Effect of serotonin antagonist mianserin on modulation of locomotion. Histograms show mean ± SEM locomotion frequency from manual observations. Worms from species showing enhanced slowing responses were incubated for 30 minutes in 20 μM mianserin hydrochloride prior to testing (see Materials & Methods). Worms not treated with mianserin are the same as shown in Figure 2. (A) C. elegans (B) C. briggsae (C) C. sp. 3 (D) O. myriophila. For all these species, there were statistically significant differences among the groups by 1-factor ANOVA (P << 0.001). Selected pairwise comparisons are indicated by lines connecting columns with significance as indicated [n.s. - not significant (P > 0.05), * - P < 0.05, ** - P < 0.01, *** - P < 0.001] Mianserin significantly attenuated the enhanced slowing response in A & D, but did not affect the locomotory rate with or without bacteria. Mianserin did not affect enhanced slowing in Caenorhabditis sp. 3 but reduced the locomotory rate on or off bacteria. Not indicated in the figure (but presented in text) are pairwise comparisons of untreated vs. treated with mianserin, well-fed worms on bacteria (C. elegans, n.s.; C. briggsae, n.s.; C. sp. 3, P < 0.001; O. myriophila, n.s.) Numbers of worms tested in mianserin treatments: C. elegans (n = 58-77), C. briggsae (n = 34-49), C. sp. 3 (n = 61-68), O. myriophila (n = 60-85).