DA exerts its effects via a D2R receptor expressed in 5-HT neurons during development of the circuit. A - C. Feeding behavior. A, Only the D2 receptor, D2R, affects feeding behavior (elavC155/pattP2, parental control; DopR and DopR2, D1 receptors). B. Neuronal knockdown of D2R expression in 2nd instar larvae has no effect. C. D2R expression must be reduced in 5-HT (DTRHGal4), but not DA (DTHGal4), neurons, to affect feeding behavior. n = 40 for behavioral analyses, from 4-6 independent experiments. *p < 0.05, ***p < 0.001. Statistical analyses were performed using one way ANOVA and Dunnet's post-test (A, C) or Student's t-test (B). D - G. Axonal fiber architecture. D - E. branching. F - G. Varicosity area. *p < 0.05, **p < 0.01, ***p < 0.001. Statistical analyses were performed using one way ANOVA and Dunnet's post-test (D, F) or Student's t-test (E, G). elavC155/pattP2, 43 fibers from 32 guts from 7 independent experiments; elavC155/DopR, 47 fibers from 37 guts from 6 independent experiments; elavC155/DopR2, 37 fibers from 29 guts from 5 independent experiments; elavC155/D2R, 37 fibers from 22 guts from 4 independent experiments; DTHGal4/pattP2, 34 fibers from 26 guts from 3 independent experiments; DTHGal4/D2R, 29 fibers from 18 guts from 5 independent experiments; DTRHGal4/pattP2, 33 fibers from 26 guts from 5 independent experiments; DTRHGal4/D2R, 27 fibers from 22 guts from 3 independent experiments. Lines above the graph depict standard error of the mean. H - K. Visual depiction of 5-HT axonal fibers from elavC155/pattP2 (H), elavC155/D2R (I), DTRHGal4/pattP2 (J) and DTRHGal4/D2R (K) guts. Scale bar = 20 μm.