Maggot learning and Synapsin function

Cognitive limits of larval Drosophila: testing for conditioned inhibition, sensory preconditioning, and second-order conditioning

Drosophila larvae are an established model system for studying the mechanisms of innate and simple forms of learned behavior. They have about 10 times fewer neurons than adult flies, and it was the low total number of their neurons that allowed for an electron microscopic reconstruction of their brain at synaptic resolution. Regarding the mushroom body, a central brain structure for many forms of associative learning in insects, it turned out that more than half of the classes of synaptic connection had previously escaped attention. Understanding the function of these circuit motifs, subsequently confirmed in adult flies, is an important current research topic. In this context, we test larval Drosophila for their cognitive abilities in three tasks that are characteristically more complex than those previously studied. Our data provide evidence for (i) conditioned inhibition, as has previously been reported for adult flies and honeybees. Unlike what is described for adult flies and honeybees, however, our data do not provide evidence for (ii) sensory preconditioning or (iii) second-order conditioning in Drosophila larvae. We discuss the methodological features of our experiments as well as four specific aspects of the organization of the larval brain that may explain why these two forms of learning are observed in adult flies and honeybees, but not in larval Drosophila.

FTD-associated mutations in Tau result in a combination of dominant and recessive phenotypes

Although mutations in the microtubules-associated protein Tau have long been connected with several neurodegenerative diseases, the underlying molecular mechanisms causing these tauopathies are still not fully understood. Studies in various models suggested that dominant gain-of-function effects underlie the pathogenicity of these mutants; however, there is also evidence that the loss of normal physiological functions of Tau plays a role in tauopathies. Previous studies on Tau in Drosophila involved expressing the human Tau protein in the background of the endogenous Tau gene in addition to inducing high expression levels. To study Tau pathology in more physiological conditions, we recently created Drosophila knock-in models that express either wildtype human Tau (hTau^WT) or disease-associated mutant hTau (hTau^V337M and hTau^K369I) in place of the endogenous Drosophila Tau (dTau). Analyzing these flies as homozygotes, we could therefore detect recessive effects of the mutations while identifying dominant effects in heterozygotes. Using memory, locomotion and sleep assays, we found that homozygous mutant hTau flies showed deficits already when quite young whereas in heterozygous flies, disease phenotypes developed with aging. Homozygotes also revealed an increase in microtubule diameter, suggesting that changes in the cytoskeleton underlie the axonal degeneration we observed in these flies. In contrast, heterozygous mutant hTau flies showed abnormal axonal targeting and no detectable changes in microtubules. However, we previously showed that heterozygosity for hTau^V337M interfered with synaptic homeostasis in central pacemaker neurons and we now show that heterozygous hTau^K369I flies have decreased levels of proteins involved in the release of synaptic vesicles. Taken together, our results demonstrate that both mutations induce a combination of dominant and recessive disease-related phenotypes that provide behavioral and molecular insights into the etiology of Tauopathies.

Associative learning in larval and adult Drosophila is impaired by the dopamine-synthesis inhibitor 3-Iodo-L-tyrosine

Across the animal kingdom, dopamine plays a crucial role in conferring reinforcement signals that teach animals about the causal structure of the world. In the fruit fly Drosophila melanogaster, dopaminergic reinforcement has largely been studied using genetics, whereas pharmacological approaches have received less attention. Here, we apply the dopamine-synthesis inhibitor 3-Iodo-L-tyrosine (3IY), which causes acute systemic inhibition of dopamine signaling, and investigate its effects on Pavlovian conditioning. We find that 3IY feeding impairs sugar-reward learning in larvae while leaving task-relevant behavioral faculties intact, and that additional feeding of a precursor of dopamine (L-3,4-dihydroxyphenylalanine, L-DOPA), rescues this impairment. Concerning a different developmental stage and for the aversive valence domain. Moreover, we demonstrate that punishment learning by activating the dopaminergic neuron PPL1-γ1pedc in adult flies is also impaired by 3IY feeding, and can likewise be rescued by L-DOPA. Our findings exemplify the advantages of using a pharmacological approach in combination with the genetic techniques available in D. melanogaster to manipulate neuronal and behavioral function.

Summary: We surveyed the effects of a dopamine-synthesis inhibitor on associative learning in larval and adult Drosophila. This approach can supplement genetic tools in investigating the conserved reinforcing function of dopamine.

Ethanol-guided behavior in Drosophila larvae

Chemosensory signals allow vertebrates and invertebrates not only to orient in its environment toward energy-rich food sources to maintain nutrition but also to avoid unpleasant or even poisonous substrates. Ethanol is a substance found in the natural environment of Drosophila melanogaster. Accordingly, D. melanogaster has evolved specific sensory systems, physiological adaptations, and associated behaviors at its larval and adult stage to perceive and process ethanol. To systematically analyze how D. melanogaster larvae respond to naturally occurring ethanol, we examined ethanol-induced behavior in great detail by reevaluating existing approaches and comparing them with new experiments. Using behavioral assays, we confirm that larvae are attracted to different concentrations of ethanol in their environment. This behavior is controlled by olfactory and other environmental cues. It is independent of previous exposure to ethanol in their food. Moreover, moderate, naturally occurring ethanol concentration of 4% results in increased larval fitness. On the contrary, higher concentrations of 10% and 20% ethanol, which rarely or never appear in nature, increase larval mortality. Finally, ethanol also serves as a positive teaching signal in learning and memory and updates valence associated with simultaneously processed odor information. Since information on how larvae perceive and process ethanol at the genetic and neuronal level is limited, the establishment of standardized assays described here is an important step towards their discovery.

Predictive olfactory learning in Drosophila

Olfactory learning and conditioning in the fruit fly is typically modelled by correlation-based associative synaptic plasticity. It was shown that the conditioning of an odor-evoked response by a shock depends on the connections from Kenyon cells (KC) to mushroom body output neurons (MBONs). Although on the behavioral level conditioning is recognized to be predictive, it remains unclear how MBONs form predictions of aversive or appetitive values (valences) of odors on the circuit level. We present behavioral experiments that are not well explained by associative plasticity between conditioned and unconditioned stimuli, and we suggest two alternative models for how predictions can be formed. In error-driven predictive plasticity, dopaminergic neurons (DANs) represent the error between the predictive odor value and the shock strength. In target-driven predictive plasticity, the DANs represent the target for the predictive MBON activity. Predictive plasticity in KC-to-MBON synapses can also explain trace-conditioning, the valence-dependent sign switch in plasticity, and the observed novelty-familiarity representation. The model offers a framework to dissect MBON circuits and interpret DAN activity during olfactory learning.

