Ingestion of artificial sweeteners leads to caloric frustration memory in Drosophila

Dynamics of two distinct memory interactions during water seeking in Drosophila

Forming and forgetting memories shape our self-awareness and help us face future challenges. Therefore, understanding how memories are formed and how different memories interact in the brain is important. Previous studies have shown that thirsty flies sense humidity through ionotropic receptors, which help them locate water sources. Here, we showed that thirsty flies can be trained to associate specific odors with humidity to form a humidity memory that lasts for 30 min after association. Humidity memory formation requires the Ir93a and Ir40a ionotropic receptors, which are essential for environmental humidity sensing. Water memory takes precedence, leading to the forgetting of humidity memory by activating a small subset of dopaminergic neurons called protocerebral anterior medial (PAM)-γ4, that project to the restricted region of the mushroom body (MB) γ lobes. Adult-stage-specific silencing of Dop2R dopaminergic receptors in MB γ neurons prolongs humidity memory for 3 h. Live-brain calcium imaging and dopamine sensor studies revealed significantly increased PAM-γ4 neural activity after odor/humidity association, suggesting its role in forgetting the humidity memory. Our results suggest that overlapping neural circuits are responsible for the acquisition of water memory and forgetting humidity memory in thirsty flies.

The impact of sugar diet on humidity preference, survival, and host landing in mosquitoes

Mosquito-borne diseases have caused more than 1 million deaths each year. There is an urgent need to develop an effective way to reduce mosquito-host interaction to mitigate disease transmission. Sugar diets have long been linked to abnormal physiology in animals, making them potential candidates for mosquito control. Here, we show the impact of sugar diets on humidity preference and survival in Aedes aegypti (Gainesville) and Culex pipiens (Buckeye). Two-choice assays with high and low relative humidity (80% and 50% RH) show that the impact of sugar diets on humidity preference is species-specific. In comparison to Cx. pipiens, various sugar diets resulted in marked reductions in humidity avidity and preference in Ae. aegypti, which exhibited significant differences. Among the sugar diets, arabinose significantly reduced the survival rate of mosquitoes at low concentrations. Moreover, we found that host landing was not impacted by feeding on different sugar types. Our study suggests that specific sugar treatments could be applied to mosquito control by dampening their humidity preference and reducing their lifespan, thus reducing mosquito-borne disease transmission.

The impact of sugar diet on humidity preference, survival, and host landing in mosquitoes

Mosquito-borne diseases have caused more than one million deaths each year. There is an urgent need to develop an effective way to reduce mosquito-host interaction to mitigate disease transmission. Sugar diets have long been linked to abnormal physiology in animals, making them potential candidates for mosquito control. Here, we show the impact of sugar diets on humidity preference and survival in Aedes aegypti and Culex pipiens . With two-choice assays between 100% and 75% relative humidity (RH), we demonstrate that the effect of sugar diets on humidity preference is species-specific where Ae. aegypti showed significant differences and the reduced effects were noted in Cx. pipiens . Among the sugar diets, arabinose significantly reduced the survival rate of mosquitoes even at low concentrations. Moreover, we found that host landing was not impacted by feeding on different sugar types. Our study suggests that specific sugar treatments could be applied to mosquito control by dampening their humidity preference and reducing their lifespan, thus reducing mosquito-borne disease transmission.

Food memory circuits regulate eating and energy balance

In mammals, learning circuits play an essential role in energy balance by creating associations between sensory cues and the rewarding qualities of food. This process is altered by diet-induced obesity, but the causes and mechanisms are poorly understood. Here, we exploited the relative simplicity and wealth of knowledge about the D. melanogaster reinforcement learning network, the mushroom body, in order to study the relationship between the dietary environment, dopamine-induced plasticity, and food associations. We show flies that are fed a high-sugar diet cannot make associations between sensory cues and the rewarding properties of sugar. This deficit was caused by diet exposure, not fat accumulation, and specifically by lower dopamine-induced plasticity onto mushroom body output neurons (MBONs) during learning. Importantly, food memories dynamically tune the output of MBONs during eating, which instead remains fixed in sugar-diet animals. Interestingly, manipulating the activity of MBONs influenced eating and fat mass, depending on the diet. Altogether, this work advances our fundamental understanding of the mechanisms, causes, and consequences of the dietary environment on reinforcement learning and ingestive behavior.

