Acetylcholine (ACh) receptors (AChR) regulate neural circuit activity in multiple contexts. In humans, mutations in ionotropic acetylcholine receptor (iAChR) genes can cause neurological disorders, including myasthenia gravis and epilepsy. In C. elegans, iAChRs play multiple roles in the locomotor circuit. The cholinergic motor neurons express an ACR-2-containing pentameric acetylcholine receptor (ACR-2R) comprised of ACR-2, ACR-3, ACR-12, UNC-38, and UNC-63 subunits. A gain-of-function mutation in the non-alpha subunit gene
acr-2 (
acr-2(gf)) causes defective locomotion as well as spontaneous convulsions. Previous studies of genetic suppressors of
acr-2(gf) have provided insights into ACR-2R composition and assembly. Here, to further understand how the ACR-2R regulates neuronal activity, we expanded the suppressor screen for
acr-2(gf)-induced convulsions. A majority of these suppressor mutations affect genes that play critical roles in synaptic transmission, including two novel mutations in the vesicular acetylcholine transporter
unc-17 In addition, we identified a role for a conserved major facilitator superfamily domain family protein
mfsd-6 in regulating neural circuit activity. We further defined a role for the sphingosine kinase,
sphk-1, in cholinergic neuron activity, independent of previously known signaling pathways. Overall, the genes identified in our study suggest that optimal modulation of synaptic activity is balanced by the differential activities of multiple pathways, and the novel alleles provide valuable reagents to further dissect neuronal mechanisms regulating the locomotor circuit.