State of the Art on Insect Nicotinic Acetylcholine Receptor Function in Learning and Memory

Assessing the effects of a commercial fungicide and an herbicide, alone and in combination, on Apis mellifera: Insights from biomarkers and cognitive analysis

Agrochemicals play a vital role in protecting crops and enhancing agricultural production by reducing threats from pests, pathogens and weeds. The toxicological status of honey bees can be influenced by a number of factors, including pesticides. While extensive research has focused on the lethal and sublethal effects of insecticides on individual bees and colonies, it is important to recognise that fungicides and herbicides can also affect bees' health. Unfortunately, in the field, honey bees are exposed to mixtures of compounds rather than single substances. This study aimed to evaluate the effects of a commercial fungicide and a commercial herbicide, both individually and in combination, on honey bees. Mortality assays, biomarkers and learning and memory tests were performed, and the results were integrated to assess the toxicological status of honey bees. Neurotoxicity (acetylcholinesterase and carboxylesterase activities), detoxification and metabolic processes (glutathione S-transferase and alkaline phosphatase activities), immune system function (lysozyme activity and haemocytes count) and genotoxicity biomarkers (Nuclear Abnormalities assay) were assessed. The fungicide Sakura® was found to activate detoxification enzymes and affect alkaline phosphatase activity. The herbicide Elegant 2FD and the combination of both pesticides showed neurotoxic effects and induced detoxification processes. Exposure to the herbicide/fungicide mixture impaired learning and memory in honey bees. This study represents a significant advance in understanding the toxicological effects of commonly used commercial pesticides in agriculture and contributes to the development of effective strategies to mitigate their adverse effects on non-target insects.

Critical roles of nicotinic acetylcholine receptors in olfactory memory formation and retrieval in crickets

Acetylcholine (ACh) is a major excitatory neurotransmitter in the insect central nervous system, and insect neurons express several types of ACh receptors (AChRs). AChRs are classified into two subgroups, muscarinic AChRs and nicotinic AChRs (nAChRs). nAChRs are also divided into two subgroups by sensitivity to α-bungarotoxin (α-BGT). The cricket Gryllus bimaculatus is one of the useful insects for studying the molecular mechanisms in olfactory learning and memory. However, the roles of nAChRs in olfactory learning and memory of the cricket are still unknown. In the present study, to investigate whether nAChRs are involved in cricket olfactory learning and memory, we tested the effects of two different AChR antagonists on long-term memory (LTM) formation and retrieval in a behavioral assay. The two AChR antagonists that we used are mecamylamine (MEC), an α-BGT-insensitive nAChR antagonist, and methyllycaconitine (MLA), an α-BGT-sensitive nAChR antagonist. In crickets, multiple-trial olfactory conditioning induced 1-day memory (LTM), whereas single-trial olfactory conditioning induced 1-h memory (mid-term memory, MTM) but not 1-day memory. Crickets injected with MEC 20 min before the retention test at 1 day after the multiple-trial conditioning exhibited no memory retrieval. This indicates that α-BGT-insensitive nAChRs participate in memory retrieval. In addition, crickets injected with MLA before the multiple-trial conditioning exhibited MTM but not LTM, indicating that α-BGT-sensitive nAChRs participate in the formation of LTM. Moreover, injection of nicotine (an nAChR agonist) before the single-trial conditioning induced LTM. Finally, the nitric oxide (NO)-cGMP signaling pathway is known to participate in the formation of LTM in crickets, and we conducted co-injection experiments with an agonist or inhibitor of the nAChR and an activator or inhibitor of the NO-cGMP signaling pathway. The results suggest that nAChR works upstream of the NO-cGMP signaling system in the LTM formation process.

Identification of CYCLE targets that contribute diverse features of circadian rhythms in the mosquito Culex pipiens

Culex pipiens demonstrates robust circadian rhythms in adult eclosion, flight activity, mating, and development. These rhythmic patterns are believed to be controlled by the endogenous light-entrainable circadian clock that consists of positive and negative regulators working in a transcription-translation feedback loop. Moreover, these mosquitoes undergo seasonal diapause in exposure to the short photoperiod of late summer or early fall. However, the exact genetic and cellular mechanism behind the clock gene-mediated activity pattern, seasonal time measurement, and subsequent diapause initiation still need to be unraveled. To determine the possible linkage between clock genes and downstream processes, here we employed ChIP-sequencing to identify the direct targets of one of the core clock proteins, Cycle (CYC). The nearest genes with peaks mapping to their 1Kb upstream region of the transcription start site were extracted and scanned for consensus E box sequences, resulting in a dataset comprising the target genes possibly regulated by CYC. Based on the highest fold enrichment and functional relevance, we identified genes relating to five gene categories of potential interest, including peptide/receptors, neurotransmission, olfaction, immunity, and reproductive growth. Of these, we validated fourteen genes with ChIP-qPCR and qRT-PCR. These genes showed a significantly high expression in dusk compared to dawn in concert with the activity level of the CYC transcription factor and are thus strong candidates for mediating circadian rhythmicity and possibly regulating seasonal shifts in mosquito reproductive activity.

Chronic nicotine exposure influences learning and memory in the honey bee

In insects, nicotine activates nicotinic acetylcholine receptors, which are expressed throughout the central nervous system. However, little work has been done to investigate the effects of chronic nicotine treatment on learning or other behaviors in non-herbivorous insects. To examine the effects of long term nicotine consumption on learning and memory, honey bees were fed nicotine containing solutions over four days. Bees were able to detect nicotine at 0.1 mM in sucrose solutions, and in a no choice assay, bees reduced food intake when nicotine was 1 mM or higher. Treatment with a low dose of nicotine decreased the proportion of bees able to form an associative memory when bees were conditioned with either a massed or spaced appetitive olfactory training paradigm. On the other hand, higher doses of nicotine increased memory retention and the proportion of bees responding to the odor during 10 min and 24 h recall tests. The reduction in nicotine containing food consumed may also impact response levels during learning and recall tests. These data suggest that long term exposure to nicotine has complex effects on learning and memory.

Multiomic interpretation of fungus-infected ant metabolomes during manipulated summit disease

Camponotus floridanus ants show altered behaviors followed by a fatal summiting phenotype when infected with manipulating Ophiocordyceps camponoti-floridani fungi. Host summiting as a strategy to increase transmission is also observed with parasite taxa beyond fungi, including aquatic and terrestrial helminths and baculoviruses. The drastic phenotypic changes can sometimes reflect significant molecular changes in gene expression and metabolite concentrations measured in manipulated hosts. Nevertheless, the underlying mechanisms still need to be fully characterized. To investigate the small molecules producing summiting behavior, we infected C. floridanus ants with O. camponoti-floridani and sampled their heads for LC-MS/MS when we observed the characteristic summiting phenotype. We link this metabolomic data with our previous genomic and transcriptomic data to propose mechanisms that underlie manipulated summiting behavior in "zombie ants." This "multiomic" evidence points toward the dysregulation of neurotransmitter levels and neuronal signaling. We propose that these processes are altered during infection and manipulation based on (1) differential expression of neurotransmitter synthesis and receptor genes, (2) altered abundance of metabolites and neurotransmitters (or their precursors) with known behavioral effects in ants and other insects, and (3) possible suppression of a connected immunity pathway. We additionally report signals for metabolic activity during manipulation related to primary metabolism, detoxification, and anti-stress protectants. Taken together, these findings suggest that host manipulation is likely a multi-faceted phenomenon, with key processes changing at multiple levels of molecular organization.

Acetylcholine titre regulation by non-neuronal acetylcholinesterase 1 and its putative roles in honey bee physiology

Similar to other insects, honey bees have two acetylcholinesterases (AChEs), AmAChE1 and AmAChE2. The primary catalytic enzyme for acetylcholine (ACh) hydrolysis in synapses is AmAChE2, which is predominantly expressed in neuronal tissues, whereas AmAChE1 is expressed in both neuronal and non-neuronal tissues, with limited catalytic activity. Unlike constitutively expressed AmAChE2, AmAChE1 expression is induced under stressful conditions such as heat shock and brood rearing suppression, but its role in regulating ACh titre remains unclear. In this paper, to elucidate the role of AmAChE1, the expression of AmAChE1 was suppressed via RNA interference (RNAi) in AmAChE1-induced worker bees. The ACh titre measurement following RNAi revealed that the expression of AmAChE1 downregulated the overall ACh titre in all tissues examined without altering AmAChE2 expression. Transcriptome analysis showed that AmAChE1 knockdown upregulated protein biosynthesis, cell respiration, and thermogenesis in the head. These findings suggest that AmAChE1 is involved in decreasing neuronal activity, enhancing energy conservation, and potentially extending longevity under stressful conditions via ACh titre regulation.

