D-Serine disrupts Cbln1 and GluD1 interaction and affects Cbln1-dependent synaptic effects and nocifensive responses...

Ionotropic glutamate receptors (iGluRs) mediate fast excitatory neurotransmission in the nervous system. In addition to NMDA receptor co-agonists, D-serine is a ligand for glutamate delta receptors (GluDs) and interacts with the ligand-binding domain with low affinity. However, D-serine binding does not lead to typical ion channel currents in GluD1 or GluD2 but may contribute to synaptic plasticity. In the developing brain, D-serine binding to GluD2 facilitates long-term depression at parallel fiber-Purkinje cell synapses. However, the influence of D-serine on GluD1's interaction with its amino terminal domain synaptogenic ligand Cbln1 and its subsequent impact on synaptic function and behavior remains unexplored. Here, we found that D-serine inhibited the interaction between Cbln1 and GluD1 in an in vitro cell-binding assay. This effect was concentration-dependent, with an IC₅₀ value of ~ 300 µM. Furthermore, in ex vivo central amygdala (CeA) slices application of recombinant Cbln1 (rCbln1), consistent with its synaptogenic property, produced a robust increase in excitatory neurotransmission and GluD1 expression. This effect of rCbln1 was partially blocked by pre-treatment with D-serine. Finally, in behavioral experiments, we observed that the pro-nociceptive effect of intra-CeA injection of rCbln1 was inhibited by pre-treatment with D-serine. In addition, the antinociceptive effect of intra-CeA rCbln1 injection in an inflammatory pain model was blocked by D-serine. Overall, these results demonstrated that D-serine binding to GluD1 reduces its interaction with Cbln1, which may be relevant to synaptic plasticity and behavior.