The roles of dopamine and related compounds in reward-seeking behavior across animal phyla

Sound the alarm: A meta-analysis on the effect of aquatic noise on fish behavior and physiology

The aquatic environment is increasingly bombarded by a wide variety of noise pollutants whose range and intensity are increasing with each passing decade. Yet, little is known about how aquatic noise affects marine communities. To determine the implications that changes to the soundscape may have on fishes, a meta-analysis was conducted focusing on the ramifications of noise on fish behavior and physiology. Our meta-analysis identified 42 studies that produced 2,354 data points, which in turn indicated that anthropogenic noise negatively affects fish behavior and physiology. The most predominate responses occurred within foraging ability, predation risk, and reproductive success. Additionally, anthropogenic noise was shown to increase the hearing thresholds and cortisol levels of numerous species while tones, biological, and environmental noise were most likely to affect complex movements and swimming abilities. These findings suggest that the majority of fish species are sensitive to changes in the aquatic soundscape, and depending on the noise source, species responses may have extreme and negative fitness consequences. As such, this global synthesis should serve as a warning of the potentially dire consequences facing marine ecosystems if alterations to aquatic soundscapes continue on their current trajectory.

Dopamine signalling in locusts and other insects

Dopamine is an important catecholamine neurotransmitter in invertebrates and vertebrates. It is biochemically derived from tyrosine via L-DOPA. It is most abundant in the central nervous system, but can also be produced in e.g. epidermal cells. Dopamine has conserved roles in the control of movement, pleasure, motivation, arousal and memory between invertebrate and vertebrate animals. It is crucial for melanisation and sclerotisation, important processes for the formation of the exoskeleton of insects and immune function. In this brief review I will discuss some general aspects of insect dopamine biosynthesis and breakdown, dopamine receptors and their pharmacology. In addition, I will provide a glance on the multitude of biological functions of dopamine in insects. More detail is provided concerning the putative roles of dopamine in phase related phenomena in locusts. Finally, molecular and pharmacological adjustments of insect dopamine signalling are discussed in the light of possible approaches towards insect pest management.

Studying emotion in invertebrates: what has been done, what can be measured and what they can provide

Until recently, whether invertebrates might exhibit emotions was unknown. This possibility has traditionally been dismissed by many as emotions are frequently defined with reference to human subjective experience, and invertebrates are often not considered to have the neural requirements for such sophisticated abilities. However, emotions are understood in humans and other vertebrates to be multifaceted brain states, comprising dissociable subjective, cognitive, behavioural and physiological components. In addition, accumulating literature is providing evidence of the impressive cognitive capacities and behavioural flexibility of invertebrates. Alongside these, within the past few years, a number of studies have adapted methods for assessing emotions in humans and other animals, to invertebrates, with intriguing results. Sea slugs, bees, crayfish, snails, crabs, flies and ants have all been shown to display various cognitive, behavioural and/or physiological phenomena that indicate internal states reminiscent of what we consider to be emotions. Given the limited neural architecture of many invertebrates, and the powerful tools available within invertebrate research, these results provide new opportunities for unveiling the neural mechanisms behind emotions and open new avenues towards the pharmacological manipulation of emotion and its genetic dissection, with advantages for disease research and therapeutic drug discovery. Here, we review the increasing evidence that invertebrates display some form of emotion, discuss the various methods used for assessing emotions in invertebrates and consider what can be garnered from further emotion research on invertebrates in terms of the evolution and underlying neural basis of emotion in a comparative context.

Depression subtyping based on evolutionary psychiatry: Proximate mechanisms and ultimate functions

Major depressive disorder constitutes one of the leading causes of disability worldwide. However, it is not a unitary disease-it is a heterogeneous syndrome, with patients differing remarkably in symptom profile, pathophysiology and treatment responsiveness. Previous attempts to subtype major depressive disorder have showed limited clinical applicability. We present a classification of major depressive disorder episodes based on the proximate mechanisms that led to the original mood change that caused the depressive episode. We identify discrete depression subtypes that are induced by: 1) infection, 2) long-term stress, 3) loneliness, 4) traumatic experience, 5) hierarchy conflict, 6) grief, 7) romantic rejection, 8) postpartum events, 9) the season, 10) chemicals, 11) somatic diseases and 12) starvation. We further examine the ultimate functions of these subtypes and show that not all types of mood changes that trigger depression are adaptive. Instead, some are clearly maladaptive and some are byproducts of other adaptations. In modern societies, low mood after adverse life events may turn into a pathological depressive state. Modern lifestyle increases susceptibility to inflammatory dysregulation and chronic stress, both of which increase the amount of proinflammatory cytokines in peripheral blood, leading to low mood and sickness behaviour. Proinflammatory cytokines may aggravate the previously adaptive short-term mood changes to a chronic maladaptive depressive state by preventing the normalization of mood after adverse life events. Subtyping depression enables an effective and intelligent long-term treatment of patients in each subtype by treating the underlying causes of depression.

Uncertainty processing in bees exposed to free choices: Lessons from vertebrates

Bees tend to avoid or to show indifference to uncertain ("risky") relative to certain ("safe") food rewards, whether in nectar volume or in nectar concentration. The unattractiveness of uncertain food rewards is also sometimes independent of the energy budget of bees. This pattern of responses seems to differ from that observed in mammals and birds, which may exhibit a strong preference for the uncertainty over the certainty of food delivery on a given trial in dual-choice tasks. Upon analysis of the conditions that determine preference and aversion for uncertain food rewards in "higher" vertebrates, I attempt to demonstrate that bees react to uncertainty in a similar way. It is argued that, because of their social organization and of the type of resources they seek, bees are essentially exposed to situations in which "higher" vertebrates find reward uncertainty unattractive as well. The nature of their representation of food distribution is discussed, and it is suggested that scout bees may differ from recruits with respect to uncertainty processing.