Impairments in learning and memory performances associated with nicotinic receptor expression in the honeybee Apis mellifera after exposure to a sublethal dose of sulfoxaflor

Sulfoxaflor is a new insecticide which acts on the nicotinic acetylcholine receptor (nAChRs) in a similar way to neonicotinoids. However, sufloxaflor (SFX) is thought to act in a different manner and is thus proposed as an alternative in crop protection. The goal of this study is to evaluate the toxicity of SFX and its sublethal effect on the honeybee Apis mellifera after acute exposure. In toxicological assay studies, the LD₅₀ value and sublethal dose (corresponding to the NOEL: no observed effect level) were 96 and 15 ng/bee, respectively. Using the proboscis extension response paradigm, we found that an SFX dose of 15 ng/bee significantly impairs learning and memory retrieval when applied 12 h before conditioning or 24 h after olfactory conditioning. SFX had no effect on honeybee olfactory performance when exposure happened after the conditioning. Relative quantitative PCR experiments performed on the six nicotinic acetylcholine receptor subunits demonstrated that they are differently expressed in the honeybee brain after SFX exposure, whether before or after conditioning. We found that intoxicated bees with learning defects showed a strong expression of the Amelβ1 subunit. They displayed overexpression of Amelα9 and Amelβ2, and down-regulation of Amelα1, Amelα3 and Amelα7 subunits. These results demonstrated for the first time that a sublethal dose of SFX could affect honeybee learning and memory performance and modulate the expression of specific nAChR subunits in the brain.

Using Citronella to Protect Bees (honeybee Apis mellifera L.) from certain Insecticides and Their Nano Formulations

Abstract Experiments were performed investigating citronella (Cymbopogon winterianus Jowitt) as a repellent to honeybee Apis mellifera (L.) (Hymenoptera: Apidae) in Egypt, it was conducted in laboratory in the Department of Entomology and Pesticides Science, Faculty of Agriculture, Cairo University, to check long-term survival of honeybee when exposed to different nano insecticides alone or combined with citronella at the same examination box for each. In this study, we used a modeling approach regarding survival data of caged worker bees under chronic exposure to four insecticides (Chloropyrophos, Nano-chloropyrophos Imidacloprid, Nano-Imidacloprid) each of them was supplemented in a box alone and in combination with citronella. Having three replicates and five concentrations (100, 200, 300, 400 and 500 ppm). Laboratory bioassay of these insecticides showed that chloropyrophos and nano chloropyrophos were the most toxic at their high dose (500 ppm) with LT50 of 120.98 and 122.02 followed by 132.14 and 136.5 minutes for Imidacloprid and Nano-Imidacloprid, respectively. No consumption occurred by bees to mixed sugar syrup with insecticides in all treatments when citronella was added. These data highly recommended that adding citronella is very effective when nicotinoid pesticides are used to longevity honeybee life and keep bee safe.

Digitally aligned ZnO nanowire array based synaptic transistors with intrinsically controlled plasticity for short-term computation and long-term memory

Digitally aligned long continuous ZnO NWs with distinct widths and microstructures are prepared and used for tuning the plasticity of synaptic transistors (STs) for the first time. Intrinsically controlled synaptic plasticity, i.e. short-term plasticity (STP) and long-term plasticity (LTP), was achieved using the same source material and post-fabrication condition for the first time, which is essential for simple and low-cost fabrication. Moreover, these versatile properties of ZnO STs enable the integration of STP and LTP as realized by multiplexed neurotransmission of different neurotransmitters: dopamine and acetylcholine, which promote learning and memory in organisms, so the device may utilize these processes in neuroelectronic devices. Devices with well-controlled synaptic plasticity can simulate the "learning-forgetting-erase" and "instant display" processes. ZnO NWs may enable the development of neuromorphic computers that can use the same material to achieve both short-term computation and long-term memory.

Robust functional expression of insect nicotinic acetylcholine receptors provides new insights into neonicotinoid actions and new opportunities for pest and vector control

Neonicotinoids are selective modulators of insect nicotinic acetylcholine receptors (nAChRs). These widely deployed insecticides interact with the orthosteric sites of nAChRs, not only to activate nAChRs on their own, but also to block the desensitizing component of nAChR responses. To date recombinant vertebrate or insect/vertebrate hybrid nAChRs have been deployed to understand the mechanism of selectivity and diversity of neonicotinoid actions as well as to show that both α/α and α/non-α interfaces are involved in the interactions with neonicotinoids. However, many of the fine details of insecticide interactions with sites on nAChRs remain to be resolved. The breakthrough of functional expression of insect nAChRs allows such questions to be addressed, not only for neonicotinoids but for other insecticides targeting insect nAChRs. © 2020 Society of Chemical Industry.

Characterization and expression analysis of tandemly duplicated nicotinic acetylcholine receptors in pearl oysters after stimulation of pathogen-related molecular patterns

Nicotinic acetylcholine receptors (nAChRs) are a class of ligand-gated ion channels that participate in signal transduction and are reported to play an important role in the immunomodulation of vertebrates and invertebrates. Previous studies have shown that the nAChRs in mollusks have undergone large-scale expansion after tandem repeats and retrotransposition, with the most expansion observed in bivalves. This study characterized the sequence of a tandem repeat nAChR unique to several bivalve mollusks and investigated its functions in Pinctada fucata martensii. Firstly, phylogenetic analysis revealed that the tandem arrays of nAChRs existed before bivalve differentiation and m ost tandem-replicated nAChR genes have a conserved genomic structure and domain combination. In present study, five tandemly duplicated nAChR genes were cloned from P. f. martensii and designated as PmnAChR-1 to PmnAChR-5. qRT-PCR analysis revealed that five PmnAChRs were specifically expressed in adult gills. In addition, after PAMP stimulation, the expression of PmnAChRs in hemocytes of P. f. martensii were strongly induced but exhibited different responses to different stimuli. PmnAChR-1, PmnAChR-4, and PmnAChR-5 exhibited strong and wide responsiveness to lipopolysaccharide (LPS) stimulation but had no response to peptidoglycan (PGN) stimulation. PmnAChR-2 expression was notably upregulated at 6 h after PGN challenge but had no response to LPS stimulation. Polyinosinic-polycytidylic acid challenge upregulated nearly all PmnAChRs, except for PmnAChR-5. Furthermore, Pm-miR-873-3p, Pm-miR-4577, Pm-miR-103a-3p, and Pm-miR-6753-3p were identified as the regulatory miRNA of PmnAChR-1, PmnAChR-3, PmnAChR-4, and PmnAChR-5, respectively. These findings suggested that these tandem arrays of nAChRs are unique to bivalves, and the tandem duplication of nAChR genes may be involved in the immune regulation process after pathogen stimulation.

The Power of Drosophila melanogaster for Modeling Neonicotinoid Effects on Pollinators and Identifying Novel Mechanisms

Neonicotinoids are the most widely used insecticides in the world and are implicated in the widespread population declines of insects including pollinators. Neonicotinoids target nicotinic acetylcholine receptors which are expressed throughout the insect central nervous system, causing a wide range of sub-lethal effects on non-target insects. Here, we review the potential of the fruit fly Drosophila melanogaster to model the sub-lethal effects of neonicotinoids on pollinators, by utilizing its well-established assays that allow rapid identification and mechanistic characterization of these effects. We compare studies on the effects of neonicotinoids on lethality, reproduction, locomotion, immunity, learning, circadian rhythms and sleep in D. melanogaster and a range of pollinators. We also highlight how the genetic tools available in D. melanogaster, such as GAL4/UAS targeted transgene expression system combined with RNAi lines to any gene in the genome including the different nicotinic acetylcholine receptor subunit genes, are set to elucidate the mechanisms that underlie the sub-lethal effects of these common pesticides. We argue that studying pollinators and D. melanogaster in tandem allows rapid elucidation of mechanisms of action, which translate well from D. melanogaster to pollinators. We focus on the recent identification of novel and important sublethal effects of neonicotinoids on circadian rhythms and sleep. The comparison of effects between D. melanogaster and pollinators and the use of genetic tools to identify mechanisms make a powerful partnership for the future discovery and testing of more specific insecticides.