Nutrient responding peptide hormone CCHamide-2 consolidates appetitive memory

CCHamide-2 (CCHa2) is a protostome excitatory peptide ortholog known for various arthropod species. In fruit flies, CCHa2 plays a crucial role in the endocrine system, allowing peripheral tissue to communicate with the central nervous system to ensure proper development and the maintenance of energy homeostasis. Since the formation of odor-sugar associative long-term memory (LTM) depends on the nutrient status in an animal, CCHa2 may play an essential role in linking memory and metabolic systems. Here we show that CCHa2 signals are important for consolidating appetitive memory by acting on the rewarding dopamine neurons. Genetic disruption of CCHa2 using mutant strains abolished appetitive LTM but not short-term memory (STM). A post-learning thermal suppression of CCHa2 expressing cells impaired LTM. In contrast, a post-learning thermal activation of CCHa2 cells stabilized STM induced by non-nutritious sugar into LTM. The receptor of CCHa2, CCHa2-R, was expressed in a subset of dopamine neurons that mediate reward for LTM. In accordance, the receptor expression in these dopamine neurons was required for LTM specifically. We thus concluded that CCHa2 conveys a sugar nutrient signal to the dopamine neurons for memory consolidation. Our finding establishes a direct interplay between brain reward and the putative endocrine system for long-term energy homeostasis.

Emotional states: Sweet relief for depressed flies

Mammals and insects appear to have emotional states with features characteristic of human depression. A new study has defined a neural circuit including serotonergic neurons that drive sugar-induced relief from a depression-like-state in Drosophila.

Octopamine mediates sugar relief from a chronic-stress-induced depression-like state in Drosophila

Chronic, uncontrollable stress can result in psychiatric syndromes, including anxiety and major depressive disorder, in humans and mammalian disease models.^1,2 Similarly, several days of chronic stress can induce depression-associated behavioral alteration in Drosophila accompanied by changes in biogenic amine levels in the adult brain.^3-6 In our chronic stress paradigm, flies are subjected to 3 days of repetitive phases of 300 Hz vibrations combined with overcrowding and food deprivation. This treatment reduces voluntary behavioral activity, including the motivation to climb wide gaps (risk taking) and to stop for sweets (anhedonia), suggesting a depression-like state (DLS). These behavioral changes correlate with decreased serotonin release to the mushroom body (MB), a major behavioral control center in the central brain of the fly.^7,8 Stressed flies are relieved from the DLS by feeding the anti-depressant serotonin precursor 5-HTP or the selective serotonin reuptake inhibitor fluoxetine. Notably, feeding sucrose to stressed flies results in elevated serotonin levels in the brain and ameliorates the DLS.³ Here, we show that this sugar relief is mediated by the neurotransmitter octopamine signaled from ventral unpaired medial neurons located in the subesophageal ganglion. The octopamine signaling of sweet sensation is transmitted to the MB via the dopaminergic PAM neurons. In addition, neuronal-silencing experiments reveal that the serotonergic dorsal paired medial (DPM) neurons innervating the MB are essential for sugar relief. Conversely, thermogenetic or optogenetic activation of DPMs can replace sweet sensation, elucidating that serotonergic signaling from DPMs takes part in positively modulating DLS-related behavioral changes.

