Taste-independent detection of the caloric content of sugar in Drosophila

Abstract

Feeding behavior is influenced primarily by two factors: nutritional needs and food palatability. However, the role of food deprivation and metabolic needs in the selection of appropriate food is poorly understood. Here, we show that the fruit fly, Drosophila melanogaster, selects calorie-rich foods following prolonged food deprivation in the absence of taste-receptor signaling. Flies mutant for the sugar receptors Gr5a and Gr64a cannot detect the taste of sugar, but still consumed sugar over plain agar after 15 h of starvation. Similarly, pox-neuro mutants that are insensitive to the taste of sugar preferentially consumed sugar over plain agar upon starvation. Moreover, when given a choice between metabolizable sugar (sucrose or D-glucose) and nonmetabolizable (zero-calorie) sugar (sucralose or L-glucose), starved Gr5a; Gr64a double mutants preferred metabolizable sugars. These findings suggest the existence of a taste-independent metabolic sensor that functions in food selection. The preference for calorie-rich food correlates with a decrease in the two main hemolymph sugars, trehalose and glucose, and in glycogen stores, indicating that this sensor is triggered when the internal energy sources are depleted. Thus, the need to replenish depleted energy stores during periods of starvation may be met through the activity of a taste-independent metabolic sensing pathway.

Fig. 1.

Gr5a; Gr64a mutants develop a preference for sucrose after prolonged periods of food deprivation. (A and C) Two-choice preference assay with sucrose versus agar after 5 h (A) and after 22 h (C) of starvation. Flies of different genotypes were given a choice between agar containing 100 mM sucrose versus plain agar. The y axis shows the average percentage of flies that ate sucrose, plain agar, or did not eat. n = 4–8 with each trial comprising 50 flies. Error bars: SEM, *P < 0.001, two-way ANOVA with Bonferroni post hoc test in this figure and all subsequent figures unless indicated otherwise. (B and D) Dissected crop and gut from 5-h (B) and 22-h (D) starved flies that were given a choice between agar containing 100 mM sucrose mixed with green dye versus plain agar mixed with red dye (A and C). (E) PER of 22-h starved flies when their labellum was stimulated by 100 mM sucrose. A full extension was given a score of 1 and a partial extension was given a score of 0.5. n = 24–37. (F) Time course for the development of sucrose preference. Flies were starved for 22 h and then given a choice between 100 mM sucrose versus agar for different durations of time (x axis). n = 4. *P < 0.001 and **P < 0.05.

Fig. 2.

The taste-independent sensor responds to different sugars. (A–H) Two-choice preference assay with agar containing 200 mM glucose (A and B), 200 mM trehalose (C and D), 400 mM galactose (E and F), or 200 mM fructose (G and H) versus plain agar. Gr5a;Gr64a¹ and Gr5a;Gr64a² double mutants and control flies—CS WT, Gr5a, Gr64a¹, and Gr64a² single mutants—were tested after 5 h (A, C, E, and G) and after 22 h of starvation (B, D, F, and H). n = 3–6. *P < 0.001.

Fig. 3.

The taste-independent sensor responds to metabolizable sugars, but not to nonmetabolizable sugars. (A) Molecular structures of sucrose and sucralose (Upper panels) and d-glucose and l-glucose (Lower panels). (B) PERs of Gr5a; Gr64a¹ mutants and CS WT flies to 0.3 mM sucralose. *P < 0.001 with one-way ANOVA. n = 37–40. (C and D) Two-choice preference assay with (C) 0.3 mM sucralose versus plain agar and (D) 0.3 mM sucralose versus 100 mM sucrose after 22 h of starvation. An amount of 0.3 mM sucralose induces comparable PER responses to 100 mM sucrose (Fig. 1E). n = 6–8. (E) PERs of Gr5a; Gr64a¹ mutants and CS WT flies to 200 mM d-glucose and 200 mM l-glucose. The same concentration of d-glucose and l-glucose leads to similar PER responses in WT flies. *P < 0.001 with one-way ANOVA. n = 22–36. (F and G) Two-choice preference assay with (F) 200 mM l-glucose versus plain agar and (G) 200 mM l-glucose versus 200 mM d-glucose after 22 h of starvation. n = 4. (H) PI for d-glucose in 5-h (light stippled bars) and 22-h (dark stippled bars) starved flies, which were given a choice between 200 mM l-glucose versus 50 mM d-glucose. Four independent WT strains—CS, Or-R, yellow white (yw), and Harwich—were used. A PI value of 0.5 indicates that flies equally prefer d- and l-glucose whereas a PI value of 0.5–1 indicates a preference for d-glucose. For the detailed calculation of PI (d-glucose), see Materials and Methods. *P < 0.001, Student's t test within each genotype; n = 4 for each trial comprising 50 flies.

Fig. 4.

Changes in the hemolymph glycemia correlate with the timing of the behavioral switch to metabolizable sugars. (A and B) The levels of (A) hemolymph glucose and trehalose and (B) glycogen as a function of starvation time. *P < 0.001 in comparison with 5-h starvation, one-way ANOVA, Bonferroni post hoc test; n = 7–10 for hemolymph glycemia and n = 9 for glycogen. (C) Time course for the induction of the taste-independent food choice behavior. Gr5a;Gr64a¹, and controls were given a choice between agar containing 100 mM sucrose versus plain agar after 5, 10, 15, and 22 h of starvation. Gr5a; Gr64a¹ mutants showed a preference for sucrose between 10 and 15 h of starvation. n = 4–6. (D and E) Concentrations of glucose and trehalose in the hemolymph of flies fed 400 mM glucose, 400 mM 2DG, or plain agar after (D) 10 h or (E) 15 h of starvation. *P < 0.001 with one-way ANOVA. n = 10–12 for 10 h of starvation and n = 9–10 for 15 h of starvation. (F and G) PI for d-glucose in flies fed 400 mM d-glucose, 400 mM 2DG, and plain agar for (F) 10 h and (G) 15 h. These flies were then given a choice between d-glucose versus l-glucose. *P < 0.001 with one-way ANOVA. n = 4.