Serotonin (5-HT) stimulates both pharyngeal pumping and egg-laying in Caenorhabditis elegans. Four distinct 5-HT receptors have been partially characterized, but little is known about their function in vivo. SER-7 exhibits most sequence identity to the mammalian 5-HT7 receptors and couples to a stimulation of adenyl cyclase when expressed in COS-7 cells. However, many 5-HT7 specific agonists have low affinity for SER-7. 5-HT fails to stimulate pharyngeal pumping and the firing of the MC motorneurons in animals containing the putative
ser-7(
tm1325) and
ser-7(
tm1728) null alleles. In addition, although pumping on bacteria is up-regulated in
ser-7(
tm1325) animals, pumping is more irregular. A similar failure to maintain "fast pumping" on bacteria also was observed in
ser-1(
ok345) and
tph-1(
mg280) animals that contain putative null alleles of a 5-HT2-like receptor and tryptophan hydroxylase, respectively, suggesting that serotonergic signaling, although not essential for the up-regulation of pumping on bacteria, "fine tunes" the process. 5-HT also fails to stimulate egg-laying in
ser-7(
tm1325),
ser-1(
ok345) and
ser-7(
tm1325);
ser-1(
ok345) animals, but only the
ser-7;
ser-1 double mutants exhibit an Egl phenotype. All of the SER-7 mutant phenotypes are rescued by the expression of full length
ser-7::gfp translational fusions.
ser-7::gfp is expressed in several pharyngeal neurons, including the MC, M2, M3, M4 and M5, and in vulval muscle. Interestingly, 5-HT inhibits egg-laying and pharyngeal pumping in
ser-7 null mutants and the 5-HT inhibition of egg-laying, but not pumping is abolished in
ser-7(
tm1325);
ser-4(
ok512) double mutants. Taken together, these results suggest that SER-7 is essential for the 5-HT stimulation of both egg-laying and pharyngeal pumping, but that other signaling pathways can probably fulfill similar roles in vivo.