Excessive energy expenditure due to acute physical restraint disrupts Drosophila motivational feeding response

To study the behavior of Drosophila, it is often necessary to restrain and mount individual flies. This requires removal from food, additional handling, anesthesia, and physical restraint. We find a strong positive correlation between the length of time flies are mounted and their subsequent reflexive feeding response, where one hour of mounting is the approximate motivational equivalent to ten hours of fasting. In an attempt to explain this correlation, we rule out anesthesia side-effects, handling, additional fasting, and desiccation. We use respirometric and metabolic techniques coupled with behavioral video scoring to assess energy expenditure in mounted and free flies. We isolate a specific behavior capable of exerting large amounts of energy in mounted flies and identify it as an attempt to escape from restraint. We present a model where physical restraint leads to elevated activity and subsequent faster nutrient storage depletion among mounted flies. This ultimately further accelerates starvation and thus increases reflexive feeding response. In addition, we show that the consequences of the physical restraint profoundly alter aerobic activity, energy depletion, taste, and feeding behavior, and suggest that careful consideration is given to the time-sensitive nature of these highly significant effects when conducting behavioral, physiological or imaging experiments that require immobilization.

Prolonged Consumption of Sweetened Beverages Lastingly Deteriorates Cognitive Functions and Reward Processing in Mice

We investigated the detrimental effects of chronic consumption of sweet or sweetened beverages in mice. We report that consumption of beverages containing small amounts of sucrose during several weeks impaired reward systems. This is evidenced by robust changes in the activation pattern of prefrontal brain regions associated with abnormal risk-taking and delayed establishment of decision-making strategy. Supporting these findings, we find that chronic consumption of low doses of artificial sweeteners such as saccharin disrupts brain regions' activity engaged in decision-making and reward processes. Consequently, this leads to the rapid development of inflexible decisions, particularly in a subset of vulnerable individuals. Our data also reveal that regular consumption, even at low doses, of sweet or sweeteners dramatically alters brain neurochemistry, i.e., dopamine content and turnover, and high cognitive functions, while sparing metabolic regulations. Our findings suggest that it would be relevant to focus on long-term consequences on the brain of sweet or sweetened beverages in humans, especially as they may go metabolically unnoticed.

The Panopticon-Assessing the Effect of Starvation on Prolonged Fly Activity and Place Preference

Animal behaviours are demonstrably governed by sensory stimulation, previous experience and internal states like hunger. With increasing hunger, priorities shift towards foraging and feeding. During foraging, flies are known to employ efficient path integration strategies. However, general long-term activity patterns for both hungry and satiated flies in conditions of foraging remain to be better understood. Similarly, little is known about how permanent contact chemosensory stimulation affects locomotion. To address these questions, we have developed a novel, simplistic fly activity tracking setup—the Panopticon. Using a 3D-printed Petri dish inset, our assay allows recording of walking behaviour, of several flies in parallel, with all arena surfaces covered by a uniform substrate layer. We tested two constellations of providing food: (i) in single patches and (ii) omnipresent within the substrate layer. Fly tracking is done with FIJI, further assessment, analysis and presentation is done with a custom-built MATLAB analysis framework. We find that starvation history leads to a long-lasting reduction in locomotion, as well as a delayed place preference for food patches which seems to be not driven by immediate hunger motivation.

Confection Confusion: Interplay Between Diet, Taste, and Nutrition

Although genetics shapes our sense of taste to prefer some foods over others, taste sensation is plastic and changes with age, disease state, and nutrition. We have known for decades that diet composition can influence the way we perceive foods, but many questions remain unanswered, particularly regarding the effects of chemosensory plasticity on feeding behavior. Here, we review recent evidence on the effects of high-nutrient diets, especially high dietary sugar, on sweet taste in vinegar flies, rodents, and humans, and discuss open questions about molecular and neural mechanisms and research priorities. We also consider ways in which diet-dependent chemosensory plasticity may influence food intake and play a role in the etiology of obesity and metabolic disease. Understanding the interplay between nutrition, taste sensation, and feeding will help us define the role of the food environment in mediating chronic disease and design better public health strategies to combat it.

Changing memories on the fly: the neural circuits of memory re-evaluation in Drosophila melanogaster

Associative learning leads to modifications in neural networks to assign valence to sensory cues. These changes not only allow the expression of learned behavior but also modulate subsequent learning events. In the brain of the adult fruit fly, Drosophila melanogaster, olfactory memories are established as dopamine-driven plasticity in the output of a highly recurrent network, the mushroom body. Recent findings have highlighted how these changes in the network can steer the strengthening, weakening and formation of parallel memories when flies are exposed to subsequent training trials, conflicting situations or the reversal of contingencies. Together, these processes provide an initial understanding of how learned information can be used to guide the re-evaluation of memories.

