a, Schematic depicting the right hemisphere of a fly brain. One DAn (dark grey circle) in the protocerebral posterior lateral 1 (PPL1) cluster synapses onto the α2α’2-MBn compartment (dark grey). Brief stimulation of PPL1-α2α’2 (058B > TrpA1) just before retrieval transiently suppressed, while blocking synaptic output (058B > Shibire) temporarily enhanced PSD-LTM expression. b, Conditioned flies were fed with RU486 to induce RNAi expression in the MBn. Knocking down DAMB, but not dDA1, blocked the PPL1-α2α’2-induced memory suppression. PPL1-α2α’2-lexA, MB-GeneSwitch > uas-RNAi, lexAop-TrpA1. Box-and-whisker plots show the range of individual data points, the interquartile spread as the box, and the median as the line bisecting each box. * P < 0.05, ** P < 0.01, *** P < 0.001, **** P < 0.0001; n = 12, two-way ANOVA with Tukey’s test. Exact P-values and comparisons are shown in Supplementary Information.