Switch-like and persistent memory formation in individual Drosophila larvae

Abstract

Associative learning allows animals to use past experience to predict future events. The circuits underlying memory formation support immediate and sustained changes in function, often in response to a single example. Larval Drosophila is a genetic model for memory formation that can be accessed at molecular, synaptic, cellular, and circuit levels, often simultaneously, but existing behavioral assays for larval learning and memory do not address individual animals, and it has been difficult to form long-lasting memories, especially those requiring synaptic reorganization. We demonstrate a new assay for learning and memory capable of tracking the changing preferences of individual larvae. We use this assay to explore how activation of a pair of reward neurons changes the response to the innately aversive gas carbon dioxide (CO2). We confirm that when coupled to CO2 presentation in appropriate temporal sequence, optogenetic reward reduces avoidance of CO2. We find that learning is switch-like: all-or-none and quantized in two states. Memories can be extinguished by repeated unrewarded exposure to CO2 but are stabilized against extinction by repeated training or overnight consolidation. Finally, we demonstrate long-lasting protein synthesis dependent and independent memory formation.

Idiosyncratic learning performance in flies

“…Bees were recently shown to be similarly idiosyncratic, but without generalization between visual and olfactory CS modalities [30]. Our results, in a genetic model organism (see also [31]), provide a basis to probe the mechanistic basis of individuality in learning. Specifically, our results hint that the biological basis for such idiosyncrasy in olfactory learning originates more centrally in the brain than sensory circuit elements dedicated to encoding either CS or US.…”

High-resolution analysis of individualDrosophila melanogasterlarvae within groups uncovers inter- and intra-individual variability in locomotion and its neurogenetic modulation

“…Interestingly, the effects of the Dop1R1-knockdown were also observed when it occurred locally only in the mushroom body, one of the central brain regions in insects [63]. The dopamine neurons innervating the mushroom body play a crucial role in signalling reward or punishment information in associative learning [29,[64][65][66][67][68]. The mushroom body-innervating neurons covered by TH-Gal4 are mostly linked to punishment signalling, with the reward-signalling dopamine neurons notably missing in the expression pattern [64,69,70].…”

“…These focus on the average behaviour of groups of animals and thus do not allow for individuals to be followed. More recent approaches have tried to combine the advantages of both types of tracking [29, 30]. The major challenge in these cases is that individual Drosophila larvae look very much alike.…”

“…As a result, when two larvae collide, their identity is lost, resulting in a multitude of separated tracks and no opportunity for the experimenter to disentangle which tracks belong to which of the animals. Two different strategies have been followed in tackling this problem: one relies on the parallel tracking of many separated individual animals [29]; the other tries to resolve the collisions between larvae and thus to keep track of the identity of individual larvae within a group of animals [30] (for an earlier approach not specific to Drosophila larvae, see [31]). The latter solution has been integrated into the FIM (Frustrated total internal reflection-based Imaging Method) tracker software which requires specific hardware to illuminate the tracked animals [23].…”

Pain is so close to pleasure: the same dopamine neurons can mediate approach and avoidance in Drosophila

“…In an operant olfactory conditioning paradigm in flies, optogenetic activation of TH-Gal4 DANs served as punishment and was able to induce an olfactory preference (Claridge-Chang et al, 2009). Also in Drosophila larvae, DANs appear to serve analogous functions (Lesar et al, 2021;Rohwedder et al, 2016;Schleyer et al, 2020Schleyer et al, , 2013. DANs and octopaminergic neurons also appear to be involved in value-based decision-making in flies (Gorostiza et al, 2016), adding additional evidence for the valuation-related function of DANs.…”