Effects of the muscarinic antagonists atropine and pirenzepine on olfactory conditioning in the honeybee

Pirenzepine Binding Sites in the Brain of the Honeybee Apis mellifera: Localization and Involvement in Non-Associative Learning

Muscarinic acetylcholine receptors (mAChRs) play a central role in learning and memory in mammals as in honeybees. The results obtained in the honeybee Apis mellifera are based on the detrimental effects of the mAChR antagonists, atropine and scopolamine, on olfactory associative memory. Binding sites for the mAChR antagonist BODIPY^® FL pirenzepine were localized in the brain of the honeybee forager. Pirenzepine binding sites were detected indifferently in several somata and neuropilar areas. The highest binding site densities were present in the central complex and in somata of the dorsomedial border of the antennal lobes. An additional binding pattern was found in somata of the subesophageal ganglion. By contrast, Kenyon cell (KC) somata were not stained. Pirenzepine (PZ) effects on non-associative learning were evaluated. Treated animals required more trials for the habituation of the proboscis extension reflex (PER) than controls, and the duration of the PER increased after PZ brain injection. These results suggest that the network mediating habituation of the PER involves PZ binding sites that are not necessarily present on the circuitry mediating olfactory conditioning of the PER.

LC-MS/MS Quantification Reveals Ample Gut Uptake and Metabolization of Dietary Phytochemicals in Honey Bees (Apis mellifera)

The honey bee pollen/nectar diet is rich in bioactive phytochemicals and recent studies have demonstrated the potential of phytochemicals to influence honey bee disease resistance. To unravel the role of dietary phytochemicals in honey bee health it is essential to understand phytochemical uptake, bioavailability, and metabolism but presently limited knowledge exists. With this study we aim to build a knowledge foundation. For 5 days, we continuously fed honey bees on eight individual phytochemicals and measured the concentrations in whole and dissected bees by HPLC-MS/MS. Ample phytochemical metabolization was observed, and only 6-30% of the consumed quantities were recovered. Clear differences in metabolization rates were evident, with atropine, aucubin, and triptolide displaying significantly slower metabolism. Phytochemical gut uptake was also demonstrated, and oral bioavailability was 4-31%, with the highest percentages observed for amygdalin, triptolide, and aucubin. We conclude that differences in the chemical properties and structure impact phytochemical uptake and metabolism.

Characterization of an A-Type Muscarinic Acetylcholine Receptor and Its Possible Non-neuronal Role in the Oriental Armyworm, Mythimna separata Walker (Lepidoptera: Noctuidae)

Muscarinic acetylcholine receptor (mAChR) regulates many neurophysiological functions in insects. In this report, a full-length cDNA encoding an A-type mAChR was cloned from the oriental armyworm, Mythimna separata. Pharmacological properties studies revealed that nanomolar to micromolar concentrations of carbachol or muscarine induced an increase of intracellular Ca²⁺ concentration ([Ca²⁺] _i ), with the EC₅₀ values of 124.6 and 388.1 nM, respectively. The increases of [Ca²⁺] _i can be greatly blocked by the antagonist atropine, with an IC₅₀ value of 0.09 nM. The receptor mRNA is expressed in all developmental stages, with great differential expression between male and female adults. The tissue expression analysis identified novel target tissues for this receptor, including ovaries and Malpighian tubules. The distribution of Ms A-type mAChR protein in the male brain may suggest the neurophysiological roles that are mediated by this receptor. However, the receptor protein was found to be distributed on the membranes of oocytes that are not innervated by neurons at all. These results indicate that Ms A-type mAChR selectively mediates intracellular Ca²⁺ mobilization. And the high level of receptor protein in the membrane of oocytes may indicate a possible non-neuronal role of A-type mAChR in the reproductive system of M. separata.