Thiamethoxam exposure deregulates short ORF gene expression in the honey bee and compromises immune response to bacteria

Maximizing crop yields relies on the use of agrochemicals to control insect pests. One of the most widely used classes of insecticides are neonicotinoids that interfere with signalling of the neurotransmitter acetylcholine, but these can also disrupt crop-pollination services provided by bees. Here, we analysed whether chronic low dose long-term exposure to the neonicotinoid thiamethoxam alters gene expression and alternative splicing in brains of Africanized honey bees, Apis mellifera, as adaptation to altered neuronal signalling. We find differentially regulated genes that show concentration-dependent responses to thiamethoxam, but no changes in alternative splicing. Most differentially expressed genes have no annotated function but encode short Open Reading Frames, a characteristic feature of anti-microbial peptides. As this suggested that immune responses may be compromised by thiamethoxam exposure, we tested the impact of thiamethoxam on bee immunity by injecting bacteria. We show that intrinsically sub-lethal thiamethoxam exposure makes bees more vulnerable to normally non-pathogenic bacteria. Our findings imply a synergistic mechanism for the observed bee population declines that concern agriculturists, conservation ecologists and the public.

Effects of insecticides used in strawberries on stingless bees Melipona quadrifasciata and Tetragonisca fiebrigi (Hymenoptera: Apidae)

The use of pesticides is considered one of the most important threats to pollinators, especially since they are widely used in agriculture for pest control. In the last years, several studies have reported severe secondary effects on various bee species, including exotic and native bees. In this study, lethal (mortality) and sublethal (locomotor activity) effects of insecticides and acaricides used in strawberries in Brazil (abamectin, novaluron, spinetoram, and thiamethoxam) were evaluated on the native stingless bees Melipona quadrifasciata and Tetragonisca fiebrigi. The results showed that the effects varied significantly according to the pesticide, type of exposure (oral or topical), and bee species. Through oral exposure, M. quadrifasciata was more susceptible to all insecticides except for abamectin, while in topical exposure, T. fiebrigi was more sensitive. Thiamethoxam followed by spinetoram and abamectin were the most lethal, regardless of species or exposure route; novaluron was not harmful at the highest tested dose. The locomotor activity of bees was altered in the presence of sublethal doses (LC₁₀ and LC₅₀) of all insecticides. Spinetoram and abamectin can be as much as toxic as thiamethoxam against M. quadrifasciata and T. fiebrigi in laboratory experiments. These findings should be confirmed in field experiments to define possibilities to combine pest control and pollinator management. In crops like strawberries, the selectivity of native pollinators should be considered.

Modulation of octopaminergic and cholinergic pathways induced by Caatinga tree Manilkara rufula chemical compounds in Nauphoeta cinerea cockroaches

The entomotoxic potential of Manilkara rufula crude extract (CEMR) and its aqueous (AFMR) and methanolic (MFMR) fractions were evaluated against Nauphoeta cinerea cockroaches. The results point out to a direct modulation of octopaminergic and cholinergic pathways in insect nervous system. CEMR induced an anti-acetylcholinesterase (AChE) effect in cockroach brain homogenates. CEMR significantly decreased the cockroach heart rate in semi-isolated heart preparations. CEMR also caused a broad disturbance in the insect behavior by reducing the exploratory activity. The decreased antennae and leg grooming activities, by different doses of CEMR, mimicked those of phentolamine activity, a selective octopaminergic receptor antagonist. The lethargy induced by CEMR was accompanied by neuromuscular failure and by a decrease of sensilla spontaneous neural compound action potentials (SNCAP) firing in in vivo and ex vivo cockroach muscle-nerve preparations, respectively. AFMR was more effective in promoting neuromuscular paralysis than its methanolic counterpart, in the same dose. These data validate the entomotoxic activity of M. rufula. The phentolamine-like modulation induced in cockroaches is the result of a potential direct inhibition of octopaminergic receptors, combined to an anti-AChE activity. In addition, the modulation of CEMR on octopaminergic and cholinergic pathways is probably the result of a synergism between AFMR and MFMR chemical compounds. Further phytochemical investigation followed by a bio-guiding protocol will improve the molecular aspects of M. rufula pharmacology and toxicology to insects.

Modulation by neonicotinoids of honeybee α1/chicken β2 hybrid nicotinic acetylcholine receptors expressed in Xenopus laevis oocytes

Neonicotinoids targeting insect nicotinic acetylcholine (ACh) receptors (insect nAChRs) are used for crop protection, but there is a concern about adverse effects on pollinators such as honeybees (Apis mellifera). Thus, we investigated the agonist actions of neonicotinoids (imidacloprid, thiacloprid and clothianidin) on A. mellifera α1 (Amα1)/chicken β2 hybrid nAChRs in Xenopus laevis oocytes according to the subunit stoichiometry of (Amα1)₃(β2)₂ and (Amα1)₂(β2)₃ using voltage-clamp electrophysiology. ACh activated (Amα1)₃(β2)₂ and (Amα1)₂(β2)₃ nAChRs with similar current amplitude. We investigated the agonist activity of imidacloprid, thiacloprid and clothianidin for the two hybrid nAChRs and found that: 1) imidacloprid showed higher affinity than clothianidin, whereas clothianidin showed higher efficacy than imidacloprid for the nAChRs; 2) Thiacloprid showed the highest agonist affinity and the lowest efficacy for the nAChRs. The Amα1/β2 subunit ratio influenced the efficacy of imidacloprid and thiacloprid, but hardly affected that of clothianidin. Hydrogen bond formation by the NH group in clothianidin with the main chain carbonyl of the loop B may account, at least in part, for the unique agonist actions of clothianidin on the hybrid nAChRs tested.

Chronic within-hive video recordings detect altered nursing behaviour and retarded larval development of neonicotinoid treated honey bees

Risk evaluations for agricultural chemicals are necessary to preserve healthy populations of honey bee colonies. Field studies on whole colonies are limited in behavioural research, while results from lab studies allow only restricted conclusions on whole colony impacts. Methods for automated long-term investigations of behaviours within comb cells, such as brood care, were hitherto missing. In the present study, we demonstrate an innovative video method that enables within-cell analysis in honey bee (Apis mellifera) observation hives to detect chronic sublethal neonicotinoid effects of clothianidin (1 and 10 ppb) and thiacloprid (200 ppb) on worker behaviour and development. In May and June, colonies which were fed 10 ppb clothianidin and 200 ppb thiacloprid in syrup over three weeks showed reduced feeding visits and duration throughout various larval development days (LDDs). On LDD 6 (capping day) total feeding duration did not differ between treatments. Behavioural adaptation was exhibited by nurses in the treatment groups in response to retarded larval development by increasing the overall feeding timespan. Using our machine learning algorithm, we demonstrate a novel method for detecting behaviours in an intact hive that can be applied in a versatile manner to conduct impact analyses of chemicals, pests and other stressors.

Copper decreases associative learning and memory in Drosophila melanogaster

Copper is an essential element to all living organisms. Repeated use of metal-enriched chemicals, fertilizers, and organic substances may cause contamination at a large scale. Altered levels of Cu²⁺ may result in harmful effects and can be associated with memory and cognitive dysfunction. Studying simple, genetically tractable organisms such as Drosophila melanogaster, can reveal important data on the neural basis of conditioning. D. melanogaster is an important alternative experimental model to assess the toxic response to metals. In the present study, the effects of copper on flies' development and in learning and memory retention in male and female adult flies were investigated. We paired an odorant to pain perception and observed the aversion behavior over time. Exposure of D. melanogaster eggs to Cu²⁺ increased mortality of larvae, pupae, and adults and decreased memory retention in adults. Moreover, male flies demonstrated to be more susceptible to Cu²⁺ toxicity than females. The results therefore, reinforce the importance of controlling the anthropogenic heavy-metals soil contamination given their hazardous effects to living organisms.

