Activating the peptidergic interneuron ALA by the epidermal growth factor LIN-3 causes quiescence of locomotion and feeding (Van Buskirk and Sternberg, 2007). While synaptic transmission is required for this effect, the neurotransmitter released by ALA to induce quiescence has been unknown. A proteomics study of the ALA neuron performed in the nematode Ascaris found peptides encoded by an Ascaris gene homologous to C. elegans
flp-13 (Jarecki et al, ACS Chem. Neurosci, 2010). We therefore hypothesized that
flp-13 is expressed in C. elegans ALA, and that
flp-13 derived neuropeptides promote behavioral quiescence induced by ALA. In support of this hypothesis, we found that 5 kb upstream of the
flp-13 start site drives expression in the ALA neuron; that over-expression of
flp-13 using a heat-shock promoter induces behavioral quiescence; that
flp-13 mutants are defective in feeding quiescence induced by EGF; and that this defect is rescued by restoring
flp-13 expression in ALA. To identify a receptor for FLP-13, we postulated that a gene encoding a FLP-13 receptor might show cyclical expression with the larval cycle, since behavioral quiescence occurs naturally during larval transitions. The gene
frpr-4, which encodes a predicted G-protein coupled neuropeptide receptor, fits this criterion. Transgenic multi-copy expression of
frpr-4 expressed under its endogenous regulatory elements induces spontaneous bouts of feeding and locomotion quiescence, which were reduced in the presence of a
flp-13 null mutation. Peptides encoded by
flp-13 potently activate FRPR-4 in a heterologous mammalian cell culture system. Based on this combination of in vivo and in vitro data, we conclude that FLP-13 neuropeptides released from the ALA neuron signal through FRPR-4 to regulate behavioral quiescence.