Fig. 1: During starvation, mushroom body neurons rely on ketone body metabolism to sustain ketone body-dependent associative memory formation.

a, Inhibition of ACAT1 expression in adult MB neurons impaired memory after massed training in starved flies (F_2,42 = 9.49, P = 0.0004), but not in fed flies (F_2,42 = 0.74, P = 0.485). b, After massed training, memory was impaired in starved flies expressing a Sln RNAi in adult MB neurons (F_2,56 = 11.33, P < 0.0001), but not in fed flies (F_2,55 = 2.09, P = 0.133). c, Images of the Laconic FRET sensor expressed in MB neurons through the Tub-Gal80^ts;VT30559 driver, obtained by two-photon microscopy in the mTFP and Venus channels with the same conditions that were used for live recordings (scale bar, 20 µm). v, vertical lobe, corresponding to one of the axon fascicles of MB neurons; s, soma of MB neurons. d, In the fed condition, application of 10 mM of acetoacetate (red dashed line) resulted in a decreased Laconic ratio followed by a plateau in the MB soma of control flies, revealing lactate efflux from MB neuronal somas after acetoacetate bath application (mean trace ± s.e.m.). Quantification of the mean Laconic ratio at the plateau was performed on the last 100 s of recording (red line). Inhibition of Sln expression in adult MB neurons impaired lactate efflux evoked by acetoacetate application (t₁₉ = 3.355, P = 0.003). n represents either a group of 40–50 flies analysed together in a behavioural assay (a and b) or the response of a single recorded fly (d). Data are expressed as the mean ± s.e.m. with dots as individual values, and analysed by one-way analysis of variance (ANOVA) with post hoc testing by Newman–Keuls pairwise comparisons test (a and b) or by unpaired two-sided t-test (d). Asterisks refer to the least-significant P value of post hoc comparison between the genotype of interest and the genotypic controls (a and b), or to the P value of the unpaired t-test comparison (d). **P < 0.01, NS, not significant.