Interaction of field realistic doses of clothianidin and Varroa destructor parasitism on adult honey bee (Apis mellifera L.) health and neural gene expression, and antagonistic effects on differentially expressed genes

While many studies have examined the effects of neonicotinoid insecticides and the parasitic mite Varroa destructor on honey bees (Apis mellifera), more information on the combined effects of such stressors on gene expression, including neural related genes, and their impact on biological pathways is needed. This study analyzed the effects of field realistic concentrations of the neonicotinoid clothianidin on adult bees infested and not infested with V. destructor over 21 consecutive days and then determined bee survivorship, weight, deformed wing virus (DWV) levels and gene expression. V. destructor parasitism with or without clothianidin exposure was significantly associated with decreased survivorship, weight loss and higher DWV levels, while clothianidin exposure was only associated with higher levels of DWV. Expression analysis of the neural genes AmNlg-1, BlCh and AmAChE-2 showed that V. destructor caused a significant down-regulation of all of them, whereas clothianidin caused a significant down-regulation of only AmNrx-1 and BlCh. An interaction was only detected for AmNrx-1 expression. RNAseq analysis showed that clothianidin exposure resulted in 6.5 times more up-regulated differentially expressed genes (DEGs) than V. destructor alone and 123 times more than clothianidin combined with V. destructor. Similar results were obtained with down-regulated DEGs, except for a higher number of DEGs shared between V. destructor and the combined stressors. KEGG (Kyoto Encyclopedia of Genes and Genomes) biological pathway analysis of the DEGs showed that the stressor linked to the highest number of KEGG pathways was clothianidin, followed by V. destructor, and then considerably fewer number of KEGG pathways with the combined stressors. The reduced numbers of DEGs and KEGG pathways associated with the DEGs for the combined stressors compared to the stressors alone indicates that the interaction of the stressors is not additive or synergistic, but antagonistic. The possible implications of the antagonistic effect on the number of DEGs are discussed.

In silico identification and assessment of insecticide target sites in the genome of the small hive beetle, Aethina tumida

BACKGROUND

The small hive beetle, Aethina tumida, is a rapidly emerging global pest of honey bee colonies. Small hive beetle infestation can be extremely destructive, which may cause honey bees to abscond and render colony infrastructure unusable. Due to the impacts small hive beetles have on honey bees, a wide variety of physical, cultural, and chemical control measures have been implemented to manage small hive beetle infestations. The use of insecticides to control small hive beetle populations is an emerging management tactic. Currently, very little genomic information exists on insecticide target sites in the small hive beetle. Therefore, the objective of this study is to utilize focused in silico comparative genomics approaches to identify and assess the potential insecticide sensitivity of the major insecticide target sites in the small hive beetle genome.

RESULTS

No previously described resistance mutations were identified in any orthologs of insecticide target sites. Alternative exon use and A-to-I RNA editing were absent in AtumSC1. The ryanodine receptor in small hive beetle (Atum_Ryr) was highly conserved and no previously described resistance mutations were identified. A total of 12 nAChR subunits were identified with similar alternative exon use in other insects. Alternative exon use and critical structural features of the GABA-gated chloride channel subunits (Atum_RDL, Atum_GRD, and Atum_LCCH3) were conserved. Five splice variants were found for the glutamate-gated chloride channel subunit. Exon 3c of Atum_GluCl may be a beetle-specific alternative exon. The co-occurrence of exons 9a and 9b in the pH-sensitive chloride channel (Atum_pHCl) is a unique combination that introduces sites of post-translational modification. The repertoire and alternative exon use for histamine-gated chloride channels (Atum-HisCl), octopamine (Atum_OctR) and tyramine receptors (Atum_TAR) were conserved.

CONCLUSIONS

The recently published small hive beetle genome likely serves as a reference for insecticide-susceptible versions of insecticide target sites. These comparative in silico studies are the first step in discovering targets that can be exploited for small hive beetle-specific control as well as tracking changes in the frequency of resistance alleles as part of a resistance monitoring program. Comparative toxicity alongside honey bees is required to verify these in silico predictions.

Massive expansion and diversity of nicotinic acetylcholine receptors in lophotrochozoans

Background

Nicotinic acetylcholine receptors (nAChRs) are among the oldest and most conserved transmembrane receptors involved in signal transduction. Despite the prevalence and significance of cholinergic signaling, the diversity and evolution of nAChRs are not fully understood.

Result

By comparative genomic analysis, we found massive expansions of nAChR genes in molluscs and some other lophotrochozoans. The expansion is particularly pronounced in stationary bivalve molluscs with simple nervous systems, with the number of nAChR genes ranging from 99 to 217 in five bivalves, compared with 10 to 29 in five ecdysozoans and vertebrates. The expanded molluscan nAChR genes tend to be intronless and in tandem arrays due to retroposition followed by tandem duplication. Phylogenetic analysis revealed diverse nAChR families in the common ancestor of bilaterians, which subsequently experienced lineage-specific expansions or contractions. The expanded molluscan nAChR genes are highly diverse in sequence, domain structure, temporal and spatial expression profiles, implying diversified functions. Some molluscan nAChR genes are expressed in early development before the development of the nervous system, while others are involved in immune and stress responses.

Conclusion

The massive expansion and diversification of nAChR genes in bivalve molluscs may be a compensation for reduced nervous systems as part of adaptation to stationary life under dynamic environments, while in vertebrates a subset of specialized nAChRs are retained to work with advanced nervous systems. The unprecedented diversity identified in molluscs broadens our view on the evolution and function of nAChRs that are critical to animal physiology and human health.

Acetylcholine and Its Receptors in Honeybees: Involvement in Development and Impairments by Neonicotinoids

Acetylcholine (ACh) is the major excitatory neurotransmitter in the insect central nervous system (CNS). However, besides the neuronal expression of ACh receptors (AChR), the existence of non-neuronal AChR in honeybees is plausible. The cholinergic system is a popular target of insecticides because the pharmacology of insect nicotinic acetylcholine receptors (nAChRs) differs substantially from their vertebrate counterparts. Neonicotinoids are agonists of the nAChR and are largely used in crop protection. In contrast to their relatively high safety for humans and livestock, neonicotinoids pose a threat to pollinating insects such as bees. In addition to its effects on behavior, it becomes increasingly evident that neonicotinoids affect developmental processes in bees that appear to be independent of neuronal AChRs. Brood food (royal jelly, worker jelly, or drone jelly) produced in the hypopharyngeal glands of nurse bees contains millimolar concentrations of ACh, which is required for proper larval development. Neonicotinoids reduce the secreted ACh-content in brood food, reduce hypopharyngeal gland size, and lead to developmental impairments within the colony. We assume that potential hazards of neonicotinoids on pollinating bees occur neuronally causing behavioral impairments on adult individuals, and non-neuronally causing developmental disturbances as well as destroying gland functioning.

The Neurophysiological Bases of the Impact of Neonicotinoid Pesticides on the Behaviour of Honeybees

Acetylcholine is the main excitatory neurotransmitter in the honeybee brain and controls a wide range of behaviours that ensure the survival of the individuals and of the entire colony. Neonicotinoid pesticides target this neurotransmission pathway and can thereby affect the behaviours under its control, even at doses far below the toxicity limit. These sublethal effects of neonicotinoids on honeybee behaviours were suggested to be partly responsible for the decline in honeybee populations. However, the neural mechanisms by which neonicotinoids influence single behaviours are still unclear. This is mainly due to the heterogeneity of the exposure pathways, doses and durations between studies. Here, we provide a review of the state of the science in this field and highlight knowledge gaps that need to be closed. We describe the agonistic effects of neonicotinoids on neurons expressing the different nicotinic acetylcholine receptors and the resulting brain structural and functional changes, which are likely responsible for the behavioural alterations reported in bees exposed to neonicotinoids.

Effect of Sub-lethal Doses of Imidacloprid on Learning and Memory Formation of Indigenous Arabian Bee (Apis mellifera jemenitica Ruttner) Adult Foragers

The indigenous bee race Apis mellifera jemenitica Ruttner of Saudi Arabia can learn and retain memories established by the classical conditioning of proboscis extension response (PER). The insecticide imidacloprid has shown a drastic effect on the olfactory behavior of A. m. jemenitica in the harsh arid climatic conditions of central Saudi Arabia. The oral feeding of single imidacloprid sub-lethal doses (1.0 ng, 0.5 ng, or 0.1 ng) under laboratory conditions significantly impaired associative learning during the 2nd and 3rd conditioning trials compared to control bees (0 ng). The memory tests also revealed significant impairment in memory formation at 1 h, 2 h, and 24 h after conditioning compared to control bees. Even the lowest dose (0.1 ng/bee) can significantly impair the bees' ability to learn and memorize. This impairment effect was dose dependent and increased with increasing doses. The higher dose (1.0 ng) completely impaired the learning but still showed a little memory and reflected the potential recovery of bees from insecticide-induced impairment with the passage of time. To our knowledge, this is the first study in A. m. jemenitica that demonstrated the drastic effect of neonicotinoids on associative learning in indigenous bees. This study further expresses the possible severity of insecticidal exposure to bees in actual field conditions and its effect on the neural functions used in important behavior involved in the foraging of bees.