Cholinergic Agonists and Antagonists Have an Effect on the Metabolism of the Beetle Tenebrio Molitor

Synthetic insecticides are still widely used in plant protection. The main target for their action is the nervous system, in which the cholinergic system plays a vital role. Currently available insecticides have low selectivity and act on the cholinergic systems of invertebrates and vertebrates. Acetylcholine, a cholinergic system neurotransmitter, acts on cells by two types of receptors: nicotinic and muscarinic. In mammals, the role of muscarinic acetylcholine receptors (mAChRs) is quite well-known but in insects, is still not enough. Based on data indicating that the muscarinic cholinergic system strongly affects mammalian metabolism, we investigated if it similarly occurs in insects. We investigated the influence of agonists (acetylcholine, carbachol, and pilocarpine) and antagonists (tropane alkaloids: atropine and scopolamine) of mAChRs on the level of selected metabolites in Tenebrio molitor beetle trophic tissues. We analyzed the glycogen content in the fat body and midgut, the total free sugar concentration in the hemolymph and the lipid amount in the fat body. Moreover, we analyzed the levels of insulin-like peptides in the hemolymph. The tested compounds significantly influenced the mentioned parameters. They increased the glycogen content in the fat body and midgut but decreased the concentration of free sugars in the hemolymph. The observed effects were tissue-specific, and were also time- and dose-dependent. We used nonligated and neck-ligated larvae (to eliminate the influence of head factors on tissue metabolism) to determine whether the observed changes are the result of direct or indirect impacts on tissues. The obtained data suggest that the cholinergic system affects the fat body and midgut indirectly and directly and a pleiotropic role for mAChRs exists in the regulation of energy metabolism in insects. Moreover, tested compounds significantly affected the level of insulin-like peptides in hemolymph. Our studies for the first time showed that mAChRs are involved in regulation of insect metabolism of trophic tissues, and act on them directly and indirectly. Improved knowledge about insect cholinergic system may help in searching more selective and environment-friendly solutions in pest management.

Muscarinic ACh Receptors Contribute to Aversive Olfactory Learning in Drosophila

The most studied form of associative learning in Drosophila consists in pairing an odorant, the conditioned stimulus (CS), with an unconditioned stimulus (US). The timely arrival of the CS and US information to a specific Drosophila brain association region, the mushroom bodies (MB), can induce new olfactory memories. Thus, the MB is considered a coincidence detector. It has been shown that olfactory information is conveyed to the MB through cholinergic inputs that activate acetylcholine (ACh) receptors, while the US is encoded by biogenic amine (BA) systems. In recent years, we have advanced our understanding on the specific neural BA pathways and receptors involved in olfactory learning and memory. However, little information exists on the contribution of cholinergic receptors to this process. Here we evaluate for the first time the proposition that, as in mammals, muscarinic ACh receptors (mAChRs) contribute to memory formation in Drosophila. Our results show that pharmacological and genetic blockade of mAChRs in MB disrupts olfactory aversive memory in larvae. This effect is not explained by an alteration in the ability of animals to respond to odorants or to execute motor programs. These results show that mAChRs in MB contribute to generating olfactory memories in Drosophila.

Improved Cholinergic Transmission is Detrimental to Behavioural Plasticity in Honeybees (Apis mellifera)

Unravelling the role of neuromessenger processes in learning and memory has long interested researchers. We investigated the effects of an acetylcholinesterase blocker, Methyl Parathion (MeP), on honeybee learning. We used visual and olfactory tasks to test whether MeP had a detrimental effect on the acquisition of new knowledge when this new knowledge contradicts previously acquired one. Our results indicate that treatment with MeP prior to conditioning was significantly detrimental to the acquisition of incongruous (but not irrelevant or congruous) new knowledge due to improved recall. The neurobiological and ecotoxicological consequences of these results are discussed.

Rho GTPase activity in the honey bee mushroom bodies is correlated with age and foraging experience

Foraging experience is correlated with structural plasticity of the mushroom bodies of the honey bee brain. While several neurotransmitter and intracellular signaling pathways have been previously implicated as mediators of these structural changes, none interact directly with the cytoskeleton, the ultimate effector of changes in neuronal morphology. The Rho family of GTPases are small, monomeric G proteins that, when activated, initiate a signaling cascade that reorganizes the neuronal cytoskeleton. In this study, we measured activity of two members of the Rho family of GTPases, Rac and RhoA, in the mushroom bodies of bees with different durations of foraging experience. A transient increase in Rac activity coupled with a transient decrease in RhoA activity was found in honey bees with 4 days foraging experience compared with same-aged new foragers. These observations are in accord with previous reports based on studies of other species of a growth supporting role for Rac and a growth opposing role for RhoA. This is the first report of Rho GTPase activation in the honey bee brain.