Genomic and transcriptomic analysis of the Asian honeybee Apis cerana provides novel insights into honeybee biology

The Asian honeybee Apis cerana is one of two bee species that have been commercially kept with immense economic value. Here we present the analysis of genomic sequence and transcriptomic exploration for A. cerana as well as the comparative genomic analysis of the Asian honeybee and the European honeybee A. mellifera. The genome and RNA-seq data yield new insights into the behavioral and physiological resistance to the parasitic mite Varroa the evolution of antimicrobial peptides, and the genetic basis for labor division in A. cerana. Comparison of genes between the two sister species revealed genes specific to A. cerana, 54.5% of which have no homology to any known proteins. The observation that A. cerana displayed significantly more vigilant grooming behaviors to the presence of Varroa than A. mellifera in conjunction with gene expression analysis suggests that parasite-defensive grooming in A. cerana is likely triggered not only by exogenous stimuli through visual and olfactory detection of the parasite, but also by genetically endogenous processes that periodically activates a bout of grooming to remove the ectoparasite. This information provides a valuable platform to facilitate the traits unique to A. cerana as well as those shared with other social bees for health improvement.

Large-scale transcriptome changes in the process of long-term visual memory formation in the bumblebee, Bombus terrestris

Many genes have been implicated in mechanisms of long-term memory formation, but there is still much to be learnt about how the genome dynamically responds, transcriptionally, during memory formation. In this study, we used high-throughput sequencing to examine how transcriptome profiles change during visual memory formation in the bumblebee (Bombus terrestris). Expression of fifty-five genes changed immediately after bees were trained to associate reward with a single coloured chip, and the upregulated genes were predominantly genes known to be involved in signal transduction. Changes in the expression of eighty-one genes were observed four hours after learning a new colour, and the majority of these were upregulated and related to transcription and translation, which suggests that the building of new proteins may be the predominant activity four hours after training. Several of the genes identified in this study (e.g. Rab10, Shank1 and Arhgap44) are interesting candidates for further investigation of the molecular mechanisms of long-term memory formation. Our data demonstrate the dynamic gene expression changes after associative colour learning and identify genes involved in each transcriptional wave, which will be useful for future studies of gene regulation in learning and long-term memory formation.

Foraging motivation favors the occurrence of Lévy walks

Lévy walks are a property of random movements often observed among foraging animals (and humans), and they might confer some advantages for survival in an unpredictable environment, in comparison with Brownian walks. In animals with a nervous system, specific neurotransmitters associated with some psychological states could play a crucial role in controlling the occurrence of Lévy walks. We argue that incentive motivation, a dopamine-dependent process that in vertebrates makes rewards and their predictive conditioned stimuli attractive, has behavioral effects that may favor their occurrence: incentive motivation is higher when food is unpredictable and it strongly underpins foraging activity. An individual-based computer model is used to determine whether changes in incentive motivation can influence the probability that Lévy walks occur among foraging agents. Our results suggest that they are produced more often under an unpredictable than a predictable food access, and more often in strongly rather than weakly motivated foragers exposed to an unpredictable food access. Also, our motivational framework indicates that the occurrence of Lévy walks are correlated with, but not causally linked to, the number of food items consumed and the ability to store fat reserves. We conclude that Lévy walks can confer some advantages for survival in an unpredictable environment, provided that they appear in foragers with a high motivation to seek food.

Origins of Aminergic Regulation of Behavior in Complex Insect Social Systems

Neuromodulators are conserved across insect taxa, but how biogenic amines and their receptors in ancestral solitary forms have been co-opted to control behaviors in derived socially complex species is largely unknown. Here we explore patterns associated with the functions of octopamine (OA), serotonin (5-HT) and dopamine (DA) in solitary ancestral insects and their derived functions in eusocial ants, bees, wasps and termites. Synthesizing current findings that reveal potential ancestral roles of monoamines in insects, we identify physiological processes and conserved behaviors under aminergic control, consider how biogenic amines may have evolved to modulate complex social behavior, and present focal research areas that warrant further study.

Roles of Retinoic Acid Signaling in Shaping the Neuronal Architecture of the Developing Amphioxus Nervous System

The morphogen retinoic acid (RA) patterns vertebrate nervous systems and drives neurogenesis, but how these functions evolved remains elusive. Here, we show that RA signaling plays stage- and tissue-specific roles during the formation of neural cell populations with serotonin, dopamine, and GABA neurotransmitter phenotypes in amphioxus, a proxy for the ancestral chordate. Our data suggest that RA signaling restricts the specification of dopamine-containing cells in the ectoderm and of GABA neurons in the neural tube, probably by regulating Hox1 and Hox3 gene expression, respectively. The two Hox genes thus appear to serve distinct functions rather than to participate in a combinatorial Hox code. We were further able to correlate the RA signaling-dependent mispatterning of hindbrain GABA neurons with concomitant motor impairments. Taken together, these data provide new insights into how RA signaling and Hox genes contribute to nervous system as well as to motor control development in amphioxus and hence shed light on the evolution of these functions within vertebrates.

The chemical signatures underlying host plant discrimination by aphids

The diversity of phytophagous insects is largely attributable to speciation involving shifts between host plants. These shifts are mediated by the close interaction between insects and plant metabolites. However, there has been limited progress in understanding the chemical signatures that underlie host preferences. We use the pea aphid (Acyrthosiphon pisum) to address this problem. Host-associated races of pea aphid discriminate between plant species in race-specific ways. We combined metabolomic profiling of multiple plant species with behavioural tests on two A. pisum races, to identify metabolites that explain variation in either acceptance or discrimination. Candidate compounds were identified using tandem mass spectrometry. Our results reveal a small number of compounds that explain a large proportion of variation in the differential acceptability of plants to A. pisum races. Two of these were identified as L-phenylalanine and L-tyrosine but it may be that metabolically-related compounds directly influence insect behaviour. The compounds implicated in differential acceptability were not related to the set correlated with general acceptability of plants to aphids, regardless of host race. Small changes in response to common metabolites may underlie host shifts. This study opens new opportunities for understanding the mechanistic basis of host discrimination and host shifts in insects.