Unravelling the Molecular Determinants of Bee Sensitivity to Neonicotinoid Insecticides

The impact of neonicotinoid insecticides on the health of bee pollinators is a topic of intensive research and considerable current debate [1]. As insecticides, certain neonicotinoids, i.e., N-nitroguanidine compounds such as imidacloprid and thiamethoxam, are as intrinsically toxic to bees as to the insect pests they target. However, this is not the case for all neonicotinoids, with honeybees orders of magnitude less sensitive to N-cyanoamidine compounds such as thiacloprid [2]. Although previous work has suggested that this is due to rapid metabolism of these compounds [2-5], the specific gene(s) or enzyme(s) involved remain unknown. Here, we show that the sensitivity of the two most economically important bee species to neonicotinoids is determined by cytochrome P450s of the CYP9Q subfamily. Radioligand binding and inhibitor assays showed that variation in honeybee sensitivity to N-nitroguanidine and N-cyanoamidine neonicotinoids does not reside in differences in their affinity for the receptor but rather in divergent metabolism by P450s. Functional expression of the entire CYP3 clade of P450s from honeybees identified a single P450, CYP9Q3, that metabolizes thiacloprid with high efficiency but has little activity against imidacloprid. We demonstrate that bumble bees also exhibit profound differences in their sensitivity to different neonicotinoids, and we identify CYP9Q4 as a functional ortholog of honeybee CYP9Q3 and a key metabolic determinant of neonicotinoid sensitivity in this species. Our results demonstrate that bee pollinators are equipped with biochemical defense systems that define their sensitivity to insecticides and this knowledge can be leveraged to safeguard bee health.

Effects of the novel pesticide flupyradifurone (Sivanto) on honeybee taste and cognition

Due to intensive agriculture honeybees are threatened by various pesticides. The use of one group of them, the neonicotinoids, was recently restricted by the European Union. These chemicals bind to the nicotinic acetylcholine receptor (nAchR) in the honeybee brain. Recently, Bayer AG released a new pesticide by the name of "Sivanto" against sucking insects. It is assumed to be harmless for honeybees, although its active ingredient, flupyradifurone, binds nAchR similar to the neonicotinoids. We investigated if this pesticide affects the taste for sugar and cognitive performance in honeybee foragers. These bees are directly exposed to the pesticide while foraging for pollen or nectar. Our results demonstrate that flupyradifurone can reduce taste and appetitive learning performance in honeybees foraging for pollen and nectar, although only the highest concentration had significant effects. Most likely, honeybee foragers will not be exposed to these high concentrations. Therefore, the appropriate use of this pesticide is considered safe for honeybees, at least with respect to the behaviors studied here.

The Biogenic Amine Tyramine and its Receptor (AmTyr1) in Olfactory Neuropils in the Honey Bee (Apis mellifera) Brain

This article describes the cellular sources for tyramine and the cellular targets of tyramine via the Tyramine Receptor 1 (AmTyr1) in the olfactory learning and memory neuropils of the honey bee brain. Clusters of approximately 160 tyramine immunoreactive neurons are the source of tyraminergic fibers with small varicosities in the optic lobes, antennal lobes, lateral protocerebrum, mushroom body (calyces and gamma lobes), tritocerebrum and subesophageal ganglion (SEG). Our tyramine mapping study shows that the primary sources of tyramine in the antennal lobe and calyx of the mushroom body are from at least two Ventral Unpaired Median neurons (VUMmd and VUMmx) with cell bodies in the SEG. To reveal AmTyr1 receptors in the brain, we used newly characterized anti-AmTyr1 antibodies. Immunolocalization studies in the antennal lobe with anti-AmTyr1 antibodies showed that the AmTyr1 expression pattern is mostly in the presynaptic sites of olfactory receptor neurons (ORNs). In the mushroom body calyx, anti-AmTyr1 mapped the presynaptic sites of uniglomerular Projection Neurons (PNs) located primarily in the microglomeruli of the lip and basal ring calyx area. Release of tyramine/octopamine from VUM (md and mx) neurons in the antennal lobe and mushroom body calyx would target AmTyr1 expressed on ORN and uniglomerular PN presynaptic terminals. The presynaptic location of AmTyr1, its structural similarity with vertebrate alpha-2 adrenergic receptors, and previous pharmacological evidence suggests that it has an important role in the presynaptic inhibitory control of neurotransmitter release.

Drosophila Studies on Autism Spectrum Disorders

In the past decade, numerous genes associated with autism spectrum disorders (ASDs) have been identified. These genes encode key regulators of synaptogenesis, synaptic function, and synaptic plasticity. Drosophila is a prominent model system for ASD studies to define novel genes linked to ASDs and decipher their molecular roles in synaptogenesis, synaptic function, synaptic plasticity, and neural circuit assembly and consolidation. Here, we review Drosophila studies on ASD genes that regulate synaptogenesis, synaptic function, and synaptic plasticity through modulating chromatin remodeling, transcription, protein synthesis and degradation, cytoskeleton dynamics, and synaptic scaffolding.

Molecular characterization and expression profiles of nicotinic acetylcholine receptors in the rice striped stem borer, Chilo suppressalis (Lepidoptera: Crambidae)

Nicotinic acetylcholine receptors (nAChRs) are members of the cys-loop ligand-gated ion channel (cysLGIC) superfamily, mediating fast synaptic cholinergic transmission in the central nervous system in insects. Insect nAChRs are the molecular targets of economically important insecticides, such as neonicotinoids and spinosad. Identification and characterization of the nAChR gene family in the rice striped stem borer, Chilo suppressalis, could provide beneficial information about this important receptor gene family and contribute to the investigation of the molecular modes of insecticide action and resistance for current and future chemical control strategies. We searched our C. suppressalis transcriptome database using Bombyx mori nAChR sequences in local BLAST searches and obtained the putative nAChR subunit complementary DNAs (cDNAs) via reverse transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends methods. Similar to B. mori, C. suppressalis possesses 12 nAChR subunits, including nine α-type and three β-type subunits. Quantitative RT-PCR analysis revealed the expression profiles of the nAChR subunits in various tissues, including the brain, subesophageal ganglion, thoracic ganglion, abdominal ganglion, hemocytes, fat body, foregut, midgut, hindgut and Malpighian tubules. Developmental expression analyses showed clear differential expression of nAChR subunits throughout the C. suppressalis life cycle. The identification of nAChR subunits in this study will provide a foundation for investigating the diverse roles played by nAChRs in C. suppressalis and for exploring specific target sites for chemicals that control agricultural pests while sparing beneficial species.

Nicotine in floral nectar pharmacologically influences bumblebee learning of floral features

Many plants defend themselves against herbivores by chemical deterrents in their tissues and the presence of such substances in floral nectar means that pollinators often encounter them when foraging. The effect of such substances on the foraging behaviour of pollinators is poorly understood. Using artificial flowers in tightly-controlled laboratory settings, we examined the effects of the alkaloid nicotine on bumblebee foraging performance. We found that bumblebees confronted simultaneously with two equally rewarded nicotine-containing and nicotine-free flower types are deterred only by unnaturally high nicotine concentrations. This deterrence disappears or even turns into attraction at lower nectar-relevant concentrations. The alkaloid has profound effects on learning in a dose-dependent manner. At a high natural dose, bees learn the colour of a nicotine-containing flower type more swiftly than a flower type with the same caloric value but without nicotine. Furthermore, after experiencing flowers containing nicotine in any tested concentration, increasing numbers of bumblebees stay more faithful to these flowers, even if they become a suboptimal choice in terms of reward. These results demonstrate that alkaloids enhance pollinator flower constancy, opening new perspectives in co-evolutionary process between plants and pollinators.