Herbal extracts and phytochemicals: plant secondary metabolites and the enhancement of human brain function

Humans consume a wide range of foods, drugs, and dietary supplements that are derived from plants and which modify the functioning of the central nervous sytem (CNS). The psychoactive properties of these substances are attributable to the presence of plant secondary metabolites, chemicals that are not required for the immediate survival of the plant but which are synthesized to increase the fitness of the plant to survive by allowing it to interact with its environment, including pathogens and herbivorous and symbiotic insects. In many cases, the effects of these phytochemicals on the human CNS might be linked either to their ecological roles in the life of the plant or to molecular and biochemical similarities in the biology of plants and higher animals. This review assesses the current evidence for the efficacy of a range of readily available plant-based extracts and chemicals that may improve brain function and which have attracted sufficient research in this regard to reach a conclusion as to their potential effectiveness as nootropics. Many of these candidate phytochemicals/extracts can be grouped by the chemical nature of their potentially active secondary metabolite constituents into alkaloids (caffeine, nicotine), terpenes (ginkgo, ginseng, valerian, Melissa officinalis, sage), and phenolic compounds (curcumin, resveratrol, epigallocatechin-3-gallate, Hypericum perforatum, soy isoflavones). They are discussed in terms of how an increased understanding of the relationship between their ecological roles and CNS effects might further the field of natural, phytochemical drug discovery.

Modulatory action of acetylcholine on the Na+-dependent action potentials in Kenyon cells isolated from the mushroom body of the cricket brain

Kenyon cells, intrinsic neurons of the insect mushroom body, have been assumed to be a site of conditioning stimulus (CS) and unconditioned stimulus (US) association in olfactory learning and memory. Acetylcholine (ACh) has been implicated to be a neurotransmitter mediating CS reception in Kenyon cells, causing rapid membrane depolarization via nicotinic ACh receptors. However, the long-term effects of ACh on the membrane excitability of Kenyon cells are not fully understood. In this study, we examined the effects of ACh on Na(+) dependent action potentials (Na(+) spikes) elicited by depolarizing current injection and on net membrane currents under the voltage clamp condition in Kenyon cells isolated from the mushroom body of the cricket Gryllus bimaculatus. Current-clamp studies using amphotericin B perforated-patch recordings showed that freshly dispersed cricket Kenyon cells could produce repetitive Na(+) spikes in response to prolonged depolarizing current injection. Bath application of ACh increased both the instantaneous frequency and the amplitudes of Na(+) spikes. This excitatory action of ACh on Kenyon cells is attenuated by the pre-treatment of the cells with the muscarinic receptor antagonists, atropine and scopolamine, but not by the nicotinic receptor antagonist mecamylamine. Voltage-clamp studies further showed that bath application of ACh caused an increase in net inward currents that are sensitive to TTX, whereas outward currents were decreased by this treatment. These results indicate that in order to mediate CS, ACh may modulate the firing properties of Na(+) spikes of Kenyon cells through muscarinic receptor activation, thus increasing Na conductance and decreasing K conductance.

Enhanced cholinergic transmission promotes recall in honeybees

The involvement of the cholinergic system in learning and memory in honeybees has been well established using olfactory conditioning. We examined the effect of Methyl Parathion (MeP), an acetylcholinesterase inhibitor of the organo-phosphate family, on the learning and recall of visual and olfactory discrimination tasks in honeybees. One of our expectations was to observe the effects induced by both the nicotinic and muscarinic systems, as the blocking of acetylcholinesterase should induce an increase in the activity of both systems. We were also interested in knowing whether the type of tasks could influence the results. The visual tasks involved learning to discriminate the orientation of gratings in a Y-maze; the olfactory task involved learning to discriminate odours in a proboscis extension reflex (PER) paradigm. The results indicate that MeP treatment enhances recall of learned tasks in the visual and olfactory domains, but it does not affect the acquisition phase in either domain. Surprisingly, MeP treatment led to muscarinic-like effects but failed to mimic the nicotinic-like effects already described in relation to learning phases in honeybees. Implications for the role of cholinergic pathways in learning and memory and the nature of their involvement are discussed, and a hypothesis relating to the organisation of the cholinergic system and the relationship between the nicotinic and muscarinic systems in honeybees is proposed. The results are also discussed in terms of their ecotoxicological consequences.