Daughterless, the Drosophila orthologue of TCF4, is required for associative learning and maintenance of the synaptic proteome

Mammalian transcription factor 4 (TCF4) has been linked to schizophrenia and intellectual disabilities, such as Pitt-Hopkins syndrome (PTHS). Here, we show that similarly to mammalian TCF4, fruit fly orthologue Daughterless (Da) is expressed widely in the Drosophila brain. Furthermore, silencing of da, using several central nervous system-specific Gal4 driver lines, impairs appetitive associative learning of the larvae and leads to decreased levels of the synaptic proteins Synapsin (Syn) and Discs large 1 (Dlg1), suggesting the involvement of Da in memory formation. Here, we demonstrate that Syn and dlg1 are direct target genes of Da in adult Drosophila heads, as Da binds to the regulatory regions of these genes and the modulation of Da levels alter the levels of Syn and dlg1 mRNA. Silencing of da also affects negative geotaxis of the adult flies, suggesting the impairment of locomotor function. Overall, our findings suggest that Da regulates Drosophila larval memory and adult negative geotaxis, possibly via its synaptic target genes Syn and dlg1 These behavioural phenotypes can be further used as a PTHS model to screen for therapeutics.This article has an associated First Person interview with the first author of the paper.

Food restriction reconfigures naïve and learned choice behavior in Drosophila larvae

In many animals, the establishment and expression of food-related memory is limited by the presence of food and promoted by its absence, implying that this behavior is driven by motivation. In the past, this has already been demonstrated in various insects including honeybees and adult Drosophila. For Drosophila larvae, which are characterized by an immense growth and the resulting need for constant food intake, however, knowledge is rather limited. Accordingly, we have analyzed whether starvation modulates larval memory formation or expression after appetitive classical olfactory conditioning, in which an odor is associated with a sugar reward. We show that odor-sugar memory of starved larvae lasts longer than in fed larvae, although the initial performance is comparable. 80 minutes after odor fructose conditioning, only starved but not fed larvae show a reliable odor-fructose memory. This is likely due to a specific increase in the stability of anesthesia-resistant memory (ARM). Furthermore, we observe that starved larvae, in contrast to fed ones, prefer sugars that offer a nutritional benefit in addition to their sweetness. Taken together our work shows that Drosophila larvae adjust the expression of learned and naïve choice behaviors in the absence of food. These effects are only short-lasting probably due to their lifestyle and their higher internal motivation to feed. In the future, the extensive use of established genetic tools will allow us to identify development-specific differences arising at the neuronal and molecular level.

Reversal learning in Drosophila larvae

Adjusting behavior to changed environmental contingencies is critical for survival, and reversal learning provides an experimental handle on such cognitive flexibility. Here, we investigate reversal learning in larval Drosophila Using odor-taste associations, we establish olfactory reversal learning in the appetitive and the aversive domain, using either fructose as a reward or high-concentration sodium chloride as a punishment, respectively. Reversal learning is demonstrated both in differential and in absolute conditioning, in either valence domain. In differential conditioning, the animals are first trained such that an odor A is paired, for example, with the reward whereas odor B is not (A+/B); this is followed by a second training phase with reversed contingencies (A/B+). In absolute conditioning, odor B is omitted, such that the animals are first trained with paired presentations of A and reward, followed by unpaired training in the second training phase. Our results reveal "true" reversal learning in that the opposite associative effects of both the first and the second training phase are detectable after reversed-contingency training. In what is a surprisingly quick, one-trial contingency adjustment in the Drosophila larva, the present study establishes a simple and genetically easy accessible study case of cognitive flexibility.

Modulations of microbehaviour by associative memory strength in Drosophila larvae

Finding food is a vital skill and a constant task for any animal, and associative learning of food-predicting cues gives an advantage in this daily struggle. The strength of the associations between cues and food depends on a number of parameters, such as the salience of the cue, the strength of the food reward and the number of joint cue-food experiences. We investigate what impact the strength of an associative odour-sugar memory has on the microbehaviour of Drosophila melanogaster larvae. We find that larvae form stronger memories with increasing concentrations of sugar or odour, and that these stronger memories manifest themselves in stronger modulations of two aspects of larval microbehaviour, the rate and the direction of lateral reorientation manoeuvres (so-called head casts). These two modulations of larval behaviour are found to be correlated to each other in every experiment performed, which is in line with a model that assumes that both modulations are controlled by a common motor output. Given that the Drosophila larva is a genetically tractable model organism that is well suited to the study of simple circuits at the single-cell level, these analyses can guide future research into the neuronal circuits underlying the translation of associative memories of different strength into behaviour, and may help to understand how these processes are organised in more complex systems.

Implications of the Sap47 null mutation for synapsin phosphorylation, longevity, climbing proficiency and behavioural plasticity in adult Drosophila

The Sap47 gene of Drosophila melanogaster encodes a highly abundant 47 kDa synaptic vesicle-associated protein. Sap47 null mutants show defects in synaptic plasticity and larval olfactory associative learning but the molecular function of Sap47 at the synapse is unknown. We demonstrate that Sap47 modulates the phosphorylation of another highly abundant conserved presynaptic protein, synapsin. Site-specific phosphorylation of Drosophila synapsin has repeatedly been shown to be important for behavioural plasticity but it was not known where these phospho-synapsin isoforms are localized in the brain. Here, we report the distribution of serine-6-phosphorylated synapsin in the adult brain and show that it is highly enriched in rings of synapses in the ellipsoid body and in large synapses near the lateral triangle. The effects of knockout of Sap47 or synapsin on olfactory associative learning/memory support the hypothesis that both proteins operate in the same molecular pathway. We therefore asked if this might also be true for other aspects of their function. We show that knockout of Sap47 but not synapsin reduces lifespan, whereas knockout of Sap47 and synapsin, either individually or together, affects climbing proficiency, as well as plasticity in circadian rhythms and sleep. Furthermore, electrophysiological assessment of synaptic properties at the larval neuromuscular junction (NMJ) reveals increased spontaneous synaptic vesicle fusion and reduced paired pulse facilitation in Sap47 and synapsin single and double mutants. Our results imply that Sap47 and synapsin cooperate non-uniformly in the control of synaptic properties in different behaviourally relevant neuronal networks of the fruitfly.

One-trial learning in larval Drosophila

Animals of many species are capable of “small data” learning, that is, of learning without repetition. Here we introduce larval Drosophila melanogaster as a relatively simple study case for such one-trial learning. Using odor-food associative conditioning, we first show that a sugar that is both sweet and nutritious (fructose) and sugars that are only sweet (arabinose) or only nutritious (sorbitol) all support appetitive one-trial learning. The same is the case for the optogenetic activation of a subset of dopaminergic neurons innervating the mushroom body, the memory center of the insects. In contrast, no one-trial learning is observed for an amino acid reward (aspartic acid). As regards the aversive domain, one-trial learning is demonstrated for high-concentration sodium chloride, but is not observed for a bitter tastant (quinine). Second, we provide follow-up, parametric analyses of odor-fructose learning. Specifically, we ascertain its dependency on the number and duration of training trials, the requirements for the behavioral expression of one-trial odor-fructose memory, its temporal stability, and the feasibility of one-trial differential conditioning. Our results set the stage for a neurogenetic analysis of one-trial learning and define the requirements for modeling mnemonic processes in the larva.