Reward foraging task and model-based analysis reveal how fruit flies learn value of available options

Foraging animals have to evaluate, compare and select food patches in order to increase their fitness. Understanding what drives foraging decisions requires careful manipulation of the value of alternative options while monitoring animals choices. Value-based decision-making tasks in combination with formal learning models have provided both an experimental and theoretical framework to study foraging decisions in lab settings. While these approaches were successfully used in the past to understand what drives choices in mammals, very little work has been done on fruit flies. This is despite the fact that fruit flies have served as model organism for many complex behavioural paradigms. To fill this gap we developed a single-animal, trial-based decision making task, where freely walking flies experienced optogenetic sugar-receptor neuron stimulation. We controlled the value of available options by manipulating the probabilities of optogenetic stimulation. We show that flies integrate reward history of chosen options and forget value of unchosen options. We further discover that flies assign higher values to rewards experienced early in the behavioural session, consistent with formal reinforcement learning models. Finally, we also show that the probabilistic rewards affect walking trajectories of flies, suggesting that accumulated value is controlling the navigation vector of flies in a graded fashion. These findings establish the fruit fly as a model organism to explore the genetic and circuit basis of reward foraging decisions.

Effects of Non-nutritive Sweeteners on Sweet Taste Processing and Neuroendocrine Regulation of Eating Behavior

PURPOSE OF REVIEW

Non-nutritive sweeteners (NNS) are increasingly used as a replacement for nutritive sugars as means to quench the desire for "sweets" while contributing few or no dietary calories. However, there is concern that NNS may uncouple the evolved relationship between sweet taste and post-ingestive neuroendocrine signaling. In this review, we examine the effects of NNS exposure on neural and peripheral systems in humans.

RECENT FINDINGS

NNS exposure during early development may influence sweet taste preferences, and NNS consumption might increase motivation for sweet foods. Neuroimaging studies provide evidence that NNS elicit differential neuronal responsivity in areas related to reward and satiation, compared with caloric sweeteners. Findings are heterogenous regarding whether NNS affect physiological responses. Additional studies are warranted regarding the consequences of NNS on metabolic outcomes and neuroendocrine pathways. Given the widespread popularity of NNS, future studies are essential to establish their role in long-term health.

Multiple neurons encode CrebB dependent appetitive long-term memory in the mushroom body circuit

Lasting changes in gene expression are critical for the formation of long-term memories (LTMs), depending on the conserved CrebB transcriptional activator. While requirement of distinct neurons in defined circuits for different learning and memory phases have been studied in detail, only little is known regarding the gene regulatory changes that occur within these neurons. We here use the fruit fly as powerful model system to study the neural circuits of CrebB-dependent appetitive olfactory LTM. We edited the CrebB locus to create a GFP-tagged CrebB conditional knockout allele, allowing us to generate mutant, post-mitotic neurons with high spatial and temporal precision. Investigating CrebB-dependence within the mushroom body (MB) circuit we show that MB α/β and α’/β’ neurons as well as MBON α3, but not in dopaminergic neurons require CrebB for LTM. Thus, transcriptional memory traces occur in different neurons within the same neural circuit.

Our brains can store different types of memories. You may have forgotten what you had for lunch yesterday, but still be able to remember a song from your childhood. Short-term memories and long-term memories form via different mechanisms. To establish long-term memories, the brain must produce new proteins, many of which are common to all members of the animal kingdom. By studying these proteins in organisms such as fruit flies, we can learn more about their role in our own memories.

Widmer et al. used this approach to explore how a protein called CrebB helps fruit flies to remember for several days that a specific odor is associated with a sugary reward. These odor-reward memories form in a brain region called the mushroom body, which has three lobes. Input neurons supply information about the odor and the reward to the region, while output neurons pass on information to other parts of the fly brain. CrebB regulates the production of new proteins required to form these long-term odor-reward memories: but where exactly does CrebB act during this process?

