Ionotropic glutamate receptors, which underlie a majority of excitatory synaptic transmission in the CNS, associate with transmembrane proteins that modify their intracellular trafficking and channel gating. Significant advances have been made in our understanding of AMPA-type glutamate receptor (AMPAR) regulation by transmembrane AMPAR regulatory proteins. Less is known about the functional influence of cornichons-unrelated AMPAR-interacting proteins, identified by proteomic analysis. Here we confirm that cornichon homologs 2 and 3 (CNIH-2 and CNIH-3), but not CNIH-1, slow the deactivation and desensitization of both GluA2-containing calcium-impermeable and GluA2-lacking calcium-permeable (CP) AMPARs expressed in tsA201 cells. CNIH-2 and -3 also enhanced the glutamate sensitivity, single-channel conductance, and calcium permeability of CP-AMPARs while decreasing their block by intracellular polyamines. We examined the potential effects of CNIHs on native AMPARs by recording from rat optic nerve oligodendrocyte precursor cells (OPCs), known to express a significant population of CP-AMPARs. These glial cells exhibited surface labeling with an anti-CNIH-2/3 antibody. Two features of their AMPAR-mediated currents-the relative efficacy of the partial agonist kainate (I(KA)/I(Glu) ratio 0.4) and a greater than fivefold potentiation of kainate responses by cyclothiazide-suggest AMPAR association with CNIHs. Additionally, overexpression of CNIH-3 in OPCs markedly slowed AMPAR desensitization. Together, our experiments support the view that CNIHs are capable of altering key properties of AMPARs and suggest that they may do so in glia.