Member of the ionotropic glutamate receptor family, which plays a crucial role in synaptic organization and signal transduction in the central nervous system. Although it shares structural features with ionotropic glutamate receptors, does not bind glutamate as a primary ligand (PubMed:38060673). Instead, forms trans-synaptic adhesion complexes with presynaptic neurexins and cerebellins, regulating NMDA and AMPA receptor activity and influencing synaptic plasticity through signal transduction (By similarity). In the presence of neurexins and cerebellins, forms cation-selective channels that are proposed to be gated by glycine and D-serine (By similarity). However, recent research disputes this ligand-gated cation channel activity (PubMed:39052831). Cation-selective ion channel can be triggered by GRM1 in dopaminergic neurons (By similarity). Also acts as a receptor for GABA, modulating inhibitory synaptic plasticity through non- ionotropic mechanisms (PubMed:38060673). {ECO:0000250|UniProtKB:O43424, ECO:0000250|UniProtKB:Q61627, ECO:0000269|PubMed:38060673, ECO:0000269|PubMed:39052831}. This is the function of GRID1 (glutamate ionotropic receptor delta type subunit 1, Ensembl gene identifier ENSG00000182771).