An amino-acid mixture can be both rewarding and punishing to larval Drosophila

Abstract

Amino acids are important nutrients for animals because they are necessary for protein synthesis in particular during growth, as well as for neurotransmission. However, little is known about how animals use past experience to guide their search for amino-acid-rich food. We reasoned that the larvae of Drosophila melanogaster are suitable for investigating this topic because they are the feeding and growth stages in the life cycle of these holometabolous insects. Specifically, we investigated whether experiencing an odour with a 20 amino-acid mixture as a semi-natural tastant during training establishes odourtastant associative memories. Across a broad concentration range (0.01-20 mmol l −1 ), such an amino-acid mixture was found to have a rewarding effect, establishing appetitive memory for the odour. To our surprise, however, manipulation of the test conditions revealed that relatively high concentrations of the amino-acid mixture (3.3 mmol l −1 and higher) in addition establish aversive memory for the odour. We then characterized both of these oppositely valenced memories in terms of their dependency on the number of training trials, their temporal stability, their modulation through starvation and the specific changes in locomotion underlying them. Collectively, and in the light of what is known about the neuronal organization of odour-food memory in larval D. melanogaster, our data suggest that these memories are established in parallel. We discuss the similarity of our results to what has been reported for sodium chloride, and the possible neurogenetic bases for concentration-dependent changes in valence when these tastants are used as reinforcers.

Optogenetically induced reward and ‘frustration’ memory in larval Drosophila melanogaster

“…Experiments were performed on 3 rd instar foraging larvae of Drosophila melanogaster raised on standard food at 25 °C, 60-70 % relative humidity, in a room running on a 12 h light/dark cycle but in vials wrapped with black cardboard to keep them in darkness. We used the split-Gal4 driver strain SS00864 (HHMI Janelia Research Campus, USA; Saumweber et al, 2018), supporting strong and reliable transgene expression in DAN-i1 of both hemispheres, plus expression from a few additional cells that is stochastic between hemispheres and preparations (Saumweber et al, 2018;Schleyer et al, 2020;Weiglein et al, 2019). As argued before, such stochastic expression is unlikely to cause systematic effects in the behavioural mass assays employed in the present study (Saumweber et al, 2018;Schleyer et al, 2020).…”

“…Second, given that a single training trial can be sufficient to establish memory for some natural tastant reinforcers (Weiglein et al, 2019), we tested for one-trial memory using DAN-i1 864 activation. We observed reward memory after one-trial forward training with DAN-i1 864 activation, but no frustration memory after one-trial backward training (Fig.…”

“…Fourth, we wondered whether reward and frustration memories established via DAN-i1 864 activation differ in their locomotor 'footprint'; that is, associative memories can modulate both rate and direction of lateral head movements (head casts, HC) (Paisios et al, 2017;Thane et al, 2019;Toshima et al, 2019). In comparison to larvae that underwent unpaired training, forward training promoted head casts when crawling away, rather than when crawling towards the odour, whereas the opposite was the case upon backward training (Fig.…”

Ethanol-guided behavior in Drosophila larvae

“…In addition, we gain increasing insight to understand what stimulation the larva classifies as rewarding. Chemosensory stimuli include ethanol, low salt concentrations, amino acids, ribonucleosides and various sugars (e.g., sucrose, glucose, maltodextrin, sorbitol, ribose), even though some cannot be metabolized (arabinose, xylose) 20 , 21 , 42 , 49 , 72 , 73 . Without a direct comparison, it is of course difficult to assess how rewarding ethanol is for the larva.…”

Ethanol-guided behavior inDrosophilalarvae

“…Nonetheless, we gain increasing insight to understand what stimulation the larva classifies as rewarding. Chemosensory stimuli include ethanol, low salt concentrations, amino acids, ribonucleosides and various sugars (e.g., sucrose, glucose, maltodextrin, sorbitol, ribose), even though some cannot be metabolized (arabinose, xylose) 20,21,42,49,66,67 .…”