Fig. 7. Chronic depletion of endogenous serotonin affects pharyngeal behavior. (A) In the absence of exogenous serotonin (1 µmoll^-1), eat-18; tph-1 worms have decreased M3 activity relative to eat-18 worms (Fig. 3B) (P<0.05), and are unresponsive to gramine (100 µmoll^-1) and octopamine (100 µmoll^-1) (P>0.2). Exogenous serotonin enhances M3 activity to levels comparable to serotonin-stimulated eat-18 worms (P<0.05). Gramine can suppress the serotonin-stimulated enhancement of M3 activity in eat-18; tph-1 worms (P<0.05). (B) In the absence of exogenous serotonin, eat-18; tph-1 worms have action potentials similar in length to those of eat-18 worms (P>0.05). Gramine and octopamine have no affect on action potential duration in the absence of added serotonin (P>0.6). Exogenous serotonin shortens the action potential of eat-18; tph-1 worms (P<0.05). This effect is blocked by gramine and octopamine (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. Hatched bars represent significant differences between measurements from eat-18; tph-1 (this figure) and those from eat-18 (Fig. 3B).