Serotonin and Go Modulate Functional States of Neurons and Muscles Controlling C. elegans Egg-Laying Behavior

AbstractFrom nematodes to humans, animals employ neuromodulators like serotonin to regulate behavioral patterns and states. In the nematode C. elegans, serotonin has been shown to act in a modulatory fashion to increase the rate and alter the temporal pattern of egg laying [1–4]. Though many candidate effectors and regulators of serotonin have been identified in genetic studies [5–16], their effects on specific neurons and muscles in the egg-laying circuitry have been difficult to determine. Using the genetically encoded Ca2+ indicator cameleon, we found that serotonin acts directly on the vulval muscles to increase the frequency of Ca2+ transients. In contrast, we found that the spontaneous activity of the egg-laying motorneurons was silenced by serotonin. Mutations in G protein α subunit genes altered the responses of both vulval muscles and egg-laying neurons to serotonin; specifically, mutations in the Gqα homolog egl-30 blocked serotonin stimulation of vulval muscle Ca2+ transients, while mutations in the Goα homolog goa-1 prevented the silencing of motorneuron activity by serotonin. These data indicate that serotonin stimulates egg laying by directly modulating the functional state of the vulval muscles. In addition, serotonin inhibits the activity of the motorneurons that release it, providing a feedback regulatory effect that may contribute to serotonin adaptation