Pilocarpine improves recognition of nestmates in young honey bees

Honey bees can distinguish nestmates from non-nestmates, directing aggressive responses toward non-nestmates and rarely attacking nestmates. Here we provide evidence that treatment with pilocarpine, a muscarinic agonist, significantly reduced the number of aggressive responses directed toward nestmates. By contrast, treatment with scopolamine, a muscarinic antagonist, significantly increased attacks on nestmates. Locomotor activity was not altered by these pharmacological treatments. When interpreted in light of known cholinergic pathways in the insect brain, our results provide the first evidence that cholinergic signaling via muscarinic receptors plays a role in olfaction-based social behavior in honey bees.

Stimulation of muscarinic receptors mimics experience-dependent plasticity in the honey bee brain

Honey bees begin life working in the hive. At approximately 3 weeks of age, they shift to visiting flowers to forage for pollen and nectar. Foraging is a complex task associated with enlargement of the mushroom bodies, a brain region important in insects for certain forms of learning and memory. We report here that foraging bees had a larger volume of mushroom body neuropil than did age-matched bees confined to the hive. This result indicates that direct experience of the world outside the hive causes mushroom body neuropil growth in bees. We also show that oral treatment of caged bees with pilocarpine, a muscarinic agonist, induced an increase in the volume of the neuropil similar to that seen after a week of foraging experience. Effects of pilocarpine were blocked by scopolamine, a muscarinic antagonist. Our results suggest that signaling in cholinergic pathways couples experience to structural brain plasticity.

Antennal tactile learning in the honeybee: Effect of nicotinic antagonists on memory dynamics

Restrained worker honeybees (Apis mellifera L.) are able to learn to associate antennal-scanning of a metal plate with a sucrose reinforcement delivered to the mouthparts. Learning occurs reliably in a single association of the two sensory stimuli. The involvement of nicotinic pathways in memory formation and retrieval processes was tested by injecting, into the whole brain through the median ocellus, either mecamylamine (0.6 microg per bee) or alpha-bungarotoxin (2.4 ng per bee). Saline served as a control. Mecamylamine injected 10 min before the retrieval test impairs the retention level tested 3 h and 24 h after single- or multi-trial learning. Retrieval tests performed at various times after the injection show that the blocking effect of mecamylamine lasts about 1 h. The drug has no effect on the reconsolidation or extinction processes. Mecamylamine injected 10 min before conditioning impairs single-trial learning but has no effect on five-trial learning and on the consolidation process. By contrast, alpha-bungarotoxin only impairs the formation of long-term memory (24 h) induced by the five-trial learning and has no effect on medium-term memory (3 h), on single-trial learning or on the retrieval process. Hence, owing to previous data, at least two kinds of nicotinic receptors seem to be involved in honeybee memory, an alpha-bungarotoxin-sensitive and an alpha-bungarotoxin-insensitive receptor. Our results extend to antennal mechanosensory conditioning the role of the cholinergic system that we had previously described for olfactory conditioning in the honeybee. Moreover, we describe here in this insect a pharmacological dissociation between alpha-bungarotoxin sensitive long-term memory and alpha-bungarotoxin insensitive medium-term memory, the last one being affected by mecamylamine.

