Testing Drosophila olfaction with a Y-maze assay

Reward expectations direct learning and drive operant matching in Drosophila

Foraging animals must use decision-making strategies that dynamically adapt to the changing availability of rewards in the environment. A wide diversity of animals do this by distributing their choices in proportion to the rewards received from each option, Herrnstein's operant matching law. Theoretical work suggests an elegant mechanistic explanation for this ubiquitous behavior, as operant matching follows automatically from simple synaptic plasticity rules acting within behaviorally relevant neural circuits. However, no past work has mapped operant matching onto plasticity mechanisms in the brain, leaving the biological relevance of the theory unclear. Here, we discovered operant matching in Drosophila and showed that it requires synaptic plasticity that acts in the mushroom body and incorporates the expectation of reward. We began by developing a dynamic foraging paradigm to measure choices from individual flies as they learn to associate odor cues with probabilistic rewards. We then built a model of the fly mushroom body to explain each fly's sequential choice behavior using a family of biologically realistic synaptic plasticity rules. As predicted by past theoretical work, we found that synaptic plasticity rules could explain fly matching behavior by incorporating stimulus expectations, reward expectations, or both. However, by optogenetically bypassing the representation of reward expectation, we abolished matching behavior and showed that the plasticity rule must specifically incorporate reward expectations. Altogether, these results reveal the first synapse-level mechanisms of operant matching and provide compelling evidence for the role of reward expectation signals in the fly brain.

Enhanced olfactory memory detection in trap-design Y-mazes allows the study of imperceptible memory traces in Drosophila

The neural basis of behavior is identified by systematically manipulating the activity of specific neurons and screening for loss or gain of phenotype. Therefore, robust, high-scoring behavioral assays are necessary for determining the neural circuits of novel behaviors. We report a simple Y-maze design for Drosophila olfactory learning and memory assay. Memory scores in our Y-mazes are considerably better and longer-lasting than scores obtained with commonly used T-mazes. Our results suggest that trapping flies to an odor choice in a Y-maze could improve scores. We postulated that the improved scores could reveal previously undetectable memory traces, enabling the study of underlying neural mechanisms. Indeed, we identified unreported protein synthesis-dependent long-term memories (LTMs), reinforced by ingestion of (1) an aversive compound and (2) a sweet but nonnutritious sugar, both 24 h after training. We also used Y-mazes to probe how using a greater reward may change memory dynamics. Our findings predict that a greater sugar reward may extend existing memory traces or reinforce additional novel ones.

Animal Models of Neurodegenerative Disease: Recent Advances in Fly Highlight Innovative Approaches to Drug Discovery

Neurodegenerative diseases represent a formidable challenge to global health. As advances in other areas of medicine grant healthy living into later decades of life, aging diseases such as Alzheimer's disease (AD) and other neurodegenerative disorders can diminish the quality of these additional years, owed largely to the lack of efficacious treatments and the absence of durable cures. Alzheimer's disease prevalence is predicted to more than double in the next 30 years, affecting nearly 15 million Americans, with AD-associated costs exceeding $1 billion by 2050. Delaying onset of AD and other neurodegenerative diseases is critical to improving the quality of life for patients and reducing the burden of disease on caregivers and healthcare systems. Significant progress has been made to model disease pathogenesis and identify points of therapeutic intervention. While some researchers have contributed to our understanding of the proteins and pathways that drive biological dysfunction in disease using in vitro and in vivo models, others have provided mathematical, biophysical, and computational technologies to identify potential therapeutic compounds using in silico modeling. The most exciting phase of the drug discovery process is now: by applying a target-directed approach that leverages the strengths of multiple techniques and validates lead hits using Drosophila as an animal model of disease, we are on the fast-track to identifying novel therapeutics to restore health to those impacted by neurodegenerative disease.

Mosquito Olfactory Response Ensemble enables pattern discovery by curating a behavioral and electrophysiological response database

Many experimental studies have examined behavioral and electrophysiological responses of mosquitoes to odors. However, the differences across studies in data collection, processing, and reporting make it difficult to perform large-scale analyses combining data from multiple studies. Here we extract and standardize data for 12 mosquito species, along with Drosophila melanogaster for comparison, from over 170 studies and curate the Mosquito Olfactory Response Ensemble (MORE), publicly available at https://neuralsystems.github.io/MORE. We demonstrate the ability of MORE in generating biological insights by finding patterns across studies. Our analyses reveal that ORs are tuned to specific ranges of several physicochemical properties of odorants; the empty-neuron recording technique for measuring OR responses is more sensitive than the Xenopus oocyte technique; there are systematic differences in the behavioral preferences reported by different types of assays; and odorants tend to become less attractive or more aversive at higher concentrations.

