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Copyright © by the authors. This article has been cited by other articles in PMC. The cholinergic system is a popular target of insecticides because the pharmacology of insect nicotinic acetylcholine receptors nAChRs differs substantially from their vertebrate counterparts.

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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. Keywords: insecticides, Apis mellifera, brood, larvae, nicotinic acetylcholine receptors, royal jelly 1. Introduction Acetylcholine ACh is an evolutionary highly-conserved signaling molecule. It preceded the appearance of the nervous system since it is expressed in bacteria, archaea, in eucaryotic unicellular organisms, and in higher organisms such as plants, fungi, and animals [ 1 ].

Therefore, the neuronal system basically utilizes the existing cholinergic system and improved frank 1968 suisse anti aging communication speed by releasing ACh from vesicles during synaptic transmission.

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However, the non-neuronal cholinergic system remains side-by-side to the neuronal cholinergic system within animals. The functional principles of both systems are basically similar. In addition to its expression in the nervous system, these components have been widely localized in epithelial and endothelial tissues [ 2 ], in reproductive organs [ 3 ], and in muscle and immune cells.

Thus, numerous cell functions can be regulated by ACh, such as gene expression, proliferation, differentiation, cytoskeletal organization, cell—cell contact, locomotion, migration, ciliary activity, electrical activity, secretion, and absorption [ 4 ]. Compared to vertebrates, the non-neuronal cholinergic system of insects is largely understudied, although it is crucial during all developmental stages, and ACh, AChE, and ChAT are present in very much higher titers than in nervous tissues [ 5 ].

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In most insect species, two AChE are present, and a large group of insecticides specifically target those esterases, such as organophosphates and carbamates [ 6 ]. Apparently, the amount of the soluble AmAChE1 is regulated by the breeding activity of honey bee colonies, which provides further evidence for the influence of the cholinergic system on reproduction in insects [ 8 ].

In Tribolium castanaeum, expression of the AChE gene Frank 1968 suisse anti aging is important during female reproduction, embryo development, and offspring growth [ 9 ]. In Drosophila melanogaster, which has only one AChE gene, a non-neuronal effect was reported in which ACh, after its transport through the hemolymph, regulates the heart rate [ 10 ]. The importance of ACh for the insect immune responses is suggested since cholinergic disrupting chemicals impair immune responses [ 1112 ].

Furthermore, initial evidence shows that hemocytes and the fatbody express nAChRs subunits in bees [ 13 ]. Cholinergic synaptic transmission has been intensively studied as it is prevalent within the insect brain. Nicotinic acetylcholine receptors have been localized in most brain neuropils, and functional nAChRs were characterized in vitro in various species cf. Their physiologies are in accordance with a function during excitatory synaptic transmission within the insect brain.

Their molecular and functional similarities e. Accordingly, cholinergic signal transduction is required for olfactory learning and memory formation in insects reviews: [ 171819 ]. Honeybee nicotinic receptors share many features of insect nAChRs with respect to localization [ 20 ], pharmacology [ 212223 ], permeability [ 15 ], and molecular identity [ 2425ecset igazi technika swiss anti aging ].

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The pharmacology differs substantially from its vertebrate counterparts [ 2728 ]. Therefore, neonicotinoids have been developed that specifically target insect nAChR-dependent synaptic transmission, acting agonistically on the receptor with high specificity [ 2930 ] see also: [ 31 ]. Neonicotinoids are a widely used group of insecticides. Not surprisingly, one of the problems with agricultural neonicotinoid applications is that they also bind to cholinergic receptors and induce currents through neuronal nAChRs [ 23323334 ] of pollinating insects, such as honeybees or bumblebees, and impair cholinergic transmission and—as a consequence—behavioral output.

Socially living bees may be affected in several ways. Firstly, as adults, during foraging, during trophallactic contact, or by consumption of stored contaminated nectar, honey, or pollen. Secondly, as larvae after been fed with contaminated brood food. Therefore, bees need to be protected from exposure to neonicotinoids.