Relapse to cocaine seeking in an invertebrate

Addiction is characterised by cycles of compulsive drug taking, periods of abstinence and episodes of relapse. The extinction/reinstatement paradigm has been extensively used in rodents to model human relapse and explore underlying mechanisms and therapeutics. However, relapse to drug seeking behaviour has not been previously demonstrated in invertebrates. Here, we used a cocaine conditioned place preference (CPP) paradigm in the flatworm, planarian, followed by extinction and reinstatement of drug seeking. Once baseline preference was established for one of two distinctly textured environments (i.e. compartments with a coarse or smooth surface), planarian received pairings of cocaine (5μM) in the non-preferred, and vehicle in the most preferred, environment, and were tested for conditioning thereafter. Cocaine produced robust CPP, measured as a significant increase in the time spent in the cocaine-paired compartment. Subsequently, planarian underwent extinction training, reverting back to their original preference within three sessions. Brief exposure to cocaine (5μM) or methamphetamine (5μM) reinstated cocaine-seeking behaviour. By contrast, the high affinity dopamine transporter inhibitor, (N-(n-butyl)-3α-[bis (4-fluorophenyl) methoxy]-tropane) (JHW007), which in rodents exhibits a neurochemical and behavioural profile distinct from cocaine, was ineffective. The present findings demonstrate for the first time reinstatement of extinguished cocaine seeking in an invertebrate model and suggest that the long-term adaptations underlying drug conditioning and relapse are highly conserved through evolution.

A plausible neural circuit for decision making and its formation based on reinforcement learning

A human's, or lower insects', behavior is dominated by its nervous system. Each stable behavior has its own inner steps and control rules, and is regulated by a neural circuit. Understanding how the brain influences perception, thought, and behavior is a central mandate of neuroscience. The phototactic flight of insects is a widely observed deterministic behavior. Since its movement is not stochastic, the behavior should be dominated by a neural circuit. Based on the basic firing characteristics of biological neurons and the neural circuit's constitution, we designed a plausible neural circuit for this phototactic behavior from logic perspective. The circuit's output layer, which generates a stable spike firing rate to encode flight commands, controls the insect's angular velocity when flying. The firing pattern and connection type of excitatory and inhibitory neurons are considered in this computational model. We simulated the circuit's information processing using a distributed PC array, and used the real-time average firing rate of output neuron clusters to drive a flying behavior simulation. In this paper, we also explored how a correct neural decision circuit is generated from network flow view through a bee's behavior experiment based on the reward and punishment feedback mechanism. The significance of this study: firstly, we designed a neural circuit to achieve the behavioral logic rules by strictly following the electrophysiological characteristics of biological neurons and anatomical facts. Secondly, our circuit's generality permits the design and implementation of behavioral logic rules based on the most general information processing and activity mode of biological neurons. Thirdly, through computer simulation, we achieved new understanding about the cooperative condition upon which multi-neurons achieve some behavioral control. Fourthly, this study aims in understanding the information encoding mechanism and how neural circuits achieve behavior control. Finally, this study also helps establish a transitional bridge between the microscopic activity of the nervous system and macroscopic animal behavior.

The Transition to Minimal Consciousness through the Evolution of Associative Learning

The minimal state of consciousness is sentience. This includes any phenomenal sensory experience - exteroceptive, such as vision and olfaction; interoceptive, such as pain and hunger; or proprioceptive, such as the sense of bodily position and movement. We propose unlimited associative learning (UAL) as the marker of the evolutionary transition to minimal consciousness (or sentience), its phylogenetically earliest sustainable manifestation and the driver of its evolution. We define and describe UAL at the behavioral and functional level and argue that the structural-anatomical implementations of this mode of learning in different taxa entail subjective feelings (sentience). We end with a discussion of the implications of our proposal for the distribution of consciousness in the animal kingdom, suggesting testable predictions, and revisiting the ongoing debate about the function of minimal consciousness in light of our approach.

New Insights Into the Roles of Retinoic Acid Signaling in Nervous System Development and the Establishment of Neurotransmitter Systems

Secreted chiefly from the underlying mesoderm, the morphogen retinoic acid (RA) is well known to contribute to the specification, patterning, and differentiation of neural progenitors in the developing vertebrate nervous system. Furthermore, RA influences the subtype identity and neurotransmitter phenotype of subsets of maturing neurons, although relatively little is known about how these functions are mediated. This review provides a comprehensive overview of the roles played by RA signaling during the formation of the central and peripheral nervous systems of vertebrates and highlights its effects on the differentiation of several neurotransmitter systems. In addition, the evolutionary history of the RA signaling system is discussed, revealing both conserved properties and alternate modes of RA action. It is proposed that comparative approaches should be employed systematically to expand our knowledge of the context-dependent cellular mechanisms controlled by the multifunctional signaling molecule RA.

A decision underlies phototaxis in an insect

Like a moth into the flame-phototaxis is an iconic example for innate preferences. Such preferences probably reflect evolutionary adaptations to predictable situations and have traditionally been conceptualized as hard-wired stimulus-response links. Perhaps for that reason, the century-old discovery of flexibility in Drosophila phototaxis has received little attention. Here, we report that across several different behavioural tests, light/dark preference tested in walking is dependent on various aspects of flight. If we temporarily compromise flying ability, walking photopreference reverses concomitantly. Neuronal activity in circuits expressing dopamine and octopamine, respectively, plays a differential role in photopreference, suggesting a potential involvement of these biogenic amines in this case of behavioural flexibility. We conclude that flies monitor their ability to fly, and that flying ability exerts a fundamental effect on action selection in Drosophila This work suggests that even behaviours which appear simple and hard-wired comprise a value-driven decision-making stage, negotiating the external situation with the animal's internal state, before an action is selected.

Unexpected rewards induce dopamine-dependent positive emotion–like state changes in bumblebees

Whether invertebrates exhibit positive emotion–like states and what mechanisms underlie such states remain poorly understood. We demonstrate that bumblebees exhibit dopamine-dependent positive emotion–like states across behavioral contexts. After training with one rewarding and one unrewarding cue, bees that received pretest sucrose responded in a positive manner toward ambiguous cues. In a second experiment, pretest consumption of sucrose solution resulted in a shorter time to reinitiate foraging after a simulated predator attack. These behavioral changes were abolished with topical application of the dopamine antagonist fluphenazine. Further experiments established that pretest sucrose does not simply cause bees to become more exploratory. Our findings present a new opportunity for understanding the fundamental neural elements of emotions and may alter the view of how emotion states affect decision-making in animals.