Identification and localization of the nicotinic acetylcholine receptor alpha3 mRNA in the brain of the honeybee, Apis mellifera

The nicotinic acetylcholine receptors are ligand-gated ion channels responsible for rapid neurotransmission and are target sites for pesticides in insects. In the honeybee Apis mellifera, pharmacological and electrophysiological studies have shown that different nicotinic acetylcholine receptor subtypes may exist in the brain. Here, we have identified a honeybee cDNA that encodes a 537 amino acid protein with features typical of nicotinic acetylcholine receptor alpha subunit, and sequence homology to human alpha3. In situ hybridization on cryosections shows that the Apisalpha3 mRNA is differently expressed in larvae and adult. In larvae, Apisalpha3 mRNA expression is restricted to the suboesophageal ganglia. In adult, it is further expressed in the optic lobes, the dorsal lobes, the antennal lobes and the calyces of mushroom bodies. Together our results suggest that Apisalpha3 shows a controlled expression pattern during development.

Regional brain variations of cytochrome oxidase staining during olfactory learning in the honeybee (Apis mellifera)

Regional brain variations of cytochrome oxidase (CO) staining were analyzed in the honeybee (Apis mellifera) after olfactory conditioning of the proboscis extension reflex. Identification of brain sites where stimuli converge was done by precise image analysis performed in antennal lobes (AL) and mushroom bodies (MB). In Experiment 1, bees received 5 odorant stimulations that induced a transient decrease of CO activity in the lateral part of the AL. In Experiment 2, bees were trained with 5-trial olfactory conditioning. CO activity transiently increased in the lips of the MB calyces. There was also a delayed increase in the lateral part of the AL. An olfactory stimulus presented alone and an odor paired to a sucrose stimulation are treated by different pathways, including both AL and MB.

Effects of imidacloprid metabolites on habituation in honeybees suggest the existence of two subtypes of nicotinic receptors differentially expressed during adult development

Habituation of the proboscis extension reflex (PER) in honeybees (Apis mellifera) is age-dependent. Very young bees (< or =7 days old) require significantly less trials to abolish the response to multiple sucrose stimulations than older bees (> or =8 days old). A nicotinic agonist, imidacloprid, modifies this behaviour by increasing the number of trials in < or =7-day-old bees and by decreasing it in older bees [Neurobiol. Learn. Mem. 76 (2001) 183.]. Here we tested our hypothesis that this effect is associated with a differential expression of two subtypes of nicotinic acetylcholine receptors (nAChRs). By testing the effects of six metabolites of imidacloprid, we show that two of them, olefin and 5-hydroxy-imidacloprid, modify the number of trials needed to habituate the PER in a contrasting manner. Olefin increases the number of trials in both age groups, whereas 5-hydroxy-imidacloprid decreases the number of trials, but only in 8-day-old individuals. We conclude that olefin and 5-hydroxy-imidacloprid are specific agonists of two subtypes of an nAChR that are differentially expressed during adult maturation of young honeybees. Olefin is the agonist of an nAChR expressed in both age groups, whereas 5-hydroxy-imidacloprid is the agonist of a late-onset nAChR that is activated in 8-day-old bees. The implications of this finding for the honeybee biology are discussed.

Contrasting effects of imidacloprid on habituation in 7- and 8-day-old honeybees (Apis mellifera)

We examined the effects of sublethal doses (0.1, 1, and 10 ng per animal) of a new neonicotinoid insecticide, Imidacloprid, on habituation of the proboscis extension reflex (PER) in honeybees (Apis mellifera) reared under laboratory conditions. In untreated honeybees, the habituation of the proboscis extension reflex is age-dependent and there is a significant increase in the number of trials required for habituation in older bees (8-10 days old) as compared to very young bees (4-7 days old). Imidacloprid alters the number of trials needed to habituate the honeybee response to multiple sucrose stimulation. In 7-day-old bees, treatment with Imidacloprid leads to an increase in the number of trials necessary to abolish the response, whereas in 8-day-old bees, it leads to a reduction in the number of trials for habituation (15 min and 1 h after treatment), and to an increase 4 h after treatment. The temporal effects of Imidacloprid in both 7- and 8-day-old bees suggest that 4h after treatment the observed effects are due to a metabolite of Imidacloprid, rather than to Imidacloprid itself. Our results suggest the existence of two distinct subtypes of nicotinic receptors in the honeybee that have different affinities to Imidacloprid and are differentially expressed in 7- and 8-day-old individuals.