Many studies investigated survival after insecticide treatment targeting the cholinergic system [ 3536373839404142 frank 1968 suisse anti aging, and numerous studies have examined their effect on brood development by registering colony constitutions of honeybees e. Furthermore, sub-lethal neonicotinoid effects on adult honeybees comprise disturbances of navigation and orientation [ 495051 ], walking behavior [ 52 ], learning and memory [ 5354 ], foraging behavior [ 55 ], and nurse-larva-interactions [ 56 ].

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For comprehensive overviews on sub-lethal neonicotinoid effects on honeybees, see [ 57585960 ]. Despite the fact that field studies largely failed to unambiguously demonstrate adverse effects of treated crop fields on whole hives the behavioral experiments clearly indicate impairments on individually treated honeybees.

It is plausible to assume that the reported deficits also occur in those hives that appear vital upon superficial inspection. The mode of action through which neonicotinoids induce these effects may be manifold. Certainly, acting via neuronal nAChRs is one of the major routes. In addition to this mechanism, neonicotinoids may affect muscarinic AChRs that frank 1968 suisse anti aging, as yet, largely uncharacterized in bees.

A third way, which we assume to be particularly important for developmental effects, is via non-neuronal AChRs.

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Here, we review the properties of honeybee nAChRs and the actions of neonicotinoids on the neuronal receptors. Frank 1968 suisse anti aging will then discuss the effects of neonicotinoids on larval and adult development and present an integrative model of frank 1968 suisse anti aging signaling and disturbances by neonicotinoids.

Acetylcholine Receptors in the Honeybee Acetylcholine is the major excitatory transmitter in the insect brain reviews in [ 14 ]. Immunolabelling of the nAChR or ChAT, as well as the histochemistry of AChE activity and in situ hybridization studies of the various nAChR α-subunits identified several pathways and neuropils that are presumably cholinergic in insects. In bees, the olfactory system frank 1968 suisse anti aging the visual neuropils probably rely mainly on cholinergic signal transmission [ 20 ].

Axons of the olfactory receptor neurons probably release ACh onto postsynaptic neurons within the antennal lobes ALsand a subpopulation of projection neurons from the AL form cholinergic synapses with Kenyon cells within the mushroom body MB lip regions. The lamina, medulla, and lobula of bees contain cholinergic neurons as well as neurons of the central complex.

The honeybee nAChR is an ionotropic receptor of the cys-loop receptor family, a pentameric receptor whose stoichiometry is as yet unknown [ 26 ].

Sequence analyses identified nine different α-subunits, Amelα1—9, and two β-subunits, Amelβ1—2 [ 242561 ]. Amelβ2 subunits are found in Kenyon cells. In the optic lobes, Amelα2, Amelα3, and Amelα7—2, expressions were identified [ 2425 ]. The native honeybee nAChRs in Kenyon cells and AL neurons are cation-selective channels with a neuronal pharmacological profile.

Pressure applications of ACh or nicotine induce rapidly activating inward currents in cultured bee neurons [ 15 ]. Currents through nAChR are blocked by nicotinergic blockers curare, methyllycaconitine, frank 1968 suisse anti aging, hexamethonium, and mecamylamine [ 15212223 ]. ACh, as well as carbamylcholine, are full agonists, whereas nicotine, epibatidine, and cytisine are partial agonists [ 21 ].

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Despite its neuronal profile, the honeybee nAChR has a rather unusual pharmacology as compared to its vertebrate counterparts. However, muscarinic agonists muscarine, pilocarpine, or oxotremorine do not induce currents through honeybee nAChRs [ 21 ].

Finally, the GABA γ-aminobutyric acid receptor blockers picrotoxin, bicuculline, and fipronil, as well as the glycine receptor blocker strychnine, act antagonistically [ 2122 ].

