Rest Is Required to Learn an Appetitively-Reinforced Operant Task in Drosophila

Abstract

Maladaptive operant conditioning contributes to development of neuropsychiatric disorders. Candidate genes have been identified that contribute to this maladaptive plasticity, but the neural basis of operant conditioning in genetic model organisms remains poorly understood. The fruit fly Drosophila melanogaster is a versatile genetic model organism that readily forms operant associations with punishment stimuli. However, operant conditioning with a food reward has not been demonstrated in flies, limiting the types of neural circuits that can be studied. Here we present the first sucrose-reinforced operant conditioning paradigm for flies. In the paradigm, flies walk along a Y-shaped track with reward locations at the terminus of each hallway. When flies turn in the reinforced direction at the center of the track, they receive a sucrose reward at the end of the hallway. Only flies that rest early in training learn the reward contingency normally. Flies rewarded independently of their behavior do not form a learned association but have the same amount of rest as trained flies, showing that rest is not driven by learning. Optogenetically-induced sleep does not promote learning, indicating that sleep itself is not sufficient for learning the operant task. We validated the sensitivity of this assay to detect the effect of genetic manipulations by testing the classic learning mutant dunce. Dunce flies are learning-impaired in the Y-Track task, indicating a likely role for cAMP in the operant coincidence detector. This novel training paradigm will provide valuable insight into the molecular mechanisms of disease and the link between sleep and learning.

Sleep benefits different stages of memory in Drosophila

“…In a novel appetitive operant conditioning task flies were rewarded with sucrose when they turned in the desired direction on a Y-shaped track ( Wiggin et al, 2021 ). It was reported that only flies who rested during training (with rest defined as 1 min or more of continuous inactivity) were able to learn and perform well in this task.…”

“…It was reported that only flies who rested during training (with rest defined as 1 min or more of continuous inactivity) were able to learn and perform well in this task. Interestingly, and perhaps counter-intuitively, optogenetically inducing sleep via activation of the dorsal FB did not enhance learning ( Wiggin et al, 2021 ). This suggests that spontaneous rest is required to perform well but not sufficient to induce learning in this task, or that the specific restorative rest that gates learning in this operant task is driven by a different neural population.…”

“…This suggests that spontaneous rest is required to perform well but not sufficient to induce learning in this task, or that the specific restorative rest that gates learning in this operant task is driven by a different neural population. In this operant task, the individual flies who tended to rest during training and therefore performed well at the task also spent more time adjacent to the sucrose reward, indicating an increased intake of food ( Wiggin et al, 2021 ). When the sucrose reward was removed, the proportion of flies that showed spontaneous rest behaviour decreased ( Wiggin et al, 2021 ), consistent with reports on increased sucrose intake being associated with increased rest ( Murphy et al, 2016 ) and a sleep suppressing effect of food deprivation ( Keene et al, 2010 ).…”

“…In this operant task, the individual flies who tended to rest during training and therefore performed well at the task also spent more time adjacent to the sucrose reward, indicating an increased intake of food ( Wiggin et al, 2021 ). When the sucrose reward was removed, the proportion of flies that showed spontaneous rest behaviour decreased ( Wiggin et al, 2021 ), consistent with reports on increased sucrose intake being associated with increased rest ( Murphy et al, 2016 ) and a sleep suppressing effect of food deprivation ( Keene et al, 2010 ). Therefore, it seems that in this specific task sucrose consumption promotes rest which, in turn, promotes learning.…”

Reward expectations direct learning and drive operant matching in Drosophila

“…To answer these questions, we designed an olfactory two-alternative forced choice (2AFC) task for individual Drosophila , inspired by earlier behavior assays for flies ( 57 – 61 ) and foraging related 2AFC tasks in vertebrates ( 10 – 12 , 14 ). The assay allows us to measure hundreds of sequential choices from individual flies as we vary the probability of reward associated with different odor cues.…”