Serotonin signals through postsynaptic Gα_q, Trio RhoGEF, and diacylglycerol to promoteC. elegansegg-laying circuit activity and behavior

Abstract

Activated Gαq signals through Phospholipase-Cβ (PLCβ) and Trio, a Rho GTPase exchange factor (RhoGEF), but how these two effector pathways promote synaptic transmission remains poorly understood. We used the egg-laying behavior circuit of C. elegans to determine whether PLCβ and Trio mediate serotonin and Gαq signaling through independent or related biochemical pathways. Using genetic rescue experiments, we find that PLCβ functions in neurons while Trio functions in both neurons and the postsynaptic vulval muscles. While Gαq, PLCβ, and Trio RhoGEF mutants all fail to lay eggs in response to serotonin, optogenetic stimulation of the serotonin releasing HSN command neurons restores egg laying only in PLCβ mutants. Vulval muscle Ca2+ activity remained in PLCβ mutants but was eliminated in strong Gαq and Trio RhoGEF mutants. Exogenous treatment with Phorbol esters that mimic Diacylglycerol (DAG), a product of PIP2 hydrolysis, rescued egg-laying circuit activity and behavior defects of Gαq signaling mutants, suggesting both Phospholipase C and Rho signaling promote synaptic transmission and egg-laying via DAG production. DAG has been proposed to activate effectors including UNC-13, however, we find that phorbol esters, but not serotonin, stimulate egg laying in unc-13 mutants. Together, these results show that serotonin signaling through Gαq and PLCβ modulates UNC-13 activity to promote neurotransmitter release. Serotonin also signals through Gαq, Trio RhoGEF, and an unidentified PMA-responsive effector to promote postsynaptic muscle excitability. Thus, the same neuromodulator serotonin can signal in distinct cells and effector pathways to activate a motor behavior circuit.