Exposure to thiamethoxam during the larval phase affects synapsin levels in the brain of the honey bee

Thiamethoxam (TMX) is a neurotoxic insecticide widely used for insect pest control. TMX and other neonicotinoids are reported to be potential causes of honey bee decline. Due to its systematic action, TMX may be recovered in pollen, bee bread, nectar, and honey, which make bees likely to be exposed to contaminated diet. In this study, we used immunolabeling to demonstrate that sublethal concentrations of TMX decrease the protein levels of synapsin in the mushroom bodies (MBs) and the antennal lobes (ALs) of pupae and newly emerged worker bees that were exposed through the food to TMX during the larval phase. A decrease in the synapsin level was observed in the MBs of pupae previously exposed to 0.001 and 1.44 ng/µL and in newly emerged bees previously exposed to 1.44 ng/µL and no changes were observed in the optical lobes (OLs). In the ALs, the decrease was observed in pupae and newly emerged bees exposed to 1.44 ng/µL. Because the MBs and ALs are brain structures involved in stimuli reception, learning, and memory consolidation and because synapsin is important for the regulation of neurotransmitter release, we hypothesize that exposure to sublethal concentrations of TMX during the larval stage may cause neurophysiological disorders in honey bees.

Connectomics and function of a memory network: the mushroom body of larval Drosophila

The Drosophila larva is a relatively simple, 10 000-neuron study case for learning and memory with enticing analytical power, combining genetic tractability, the availability of robust behavioral assays, the opportunity for single-cell transgenic manipulation, and an emerging synaptic connectome of its complete central nervous system. Indeed, although the insect mushroom body is a much-studied memory network, the connectome revealed that more than half of the classes of connection within the mushroom body had escaped attention. The connectome also revealed circuitry that integrates, both within and across brain hemispheres, higher-order sensory input, intersecting valence signals, and output neurons that instruct behavior. Further, it was found that activating individual dopaminergic mushroom body input neurons can have a rewarding or a punishing effect on olfactory stimuli associated with it, depending on the relative timing of this activation, and that larvae form molecularly dissociable short-term, long-term, and amnesia-resistant memories. Together, the larval mushroom body is a suitable study case to achieve a nuanced account of molecular function in a behaviorally meaningful memory network.

Memory enhancement by ferulic acid ester across species

Cognitive impairments can be devastating for quality of life, and thus, preventing or counteracting them is of great value. To this end, the present study exploits the potential of the plant Rhodiola rosea and identifies the constituent ferulic acid eicosyl ester [icosyl-(2E)-3-(4-hydroxy-3-methoxyphenyl)-prop-2-enoate (FAE-20)] as a memory enhancer. We show that food supplementation with dried root material from R. rosea dose-dependently improves odor-taste reward associative memory scores in larval Drosophila and prevents the age-related decline of this appetitive memory in adult flies. Task-relevant sensorimotor faculties remain unaltered. From a parallel approach, a list of candidate compounds has been derived, including R. rosea-derived FAE-20. Here, we show that both R. rosea-derived FAE-20 and synthetic FAE-20 are effective as memory enhancers in larval Drosophila. Synthetic FAE-20 also partially compensates for age-related memory decline in adult flies, as well as genetically induced early-onset loss of memory function in young flies. Furthermore, it increases excitability in mouse hippocampal CA1 neurons, leads to more stable context-shock aversive associative memory in young adult (3-month-old) mice, and increases memory scores in old (>2-year-old) mice. Given these effects, and given the utility of R. rosea-the plant from which we discovered FAE-20-as a memory enhancer, these results may hold potential for clinical applications.

Associative Learning of Stimuli Paired and Unpaired With Reinforcement: Evaluating Evidence From Maggots, Flies, Bees, and Rats

Finding rewards and avoiding punishments are powerful goals of behavior. To maximize reward and minimize punishment, it is beneficial to learn about the stimuli that predict their occurrence, and decades of research have provided insight into the brain processes underlying such associative reinforcement learning. In addition, it is well known in experimental psychology, yet often unacknowledged in neighboring scientific disciplines, that subjects also learn about the stimuli that predict the absence of reinforcement. Here we evaluate evidence for both these learning processes. We focus on two study cases that both provide a baseline level of behavior against which the effects of associative learning can be assessed. Firstly, we report pertinent evidence from Drosophila larvae. A re-analysis of the literature reveals that through paired presentations of an odor A and a sugar reward (A+) the animals learn that the reward can be found where the odor is, and therefore show an above-baseline preference for the odor. In contrast, through unpaired training (A/+) the animals learn that the reward can be found precisely where the odor is not, and accordingly these larvae show a below-baseline preference for it (the same is the case, with inverted signs, for learning through taste punishment). In addition, we present previously unpublished data demonstrating that also during a two-odor, differential conditioning protocol (A+/B) both these learning processes take place in larvae, i.e., learning about both the rewarded stimulus A and the non-rewarded stimulus B (again, this is likewise the case for differential conditioning with taste punishment). Secondly, after briefly discussing published evidence from adult Drosophila, honeybees, and rats, we report an unpublished data set showing that relative to baseline behavior after truly random presentations of a visual stimulus A and punishment, rats exhibit memories of opposite valence upon paired and unpaired training. Collectively, the evidence conforms to classical findings in experimental psychology and suggests that across species animals associatively learn both through paired and through unpaired presentations of stimuli with reinforcement – with opposite valence. While the brain mechanisms of unpaired learning for the most part still need to be uncovered, the immediate implication is that using unpaired procedures as a mnemonically neutral control for associative reinforcement learning may be leading analyses astray.

Nociceptive Biology of Molluscs and Arthropods: Evolutionary Clues About Functions and Mechanisms Potentially Related to Pain

Important insights into the selection pressures and core molecular modules contributing to the evolution of pain-related processes have come from studies of nociceptive systems in several molluscan and arthropod species. These phyla, and the chordates that include humans, last shared a common ancestor approximately 550 million years ago. Since then, animals in these phyla have continued to be subject to traumatic injury, often from predators, which has led to similar adaptive behaviors (e.g., withdrawal, escape, recuperative behavior) and physiological responses to injury in each group. Comparisons across these taxa provide clues about the contributions of convergent evolution and of conservation of ancient adaptive mechanisms to general nociceptive and pain-related functions. Primary nociceptors have been investigated extensively in a few molluscan and arthropod species, with studies of long-lasting nociceptive sensitization in the gastropod, Aplysia, and the insect, Drosophila, being especially fruitful. In Aplysia, nociceptive sensitization has been investigated as a model for aversive memory and for hyperalgesia. Neuromodulator-induced, activity-dependent, and axotomy-induced plasticity mechanisms have been defined in synapses, cell bodies, and axons of Aplysia primary nociceptors. Studies of nociceptive sensitization in Drosophila larvae have revealed numerous molecular contributors in primary nociceptors and interacting cells. Interestingly, molecular contributors examined thus far in Aplysia and Drosophila are largely different, but both sets overlap extensively with those in mammalian pain-related pathways. In contrast to results from Aplysia and Drosophila, nociceptive sensitization examined in moth larvae (Manduca) disclosed central hyperactivity but no obvious peripheral sensitization of nociceptive responses. Squid (Doryteuthis) show injury-induced sensitization manifested as behavioral hypersensitivity to tactile and especially visual stimuli, and as hypersensitivity and spontaneous activity in nociceptor terminals. Temporary blockade of nociceptor activity during injury subsequently increased mortality when injured squid were exposed to fish predators, providing the first demonstration in any animal of the adaptiveness of nociceptive sensitization. Immediate responses to noxious stimulation and nociceptive sensitization have also been examined behaviorally and physiologically in a snail (Helix), octopus (Adopus), crayfish (Astacus), hermit crab (Pagurus), and shore crab (Hemigrapsus). Molluscs and arthropods have systems that suppress nociceptive responses, but whether opioid systems play antinociceptive roles in these phyla is uncertain.

