Major depression (MD), one of the most common mental disorders, is highly heritable and is commonly treated with selective serotonin reuptake inhibitors (SSRIs), which increase serotonin signaling in the brain. Yet, genome-wide association studies have failed to definitively implicate genetic defects in serotonin signaling as the cause of MD.1 We are studying the mechanism by which serotonin activates egg laying in C. elegans as a model for understanding the role of serotonin signaling in the activity of neural circuits. The C. elegans egg-laying system is well-suited for studying 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.2 Interestingly, while egg laying requires the serotonergic HSNs3 and can be hyperactivated with an SSRI4, mutations knocking out serotonin biosynthesis have only weak effects on egg laying. This puzzling result draws striking parallels with the role of serotonin signaling in MD. We found that the HSNs release a neuropeptide in addition to serotonin to stimulate egg laying. The
nlp-3 neuropeptide gene as well as the enzymes necessary for serotonin biosynthesis and release are all expressed in the HSNs. Knocking out either
nlp-3 or serotonin biosynthesis caused mild egg-laying defects, but knocking out both caused a strong egg-laying defect that phenocopies that of animals lacking HSNs. Further, we optogenetically stimulated the HSNs and observed that the resulting egg-laying behavior is profoundly dependent on
nlp-3, and is completely silenced in animals lacking both NLP-3 neuropeptides and serotonin. We propose that the HSN releases both serotonin and NLP-3 neuropeptides, which act cooperatively in a partially redundant fashion to stimulate egg laying, explaining the absence of strong egg-laying defects in mutants lacking serotonin. Serotonin receptors are found only on the muscles within this circuit, and sensitize them to contract in response to acetylcholine.2 We are currently analyzing the Ca2+ activity in the circuit to determine roles for NLP-3 neuropeptides in circuit activity. Co-release of serotonin and neuropeptides also occurs in the mammalian brain, and by analogy with the worm egg-laying circuit may be an important feature in explaining the absence of a strong genetic link between MD and serotonin. 1Flint and Kindler. Neuron. 2014. 2Collins, et al. eLife. 2016. 3Trent, et al. Genetics. 1983. 4Weinshenker, et al. J. Neurosci. 1995.