Using a gene editing technique called CRISPR, Widmer et al. generated mutant flies. In these insects CrebB could be easily deactivated ‘at will’ in either the entire brain, the whole mushroom body, each of the three lobes or in specific output neurons. The flies were then trained on the odor-reward task, and their memory tested 24 hours later. The results revealed that for the memories to form, CrebB is only required in two of the three lobes of the mushroom body, and in certain output neurons. Future studies can now focus on the cells shown to need CrebB to create long-term memories, and identify the other proteins involved in this process.

In humans, defects in CrebB are associated with intellectual disability, addiction and depression. The mutant fly created by Widmer et al. could be a useful model in which to investigate how the protein may play a role in these conditions.

Identification and characterization of mushroom body neurons that regulate fat storage in Drosophila

Background

In an earlier study, we identified two neuronal populations, c673a and Fru-GAL4, that regulate fat storage in fruit flies. Both populations partially overlap with a structure in the insect brain known as the mushroom body (MB), which plays a critical role in memory formation. This overlap prompted us to examine whether the MB is also involved in fat storage homeostasis.

Methods

Using a variety of transgenic agents, we selectively manipulated the neural activity of different portions of the MB and associated neurons to decipher their roles in fat storage regulation.

Results

Our data show that silencing of MB neurons that project into the α’β’ lobes decreases de novo fatty acid synthesis and causes leanness, while sustained hyperactivation of the same neurons causes overfeeding and produces obesity. The α’β’ neurons oppose and dominate the fat regulating functions of the c673a and Fru-GAL4 neurons. We also show that MB neurons that project into the γ lobe also regulate fat storage, probably because they are a subset of the Fru neurons. We were able to identify input and output neurons whose activity affects fat storage, feeding, and metabolism. The activity of cholinergic output neurons that innervating the β’2 compartment (MBON-β’2mp and MBON-γ5β’2a) regulates food consumption, while glutamatergic output neurons innervating α’ compartments (MBON-γ2α’1 and MBON-α’2) control fat metabolism.

Conclusions

We identified a new fat storage regulating center, the α’β’ lobes of the MB. We also delineated the neuronal circuits involved in the actions of the α’β’ lobes, and showed that food intake and fat metabolism are controlled by separate sets of postsynaptic neurons that are segregated into different output pathways.

Electronic supplementary material

The online version of this article (10.1186/s13064-018-0116-7) contains supplementary material, which is available to authorized users.

Feeding-State-Dependent Modulation of Temperature Preference Requires Insulin Signaling in Drosophila Warm-Sensing Neurons

Starvation is life-threatening and therefore strongly modulates many aspects of animal behavior and physiology [1]. In mammals, hunger causes a reduction in body temperature and metabolism [2], resulting in conservation of energy for survival. However, the molecular basis of the modulation of thermoregulation by starvation remains largely unclear. Whereas mammals control their body temperature internally, small ectotherms, such as Drosophila, set their body temperature by selecting an ideal environmental temperature through temperature preference behaviors [3, 4]. Here, we demonstrate in Drosophila that starvation results in a lower preferred temperature, which parallels the reduction in body temperature in mammals. The insulin/insulin-like growth factor (IGF) signaling (IIS) pathway is involved in starvation-induced behaviors and physiology and is well conserved in vertebrates and invertebrates [5-7]. We show that insulin-like peptide 6 (Ilp6) in the fat body (fly liver and adipose tissues) is responsible for the starvation-induced reduction in preferred temperature (Tp). Temperature preference behavior is controlled by the anterior cells (ACs), which respond to warm temperatures via transient receptor potential A1 (TrpA1) [4]. We demonstrate that starvation decreases the responding temperature of ACs via insulin signaling, resulting in a lower Tp than in nutrient-rich conditions. Thus, we show that hunger information is conveyed from fat tissues via Ilp6 and influences the sensitivity of warm-sensing neurons in the brain, resulting in a lower temperature set point. Because starvation commonly results in a lower body temperature in both flies and mammals, we propose that insulin signaling is an ancient mediator of starvation-induced thermoregulation.