Knock down of Whitefly Gut Gene Expression and Mortality by Orally Delivered Gut Gene-Specific dsRNAs

Control of the whitefly Bemisia tabaci (Genn.) agricultural pest and plant virus vector relies on the use of chemical insecticides. RNA-interference (RNAi) is a homology-dependent innate immune response in eukaryotes, including insects, which results in degradation of the corresponding transcript following its recognition by a double-stranded RNA (dsRNA) that shares 100% sequence homology. In this study, six whitefly 'gut' genes were selected from an in silico-annotated transcriptome library constructed from the whitefly alimentary canal or 'gut' of the B biotype of B. tabaci, and tested for knock down efficacy, post-ingestion of dsRNAs that share 100% sequence homology to each respective gene target. Candidate genes were: Acetylcholine receptor subunit α, Alpha glucosidase 1, Aquaporin 1, Heat shock protein 70, Trehalase1, and Trehalose transporter1. The efficacy of RNAi knock down was further tested in a gene-specific functional bioassay, and mortality was recorded in 24 hr intervals, six days, post-treatment. Based on qPCR analysis, all six genes tested showed significantly reduced gene expression. Moderate-to-high whitefly mortality was associated with the down-regulation of osmoregulation, sugar metabolism and sugar transport-associated genes, demonstrating that whitefly survivability was linked with RNAi results. Silenced Acetylcholine receptor subunit α and Heat shock protein 70 genes showed an initial low whitefly mortality, however, following insecticide or high temperature treatments, respectively, significantly increased knockdown efficacy and death was observed, indicating enhanced post-knockdown sensitivity perhaps related to systemic silencing. The oral delivery of gut-specific dsRNAs, when combined with qPCR analysis of gene expression and a corresponding gene-specific bioassay that relates knockdown and mortality, offers a viable approach for functional genomics analysis and the discovery of prospective dsRNA biopesticide targets. The approach can be applied to functional genomics analyses to facilitate, species-specific dsRNA-mediated control of other non-model hemipterans.

Putative nicotinic acetylcholine receptor subunits express differentially through the life cycle of codling moth, Cydia pomonella (Lepidoptera: Tortricidae)

Nicotinic acetylcholine receptors (nAChRs) are the targets of neonicotinoids and spinosads, two insecticides used in orchards to effectively control codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae). Orchardists in Washington State are concerned about the possibility of codling moth field populations developing resistance to these two insecticides. In an effort to help mitigate this issue, we initiated a project to identify and characterize codling moth nAChR subunits expressed in heads. This study had two main goals; (i) identify transcripts from a codling moth head transcriptome that encode for nAChR subunits, and (ii) determine nAChR subunit expression profiles in various life stages of codling moth. From a codling moth head transcriptome, 24 transcripts encoding for 12 putative nAChR subunit classes were identified and verified by PCR amplification, cloning, and sequence determination. Characterization of the deduced protein sequences encoded by putative nAChR transcripts revealed that they share the distinguishing features of the cys-loop ligand-gated ion channel superfamily with 9 α-type subunits and 3 β-type subunits identified. Phylogenetic analysis comparing these protein sequences to those of other insect nAChR subunits supports the identification of these proteins as nAChR subunits. Stage expression studies determined that there is clear differential expression of many of these subunits throughout the codling moth life cycle. The information from this study will be used in the future to monitor for potential target-site resistance mechanisms to neonicotinoids and spinosads in tolerant codling moth populations.

A Locomotor Deficit Induced by Sublethal Doses of Pyrethroid and Neonicotinoid Insecticides in the Honeybee Apis mellifera

The toxicity of pesticides used in agriculture towards non-targeted organisms and especially pollinators has recently drawn the attention from a broad scientific community. Increased honeybee mortality observed worldwide certainly contributes to this interest. The potential role of several neurotoxic insecticides in triggering or potentiating honeybee mortality was considered, in particular phenylpyrazoles and neonicotinoids, given that they are widely used and highly toxic for insects. Along with their ability to kill insects at lethal doses, they can compromise survival at sublethal doses by producing subtle deleterious effects. In this study, we compared the bee's locomotor ability, which is crucial for many tasks within the hive (e.g. cleaning brood cells, feeding larvae…), before and after an acute sublethal exposure to one insecticide belonging to the two insecticide classes, fipronil and thiamethoxam. Additionally, we examined the locomotor ability after exposure to pyrethroids, an older chemical insecticide class still widely used and known to be highly toxic to bees as well. Our study focused on young bees (day 1 after emergence) since (i) few studies are available on locomotion at this stage and (ii) in recent years, pesticides have been reported to accumulate in different hive matrices, where young bees undergo their early development. At sublethal doses (SLD48h, i.e. causing no mortality at 48 h), three pyrethroids, namely cypermethrin (2.5 ng/bee), tetramethrin (70 ng/bee), tau-fluvalinate (33 ng/bee) and the neonicotinoid thiamethoxam (3.8 ng/bee) caused a locomotor deficit in honeybees. While the SLD48h of fipronil (a phenylpyrazole, 0.5 ng/bee) had no measurable effect on locomotion, we observed high mortality several days after exposure, an effect that was not observed with the other insecticides. Although locomotor deficits observed in the sublethal range of pyrethroids and thiamethoxam would suggest deleterious effects in the field, the case of fipronil demonstrates that toxicity evaluation requires information on multiple endpoints (e.g. long term survival) to fully address pesticides risks for honeybees. Pyrethroid-induced locomotor deficits are discussed in light of recent advances regarding their mode of action on honeybee ion channels and current structure-function studies.

The immunomodulation of nicotinic acetylcholine receptor subunits in Zhikong scallop Chlamys farreri

Nicotinic acetylcholine receptor (nAChR), the best-studied ionotropic neuron receptor protein, is a key player in neuronal communication, and it has been reported to play an important role in immunomodulation of vertebrates. Although nAChRs have also been identified in most invertebrates, the knowledge about their immunomodulation is still limited. In the present study, two scallop nAChR genes were identified from Chlamys farreri (designed as CfnAChR1 and CfnAChR2), which encoded 384 and 443 amino acids, respectively. The conserved disulfide-linked cystines, ion selectivity residues and the hydrophobic gating residues (L251, V255 and V259) were identified in CfnAChR1 and CfnAChR2. The immunoreactivities of CfnAChR1 and CfnAChR2 were observed in all the tested scallop tissues, including adductor muscle, mantle, gill, hepatopancreas, kidney and gonad. After LPS (0.5 mg mL(-1)) stimulation, the expression of CfnAChR1 mRNA in haemocytes increased significantly by 9.83-fold (P < 0.05) and 12.93-fold (P < 0.05) at 3 h and 24 h, respectively. While the expression level of CfnAChR2 mRNA increased 43.94% at 12 h after LPS stimulation (P < 0.05). After TNF-α (50 ng mL(-1)) stimulation, the expression levels of CfnAChR1 and CfnAChR2 both increased significantly at 1 h, which were 21.33-fold (P < 0.05) and 2.44-fold (P < 0.05) of that in the PBS group, respectively. The results collectively indicated that the cholinergic nervous system in scallops could be activated by immune stimulations through CfnAChR1 and CfnAChR2, which function as the links between the cholinergic nervous system and immune system.

Low doses of neonicotinoid pesticides in food rewards impair short-term olfactory memory in foraging-age honeybees

Neonicotinoids are often applied as systemic seed treatments to crops and have reported negative impact on pollinators when they appear in floral nectar and pollen. Recently, we found that bees in a two-choice assay prefer to consume solutions containing field-relevant doses of the neonicotinoid pesticides, imidacloprid (IMD) and thiamethoxam (TMX), to sucrose alone. This suggests that neonicotinoids enhance the rewarding properties of sucrose and that low, acute doses could improve learning and memory in bees. To test this, we trained foraging-age honeybees to learn to associate floral scent with a reward containing nectar-relevant concentrations of IMD and TMX and tested their short (STM) and long-term (LTM) olfactory memories. Contrary to our predictions, we found that none of the solutions enhanced the rate of olfactory learning and some of them impaired it. In particular, the effect of 10 nM IMD was observed by the second conditioning trial and persisted 24 h later. In most other groups, exposure to IMD and TMX affected STM but not LTM. Our data show that negative impacts of low doses of IMD and TMX do not require long-term exposure and suggest that impacts of neonicotinoids on olfaction are greater than their effects on rewarding memories.

Injection of insect membrane in Xenopus oocyte: An original method for the pharmacological characterization of neonicotinoid insecticides

INTRODUCTION

Insect nicotinic acetylcholine receptors (nAChRs) represent a major target of insecticides, belonging to the neonicotinoid family. However, the pharmacological profile of native nAChRs is poorly documented, mainly because of a lack of knowledge of their subunit stoichiometry, their tissue distribution and the weak access to nAChR-expressing cells. In addition, the expression of insect nAChRs in heterologous systems remains hard to achieve. Therefore, the structure-activity characterization of nAChR-targeting insecticides is made difficult. The objective of the present study was to characterize insect nAChRs by an electrophysiological approach in a heterologous system naturally devoid of these receptors to allow a molecular/cellular investigation of the mode of action of neonicotinoids. Methods To overcome impediments linked to the expression of insect nAChR mRNA or cDNA, we chose to inject insect membranes from the pea aphid (Acyrthosiphon pisum) into Xenopus oocytes. This microtransplantation technique was designed to gain access to native nAChRs embedded in their membrane, through direct stimulation with nicotinic agonists. Results We provide evidence that an enriched-nAChR membrane allows us to characterize native receptors. The presence of such receptors was confirmed with fluorescent α-BgTX labeling. Electrophysiological recordings of nicotine-induced inward currents allowed us to challenge the presence of functional nAChR. We compared the effect of nicotine (NIC) with clothianidin (CLO) and we assessed the effect of thiamethoxam (TMX). Discussion This technique has been recently highlighted with mammalian and human material as a powerful functional approach, but has, to our knowledge, never been used with insect membrane. In addition, the use of the insect membrane microtransplantation opens a new and original way for pharmacological screening of neurotoxic insecticides, including neonicotinoids. Moreover, it might also be a powerful tool to investigate the pharmacological properties of insect nAChR.