Neonicotinoids are agonists of the insect nAChRs review: [ 66 ]. Due to their low human toxicity and their relative specificity to insect over vertebrate nAChR, they represent a commercially very successful insecticide group [ 6768 ].

Imidacloprid is a partial agonist of the honeybee nAChR Kenyon cells: [ 233334 ]; antennal lobe neurons: [ 223262 ]and clothianidin acts as a full agonist [ 34 ]. Behavioral pharmacological studies indicate that nAChR are involved during various phases of classical conditioning, memory formation, and retrieval review: [ 1819 ].

However, the effects caused by nicotinic antagonists are complex and often contradictory. Interestingly, odor information processing appears to be largely unaffected by pharmacological treatments since odor learning, per se, is not impaired while imidacloprid perfusions diminish odor signals in antennal lobe glomeruli [ 71 ].

At least several different receptors with differing pharmacologies probably also different stoichiometries are expressed in honeybees. Therefore, care is needed by assigning the effects of various drugs or insecticides to certain frank 1968 suisse anti aging. Unfortunately, pharmacological experiments on the functions of nAChRs within the honeybee visual system are missing, although the optic lobes express nAChRs.

Muscarinic acetylcholine receptors are G protein-coupled receptors with seven transmembrane domains. In humans, five mAChRs m1—m5 have been characterized that are expressed not only in the peripheral and central nervous system but also in epithelial airway, skin, intestine, ovary, urotheliumendothelial pulmonary vesselsimmune, and mesenchymal fibroblasts, tenocytes, smooth muscle fibres cells [ 2 ].

The insect mAChRs are less well-studied. Two mAChRs were cloned and described in D. Both receptors have been identified in all arthropods with a sequenced genome [ 72 ]. Acetylcholine in Bee Development In several vertebrate tissues, ACh demonstrates a proliferative, trophic effect via nicotinergic and muscarinic receptors [ 3 ]. In insects, the cholinergic system is crucial during all developmental stages, and ACh, AChE, and ChAT are present in very much higher titers as compared to vertebrates [ 5 ].

Apart from its occurrence in honey and bee bread [ 7475767778 ], bees apparently feed ACh to developing larvae, as it was found in millimolar concentrations in larval food [ 76 ]. This recent study confirms and extends earlier studies reporting surprisingly high ACh concentrations in brood food see below. This shift has also been reported for ACh content in worker nutrition. While larvae below 5 mg weight receive a relatively high ACh amount in their food 1.

The oldest larvae received modified worker jelly with the least amount of ACh 0. This is generally consistent with the study by Wessler et al.

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Drone food also contains relatively high ACh concentrations 1. The reduction in the ACh content during worker development is conceivable since gland secretion decreases in favor of sugar containing food from the honey stomach [ 79 ], and ACh is synthesized in hypopharyngeal canal cells via membrane-bound ChAT [ 76 ].

The synthesis during jelly excretion and the surrounding acidity of pH 4. ACh in brood food can even be preserved after two hours of boiling in water [ 74 ].

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Royal jelly also contains high ACh amounts. It is fed to developing honey bee queens and, compared to worker nutrition, contains a higher amount of sugar [ 79 ]. According to [ 81 ], ACh content decreases from 1. HPLC analyses quantified 8 mM 1. Experimentally reducing the ACh content in artificial brood food increased larval mortality [ 76 ].

ACh-uptake by larvae is, therefore, required for the proper development of queens, workers, and drones. It is produced from non-neuronal tissue and probably acts via non-neuronal AChRs. Neonicotinoids Affect Larval and Adult Development Although ACh is important during larval development, only a few studies exist that have investigated neonicotinoid effects on the bee ontogeny.

However, most of the available studies investigating larval development describe a developmental delay and some abnormal appearances. Also, there is reason to assume that some effects occur in those adults that were exposed to neonicotinoids as larvae e. Repeated administration frank 1968 suisse anti aging 0. However, the pupal development time was not affected [ 40 ]. Furthermore, high dosages of clothianidin ppb and thiacloprid 8.

