Wild-type worms chemotax to NaCl and various other water soluble attractants [11]. Worms previously starved on plates of NaCl for 4 h learn to avoid it on subsequent choice tests up to an hour later [12]. Learned aversion to NaCl can also occur in the presence of food, following repeated pairings with aversive stimuli (e.g. glycerol) [13]. Wen et al. [14] conducted a genetic screen and identified the first two C. elegans learning mutants, lrn-1 and lrn-2. Both mutants displayed deficits in attractive and aversive NaCl conditioning,

but the mutations have never been cloned [14]. Using reverse genetics, several molecules acting in multiple parallel pathways have GSK1120212 been implicated in the acquisition and retention of NaCl conditioning 13, 15, 16, 17, 18 and 19.

Hukema et al. [16] propose that ASE mediates naïve attraction to NaCl, but starvation training causes ASE to release a signal that sensitizes the ADF, ADL, ASI and ASH chemosensory neurons resulting in avoidance of previously attractive NaCl concentrations. Saeki et al. [12] reported that males were worse at learning to avoid NaCl paired with starvation than hermaphrodites. Based on this and the known differences in ILP functioning between males and hermaphrodites, Vellai et al. [20] investigated, and found a role for INS-1 in NaCl aversive conditioning. In a similar study Tomioka et al. [21•] found that INS-1

secreted from the AIA interneurons signaled through DAF-2 receptors on salt-sensing ASER to only modulate chemotaxis. Recently, a signaling Fulvestrant in vitro molecule similar in structure to mammalian vasopressin (VP) and oxytocin (OT) has also been implicated in NaCl-starvation learning. Through the use of in silico data mining Beets et al. [22••] identified two G-coupled protein receptors with amino acid residues necessary for vasopressin and oxytocin binding. The receptors, dubbed NTR-1 and NTR-2 were then expressed in host cells and challenged with 262 C. elegans peptides in order to identify an endogenous ligand. NTR-1 expressing cells responded to a single VP/OT like peptide, later named nematocin, NTC-1. NTR-2 did not respond to any of the peptides tested [22••]. GFP reporter constructs revealed expression of NTR-1 in neurons known to be involved in gustatory plasticity, including ASEL, ASH and ADF. Loss of nematocin or its receptor did not disrupt NaCl chemotaxis, but did impair the learned aversion. Cell-specific rescue experiments demonstrated that NTC-1 released from AVK interneurons act on NTR-1 in ASEL to modulate NaCl chemotaxis. Double mutants suggested that VP/OT like signaling interacts with molecules previously implicated in gustatory plasticity, including the Gγ subunit GPC-1, TRPV channel OSM-9, and dopamine and serotonin 13, 16 and 22••.