Maggot Instructor: Semi-Automated Analysis of Learning and Memory in Drosophila Larvae

For several decades, Drosophila has been widely used as a suitable model organism to study the fundamental processes of associative olfactory learning and memory. More recently, this condition also became true for the Drosophila larva, which has become a focus for learning and memory studies based on a number of technical advances in the field of anatomical, molecular, and neuronal analyses. The ongoing efforts should be mentioned to reconstruct the complete connectome of the larval brain featuring a total of about 10,000 neurons and the development of neurogenic tools that allow individual manipulation of each neuron. By contrast, standardized behavioral assays that are commonly used to analyze learning and memory in Drosophila larvae exhibit no such technical development. Most commonly, a simple assay with Petri dishes and odor containers is used; in this method, the animals must be manually transferred in several steps. The behavioral approach is therefore labor-intensive and limits the capacity to conduct large-scale genetic screenings in small laboratories. To circumvent these limitations, we introduce a training device called the Maggot Instructor. This device allows automatic training up to 10 groups of larvae in parallel. To achieve such goal, we used fully automated, computer-controlled optogenetic activation of single olfactory neurons in combination with the application of electric shocks. We showed that Drosophila larvae trained with the Maggot Instructor establish an odor-specific memory, which is independent of handling and non-associative effects. The Maggot Instructor will allow to investigate the large collections of genetically modified larvae in a short period and with minimal human resources. Therefore, the Maggot Instructor should be able to help extensive behavioral experiments in Drosophila larvae to keep up with the current technical advancements. In the longer term, this condition will lead to a better understanding of how learning and memory are organized at the cellular, synaptic, and molecular levels in Drosophila larvae.

Reinforcement signaling of punishment versus relief in fruit flies

Painful events establish opponent memories: cues that precede pain are remembered negatively, whereas cues that follow pain, thus coinciding with relief are recalled positively. How do individual reinforcement-signaling neurons contribute to this “timing-dependent valence-reversal?” We addressed this question using an optogenetic approach in the fruit fly. Two types of fly dopaminergic neuron, each comprising just one paired cell, indeed established learned avoidance of odors that preceded their photostimulation during training, and learned approach to odors that followed the photostimulation. This is in striking parallel to punishment versus relief memories reinforced by a real noxious event. For only one of these neuron types, both effects were strong enough for further analyses. Notably, interfering with dopamine biosynthesis in these neurons partially impaired the punishing effect, but not the relieving after-effect of their photostimulation. We discuss how this finding constraints existing computational models of punishment versus relief memories and introduce a new model, which also incorporates findings from mammals. Furthermore, whether using dopaminergic neuron photostimulation or a real noxious event, more prolonged punishment led to stronger relief. This parametric feature of relief may also apply to other animals and may explain particular aspects of related behavioral dysfunction in humans.

A Fly's Eye View of Natural and Drug Reward

Animals encounter multiple stimuli each day. Some of these stimuli are innately appetitive or aversive, while others are assigned valence based on experience. Drugs like ethanol can elicit aversion in the short term and attraction in the long term. The reward system encodes the predictive value for different stimuli, mediating anticipation for attractive or punishing stimuli and driving animal behavior to approach or avoid conditioned stimuli. The neurochemistry and neurocircuitry of the reward system is partly evolutionarily conserved. In both vertebrates and invertebrates, including Drosophila melanogaster, dopamine is at the center of a network of neurotransmitters and neuromodulators acting in concert to encode rewards. Behavioral assays in D. melanogaster have become increasingly sophisticated, allowing more direct comparison with mammalian research. Moreover, recent evidence has established the functional modularity of the reward neural circuits in Drosophila. This functional modularity resembles the organization of reward circuits in mammals. The powerful genetic and molecular tools for D. melanogaster allow characterization and manipulation at the single-cell level. These tools are being used to construct a detailed map of the neural circuits mediating specific rewarding stimuli and have allowed for the identification of multiple genes and molecular pathways that mediate the effects of reinforcing stimuli, including their rewarding effects. This report provides an overview of the research on natural and drug reward in D. melanogaster, including natural rewards such as sugar and other food nutrients, and drug rewards including ethanol, cocaine, amphetamine, methamphetamine, and nicotine. We focused mainly on the known genetic and neural mechanisms underlying appetitive reward for sugar and reward for ethanol. We also include genes, molecular pathways, and neural circuits that have been identified using assays that test the palatability of the rewarding stimulus, the preference for the rewarding stimulus, or other effects of the stimulus that indicate how it can modify behavior. Commonalities between mechanisms of natural and drug reward are highlighted and future directions are presented, putting forward questions best suited for research using D. melanogaster as a model organism.

Functional architecture of reward learning in mushroom body extrinsic neurons of larval Drosophila

The brain adaptively integrates present sensory input, past experience, and options for future action. The insect mushroom body exemplifies how a central brain structure brings about such integration. Here we use a combination of systematic single-cell labeling, connectomics, transgenic silencing, and activation experiments to study the mushroom body at single-cell resolution, focusing on the behavioral architecture of its input and output neurons (MBINs and MBONs), and of the mushroom body intrinsic APL neuron. Our results reveal the identity and morphology of almost all of these 44 neurons in stage 3 Drosophila larvae. Upon an initial screen, functional analyses focusing on the mushroom body medial lobe uncover sparse and specific functions of its dopaminergic MBINs, its MBONs, and of the GABAergic APL neuron across three behavioral tasks, namely odor preference, taste preference, and associative learning between odor and taste. Our results thus provide a cellular-resolution study case of how brains organize behavior.

Behavioral Evidence for Enhanced Processing of the Minor Component of Binary Odor Mixtures in Larval Drosophila

A fundamental problem in deciding between mutually exclusive options is that the decision needs to be categorical although the properties of the options often differ but in grade. We developed an experimental handle to study this aspect of behavior organization. Larval Drosophila were trained such that in one set of animals odor A was rewarded, but odor B was not (A+/B), whereas a second set of animals was trained reciprocally (A/B+). We then measured the preference of the larvae either for A, or for B, or for "morphed" mixtures of A and B, that is for mixtures differing in the ratio of the two components. As expected, the larvae showed higher preference when only the previously rewarded odor was presented than when only the previously unrewarded odor was presented. For mixtures of A and B that differed in the ratio of the two components, the major component dominated preference behavior-but it dominated less than expected from a linear relationship between mixture ratio and preference behavior. This suggests that a minor component can have an enhanced impact in a mixture, relative to such a linear expectation. The current paradigm may prove useful in understanding how nervous systems generate discrete outputs in the face of inputs that differ only gradually.

The complete connectome of a learning and memory centre in an insect brain

Associating stimuli with positive or negative reinforcement is essential for survival, but a complete wiring diagram of a higher-order circuit supporting associative memory has not been previously available. Here we reconstruct one such circuit at synaptic resolution, the Drosophila larval mushroom body. We find that most Kenyon cells integrate random combinations of inputs but that a subset receives stereotyped inputs from single projection neurons. This organization maximizes performance of a model output neuron on a stimulus discrimination task. We also report a novel canonical circuit in each mushroom body compartment with previously unidentified connections: reciprocal Kenyon cell to modulatory neuron connections, modulatory neuron to output neuron connections, and a surprisingly high number of recurrent connections between Kenyon cells. Stereotyped connections found between output neurons could enhance the selection of learned behaviours. The complete circuit map of the mushroom body should guide future functional studies of this learning and memory centre.