Morphine addiction in ants: a new model for self-administration and neurochemical analysis

Conventional definitions of drug addiction are focused on characterizing the neurophysiological and behavioral responses of mammals. Although mammalian models have been invaluable in studying specific and complex aspects of addiction, invertebrate systems have proven advantageous in investigating how drugs of abuse corrupt the most basic motivational and neurochemical systems. It has recently been shown that invertebrates and mammals have remarkable similarities in their behavioral and neurochemical responses to drugs of abuse. However, until now only mammals have demonstrated drug seeking and self-administration without the concurrent presence of a natural reward, e.g. sucrose. Using a sucrose-fading paradigm, followed by a two-dish choice test, we establish ants as an invertebrate model of opioid addiction. The ant species Camponotus floridanus actively seeks and self-administers morphine even in the absence of caloric value or additional natural reward. Using HPLC equipped with electrochemical detection, the neurochemicals serotonin, octopamine and dopamine were identified and subsequently quantified, establishing the concurrent neurochemical response to the opioid morphine within the invertebrate brain. With this study, we demonstrate dopamine to be governing opioid addiction in the brains of ants. Thus, this study establishes ants as the first non-mammalian model of self-administration that is truly analogous to mammals.

Anticlockwise or clockwise? A dynamic Perception-Action-Laterality model for directionality bias in visuospatial functioning

Orientation bias and directionality bias are two fundamental functional characteristics of the visual system. Reviewing the relevant literature in visual psychophysics and visual neuroscience we propose here a three-stage model of directionality bias in visuospatial functioning. We call this model the 'Perception-Action-Laterality' (PAL) hypothesis. We analyzed the research findings for a wide range of visuospatial tasks, showing that there are two major directionality trends in perceptual preference: clockwise versus anticlockwise. It appears these preferences are combinatorial, such that a majority of people fall in the first category demonstrating a preference for stimuli/objects arranged from left-to-right rather than from right-to-left, while people in the second category show an opposite trend. These perceptual biases can guide sensorimotor integration and action, creating two corresponding turner groups in the population. In support of PAL, we propose another model explaining the origins of the biases - how the neurogenetic factors and the cultural factors interact in a biased competition framework to determine the direction and extent of biases. This dynamic model can explain not only the two major categories of biases in terms of direction and strength, but also the unbiased, unreliably biased or mildly biased cases in visuosptial functioning.

Identification of Multiple Loci Associated with Social Parasitism in Honeybees

In colonies of the honeybee Apis mellifera, the queen is usually the only reproductive female, which produces new females (queens and workers) by laying fertilized eggs. However, in one subspecies of A. mellifera, known as the Cape bee (A. m. capensis), worker bees reproduce asexually by thelytoky, an abnormal form of meiosis where two daughter nucleii fuse to form single diploid eggs, which develop into females without being fertilized. The Cape bee also exhibits a suite of phenotypes that facilitate social parasitism whereby workers lay such eggs in foreign colonies so their offspring can exploit their resources. The genetic basis of this switch to social parasitism in the Cape bee is unknown. To address this, we compared genome variation in a sample of Cape bees with other African populations. We find genetic divergence between these populations to be very low on average but identify several regions of the genome with extreme differentiation. The regions are strongly enriched for signals of selection in Cape bees, indicating that increased levels of positive selection have produced the unique set of derived phenotypic traits in this subspecies. Genetic variation within these regions allows unambiguous genetic identification of Cape bees and likely underlies the genetic basis of social parasitism. The candidate loci include genes involved in ecdysteroid signaling and juvenile hormone and dopamine biosynthesis, which may regulate worker ovary activation and others whose products localize at the centrosome and are implicated in chromosomal segregation during meiosis. Functional analysis of these loci will yield insights into the processes of reproduction and chemical signaling in both parasitic and non-parasitic populations and advance understanding of the process of normal and atypical meiosis.

Catecholaminergic System of Invertebrates: Comparative and Evolutionary Aspects in Comparison With the Octopaminergic System

In this review we examined the catecholaminergic system of invertebrates, starting from protists and getting to chordates. Different techniques used by numerous researchers revealed, in most examined phyla, the presence of catecholamines dopamine, noradrenaline, and adrenaline or of the enzymes involved in their synthesis. The catecholamines are generally linked to the nervous system and they can act as neurotransmitters, neuromodulators, and hormones; moreover they play a very important role as regards the response to a large number of stress situations. Nevertheless, in some invertebrate phyla belonging to Protostoma, the monoamine octopamine is the main biogenic amine. The presence of catecholamines in some protists suggests a role as intracellular or interorganismal signaling molecules and an ancient origin of their synthetic pathways. The catecholamines appear also involved in the regulation of bioluminescence and in the control of larval development and metamorphosis in some marine invertebrate phyla.

Differences in the spatiotemporal expression and epistatic gene regulation of the mesodiencephalic dopaminergic precursor marker PITX3 during chicken and mouse development

Mesodiencephalic dopaminergic (mdDA) neurons are located in the ventral mesencephalon and caudal diencephalon of all tetrapod species studied so far. They are the most prominent DA neuronal population and are implicated in control and modulation of motor, cognitive and rewarding/affective behaviors. Their degeneration or dysfunction is intimately linked to several neurological and neuropsychiatric human diseases. To gain further insights into their generation, we studied spatiotemporal expression patterns and epistatic interactions in chick embryos of selected marker genes and signaling pathways associated with mdDA neuron development in mouse. We detected striking differences in the expression patterns of the chick orthologs of the mouse mdDA marker genes Pitx3 and Aldh1a1, which suggests important differences between the species in the generation/generating of these cells. We also discovered that the Sonic hedgehog signaling pathway is both, necessary and sufficient for the induction of ectopic PITX3 expression in chick mesencephalon downstream of WNT9A induced LMX1a transcription. These aspects of early chicken development resemble the ontogeny of zebrafish diencephalic DA neuronal populations, and suggest a divergence between birds and mammals during evolution.

