5-HT regulates diverse central and peripheral responses in mammals. However, although all of the mammalian 5-HT receptors have been cloned and characterized, their integration into complex multi-level signaling pathways is only poorly understood. 5-HT also modulates most key behaviors in the nematode model, Caenorhabditis elegans. To characterize 5-HT receptor interactions in C. elegans, key 5-HT dependent phenotypes were examined in
ser-4(
ok512);
mod-1(
ok103);
ser-7(
tm1325)
ser-1(
ok345) animals that contained null alleles of the four previously identified 5-HT receptors. Surprisingly, these quadruple null mutants were viable and superficially normal, although they failed to respond to 5-HT in most locomotory, feeding, and egg-laying assays. In contrast, in the presence of bacteria, these mutant animals appeared to move normally and up-regulated pumping and egg-laying, suggesting the operation of redundant regulatory pathways. In fact,
ser-4;
mod-1 and
ser-7 ser-1 double mutants exhibited more obvious behavioral defects than
ser-4;
mod-1;
ser-7 ser-1 animals, emphasizing the delicate balance of serotonergic signaling that regulates these key processes. Multiple 5-HT receptors operate at multiple levels and can provide both excitatory and inhibitory serotonergic inputs into the same process. For example, SER-4/MOD-1 provide inhibitory serotonergic input into pharyngeal pumping, locomotion and egg-laying. In contrast, SER-7 and SER-1 facilitate pumping and egg-laying. Interestingly,
tph-1 null animals that lack detectable 5-HT exhibited more obvious phenotypes than the
ser-4;
mod-1;
ser-7 ser-1 animals. Whether this difference resulted from unidentified 5-HT receptors, the differential action of low endogenous levels of 5-HT or constitutive activity of the 5-HT receptors remains to be determined.