The Ol1mpiad: concordance of behavioural faculties of stage 1 and stage 3 Drosophila larvae

Mapping brain function to brain structure is a fundamental task for neuroscience. For such an endeavour, the Drosophila larva is simple enough to be tractable, yet complex enough to be interesting. It features about 10,000 neurons and is capable of various taxes, kineses and Pavlovian conditioning. All its neurons are currently being mapped into a light-microscopical atlas, and Gal4 strains are being generated to experimentally access neurons one at a time. In addition, an electron microscopic reconstruction of its nervous system seems within reach. Notably, this electron microscope-based connectome is being drafted for a stage 1 larva – because stage 1 larvae are much smaller than stage 3 larvae. However, most behaviour analyses have been performed for stage 3 larvae because their larger size makes them easier to handle and observe. It is therefore warranted to either redo the electron microscopic reconstruction for a stage 3 larva or to survey the behavioural faculties of stage 1 larvae. We provide the latter. In a community-based approach we called the Ol1mpiad, we probed stage 1 Drosophila larvae for free locomotion, feeding, responsiveness to substrate vibration, gentle and nociceptive touch, burrowing, olfactory preference and thermotaxis, light avoidance, gustatory choice of various tastants plus odour–taste associative learning, as well as light/dark–electric shock associative learning. Quantitatively, stage 1 larvae show lower scores in most tasks, arguably because of their smaller size and lower speed. Qualitatively, however, stage 1 larvae perform strikingly similar to stage 3 larvae in almost all cases. These results bolster confidence in mapping brain structure and behaviour across developmental stages.

Pavlovian Conditioning of Larval Drosophila: An Illustrated, Multilingual, Hands-On Manual for Odor-Taste Associative Learning in Maggots

Larval Drosophila offer a study case for behavioral neurogenetics that is simple enough to be experimentally tractable, yet complex enough to be worth the effort. We provide a detailed, hands-on manual for Pavlovian odor-reward learning in these animals. Given the versatility of Drosophila for genetic analyses, combined with the evolutionarily shared genetic heritage with humans, the paradigm has utility not only in behavioral neurogenetics and experimental psychology, but for translational biomedicine as well. Together with the upcoming total synaptic connectome of the Drosophila nervous system and the possibilities of single-cell-specific transgene expression, it offers enticing opportunities for research. Indeed, the paradigm has already been adopted by a number of labs and is robust enough to be used for teaching in classroom settings. This has given rise to a demand for a detailed, hands-on manual directed at newcomers and/or at laboratory novices, and this is what we here provide.

The paradigm and the present manual have a unique set of features:

The present manual can thus foster science education at an earlier age and enable research by a broader community than has been the case to date.

Common microbehavioral "footprint" of two distinct classes of conditioned aversion

Avoiding unfavorable situations is a vital skill and a constant task for any animal. Situations can be unfavorable because they feature something that the animal wants to escape from, or because they do not feature something that it seeks to obtain. We investigate whether the microbehavioral mechanisms by which these two classes of aversion come about are shared or distinct. We find that larval Drosophila avoid odors either previously associated with a punishment, or previously associated with the lack of a reward. These two classes of conditioned aversion are found to be strikingly alike at the microbehavioral level. In both cases larvae show more head casts when oriented toward the odor source than when oriented away, and direct fewer of their head casts toward the odor than away when oriented obliquely to it. Thus, conditioned aversion serving two qualitatively different functions—escape from a punishment or search for a reward—is implemented by the modulation of the same microbehavioral features. These features also underlie conditioned approach, albeit with opposite sign. That is, the larvae show conditioned approach toward odors previously associated with a reward, or with the lack of a punishment. In order to accomplish both these classes of conditioned approach the larvae show fewer head casts when oriented toward an odor, and direct more of their head casts toward it when they are headed obliquely. Given that the Drosophila larva is a genetically tractable model organism that is well suited to study simple circuits at the single-cell level, these analyses can guide future research into the neuronal circuits underlying conditioned approach and aversion, and the computational principles of conditioned search and escape.

Neural Circuits Underlying Fly Larval Locomotion

Locomotion is a complex motor behavior that may be expressed in different ways using a variety of strategies depending upon species and pathological or environmental conditions. Quadrupedal or bipedal walking, running, swimming, flying and gliding constitute some of the locomotor modes enabling the body, in all cases, to move from one place to another. Despite these apparent differences in modes of locomotion, both vertebrate and invertebrate species share, at least in part, comparable neural control mechanisms for locomotor rhythm and pattern generation and modulation. Significant advances have been made in recent years in studies of the genetic aspects of these control systems. Findings made specifically using Drosophila (fruit fly) models and preparations have contributed to further understanding of the key role of genes in locomotion. This review focuses on some of the main findings made in larval fruit flies while briefly summarizing the basic advantages of using this powerful animal model for studying the neural locomotor system.

Synapsin-based approaches to brain plasticity in adult social insects

Development of the mushroom bodies continues after adult eclosion in social insects. Synapsins, phosphoproteins abundant in presynaptic boutons, are not required for development of the nervous system but have as their primary function modulation of synaptic transmission. A monoclonal antibody against a conserved region of Drosophila synapsin labels synaptic structures called microglomeruli in the mushroom bodies of adult social insects, permitting studies of microglomerular volume, density, and number. The results point to multiple forms of brain plasticity in social insects: age-based and experience-based maturation that results in a decrease in density coupled with an increase in volume of individual microglomeruli in simultaneous operation with shorter term changes in density produced by specific life experiences.

Continuous lateral oscillations as a core mechanism for taxis in Drosophila larvae

Taxis behaviour in Drosophila larva is thought to consist of distinct control mechanisms triggering specific actions. Here, we support a simpler hypothesis: that taxis results from direct sensory modulation of continuous lateral oscillations of the anterior body, sparing the need for ‘action selection’. Our analysis of larvae motion reveals a rhythmic, continuous lateral oscillation of the anterior body, encompassing all head-sweeps, small or large, without breaking the oscillatory rhythm. Further, we show that an agent-model that embeds this hypothesis reproduces a surprising number of taxis signatures observed in larvae. Also, by coupling the sensory input to a neural oscillator in continuous time, we show that the mechanism is robust and biologically plausible. The mechanism provides a simple architecture for combining information across modalities, and explaining how learnt associations modulate taxis. We discuss the results in the light of larval neural circuitry and make testable predictions.