Omega-3 deficiency impairs honey bee learning

Deficiency in essential omega-3 polyunsaturated fatty acids (PUFAs), particularly the long-chain form of docosahexaenoic acid (DHA), has been linked to health problems in mammals, including many mental disorders and reduced cognitive performance. Insects have very low long-chain PUFA concentrations, and the effect of omega-3 deficiency on cognition in insects has not been studied. We show a low omega-6:3 ratio of pollen collected by honey bee colonies in heterogenous landscapes and in many hand-collected pollens that we analyzed. We identified Eucalyptus as an important bee-forage plant particularly poor in omega-3 and high in the omega-6:3 ratio. We tested the effect of dietary omega-3 deficiency on olfactory and tactile associative learning of the economically highly valued honey bee. Bees fed either of two omega-3-poor diets, or Eucalyptus pollen, showed greatly reduced learning abilities in conditioned proboscis-extension assays compared with those fed omega-3-rich diets, or omega-3-rich pollen mixture. The effect on performance was not due to reduced sucrose sensitivity. Omega-3 deficiency also led to smaller hypopharyngeal glands. Bee brains contained high omega-3 concentrations, which were only slightly affected by diet, suggesting additional peripheral effects on learning. The shift from a low to high omega-6:3 ratio in the Western human diet is deemed a primary cause of many diseases and reduced mental health. A similar shift seems to be occurring in bee forage, possibly an important factor in colony declines. Our study shows the detrimental effect on cognitive performance of omega-3 deficiency in a nonmammal.

Association of Amine-Receptor DNA Sequence Variants with Associative Learning in the Honeybee

Octopamine- and dopamine-based neuromodulatory systems play a critical role in learning and learning-related behaviour in insects. To further our understanding of these systems and resulting phenotypes, we quantified DNA sequence variations at six loci coding octopamine-and dopamine-receptors and their association with aversive and appetitive learning traits in a population of honeybees. We identified 79 polymorphic sequence markers (mostly SNPs and a few insertions/deletions) located within or close to six candidate genes. Intriguingly, we found that levels of sequence variation in the protein-coding regions studied were low, indicating that sequence variation in the coding regions of receptor genes critical to learning and memory is strongly selected against. Non-coding and upstream regions of the same genes, however, were less conserved and sequence variations in these regions were weakly associated with between-individual differences in learning-related traits. While these associations do not directly imply a specific molecular mechanism, they suggest that the cross-talk between dopamine and octopamine signalling pathways may influence olfactory learning and memory in the honeybee.

Perturbations in dopamine synthesis lead to discrete physiological effects and impact oxidative stress response in Drosophila

The impact of mutations in four essential genes involved in dopamine (DA) synthesis and transport on longevity, motor behavior, and resistance to oxidative stress was monitored in Drosophila melanogaster. The fly lines used for this study were: (i) a loss of function mutation in Catecholamines up (Catsup(26)), which is a negative regulator of the rate limiting enzyme for DA synthesis, (ii) a mutant for the gene pale (ple(2)) that encodes for the rate limiting enzyme tyrosine hydroxylase (TH), (iii) a mutant for the gene Punch (Pu(Z22)) that encodes guanosine triphosphate cyclohydrolase, required for TH activity, and (iv) a mutant in the vesicular monoamine transporter (VMAT(Δ14)), which is required for packaging of DA as vesicles inside DA neurons. Median lifespans of ple(2), Pu(Z22) and VMAT(Δ14) mutants were significantly decreased compared to Catsup(26) and wild type controls that did not significantly differ between each other. Catsup(26) flies survived longer when exposed to hydrogen peroxide (80 μM) or paraquat (10mM) compared to ple(2), Pu(Z22) or VMAT(Δ14) and controls. These flies also exhibited significantly higher negative geotaxis activity compared to ple(2), Pu(Z22), VMAT(Δ14) and controls. All mutant flies demonstrated rhythmic circadian locomotor activity in general, albeit Catsup(26) and VMAT(Δ14) flies had slightly weaker rhythms. Expression analysis of some key antioxidant genes revealed that glutathione S-transferase Omega-1 (GSTO1) expression was significantly up-regulated in all DA synthesis pathway mutants and especially in Catsup(26) and VMAT(Δ14) flies at both mRNA and protein levels. Taken together, we hypothesize that DA could directly influence GSTO1 transcription and thus play a significant role in the regulation of response to oxidative stress. Additionally, perturbations in DA synthesis do not appear to have a significant impact on circadian locomotor activity rhythms per se, but do have an influence on general locomotor activity levels.

Exploration versus exploitation in space, mind, and society

Search is a ubiquitous property of life. Although diverse domains have worked on search problems largely in isolation, recent trends across disciplines indicate that the formal properties of these problems share similar structures and, often, similar solutions. Moreover, internal search (e.g., memory search) shows similar characteristics to external search (e.g., spatial foraging), including shared neural mechanisms consistent with a common evolutionary origin across species. Search problems and their solutions also scale from individuals to societies, underlying and constraining problem solving, memory, information search, and scientific and cultural innovation. In summary, search represents a core feature of cognition, with a vast influence on its evolution and processes across contexts and requiring input from multiple domains to understand its implications and scope.

Cocaine affects foraging behaviour and biogenic amine modulated behavioural reflexes in honey bees

In humans and other mammals, drugs of abuse alter the function of biogenic amine pathways in the brain leading to the subjective experience of reward and euphoria. Biogenic amine pathways are involved in reward processing across diverse animal phyla, however whether cocaine acts on these neurochemical pathways to cause similar rewarding behavioural effects in animal phyla other than mammals is unclear. Previously, it has been shown that bees are more likely to dance (a signal of perceived reward) when returning from a sucrose feeder after cocaine treatment. Here we examined more broadly whether cocaine altered reward-related behaviour, and biogenic amine modulated behavioural responses in bees. Bees developed a preference for locations at which they received cocaine, and when foraging at low quality sucrose feeders increase their foraging rate in response to cocaine treatment. Cocaine also increased reflexive proboscis extension to sucrose, and sting extension to electric shock. Both of these simple reflexes are modulated by biogenic amines. This shows that systemic cocaine treatment alters behavioural responses that are modulated by biogenic amines in insects. Since insect reward responses involve both octopamine and dopamine signalling, we conclude that cocaine treatment altered diverse reward-related aspects of behaviour in bees. We discuss the implications of these results for understanding the ecology of cocaine as a plant defence compound. Our findings further validate the honey bee as a model system for understanding the behavioural impacts of cocaine, and potentially other drugs of abuse.

