(A)–(B) Feeding rates of wild-type(+) and ser-7(tm1325) on HB101 (A) and DA1878 (B) after a 7- to 8-hr interval from training the animals on one or the other bacterium. Wild-type worms feed more actively on familiar food than novel food. On novel food the feeding rate of wild-type is slightly higher than that of the ser-7 null mutant. The difference may be due to constitutive activity of SER-7; that is, SER-7 is active to some extent even in absence of its ligand, serotonin (Hobson et al., 2003). (C)–(D) Feeding rates of ser-4(ok512); mod-1(ok103); ser-7(tm1325) and ser-7(tm1325) on HB101 (C) and DA1878 (D) after a 7- to 8-hr interval from training the animals on one or the other bacterium. ser-7(tm1325) is defective in increasing the feeding response to familiar food compared to novel food. Familiarity of food does not alter feeding rates in ser-4(ok512); mod-1(ok103); ser-7(tm1325). Like the positive SER-7-mediated signal, the inhibitory SER-4- and MOD-1-mediated serotonin signaling is more active on familiar food than novel food, but it decreases the feeding rate. (E) A simple linear model explaining how different serotonin receptors might contribute to the regulation of pumping on familiar food and on novel food. There are three effects: B: Basal activity of SER-7, S: Serotonin-stimulated activity of SER-7, and −I: Serotonin-stimulated activity of inhibitory serotonin receptors SER-4 and MOD-1. The net effect of serotonin on wild-type(+) pumping is S + B − I; the net effect on pumping in a mutant lacking SER-7 is −I. While it is presented as an aid to thinking about the results, none of the results presented in the paper depend on this model. Figure 4H, in particular, is a direct measurement of the effect of SER-7 under differing conditions, calculated as the difference in feeding rates between wild-type(+) and the ser-7 null mutant worms. A change in this number suggests the action of serotonin via SER-7. We use this as the measure of serotonin action via SER-7 because it is model-independent and robust. (F) Serotonin signaling via SER-4 and MOD-1 that suppresses the feeding response on familiar food is more active on HB101 than DA1878. The y axis indicates the difference in the feeding rates between ser-4(ok512); mod-1(ok103); ser-7(tm1325) and ser-7(tm1325) animals. Each value corresponds to the difference in the feeding rates between the triple null mutant and the ser-7null mutant presented in (C) and (D). For (A–D) and (F), data shown as mean ± SEM, n.s., not significant (p≥0.05), *p<0.05, ***p<0.001; for (A–D), one-way ANOVA, post hoc Tukey test and for (F), Student's t test. The number of animals tested (n ≥ 3 independent assays per group) is shown in parentheses.

DOI: http://dx.doi.org/10.7554/eLife.00329.009