The neonicotinoid imidacloprid impairs honey bee aversive learning of simulated predation

Neonicotinoid insecticides can impair bee learning and memory--cognitive features that play a key role in colony fitness because they facilitate foraging. For example, the commonly used neonicotinoid imidacloprid reduces honey bee olfactory learning. However, no studies have previously determined whether imidacloprid can impair aversive associative learning, although such learning should enhance bee survival by allowing bees to avoid dangerous foraging sites. To mimic attempted predation of foragers, we developed an electro-mechanical predator that consistently attacked foragers with a pinching bite at a fixed force and elicited aversive olfactory learning in a sting extension response (SER) assay. We show that chronic exposure to a sublethal concentration of imidacloprid (25.6 µg l(-1)=20.8 ppb) over 4 days (mean of 1.5 ng per bee day(-1)), significantly impaired aversive short-term learning and memory retention. Imidacloprid treatment reduced short-term learning by 87% and memory retention by 85% in comparison with control bees. Imidacloprid therefore impairs the ability of honey bees to associate a naturalistic predation stimulus--biting--with floral odor compounds. Such learning should enhance bee survival, suggesting that xenobiotics could alter more complex ecological interactions such as predator-prey relationships.

Wings as a new route of exposure to pesticides in the honey bee

In pesticide risk assessment, estimating the routes and levels of exposure is critical. For honey bees subjected to pesticide spray, toxicity is assessed by thorax contact to account for all possible contact exposures. In the present study, the authors tested 6 active substances with different hydrophobicity. For the first time, the authors demonstrated that it is possible to induce mortality by pesticide contact with only the wings of the honey bee. The toxicities induced by contact with the wings and thorax were similar, with the wing median lethal dose (LD50) being 0.99 to 2.23 times higher than that of the thorax. This finding demonstrates that the wings represent a relevant route of exposure in the honey bee. In a second approach, the authors estimated the air volume displaced by the wings during 1 beating cycle to be 0.51 ± 0.03 cm(3), which corresponds to a volume of 116.8 ± 5.8 cm(3)  s(-1) at a wing beat frequency of 230 Hz. The authors then tested realistic scenarios of exposure for bees flying through a pesticide cloud at different concentrations. In the worst-case scenario, the dose accumulated during the flight reached 525 ng bee(-1)  s(-1). These results show that the procedure used to assess the risk posed by contact with pesticides could be improved by accounting for wing exposure.

Effects of sublethal doses of glyphosate on honeybee navigation

Glyphosate (GLY) is a herbicide that is widely used in agriculture for weed control. Although reports about the impact of GLY in snails, crustaceans and amphibians exist, few studies have investigated its sublethal effects in non-target organisms such as the honeybee Apis mellifera, the main pollen vector in commercial crops. Here, we tested whether exposure to three sublethal concentrations of GLY (2.5, 5 and 10 mg l(-1): corresponding to 0.125, 0.250 and 0.500 μg per animal) affects the homeward flight path of honeybees in an open field. We performed an experiment in which forager honeybees were trained to an artificial feeder, and then captured, fed with sugar solution containing traces of GLY and released from a novel site either once or twice. Their homeward trajectories were tracked using harmonic radar technology. We found that honeybees that had been fed with solution containing 10 mg l(-1) GLY spent more time performing homeward flights than control bees or bees treated with lower concentrations. They also performed more indirect homing flights. Moreover, the proportion of direct homeward flights performed after a second release from the same site increased in control bees but not in treated bees. These results suggest that, in honeybees, exposure to levels of GLY commonly found in agricultural settings impairs the cognitive capacities needed to retrieve and integrate spatial information for a successful return to the hive. Therefore, honeybee navigation is affected by ingesting traces of the most widely used herbicide worldwide, with potential long-term negative consequences for colony foraging success.

Neonicotinoid-Coated Zea mays Seeds Indirectly Affect Honeybee Performance and Pathogen Susceptibility in Field Trials

Thirty-two honeybee (Apis mellifera) colonies were studied in order to detect and measure potential in vivo effects of neonicotinoid pesticides used in cornfields (Zea mays spp) on honeybee health. Honeybee colonies were randomly split on four different agricultural cornfield areas located near Quebec City, Canada. Two locations contained cornfields treated with a seed-coated systemic neonicotinoid insecticide while the two others were organic cornfields used as control treatments. Hives were extensively monitored for their performance and health traits over a period of two years. Honeybee viruses (brood queen cell virus BQCV, deformed wing virus DWV, and Israeli acute paralysis virus IAPV) and the brain specific expression of a biomarker of host physiological stress, the Acetylcholinesterase gene AChE, were investigated using RT-qPCR. Liquid chromatography-mass spectrometry (LC-MS) was performed to detect pesticide residues in adult bees, honey, pollen, and corn flowers collected from the studied hives in each location. In addition, general hive conditions were assessed by monitoring colony weight and brood development. Neonicotinoids were only identified in corn flowers at low concentrations. However, honeybee colonies located in neonicotinoid treated cornfields expressed significantly higher pathogen infection than those located in untreated cornfields. AChE levels showed elevated levels among honeybees that collected corn pollen from treated fields. Positive correlations were recorded between pathogens and the treated locations. Our data suggests that neonicotinoids indirectly weaken honeybee health by inducing physiological stress and increasing pathogen loads.

The integrative analysis of microRNA and mRNA expression in Apis mellifera following maze-based visual pattern learning

The honeybee (Apis mellifera) is a social insect with strong sensory capacity and diverse behavioral repertoire and is recognized as a good model organism for studying the neurobiological basis of learning and memory. In this study, we analyzed the changes in microRNA (miRNA) and messenger RNA (mRNA) following maze-based visual learning using next-generation small RNA sequencing and Solexa/lllumina Digital Gene Expression tag profiling (DGE). For small RNA sequencing, we obtained 13 367 770 and 13 132 655 clean tags from the maze and control groups, respectively. A total of 40 differentially expressed known miRNAs were detected between these two samples, and all of them were up-regulated in the maze group compared to the control group. For DGE, 5 681 320 and 5 939 855 clean tags were detected from the maze and control groups, respectively. There were a total of 388 differentially expressed genes between these two samples, with 45 genes up-regulated and 343 genes down-regulated in the maze group, compared to the control group. Additionally, the expression levels of 10 differentially expressed genes were confirmed by quantitative reverse transcription polymerase chain reaction (qRT-PCR) and the expression trends of eight of them were consistent with the DGE result, although the degree of change was lower in amplitude. The integrative analysis of miRNA and mRNA expression showed that, among the 40 differentially expressed known miRNAs and 388 differentially expressed genes, 60 pairs of miRNA/mRNA were identified as co-expressed in our present study. These results suggest that both miRNA and mRNA may play a pivotal role in the process of learning and memory in honeybees. Our sequencing data provide comprehensive miRNA and gene expression information for maze-based visual learning, which will facilitate understanding of the molecular mechanisms of honeybee learning and memory.

Larval Helicoverpa zea Transcriptional, Growth and Behavioral Responses to Nicotine and Nicotiana tabacum

The polyphagous feeding habits of the corn earworm, Helicoverpa zea (Boddie), underscore its status as a major agricultural pest with a wide geographic distribution and host plant repertoire. To study the transcriptomic response to toxins in diet, we conducted a microarray analysis of H. zea caterpillars feeding on artificial diet, diet laced with nicotine and Nicotiana tabacum (L.) plants. We supplemented our analysis with growth and aversion bioassays. The transcriptome reflects an abundant expression of proteases, chitin, cytochrome P450 and immune-related genes, many of which are shared between the two experimental treatments. However, the tobacco treatment tended to elicit stronger transcriptional responses than nicotine-laced diet. The salivary factor glucose oxidase, known to suppress nicotine induction in the plant, was upregulated by H. zea in response to tobacco but not to nicotine-laced diet. Reduced caterpillar growth rates accompanied the broad regulation of genes associated with growth, such as juvenile hormone epoxide hydrolase. The differential expression of chemosensory proteins, such as odorant binding-protein-2 precursor, as well as the neurotransmitter nicotinic-acetylcholine-receptor subunit 9, highlights candidate genes regulating aversive behavior towards nicotine. We suggest that an observed coincidental rise in cannibalistic behavior and regulation of proteases and protease inhibitors in H. zea larvae signify a compensatory response to induced plant defenses.

