GluD1 is localized at cholinergic synapses and is an acetylcholine receptor

The GluD1 receptor has many unusual features including expression at both excitatory and inhibitory synapses and a ligand binding domain capable of binding both D-serine and GABA. We have previously demonstrated that striatal GluD1 is critical for the regulation of behavioral flexibility, a phenotype dependent on the cholinergic system. Here, we found that GluD1 is enriched postsynaptically at cholinergic synapses in the mouse and monkey dorsal striatum. Further, loss of GluD1 reduces the abundance of cholinergic terminals, excitatory responses at cholinergic synapses as well as muscarinic receptor-induced plasticity. In addition, optogenetic stimulation of cholinergic interneurons or puff-application of ACh, in the presence of cholinergic and AMPA/GABA_A receptor blockers, produced current responses in medium spiny neurons (MSNs) that were sensitive to the GluD1-channel blocker NASPM. These responses were absent in GluD1 KO and overexpression of GluD1 on KO background rescued Ach puff-induced currents suggesting potential conductance via GluD1. Finally, using GluD1-Cbln1 interaction assay as an indirect method to evaluate ligand binding interaction, we found that ACh can bind GluD1 and induce conformational changes. A similar ACh-induced conformational change was observed for GluD2 in the cell binding assay. Importantly, molecular dynamics simulations and mutagenesis analysis demonstrated that ACh binding orientation in the GluD1 ligand binding domain is different from D-serine and GABA. Overall, our results identified an unprecedented feature of GluD1 in the regulation of cholinergic synapses.