A fighter's comeback: dopamine is necessary for recovery of aggression after social defeat in crickets

Social defeat, i.e. losing an agonistic dispute with a conspecific, is followed by a period of suppressed aggressiveness in many animal species, and is generally regarded as a major stressor, which may play a role in psychiatric disorders such as depression and post-traumatic stress disorder. Despite numerous animal models, the mechanisms underlying loser depression and subsequent recovery are largely unknown. This study on crickets is the first to show that a neuromodulator, dopamine (DA), is necessary for recovery of aggression after social defeat. Crickets avoid any conspecific male just after defeat, but regain their aggressiveness over 3 h. This recovery was prohibited after depleting nervous stores of DA and octopamine (OA, the invertebrate analogue of noradrenaline) with α-methyl-tyrosine (AMT). Loser recovery was also prohibited by the insect DA-receptor (DAR) antagonist fluphenazine, but not the OA-receptor (OAR) blocker epinastine, or yohimbine, which blocks receptors for OA's precursor tyramine. Conversely, aggression was restored prematurely in both untreated and amine depleted losers given either chlordimeform (CDM), a tissue permeable OAR-agonist, or the DA-metabolite homovanillyl alcohol (HVA), a component of the honeybee queen mandibular pheromone. As in honeybees, HVA acts in crickets as a DAR-agonist since its aggression promoting effect on losers was selectively blocked by the DAR-antagonist, but not by the OAR-antagonist. Conversely, CDM's aggression promoting effect was selectively blocked by the OAR-antagonist, but not the DAR-antagonist. Hence, only DA is necessary for recovery of aggressiveness after social defeat, although OA can promote loser aggression independently to enable experience dependent adaptive responses.

Neurogenomics of behavioral plasticity

Across animals, there is remarkable diversity in behavior. Modern genomic approaches have made it possible to identify the molecular underpinnings of varied behavioral phenotypes. By examining species with plastic phenotypes we have begun to understand the dynamic and flexible nature of neural transcriptomes and identified gene modules associated with variation in social and reproductive behaviors in diverse species. Importantly, it is becoming increasingly clear that some candidate genes and gene networks are involved in complex social behaviors across even divergent species, yet few comparative transcriptomics studies have been conducted that examine a specific behavior across species. We discuss the implications of a range of important and insightful studies that have increased our understanding of the neurogenomics of behavioral plasticity. Despite its successes, behavioral genomics has been criticized for its lack of hypotheses and causative insights. We propose here a novel avenue to overcome some of these short-comings by complementing "forward genomics" studies (i.e., from phenotype to behaviorally relevant gene modules) with a "reverse genomics" approach (i.e., manipulating novel gene modules to examine effects on behavior, hormones, and the genome itself) to examine the functional causes and consequences of differential gene expression patterns. We discuss how several established approaches (such as pharmacological manipulations of a novel candidate pathway, fine scale mapping of novel candidate gene expression in the brain, or identifying direct targets of a novel transcription factor of interest) can be used in combination with the analysis of the accompanying neurogenomic responses to reveal unexpected biological processes. The integration of forward and reverse genomics will move the field beyond statistical associations and yield great insights into the neural and molecular control of social behavior and its evolution.

Molecular, pharmacological, and signaling properties of octopamine receptors from honeybee (Apis mellifera) brain

G protein-coupled receptors are important regulators of cellular signaling processes. Within the large family of rhodopsin-like receptors, those binding to biogenic amines form a discrete subgroup. Activation of biogenic amine receptors leads to transient changes of intracellular Ca²⁺-([Ca²⁺](i)) or 3',5'-cyclic adenosine monophosphate ([cAMP](i)) concentrations. Both second messengers modulate cellular signaling processes and thereby contribute to long-lasting behavioral effects in an organism. In vivo pharmacology has helped to reveal the functional effects of different biogenic amines in honeybees. The phenolamine octopamine is an important modulator of behavior. Binding of octopamine to its receptors causes elevation of [Ca²⁺](i) or [cAMP](i). To date, only one honeybee octopamine receptor that induces Ca²⁺ signals has been molecularly and pharmacologically characterized. Here, we examined the pharmacological properties of four additional honeybee octopamine receptors. When heterologously expressed, all receptors induced cAMP production after binding to octopamine with EC₅₀(s) in the nanomolar range. Receptor activity was most efficiently blocked by mianserin, a substance with antidepressant activity in vertebrates. The rank order of inhibitory potency for potential receptor antagonists was very similar on all four honeybee receptors with mianserin >> cyproheptadine > metoclopramide > chlorpromazine > phentolamine. The subroot of octopamine receptors activating adenylyl cyclases is the largest that has so far been characterized in arthropods, and it should now be possible to unravel the contribution of individual receptors to the physiology and behavior of honeybees.