Cyclic nucleotide-gated channels, calmodulin, adenylyl cyclase, and calcium/calmodulin-dependent protein kinase II are required for late, but not early, long-term memory formation in the honeybee

Memory is a dynamic process that allows encoding, storage, and retrieval of information acquired through individual experience. In the honeybee Apis mellifera, olfactory conditioning of the proboscis extension response (PER) has shown that besides short-term memory (STM) and mid-term memory (MTM), two phases of long-term memory (LTM) are formed upon multiple-trial conditioning: an early phase (e-LTM) which depends on translation from already available mRNA, and a late phase (l-LTM) which requires de novo transcription and translation. Here we combined olfactory PER conditioning and neuropharmacological inhibition and studied the involvement of the NO-cGMP pathway, and of specific molecules, such as cyclic nucleotide-gated channels (CNG), calmodulin (CaM), adenylyl cyclase (AC), and Ca(2+)/calmodulin-dependent protein kinase (CaMKII), in the formation of olfactory LTM in bees. We show that in addition to NO-cGMP and cAMP-PKA, CNG channels, CaM, AC, and CaMKII also participate in the formation of a l-LTM (72-h post-conditioning) that is specific for the learned odor. Importantly, the same molecules are dispensable for olfactory learning and for the formation of both MTM (in the minute and hour range) and e-LTM (24-h post-conditioning), thus suggesting that the signaling pathways leading to l-LTM or e-LTM involve different molecular actors.

Chronic exposure of imidacloprid and clothianidin reduce queen survival, foraging, and nectar storing in colonies of Bombus impatiens

In an 11-week greenhouse study, caged queenright colonies of Bombus impatiens Cresson, were fed treatments of 0 (0 ppb actual residue I, imidacloprid; C, clothianidin), 10 (14 I, 9 C), 20 (16 I, 17C), 50 (71 I, 39 C) and 100 (127 I, 76 C) ppb imidacloprid or clothianidin in sugar syrup (50%). These treatments overlapped the residue levels found in pollen and nectar of many crops and landscape plants, which have higher residue levels than seed-treated crops (less than 10 ppb, corn, canola and sunflower). At 6 weeks, queen mortality was significantly higher in 50 ppb and 100 ppb and by 11 weeks in 20 ppb-100 ppb neonicotinyl-treated colonies. The largest impact for both neonicotinyls starting at 20 (16 I, 17 C) ppb was the statistically significant reduction in queen survival (37% I, 56% C) ppb, worker movement, colony consumption, and colony weight compared to 0 ppb treatments. Bees at feeders flew back to the nest box so it appears that only a few workers were collecting syrup in the flight box and returning the syrup to the nest. The majority of the workers sat immobilized for weeks on the floor of the flight box without moving to fed at sugar syrup feeders. Neonicotinyl residues were lower in wax pots in the nest than in the sugar syrup that was provided. At 10 (14) ppb I and 50 (39) ppb C, fewer males were produced by the workers, but queens continued to invest in queen production which was similar among treatments. Feeding on imidacloprid and clothianidin can cause changes in behavior (reduced worker movement, consumption, wax pot production, and nectar storage) that result in detrimental effects on colonies (queen survival and colony weight). Wild bumblebees depending on foraging workers can be negatively impacted by chronic neonicotinyl exposure at 20 ppb.

Use of primary cultures of Kenyon cells from bumblebee brains to assess pesticide side effects

Bumblebees are important pollinators in natural and agricultural ecosystems. The latter results in the frequent exposure of bumblebees to pesticides. We report here on a new bioassay that uses primary cultures of neurons derived from adult bumblebee workers to evaluate possible side-effects of the neonicotinoid pesticide imidacloprid. Mushroom bodies (MBs) from the brains of bumblebee workers were dissected and dissociated to produce cultures of Kenyon cells (KCs). Cultured KCs typically extend branched, dendrite-like processes called neurites, with substantial growth evident 24-48 h after culture initiation. Exposure of cultured KCs obtained from newly eclosed adult workers to 2.5 parts per billion (ppb) imidacloprid, an environmentally relevant concentration of pesticide, did not have a detectable effect on neurite outgrowth. By contrast, in cultures prepared from newly eclosed adult bumblebees, inhibitory effects of imidacloprid were evident when the medium contained 25 ppb imidacloprid, and no growth was observed at 2,500 ppb. The KCs of older workers (13-day-old nurses and foragers) appeared to be more sensitive to imidacloprid than newly eclosed adults, as strong effects on KCs obtained from older nurses and foragers were also evident at 2.5 ppb imidacloprid. In conclusion, primary cultures using KCs of bumblebee worker brains offer a tool to assess sublethal effects of neurotoxic pesticides in vitro. Such studies also have the potential to contribute to the understanding of mechanisms of plasticity in the adult bumblebee brain.

Acetylcholinesterase in honey bees (Apis mellifera) exposed to neonicotinoids, atrazine and glyphosate: laboratory and field experiments

In Québec, as observed globally, abnormally high honey bee mortality rates have been reported recently. Several potential contributing factors have been identified, and exposure to pesticides is of increasing concern. In maize fields, foraging bees are exposed to residual concentrations of insecticides such as neonicotinoids used for seed coating. Highly toxic to bees, neonicotinoids are also reported to increase AChE activity in other invertebrates exposed to sub-lethal doses. The purpose of this study was therefore to test if the honey bee's AChE activity could be altered by neonicotinoid compounds and to explore possible effects of other common products used in maize fields: atrazine and glyphosate. One week prior to pollen shedding, beehives were placed near three different field types: certified organically grown maize, conventionally grown maize or non-cultivated. At the same time, caged bees were exposed to increasing sub-lethal doses of neonicotinoid insecticides (imidacloprid and clothianidin) and herbicides (atrazine and glyphosate) under controlled conditions. While increased AChE activity was found in all fields after 2 weeks of exposure, bees close to conventional maize crops showed values higher than those in both organic maize fields and non-cultivated areas. In caged bees, AChE activity increased in response to neonicotinoids, and a slight decrease was observed by glyphosate. These results are discussed with regard to AChE activity as a potential biomarker of exposure for neonicotinoids.

Involvement of α7 nAChR signaling cascade in epigallocatechin gallate suppression of β-amyloid-induced apoptotic cortical neuronal insults

Excessive generation and accumulation of the β-amyloid (Aβ) peptide in selectively vulnerable brain regions is a key pathogenic event in the Alzheimer's disease (AD), while epigallocatechin gallate (EGCG) is a very promising chemical to suppress a variety of Aβ-induced neurodegenerative disorders. However, the precise molecular mechanism of EGCG responsible for protection against neurotoxicity still remains elusive. To validate and further investigate the possible mechanism involved, we explored whether EGCG neuroprotection against neurotoxicity of Aβ is mediated through the α7 nicotinic acetylcholine receptor (α7 nAChR) signaling cascade. It was shown in rat primary cortical neurons that short-term treatment with EGCG significantly attenuated the neurotoxicity of Aβ1-42, as demonstrated by increased cell viability, reduced number of apoptotic cells, decreased reactive oxygen species (ROS) generation, and downregulated caspase-3 levels after treatment with 25-μM Aβ1-42. In addition, EGCG markedly strengthened activation of α7nAChR as well as its downstream pathway signaling molecules phosphatidylinositol 3-kinase (PI3K) and Akt, subsequently leading to suppression of Bcl-2 downregulation in Aβ-treated neurons. Conversely, administration of α7nAChR antagonist methyllycaconitine (MLA; 20 μM) to neuronal cultures significantly attenuated the neuroprotection of EGCG against Aβ-induced neurototoxicity, thus presenting new evidence that the α7nAChR activity together with PI3K/Akt transduction signaling may contribute to the molecular mechanism underlying the neuroprotective effects of EGCG against Aβ-induced cell death.