Serotonin signals through postsynaptic Gαq, Trio RhoGEF, and diacylglycerol to promote Caenorhabditis elegans egg-laying circuit activity and behavior

Activated G alpha(q) signals through phospholipase-C beta and Trio, a Rho GTPase exchange factor (RhoGEF), but how these distinct effector pathways promote cellular responses to neurotransmitters like serotonin remains poorly understood. We used the egg-laying behavior circuit of Caenorhabditis elegans to determine whether phospholipase-C beta and Trio mediate serotonin and G alpha(q) signaling through independent or related biochemical pathways. Our genetic rescue experiments suggest that phospholipase-C beta functions in neurons while Trio Rho GTPase exchange factor functions in both neurons and the postsynaptic vulval muscles. While G alpha(q), phospholipase-C beta, and Trio Rho GTPase exchange factor mutants fail to lay eggs in response to serotonin, optogenetic stimulation of the serotonin-releasing HSN neurons restores egg laying only in phospholipase-C beta mutants. Phospholipase-C beta mutants showed vulval muscle Ca2+ transients while strong G alpha(q) and Trio Rho GTPase exchange factor mutants had little or no vulval muscle Ca2+ activity. Treatment with phorbol 12-myristate 13-acetate that mimics 1,2-diacylglycerol, a product of PIP2 hydrolysis, rescued egg-laying circuit activity and behavior defects of G alpha(q) signaling mutants, suggesting both phospholipase-C and Rho signaling promote synaptic transmission and egg laying via modulation of 1,2-diacylglycerol levels. 1,2-Diacylglycerol activates effectors including UNC-13; however, we find that phorbol esters, but not serotonin, stimulate egg laying in unc-13 and phospholipase-C beta mutants. These results support a model where serotonin signaling through G alpha(q), phospholipase-C beta, and UNC-13 promotes neurotransmitter release, and that serotonin also signals through G alpha(q), Trio Rho GTPase exchange factor, and an unidentified, phorbol 12-myristate 13-acetate-responsive effector to promote postsynaptic muscle excitability. Thus, the same neuromodulator serotonin can signal in distinct cells and effector pathways to coordinate activation of a motor behavior circuit.