Serotonin modulates locomotory behavior and coordinates egg-laying and movement in Caenorhabditis elegans

Biogenic amines have been implicated in the modulation of neural circuits involved in diverse behaviors in a wide variety of organisms. In the nematode C. elegans, serotonin has been shown to modulate the temporal pattern of egg-laying behavior. Here we show that serotonergic neurotransmission is also required for modulation of the timing of behavioral events associated with locomotion and for coordinating locomotive behavior with egg-laying. Using an automated tracking system to record locomotory behavior over long time periods, we determined that both the direction and velocity of movement fluctuate in a stochastic pattern in wild-type worms. During periods of active egg-laying, the patterns of reversals and velocity were altered: velocity increased transiently before egg-laying events, while reversals increased in frequency following egg-laying events. The temporal coordination between egg-laying and locomotion was dependent on the serotonergic HSN egg-laying motorneurons as well as the decision-making AVF interneurons, which receive synaptic input from the HSNs. Serotonin-deficient mutants also failed to coordinate egg-laying and locomotion and exhibited an abnormally low overall reversal frequency. Thus, serotonin appears to function specifically to facilitate increased locomotion during periods of active egg-laying, and to function generally to modulate decision-making neurons that promote forward movement.