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MORE is a database of behavioral and electrophysiological responses to odors

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MORE includes data from 170 studies covering 12 species of mosquitoes along with flies

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MORE shows differences in odor preferences measured with different assays

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Empty-neuron technique measures responses more sensitively than the oocyte technique

Preference and detrimental effects of high fat, sugar, and salt diet in wild-caught Drosophila simulans are reversed by flight exercise

High saturated fat, sugar, and salt contents are a staple of a Western diet (WD), contributing to obesity, metabolic syndrome, and a plethora of other health risks. However, the combinatorial effects of these ingredients have not been fully evaluated. Here, using the wild-caught Drosophila simulans, we show that a diet enriched with saturated fat, sugar, and salt is more detrimental than each ingredient separately, resulting in a significantly decreased lifespan, locomotor activity, sleep, reproductive function, and mitochondrial function. These detrimental effects were more pronounced in female than in male flies. Adding regular flight exercise to flies on the WD markedly negated the adverse effects of a WD. At the molecular level, the WD significantly increased levels of triglycerides and caused mitochondrial dysfunction, while exercise counterbalanced these effects. Interestingly, fruit flies developed a preference for the WD after pre-exposure, which was averted by flight exercise. The results demonstrate that regular aerobic exercise can mitigate adverse dietary effects on fly mitochondrial function, physiology, and feeding behavior. Our data establish Drosophila simulans as a novel model of diet-exercise interaction that bears a strong similarity to the pathophysiology of obesity and eating disorders in humans.

Functional Analysis of Hydrolethalus Syndrome Protein HYLS1 in Ciliogenesis and Spermatogenesis in Drosophila

Cilia and flagella are conserved subcellular organelles, which arise from centrioles and play critical roles in development and reproduction of eukaryotes. Dysfunction of cilia leads to life-threatening ciliopathies. HYLS1 is an evolutionarily conserved centriole protein, which is critical for ciliogenesis, and its mutation causes ciliopathy–hydrolethalus syndrome. However, the molecular function of HYLS1 remains elusive. Here, we investigated the function of HYLS1 in cilia formation using the Drosophila model. We demonstrated that Drosophila HYLS1 is a conserved centriole and basal body protein. Deletion of HYLS1 led to sensory cilia dysfunction and spermatogenesis abnormality. Importantly, we found that Drosophila HYLS1 is essential for giant centriole/basal body elongation in spermatocytes and is required for spermatocyte centriole to efficiently recruit pericentriolar material and for spermatids to assemble the proximal centriole-like structure (the precursor of the second centriole for zygote division). Hence, by taking advantage of the giant centriole/basal body of Drosophila spermatocyte, we uncover previously uncharacterized roles of HYLS1 in centriole elongation and assembly.

Olfactory detection of a bacterial short-chain fatty acid acts as an orexigenic signal in Drosophila melanogaster larvae

Microorganisms inhabiting fermenting fruit produce chemicals that elicit strong behavioral responses in flies. Depending on their ecological niche, individuals confer a positive or a negative valence to a chemical and, accordingly, they trigger either attractive or repulsive behaviors. We studied the case of bacterial short-chain fatty acids (SCFA) that trigger opposite behaviors in adult and larvae of Drosophila melanogaster. We determined that SCFA-attractive responses depend on two larval exclusive chemoreceptors, Or30a and Or94b. Of those SCFA, propionic acid improves larval survival in suboptimal rearing conditions and supports growth. Olfactory detection of propionic acid specifically is sufficient to trigger feeding behaviors, and this effect requires the correct activity of Or30a⁺ and Or94b⁺ olfactory sensory neurons. Additionally, we studied the case of the invasive pest Drosophila suzukii that lives on undamaged ripe fruit with less SCFA production. Contrary to D. melanogaster, D. suzukii larvae show reduced attraction towards propionic acid, which does not trigger feeding behavior in this invasive species. Our results demonstrate the relevance of propionic acid as an orexigenic signal in D. melanogaster larvae. Moreover, this study underlines that the changes on ecological niche are accompanied with alterations of olfactory preferences and vital olfactory driven behaviors.