Interestingly, clothianidin increased pupal development time, whereas thiacloprid decreased it [ 56 ]. In the stingless bee Scaptotrigona aff. These in vitro insecticide experiments show that the cholinergic system is important for larval development and is disturbed by neonicotinoids. However, a direct delivery of neonicotinoids from nurse bees to larvae via brood food appears not to occur under field conditions.

Only 0. This is consistent with distributions of other insecticides within workers. Low radioactivity was measured in hypopharyngeal glands after frank 1968 suisse anti aging were fed with radiolabelled carbaryl and diflubenzuron [ 89 ] or carbofuran and dimethoate [ 90 ]. If direct transmission by honey was the main reason for residues in royal jelly, worker larvae would receive very little neonicotinoids since royal jelly seems to contain more sugar than worker jelly [ 79 ]. However, this may increase as the older worker larvae are fed modified royal jelly.

As several studies demonstrate delayed larval development within honeybee colonies after chronic neonicotinoid treatment e. As a consequence, cholinergic transmission could also be important for the development and maturation of adult bees. The cholinergic system continues to develop in the adult honeybee. During the first week after adult eclosion, the brain activity of AChE increases and remains at this level until old age [ 91 ].

Due to the plasticity of each individual worker, the bee colony as a whole is very adaptive in its division of labor and can shift nursing and foraging activity due to environmental and colony demands. This adaptation is under the influence of various internal e. The subsequent development or degradation of the hypopharyngeal glands [ 93 ] is hormone-dependent.

Newly emerged workers have undeveloped glands with small acini, and with nursing activity, they increase in size and produce the protein-rich jelly. The glands decrease in size and activity again when the worker starts to forage.

As the nursing workers feed the young, other nest mates and the queen [ 9495 ], glandular ACh could be consumed by all individuals of the colony throughout their whole life span. Therefore, a regulatory influence of ACh on adult development and behavior is plausible.

If the exchanged food contains differing amounts of ACh, cholinergic social signaling is likely to contribute to gland development and function and would explain the disturbing effects of neonicotinoids and other xenobiotics described below.

Similarly, after one week of 1 ppb imidacloprid application in sugar syrup to colonies, day old workers displayed reduced acini diameter sizes [ 97 ]. Similar effects were present when bees were exposed to 48 and 72 h treatment with 0.

In addition to these morphological abnormalities, neonicotinoid exposure also impairs hypopharyngeal gland function because acetylcholine secretion into the brood food diminishes after clothianidin or thiacloprid feeding [ 76 ].

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This probably leads frank 1968 suisse anti aging brood impairments in small hives under semi-field conditions. Expression of Amelα3 and Amelα4 nAChR subunits in the hypopharyngeal glands indicates an ACh-dependency of the glandular function, although ChAT activity is not acutely blocked by neonicotinoids, but requires chronic exposure [ 76 ]. Obviously, other xenobiotics that interfere with cholinergic signaling in honeybees may lead to similar disruptions in glandular function.

Among them are the acaricide coumaphos, an organophosphate that inhibits the AChE, and other insecticides like the carbamate fenoxycarb for review see: [ ]. Effects on hypopharyngeal glands were also reported for the GABA receptor blocker fipronil, the herbicide glyphosate, or the fungicide pyraclostrobin [ ]. Several mechanisms may underlie the delays in development, including impairments of the endocrine system, altered gene expression of metabolic pathways, and an increased energy use due to detoxification mechanisms.

Some studies suggest that neonicotinoids interfere with the honey bee endocrine system [, ]. Juvenile hormone slows down larval development, as ligaturing frank 1968 suisse anti aging corpora allata results in shortened worker ontogenesis [ ].

Therefore, neonicotinoids may increase juvenile hormone titers in bees.