Fig. 1: SELKs are anatomically and functionally connected to both bitter and sweet GRNs.

a, b Staining the central brain with an antibody against LK (green) labels a pair of neurons in the lateral horn (LHLKs, arrow) and a pair in the ventral SEZ (SELKs, arrowhead). The nuclei of the postsynaptic partners of bitter-GRNs (a) and sweet-GRNs (b) as revealed by trans-Tango, are shown in magenta. The merge images in the right panels reveal colocalization of the LK immunostaining with the postsynaptic trans-Tango signal in SELKs but not in LHLKs. Zoomed-in images of the SEZ are shown at the bottom panels of (a, b). Scale bars: 50 μm for top and 10 μm for bottom panels. Neuropil staining is shown in blue. Optogenetic activation of bitter GRNs (c–e), or sweet GRNs (f–h) in conjunction with calcium imaging of SELKs. Schematics of the experiments in which bitter GRNs (c), or sweet GRNs (f) are optogenetically activated, and activity of SELKs is monitored by in vivo calcium imaging. Individual traces from imaging sessions of SELKs while either bitter GRNs (d), or sweet GRNs (g) are optogenetically activated (n = 16 and 13, respectively). The blue-shaded area indicates the time frame in which the light is on. Dotted lines indicate the actual data points, and the solid lines indicate the smoothened curve. e, h Note the significant increases in the maximum fluorescence levels in SELKs during light stimulation of both bitter GRNs (n = 15, biological replicates, two-tailed P = 0.0077, e) and sweet GRNs (n = 13, biological replicates, two-tailed P = 0.0142, h) compared to the baseline before light stimulation. Paired t-test. i–n Calcium imaging of SELKs during administration of either 10 mM lobeline (a bitter compound) (i–k), or 1 M sucrose (a sweet compound) (l–n), to the fly’s proboscis. In fed flies, SELKs exhibit elevated Ca⁺⁺ levels in response to bitter (n = 7, biological replicates, two-tailed P = 0.0255, j) but not to sweet tastants (n = 6, biological replicates, two-tailed P = 0.0924, m). By contrast, in starved flies, there is a significant increase in the GCaMP signal upon delivery of bitter (n = 7, biological replicates, two-tailed P = 0.0031, k), or sweet (n = 5, biological replicates, two-tailed P = 0.0167, n) tastants. Paired t-test. P-values for all statistical tests: *≤ 0.05 and **≤ 0.01. Boxplots indicate median (middle line), 25th and 75th percentile (box), bars represent maximum and minimum. Source data are provided as a Source data file.