Figure 7. Activation of Gr43a^GAL4 brain neurons assigns satiety-dependent valence.

(A) Schematic diagram of the olfactory conditioning assay. Flies are exposed to odor A, while their Gr43a^GAL4 brain neurons are activated using trpA1 (29°C). After a brief rest period in an odorless vial, they are exposed to odor B in the absence of neural activation (23°C), followed by another rest period in an odorless vial. This training session is repeated once more before the flies are tested in a T-Maize assay for acquired odor preference.

(B) Olfactory conditioning assay of starved and satiated flies. Prior conditioning, flies were kept in agarose or agarose containing 250 mM sucrose for 18-24 hours to induce starvation and satiation, respectively. Gr43a^GAL4/UAS-trpA1;Cha^7.4kb-GAL80/+ flies assign positive valence when hungry (pleasant; left graph), but negative valence when satiated (unpleasant; right graph). Control flies were Gr43a^GAL4/+; Cha^7.4kb-GAL80/+ and UAS-trpA1/+. *p < 0.05; **p<0.01; ANOVA. 12≤n≤18.

(C) Model: The fly’s major blood sugars, glucose and trehalose are kept at a fairly constant, relatively high level. Conversely, internal fructose level is very low, but fluctuates in response to feeding of nutritious sugars. Since nutritious carbohydrates can be converted into fructose, the activity of Gr43a^GAL4 brain neurons depends on the nutritious value of the ingested food. Activation of Gr43a^GAL4 brain neurons, in combination with the state of satiety, leads either to a pleasant sensation in hungry flies, reinforcing feeding behavior, or is perceived as unpleasant, thereby terminating feeding behavior.