DOI:http://dx.doi.org/10.7554/eLife.15504.001

DISC1 causes associative memory and neurodevelopmental defects in fruit flies

Originally found in a Scottish family with diverse mental disorders, the DISC1 protein has been characterized as an intracellular scaffold protein that associates with diverse binding partners in neural development. To explore its functions in a genetically tractable system, we expressed the human DISC1 in fruit flies (Drosophila melanogaster). As in mammalian neurons, DISC1 is localized to diverse subcellular domains of developing fly neurons including the nuclei, axons and dendrites. Overexpression of DISC1 impairs associative memory. Experiments with deletion/mutation constructs have revealed the importance of amino-terminal domain (46-290) for memory suppression whereas carboxyl domain (598-854) and the amino-terminal residues (1-45) including the nuclear localization signal (NLS1) are dispensable. DISC1 overexpression also causes suppression of axonal and dendritic branching of mushroom body neurons, which mediate a variety of cognitive functions in the fly brain. Analyses with deletion/mutation constructs reveal that protein domains 598-854 and 349-402 are both required for the suppression of axonal branching, while amino-terminal domains including NLS1 are dispensable. In contrast, NLS1 was required for the suppression of dendritic branching, suggesting a mechanism involving gene expression. Moreover, domain 403-596 is also required for the suppression of dendritic branching. We also show that overexpression of DISC1 suppresses glutamatergic synaptogenesis in developing neuromuscular junctions. Deletion/mutation experiments have revealed the importance of protein domains 403-596 and 349-402 for synaptic suppression, while amino-terminal domains including NLS1 are dispensable. Finally, we show that DISC1 functionally interacts with the fly homolog of Dysbindin (DTNBP1) via direct protein-protein interaction in developing synapses.

Artificial Induction of Associative Olfactory Memory by Optogenetic and Thermogenetic Activation of Olfactory Sensory Neurons and Octopaminergic Neurons in Drosophila Larvae

The larval brain of Drosophila melanogaster provides an excellent system for the study of the neurocircuitry mechanism of memory. Recent development of neurogenetic techniques in fruit flies enables manipulations of neuronal activities in freely behaving animals. This protocol describes detailed steps for artificial induction of olfactory associative memory in Drosophila larvae. In this protocol, the natural reward signal is substituted by thermogenetic activation of octopaminergic neurons in the brain. In parallel, the odor signal is substituted by optogenetic activation of a specific class of olfactory receptor neurons. Association of reward and odor stimuli is achieved with the concomitant application of blue light and heat that leads to activation of both sets of neurons in living transgenic larvae. Given its operational simplicity and robustness, this method could be utilized to further our knowledge on the neurocircuitry mechanism of memory in the fly brain.

Involvement of phosphorylated Apis mellifera CREB in gating a honeybee's behavioral response to an external stimulus

The transcription factor cAMP-response element-binding protein (CREB) is involved in neuronal plasticity. Phosphorylation activates CREB and an increased level of phosphorylated CREB is regarded as an indicator of CREB-dependent transcriptional activation. In honeybees(Apis mellifera)we recently demonstrated a particular high abundance of the phosphorylated honeybee CREB homolog (pAmCREB) in the central brain and in a subpopulation of mushroom body neurons. We hypothesize that these high pAmCREB levels are related to learning and memory formation. Here, we tested this hypothesis by analyzing brain pAmCREB levels in classically conditioned bees and bees experiencing unpaired presentations of conditioned stimulus (CS) and unconditioned stimulus (US). We demonstrate that both behavioral protocols display differences in memory formation but do not alter the level of pAmCREB in bee brains directly after training. Nevertheless, we report that bees responding to the CS during unpaired stimulus presentations exhibit higher levels of pAmCREB than nonresponding bees. In addition, Trichostatin A, a histone deacetylase inhibitor that is thought to enhance histone acetylation by CREB-binding protein, increases the bees' CS responsiveness. We conclude that pAmCREB is involved in gating a bee's behavioral response driven by an external stimulus.

Changes in Synapsin Levels in the Millipede Gymnostreptus olivaceus Schubart, 1944 Exposed to Different Concentrations of Deltamethrin

Millipedes are ecologically important soil organisms and may also be an economically threatening species in rural and urban areas when population outbreaks occur. In order to control infestations commercial formulations of deltamethrin have been commonly applied, even though there are few studies about the effects of such insecticide on millipedes. This paper describes the effects of this insecticide on millipedes showing neurotoxic effects assessed by synapsin labeling and confocal microscopy. Deltamethrin concentrations related to the DL50 of the active ingredient and a field concentration were applied topically in the diplopod Gymnostreptus olivaceus to evaluate the behavior, mortality rate, and synapsin levels in the brain 12, 24, and 48h after contact with deltamethin. The insecticide caused mortality at the higher concentrations employed, in which no change was observed in neurotransmission in the survivors. In contrast, at field concentrations, deltamethrin did not cause any deaths, but triggered significant changes in synapsin levels. The results obtained form the synapsin labeling provide several interpretations suggesting that the isolated application of this tool must be associated with additional tools in order to evaluate biologically induced effects of deltamethrin in an accurate way. In addition, the feasibility of chemical control of millipedes with deltamethrin is questioned.

Synapsin is required to "boost" memory strength for highly salient events

Synapsin is an evolutionarily conserved presynaptic phosphoprotein. It is encoded by only one gene in the Drosophila genome and is expressed throughout the nervous system. It regulates the balance between reserve and releasable vesicles, is required to maintain transmission upon heavy demand, and is essential for proper memory function at the behavioral level. Task-relevant sensorimotor functions, however, remain intact in the absence of Synapsin. Using an odor–sugar reward associative learning paradigm in larval Drosophila, we show that memory scores in mutants lacking Synapsin (syn⁹⁷) are lower than in wild-type animals only when more salient, higher concentrations of odor or of the sugar reward are used. Furthermore, we show that Synapsin is selectively required for larval short-term memory. Thus, without Synapsin Drosophila larvae can learn and remember, but Synapsin is required to form memories that match in strength to event salience—in particular to a high saliency of odors, of rewards, or the salient recency of an event. We further show that the residual memory scores upon a lack of Synapsin are not further decreased by an additional lack of the Sap47 protein. In combination with mass spectrometry data showing an up-regulated phosphorylation of Synapsin in the larval nervous system upon a lack of Sap47, this is suggestive of a functional interdependence of Synapsin and Sap47.

The Identification of Congeners and Aliens by Drosophila Larvae

We investigated the role of Drosophila larva olfactory system in identification of congeners and aliens. We discuss the importance of these activities in larva navigation across substrates, and the implications for allocation of space and food among species of similar ecologies. Wild type larvae of cosmopolitan D. melanogaster and endemic D. pavani, which cohabit the same breeding sites, used species-specific volatiles to identify conspecifics and aliens moving toward larvae of their species. D. gaucha larvae, a sibling species of D. pavani that is ecologically isolated from D. melanogaster, did not respond to melanogaster odor cues. Similar to D. pavani larvae, the navigation of pavani female x gaucha male hybrids was influenced by conspecific and alien odors, whereas gaucha female x pavani male hybrid larvae exhibited behavior similar to the D. gaucha parent. The two sibling species exhibited substantial evolutionary divergence in processing the odor inputs necessary to identify conspecifics. Orco (Or83b) mutant larvae of D. melanogaster, which exhibit a loss of sense of smell, did not distinguish conspecific from alien larvae, instead moving across the substrate. Syn97CS and rut larvae of D. melanogaster, which are unable to learn but can smell, moved across the substrate as well. The Orco (Or83b), Syn97CS and rut loci are necessary to orient navigation by D. melanogaster larvae. Individuals of the Trana strain of D. melanogaster did not respond to conspecific and alien larval volatiles and therefore navigated randomly across the substrate. By contrast, larvae of the Til-Til strain used larval volatiles to orient their movement. Natural populations of D. melanogaster may exhibit differences in identification of conspecific and alien larvae. Larval locomotion was not affected by the volatiles.

