain genes has allowed extrapolation of certain knowledge obtained in animals. Drosophila melanogaster, studied for decades, has been widely used as model for human diseases as well as to study responses associated with the consumption of several substances. In the present work we explore the impact of tobacco consumption on a model of 'smoking flies'. Throughout these experiments, we aim to provide information about the effects of tobacco consumption on cardiac physiology. We assessed intracellular calcium handling, a phenomenon underlying cardiac contraction and relaxation. Flies chronically exposed to tobacco smoke exhibited an increased heart rate and alterations in the dynamics of the transient increase of intracellular calcium in myocardial cells. These effects were also evident under acute exposure to nicotine of the heart, in a semi-intact preparation. Moreover, the alpha 1 and 7 subunits of the nicotinic receptors are involved in the heart response to tobacco and nicotine under chronic (in the intact fly) as well as acute exposure (in the semi-intact preparation). The present data elucidate the implication of the intracellular cardiac pathways affected by nicotine on the heart tissue. Based on the probed genetic and physiological similarity between the fly and human heart, cardiac effects exerted by tobacco smoke in Drosophila advances our understanding of the impact of it in the human heart. Additionally, it may also provide information on how nicotine-like substances, e.g. neonicotinoids used as insecticides, affect cardiac function.This article has an associated First Person interview with the first author of the paper.

In Silico Studies of Lamiaceae Diterpenes with Bioinsecticide Potential against Aphis gossypii and Drosophila melanogaster

Background: The growing demand for agricultural products has led to the misuse/overuse of insecticides; resulting in the use of higher concentrations and the need for ever more toxic products. Ecologically, bioinsecticides are considered better and safer than synthetic insecticides; they must be toxic to the target organism, yet with low or no toxicity to non-target organisms. Many plant extracts have seen their high insecticide potential confirmed under laboratory conditions, and in the search for plant compounds with bioinsecticidal activity, the Lamiaceae family has yielded satisfactory results. Objective: The aim of our study was to develop computer-assisted predictions for compounds with known insecticidal activity against Aphis gossypii and Drosophila melanogaster. Results and conclusion: Structure analysis revealed ent-kaurane, kaurene, and clerodane diterpenes as the most active, showing excellent results. We also found that the interactions formed by these compounds were more stable, or presented similar stability to the commercialized insecticides tested. Overall, we concluded that the compounds bistenuifolin L (1836) and bistenuifolin K (1931), were potentially active against A. gossypii enzymes; and salvisplendin C (1086) and salvixalapadiene (1195), are potentially active against D. melanogaster. We observed and highlight that the diterpenes bistenuifolin L (1836), bistenuifolin K (1931), salvisplendin C (1086), and salvixalapadiene (1195), present a high probability of activity and low toxicity against the species studied.

Identification and Expression Analysis of G Protein-Coupled Receptors in the Miridae Insect Apolygus lucorum

G protein-coupled receptors (GPCRs) are the largest and most versatile family of transmembrane receptors in the cell and they play a vital role in the regulation of multiple physiological processes. The family Miridae (Hemiptera: Heteroptera) is one of the most diverse families of insects. Until now, information on GPCRs has been lacking in Miridae. Apolygus lucorum, a representative species of the Miridae, is an omnivorous pest that occurs worldwide and is notorious for causing serious damage to various crops and substantial economic losses. By searching the genome, 133 GPCRs were identified in A. lucorum. Compared with other model insects, we have observed GPCR genes to be remarkably expanded in A. lucorum, especially focusing on biogenic amine receptors and neuropeptide receptors. Among these, there is a novel large clade duplicated from known FMRFamide receptors (FMRFaRs). Moreover, the temporal and spatial expression profiles of the 133 genes across developmental stages were determined by transcriptome analysis. Most GPCR genes showed a low expression level in the whole organism of A. lucorum. However, there were a few highly expressed GPCR genes. The highly expressed LW opsins in the head probably relate to nocturning of A. lucorum, and the expression of Cirl at different times and in different tissues indicated it may be involved in growth and development of A. lucorum. We also found C2 leucine-rich repeat-containing GPCRs (LGRs) were mainly distributed in Hemiptera and Phthiraptera among insects. Our study was the first investigation on GPCRs in A. lucorum and it provided a molecular target for the regulation and control of Miridae pests.

Vesicular neurotransmitter transporters in Drosophila melanogaster

Drosophila melanogaster express vesicular transporters for the storage of neurotransmitters acetylcholine, biogenic amines, GABA, and glutamate. The large array of powerful molecular-genetic tools available in Drosophila enhances the use of this model organism for studying transporter function and regulation.

Muscarinic Modulation of Antennal Lobe GABAergic Local Neurons Shapes Odor Coding and Behavior

In the antennal lobe (AL), the first olfactory relay of Drosophila, excitatory neurons are predominantly cholinergic. Ionotropic nicotinic receptors play a vital role in the effects of acetylcholine in the AL. However, the AL also has a high expression level of metabotropic muscarinic acetylcholine receptors type A (mAChRs-A). Nevertheless, the neurons expressing them and their role in the AL are unknown. Elucidating their function may reveal principles in olfactory modulation. Here, we show that mAChRs-A shape AL output and affect behavior. We localized mAChRs-A effects to a sub-population of GABAergic local neurons (iLNs), where they play a dual role: direct excitation of iLNs and stabilization of the synapse between receptor neurons and iLNs, which undergoes strong short-term depression. Our results reveal modulatory functions of the AL main excitatory neurotransmitter. Striking similarities to the mammalian olfactory system predict that mammalian glutamatergic metabotropic receptors could be associated with similar modulations.