Changes in signaling by the neurotransmitter serotonin may underlie depression in humans. To determine how serotonin signaling is modulated, we analyzed how C. elegans regulates function of the serotonergic HSN motor neurons that stimulate egg-laying behavior. Egg laying is inhibited by the G protein G<font face=symbol>a</font><sub>o</sub> (known as GOA-1 in C. elegans) and activated by the G protein G<font face=symbol>a</font><sub>q</sub> (known as EGL-30 in C. elegans). We found that G<font face=symbol>a</font><sub>o</sub> and G<font face=symbol>a</font><sub>q</sub> act directly in the HSN motor neurons to control egg laying. We used quantitative confocal microscopy to examine expression and localization of a variety of fluorescent reporter proteins in the HSNs to look for effects of the G proteins on HSN structure and function. The G protein mutants showed no detectable changes in the structure of the HSN, its synapses, or in localization of the synaptic vesicle priming protein UNC-13S, all of which have been suggested to be affected by the G proteins in other neurons. However, we found that the G proteins do act in the HSNs to have opposing effects on transcription of the tryptophan hydroxylase gene
tph-1, which encodes the rate-limiting enzyme for serotonin biosynthesis. Anti-serotonin staining confirmed that G<font face=symbol>a</font><sub>o</sub> and G<font face=symbol>a</font><sub>q</sub> antagonistically affect serotonin levels in the HSNs. Altering
tph-1 gene dosage showed that small changes in
tph-1 expression were sufficient to affect egg-laying behavior. Our results suggest that serotonin signaling is regulated at the level of serotonin biosynthesis. G<font face=symbol>a</font><sub>o</sub> and G<font face=symbol>a</font><sub>q</sub> are expresed in all neurons and have opposing effects on many behaviors besides egg laying. These two G proteins may generally have their opposing effects by antagonizing each other directly in the same neurons, as they do in the HSNs. Thus they may integrate information from multiple signals and set levels of neurotransmitter release, in part through antagonistic effects on transcription of neural genes.