Synapsin determines memory strength after punishment- and relief-learning

Adverse life events can induce two kinds of memory with opposite valence, dependent on timing: "negative" memories for stimuli preceding them and "positive" memories for stimuli experienced at the moment of "relief." Such punishment memory and relief memory are found in insects, rats, and man. For example, fruit flies (Drosophila melanogaster) avoid an odor after odor-shock training ("forward conditioning" of the odor), whereas after shock-odor training ("backward conditioning" of the odor) they approach it. Do these timing-dependent associative processes share molecular determinants? We focus on the role of Synapsin, a conserved presynaptic phosphoprotein regulating the balance between the reserve pool and the readily releasable pool of synaptic vesicles. We find that a lack of Synapsin leaves task-relevant sensory and motor faculties unaffected. In contrast, both punishment memory and relief memory scores are reduced. These defects reflect a true lessening of associative memory strength, as distortions in nonassociative processing (e.g., susceptibility to handling, adaptation, habituation, sensitization), discrimination ability, and changes in the time course of coincidence detection can be ruled out as alternative explanations. Reductions in punishment- and relief-memory strength are also observed upon an RNAi-mediated knock-down of Synapsin, and are rescued both by acutely restoring Synapsin and by locally restoring it in the mushroom bodies of mutant flies. Thus, both punishment memory and relief memory require the Synapsin protein and in this sense share genetic and molecular determinants. We note that corresponding molecular commonalities between punishment memory and relief memory in humans would constrain pharmacological attempts to selectively interfere with excessive associative punishment memories, e.g., after traumatic experiences.

The impact of odor-reward memory on chemotaxis in larval Drosophila

How do animals adaptively integrate innate with learned behavioral tendencies? We tackle this question using chemotaxis as a paradigm. Chemotaxis in the Drosophila larva largely results from a sequence of runs and oriented turns. Thus, the larvae minimally need to determine (i) how fast to run, (ii) when to initiate a turn, and (iii) where to direct a turn. We first report how odor-source intensities modulate these decisions to bring about higher levels of chemotactic performance for higher odor-source intensities during innate chemotaxis. We then examine whether the same modulations are responsible for alterations of chemotactic performance by learned odor “valence” (understood throughout as level of attractiveness). We find that run speed (i) is neither modulated by the innate nor by the learned valence of an odor. Turn rate (ii), however, is modulated by both: the higher the innate or learned valence of the odor, the less often larvae turn whenever heading toward the odor source, and the more often they turn when heading away. Likewise, turning direction (iii) is modulated concordantly by innate and learned valence: turning is biased more strongly toward the odor source when either innate or learned valence is high. Using numerical simulations, we show that a modulation of both turn rate and of turning direction is sufficient to account for the empirically found differences in preference scores across experimental conditions. Our results suggest that innate and learned valence organize adaptive olfactory search behavior by their summed effects on turn rate and turning direction, but not on run speed. This work should aid studies into the neural mechanisms by which memory impacts specific aspects of behavior.

Generalization and discrimination tasks yield concordant measures of perceived distance between odours and their binary mixtures in larval Drosophila

Similarity between odours is notoriously difficult to measure. Widely used behavioural approaches in insect olfaction research are cross-adaptation, masking, as well as associative tasks based on olfactory learning and the subsequent testing for how specific the established memory is. A concern with such memory-based approaches is that the learning process required to establish an odour memory may alter the way the odour is processed, such that measures of perception taken at the test are distorted. The present study was therefore designed to see whether behavioural judgements of perceptual distance are different for two different memory-based tasks, namely generalization and discrimination. We used odour–reward learning in larval Drosophila as a study case. In order to challenge the larvae's olfactory system, we chose to work with binary mixtures and their elements (1-octanol, n-amyl acetate, 3-octanol, benzaldehyde and hexyl acetate). We determined the perceptual distance between each mixture and its elements, first in a generalization task, and then in a discrimination task. It turns out that scores of perceptual distance are correlated between both tasks. A re-analysis of published studies looking at element-to-element perceptual distances in larval reward learning and in adult punishment learning confirms this result. We therefore suggest that across a given set of olfactory stimuli, associative training does not grossly alter the pattern of perceptual distances.

Learning the specific quality of taste reinforcement in larval Drosophila

The only property of reinforcement insects are commonly thought to learn about is its value. We show that larval Drosophila not only remember the value of reinforcement (How much?), but also its quality (What?). This is demonstrated both within the appetitive domain by using sugar vs amino acid as different reward qualities, and within the aversive domain by using bitter vs high-concentration salt as different qualities of punishment. From the available literature, such nuanced memories for the quality of reinforcement are unexpected and pose a challenge to present models of how insect memory is organized. Given that animals as simple as larval Drosophila, endowed with but 10,000 neurons, operate with both reinforcement value and quality, we suggest that both are fundamental aspects of mnemonic processing-in any brain.

The effect of stress on motor function in Drosophila

Exposure to unpredictable and uncontrollable conditions causes animals to perceive stress and change their behavior. It is unclear how the perception of stress modifies the motor components of behavior and which molecular pathways affect the behavioral change. In order to understand how stress affects motor function, we developed an experimental platform that quantifies walking motions in Drosophila. We found that stress induction using electrical shock results in backwards motions of the forelegs at the end of walking strides. These leg retrogressions persisted during repeated stimulation, although they habituated substantially. The motions also continued for several strides after the end of the shock, indicating that stress induces a behavioral aftereffect. Such aftereffect could also be induced by restricting the motion of the flies via wing suspension. Further, the long-term effects could be amplified by combining either immobilization or electric shock with additional stressors. Thus, retrogression is a lingering form of response to a broad range of stressful conditions, which cause the fly to search for a foothold when it faces extreme and unexpected challenges. Mutants in the cAMP signaling pathway enhanced the stress response, indicating that this pathway regulates the behavioral response to stress. Our findings identify the effect of stress on a specific motor component of behavior and define the role of cAMP signaling in this stress response.

Immediate and punitive impact of mechanosensory disturbance on olfactory behaviour of larval Drosophila

The ability to respond to and to learn about mechanosensory disturbance is widespread among animals. Using Drosophila larvae, we describe how the frequency of mechanosensory disturbance ('buzz') affects three aspects of behaviour: free locomotion, innate olfactory preference, and potency as a punishment. We report that (i) during 2-3 seconds after buzz onset the larvae slowed down and then turned, arguably to escape this situation; this was seen for buzz frequencies of 10, 100, and 1000 Hz, (ii) innate olfactory preference was reduced when tested in the presence of the buzz; this effect was strongest for the 100 Hz frequency, (iii) after odour-buzz associative training, we observed escape from the buzz-associated odour; this effect was apparent for 10 and 100, but not for 1000 Hz. We discuss the multiple behavioural effects of mechanosensation and stress that the immediate effects on locomotion and the impact as punishment differ in their frequency-dependence. Similar dissociations between immediate, reflexive behavioural effects and reinforcement potency were previously reported for sweet, salty and bitter tastants. It should be interesting to see how these features map onto the organization of sensory, ascending pathways.