Nicotinic acetylcholine receptor ligands differently affect cytochrome oxidase in the Honeybee brain

The objective of this study was to determine if nicotinic receptor antagonists known for their ability to impair memory in the honeybee could induce changes in brain metabolism. We tested the effect of antagonists [hexamethonium, mecamylamine, alpha-bungarotoxin (alpha-BTX)] and agonist (nicotine) brain injections on cytochrome oxidase (CO) histochemistry. Within as little as 30 min following nicotine injection, an increase of the staining was observed in almost all the structures analyzed. The increase was limited to the alpha-lobe after alpha-BTX injection. In contrast, the antagonists hexamethonium and mecamylamine reduced CO staining in this structure that seems to be involved in information retrieval. These results suggest that the decrease of metabolism in the alpha-lobe obtained with hexamethonium and mecamylamine injections could be related to the impairment of retrieval processes previously observed with these drugs.

Functional cytochrome oxidase histochemistry in the honeybee brain

The variations of neural metabolism induced by surgical and chemical treatments were studied in the honeybee brain by the means of cytochrome oxidase (CO) histochemistry. CO staining is considerably reduced in the alpha-lobe by antennal input deprivation. Chemical stimulation (50 mM K(+)) was linked to an increase of CO staining in antennal lobes (AL) and to a decrease in the basal ring of calyces (Cal). Application of the nicotinic ligand imidacloprid (10(-4) M) resulted in increased CO labelling within 30 min in all the structures analysed. Treatment with a lower concentration (10(-8) M) resulted in reduced staining in Cal and central body (CB). We conclude that CO histochemistry can be used to identify the target structures of cholinergic ligands in the honeybee brain.

Histochemistry of classical neurotransmitters in antennal lobes and mushroom bodies of the honeybee

This paper summarizes histochemical and immunocytochemical investigations of cholinergic, GABAergic, and glutamatergic pathways in the central brain and suboesophageal ganglion of the honeybee. Acetylcholinesterase histochemistry, immunocytochemical staining for nicotinic acetylcholine receptors, and mapping for alpha-bungarotoxin binding sites indicate cholinergic synaptic interactions in the antennal lobe and a cholinergic pathway via a subset of olfactory projection neurons into the mushroom bodies. Calcium imaging experiments in cell cultures prepared from mushroom bodies demonstrate the expression of nicotinic cholinergic receptors on Kenyon cells. Neurons synthesizing GABA and glutamate are stained with well-defined polyclonal antisera against the amino acids. GABA-immunoreactivity is mainly localized in local interneurons of the antennal lobe and in extrinsic neurons innervating the mushroom bodies. High levels of glutamate-immunoreactivity are found in motoneurons of the suboesophageal ganglion, the dorsal lobe, and also in interneurons. A subgroup of the Kenyon cells shows distinct but weaker levels of glutamate-immunoreactivity. The detailed knowledge about the chemical neuroanatomy of the bee provides a framework for behavioral pharmacological approaches, which implicate the involvement of cholinergic mechanisms in olfactory learning and GABAergic mechanisms in odor discrimination.

A new attempt to assess the effect of learning processes on the cholinergic system: studies on fruitflies and honeybees

The effect of training on the functioning of the cholinergic system was investigated in fruitflies and in honeybees. Drosophila were submitted to a passive avoidance conditioning of the proboscis extension response (PER). Flies had to learn to suppress the sugar-induced PER to avoid an aversive quinine reinforcement. In a yoked control group, the punishment was administered with no relation to the response displayed. Honeybees underwent a five-trial olfactory conditioning of the PER elicited by an antennal gustatory stimulation. In the control group, olfactory and gustatory stimulations were unpaired to prevent a learning process from developing. Immediately at the end of the learning session, acetylcholinesterase (AChE) activity was individually measured on the whole animal for Drosophila and on the head for the honeybee in experimental and in control groups. In fruitflies and honeybees, the AChE rate did not differ between the experimental group and its respective control group. Moreover, no significant correlation could be found individually between the learning performance and the AChE rate in either Drosophila or in honeybees. This experiment did not reveal any modulatory effect of the learning acquisition level on the AChE activity in insects as was previously reported in honeybees.