Effect of a neuropeptide gene on behavioral states in Caenorhabditis elegans egg-laying

Egg-laying behavior in the nematode Caenorhabditis elegans involves fluctua tion between alternative behavioral states: an inactive state, during which eggs are retained in the uterus, and an active state, during which eggs ar e laid in bursts. We have found that the flp-1 gene, which encodes a group of structurally related neuropeptides, functions specifically to promote th e switch from the inactive to the active egg-laying state. Recessive mutati ons in flp-1 caused a significant increase in the duration of the inactive phase, Vet egg-laying within the active phase was normal. This pattern rese mbled that previously observed in mutants defective in the biosynthesis of serotonin, a neuromodulator implicated in induction of the active phase. Al though flp-1 mutants were sensitive to stimulation of egg-laying by seroton in, the magnitude of their serotonin response was abnormally low. Thus, the flp-1-encoded peptides and serotonin function most likely function in conc ert to facilitate the onset of the active egg-laying phase. Interestingly, we observed that flp-1 is necessary for animals to down-regulate their rate of egg-laying in the absence of food. Because flp-1 is known to be express ed in interneurons that are postsynaptic to a variety of chemosensory cells , the FLP-1 peptides may function to regulate the activity of the egg-layin g circuitry in response to sensory cues.