Catecholamine levels in groundwater and stream amphipods and their response to temperature stress

Temperature stress in invertebrates is known to be reflected by changes in catecholamine levels. However, the mechanisms of stress response are not fully understood. Groundwater and surface water amphipods are expected to be differently adapted to temperature elevations due to the different temperature regimes in their habitats and consequently, show a different stress response. No data have been published so far regarding the effects of stress on catecholamine patterns in groundwater invertebrates and accordingly, comparisons with surface water fauna are also missing. In this study, we compared the average catecholamine levels in two taxonomically related amphipod species: Niphargus inopinatus, living in groundwater with constant water temperatures throughout the year, and Gammarus pulex, a surface water stream amphipod frequently exposed to diurnal and seasonal temperature fluctuations. Furthermore, we tracked the immediate changes in whole-animal catecholamine levels in response to heat stress in both species. Pronounced differences in the catecholamine levels of the two species became apparent, with the average dopamine (DA) level of N. inopinatus being almost 1000 times higher than that in G. pulex. The noradrenaline (NA) concentrations in N. inopinatus were on average two orders of magnitude higher than in G. pulex, and for adrenaline (A), the difference constituted one order of magnitude. When exposed to short-term heat stress, both species showed a response in terms of catecholamine levels, but the observed patterns were different. In N. inopinatus, temperature stress was reflected by the appearance of adrenaline, while in G. pulex a significant increase in noradrenaline levels occurred in the treatment with the highest temperature elevation.

Invertebrate models in addiction research

While drug addiction is a uniquely human problem, most research examining the biological mechanisms of the transition from substance use to addiction is conducted with vertebrate animal models. Many other fields of neuroscience have greatly benefitted from contributions from simple and manipulable invertebrate model systems. However, the potential of invertebrate research has yet to be fully capitalised on in the field of addiction neuroscience. This may be because of the complexity of addiction and the clinical imperative of addiction research. We argue that the homocentric diagnostic criteria of addiction are no more a hindrance to the use of invertebrate models than they are to vertebrate models. We highlight the strengths of the diversity of different invertebrate model systems in terms of neuroanatomy and molecular machinery, and stress that working with a range of different models will aid in understanding addiction and not be a disadvantage. Finally, we discuss the specific advantages of utilising invertebrate animals for addiction research and highlight key areas in which invertebrates are suited for making unique and meaningful contributions to this field.

Dopamine interferes with appetitive long-term memory formation in honey bees

Studies in vertebrates and invertebrates have proved the instructive role that different biogenic amines play in the neural representation of rewards and punishments during associative learning. Results from diverse arthropods and using different learning paradigms initially agreed that dopamine (DA) is needed for aversive learning and octopamine (OA) is needed for appetitive learning. However, the notion that both amines constitute separate pathways for appetitive and aversive learning is changing. Here, we asked whether DA, so far only involved in aversive memory formation in honey bees, does also modulate appetitive memory. Using the well characterized appetitive olfactory conditioning of the proboscis extension reflex (PER), we show that DA impairs appetitive memory consolidation. In addition, we found that blocking DA receptors enhances appetitive memory. These results are consistent with the view that aversive and appetitive components interact during learning and memory formation to ensure adaptive behavior.

Invertebrate learning and cognition: relating phenomena to neural substrate

Diverse invertebrate species have been used for studies of learning and comparative cognition. Although we have gained invaluable information from this, in this study we argue that our approach to comparative learning research is rather deficient. Generally invertebrate learning research has focused mainly on arthropods, and most of that within the Hymenoptera and Diptera. Any true comparative analysis of the distribution of comparative cognitive abilities across phyla is hampered by this bias, and more fundamentally by a reporting bias toward positive results. To understand the limits of learning and cognition for a species, knowing what animals cannot do is at least as important as reporting what they can. Finally, much more effort needs to be focused on the neurobiological analysis of different types of learning to truly understand the differences and similarities of learning types. In this review, we first give a brief overview of the various forms of learning in invertebrates. We also suggest areas where further study is needed for a more comparative understanding of learning. Finally, using what is known of learning in honeybees and the well-studied honeybee brain, we present a model of how various complex forms of learning may be accounted for with the same neural circuitry required for so-called simple learning types. At the neurobiological level, different learning phenomena are unlikely to be independent, and without considering this it is very difficult to correctly interpret the phylogenetic distribution of learning and cognitive abilities. WIREs Cogn Sci 2013, 4:561-582. doi: 10.1002/wcs.1248 For further resources related to this article, please visit the WIREs website.

Dispensable, redundant, complementary, and cooperative roles of dopamine, octopamine, and serotonin in Drosophila melanogaster

To investigate the regulation of Drosophila melanogaster behavior by biogenic amines, we have exploited the broad requirement of the vesicular monoamine transporter (VMAT) for the vesicular storage and exocytotic release of all monoamine neurotransmitters. We used the Drosophila VMAT (dVMAT) null mutant to globally ablate exocytotic amine release and then restored DVMAT activity in either individual or multiple aminergic systems, using transgenic rescue techniques. We find that larval survival, larval locomotion, and female fertility rely predominantly on octopaminergic circuits with little apparent input from the vesicular release of serotonin or dopamine. In contrast, male courtship and fertility can be rescued by expressing DVMAT in octopaminergic or dopaminergic neurons, suggesting potentially redundant circuits. Rescue of major aspects of adult locomotion and startle behavior required octopamine, but a complementary role was observed for serotonin. Interestingly, adult circadian behavior could not be rescued by expression of DVMAT in a single subtype of aminergic neurons, but required at least two systems, suggesting the possibility of unexpected cooperative interactions. Further experiments using this model will help determine how multiple aminergic systems may contribute to the regulation of other behaviors. Our data also highlight potential differences between behaviors regulated by standard exocytotic release and those regulated by other mechanisms.

Neural mechanisms of reward in insects

Reward seeking is a major motivator and organizer of behavior, and animals readily learn to modify their behavior to more easily obtain reward, or to respond to stimuli that are predictive of reward. Here, we compare what is known of reward processing mechanisms in insects with the well-studied vertebrate reward systems. In insects almost all of what is known of reward processing is derived from studies of reward learning. This is localized to the mushroom bodies and antennal lobes and organized by a network of hierarchically arranged modulatory circuits, especially those involving octopamine and dopamine. Neurogenetic studies with Drosophila have identified distinct circuit elements for reward learning, "wanting," and possibly "liking" in Drosophila, suggesting a modular structure to the insect reward processing system, which broadly parallels that of the mammals in terms of functional organization.

