Search

Background Proper regulation of feeding is important for an organism’s well-being and survival and involves a motivational component directing the search for food. Dissecting the molecular and neural mechanisms of motivated feeding behavior requires assays that allow quantification of both motivation and food intake. Measurements of motivated behavior usually involve assessing physical effort or overcoming an aversive stimulus. Food intake in Drosophila can be determined in a number of ways, including by measuring the time a fly’s proboscis interacts with a food source associated with an electrical current in the fly liquid-food interaction counter (FLIC). Here, we show that electrical current flowing through flies during this interaction is aversive, and we describe a modified assay to measure motivation in Drosophila. Results Food intake is reduced during the interaction with FLIC when the electrical current is turned on, which provides a confounding variable in studies of motivated behavior. Based on the FLIC, we engineer a novel assay, the fly liquid-food electroshock assay (FLEA), which allows for current adjustments for each feeding well. Using the FLEA, we show that both external incentives and internal motivational state can serve as drivers for flies to overcome higher current (electric shock) to obtain superior food. Unlike similar assays in which bitterness is the aversive stimulus for the fly to overcome, we show that current perception is not discounted as flies become more food-deprived. Finally, we use genetically manipulated flies to show that neuropeptide F, an orthologue of mammalian NPY previously implicated in regulation of feeding motivation, is required for sensory processing of electrical current. Conclusion The FLEA is therefore a novel assay to accurately measure incentive motivation in Drosophila. Using the FLEA, we also show that neuropeptide F is required for proper perception or processing of an electroshock, a novel function for this neuropeptide involved in the processing of external and internal stimuli.

Proper regulation of feeding is important for an organism’s well-being and survival. Food intake in Drosophila can be determined in a number of ways, including by measuring the time a fly’s proboscis interacts with a food source in the fly liquid-food interaction counter (FLIC). Here, we show that electrical current flowing through flies during this interaction is aversive and leads to a reduction in food intake. Based on the FLIC, we engineer a novel assay, the fly liquid-food electroshock assay (FLEA), which allows for current adjustments for each feeding well. Using the FLEA, we show that both external incentives as well as internal motivational state can serve as drivers for flies to overcome higher current (electric shock) to obtain superior food. Unlike similar assays in which bitterness is the aversive stimulus for the fly to overcome, we show that current perception is not discounted as flies become more food-deprived. The FLEA is therefore a novel assay to accurately measure incentive motivation in Drosophila. Using the FLEA, we also show that neuropeptide F is required for proper perception or processing of an electroshock, a novel function for this neuropeptide involved in processing of external and internal stimuli.Significance StatementMany neuropsychiatric disorders, such as depression or addiction, are associated with alterations in motivated behavior. Assays measuring incentive motivation determine how driven an organism is to attain a goal, like food, or how attractive an incentive is. These tests require the animal to put effort into obtaining the reward, which can include physical work or overcoming an aversive stimulus. Such assays for Drosophila feeding have relied on flies overcoming bitterness to obtain their food. However, the perception of bitterness is discounted as flies become food deprived, confounding the interpretation. Here, we developed a novel assay that does not suffer from the same shortcomings and thus allows for more accurate assessments of incentive motivation in this widely used model organism.

Background Long-lasting transcriptional changes underlie a number of adaptations that contribute to alcohol use disorders (AUD). Chromatin remodeling, including histone methylation, can confer distinct, long-lasting transcriptional changes, and histone methylases are known to play a role in the development of addiction. Conversely, little is known about the relevance of Jumonji (JmjC) domain-containing demethylases in AUDs. We systematically surveyed the alcohol-induced phenotypes of null mutations in all 13 Drosophila JmjC genes. Methods We used a collection of JmjC mutants, the majority of which we generated by homologous recombination, and assayed them in the Booze-o-mat to determine their naive sensitivity to sedation and their tolerance (change in sensitivity upon repeat exposure). Mutants with reproducible phenotypes had their phenotypes rescued with tagged genomic transgenes, and/or phenocopied by nervous system-specific knockdown using RNA interference (RNAi). Results Four of the 13 JmjC genes (KDM3, lid, NO66, and HSPBAP1) showed reproducible ethanol (EtOH) sensitivity phenotypes. Some of the phenotypes were observed across doses, for example, the enhanced EtOH sensitivity of KDM3KO and NO66KO, but others were dose dependent, such as the reduced EtOH sensitivity of HSPBAP1KO, or the enhanced EtOH tolerance of NO66KO. These phenotypes were rescued by their respective genomic transgenes in KDM3KO and NO66KO mutants. While we were unable to rescue lidk mutants, knockdown of lid in the nervous system recapitulated the lidk phenotype, as was observed for KDM3KO and NO66KO RNAi-mediated knockdown. Conclusions Our study reveals that the Drosophila JmjC-domain histone demethylases Lid, KDM3, NO66, and HSPBAP1 are required for normal EtOH-induced sedation and tolerance. Three of 3 tested of those 4 JmjC genes are required in the nervous system for normal alcohol-induced behavioral responses, suggesting that this gene family is an intriguing avenue for future research.