'Peer pressure' in larval Drosophila?

Understanding social behaviour requires a study case that is simple enough to be tractable, yet complex enough to remain interesting. Do larval Drosophila meet these requirements? In a broad sense, this question can refer to effects of the mere presence of other larvae on the behaviour of a target individual. Here we focused in a more strict sense on ‘peer pressure’, that is on the question of whether the behaviour of a target individual larva is affected by what a surrounding group of larvae is doing. We found that innate olfactory preference of a target individual was neither affected (i) by the level of innate olfactory preference in the surrounding group nor (ii) by the expression of learned olfactory preference in the group. Likewise, learned olfactory preference of a target individual was neither affected (iii) by the level of innate olfactory preference of the surrounding group nor (iv) by the learned olfactory preference the group was expressing. We conclude that larval Drosophila thus do not take note of specifically what surrounding larvae are doing. This implies that in a strict sense, and to the extent tested, there is no social interaction between larvae. These results validate widely used en mass approaches to the behaviour of larval Drosophila.

Bitter-sweet processing in larval Drosophila

"Sweet-" and "bitter-" tasting substances distinctively support attractive and aversive choice behavior, respectively, and therefore are thought to be processed by distinct pathways. Interestingly, electrophysiological recordings in adult Drosophila suggest that bitter and salty tastants, in addition to activating bitter, salt, or bitter/salt sensory neurons, can also inhibit sweet-sensory neurons. However, the behavioral significance of such a potential for combinatorial coding is little understood. Using larval Drosophila as a study case, we find that the preference towards fructose is inhibited when assayed in the background of the bitter tastant quinine. When testing the influence of quinine on the preference to other, equally preferred sweet tastants, we find that these sweet tastants differ in their susceptibility to be inhibited by quinine. Such stimulus specificity argues that the inhibitory effect of quinine is not due to general effects on locomotion or nausea. In turn, not all bitter tastants have the same potency to inhibit sweet preference; notably, their inhibitory potency is not determined by the strength of the avoidance of them. Likewise, equally avoided concentrations of sodium chloride differ in their potency to inhibit sugar preference. Furthermore, Gr33a-Gal4-positive neurons, while being necessary for bitter avoidance, are dispensable for inhibition of the sweet pathway. Thus, interactions across taste modalities are behaviorally significant and, as we discuss, arguably diverse in mechanism. These results suggest that the coding of tastants and the organization of gustatory behavior may be more combinatorial than is generally acknowledged.

Could microbe stimulated maggots become a targeted natural antibiotics family?

Maggot debridement therapy plays an important role in treatment of diabetic foot ulcers and other chronic infectious wounds, cause of this is its extremely low drug resistance. However, the microbe stimulated maggot, we may call it a derivative of normal sterile maggot, could exhibit stronger bacterial or bactericidal effects. Methods of the pretreatment on maggot was different germ solution were artificially mixed and added with originally sterile maggots, the novel secretions were collected. Some of this have been demonstrated by plate test and telescope analysis. Thus, we hypothesize that maggot especially the larvae of Lucilia sericata was conducted as the germ irritant receptor, and diverse germs interacted with it, at last, novel secretions/excretions we got will offer a great help to the general surgery clinicians as well as researchers who are interested in novel antibiotics discovery.

Pain-relief learning in flies, rats, and man: basic research and applied perspectives

Memories relating to a painful, negative event are adaptive and can be stored for a lifetime to support preemptive avoidance, escape, or attack behavior. However, under unfavorable circumstances such memories can become overwhelmingly powerful. They may trigger excessively negative psychological states and uncontrollable avoidance of locations, objects, or social interactions. It is therefore obvious that any process to counteract such effects will be of value. In this context, we stress from a basic-research perspective that painful, negative events are "Janus-faced" in the sense that there are actually two aspects about them that are worth remembering: What made them happen and what made them cease. We review published findings from fruit flies, rats, and man showing that both aspects, respectively related to the onset and the offset of the negative event, induce distinct and oppositely valenced memories: Stimuli experienced before an electric shock acquire negative valence as they signal upcoming punishment, whereas stimuli experienced after an electric shock acquire positive valence because of their association with the relieving cessation of pain. We discuss how memories for such punishment- and relief-learning are organized, how this organization fits into the threat-imminence model of defensive behavior, and what perspectives these considerations offer for applied psychology in the context of trauma, panic, and nonsuicidal self-injury.

Synapsin function in GABA-ergic interneurons is required for short-term olfactory habituation

In Drosophila, short-term (STH) and long-term habituation (LTH) of olfactory avoidance behavior are believed to arise from the selective potentiation of GABAergic synapses between multiglomerular local circuit interneurons (LNs) and projection neurons in the antennal lobe. However, the underlying mechanisms remain poorly understood. Here, we show that synapsin (syn) function is necessary for STH and that syn(97)-null mutant defects in STH can be rescued by syn(+) cDNA expression solely in the LN1 subset of GABAergic local interneurons. As synapsin is a synaptic vesicle-clustering phosphoprotein, these observations identify a presynaptic mechanism for STH as well as the inhibitory interneurons in which this mechanism is deployed. Serine residues 6 and/or 533, potential kinase target sites of synapsin, are necessary for synapsin function suggesting that synapsin phosphorylation is essential for STH. Consistently, biochemical analyses using a phospho-synapsin-specific antiserum show that synapsin is a target of Ca(2+) calmodulin-dependent kinase II (CaMKII) phosphorylation in vivo. Additional behavioral and genetic observations demonstrate that CaMKII function is necessary in LNs for STH. Together, these data support a model in which CaMKII-mediated synapsin phosphorylation in LNs induces synaptic vesicle mobilization and thereby presynaptic facilitation of GABA release that underlies olfactory STH. Finally, the striking observation that LTH occurs normally in syn(97) mutants indicates that signaling pathways for STH and LTH diverge upstream of synapsin function in GABAergic interneurons.

Olfactory memories are intensity specific in larval Drosophila

Learning can rely on stimulus quality, stimulus intensity, or a combination of these. Regarding olfaction, the coding of odour quality is often proposed to be combinatorial along the olfactory pathway, and working hypotheses are available concerning short-term associative memory trace formation of odour quality. However, it is less clear how odour intensity is coded, and whether olfactory memory traces include information about the intensity of the learnt odour. Using odour-sugar associative conditioning in larval Drosophila, we first describe the dose-effect curves of learnability across odour intensities for four different odours (n-amyl acetate, 3-octanol, 1-octen-3-ol and benzaldehyde). We then chose odour intensities such that larvae were trained at an intermediate odour intensity, but were tested for retention with either that trained intermediate odour intensity, or with respectively higher or lower intensities. We observed a specificity of retention for the trained intensity for all four odours used. This adds to the appreciation of the richness in 'content' of olfactory short-term memory traces, even in a system as simple as larval Drosophila, and to define the demands on computational models of associative olfactory memory trace formation. We suggest two kinds of circuit architecture that have the potential to accommodate intensity learning, and discuss how they may be implemented in the insect brain.