Immunological and pharmacological identification of the dopamine D1 receptor in the CNS of the pond snail, Lymnaea stagnalis

We investigated the presence and distribution of the D1 dopamine receptor in the CNS of Lymnaea stagnalis applying immunobloting and immunocytochemistry. We also investigated the effect of dopamine as well as the specific D1 receptor blocker, SCH23390, on the firing activity of the feeding modulator serotonergic neuron, CGC, which displayed D1 immunoreactivity. Immunoblot experiments showed one specifically labeled band with 62 kDa mw which is close to that of the mammalian D1 receptor. Neurons displaying D1-like immunoreactivity can be observed in each ganglion of the CNS but particularly in the pedal ganglia which are the center for locomotion. Dopamine regularly evokes burst activity in the serotonergic CGC at 1 mM and this effect could be antagonized by SCH23390. These observations suggest that a D1-like receptor molecule is present in the CNS of Lymnaea.

The decision to fight or flee - insights into underlying mechanism in crickets

Ritualized fighting between conspecifics is an inherently dangerous behavioral strategy, optimized to secure limited resources at minimal cost and risk. To be adaptive, potential rewards, and costs of aggression must be assessed to decide when it would be more opportune to fight or flee. We summarize insights into the proximate mechanisms underlying this decision-making process in field crickets. As in other animals, cricket aggression is enhanced dramatically by motor activity, winning, and the possession of resources. Pharmacological manipulations provide evidence that these cases of experience dependent enhancement of aggression are each mediated by octopamine, the invertebrate counterpart to adrenaline/noradrenaline. The data suggest that both physical exertion and rewarding aspects of experiences can activate the octopaminergic system, which increases the propensity to fight. Octopamine thus represents the motivational component of aggression in insects. For the decision to flee, animals are thought to assess information from agonistic signals exchanged during fighting. Cricket fights conform to the cumulative assessment model, in that they persist in fighting until the sum of their opponent’s actions accumulates to some threshold at which they withdraw. We discuss evidence that serotonin, nitric oxide, and some neuropeptides may promote an insect’s tendency to flee. We propose that the decision to fight or flee in crickets is controlled simply by relative behavioral thresholds. Rewarding experiences increase the propensity to fight to a level determined by the modulatory action of octopamine. The animal will then flee only when the accumulated sum of the opponent’s actions surpasses this level; serotonin and nitric oxide may be involved in this process. This concept is in line with the roles proposed for noradrenaline, serotonin, and nitric oxide in mammals and suggests that basic mechanisms of aggressive modulation may be conserved in phylogeny.

A novel and conserved protein AHO-3 is required for thermotactic plasticity associated with feeding states in Caenorhabditis elegans

Although a large proportion of molecules expressed in the nervous system are conserved from invertebrate to vertebrate, functional properties of such molecules are less characterized. Here, we show that highly conserved hydrolase AHO-3 acts as a novel regulator of starvation-induced thermotactic plasticity in Caenorhabditis elegans. As wild-type animals, aho-3 mutants migrated to the cultivation temperature on a linear thermal gradient after cultivation at a particular temperature with food. Whereas wild-type animals cultivated under food-deprived condition showed dispersed distribution on the gradient, aho-3 mutants exhibited tendency to migrate toward higher temperature. Such an abnormal behavior was completely rescued by the expression of human homologue of AHO-3, indicating that the molecular function of AHO-3 is highly conserved between nematode and human. The behavioral regulation by AHO-3 requires the N-terminal cysteine cluster, which ensures the proper subcellular localization of AHO-3 to sensory endings. Double-mutant analysis suggested that AHO-3 acts in the same pathway with ODR-3, a heterotrimeric G protein alpha subunit. Our results unveiled a novel neural protein in C. elegans, confirming its conserved role in behavioral regulation.

Winning fights induces hyperaggression via the action of the biogenic amine octopamine in crickets

Winning an agonistic interaction against a conspecific is known to heighten aggressiveness, but the underlying events and mechanism are poorly understood. We quantified the effect of experiencing successive wins on aggression in adult male crickets (Gryllus bimaculatus) by staging knockout tournaments and investigated its dependence on biogenic amines by treatment with amine receptor antagonists. For an inter-fight interval of 5 min, fights between winners escalated to higher levels of aggression and lasted significantly longer than the preceding round. This winner effect is transient, and no longer evident for an inter-fight interval of 20 min, indicating that it does not result from selecting individuals that were hyper-aggressive from the outset. A winner effect was also evident in crickets that experienced wins without physical exertion, or that engaged in fights that were interrupted before a win was experienced. Finally, the winner effect was abolished by prior treatment with epinastine, a highly selective octopamine receptor blocker, but not by propranolol, a ß-adrenergic receptor antagonist, nor by yohimbine, an insect tyramine receptor blocker nor by fluphenazine an insect dopamine-receptor blocker. Taken together our study in the cricket indicates that the physical exertion of fighting, together with some rewarding aspect of the actual winning experience, leads to a transient increase in aggressive motivation via activation of the octopaminergic system, the invertebrate equivalent to the adrenergic system of vertebrates.

Dopamine and octopamine influence avoidance learning of honey bees in a place preference assay

Biogenic amines are widely characterized in pathways evaluating reward and punishment, resulting in appropriate aversive or appetitive responses of vertebrates and invertebrates. We utilized the honey bee model and a newly developed spatial avoidance conditioning assay to probe effects of biogenic amines octopamine (OA) and dopamine (DA) on avoidance learning. In this new protocol non-harnessed bees associate a spatial color cue with mild electric shock punishment. After a number of experiences with color and shock the bees no longer enter the compartment associated with punishment. Intrinsic aspects of avoidance conditioning are associated with natural behavior of bees such as punishment (lack of food, explosive pollination mechanisms, danger of predation, heat, etc.) and their association to floral traits or other spatial cues during foraging. The results show that DA reduces the punishment received whereas octopamine OA increases the punishment received. These effects are dose-dependent and specific to the acquisition phase of training. The effects during acquisition are specific as shown in experiments using the antagonists Pimozide and Mianserin for DA and OA receptors, respectively. This study demonstrates the integrative role of biogenic amines in aversive learning in the honey bee as modeled in a novel non-appetitive avoidance learning assay.