Learning the intensity of past stimulus, e.g. a concentration of a chemical substance, is a widely conserved function of the nervous system. However, how the memory is encoded in molecules and decoded through synaptic transmission is not comprehensively understood. C. elegans memorizes experienced NaCl concentration and migrates toward the learned concentration. The canonical signaling pathway underlying this behavior is the diacylglycerol(DAG)/protein kinase C(PKC) pathway, which functions in the NaCl-sensing neuron ASER. Previous reports observed DAG dynamics during changes in ambient NaCl concentration and revealed that DAG decreased with increasing NaCl and DAG increased in response to NaCl decrease. Subsequently, once activity of PKC-1 is changed, the direction of chemotaxis on the NaCl gradient is biased: when PKC-1 is activated, the worm migrates towards higher NaCl, whereas inactivation causes migration towards lower NaCl. As a consequence, worms migrate towards preferred concentration of NaCl. However, how PKC-1 generates migration bias is yet unknown. To identify the molecule that acts downstream of PKC-1, we performed nervous system-specific phosphoproteomic analysis, using a nonspecific biotinylation enzyme, TurboID. Among phospho-sites upregulated in
pkc-1(gf)-expressing worms, we found a phosphorylation of UNC-64/Syntaxin 1A at Ser65. The Ser65 phosphorylation-deficient mutant of
unc-64,
unc-64(S65A), showed a migration bias in the same direction as
pkc-1(lf).
unc-64(S65A) showed reduced release of glutamate following activation of ASER. Furthermore, we investigated how
unc-64(S65A), or a reduction in glutamate release, results in migration bias. As previous studies from our lab showed that the AIB interneurons respond to a change in NaCl concentration via the glutamatergic transmission from ASER, we focused on the response of AIB. Both in
unc-64(S65A) and
pkc-1(lf), AIB responses were reversed compared to wild type. However, glutamate release from ASER increased by activation of ASER in both wild type and
pkc-1(lf), suggesting changes in glutamate release alone cannot explain the reversed AIB response. This led us to examine whether the basal synaptic release, or basal glutamate level may affect the response of AIB. Indeed, direct exposure of AIB neurons to glutamate revealed that the response of AIB to increase of glutamate is reversed according to the glutamate concentration in the external bath solution. We now hypothesize that this is due to a difference of sensitivity between inhibitory and excitatory glutamate receptors.