It remains unclear exactly how serotonin regulates neural circuit activity in the brain or how changes in serotonin signaling might cause mental disorders. The C. elegans egg-laying system is well-suited for studying the basics of how serotonin functions in a circuit. The serotonergic HSN motor neurons initiate ~2.5 minute egg-laying active phases that occur every ~20 minutes. The evidence for this is that rhythmic HSN Ca2+ activity measured using a GCaMP reporter always accompanies egg-laying active phases, and optogenetic activation of HSN neurons is sufficient to stimulate activity of the egg-laying circuit.We knocked out serotonin biosynthesis and showed that optogenetically stimulating the HSNs still caused animals to lay eggs. We hypothesized that the HSNs release a signal in addition to serotonin to stimulate egg laying. We found that neuropeptides encoded by the nlp-3 gene, which is known to be expressed in the HSNs, are such a signal. Knocking out either nlp-3 or serotonin biosynthesis alone caused mild egg-laying defects, but knocking out both caused a strong egg-laying defect that phenocopies that of animals lacking HSNs. Interestingly, co-release of serotonin and a neuropeptide also occurs in the mammalian brain, although the function of such co-release is unknown.Serotonin stimulates egg-laying through the G-protein coupled receptors (GPCRs) SER-1 and SER-7 that are expressed on the egg-laying muscles.1,2,4 Consistent with this previous work, we found that double-mutant worms lacking NLP-3 and either SER-1 or SER-7 show a strong synthetic egg-laying defect. NLP-3 neuropeptides were previously reported to act in an aversive response circuit through the GPCR NPR-17.3 Using a fosmid-based GFP reporter, we observed additional previously unreported expression of NPR-17 in cells of the egg-laying circuit.We found that animals lacking HSNs, serotonin, or NLP-3 neuropeptides, which all lay eggs less frequently than wild type, surprisingly all show increased activity of the egg-laying circuit as measured using GCaMP recordings in freely-behaving animals. We therefore hypothesize that co-release of serotonin and NLP-3 from the HSNs is not essential for activity of the circuit, but is required to coordinate muscle activity, allowing the animal to productively lay eggs.1Carnell et al. (2005) J. Neurosci. 25:10671-81; 2Hapiak et al. (2009) Genetics 181:153-63; 3Harris et al. (2010) J. Neurosci. 30:7889-99; 4Hobson et al. (2006) Genetics 172:159-69.

Affiliation:
- Dept. of MB&B, Yale University, New Haven, CT