Fig. 4. Gramine suppresses serotonin-stimulated pumping in wild-type worms. 1 µmol l^-1 serotonin induces rapid pharyngeal pumping (P<0.0001). Addition of 100 µmol l^-1 gramine suppresses serotonin-stimulated pumping (P<0.0001). However, serotonin still has residual effects on pumping rate in the presence of 100 µmol l^-1 gramine (P<0.005). 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 µmol l^-1 serotonin.