Fig. 3. (A) Serotonin, gramine and octopamine affect the electropharyngeograms (EPGs) of eat-18 mutants. Serotonin (1 µmoll^-1) enhances M3 activity and decreases the action potential duration of eat-18 mutant pharynxes. Gramine (100 µmoll^-1) and octopamine (100 µmoll^-1) increase action potential duration and suppress M3 activity in the presence and absence of exogenous serotonin. EPGs shown for each drug condition are statistically representative of the average action potential duration and M3 activity of all recordings. (B) Quantitative analysis of M3 activity and action potential duration in eat-18 mutants. Serotonin (1 µmoll^-1) significantly decreases action potential duration and enhances M3 activity (P<0.005). Gramine (100 µmoll^-1) and octopamine (100 µmoll^-1) block the serotonin-stimulated decrease in action potential duration and enhancement of M3 activity (P<0.001). Gramine and octopamine also increase action potential duration and suppress M3 activity in the absence of exogenous serotonin (P<0.05). Values are means + S.E.M. Significant differences between the indicated measurement and data recorded ^* in the absence of drug or in the presence of 1 µmoll^-1 serotonin.